Drapery motor AC120V - mechanical limit switch convert to current sense

[pico]

I have a Lightwood (also sold under the Wells name) draper motor that has been in service for a good 15 years. The mechanical limit switch no longer detects when the drapes are fully closed so the whole thing becomes a boat anchor... unless... I can convert the mechanical limit switch to something electronic.

These motors mount near the floor and pull the cord for you. They are very robust and work very well. This had been one of the best things about my automated home. It is a testament to the design that it lasted 15 years but then again it only opens and closes once per day. The original patents for the unit date back to the 1950's. The design was eventually sold to Somfy, but the newer versions went to current sensing and the most recent is a direct drive system with no gearbox.

I was going to use an optical pulse count system to detect when the motor slowed down before stopping but someone said it would not be reliable and suggested current detection as an alternative. I agreed and found the Allegro ACS704ECL-005 sensor (5Amp) which I thought might be suitable.

Unfortunately the measured current barely changes under any operating conditions and with the gearing it is very difficult to jam this motor.

Normal load is 660mA
End of track is 680mA
Jam condition >700mA

Realistically can this small current be measured with an Allegro current sensor ? I wish they had a 1Amp model but..oh well.

Here is a couple pics of the motor and the schematic. You can see the black plastic actuator that is pushed sideways by the force of the draw cord and the spring loaded snap type limit switch which is where the problem lies.

 

[tunedwolf]

I think I would be looking for an electro-mechanical solution to this problem rather than an electronic one, perhaps two long lever micro switches fitted back to back or something...but as long as you're in the mood for an electronice one, can you not just detect the lever change over action with a couple of photo detectors ? If you can, then it's a simple matter of constructing a change over with a relay instead of the switch...
Current draw is perhaps one way of detecting when it's stalling, however anything that causes it to draw more current will upset your circuit.

 

[pico]

I appreciate the ides. Yes, there are some problems with current detection, but it seems better than my optical idea.

What would be very good is some kind of pressure sensor that can give me a variable output based on how much tension is on the curtain cord is when it is pulled tight at fully opened or fully closed.

Close-ups of click-type limit switch and the actuator.



Anyone ? Please be candid I'm open to any workable idea.

 

[philba]

I'd think photo detectors would be problematic - varying light conditions and all.

You might take a look at Hall Effect devices. They are triggered by a magnet and can be used to drive an NPN that in turn drives a relay to mimic the limit switches. Glue a magnet to the carriage (is that the correct term) and when it gets to the HE device, it trips the relay.

I am looking at building a motorized mask for my projector's screen. It will have multiple stops so the HE approach makes a lot of sense.

 

[pico]

What kills me is the switch and all look fine and the contacts are good. I might be the curved spring that can be seen on the limit swiitch because the switch works well in one direction but does't flip/click to the other direction with the same amount of force. I can't seem to duplicate this when it is apart.

philba: the unit is enclosed so the ambient light would not have any effect.

Update:
I just finished pulling apart my other motor which is a newer Somfy DR1000. I was surprised to see an optical encoder wheel and PIC chip inside. This is exactly what I had thought about doing in the first place.

A sensor that can detect the "torque" on the motor or on the cord would probably be an even better solution.