Arduino millis max value8/8/2023 ![]() Off the top of my head: feed forward, reset tiebacks, integer math, different pid forms, using velocity instead of position. There are many other issues that I intentionally left out in the name of simplicity. I may have missed something, or might just need to be clearer in my explanation. If something in this series looks wrong please let me know.For those of you writing your own PID, I hope you were able to glean a few ideas that save you some cycles down the road. For those readers that were looking for a detailed explanation of the PID Library, I hope you got what you came for. We’ve turned “The Beginner’s PID” into the most robust controller I know how to make at this time. Void SetControllerDirection(int Direction)Īnd that about wraps it up. Void SetOutputLimits(double Min, double Max) SampleTime = (unsigned long)NewSampleTime *Compute all the working error variables*/ this ensures that the parameters all have the same sign, and hopefully makes things more intuitive. If the user is connected to a reverse process, they specify that separately using the SetControllerDirection function. To make the process a little simpler, I require that kp, ki, and kd all be >=0. ![]() This isn’t a problem per se, but the user must choose the correct sign, and make sure that all the parameters have the same sign. To make the beginner PID work with a reverse process, the signs of kp, ki, and kd all must be negative. In a refrigerator for example, an increase in cooling causes the temperature to go down. For reverse acting processes the opposite is true. That is, an increase in the output causes an increase in the input. ![]() All the examples I’ve shown so far have been direct acting. The processes the PID will be connected to fall into two groups: direct acting and reverse acting. ![]() (This is the last modification in a larger series on writing a solid PID algorithm) The Problem Even if your loop time is less than 1 mSec (say it’s 0.7 mSec,) Compute only gets called once per loop, so the pid will actually be evaluated every 1.4 mSec.īottom line, if you’re looking to evaluate the pid really quickly, you need to be careful to avoid weird performance.All together it may add up to more than 1mSec Looking at these results, your first inclination might be to make the sample time as small as possible (1 mSec.) After all, that’s WAY bigger than the 0.08 mSec required. Unless I’m missing something (please tell me if I am,) it looks like the Compute function is only slightly slower than an analog read or an analog write! Some Closing Thoughts So the PID was only responsible for about a third of the total time. Now just for fun, I commented out the read and write code (lines 21 & 23) to see how much of the 0.21 was due to the Compute function: I couldn’t think of a situation where you’d have a PID without also having those functions somewhere in the loop, so I included them in the test. Notice that analogRead and analogWrite are in the for loop. That ensured that every time Compute() was called, the PID equation was evaluated. Not pictured in this code, I also went into PID_v1.cpp and made SampleTime=0. Dividing the resulting elapsed time by 10000 gives us precision down into the microsecond range. Rather than do the pid stuff once though, it calls it 10000 times in a row. ![]() The code above is a modification of the Basic PID example. PID myPID(&Input, &Output, &Setpoint,2,5,1, DIRECT) (Tests were done on a Duemilanove with the ATMega168.) The Test Since I had no idea, I decided I should do a little benchmarking. A couple people have asked me about speed since v1 was released. ![]()
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