Motor Control app ...how to sense current non invasively?

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I'm gonna be driving DC motors in the range of 24 to 100+ volts using
an H bridge. Due to the potentially high voltage components, i'm
isolating the drive circuit from the PIC microcontroller with
optocouplers.

However, I need a way to sense the motor current. I'm thinking of
putting a current sense resistor, and the traditional op amp, etc, and
feeding this to a voltage to frequency converter chip. The output of
the VtoF chip would then go into the optocoupler input, and out to the
Capture/Compare pin on the PIC.

The above looks too tedious and has too much parts count. Is it
possible to use a "current sense transformer" or even so called "hall
effect sensor" from Allegro, where I would thread the motor cable
through and it gives me the voltage? I would be clocking the PWM at 32
khz to 64khz.

THanks,
Mike

Re: Motor Control app ...how to sense current non invasively?
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That's not high voltage, really now! Think analog and get a data sheet
for the IL300. IIRC it has full circuits for just such as this.

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Sensing would be fairly slow. All your output devices could be dead
before the PIC heard them squeal.

Paul Burke


Re: Motor Control app ...how to sense current non invasively?
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This is way too fast for your pic to do reliably, why do you need to chop so
fast anyway? As a general rule you should run at the lowest frequency that you
can get away with. You can sence the drive current with a resistor/amp, or use
a transformer, coupled to a comparator and fed back to 1 of your pic interrupt
inputs. Be aware that using software to directly control the mosfets can lead
to problems/failures if you have a crash/bug. Most people use hardware to do
this sort of thing, you can find lots of circuits all over the web.

Re: Motor Control app ...how to sense current non invasively?
Does your PIC have an ADC ?
The best (although not inexpensive) solution is an isolated closed loop
hall-effect current transducer (www.lem.com)

You'd also probably want hardware overcurrent comparators to quickly disable
the FET's/IGBT's in the bridge in the case of an overcurrent

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Re: Motor Control app ...how to sense current non invasively?
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If you can manage to stay below 55V then the LMD18245 from National
Semiconductors may be your ticket. It has an integrated current sense
amplifier, output readily available.

Rob



Re: Motor Control app ...how to sense current non invasively?
Thanks, can't use this though, because the supplied motor may be one
rated at 120V. So i'll be rectifying the winding from a 100VAC
transformer.
-Mike

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Re: Motor Control app ...how to sense current non invasively?
snipped-for-privacy@yahoo.com (Mike V.) wrote in message
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There is on opto device that transmitts voltage levels across the
isolation barrier.  CP Clair perhaps.  You could put in a current
sense resistor then this opto and have an isolated voltage
proportional to current.

George

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