Treads, Tracks, Tank system Talk/Help

The Background:
Alright, I would like to say to start off I love treads/ tracks/tank drive and feel the need to please the tank gods. My team has had a tank drive system for one year and support after that year for tank type drive systems have died and have be replaced with others systems aka wheels, omnis, mecanum, etc. The robot equipped with the tank drive system was for a while my roommate as i had made repairs but found out why then bot had so much issues like fractured lex sand framework and really bad gearboxes but yet it holds true to my heart. I’m revamping its design which is near completion but has some questions that need answering before it can be completed and also it may have more questions that i may have not thought of.

The Question:

  1. What gear ratios should i have for the tank tank system (motor to gearbox output)?
    Note: I’m not stupid i know there is a series of equations to find the right ratio but I’m looking mainly for what others have done that might work for me.
  2. How to solve the walking effect where i seen some robots with treads hobble as they turn and even become unbalanced that the driver has to stop?
    I assume this is from the tracks not being held center because of certain belt track systems.
  3. How to correctly tension the tank system, such as spring loaded or motor controller tension or to have the tracks at a fixed tension set by the frame?

General Talk:
I’m also looking for just a general what you may have done in this field of drive train and certain things you may have experienced that may help me and everyone have a better idea on how to produce tank systems to please the tank gods.

997 has a very impressive tank drive, might want to ask them.

as do the RoboWizards 522. Feel free to contact any of their active CD members.

Some of the issues you are asking about are typical of tank tread systems. One that you have not mentioned is the bane of tank drive using treads or locked wheels, i.e.stalled motors.
When using treads on carpet, the side friction (during turns) with the carpet is so high that robot has to hop to break free from the carpet surface. Since the motor/transmission is usually capable of enough torque to overcome the friction, the robot hops when turning. However, during these moves the motors are at, or very near, stall. In a four motor drive using the CIMs, that is potentially over 400 amps. In the IFI control system, this causes the battery voltage to temporarily fall below the drop out voltage of 8 volts, and the controller goes into protect mode disabling all outputs and switching to the backup battery. (We don’t have any data yet on what the new controllers might do under these conditions.) A wheeled robot in the same kind of drive could have a little less side friction but the problem remains and they also will hop to some extent in turns. Drivers can minimize the hopping by turning in conjunction with forward or backward motion. The AndyMark style wheels that have no side friction were developed for this reason. Everyone tries tank treads from time to time, but the tradeoffs are usually significant enough that teams go back to wheeled drive systems. Any game in which turning is required will have a minimum number of robots with treads. This year’s game is a good example.

I have no robot experience with tank treads, but most tractors used a single powersource into a differential with turning brakes. This is common on wheeled tractors too (they are often called plow brakes). Using the differential and side brakes essentially allows for torque vectoring and keeping from a stall condition. A couple of teams did this style of drive system this year to help with Car-style steering robots. This probably won’t help with the side traction hopping, but it may help keep the current loads lower.

I watched a couple of shows on Discovery (one was JYW where their task was to build a tank) and I believe actual tanks used to be built the same way.

An idea I played around in CAD a few months ago took the 6WD rocking-style drive train and converted the front four wheels into tank treads. The design put most of the weight in the back so the 'bot never really rocked forward onto the treads, however the tank treads would greatly improve the ability to climb ramps. Maybe it’s worth exploring, maybe not…I didn’t have time to fully CAD it out and consider the possibilities.

You can greatly increase the mobility of the tread system if you decrease the surface contact.

Take team 522 for example. Year after year they produce a successful tread system.

http://www.chiefdelphi.com/media/photos/17294

Their surface contact basically creates a square profile on the ground and allows the robot to turn with more ease.

Thinking about tank treads and other drive configurations is probably a good idea. The past few years many teams have gone to 6 small wheel designs. For the most part the games have been played on flat surfaces. This has not always been true. What would your team do for a drive system if this years game required going up stairs or getting up on a platform?

Your center of gravity has a very impact on this issue also. Assume you have a four cog setup (three similar to a six wheel drive setup and one tensioner) and your weight is either towards the front cog or the back cog you will run into this issue. However by finding the happy medium between center cog drop and your center of gravity as close to the center/low as possible many track systems level out on that center wheel as they reach. My understanding is that this is due to the tread actually sinking a little. This gives the robot a slightly larger footprint to balance on. Account for that and the length of the belt touching the carpet,its like turning a very short but wide wheel base. Another thing that drivers tend to do is not let the robot settle for a second before trying to turn this often throws off the center of gravity way over to the front/rear cogs. To settle the robot you don’t necessarily have to stop the machine but just let up slightly to bring down the robots acceleration and allow the robot to tilt back on plane with the center cogs. However I do agree with Al in saying if it is not necessary for the game why implement it. Tread’s are superb and often faster at vertical obstacles(stairs, bumps, etc.) but in a flat floor game it tends to be prohibitive.

Best of luck,

Pat

Pull yourself up to it from the floor :smiley:

I’d suggest triwheels.

But I’m just a programmer, so when it comes to mechanical design I don’t get too vocal about my preferences.

We did that once with a 2-wheel-drive setup. It all depends on the specifics.

Well first off, half the things we do in life is what we want and the other half is what we do. I personally choose the tank system for its ability to have the power to climb over any planed or unplanned obstacles. Tracks provided a larger footprint to the floor allowing for greater traction that no other dive can compare with equivalent materials (such at same gear ratio and tread material). Tracks due to their larger footprint can be harder to push around and with the right gear setup push others that may be friend or foe.

If you ever sit down and think about these robots as if part of a video game where you have different characters with different abilities you see all have advantages and disadvantages you never thought of but depending on how you do it could have some unexpected combination. Depending on the parts used, a tank robot could drive just as fast as any other bot but could never match the maneuverability of mecanum, swerve, 4(or 3) way omni, yet anyone can push around mecanum or 4(or 3) way omni, but is is far less likely for a Track system to be pushed around. So in the end what is truely necessary for a great machine because from what i see tracks offer no less than any other drive system.
Note: I’m not saying tracks are the best since their greatest disadvantages is weight and turning but yet this all depends on how you do things. Yet when you say something like “it not necessary” i can’t agree. Please think about the world from both views as in the glass in half empty and half full.

Actually, it’s twice as big as it needs to be.

You have to consider this from all angles. The “cool” factor, pushing power, maneuverability, speed, maintainability, simplicity if you like the KISS principle… All of those are going to affect whether or not a tread design will be better than a 6WD/mecanum/3WD omni/swerve system for a given application. Demo robots, who cares. “Cool” factor will probably win. Prototype/practice will have a different dominant factor. Competition, yet another.

From what I understand, you’re looking at doing a prototype/competition treaded robot design, not that you’re actually going to use it, but that you’ll have it in your bag of tricks. Whether or not you use it in competition is up to you, and you’ll have to decide whether it’ll fit the game requirements when the time comes. For now, design help:

I’m not a tread drive expert, but I believe that the gearing is similar to that of a normal skid-steer robot. Look at it this way: The robot lays down its floor, rolls on it, and then picks it up.

Correct tensioning: have an idler wheel that is adjustable. Slide up or down to tension. That’s one way; there are others.

Oh, and for Gdeaver: 6WD (and the other systems, for the most part) is plenty good at ramps. Stairs are a different problem, but can be solved in many creative ways.

when I was a mentor with 226 we had a tank drive for two years. We used brecoflex belts for tank treads. The hobbling that we experienced was from the track trying to scrub off the friction while turning. We ended up using an adjustable center wheel that was lower than the rest to provide better turning similar to what is used on 6-wheel bots. Not sure how low it ended up being, but I know it was between 1/8" and 1/4" lower than the rest. Gear ratios depend on the game for that year. We used a two speed shifter to give us more flexibility. Older games tended to need a lot of traction and had a lot of pushing/pulling, so we had one gear that was real low. We also had a high gear that provided around 12fps. As far as belt tension, we used a wheel that pushed down on the belt that adjusted using a jack bolt. We would tighten the bolt until the wheel was putting the right tension on the tread and then tighten a jam nut so it would not come loose. Feel free to PM me if you want any other info.

Nicely put EricH.

Power depends on the motors and any mechanical losses in the drive system. All else being equal, wheels and tank treads should have no significant difference in pushing or climbing power. Tanks are better at dealing with unstable surfaces, but unless what you’re driving on moves or shifts there probably isn’t a good reason to use them.

A larger footprint does not have any theoretical advantage in traction. With the same coefficent of friction and the same weight, the amount of surface contact is not a factor.

With carpet deforming more under the concentrated weight of wheels, the actual traction of a FRC robot is somewhat complicated to predict in advance. Experiments seem to show that wider wheels are better in practice than narrower ones, but long tank treads don’t give the benefit you attribute to them. On the contrary, the difficulty they create in turning is usually considered to be a good reason to avoid them.

This is sort of true. With compliant surfaces like carpet, rubber tires and treads there is some degree of mechanical interlock that can change the grip level to normal load.
While you can generally equate a coefficient of friction as a constant, there are noticeable changes relative to load. I think 494 did a neat project on this.
%slip also makes a huge difference.

Any students interested in a good/simple science fair project, this is a solid one. Map the coefficient of friction between two materials (let’s say carpet and brown ruff belting) vs. contact area and load. You will get a really cool 3D curve and likely win some scholarship money (and might even learn something) in the process.

Has there been a game yet that REQUIRED teams to climb obstacles? Seems a lot of the successful teams from 1999 and 2004 found clever ways to avoid climbing things.

There have been games with obstacles that needed to be climbed. But I understand what your saying.

But treads, WHEN DONE RIGHT, are a great option for maneuvering up and over obstacles such as steps, ramps, etc…

There is a reason why tanks use treads.