[Robotgroup] Safety Circuit Design Question

[Michael Boswell]

OK, the reasoning for supply side makes lots of sense. Thanks for your and
Andre's response and clear logic.  I also agree that a NC circuit with a
non-latching relay would be even safer, but I am concerned about the
constant current drain from holding the relay closed. If it was plugged in
the wall, no problem but on battery power it might add up quickly. I confess
though that I have not actually calculated how much power this would take in
terms of reduced run time. I am just thinking to minimize power consumption.


Another advantage of a N.O. circuit is that it can be and effective OR
circuit of a Multiple trigger sources. In other words, I could have several
bump switches wired in parallel and into the same coil input, I could also
easily add a Microprocessor controlled signal so that the processor could
also initiate a shutdown. This would seem to be more difficult with a N.C.
design. 

Michael Boswell
Austin End Of the Line Kite Team - Kite #4
http://Austineol.com
Picture Gallery at http://www.pbase.com/mboswell

-----Original Message-----
From: robotgroup-bounces at puremagic.com
[mailto:robotgroup-bounces at puremagic.com] On Behalf Of Paul Atkinson
Sent: Friday, May 02, 2008 6:52 PM
To: The Robot Group Mailing List
Subject: Re: [Robotgroup] Safety Circuit Design Question

I agree with Andre' and the examples he gave. I'd put the break in the
supply (or positive side) and leave the ground attached all the time. For me
it's just easier to do it that way. It also matches the electrical system in
your car and hopefully your house (where you switch the hot lead and neutral
is always connected).

I might use normally closed switches so that if they open the relay circuit
is broken and power is removed from the motors/load. This way the
motors/load are only energized when the relay power is on and the switches
are closed. In my mind this is safer because everything (in the safety
circuit) *has* to be working in order to energize the load.

The other way, the load is energized and if power to the relays doesn't work
for some reason, you can't turn off the load.

Hope that's not too confusing.

Paul

On Fri, May 2, 2008 at 6:35 PM, Andre Lamothe <ceo at nurve.net> wrote:

> Put it on the supply side.
>
> You never want to break ground since there might be another ground or
> short
> and thus the thing still works, but there is only 1 supply line. Just like
> when your jump a car, you put the + on first since there is no way to
> create
> a short with the - since the whole car is - (ground). If you put the - on
> first then you touch + to the chassis or body then you have 300-1200 amps
> flowing thru the short and something is getting vaporized.
>
> So I would say supply side, always a better idea. Also, if anything has
> any
> charge left in it, it can always go to ground, if you cut the ground side,
> then it can act like a capacitor and discharge later -- also, if you cut
> ground, philosophically it still has power, just too much impedance (air)
> for current flow, its still hot relative to another ground.
>
> Andre'
>
> ----- Original Message -----
> From: "Michael Boswell" <Michael at hilltopcafe.net>
> To: "'The Robot Group Mailing List'" <robotgroup at puremagic.com>
> Sent: Friday, May 02, 2008 5:22 PM
> Subject: [Robotgroup] Safety Circuit Design Question
>
>
> Awhile back there was a discussion about the importance of a safety
> cut-off
> circuit in robot designs.  The video of the 300# robot going straight at
> people was very sobering. While I am not contemplating working on anything
> that big and deadly, it still seems like a good idea to have some sort of
> safety circuit.
>
>
>
> I have a dual coil latching relay that I am planning to put into the main
> motor power buss. This will be after the feed for the low voltage
> regulators
> so the processor and any sensors will not be effected by the state of the
> power relay but the Motor of course will only get 12vdc or 24 vdc when the
> relay is latched.  My question is around the trigger circuit for the
> relay.
> I would expect to put some sort of N.O. switch(s) like Bump Switches on
> the
> chassis that will close on contact and energize the RESET coil to open the
> relay.  The coils in the relay I found are 4.5v nominal. Now for my
> philosophical question. Should I put the switch part of the circuit on the
> supply side or ground side of the relay coil?  Is there any standard for
> this or reason to go one way or the other ?
>
>
>
> Thanks for any thoughts you all have on the subject.
>
>
>
> Michael Boswell
>
> Austin End Of the Line Kite Team - Kite #4
>
> http://Austineol.com
>
> Picture Gallery at http://www.pbase.com/mboswell
>
>
>
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