Lenze MC3000 Series - Manual (Page 73)

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In order to use TB-5A or TB-5B for a setpoint input, one of the TB-13 terminals must
be programmed for the appropriate signal . Closing the TB-13 terminal to TB-2 will then
select that signal as the setpoint reference . If the contact closure is not made to TB-2,
the setpoint reference source will default to the keypad . Refer to Parameters 47 - TB13A,
48 - TB13B, and 49 - TB13C .
Remote setpoint reference inputs at TB-5A and TB-5B can only be used if that terminal
is NOT being used for the process feedback signal from a transducer . The MC3000
has only one analog input of each type, so the same type of signal cannot be used for
transducer feedback and setpoint reference . For example, a 4-20 mA signal from a
transducer could not be used as a feedback signal if the setpoint 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 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 ACC . 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 ACC is set to 0 .0 seconds,
it is effectively disabled .