S2 Presentation Stepper Motors

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S2 Presentation Stepper Motors Powered By Docstoc
					S2 Presentation: Stepper Motors
                        Wildi Text Ch 19, Sec 1 to 8, Q&P 1 to 10
   basic construction - two designs
   current pulses
   prediction of operation
   half-step operation
   reducing oscillations
   exploded view of a motor
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
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