How does the stalled() function work in Pybricks-MicroPython

Question

MicroPython 2.0 beta 5

Trying to understand how the stalled() function on the motor works. I run a motor at dc of 100, and hold the wheel so that it cannot move.

But the stalled function doesn't fire, indeed whatever I do I don't seem able to get it to return True?

I tried with less power, but still not able to get anything out of this function.

#!/usr/bin/env pybricks-micropython
from pybricks import ev3brick as brick
from pybricks.ev3devices import Motor
from pybricks.parameters import Port, Stop

left_motor = Motor(Port.B) 

speed = 800

# option 1
left_motor.dc(100)

# option 2
#left_motor.run_until_stalled(speed, Stop.HOLD, 100)

while True:
    if left_motor.stalled():
        print("stalled")

If I use option 1: the motor runs, I hold it until it stops, nothing reported. I let go and off it goes again.

If I use option 2: the motor runs, I hold it, it stops. But at no point do I see a report saying it stalled.

Answer 1

A motor is stalled when it can't reach its target speed or angle, despite applying the maximum duty cycle.

Your example can be adapted like this:

#!/usr/bin/env pybricks-micropython
from pybricks.ev3devices import Motor
from pybricks.parameters import Port
from pybricks.tools import wait

# Initialize the motor
left_motor = Motor(Port.B)

# Start running the motor at 500 deg/s
left_motor.run(500)

# Do nothing until we are stalled
while not left_motor.stalled():
    wait(10)

# Stop the motor
left_motor.stop()

This example is equivalent to the one-liner left_motor.run_until_stalled(500). The manual approach can be useful if you want to extend it to multiple motors.

The dc() method in the question does not set a target speed or angle; it sets the duty cycle directly, so there is no stall information.

Note: the left_motor.stalled() method is instead accessible through left_motor.control.stalled() as of Pybricks version Pybricks 2.0. It is in public beta only as of March 2020, so I'm not sure the version reported in the original post in August 2019 is correct.