Magnetic Sensor controlling a small winch

[DanSteely]

Hi All,

 

I am considering methods of limiting the travel of a mains powered stage truck so that it stops every time in the right place. The truck will be winched by a mains powered motor which will pull the truck into place. The truck will travel in a countersunk track in the stage.

 

I am looking at magnetic reed type sensor in the stage and a permanent magnet in the truck. The reed switch sensor would then control a DC relay, which in turn switches the mains powered winch.

 

The winch takes about 6 amps so controlling this directly by a sensor is not practical, therefore the 6A switching will be done by a 12V relay.

 

The 12V relay in question pulls about 160 mA so my question is will a small magnetic sensor like this do the job?

 

http://cpc.farnell.com/hamlin/59135-030/sensor-proximity-magnetic-spdt/dp/SN36685

 

Obviously there will be dead mans switches in this arrangement as a plan B.

 

Any thoughts gratefully received.

 

 

[jonathanhill]

I would use a Flint's style hand winch with a TruMeter on it. That way, when the delicate reed switch gets broken, the truck doesn't career towards the pit with alarming velocity.

 

That switch you linked to has a sensing distance of around 29mm.

[Jivemaster]

If I wanted to trust a reed switch I would also look at the encapsulated ones for garage doors. They have a big protective alloy block and thick wires. In reality I would want a real tech controlling the truck, maybe with assistance from technology. Fully auto trucks include some highly visible failure modes.

[ImagineerTom]

Once you get in to the world of stops / safetys and position sensors for big moving things that can easily damage people and other stuff a) you need to start designing systems with multiple redundencies and with inherent safety features built in and b) you definitly need to be looking at physical interfaces for positioning. Magnets have different properties across their life (and different magnets in different trucks or after being repaired) and are effected by all sorts of external factors (someone places a speaker cab near your reed switch that causes it to jam or become desensitised for example) that will result in the stop position changing significantly.

 

Professional automation systems use encoders as the primary positioning / limiting control with backup systems of physical switches built in to the track. Some industrial microprocessors running a stable logic system sits in the background constantly monitoring ALL the sensors and if any one ever produces and error or deviates from optimum condition at any time then everything is instantly halted.... all this in addition (but entirely able to override) whatever actual control system you then use for the day-to-day movements. If you're installing some automation that is the absolute minimum level of safe control system you should be thinking about; you also need to be doing a lot of RA's for the whole setup as there's a wide range of potential circumstances where what would normally be a minor issue suddenly becomes a major incident with blood and broken bones because a hundred kilos of unstoppable automated truck is involved.

[pstewart]

Wouldn't a limit switch be more appropriate for this?

[DanSteely]

...Sorry ... been v busy.. Many thanks for all your comments and suggestions on this...

[KevinE]

Agree, an industrial limit switch with whatever actuator suits your situation. It's less likely to lose a magnet or stick and these sort of thing are happily acting as overtravel safety switches in lifts, fairground rides and factories producing everything everywhere.