# S2 Presentation Stepper Motors

Document Sample

```					S2 Presentation: Stepper Motors
Wildi Text Ch 19, Sec 1 to 8, Q&P 1 to 10
   characteristics
   basic construction - two designs
   current pulses
   prediction of operation
   half-step operation
   reducing oscillations
   accuracy
   exploded view of a motor
Characteristics
stepper motors have salient poles on both stator and rotor

stator poles wound with coils - carry rel long pulses of current

rotor poles - either magnetically soft or hard iron
variable reluctance (VR) motors - soft iron rotors
permanent magnet (PM) motors - hard iron rotors - magnetized
advantage of PM - motor stays after power disconnected

no of stator poles (NS) - different from no of rotor poles (NR)

two figures following - main difference is no of poles (NS and NR) .
VR Stepper Motor NS = 3 NR = 2 W Fig 19.1
PM Stepper Motor NS = 4 NR = 6 W Fig 19.10
Control and Stator Current Pulses (NS = 3)
Note the power supply and switches A, B, C in Wildi Fig 19.1
are replaced by the alternating pulses below.
Stepper Motors Operated in Two Modes
(a) displacement advancing by fixed angle D corresponding to
specific number of control pulses NP

(b) rotation     at a constant speed n corresponding to a specific
control pulse repetition rate f (pps)
Prediction of stepper motor operation
A basic design parameter is step size:
360 (
S =           degrees
N S NR

A related performance measure is step resolution:
360 (
S=       = NS NR steps/rev
S
In mode (a) the displacement is:
D = S × NP degrees

In mode (b) the speed is:
60 f      60 f
n =            =       RPM
N S NR       S
Half-step Operation
 double step resolution by pulsing first one pole, then two
together, then next one

 create additional poles between the actual poles
 if pulse repetition rate (frequency) is doubled at same time,
speed remains the same

using 3-pole stator in Fig 19.1 as an example
sequence for pulsing stator poles is:
1, 1&2, 2, 2&3, 3, 3&1, repeat             (1, 2, 3 for full-step)
Reducing Oscillations
 is important where step changes occur

 motors and/or loads with high inertia tend to result in overshoot
and oscillation

 stepping motors have best performance where inertia is low or
viscous damping used to mitigate against oscillation
Accuracy is the ability of the rotor to follow the pulse stream
stepper motor is digital unlike other motors which are analog
 maximum pulse repetition frequency - depends on both load
torque and inertia above which errors occur - maximum
frequency is higher for rotation mode (also called slewing)
relative to displacement mode because inertia effects disappear
at constant speed.
 maximum torque that a stepper motor can exert while moving
from one position to the next is called pull-over torque
 ramping is used to describe acceleration and deceleration of
a stepper motor - again there is a maximum value, above which
accuracy is lost
Exploded View of Stepping Motor - base, rotor, stator, cover

File ee2mm3/s2_pres.wpd Alden November 24, 1998

```
DOCUMENT INFO
Shared By:
Categories:
Stats:
 views: 24 posted: 12/23/2009 language: English pages: 12