Capturing the range of an oscillation


Thread Starter


We are currently exploring changing the way we control our evaporators, using motor loads. The amps oscillate in a fairly regular manner, and we'd like to set up logic to compare the most recent minima and maxima for the motor load. That is, is there any way to find the range of the last cycle in the motor load? We're using ABB Composer, and any help would be appreciated!


William Sturm

If you can take the Derivative of the motor load, you should be able to determine when the slope changes from pos to neg and back. You will probably need to filter the load signal, unless it is very smooth. Taking the Derivative of noise is very misleading.

Bill Sturm
Abbeytronics LLC
Caitlin... unfortunately using amperes alone only works if voltage is not varying!

I suggeest you use kW as the variable to monitor.

Regards, Phil Corso (cepsicon [at]

Look at ....

OK, yes, its complete overkill if you want to monitor a couple of motors or PID loops. However, this kind of project rapidly gets out of control and starts growing as your production and management staff realise how important this sort of stuff is.

Trying to nail down sources of oscillation in an evaporator system (or any complex interconnected plant) can be a time consuming affair and by the time you do the analysis on the motor loads you'll potentially find they are oscillating as well but are not the root cause.

Inside the DCS or PLC controller really isn't the best places to be writing an oscillation detection & analysis routine anyway.