Build Your Own Combat Robot

282 Build Your Own Combat Robot
FIGURE 13-2
Internal gears to a
standard R/C servo.
You’ll notice a small brass shaft from a potentiometer. The potentiometer is
used to monitor the actual position of the servo’s output shaft. The next step is to
cut the link between the potentiometer and the output shaft. By doing this, you
can trick the servo into acting like a gearmotor.
First, you’ll have to modify the output gear. There is a small, black tab on the
top of the gear, as shown in Figure 13-3. Use a sharp knife to cut the tab off. Make
sure you don’t get any cuttings caught in the teeth of this gear. Now turn the gear
upside-down, and look inside it. If you see a metal ring and a small removable elliptical
retainer plate that grabbed onto the potentiometer’s shaft, remove the
metal ring and then remove the retainer. After the retainer has been removed, replace
the ring back into the gear. This ring acts like a bearing, so be careful not to
damage it. Figure 13-4 shows what this configuration looks like.
If your gear doesn’t have this metal ring and elliptical retainer plate, then you’ll
need to cut off the output shaft of the brass potentiometer. Figure 13-5 shows how
to do this with a pair of wire cutters. A Dremel cut-off wheel would work also
here. Just make sure that no cuttings get inside the gearbox. Cut the shaft flush to
the top of the gear support.
After both of these modifications, the output gear should freely rotate 360 degrees.
Now it is time to calibrate the servo.
Remove the other two gears, and place them in the top of the servo case. Plug
the servo into an R/C receiver, and turn everything on. You will probably notice
the motor spinning in one direction. On the radio transmitter, move the stick to
the center position. Then with a pair of needle-nose pliers, rotate the remaining
output shaft from the potentiometer until the motor stops turning. At this point,
you have calibrated the servo to not move when it sees an approximate 1.5 ms
pulse width. Now if you move the stick on the radio transmitter, you will notice