Concept of PID explained

[billbo911]

Originally Posted by eugenebrooks

The range of PosError3, declared as a char, is -128 to 127.
The range of the expression assigned to it is larger. When you
assign 254, for instance, and read the value back from PosError3,
you will get -2, not the value intended. Looking at your code,
it looks like the result would be a pwm signal in the dead band,
instead of full forward. Assigning 128 will be read as -128,
and in this case your code will cause the motor to go full speed
in the wrong direction.

In the case of DrvCmd3, you have the same problem, but you are
saved by the fact that the computer takes the misinterpreted 8 bits
and blindly shoves them into the unsigned char, pwm03, so the result is
the one you expect.

As I noted, the issue a visible bug for PosError3, but does not manifest
itself as a bug for DrvCmd3. If you, or someone else, were to modify
the code to somehow test the value of DrvCmd3 and manipulate it
before assigning it to pwm03, the additional bug would surface.

A table of widths, and range, of integer types on the robot controller
can be found in "An introduction to C programming for FIRST robotics
applications." See "C programming for the robot controller"
at the web address
http://srvhsrobotics.org/index.php?g...tarticle&cat=2
for the latest version.

I hope this is helpful...

Eugene

After reading through the Integer Types and their Properties section of the paper you linked to and re-reading the IFI Programming Reference Guide and the MPLAB Compiler Users Guide, I see my error . With out a doubt, "int" should be used for both DrvCmd3 and PosError3.


Curiosity question here.
I noted significant differences between your paper and the IFI documents when it came to variable types and their ranges, primarily "int". Again I'm pleading ignorance here as I'm so new to coding. Why are they different?

Again, Thanks for your help! It would not have been fun trying to figure out where we/I had gone wrong with the code with out your input.