Holonomic Wheels on Cars

[DrJB]

The word holonomic was alien to me ... until I took a course in Graduate Dynamics years ago ... and then fully understood what it meant.

In any event, this is not about 2.032 ... I have seen many Lego vehicles use omni-wheels in-place-of regular wheels (e.g. Mahjqa's Grudge), and yet one ponders how come such wheels are not more widely used in real life.

Can you think of Pros/Cons for such wheels? (besides the obvious complexity) ... etc

I'll start with this: If you push laterally on a regular wheel, it does not move, if you push on an omni-wheel ... it moves freely. In tire-dynamics jargon, Omni-wheels have ZERO 'Cornering Stiffness'. What this really means is, while driving on the highway at 60 km/h, a small gust of wind, or a slight turn would cause the vehicle to drift to the side. The vehicle simply cannot take turns, it will change its heading, but its center of mass will continue on a straight line, no matter what you do with the steering wheel. That's pretty dangerous! Also, if you're driving on a road that has a small bank, the vehicle would roll towards the steepest slope, no matter what you do with the gas pedal and steering ...

With all that, it makes sense why we do not see such holonomic wheels in wide-spread applications (besides conveyors) ... it's a nice toy, interesting kinematics, but not much more.

You may ask what this has to do with Technic ... Well, I have collected enough parts and now built both omni and mecanum wheels, and now need an application for them. Don't get me wrong, they make nice toys.

Edited February 20, 2014 by DrJB

[Zerobricks]

For mecanum wheels you can always make an omni directional forklift :)

[Choops]

Forklift.

[weavil]

They even have a scissor lift.

Edited February 20, 2014 by weavil

[DrJB]

Correct, I'm fully aware of the mecanum forklift ... but that works best on flat surfaces. On inclined planes, the lateral friction from such wheels is smaller than that of regular wheels ...

[weavil]

There is a way to make the steering work. Add power/drive to each roller on each wheel unit.

http://www.danielbauen.com/holonomic/index.htm

But are are talking no steering or no steering?

[DrJB]

I like the power-drive concept. My main argument (initial post) is that the free-rolling wheels on both omni/mecanum are an uncontrollable (troublesome) degree-of-freedom. The simplest experiment I can think of is a car with 4 omni-wheels going on a straight line ... on a bank (road tilt left to right). The car will simply drift down the slope and the free-rolling small wheels on the omni won't prevent it from doing so, as such tires have ZERO cornering stiffness.

[weavil]

They're designed for low speed and ZERO-impact neutral steer. Drive, stop rotate 90°, and continue on. But for an actual vehicle, each "roller" needs a brake, power, or another means of controlling when they can move. However, that would be easy for any light thing (i.e. a robot), but for a 3,000lb vehicle it would tremendous force to keep the "roller" from moving. Other wise stability of any kind would not be achieved.

[Aswin]

There are other issues as well.

You need to have all the wheels propelled and synchronized to have a controlled ride. This is hard when the surface is not flat, when the wheels are suspended or carry different loads.

[DrJB]

Thank you, that's exactly the kind of observations I'm looking for. On a different note, I built your 3-omni-wheel robot, used the new EV3 motors (2×) and one NXT motor .... only about 90% into the build, I realized the pins locations are not compatible on the NXT/EV3 motors ... They both looked the same for as long as I've had them. Now I realize they are not. Two options for me: buy an EV3 motor ... or build the NXT version.

[Aswin]

Thank you, that's exactly the kind of observations I'm looking for. On a different note, I built your 3-omni-wheel robot, used the new EV3 motors (2×) and one NXT motor .... only about 90% into the build, I realized the pins locations are not compatible on the NXT/EV3 motors ... They both looked the same for as long as I've had them. Now I realize they are not. Two options for me: buy an EV3 motor ... or build the NXT version.

Or use the Medium EV3 motor in the third leg!

It think this is rather easy as you can just build the leg frame without the large motor in it. You can then attach the medium motor underneath the frame and connect it directly, without gearing, to the wheel. Add one or two extra liftarms to make the frame stiffer and you are done.

Thank you, that's exactly the kind of observations I'm looking for. On a different note, I built your 3-omni-wheel robot, used the new EV3 motors (2×) and one NXT motor .... only about 90% into the build, I realized the pins locations are not compatible on the NXT/EV3 motors ... They both looked the same for as long as I've had them. Now I realize they are not. Two options for me: buy an EV3 motor ... or build the NXT version.

[sanjy009]

Hello,

I'm building my own custom LEGO holonomic wheels in an EV3 Killough Platform (see previous post here: http://www.eurobricks.com/forum/index.php?showtopic=86692&st=100#entry1718813). I've used 3 large EV3 motors, but my own design (following LEGO dimensions as much as possible) of an omni-wheel. You can see how 'bouncy' the ride is, they are built for manoeuvring rather than speed, though I can see a real life use for motorised wheelchairs/ mobility scooters.

On other forum topics (especially your detailed one Aswin) there is some discussion of steering controls. I've found the Mindstorms android app is really great to use on a tablet to control the 3 motors, I just wish you could fit more onto the screen.