Page 660 - Mechanical Engineers' Handbook (Volume 2)

5 Prefilters and Feedforward 651

ing structure and its body attaches to the load. If a mass is then attached to the control valve
spool and the spool is aligned with the actuator centerline, a form of acceleration feedback
can be achieved (valve porting must be arranged to give proper feedback polarity). Mechan-
ical feedback schemes offer the potential advantages of reduced costs and complexity, as
well as alleviating the need for high open-loop bandwidth in the actuator and controller.
However, the design of servos using these techniques often requires manipulation of rather
complex physical models, which is beyond the scope of this chapter.

5 PREFILTERS AND FEEDFORWARD

As can be seen from Sections 3 and 4, the business of obtaining good closed-loop perform-
ance can become rather complex. Sometimes feedback loops and frequency compensation
are optimized to achieve the desired stability and accuracy, but the closed-loop response to
commands is not particularly desirable. Rather than compromise stability or accuracy by
altering the servoloop characteristics, it is often easier to shape the command signal before
it enters the servoloop. The following sections discuss some commonly used techniques for
accomplishing this, together with their limitations.

5.1 Lag Prefilters
High-gain servoloops are required to achieve static accuracy and rejection of load transient
disturbances, but rapid response to commands is often unnecessary or undesirable. In this
case, the addition of a simple lag prefilter will often provide the desired result:
1
G (24)
pf
s 1
pf
If this prefilter is placed in the command path, as illustrated in Fig. 27, can be made
pf
large enough that G dominates the U/U response, with an appropriate rise time.
pf
c
Another way that lag prefilters can be used is to provide a particular set of dynamics
for U/U , which are easily settable and do not change with variations in the forward-loop
c
parameters. Often called ‘‘model following,’’ this technique requires the use of high-bandpass
servoloops, so that the dynamics of the prefilter model (G ) dominate the U/U response.
pf
c
In concept, this makes it very easy to obtain any desired U/U . transfer function by simply
c
changing the electronic prefilter model. However, to have U/U faithfully reflect the model
c
dynamics, it is often necessary for the bandpass of the servoloop to be an order of magnitude
higher than the highest frequency singularity in the model transfer function. This is often
impractical. If inner feedback loops are needed to achieve the desired servoloop bandpass,
it may be more effective to tailor feedback 1oops to provide a combined feedback transfer

Figure 27 Generalized use of prefilters.