[Robotgroup] Safety Circuit Design Question

Mon May 5 17:06:46 PDT 2008

Your right that I should get out the calculator and to the energy
calculations to see what the impact would really be. I have decided that if
I do use a dual coil relay that I will allow the process to turn OFF the
motor circuit but will not connect the SET coil to the microprocessor. I too
have seen processors do some very improbable things when they crash (random
code execution) or when the power is low so I don't want to give it a chance
to energize the motor circuit. Once the motor circuit has been disconnected
it should take a manual button push to reset it. If I go with a single coil
non latching relay then it may be a bit trickier to allow the process to
command a shutdown  but the rest of the logic will be just as easy.

Thanks for yours and everyone else's thoughts and comments on the design
elements.

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 Gray Mack
Sent: Monday, May 05, 2008 11:33 AM
To: The Robot Group Mailing List
Subject: Re: [Robotgroup] Safety Circuit Design Question

Well, it depends, if you have a car battery then
driving a little relay for hours is no biggie.
Something smaller than a hatbox that runs off 4AA,
power might be more of a concern and safety less of a
requirement. 

There are some low power relays out there, for
example, Marvin has a big RC car that uses relays
instead of an h-bridge circuit to drive the rather
large motor (I guess it was cheaper than a 4 amp
h-bridge) and the power drain is probably much less
than the motor. Some relays use less than 30mA, not
too much for a 7200mAh battery.

One good reason for holding the relay closed is that
circuits often do weird stuff when the battery gets
low. Processors start brown-out resetting, etc. In
that case the logic controlled relay might disengage
naturally and to energize it would be impossible. Your
motors might be on a different battery and be able to
continue running if they weren't turned off.

Some relay info
http://www.kpsec.freeuk.com/components/relay.htm

With multiple trigger sources you are getting into
digital logic as Paul suggested, you can usually use
basic gates or "transistor resistor logic" (google it)
to do that and hopefully not need to result to
"karnaught maps" (you don't want to know) :)

-Gray

--- Michael Boswell <Michael at hilltopcafe.net> wrote:

> 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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