spacer T-Pro SG90 Micro Servo
Continuous Rotation Modification

I'm designing a March Break project for my son and his friends. It's a small robot that uses servos that have been modified for continuous rotation to drive the wheels. While looking for parts I stumbled across the T-Pro SG90 micro servo. They weigh a mere 9 grams and according to the description deliver a respectable torque. I found that HobbyPartz.com is offering these for $2.77 US a piece (as of March 9, 2011) so I figured that I couldn't go wrong. What follows is my experience modifying these servos for continuous rotation.

Size comparison to standard servo
Size comparison to a "standard" servo

Small Philips screws hold it togther

The servo's body consists of three parts. The labels span these sections so the first step is to peel the labels off. The servos are held together by four small (but long) Philips screws. You'll need a sharp jeweler's screw driver to get these out.

Servo in three parts

The top and bottom of the servo pop off revealing the gears and electronics. Flipping the servo over we can get a good look at the electronics.

Feedback wires

Here you can see the circuit board that runs the servo. The three wires marked by an arrow in the photo connect to the feedback potentiometer. This potentiometer is turned by the gears and reports the servo's position. Our goal will be to remove this potentiometer and replace it with a fixed resistor network. Then when you tell the servo to move from centre, the motor will rotate continuously since the resistor network keeps telling the controller that the servo hasn't reached the destination position yet.

Feedback wires removed

Here I've removed the feedback wires. Afterwards I found that you can leave the wires attached to the potentiometer as they'll be easier to remove once we pop the potentiometer out of it's housing.

Remove central stack of gears first

In order to remove the potentiometer and main gears we have to first remove the central stack of gears. This is most easily done by gripping the shaft (marked by the red arrow) and lifting it by a mm or so. The whole stack of gears slides out sideways.

Potentiometer and gears push out

Next, by pressing on the bottom of the potentiometer (from the inside of the servo) you can pop it and the attached gears out the top in one piece.

Here's the potentiometer and attached gears. There are two mechanical stops that prevent full rotation. One is the plastic nib on the upper gear. The other is two metal fingers inside the potentiometer. More disassembly is required.

Cut of feet and remove shaft grip

This is a good time to cut off the potentiometer's feet. Also we need to pry open and remove the "shaft grip" (I don't know what this is really called). I used a small jeweler's flat head screw driver to bend open the two arms marked by the arrows. Then a sharp mini-clipper to free the shaft.

Remove fingers and nib

To remove the metal fingers I just used my mini-clipper to cut them off. The nib come off with a sharp razor knife. Be careful to slide the lower gear as far away from the upper gear as possible when you cut off the nib. There's a small gear in between that could be damaged if you are careless.

Stops removed

The stops have been removed so it's time to reassemble the potentiometer and gears. First, reseat the potentiometer into its well in its original orientation. Next drop in the main gear stack. Finally slide the middle gear stack into place and press down on the shaft. Everything should hold in place.

Gears back in place

Now it's time to add the fixed resistors. The original potentiometer has a 5K resistance. Ideally we'll use two 2.5K resistors to mimic the potentiometer. In practice I've used 2K to 3.5K resistors depending on what I had available.

Where to add resistors

These are the pads that were attached to the potentiometer. We'll span the two gaps using small surface mount resistors. 1206 are too big. I found that 804 sized resistors fit nicely. I scavenged these resistors from old boards. If you can find precision resistors these will work better. For example resistors marked 2001 and 202 are both 2K resistors but the 2001 will be more precise. Since we want both resistors to be identical, using more precise resistors gives us a better chance of achieving this. In addition to 2001 and 202 resistors I've also used some 352 (3.5K) resistors and these worked just as well.

New resistors in place

You'll need a sharp soldering iron to solder the resistor's in place. Adding some solder flux will also help. Here the resistors are in place. Now is a good time to test the servos. If you have a tester, hook up the servo and you should find that you can rotate it continuously in either direction.

The servo is quite delicate so even small amounts of grit or debris on the gears can cause binding or irregular running. If you notice this, check out the gears for dirt.

Finally slip the board back into the case and you're ready to close.

Hope you find this guide useful.