Derivative / Rate of Change

[klawson2012]

Hi,

I have a ControlLogix processor and I'm using RSLogix 5k V19. I am trying to find the derivative or the rate of change of a signal every second across a two minute period. Is this possible with the derivative block? I have not been able to get this block to work, does anyone have samples?

Thanks.

 

[dmargineau]

Could you try to elaborate with regard to the "...have not been able to get this block to work..." statement?
If you have a valid TechConnect contract, refer to https://rockwellautomation.custhelp.com/app/answers/detail/a_id/22461/related/1
for the DERV instruction functionality and examples of implementation.

 

[klawson2012]

I setup a value that increments by 1 every second until it reaches 100, then it resets to 0. I am expecting to receive a derivative of 1 but the derivative block is outputting around 5.12 and it seems to be outputting the value sporadically instead of every second. I have read the derivative help guide Rockwell supplies but its not clear to me how to perform what I need.

 

[dmargineau]

Could you post a screenshot of the DERV configuration block?
Where is the instruction being examined, Main Task routine or Periodic Task?
If the latter, what is the Periodic Task configured execution period?
What is the configured RPI of the module containing the DERV instruction's Input data?

 

[klawson2012]

The attachment below is how I have it setup. I thought I need to use the periodic Timing Mode.

This is in a function block diagram that is using a JSR from the main routine.

 

[klawson2012]

It is in the Main Task of the program.

 

[dmargineau]

You will have to remember that the Main Task (Continuous) of any Logix application has the lowest execution priority and its execution timing is variable, depending of the CPU's "workload".
The point I am trying to make is that the Main Task is a poor choice of locating of any accurate "math" code especially when the CPU's RTC (Real Time Clock) is not "invoked".
I your particular case, if the DERV FBD subroutine is located within the Continuous Task, the Δ Time used by the block's functionality equation

Rate = Δ Value / Δ Time

is equal to the elapsed time since the last execution, time which, again, is variable in case of your applications's Main Task; this is the reason the block's Output results are not what one would expect.

I would locate the DERV subroutine within a Periodic Task with a Period equal to 1000ms (one second)and give it one more try.
Otherwise, if your process Input variable has an update timestamp, you could leave the DERV subroutine within the Continuous Task, however, use the Real Time Sampling DERV "timing" mode instead of the Periodic one.