| Modifying Servo for Continuous Rotation |
Last Modified: 2006-10-20
Modifying aircraft servos for continuous rotation is a common and well documented hack to create an easy-to-interface and inexpensive gearmotor.
There are a number of websites that detail how to modify servos for continuous rotation. If you would like to take a look at some of the other approaches used, try Mechatronix or DPRG.
Several books also discuss continuous servo modification including "Mobile Robots" and Gordon McComb's "Robot Builder's Bonanza". Again, you may want to look into these books for more information on modifying servos for continuous rotation.
At Acroname, we have a good deal of experience modifying servos for our own robots, prototypes, and as products. We have looked at several types and manufacturers servos and find the Hitech HS-300 or HS-305 a great choice for inexpensive servos ideally suited to modification for continuous rotation. While these instructions are specifically for the Hitech HS-300, other servos can be modified using similar methods. We try to include all the tips and tricks we have learned from piles of servos over the past decade.
Tools you will need:
Below are the steps we take to modify the Hitech HS-300 or HS-305 servos.
Remove the single black screw holding the white servo output wheel and remove the white wheel. Take care to hold the white wheel to prevent the servo from moving while you remove the screw. Once the screw is removed the white wheel is pretty snug and fits on a splined shaft so it may take some force to remove (after the screw has been taken out). Sometimes rocking the wheel a bit can help.
Remove the four screws from the bottom of the servo. These screws are quite long reaching the entire height of the servo. Take care as these screws can easily be stripped.
Now, carefully remove the cover of the servo. The geartrain will be exposed and don't be surprised if some of the gears fall out. Set the lid upside down on your work surface and store the servo wheel, black screw, four long screws and the gears in the lid. The last gear will be in the middle and you can rock the post it sits on back and forth slightly while pulling to remove both the post and the gear.
Using needle-nosed pliers, remove the hex nut that holds the potentiometer in place. This can typically be loosened with the pliers and spun off with your fingers. Set this hex nut into the bottom lid cover of the servo.
Pulling gently on the wires, remove the PCB from the bottom of the servo and let it hang free on the red, green, yellow, brown and orange wires.
Push the potentiometer down through the servo cavity and out. Take care to keep track of the small brass washer that was underneath the hex nut. The washer will typically stay in place due to the grease but may come loose. Once the potentiometer is out, remove the brass washer and place it with the hex nut in the servo bottom.
Now locate the three wires between the potentiometer and the PCB. The wires are green, yellow, and red. Cut these wires near the potentiometer completely separating the PCB from the potentiometer.
Locate the three small metal tabs bent over the black potentiometer back. Bend these three tabs away to allow the removal of the black potentiometer back. Once the tabs are out of the way, remove the black back.
Use the wire cutters to remove the three tabs you just bent back. The black back can be discarded. Also, there are three little fingers from the potentiometer sweep brushes, clip these out as well.
Now look into the interior of the potentiometer. There is a white disk with a flat on one side that rotates. Spin this around until the punched in stop is centered over the flat. This punched in stop needs to be cut and removed. Use the wire cutters and try to cut as flush with the outer wall of the potentiometer as possible. This typically results in a small point that nearly touches the outside wall of the potentiometer.
Using a very small flathead screwdriver, drive this stop out of the interior of the potentiometer so that it can be easily cut off. The tape on the outside of the potentiometer can be removed if it is in the way. Cut off the stop entirely so that the remains don't protrude outside the potentiometer body and the interior white disk and armature spin freely and without any hitches. This armature acts as part of the axle bearing surface for the modified servo.
Re-insert the potentiometer into the servo cavity taking care to align the key tab on the servo body with the hole in the servo cavity. Once seated, reinstall the brass washer on the top-side of the potentiometer and re-install the hex nut and tighten snugly to finish the potentiometer modification.
Take the two 2.4K Ohm resistors and twist the ends of one side together. Solder this joint and clip one of the soldered legs making something of a Y shaped resistor ladder.
Now, desolder the yellow and red wires you cut earlier from the PCB. Leave the green wire in place for now. Take care to remove enough solder that the PCB holes where the wires were are left open. Also, the yellow wire's solder pad shares that of a small SMT resistor. Take care to not remove or damage this tiny resistor on the bottom of the board as you work.
Solder the common lead from the resistor "Y" you made earlier into the hole with the SMT resistor. This hole is nearest the green wire that remains in the board.
Desolder the green wire, again removing enough solder to allow insertion of the resistor lead.
Bend the other two top ends of the "Y" over and down into the two remaining holes (formerly occupied by the green and red wires). You should be able to bend these resistors nearly flat on top of the IC and the resistor leads should not touch any of the IC pins. Solder these two leads in place and clip off the extra lead length protruding from the bottom of the board.
Insert the PCB back into the bottom of the servo cavity taking care to route the brown and orange wires near the corners and the motor to avoid pinching them with the PCB.
The biggest gear, the one with the output splines has a small plastic stop on it. Using either a tiny saw, dremel, wire clippers or other tool, remove this plastic stop to allow this gear to rotate continously.
Re-assemble the geartrain as follows:
Replace the plastic donut bearing with a ball bearing race. The ball bearings don't have a top or a bottom and can be installed either way. The fit is tight so it may take a bit of pressure to get the bearing to seat down on the gear. Once you get it aligned, it will typically snap down in place.
Making sure the gears are aligned and the PCB is not pinching any wires re-install the top of the servo and the 4 long screws to hold the servo together.
The servo is done! Test it out to make sure it spins freely and can be run in both directions. If you only get continuous rotation in one direction, regardless of the input, the resistors you installed are likely shorting against something inside. Take the servo apart again and make sure they are not contacting anything else. If there are problems, try verifying the input to the servo with another servo before taking it apart again. You may want to re-install the white output servo wheel depending on your intended use.
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