Accuracy of resolver required for PMAC motor computation

[mook johnson]

Most of the motors we deal with have motor full blown resolvers for
motor commutation. The design engineers are wringing their hands about
the required resolution for the R2D conversion. I know it doesn't
require 16 bit resolution but how low can you go.

Assume 10,000 RPM with a 10KHz PWM and sample rate. This will be FOC
controlled with a inner PID phase current loop and external PID speed loop.

How much quantization error can I tolerate in the resolver before it
shows up as inefficiency > ~5% or excessive torque ripple?

 

[Spehro Pefhany]

Most of the motors we deal with have motor full blown resolvers for
motor commutation. The design engineers are wringing their hands about
the required resolution for the R2D conversion. I know it doesn't
require 16 bit resolution but how low can you go.

Assume 10,000 RPM with a 10KHz PWM and sample rate. This will be FOC
controlled with a inner PID phase current loop and external PID speed loop.

How much quantization error can I tolerate in the resolver before it
shows up as inefficiency > ~5% or excessive torque ripple?

Well, I'd guess 1 _electrical_ degree would be more than sufficient,
so 7 bits + log2(pole count) but it may be possible to use algorithms
to deal with lower resolution. There are a bunch of papers on dealing
with misaligned Hall sensors and you could also look at the claimed
sensor alignment of high quality motors (eg. Portescap).


Best regards,
Spehro Pefhany

 

[Spehro Pefhany]

My intuition tells me that's way more precision than you need -- I
suspect that the efficiency of that part of the thing is going to go with
the cosine of the angular error, which means that at 95% efficiency
you're at a bit over 18 electrical degrees. So more like 5 bits + log_2
(poles).

I'd have to sit down and do some real live math to figure it out for sure.

You may be right. ISTR seeing an accuracy spec of 5 degrees for the
Hall sensors on some high end Swiss motors (which would be right in
the middle of our guesstimates), but I could be remembering wrong. The
quantization might be more (or maybe less) objectionable than the type
of errors you get with Hall sensor misalignment.

So, assuming a 2-pole motor, 6 to 8 bits?


Best regards,
Spehro Pefhany

 

[mook johnson]

Y'know, thinking about it, 5 bits for a two-pole motor. At absolute
minimum -- mo is better, of course.

Given that 12-bit ADCs are practically free on microcontrollers these
days, and that even when you figure that's 10 bits of accuracy and two
"marketing bits", I think Mr. Mook is home free on this one.

But -- I'd have to run them numbers.

Mook -- please tell me they're not messing around with hardware R to D
converters.

You got it. Firmware guy wants a 16 bit RDC that ADI sold him on.

You are also correct the internal ADC is good for ~ 9 bits with a few
marketing bits....I like that. I gotta remember that one.

From foggy memories I suspected there was a cosine theta in there based
on the magnetic field vector angle to the stator field vector being 90
degrees for max torque (makes since). Either side of that was flux
weakening or something else but you got less torque per amp in either
case.


Answers my questions and suspicions to a T. Thanks guys!!!!!!