LEESON Micro Series Compact Inverters - Manual (Page 81)

each type, so the same type of signal cannot be used for transducer feedback and set point reference. For
example, a 4-20 mA signal from a transducer could not be used as a feedback signal if the set point is being
controlled by a 4-20 mA signal from a PLC .
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 set point .
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 (set point 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 set point. 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 ACC . This
sets the acceleration (and deceleration) rate of the set point reference into the PID unit. When the set point
changes, this function will “filter” the input to the PID unit by ramping the set point 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 set point, resulting in smoother operation . If PID ACC is set to 0 .0
seconds, it is effectively disabled .
19.6 MICRO SERIES DISPLAY IN PID MODE
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