Build Log: Servo Drive and VFD with vector control

[Jonathan]

Almost forgot about this thread ... too many things going on.

I'm currently re-designing the PCB for the motor controller. I've split it into two boards - one for the power electronics and one for the controller and IO. They'll stack so the end result will be much more compact, not that that's a priority. I'll hopefully have that done within a few weeks, then I'll send off to get maybe 10-20 of the boards made. That's enough for my needs and hopefully some other people if I make it open source and they're interested in helping with the code - be that firmware or software.

I've also spent a little time testing a sensorless control algorithm, as it'll be good for applications where you don't need such high accuracy - e.g. spindle motors. The method I'm trying is based on the fact the machine inductance is a function of rotor position (in my case due to saturation not saliency), so if you can measure the inductance in real time you can infer the rotor position. To measure the inductance I'm injecting a high frequency voltage on top of the existing signal, then filtering and processing the current readings to isolate that signal. I get promising results when analysing the data in MATLAB, but I didn't have an encoder on the motor I tested so it's hard to compare the estimated position to the actual position. I'll try again some time with the encoder attached...

Video - or it never happened...

Coming soon - I connected it back up last week and found the encoder was misaligned, so once I've fixed that I'll take a video.

Jonathan, that's some mightily impressive work for an undergraduate 3rd year project in 8months. Kudos to you.

Thank you. I got the prize for the best project, so the people in charge agreed. It was 4th year though - I did MEng not BEng and I'm glad I did as most of the modules in 4th year were useful.

I was previously under the impression that AC servos had the highest power density, but after seeing this (and your new X3 thread), my entire world is a lie!
Are you able to point me in the right direction to learn more about this sort of motor? I'm unable to find anything useful on them.

Out of types of servo motors they have the highest power density (excluding maybe some esoteric designs), but out of motors in general it's unlikely. Power density is strongly dependent on heat, so since most servo motors tend to use passive cooling they're not going to achieve particularly high power densities. Just search on google 'permanent magnet synchronous motor' (pmsm) and you should find plenty of information. The less common feature of the one I used is the rotor being on the outside, which makes things interesting from a cooling point of view...

One initially confusing thing is people talk of 'Brushless DC' and 'Brushless AC' motors as if they're significantly different - when in fact they're both PMSMs, but with different control algorithms and shaped differently to get trapezoidal or sinusoidal back-emf.

Why don't I see these sort of motors replacing spindles?

You do:
http://www.controleng.com/single-art...abac0ac6d.html

But they are less common than induction motors. Induction motors, whilst (perhaps) not having the best power density, do have the advantage of being an inherently robust design. Also bear in mind it's only relatively recently that it's become cheap to implement vector control in a microcontroller - e.g. the PIC I used is less than £5 and it's by no means the cheapest one that is capable. Ten years ago the cost was a lot greater...

- Why don't I see these motors in non-hobby applications?

Part of the problem for companies using these is the potential for large fluctuations in the magnet price. These motors use neodymium magnets and the vast majority of neodymium is mined in China. The Chinese government therefore controls the export prices, so the magnet cost can increase suddenly without notice. That clearly makes it difficult to justify economically, especially in consumer products with low profit margin. Apparently motors with these magnets are currently used in some military applications, but for these reasons there's research into finding alternatives. You do find them in some consumer products - e.g. direct drive washing machines and hybrid cars.