Resolver motor commutation (motor guru help)

If the motor (I assume it's brushless) is going at an exactly steady speed, with an exactly steady torque, you never need to sample the resolver at all -- you just need to drive it with what you have been driving it with (remember it's already going a steady speed) forever.

More important that asking what rate you need to sample your resolver at vs. the speed of the motor, you should be asking what rate you need to sample your resolver at vs. the bandwidth of the control loop, and the bandwidth of any speed disturbances you expect to see on the motor. Taken that way, you can get away with sampling the resolver at quite low rates, although sampling close to or below 2x your inverter frequency will become inconvenient.

For me, if this were a short-run or low volume product I'd go ahead and sample at 10kHz and make sure I was buying a processor (you are doing this with a processor, yes?) that could handle the math. One month's worth of a full-time engineer will buy several hundred $25 processors, after all.

OTOH, if you're talking about something that's going into thousands of units a year, then sample at 600Hz + 4x (control loop bandwidth). You'll spend up to a man-month tearing your hair out over algorithms, but you'll save $$ on the processor, so it'll work out.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

If it's an induction motor driven at a constant frequency, what's to control? Inverter frequency?

At 6 grand, the motor will have a lot of inertia. You could close a pretty tight control loop at 100 samples per second. Any more is gravy.

John

It's a Permenant magnet AC motor with sinuoidal BEMF. It's about 1.7" diameter x 6" long, so the intertia isn't extremely high but is considerable.

I'm doing this with a 100MIPs DSP so I have plenty of processing power. The resolver angle calculated every 100uS determines the phase angle of the three output voltage waveforms. The calculated angle fed into the inv Park / inv Clark transforms to determine the three phase output voltage levels. So each time the resolver is read it directly controls the output volage to thet motor.

For commutation, you need to establish absolute position.

Most, if not all, commercial designs using PM 3-phase motors use encoders, rather than resolvers, with a 4-bit Gray code commutation signal,repeated for each pole pair, and separate quadrature plus one-revolution pulse signals for velocity/position. There are lots of encoders on the market, which are relatively cheap. That does away with the need for stable excitation, too.

Resolvers, at least rotary ones are uncommon, now. Linear ones are still sometimes used for secondary position feedback in precision systems. Rotary resolvers used to be used mainly with DC motors having analog control.

IOW, the world does the job with 16 discrete steps per pole pair. You should be able to get an idea of how fast you need to sample from that.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

So the 4 bit encoders provide 16 postion steps within one electrical cycle of the motor. My application with resolvers allows me to sample the position 33.3 times per cycle. So I chould be good. Correct?

If I take 16 resolver samples per electrical cycle, them the max speed I could achieve with that setp (assuming sufficient bus voltage) is a little over 12K RPM. Correct?

thanks,

All of the encoders I had seen were 10bit 1024 step type which was way beyond what I could achieve.

If you're looking for absolute encoders?, look at Gray code units. Those can give you precise position with in it's own resolution. most common units are 4 wire out, this gives you 16 combinations of data that you can use to know exactly where the rotor is. We have a brushless DC motors at work using this method.

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

Tim, You're correct it is a "brushless" 3 phase PMAC motor with sinusoidal BEMF.

The torque is not constant but doesn't move extremely fast(something like

1Hz bw.)

I'm using a 100MIPs processor with a 120nS A2D sample rate so I should have plenty of processor for this task. The resolver angle is used to determine the necessary phase angle of the inverter output voltage to keep the motor turning. This requires several updates during the electrical cycle to create a clean sinusoidal output for the motor. Since the RPM of motor varies depending on the setpoint (0 - 6000RPM) and the resolver reference frequency is fixed, the faster the motor turns, the fewer samples you get for each electrical cycle of the motor.

I'm wondering what would be the minimum number of samples / cycle to maintain a sinusuodal shaped phase currents?

I do know that a brushless DC motor operates with only 6 commutation points to turn the motor with trapiziodal drive. Others in this discussion have suggested that 16 points is what the industry uses via gray code encoders.

You still won't have *absolute* position detection, relative to a pole pair. The control system needs to know where the rotor is, relative to the stator, immediately on startup, with the rotor still stationary. Resolvers don't do that.

You haven't said what the characteristics of the motor are, but 12000 RPM seems a little optimistic. Most AC servo motors weigh in at 2000 - 3000 RPM, maximum.

A little more knowledge of the intended application might help. It may be that you don't need controllable positioning, just speed and torque, in which case you'd be better off with an induction motor. A resolver *would* work with that.

I've never seen a 10 bit encoder on a servo. They're almost invariably 4 bit for commutation, plus 2 quadrature pulse channels at 2000, 2048,

3000, etc. etc, pulses per rev. and a third channel giving one pulse per rev. There must be a couple dozen firms out there making them.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

Yes we measure the resolver angle with a constant current DC flowing through the phases at startup. This gives us the offset between the resolver position and the BEMF.

12,000 RPM was a theoretical number, we'll be running at 6000RPM for this application. We don't need position control of the motor beyond what is need to commutate it. We do need the small size and power provided by a rare earth magnet PMSM vs an induction motor. We require a small diameter motor is, I presume, that one comes a little more expensive, but is more

-snip-

If I read you right, that means you don't mind a bit of uncontrolled movement at startup. I was assuming this was a servo, where you'd want the motor to stay stationary until commanded

-snip-

Resolvers do do that. Incremental encoders dont.

Even the crappiest ones do 4000, not many go above 6000.

Once again your way off, stick to what you know about.

Most CNC axis drives 2000RPM

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

That maybe so but it has nothing to do with the typical specification for a brushless servo motor.

If you discount the really tiny stuff, it's a major slice of the market.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

I dont know what point that statement is supposed to be making.

You used the word "typical".

-- "Electricity is of two kinds, positive and negative. The difference is, I presume, that one comes a little more expensive, but is more durable; the other is a cheaper thing, but the moths get into it." (Stephen Leacock)