Questions to ask people at demos about URSA

Q: When driving forward at a steady speed, do you think URSA needs to lean forward, stand balanced, or lean backwards? (Maybe demonstrate holding a disabled URSA).

A: Standing balanced is correct. When URSA isn't changing the speed that it is driving at, it doesn't need to lean. The more the robot tips, the more it has to accelerate to stay at the same angle without falling. When URSA is driving at a constant velocity, in physics terms it can be said to be at rest meaning no unbalanced forces change the speed or direction it is traveling in. To remain at rest, even when it is traveling, the robot needs to stay balanced just like if it was balancing in place without moving.

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Q: If URSA starts falling forwards, how do you think it should change its speed to get balanced again?

A: It should drive forwards faster, to get the wheels back under the robot. (Accelerate backwards for falling backwards). It has to accelerate to a faster speed if already moving, not just drive forwards. What matters for tipping is changes in speed.

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Q: If URSA is driving at a constant speed, how should it change its speed to slow down?

A: For URSA to slow down, it needs to lean away from the direction that it is traveling in. To lean, URSA first has to speed up in the direction it's traveling in so the wheels are in front of the tipped back rest of the robot. Then, to keep from tipping over backwards, URSA can slow down.

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Q: Want to learn the URSA walk? (Dislcaimer: you will look very silly and you might fall on your face)

A: We naturally start walking by putting ourselves off balance and then stepping forwards. Here's a way to walk that helps you notice how you stay balanced, in a similar way to URSA. Standing on your toes, keeping your body straight and taking very small steps close together is like URSA's wheels that only touch the ground at two points.

Try standing still, stepping forwards and backwards as necessary to stay balanced upright. What do you notice about how you have to step to correct your angle? (stepping forwards tips feet forwards head back, stepping backwards brings feet back and head forwards)

Start leaning slightly, what do you notice about what your speed has to be if you keep that angle? (Increases)

If you walk at a constant speed, what angle do you need to walk at? (Vertical)

Starting while standing still, what direction do you have to step first so that you can start moving forwards? (First you have to take a step back so you are leaning forwards, once you're at the speed you want, speed up so your feet come under you and you are balanced, and then you just walk vertically at a constant speed)

If you're walking at a constant speed, how do you need to change your walking speed so that you can slow down? (It sounds like a trick question, but you actually have to speed up first so that you start leaning backwards, then you can slow down)

All of these movements are just like how URSA drives!

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Q: Can you balance this ruler on its end in your hand (or any safely light object about a foot high)? A: This ruler falls over about as quickly as URSA does when it's off. Keeping something this small balanced requires a very fast reaction time because there is very little time before it falls over. URSA's computer calculates how to move 100 times per second so it can easily stay stably balanced.

A taller object takes longer to fall and is easier to keep balanced, that's why the batteries are at the top of the robot-it raises the center of mass and makes it fall more slowly.