Seeing how fair a telescopic arm has extended in code

What are some of the best ways to see how far a telescopic arm has extended? Using encoder values can lead to inconsistency. I was thinking of putting an ultrasonic sensor at the end and measuring the distance between the end and start of the arm. Teams that have successfully measured this can you please share the sensors that you used?

It depends on how it is extended or retracted. If it is belt or chain driven it is hard to beat encoders.

I have seen some use string pots, Hall effect sensors with magnets placed along the extension, and some time of flight sensors. I still think an encoder is likely the most fail proof.

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Andy Mark sells these or you can find string potentiometers from other suppliers like digikey if that particular model doesn’t suit your needs. However unless you have a lot of slip in your telescope system, you can likely use an encoder without issue.

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I don’t know what arm my team is using but it has a Falcon 500 attached to a retract. If motor runs to retract the arm and it’s fully retracted the motor will keep spinning resulting to encoder value change. Wouldn’t this be a problem?

If there is no movement of the arm and the motor is spinning then yes there would be some error if using the motor built in encoder. If you could share some pictures or sketches of your teams system that would be helpful. It might be possible to have a limit switch that is held down when the arm retracts and as soon as it is released it resets the encoder count.

I will, thank you. That’s a smart idea of using a limit switch and encoder values!
Edit: My team is using the Andymark Climber in a Box

If you’re using a brushless motor with a built in encoder that will work, though you will want to use a limit switch on at least one end of your travel to zero that encoder since they will reset value every power cycle.

Otherwise, adding your own encoder (preferably absolute) or a potentiometer (either a string pot or attached to the motor output in some way) would be recommended. If you choose the later, make sure you set it up so you will out go past the range of motion of the pot.

3468 is planning to use a couple linear actuators to push/pull an arm and our plan is to put an MA3 Absolute Encoder at the pivot point of the arm, wired to the analog in of the Talon SRX connected to the Linear Actuator motor so we can use it for PID control.

Are you using a wheel pressed against the arm to move it in and out?

I’d agree that the string pot is the best solution to this. The limit switch to zero the encoder would also work, but now you need to rely on 2 sensors functioning correctly to get a reliable output.

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And you have to make sure the limit switch is not also functioning as an endstop. It might decide to explode and stop functioning properly.

If you are using a wheel and friction, you will definitely want to rely on something other than just an encoder. That setup is probably the most likely to experience motor slip relative to the arm extension.

Thank you. That’s what we decided, a string pot.

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