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Regarding Precision Navigation

John Whitten

[Editors note: This article is an extremely interesting bit of email that was sent in reply to another robot builders question about precision in positioning. John has a fairly well thought out opinion on the subject that is worth a look.]

Its my experience that the more "precision" I try to introduce into my projects, the more trouble I have trying to get them to act in a "precise" way.

In thinking through the problem, there seems to be a number of factors that aren't immediately apparent- why, for example, can one construct, say, a digital plotter that is easily capable of precise movements (even when constructed haphazardly!), when, using the same equipment, its very difficult to get a "robot" to move- even to within a few _inches_ - of a previously followed path???

I think the answers lie in the stuff that we don't know we're bringing to the table... for instance- when you construct a pen plotter, do you consider the fact that you aren't really "moving" much of anything? That the "carriage" is being positioned back and forth basically along two pre-fixed paths, who's intersection just happens to include the domain of the area in which we would like to plot...!? A useful side-effect to be sure, and yet, the motion of the carriage is essentially "fixed"- in the sense that it can _only_ travel along its two sliding axis rails. The real "precision" factor in this case boils down to how "precisely" can one control the steppers and how much "slop" is tolerated in the various mechanical components (eg. the positioning actuators (often steel wires or similar), the assembly that rolls along the carriage-rails (often ball-bearing, or even simply a teflon sleeve wrapped around the rail), or the amount of "wobble" allowed at the point of the pen). Indeed, it would seem that the "restricted nature" of the design ensures at least a certain amount of "precision" no matter what. Even a crudely constructed device would be capable of producing output that is recognizable at worst!

So, what's so different when we take essentially the same components and try to incorporate them into a robot? Why won't the robot "move" with the same level of precision... the answer is that it does! It does move with the same level of precision- you just changed the parameters of the problem- and so your comparion of what's "precise" is not accurate. Ok- so that's a hokey answer- I'll admit it- what does "changing the parameters" have to do with precision? The answer is that it changes the very basis of the relationship expressed by the term "precision"- namely that of "compared to what".

That is really the question- compared to what...?

In the digital plotter setup above, the "compared to what" factors are- the fixed nature of the guide rails, the orbital "resolution" of the positioning motors, the tensioning (if any) in the guide cables (wires, ropes, whatever), the mechanical interplay between the carriage positioning assembly, and of course, the amount of "wobbling" the pen tip is allowed to have.... all these factors together are used to determine the "precision" of the device.

Mobile robots, generally-speaking, do _not_ have "fixed" positioning rails- which is a major component of the plotter's ability to accurately repeat positioning movements. Another difference will probably be the way the device is propelled (not to mention steered). Wheels, treads, even legs all introduce "slippage" factors which may be randomly introduced in almost any given direction, sometimes even in multiple directions simultaneously. Certainly one can perform various types of vehicle analysis to determine the most likely points of "slippage" given a particular wheel, track, leg (or whatever propulsion) arrangement, and can design compensating features- but this does not totally account for all types of "slippage" (or other mechanical "slop" factors) that can (and will) affect the device.

One incredibly good example here of how "slippage" can affect real-wold vehicles is the Airplane. Airplanes are absolutely dependent on their ability to produce lift and control their movement through vectoring of air pressure/flow (one way or another). If anything interferes with either (or both) of these two abilities a crash is very likely to result. Even though you may understand the physics, dynamics, and characteristics involved in flying an aircraft- it is impossible to predict every sort of air pressure, or turbulance conditions that (will) exist at any particular point in space- all the time. If it _were_ possible, we really wouldn't need pilots to fly the plane! You may say- we don't need pilots now, and that may be true- 99 times out of a 100. But I'll bet if you're on that 100th flight you would be very glad indeed the pilots were present to compensate for something that the aircraft designers couldn't possibly have forseen!

You must also account for the inherent "imprecision" of the other components used in the robot (as compared to other similar components that could have been employed assuming they were cheap enough, small enough, efficient enough- etc- you have to keep the "compared to what" factor in mind all throughout the component selection and specification process). Of course, only a select few _really_ have the ability to shop with an unlimited budget, have NASA or JPL at their convenient disposal, and have the manufacturing capabilities to truly build whatever marvel may be needed. The rest of us have to deal with IN-precision. (Actually, so do NASA and JPL- its just a fact of life ;)

So, basically, the problem becomes one of NOT how much precision can I afford- but instead- How can I deal with the inherent IMPRECISION that I am forced to accept? In my own life, I have found that when I'm banging my head against the wall, I'm generally looking at the problem all wrong.

Because there is inherent imprecison in mobile robots, the real challenge becomes two-fold: A) How can I get as much precision as possible, given my (however) limited means, abilities, and ambitions, and B) How can I compensate for the imprecision that is left-over?

IMO, the first part (A) is pretty much self-apparent: You do what you can with what you've got or can afford to buy, build, or create. Dealing with the second part (B) is where all the problems come in- "the devil's in the details" as they say ;)

It might be useful at this point to take a look at humans for a moment- how do WE compensate for imprecision? We have SO darned many ways to make up for our deficiencies that it would take an incredible number of pages to list them all! For example, when you're trying to remember someone's name- do you go down the list of everyone you've ever met? I doubt it- I suspect instead you use some sort of temporal-associative "hashing" type function (to try to give the process a label) to arrive at the name and phone number of the "girl- in- the- red- dress- you- met- last- week- at- the- party". Likewise, when you walk across the room to pick up the phone to call her, you give not the slightest thought to "getting to the phone".

How is it we so effortlessly pick our way through crowded (and sometimes dangerous) space to reach the goal we have set for ourselves? We are not built from such precise components. We don't (apparently) give any real thought to accomplishing the goal- and yet we do it so well.... or do we? How well do you suppose we would navigate the SAME room with our eyes closed? Even if we have "known" the room all of our lives we would probably still have problems- especially, if unbeknownst to us, someone had rearranged all the furniture... and yet- with our eyes open- even that wouldn't significantly hinder our progress to the phone. We might still make it- though if we do, we will have to "concentrate" very hard, using our other senses, and our memories (if possible) to fill in the gap(s), and employ other "sensing" techniques to figure out where we are relative to where we are going. But- the main thing to keep in mind here is that we are _consciously_ applying more "thought-power" to solving the problem "manually" (and probably pushing aside other "thoughts" to "free up" the "processing ability" to do it). This implies A) that the normal mechanism for accomplishing this supposedly simple thing is hidden from our normal "awareness" and B) is actually more sophisticated than we usually give it credit for.

So here's an astute observation for you... Whatever it is that we do naturally as humans- the robots we build aren't doing it. ;)

Personally, I think the way we "do it" is by utilizing a number of analysis proceedures. First, we have "in mind" the goal- reaching the phone. Second, we know (from experience perhaps) that in order to achieve the goal we must perform an action, namely to navigate from "here to there". Third, we "scan the environment to "take in" the available routes and note possible obstacles. We also probably utilize prior-knowledge, when possible, of the environment to make "predictions" about the paths and obstacles, as well as recalling previously-encountered "hidden" or "unseen" problems. Finally, we use all this information to plan, plot, and implement a path to the goal while applying various course corrections along the way to account for obstacles or dangers, both planned and unplanned. It is really important to notice here that is is this analyse, strategize, improvise control system that enable us humans to cope with our inefficiencies so well.

I think it is interesting to point out that humans seem to have a disproportionately-large amount of processing power devoted to A) Cataloging things, events, places, AND _sequences_. B) Recognizing things, events, places, and sequences. And C) Moving things (including ourselves) from one place to another utilizing the abilities presented by (A) and (B). Comparatively speaking, the remaining amount of "brain-power", devoted to the when and why is quite puny. We are temporal-spatial beings posessing the ability to manipulate our environment for a purpose, and we expect our "robots" to be also.

In contrast to the digital plotter, people achieve "positional-accuracy" (aka "precision") through the use of "feedback" and re-calibration/re-registration [of the goal] while underway. So instead of being attached to "fixed rails" and using ultra-precise motors, gears, sliders, or what-have-you, our control mechanism is less-concerned with knowing _precisely_ where we are and instead is more concerned with knowing "more or less" _where we are headed_. A very UN-precise thing indeed. We don't generally pre-plan our routes to the centimeter, nor stand stalled awe-struck over the incredible multitude of potential paths and implementation strategies, we simply pick one and go with it. It may or may not be the best path to the solution; in fact, my own personal suspicion is that it usually isn't. Sometimes it gets us in trouble, but most of the time it does not. But, when we _do_ get into trouble, we simply push a new, more immediate goal onto our "goal stack"- namely getting ourselves out of our present jam- until the situation is resolved _adequately_ (not necessarily perfectly!) to allow us to re-compute our path and re-acquire the goal.

Here's another thing to consider- You invite a friend over to see your latest robotics project and he (or she, no offense to the ladies ;) asks you a simple question- "How do I get there?" What do _you_ do? You hand out directions! Simple enough- yet- consider what those directions really are- Unless you are the consumate gearhead (or flying the aforementioned aircraft ;) you probably DON'T say this:

1) Start at Longitude 76.216, Latitude 36.9202, oriented 54.76 degrees from True North
2) Travel 9,738.16 units (1-foot increments) in a straight line
3) Change heading to 86.18 degrees from True North
4) Travel 6,283 units (exactly) in a straight line
5) Change heading to 17.06 degrees from True North
6) Travel 14.3 units in a straight line
7) ...

Even though these directions ARE precise, and even though they may be exactly correct- you _still_ probably don't give directions like this! (At least, I hope you never invite _me_ over with these directions! ;) You are much more likely to say:

1) Get to Highway 14 going West
2) Go about 2 miles until you come to 1st street
3) Hang a left on 1st street and go two blocks
4) My house is the third house on the left (its blue)
5) The address just in case you get lost is: "105 Somewhere Street"
    or call me at '555-1212'

These are HUMAN-oriented directions, and they work just fine for us- and guess what? They are not even CLOSE to being precise. In fact, there are precious few precise elements in the above directions and yet _you_ would have an easier time following them than the previous set I think.

So what are the salient elements of the second set of directions that make them easier to follow? The first point, I think, to consider is that the second set is "interpreted" by the recipient as opposed to being "absolute and inflexible". Interpretation allows imprecise information to be conveyed (possibly conveyed imprecisely as well) and yet still the correct, desired outcome is capable of being achieved. Let's pretend we have the problem licked- and are "parsing" the above list through our "interpreter"- what might it key in on and what suppositions might it require in order to complete the task of following the directions?

First of all, it is useful to note that the set of directions is actually a "sequence" of steps that are necessary to accomplish the goal. So let's use an obvious sequencing notation. Next, it might be interesting to notice the action(s) that are expected to be performed in the execution of the sequence. Then we would probably want to notice that there is key information presented to give us hints and clues regarding the parameters & bounds of that action. And finally, extraneous information that may help to resolve ambiguities.

Seq-Step #1. [Get to] ( Highway 14, going West )

Seq-Step #2. [Go] ( about 2 miles ) until { [you come] ( to 1st street ) }

Seq-Step #3. [Hang a left] (on 1st street ) [and go] ( two blocks )

Goal-Info-Tip: [ My house,  is the third house,  on the left,  (its blue) ]

Ambiguity-Resolution-Info: [ The address just in case you get lost is: "105 Somewhere Street", or call me at '555-1212' ]

This, of course, is a rather simplistic view but adequate for our discussion- To follow the directions you must perform the following: "Get to", "go-until", "hang a left", "go-until (implied)". The rest of the list simply conveys the details of these operations. However, there is still much that must be pre-supposed in order to properly execute the sequence, and we'll not even worry about the problems of locomotion- The processor must also be able to _recognize_ when various conditions have been met, such as reaching 1st street, or counting houses until the count becomes three (not two, excepting that the count then becomes three, and the count may not continue to four, and five is right out! ;) This is the basis of "way-point" (Landmark-based) navigation which allows for a simpler control-command structure/syntax but requires that the processing elements interpret the instructions using "pre-learned" (canned) sequences of things, events, places and even other sequences. You might say you need "sloppy" processors to deal with "sloppy" instructions... (hmmm- this sounds like a job for Microsoft... ;)

But you may say- all this is well and good- but something low-level _still_ needs to be able to "sense" information, calculate position, and recalibrate & realign the mission to the goal. And you would be correct. You still need sensors, you still need positioning information, you still need the ability to recognize details, and you still need the ability to make sense out of the data you receive. The difference though is instead of dealing with the low-level data directly, various intermediate "sub-systems" analyze, condense, shape, refine, associate and discriminate, or otherwise transmute (or likewise fold, spindle and mutilate) the data into a more "meaningful" (read: generally more abstracted) form that the "sloppy" processors can digest. The output of the "sloppy" processors is then directed back through the "pipeline" where it is reverse-transmuted (or whatever) into a form more suited to the low-level components (Most of this happens on a totally unconscious level in humans).

Another useful (if not required) component is the ability to "predict" what the expected outcome of a particular action (in a sequence) should be, and to apply the predicted result to the actual result to obtain a correction factor. Many mobile robots already do this sort of processing, say, in their wheel-shaft encoders, counting pulses, or timing pulses, or something similar- if the pulses or time-value (or whatever) isn't within a pre-computed range, apply a correction factor.

It is no different for the "sloppy" processor. It too must make predictions but on a more general level- such as if I turn left here, I will be on 2nd street, not 1st street. However, since I was on 3rd street previously and I'm now at 2nd street, it is likely that if I continue in this direction I will next come to 1st street, which is part of my goal-fullfillment list. "Sloppy" processors are concerned very little with the actual "mechanical" aspects of performing a task, they deal almost exclusively with the abstract components. Its primary obstacle is getting over-burdened by details- quite the opposite of the low-level system which needs all the details it can get! In my scenario here- the "high-level" ("sloppy" processor) system is in charge of knowing "why", "when", and "where", while the low-level system is concerned with the "how".

"Are we there yet?" At each step in the sequence- this is the condition that must be satisfied before the robot can proceed to the next step. Other sequences may have other conditions, such as "Is it full yet?" for refilling a gasoline tank, or "Is it clean yet?" for vacuuming your carpet, etc. Generally speaking (at least IMO), these types of conditions are "high-level" conditions- problems for the "sloppy" processors to worry about. Supplying (and transmuting) the information to test these conditions however is a low-level problem.

So how many "levels" of processing are required? I dunno. That's what I'm trying to figure out myself. A few I
suspect- perhaps more than a few. But as a general statement- I'd say the abstraction factor increases as information flows "higher" in the processing system. Where does the low-level end and the high-level start? I dunno that either- another thing I'd like to know myself ;) Maybe there isn't even a discrete number of "levels" at all- but simply various pathways that data can flow in order to be processed for whatever purpose its needed. Maybe processing elements are semi-dedicated towards a particular purpose (or class of purposes) but can be "shanghied" on an as-needed basis to do more general processing? I dunno. I've been pondering many types of control schemes for a long time and I don't have any concrete answers yet, just opinions and directions to ponder for further clues.... ;) I personally lean toward the idea that there are many "agencies" (like Minsky talks about) that each do some sort of fundamental data processing or transformation, that interact, perhaps intermittantly, to achieve a result.

I also find it interesting that so many elements in our own human society are arranged, perhaps innocently, much like we ourselves may in fact be arranged-- our social circles, for example, are almost always hierarchical in nature- even if its a loosely formed hierarchy. Our businesses are almost always arranged in a hierarchal manner, our families are generally hierarchal, our friendships & allegiances, our governments, our militaries, our educational facilities are hierarchal, even our religions are very often hierarchical in nature. We naturally seem to form hiearchical structures which may, even accidently, reflect strong aspects of our own internal organization and processing abilities. Additionally, most of these same structures also exhibit the characteristics of a "massively parallel" machine, which, I'm betting, behaves in a manner remarkably similar to our own internal processes- on an abstract level at least. I strongly suspect that all this is no coincidence, but is a huge, neon-lit clue to our own internal workings- that we naturally gravitate into elements that reflect the way we work because that's how we intuitively understand to do things.

Ok, I suppose this is akin to saying that a giraffe is more inclined to look up because it is not a horse. Who says behaviors and mannerisms aren't evolved in the same manner as other biological characteristics anyway? Behaviors that are beneficial increase the tendancy for survival and reproduction, behaviors that don't affect either are irrelevant...? You never see anything on Discover or TLC about that first giraffe on the sweltering african savanah that happened to look up for food one day instead of down. That, in and of itself, would have been the spark of an entire series of behaviors and mannerisms characteristic for animals that look up for food. This seems like such a simple idea when confronted with it- but it is something very subtle that few people actually take in consideration when they ponder the evolution of a thing- at least it seems to me so ;) While our bodies where busy evolving to fill a niche, our brains were busy evolving to support the body that fills the niche... a sort of bio-feedback-engineering I suppose. If you ever find yourself with a few million years to kill with nothing better to do- I suggest you give it a try- can have interesting results ;) ;)

So ok- getting back on track (if indeed there was one...?) -- there's one big difference right there. Nature gets to play with "billions and billions" of neurons (processing elements) to get a job done. Since it has so darned many neurons to work with- its no big deal to allocate a few for this task or that task or whatever. It can even afford to allocate quite a few for some tasks. We, on the other hand, are not so lucky. Until Motorola produces the bionic MC68HC11 we're stuck with the old silicon version.... but that does NOT mean we're completely out of luck.

There are lots of ways to increase processing power- and you can arrange your processors in lots of ways- even on-the-fly schemes can be incorporated if you really want. That in fact, may not be such a bad idea. So that, in an extreme instance, the type of data being processed "configures" the processor doing the processing. Von Neuman style processing can be so limiting. It really dampens ones thoughts to other ways of achieving results. You can use PIC processors- LOTS of Pic processors- PIC's are CHEAP CHEAP CHEAP CHEAP- did I mention they are CHEAP? They are little sequence-processors in a box. You tell 'em what to do- which pins to toggle when and under what circumstances, and toggle they will. They'll toggle their little hearts out. And, if you treat them really really nice- they might even let you re-program them on-the-fly so they can do other types of processing jobs as the situation dictates. PIC's are so cheap, you can afford to chew through them like popcorn.

Another neat thing about PIC's is that they can support serial operations which makes hookup really really easy. You can have clusters of PICs working together to get a job done report to a more "centralized" PIC which concentrates the data and sends it upstream- even this can be serial- as by this time, you have had plenty of processing power to handle all sorts of low-level problems, all you really need to report is what happened and get new instructions, if any... so the PICs work together, much like the neuron-groups in our own biological brains- to accomplish simple, low-level tasks, and report them to (and be directed by) higher-level functions.

Does this leave out MC68HC11's? Heck no. They're really useful for lots of things. It doesn't rule out other processors either. Feel free to mix and match (though kitbashing in an x86 involves less kit and more bashing). Processors are cheap. If you're not comfortable at the processor core level- _get_ comfortable- its where all the action is. The goal is to create a processing-machine that meets YOUR (or your projects) processing needs. It doesn't have to be one processor. You don't have to shoe-horn it all into one box. You can use additional bolt-on processors to handle specific tasks (useful for handling things that require lots of processing in a hurry- like navigation, recognition, or communication).

And most of all you DON'T need precision! Precision to a point is useful, but after that continued insistance is moving your thoughts away from the real problem of designing an adequate control system to account for inaccuracies and errors. The more "fault-tolerant" the control system, the more flexible the resulting processing and control engine will be. If you're having problems getting enough precision- ignore it- find some other way of getting the job done- in all probability there are plenty of other ways that will work, you just need to rethink the problem and be open to creative solutions. Who cares if they are the most efficient, most cost-effective, prettiest, or most elegant- as long as they WORK- that's what counts- elegance will emerge on its own when its time.

In short, instead of forcing the solution on the result set, try using the result set to derive the solution.

Ok- enough rambling- go make robots ;)

John Whitten