How to Replace a Shoulder Servo on the Hand-Eye System David S. Touretzky version of March 9, 2009 These instructions explain how to replace the shoulder servo on a Tekkotsu Planar Hand-Eye system. The instructions for replacing servos in other positions are similar. PART I: Preparation for Servo Replacement 1. Inspect the replacement servo to make sure it is in good condition. 2. The shoulder servo should have ID 1. Fortunately, this is the default ID for new servos purchased a la carte. However, if you're using a servo from a kit, or one that was previously installed in a different position, it may have a different ID. Therefore you should verify the ID and set it before proceding further. To do this, begin by disconnecting the servo cable that runs from the mast to the shoulder servo, and also disconnecting the servo cable from the mast to the pan/tilt. To remove a servo cable, pinch all three wires between thumb and forefinger at the base of the plug, and apply gentle pulling force until the plug pops out of the socket. You can also use a small straight-edge screwdriver to apply upward pressure on a flange of the plug to help ease it out of the socket. 3. Now all the servos should be isolated from the mast. Plug one of the servo cables from the mast into your replacement servo. It doesn't matter which of the two sockets you plug the cable into. 4. To verify the ID of the replacement servo, make sure that the hand-eye system is powered up but Tekkotsu is NOT running, and type the following in a shell: /usr/local/Tekkotsu/tools/Dynamixel scan 1 If no servo with ID 1 is found, set the servo ID by typing: /usr/local/Tekkotsu/tools/Dynamixel set all 1 Then repeat the scan to verify that the servo ID is correct. ================ Part II: Replacing The Shoulder Servo 1. Unplug the power adapter and USB cables from the interface module on the back of the mast, and move them out of the way. 2. Unplug the servo cables from the shoulder servo. To remove a servo cable, pinch all three wires between thumb and forefinger at the base of the plug, and apply gentle pulling force until the plug pops out of the socket. You can also use a small straight-edge screwdriver to apply upward pressure on a flange of the plug to help ease it out of the socket. 3. Loosen the C-clamp holding the mast to the table, and gently lay the mast down on the table, taking care not to bash the pan/tilt and camera into the tabletop. 4 Using a small Phillips head screwdriver, remove the four small (size M2) screws that fasten the shoulder servo to F3 frame (i.e., the bracket) at the base of the mast. Put these in a dish so you don't lose them. If you have trouble getting the screws to pop out once you've unscrewed them, you may need to use a tweezers or pliers to extract them, or use a small straight-edge screwdriver to ease them out. 5. Rotate the arm so that it points straight out from the shoulder servo. You should notice that a notch at the top of the servo horn (the round piece that rotates when the servo runs) is aligned with the notch at the top of the servo housing. 6. This is the only tricky step of the proceudure. Using a small Phillips head screwdriver, unscrew the four screws that connect the F4 frame (long U-shaped bracket) to the face of the servo horn. The tricky part is the following: these screws are threaded into nuts that rest in niches inside the servo horn. The screws are fixed with Loctite thread locker, which makes them hard to turn. Because they're hard to turn, you will want to bear down on the screw, applying pressure to keep the screwdriver engaged with the screw head. But if you keep applying this pressure as you back the screw out, you will pop the nut out of its niche inside the servo horn. Now you have loose nuts rattling around inside the servo horn, and when it's time to reassemble the arm and you put the screws back in, there will be nothing for them to engage with. Typically only one or two nuts pop out, so you may get half-way through reassembly before noticing that the screw you're trying to insert keeps turning forever instead of tightening up. If this happens, you will have to remove the servo horn, put the nuts back in their niches, reattach the servo horn, and then proceed with reassembly. See instructions below for how to do that. If you can avoid pressing down hard on the screw when removing it, you won't need to take those extra steps. 7. Using a medium Phillips head screwdriver, unscrew the large (size M3) screw that attaches the bottom flange of the F4 frame to the back side of the servo. Then pull out the plastic bushing that this screw held in place. The servo should now come free from the frame, and a little white plastic ring will drop onto the table. Put these components in your dish with the others. Put a piece of colored tape on the shoulder servo to mark it as "bad', and set it aside. 8. Examine your replacement servo to make sure that the notch on the horn is aligned with the notch on the case. If not, grasp the horn firmly and slowly rotate it into the correct position; this takes a fair amout of pressure. The alignment doesn't have to be precise; anywhere within +/- 10 degrees is fine. 9. Insert the white plastic bushing into the bottom hole of the F4 frame. Drop the plastic ring onto the bushing. Slide the frame onto the servo; you may feel a slight pop as the white ring snaps into position between the frame and the lip of the servo. 10. Apply a drop of Loctite thread locking compound to the large M3 screw, insert it into the bushing, and screw it into the servo. Looking from bottom to top, the screw should be going through the bushing, then through the F4 frame, then through the white ring that sits between the frame and the servo, and then into the hole in the back side of the servo. Screw it down firmly but don't overtighten it. Warning: Loctite will eat some types of plastic, so use a glass dish or cardboard plate to hold your parts, and don't let the Loctite get smeared onto other random surfaces. 11. Rotate the arm so that it sticks out straight from the shoulder servo. Then apply Loctite to four small M2 screws, and use them to fasten the top flange of the F4 frame to the servo horn. Note 1: You may have to rotate the arm by a few degrees to get the first screw to drop into place in the servo horn. But you should not have to rotate it by much. If you rotate by close to 90 degrees then the screw will go into the wrong hole and the arm will be out of alignment. Note 2: If you turn a screw and it never gets tight, then the nut has popped out of its niche inside the horn and you will need to undo steps 9-11, fix the problem, and then repeat those steps. 12. Once all the screws are tight, gently move the arm back and forth a bit to verify that the shoulder turns smoothly. If you hear a squeaking sound, you've overtightened the M3 screw at the back of the servo; loosen it by a tiny bit. 13. Insert the shoulder servo into the F3 frame at the base of the mast. Apply Loctite to the remaining four M2 screws and use them to fasten the shoulder servo to the frame. 14. Plug the servo cables into the shoulder servo. It doesn't matter which cable goes in which slot. If you previously disconnected the pan/tilt because you were setting servo IDs, reattach that servo cable as well. 15. Stand the mast back up and fix it in place with the C-clamp. Plug in the power adapter and USB cables. 16. Run Tekkotsu and use the Arm Remote Control to verify that the arm is functioning correctly. If not, see the Hand-Eye Troubleshooting guide for next steps. ================ Part III: Fixing the Nuts In a Servo Horn 1. Remove anything that is partially attached to the servo horn. 2. Rotate the servo horn so that the notch in the horn is PRECISELY aligned with the notch in the case. 3. Using a medium size Phillips screwdriver, remove the large screw at the center of the horn. Note that this is not the same kind of screw as the M3 screw that goes into the back of the servo; it's slightly smaller. Keep the two separate so you don't confuse them. 4. Using a small straight-edge screwdriver and your fingers. gently pull the horn loose from the servo. Pull it straight out; do not allow it to rotate. If it does rotate, then rotate it back so the notches are properly aligned. Warning: when you pull the horn off, nuts may drop out; be prepared for this so you don't lose any. If you do lose some, the M2 nuts that come with the replacement servo can be used to replace them. 5. Turn the horn over and reinsert any loose nuts into their niches. Use a small Phillips head screwdriver to press the nuts into place. 6. Place the horn back on the servo shaft, making sure that the notches are properly aligned before pressing the horn into place. Reinsert the large screw that holds the horn fixed on the shaft. It is not necessary to use Loctite with this screw.