Lenze MCH Series Manual do Utilizador descarregar pdf (Página 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 Dierential 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 xed and process variable has settled

to a xed 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 aects the speed command.

The Integral Gain aects 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 ne-tune the system.

However, it may be necessary to use Dierential Gain (Parameter 79) to further

stabilize the system, especially when quick responses are required. The Dierential

term responds to the rate of change of the error, not the actual error itself. Dierential

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 Dierential 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 aects 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 “lter” 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 eectively disabled.

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Lenze MCH Series Manual do Utilizador Página 78