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Motor reverse safety

Former Member

I'm working on a dog door project that behaves similarly to a garage door. This is my first project with motors and I've hit a wall in my research. I want the door to be rugged so it would take considerable effort for other animals to get in but I don't want the door to injure the dogs if they try going through at the wrong time. How can I detect resistance in the doors movement so that I can reverse automatically? Currently working with a Titan 550 brush motor that I picked up at a hobby store.

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  • Joshua,  apart from a mechanical clutch, a control solution would be to monitor the current the motor draws.  I had a quick look at that motor on line and it is going to need a considerable reduction gearbox to reduce its output speed.  Such a gearbox magnifies the available torque on the output shaft and so it may be quiet difficult to accurately monitor the current.  Something like the ACS17 current monitor modules available on EBAY may suit your needs.

    For your application I would tend to look for a motor / gearbox unit with slow output speed, such an old windscreen wiper unit off a car.  Try monitoring the current when the motor is running, then applying a stalling load to its output shaft and see how the load current changes.  Then monitor the current and trip the motor at a safe level.

     

    Another alternative is a mechanical linkage with a spring in it such that the linkage is extended or compressed under load.  The movement in the linkage can be monitored by either a microswitch,  a linear potentiometer, a reed switch or a hall effect switch.  It all depends on the degree of movement you want to see.  These various devices can be input to an input with interrupt capability for an instant response.

     

    The mass of the door will have a bearing on the control and monitoring method too.

     

    PLEASE take great care that your device cannot injure any creature that uses it either by intent or accidentally.

     

    hope that helps

     

    Bob

  • in reply to bobcroft

    Thanks bobcroft! You have some great points. I will look into a safer motor / gearbox unit and pay around with monitoring the current.

     

    Good point on the faulty driver. I may have to rethink the opening mechanism to ensure that it's still safe even when faulty.

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