[phkahler]

>> And the CPU even has SIMD instructions!

Yes, but it looks like there is no hardware floating point. The description of the CORDIC module indicates fixed-point calculations, which is consistent with the lack of any reference to floating point.

I am happy the have CAN-FD and Motor PWM module, but nowhere did I see conversion times listed for the ADC. For motor control I demand 1uS conversion time or less, and in the last year I've switched from fixed point to floating point after holding off on that switch for ~15 years.

[polpo]

From the ESP32-S31 datasheet: "Single-precision floating-point unit (FPU) per core"

[BenjiWiebe]

The datasheet apparently doesn't say, but judging by their other products' listed 12 bit SAR ADC sampling rates (and assuming this one is similar to what appears to be their standard ADC ) the conversion time will be on the order of 10uS.

[BenjiWiebe]

Also why do you need 1uS for motor control? 1uS is 0.1 degrees of rotation at 16,666 RPM if I did the math right.

I don't know much about motor control, is it normal to need that fast of feedback?

[phkahler]

>> Also why do you need 1uS for motor control?

It's not that important if you use current sensors on the motor phases. But then you're looking at HALL sensors or a shunt with a very high gain amplifier with good common mode rejection - looking for mV signals on top of a +12V or +48V square wave at PWM frequency.

By using low-side shunts under each half-bridge you don't need the common mode rejection, but you can only measure phase current while the low side FET for that phase is on. That means limiting the PWM duty cycle to ensure that FET is on long enough to measure current, so we trade available voltage range for sample time.

I've also written code to measure all phase voltages with a single low-side current shunt under the whole 3-phase bridge. That requires careful phase shifting of the PWM signals and very fast conversion time, but you don't have to compromise available voltage range 0-100 percent duty cycle is possible.

Typically we run the control loop at PWM frequency, but the measurements need to be faster than that.

[namibj]

You can also have two shunts per phase, one low side and one high side. The "hall" "shunts" are pretty good though.

[actionfromafar]

That's wild. Would never have guessed.

[topspin]

Field-oriented Control schemes modulate phase currents at high frequency; the feedback loop must be much faster than the motor phases. Until fairly recently, this stuff was the exclusive province of dedicated ICs (Trinamic et al.) and FPGA. Today, FoC can be done in (mostly) software with MCUs.

Fast feedback loops are also necessary in SMPS, another area where precision, low latency MCU peripherals and software are actively displacing traditional approaches.

[imtringued]

But even if you update your PWM signal on every PWM cycle, you won't go much beyond 30kHz. At some point you're running into high switching losses on your MOSFETs.

[namibj]

There are solutions to those worries.... https://epc-co.com/epc/products/gan-fets-and-ics/epc23102

[tomcam]

I didn’t know that. Thanks for letting me… meet the FOCers

I’ll see myself out of the Internet now.

[PowerElectronix]

The closed loop experiences a phase margin loss that is exponential with the frequency. At lower frecuencies it is negligible, but if you get close to the frequency of the delay the phase margin reduction becomes dramatic and the control goes from stable to unstable very fast.

If the sensor has a limited bandwidth, you add the conversion delay and then the computation delay on top of that you end up with a max workable loop bandwidth in the low tens of kHz and anything higher will have overshoots, oscillations, etc.

[topspin]

You see this in low cost products like MKS SERVO42x, where they're doing FoC with a GD32 MCU. It works; the motor runs cool, smooth and quiet, but the system is limited to 3000 RPM, and struggles with rapid acceleration because the control loop is too slow.

[mianos]

I have tried one. It has no torque. For what looks like an awesome product, it does not have the power to drive a peristaltic pump. I used the same motor on a TMC stepper controller and it's completely silent and works. It's open loop, so comparing apples to oranges but I am not sure what the MKS servo driver on a motor could actually do, aside from spin unloaded.

[topspin]

These can deliver 2.5-3A/phase, which should ample for a pump. Respectfully, I wonder what motor was involved and whether the current was configured: they come out of the box with conservative configuration so people don't burn up motors.

[mianos]

Nema 17

[GeorgeTirebiter]

This is exactly correct. Low 10s of kHz is quite functional for machines moving in human space / speed.

If one is trying to do some assembly line (max # of operations per second), the power requirements alone get hard to manage. And then you're managing back EMF, eddy currents, heck, air resistance!

My rule: have dedicated low-level hw provide smooth PID response, mostly on the P term; and have a higher-level control produce the setpoint. Faster response means less need to rely on I or D terms as much (just because delta-T is so relatively small).

[nathanfries]

I similarly don't know much about motor control or hardware in general, but would this maybe open up multiplexing options?

[greenavocado]

People will always find a reason to complain or pretend they are controlling rocket motor servos with their ESP32

[andynerd]

Both HP cores have single-precision FPU. But only HP core 1 has SIMD, unlike S3 and P4.

[NooneAtAll3]

where did you find cordic mention?