# Linear Motion Control - Servo?

Hi. Please forgive my total ignorance here. My daughter is working on a project and I'm trying to put her on the right track. The project is:
Build a mechanical "mirror". The mirror is composed of maybe 1000 small square or rectangular pieces of something. Each of these pieces is a pixel in the image that is produced on this mirror. Each piece must be controlled independently (on or off - tilted or not - in or out - etc.) by a computer. The computer will be receiving the incoming image from a video camera. So a person stands in front of the camera and the mechanical mirror responds by turning each pixel on or off (or tilting or not). So...I'm thinking that she might start by looking at the world of servos. Any thoughts as to a good place to check out servos that could, let's say, push a wooden tile up and then release the tile. (Thus the tile would be able to look "appear" two different ways.) I'm thinking that she needs a servo that can do linear motion. Or, in other words, how can we translate the circular motion of servos into a linear motion that could push the tiles forward or backward. She needs to just get one tile going to test. She has limited access to "build anything from scratch" tools, so we want to get a hold of something that is pre-made to do the linear thing. Like a servo, a gear thingo and a rod attached to the gear thingo. Any "getting started" help on this would be most appreciated. Thanks, Rich the Dad.
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Servos are commonly used for such tasks - like pushing an airplane flap up and down or a wheel left and right - that's what they were designed for. You simply use a rod as a linkage from the horn on the servo to the hinged flap. Just look at the inside of any RC model and you will see how they are used for the task. The servo would allow you to control the amount of tilt on each mirror - which sounds like overkill for you application.
However, if you actually intend to do this with 1000 mirrors - it will cost a fortune (many thousands of dollars).
A better way would be to do something like those mechanical road signs use. Make the tile you are trying to control magnetic, and put a coil under it to attract or repel it. The road signs mechanically lock into one of two positions by using something like a spring so the coil only needs to be energized one way or the other to make it flip - it doesn't have the be held on constantly so it saves power. That allows the system to be driven a column at a time making the drive electronics simpler. But it's slow to flip to a new configuration. If you actually want to drive it as video speeds, you will need a lot of power and a lot of drive electronics.
Or, you can just by the DLP chip that does the same thing on a very small but high resolution scale and not build anything. :)
http://en.wikipedia.org/wiki/DLP
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Curt Welch http://CurtWelch.Com /
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On Thu, 20 Jul 2006 21:28:43 GMT, "Richard Greenberg"

You may want to see the below link.
http://www.smoothware.com/danny/mirror.html
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shb*NO*SPAM*@comporium.net (Si Ballenger) wrote:

That's cool. I thought it was made out of mirror tiles at first since that is what the first poster talked about, then I realized it was nothing but pieces of wood being moved. What a cool idea. I'd like to have one of those in my home.... hum.....
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Curt Welch http://CurtWelch.Com /
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Hi. Thanks for the responses. The mirrors that were built by Daniel Rozin is actually the inspiration behind the project that my daughter is working on. You're right in that it is expensive. The team she is on has about \$20,000 to build the "mirror". They looked into a device known as a linear actuator the other day and found them to be very expensive (when the cost of each one is multiplied by 1000).
I wish I could come up with a way that a small charge (or whatever you would call the electricity that moves from a computer to a chip) could be applied to something that is stationary and would change its appearance quickly.
Rich

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As in "LCD" ? ;-)
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Video monitors. :) You could create one of his "software mirrors" that way. :)
His wooden mirror seemed to actually tilt the block at different angles to represent the different shades of gray. That also means you need the correct overhead lighting in a dark room for the effect to work best. But you would need a real servo to make that work.
\$20K is probably enough of a budget to do it. The cheapest servos can be found for about \$10 and in the volume you are talking you might find an even better deal.
You will need someone with a good background in electronics and computer programming knowledge however to design and build the circuits to allow the computer to drive 1000 servos and to write the software to translate the video data into commands to move the servos. It's going to consume a good little bit of power as well.
If you want to create a device that only has two positions for each element (light and dark), then you can just use a solenoid or simple magnetic coil of some type. That could be put together for less money but probably requires more electronics experience to create the driver for it. You might even be able to make your own coils for less than you can buy them.
You can create a similar low resolution effect simply by using lights (LEDs?) in a large grid. That's how the large outdoor video displays work.
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Curt Welch http://CurtWelch.Com /
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On Fri, 21 Jul 2006 21:19:56 GMT, "Richard Greenberg"

You might want to look into LEDs as there is a good bit of info on the net about controlling arrays of them.
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Thanks again for your added responses. One of the fundamental goals of the project is to produce a structure that is attractive. Another goal is to use an approach that is "different". Different to, that is, the approaches taken already by Daniel Rozin. Another goal is to come up with an approach that the team can handle. The team is composed of a few undergrad students in the computer department. They have limited access to "building things" people, so they need to come up with something simple, elegant, and unique. Of course asking Dads to help is fine.

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On Sat, 22 Jul 2006 12:33:57 GMT, "Richard Greenberg"

If you have a few hundred computer under grads, then why don't they make a computer controlled geek "mirror" section for football games. They could use the flash cards to spell out words or make images. Each could have an RF receiver with an LED or vibrator motor that when activated would have them flip the cards. They could scroll things like "geeks like beer too" or have somebody roaming around with a cam taking shots of the babes for the computer science "jumbotron". Even the jocks would be impressed.
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Richard Greenberg wrote:

Here's what you need:
http://www.flashstar.com.tw/showroom.html
Each of these meters has a moving-coil electromagnetic movement that requires only a tiny amount of power to activate. Remove the plastic cover and glue a piece of paper or aluminum foil to the indicator needle and you're off. You'll probably have to operate them with the meter and paper facing downward or you'll have trouble with stability because the weight of the paper will unbalance the meter movement. Price: I believe they're under a dollar for the small ones. Downside: you have to order from China. Try requesting a few samples. You can also find these things in surplus stores and on eBay.
-- Joe Legris
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Joe Legris

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Hi again. I spoke to my daugther today and passed on your comments/suggestions etc. Anyway, we started talking about the idea of doing a spinning thing. If there is a variable speed motor (or even a one or two speed motor) attached to a spinning somethingorother with a pattern of some sort on it, then there could be a number of "appearances" for each of the round "pixels". Any thoughts as to what type of motor would be most suitable to such an application? Each motor of course would have to be able to be individually addressed by the computer. Thanks again. Rich the DAD.

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Well, if you want it to rotate to different fixed positions, then again, you need a servo. This is, if you want the computer to tell the motor to rotate the card to the 90 deg position, or to the 48 degree position etc. I think you could do some cool things with that by having a dark half-disk covering a white half-disk background. Different positions would cause different amounts of the white background to show through. There are multiple configurations like that which could work.
But, using the modified meter technique could work as well. They are very low power but they are in effect a motor attached to a spring. Their position will be a function of how much power you push through them. They are very delicate, but they might be strong enough to cause a very light weight paper flag to move.
If, on the other hand, you are talking about something spinning constantly at different speeds, then you can use cheap toy motors that can be found for about a dollar each. Or I guess, you could just put a bar on the shaft that would hit stops when it tried to spin. You could then make the computer spin it forward or backwards to force it to hit the clockwise or counter clockwise stop. Trying to position it between the two stops would not be very reliable.
The complexity is that the computer can tell a motor to spin, but it can't know how far the motor has rotated without some type of feedback device sending a signal back to the computer telling the computer the position of the shaft. This is what a servo has - a motor combined with gears, a pot, and electronics that makes the motor spin until it's reached the correct position and then it stops the motor. This allows the computer to simply tell the servo what position it wants the motor moved to and the servo gets the job done. A motor alone doesn't work that way.
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Curt Welch http://CurtWelch.Com /
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But could a simple motor be told to spin or not to spin from a command from a computer? I would think so. What we're thinking is that the motor would spin and that would be one state, and stop and that would be the other state. The design on the disk attached to the motor would determine the appearance of the 2 states.
Or....the motor could spin spow, spin fast, not spin and then we would have 3 states of appearance. The simple spinning motor idea has the advantage (I think) of cost and possible simpler deployment.
Or.. the motor could be variably spun, producing many subtle states of appearance.
The problem I see with the meter approach is that they would have to attach something very light weight, like paper, to the needle. They really need to make something more substantial in appearance I believe. It is meant to be displayed (interacted with) eventually in a lobby.
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Yes, very easily.

That might be cool. As I said, you can buy small cheap motors for a dollar or less. If you buy 1000 of them you can probably do even better. If you want them to spin in only one direction, the electronics are cheaper and simple as well.

Yes, that can be done as well without too much cost. I'm not sure how much the eye will be able to detect different spinning rates however. You will just have to experiment.

Yeah, with the meters, you would have to be careful transporting it to make sure you didn't bend or damage the needles. Small motors would be far more sturdy.
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Curt Welch http://CurtWelch.Com /
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I like this idea a lot. Use centrifical force to move "something" farther as the motor spins faster, and a spring to retract the "something" as the motor slows down. The "something" could open and close around a ball possibly containing the motor, it could open a "fan" like item like a peacocks tail feathers, it could make something larger or smaller, or many other types of changes in shape and color.
You might need to feed the very small computer signal to a "transistor" for each motor to control the higher current for each motor, but the cost is almost negligable. I quoted "transistor" because many different semiconductor devices could be used, not just transistors.
Interesting project.
HobbyBot
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One thing to consider (if this is the "right track" to lead the daughter...;), is that spinning objects have angular momentum - which may not be an issue when rendering still images. With video, there should be a minimum speed and this should be factored into the mechanical design to minimise inertia and into electronics - spinning down fast is a little harder than spinning up fast.
My vote would be to use servos (like other posters mentioned) which cost under \$10/ea. Forget linkages. Keep the output shaft stationary and use the body of the servo to mount your aesthetically pleasing material of choice. Keep the modules small and self contained like Legos. Use a controller like the one offered by Lynxmotion - which controls 32 at a time (and allows for synchronized group moves..) and costs ~\$40/ea viz a little over a dollar per "pixel".
http://www.lynxmotion.com/Product.aspx?productID95&CategoryIDR
Hope you have a fun and memorable time.
Kevin
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I'd like to suggest a completely different way of doing it. Drill a hole through each tile horizontally, so it can spin on a wire axle. Cut the tiles fairly thick, and drill a shallow 3mm hole through the axis from the front to the back. Mount a small rare earth magnet, like a 3mm cylinder and 5mm long, in both front and back of each tile. Then you can mount a coil behind each tile, the kind of coil that would be used in reed relays.
Then a +ve pulse will spin the tile to face forward, and a -ve pulse will make it face back. The coils get wired between a horizontal and a vertical drive wire, so that you can do row/column multiplexing. Then all you need is a half-bridge for each drive wire, and you have a complete grid where you can spin every tile individually.
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Thanks for your ideas and thoughts, I'm trying to comprehend it in its entirety. I understand that you're suggesting that a pulse will create a magnetic force and spin the tile. Do you think that this force would/could be enough to spin a tile?

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Of course. The only question is how much current it will take. Depending on the design, it might take a lot of current.
What he suggested is not that different in concept to how a compass works. You run current through the coil to make the top end the N pole, or run the current the other direction to make it the S pole. The magnet and tile would flip one way or the other just like a compass needle would point one direction or the other in response to the magnetic field created by the coil.
The problem is that if you turn the current off, and the tile is still moving from the flip, it might simply keep spinning instead of stopping. If you keep the current on, it will hold the tile, but it might oscillate back and forth for some period of time. How long it will take to flip, and how long it will take to stabilize before you can turn the current off will be a function of how much drag the tile's pivot has. To try and make this display moving video images, you would have to be flipping the tiles very quickly. And if you want to you the cheaper row/column multiplexing he suggested, it means you can only flip the tiles on one row or one column at a time. So it will take some time to flip the tiles in the entire array. If you have to apply current and wait 1/4 second for the tile to flip and stabilize before you can turn the current off and move on to the next row, and you have 30 rows, then it will take 7.5 seconds to change all the tiles just once. That's not much of "video" effect.
But, you could multiplex the coils very quickly scanning all of them in a fraction of a second. But then each coil will only be on for 1/30th of the time. That can work if you use enough current while it's own to produce a strong enough field. But to move the tiles quick enough to create any sort of workable video effect, you might have to use a lot of current with some very high power drivers. It's all a function of how light weight you can make the tiles. If their are actually mirror tiles, they are going to be heavy. If you can use some other light weight material like plastic or a thin sheet of metal then you can use less current to make it flip.
This is similar the idea was was getting at in one of my earlier posts. It's how the electromechanical road signs work. Instead of a spinning tile, they use a hinged tile flap. Basically, the areas you want to turn black or yellow (as used on those signs) has a hinged flap with the hinge in the middle of each pixel. One side of the flap is yellow, the other is black. The background is also yellow on one side and black on the other. When flipped one way, it's hiding the yellow, and showing all black, when it's flipped the other way, it's hiding the black, and showing the yellow.
The "flap" is magnetic, and the coil underneath it causes it to flip one way or the other just like the spinning tile. But because it's a flap, it can't keep spinning. It just flips one way or the other. You could make this flap using the sheets of magnetic material they use for refrigerator magnets (like the magnetic business cards). With a metal board to mound this on, the flap would just naturally stick once the coil was turned off.
This is something like how those changeable road signs work.
This design is fine for creating signs that change slowly. To create a video-like effect, would harder. You don't get any sort of shades of grey, you just get black/white.

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Curt Welch http://CurtWelch.Com /
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