The Famous Servo Hack
This page documents the steps required to modify a radio control servo to provide continuous rotation such that the servo can be used as a drive motor for a small robot. Most pictures can be clicked to get a larger version.
This modification procedure is directed towards the Cirrus CS-60 2BB servo. This servo is available from Hobby People (www.hobbypeople.net) where you can order it on-line; or find a store near you. This servo is relatively inexpensive and is suitable for the continuous rotation modification as well as modification to add a sensor to provide an encoder function. Click here for more technical data on the CS-60 2BB.
There are many other servos which may be modified for continuous rotation in a similar (but usually not identical) process. You can find other sets of instructions by searching the web. This servo has a little more torque than the "standard" servo, is rated at 6.0 vdc (which gives more speed and torque) and has a transparent case which is convenient for inspecting the gears.
If you want to use a different part number servo, be aware that some servos (including the Cirrus CS-50 and CS-400BB) do not have a full 360 degrees of teeth on the output gear. This is apparently a cost savings measure since the servo is only designed to rotate slightly over 180 degrees. Hence, the clear case of the CS-60 and CS-80 series is convenient for verifying the presence of a 360 degree gear before buying the servo.
The Cirrus servos are available with 3 sets of features. The "standard" servo has all plastic gears and plastic or metal bushings for bearings. The "BB" models have at least one ball bearing on the output shaft while the "2BB" servos have two ball bearings. The "MG" option replaces some or all of the plastic gears with metal gears. The metal gear option provides a lot more durability to the servo.
PARTS AND TOOLS REQUIRED:
2 Servos, Cirrus CS-60 2BB Hi-Torque
Super glue (optional)
Small Phillips head screw driver
Razor blade or equivalent to cut tab off.
1. Remove the output wheel on the output shaft (if installed). Use a small Phillips head screwdriver to remove the screw in the center of the output wheel; then just pull the wheel off.
2. Remove the four housing screws from the bottom corners of the servo Try to keep the bottom plastic cover in place, but it will probably come loose anyway. No problem, it easily fits back in place.
3. Split the case at the line just below the gears and the mounting flanges. The output gear and an intermediate gear will come off with the top of the case. Keep an eye on the intermediate gear in the top as it isn't well attached and could get lost.
4. In the lower case, beneath where the output gear rested, you will see a ball bearing with a brass, flatted shaft in the center with a white plastic clip around the brass shaft. This white clip is the coupling between the output gear and the position potentiometer on the other end of the brass shaft. The pot can be seen by looking through the side of the transparent case. Remove the white coupling clip. It is just pressed on over the brass shaft and can be pried off easily. This disconnects the potentiometer so that it will not turn as the servo output gear turns.
5. Some servo hacks require the pot to be removed and replaced by resistors. In this case, removal is not necessary. However, the pot must be moved to its center position. The brass shaft may be turned with your fingers to determine the maximum right and left positions; and then return it to the center as close as you can estimate. This will probably result in some error, but any offset can be accounted for in software. An alternate scheme to determine the center point is to attach the servo to a controller which is sending out a 1.5 millisecond pulse (center position). You can then find the center position of the pot by rotating the brass shaft until the motor stops moving.
You may want to lock the brass shaft into position after placing it in the proper center point. This step is optional, as with the white plastic clip removed, the output gear does not touch the brass shaft and it tends to remain where you put it. If you want to lock it in place, apply a small drop of super glue to the side of the brass shaft so it can run down the side. DO NOT get any super glue into the ball bearing around the shaft. After a few minutes, test the glue by attempting to gently rotate the brass shaft. Do not apply too much force or the glue bond may break. Leaving the shaft unlocked has the advantage that you can restore the servo to the normal configuration.
6. Press the output gear out of the top housing. This can be done by holding the housing upside down so that the shaft is against a table, then just press down on the housing and the shaft will press out. Remove the output gear. Also, set the intermediate gear aside as it will now be loose. Below the ball bearing, you will see a small plastic tab on the gear (at bottom of adjoining picture). This tab is to ensure that the servo does not travel more than 180 degrees and must be removed to allow continuous rotation. (the stops on the upper housing, which this tab runs into, do not have to be removed).
Use your favorite tool to remove the plastic tab: Wire cutters, Exacto knife, Dremel tool, etc. Try to get it all off as even a small piece may hit the tabs on the housing and cause the servo to jam.
7. Now the servo can be reassembled. Press the output gear back into the upper housing using your fingers. Make sure the intermediate gear is in place above it.
8. The top servo housing can now be reattached to the lower housing. make sure that the shaft for the intermediate gear properly goes into the gear; and gently align the upper and lower housings so that the gears will mesh together. Turn the output shaft a bit, if necessary, to get the gear teeth to line up. When the two housings are together, you can place an output wheel on the servo output shaft and rotate it gently (may take some force, but go slowly) to ensure that the gears are all engaged and moving freely. Don't push too hard if it feels jammed, or you might strip the gears. A safer test is to hook it up to a controller and try to drive it. If it is jammed, it won't turn...and won't strip.
9. Reinstall the four screws into the bottom of the servo and the modification is complete. Congratulations.