Let's Make Robots!

Tri-wheel stair climber

It seems to me that for a robot to be really useful in the home or workplace then it must be able to handle stairs. With this in mind I want to design a stair climbing robot chassis. This robot is my first experiment with Tri-wheels.

The chassis is just 4x Wild Thumper 75:1 motors driven by a Wild Thumper controller in a clear acrylic base. As this is only an experimental platform for the wheels I am controlling it with a TV remote and an IR receiver.

This first set of Tri-wheels is made using a new 80mm diameter wheel that has been produced by DAGU. The gears are steel to handle the high torque required when climbing.

The clear triangular frame is made from 3mm thick milled polycarbonate. I used CNC milled polycarbonate instead of laser cut acrylic because it is tougher. The acrylic is fine for the chassis but too brittle for the wheels. All shafts are ball raced to reduce friction.

You can see here that the chassis consist of 4 ribs, a top plate and a bottom plate. They are all held together by 56 small steel angle brackets and 112 small M2x5 screws that give the chassis good strength and rigidity. In hind sight I could have used less but this is just a prototype.

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As I explained to Gabriel, because there is no load on the back wheels, the small wheels are not locking up when they hit the stair. instead they climb it causeing the whole mechanism to spin backward. When under load, the small wheels would lock when they hit the corner and the entire mechanism would rotate forward.

Interesting results, how many inches of clearance do you have to the bottom of the chassis?  You think the clearance will fix the issue?   Most videos of this type of setup usually only climb fake steps that aren't as steep, it's nice to see someone attempting the real thing.  I too want to build a tri-wheel design, I was thinking about driving the tri-part seperately from the wheel driver.  I realize one benefit of the design is being able to use the same drive motors and the gearing allows it to turn it around, but having a seperate motor might give other benefits as well, at the cost of complicating the design.

I can't find if you posted it elsewhere, are the wheels around 3 inches? and the diameter to the outter wheel centers is 6 inches?

I look forward to seeing it climb those steps, good luck.

This is the problem, the clearance on flat ground is only half of the step height. I plan to replace these wheels with Wild Thumper wheels. That will only increase the height by about 20mm (¾ of an inch) but it will also close the gap between the front and back wheels.

The wheels I'm using now are 80mm diameter (3¼inches) the Wild Thumper wheels are 125mm diameter (5inch). The diameter of the outer wheel centers is 170mm (6¾inch).

I might just be mistaken or misunderstand, but wouldn't it climb stairs a lot better if the "rear" wheels went in the opposite direction when it's trying to climb?

No, the wheels are working correctly, the stairs are simply too high for this robot. The reason the rear wheels appear to be driving backward is because there is no weight on the back wheels at one point so that as the small wheels drive forward and then up they cause the triangular frame to spin backwards.

If there was enough weight on the back wheels then the small wheels would lock when they get into a corner. this would cause the entire triangular frame to rotate forward rather than backward.

yeah, OddBot could try to add another gear on each arm to make them flip forward instead of backwards when they loose grip.

Looking closer at the gears reveals that it's actually done right.

Perhaps gear up the wheels, would give the tri-arm rotation more torque, making it more likely to flip over forward.

As I explained to Gabriel, it was due to there being no load on the back wheels. Rather than lock when they hit the stair, they simply climb up the stair causing the whole tri-wheel to flip backward.

As the Tri-wheel is a differential drive system, if the small wheels can spin then they will. Once the load becomes great enough, the entire mechanism rotates instead.

Very nice! Finally, an old Lego design that makes it to the non-Lego robots! I had troube steering with my Lego Tri-Star Wheeler (built after Doug's example, but with the large motorcycle wheels) because the Lego wheels have an inclredible grip and everything was shaking when turning. I ended up with 2 tri-star wheels in the front and a tail wheel and it was able to climb small Lego made stairs (just like in the above link). I was supposed to build a real stair climbing robot so the robot had to climb human height stairs then come back down. After seeing how it performs, I scraped this idea and used 2 rack-and-pinion jacks to lift the robot up the stairs and back down (with no turning at the top). But that was good enough for the competition. In real life the stairs have the bull nose that makes trouble for the robots climbing them. Anyway, I look forward to see the balancing tri-star robot! Cool, OddBot!

As you say, real life provides more chalenges such as bull nose steps (overhang) and cornering on small landings. I have some ideas that I am keeping secret for now. We will see how this first prototype goes.