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Minibot falling off pole at start

Minibot falling off pole at start. Team 1640 – Sab-BOT-age Clem McKown 1/24/2011. Problem. The “Magnetic Minibot” fell off the pole when motor started Top wheel left the pole Magnet – pole distance increased Robot fell off What are the parameters which determine this?

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Minibot falling off pole at start

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  1. Minibot falling off pole at start Team 1640 – Sab-BOT-age Clem McKown 1/24/2011

  2. Problem • The “Magnetic Minibot” fell off the pole when motor started • Top wheel left the pole • Magnet – pole distance increased • Robot fell off • What are the parameters which determine this? • This is a variation on the old “will my robot tip over” analysis

  3. The System at rest y CoM CoF Fn Fa pole rear wheel x

  4. The System at rest y CoM CoF Fn Fa pole rear wheel x CoM – Center of Mass CoF – Center of Normal Force holding robot to pole Fa = Force due to acceleration & gravitational vector Fn = Force holding robot to pole – note that with magnets, this force declines rapidly as the magnet moves away from the pole

  5. The System at rest y CoM CoF Fn Fa Fa, Fn and drive torque contribute to torque to rotate the robot about the rear axle. If this torque is greater than zero, the robot will rotate. pole rear wheel t x

  6. The System at rest y CoM CoF Fn The system can be analyzed For the system at rest below with 5 lb robot and 10 lb normal force, rotational torque is negative (-2 ft lb), so the robot remains on the pole Fa pole rear wheel x (0,0)

  7. The System dynamic y CoM CoF Fn But turn the motors on and this system changes. Fa climbs due to robot acceleration and the wheel drive torque adds its contribution. Rotational torque goes positive (0.4 ft lb)… Fa pole rear wheel x (0,0)

  8. The System dynamic y CoF Fn CoM pole rear wheel x Fa (0,0) …and the robot falls off.

  9. What to do? • Increase Fn • Increase distance from CoF to rear axle • Minimize Mass • Move CoM closer to pole • Move CoM closer to rear axle • Decouple wheels so that torque from only 1 motor is available to rear wheel • Soft-start is possible, but this adds mass • Work with wheel coefficient of friction, a little slip at the start will reduce drive torque contribution

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