Hello,

I'm working on a project that uses a linear actuator, remote controlled relay board and 2 limit switches. There are two relays configured in an H-Bridge remotely controlled. When I hold the "UP" button (I have to keep it depressed) on the remote the linear actuator lifts a table upward until it hits the upper limit switch at which point it stops. When I hold the "Down" button on the remote the linear actuator lowers the table until it hits the lower limit switch at which point it stops. So it seem the wiring is working properly but I have a problem. When either limit switch is activated, the power to the motor gets cut, but the motor is coasting an extra 5-10mm and crushing the limit switch. Here is what is confusing me. Let’s say I am holding the "Up" button and the lift is moving upward, before it hits the limit switch if I let go the button the motor stops immediately(dead stop) and does not coast. I'm trying to figure out how to stop the coasting and make the motor stop when it hits the limit switches. Why does it stop immediately in one circumstance but not with the limit switches? Take a look at the wiring diagram attached. Any feedback is appreciated. Thanks. Frank

 

It's because the motor terminals are shorted together when either relay drops out before the limit switch is engaged. A form of electrical braking.
SG

 

Two options come to mind, the first is to adjust the limit switches so that the coasting does not crush them. Limit switches should never be in a position to be damaged by over travel. The second thought is to arrange for the H-bridge to short-circuit the motor when it is stopped, which should stop it much more rapidly with no coasting. Either approach should solve the problem.But dynamic braking does put a bit of stress on motors.

 

Thanks for the information. How would I arrange the H-bridge to short-circuit the motor when it is stopped by hitting the limit switch?

 

If you use the switches to control the relay coils instead of the power to the motor, with both relays dropped out, shorting the motor leads will result in dynamic breaking which will help to stop the actuator sooner.. The UP limit could be placed in series with the UP push button and the DOWN limit could be place in series with the DOWN switch.

 

If you use the switches to control the relay coils instead of the power to the motor, with both relays dropped out, shorting the motor leads will result in dynamic breaking which will help to stop the actuator sooner.. The UP limit could be placed in series with the UP push button and the DOWN limit could be place in series with the DOWN switch.

I understand what you are proposing and it makes sense. The current relay board I have has its own power supply which feeds both relays so I cannot individually control the power to each relay separately without hacking the pcb board they are on (maybe an option). I'm going to have to look at some other options. There weren't many that I found that had a remote interface. Thanks.

 

Thanks for the information. How would I arrange the H-bridge to short-circuit the motor when it is stopped by hitting the limit switch?

Do the limit switches have NO and NC contacts?
SG

 

Do the limit switches have NO and NC contacts?
SG

Yes, thy have both NO and NC tabs. They are OMROM 2691RA VX-015-1A3 30VDC..

 

Yes, thy have both NO and NC tabs.

You can rewire the limit switches to provide a brake.
SG

 

You can rewire the limit switches to provide a brake.
SG
View attachment 171585

Thanks for the diagram. Where would my relays connect. Sorry I'm new to reading these diagrams.

 

Thanks for the diagram. Where would my relays connect.

The relay wiring remains the same. You just need to add the connections drawn in RED. I will redraw your layout as well.
SG

 

Add wire as shown between NO contact of limit switches to the red wire from the Linear Actuator.
SG

 

That looks so simple. I'm going to go give it a try. I will let you know the outcome.
Thanks.

 

That looks so simple

When you do double check the wiring connections on the Limit Switches as far as C, NO and NC.
SG

 

I checked all connection for NO/C/NC. I just crimped some wire and made the connections you mentioned above. So the lower limit switch now stops on a dime perfectly so that corrected lowering the table. However the upper limit switch is still making the motor coast.

 

However the upper limit switch is still making the motor coast.

OK let me take a another look.
SG

 

OK let me take a another look.
SG

Found the problem. Your diagram is correct. My upper limit switch is inverted so the NC and NO were reversed and my eyes aren't as good as they use to be!! It works as you mentioned. Thank you very much!

 

A better way, which would also have eliminated any arcing of the contacts due to motor inductance, might have been to add a diode bridge to carry the motor current when any switch opens, as shown below

 

Thanks for the information. How would I arrange the H-bridge to short-circuit the motor when it is stopped by hitting the limit switch?

If both lower sections of the H-bridge were switched on at the same time that would provide the short circuit. If the released sides of the relay switching circuit driving the motor were connected that could also provide the short-circuiting function. BUT you must be careful to avoid accidentally short-circuiting the supply voltage. So the circuit arrangement will need some careful thinking. And now, after seeing the most recent posts, it seems that there could also have been a connections in the wiring issue.

 

A better way, which would also have eliminated any arcing of the contacts due to motor inductance, might have been to add a diode bridge to carry the motor current when any switch opens, as shown below
View attachment 171591

Thanks for the alternate concept. I will look into this as I read about using diodes but wasn't sure how to apply it to my specific setup. In the longer term I would want to prevent any arcing...