Lenze MC1000 Series - Manual (Page 76)

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19.4 SET POINT REFERENCE SOURCES
The set point reference input can be one of the following:
1. Keypad
2. 4-20 mA signal at TB-5B
3. 0-10 VDC signal at TB-5A
4. Potentiometer signal at TB-5A
In order to use TB-5A or TB-5B for a set point input, one of the terminals must be
programmed for the appropriate signal. Closing the TB-13 terminal to TB-2 will then
select that signal as the set point reference. If the contact closure is not made to
TB-2, the set point reference source will default to the keypad. See Parameters 47
- TB13A, 48 - TB13B, and 49 - TB13C.
Remote set point 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
MC1000 has only one analog input of 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.