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Stepper motor and driver selection

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Stepper motor and driver selection Powered By Docstoc
					Industrial Circuit Application Note

Stepper motor and driver selection
Stepper motors are used in many different           In data sheets for stepper motors,      57mm PM stepper is illustrated in
types of applications this makes it difficult    the pull-in and pull-out torque are        figure 1. In both cases the winding
to recommend a general step-by-step design       given, as functions of stepping rate,      and driver combination have been
flow chart. The design process is more an        for different types of motor and driver    designed to drive the maximum
iterative process, involving experience, cal-    combinations. The pull-in torque           current through the winding at stand
culation and experimentation. The purpose        curve shows the maximum friction           still without exceeding the maximum
of this application note is to show how          torque with which, the motor can           7-watt power dissipation for this type
system performance is affected by motor and      start, at different stepping rates,        of motor.
driver selection. Some popular motors and        without losing any step. In an actual         From the chart, we see that the
drivers are dealt with, as well as the im-       application, this curve has to be          output power of the motor can be
portance of the gearing between the motor        modified to account for the load           increased by a factor of six, through
and the load.                                    inertia.                                   the use of a bipolar constant current
                                                    The pull-out curve is of more inter-    driver, compared to the basic unipolar
                                                 est, because it shows the total avail-     L/R-driver. The increased output
Limits to system                                 able torque when the motor runs at         power is a function of both the
performance                                      constant speed at a given frequency.       increased over-all pull-out torque and
                                                 In an application, this torque is used     the increased stepping frequency
Torque and output power                          for overcoming the load fiction torque     range.
The output torque and power from a               and for accelerating the load and             As we can see from the figure, the
stepper motor are functions of the               motor inertia.                             maximum output power is available
motor size, motor heat sinking, work-               One problem when selecting the          at relatively high stepping rates,
ing duty cycle, motor winding, and               right motor type and size is the big       compared to the maximum pull-in
the type of driver used. In appli-               influence that the driver has on the       frequency, for this type of motor
cations with low damping, the usable             output torque and power. The               (approximately 150 to 400Hz, for
torque from the stepper motor can be             difference in output torque, power,        zero-load inertia, depending on driver
drastically reduced by resonances.               and system efficiency for a 7.5-degree     circuit). This fact, which is true for


                        Pull-out torque [mNm]                                              Output power [W]
                        80                                                                             6

                        70
                                                            4.8W
                        60                                                                             4,5

                        50

                        40                                                                             3

                        30

                        20                                                                             1,5

                        10
                                       0.8W
                          0                                                                           0
                              0            500            1000          1500        2000          2500
                                                     Full-step stepping rate [Hz]

                                         Torque PBL3770:                Output Power PBL3770:
                                         Torque L/R                     Output Power L/R

Figure 1. Pull-out torque and output power for a 57 mm PM stepper driven by a unipolar L/R-driver and a PBL 3770A bipolar
constant current driver.



                                                                                                                                 1
                                            most stepper applications, shows that,    bearing friction and magnetic losses.
    Pull-out torque [mNm]                   to be able to get a high-performance         Some driver and motor combina-
    Efficiency [%]     Output power [W]     stepper motor system, we have to use      tions have such low damping, at
    80                              6       ramping up/down when we start and         certain stepping rates, that they do
    70           Motor is no-load           stop the motor and load. The use of       not run without a high-damping load.
    60           unstable for               ramping opens up stepper motors for       This condition is known as no-load
                                      4.5
                 stepping rates                                                       instability.
    50           above 325Hz.
                                            power output applications, and does
                                       3
                                            not limit the usage of steppers to low-
    40
                                            performing low-output power system.       Resolution and positioning accuracy
    30                                                                                The resolution of a stepper motor
    20                                1.5   Damping and resonances                    system is affected by several factors—
    10                                      In applications with low system           the stepper motor full-step length,
           0.8W                                                                       the selected driver mode (full-step,
     0                                  0   damping, the available output torque
         0        500       1000     1500                                             half-step or microstepping), and the
                                            and power can be drastically reduced
          Full-step stepping rate [Hz]
                                            by resonance. Resonances in stepper       gear rate. This means that there are
                     Pull-out Torque                                                  several different combinations which
                     Efficiency
                                            motor systems can arise at low-, mid-,
                     Output Power           and high stepping rates. As a rule,       can be used to get the desired resolu-
                                            constant current drivers have the most    tion. Because of this, the resolution
                                            problems with resonances in the low-      problem of a stepper design can
Figure 2. Performance curves for a 100                                                normally be dealt with after the
                                            frequency region. These resonances
ohm unipolar 57 mm PM-motor driven          can often be eliminated by using half-    motor size and driver type have been
by a 20V L/R constant voltage driver.       stepping or microstepping. Constant       established.
                                            voltage drivers normally have problems
                                            with resonances at medium and/or          Design time
                                            high frequencies. At these frequen-       Even though customization of step
                                            cies, neither half- nor microstepping     motors is possible, it requires both
                                            can reduce the resonances. This limits    engineering time and time for manu-
                                            the usage of this type of drivers at      facturing stepper motor samples.
                                            medium and high frequencies to            Using a more-flexible driver circuit,
                                            driving high-damping loads.               like the chopper constant current
                                               Damping also depends on the            driver can make it possible to select a
                                            motor type—PM-motors have higher          standard motor with no performance
                                               damping than hybrids, due to slide     loss.


Table 1. Unipolar constant voltage driver attributes

Features                                     Drawbacks                                Applications
• Low electronic component cost.            • Lowest motor output power.              • Low speed and low power applica-
• For small motors very low cost            • Maximum power dissipation at              tions were the motor mainly is
  transistor arrays can be used.              stand still.                              used to produce a torque.
                                                                                      • Normally only used with small size
• Low electrical noise level.               • Higher motor cost and larger size
                                                                                        motors.
                                              for the same output power as from
                                              other drives.
                                            • Driver transistors have to withstand
                                              twice the maximum supply voltage.
                                            • Windings must be designed for the
                                              used supply voltage.
                                            • Regulated power supply normally
                                              required.
                                            • Holding torque depends on supply
                                              voltage and motor temperature.
                                            • Large torque ripple when driven in
                                              half-step mode.


2
Cost                                        Performance of drivers
In high-volume applications, the major      In the following section, the perfor-       Pull-out torque [mNm]
cost is the hardware—including              mance of some commonly-used driver          Efficiency [%]      Output power [W]
power supply, driver, wiring, motor,        configurations are compared when            80                               6
and gearing. In this case, the engine-      they drive a 57mm 7.5-degree PM-            70            Motor is no-load
ering cost is less important. In many                                                                 unstable for
                                            stepper motor. Driving voltage/             60            stepping rates     4.5
applications, it is possible to lower the   currents are selected so the stand-still    50            above 400Hz.
total system cost and increase the          motor losses are kept at maximum            40                               3
performance by using a more-complex         rated 7 W. The performance curves
driver (with a slightly higher cost) and                                                30
                                            show the pull-out torque, output
less-costly motor and power supply.                                                     20                                    1.5
                                            power (at the motor shaft), and the
   In low- and medium-volume appli-         system efficiency. Efficiency is defined    10           1.7W
cations, the engineering cost becomes       as the mechanical output power from          0                                    0
a larger part of the total cost. In this                                                     0           500      1000 1500
                                            the motor divided by the input power                 Full-step stepping rate [Hz]
case the flexibility and high integra-      to the driver. For each driver, features
tion of a constant current driver can       and drawbacks are also listed.                                  Pull-out Torque
help save engineering time and cost.                                                                        Efficiency
                                                                                                            Output Power
                                            Unipolar constant voltage
Dynamic characteristics                     This is the classic low-end driver. It
In applications were the stepper must                                                  Figure 3. Performance curves for a 100
                                            offers the lowest price for the driver
move from one position to another                                                      ohm unipolar 57mm PM-motor driven by
                                            electronics—only four transistors are      a 40V L/2R constant-voltage driver
then stop in the shortest possible          used. To drive small-sized motors, a
time, the settling time becomes a very                                                 (2 × 100ohm external series resistors).
                                            transistor array of ULN 2003 or
important factor. If the system is          similar type can be used. For mid-
designed properly, the settling time        sized motors, power darlington
can be kept to a minimum—if not,            transistors, or transistor arrays can be
the settling time can easily require        used. In figure 2, the performance of
several hundred milliseconds.               this type of driver is shown. A motor
   To get good dynamic behavior in          winding with 100 ohm phase
an open loop system, it is important        resistance has been selected. This
to have the correct gear rate and           gives good control of winding current
precise control of the motor running        and low losses in power transistors.
and holding torque. With well-              With this driver, the motor has
designed gearing, it is possible to         problems with no-load instabilities at
handle variations in both load inertia      stepping rates above 325Hz.
and friction.


 Table 2. Unipolar L/NR constant voltage driver attributes

 Features                                    Drawbacks                                  Applications
 • Low component cost                       • Low or very low efficiency. Lower        • Low and medium speed and low
                                              efficiency the higher Rext/R ratio.        power applications.
 • Low electrical noise level.
                                            • Problems with heat dissipation
                                              from the series resistors.
                                            • Maximum power dissipation at
                                              stand still increase by the L/nR
                                              ratio compared to the normal L/R
                                              driver.
                                            • Large torque ripple in half step
                                              mode.
                                            • Holding torque depends on supply
                                              voltage and winding temperature.




                                                                                                                                    3
                                               Unipolar L/nR constant voltage             above 400Hz. This limits the appli-
    Pull-out torque [mNm]                      This driver is similar to the unipolar     cations, at high frequencies, to
    Efficiency [%]      Output power [W]       constant voltage but has external          driving high-damping loads or to
    80                               6         series resistors in series with the        operating in ramp up/down applica-
                Motor is no-load
    70          unstable for                   motor windings. This driver can be         tions, were the motor does not run at
    60          stepping rates from 4.5        configured with different L/R ratios.      constant speed. It is possible to ramp
                525 to 850Hz.                  L/2R means that the total resistance is    through unstable frequencies, and use
    50
    40                                   3
                                               equal to two times the motor’s             the full pull-out torque (with normal
                       2.5W
                                               internal resistance. A higher L/R-ratio    safety margin) if the motor only runs
    30
                                               increases high-stepping-rate output        a limited number of steps in the
    20                                   1.5   torque, but reduces the system effici-     unstable range.
    10                                         ency. Figure 3 shows the performance
     0                                    0    of this driver in the L/2R-mode,           Unipolar timed Bi-level
         0          500     1000       1500    driving the same 100 ohm unipolar          This driver uses two voltage levels to
             Full-step stepping rate [Hz]
                                               motor.                                     increase motor utilization. At every
                      Pull-out torque             Compared to the L/R-driver we           step taken, the voltage across the
                      Efficiency               now gain higher output torque and          winding is raised, for a short time, to
                      Output Power             power. The maximum output power            a higher level compared to the
                                               has doubled, but the peak system effi-     nominal voltage used at stand still.
Figure 4. Performance curves for a 100
                                               ciency has decreased.                      During the remaining time, the
ohm unipolar 57mm PM-motor driven by
a 40/20V Bi-level constant-voltage driver         This drive also shows the no-load       nominal voltage is used. This driver
(High-voltage-on time = 4ms).                  instability, here for stepping rates       can also be configured in the “run/


Table 3. Unipolar timed bi-level driver attributes
Features                                       Drawbacks                                  Applications
• Medium electronic component                  • Timing circuit or extra CPU over-        • Low to medium speed and low to
  cost                                           head needed to control high voltage        medium power applications.
• Medium electrical noice level.                 on time.
                                               • 6 power transistors needed
                                                 compared to 4 for the standard and
                                                 L/nR unipolar drives.
                                               • If large high to low driving voltage
                                                 ratio is used the control off holding
                                                 torque and step accuracy becomes
                                                 difficult as a result of variations in
                                                 winding currents.
                                               • Holding torque depends on
                                                 winding temperature and supply
                                                 voltage.
Table 4. Unipolar constant current driver attributes
Features                                       Drawbacks                                  Applications
 .
• Nearly the same high speed torque            • Only 70% of holding and low speed        • High speed and medium power
   as bipolar chopper drive                      torque compared to bipolar constant        applications.
                                                 current.
• Uses 6 power transistors
  compared to 8 for bipolar                    • Power transistors have to withstand
  constant current.                              twice the maximum supply voltage.
• Half stepping without torque                 • Winding leakage inductance have
  ripple possible.                               to be considered when snubbing
                                                 circuit is designed.
                                               • 6 lead wires add cost and space for
                                                 motor connectors and flexible
                                                 cables.



4
stop” bi-level mode, were the high        efficiency is higher—and is not de-
voltage is used while the motor is        creased by losses in series resistors as     Pull-out torque [mNm]
stepped and the low voltage is used at    the ratio Uhigh/Unom is increased. This      Efficiency [%]     Output power [W]
stand still. This driver can also be      driver also has problems with no-load        80                                 6
combined with L/nR-series resistors       instability, but in this case only the       70            4.8W
to give higher flexibility in selecting   mid frequencies are affected. If used        60                                 4.5
stand-by holding torque. Ericsson’s       in a ramp up/down application, this          50
PBD 3517 is a fully-integrated, bi-       does not cause any problems, if the                                             3
                                                                                       40
level driver intended for use with        constant speed is selected in the stable
                                                                                       30
small-sized motors. In figure 4, the      area above 850Hz.
performance of the L/2R driver is                                                      20                                 1.5
shown while driving the same 100-         Unipolar constant current                    10
ohm unipolar PM stepper. The torque       This driver gives the best performance        0                                  0
                                                                                            0            1250          2500
curve for a given motor is a function     of the unipolar drives—but it is lower             Full-step stepping rate [Hz]
of both the high-voltage level and the    than for the bipolar chopper driver.                          Pull-out Torque
high-voltage-on time. In this example     The efficiency is reduced as a result of                      Efficiency
the high voltage is 40V (2 times the      higher resistive losses caused by using                       Output Power
nominal voltage) and the high voltage     only half of the windings at a time.
on time is 4ms. Compared to the           At higher frequencies, power losses        Figure 5. Performance curves for a 3.75
original L/R-driver, the maximum          caused by leakage inductance and           ohm bipolar 57mm PM-motor driven by
output power is three times higher.       snubbing circuits also appear.             PBL 3770A constant-current driver
Compared to the L/nR-driver, the                                                     (Chopper voltage 20V, winding current
                                                                                     960mA).
Table 5. Bipolar constant current driver attributes
Features                                  Drawbacks                                  Applications
• Maximum motor utilisation and           • 8 power transistors needed to drive      • For small and medium size motors
  high efficiency.                          a motor.                                   highly integrated drivers are
                                                                                       available.
• Maximum torque at low and high          • Problems with electrical noise and
  stepping rates.                           interference can occur.                  • High speed and high power
                                                                                       applications.
• Low losses stand bye mode               • Power losses in current sensing
  possible.                                 resistors.
• 8-lead motors can be configured
  for 3 different operating currents.
• No snubbing circuits required and
  current turn off can be selected for
  fast (return to power supply) or
  slow.
• Highly integrated drivers avail-
  able, second sourced drives also
  available.

Table 6. Bipolar constant current microstepping driver attributes
Features                                  Drawbacks                                  Applications
• Same as for the bipolar constant        • Same as for the bipolar constant         • For small and medium size motors
  current, plus:                            current, plus:                             highly integrated drivers are
                                                                                       available.
• Resonance free movement on low          • Higher cost for the current control
  step rates.                               electronics than for normal bipolar      • High speed and high power
                                            drive.                                     applications.
• Increased stop position resolution.
                                                                                     • Applications were increased
                                                                                       resolution is required.
                                                                                     • Applications were resonance free
                                                                                       low speed characteristics is needed.


                                                                                                                                5
Bipolar constant current                   driver, but extra electronics for           Snubbing and current turn off circuits
The highest output power and motor         setting the sine/cosine current levels      To assure trouble-free functioning of
utilization for a given motor is           are used. Microstepping can be used         all unipolar drives, especially when
achieved with the bipolar constant         with different microstep lengths. A         larger size motors are used, the
current driver. DC-losses is kept at a     shorter step length than 1⁄32 of a full-    winding and current turn-off circuit
minimum due to maximum utiliza-            step normally does not make any             has to be properly-designed.
tion of the copper in the winding and      further improvement in the motor’s             It is important that a unipolar
no power losses from leakage induc-        motion. With most microstepping             winding is bifilar wound—this means
tance and snubbing circuits since          controllers, is it also possible to run     the two wires that build up the coil
every winding only consists of one         the normal full- and half-step modes.       on each motor pole are wounded in
part.                                         Microstepping can also increase          parallel. This way, the leakage induc-
   In figure 5, the performance for        motor resolution and step accuracy.         tance is kept to a minimum, even
this type of driver is shown driving                                                   though the energy stored in the
the same type 57mm PM-motor. Here                                                      winding has to be taken care of (or
a motor with a constant-current-           General driver aspects                      moved to the other winding half)
adapted winding resistance of 3.75                                                     when the current is turned off. This is
ohms has been selected. The winding        Power supply design                         done by a current-turn-off circuit or a
current is selected to give the same       For all drivers of constant-voltage         snubbing circuit. If the current-turn-
resistive losses in the winding at stand   type, regulated power supplies are          off circuit works on the principle of
still as for the unipolar drives tested    normally required. This means that          current commutation from one
above. From the chart, we can see the      the over-all system efficiency will         winding half to the other, the energy
increase of output power, maximum          decrease further, compared to the           stored in the leakage inductance is
stepping rate, and system efficiency.      values shown in the figures above, due      handled by a snubbing circuit.
Due to the better utilization of the       to losses in the power supply. This            In the case of bipolar drive, separate
winding, the holding torque is also        will increase transformer cost and          snubbering circuits are never needed,
raised.                                    heating problems. If unregulated            since the windings only consist of one
   The no-load instabilities in the        supplies are used, large variations of      part each and no leakage inductance
mid- and high-stepping rate regions        holding and running torques occurs,         can occur. The current-turn-off circuit
are no longer present. This increases      thus making stop-time minimizing            is of four diodes in opposition to the
the flexibility in selecting constant-     more difficult or impossible. An            four power transistors in the H-
speed running frequencies. However,        unregulated power supply for a              bridge.
a resonance at 100Hz is present. In a      constant voltage driver also affects the
ramp up/down application, this does        motor power dissipation making good         Hysteresis losses in motors
not cause any problems as long as this     motor utilization impossible.               With some low-inductance motors,
frequency is not used as constant             For a constant-current driver, it is     chopper-type drivers can generate
speed frequency.                           normally possible to use an unregu-         increased iron losses, caused by the
   During the last 10 years, progress      lated supply voltage. The motor             winding current ripple. To minimize
in IC-technology has made it possible      current, and thereby also holding           this problem, use a high chopping
to develop fully-integrated bipolar        torque and power dissipation, is            frequency and do not use a lower in-
constant-current drivers, making this      controlled by the driver itself. The        ductance than needed to get maxi-
type of driver cost-effective for          pull-out torque at high stepping rates      mum-required step rate—it is also
driving small- and medium-sized            is affected by the supply voltage but       possible to use a lower chopping
motors.                                    at low step rates, its influence is         voltage. In most applications, the
                                           small.                                      hysteresis loss related to the chopping
Bipolar constant current microstepping        It is difficult to calculate the power   current ripple is low compared to the
This is an improved version of the         consumption for a particular applica-       hysteresis loss related to the stepping
basic full- and half- stepping bipolar     tion. The best way to get this              current changes. If chopping current
constant-current driver. Here, the         information is to make a prototype          ripple is kept at or below 10% of the
winding currents form a sine/cosine        and measure the driver input current        nominal current, this normally
pair. This greatly improves low-           under different driving conditions.         doesn’t cause a problem.
frequency stepping by eliminating          Remember that the power
overshot movements, ringing, and           consumption depends on input                Interference problems
resonances. Performance at medium-         voltage, current levels (if constant        For all chopper-type drives, the in-
and high-stepping rates are close to       current mode), load, motor                  creased risk of different interference
that of full- and half-stepping.           temperature, duty cycle and so on.          problems has to be considered. Sep-
   This driver uses the same power                                                     arate and wide grounding lines, as
stage as the bipolar constant-current                                                  well as physical separation from sen-



6
sitive electronics on the PCB, can help      The 57mm PM-motor is. for in-                          is plotted as a function of the
to avoid interference. Stepper lead       stance, suitable to use as paper feed                     stepping frequency. This motor is
wires should also be separated from       and carriage drive motor in medium-                       rated at 7 watts maximum power
sensitive signal wires to reduce          performance matrix or daisy printers                      dissipation. The chart shows the
capacitive and inductive coupling. In a   and in typewriters. Other applications                    power dissipation of the motor and
chopper application the capacitive        are fax machines, sewing machines,                        driver together. At low step rates
coupling of the chopped voltage (this     valve controls, and plotters.                             about a 3W-loss in the two PBL
is a square wave signal with the             Other popular PM-motor sizes are                       3770A circuits can be expected, as
amplitude equal to the supply voltage     35mm and 42mm. 20mm, 25mm and                             well as an additional 1W in the
and the frequency equal to the            63mm motors are also common PM                            current sensing resistors and approx-
chopping frequency) present at the        motor sizes. The 20mm motor is pop-                       imately 1W in the external diodes. At
motor lead wires can cause serious        ular as a head driver in 31⁄2" floppy-                    higher stepping rates, the driver losses
problems if not handled.                  disk drive applications. Commonly-                        decrease as the winding current de-
                                          available full-step angles are 7.5 and                    creases and the switching stops. At
                                          15 degrees but others are also avail-                     low step rates this gives a 7-watt loss
Performance of motors                     able (9, 11.25, and 18 degrees, for                       in the motor. At higher step rates, the
The maximum output torque and             examples).                                                total loss decreases indicating the
power from a stepper motor is limited        In figure 6, the performance of this                   ability to get a higher output power
by the power losses of the motor. For     motor is shown again. The power loss                      without exceeding the maximum
low stepping rates, most of the losses
are related to resistive losses in the
motor winding. At higher stepping           Pull-out torque [mNm]                                                            Output power [W]
rates the hysteresis and eddy-current       Efficiency [%]                                                                   Power losses [W]
                                                                    12W
losses become the major ones.               120                                                                                          12
Especially for low-cost tin-can PM-         100                                                                                          10
steppers, these losses can be high—           80                                                                                           8
because of the absence of laminations
and the use of low performing                 60                             4.8W
                                                                                                                                           6
magnetic materials of the stator and          40                                                                                           4
                                                                                                          37%
rotor flow path.                              20                                                                                           2
   From the above driver comparisons,                                                                                                    0
                                                0
we can see that the maximum torque,                 0           500                 1000              1500           2000            2500
efficiency, and output power from a                                               Full-step stepping rate [Hz]
                                                           Pull-out Torque         Efficiency     Output Power       Power losses
given motor is achieved with the bi-
polar chopper driver. We will now
examine the performance of some           Figure 6. Performance curves for a 3.75 ohm bipolar 57 mm PM-motor driven by PBL
commonly-used stepper motor types         3770A constant-current driver. Power losses in motor and driver are also shown
when they are driven with a bipolar       (Chopper voltage 20 V, winding current 960 mA).
chopper drive.
   A drop in performance, similar that
of the 57mm PM-motor used above,
can be expected when other types of         Pull-out torque [mNm]                                                           Output power [W]
drivers are used.                           Efficiency [%]                                                                  Power losses [W]
                                            120                                                                                          12

57mm PM motor                               100                                                                                          10
PM-motors are a cost-effective alter-        80                              7.3W                                                         8
native in many low- and medium-
                                             60                                               65%
                                                                                                                                          6
performing applications. The motors
uses slide bearings and a simple             40                                                                                           4
mechanical design to keep cost low.          20                                                                                           2
Compared to hybrid motors, the life                                                                                                      0
                                              0
expectancy is shorter, step accuracy              0           1000                 2000               3000           4000            5000
and efficiency is lower. The slide                                                Full-step stepping rate [Hz]

bearing can also cause problems if a                            Pull-out Torque        Efficiency     Output Power   Power losses

belt drive is applied directly to the
motor shaft.                              Figure 7. Performance curves for a 25 ohm bipolar 42 mm square hybrid stepper driven
                                          by PBL3770A constant-current driver (Chopper voltage 40V, winding current 280mA)



                                                                                                                                                7
allowed motor losses of 7W. If a lower           main feature of this type of motor,            stand still. This motor type is rated
duty cycle or better heat sinking is             compared to the 57mm PM-motor, is              for 4 to 6W losses depending on
applied to this motor a peak output              higher efficiency and step accuracy. In        manufacturer. Compared to the
power of at least 10W can be                     many applications, the ball-bearings           57mm PM motor in figure 6, nearly
achieved.                                        offer higher life expectancy and make          doubled system efficiency is the most
   PM-motors have one advantage                  the design of the gearing and                  interesting difference. From the power
over hybrid motors, they have a                  mechanics easier. This type of stepper         losses curve, we see that at higher
higher internal damping and offer, in            became very popular some years ago             stepping rates the losses decrease.
some applications, a more-noise-free             as head driver for 51⁄4" floppy and            This indicates that an even-higher
operation than the hybrid motors.                hard disk drives. It is suitable as a          high-frequency performance can be
                                                 carriage driver for printers and               achieved with a higher chopping
42mm square motor                                plotters, and for driving the print            voltage or with a lower-inductance
This motor is normally manufactures              wheel in typewriters and daisy wheel           winding.
with 3.6-, 1.8- and 0.9-degree step              printers. It is also a competitor to the
angle. Step accuracy is ±3% to ±7%               small-sized PM-motors, if the appli-           57mm (size 23) hybrid motor
of a full-step. The motor uses ball-             cation requires higher efficiency or           This type of hybrid stepper motor is
bearings to maintain the very small              ball-bearings                                  normally available with a 1.8- and
air-gap required for high efficiency.               Figure 7 shows the performance of           0.9-degree step angle and in a number
   This type of stepper motor is avail-          a 25-ohm bipolar 3.6-degree 42mm               of different lengths from 40mm to
able from many manufacturers at a                square motor driven by a constant-             100mm. This motor is more expen-
reasonable price, but the price is               current driver. The current level is           sive than the two other types des-
higher than the PM-motors. The                   selected to give 4W resistive losses at        cribed above. On the other hand, a
                                                                                                much higher torque and output power
                                                                                                is available.
                                                                                                   The performance of this motor type
    Pull-out torque [mNm]                                                    Output power [W]   is plotted in figure 10. A motor with
    Efficiency [%]                                             31W           Power losses [W]
    160                                                                                 32      5-degree step angle, 2.8-ohm bipolar
    140                                                                                 28      winding and with 42mm length has
    120                                                                                 24      been selected. This is the smallest
    100                                                                                 20      motor size of this class. The 5-degrees
     80                        66%
                                                                                        16      step angle is interesting when high
                                                                65%
     60                                                                                 12      shaft speed is more important than
     40                                                                                  8      high holding torque.
     20                                                                                  4         The diagram shows the same high-
      0                                                                                  0      efficiency as for the 42mm square
        0             1000          2000           3000               4000          5000
                                Full-step stepping rate [Hz]
                                                                                                motor, but four-times-higher output
            Pull-out torque    Efficiency        Output Power            Power losses
                                                                                                power. A maximum of over 30W is
                                                                                                achieved in the area of 3000 to
                                                                                                3500Hz. At high step rates, the
Figure 8. Performance curves for a 2.8 ohm bipolar 57 mm hybrid stepper (length 42mm)           power losses of the motor is approx.
driven by PBL 3770A constant-current driver (Chopper voltage 40V, winding current               12W (including driver 16W). This is
75mA).                                                                                          acceptable with normal cooling of the
                                                                                                motor and 100% duty cycle. At low
                                                                                                step rates, the losses decrease and at
 Holding torque [mNm]                                                         DC-losses [W]
 450                                                                                     18     standstill the losses are only 3W. This
              Hybrid holding torque                                                             shows the ability to increase the
              Hybrid DC-losses                                                                  motor current to get even higher
 300          PM holding torque                                                           12
                                                           284mNm                               output torque and power at low step
              PM DC-losses
                                                                                                rates. On the other hand, decreasing
    150                                                                                     6
                                                                                                the low frequency torque can be a way
                              84mNm                                                             of decreasing noise levels and vibra-
                                                                                                tions in applications where the load
      0                                                                                     0   friction torque consumes a larger part
      0        200      400   600      800 1000 1200                  1400   1600    1800
                                      Winding current                                           of the motor torque than the load
                                                                                                inertia.
Figure 9. Holding torque and DC-loss as functions of winding currents for a 57mm PM                This motor is suitable for paper
motor and for a 57mm hybrid motor.                                                              handling and carriage driving in high-



8
performance printers and plotters, or      beyond the maximum rating without                     are the characteristics of the load?”
industrial motion control. The 5-          causing too much saturation effect. In                Too often, this question is given too
degree stepper, with per-formance          figure 8, the torque from this motor                  little consideration. To get the best
shown in figure 8, is suit-able for        shows a relativly-flat torque character-              performance, it is important to do an
driving the print mechanism of laser       istic for stepping rates below 3kHz.                  analysis before selecting motor and
printers. PBL3770A is a suit-able          This is a result of the 750mA current                 driver and before designing the
driver for this size of stepper motor.     level not using the full low-speed                    transmission and mechanical system.
                                           capabilities of this motor. Increased
Power losses and holding torque            current will raise the output torque at               Friction or inertia loads
The limiting factor in high-perform-       low speeds and make the region with                   If the system will have high dynamic
ance stepper motor designs is the          maximum output power wider (to-                       performance, (high acceleration/
stepper power dissipation.                 wards low frequencies), but it will                   retardation), then most of the output
   Stepper motor manufacturers often       only increase the peak output power                   torque from the motor will be used to
specify the stepper motor windings by      marginally.                                           accelerate the system’s inertia. To get
the maximum-allowed power disipa-                                                                the maximum performance from this
tion at stand still. This gives the                                                              type of system, the gear rate should
nominal winding voltage and current        Designing a system                                    normally be designed so that the load
levels. In an application, the optimum                                                           inertia seen by the motor is close to
performance often is achieved at           Analyzing the load                                    the motor internal inertia. The load
different voltage and current levels. In   When designing a stepper motor sys-                   inertia seen by the motor is:
figure 9, the holding torques of the       tem, the first question to ask is “What
57mm PM and 57mm hybrid motors,
described above, are plotted as func-
tions of the the 2-phase-on current, as      Pull-out torque [mNm]                                                        Output power [W]
                                             Efficiency [%]                                                               Power losses [W]
are the resistive power losses in the        120                                                                                       12
windings.                                                                                                                              10
                                             100
   From the diagram, we can see that                                                  8.4W

for the PM-motor, the holding torque          80                                                                                        8
curve shows a knee at 600mA—indi-             60                                                                                        6
cating that magnetic saturation starts                                                         4.3W
                                              40                                                                                        4
to occur at this current level, even                                                              34%
though the resistive losses in the            20                                                                                        2
winding is only 3W, compared to the            0                                                                                      0
                                                0              500               1000              1500           2000            2500
specified 7W. This indicates that                                                Full-step stepping rate [Hz]
using the specified current level of                         Pull-out Torque     Efficiency     Output Power    Power losses
960mA does not give the optimum
performance on low stepping rates.         Figure 10. Performance curves for a 3.75ohm bipolar 57mm PM-motor driven by
   Figure 10 shows the affect on           PBL3770A constant-current driver. (Chopper voltage 20V, winding current 480mA).
motor and drivier performance when
the winding current is decreased to
480mA. (50% of the value used in            Pull-out torque [mNm]                                                        Output power [W]
figure 6.) Comparing figure 6 and 10        Efficiency [%]
                                                                                               12.1W
                                                                                                                         Power losses [W]
shows the improved low-frequency             120                                                                                     12
performance. Low-speed losses are            100                                                                                     10
decreased to less than 50% and low-           80                                                                                        8
speed torque only dropps to 80%. In                                                            5.5W
the high-stepping-rate region, only a         60                                                                                        6
small loss in torque apears. In figure        40                                                        34%                             4
13 another combination of driving             20                                                                                        2
current and voltage is used to increase
                                                0                                                                                       0
the output power to 5.5W with the                0             500               1000             1500           2000            2500
same maximum losses as in figure 6.                                            Full-step stepping rate [Hz]

Now the losses occur where they are                          Pull-out Torque     Efficiency     Output Power    Power losses

more motivated at the stepping rate
where the maximum output power
appears.                                   Figure 11. Performance curves for a 3.75ohm bipolar 57mm PM-motor driven by
   For the hybrid motor, we see that       PBL3770A constant-current driver. (Chopper voltage 25V, winding current 600mA).
the winding currents can be increased


                                                                                                                                             9
Jlm = Jl ÷ Gr2 where:                       However, in some applications, an          compact mechanical design easier. A
                                            increased inertia can be used to move      smaller motor can also, if the motor is
Jl = load inertia without gearing
                                            a resonance to a lower frequency.          in a moving part of the mechanism,
Gr = the gear rate.                                                                    make the design of the motion system
                                            Selecting concept                          easier.
A friction torque is reduced by the
                                            After analyzing the load, we know the         In applications where long life ex-
factor 1⁄Gr by a gear mechanism.
                                            output power needed, the maximum           pectancy is needed, motors with ball
Friction torque/load power consumption      and minimum stepping rates, and the        bearings are required. Hybrid motors
To select the right motor size and          resolution needed.                         use ball-bearings as a standard (to
driver type, it is necessary to calculate      Depending on the importance of          maintain the narrow air-gap), but
or measure the load friction torque.        the different demands and the ability      small-sized PM motors usually use
For most type of loads, this is fairly      to fulfill them, the designer has a        slide bearings. PM motors, with ball
constant at different speeds, which         range of options in combining motor        bearing as an option, are supplied by
makes measuring easy. If the system         gearing and driver in a system.            some manufacturers—but the addi-
involves a linear motion, a spring             The design is normally an iterative     tional cost for this is rather high.
scale can be used—and for a rotating        process, with calculation and experi-         If the motor drives a belt gearing or
system, a torque watch can be used.         mentation. If highest-performance or       a belt transmission directly, ball
From the measured force or torque,          lowest-cost for a given performance is     bearings are strongly recommended.
and information about the maximum           essential, it is a good idea to compare    This ensures proper lifetime and re-
speed of the motion, the maximum-           a few different combinations of motor      duces torque loss due to bearing
needed load power can be calculated:        driver and gearing.                        friction caused by the belt tension.
                                               A higher-step-rate driver and a
P[W] =v[m/s] × F[N]                                                                    Cost
                                            smaller motor, together with a
for linear systems and                                                                 The motor cost depends on motor
                                            suitable gearing, often gives better
P[W] = ω[radians/s] × T[Nm]                                                            type and size. Winding type and
                                            performance —in efficiency and
for rotating systems.                                                                  resistance do not affect the cost. As a
                                            output power—than a large motor
   Another way of estimating the load       driving the load directly.                 rule, hybrid motors are more expen-
power consumption is to replace the                                                    sive than PM-motors. The motor cost
motor or motor and gearbox with a                                                      normally increases with motor size.
DC-motor with known current-to-             Motor selection                            Another factor that influences the
torque function and drive the motor                                                    motor cost is the production volumes
at the desired speed while measuring        Output power                               of a certain motor and the number of
the current consumption. If this            This is the most important design          manufacturers of that motor. This
technique is used, it is possible to        criteria in getting the best price/        means that many times a “popular”
measure the power consumption at            performance of a stepper motor             type and size motor is the best choice
different speeds.                           system. Compare the power require-         even if the motor output power is a
                                            ments of the load with the data given      little higher than required.
Damping                                     above, or with the data in the manu-
As noted earlier, the usable torque         facturer’s data sheet. If the manu-        Customizing the motor
from a stepper motor can decrease at        facturer’s data sheet is used be aware     In medium- and high-volume appli-
certain stepping rates due to reson-        of the big differences in performance      cations, it is possible to customize the
ances. At which step rates, and to          of the stepper motors due to different     motor. Most manufacturers offers
what extent, this torque reduction          drivers. Also remember that measur-        customization on the following items.
appears depends on the application          ing stepper motor pull-out and pull-
damping and inertia. The damping of                                                    Shaft       Single- or double-sided
                                            in torque is tricky. The measurement
the driver also influence the torque                                                               Length
                                            is easily influenced by inertia and
reduction.                                                                                         Pinions
                                            resonances in the measuring system,
   Resonances at low stepping rates         and the inertia and damping of the         Winding Resistance
can normally be reduced by lowering         application is normally different. As a            Inductance
driver current and voltage levels, or       result, the pull-out curves in the data
by selecting half- or microstepping                                                    Rotor       Type of magnets
                                            sheet are not always valid for an actual               hybrid air-gap distance
mode drivers. At medium step rates          application.
the constant-current drivers normally                                                  Lead wires Length
have the least problems with reson-         Mechanical aspects                                    Connector
ances, but here the characteristics of      The physical dimension and weight of
the load have large impact.                 the motor are important criteria when        From some manufacturers, other
   Low system inertia normally creates      a motor is selected. Often the choice      parameters such as shaft diameter,
fewer problems with resonances.             of a smaller motor can make a              bearing types, mounting flange can be


10
customized but this is normally appli-
cable only in high-volume applica-          Pull-out torque [mNm]                                                           Output power [W]
tions.                                      Efficiency [%]                                                                  Power losses [W]
                                            120                                                                                          12
                                            100                                                                                          10
Driver design                                80                                        7.1W                                                  8

Selecting driver type                        60                                                                                              6
                                                                                       3.8W
The performance curves at the begin-         40                                                                                              4
ning show the effect of the driver on                                                          36%
                                             20                                                                                              2
the system. If only low stepping rates
                                              0                                                                                              0
are used and the use of gearing is not         0              500              1000              1500               2000           2500
a solution, the unipolar L/R-driver                                    Equivalent full-step stepping rate [Hz]
offers the lowest cost for the electro-                  Pull-out Torque     Efficiency       Output Power        Power losses
nics for a given output torque.
   As demand for output power from        Figure 12. Performance curves for a 3.75ohm bipolar 57mm PM-motor driven by
the stepper increases, more-effective     PBL3770A constant-current driver. (Half-step mode fast current decay. Chopper voltage
drivers offer the best price perfor-      20V, 2-phase-on current 480mA).
mance ratio. The best motor utiliza-
tion is achieved with the bipolar
constant current driver and this driver     Pull-out torque [mNm]                                                       Output power [W]
is the obvious choice for all high-         120                                                                                       12
power applications.                                                                           Pull-out 40V             Output 40V
   For applications in the low- and         100                                                                                       10
                                                                                              Pull-out 30V             Output 30V
medium-power range, several alter-            80                                7.3W                                                     8
natives exists. If system efficiency is
important, then the bipolar constant          60                                                                                         6
current driver is the best choice. This                             5.3W
                                              40                                                                                         4
driver offers higher flexibility in
selecting the motor winding, since            20                                                                                         2
both the chopper voltage and the               0                                                                                         0
current in the winding can be                   0            1000              2000           3000                4000            5000
changed to get the desired pull-out                                         Full-step stepping rate [Hz]
torque curve from the motor. Power-
supply design gets easier and power-      Figure 13. Performance as a function of chopper voltage for a 25ohm bipolar 42mm
supply losses decrease since regulated    square hybrid stepper driven by PBL3770A constant-current driver (Chopper
supply normally is not needed for         voltage 40/30V, winding current 280mA).
constant current drivers.
   If minimum cost for the driver
electronics is the most important
design criteria, rather than the over      Pull-out torque [mNm]                                                         Power losses [W]
all system performance, then the           120                                                                                         12
different unipolar driver can be the       100                                                               Pull-out 280mA              10
best choice.                                                                                                 Pull-out 200mA
                                             80                                                              Losses 280mA                 8
Selecting driver mode                                                                                        Losses 200mA
FULL-STEP MODE: This is the basic            60                                                                                           6
stepper driving mode, it offers the          40                                                                                           4
simplest control electronics and it is
recommended for high- and medium-            20                                                                                           2
frequency operation. At these frequen-        0                                                                                              0
cies, the inertia of the motor and the            0           1000             2000            3000                 4000           5000
load smooth out the torque, resulting                                          Full-step stepping rate [Hz]
in less vibration and noise compared
to low-speed operation.                   Figure 14. Performance as a function of winding current for a 25ohm bipolar 42mm
                                          square hybrid stepper driven by PBL3770A constant-current driver (Chopper voltage
                                          40V, winding current 280/200mA).



                                                                                                                                              11
HALF-STEP MODE:    Half stepping with     force) of the winding The optimum         Selecting the current level
140% 1-phase-on current gives             motor performance efficiency and out-     In a constant-current driver the
smoother movement at low step rates       put power is achieved close to the step   driver-current level mainly affects the
compared to full stepping and can be      frequency where the EMF peak value        torque at the low frequencies.
used to lower resonances at low           is equal to the driving voltage (chopp-   Depending on the load torque de-
speeds. Half stepping also doubles the    er voltage in the case of constant        mand (friction and inertia) as a func-
system resolution. Observe that for       current drive). As an example, the        tion of stepping rate, it is often a
most steppers, the step accuracy speci-   42mm square motor, with perform-          good idea to reduce the current level
fication only is valid for 2-phase-on     ance as shown in figure 7, has an EMF     to get a more-flat torque character-
positions. The accuracy is lower and      constant of 20mV/Hz (full-step            istics from the motor. This normally
the stop-position hysteresis is larger    frequency) With a 40-volt chopping        decrease resonances and power losses
for 1-phase-on positions.                 voltage, this gives a optimum stepp-      and allows a lower-rated driver
   Figure 12 shows the effects on per-    ing rate of 2kHz. From figure 7, we       circuit.
formance of the 57mm PM-motor             see that at 2kHz both the efficiency         In Figure 14, the effect of decreased
when half stepping is applied to this     and the output power are at their         winding current is shown—from the
motor. Compared to to full stepping       maximum values. To design a wind-         curve we can see that only the low
(reffer to figure 10 for the same         ing for 20 volts, with a maximum          and medium frequencies are affected
driving conditions), a slightly-higher    output at the same stepping rate, a       by the lower current. Power losses at
torque at low speed and a small de-       winding with 10mV/Hz EMF                  low step rates have also decreased.
crease at higher step rates. The main     constant should be used. This winding     The peak output power, however, is
advantage is the lowered noise and        will have half the number of turns and    not affected as the torque at 2kHz is
vibrations at low stepping rates. If      thus 1⁄4 of the resistance and induc-     not decreased.
maximum performance at both low           tance of the original winding. To get
and high step rates is essential, a       the same holding torque and low-
switch to full-step mode can be done      frequency performance the winding         Summing up
at a suitable frequency. Change the       current has to be raised to twice the     The unipolar L/R-driver offers the
stepping mode this way will also          original value.                           lowest cost for the electronics for a
lowers CPU-time requirement (step            It is not possible to increase the     given output torque, if the step rate is
rate reduced by 50% at high speeds) if    optimum stepping rate for a motor to      low.
the system use a microprocessor as        very high values since then hysteris         As demand for output power from
control unit.                             loss and rotor leakage inductance will    the stepper increases, more effective
MICROSTEPPING: The smoothest move-        decrease the efficiency.                  drivers offers the best price perfor-
ments at low frequencies is achieved         The EMF constant for a motor is        mance ratio. The best motor utiliza-
with microstepping. Higher resolu-        measured by conecting the motor           tion is achieved with the bipolar
tion is also offered. If resonance-free   winding to an osciloscope and rotating    constant current driver and this driver
movement at low step rates is impor-      the rotor at a constant speed (by means   is the obvious choice for all high-
tant, the microstepping driver is the     of a DC-motor for instance) and           power applications.
best choice. Microstepping can also be    measuring the peak value and the
used to increase stop position accuracy   frequency of the generated signal. The
beyond the normal motor limits.           generated frequency corresponds to a
                                          four-times-higher full-stepping rate.
                                          From this the EMF constant can be
Designing the winding                     calculated.
For a constant current chopper type          Figure 13 shows the affect on the
driver the winding design depends on      torque and output power of the 42mm
the desired output power, maximum         hybrid motor when the chopping
operation frequency, and chopper          voltage is decreased. From the figure,
voltage. A simplified design method,      we can see that the optimum
which in most cases when high             operating frequency moves from
output power is important, gives a        approximatly 2kHz to 1.5kHz when
good results is described below.          the chopping voltage is decreased
                                          from 40 to 30 volts. Using the EMF-
EMF selection                             rule we get the same result:
A good design criteria for winding
design is the EMF (electromotive          20mV/Hz × 1.5kHz = 30 Volts.




12

				
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