National Semiconductor LM628, Programming Manual

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III. Tuning the PID Filter
(Continued)
Observations
What follows are example oscilloscope traces of the step
response of the reference system.
Note 8: All traces were generated using the circuit of Figure 20 .
Note 9: All traces were generated using the following “step” trajectory pa-
rameters: relative position, 200 counts; absolute velocity, 500,000 counts/
sample; acceleration, 300,000 counts/sample/sample. These values gener-
ated a good small-signal step input for the reference system; other systems
will require different trajectory parameters. In general, step trajectory param-
eters consist of a small relative position, a high velocity, and a high accelera-
tion.
The position parameter must be relative. Otherwise, a define home command
(DFH) must be added to the main loop of the step generation section — filter
tuning program. See
Figure 19 .
The circuit for viewing the system step response uses an 8-bit
analog-to-digital converter. See Figure 20 . To prevent converter overflow, the
step position parameter must not be set higher than 200 counts.
Note 10: The circuit of Figure 20 produces an “inverted” step response
graph. The oscilloscope input was inverted to produce a positive-going (more
familiar) step response graph.
Figure 21 represents the step response of an under damped
control system; this response exhibits excessive overshoot
and long settling time. The filter parameters used to gener-
ate this response were as follows: k
p
, 35; k
i
, 5; k
d
, 600; d
s
, 4;
i
l
, 1000. Figure 21 indicates the need to increase k
d
, the
derivative gain coefficient.
Figure 22 represents the step response of an over damped
control system; this response exhibits excessive rise time
which indicates a sluggish system. The filter parameters
used to generate this response were as follows: k
p
, 35; k
i
, 5;
k
d
, 10,000; d
s
, 7; i
l
, 1000. Figure 22 indicates the need to
decrease k
d
and d
s
.
Figure 23 represents the step response of a critically
damped control system; this response exhibits virtually zero
overshoot and short rise time. The filter parameters used to
generate this response were as follows: k
p
, 40; k
i
, 5; k
d
,
4000; d
s
, 4; i
l
, 1000.
01086023
FIGURE 20. Circuit for Viewing the System Step Response with an Oscilloscope
01086024
FIGURE 21. The Step Response of an Under Damped Control System
AN-693
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