cvra / pid

A PID controller implementation

PID Controller

This modules implements a PID controller in parallel form for use in various control loops. It has the following features :

• Discrete PID in parallel form.
• Maximum integrator value (ARW).

Usage

pid_ctrl_t pid;
pid_init(&pid);

/* PD controller. */
pid_set_gains(&pid, 10., 0, 4.);

while (1) {
    error = motor_position - setpoint;
    motor_pwm = pid_process(&pid, error);
}

Frequency compensation

The user of this module can specify a frequency for the PID loop and the gains will then be adjusted to allow the same gains to be used for various frequencies.

This is done using the function pid_set_frequency. By default there is no compensation for the frequency of the PID.

Dependencies

None

pid's Issues

Synchronization

Setters and getters are not synchronized with the PID control function.
How should we handle synchronization of such functions?

One solution would be to restrict the code to processor architectures where float read & writes are atomic. This would reduce the problem to a few places where multiple floats must absolutely be written/read atomically.
But after the discussion about stdints the general attitude seems to be in favor of portability.

Add a limit to the integrator value

We need a way to have a maximum integrator value (ARW) and have a way to reset it.

Minor arithmetic compiler warning

Hello,
I am playing around with this cool project, and firstly want to say thank you.
Also I would like to report a compiler warning which should be handled gracefully:

pid-master\pid.c(11): warning C4756: overflow in constant arithmetic

The compiler used is MSVC 2015 with the CTP November 2013 tool chain.

Best,
François

Add a frequency parameter

So @Stapelzeiger suggested that we should add a frequency or period parameter which would allow us to have frequency - independant behaviour.

I think it is a good idea, but I don't know if it is simply diving Ki and multiplying Kd by the frequency or if it is more complex.

CI build broken

https://travis-ci.org/cvra/pid/builds/168674816

derivative filter

It would be useful to be able to filter the derivative term of the PID. I suggest a simple moving average, since it can be implemented very efficiently.

Check malloc return value

In pid_create we should check if malloc returns NULL, but what should we do then ? panic() ? return NULL ?

undefined reference to `pid_init(pid_filter_t*)'

    pid_filter_t pid;
    pid_init(&pid);

undefined reference to `pid_init(pid_filter_t*)'

Initial code review

Please review :)

Sign / (scaling) factor for filter output

Sometimes we need to flip the sign of the filter output because of how our hardware is constructed.
Should this be a property of our filter or some other abstraction layer?