Will Pybricks PID control over correct?

[fancooling]

Pybricks PID algorithm seems to over correct if robot makes a turn then goes straight. Does anyone know why?

In a test, I made robot move a distance, make a 90 degree turn, then move a short distance.

# Replace drive_base.turn() when we need accurate turn.
async def TurnByWheel(prime_hub, drive_base, left_wheel, right_wheel, target_angle, wheel_speed=30, angle_error=1):
    drive_base.stop()
    print(f"AccurateTurnWithWheel target_angle={target_angle}")
    repeated = 0
    delta_heading = 0
    repeated = 0
    delta_heading = target_angle - prime_hub.imu.heading()
    print(f"init heading={prime_hub.imu.heading()}")
    print(f"init delta angle={delta_heading}")
    while not -angle_error <= delta_heading <= angle_error:
        if delta_heading > 0: # clockwise turn
            left_wheel.run(wheel_speed)
            right_wheel.run(-wheel_speed)
        else: # counter-clockwise turn
            left_wheel.run(-wheel_speed)
            right_wheel.run(wheel_speed)
        await wait(10)        
        delta_heading = target_angle - prime_hub.imu.heading()        
    left_wheel.stop()
    right_wheel.stop()

await drive_base.straight(250)
await TurnByWheel(prime_hub, drive_base, leftwheel, rightwheel, 90)
# Heading after turn
print(f"heading after turn {prime_hub.imu.heading()} drive_base angle {drive_base.angle()}")
await drive_base.straight(100)
# Heading after turn => straight
print(f"heading after straight {prime_hub.imu.heading()} drive_base angle {drive_base.angle()}")

I realized two things:

1. prime_hub.imu.heading() and drive_base.angle() do not match. In the first print, they are off by ~0.5 degree. In the second print, it can be as big as 3 degree difference.
2. The first print shows the heading is very close to 90 degree, usually 89.5-ish. In the 2nd print, it can be as big as 97 degree. Apparently when straight() after a turn, PID made the robot turn further. This over-turn was visually observable.

I made a little change to the code, set PID parameters very small to essentially disable PID.

await drive_base.straight(250)
await TurnByWheel(prime_hub, drive_base, leftwheel, rightwheel, 90)
# Heading after turn
print(f"heading after turn {prime_hub.imu.heading()} drive_base angle {drive_base.angle()}")
pid = drive_base.heading_control.pid()
# Set very small PID parameters to essentially disable PID
drive_base.heading_control.pid(pid[0]/10, 0, 0, pid[3], pid[4])
await drive_base.straight(100)
# Heading after turn => straight
print(f"heading after straight {prime_hub.imu.heading()} drive_base angle {drive_base.angle()}")

Without PID, the heading after drive_base.straight(100) seems to be very close to 90 degree, ~91 degree. It still grew from 89.5 to 91, but much better.

This behavior caused a lot of inconsistency in our recent competition.

Any thought?

[laurensvalk]

For many teams, the default values work well enough. When adjustment was needed, the majority of teams reported they wanted to increase the parameters, so it depends on your robot.

What you may be referring to is maintaining the angle after a turn or straight. This is also a default requested by most teams, but you can turn it off by using then=Stop.Coast for your movements.

If you've found that it still works quite well even with PID nearly turned off, that's a pretty good sign, because that's close to perfection. We recommend keeping it on though, so that it can account for errors when they do occur.