Introduction: Modify a Futaba S3001 Servo for Continuous Rotation

In this instructable I well show you how to modify a Futaba S3001 dual ball bearing servo for continuous rotation. Why you might ask, you can get already modified servos from Parralax? Two reasons, one I like to tinker with things and two my local hobby shop had a box of these for $15 each so I probably saved a couple of dollars by not having to pay for shipping.

Step 1: Materials and Tools

Materials:
1 - Futaba S3001 servo

Tools:
- #1 Phillips screwdriver
- Dremal with a cutoff wheel
- Needle nose pliers

Step 2: Disassemble

Lets begin by disassembling the servo:
1) Use the screwdriver to remove the four screws holding the servo casing together.
2) Gently separate the two halves of the case being careful not to lose any of the pieces.

Step 3: Modify the Servo(disengaging the Potentiometer)

To get a servo to rotate continuously you have to do two thing, disengage the potentiometer that tells the servo the position of the control arm and remove the mechanical stop that keeps the servo from rotating too far and damaging the pot. To disengage the pot begin by looking at the inside of the top of the case where you well see three gears, two small and one larger gear with a metal ring in the center. If you notice, under that ring is a bit with a flattened out hole in it, this is what engages the pot to tell the servo the position of the arm. To remove it you simply have to pop the metal ring out and pop the bit inside with the flattened hole in it out. Once the drive bit is out replace the metal ring in the gear, this engaged the bearing.

Step 4: Modify the Servo(removing the Mechanical Stop)

Now that the servos electronics well allow it to continuously rotate we have to make it so that it well continuously rotate mechanically. All servos have a mechanism to keep it from rotating to far and damaging the pot, no longer a problem in our case.
- To begin, remove the screw holding the control horn on and pull the control horn off the out put shaft.
- Now that the control horn is removed, slide the output gear out of the top of the case by pushing on the shaft until it pops out.
- On the shaft you well notice there is a bit that protrudes from the shaft, if you look on the inside of the case where the gear and shaft were there are two bits that stick out and catch the bit that is protruding from the shaft, this is the mechanical stop.
- Take the Dremal with the cutoff wheel and carefully cut the bit that is protruding from the shaft away, being careful not to damage the gear itself.
- Put the gear back into the top of the case and check that it rotates all the way around without binding.
- If it doesn't bind carefully reassemble the servo, taking care to get the gears and shafts lined up properly and your done. If it does bind up still, you well need to remove more of the protruding bit and try again.

Comments

author
Hoolaput made it!(author)2009-07-21

awesome this is one of the simplest servo mods i've seen one question does it work for all servos???

author
Ohm made it!(author)2009-07-21

The principal is the same but each servo is different so the stop mechanism well be different for each.

author
Hoolaput made it!(author)2009-07-21

how much does these things normally cost?

author
Gelfling6 made it!(author)2011-10-02

Standard servos, anywhere from $12.95 (USD) to $50.95 (USD) (depends on gearing, torque, size.) I've gotten Futaba S3004's (standard torque, ball-bearing) for $14.95 (USD) at a local hobby store. Parallax charges $12.99 (USD) each, equivilent to the S3001's, potentiometers are changed to a small, PC mount 270-degree turn 5K potentiometer, which you can adjust with a small flat-blade screwdriver through a hole drilled through the side.

author
mrgreencool made it!(author)2009-10-24

I Don't think it will work with servo mg995 (steel gear), i did remove the blocker...but once i remove it, when it turn the rheostat in the servo will follow the cycle and break the wire...and walaaaa i need to buy a new one!!!...-_- ....rheostat can move 180 deg only...

author
Gelfling6 made it!(author)2011-10-02

As others have said, you need to remove the connection between the final drive gear, and the potentiometer (rheostat, refers to wire-wound, mainly for AC voltage, but you've got the right idea.) This will entail removing the potentiometer completely, or the inner workings, so it can rotate without hitting stops. (and replacing the wiring to it, with a pair of 2.2K resistors, in series, but the center connection to the 'wiper' wire for the pot.

author
manmelvin made it!(author)2011-05-10

Can the servo direction be reversed? I just did it with one of mine and it spins fine one way but just buzzes when I try and reverse the direction.

author
Gelfling6 made it!(author)2011-10-02

As the other person below mentioned, One missing step. The potentiometer (under the final drive gear) needs to be replaced with a pair of 2.2K resistors to simulate the potentiometer being centered. I've modified Futaba 3004's (same as the 3001's, but ball-bearing on the final drive gear.), by "S" bending the leads to the potentiometer, and sticking it out the side of the case, so I can tweak it by hand. It sounds like, if you didn't replace or center the potentiometers, they're still in one direction (not at center).. The dremel is a bit overkill.. Most of the time, a pair of diagonal cutters and a file (or careful whittling with a utility knife) to get rid of the stop point.

author
dwalton made it!(author)2010-08-09

Nice job on the pictures! This is very well done.

Unfortunately there is one more step to creating a continuous rotation servo. You need to hold the potentiometer in a centered position or replace it with a resistor divider. I learned this the hard way. If the potentiometer is not centered, the servo will turn with a 1.5 ms pulse when it should be still and will turn faster in one direction than the other.  If the potentiometer is turned too far, your servo will only turn in one direction, no matter how wide the input pulse is.

The Parallax continuous rotation servos have moved the potentiometer so you can access it from the side of the servo without having to disassemble it. This allows you to send a 1.5 ms pulse and adjust the potentiometer with a screwdriver so that the servo is still.

I think there is a simple solution to the problem.  I have been able to drive a 1.5 ms pulse to the servo with the gears exposed (like in step 3) and turn the potentiometer by hand until the servo was still.  I then reassembled the potentiometer and tested to make sure the servo was still with a 1.5 ms pulse.  There is still a chance the potentiometer could get vibrated or bumped into a non-centered position, but so far it has worked.

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