On Mon, Apr 05, 2010 at 07:56:51AM -0700, Peter C. Wallace wrote:
> On Mon, 5 Apr 2010, Erik Christiansen wrote:
> >
> > A) Is it safe to interrupt Motor Power to my 7i40s, with hairy great
> > inductive motors on their outputs? (OK, mostly resistive under load,
> > but then back EMF might also be deadly if V+ is open-circuited?)
>
> Should be, the 7I40 has overvoltage protection, but this cannot be
> depended on for repeated faults (board thermal limit)
That scared me off at first, but reading the manual's description of the
in-built load dump protection, I'm sufficiently comforted to give it a
go. [1]
> A good Estop system is to disconnect servo primary power and at the
> same time discharge the servo filter capacitors with a resistor for .5
> - 1 second decay time. This will brake the motors so the voltage decay
> should not be too fast.
Realising that the back-emf sees the parasitic FET diodes in the
H-bridge as a bridge rectifier, much like the one Andy used to protect
exploding relays, It finally sinks in that that provides dynamic braking
of all motors with one shunt. This scheme should also keep the 7i40
powered up for the peak of the load dump, even with the supply
disconnected, so I don't need an external load dump protector.
Even using a FET or two (held on by a contact on the estop relay) to
interrupt the total supply current (which my relays won't handle) this
is neater than motor-side protection. Your "at the same time" advice is
just sinking in. There's no need for dead-time between supply
disconnection and shunt application. (It just get simpler. :-)
One second decay seems a bit slow, to this novice. The motors would stop
well before zero volts, but every 0.5s delay moves the hard limit
switches further from the end of travel, reducing usable table area. I
don't think I'd want soft limit switches even further in, then.
Time to just build and try it now, I think.
Many thanks for the sagacious advice.
Erik
[1] Found a bunch of 29mm square self-adhesive Al heatsinks, just made
to stick onto each quartet of FETs on the 7i40. OK, they're only
contacting the cases, but must provide some additional thermal
margin.
--
The advantage of a bad memory is that one enjoys several times the same
good things for the first time.
- Friedrich Nietzsche
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