Hacker News Reader

Comment by lrizzo | Hacker News Reader

I really don't understand why for this use case people do not use a third wheel and save the energy consumed to stand still with self balancing. Not just this segway, also the double robot (the latter, at least, can claim aesthetic motivations given the barrel shape).

View on HN

Tri-wheel robots wobble really badly. For telepresence, the screen is at the top of a mast so wobbles are amplified. The balancing robots have very smooth motion.

Wouldn't it be easier to stabilize the screen than the whole robot?

No, it's very easy and inexpensive to balance that robot at this point for Segway. They've already solved those problems, it's a natural extension to use their existing technology for stabilizing the robot. Further, it's not an option to not stabilize the whole robot, you have to or the whole concept implodes. Put another way, they were going down that road regardless.

They still have the problem of falling over when the battery dies.

Put it this way: The head on the end of the mast is a significant mass at the end of a long lever. To dynamically stabilize that, you're going to need big, hefty, expensive motors. Well, you already have big, hefty expensive motors to move the thing around in the first place, so it actually costs less money to dynamically stabilize the whole robot.

Check out the inverted pendulum on a cart problem, or go balance a couple bats on your hand. When near vertical the inertia of the pendulum mass helps you out, it's the inertia of the cart the motors need to overcome. The better the control loop the fewer and smaller the corrections will be, tending towards zero.

I have a video camera with optical image stabilization. It uses teeny, tiny cheap motors to move some optics around to stabilize the image. It is not obvious that stabilizing video justifies giving up the passive stability of a tripod.

What if they had a tiny kickstand for when they're stopped?

I guess it's aesthetics either way, having only two wheels allows the robot to turn on the spot as well without risk of hitting anything/anyone...

Plus isn't self-balancing kind of segway's thing?!

Active balancing is actually really cheap, if I'm remembering correctly. In an ideal situation the cost should be zero as the corrections required would be infinitesimal. That case isn't really reached, but the robot I worked on massed 150 kilos and probably spent way more power on computational homeostasis then it did on balance.

Agreed, I did a short bit of research on simulation, and in this case (rotational dynamics) moment of inertia is substituted for mass in the force calculation, and when standing still, that is always near zero. Therefore very little energy should be necessary to maintain it.

I don't know much about them, but wouldn't inclines be harder for anything with more than two wheels? With two wheels, you can easily maintain balance, but with three you would have to have another control (or two) to make sure the 3rd wheel is in the correct direction and I presume change the angel as well.

Did you see any video or press from the actual Segway event? The "robot" here is actually a rideable mini-segway. You fold its head sideways and awkwardly put your crotch over it as you step on its wheeled "legs."

Its a strange concept. Am I buying a mini segway or a robot? Do I really want to ride my robot around? I imagine for the price this thing is going to sell for, the answer will probably be, "Hell yeah, for this kind of money it better carry me around."

I think a proper home robot wouldn't have these capabilities and a third wheel or four wheel platform would be more likely. The problem is that Segway can't make a robot that isn't, well, a segway.

Balancing enables it to be any height without increasing footprint or hence weight. The camera can be at human height, but only take up as much floor space as a human. As an example, the Beam Pro does not balance, has 4 wheels and weighs over 100 pounds with a camera at 5'6". Anybots QB weighs about 35 pounds and can have the camera at 6'2".

It also enables added mass without changing the drive system. So instruments and equipment can be added.

It's got a better chance of falling down curbs and stairs more gracefully with two wheels? Getting back up is another problem, though.

Also, it might be able to navigate spaces a bit easier.

Finally, it looks slightly more human since we're bipedal and it's bi-wheel.

The double robot has a little kickstand for when it's going to stay in one place a while.

My guess is that it saves the trouble of making a wheel that can rotate along the vertical axis (to turn left and right).

That greatly simplifies the design and make the whole thing more robust.

Home Depot has decent swivel wheels for maybe eight bucks retail. The wheel that rotates doesn't have to be active.

Hum, I think a free swivel wheel would only work for very cheap designs. If you're not convinced, you can search for 3 wheels vehicles on google image [1]: none of the vehicles there have the 2 wheels + 1 free rotating wheel design that you recommend.

https://www.google.com/search?q=3wheels&biw=1673&bih=1124&so...

Edit: actually, some baby carts have it .. alright

http://www.aliexpress.com/store/product/EMS-Free-shipping-Go...

I think that's more of a speed thing than a cost thing. At higher speeds you want to control all the wheels to ensure you don't go sliding off into a utility pole or something.

Yes, by "cheap" I mean "low quality with low requirements".

Reply on news.ycombinator.com