Quote:
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?
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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
Quote:
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?
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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.