Steering characteristics of a 4-wheeled platform

This is the "skid steer" approach popular with skip loaders, like the Bobcat. The point of steering is actually between the wheels, so the vehicle makes turns by wheel slippage over the ground, similar to a tank. Similart design principles apply.

I've designed several of these, and you can see basic pics and read the dimensions on my site:

?shop=1&cat=116& Like tank designs, it's best to keep the vehicle width narrow. This affords better steering control. It doesn't matter to change the speed or direction of the motors that are on the same side. For practical purposes, the two wheels on each side act as one. I happen to use separate motors because it's actually cheaper to do it this way, rather than to buy or revise a gear train. However for larger vehicles you may find a single motor per side, driving a chain or gear, is the more affordable solution.

Wheel surface is critical. On the first link above (trhe 4WD unit), the nubs in the tires pretty much prevent the vehicle from running over carpet. You need to clip the nubs off and run on smooth rubber. I leave the nubs on the wheels for those who will always run it over dirt or concrete.

The Rigel wheels are harder plastic, which can be a benefit for application over different surfaces without modifying the tire. The wheels slide a bit more readily, which obviously has its pros and cons.

Tracked and 4WD vehicles require more horsepower to turn than the traditionally-steered vehicle, so be sure the motors are well suited for the job. Both of the vehicles above use four 57 oz-in servos, which for the size and weight of the vehicles gives good oopmph.

-- Gordon

Can you explain your reasoning for that? I would expect the opposite to be true, that the shorter and wider the vehicle, the better the steering control. If you have a long narrow wheelbase the wheels have to travel almost sideways when turning and produce little turning torque; if you have a wide, short wheelbase they travel almost forward/backward and produce high turning torque.

-chris

I have been experimenting with 4 wheel drive differential / skid steering using Vex parts. I found that as the distance between the front and rear wheels increases, the steering becomes less effective. I have tried web searching for any guidelines that describe the optimal dimensions for this style platform, but the closest I found is this thesis paper:

While it discusses the forces involved for differential steering, it does not identify any optimal conditions.

Anyone know if there are some established guidelines as to what the front-to-rear and left-to-right dimensions should be for skid steering?

Experimentally I have found that I get better operation if the separation between the left / right wheels is greater than the separation between the front / rear wheels. Can anyone else confirm this finding?

Mike

It's straightforward to build a skid-steer machine, and it will work, but don't try to estimate heading from odometry. You'll need an IMU/compass combo.

It's a reasonable approach for operation on dirt. For floors or pavement, probably not.

John Nagle Animats

There are several 4WD skid-steered platforms around, in addition to Gordon's Rigel ...

Despite what Gordon said, the wheelbase on the Rigel is a lot shorter than the distance between wheels side to side [roughly 3" versus 6"], and I think this does aid turning. I found the Rigel to turn well on most surfaces, including deep-pile carpets. OTOH, on my Radio Shack A1 tank, the grounded tread length is 7" front-to-back, and the side-to-side distance between tracks is 4.5", so I'm not sure what the final conclusion is. The tank treads are hard plastic, and tread "bars" do run sideways which will aid sideways slippage on turns.

- dan michaels

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Correct that a short, wide wheelbase sees less drag in a turn, but that's indirectly related to what I was referring to. Of course "narrow" is a relative term; in the two example robots I provided the width to length of the wheelbase represents a square-ish box. In both robots the width between the wheels is wider than pitch between the two wheels on each side. The vehicle chassis itself is narrow, but the contact area of the wheels is a box that is wider than it is long. To get these wheel dimensions, which seem to be a good compromise for at least these particular designs, the chassis of the vehicle has to be "narrow."

What I've found is that the control of steering is more difficult when the width greatly exceeds the length (I'm talking wheelbases here, not chassis dimensions). I imagine this is precisely because of drag that the vehicle tends to go straight even with slight variations in the motor speeds on each side. Maybe steering control isn't a good phrase. More like heading control, or something. If you're just R/Cing it, I guess it doesn't really matter. You can manually adjust the heading as you steer the thing around. Odometers on the motors to keep their speeds constant would help in computer control, but not everyone uses these. And odometers for calculating bearing are fairly inaccurate on any skid-steer vehicle (in my experience).

FWIW, I tried to design the overall dimensions of the wheelbase after the Bobcat, in reduced scale, of course. I figured it's a tried and true design. For those interested, the wheel width to length is a ratio of about 1.3:1. On the larger (newer) 4WD I recenrly did, the ratio is exactly the same. On the Rigel it's about 1.1:1. It's probably not as critical with that robot because of the plastic (rather than rubber) wheels.

-- Gordon

Sorry...I think I used some poor terminology. See my other message which hopefully clears up what I meant.

I don't have a made-up Rigel on hand at the moment, but I recall the centerline width between wheels should be more like 5" wide, by about 3" in length. That equates to more or less 1.6:1 ratio. (I had remembed it more like 1.15:1, but I think 5x3 is correct) The Bobcat, my real-life model, is 1.3:1, as is the "Rigel XL," the new 4WD drive unit I did.

Both of the Rigel chassis, though, are "narrow." Sigh.

-- Gordon

For wheeled skid-steer I think the best teachers are the skip loaders and other vehicles on the commercial market, most of which are provided with wheel-base specifications. Just go to the company Web sites. I centered on the Bobcat and its 1.3:1 ratio, because it's iconic. Though I'm sure there are others, with variations in design. These would be good starting points.

I'm afraid I did confuse things a bit in my other messsage to Tim, but indeed, a >1:1 width-to-length ratio provides better results. This doesn't mean something like a 3:1 width-to-length would have superior performance; in my tests I found those harder to steer, as in keep straight. I'm sure someone better at math could explain why. (Or maybe it's just me! )

For tracked skid-steer, I have never found anything, and have instead relied on dimensional specifications of military tanks, and also treaded skip loaders and construction vehicles. Specs for these are similarly available.

-- Gordon

Neither do I, so I guesstimated off of your own picture ;-) ... bottom view.

Note how Gordon mounted the rear wheels further apart using a spacer to give the base a rakish look.

but I recall the

..........

He, he. I do have your servo tank bot, which I just bought a week or so ago, and the front-to rear length of tread on the floor is just under

3", while the side-side width between tread centers is 4.25", so the ratio is 1 : 1.5 :: length : width.

OTOH, the Radio Shack tank I mentioned is just the opposite at 1 : 0.68 [7" tread base to 4.75" width between centers]. We're certainly all over the place.

Offhand, do you know the ratio on the original Tamiya treaded base that uses the same treads?

The servo tank bot is pretty cool, although I made a few mechanical mods so far :).

Ok. What I mean by control is the predictability of (relatively difficult to measure) robot motion as a function of (relatively easy to measure) actuator motion. My interest in this is that I am presently building a tank-tread robot; I am using servos (i.e. pm dc brushmotors with optical encoders, not rc servos) to drive the treads, however, so I do not expect driving straight to be a problem. That is admittedly not an option for "budget" robots. (then again, you can get a nice pittman servo on ebay for about the same price of an rc servo in a store...)

As width goes from much less than to much greater than length, mobility can be classified in roughly three categories:

1. platform cannot steer at all, at least on high-friction terrain
2. platform can steer effectively, but not predictably
3. platform can steer predictably

Going for #3 is probably not worthwhile; for dead reckoning to be more accurate than magnetic or inertial reference sensors the geometry would be awful for other reasons. On the other hand, #1 can be a very real problem. I had trouble with that on an old robot

while driving on certain carpets, though that platform was admittedly underpowered and overloaded.

In any case, it's also important to remember that the ideal shape for a robot that operates in an enclosed area is exactly round, as that shape optimizes the ratio of footprint to area swept while turning in place. As such, and since there was no compelling reason to alter the tread geometry one way or the other, I ended up making my new robot exactly square.

-chris.

Never was able to determine if the Rakish Look had any bearing on steerability, but it looks cool! Seriously, I got the idea it from (I think) an iRobot design, though I note there are just as many 4WD vehicles without it. In any case it can be assembled without the spacer.

I remember now why I thought the ratio was 1.1:1 or so: In the original I had larger wheels, so the servos were turned outward. That increases the front-to-back wheelbase distance (as the servo shafts are off-center). When I found the current wheels, which I liked better, I discovered I could keep the design and just turn the servos inward. That had the effect of increasing the width-to-length ratio. Some things just work out!

-- Gordon

For the regular Tankbot (the DC motor one), I pretty much kept things exactly the way Tamiya has them in their kits that use the same track. This is by necessity, as it uses the Twin Motor, and the supplied steel axles. I kept the overall standard track lengths as used on the Tamiya bulldozer, etc. As you know you can mix-and-match the links to make a couple of different lengths.

For the Tankbot Servo, the stance is about an inch wider, because of the depth of the motors. The track is the same length, with about the same amount of track on the ground. That makes the width slightly wider on this version.

On the new tank I have

, I provide framing so that you can build it any of four ways. The first is the traditional "long track" method. The track patch is fairly long, without "lifting" any of the idlers. Probably similar to the Radio Shack tank you measured.

On the other, you can use it in any of three ways: with 2 or 4 idlers on the ground, or 6 idlers. In this version, and using the 6 idlers, the two outer idlers on are stepped up 1/16". This prevents the whole length of the track from sitting flat on the ground, exposing a considerable surface area that has to be slid over. This is what some tank designs do to minimize the track patch, when that is a critical factor (to be honest, it isn't often for dirt or sand).

Because I believe in people deciding for themselves what works best, I'll note that these new designs are constructed in a way where you can widen the stance how ever you want. Each use independent "side rails" where the motors attach to, and a center piece that holds things together. You can use your own center piece of any width. Right now, the width is about the minimum it can be, due to the dimensions of the motors. So one can experiment by making the stance wider, just by attaching their own piece of plastic, metal, or wood as the center. Maybe someday I'll provide pre-cut/pre-drilled pieces with different widths, but that'll need to wait until I can add some more of the bases I'm working on.

-- Gordon

Excellent points all. I just wanted to note that I agree the surplus/RFE Pittmans, especially the ones with an encoder, are absolutely fantastic. But the price starts accumulating when you add on A) some form of hub to attach the motor shaft to the drive sprocket and B) a suitable H-bridge. Of course, if one has to buy the Pittman motors new, the price skyrockets. I was quoted over $100 each, in quantities I don't even want to think about!

Very true.

-- Gordon

Thanks John I figured odometry might be a bit messed up. I've been using dead reckoning by counting motor steps with the robots I've built so far. Still, the 4x4 would be good for teleoperation.

____________________________________________________ "I like to be organised. A place for everything. And everything all over the place."

Thanks Gordon I hadn't seen your site before. The discussion further down this thread gives me some ideas. I was likely to end up with a fairly square-ish robot, since I've used the case of a CD ROM drive as the starting point. I think I'll leave it as a 3-wheeler for now but knock up some hardware for later.

BTW the PIC and MOSFETS stepper motor driver works well. I'm running the motors off a 12V SLA battery and they're unstoppable.

____________________________________________________ "I like to be organised. A place for everything. And everything all over the place."

Hi Gordon. Your new tank bots look pretty cool, since they're longer and lower than the original servo tankbot. Do they still use the same Tamiya treads, or a different tread? It's not quite clear from the photos on your site.

The downside to having a longer treadbase is the ability to turn, as you note on your site. I would imagine the base with 6 idler rollers will have a lot of trouble turning on nasty carpets, given my experience with your original servo tank with 3 idlers and 3" treadbase.

The first thing I did was set the servo tank to the fastest turn speed [with both treads moving opposite directions, of course], and put it down onto several different surfaces. I have one throw rug with a very tough woven underside, which proved the worst case, and the treads would derail, until I played a few games with the iders.

The derailing would occur on the rearmost idler with the long throw back to the drive gear, and I found that changing that idler to a toothed idler helped a lot. It could partially derail and still recover.

I imagine there might be a similar derailing problem on your Tankbot XL with flip base, since it also has long throws on the endmost idlers. Of course, besides my "mods", the other way to deal with derailing is to turn slower :).

- dan ===========

Not the Tamiya treads, but brand new and exclusive to us. I'll add an explicit note about these new treads this weekend. (Though I would have thought just by appearances in the photos people familiar with the Tamiya treads would know these aren't Tamiya.)

Carpet is the bane of any rubber tread, but it depends on the carpet.

The new treads have an inner cog that positively engages into the idlers

-- all of the idlers are sprocketed, and rear idler has keepers inside and out that prevent the track from jumping there. You can see the cog in the bottom picture of the catalog page:

It sounds like you may be over-weighting your robot, and/or should play with the tension of the treads. Rubber treads will elongate enough to detrack in turns if you put too much weight on the vehicle. This is especially true of the Tamiya tracks. Change the tensioning to keep them from stretching too much that they'll detrack. If they've been assembled properly they won't easily come apart, but if you're worried about that, you can dab on silicone caulking at the joints. Mine have not come apart in 2-3 years of play. Be sure to smooth out the caulking so that there isn't a rough spot there that will interfere with the drive sprocket.

Since it's impossible to know what surfaces people will be using, it's not possible to know for sure if the tracks will jump, which is why the XL can be built in different configurations. But I rather doubt it'll happen much especially with the flip base.

The biggest problem is that people want to use tracked bases when they shouldn't. Tracked bases with rubber treads are best on non-compliant surfaces (i.e. concrete, low nap commercial carpeting, closed loop Berber carpets, etc.). Problems arise when the tread is compliant and so is the surface it's going over. Friction is very high in turns, so even if the treads don't detrack, it takes a lot of torque to negotiate the turn, and often the turn results in skiddering and other conditions not helpful in an autonomous vehicle. You want to avoid this condition on many levels.

For some time I've offered a track base with hard plastic (ABS) treads, the Tracked Drive, that navigates over the toughest of carpets, for those people who want a rugbot. The treads absolutely cannot stretch and detrack. However, the hard plastic is very slippery on hard surfaces, so traction in those cases isn't very good. I provide a length of rubbery non-slip tape you can put on the treads, for these situations.

-- Gordon

Excellent. I thought the treads on the new tanks look different, but the web pictures are a little too dark to tell, which is why I asked.

However, I just took the sprocket pic and jiggered it using Photostyler, and can easily see the new treads have keeper/extensions in the center of the treads, which will fit into cutouts on the sprockets. This is how my Radio Shack tank treads are designed. Much better for reducing chance of derailing, I think.

Are you going to be selling the treads alone? Are these ABS?

...........

I've not put any more weight than just batteries and a small contorller board on the tank so far. After I adjusted the tread tension better, the advent of derailing when way down. Also, changing the rearmost idler from smooth to toothed helped a lot.

The tread tension adjustment is really quite moderate, as I've only moved the servos back 2mm in the slots. The one thing I do dislike a bit with the tankbot servo is that, even at moderately low tread tension, this pulls the large front idler back at any angle [not too large]. I imagine this doesn't happen on the original Tamiya base, since it uses a rod that extends all the way acorss to hold the idlers, rather than screws. Just some constructive feedback.

Once the tread tension was increased, I super-glued [CA] the links to make them permanent. Seems to be ok.

...............

Are the new treads for the XL tanks also hard ABS, or a softer variety?