NIST Interschool Tactical Robotics Organization

NIST Interschool Tactical Robotics Organization

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Servo Modification

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How to Modify a Servo

We can control a robot using R/C technology, but it would usually require a MOTOR SPEED CONTROLLER, another set of BATTERIES, as well as a set of MOTORS. This adds up to extra weight and extra cost. One way to work around this problem is to use servos as drive motors.

When you buy a controller, you should always get something with more than 2 channels. I was being cheap so I bought a 2-channel AM radio, which means I do not have the space to expand and add weapons. Too bad. I think it's a good idea to invest in a good radio, as well as a couple of crystals, because they will never get broken and you can use them in each robot you build.

But be warned, servos are not designed to be drive motors for robots, so they will be slow. This means in a real competitive environment, you'll have the other robots running circles around you. However, it is a good place to start because many controllers come with servos anyways, and so you can put them to good use.

A servo is a computer controlled motor used (usually) to move the control surfaces on a model plane or the steering on a model car. It has an output shaft that rotates 90 degrees back and forth, based on the position of the stick on your remote. While this is fine for planes, if we want to use them to drive a bot directly, we need to do a little work on them.

A servo works by comparing the position of the output shaft to the position requested by your remote. If your stick is at 78 degrees, and the servo shaft is at 30 degrees, the electronics in the servo rotate the shaft until the two values match. There are 2 steps to modifying a servo. The first is to remove any physical stops that keep the shaft from rotating 360 degrees. The second is to fool the electronics in the servo into thinking the shaft is always centered. Once you do this, if the remote commands a position of 30 degrees, and the (fooled) servo electronics see 0 degrees (centered), the servo is going to rotate the shaft to match them. Silly servo. It doesn't realize that because of your modifications, it will never get there. So, it just keeps happily spinning. Same for the other direction.

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Name:

Frank (Tha-An) Lin

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Remember that there are many types of servos out there, and these directions are what I have done to modify my Futaba S3003 servos that came with my AM radio set. You can search for "servo modification" on the internet for directions for your particular servo.

1.) First, we take our servo, and open it up. We do this by unscrweing the screws located on the bottom of the servo. Be careful when you're taking it apart, remember to remove the control horn, first (that X-shaped thing on top).

2.) Then, you can look at all the complicated gears inside and go "wow, that looks complicated". Don't be scared to take them apart, just remember what position they were in originally. Try not to lose any of the gears. That would be bad.

3.) In these newer Futaba S3003 servos, the gear we need to look at is black, and it's got a little notch. We need to file that notch off because it stops the output shaft from turning 360 degrees. I used a triangular file that did the job perfectly. Try not to do what I did and come close to destroying the gear. Take it slowly and make it look pretty, because it can effect the gear's performance.

4.) In the servo, there is a potentiometer that tells the electronics where the shaft is pointing. Since we want the electronics to think the shaft is always centered, we need to remove anything that connects the potentiometer to the gears. I did this by reaming out the inside of the black gear using a triangular file. You need to make sure that the potentiometer shaft no longer catches on the gear.

5.) When the black gear can turn without catching on the potentiometer or the notches in the cover, we need to calibrate the servos. We do this by turning on the receiver and the transmitter. The servo should start spinning like there's no tommorow. Using your fingers, turn the potentiometer shaft until it calms down and comes as close as possible to being stationary.

6.) You can now quickly put the servo back together, making sure everything is in the right place. If the servos refuse to stop turning, you can use the manual adjustment sliders on the controller to fix the problem. Voila! You now have the perfect high-torque drive motor for your antweight robot.

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