Redesigning multirotor ESC's

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It's been quiet on my front, but that was because I was redesigning ESC's (for multirotors and AP's).

Most of the ESC's for multirotor use the SimonK firmware on a relatively simple Atmel microcontroller. There's a single control wire running from the autopilot to the ESC, which is a signal proportionally dictates how long the mosfets are left open and as such command the torque on the motors.

And that's pretty much all there is to an ESC... No signal/wire coming back to tell the autopilot how that particular motor is doing or what the rpm or current is, it's just a "command wire". That sounds a bit antiquated for 2014.

So this picture is of an ESC dev board I first started on, here using the Allegro A4960 chip for simplicity. Shipping to Brazil takes time, so before it arrived the design already morphed into something new, so that's why the board looks unused. Both the MCU and driver chip changed on the newest development board version and I introduced testing points for oscilloscope readings; this project is about to get serious!

What are the features that I think an ESC for a modern multirotor should have?

1. Send the rpm back to the AP; for logging. I see people posting logs to request help figuring out what went wrong, but the log only states the "pwm out" for each motor, which is in no way a guarantee that the motor actually did that. So we need some feedback that states what the motor was actually doing, not what it was commanded to do.
2. Overload detection; the ESC's know what the current is and warn for overload situations.
3. Current & velocity control; neither current nor rpm is actively controlled as a proportional measure to the input PWM signal. So the control loop for the AP spans the IMU, motors, ESC and props, which is a large loop with lots of variables. This ESC will run one or two 'inner loops' and become responsible for achieving either torque or lift and run at a much higher frequency than 500Hz. What you get is that some variables no longer impact the control loop of the AP directly, which should make the vehicle more stable and likely more responsive.
4. Field Oriented Control; The flyback diodes next to mosfets typically burn energy in trapezoidal drive implementations, which  increases the heat on those mosfets. This happens because the mosfets close suddenly. The motor coil wants to resist that change, so you have a current that has nowhere to go except through that diode. In sinusoidal control, there's always one mosfet open for any coil, so the current always has somewhere to go, which means the flyback diodes won't get used, so you don't lose the heat.
5. FOC; better efficiency, because the current is always perpendicular to the magnetic field. This may come at the cost of max. torque (related to motor inductance and then only about 5%).
6. FOC; lower torque ripple (1/2-1/3) vs. trapezoidal drive, so hopefully less vibrations, less whistling.
7. Send current readings back to the AP; another opportunity for precisely logging what goes on near the motors. This could be helpful to detect ESC/motor/prop health (bad bearings, prop drag, etc)
8. Configuration; the AP can reconfigure ESC's prior to flight or when in maintenance to tune it for a specific motor.
9. Motor monitoring; if the motor stopped, shorted or the mosfets misbehaved, the ESC can shut down immediately and advise the AP. The AP can then take additional action.
10. Opportunities for automated ESC tuning specific to the motor/prop in use.

The way I see this ESC make a difference is when abnormal situations occur. The current AP's cannot be informed of failure, so it would simply send a signal to "run faster", which, guaranteed, has a disastrous effect to mosfet or motor and could therefore worsen the situation. Soon as the AP is informed something is wrong, it could sound an alarm, activate a chute, disable the counter motor... you suddenly have options!


To spur innovation in this area, I'm considering to setup a kickstarter and actually manufacture around 1.000 or so at a professional PCB house. Aren't these features indispensable for an ESC made in 2014? Would you back it?

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Comments

  • +1 interested. Any Kickstarter project so far?

  • Hi Gerard,

    I am quite interrested in your new ESC design. Any progress recently?

    Also when you mentionned the noise reduction with the sinusoidal drive (FOC), I guess the noise of the motor/ESC is reduced but in multi-rotor the noise comes mainly from the prop, does the FOC also help to reduce the prop harmonics noise?

    Regards and congrats for the work so far.

    Dimitri

  • And here we go... The ESC in action. Grinding noise to be taken care of, but inbetween you how silent the motor functions. Next steps are to look at voltage waveforms, check current consumption at specific rpm's vs. conventional ESC's and work on responsive rpm/torque control.

  • Vector control implementations are pretty hard to implement, especially on processors where there's a lot of things going on at the same time. I decided on making a simulation package where I could verify the algorithm through a simulator first.

    This simulator is available on github: https://github.com/gtoonstra/bldc-sim

    Video demonstrating the results:

    gtoonstra/bldc-sim
    A simulator written in python with some basic diagramming functions for BLDC motor control analysis - gtoonstra/bldc-sim
  • yes right... I meant to say normal air flow, but I guess normal Jedec PCB airflow conditions don't really apply here.

  • Your biggest problem will be getting this power out of the enclosure. Good heat transfer from PCB to air is useless if you have 80 deg C air...

  • Your biggest problem will be getting this power out of the enclosure. Good heat transfer from PCB to air is useless if you have 80 deg C air...

  • what is the rule of thumb calculation for the volume of copper required to dissipate 0.4W of power in a still air environment. I am going to be stuffing my ESCs inside an airtight plastic frame ...it was the most durable one I could find.

  • TI's DRV830x? I am using DRV8301, Freescale's 56f8037 (most familiar to me for motor control), 1 mOhm shunts and these MOSFETs: Infineon TOLL. ESC is a bit bigger (75x50 mm) and runs sensorless FOC. It runs smooth even at low currents (2 - 3A). I am planning to use absolute magnetic encoder with this sensor so I will be able to park rotor in desired position, or (in the future) to be able to use RC motor as a servo drive.

  • TI's DRV830x? I am using DRV8301, Freescale's 56f8037 (most familiar to me for motor control), 1 mOhm shunts and these MOSFETs: Infineon TOLL. ESC is a bit bigger (75x50 mm) and runs sensorless FOC. It runs smooth even at low currents (2 - 3A). I am planning to use absolute magnetic encoder with this sensor so I will be able to park rotor in desired position, or (in the future) to be able to use RC motor as a servo drive.

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