Face Vector Modulation, Part I: Motivation

Everybody knows that I love brushless motors. They have essentially taken over DC brushed motors in just about every high performance application by virtue of being much more power dense and efficient. They have a fundamentally stronger physical construction (no spinning bundle of wire), less friction (no brushes), are simpler (no commutator), and can spin at speeds in the six digit range.  And are just… awesome.

What’s not awesome about them is control. A DC motor can be controlled by touching wires to battery terminals. A brushless motor opts to replace the mechanical segmented commutator with power semiconductors, control logic, and embedded software.

Wait a second. I hate software, semiconductors, and control logic. Those are electronics. They are my archnemesis. I want my DC motors back.

Historically speaking, control has been one of the roadblocks to widespread BLDC adoption by hobbyists. The issues of motor control are inevitably tied into motor application. There are a few characteristic traits represenative of the majority of BLDC motor controllers that are for sale to the general public:

• They tend to be very optimized. R/C airplane motor controllers are cheap and power-dense, but being built for use with R/C equipment, they are completely run by (and calibrated by) the industry standard 1 to 2ms pulse duration modulation signal. They are designed for low torque (and shock), high speed loads, like a plane propellor, and have very poor torque characteristics otherwise.
• “Industrial” controllers tend to be massively built and extensively featured. Field oriented control, precision tachometry, programmable curves for every variable motor parameter, feedback and servo amplification – you name it. The net result is “EXPENSIVE”, and usually low powered, for reasons including the fact that industrial motors tend to do the same thing all the time.

I approach this from the perspective of small electric vehicle hacking. Using BLDC motors in drivetrains presents issues which are (usually) not present in either a model aircraft or industrial setting:

• Widely varying torque and speed requirements, especially high torques at low speeds
• Shock loading and sudden disturbances
• Commandable reversibility

Many small EVs such as scooters and bikes have been built with model aircraft controllers, including mine, and suffer from  the problem of launching from standstill. These controllers are what’s known as back-EMF commutated or sensorless controllers. They require the motor to be moving a bit so they can sense the phase voltages in order to know where the rotor is. A slight chicken-and-egg issue if I may say so myself. So unlike DC brush motors, you can’t usually stand on one of these and just take off. Motors with rotor sensors, usually Hall-effect type, and the controllers that can read them, can much more closely emulate the torque curve of a DC brush motor.

The other problem is one of signal interfacing. Because they are designed as black boxes, they are compatible with the industry standard, 50 hertz refresh rate 1 to 2 millisecond duration command pulse used by servo motors and receivers. The vast majority of EV throttle interfaces just use a potentiometer to output a varying voltage. To bridge the two requires some degree of electronic trickery or software hackery, or a product such as a servo tester.

The third problem with model aircraft controllers is their annoying startup and calibration routines. In the field, pilots tend to have exactly one tool to communicate with the model – the transmitter. Consequently, elaborate systems of changing controller settings involving varying the stick position (i.e. pulse duration) exist. So if your signal interface is even marginally wrong, the controller may become confused and not run at all. The majority of these settings only product noticeable effects at high RPMs.

Recently, companies such as Kelly Controls have introduced low-cost, EV specialized controllers that address both the sensor and interface shortcomings. Besides being of questionable quality, however, the controllers tack on all sorts of frills that are useful only to legitimate EVs, such as contactor drivers, reverse beepers, light controllers, programmable throttle and brake curves, current limiting, etc. While nice to have, they are overkill for most small one-offs like scooters and e-bikes, and are enormous for the power they handle.

At the end of the day, I just want something that will run my Robot Jesus damned motor. It doesn’t have to do it with maximum efficiency, nor with minimal torque ripple, nor with phase current control. The best thing in the world would be a model aircraft ESC, with its extremely high power density, crossed with an EV controller that uses sensored motors and an analog throttle.

The goal of Face Vector Modulation is to produce a hardware kernel that will commutate a brushless DC motor with Hall-effect rotor sensors. It should not require specialized parts, proprietary ICs, or programming microcontrollers. This kernel can be extended and built upon, like an operating system kernel, to produce an EV (or robot, or bizarre kinetic sculpture) controller of the builders’ choosing, to arbitrary power limits.

The goal of a controller that I will make using the FVM kernel will be a small EV traction drive that can operate from 0 speed to the maximum speed of a not-yet determined motor, have high torque at low speeds, and interfaces to the user through a standard 5 kilohm throttle potentiometer.