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  1. #1

    Control traveling and ensuring safety in linear actuator using NEMA stepper?

    I am working on a linear actuator prototype with a partner that we plan to eventually mass produce and market. Our vision is to use them for assembly or stacking several of them in multiple axis configurations.

    In our design for the single axis linear actuator, we are currently using a set of small limit switches. The switches are positioned to the side of the axis of motion and use signal lines to/from an Arduino controller to stop (via software) the actuator's travel.

    We recently integrated a second set of external limit switches just a short distance beyond each of the first set. This second set runs a voltage line of the motor through these second switches before it reaches the motor. We mean for this to act as a safety feature that will shut off the motor if the traveling component goes beyond either of the intended first switch set positions.

    For a number of reasons, this extra set of switches will not work in the final design of our product. Therefore, we need some other alternative to limit the travel while also ensuring safety with using the high torque motor. Ultimately, we would like to have a single limit switch or other electromechanical component that limits the travel length while also ensuring safety under operation from the high torque motor.

    My question is:

    How do other similar commercially available products that use high torque motors accomplish this while ensuring safety? Are additional fuses, switches, or relays typically used in combination to prevent catastrophic failures? How can we guarantee the motor will stop when it hits the switch or component, even if there is a hardware failure or software error?

    Thank you for reading my post.

  2. #2
    Community Moderator Jim Dawson's Avatar
    Join Date
    Dec 2013

    Re: Control traveling and ensuring safety in linear actuator using NEMA stepper?

    Typically the travel limits are connected to the controller or drive. The hard stops should be strong enough to resist the torque of the motor. In the case of a servo motor, the drives normally have over-torque and/or encoder error protection built in and that should disable the drive. In the case of stepper motors, when they encounter excssive torque, they just magnetically decouple and stall without damage.

    In the case of a run away motor, there is not much you can do besides hope that the hard stop holds up to the moving mass.
    Jim Dawson
    Sandy, Oregon, USA

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