in
minimum time. When the current reaches its predetermined maximum value the high voltage is reduced to a lower
"maintenance" voltage for the remainder of the step.
5.5 Chopper Drivers
The chopper drive system limits the motor current by means of voltage modulation. It is a highly efficient means of motor control
which permits an initial high voltage (10 to 20 times rated voltage) to energize the motor until the desired current is obtained.
The voltage is then switched off until the current decays to a predetermined level after which the voltage is again turned-on. This
cycle of on-off voltage (chopping) is continued through the drive pulse time. This method of motor drive is well suited for stop
start operation.
5.6 Closed Loop Considerations
The performance of a step motor incremental control system can be greatly improved through the use of position and/or velocity
feedback. A feedback system can more precisely determine correct phase switching for optimum motor control and is
recommended whenever maximum motor performance is required or whenever velocity and/or position verification is necessary
for system performance. Most closed-loop control systems utilize the feedback of motor displacement Information. The usual
methods of feedback (analog, digital, phase locked loop, etc.) may be employed depending upon system requirements. Since the
step motor is sort of a digital device and its associated control circuitry is basically digital, the use of a digital encoder is very
convenient.
Many more sophisticated motor control schemes employ the use of microprocessors in order to apply step motors to more
demanding applications, where interpretation and command modification may be required to perform a given end result.
6.0 CONSIDERATIONS FOR STEP MOTOR APPLICATIONS
Knowledge of the load to be driven is obviously imperative. High inertial loads require large amounts of power to accelerate and
decelerate. Proper inertia matching is necessary with 3:1 being close to ideal. Never should the load inertia be greater than 10
times the rotor inertia. Friction is both good and bad. Too little, and a system may be underdamped. Too much friction will cause
position error and power loss.
The step angle is an important consideration because it must be a multiple of the desired incremental motion. Half stepping and
microstepping techniques allow for a greater range of step angles and position resolution.
Many means are available for coupling the motor to its load. Several of these are:
Lead Screw
Pulley & Cable
Metal Band
Timing Belt
Direct Drive
The lead screw combined with a step motor becomes a very effective linear actuator and has been very successfully used as
such in various applications in the machine tool and computer peripheral equipment industries.
Major areas to be considered in the proper selection and application of a step motor are:
Motion to be achieved (displacement, acceleration, deceleration, etc.)
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Motor size (torque, power, etc.)
Load (inertia and friction)
Step angle
Motor temperature
Control and damping techniques
Motor to load matching method (direct, screw, belt, etc.)
System cost
REFERENCES FOR FURTHER READING
Sigma Stepping Motor Handbook - Sigma Instruments Inc., 1972.
Warner Electrics Guide To Selecting And Controlling Step Motors - Warner Electric Clutch
And Brake Co., Beloit, Wis., 1979.
Stepping Motor Selection Using Normalized Start-Stop Rate Curves C. K. Taft and R. G.
Gauthier, College of Engineering and Physical Sciences, University of New Hampshire.
Tutorial On The Control Of Stepping Motors - The Superior Electric Co., Bristol, Ct.
Incremental Motion Control Symposium Proceedings, 1974-1981.
Incremental Motion Control - Volume ll - Benjamin Kuo and Jacob Tol, SRL Publishing Co.


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