How do i make a servomotor using a 3-phase brushless?

[jpoopdog]

what are application notes and what am i meant to aquire from them? i feel like your suggesting i 1-chip solutions from Ti for this, which to an extent would be ok. or do you just mean study their designs?

 

[dknguyen]

No, lots of companies, especially microcontroller manufacturers, have whitenotes on how to set up a motor drive using their microcontrollers.

Many of them also have accompanying code or sample projects. Just google something like "sensorless BLDC" and you'll come up with whackloads of white papers. It's easy adapt a sensor to a sensorless design if you want to go that route.

 

[dknguyen]

https://www.microchip.com/wwwAppNotes/AppNotes.aspx?appnote=en546013

https://www.microchip.com/wwwAppNotes/AppNotes.aspx?appnote=en533912

https://www.ti.com/solution/bldc_motor_drives

**broken link removed**

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=20&ved=2ahUKEwjvpduazLHdAhXEwMQHHSBpBTUQFjATegQIARAC&url=https://www.zilog.com/force_download.php?filepath=YUhSMGNEb3ZMM2QzZHk1NmFXeHZaeTVqYjIwdlpHOWpjeTloY0hCdWIzUmxjeTloYmpBeU1qWXVjR1Jt&usg=AOvVaw1b47F20J1Epa8ocpsvsC2O

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=12&ved=2ahUKEwiG1Y2JzbHdAhWorVQKHeweCQAQFjALegQICBAC&url=https://www.st.com/resource/en/application_note/cd00020086.pdf&usg=AOvVaw30Qr150qo3JIrgOXDcMrf0

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=2ahUKEwjhn8OtzbHdAhWrxVQKHSZkDxEQFjABegQICRAC&url=https://www.nxp.com/docs/en/reference-manual/DRM086.pdf&usg=AOvVaw22vL6XjaTBeLFxarctonXr

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=2ahUKEwjhn8OtzbHdAhWrxVQKHSZkDxEQFjAAegQIAxAC&url=https://www.nxp.com/docs/en/reference-manual/DRM117.pdf&usg=AOvVaw2wfzFCECQwfga2df5_fz9A

https://www.nxp.com/doc/DRM151

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&uact=8&ved=2ahUKEwjhn8OtzbHdAhWrxVQKHSZkDxEQFjAEegQIBhAC&url=https://cache.freescale.com/files/product/doc/AN1914.pdf&usg=AOvVaw3bOO3KZZ9akE4GwIHRXhW-

**broken link removed**

 

[jpoopdog]

ill be using a fairly decent optical encoder along with back emf for startup calibration, im not too concerned about pole allignment. the hard part will be writing code to control it in a closed loop, since apparently ill be starting from scratch as nobody has ever built a servo before it seems.

-edit-
thanks for that list

 

[dknguyen]

ill be using a fairly decent optical encoder along with back emf for startup calibration, im not too concerned about pole allignment. the hard part will be writing code to control it in a closed loop, since apparently ill be starting from scratch as nobody has ever built a servo before it seems.

-edit-
thanks for that list

No, you need to know the stator pole alignment relative to the encoder if you want to use the quadrature encoder for commutating the motor or else it will lock-up. I'm not talking about using the encoder for servoing for the shaft position. I'm talking about actually commutating the windings to spin the motor.

If you go with a sensorless design, then you obviously don't since an encoder was never expected to available for commutation.

 

[jpoopdog]

i know, i said though that would be easy done so im not concerned. the difficulty does not concern me much.
At least for the initial startup calibration, i understand its importance to assign the poles to a known encoder value

 

[rjenkinsgb]

Have a look at the DSPIC motor control examples.

They provide full example software and test circuits for several types of motor control.
This is a manual for one of their development kits for motor control, with full circuits etc. and you can download the various files related to it.
https://ww1.microchip.com/downloads/en/DeviceDoc/70098A.pdf

 

[tomizett]

Alright - without wanting to take the thread off-topic - may I ask a stoopid question? I'm quite naive about electric motors (I blame this on not having a proper workshop).

May I ask what kind of motor we're discussing here? To me, a "brushless DC" (BLDC) motor has always meant an AC motor (of some sort) with integrated electronics to make it behave like a DC motor. This is, you apply volts and it spins - as in the 12V computer fans we are all familiar with.

Sounds like an interesting project; I'll be watching.

 

[Nigel Goodwin]

May I ask what kind of motor we're discussing here? To me, a "brushless DC" (BLDC) motor has always meant an AC motor (of some sort) with integrated electronics to make it behave like a DC motor. This is, you apply volts and it spins - as in the 12V computer fans we are all familiar with.

That's my understanding as well, but it tends to have been adopted for simple motors that require external electronics.

 

[rjenkinsgb]

The tiny "brushless DC" type as used in PC fans etc. use a couple of coils and hall effect sensors with transistor switches, so that class are brushless but just self-running with little control, rather different to servo motors.

Brushless servo motors are built with something similar to an AC motor stator - fixed coils with three sets of windings and pole pieces - but with a permanent magnet rotor.

They also have a position sensor, often just a magnet and three hall sensors at 120 degree spacing, so the control system knows which coil or combination it needs to apply power to for the required torque.

This is a typical large machine tool servo motor; It's about 18" / 500mm long :
https://assets.suredone.com/1726/me...0-n2l-110ga0-permanent-magnet-motor-3220a.jpg

This is a drive to go with that make of motor:
**broken link removed**

They typically run at 400 - 600V DC at anything from 20 to 70 amps.

BLDC motors come in all sizes, everything from much bigger than that example down to tiny stuff like in models and toys.

Some small ones have external control electronics but are used without position feedback if the application only needs speed control rather than position control - eg. the ones used on model drones.

 

[tomizett]

Thanks for the clarification Mr Jenkins. So we're talking about a permanent magnet three-phase motor, rather than a three phase induction motor such as would be used for fixed-speed or variable-speed-but-not-including-stopped operation. Because, I imagine, you can't run an induction motor down to 0 frequency.

So would it be correct to say that this kind of BLDC motor could also be described as a three-pole permanent-magnet stepper motor? Presumably they're driven with a PWM synthesised sine wave, which seems like a similar technique to micro-stepping a stepper motor. Is that correct?

 

[MaxHeadRoom78]

That's my understanding as well, but it tends to have been adopted for simple motors that require external electronics.

With Electronically commutated motors such as BLDC and AC versions, the motors are virtually identical, the difference is in the commutation, BLDC (brushless DC) because only two of the three windings are energized at any one time, similar to a DC motor turned inside out, hence BLDC.
The identical 3ph AC has a 3phase signal applied simultaneously to all 3 windings.
Other than the commutation electronics, the motors themselves are virtually identical, with a 3 winding stator and a P.M. rotor.
P.C. fans are a rather simpler version.
Max.

 

[dknguyen]

Thanks for the clarification Mr Jenkins. So we're talking about a permanent magnet three-phase motor, rather than a three phase induction motor such as would be used for fixed-speed or variable-speed-but-not-including-stopped operation. Because, I imagine, you can't run an induction motor down to 0 frequency.

So would it be correct to say that this kind of BLDC motor could also be described as a three-pole permanent-magnet stepper motor? Presumably they're driven with a PWM synthesised sine wave, which seems like a similar technique to micro-stepping a stepper motor. Is that correct?

No, a stepper is something else. More like a PM synchronous motor.

 

[tomizett]

Well I've a lot to learn on the topic, I think.... The info is appreciated, as ever!

 

[dknguyen]

Well I've a lot to learn on the topic, I think.... The info is appreciated, as ever!

To be fair, they look really really similar. I think the phases/windings arranged and connected differently.

 

[tomizett]

That's interesting. The steppers that I generally see have two only circuits accessible - I take it that each circuit is a string of coils in series. I believe that that makes them "two phase" motors.
But I've certainly seen older/larger motors with whole bunches of wires coming out of them - they could have been one-circuit-per-coil.
If I'm getting this right, a two-phase stepper motor would have to be driven in less than half steps, because otherwise you'd not be able to impart any direction - it could just as easily jump forwards as backwards.

 

[MaxHeadRoom78]

That's interesting. The steppers that I generally see have two only circuits accessible -.

There is a subtle difference in the winding configuration between Bi-polar and Unipolar versions. this determines the way they are commutated.
Max.

 

[dr pepper]

If your doing a one off use a Rc type Esc, you can get a microcontroller to drive a brushless motor, its pretty tricky though.
I have done a couple of projects that use Bldc motors as servo's, however they are a slow servo, the motor only runs 200 rpm at the most.
You can also get cheap boards from china to run Hdd motors which are Bldc, you could beef up the o/p on one of these with an outboard set of fets, speed control is done by a reference voltage.