Building A One-ton Linear Servo


A while back, [Windell] from Evil Mad Scientist Laboratories wrote up an article for Make Magazine detailing how he built a one-ton, servo-controlled scissor jack for under $100. He dropped us a line to let us know that the project details have been released for free at Make Projects, so we stopped by to take a look.

The project starts out by pulling apart an electronic scissor jack to get access to the solder pads for the up and down buttons. Once wires are added there, a servo is the next victim. [Windell] recommends using an old servo with a busted motor, but you can use a good one just the same. The servo’s pots are replaced with 10 turn pots, and then wired up to a controller board, to which the jack is also connected. Then, to provide feedback to the servo, a string is looped around the top of the jack, which is used to turn the pots added in the previous step.

[Windell] says that the setup works quite well, though we imagine the duty cycle might be a bit short before adjustments are required. Regardless, it’s a quick way to get a heavy load lifted with servo precision.

13 thoughts on “Building A One-ton Linear Servo

  1. There is something to be said for simplicity and working with what you have, but a simple rotary encoder attached to the main screw would be a lot more reliable in the long run.

    I can set a lot of uses for some heavy duty servos/actuators.

  2. I’ve always liked scissor jacks, because you can get them at the junkyard for peanuts, and they can be blasted up and down with an air wrench… Unfortunately they wear out quickly in this sort of duty. I wonder if that will be a problem here.

  3. @raidscsi The problem with a rotary encoder is that you would only be able to know the rotation, not the actual position. You would either always have to start the control with jack at the lowest point or store the position in memory. with a pot you can just read the position at powerup.

    That said, there has to be a better way to link the pot to the jack.

  4. Why not just attach the pot directly to the pivot joint of the jack, or even just use a small arm as used on car/truck air suspension sensors? I can’t see you need a 10-turn-pot’s worth of accuracy from your bit of string & elastic band.

  5. A rotary encoder on the shaft has another problem (besides position) – it’s not linear with respect to the height of the jack. The twine and 10-turn pot is a simple, if inelegant, solution.

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