This instructable teaches how to convert a digital servo, specifically the TowerPro MG995 (however it should work to some degree with other servos), into a fully controllable linear actuator, converting its angular motion into linear motion. The length of the arm's extension is virtually unrestricted, so you can fashion linear actuators as long as you wish. This linear actuator's design enables control of both speed and direction of movement.
In order to complete this guide, you will need the following:
Tools and materials:
3/16" threaded rod and a matching nut
Metal tube whose diameter is as close as possible (yet slightly larger) to the rod's diameter
A saw (even hack saws are OK)
Soldering iron and some solder
Small cordless drill / screwdriver
Your standard work tools (pliers, cutters, screwdrivers...)
TowerPro MG995 digital servo
DC motor driver (I'll be using the L293D)
Step 1: Disassemble the Servo #1
Using a small Phillips screwdriver, unscrew the four bottom screws to expose the servo's internals. Pull out the upper and bottom cases, exposing both the electrical circuitry in the bottom and the metal gears at the top. It is recommended to wear protective gloves, as the gears are covered in grease.
Take a picture (or use the one included here) if you regret and wish to re-assemble the servo. Then, take the gears apart and place them on a table. It is recommended to wipe both the servo and the gears clean from the grease.
Step 2: Disassemble the Servo #2
Using a soldering iron, detach the 8 wires connected to the small integrated circuit (the 3 external wires, 2 wires going into the DC motor and 3 red wires in the back going into the potentiometer).
Remove the potentiometer (the "pot") completely from the servo. It it held tight with two clips, so use a small flat screwdriver to pry it out.
Finally, remove the top plastic ring located over the motor's shaft. First, pop out the motor from the bottom to avoid damaging the motor's gear. The top ring is glued to the rest of the servo, so force is required here. You may use a blow dryer to weaken the glue before pulling the ring out, however it is not necessary.
Step 3: Prepare the Actuator Parts
In order to complete the build, you will need a threaded rod, a matching nut and a metal tube with a diameter as close as possible to the rod's diameter. The rod's length determines the extension length of the actuator.
In my build, I disassembled a back scratcher (rest its spirit) with a telescopic arm and picked the tube that was most suitable. I purchased a long threaded rod and shortened it with a small hack saw, and got matching nuts (the cost of all the parts described here was around $2). The metal tube was shortened was well to match the length of the rod.
Step 4: Glue the Actuator Parts Together
Glue all the actuator parts together. Since most (to all) parts are metallic, use an epoxy resin or carefully weld the parts together. Glues that are not suitable for metals will not work, so don't even try.
First, glue the nut to the bottom of the metal tube. Make sure it is completely centered!
Then, take the flattest metal gear (the one that was directly moved by the motor) and the one that looks like a 3-story cake. Place the taller gear above the flat one and center them by placing one of the metallic rods that held the gears through both of them, and glue them together with epoxy resin. The goal is to create a gear that will rotated by the motor, but has enough surface area to hold the threaded rod.
Finally, glue the threaded rod (after it has been trimmed to the size you wish) to the top of the combined metal gears.
It is important to make sure that all parts were glued completely straight, otherwise the actuator will be crooked.
Step 5: Modify the Electronics
This step is highly recommended, as it turns your servo into an autonomous linear actuator that can work with any microcontroller. If you do not wish to perform this step, simply solder both ends of the DC motor to the power cable that came with the servo.
This step turns the simple DC motor into a fully controllable motor, allowing you to set its speed and direction, using a L293D DC motor driver. Its schematics can be found here.
For the servo to become a fully controllable linear actuator, you will need 5 wires instead of 3. I took 2 servos apart and joined their wires, but you can use any 5 wires. Two of the wires will serve as 5V and GND, and the other three will serve as enable and direction control pins.
Solder the L293D's pins as follows:
5V - pins 8 + 16 (solder 2 small wires to the wire going out of the servo and connect each to a matching pin)
GND - pin 4
Motor - pins 3 + 6
Enable pin (EN) - pin 1
Control pin #1 (In1) - pin 2
Control pin #2 (In2) - pin 7
Once you're done, cover adjacent solder points with electrical tape and push the driver into the vacant area in the casing (which used to house the feedback system). Make sure you can close the bottom afterwards, it may require some force but you should be able to close it properly. Remember that the bottom will be screwed shut when we're done, so don't worry if the bottom "fights back" a little.
If you cannot fit the driver inside the case, try trimming the chip's pins so that it occupies less space.
Step 6: Putting It All Together and Testing
Before we modify the top and close up, you should make sure that everything is working.
The gears were originally held in place by two metallic rods. Looking at the top part of the servo, take the shorter one of the two and place it in the middle. Then, use it to slide the combined metal gear we created before into place. Screw the threaded rod into the metal tube.
Now, hold both the servo and the metal tube, slightly pushing the tube downwards to simulate the weight it will need to support. Run the motor in both directions and see if the rod goes up and down. If anything is unbalanced, go back a step and fix whatever needs to be fixed. Only if everything works as planned, continue to the last step.
Step 7: Close Everything Up
Drill a hole in the upper case of the servo directly above the metal gear attached to the threaded rod. You can simply keep the original hole, or cover it up with a small piece of black electrical tape.
Place the top case back on top of the servo, and close the casing with the four long screws.
Connect your new linear actuator to a microcontroller and enjoy!