Check the units of the BuilderBot differential drive system

[allsey87]

The following code should drive the BuilderBot forward at 5 cm/s. However, the hardware implementation may not yet be updated and may try to take off at 5 m/s instead.

function step()
  -- drive forwards at 5 centimeters per second
  robot.differential_drive.set_target_velocity(5, 5)
end