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From: Winfield Hill
Subject: Re: Help - Power mosfets - difficult load
Date: 28 Oct 2002 10:03:53 -0800
Organization: Rowland Institute
References: <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
X-Newsreader: Direct Read News 2.98
Bill Allison wrote...
> But you've triggered another idea...
> I could put some resistance in series with the motor and arrange to
> short it out as power demanded approached 100%. That would soften
> transients and would still give me 50Hz impulses to overcome stiction
> and make fine control near zero speed better.
That prompts me to mention another scheme I find more interesting,
current-source control. First consider this: Q. How to add an
electronic fuse or circuit breaker for fault conditions? A. add
a common 100A 50-millivolt shunt resistor (R = 0.5 milliohms), and
upon detecting more than say 100mV (200A) for longer than say 3 sec,
set a fault flip-flop to kill the FET gate drive.
Q. What other cool thing can I do with this shunt resistor?
A. Use it with circuitry to turn the power MOSFETs into a say
0 to 50A linear current source to run the motor in a programmable
torque mode when starting. Consider, 0 to 50A would be 0 to 25mV,
a very managable voltage range. A difference amplifier with 0.1mV
offset could be used to control the servo; 0.1mV offset creates an
acceptable uncertainty of only 0.2 amps. And etc.
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