Page 578 - Mechanical Engineers' Handbook (Volume 2)

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6 Electrical Modulators 569

Since the time allowed for the acceleration of the load is 0. 1 s, which is more than an order
of magnitude greater than either time constant, the motor should have adequate dynamic
response.
For a more exact dynamic response determination, the motor parameters may be deter-
mined experimentally (as described in Chapter 6 of Ref. 20) and used in the dynamic models.

6 ELECTRICAL MODULATORS

This section describes electrical modulators used for the various types of servomotors de-
scribed in Sections 3–5. The term ‘‘modulator,’’ as deﬁned earlier, designates components
employed for conversion of the command signal to appropriate means to modulate the power
ﬂow to the servomotor (see Fig. 1). Modulators for the various types of electrical servomotors
differ signiﬁcantly.

6.1 Direct-Current Motor Modulators
Modulators used in servoactuator applications with dc motors usually contain two stages of
ampliﬁcation: a ﬁrst-stage voltage ampliﬁed followed by a second-stage power ampliﬁer.
Voltage ampliﬁers are generally quite linear in performance. Power ampliﬁers are used in
two different conﬁgurations, type T and type H, as shown in Fig. 18. The type T conﬁgu-
ration employs two power sources and only two power transistors. The type H conﬁguration
uses only one power source but four transistors. The type T conﬁguration lends itself readily
to current feedback schemes. However, the type H is more commonly used because of a
single power source requirement. Type H conﬁgurations can be operated in two different
modes, resulting in bipolar and unipolar drives.

Linear Ampliﬁers
Linear ampliﬁers that are dc ampliﬁers used in the output stage (as H or T conﬁguration)
provide gains typically in the range of 2–10. At power levels above 200 W, these ampliﬁers
require external cooling (e.g., fans) to overcome excessive heat generation. This problem is
particularly severe in high-performance servo applications that typically have low-impedance
rotors operating at low-speed and high-torque conditions. Sometimes this problem is over-
come by incorporating a dual-mode current-limiting device which permits high currents for
short durations (for overcoming inertia) and then imposes a lower limit for longer durations.
Linear ampliﬁers are generally conﬁgured as voltage ampliﬁers, but in some applications
a current source conﬁguration is employed. Reference 5 discusses the details of voltage and
current source conﬁgurations, the associated mathematical models, and the inﬂuence of the
two conﬁgurations on the dynamic response of the motor.
Switching ampliﬁers overcome heat generation problems by switching the voltage on
and off at high frequencies. This switching limits the maximum allowable inductance and
results in shorter time constants and increased bandwidth. One drawback is that the RFI
noise level generated is much higher than with linear ampliﬁers. There are predominantly
two types of switching used in servoactuator applications: (1) PWM and (2) PFM. PWM
ampliﬁers are most commonly used.

PWM Ampliﬁers
In PWM ampliﬁers, the voltage applied to the servomotor is varied by changing the pulse
width of a high, constant-frequency (typically 10-kHz) train of pulses. Figure 19 shows a
schematic of a PWM ampliﬁer. Figure 20a shows a photograph of a typical PWM ampliﬁer

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