Holonomic drive train and field-based control

[Dan Petrovic]

Originally Posted by tseres

here's a question for people who have done omni-directional drive systems before:
did you use sensors, and what were they? personally i think omni wheels should have a gyro, but other people think it would be better without one. also, wouldn't you need a gyro or sensors to do autonomous driving?

We used encoders to calibrate the motors and potentially be used for autonomous. No gyro as we didn't have a field based control.

[T3_1565]

how well do you think an omni bot would drive without sensors? Like a frame omni wheels and the motors (and obviously programmed). has anyone actually tried it without encoders or gyros? (our buget it limited A LOT)

[Lil' Lavery]

Originally Posted by T3_1565

how well do you think an omni bot would drive without sensors? Like a frame omni wheels and the motors (and obviously programmed). has anyone actually tried it without encoders or gyros? (our buget it limited A LOT)

It can drive quite well, you would just have to account for any mechanical inefficiencies in your code and have a "robot-centric" control system. Just like any open-loop system, your driver would have to account for anything that goes wrong and anything that doesn't work properly.

[T3_1565]

Originally Posted by Lil' Lavery

It can drive quite well, you would just have to account for any mechanical inefficiencies in your code and have a "robot-centric" control system. Just like any open-loop system, your driver would have to account for anything that goes wrong and anything that doesn't work properly.

Ok, so its not a huge problem, unless built badly, thats good to hear thanks a lot

[rachal]

Originally Posted by dmlawrence

What is the approximate cost of the materials needed?
How significant is the reduction in pushing power?
What are the (dis)advantages of field-based control?
Overall, is it worth it?

1. Depends whether you plan to buy the wheels or make your own. My team (1072) made our own last year, the main difference between ours and AM’s being that our rollers were on the outside. We just used rubber lab stops for the rollers, which cut costs a bit. I’d say about $300 for the wheels themselves, and another $100 for the suspension (which we overengineered, I'm sure you could do it for much cheaper).

2. You get pushed by literally everything except other holonomics. Mecanums have a slight advantage over omnis in terms of traction, but if you actually want to count on pushing it’s not enough to matter.

3. Field-based control is amazing for the driver, and not so much for the programmer. We had field-oriented driving working for a while with a magnetic compass chip, but had to disable it at some point during competition. When we were testing it at school during build the robot would go crazy and bump into walls every time it passed a certain door. We eventually realized the school’s backup generator was probably hidden there

4. Heavily dependent on the game and your machining/designing skill, and in most cases no. For example, we were able to align ourselves with ramps without wasting too much time maneuvering behind the home zone line, but we got pushed around too much out in the middle of the field. So even though we could easily get in front of another bot to block, if their arm was long enough to score over us they could just shove us into the rack. Definitely not worth it for the 2007 game. However, building a holonomic is a really fun experience. Everyone should try it once, even if not for the actual competition bot

Originally Posted by tseres

here's a question for people who have done omni-directional drive systems before:
did you use sensors, and what were they? personally i think omni wheels should have a gyro, but other people think it would be better without one. also, wouldn't you need a gyro or sensors to do autonomous driving?

We just had an encoder on each wheel, and a magnetic compass for field-oriented control.

One of the downsides of mecanum is that if a wheel stops working/loses contact with the ground, the exerted forces no longer balance out, but you can still drive with two. So at some point, our programmer added code so that if a wheel went offline, its mirrored wheel would shut down.

[EricH]

Originally Posted by rachal

4. Heavily dependent on the game and your machining/designing skill, and in most cases no. For example, we were able to align ourselves with ramps without wasting too much time maneuvering behind the home zone line, but we got pushed around too much out in the middle of the field. So even though we could easily get in front of another bot to block, if their arm was long enough to score over us they could just shove us into the rack. Definitely not worth it for the 2007 game. However, building a holonomic is a really fun experience. Everyone should try it once, even if not for the actual competition bot

We tried it in 2005, and if you looked closely, the robot had space for another set of trannies. We weren't sure until Week 5 or 6 whether or not we were going to use mecanums or 6-wheel, so we designed for both. We ended up going with the 6-wheel, for reasons you found out. (I think we were one of the teams that went over you this year.) Testing revealed that a simple 4-wheeled robot could (without turning) easily block a mecanum from going through a small target area.

Originally Posted by rachal

One of the downsides of mecanum is that if a wheel stops working/loses contact with the ground, the exerted forces no longer balance out, but you can still drive with two. So at some point, our programmer added code so that if a wheel went offline, its mirrored wheel would shut down.

Another downside happens when you have a decent weight held away from the robot. You effectively lose two wheels, and may start turning unintentionally while going sideways. Not good. It wouldn't have had as much effect this year, as a tetra is significatly heavier than a tube, but it is something to consider.