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(Note - For cascade control it is best to set the Integral Minimum = 0 and Integral
Maximum = 32767. This is because it is necessary to maintain a positive pressure on the
dome of the regulator to maintain output pressure from the system.)
In the Signal Generator, set 'Setpoint 1' & 'Setpoint 2' to 625 and 1875, which is 25%
and 75%. Make sure that the plot screen is set up to show the entire tuning range. For
example, with the above setpoints, the display should be set up for 0 to 2500.
Since tuning is generally done using step changes, set up 'Wave Type' for option
'TOGGLE'.
Now click on the "Setpoint 1/Setpoint 2" in the Signal Generator to toggle between the
two setpoints.
With the proportional term set to 200, the response will be relatively slow. The first step
in tuning is to see how far the proportional term can be increased. In general, increasing
the proportional term will decrease the response time (i.e. - make the response wave
more square), however a point will be reached at which the effect becomes detrimental.
That is, too much proportional term will result in overshoot and possibly ringing of the
response. Also, it should be noted that the effect of the tuning parameters (PID) tends to
be logarithmic. Thus a possible sequence of values to try for the proportional term might
be 200, 400, 800, 1600, until ringing and/or overshoot occurs and then narrow down to
an optimal value, one which gives just a slight amount of overshoot and ringing.
Next the derivative term should be increased so as to reduce the overshoot and ringing in
the system. However, too much derivative term may result in a noisy output.
Once the optimal proportional and derivative terms are found, the integral term should
be adjusted. The integral term has the effect of eliminating DC offsets, however too
much integral will lead to instability (overshoot and ringing). Again the effect is
logarithmic so values such as 10, 20, 50, 100, 200, 500, 1000 ... can be tried until an
optimal value is obtained.
At this point it should be noted that the ER3000 provides a unique integral limiting
feature ('Minimum Outer Integral' and 'Maximum Outer Integral'). This allows for using
large integral terms without creating excessive overshoot. It should be realized that the
integral sum is what holds the valves open even when there is zero error. For closed-
ended systems the minimum and maximum integrals can be very small. However, for
systems requiring flow it is necessary to keep these values higher. To determine how
small they can be made (remember - smaller is better as long as proper flow is
maintained) the setpoint should be set to the maximum level and the minimum and
maximum reduced until flow is no longer maintained, then slightly increase these values.
Often, for closed ended systems a value of one or two is sufficient.
After performing the steps mentioned above, tuning can often be improved by iterating
between the various parameters. Although performing basic tuning can be simple,
learning to fully optimize the tuning requires experience, so experiment!
6. Tuning Procedure 35
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