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Modifying Servos for Continuous Rotation

Originally posted January 17, 2008:

In my recent delve into simple robotics, one of the major pains in the rump has been modifying servos to spin a full 360 degrees, making them into little cheap gear motors to push my little creations around the house.  The biggest hassle to the process is getting the servo potentiometer to stay at the "90 degree" position, tricking the internal electronics into stopping the motor when I send it a pulse to head for 90 degrees.  The standard method involves cutting out the mechanical stop on the final gear, drilling out the tab that locks the same gear to the potentiometer head, and gluing the pot in place after adjusting it to the proper position.  The issues that are inherent in this process are many, but include having to break apart the glue and re-adjust the pot if your voltage source ever changes, and having it dry just out of position, as these pots (at least in the HS-311 servos) are really, really touchy.

When I went looking for a better solution to the problems I was seeing, I found a number of people that recommended using a pair of 3.3k resistors tied together to produce a three legged fixed resistor network in place of the pot.  In theory, this would mean that the resistance from the center leg to either other leg would be equal, fooling the control circuit into thinking it had a perfectly centered pot.  I thought this an ingenious approach, and that fact that it would be immune to bumping and such was a great bonus.  In practice, finding two exactly identical resistors, soldering them perfectly so as not to add undue resistance, and accounting for the internal resistance in the rest of the circuit that apparently throws things out of whack was a major pain in my bootie.  I needed a better approach.

Searching Digi-Key, I decided to try a large turn pot that would allow me to fine-tune the resistance as perfectly as I could.  I settled on this guy here, a 5k 25 turn pot small enough to fit comfortably in the case.  After posting about this plan on the Society of Robots forums, another poster suggested to drill a hole in the side of the servo for the screw adjustment to protrude though for easy access to adjust later.

Once the parts came in (always fast through Digi-Key), I popped open my servos and cut out the stock pots.  Soldering the original wires to the legs of the new pot, I ended up with a nice, simple mod.

Then I simply drilled the hole for the screw to poke into, and closed everything up after a quick test.  The pot is held in place internally with a little bit of super glue (use the gel type, it won't flow everywhere), and the mod looks very clean.  You can see the small gold screw adjusters from the back of ADAM.

These are now super easy to adjust when testing, and with the amount of changes I'm always making, a really nice time saver.  The bonus to doing this in the future to other servos is not having to drill the plastic out of the center of the large gear riding over the stock pot, since the pot can be completely removed with just one screw.  Some servos have inserts for this gear that can be popped out easily with a screwdriver, but the HS-311 does not.  Total time from a stock servo to a sweet cheap gear motor with this method: less then 10 minutes (plus dry time for the glue if used).  I wish I had done this sooner.