A high Kp makes the system respond strongly to the error. However, the response can be too large, and can lead to long settling
times. A high Kp may make operation unstable.
A low Kp makes the system respond more weakly to the error. A low Kp reduces the settling time. However, the response can be too
small, and therefore ineffective.
Integral part
The integral part eliminates the steady-state error.
The integral part is determined by:
• The amplified error
• The integral action time (Ti)
• The error history
Ti is the time it takes for the contribution of the integral part to be equal to the contribution of the proportional part. If Ti is shorter, the
contribution of the integral part is higher.
Do not set Ti too low. This can make the operation unstable (the effect is similar to a very high gain).
The figures below show the effect of Ti (a constant error is used to simplify the example).
When the system is far away from the reference point the integral part will have a large contribution to the correction. When the
system is close to the reference value, the integral part will have a small contribution to the correction.
1 20 Time [s]
Contribution
Error
P contribution
I contribution
3
Ti
1 20 Time [s]
Contribution
Error
P contribution
I contribution
3
Ti
The error history is calculated using integration.
INFO
Set Ti to zero to turn off the integral part.
Derivative part
The derivative part stabilises operation, allowing higher gain and lower integral action times. The derivative part can improve the
settling time.
The derivative part is determined by:
• The amplified error
• The derivative action time (Td)
• The current rate of change of the error
The derivative part uses a linear extrapolation of the current rate of change over Td to predict the future error. This works well for
slower systems, since they are easier to predict than faster systems. If Td is higher than the optimal time, the settling time can be
very long. For very high values, the system might not be able to settle at the reference value (the effect is similar to a very high
gain).
DESIGNER'S HANDBOOK 4189340911K UK Page 170 of 521
To Next Page
Loading...
