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Question How is BLDC torque controlled with an ESC?

Discussion in 'Motor actuators and drivers' started by TheSnowGuy, Mar 18, 2022.

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  1. TheSnowGuy

    TheSnowGuy New Member

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    Hello everyone, I'm going to be making my own FFB wheel and I'm confused about how the existing wheels in the market control torque. I know that BLDCs use ESCs for speed control based on PWM input, but the issue I see is that even if you apply 5% of max speed, if you're holding the wheel still, torque will ramp until the wheel achieves 5% speed due to the ESCs feedback loop. Since real cars will have variable torque on the wheel and it seems all commercial FFB wheels achieve this effect, how exactly do they use an ESC to control the torque? With a brushed DC motor it seems as though you can limit torque since you're limiting the average voltage (and subsequently current) with PWM, but with a BLDC motor and an ESC I'm under the impression that the ESC will feed as much current to the motor as possible to reach a speed corresponding to your PWM input.

    I'm more of a firmware person so my knowledge in analog isn't that great but I do know a bit about the hardware side of things so don't hold back!
  2. TheSnowGuy

    TheSnowGuy New Member

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    Okay so I've learned that you don't really control torque with an ESC. From reading other threads, it appears the dynamic nature of ffb means that you can get a good approximation with PWM even though it ramps to max torque, and that you need to use a servo driver in torque mode to truly control torque (but those are expensive...)

    I have a follow up question though. I'm seeing that at stall the current is incredibly high and will burn the motors, which makes sense. What doesn't make sense to me is how with a FFB wheel, the motor is almost constantly at stall yet it doesn't burn out. Does this have something to do with the driver? Is it the pwm that keeps the motor from burning?
  3. Gadget999

    Gadget999 Well-Known Member

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    My Motion Simulator:
    2DOF, DC motor, Arduino, 6DOF
    a brushless motor is normally a 3 phase motor with coils that are activated in sequence as the motor spins

    the esc knows the position of the coils and energises them accordingly

    they work pretty good - but a brushed motor can deliver more torque at low rpm

    brushless motors have lots of advantages but brushed motors have much higher starting torques which makes them useful in simulation where you are constantly moving back and forards from stall

    https://www.parvalux.com/what-is-the-difference-between-brushed-and-brushless-dc-motors/
  4. Thanos

    Thanos Building the Future one AC Servo at a time... or 6

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    My Motion Simulator:
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    Brushless servos with drives that can handle back-emf, are expensive... the ESC works because the motor does NOT change direction constantly, only one direction mostly with constant speed (e-bikes, rc vehicles, drones etc).

    Certainly can't do that:

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  5. TheSnowGuy

    TheSnowGuy New Member

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    Thank you both for your insights.

    So this would make the two realistic options between motors for a FFB wheel brushed DC motors and Servos? If I'm reading what you both said correctly, BLDCs will only really be useful in a FFB sim-racing wheel when used with a servo driver and since those are expensive, to make a cost-effective FFB wheel a brushed DC motor is the best choice?

    Also I'm curious how these motors aren't burning out at stall as I've been led to believe would happen with any DC motor.
  6. TheSnowGuy

    TheSnowGuy New Member

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    Update: I figured it out. I had no idea most motor controllers have a current sensor voltage I could read from. That alone basically answers all my questions. I'm excited to make this happen. Gonna test everything out with a small DC motor to make sure I've got everything working and I'll make an update post somewhere with my progress. Might even try to make the worlds smallest FFB wheel...
  7. Thanos

    Thanos Building the Future one AC Servo at a time... or 6

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    My Motion Simulator:
    AC motor, Motion platform, 4DOF, 6DOF
    Already made one years before. 24v tiny motor, running at 18v via 2A step-up power circuit from 3.7v battery. Motor stall current is 0.7A and the hbridge used is 1A max before the cutoff.



    See how small is this motor:


    You may want to read more on about the specs here:
    https://www.thingiverse.com/thing:3012316
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    Last edited: Mar 19, 2022
  8. fzxj520

    fzxj520 Active Member

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  9. Thanos

    Thanos Building the Future one AC Servo at a time... or 6

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    My Motion Simulator:
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    The VESC is using sensors for sensing the angle position of the motor shaft (vectors) and save the motor coils or driver mosfets from burning while its holding position.

    ESC is sensorless
  10. fzxj520

    fzxj520 Active Member

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    thank you! point out:)
  11. TheSnowGuy

    TheSnowGuy New Member

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    I appreciate pointing those out. Although, I'm going to be programming all control myself so I'm going to use as little existing stuff (outside of motor drivers with current sensing) as possible. I've settled in my project on using a brushed DC motor for now with a motor drive that provides current sense. Just finished programming the USB side of things for the pedals, shifter, and handbrake.
  12. TheSnowGuy

    TheSnowGuy New Member

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    That's pretty sweet! I got these tiny 12v dc motors right now that I'm gonna prototype with. Might compete for the title of smallest sim rig ;)
  13. fzxj520

    fzxj520 Active Member

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    My Motion Simulator:
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    Brushed DC Motor Driver with Current Sensing
    This is a great idea! Hope he is easy to build
    :thumbs
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