Provided you have the tools and the servo you can built this for under a couple of bucks. The actuator extends with a rate of about 50mm/min. It is rather slow but very powerful. Watch my video at the end of the post where the small actuator lifts 10kg vertically.
- hobby servo
- standard hobby brass tubing
-OD: 4.0mm, ID: 3.4mm
-OD: 5.8mm, ID: 4.5mm
- standard hobby styrene tubing
-OD: 4.8mm, ID: 3.5mm
- M4 studding
- 2 x M5 washers
- 2 x M4 nuts
- 5 minute epoxy
- multi-strand cables
- heat-shrink tubing
- standard tools – screwdrivers, scalpel, files etc.
- dremmel multi-tool with ceramic abrasive disk, or similar
- hand-drill + 4.9mm + 2.5mm drill-bits
- M3 tap
- M4 tap
- soldering iron
- glue gun
- small vice
- small saw
- sanding paper (relatively fine)
- small flame torch
- I will be giving instructions based on the dimensional parameters of the Hitec HS-300. The procedure remains the same for any type servo. I strongly recommend you read the whole post before you start. So lets make a start, shall we?
- Open your hobby servo, remove control electronics, feedback potentiometer and mechanical stop on the servo’s output gear.
- Solder new cables on the servo motor’s leads.
- Drill two 4.9mm holes on the servo case bottom cover. These should be located longitudinally along the centre line and 9.5 mm from each end (this applies on the Hitec HS-300 and is also true for many standard servos but depending on your servo type there might be differences). The M4 thread will come out from the servo body using one of these two so this hole must be located directly below the centre of rotation of the servo’s output gear. Be very careful since this alignment is very important! If you don’t get it right you might have to use a new servo! The more accurate you are, the longer your servo will endure.
- Measure the dimensions of the rotating shaft of the potentiometer on the servo’s original electronics – note the geometry in general. The shaft should be flattened right at the tip in order to prevent it from freely-rotating once inserted into the servo’s output gear.
- Take the M4 studding (M4 thread) pick one end and by using the dremmel and the abrasive wheel tool, replicate the tip of the servo’s potentiometer on that end. Start by decreasing the diameter of the thread, rotating it steadily by hand against the abrasive disk (normally to 3.5mm in diameter and at least 6mm in length). Try to think of your fingers as the chuck of a slow-turning lathe. Once the diameter of the thread is down to the pot’s shaft diameter, flatten the tip according to the potentiometer’s tip. The idea is that the thread must be inserted in the servo’s output gear in the same way the potentiometer did before. The better the fit the longer your servo will endure.
- On the flat tip of the M4 thread, screw the two M4 nuts approximately 20mm down its length. Following that, insert the two M5 washers.
- Insert the thread inside the servo and adjust the distance of the nuts and washers down the thread such that the servo case bottom cover closes properly and the motor rotates efficiently. Basically, you have to make sure that once the thread and the servo are assembled there is no pressure between the servo case bottom cover and the nut-washer assembly. Similarly, you have to make sure that once the thread and the servo are assembled there is no gap between the servo case bottom cover and the nut-washer assembly. Once again, the better the fit the more your linear actuator will endure.
- Once you find the optimum position carefully disassemble the servo, remove the washers from the thread and use a drop of cyanoacrylate on the side of the nut that was in contact with the washers in the assembly. Let the glue to settle for 5 minutes. Unscrew the second nut by 10mm towards the flat end of the thread, and prepare a small epoxy mix.
- Put the mix between the two nuts and screw the second nut back in place. Once in place also use some epoxy on the back of the second nut as well. Ideally you should sand all contacting areas before you apply the epoxy glue. Leave to settle for at least 6 hours (even if you use a 5 min epoxy).
- Secure tightly the 4mm diameter brass tube onto a vice by flattening the mounting end and use the M4 tap VERY carefully tapping as deep as possible (at least 15mm). Using the dremmel cut 10mm out of the threaded part of the tube and then verify that the created thread runs along the whole length of the small threaded tube by screwing it onto an M4 screw. Keep the 4mm threaded tube on the screw for handling purposes. Apply a layer of solder on the outside surface.
- Take the 5.8mm diameter brass tube pick one end and try to sand at least 5mm into the tube (on the inside). Mount the brass tubing on the vice without squishing it and apply a thin layer of solder on the inside.
- Ignite the flame torch, take the 4mm threaded tube (holding it by the screw) and move it on the soldered end of the 5.8mm diameter brass tube which should still be mounted on the vice. Using the flame torch heat-up both tubes and carefully insert the 4mm threaded tubing inside the 5.8mm tubing until is fully inside. Use a pair of pliers and insert the brass tube by holding the end of the screw that sticks out. Hold the threaded tube levelled inside the 5.8mm tube until the solder settles. If you do not have a flame torch use a candle, your soldering iron and your patience J. Remove the screw. The end result will be the cylinder of your linear actuator.
- The cylinder length should be equal to: the actuator’s desired working length (stroke) + length of the 4mm threaded tube which is inside the 5.8mm tube + 10mm for the mounting hinge at the cylinder end.
- The thread length should be: the actuator’s desired working length (stroke) + length of threaded tube which is inside the 5.8mm tube + length of the thread which resides inside the servo casing, which is model-dependant.
- Take the non-threaded/non-soldered side of the cylinder and drill a 2.5mm hole through, 5mm from the tip.
- Cover the entire length of the cylinder with heat-shrinking tube and cut-off any excess bits. The 2.5mm through holes made earlier on the non-threaded side of the cylinder are now covered. Use the drill again to expose them and tap them through, using the M3 tap. Screw a 20mm long M3 studding or simply cut-off the head of a 20mm long M3 screw. This will act as your cylinder mounting hinge.
- Take the 4.8mm styrene tubing and M4 tap it 10mm deep. Cut a small ring 5mm in length and screw it in the M4 thread fully, from the side of the nut that was in contact with the washers (long side of the M4 thread). This will act as bushing between the thread and the servo’s case bottom cover. Ideally you should use nylon, copper or metal bushing.
- Secure the motor cables inside the servo casing using a glue-gun and use heat-shrinking tube to cover them. Assemble the servo including the thread, the styrene bushing and the washers.
- Screw-on the cylinder and you are good to go!
Watch a video of the small linear actuator lifting 10kg: http://www.youtube.com/watch?v=1fUwPSn32T8
The next tutorial describes how you can add 10-bit absolute feedback to your servo using a simple magnetic encoder.