Hi, I have a linear actuator which I purchased to open my gate remotely. This has automatic stops at the extended and retracted position. The current to the 12v motor is reversed to change from opening to closing.
I have found it impossible to match the stops at the fully opened and fully closed position.
I have decided on an excellent way to stop power to the actuator when the gate is closed or opened. But it needs a smart circuit design which is beyond me. I have tried flip flops, latching ics etc not physically but at a design stage and have got hopelessly confused.
The first obstruction to the gate movement is at the gate closed position where it cannot move any further. I can make an obstruction at the gate open position. I have used a 1 amp fuse in the circuit which blows when the gate meets an obstruction protecting the motor. When a dc motor is overloaded it draws more current.
I want this smart circuit to stop power to the actuator when the current to the motor exceeds 1 amp. When the current to the motor is reversed I want power to the actuator to be restored thereby moving the gate in the other direction.
Such a circuit may already be available or if not can any of you smart guys on the forum design such a circuit for me? Thanks in advance.

 

It would be a lot easier if you can add limit switches at the open and closed positions.

 

I would think that constantly forcing the motor to stall and exceed its normal operating current would shorten the life of the motor, gear train, or something, but there is nothing wrong in theory with sensing the stall current and using using that signal to control the direction circuits.

What is the motor's operating current rating?

What type of power switching devices do you need - relay's, power transistors, etc?

Is this what you want:

Motor is running in the open direction.
Gate hits an obstruction and the motor stalls.
Circuit cuts power to the motor.
Gate sits open indefinitely, until ...
Operator flips the reversing switch.
Motor runs in the close direction
Gate hits an obstruction and the motor stalls.
Circuit cuts power to the motor.
Gate sits closed indefinitely, until ...
Operator flips the reversing switch.
Repeat

yes / no

ak

 

Something to be allowed for, if you sense the 'stall' condition, is differentiating between stalling due to hitting an obstruction and 'stalling' due to normal start-up.

 

Hopefully, startup current and stall current are different enough to differentiate. Also, a solid state motor driver can implement a soft-start current control.

ak

 

If you want to go the Stall version, replace the fuse with a Polyswitch and a reverse diode of sufficient size.
When the motor stalls, the polyswitch heats up and goes high resistance, limiting the current. The motor can be run in the opposite direction because of the reverse diode across the polyswitch. When the polyswitch cools down, it resets.
I would add these polyswitch setups AND limit switches. Limit switches are the best way to go as it does remove the strain from the motor. And the ployswitch will stop the motor if there is an obstruction.

 

Hi, I have a linear actuator which I purchased to open my gate remotely. This has automatic stops at the extended and retracted position. The current to the 12v motor is reversed to change from opening to closing.
I have found it impossible to match the stops at the fully opened and fully closed position.
I have decided on an excellent way to stop power to the actuator when the gate is closed or opened. But it needs a smart circuit design which is beyond me. I have tried flip flops, latching ics etc not physically but at a design stage and have got hopelessly confused.
The first obstruction to the gate movement is at the gate closed position where it cannot move any further. I can make an obstruction at the gate open position. I have used a 1 amp fuse in the circuit which blows when the gate meets an obstruction protecting the motor. When a dc motor is overloaded it draws more current.
I want this smart circuit to stop power to the actuator when the current to the motor exceeds 1 amp. When the current to the motor is reversed I want power to the actuator to be restored thereby moving the gate in the other direction.
Such a circuit may already be available or if not can any of you smart guys on the forum design such a circuit for me? Thanks in advance.

What is the Model # of this actuator?
Please post the URL ...

 

I would think that constantly forcing the motor to stall and exceed its normal operating current would shorten the life of the motor, gear train, or something, but there is nothing wrong in theory with sensing the stall current and using using that signal to control the direction circuits.

What is the motor's operating current rating?

What type of power switching devices do you need - relay's, power transistors, etc?

Is this what you want:

Motor is running in the open direction.
Gate hits an obstruction and the motor stalls.
Circuit cuts power to the motor.
Gate sits open indefinitely, until ...
Operator flips the reversing switch.
Motor runs in the close direction
Gate hits an obstruction and the motor stalls.
Circuit cuts power to the motor.
Gate sits closed indefinitely, until ...
Operator flips the reversing switch.
Repeat

yes / no

ak

analog kid, yes that's exactly what I want. not necessary to know the operating current of the motor. the dc current to the motor is less than 1 amp. when the motor under load it will draw more current which can be used to stop power to the motor when an obstacle is encountered.
Have used 1 amp fuse a few times now. no damage to motor.
I have a relay which is capable of taking reversing currents but there may be another way to do it without a relay.
It would be great if you could design a circuit for me analog kid. I too work mainly on analog circuits because in the end you always have to use analog parts even if you have a microcontroller.

If you want to go the Stall version, replace the fuse with a Polyswitch and a reverse diode of sufficient size.
When the motor stalls, the polyswitch heats up and goes high resistance, limiting the current. The motor can be run in the opposite direction because of the reverse diode across the polyswitch. When the polyswitch cools down, it resets.
I would add these polyswitch setups AND limit switches. Limit switches are the best way to go as it does remove the strain from the motor. And the ployswitch will stop the motor if there is an obstruction.
View attachment 178049

limit switch no good because I have a remote operated lock on the gate. it would be near impossible to match the switch with the lock. the gate shut position is an obstacle which exactly aligns with the lock. further advantage of overcurrent protection is if a child or person is the obstacle.

 

Hopefully, startup current and stall current are different enough to differentiate. Also, a solid state motor driver can implement a soft-start current control.

ak

the motor does not actually stall in the sense that it stops no damage done. without the 1 amp fuse the motor would burn out.

 

Hopefully, startup current and stall current are different enough to differentiate. Also, a solid state motor driver can implement a soft-start current control.

ak

the start up current is less than 1 amp otherwise the gate would not have opened because the fuse would have blown at the start.

 

not necessary to know the operating current of the motor..

Yes, it is.

ak

 

Yes, it is.

ak

current extending is 480ma. for retracting 400ma. start up current only slightly higher. for manual control of the actuator I used a 3 position switch. centre is off, and other 2 positions are for forward and reverse current.
for remote control I would use a relay. when coil is activated current flows in 1 direction. when deactivated it flows in the opposite direction. there is no off position.
All available on market swing gate openers do not come with an electronic lock. It is easy to do with a sliding gate opener but not with a swing gate. I had one before. A lock is essential as otherwise the gate is easily forced open damaging the gears on the opener which happened to mine.
This time I got the electronic lock first which is good on its own being remote control.
If you or someone else on the forum could design the circuit to do what you have precisely stated AnalogKid that will be great. I can see it is not an easy project and has got me stumped. Really grateful for your help. If you need any more info please let me know.

 

Here is a very simple circuit that should do what you want.

In operation the motor direction switch powers the motor with the 0V return path via the 1 ohm resistor and transistor Q1, controlled by the op-amp IC1.

The non-inverting input of the op-amp is held at 0.7V by diode D1, if the inverting input (set by VR1) rises above this level then the op-amp will switch off Q1 cutting power to the motor.

The 1 ohm resistor should be at least 1W.

Capacitor C1 should be as large as possible, giving a slight time delay in operation of the current trip. You could replace VR1 & the 10kΩ resistor with values of 100KΩ to increase the time delay operation of the trip (to minimise start up/nuisance tripping of the circuit).

 

start up current only slightly higher

The start-up duration may be very short if the actuator is a worm-drive or ball-screw type, but I find it hard to believe that the start-up (stall) current is only slightly higher than the normal running current. A multimeter may not register the brief start-up current pulse accurately. How did you measure it?

 

Here is a very simple circuit that should do what you want.

In operation the motor direction switch powers the motor with the 0V return path via the 1 ohm resistor and transistor Q1, controlled by the op-amp IC1.

The non-inverting input of the op-amp is held at 0.7V by diode D1, if the inverting input (set by VR1) rises above this level then the op-amp will switch off Q1 cutting power to the motor.

The 1 ohm resistor should be at least 1W.

Capacitor C1 should be as large as possible, giving a slight time delay in operation of the current trip. You could replace VR1 & the 10kΩ resistor with values of 100KΩ to increase the time delay operation of the trip (to minimise start up/nuisance tripping of the circuit).

 

The circuit must be automatic and in your circuit you have to press a button to override the current limit turn off. Also the RT424012 relay does not have a power turn off position. No power to the relay coil directs the current in 1 direction, power directs the current in the opposite direction. It is a dpdt relay. Further there should be no delay to turning off the power to prevent damage to the motor.

 

Yes, it is.

ak

You really need more info from me to solve this problem. Here it is. For remote control I have 4 remote control operated relays.1turns on the gate lock, press again to release gate lock.
I can use another remote relay to operate the linear actuator. I have a dpdt RT424012 relay. For extending the actuator the ground is applied to the red of the actuator and + to the black. For retraction its + to red and ground to black.
No power to the coil of the relay sets + to the red of the motor, G to black. Power sets + to black of motor and G to red. There is no power off position as with my manual switch.
Hoping this further info may aid in a circuit design.

Yes, it is.

ak

 

The standard problem is the "chicken coop door" or 'door lock actuator", but it does not allow obstacle detection. It does implement dynamic braking which means the motor does not overrun it's limits. I did design a comprehensive interface for AAC "greenhouse window" that can detect "moving", and at each limit.

The "standard" door lock circuit uses two automotive relays. When one coil is engaged you get an open. when the other is engaged, you get a close. When both or none are engages, you get a stop/brake action.

For fun, way back when and maybe about the same currents, I did a very quck protection for a model railroad gantry crane. I allowed a brief pulse to be OK, but then as an overload happened. I put the regulator in shutdown 1.2 V out. not the best, but it did work.

There are some reference designs out there that are for power window circuits. Thisisultimately what you need. I believe that they do incorporate hall effect switches and they don't really need limit switches.

I cvna't find a link for a door controller that was pretty simple. I think it worked on 24 VDC motors. It was not in the US.

EDIT: Found it
https://electromen.com/en/products/item/power-controllers/230vac/EM-231B


I just found this: https://media.distributordatasoluti.../8e87184009fddd327cd8b5efb6bc38e5da1d41cb.pdf

 

The circuit must be automatic and in your circuit you have to press a button to override the current limit turn off. Also the RT424012 relay does not have a power turn off position. No power to the relay coil directs the current in 1 direction, power directs the current in the opposite direction. It is a dpdt relay. Further there should be no delay to turning off the power to prevent damage to the motor.

In operation, power must be removed from the motor otherwise the motor will always be driving in one or other direction.

When power is removed from the motor, the overcurrent circuit will automatically reset/switch off. The overcurrent bypass was provided as a manual operation (if required).

If you want to avoid any delay in operation of the overcurrent protection, reduce the value of capacitor C1, or remove it.