\r\n#include <PID_v1.h>\r\n\r\ndouble Setpoint, Input, Output;\r\nPID myPID(&Input, &Output, &Setpoint,2,5,1, DIRECT);\r\n\r\nvoid setup()\r\n{\r\n Input = analogRead(0);\r\n Setpoint = 100;\r\n\r\n myPID.SetMode(AUTOMATIC);\r\n Serial.begin(9600);\r\n}\r\n\r\nvoid loop()\r\n{\r\n unsigned long startTime = millis();\r\n\/*Do a PID calculation 10000 times*\/\r\n for(int i=0;i<10000;i++)\r\n {\r\n Input = analogRead(0);\r\n myPID.Compute();\r\n analogWrite(3,Output);\r\n }\r\n\/*print the elapsed time*\/\r\n Serial.println(millis()-startTime);\r\n}\r\n<\/pre>\nThe code above is a modification of the Basic PID example<\/a>. Rather than do the pid stuff once though, it calls it 10000 times in a row. Dividing the resulting elapsed time by 10000 gives us precision down into the microsecond range.<\/p>\nNot pictured in this code, I also went into PID_v1.cpp and made SampleTime=0. That ensured that every time Compute() was called, the PID equation was evaluated.<\/p>\n
One last note. Notice that analogRead and analogWrite are in the for loop. 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.<\/p>\n
The Results<\/h3>\n
![\"\"](\"http:\/\/brettbeauregard.com\/blog\/wp-content\/uploads\/2011\/04\/BasicSpeedResults.png\" "\\"BasicSpeedResults\\"")
<\/a><\/p>\n2088 mSec \/ 10000 = 0.21 mSec<\/b>
\nOk. Pretty fast. 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:<\/p>\n
![\"\"](\"http:\/\/brettbeauregard.com\/blog\/wp-content\/uploads\/2011\/04\/TrimSpeedResults.png\" "\\"TrimSpeedResults\\"")
<\/a>
\n826mSec \/ 10000 = 0.08 mSec<\/b>
\nSo the PID was only responsible for about a third of the total time. 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! <\/p>\nSome Closing Thoughts<\/h3>\n
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. 2 things to remember:<\/p>\n
\n- There’s other code in your program. All together it may add up to more than 1mSec<\/li>\n
- 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.\n<\/ol>\n
Bottom line, if you’re looking to evaluate the pid really quickly, you need to be careful to avoid weird performance.<\/p>\n","protected":false},"excerpt":{"rendered":"
A couple people have asked me about speed since v1 was released. Since I had no idea, I decided I should do a little benchmarking. (Tests were done on a Duemilanove with the ATMega168.) The Test #include <PID_v1.h> double Setpoint, … Continue reading