Page 356 - Robot Builder's Bonanza

LEG POWER 325

Figure 27- 6 The Mini SSC II, a
popular serial servo controller capable
of operating up to eight R/C servo
motors simultaneously. (Photo courtesy
Scott Edwards Electronics.)

from a separate battery supply. When using this arrangement, be absolutely sure that the
ground connections of the two battery supplies are connected, or your robot will not function
properly.

USING A DEDICATED SERVO CONTROLLER
Legged robots require lots of servos. Rather than hogtie the robot’s main processor with the
job of running them all, you can hand it off to a coprocessor instead. That’s the idea of the
serial servo controller, or SSC (see Figure 27- 6). These compact circuit boards are designed
to receive one- time instructions from your robot’s microcontroller on which servos to activate
and what position to move them to. The SSC then independently controls the servos without
any further intervention by the robot’s controller.
It’s called a serial servo controller because the microcontroller communicates with the SSC
via a simple serial communications link. The link can be one- way or two- way. Most microcon-
trollers provide a simple command structure for sending serial data to another device. For ex-
ample, the BASIC Stamp offers the serout command, and the Arduino has a SoftwareSerial
library that can use any I/O pin as a serial line.

ON THE
Check out the RBB Online Support site (see Appendix A for details) for several hands- on
examples of using popular SSCs with the Arduino and other microcontrollers.
W E B
ANALOG VERSUS DIGITAL SERVOS
As you read in Chapter 23, the most common radio- controlled servos are analog. Digital
servos use onboard microcontrollers to enhance their operation. Because of the way digital
servos work, they tend to provide more torque and are naturally better suited for use on walk-
ing robots. Digital servos are more expensive (sometimes much more expensive), so you’ll

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