I needed to modify a number of Parallax standard servos to continuous rotation for a kids' robot workshop project. I came up with a method that requires no soldering and allows the servo to be changed back to position operation if desired. The following procedure is specific to the Parallax servo although the concept may work for other brands if their construction allows it.
What I did was to adjust the position feedback potentiometer (pot) to its electrical center and reposition the pot so it was mechanically decoupled from the gear train. I also cut the stop nub off one of the gears. After modification, any commanded position ahead or behind center causes continuous rotation. You can return the servo to standard service by restoring the pot to its original orientation. A small plastic pin or screw stop can be added if desired.
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Step 1: Disassembling the Top and Bottom
• Unscrew and remove the servo horn if attached. Set it aside.
• Remove the four long screws that hold the servo assembly together.
• Pull off the bottom and top covers. Be careful. Some of the gears may come out when the top cover is removed. They are often stuck together with the lubricating grease.
• Remove the gears. The large gear at the bottom may be left in place. Preserve as much of the grease as possible.
• Remove the two short screws that were exposed when the gears were removed. This detaches the board and motor assembly from the case.
Step 2: Removing the Board
• Carefully pry the board up around its edges until you can slide out the entire assembly. It may take several rounds of prying a little at a time before the board comes loose. Set the case aside.
• Notice that the servo feedback pot is mounted on long springy leads.
Step 3: Setting the Servo Feedback Pot and Changing Its Orientation
• Turn the feedback position pot from one stop to the other and then to its center. The flat sides of the pot shaft will be aligned with sides of the body at dead center. The leads on the pot in the picture were partially bent before I remembered to capture the image.
• The pot will be physically repositioned in the next step.
Before proceeding, look inside the empty case and note the square cavity for the pot on one side. Keep in mind that the body of the pot will have to fit into that cavity when you reassemble the servo after repositioning the pot.
Step 4: Repositioning the Servo Feedback Potentiometer
• Use a pair of needle-nose pliers to bend the leads of the pot such that the shaft points down towards the board and is nestled in next to the electrolytic capacitor. I found that first straightening the leads and then bending them as needed worked best. Align the pot body so it will fit into the case cavity upon reassembly. The springiness of the leads will forgive a little misalignment. Get the pot down so that the shaft almost touches the board. Be sure the leads do not touch the metal capacitor body.
Step 5: Removing the Gear Stop Nub
• Select the tall gear with a slotted hole and a metal bearing on one end.
• Cut the stop nub off the gear, being careful not to damage the gear teeth. The nub must be completely removed down to the surface of the gear. I used a sharp Exacto knife to clean up the surfaces after nipping off most of the nub with side cutters. A jeweler’s file should also work well for cleaning up the surfaces.
Step 6: Reassembling and Kicking Back
• Slide the board assembly back into the case, being careful that the potentiometer fits into its cavity.
• Put in the two small screws on either side of the motor shaft and reassemble the gear train.
• Put on the top and bottom of the case and secure them with the four long screws.
Congratulations, you’re done!
Any commanded position ahead or behind center will cause the servo to continuously rotate.
Restoration of servo function
You can return the servo to standard service by restoring the pot to its original orientation. A small plastic pin or screw stop can be added if desired.