Instructables

Reversible No-Soldering Modification of Parallax Standard Servo to Continuous Rotation

Picture of Reversible No-Soldering Modification of Parallax Standard Servo to Continuous Rotation
I needed to modify a number of Parallax standard servos to continuous rotation for a kids' robot workshop project.  I came up with a method that requires no soldering and allows the servo to be changed back to position operation if desired.  The following procedure is specific to the Parallax servo although the concept may work for other brands if their construction allows it. 

What I did was to adjust the position feedback potentiometer (pot) to its electrical center and reposition the pot so it was mechanically decoupled from the gear train.  I also cut the stop nub off one of the gears. After modification, any commanded position ahead or behind center causes continuous rotation. You can return the servo to standard service by restoring the pot to its original orientation.  A small plastic pin or screw stop can be added if desired.
 
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Step 1: Disassembling the top and bottom

Picture of Disassembling the top and bottom
Servo Apart 3.jpg
NakedTop.jpg
NakedBottom.jpg
• Unscrew and remove the servo horn if attached. Set it aside.

• Remove the four long screws that hold the servo assembly together.

• Pull off the bottom and top covers. Be careful.  Some of the gears may come out when the top cover is removed.  They are often stuck together with the lubricating grease.

• Remove the gears. The large gear at the bottom may be left in place. Preserve as much of the grease as possible.

• Remove the two short screws that were exposed when the gears were removed.  This detaches the board and motor assembly from the case.

Step 2: Removing the board

Picture of Removing the board
• Carefully pry the board up around its edges until you can slide out the entire assembly.  It may take several rounds of prying a little at a time before the board comes loose.  Set the case aside.

• Notice that the servo feedback pot is mounted on long springy leads.
egranlund1 year ago
Thanks for this!

I had a lot of problems getting the pot to stay centered when I bent it back like you mentioned.

Upon further inspection I noticed you can remove the little "catch" that grabs the top of the pot out of the gear that has the nub on it. It just slips right off. This makes it so you don't have to even crack open the bottom half of the servo. I ended up doing this and now it works great :D

Just have to be careful not to loose that piece :)
danzee (author) 1 year ago
Thanks for the info. I include a selectable wait period (default 5 secs) at the beginning of all my Bot programs to allow placement or alignment before the Bot stars running. WIth CR servos there is really no zeroing to be done. I certainly agree there is utility in having a zero commanded initial period with position servos. I did that on a walker to get the legs set right, but I'll keep it in mind for the next project with externally tweakable position servos. Cheers.
I have done similar modifications on the T-Pro SG-5010 servos from Hobbypartz.com.

One small refinement is to cut a hole in the back of the case and position the potentiometer's shaft  out the hole. This allows you to zero the servo externally, similar to the Parallax continuous rotation servos.

Since the servo zero can drift slightly over time due to voltage and temperature differences this is a handy way to zero your robot before a run. When using servos in a robot, I always include a small initialization routine in the beginning of the robot's program that checks the zero position of the servos. The routine places the servos at "zero" position for a given time or until the start button is pushed to allow a zero check.
servo_mod.jpg
danzee (author) 1 year ago
Thanks for the info. We had a quantity of standard servos and needed continuous rotation ones. Otherwise I would certainly have bought the Parallax/Futuba CR ones you mention. Scrunching the pot around to the side with a hole for trimming is a nice touch. I guess the back would work too. However, I command the modified servo with either 1 or 2 ms for reverse and forward, so trimming the pot shouldn't be necessary for me. I find it goes full bore except near center where I can make it growl at slow speed :)
Gelfling61 year ago
I've done similar, converting them to constant rotation.. Actually, they're not proprietary Parallax's.. they're actually re-labeled Futaba S3004's, . (identical inside!) they simply remove the POT, grind/cut out the stop nub, and replace the POT with a 5K PC-Mount POT ( http://i00.i.aliimg.com/wsphoto/v0/692709740/Free-shipping-50pcs-vertical-mount-font-b-10k-b-font-font-b-Ohm-b-font-Mini.jpg as example) for their CR's.. (such as the BOE-Bot's main drives, which I replaced with two modified 3004's).. the modification I do, no soldering, I drill a hole through the side of the case, about mid-point of the side with the POT, then "Z"-bend the connection strips, so the POT sits 90-degrees, and the post will protrude through the hole in the side.. result, instant trim,) I still had to grind down the stop, same as you.. It's amazing, how much space is open inside!