Micro Servo Modding.
Servos have many uses outside of remote controlled models, one of the most popular being as a drive system for wheels on robots. Better still, you can get some very small servos that allow you to make equally small robots! However, to use them for driving robot wheels they need to be modified to allow continuous rotation, as servos are limited to only a semicircle of revolution. Therefore, this page describes how a small 9g micro servo was modified, as it may help you modify yours. But please remember that servos will vary widely in detail so you may need to adapt this page to suit your needs.
There's a very good article on instructables.com that was very useful in working out how to modify the servo used here, but it involves removing the base of the servo as well as the top. That makes it a very fiddly job, especially for first timers, so it's lucky that we found an easier way to do it without needing to take off the base, remove the circuit board, or get frustrated putting it back together. If it doesn't work for you, because your servo is different, you can always head over to instructables.com for more help, but you'll still need to do most of what's described on this page.
The first thing that needs doing is to remove the four small screws from the bottom of the servo using a very fine screwdriver, which needs to be done carefully as those screwdrivers can cause a nasty injury if they slip. If you've read on other sites that you need to cut or remove the stickers, please don't unless you really want to remove the servo base. The four screws that need removing are shown in Figure 1.
Figure 1. A micro servo and the screws that need to be removed.
Once all four screws are removed, the top of the servo case should be fairly easily removable. Removing it exposes the gears and it's very important to make a note of each, including their position, as you need to remove all four gears and remember where to put them later when you reassemble the servo. Two of the gears need modifying, these being the one with a tab underneath and the gear below it. Both need to have their central holes enlarged very slightly so they rotate freely, but not sloppily, on their metal shaft. Also, the plastic tab marked in Figure 2 needs to be removed. A Dremel hobby drill, with a fine drill bit and set to a slow rotation speed, works well for enlarging the central holes. To remove the tab a small wire cutter can be used, and a Dremel can again be used to clean away any remnants of the tab, but you need to be very careful not to damage the gear. When you remove the other two gears be aware that their shaft may be loosely fixed into the plastic case, so may be easily lost if you don't keep it somewhere safe.
Figure 2. The open servo showing the gears and the tab that needs removing.
Having removed the gears, the micro servo should look something like Figure 3. It's now that you should be able to tell whether you can modify the servo without removing the bottom of the case, or head across to instructables.com to find out more on how to remove the servo inners for a more complicated mod. If you can see the pot (a.k.a. a potentiometer or variable resistor) with an exposed rotor, as shown on the left of Figure 3, then you should be able to do a fairly simple mod. However, remember that servo mods are quite fiddly and so things can easily go wrong, so for your first time it's best to avoid modding expensive servos and try it first with a cheap one you don't mind trashing. Two things are needed for the mod now: some glue and an arduino to ensure that the pot is adjusted so that the motor isn't rotating.
Figure 3. The servo with the top of the case and gears removed.
The first thing to do is to load some code into the Arduino that sets the servo angle to ninety degrees, which you can get by clicking here. Then, connect your servo to the Arduino and very slowly and carefully rotate the pot shaft until you find the position where the motor stops turning. You'll need to make sure the pot stays in that position until the glue is dry, so best to keep the servo and Arduino working together until the mod is complete. For the servo used here Araldite two-part glue was used, which sets in a few minutes. With the pot position set the glue was carefully placed onto the rotor in the pot to prevent it rotating, using a small wooden stick. It's very important not to end up with too much glue, as it might prevent the reinstalling of the gears, nor to get glue in the screw holes (which you could cover temporarily with small pieces of tape just in case). Now you just have to wait for the glue to set. Of course, if you touched the pot when putting the glue in then the motor may have started turning, in which case you will need to carefully tweak the pot to re-find it's no-rotation position.
Of course, there's no guarantee that your servo won't rotate a liitle bit when later connected to your microcontroller and set to ninety degrees. Unfortunately, servo mods can be a bit hit-and-miss when it comes to ensuring you can prevent all rotation, such as in a robot where you want to be able to ensure it stays in one position. For more advanced modders, some sites recommend disconnecting the pot wires from the servo circuit board and running new wires to an external pot to allow later tweaking. Alternatively, if it's a problem for you, you could power your servos through a high power transistor, which could be switched off by the microcontroller when you want to prevent movement. However, that doesn't prevent some movement when switching the servo power on again. If your robot is hyper-active, and so always moving around even just a little, then if you're careful in keeping the pot position set while the glue is drying then you'll probably not be too worried about such problems.
That discussion over, hopefully your glue is now completely hardened. If so, you can disconnect the servo from your Arduino, trim off any excess glue carefully with a sharp knife, and put the gears back in place. It's important to check that the two on the pot shaft, which were drilled to have a slightly larger central hole, do rotate freely. If not, it's wise to rectify that now. With the gears in place, and maybe a little servo grease put in if needed, the top of the case, and the four screws, can now be reassembled. Once that's done you can try out your newly modded servo using the code you used to centre it: try setting angles within a few degrees of ninety degrees, and if everything went well the servo should now continuously rotate both clockwise and counter-clockwise.