PHOTO 2. Gear
hardware and the values of other channels. The pulses do
not all come out at the same moment, however. They are
staggered. On the JR, it’s sequentially in channel order. The
staggering is not necessary electronically but nice for
current loading on the servos so they don’t all jump at the
Anyway, each pulse is divided into two segments. We’ll
call these a ‘base’ pulse and a ‘data’ pulse. The base
segment is always the same duration in time. In the case of
the JR-XP6102 radio, it’s about 900 microseconds measured
from the beginning of the 0 to 1 transition. The data
segment stretches the base from 0.9 µs up to 2. 4 µs. The
time period of the data segment is proportional to the
deflection of the servo. A shorter data pulse yields a smaller
angle of deflection. Figure 1 shows this graphically.
Let’s jump from the servo all the way back to the
joystick on the R/C transmitter. The joystick is attached to a
potentiometer which creates a voltage proportional to the
stick deflection. Through the magic of electronics, this
voltage disappears into a set of converters and time
multiplexing circuits, then out the transmitter on an RF
signal. Your receiver picks up the signal, sorts it all out, and
generates the PWM pulses out of the appropriate channel.
FIGURE 1. Servo deflection with PWM.
So, this whole system changes the voltage at a
pot on the transmitter into a sequence of PWM pulses on
the receiver. On my JR transmitter — and most other six or
greater channel systems — there are channels that can be
devoted to a binary type signal. I say ‘can be’ because you
have to set up the transmitter to do it. These channels can
also be slaved to other analog control channels. For
example, in a helicopter configuration, the throttle channel
might be slaved to the collective so as the blade angle of
attack increases, the throttle is opened up to add power.
However, for a simple airplane configuration these
channels could be assigned to the gear and flap control.
The function of these controls is to extend and retract the
landing gear or flaps of an R/C plane. We typically don’t
put landing gear halfway down or partially up, so they
really only need two positions. They are driven by a two-position toggle switch on the transmitter. With only two
positions, the PWM signal coming out of the receiver will
only have two data segment lengths: long and short. Those
are now 1 and 0.
The transmitter functions as a free-running loop that
SCHEMATIC 1. Transmitter/Encoder CPU.
56 SERVO 04.2010