Real-time control PLC connected to Simulink is faulty

Hi, everyone

I am trying to build a stepper motor speed control model using simulink with PLC using OPC toolbox. I set Sample Time = 0.01s for all blocks. OPC Read and OPC Write block (Asyn. mode). The PLC Encoder sampling rate is 100ms. I have identified the transfer function as shown in the red circle. The problem with the PID here is that when running a real-time model that only matches (100,110,30), Settling Time = 8s. According to theory, the better set of numbers (16593,31551,4087) is Settling Time = 2s, but when running the real-time model, it is not true when running the simulation. Does anyone know why?

Answer 1

[MULI]

Hi Hoang,

It seems like the issue you are facing arises from the mismatch between the sampling rates of your system components. It can arise due to following reasons:

• Your Simulink model expects data updates every 10ms, but the encoder only provides updates every 100ms. This can lead to inaccurate control signals from the PID controller.
• Simulations run in sync, but real-time systems experience delays due to hardware, causing mismatches in behaviour.

You can follow the below suggestions to resolve the issue:

1. Increase Sample Time in Simulink:

Set the Simulink model's sample time to 100ms to match the encoder. This synchronizes the model with the encoder data.

2. Implement Data Interpolation:

Use interpolation (e.g., linear interpolation) to estimate encoder values between updates, allowing Simulink to work with estimated data at 10ms.

3.Tune the PID Controller for Real-Time:

Re-tune the PID controller to account for the slower 100ms data updates. Use less aggressive parameters to improve real-time performance.

https://www.mathworks.com/help/slcontrol/cat-scd-pid-controller-tuning.html