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Re: [PSUBS-MAILIST] article on the OSS
Hi John,
Let me give you some answers... First the advantage of
pwm is to give you flexibility and reduce the energy
consumption (compared to a high power potentiometer).
Have you tried to switch (even with a relay) high
current? If you're not choosing expensive relays, the
spark generated every switch on/off will solder or
destroy your contacts. Don't forget that your motor is
mainly an inductor and when you're opening/closing the
circuit you generate high current peak...
You can use either power mosfet or igbt in parallel,
depending on the current you need to drive. My motor
controller board is driven by a microcontroller (I use
both 812 Microconverter from Analog devices and
C8051F020 from Cygnal for the PWM control,
instrumentation (pressure, depth gauge, O2, H2, CO2,
water detection, valves control,...) connected
together with a RS485 bus for noise immunity and an
option for optical and radio communication to avoid
through hull connection). Both of them are Intel 8051
compatible but much much more faster (up to 25MIPS for
the cygnal) and include memory, Analog to Digital
converter, Digital to Analog converter, PWM generator,
Temperature sensor, Comparator....
I've chosen this solution because:
1/ I wanted to be able to ramp up the current to drive
the motor at each start to avoid current peak and high
mechanical constraint.
2/ I wanted to have a system flexible enough to be
controlled with both system: a computer for
autocruise, and a joystick in manual mode.
3/ As I'm going to use up to 6 motors, I want to have
a modular system. Each of these boards is also
monitoring the motor temperature, water leaking, and
pressure compensation. I can drive up to 120A under 12
or 24V on each board.
4/ Compared to a high power relay board, mine is 2/3
to 1/2 smaller (including the heat sink)
Laurent
--- John Rogers <djohnndjinn@yahoo.com> wrote:
>
> I dont know about using PWM for something as large
> as a main drive for a Psub. Correct me if I am
> mistaken, but these size motors would use something
> on the order of 40 amps. This would require some
> considerably expensive FETs (with significant heat
> sinks). I would have to look at a catalog but I
> recall transistors of this size being sold for more
> than $80 each. And you would need 4 per thruster.
> I guess only the main thruster would require that
> kind of power though ; you could build smaller units
> for steering thrusters. Plus these things would
> waste quite a bit of energy as heat.
> Do most PSubs use relay-switched banks of batteries
> to control the speed of their main drive? This
> would have the advantage of nearly complete
> transmission of power to the motor, with the
> disadvantage of mechanical and control complexity.
> Could someone give me an estimate of the power
> requirements for a main drive motor for a ship this
> size and speed? I could come back with some hard
> data.
> thanks,
> --John
> Warrend Greenway <dub@linuxmail.org>
> wrote:Actually, a PWM motor controller is pretty
> dang simple. You could almost
> call it a no brainer. It does get more difficult as
> the current increases,
> because of the isolation, but it isn't rocket
> science. One of my first
> electronics projects was to make a PWM motor
> controller with a 555 timer.
> Reference TTL Cookbook. The MSP430 has a PWM built
> in, all you need to
> do is supply isolation and a couple FETs or SCRs.
> So, Dale wasn't that
> far off. This is also why I said that there were no
> if-and-or-buts in
> the motor controller method. PWM is easier, cheaper,
> more reliable, and
> vastly more efficient then manual switching or
> resistive methods.
>
> Warren.
>
> > Dale,
> >
> > At ease, you didn't offend anyone and it's
> possible I misread your article. I
> > was just making a comment that designing and
> building a PWM motor controller
> > is not an easy task.
> >
> > Sorry if you thought I was offended.
> >
> > Have fun,
> >
> > Ken Martindale
>
> --
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=====
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From: Laurent ROUFFIGNAC
444 Saratoga Avenue, Apt 8G
Santa Clara, CA 95050 USA
Home: (408) 244-1285 Office: (408) 546-4704
Email: laurent_rouffignac@yahoo.com
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