DPRG mailing list archives: [DPRG] Motor backlash / Back EMF

Sun Jan 6 14:58:20 CST 2008

At 11:15 PM 1/5/2008, scott at lighthouse21.com wrote:
>The L293D and its associated relays are physically located on a separate
>breadboard from the main Atmel board.  Assuming the backlash is coming
>from the relays and not the steering motor (which is at the opposite end
>of the chasis and only has a common ground) then I can decouple the relay
>driver board at that point.

I wasn't clear, I suspect your problem has nothing at all to do with 
the relays and the LM293D. What I suspect is happening is that your 
steering motor is putting a significant load on its power supply 
(relative to the source impedance of your power supply here) and that 
when the steering motor disengages that causes the power supply of 
the steering motor to "bump" and it pulls its ground level up as it 
bumps. (this is called ground bounce by the way). There are a number 
of factors that can cause this the most common is a lot of inductance 
in the wiring between the power supply and the load.

Anyway, since your ATMEL shares that ground it also sees the "bump" 
and its ground goes up relative to its power supply. You can often 
measure this with an oscilloscope by putting it in "AC" coupling 
across the power supply of the ATMEL chip and then capture single 
events. Turn your steering motor on and off and "poof" you will see a 
bump on your scope trace.

If the magnitude of the bounce is high enough, the ATMEL sees it as a 
brownout and resets.

The fix is to prevent the bounce from coming through the ground plane 
to your CPU. And the easiest way to do that is to put an inductor 
between the CPU's ground and the common ground that the motors are 
on. This inductor will resist the change in current (which is the 
ultimate cause of the ground bounce) and so the CPU won't see the 
brownout and reset. If your CPU is on its own PCB or breadboard this 
is an easy fix to try out.

--Chuck