Page 679 - Mechanical Engineers' Handbook (Volume 2)

670 Controller Design

hundred kilohertz and virtually infinite resolution. Because of this, they are free of the
sampling delays, phase lags, and resolution problems associated with microprocessor-based
controllers. Furthermore, they are relatively tolerant of electrical noise, and troubleshooting
can be accomplished with simple equipment. On the other hand, analog implementation of
complex controller functions such as nonlinearities, automatic gain changing, elaborate com-
mand processing, and complex failure detection can cause the electronic circuitry to become
extremely complicated. In addition, analog controllers typically require periodic adjustment
and calibration.
The basic functional elements of a typical analog controller are shown in Fig. 44. The
functions of prefilters and compensation networks have been discussed previously in this
chapter. Some form of signal conditioning is usually required for transducers. In the case of
a simple dc transducer such as a potentiometer, the conditioning may consist of dc excitation
together with an output buffer amplifier. Buffer amplifiers can be designed to protect against
large voltages erroneously connected to the electronics, to provide consistent loading of the
transducer output, to reject electrical cabling noise (electromagnetic interference), and to
filter transducer ripple. Low-output transducers, such as those that employ strain gage ele-
ments, require high-gain, low-drift amplifiers. Linear variable differential transformers
(LVDTs), resolvers, and other ac transducers require ac excitation, demodulation, and filter-
ing to remove ripple. Greatly increased servo accuracy can be obtained with a combination
of ac command generation and ac feedback, such as when synchros are utilized. However,
this approach has largely been replaced by the use of digital controllers and transducers. In
any case, transducer specifications should be carefully studied to determine the proper signal
conditioner characteristics. The dynamic characteristics of the signal conditioners, as well as
the transducers themselves, can have significant impact on the stability and performance of
the servoloops, as explained previously in this chapter.
There are many sources of information concerning the design of analog controllers.
References 8 and 9 are excellent sources for the design of operational amplifier circuits for
a wide variety of purposes, including compensation and signal conditioning. Furthermore,
most manufacturers of operational amplifiers publish useful application handbooks. Also,
application literature from transducer manufacturers often discusses signal-conditioning tech-
niques in some detail.
The design of power amplifiers varies widely with the type of servoactuator being driven.
In the case of conventional electrohydraulic servoactuators and other types requiring low-
power electrical inputs, the power amplifier can be a very simple linear (proportional) circuit.
Typically, it utilizes an operational amplifier and a power boost stage consisting of a com-
plementary pair of transistors. Some operational amplifiers have enough output capability to
provide the required electrical input directly. In the case of electromechanical devices that
must provide a direct electrical-to-mechanical energy conversion, such as brushless servo-
motors, the power amplifier can become very complex. In this case, its design should be left
to an experienced electronics engineer. Linear amplifiers are still the most straightforward
and offer the best servo performance but are severely limited in the size of the motor they
can control because of the large amount of heat they must dissipate. Since switching tran-
sistors typically generate little heat in their full-on or full-off states, various time-modulated
on–off power drivers have been developed. The most popular type for servocontrol appli-
cations seems to be pulse width modulation (PWM). In this approach, the power devices are
switched on and off at a very high fixed frequency. The percent on-time during each cycle
is proportional to the dc input voltage from the upstream analog controller circuitry. If the
PWM frequency is high enough, only the average cycle voltage will affect the servoactuator
output, thereby resulting in nearly proportional control. To accomplish this, the PWM fre-
quency must usually be at least one order of magnitude higher than the bandwidth of the