If you’re giving up the position sensing of a servo, why not just attach the leadscrew to a cheaper & faster/more powerful gear motor? If all you have lying around are servos it works, just don’t go our of your way to get one if you need a linear actuator.

Since we are talking about converting things, this also is interesting Convet DC motor to a servo

@Kevin – I know. It’s tragic how some people can’t spell. The world is so unfair.

Awsome, this will bring the price of heliostats way down ;-)

Awesome! A whole new way to make a Sybian…

Crap! He beat me to it. I have been working on one of these for a while. One suggestion for anyone planning on building one. Spring for ‘telescoping tube’. Smallparts has it.

@svofski
Having a quick peek in The Laminated Book of Dreams (read: the mail order component catalogue), gear motors of an equivalent power to a servo are about half as expensive. Albeit these are slightly larger motors, not tiny hobby servos.
@Mic
The servo he gutted used a potentiometer for position sensing (which was removed). If you had a rotary encoder servo, then yes you can make a precision linear actuator. This is what’s used in CNC mills, though using ballscrews instead of leadscrews, and much beefier servos.

You could probably retrofit an existing motor with a cheap rotary encoder and a PID controller and get a high power precision linear actuator. Measure it’s uncontrolled motion for a bit, whack the measurements into EurEQa (or enjoy matlab-induced aneurysms), and add the controller equation to whatever you’re using to drive the thing. Or if you want a standalone part, check this out: http://www.reprap.org/bin/view/Main/Magnetic_Rotary_Encoder_1_0

Seems to me like there should be a mechanical way to link the pot to the linear device and retain feedback. Easy but big way is a slide pot…

@edz: care to share your source of cheap gearmotors? The cheapest gearmotors I’ve seen are about $8, which more than half the cost of an average servo (about $12), and that’s comparing surplus gearmotors of random speed to “new, brandname, get it anytime” servos. Thus the many web pages to convert servos to general purpose gearmotors. (“new” gearmotors, where you get to specify the RPMs and such, tend to be outrageously expensive for some reason.)

Hi all,

Let me make a start by saying that the reason I decided to post this method is not because I found the answer to every operational aspect of the hobby servo – linear actuator combo :)

As EdZ correctly pointed out, you have to get rid off the pot and hence the feedback element, but coming to think of it, when it comes to continuous rotation the pot is not of much use anyway since there is a dead band of about 10 degrees, which leaves the pot’s output floating – not a very good I/P for the ADC of your micro-controller. If one insisted to use a pot they could very easily affix a potentiometer on the output shaft of the servo. Of course it would be better to use an absolute encoder:

http://01mech.com/MagEnc :))

I believe that servos are great for hobby robotics ventures since are way cheaper than standard gearmotors, all the components come enclosed up to IP64 standard, (protection against low pressure spraying water), they come in a small form factor with mounting flanges ready-fitted.

The idea came to me from my planetary exploration rover (my PhD), where I had to make the linear actuators myself using standard DC gearmotors parallel gearboxes quadrature encoders etc etc
Have a look:

The small servo hack would be extremely useful in making a scale model of my rover. I intend to release the rover designs to the community – the servo linear actuator is a first attempt to draw attention :)

The next step is to hack the servo into a PID speed/position controlled continuous rotation gearmotor with absolute feedback (1024ppr). Schematics are already freely available:

http://01mech.com/supermodified

@ramsay
Wow, just wow, I hope it was some sort of joke or translation error on that sun around the earth thing, else it’s the most embarrassing thing ever, especially since he’s dutch and not american I gather. (dutch are largely non-religious and generally have some decent basic education)

Crazy cool! I used the PSC belt actuator from http://www.macrondynamics.com to make a bigger suspension bridge. It works ok, but I like this better