MoveDifferential.on_to_coordinates in reverse doesn't work

[kpu979]

There may be times when driving backwards to a coordinate is appropriate.

(our tank's wheels are mounted backwards to save space, hence the 'inversed' polarity)

This code drives the tank in a square (almost perfectly...almost). It starts north, then east, south and west. It then turns 90deg to end up in the same position it started.... almost.

class MCTire(Wheel):
def init(self):
Wheel.init(self, 100.3, 17)

mdiff = MoveDifferential(OUTPUT_C, OUTPUT_B, MCTire, 129.3)
mdiff.set_polarity('inversed')
mdiff.odometry_start()
side=200
speed=30
mdiff.on_to_coordinates(SpeedRPM(speed), 0, side,brake='false')
mdiff.on_to_coordinates(SpeedRPM(speed), side, side,brake='false')
mdiff.on_to_coordinates(SpeedRPM(speed), side, 0,brake='false')
mdiff.on_to_coordinates(SpeedRPM(speed), 0, 0,brake='false')
mdiff.turn_right(SpeedRPM(speed), 90)

mdiff.odometry_stop()

HOWEVER, when I change the speed=-30, I would expect the robot to traverse the same path, only backwards, but instead, it took a much different path. It rolls to (0,-200), then makes a ~30deg turn rolls to about (100,-300), then to about (150,-350) etc. (this is the point where it hits a wall, or closet , or sleeping dog)

I hope this explains the issue, but if not, let me know. Perhaps a video would work?

**ev3dev version: 4.14.117-ev3dev-2.3.4-ev3
**ev3dev-lang-python version:
+++-====================================-=======================-=======================-==============================================================================
ii micropython-ev3dev2 2.0.0 all Python language bindings for ev3dev for MicroPython
ii python3-ev3dev 1.2.0 all Python language bindings for ev3dev
ii python3-ev3dev2 2.0.0 all Python language bindings for ev3dev

[dwalton76]

Hmm, I am sure the combo of negative speeds with on_to_coordinates is something I did not test when working on odometry. It could be made to work though...we start out by calculating what angle to turn to before driving forwards, we would just need to add 180 to that before driving backwards.

For now I would stick to positive speed...the dog will be happier that way :)

[kpu979]

quick q: where is the log file that gets written to by odometry? I'd like to understand this, and perhaps make an attempt at getting this fixed.

I think it's more than calculating the angle to turn. _odometry_monitor need to know the speed (positive or negative) to calculate the true positional movement across a grid.

[dwalton76]

https://stackoverflow.com/questions/6386698/how-to-write-to-a-file-using-the-logging-python-module

Is what you want to do in your program