How to use RLIM

Introduction to How to use RLIM

The ControlLogix Rate Limiter (RLIM) slows the rate of change coming from a tag. Specifically, we usually use this for a set point. When an operator changes the setpoint on a PID instruction, there is an immediate change in the error. This change in error might cause a large “bump” of output on the PID instruction. Another example would be to slow the rate of position change to a valve to minimize the water hammer effect.

Generally, we’ll bring a tag into the RLIM instruction. Then we’ll provide an output that increases or decreases at a set rate.

For example, we’ll create two tags. Setpoint will be the value the operator enters from an HMI. RLimSetpoint will be the slowed value.

Set up the RLIM

Up to the present time, the RLIM instruction is not available in ladder logic. We need to set this up in a Function Block or Structured Text routine. In this case, we’ll set this up in a Function Block Routine. Rockwell recommends that you place the instruction into a periodic task, so it will have a stable time base

We’ll set up the RLIM instruction as follows.

Notice we also have a bypass if needed. If this goes true, the Setpoint will be fed directly to the RLimSetpoint.

Moreover, we’ll configure the limits on the rate change. At this time, click the ellipsis (three dots) to configure the RLIM instruction.

We’ll set up the IncRate and DecRate tags. In this case, we’ll allow the output to increment or decrement by a value of 5 every second. We’ll leave the timing mode at default for this example. For more details on each parameter of the RLIM, visit Rockwell’s General Instruction Set Reference Manual.

Test your work

At this point, I’ll download the project and change the setpoint from 0 to 100. Remember that if we change the setpoint by 100, the output will take 20 seconds to arrive at this value (5/second).

Clearly, we see the value ramping up.

Don’t forget to visit the ControlLogix page for other tutorials!

— Ricky Bryce

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