Alt-BEAM Archive

Message #05665



To: Richard Piotter richfile@rconnect.com, BEAM beam@sgiblab.sgi.com, Twin Cities Robotics Group tcrobots@orbis.net
From: Rj59@aol.com
Date: Thu, 12 Aug 1999 12:21:05 EDT
Subject: [alt-beam] Re: New robot pictures!


I like your spider... what kind of motors are you using and where could I buy them? I've wanted to make something like that too!
Thanks



5666 Thu, 12 Aug 1999 10:25:14 -0700 [alt-beam] Re: LEM videos now on-line. beam@sgiblab.sgi.com Sean Rigter No time right now for a full reply, but first thanks for the clarification : it help me
understand the error of my ways.

A couple of comments:

I assumed the LEM wheels where rolling or skidding but it would appear that the LEM is
airborne for some of the time as it "micro bounces" off the gear teeth. When the gears lose
contact with the surface (skipping), the friction to linear motion is momentarily
suspended. I did not have an idea of the Bicore time constants making it even more
difficult to predict motion behaviour.
Like the Unicore design there is a enormous range of behaviour (phasespace) where the time
constants of the electronic and mechanical components interact in differents modes, resonate
etc.

My triwheeler was designed to investigate linear rolling motion and has now evolved in a
different direction.(Killough linear wheel) The tricore circuit was just a convient source
of drive signals and not meant to emulate the LEM " horse and rider" architecture.

The LEM gear wheels remind me a bit of a oil rig drilling bit and my guess is that a soft
or resilient surface runs some risk of being chewed up.

What about obstacles? How does the LEM handle those? A smooth incline is a possibility but
what about a small ridge like ruler or a pencil?

regards

wilf

Bob Shannon wrote:

> Sean Rigter wrote:
>
> > Congratulations Bob,
> >
> > Your whole LEM project (ie the physical design, shape and symmetry, sensors, the
> > documentation and video) shows great finesse. Since so far there have been only
> > kudos, I hope you are ready for some critical comments.
>
> You mean some technical questions, and discussion of the design!?
>
> Fantastic!
>
> > I found the concept of a net linear displacement (ie the useful motion to move the
> > LEM from point A to point B) of a triaxial wheeled platform difficult to grasp
> > until I realized it is similar to Steven Bolt's 2 wheel design of a pivoting motion
> > in which one wheel turns while the other is stalled. In Bolt's design, the 2
> > unidirectional motors are alternately turned on (in one direction) for a 180 degree
> > pivot around the stalled wheel resulting in a somewhat wobbly but on average linear
> > motion while the platform is continuously spinning around it's center (in the same
> > direction). This pivoting linear motion can be approximated with a conventional 2
> > wheel platform which reciprocates (not spin) around it's center. The main advantage
> > of Bolt's design is that it can reverse by turning on both motors and spinning in
> > place around it's vertical center and can therefore "back up" from walls and
> > corners. A conventional 2 wheeler with unidirectional motors can reverse only by
> > keeping one motor on for 360 degrees instead of 180 degrees and which does not allow
> > it to "back out" of a corner.
>
> Not quite right.
>
> LEM's 'Horse' layer is a very capable mobile platform even without the 'Rider'. Without
>
> the Rider board to disable one of the three Bicore's in turn, it still moves quite well,
> but
> not in exactly the manner to seem to be describing here.
>
> What happens is that the Bicores start to move, lets say all in the same direction. We
> store this
> energy in rotational inerita. At the point where one of the Bicores reverses direction
> before the
> others, this uniform rotational inertia gets transfered into linear motion. I think
> this is a bit different than reciprical motion with 2 coaxial wheels.
>
> Because no wheels share a common axis, this design will not roll down-slope even with
> the motors unpowered. Without using gearmotors, coaxially mounted wheels will. Part of
> what LEM does, is to use the transfer of rotational intertia to linear motion based an
> asymetries in how that rotational torque is applied to the chassis. This design can
> turn around in place, but it does not need to in order to escape from a corner like
> Bolt's design, it will simply start moving in a different direction. I dont see any
> clear advantage in either case.
>
> (Actually, I beleive that the photopopper design uses this form of motion before Steven
> Bolt, its really only a matter of how much capactiance you charge to what voltage that
> sets the swing angle. My most 'effieicnt' (fastest) photopopper design turns 1bout 120
> degrees per step, pivioting around the unpowered wheel.)
>
> As for the slipping and sticking of the wheels, please notice the sounds LEM produces.
> This is
> caused by many small splines on the motor shaft gears being used as wheels. Because
> these splines are parallel to the motor shafts, the 'wheels' can be easily dragged along
> the axis of the
> motor shaft with very little friction. LEM 'skipps' over the surface its moving on.
>
> As the other 2 motors also have the same type of wheel, there is a fair ammount of
> vertical, high frequency viberation of the chassis. Should a unpowered wheel catch on
> something, its quickly nudged over the obstruction. Also the design rarely moves in any
> given direction, so as a result
> of all these factors, frictional losses are quite managable overall.
>
> Using gearmotors and larger wheels, it would be critical to adjust the timing of the
> motor drivers to match the mechanical resonance of the chassis in order to acheive the
> same form of motion that LEM is using. Part of the whole idea of LEM was to develop a
> way to apply non-gear reduced motors to drive a 'bot over terrain normally thought to be
> impassable to direct drive designs.
>
> The key to this, is to store energy, either in some form of suspension, or in this case,
> as rotational inertia, and then apply it in small bursts of motion with more energy than
> the motors could apply directly. It clearly works, but I admit it could be made much
> more efficient by adding a more conventional suspension system to each motor.
>
> > So in a 3 wheel platform one can approximate Bolt's linear motion with two wheels
> > spinning in the same direction while the third is reversed causing pivoting around
> > an off-center pivot point and resulting in a net linear displacement.
>
> That seems to be a very loose approximation, but basically correct, but only for a tiny
> fraction of the time that LEM is moving. The motors reverse direction much faster,
> keeping the LEM 'aloft'
> in a fast 'skipping-like' motion.
>
> > While it was possible to conceive of such linear motion, I had problems
> > understanding the frictional losses in such a 3 wheel design so I decided to build
> > one. I used 3 gear motors mounted horizonally with 1 inch diameter toy wheels
> > attached on the perimeter of a 5 inch circular platform. I used a simple tricore
> > with the 3 outputs connected to 3 bridge circuits. This results in a simple sequence
> > of 2 motors Forward (F=1,2)and one motor Reversed (3), followed by F=2,3 R=1 then
> > followed by F=3,1 and R=2 . The Tricore timing was adjusted to a rotation of
> > approximately 120 degrees around the pivot point
>
> This does not approximate LEM at all. LEM's basic form of motion does not require that
> I control the timing of the Bicores in any way. You may be confusing functions of my
> Horse
> and Rider layers here.
>
> I only disable a single Bicore at a time in order to save power, but this does not
> effect the basic
> gait of the machine. The translation of rotational inertial to linear motion is only a
> function of the relative timing between the bicores themselves. No Rider board is
> needed at all, so the sequencing of the motors is not as you described above. Imagine
> that each motor is driven by a free wheeling Bicore using the standard .22 uf caps,
> etc. They oscillate rather quickly.
>
> The speed of the Bicores is such that they may reverse several times per linear 'step',
> so the thing actually sort of skipps and glides over smooth surfaces, but digs in and
> crawls over rougher surfaces or small obstacles. On a really smooth surface, you can
> see this by giving the thing a good push with your hand, it spins and glides like an
> air-hocky puck, regains traction, and comes right back at you, acting really mad about
> it too, in a very life-like way. I cannot see how you can
> be close to this with gearmotors.
>
> > I found that net linear motion does results in my 3 motor design but is very
> > inefficient with a lot of wheel dragging and slipping resulting in high frictional
> > losses compared to, for example, Bolt's 2 motor design! These losses are caused by
> > the "toe in" of the 3 wheels which oppose all net linear motion. Even if one wheel
> > is held perfectly stationary causing the platform to rotate about that relatively
> > low friction pivot point , the sideways dragging and slipping between wheel and
> > surface (ie friction losses) of the other 2 wheels is very high.
> >
> > So my question are:
> >
> > Am I missing something?
>
> Yes.
>
> First, I have to admit that LEM is not as efficient as a 2 motor rover. Nothing is,
> except a unicycle in theory. No walker, nor any other design will ever match a wheeled
> rover for efficiency, thats why we invented bicycles after all.
>
> But LEM moves a lot of weight over a wide range of terrain without gear reduction. It
> does draw some current, but it will run a long while on those 280 maH NiCads, and its
> tossing solar designs aside the whole time. So I would not call LEM particularlly
> inefficient either.
>
> But what you are missing is simply that the wheels are not intended to have solid
> traction while its in motion. I've experimented with a number of wheel designs on LEM,
> and even high friction rubber wheels work better than I had expected. The small gear
> splines however, work far far better than I could have imagined. The secret here is in
> the sound it makes.
>
> LEM is not a true roller at all, thats why it does not roll down hill when it sleeps.
> Try this with
> a direct drive, coaxial motor design.
>
> I think that the really poor frame rate of the videos is causing some confusion here.
> The NTSC video tape alreay looked slightly choppy because LEM really moves much
> faster, and in a much more complex way than the compressed videos show. After
> compression,
> even at the 128K/second rate, it looks (to me) like very bad clay-mation.
>
> The original, uncompressed AVI file is about 142 megabytes, so you can see that a lot of
> detail is
> lost by the time you see the RealVideo versions over the web site.
>
> If anyone really wants to see the full 142 meg AVI version, I can try to make it
> available.
>
> > Do you observe similar "wheel/surface" drag and slip by peeking on the "skirt" of
> > the LEM? (it is impossible for us to see this on the video)
>
> Yes, I tried, but it moves so quickly that this was impossible to capture on NTSC video,
> much
> less any compressed form. I suppose I can make you a very short AVI file, with a
> close-up of
> one wheel-footie-thing, and see just how bad it turns out. Maybe it will suggest the
> details of
> the wheel to floor interaction, but I really think that the sounds it makes tells you
> whats happening
> here better than even the naked eye can do.
>
> > In your opinion, would you say that the LEM design has an efficient linear motion (
> > getting from point A to point B)?
>
> Given its weight, and the work (expoloration) it does along the way, its nominally
> efficient I think.
>
> But its design includes a lot more than moving from point A to B, if that were the goal,
> I would use
> a different design, a conventional wheeled rover. LEM was designed for the form of
> motion contest, where praticality was the stated goal. Turn LEM loose in your RJP, and
> see how practical it is compared with convetional BEAM designs.
>
> And its efficient enough to carry along a solar recharger system, giving it a greater
> survival
> space than a lot of other (more efficient) designs.
>
> > What do you think is the efficiency of useful linear motion of your 3 motor
> > triaxial LEM compared to a 2 motor axial + one caster platform controlled, for
> > example, by a Unicore circuit which has a similar Horse and Rider (embedded
> > Bicores) architecture.?
>
> Ahhhh, now you have gotten to the real point behind this design!!!!
>
> Its all about the control system design philosophy. If you design a geometric resonance
>
> between the control system, sensors and effectors, your going to get something very
> interesting.
>
> My philosophy with LEM is that BEAM tech is much better applied if we try to emulate
> much lower forms of life that is common. We compare BEAM designs with ants, clearly
> absurd! We should be trying to build a starfish or similar design first. In these
> creatures, we see no neurons at all, yet we have very complex, directed behaviors
> emerging from an array of rather simple subsystems in a geometric design.
>
> If efficiency is a goal, you could try a true roller design, if you constrain the
> environment to smoother surfaces, or use larger diameter wheels and gear reduction. The
> controller time constants would have to be altered, and I might reccomend spherical
> wheels as well...
>
> If robustness is more important (as in a predatory design) then don't be afraid to burn
> the milliwatts as long as its getting work done fast. LEM does this well enough I
> think.
>
> > Do you think that by extrapolating the LEM design to a 2 motor or 4 motor design,
> > would this result in a more or less efficient linear motion?
>
> Its much more complex than this. What is your working definition of more or less
> efficient?
>
> If its milliwatts per centimeter over time, the number of motors is not the main
> factor. If I used
> a turbot-LEM design, I may have many motors, but few ever get power. Those that do may
> have a good enough match between their gear ratios and mechanical advantges of the arms
> that while very slow, its also very very efficient. In this case, it may indeed be more
> efficient than a design with fewer motors and high frictional losses.
>
> Personally, I think the most practical design might be a 4 motor tetrahedral turbot,
> able to operate efficiently in any orientation, over most any terrain.
>
> > I look forward to comparing my tentative results with your experience (but of course
> > you have the right to remain silent - grin)
> >
> > regards
> >
> > wilf
>
> Oh no I dont!
>
> This is an open contest, to enter, I must disclose my design fully I beleive. Anything
> less would
> not be fair to the community.
>
> Remember, its not a conventional roller design, thats why the motors are not coaxial.
> Maybe its no model of efficiency, but its cheap and practical, needing no hard to get
> (win)
> gearmotors, yet it performs in a very interesting way.
>
> Try to test the actual Horse design, with direct drive motors and totally seperate
> Bicores.
> You can have a lot of fun just testing out what all the different mappings between the
> photodiodes and bicores do. It gets very complex, even without any 'Rider' at all.
>
> What your testing right now does not seem to desribe LEM's form of motion very well.

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