Motor synchronization

[WasabiFan]

There's discussion that has been happening in ev3dev/ev3dev#454 and ev3dev/ev3dev#853 around synchronization of motors, mainly for wheeled robots. While kernel support for "proper" synchronization is not going to be arriving for a while, I think it's reasonable to implement this as best as possible in this library and adapt to the kernel model later.

I'm thinking that a MotorPair class which operates on given motor objects and/or port names would be reasonable as a Python implementation. It would be nice to provide functions for steering, potentially similar to the EV3-G functions.

Thoughts?

[ddemidov]

Something like Tank class in helper.py?

[ddemidov]

Hmm, that class is not what I thought it was. I thought @dwalton76 had something like this implemented.

[WasabiFan]

In particular, one of the features of this class is that it would be as efficient as it can be for a library written in Python. That primarily means we'd set all the motor properties together and then send the commands sequentially to start them at the same time, which makes a surprisingly large difference in my brief testing.

[dwalton76]

I can't think of any builds I've done where I needed a pair of motors to start at the exact same time...I don't think it was me :)

[dwalton76]

@ddemidov maybe you are thinking of TankWebHandler in https://github.com/rhempel/ev3dev-lang-python/blob/develop/ev3dev/webserver.py , specifically around line 500 where it is starting two motors but with different speeds in order to turn the tank? This was the tank that uses a web interface with a little "joystick" dot that you move around inside a larger circle to control the speed/direction of the tank.

[ddemidov]

So what would be the interface?

class MotorPair(object):
  def __init__(self, m1, m2):
    ...
  def drive(self, power, direction):
    ...

What else?

[WasabiFan]

The ones I see are:

• The ability to control them in "tank" mode (individual powers for the two motors) as opposed to the current "steering" mode
• Options for both power and speed control
• A method to stop both motors
• An interface to set arbitrary properties on both motors

Additionally, it would be interesting to see if we could implement it such that the interface was interchangeable with a normal motor, allowing you to pair MotorPairs (e.g., if you have four motors for each corner of a 'bot). That would be a stretch goal though.

[ensonic]

+1 for this. In openroberta I first set all attributes and then start the motors like this:
https://github.com/OpenRoberta/robertalab-ev3dev/blob/develop/roberta/ev3.py#L374
which is a pita.

[WasabiFan]

One other thought: In case it wasn't clear from my earlier comments, I'd like to make sure that you can optionally initialize a MotorPair with just port names. The caller could do this:

drive = ev3.MotorPair('outA', 'outD')

Internally, we'd do this:

if type(motor1) is str:
    motor1 = LargeMotor(motor1)
assert isinstance(motor1, BaseMotor), "A motor object or string must be provided"

This would also mean that we should have a documented way of accessing the internal motor objects.

[dwalton76]

I've started working on this but named the class MotorSet, you can have as many motors as you want (all wheel drive 4-wheeled vehicles, etc). My first stab is here:
https://github.com/dwalton76/ev3dev-lang-python/blob/MotorPair/ev3dev/helper.py#L150

The user can make any sort of motor pair they want but I created LargeMotorPair and MediumMotorPair child classes since those are the two that will be most common. The user would do:

tank = LargeMotorPair(OUTPUT_A, OUTPUT_C)
tank.reset()
tank.run_to_rel_pos(speed_sp=100, position_sp=360)
tank.wait_until_not_moving()

This isn't working yet though, when I call tank.run_to_rel_pos(speed_sp=100, position_sp=360) the first motor correctly rotates at speed 100 for 360 degrees while the second motor rotates much faster and only stops once the first motor gets to 360. I need to narrow down if this is a python issue or not...my gut says no.

[dwalton76]

Turns out I had a bad motor, this is actually working quite nicely. Will post a PR.

[dwalton76]

Fixed