«

»

Nov
27

Saving PID Tuning When Replacing a Control System

When replacing an existing control system, the startup of the new system can often be made much smoother by re-using the PID tuning that has been established over the years with the existing system. If you are lucky, the form of the PID and units of the tuning parameters of the old and new systems match and you can just re-enter the tuning values directly into the new control system. However, the form of the PID and the units of the PID tuning parameters often vary between manufacturers and thus it may be necessary to convert the tuning values before they can be used in the new control system. If you find the PID form and units of the tuning parameter for the old and new control systems do not match, then you will need to convert the tuning parameter values to obtain the same dynamic response in the new control system.

If the converted loops only use PI control i.e. derivative (rate) gain = 0, then whether the form of the PID is series or standard will have no impact on the tuning. The tuning of the standard and series PID (for the same dynamic response) vary only when derivative (rate) is used in control. When derivative action has been used in the older system, then it is always possible to convert series tuning to the equivalent tuning for the standard form of the PID.

The conversion of existing tuning should always take in account for the units of the tuning parameters. Examples of unit variations you may encounter in commercial products are:

Proportional Gain: %/% or Proportional Band

Integral (reset): repeats/min, repeats/sec, min/repeat, second/repeat

Derivation(rate): minute, second

Accounting for difference in gain units can be done independent of the form of the PID. Conversion between two commonly used units may be done as following:

Proportional Gain (%/%) = 100/Proportional Band

If the form of the PID in the old and new system is the same or derivative action is not used in the loop, then it is only necessary to consider the units of the tuning parameters. For example, if the reset unit used in the older system is repeats/min and the new system reset unit is min/repeat, then the reset value for the new system is just the reciprocal of the old reset value:

Reset (min/repeat) = 1/Reset (repeats/min)

Often you may find it will save time to create a spread sheet to do the calculations needed to convert tuning parameters. For example, the following spreadsheet was created to support conversion from series form with units of Gain(%/%), Reset (repeats/min), Rate(min) to either standard or series from where the units are Gain(%/%), Reset (second/repeat), Rate(sec)

Example Conversion of PID Tuning