Redesigning multirotor ESC's


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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  • Awesome!

    Can you expand a bit on "4. Field Oriented Control" and "flyback diodes next to mosfets"?

    Seems you are alluding to active freewheeling, avoiding diode energy waste by directing current back to motors with one mostfet on at all times?  Not sure I am reading you completely right ...

  • Very nice - but sorry to say, nothing new...

    The autoquad project does closed loop control with CAN-bus and ESC Datalogging and thrust control for some time now.

    Take a look at maybe you can use the ESC32 for first testing of your ideas. Software is OpenSource, hardware is closed.


    - STM32F103 72MHz 32bit ARM
    – All N-FET design with gate drivers
    – 2S through 5S battery voltage
    – Option to power logic side via UART or PWM IN +5v
    – CAN transceiver hardware support onboard
    – Firmware written completely in C
    – Cortex SWD connector pads for real-time debugging
    – Communications ports: PWM IN / UART / I2C / CAN Bus
    – Communications protocols: PWM IN / CLI / binary / 1-wire / CAN / I2C**
    – 4KHz to 64KHz PWM out
    – Current sensing / limiting with real shunt resistor
    – Virtual current limiter
    – Regenerative braking (experimental)
    – Closed loop control modes
    - Lot of available RAM / FLASH for experimentation and development

    Price is 39€ (~ 50$]

    This would be definitely a big step for Arducopter!!!

  • Stunning stuff - take my money!

  • This is a great idea and get ready for a huge software group this can be modified in unlimited ways if designed correctly would use this as a main board for some small 1or2-motor applications.

  • Moderator

    Hi Gerard

    Great forward thinking idea. not only for security but for tuning and operation as well. not for everyone on a day by day basis but for great for a developer to improve his machine with results not guesses. 

    Put my name down. I'm all for more efficiency and information. 

  • This was about time :) I will back it
  • Brian, you have a good point, however keep in mind the commercial market is only going to grow. These may not end up being well suited to hobbyists (though it would be nice) - but there will still be a market for them.

  • Have you estimated what these ESCs might cost in production?  I think most of others that have tried this idea (and there have been many) have decided that the cost difference between the commodity ESCs and what they could produce would be more than most hobbyists would be willing to spend.

    On the other hand, I would love to see a new standard for ESCs that used a more modern interface.

  • spectacular Idea! Just recently I was discussing the need for accurate RPM to be transmitted back to the kernel so that a dynamic vario-pitch assembly could be incorporated and that the code could also support it. For a while I have been playing around with variable pitch setups that are direct drive reducing gearing, however the accomplishment in mechanics was then quickly dwarfed by the inability for the code to really capitalize on the capability.

    Ever since Rob Lefevre did his talk about Momentum Theory I have been wanting to explore capabilities of stream tube dynamics in coaxial steups. In just searching for some references I stumbled on this,


    just a simple drawing of a vortex tube, however I have used it as inspiration to incorporate into rotor hub to spot cool the assembly. I plan on using a vortex tube to the motor. This way if we add a motor temperature we could further control temperature via a valve.

    I see allot of possibilities with this and just like Rob Said, Take my money, and happy to help anyway I can as it allows me to further this crazy quest I seem to have.

    Understanding the physics involved has driven me countless time to try and extract power in different way from what is like a "spring loaded effect" but with gases. The theorem -A fully expanded gas would approach a Mach number of infinity as it's temperature drops to absolute zero.- has always been intriguing, and with the ability to possibly engage just portions of that force could be very rewarding.  



  • Also think you could crowdfund this!

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