Lenze MCH Series - Manual (Page 78)

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19.5 TUNING THE PID CONTROL
Once the PID control is set up correctly, it needs to be tuned in order to maintain the
process setpoint. First, set the Integral and Differential Gains to zero, and increase the
Proportional Gain (Parameter 77) until the system becomes unstable, then lower the
gain until the system stabilizes again. Set the Proportional Gain about 15% less than
that value that stabilizes the system. If only Proportional Gain is used, and the system is
operating in a steady-state condition (setpoint is fixed and process variable has settled
to a fixed value), there will always be a certain amount of error in the system. This is
called the steady-state error.
Integral Gain (Parameter 78) is used to force the steady-state error to zero by increasing
the output speed command with respect to time. Over time, the error will be forced to
zero because the Integral term will continue to change the speed command, even after
the Proportional term reaches steady state and no longer affects the speed command.
The Integral Gain affects the rate of rise of the output speed command from the Integral
term. Small amounts of Integral Gain can cause large changes in PID performance, so
care must be taken when adjusting Integral Gain. Too much Integral Gain will result in
overshoots, especially if large step changes in error occur.
Typically, Proportional and Integral Gain are all that is needed to fine-tune the system.
However, it may be necessary to use Differential Gain (Parameter 79) to further
stabilize the system, especially when quick responses are required. The Differential
term responds to the rate of change of the error, not the actual error itself. Differential
Gain acts like a “shock-absorber” to dampen overshoots that can occur when the PID
tries to react quickly to changes in error or setpoint. This allows fast PID response,
with reduced risk of becoming unstable due to overshoots. The Differential term is very
sensitive to electrical noise on the feedback signal and to digitizing errors, so it must be
used with caution.
The other parameter setting that affects the response of the PID control is Parameter
80 - PID ACCEL. This sets the acceleration (and deceleration) rate of the setpoint
reference into the PID unit. When the setpoint changes, this function will “filter” the input
to the PID unit by ramping the setpoint reference from the previous value to the new
value. This will help prevent overshoots that can occur when the PID control attempts
to respond to step changes in setpoint, resulting in smoother operation. If PID ACCEL
is set to 0.0 seconds, it is effectively disabled.