Actually, why PID controller in this system is needed?
Closed loop control is required for speed control. However, if the OP doesn't care how the load stops then ramping down in open loop will work too.
Can you please show me the ramps on #13 curves?
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There aren't any. So? What does that have to do with over heating or going in to saturation when ramping down?
About the plot.
Does that 5-minute plot look right to you? It doesn't look right to me.
1 I don't trust Rockwell trends for motion because the sample times aren't fast enough and not guaranteed to be at consistent intervals.
2. If this is a high inertia load, why is the speed changing so quickly? Is the speed really changing that quickly?
3. Could it be a sampling and trending problem? Uneven sampling will make it hard to calculate an accurate velocity.
It isn't clear to me if this motor what is generating the green control signal. Is the control signal proportional to speed or acceleration? If it comes from the PLC then it is probably the frequency as stated in #1. Then green line looks like it comes from a PLC register. Why is this value changing so much if it is a frequency? Is this value sent to a drive that actually does the PID control or is the SLC trying to control the speed reference? If so, NOOOOO!
Speed control only requires a PI controller, but it is important that an accurate velocity can be derived from the encoder. This is where many speed control systems fail. If the PLC is trying to do the speed control, how is it computing the speed?