[Robotgroup] Safety Circuit Design Question

[Andre Lamothe]

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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