Using Discrete PID Significantly Slows Down Simulation

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Aziz Alawadhi
Aziz Alawadhi 2020-5-21
编辑： David Balbuena 2020-6-9
Hello,
I am currently modeling a complex system with numerous joints and motors (from sim mechanics) and I ran into an issue while trying to discretize the system. To start, I modeled my PID controllers as continuos (I have 12 of them). I now wish to turn them into discrete-time PIDs.
However, when I switch the PID's to discrete, the simulation that used to take a couple of minutes to run now takes hours. Furthermore, as the simulation goes on, it only gets slower. When I switched the PID's back to continous, the simulation ran in minutes as it did before. I even tried to switch only one of the 12 PID controllers into discrete to see what effect it would have and the same thing ended up happening - the simulation would take hours. I tried this with two different solver types (ode15s and ode23t) and tried to reduce the solver's relative tolerance from 1e-3 to 1e-2 but it did not help.
Initially I used a PID sample time of 0.001 but this ran way too slow so i switched it to 0.01 and it started running faster but the simulation would still take hours.
Is there a reason why this is happening? And is there a way to get around this?
Thanks!

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回答（1 个）

David Balbuena
David Balbuena 2020-6-9
编辑：David Balbuena 2020-6-9
This is most likely happening because you have a discrete controller driving a continuous variable-step plant. As mentioned in this documentation page,
https://www.mathworks.com/help/releases/R2020a/simulink/ug/solver-resets.html#mw_85fe4463-d903-444c-8420-7d15196bf1b9
this scenario will trigger Solver Resets every time the discrete signal changes, which can impact the performance of your model. In R2019b, there was a new "First Order Hold" block added for this specific use-case (discrete controller driving continuous plant):
https://www.mathworks.com/help/releases/R2020a/simulink/slref/firstorderhold.html
This block is introduced in greater detail in this blog post:
https://blogs.mathworks.com/simulink/2020/01/17/a-new-first-order-hold/?s_tid=srchtitle
If you are in R2019b or later, feel free to give the First Order Hold block a try and see how that works.

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