`ValueError: math domain error` triggered in drive_xy

Using the following testing script

from brachiograph import BrachioGraph
from time import sleep

# The servo_x_degree_ms signs need flipping as my motors
# go in the opposite direction to those in the article
plotter = BrachioGraph(
  inner_arm=8.2,
  outer_arm=8.85,
  servo_1_degree_ms=10,
  servo_2_degree_ms=-10,
)
print("Resetting arm position")
plotter.set_angles(angle_1=-90, angle_2=90)
print("Set up complete")

plotter.drive_xy()

and moving around with a,d,l,k I can consistently trigger a math domain error. I think this happens when the hypotenuse becomes very small or perhaps negative?

Here’s the stack trace

Traceback (most recent call last):
File "test.py", line 23, in <module>
plotter.drive_xy()
File "/home/pi/branchiograph/brachiograph.py", line 698, in drive_xy
self.xy(self.current_x, self.current_y)
File "/home/pi/branchiograph/brachiograph.py", line 434, in xy
(angle_1, angle_2) = self.xy_to_angles(x, y)
File "/home/pi/branchiograph/brachiograph.py", line 590, in xy_to_angles
(hypotenuse**2+self.INNER_ARM**2-self.OUTER_ARM**2)/(2*hypotenuse*self.INNER_ARM)
ValueError: math domain error

and here are the values printed during driving the plotter

Resetting arm position
Set up complete
-9.2 8.85
-10.2 8.85
-11.2 8.85
-12.2 8.85
-13.2 8.85
-14.2 8.85
-13.2 8.85
-12.2 8.85
-11.2 8.85
-12.2 8.85
-13.2 8.85
-14.2 8.85
-13.2 8.85
-12.2 8.85
-11.2 8.85
-11.2 9.85
-11.2 10.85
-11.2 11.85
-11.2 12.85
-11.2 13.85

[willprice]

I’ve added a bit of code to catch this issue and inform the user. https://github.com/willprice/BrachioGraph/commit/c8d764d3d50916e7ea4408d3d27ce832a613a0e8 In that branch I’ve turned all the return str messages into exceptions too.

[willprice]

That looks super useful. Thanks @evildmp, I’ll check it out.