"Hopping" With tank style turning.
 

Alright. Our robot is configured to have all 4 wheels being driven, two motors on each side drive each sides wheels. To steer, it obviously slows down one side, or to spin on a dime, one side goes forward, the other backwards. The problem is... the robot seems to "hop", and the motors get really loaded when turning and especially when trying to spin. We are using the IFI Robot Wheels which are extremely grippy. I notice that alot of teams steer like this... and i'd like to know how they overcome the probelm we are having. Thanks.

Re: "Hopping" With tank style turning.
 

There are a couple ways to do this:
1) use less "grippy" wheels. Since the wheels will slide easier, it won't hop
2) widen your wheelbase. If a wheelbase uses 4 identical wheels like you have, and it is longer than wide, it will hop. Typically, robots that are wider than they are long will not have this problem. Note that for this purpose, you measure from where the contacts the floor.

At this point in the season, #1 would obviously be easier for you to do.

There may be other ways around this problem, but this is all I can think of off the top of my head.

This whitepaper discusses this problem and shows you how to fix it.

Good Luck!

Re: "Hopping" With tank style turning.
 

Along with Ryan's advice, many teams use Omni-wheels to make their front pair* of wheels slide laterally with ease. You will still have good front to back pushing power. Also, your resistance to getting pushed will still be good on the end of your robot where the Traction Wheels are located. You will lose resistance to being pushed sideways at omni-wheel end of your robot.

Teams have been using this technique to steer better since '98.

#: or back pair, depending on which end is carrying the most weight. You want the traction wheels carrying the heavier load.

Andy B.

Re: "Hopping" With tank style turning.
 

Another option is to replace all the wheels with mecanum wheels and drive them all with seperate motors. These wheels are similiar to Omni-Wheels but have the rollers at 45 degrees making it so you can drive holonomically. This idea is probably not feasable at this stage since it took us 4 weeks to make ours last year and a lot of programming to make them work perfectly.

Re: "Hopping" With tank style turning.
 

what are we up to now? Plan D?

Plan D: use different wheels on the front that are not as grippy. They will slide sideways easier. The back wheels will dominate the steering, and the front wheels will slide.

Plan E: make a pneumatic wheelie-castor that raises the front wheels off the ground using a castor and a pneumatic cylinder. When you want to turn easily you push the castor down, turn, then retract it.

Re: "Hopping" With tank style turning.
 

I'm guessing it's a bit late to go to a wider frame or change the drive around.

And if you don't have the money or machine tools to get omni wheels (which are a great idea at this point for ya), then you can do the cheap fix.

Wrap the wheels on the light side of the robot in something slippery - we use rip-stop nylon zip tied onto the wheel. Heck, last year we PLANNED on using replaceable rip-stop nylon wheel covers. They worked well, and cost about $2.

Omni wheels are, of course, better if you can afford them.

Re: "Hopping" With tank style turning.
 

The pneumatic pop caster has worked flawlessly for us - you get the best of both worlds: turning on a dime, and four high traction wheels on the ground when you need them.

Re: "Hopping" With tank style turning.
 

This is exactly what happened to us at VCU last year. See, with your robot's drive geometry it can't turn. What's happening is the insane torque of the drivetrain is twisting the chassis so one wheel is no longer in contact with the ground. Since that wheel now has zero traction and no real effect on turning, it becomes a 3WD that can turn. Then, the chassis flexes back, since you don't have the force to keep it up anymore, starting the cycle all over again.

Now, as to fixing it, we packed our treads with epoxy clay last year, and it worked like a charm. After a while, though, it started to crack and fall off (epoxy clay is brittle when dry), so we reapplied at the championship. It also looked way better than a bunch of zip ties wrapped around the wheels.

Re: "Hopping" With tank style turning.
 

We had the same problem last year, too much traction, it seams can also be a bad thing. So we simply wrapped the two front tires in a bunch of zip ties (not the best looking) but elegant in it's suplicity. Hind sight, being what it is, you could have always gone for real grippy front or rear tires, and gone for some of thoose fancy omni wheels on the other set. Still provides great forward traction, and at the same time, allows you to turn without causing as much load on the motors and drivetrain.

Re: "Hopping" With tank style turning.
 

We've used zip ties all around the rear wheels before,we have also cut grooves in the tires but i don't remember if that was for traction or turning

Re: "Hopping" With tank style turning.
 

We had that problem last year with our massive 1.5" brecoflex 4-wheel drive. This year we switched to a six-wheel drive. With the center wheels mounted about a quarter inch lower than the front and rear pairs, most of the weight is on the center wheel, which allows for insane turning, and insane traction at the same time. In fact, we just got the chassis driving last week and we were having so much fun just spinning the thing in high gear circles, something that wouldn't have come close to working last year.

So yeah, it's probably a bit late to add another pair of drive wheels to your chassis, but for next year, definitely go 6-wheel. Last year we partially fixed the problem by using the beige diamond-shaped tread on the back wheels and the super grippy black tread on the front wheels.

Re: "Hopping" With tank style turning.
 

Are your IFI wheels 8"?
You could try replacing the front or back wheels with the ktibot skyway 8" wheels. Since they have less traction, this would be a quick way to experiment and see if wheels with less traction would solve the hopping problem.

Re: "Hopping" With tank style turning.
 

Quantum,
As you see, this is a common problem with tank style drives. The side friction causes incredible (near stall) current demands on the elctrical system and the hopping is due to a variety of factors including robot frame flexing. The best alternative at this stage (late in the build season) is to use omni wheels. A variety of types are available including the types found here. http://www.andymark.biz/plastic-omni.htm which are easily available and designed for First robot use. Once installed you will find your robot is happier, your electrical system is happier and drivers are happier.

Re: "Hopping" With tank style turning.
 

Try to find some wheels of the same diameter with less traction, at least on one end. Even with that, you may need to bump steer. One year, our drivers had to reverse one side and go full forward with the other in order to turn worth a darn.

Re: "Hopping" With tank style turning.
 

The easiest way to cut down the traction on the fron wheels without spending too much more money is to buy th pool hose (like for pool filters) and cut it in half then zip tie it to your wheels. If you want to see what i mean find pictures or 121 in 2004 and look at their front wheels. Try http://www.Joemenassa.com

Re: "Hopping" With tank style turning.
 

I disagree. It might take a while for them to arrive, they are moderately expensive, and the downside to omni-wheels is that you lose a lot of pushing power. If someone wants to push you at an angle, you might as well throw in the towel. A much better solution is just to get yourself a bit less traction on 2 or 4 of your wheels by applying zip-ties, tubing, nylon, whatever you have lying around (experimentation will be needed here). Like I said, we just picked a different tread for two of our wheels last year and it drastically cut down on hoppage.

EDIT: sorry for the not-so-good wording, I'm talking about loss of pushing power in directions other than forward, which will probably end up being a high percentage of the total pushing.

Re: "Hopping" With tank style turning.
 

How so? Omniwheels are designed to have little resistance to sideways motion, but they have plenty of traction in the forward/backward direction. They don't make you lose any power. In a straight pushing match, you won't automatically be outmatched just because you're using omniwheels.

You might be thinking of a holonomic drive configuration, where the wheels are mounted at an angle to the direction of travel and thus use up some of the power to spin the rollers during normal operation, but that's not the case being suggested here.

Re: "Hopping" With tank style turning.
 

I think he means if the opposing team sees omniwheels in the back, that's where they're going to bump you (from the side) to screw up your aim ;)

Re: "Hopping" With tank style turning.
 

You can add some deployable skids on one side of the robot.

Anyobody ever experimented with toe?

Re: "Hopping" With tank style turning.
 

Guys,
So everyone understands, the "hopping robot" is a symptom of a potentially serious problem for the electrical health of the robot which is why I address this issue in a number of threads. Your robot will stop hopping when the battery becomes so drained it can no longer cause the motors to overcome the friction with the floor and break loose their hold with earth. During turns with this type of drive design, we have recorded currents above 250 amps. (Remember that the small Chalupa motors have a stall current in excess of 100 amps each, and you are forcing the motors to near stall in a turn.) Electrically speaking, the battery has an internal resistance of about .011 ohms. Doing the math on Ohm's law, that amounts to almost a 3 volt drop on a good battery. Since the RC will cutout below 8 volts, your robot is in danger of going to sleep every time you turn. By now the main and branch circuit breakers have warmed to the point of failure. Add a few more amps for other accessories on your robot and you won't need to worry about getting into a pushing match. Your alliance partners will just push over to the side to get your hardware out of the way of operating robots.
Yes, that is an extreme situation but it does happen all too often. If you ask me for help, you might not like the answer but I am going to give it to you straight. If you come to play, I am bound to help you play.

Re: "Hopping" With tank style turning.
 

Heres an idea, put two casters on the end of pneumatic cylinders and mount the cylinders in one corner of the robot near the front or back. Then when you want to turn extend the pneumatic cylinders and you will raise up the front of your robot a little so those two wheels don't touch the ground. Then when you want insane traction you can retract the cylinders and your robot is back on the ground. We did something like this in 2004 to help get on the ramp and it worked quite well.

Re: "Hopping" With tank style turning.
 

raise one of the wheels about 1/8". You will drive on 3 under normal circumstances. This is why mant teams have gone to 6 wheels with the middle set of wheels slightly lower than the four outer corner wheels.

Re: "Hopping" With tank style turning.
 

This year our 4 WD robot has a tendency to hop because the tires are very sticky and we reduced air pressure slightly to better climb the ramp. We used this to advantage in a scrimmage yesterday to free up a ball jam in out hopper. It makes it a real bear to drive, but we were able to push any other robot on the field quite easily.