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Maximization of motor speed/torque

  1. Feb 12, 2016 #1
    For an upcoming competition, I must construct a vehicle that is capable of travelling at high speeds and can travel precise predetermined distances. While any electrical components may be used, any and all circuitry can only be powered by a single 9V battery.

    I am currently using a rather rudimentary set-up: 2 DC motors controlled by an Arduino microcontroller, powered in parallel by a 9V duracell battery. I was wondering if any of you guys had any ideas on how I could improve such a system in terms of circuitry/electrical components.

    Some ideas:

    Better battery - Other battery types may allow me to draw higher current and thus greater power. Any suggestions on specific types?

    Better motors - A DC motor isn't incredibly fast not precise. I was considering a brushless DC motor simply because they provide very high rpm - can stepper motors provide similar high speeds along with their inherent high precision? A vast majority of brushless motors available on the internet don't seem to come at 9V - would it be worth it to step up the voltage to use higher voltage motors, despite the loss in power?

    Any other suggestions? I am not the most familiar with circuitry, so I would appreciate any suggestions. Thank you.
  2. jcsd
  3. Feb 13, 2016 #2
    I would look at brushless/stepper motors. They have good torque. Also they can back up if needed to make your end mark. They can be geared if you need lots of RPMs, but I read 1000s RPMs is not unreasonable for an ungeared stepper. That seems excessive for wheels to me, but I don't know your application.

    They do require more programming and design work.
  4. Feb 13, 2016 #3


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    Are there any limits on the battery? Some can deliver 100+ amps so I would be surprised if there weren't some other limits on what you can use in the rules. You need to define the battery before you look for a motor.
  5. Feb 13, 2016 #4
    Well, the only limit I have on the battery is its price, haha. As defined in the rules:
    "The vehicle may use no more than 6 individual cells (labelled 1.5V or less each), a single 9V battery, or a single batery pack (labeled 7.2 V or less) during the run time"
    What would be the highest mA-hr battery (also reasonably priced) I could get that is commercially sold?

    To control the motors, I am currently using an arduino microcontroller with an adafruit motor shield. Unfortunately, the motor shield cannot be used to control brushless motors (but can control stepper motors, which I always though were a subset of brushless DC motors). Anyway, my options are reduced to brushed motors and stepper motors.

    I think my biggest problem I'm facing in this project is actually finding the parts I need. I will most likely purchase one of the brushed DC motors from this website:
    http://www.maxonmotorusa.com/maxon/view/content/products - are there any other suggestions for good motor suppliers?

    Any recommended motors in the following list?

    I appreciate any help.
  6. Feb 13, 2016 #5
    How fast is fast? How long does it need to run? how heavy is the whole thing?

    How precisely do you need to move?... if you need to move within a couple mm's, a brushed motor will do very well, if you want to move within a 1/10th of a mm, you may be better off with a stepper. The nice thing about a brushed DC motor is it's pretty simple to control. Benefit to a stepper is they have the best torque at low speeds, which makes them good candidates to direct drive the wheels, saving on gearing. I have a stepper motor controller powered by a STmicro 6470 chip over SPI interface.. it's made by Phi Robotics (got it through robotshop)

    For a battery, I'd check out cordless drill /screwdriver batteries... Buy a cordless drill (it might even be handy to build the project), then use the battery to power it.. Even an old laptop battery will have 4V Lithium cells, and a few of them will probably still be good.
    If you really want to go for broke, you might find 6V lead/acid batteries for motorcycle use, they aren't exaggeratedly big and pack pretty good power.

    You might want to check out www.robotshop.com for steppers, www.sparkfun.com for controllers... I get a lot of stuff on Amazon, and of course Ebay... Adafruit may have something as well.
  7. Feb 13, 2016 #6
    So my goal is to try and move the vehicle across 8 m in less than 2 seconds, but less than 3 seconds is acceptable as well. The vehicle must then stop at some designated point after the 8 m, to a precision of less that 1 cm.

    To control the motors, I'm using the adafruit motor shield, which simplifies the process considerably. The program I am currently using runs the motors at max speed for the initial 8 meters, then slow down considerably and crawls up to the designated stopping line. Another program I have has the vehicle run at full speed past the stop line and stop at 12 meters - then it moves in reverse back to the stop line at a very slow speed.
  8. Feb 13, 2016 #7
  9. Feb 14, 2016 #8


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    Some Lipo packs are rated at 7.4V 4800mAH and 90C for short bursts..


    However that battery can probably deliver way more power than your wheels can (without them just spinning like crazy). I think it would be worth contacting a model car racing club to seek advice on how to maximise traction on the competition floor surface.

    If the run time only has to be <10 seconds then then the mHA rating is almost irrelevant - except that the max current they will deliver is usually higher for larger capacity cells than smaller ones.

    I think weight will also be a factor - it's one thing to accelerate as fast as possible but you also have to slow down and stop without skidding out of the room. I'm thinking you probably don't want a massive battery, just one that is light but which can still deliver reasonably high current?

    PS The battery pack above is marked 7.4V not 7.2V so wouldn't be allowed. That's because it's a Lipo pack. You may have to choose between a lower voltage lipo or a NiMH pack.
  10. Feb 14, 2016 #9


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    You've really started in the wrong place in the design of your vehicle .

    More than adequate batteries and motors can probably be found easy enough . Your problem is in getting max available power through the wheels without slip and making vehicle run in a straight line .

    So consider things like :

    (1) Basic layout of vehicle components and weight distribution .

    (2) Wheel size , tyre material and steering arrangement .
    Last edited: Feb 14, 2016
  11. Feb 14, 2016 #10
    As of now, my actual vehicle layout is somewhat set in stone, but if you have any suggestions, those are appreciated as well.

    I am currently using a 9'' by 5'' aluminum sheet to mount all the parts. I am using Banebots wheels with 2-7/8'' inch diameter as seen in the following link:

    The two motors used to drive the vehicle will be placed in the very front of the aluminum sheet, with two unpowered wheels in the rear. Any other components (microcontroller, battery, etc.) will be placed roughly along the middle so the center of mass is still in the center of the vehicle. With my current setup, using 4 DC motors, a microcontroller, and a duracell 9V battery, the total weight of the vehicle is around 500 g.

    I should also mention that skidding shouldn't be as much of an issue. I plan on slowing down gradually well before the actual stop line - the vehicle will then move forwards slowly and stop. Since the speed only matters during the initial 8 meters, whatever happens after that isn't important, so long as it stops at the stop line.
  12. Feb 15, 2016 #11
    Well, we gotta figure out the minimum acceleration.. Now if the stop line requires you to start slowing down before the 8m mark, you'll have to increase the acceleration, but for now lets not bother with that
    So x = ½a(Δt)2
    x = 8, Δt = 3
    a = 16/9 or roughly 3m/s2
    Max speed will be about 9m/s (3m/s^2 for 3 seconds)... If you can accelerate faster you can reduce the top speed

    Minimum coefficient of friction will be 3/9.8 or about .3, which should be fairly easy to achieve, though driving the rear wheels would greatly help!

    Now you say you have 2 7/8"(0.0365m radius) wheels, or 0.23m circumference, at 9m/s that's 40 r/s, or 2400 RPM.
    I'm going to go with this motor for calculations... since you're already dealing with them it seems
    7.3V, 16000RPM no load speed, peak efficiency at 13000 RPM, stall torque = 11 in/lb, peak efficiency torque = 2in/lb..
    so you'll need to gear it down 13000/2400 or about 5.5:1...

    Banebots has a 5:1 gearbox to fit that motor... lets go with it.. for a 5:1 reduction you could maybe get a more affordable solution with a belt drive?

    Lets go with a middle-of-the-road torque of 7in/lb (.8nm), multiplied by 5 for the reduction gear puts it at 4nm torque at the wheel
    Work it out to actual force, torque/radius = 4/.0365 = 110n

    I'm going to be generous with the weight and double it for 1 kg total, or .25kg per wheel.. which means a force of 2.5N

    I don't have to do much math with the coefficient of friction to see that this motor will burn rubber if it's capable of 110N of driving force with only 2.5N of downforce.. It actually looks like you could direct drive it, and even just use one motor, unless steering is done by driving one side more than the other.
  13. Feb 15, 2016 #12

    jim hardy

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    To achieve that precision in distance, 1cm out of 8meters, 0.125%
    will you count wheel turns? Are you allowed to place a target at the destination?
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