Student Built Robot Chassis Has Something You Can Learn From

This is a four-wheeled robot chassis built by high school students over the summer. They were participating in workshops put on by xbot robotics in Seattle, Washington. The goal is to get them participating in events like FIRST Robotics and LEGO league, and eventually into science related careers.

At first glance we thought: oh, that’s a nice chassis build… on to the next tip. But then the difference in front and rear wheel types caught our eye. The problem with four-wheeled designs is that you need differential steering to overcome the skidding issue when turning. This usually means two independently powered rear wheels and one unpowered front wheel that can swivel. One way to overcome this is to use three omniwheels, each with their own motor. And more recently we have seen four-wheelers that use mechanum wheels to get around the issue… but that takes four motors.

The design seen above uses just two motors, each with a chain to drive both wheels on one side. The rear wheels have rubber grippers which give them great traction. The front wheels are omni-wheels which allow them to move side to side easily during turns while aiding in forward progress when not turning. This gives the robot enough grip to push object around, like you can see in the video after the break.


27 thoughts on “Student Built Robot Chassis Has Something You Can Learn From

  1. i would suggest putting the main wheels in the center of the robot, so the center of gravity would be right over the center “axle”. That would not only give move traction, but also make the robot much more nimble for turning. then just have some more omni wheels for extra support, or even go with a 6 wheel method.

    1. Agreed. Another trick I’ve seen with the six wheel design is to raise the non-drive wheels on the front and back a fraction of an inch to ensure that the center drive wheels are carrying most of the weight.

  2. I’m part of a team who is just starting a robotics club at my high school, and we actually chose 2 omni wheels and two regular wheels for our wheels.

    In the picture above, they have Tetrix (motor and mount, both types of wheels, and battery), LEGO (NXT, HiTechnic motor driver) and then MircoRAX for the rest of the chassis.

    I wonder why they decided to mix materials instead of just sticking with Tetrix of MicroRAX exclusively?

    1. I’m pretty sure that no one is controlling it, and that it is just following its programming, which if they are early development, still needs some improvement. When I did FIRST, our code almost double in length during testing as we identified and fixed all the different ways the robot could get lost or confused.

      1. God the sound in that video is terrible.

        Admittedly, skipping the chain might be a good thing. Last time I used those Tetrix motors for anything they had a nasty habit of cooking if you stalled them for less than a second or so. The rest of the machine is pretty clean looking though.

    1. I guess for the maximum amount of traction they can get with those two not-so-tractiony wheels, the chain wrap is good enough.

      But as roboman2444 points out, most experienced teams opt for a six-wheel platform with low-traction wheels at the front and back and traction wheels in the middle (lowered by ~1/8″) for better maneuverability and pushing power.

      1. Just so someone reading this doesn’t think 1/8″ is a magic number… FRC competitions are GENERALLY run on carpet and it has been found that between 3/32 and 5/32 of drop on your center/center two wheels provides enough rock to effectively shorten your wheel base. There are, of course, alternate methods of turning with a longer wheel base. One team from Jersey cuts their wheels in such a way that they provide less traction when pushed sideways. This prevents them from having to drop their center wheels.

        Now, if you are running on dirt the drop may or may not be needed. One possible solution would be to use pneumatic wheels (mountain board wheels are popular) and over inflate the center wheels. This has ~the same effect but allows you to adjust your “drop” easily.

  3. I’m with Alabama Lunabotics. We have to design a robot to collect lunar regolith(soil), transport it, and dump it. One of our main concerns is with turning and how to do it without digging a hole (this stuff is powdery!) We actually have a design challenge open on our website that anyone is welcome to register for. Winner gets a prize and we will try to build the winning design. if anyone is interested.

  4. My team used a crab drive last year, with 4 independently powered wheels and 2 motors controlling turning of front and back wheels respectively, similar to a car. I found that even though its complex, if you have a good operator it leads to the best control. It is very difficult to program and set up though because of the sensors and having to constantly have input to limit the wheels so they don’t turn beyond the radius allowed by the wires.

  5. Omniwheels are useful ONLY on clean smooth floors. they are EPIC FAIL in dirt.

    the correct solution is a 6 wheeled that uses a small motor to lift the front and rear wheels slightly to reduce traction when doing a rotate maneuver. this works incredibly well.

    If your robot cant drive across the lawn, it’s not a robot, it’s a early beta prototype.

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