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Sizing DC Motor for Large Robot (350 lb)

  1. Nov 25, 2014 #1
    I'm working on a senior design project. Currently I'm having some issues with sizing a motor for a large robot; everywhere I look I get different advice. I have a motor that I think will work but would appreciate some input.

    The specs are for our robot are as follows:

    mass = 350 lb = 158.8 kg
    Max velocity = 4 mph = 1.788 m/s
    angle = 4.8 degrees = 0.0837 radians
    acceleration = (2*v_max)/t + v_i = 1.788 m/s^2
    wheel radius = 10 inches = 0.254 m
    time = 5 seconds

    Surface = smooth indoor flooring

    where v_max is the maximum velocity and v_i is the initial velocity (0 m/s), t is the time it takes for the robot to accelerate from 0 to v_max

    I did a free body analysis (FBD) of the robot on an inclined plane (0.0837 radians) and got the following equation for the torque:

    Torque = m*r*(a+g*sin(theta))

    where m is the mass, a is the acceleration, g is gravity (9.81 m/s^2) and theta is the angle of the inclined plane.

    This gives me a torque of 31.7 N-m. I think that this motor:

    http://www.superdroidrobots.com/shop/item.aspx/dg-158-24vdc-135-rpm-wheel-chair-motor-pair/1531/

    may work (the specs given are for the motors together). But the specs are confusing me a little. I don't understand where the 'Power Output (W) =200' spec is coming from or what the 'Gear Rated Motor Torque = 16.95 N-m)' means. As I understand it:

    Power = torque*angular velocity

    So, for the specs given for the motor:

    w_motor = 135 RPM *2*pi/60 = 14.13 rad/s

    Power_motor = 16.95 N-m*14.13 rad/s = 239.5 W

    The issues is that for us to move the robot we need the following power:

    w_robot= v_max/r = (1.788m/s)/(0.13 m) = 13.75 rad/s

    where we rounded the radius of the wheel from 0.254 m to 0.13m.

    Power_robot = 31.7 N-m*13.75 rad/s = 435.875 W

    Additionally, in the images given for this motor there is a speed (RPM) vs torque graph which plots efficiency, current and speed (RPM). Based on the speed curve I re-plotted the points in excel and used a curve of best fit to find that the stall torque was approximately 60 N-m. So, we'd be operating these motors at 50% of their stall torque, but most sources I've read indicate that you should operate dc motors between 10 and 33% of their stall torque.

    Any input about whether I'm looking in the right direction would be appreciated. Thanks
     
  2. jcsd
  3. Nov 25, 2014 #2

    Danger

    User Avatar
    Gold Member

    I know nothing of math or even actual engineering. When faced with a high-torque application, I always fall back on the good old automobile starter motor.
    Also, you could look into the motors used on my power-chair (Jazzy Jet 3). It's like a combination of easy chair and motorized wheelchair. 24VDC gearhead motors. You can get them through eBay for a couple of hundred bucks, but they cost over $1,500 new.
     
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