Re: Do you think NI can fix my PLL?

Mike Monett wrote:
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.

I think the arrangement would be to work closely together, where I would learn how the loop was optimized so I could probably handle future adjustments.

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.

The requirements are not terribly difficult initially. About +/-0.5 degreees jitter is tolerable. But there is another dimension to the project to explore syncing two wheels together. This will need to be about 0.01 degrees p-p.

My problem is that when I did it with a PM motor, it worked very well. Hardware behaved with a few % of the sim. Moving to brushless is faltering, though the circuit is basically the same.

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.

Yeah, that's right, which is why I am surprized it is proving cantankerous.

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.

Mike, it wouldn't be Windows. It would by LabVIEW running on a dedicated real time CPU or FPGA. Windows might be involved at a higher level of "stop/go" that sort of thing.

Don't get me wrong, I still think it's a non-ideal approach.

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.

Hmm. Will have to try a bit harder to see what can be done here.


Thanks for the response.


--
Good day!

________________________________________
Christopher R. Carlen
Principal Laser&Electronics Technologist
Sandia National Laboratories CA USA
crcarleRemoveThis@xxxxxxxxxxxxxxx
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