Author Topic: Radio-Controlled E-stop  (Read 1578 times)

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Offline ArcManTopic starter

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Radio-Controlled E-stop
« on: August 12, 2009, 03:08:43 PM »
I want to implement a radio-controlled E-stop for my radio-controlled yard robot.  It currently has an E-stop circuit activated by an E-stop pushbutton on the control panel, but I also want to E-stop it remotely by shutting off my radio transmitter.  I'm looking for ideas/help/feedback on this.

For those not familiar with E-stops, the key point is to stop a machine very reliably.  Component failures should cause the machine to stop.  My E-stop relay drops out power to the drive motors and supresses the spark on the gasoline lawn mower engine.  I don't have anything fancy like a safety relay with contact weld sensing, etc., but I want a fairly reliable safety circuit.

My current idea is to run the radio receiver to a micro which will time out if a pulse is not received for 100ms.  The micro would drive an AC coil relay whose contact will drop out the E-stop circuit.  The micro would produce a 60 Hz square wave output, which would be converted to a 60 Hz sine wave to drive the coil.  Since the relay has an AC coil, if the micro fails the relay will drop out regardless of the output state upon failure (since it will be DC).  If the radio receiver or transmitter fails, the pulses will cease and the micro will turn off the relay.

Any better ideas?  If not, what is the best way to convert a square wave to a sine wave to drive a relay?  Maybe I don't even have to convert to a sine wave?  I got lots of Google hits, but nothing really gives me any practical, useful advice.

Offline Soeren

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Re: Radio-Controlled E-stop
« Reply #1 on: August 12, 2009, 03:43:07 PM »
Hi,

Any better ideas?
Just use a "missing pulse detector" with a 555 and a few other bits. The less components you use, the less chance of a component failure in your emergency circuit.

Using a micro like you want, it could get into a loop (if hit by ESD or similar), keeping up the AC even if the TX or RX stops working. Keep it as simple as possible and remember that nothing can be made 100% stable (or emergency kill switches wouldn't be needed at all).

You could use a spare channel for a Dead Mans Handle, which would make it stop if you dropped the TX-box, felt asleep or whatever.

You don't need a sine for an AC relay, a square wave should be fine, but it might ruin your plans that it will hang on DC as well, unless modified with a large pass through capacitor.

If you want to improve the failsafe idea, you should use eg. 2 series connected transistors for controlling the relay, as the typical failure mode for a transistor is shorted. with 2 of them, it would be less likely to get a total shunt (but you'd need to check both transistors on a regular basis).
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Joker94

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Re: Radio-Controlled E-stop
« Reply #2 on: August 13, 2009, 05:35:10 AM »
some more expensive units will have built in e-stops, if it does it will say in the instructions

Offline ArcManTopic starter

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Re: Radio-Controlled E-stop
« Reply #3 on: August 13, 2009, 01:58:59 PM »
Thanks for the input.

Especially about the square wave being able to drive and AC relay.  I was planning on using a pass-through capacitor to ensure that a DC signal couldn't operate the relay coil.  How big do you think the cap has to be for 60 Hz?

Offline Admin

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Re: Radio-Controlled E-stop
« Reply #4 on: August 13, 2009, 02:44:07 PM »
This is the easiest most reliable most efficient wireless kill switch I know. No programming, circuits, or anything else needed. And doesn't matter what your current draw or voltage is.

Get a RC controller, a receiver, and a servo. Get the cheapest out there, probably costing you $50 including shipping. You probably already have these so it won't cost you a penny . . .

The servo is then physically connected to a power switch.

Done! :P

 


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