Re: Do you think NI can fix my PLL?

Chris Carlen <crcarleRemoveThis@xxxxxxxxxxxxxxx> 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.

This is a critical portion that you need to know inside and out. If you
outsource, you will be constantly calling the guy whenever something goes
wrong or doesn't work quite right.

What kind of problems are you encountering? If the loop is wildly unstable,
it obviously needs to be fixed. If you are asking for extremely tight
control, i.e. picoseconds of phase error control in milliseconds of
rotational period, maybe the requirements are too tight and need to be
relaxed.

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.

Small errors like this should have little effect. In a normal loop, changes
in the response go as the square root of gain. For example, you can run a
varactor pll from one end of the range to the other with satisfactory loop
response even though the vco gain changes quite a bit.

A co-worker involved in the project has contacted National Instruments,
and they will come in next week to tell us "what we need". I have
serious doubts that NI solutions will either be appropriate from a
design perspective, nor able to achieve results quicker/less costly than
if I get a control expert on the task.

I suspect from what I've heard my co-worker say (he is a LabView
programmer primarily) that what NI has to offer which applies to PLL
implementations is a LabVIEW-FPGA platform. This would implement
digital filtering of the sort needed to stabilize the loop. Of course,
then also some digital IO would be needed to accept the reference and
wheel position sensor signals, and D/A output to drive the motor power
drive.

What is the view of "the SED community" on the appropriateness and
likelyhood of success with this approach?

Putting Windows software in the feedback loop is a recipie for disaster.
Any consultant that recommends relying on Windows has no concept of system
reliability and should be shot without benefit of blindfold.

My assertion has been that PLL stabilization is not something that can
be done the way most controls are handled around here (usually PID)
where you can use heuristic algorithms to getting it to work). Rather a
PLL must be computed via classical analysis and servo loop design
methods, ie, do the math.

You are absolutely correct. Any other seat-of-the-pants approach will not
work. I've tried. You cannot possibly arrive at the proper loop components
by guessing.

After doing the math and simulations and getting the loop operating
correctly, you can trim the values to optimize the response at a desired
operating point. But not before.

Since the LabVIEW guy has no such experience the only possible way this
could work using the NI approach is if:

1. NI has PLL tuning algorithms that can "autotune" successfully.

Unlikely.

2. OR NI will also provide us with contracted design assistance to
have one of their experts tune and set up the digital filtering LabVIEW
code.

Highly recommend do not rely on Windows software to close the loop.

4. AND I am wrong about the need for analysis to solve this PLL's loop
stabilization requirements.

This seems to be the root of the problem. Instinct tells me there is
something being overlooked. PLL motor speed control is not that difficult.

3. AND I am wrong about the non-linearities being the root of
difficulties. Although, a digital platform might actually help in this
regard if some sort of linearizing function must be applied. I suspect
however, that this would be better solved by a simpler minor-loop
synthesis approach. Ie, run the motor in speed servo mode using the
drive and a speed feedback sensor (which should then be highly linear),
and build the PLL around that.

Extreme linearity is not needed. You can get the loop operating fairly
well, then trim to optimize the response.

What do you think?

I am quite opposed to this situation of employing NI, but need to keep
an open mind. But from an engineering perspective, I think it is absurd
even if it ultimately works to employ vastly complecx FPGAs and ultra
high-level programming software, DAQ systems etc. to do the job of
something that should be doable with a couple op-amps and a handful of
resistors and capacitors.

Follow your instincts. You are correct.

Thanks for input.

Regards,

Mike Monett
.

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  • Re: Do you think NI can fix my PLL?
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  • Do you think NI can fix my PLL?
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