Specifications

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 AC400 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.