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Single wheel for front of robot to turn — Parallax Forums

Single wheel for front of robot to turn

NWCCTVNWCCTV Posts: 3,629
edited 2012-10-11 20:40 in General Discussion
So, I have 2 ideas for the front end of my bot. The first is a single wheel that would be about half the size of the rear wheels. My question is, should I put a small motor for steering purposes only at the top of this or just let it roam freely and let the rear wheels control steering? For some reason this just does not sem like it would be the best approach. My second idea is to create a suspension system similar to an RC car but with a wider turning radius which would have a motor to control steering. I would like to keep it simple but it also needs to function correctly. The rear wheels I had intended on using were lawn mower tires. However, after reading ercos comments in a different thread I have since decided to go with either 8 inch or 10 inch pneumatic wheels.

Comments

  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-10-11 16:42
    Your rear wheels could control steering but be aware that a "freely roaming" front wheel, AKA caster wheel, will impart a little bit of slop into your guidance system every time you change direction. Depending on how long the "arm" is on your caster, this slop can be small or it can cause your little buddy to wiggle around in all kinds of odd ways before it gets on a reasonable-looking track. If you need to steer around in tight places, the caster system might be too wiggly to be of use.
  • NWCCTVNWCCTV Posts: 3,629
    edited 2012-10-11 16:45
    ElectricAye wrote: »
    If you need to steer around in tight places, the caster system might be too wiggly to be of use.

    This is why I was wondering if I should use motor control. I was thinking of forks similar to a motorcycle that has a motor attached to it for steering control.
  • ercoerco Posts: 20,260
    edited 2012-10-11 17:10
    Another nice option on a long chassis is to use a universal wheel (omniwheel) mounted with its axis pointing forward. It rolls on one of the many small outer rollers when the bot is driving straight ahead. This way, it controls all the steering and the rear drive wheel(s) control forward & back. You rotate the front omniwheel to steer left or right, even with your rear drive wheel off. You can turn in place with this arrangement. I have an older bot at home that uses this, I can post a pic later if my description isn't clear.

    http://www.vexrobotics.com/276-1902.html
  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-10-11 17:15
    NWCCTV wrote: »
    This is why I was wondering if I should use motor control. I was thinking of forks similar to a motorcycle that has a motor attached to it for steering control.

    Ten inch wheels, sounds like you want to build a somewhat large robot. Have you ever built a smaller version of what you have in mind? It's amazing what you learn from building the little 'bots. You could build a small prototype out of wood and tinker with it to see what to expect from your caster wheel. Another option could be a track system. I've seen people build small model radio-controlled tanks that can turn on a dime.
  • NWCCTVNWCCTV Posts: 3,629
    edited 2012-10-11 17:27
    Yes, I have built smaller bots, just not this type. This one is going to be mostly outdoors as opposed to others I have done for indoor use. My design is pretty much complete with the front end exception. I do not want to use tracks. My smaller bots have the "Ball" turning concept. I like this but worry about the fact that one could bind up in rough terrain. ercos omniwheel idea is a good one, but there again I would be worried about it binding up. I think I will give my single wheel with motor control a shot and if it does not work I can always change.
  • xanaduxanadu Posts: 3,347
    edited 2012-10-11 20:04
    It would be cool if you could put a motor to steer your caster. Like an active caster. Maybe some fast linear slides would work. Or maybe they wouldn't. With a caster you get 360 range of movement. If you need that then let it roam free. If not a steering mechanism would be better.

    It depends a lot on how heavy the front caster is. On my bot it is very heavy and it adversely steers the robot often under certain conditions. I don't have encoders on the main wheels. At slow speeds the caster will take time to straighten out from a turn to go straight. I get a little left over turn when going forward but it is easy to compensate for. The other thing I noticed is on certain angles of terrain the tire can swing out and throw the robots direction off by around 30 degrees, which is kind of annoying, but this is only at low low speeds with a big heavy tire. It is so inexpensive and easy to throw a caster on a robot the movement issues make sense and for the most part are easy to overcome.
  • NWCCTVNWCCTV Posts: 3,629
    edited 2012-10-11 20:12
    xanadu, I like the linear slider idea. I was thinking of a splined aluminum round bar that a stepper motor gear would fit in to. I have a ton of them and several have very high torque values. I would then build a "fork"" similar to a motorcycle. Might be easier your way.
  • xanaduxanadu Posts: 3,347
    edited 2012-10-11 20:32
    If you have the stuff laying around it's worth a shot and probably a good experience. I never messed with linear slides because the caster was $50. The wide turn radius would really only be good for something fast. A caster transitions faster under power than the linear slides and plus since the tire is in contact with the ground it stays in tune with the movement of the robot. If you use power steering on a caster you might slide or skid just as much as if you used a regular ol caster wheel.

    A free moving caster is kind of smart on its own. There are so many pros and cons but I'm glad I slapped a $50 pneumatic caster on my first scratch built robot. It let me get past build issues and start driving around. Now I have tons of data on the setup and know what to do next. I think the same thing can apply to any build aspect, front tires with power steering can appreciate the experience gained from a caster setup too.

    Oh wow I'm getting all caster happy... There should be a rule you must own at least one robot with a front or rear caster.
  • NWCCTVNWCCTV Posts: 3,629
    edited 2012-10-11 20:40
    Well, I thought about it and I think I have another solution figured out. Rather than using a caster, I am going to build a set of forks around my pneumatic tire. Then build a gooseneck similar to a bicycle that comes straight up. I have 2 options in mind at this point. Either use a belt drive with a gear at the top of the goose neck and plae the motor a few inches away with an adjuster or just go direct drive to a gear attached to the goos neck. I actually think the latter would be more practical for this application. I have everything I need to go either way. I could then cover the whole unit and it would not be seen and would be out of the weather.
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