Brushless gimbal motors

Other Hardware Motors, Mechanics, Power and CNC
1

I've been looking again at "brushless gimbal motors", and how they actually work.

These are the motors used for pan/tilt/stabilisation mechanisms for camera on quadcopters, among other uses.

A brushless motor normally works by having an ESC creating a rotating magnetic field using three phases of windings, which drags an assemblage of permanent magnets around with it, at high speed. They don't need brushes because the electromagnet part is fixed and the permanent magnets are on the rotating part. The speed is controlled by varying the speed at which you are synthesising the three phase magnetic field.

It seems that with these "brushless gimbal motors", people disassemble the brushless motors and rewire them with thinner magnet wire to increase the winding resistance to limit the current.

These devices seem to have 12 electromagnet poles ( 4 for each phase ), and 14 permanent magnet poles. Therefore there is an intentional mis-alignment between the electromagnet poles and the permanent magnet poles . The electromagnet poles are 30 degrees apart and the permanent magnet poles are 25.7 degrees apart. If you energise one of the phase windings with a DC current ( which is why they are rewound with thinner wire, to limit the current ), one pair of the permanent magnets will be pulled into alignment with the electromagnetic field. If you then energise another set of the phase windings, a different pair of magnetic poles will be pulled into alignment and the device will rotate 4 degrees.

What they have done here, is created ( or re-invented ) a stepper motor, haven't they ?

Obviously if you are trying to control camera alignment, you need to be able to move the camera by small amounts, not spin it around at thousands of rpm. In other words, what a servo or a stepper motor could potential do.

There seem to be various little driver boards to control these "brushless gimbal motors", to perform their servo-like function, but I have not been able to find any description of how they work. And they are quite expensive. I've also seen references to controlling them with an ESC from an RC plane, which would seem to be impossible.

What I am looking to do here, is to drive one of these devices as one would drive a stepper motor, that is, energise the windings one at a time in polarity and sequence so as to cause the rotating part to advance forwards or backwards one step at a time.

Is anyone familiar with these devices, is that how they actually work ?

2

They are basically 3-phase BLDCs (brushless DC motor). Normally these would have
hall-sensors to allow commutation at low-speed or stationary, and perform as general
motors.

However I suspect you are right they are used open-loop like steppers for this application,
possibly with another control loop on top to allow for slippage.

The high power brushless RC motors normally come with windings of a few dozen
milli-ohms and run sensorless on very high current, rewinding enables a saner amount
of current and power.

I've managed in the past to add hall-sensors to one of the larger RC motors as
a fun exercise to turn it into a sensored motor, but they are very coggy.

I reckon a small stepper motor (NEMA11 or so) would be a more sensible choice
if they weren't expensive.

3

Well I am going to buy one and play with it. The motors are c. $20 and the controller is $180.00 so I am not going to buy the controller. Unless I have failed to choose the magic combination of google keywords, there is very little formal information available about them.

I don't know what advantage these have over a stepper motor. Speed of response for the camera stabilisation application ? Low mass and colourful designs ? Convenience of the "outrunner" design for the mechanical implementation ?

It might simply be the case that the bleeding edge quadcoptarians who invent this sort of stuff, work with devices that they are most familiar with.

4

BLDC(*) is the standard motor for use as a position-driven servo in industry, so
probably just following that example. I am wondering if they are sensored though.

(*) OK, many are fully sinusoidal AC motors, but the principle is similar, 3-phase
windings and PM rotor.

5

Which industry and/or application to you have in mind ?

I havem't seen any in the industries I worked in. There are a lot of devices that use gearboxes and screw actuators to use motors that run at normal speeds to drive actuators. In general, it is not straighforward to run motors very slowly.

6

All high performance position control systems use servo motors! Steppers are
their cheap cousin for undemanding applications.