Re: Linear Motion Control - Servo?
- From: curt@xxxxxxxx (Curt Welch)
- Date: 25 Jul 2006 05:56:04 GMT
"Richard Greenberg" <richieg@xxxxxxxxxxxxxxxx> wrote:
Hi all. We are most intrigued with the concept of the spinning motors. We
think that by creating the "different" states by way of spin (along with
interchangable faces on the disks) we open up a whole dimension of
creativity (that is not a direct copy of the existing "mirrors" out
there). That is, the image could look very different depending on which
set of disks are used. Also, our feeling is that the cost of going this
route would not be too great. Also, the construction seems simpler.
Anyway, here are the questions at hand.
1. What kind of motors do you recommend?
small cheap ones? I'm no expert on motors but you can search around for
robotics and motors and find many sights selling small cheap DC brush
motors. These have permanent magnets in them and run off of DC voltage.
The direction of spin is controlled by the direction you apply the voltage.
I'm not sure how these are normally mounted. I think some might come with
taped screw holes but I think most are expected to be clamped into place
somehow. Many someone here with more practical experience with them can
provide some insight.
2. How do we address motors individually from a computer (what are the
main components to pull this off)?
What type of computer are thinking of using? A normal PC or some type of
embedded processor? There's a lot of different ways to go with that and it
will depend a lot on the type of experience your team has.
No matter what you do, you will need some type of I/O port on the computer
connected to some type of drivers to power the motor. The I/O ports on
computers won't in general have the power needed to drive a motor directly.
So you need some type of driver circuit to basically amplify the signals
from the computer to provide the current to the motor.
One class of such devices is called an H-Bridge. These are devices that
not only allow you power the motor, but to also switch the direction of
current to make the motor spin in either direction. They also allow you to
control the speed of the motor. They normally do that by turning the power
on and off very quickly with different ratios of on time vs off time.
There are multiple options on how a computer might connect to the H-Bridge
driver. The small embedded processors have digital I/O lines. These are a
single wire coming from the processor can have it's binary value set to 1
or 0. At 1, it's held at something like +5 Volts. For 0, it's held at
ground. The computer can turn these digital I/O ports on and off in any
sequence you want to program and it can do it fairly quickly (thousands of
times per second).
Some H-Bridge devices are meant to be driven straight from a I/O lines like
these. You might use two of them, and depending on the combination of 1
and 0 values the computer has output to these I/O lines, the H-Bridge will
make the motor spin forward, or reverse, or act as a brake to make it stop.
Other H-Bridges can be driven with the pulse width signals used to drive
servos.
If you are using a standard PC, the closest there is to single I/O lines is
a parallel port (which newer PCs don't even have any more).
There are devices for controlling H-Bridges with serial ports from
computers, and there are probably some USB options. Though I don't know
what's available that way.
Your problem however is how to control a large number of motors without
costing a fortune. You could buy 1000 H-bridge devices and find some way
to make your computer send signals to all 1000 H-Bridge devices - but
that's going to be expensive because a H-Bridge is not all that cheap.
A quick search on the web turns up this:
http://www.robotstore.com/store/product.asp?pid=101&catid=1586
Which is a device you can hook to a serial port on your PC. You then buy
one of the their dual H-Bridge circuits to connect to the serial board.
With those two boards, you could write software on a PC to send data out a
serial port, which would control only 2 DC motors. And those two boards
would cost you $100 total. So that's $50 per motor.
That serial board is designed so you can hook many to the same serial port
and end up controlling a total 254 motors. Using that with 4 serial ports
on a PC and you could control 1024 motors. But it would cost you $50 per
motor.
That H-bridge however is larger and more expensive than what you will need.
But there are probably multiple options like the above, that you could buy
off the shelf parts and not have to do any electronics other than wire
parts together. Search for things like robots and h-bridge on the Internet
to find more about these things.
Now, the other option you have, is to develop a custom circuit for making
this work for less money. How much of this you will be able to do depends
of anyone on your team has any electronics experience (or if you can get
someone that does). There might be people in this group that could even
hire to do some work for you at a low rate. I know the theory, but don't
have the practical experience to do anything other than give you some
ideas.
What you can do is hook all your motors into a large X-Y grid where you
have row and column wires where the motor is attached at the intersection
of each row and column wire. You will also need a diode in series with
each motor. With this configuration, you can build a circuit to apply
power to one row wire, and one column wire at a time. Doing that will
activate the motor located at the intersection of that row and that column
wire.
A custom driver circuit would then have to be built to apply the power to
the row and column wires in response to the signals from the computer. But
this could be made much cheaper, than using 1000 individual motor controls.
With this circuit, you could only spin the motors in one direction. If you
want to spin them in two directions the same idea could be used, but with
more column wires and drivers.
The computer would have to be programmed to do a complex pattern of turning
row and column power drivers on and off to make everything spin correctly.
But to some extent, you could could control the speed by how often you
turned motors on and how long you left them off. The programming could be
rather tricky.
So that's some ideas to get you started.
3. Do motors have different "end types"? Such as...Are there motors with
simple female threaded ends?
Most the small cheap motors just have a small shaft sticking out with no
threads or any way to attach anything. You have to either pressure fit
something on to it, or weld or glue it, or use a collar with a setscrew.
But if you check around, you might find other options.
4. How do we control the speed of each motor independently? (We are
thinking that having variable speed control will make things more
interesting, but we are open to the idea of the on/off binary situation
as well.)
As above. The H-bridge circuits allow you to control speed by turning the
motor on and off quickly. But even the cheap H-bridge circuits that you
can buy off the shelf will cost you a lot if you use a separate H-bridge
for every motor.
So, we know this is asking alot, but if any of you have the time to
address these questions with answers that we may understand, we would be
most appeciative. We really just need some "basic" concepts/components
info at this point.
Thanks, Em and DAD
That's a start for you.
If you search the Internet for robot stores that sell robot kit parts and
search for H-bridge and motor speed control and start reading everything
you find you will start to get an idea of what's out there and you will
find a lot of sites that offer introduction information to all the
technology. What you will be able to do has a lot to do with how much
electronics experience and computer experience your team can put together
so you can custom create a solution instead of having to try and make it
work with off the shelf plug-n-play devices that will cost you a fortune.
Maybe there is some product that I've not heard about for controlling a
matrix of motors - but it's such an odd application that I would doubt it.
But maybe you can find info on controlling a matrix of lights that you can
adapt to your project?
--
Curt Welch http://CurtWelch.Com/
curt@xxxxxxxx http://NewsReader.Com/
.
- References:
- Linear Motion Control - Servo?
- From: Richard Greenberg
- Re: Linear Motion Control - Servo?
- From: Richard Greenberg
- Linear Motion Control - Servo?
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