Re: Do you think NI can fix my PLL?

On Fri, 02 Jun 2006 17:58:10 -0700, Tim Wescott <tim@xxxxxxxxxxxxxxxx>
wrote:

Chris Carlen wrote:

Hi:


I'm attempting to build another motor PLL system and running into some
difficulties stabilizing the loop. Since there is a lot of work to do,
I have considered contracting out the PLL design to a well known control
expert outside of my company, so I can focus on building other
subsystems of the project.

Basically, the PLL is to lock a 136mmx2.54mm Al wheel to a 400Hz
reference (24kRPM, 1 pulse/rev position sensor). Must be 2nd order PLL
yielding zero phase error with constant frequency input. Wheel to be
driven directly by a Maxon 200W brushless DC motor with an Advanced
Motion Controls B15A8 PWM servo driver running in open loop mode. I
have found that the open-loop mode of the motor drive results in not
very linear DC transfer of ref. voltage in to motor phase voltage out,
as well as not yielding a very linear dynamic response as well (rise
time != fall time, but only by about 10-20%).

I have suspected that this may be the root of why the PLL behaves quite
a bit less stable than my modeling predicts.

-- snarping snipped --

Thanks for input.


I think the root of your problem lies in the amplifier.

Your brushed DC motor system has magnets attached to the case, a thingy
wound with coils attached to the shaft, an amplifier that delivers
current to a switch, and a switch that directs that current to the
appropriate coils depending on the angle of the shaft with relation to
the case (and hence the magnets with relation to the coils).

Your brushless "DC" motor system has magnets attached to the shaft, a
thingy wound with coils attached to the case, sensors that tell you
where the shaft is in relation to the case, and what sounds like this
gawdaful mess of electronics interposed between your torque command and
your coils.

You say you're driving the brushed motor with a power op-amp that's
being driven by your torque command -- why not do the same thing with
your brushless motor? Get three power op amps that are big enough to
handle the current, take the signals from your motor's position sensors
and commutate your drive current (or drive voltage) command with an
appropriate complement of 405x switches, and do the whole thing
linearly. You won't have any concerns about the internal PWM of the
drive doing weird things because there won't be any. You won't have to
worry about internal sampling in the controller, or weirdo loops, or any
of that, either.

I suspect that if you do this you will find out two things: one, the
room will be warmer when the system is running, and two, the brushless
motor/amplifier system will act a whole lot more like the brushed
motor/amplifier system did.

What's this PWM crap, it's slow enough, why not an EC motor driven by
a VCO?

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
.

Relevant Pages

  • Re: Do you think NI can fix my PLL?
    ... I'm attempting to build another motor PLL system and running into some ... I have considered contracting out the PLL design to a well known control ... Motion Controls B15A8 PWM servo driver running in open loop mode. ... certain frequency determined by mass of the flywheel and the series ...
    (sci.electronics.design)
  • Re: Do you think NI can fix my PLL?
    ... 90' phase shift within the PLL loop so the maths is not like an ordinary ... electronic PLL, the conversion of rpm to phase difference unavoidably shifts ... certain frequency determined by mass of the flywheel and the series ... Yes, I have modeled the motor as an electronic equivalent circuit, and set up loop compensation to provide>45 deg phase margin at unity, also -6dB/oct slope through unity. ...
    (sci.electronics.design)
  • Re: Do you think NI can fix my PLL?
    ... Since there is a lot of work to do, I have considered contracting out the PLL design to a well known control expert outside of my company, so I can focus on building other subsystems of the project. ... Wheel to be driven directly by a Maxon 200W brushless DC motor with an Advanced Motion Controls B15A8 PWM servo driver running in open loop mode. ...
    (sci.electronics.design)
  • Re: Do you think NI can fix my PLL?
    ... I'm attempting to build another motor PLL system and running into some difficulties stabilizing the loop. ... Since there is a lot of work to do, I have considered contracting out the PLL design to a well known control expert outside of my company, so I can focus on building other subsystems of the project. ... Wheel to be driven directly by a Maxon 200W brushless DC motor with an Advanced Motion Controls B15A8 PWM servo driver running in open loop mode. ...
    (sci.electronics.design)
  • Re: Do you think NI can fix my PLL?
    ... Since there is a lot of work to do, I have considered contracting out the PLL design to a well known control expert outside of my company, so I can focus on building other subsystems of the project. ... Wheel to be driven directly by a Maxon 200W brushless DC motor with an Advanced Motion Controls B15A8 PWM servo driver running in open loop mode. ... Your brushed DC motor system has magnets attached to the case, a thingy wound with coils attached to the shaft, an amplifier that delivers current to a switch, and a switch that directs that current to the appropriate coils depending on the angle of the shaft with relation to the case. ...
    (sci.electronics.design)