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Hacker.instanet.net • View topic - Continious Rotation Servo

Continious Rotation Servo

Anything Arduino

Continious Rotation Servo

Postby Tim Laren » Thu Nov 08, 2012 7:37 pm

Hobby Servos are great for steering a car or airplane and very easy to control from your Arduino. But they only provide about 180 degrees of rotation. The thing is these servos have a motor, gearbox and controller all in a little package. With a little modification you can make these guys an adjustable speed and by-directional drive motor. They are going to be reasonably strong but not too fast. Well depending on what servo you start with both of these values are based on the unit you pick. Some servos are fast and other are strong and some are a little of both.

This is the servo I am going to modify today:
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The servo has four screws on the bottom that holds the top cover and bottom cover onto the main unit.
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Remove these 4 screws being sure that the servo does not fall apart in your hands. The servo pretty much stays together but remember we have to put it back together when we are done the mod.
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We now have to remove the first, top gear. It is press fit on the shaft. I used a knife to get it off. Here is the top cover and the top gear.
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There is a potentiometer that senses the position of the shaft that we will work on later, but it would be damaged if turned too far. The servo has a stop to prevent damage, some in the cover but on this servo it is on the gear.
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You can see the stop, I circled it on red. You have to remove this without damaging the rest of the gear. I used a pair of sharp flush cutters but have also used a X-Acto Knife in the past.

You can see I removed the stop and cut it very flush, like it was never there to start.
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We need to remove the rest of the gears. They are loose on the shaft and can be simply lifted off. Remember the order they came off as we have to replace them in the same order. After removing the gears we, press out the potentiometer from the bottom.
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The potentiometer also has to modify. It has two stops that we will remove.
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It has three wires attached that you can either unsolder or cut. We will not be using these wires. I used my flat cutters to remove the wires and the tabs that the wires were soldered to also. I cut off the two stops the same way.
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The potentiometer also has a wiper that need to be removed.
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Now we need to work on the controller board. We start by removing the three wires.
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We will be replacing the wires with two resistors. We need two resistors of equal value. The potentiometer in the servo is 5K, and we want to be in the center position so 2 2.5K resistors will work well. Actually any two equal valued resistors will work. I used 2 3K resistors because there were handy and close to the 2.5K. We will solder the two together and connect the two separate ends end to the outside location where the wires were removed from. The lead where the two resistors are connected together will be connected to the center position of the three wires.
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Now press the potentiometer back into the main housing and replace the insulating tab (sticker) that was between the back side of the potentiometer and the controller board.
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Then put the controller back in, it goes at an angle with the ribbon cable on the side away from the motor and inserted first towards the back of the potentiometer. Make sure that the two black motor wires are not cut or shorted on the controller or motor and clear of the bottom cover. Replace the bottom cover allowing the ribbon cable to pass through the cutout on the bottom cover. Make sure there are no wires sticking out between the bottom cover and the main body.

Replace all the gears in the reverse order you removed them. Remember that the last, top gear is press fitted onto the shaft of the potentiometer. You will find some extra grease in the top cover that you can spread on the gears. Remember that the gears closer to the motor spin several times more then the output gear and require as much grease as possible.

Replace the top cover and make sure everything aligns. Now insert the screws. I little tidbit of knowledge: "If you first turn the screws backwards slowly, you will hear a little click when the screw is aligned with the threads. If you then turn the screw in the tighten direction, it will use the existing threads rather than cutting new threads in the plastic." This is important if you are opening and closing anything that has a plastic case and used self taping screws. If you open and close the case many time without doing this, all the plastic will eventually get eaten up and you will not be able to tighten the screws.

Here is the final product. You may want to park this servo in some way because it looks almost exactly the same as an un-modified servo.
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In my next article i will test the servo with an Arduino.
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Tim Laren
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