How to hack the "servo" out of "servomotor"...

By Ted Pavlic

Pictures taken from "How to modify a servo"(defunct) webpage

"How to Hack a Servo" link on FEH webpage...

Some of you may have already noticed that there is a link on the FEH webpage telling you How to Hack a Servo (now defunct), but if you follow the steps there you probably aren't going to be happy (if you've already followed those instructions, don't worry; there's still hope).

The instructions listed there are for turning a typical Futaba S-148 servomotor (similar the ones you have in class, Futaba S-3003, the model which replaced the S-148) into a "continuous rotation servomotor." These sorts of servos can be used for locomotion on a robot, but they still are basically servomotors. That is, they have special control circuitry in them with which your Handy Board communicates data which tells them how to control the motor.

This means that your new continuous rotation servomotor needs to be plugged into a digital port, NOT a motor port, and must run off the regulated 5V provided by the digital ports.

Unfortunately, it is difficult to use more than one servomotor with the Handy Board, and near impossible to use more than two. Most who use servomotors in their drive train will need two servomotors just to drive and perhaps another for other utilities on the robot. Because of this, this "continuous rotation servomotor" is not necessarily the most appropriate.

The hack I suggest here has a key step that is much different from the other hack which actually removes the "servo" part of "servomotor" making the unit simply a DC gearhead motor. This allows you to plug the motor directly into the motor ports and regulate the voltage from them however you want.

So how do I do it?

I encourage you to read the How to Hack a Servo information to gain a better understanding about how a servomotor works. It should also help you anticipate what needs to be done to turn the unit into a true DC gearhead motor.

Simply, follow every step in that information, but STOP at the "Now for the actual modifications" step. That is, do:

Open the case by removing the four long screws at the bottom of the unit. It should be easy to remove the bottom and the top of the housing.

Remove the gear assembly, but be careful to remember how they were originally arranged.

The next step will vary depending on what type of servomotor you have. Look for small screws that attach the drive motor to the housing. Hopefully there will be two small phillips-head screws going into the top of the motor, but there are no guarantees.
Remove the motor and circuit board from the housing. You may have to apply a lot of force to the top of the potentiometer sticking out of the top of the housing in order to pop the circuit board out. You don't have to worry TOO much about breaking anything as we shall find out in a second that the circuit board matters very little. Be sure not to break any of the gears or the housing, however.

At this point you should have a bunch of gears, a plastic housing, and a circuit board with a DC motor attached.
Regardless of your objections, take some sort of cutting device (like some small diagonal cutters, for example) and clip the circuit board in half. That is, get rid of the important part of the circuit board next to the motor. The only thing we care about is the motor. You can leave the half of the board mounted onto the motor, but get rid of the stuff next to it (really this is a formality; I suggest it for safety).

The control circuitry you just clipped off should have three wires soldered to it. You probably want to use these wires, so remove them from the control circuitry. It is not difficult to simply use a soldering iron to heat up the existing solder to the point where you can slide the wires right out.

Since you only need two wires, you can remove one of the wires if you want.

Now solder the two remaining wires to the two leads coming from the bottom of the motor. These leads will be protruding through the bottom of the circuit board remaining connected to the motor. To each lead coming from the motor, solder one wire. These wires will provided the PWM-regulated power from the Handy Board motor ports.

You can start using the steps from the previous information at this point. Start at the "Now, reassemble ..." step. That is, do:

Insert the motor (now without the control circuitry) back into the housing. You should notice that now the potentiometer will NOT protrude out the top because it is not there anymore :). Only the motor will protrude out the top of the housing.
Fasten the motor as necessary (that is, put the proper screws back or do whatever steps are necessary to fasten the motor inside the housing).
Before reassembling the gears, you will have to remove a little plastic "stop." It's a small nub of plastic on one of the gears that should obviously keep the gears from turning a full 360 degrees. Simply clip or cut this stop off with diagonal cutters or some sort of knife. Make the gear smooth so that there will be nothing keeping it from turning 360 degrees.

Reassemble the gears and place them back onto the housing on top of the motor.
Put the housing back together. You now have a DC gearhead motor.
Try turning the motor. It should be able to turn 360 degrees with only the resistance of the motor. If there is any extra resistance, open it back up and be sure you removed the mechanical stop all the way.
NOTE: You may want to add some voltage regulation in line with the two wires of this cable in order to ensure the proper rated voltage goes to the DC motor. You will have to seek separate documentation for this step.

NOTE: Feel free to ask for other TA's experience with "hacked servo" drive trains, but with this particular hack typically there is not often a problem simply running the motor "open." That is, typically groups are able to run the DC gearhead motor produced here directly off of the battery voltage with no voltage regulation. If you wish to take the [relatively low] risk of burning out your motor and/or, drawing enough current to reset the Handy Board during normal operation, simply modify the cable to plug directly into the motor ports. Either by removing the female header at the end of the existing servo wires and putting on an appropriate male header, or by simply creating a male header which can be plugged into the female header, modify the servo power cable so that it can be plugged into the Handy Board motor ports. Remember that the motor ports provide voltage and ground separated by one space. That is, the connector is three-wide.

	Open the case by removing the four long screws at the bottom of the unit. It should be easy to remove the bottom and the top of the housing.
	Remove the gear assembly, but be careful to remember how they were originally arranged.
	The next step will vary depending on what type of servomotor you have. Look for small screws that attach the drive motor to the housing. Hopefully there will be two small phillips-head screws going into the top of the motor, but there are no guarantees.
	Remove the motor and circuit board from the housing. You may have to apply a lot of force to the top of the potentiometer sticking out of the top of the housing in order to pop the circuit board out. You don't have to worry TOO much about breaking anything as we shall find out in a second that the circuit board matters very little. Be sure not to break any of the gears or the housing, however.
	At this point you should have a bunch of gears, a plastic housing, and a circuit board with a DC motor attached.

	Insert the motor (now without the control circuitry) back into the housing. You should notice that now the potentiometer will NOT protrude out the top because it is not there anymore :). Only the motor will protrude out the top of the housing.
	Fasten the motor as necessary (that is, put the proper screws back or do whatever steps are necessary to fasten the motor inside the housing).
	Before reassembling the gears, you will have to remove a little plastic "stop." It's a small nub of plastic on one of the gears that should obviously keep the gears from turning a full 360 degrees. Simply clip or cut this stop off with diagonal cutters or some sort of knife. Make the gear smooth so that there will be nothing keeping it from turning 360 degrees.
	Reassemble the gears and place them back onto the housing on top of the motor.
	Put the housing back together. You now have a DC gearhead motor.
	Try turning the motor. It should be able to turn 360 degrees with only the resistance of the motor. If there is any extra resistance, open it back up and be sure you removed the mechanical stop all the way.