# Many small and cheap linear actuators [closed]

Me and my friends are trying to build a linear actuator matrix. It's supposed to be a grid of pins that go up and down at reasonably fast speed (more than 1cm/s, the faster the better). Problem is, these things cost money and we haven't found something cheap enough. We have about $300 to spare, and would like to have ideally 1000 of them, but we can make do with about 300 (again, the more the better, and even better if we save some money). We haven't found a cheap enough linear actuator that is small enough for our needs (2mm to 10mm diameter, the closer to 2mm the better) and we would like your help. EDIT: To all your questions: I'm building a tactile electronic display (which will look like a bed of nails) that people are supposed to touch. I'm not sure how force the pins should withstand, but they are not going to be pushed, only touched by blind people, I presume it's less than 100N but I also think it is way less, I just don't have an accurate number. As for the length I need the actuator to travel, about 5 to 15 millimeters. To the guy who asked if the pins need to be either up or down or at different positions: There is a small preference towards intermediate positions (no specific number of positions required) but if the price doesn't allow it, we'll make do with up-down. ## closed as off-topic by Michael Karas, Daniel Grillo, Ricardo, nidhin, Dave Tweed♦Apr 17 '15 at 21:07 This question appears to be off-topic. The users who voted to close gave this specific reason: If this question can be reworded to fit the rules in the help center, please edit the question. • How far does each actuator need to travel? How much force do they need to produce? Are you building some sort of haptic (touchable) interface? Piezoelectric "squiggle" motors are that small and that fast, but I don't think you can get them for$0.30 - $1.00 each. – Dave Tweed Apr 16 '15 at 14:53 • You can get small vibration motors in that price range. – Wouter van Ooijen Apr 16 '15 at 15:46 • Do the pins only need to be either up or down, or do you need intermediate positions? If so, how many steps and what spacing? Is this like a bed-of-nails? Do you need many/all actuators to be move simultaneously, or could you set the position of each of them one at a time? – bigjosh Apr 16 '15 at 16:10 • Muscle wire, with a lever to move the pin? Small$3 servos from eBay are probably the cheapest actuator though. – tomnexus Apr 16 '15 at 16:14
• @DaveTweed I know, I've seen squiggle motors and they seem great, maybe there is a way to get 1000 of them at once and get a discount you know of? (Also: see my edit) – shoham Apr 16 '15 at 18:58

Consider building your own tiny solenoids. I think this would make it possible to reach your functional and cost goals.

Your bed of nails would be made from an array of actual iron nails. Each nail would hang inside of a tubular coil with the nail head flush with the top of the coil and the bottom of the nail would protrude down beyond the bottom of the coil. When the coil was energized, it would pull the nail up into the coil and push the head up above the top of the coil.

To build each solenoid, I would start with a plastic rod that was slightly thicker than your nail shaft. I wind very thin magnet wire around the rod using an electric drill, then solidify the coil using nail polish or lacquer or glue. Once hardened, slide the finished coil off the rod. Repeat 1,000 times.

You would need to experiment to find the optimal number of turns, wire thickness, and coil length for your application.

Attach a suitable flyback diode across the two leads of each coil, then wire the coils in a matrix so that you have one lead for each row and one for each column coming out of the matrix.

You can now drive this coil matrix by scanning across the rows and columns at high speed. You might want to use high current shift registers to make it easier to interface to something like an Arduino.

It will take a lot of work, but I think you could something working your limited budget. Winding the coils actually gets fun after the first 20 or so - then less fun after the 100th... :)

This is just an overview of the strategy. Feel free to ask for details if you actually try to do it!

• If you have more money than time, you can get some surplus solenoids for \$2.50 each that should work well. – bigjosh Apr 16 '15 at 21:19
• It's interesting, I'll consult my friends to see if they're up for the job. – shoham Apr 16 '15 at 21:47

Here is a similar device, using 30x30 pins.

The system uses 30 x 30 motorized white polystyrene pins, in a 381 x 381 mm area. The pins have a 9.525 mm² footprint, with 3.175 mm inter-pin spacing, and can extend up to 100 mm from the surface. Push-Pull rods are used to link each pin with an actuator, to enable a dense pin arrangement independent of actuator size, giving the system a height of 1100 mm. The linkage, a nylon rod inside a plastic housing (Sullivan Gold-N-Rods), transmits bi-directional force from a motorized slide potentiometer (ALPS RSA0N11M9A07), through a bend.

I think the most important thing is that they make the size of the display independent from the size of the actuators by connecting each pin and its actuator with a rod. This gives more freedom of choice when looking for actuators.

I agree with @tomnexus that small servos are probably your cheapest option. But even considering a reduced price due to bulk ordering them, I doubt that you can build the device with your budget.

You have to keep in mind that you need wiring and electronics to control all these servos, too. I can tell from experience that these cheap servos are cheap for a reason. They cannot take a lot of load. If the desired user experience is not hearing plastic gears break, you probably want to go for a different solution.

I think that you have to either reduce the number of pins or raise your budget.

I highly recommend building a prototype of say 4x4 (or even less) pins.