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324 BUILD ROBOTS WITH LEGS

Full load on
motor shaft Share of load Reduced load

Yoke
Direction Figure 27- 5 A yoke mechanism
of force
distributes weight or force so that
it’s not all against the shaft of the
motor. This helps the motor work
Direction
of force more efficiently and last longer.
robots that see only occasional use and aren’t too heavy. But it’s something you’ll want to
consider as your walkerbots gain weight or are frequently used for show- and- tell.
The most common way to alleviate the strain on the motor is by using a yoke or double- sided
bracket (see Figure 27- 5). Here, the load on the motor is distributed evenly between the motor
shaft and a secondary (and passive) shaft. The two shafts are in line with one another. The
second passive shaft can be a steel or aluminum rod, or even a machine screw. The shaft may
use ball bearings or bushings to ensure unimpeded rotation.
Note that the X- Y joint plans that appear later in this chapter are sans support. You can
add a yoke- style arrangement with nothing more than a third piece of plastic (or wood if that’s
your material of choice) that screws onto the bracket opposite the motor shaft.

Leg Power

When it comes to the muscles of a walking robot, radio- controlled servos are the preferred
choice; they’re compact and widely available, and they require no special interfacing or drive
electronics connected to your robot’s central computer.
Where rolling robots can make do with just about any standard- size R/C servo, legged
robots need a bit more attention to the details. Specifically, you need servo motors that pro-
vide enough torque to lift the legs and move the robot. It’s not unusual for a six- legged hexa-
pod to weigh several pounds. With underpowered servos, the robot will just sag to the ground,
unable to sustain its own weight.

CONTROLLING SERVOS
Recall from Chapter 23, “Using Servo Motors,” that servos require a special pulse signal in
order to operate. Typical operating voltage for servos is between 4.8 and 6.0 volts, though
many robot builders push to the edges of the envelope and supply 7.2 volts— this increases
the torque of the motor by as much as 30 or 40 percent.
Overvolting an R/C servo motor has the potential of burning it out, so you must exercise
care if you wish to experiment with this technique. Some brands, and even models within
each brand, are more tolerant of the higher voltages than others, so you need to experiment.
When using a higher- than- normal voltage with your servos, give them a periodic touch test to
make sure they aren’t overheating. Immediately disconnect the servo if it seems to be getting
too hot or is putting out an unusual smell.
Because of the heavy demands of the motors in a walking robot, you need to run them

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