Drive Base Questions

I have a couple questions regarding drive design:

  1. To prevent being pushed from the side, If a bot doesn’t have room for a 6 wheel drive with locked omnis in the middle, could it be effective to have a 4 wheel drive with free spinning locked omnis separate from the driven wheels? (kind of like encoder wheels)

  2. For odometry, how many tracking wheels does a tank drive need? I don’t understand the purpose of more than 1 tracking wheel cause an inertial sensor can get the absolute angle of a bot so wouldn’t tracking wheels only be needed for lateral movement?

  3. Why are encoders used for wheels while potentiometers are used for other applications such as lifts? Why not just use encoders for everything?

2 Likes

Yes, but the better traction those have, the worse traction the rest of the wheels will have. Better get yourself some ramps on the side of the bot so that opponents wind up driving up you before they would get to push.

Wheel slippage can occur, requiring one for side to side and one for forward and back motion. Just two of them can get data with the use of an inertial sensor. Otherwise you would need 3 to track rotation.

Encoders were classically very large and had less resolution than the potentiometers, which gave like 4 thousand values across 200 degrees. Additionally potentiometers were absolute, not relative. It means when you turned it off and back on, the pot was still reading the same value, while the quad encoder has been reset.

7 Likes

This should work just fine, just make sure that the free spinning wheels aren’t pressing into the ground harder than the powered wheels or they could lose some traction.

I’m no expert on odom, but to my knowledge you theoretically only need one tracking wheel if you use an inertial sensor to get your heading. But that assumes that a tank drive will never drift side to side, which can and does happen, especially with an all omni wheel setup. To account for that I think you need one forward wheel, and one sideways one. If you don’t use an inertial sensor, 3 tracking wheels is the best option because you need 2 forward facing wheels to find the robot’s heading and forward displacement, and one sideways wheel to find the robots horizontal displacement (much more important on a holonomic drive, but tank drives do still drift sideways slightly which can throw off your values if you don’t measure that drift)

potentiometers can be more useful because they are more compact, and they have an absolute position, not a relative one. Which means that the sensor will always report the same value in the same position, regardless of what position it is in when you start the program, while encoders always start the program at the value 0, which means you can find rotational displacement, but not absolute angle with them.

2 Likes

For more information on what Xenon is talking about (and position tracking odometry in general), I would highly recommend reading through 5225a’s position tracking document.

5 Likes

@Xenon27 jinx

20char

This topic was automatically closed 365 days after the last reply. New replies are no longer allowed.