Detecting wheel slippage/not moving

I was wondering if there was a way to detect if the wheels are slipping or not moving on the Stretch? I plan to do some collision detection with the base and couldn’t figure out if there was any way to get this information. Thanks in advance! :slight_smile:

Hello Daphne, welcome to the Stretch Forum!

We’ve done only preliminary exploration of collision detection on the base. It is something that we do plan to provide as a standard signal exposed by Stretch Body (tbd when we will however).

Perhaps if you figure out a robust collision detector you can share your results back here. There are a number of signals coming out of Stretch Body that may be useful:

Motor current: The current signal (robot.base.right_wheel.status['current'] and robot.base.left_wheel.status['current'] will exceed the normal value when the robot stalls or hits a large obstacle. However, we don’t yet have a dynamic model of the robot base (which will be necessary to distinguish between internal loads and external disturbances) – although this is coming soon.

Base accelerometer: The IMU in the base includes an accelerometer which can be used to infer momentary contact events. We expose robot.pimu.status['bump_event_cnt'] which increments on a bump event. Bump events are triggered when the sum-of-the-squares of the acceleration values exceed a YAML specified threshold. You should be able to see the bump_event_cnt increment if you knock the base somewhat hard with your hand.

Trajectory tracking: The steppers in the base track position and velocity fairly well in most circumstances (less than 1 degree error RMS or so). You can monitor the tracking error (eg robot.base.right_wheel.status['err']) of each motor. If the error value exceeds the normal range it likely indicates that the wheel is experiencing a disturbance such as a collision.

I’d suggest exploring these signals and possibly building a detector that fuses the information in some manner (as each signal may capture different aspects of a collision event). The tool provides a good example of how to plot signal values in a real-time oscilloscope. In addition, you can check the bump_event values by simply running: --bump

I hope this helps. Let us know how it goes!

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