From BEAM Robotics Wiki
[edit] Introduction
Off the rack, servo motors have a 3 conductor cable coming out of one side. Two of the wires are for supply and one is for control. Internally, the servo contains a feedback pot whose resistance varies as the motor shaft turns. An unmodified servo can make a maximum rotation of about 270 degrees.
Hacking the servo involves removing the internal PCB and, if you want continuous rotation, removing the pot and the stop tab.
[edit] Tools Needed
You may need some tape to prevent the gears from falling off when the servo is opened. This is not needed if you intend to remove the pot and use the space for another circuitry. Also a Phillips screw driver and a blade.
[edit] Hacking
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Loosen the screws on the rear of the servo and remove the cover.
Once open, the inside of the servo will look something like what you see in Figure 2.
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[edit] Accessing the Gears
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Next, remove the top cover to expose the geartrain.
If you intend to leave the pot in the servo you can put a piece of tape over the gears to hold them in place.
If however you intend to remove the pot, then the gears will need to be removed first. But remember, or better yet write down, the gear sequence so that you can reassemble them in the proper order.
HITEC servos have a number on each gear, so reassembly was easier.
- Note: The red arrow seen in Figure 3 is pointing to a mechanical stop that will need to be removed to allow full rotation of the servo
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To allow for full servo rotation, you might have to take out the pot. Some servos allow the pot to be mechanically disconnected from the geartrain, but not all. To remove the pot you will need to loosen and remove the nut (indicated by the red arrow in Figure 4) holding it in place. If this is removed the pot can be removed from the other side of the servo.
Next, remove the PC board (PCB), This will require desoldering or cutting (desoldering is the recommended method) the wires or terminals connecting the motor and pot (if it is to be left intact) to the board.
Discard the PCB, or better yet, save it aside for future use. The PCB, or at least the IC on it, can be adapted for use as a motor driver with a different motor.
[edit] Stop Tab
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To complete the modification of the servo for full rotation, carefully use a knife to remove the stop tab (indicated by the red arrow in Figure 3 and Figure 5) on the inner side of the gear to which the servo horn is normally attached.
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Solder two wires to the motor leads and reassemble the servo.
If full rotation is not required, the servopot can be left intact, and used to provide feedback (shaft position information) to the circuit controlling the robot. If this option is desired, be sure to solder wires to the pot terminals and brought out of the servo housing. In fact, do so even if you do not have a use for the pot at this time. That way it will be available for future use should the need or desire arise.
If possible, The pot wires, and the motor wires should not overlap, and should exit the servo from opposite sides of the servo housing. This will require cutting an additional hole in the servo housing to accommodate the extra wires, but it also reduces the likelihood that the pot wires will pick up noise from the motor wires, thereby eliminating (or at least reducing) potential problems before they occur.
[edit] Finishing The Job
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Now you can reassemble the Servo. Start by putting the gears back in their orignal order. Next place the top cover back on. It can sometimes takes a bit of play to make everything under the top line up properly so that the cover fits right.
If you have soldered wires to the pot it may be necessary to use a knife to slightly enlarge the wire hole to account for the extra wires. Do not make the hole to large. The wires should fit snuggly. Not so tight as to cut into the insulation, nor so loose that moving the wires will evetually cause one or more to break free from the motor or pot.
Lastly, replace the rear cover, put the screws back in place and tighten them down. Do not over tighten the screws, the case is only plastic after all.
There you have it. Job done!!!
[edit] External References
[edit] Hacking
[edit] Useful Information
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