# 3 axis propulsion - how do we do it properly?

We are in need of intelligent hints/ideas to allow us to continue development of our Junior Robocup custom robot. System specifications - 3x 4cm omni directional wheels, direct coupling to
COPAL 16mm gearhead 12v motors (63mm total length). We have looked inside the COPAL gearheads and found the planetary system to be plastic based gears.Robot base diameter is 22cm, wheels mounted 120 degrees apart. Control is via PWM motor controllers, servo to PWM convertors and MIT Handyboard.Total system weight is 5lbs, height is 20cm. Problem: far too much force on the geartrain/shaft is giving us a very short gearhead motor lifetime, the planetary gears are stripping teeth and supports being broken in the gear train.
Attempted solution: we made an aluminium bracket with two mounting holes for the gearhead motor system. This provides much more support for the gearhead but only extends the overall lifetime by a small amount.
We are still at high school, not studying mechanics and we are involved in subjects that are way beyond our teachers knowledge skills. We need hints or to be pointed in the right direction to find out about these things:
a) How can we calculate the radial and axial forces involved when this robot is in motion? Would it require special test instruments?
If we had this information, we could match it to the gearhead motor specifications which we feel are currently being well exceeded.
b) When working, the movement of the robot 3 axis system is beautiful, however as the motor is constantly being changed in speed of rotation and direction (often in a fraction of a second) - is this a special situation we need to take into account when choosing a motor? The motor can be rotating at 5000rpm and then almost instantly be reversing - this must be creating a great deal of stress on the geartrain? Can we reduce this strain? Would making the geartrain stop before reversing help much? and wouldnt this make for a rather 'jerky'movement?
c) If we redesigned the drive system to an axle type - make the gearhead shaft longer and supported by another bracket and mount the wheel in the centre of this drive shaft. Would this not greatly reduce the radial and axial forces the gearhead is subjected to?
d) How do we work out the change of forces from a 'standard' motor system (2 wheels) which are driven 'straight ahead' and just turn by having more or less power applied to one wheel? The omni system has each wheel set at 120 degrees in a circle format and when moving straight forward, the wheels are actually moving at an angle but forward.This must be substantially increasing the radial force being applied to the drive shaft?
e) If we use something like the Faulhaber gearhead motor system (www.erni.com.au) say the series 20/1 with 66:1 gear reduction and the 1727-012C motor, this has metal gears but will we be better off or will it just delay the teeth stripping a little bit?
I hope this posting has enough information for ideas and looking forward to your input. (We just finished competing in Padua Italy and got to the quarter finals fine but then the motors (all of them!) geartrain failed and the replacements just didnt do the job well enough.)
TWAIN Sydney Australia Jeffery Sinclair
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Anyone got some decent motor suppliers they can think of?? I need motor suppliers and specifications - forces etc... So far have looked at Mabuchi,Copal and Faulhaber - the latter looked so good but they cant supply for 8 weeks!! (thank goodness we dont use them in our machines at work!) - we dont care where the supplier is - just must have these rough specs upto 30mm diameter motor/gearhead assembly Maximum length would be around 65mm Torque must be at least 1Kg.cm Radial and axial maximum allowed 150N Running 12volts PWM control - must be able to reverse. Our current motor controllers allow us to run upto 2A on each motor - if the stall current is higher then we can always upgrade our controllers - we need a decent motor to start with.
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Hi
There are only 2 small ones that I know of, http://rswww.com and put 748601 in the search box.
Good luck anyway.
best regards
Robin G Hewitt
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Your responds will not be slowed down. Only a full reverse takes 1.5 to 2 seconds. If you slow down one side 10% and acc. the other side 10% you will make a curve. This only take 1/10 of a second.

If you implement is, you will see the movements will go more smooth, but still very controllable.
Good luck,
Peter
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If you want to jerk things around rapidly, start by reducing the inertia as much as possible. A 5lb mass sounds on the high side - do your utmost to reduce this figure.
Gearmotors are not a good choice for this kind of application. Amongst other things, the gears must be sized to fit in a very small space which means that they will not stand up to being ill treated.
I haven't used the type of wheels you have, but I imagine that the frictional losses will be high - lots of plastic/plastic bearings. Note also that gearmotors are not very efficient either.
I would lean in the direction of using low inertia servo type motors (also called 'ironless' or 'coreless' - but not to be confused with RC servos !), which are designed to generate torque rather than revs, together with a modest gear ratio - something like (say) 6:1. Sources of such motors include Maxon and Escap. Read the motor specs very carefully. Most will allow you a very high current for a short period to provide a 'kick'. These motors can be expensive, but on the other side you will not need to buy a (usually very expensive) gearhead !
For this job you could use plastic or metal gears - plastic gears may work ok (and be light !) if they are sufficiently over sized, but of course the pinion will need to be metal. Another possibility is small toothed belt drives, which have advantages in absorbing shock and a certain amount of mis-alignment. If you only have limited engineering skills, then the toothed belt is probably the simplest option.
For this kind of competition robot, you need to ensure that you will survive anything that can happen. The drive system will take the brunt of any collisions, so making it over strength will buy you peace of mind ..
good luck
Dave Garnett

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as much as possible. A 5lb mass sounds on the high side -

other things, the gears must be sized to fit in a very small

Hi Dave
I get the impression that you actually know what you're talking about, but why aren't you restricting yourself to the kit/modular/'where do I buy that' approach to this question? :o)
I worry that one day someone might suggest cutting a few gears or making a slip clutch and this group will have a collective nervous breakdown.
best regards
Robin G Hewitt
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It''s called engineering, and it seems to be going out of fashion. For many purposes the hack approach is perfectly adequate of course, but if you really want to win competitions then everything matters.
[ I've been involved with some robots for the FIRST competition, and it is kind of hard to persuade teams that part of the testing should include driving their masterpiece into a concrete wall at full speed ...]
regards, Dave
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that'
I don't know... the original posting suggests the people working on this project have the ability to produce the mechanics suggested.

for
They also seem well funded based on their consideration of building/buying new in place of fixing the current drive train, and the fact they compete internationally.
Mechanical solutions that are not "off the shelf" look to be valuable in this case. It is amazing what you can do in high school!
Timing belts might be a light and easy to implement alternative to chain suggested by Dave.
-andy
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snowdy wrote:

Maybe you need to provide some point of reference on what your budget is. You're able to spend \$600 on wheels but can't afford gearhead motors with metal gears?
Have you seen what Acroname (http://www.acroname.com/) offers? They have omni wheels as well as nice Maxon gearhead motors with encoders (\$119 each). They usually have these things in stock.
Mitch Berkson
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Well, the way I see it, being a non-experimenter and more of an enthusiast and also a repairman of other machines, the best thing would be to eliminate the weak link in the system. Seems the weak link is the plastic (nylon?) gearing system. Perhaps the units could be disassembled and pieces replaced? If so, disassemble the gears, make detailed drawings if possible, and see if a machine shop or parts house might be able to supply aluminum duplicates or even fabricate custom parts for you. Maybe they might even be able to design something more effective for your use. Try looking up an automotive machine shop or engine rebuilding shop in the phone directory. They have all kinds of precision cutting and milling machinery that might be able to manufacture the high-endurance components you need. The fact you're high-school students and machine enthusiasts can't hurt in persuading someone to take on such an oddball job request. Once you have 'em, keep them lubricated with either a sealed oil chamber or an oil spray arrangement and they could likely outlast the rest of the components altogether. On cars, usually the transmission is the strongest part, as it has to take the most abuse. Think in terms of designing a proper transmission and that might help you find your solutions quicker.
sean
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