Control of Mobile Robots is a course that focuses on the application of modern control theory to the problem of making robots move around in safe and effective ways. The structure of this class is somewhat unusual since it involves many moving parts - to do robotics right, one has to go from basic theory all the way to an actual robot moving around in the real world, which is the challenge we have set out to address through the different pieces in the course.
A Clever Trick
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Del curso dictado por Georgia Institute of Technology
Control of Mobile Robots
1019 calificaciones
De la lección
Putting It All Together
Conoce a los instructores
Dr. Magnus EgerstedtProfessor
School of Electrical and Computer Engineering
The outcome of the last handful of lectures was that we can really use a
layered architecture to make our robots act like a simpler system.
Like x dot = u, in our case. And they key idea was to plan using the
simple dynamics. And then track using some clever
controller. And what we saw last time was we could
use a PID regulator, for instance for, controlling the heading of the robot, and
then simply set the velocity of the robot equal to the magnitude of the planned
velocity, u. Well, today, I want to discuss another
twist to this theme. In fact, I want to be even more clever.
And there is a particularly eloquent and cute way you can think about unicycles.
And in general I'm not a big fan of clever tricks for the sake of clever
tricks, but because differential drive mobile robots are so common.
And we know that unicycles model them well, then this trick becomes more than
just a piece of mathematical curiosity, becomes an effective way of thinking
about a large, a large and rich class of mobile robots.
So, instead of doing tracking, we're going to transform the unicycle dynamics.
So recall that we have the position xy, we have the heading phi, and we have the
dynamics as seen here. Well, typically, what we've seen so far
is, we want the robot not to slam into things, we want the robot to end up on
top of goal points we want the robot to folow walls.
We don't particularly care which way it's pointing, so the first thing we're going
to do is say, you know what let's just pretend that we don't worry about
orientation whatsoever. Well, we still have a rather awkward
looking nonlinear differential equation but let, now, pick another point.
Let's not pick x, y, which as you remember, was really the point right in
between the wheels of the differential-drive mobile robot.
What if we picked another point, instead? Let's pick this point.
The red point here. And let's say, that the distance between,
the red point and the black point. That's, that's some distance l, where l
is some small number. What would happen if we looked at this
new point, instead? Well, let's place the new point at x tilde y tilde The next
tilde is simply x + l cosine phi. Simple geometry.
And y tilde is y + l sine phi. Where, again, l is the distance removing
our point along the heading of, of the robot.
Well, let's see what the dynamics of this new point.
The tilde point actually looks like. Well, this is our old dynamics this how X
and Y and phata actually behaves. This is our new point and now, what we
need to do is put dots over the tildes to see.
What do the time derivatives actually look like? Well, it's not so complicated.
X till the dot, well, it's x dot + the time derivative of this thing, and, the
time derivative of cosine phi, and remember phi is a function of t, if I
take ddt of this thing, then. What I get, is, I get - phi dot * sin
phi. Well, that's what we have here, x tilde
dot, is x dot - l phi dot sin phi, and similarly for y, tilde dot.
Alright, this still doesn't seem all that helpful, to be completely honest, so lets
simplify this a little bit. Well, first of all, I know what x dot is.
It's this thing here, so instead of that, I'm just plugging in v cosine phi.
I also know what phi dot is. It's, it's omega.
So, now I end up with a slightly, less scary-looking expression.
And, then we do the same thing for y tilde dot.
Well okay, where does this lead us? Well, let's assume for a second that we can
control this new point directly. That this new point is indeed, the point
that we before have called x dot = to u, as a way of generating reference
velocities. Let's assume that we can actually
immediately control this new point. What would that tell us? Well, x tilde
dot as you recall, was given by this mouthful of an expression.
Well, let's set this equal to u1 which is our desired velocity in the x direction.
And similarly, we set y tilde dot = u2. Now, all we need to do at least, well I
can say all we need to do, I don't know if we can do it.
But, all we would like to do is actually solve this equation with respect to V and
omega. Because, if we did that, we would relate
the actual velocities of the unicycle to these u1 and u2 velocities, which are The
planned velocities. So let's see if we can do that.
Well the first thing we do is we pull out the cosine and the sine terms.
So we get that cosine * v - sine * l omega well that's that thing, that's
equal to u1. And similarly for the y tilde dot, or the
u2 dot, sorry, the u2 equation. I don't know, this equation still looks a
little hairy but, as you hopefully recall, this matrix right here is a
rather nice matrix. It is, in fact, a rotation matrix.
All it's doing is it's taking a vector and rotating it theta degrees.
Actually, it's rotating it phi degrees, but it is a very well behaved and well
understood matrix, okay?
I don't particularly like l there, because l is not part of what I would
like. I would like v and omega and, no l.
So, what I can do, is I can rewrite this thing as, a matrix, a diagonal matrix,
with one on the first diagonal, and l on the second diagonal, simply times v, and
omega. And if I do that,
I get the following expression. Rotation matrix phi, sorry, yeah, phi, *
this matrix, v omega. Well, these are all invertable.
What is the inverse of the rotation matrix? Well, the rotation matrix rotates
something phi degrees. Well, the inverse would be to rotate it
back, right? Then you're back where you started.
That's -5. So, R inverse phi always exists and it
corresponds to rotating it back five degrees.
So, we can invert this matrix easily. Well, this is a diagonal matrix, and the
inverse of this thing is simply the inverse of the diagonal, 1 / 1 00 1 / l.
So the inverse of this thing is also trivial.
That means that I can actually solve directly for V and omega, no problem.
No questions asked, no tracking, no approximation, just bam, I get the right
velocities out. So if I have my low level plan producing
U1 and U2, then I can transform that into V and omega directly.
And this seems almost too good to be true, and it is too good to be true,
because remember We're actually not making x y behave perfectly, we're making
this new point that we call x tilde y tilde, we're making that point behave
perfectly. And then we have l, that tells us how far
away we are, and by making l small, we're getting closer to x and y, so you
practice. You pick l tiny and then you go to work.
So what does this mean, it means that before we had a planner we used our
complicated navigation algorithm to spid out u1 and u2 all right and then we had a
tracker and in our case we use the PID tracker for the heading and simply set
the velocity equal to the magnitude of u and from that we got v and omega out.
Well now, we don't have to deal with trackers.
We've actually replaced second part with a transformation of the unicycle where
we're simply saying that V and omega, or these 2 matrices * u1, u2.
And again, these matrices are, not particularly complicated to compute.
This, is simply the rotation matrix and phi of course we have.
This is one, easy to compute, it's just one.
And ;; is something that we decide, so what we put here, this is 1 / l, l should
be small so 1 / l should be some large number, right? And the larger this number
becomes the closer the tilde the point and the actual point approach each other.
I should put out a little warning though that if you make this number too large,
then you get a behavior where the robot starts turning quite a bit, because this
going to hit omega directly. Okay but, this gives us a direct
transformation, instead of a tracker, to produce the desired velocities.
And the reason why I like this trick, and why this trick shouldn't be thought of as
simply, an interesting little curiosity because it is kind of interesting is that
so many robots are differential drive robots, and this transformation applies
to all of them, because all of them can be mapped onto the unicycle dynamics.
So, that takes care of, how do we actually go from low-level plans.
To executable, desired velocities for differential drive mobile robots.
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