Page 582 - Mechanical Engineers' Handbook (Volume 2)

6 Electrical Modulators 573

t v
v ma as (36)
t p
and
i
t
v t
sa
s
L 1 t p (37)
For operating conditions where the instantaneous motor current becomes zero for a part
of the cycle, the mathematical models can be found in Refs. 5, 20, 23, and 24.

6.2 Stepper Motor Modulators

Modulators for stepper motors are also called ‘‘drives.’’ There are three types: oscillator-
translators, indexers, and microstepping controls. Oscillators provide a variable-frequency
pulse train to the translator. The function of the translator is to convert the pulse train into
appropriate signals to operate the power transistors, thereby directing power to the stepper
motor. The variable-frequency (3000–15,000 pulses per second) capability of the oscillators
provides for accurate manual control of speed. Similarly, a pulse generator in conjunction
with a translator provides manual position control. Figure 22 shows a typical oscillator-
translator package. Most commercial units also provide for half-stepping and electronic
damping.
Indexers are generally programmable microprocessors that perform the functions of the
oscillator-translator package, in addition to providing features such as numerical processing,
programming, and communications (RS232 and RS274) with computers. The software con-
trols provide such features as setting upper and lower limits of stepper motion and controlling
slewing rate. Figure 23 shows a typical indexer. Indexers generally operate on 20–100 V dc
power.

(a) (b)
Figure 22 Oscillator-translator modulator for stepper motor control (courtesy of Superior Electric Co.,
Bristol, CT): (a) photograph of modulator; (b) Photograph of circuit card.