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

Intro: 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.

STEP 1: Disassembling the Top and Bottom

• 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

• 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.

STEP 3: Setting the Servo Feedback Pot and Changing Its Orientation

• Turn the feedback position pot from one stop to the other and then to its center. The flat sides of the pot shaft will be aligned with sides of the body at dead center.  The leads on the pot in the picture were partially bent before I remembered to capture the image.

• The pot will be physically repositioned in the next step.  

Before proceeding, look inside the empty case and note the square cavity for the pot on one side.  Keep in mind that the body of the pot will have to fit into that cavity when you reassemble the servo after repositioning the pot.

STEP 4: Repositioning the Servo Feedback Potentiometer

• Use a pair of needle-nose pliers to bend the leads of the pot such that the shaft points down towards the board and is nestled in next to the electrolytic capacitor.  I found that first straightening the leads and then bending them as needed worked best.  Align the pot body so it will fit into the case cavity upon reassembly.  The springiness of the leads will forgive a little misalignment.   Get the pot down so that the shaft almost touches the board.  Be sure the leads do not touch the metal capacitor body.  

STEP 5: Removing the Gear Stop Nub

• Select the tall gear with a slotted hole and a metal bearing on one end.

• Cut the stop nub off the gear, being careful not to damage the gear teeth.  The nub must be completely removed down to the surface of the gear.  I used a sharp Exacto knife to clean up the surfaces after nipping off most of the nub with side cutters.  A jeweler’s file should also work well for cleaning up the surfaces.

STEP 6: Reassembling and Kicking Back

• Slide the board assembly back into the case, being careful that the potentiometer fits into its cavity.

• Put in the two small screws on either side of the motor shaft and reassemble the gear train.

• Put on the top and bottom of the case and secure them with the four long screws.

Congratulations, you’re done!
Any commanded position ahead or behind center will cause the servo to continuously rotate.

Restoration of servo function
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.

Comments

[egranlund]

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]

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.

[paradug]

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.

[danzee]

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 :)

[Gelfling6]

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!