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chapter 7

Servo Motors and

Extending the Servo

Control System

Introduction

In this chapter, I will show you what makes up a standard R/C servo motor, as well as how
to control one. I have already provided a considerable amount of information in the three
previous chapters on how pulse signals are used to control servos. Now it is time to reveal
the inner workings of a servo motor so that you will understand how and why it operates as
it does and be aware of its limitations and constraints when you are using it. This “reveal”
will focus on how a specific pulse width translates into a specific servo-motor motion.
I will also discuss how a standard servo motor can be converted into a continuous rotation
(CR) servo motor. CR motors operate a bit differently than standard ones do. The CR motor
has the pulse width that directly controls the continuous angular speed or rotation instead
of providing a limited angular motion as the standard servo motor does. CR servo motors
are often used as replacements for conventional motors in which low torque requirements
exist, such as for powering small R/C cars or boats. I always use CR servos to power my
robotic projects, and they seem to function quite nicely.
The next section describes how I built a system to measure the pulse widths for up to
three of the R/C channels. I will also show you how to display the results on a 4 × 20 LCD
character display. This system uses the Parallax Board of Education (BOE) and can be made
totally portable by powering it all from a standard 9-V battery. My discussion of the software
includes quite a bit of information regarding pointers and indirection, which are often a
source of confusion for beginning programmers. Also, in the software, I point out how a Spin
program measures pulse width in a way that has not been previously shown in this book.
This chapter concludes with a discussion on two ways to extend the standard servo-
control system to accomplish functions that enhance the Elev-8 platform. The first one
controls the onboard LED lighting strips mounted on the bottom of the Elev-8 booms. The
second one controls a tilting mechanism for a remote-controlled, first-person viewer (FPV),
which can be attached to the bottom of the Elev-8. The actual FPV will be described in a later
chapter. Now, I want to focus on only the servo-control aspect of this system.

Exploring a Standard R/C Analog Servo Motor

Figure 7.1 is a partially transparent view of the inner workings of a standard R/C servo
motor. I would like to point out five components in this figure:

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