Tuesday, January 22, 2013

Introducing the BE3P

In a bit of a departure from my typical posts, I'm going to write one about something that doesn't actually physically exist yet: a brushless 3-phase motor controller I've spent the last week working on with my friend Erik; the BE3P (Banks Erik 3-Phase). Traditional brushed motor control effectively provides the motor with an average DC voltage and then brush contacts in the motor shift as it rotates, causing the magnetic field generated by the coils to always attract the next set of magnet poles, producing torque. Brushless control is a bit trickier, because this switching of the field needs to be handled in software, i.e. the controller needs to determine where the motor coils are in relation to the magnets in order to switch current through them accordingly. We got a bit of a crash-course in brushless motor control from our friend Shane as well as a few online resources, then we immediately started browsing for the major components. We decided to make the controller optimized for the hub motor I built, which probably isn't able to handle more than about 1200 Watts peak. My battery runs around 40V, meaning we're aiming for ~30A peak. Since exploding your components is kind of bad, we over-specced everything a bit (especially the MOSFETs) in order to (hopefully) be able to handle 30A at 40V peak without substantial heatsinking. We centered our design around an Atmega328 microcontroller, an HCPL3120 optocoupled gate drive, and surface-mount MOSFETs rated to 120A at 60V.

Just to warn you, I'm going to try to start basic but this post is necessarily going to get progressively more difficult to understand if you don't already have some idea of motor control theory.

Okay, quick intro: the easiest way to control the average voltage (voltage relates to speed) given to a motor is by taking your battery voltage and pulsing it (relatively quickly, on the order of 10's of kHz) across the motor. You can vary the exact average by controlling the duty cycle (percentage of time that you're applying battery voltage) of the pulses. This type of voltage control is called PWM. So, how do you quickly switch between battery voltage and ground? Transistors! A MOSFET (metal-oxide-semiconductor field-effect transistor) is a type of transistor that can turn on quickly and takes relatively little power to do so. Effectively, the MOSFET has a capacitor ("gate") inside it that turns the "switch" on when you give it a voltage on the order of ~5-15V. "Okay, sweet, my Arduino can do that!", you say? No, not exactly. The problem is that the MOSFET doesn't behave nearly as nicely while the gate capacitance lies between 0V and the "turn-on" voltage of ~5-15V. While your Arduino would (actually it wouldn't, Arduinos have fairly fragile output current limits) be able to drive an "ideal" MOSFET (zero resistance between your output pin and the gate capacitor), in reality there's some amount of resistance along that path, which means that there'll some nonzero amount of time required to charge the gate up. Why? Physics. It ends up being much more efficient to drive the gate with higher voltages (~12-15V) in order to throw higher currents (remember that current is rate of charge flow) into your gate, charging it faster.

So now we need something that can quickly switch between 0 and 15V to drive our MOSFET which can quickly switch between 0 and ~40V. What, you thought this was going to be simple? (I'm not even going to go into other problems like the necessity of bootstrapping your high-side nFETs to enforce gate drive voltage between the gate and the source by leaving the source voltage floating). Well, this is where gate drivers come into play. Gate drivers are devices that can throw quick bursts of charge onto your MOSFET gates. Perfect! The gate drivers we picked out are opto-coupled, which basically means they have a tiny LED inside them and they dump charge onto your FET gates whenever the LED is turned on. This is great, because it means that all your little 5V microcontroller has to do is turn on an LED in order to drive the motor. Additionally, it means that your 5V systems and your 15V systems are only connected by light, which protects your low-voltage electronics and prevents high noise levels on your signal wires.

All of the above basically needs to be copy/pasted 3 times for brushless 3-phase control, which I'm not going to try to explain too heavily so as to avoid turning this blog post into a dissertation. The main important thing is that for "sensorless" control, meaning the controller doesn't directly know where the motor coils are relative to its magnets, it needs to have voltage sensors on each of the phases such that it can turn one off at a time and sense the voltage coming back out of it, from which you can determine the "electrical position" (basically the position of the coils relative to the magnet poles) of the motor, which in turn lets you drive it to the next desired position.

Okay, now that we've gotten physics/theory out of the way, let's move on to the interesting part: actually designing. Not only is this my first foray into brushless motor control, it's also my first time using Eagle CAD for a major board as well as using surface mount parts at all. This was basically a "learn everything as you go" style project for me, so I just kind of jumped into Eagle and started designing.

Here's an early schematic which shows the absolute basics. I'll explain starting from the far left, because that's sort of the direction of flow for this design. First there's a 15V switching regulator, which converts the 40 volts off of the battery down to something more manageable. Next, a feedback and filtering circuit which allows the 15V regulator to operate and cleans up its output a bit. I then have a 5V linear regulator which converts the 15V down to 5V in order to power the microcontroller. The giant box with lots of pins is the microcontroller itself (Atmega328), which then connects to a couple of status LEDs, as well as the three giant boxes you see in the middle of the schematic. The boxes are inverters and they along with the little resistor and capacitor to the right of them form a little circuit that takes the PWM signal from the microcontroller, splits it into two opposite signals and adds a little delay between turning each side off and turning the next on. This delay is a form of shoot-through protection, which means it avoids allowing the high-side and the low-side to turn on at the same time, which would directly short your battery through both FETs and explode everything. To the right of this inverting circuit you can see all 6 gate drivers, and finally the 6 FETs that they're driving.

Next, I moved on to adding the voltage and current sensing circuits. The current sensing consists of a very small (.001ohm), high current resistor in series with the motor phase, connected to a sensitive differential amplifier, which determines the voltage drop across the resistor and thus allows you to calculate current passing through the whole phase. Current sensing is extremely useful for a variety of reasons: (warning: physics) it allows you to calculate the phase-lag between your drive voltage and the current response of your phase (because your motor is an RL circuit), and since current is what you really care about in terms the position of the motor (because current creates flux which in combination with the field of the magnets applies torque), you need to offset the voltage by that detected phase-lag in order to time the torque with your phases switching electrical positions. Current sensing also allows you to do some nice things like over-current protection, constant-torque control, and the ability to detect shorts between the outputs and (hopefully) save your FETs from detonation.

As for voltage sensing, it's absolutely essential for DC sensorless control because, as I said above, it's the only way you can determine the electrical position of the motor.

Here you can see the current sensors (small boxes to the right of the FETs, and only on 2 phases) followed by the voltage sensors to the far right. The voltage reading is passed through a 2-stage RC low-pass filter, because there are lots of high-frequency voltage transients across the undriven phase due to the PWM on the other two and you want a smooth signal to determine the electical position.

With the absolute basics out of the way, I moved on to adding various extra stuff, including bus capacitors to smooth the voltage across the FETs/provide them with clean-ish power, a battery voltage sensor, a 20MHz oscillator for the microcontroller, an ISP header so I can actually program the microcontroller, throttle/extra input (or output) pins, and finally an XBee header so that the controller will be able to support wireless data transmission.

Yay! With the schematic finished (after like 4 days of staring at datasheets), I moved on to actually laying out the board. I definitely wanted it to be able to fit in my scooter, so I decided to make the outline 2.75" by 3.75".

I'd never laid out a board before, so this was my chance to learn Eagle's board layout features. I experimented with a couple of different general layouts before I settled on one I actually liked, the picture above is one of those early layouts.

This is the general layout I ended up settling on, you can see the massive power and ground traces on the left side of the board, and away from everything but the FETs and bus capacitors.

Here's a snapshot from the end of the first day of laying out, I got a little panicked about how much space things were taking and started trying to cram everything together towards the bottom, which in retrospect was a little bit of a mistake. It turns out I had more space than I thought, so cramming made my layout look pretty disorganized. On the plus side, however, this meant that the last third or so of routing went extremely quickly because there was an abundance of space available.

Here's the finished board design with everything routed, and I also added a logo on the back as well as name/date/credits on the front middle.

Erik and I each independently created our own boards and schematics because we wanted the additional practice, and for our PCB order we just put 2 of each of our boards on a panel. We also ordered enough components for 4 boards, so if all goes well we'll end up with 2 controllers each.

For now, it's time to wait for parts to arrive, which we can fill by starting on the code for the microcontroller. Basically, I'm going to enjoy spending a few days staring at massive spreadsheets which tell me which registers do what.

Hopefully that was at least somewhat intelligible for people of varying levels of experience, let me know if you have questions about the design/components/whatever.