THE WORKINGS OF RADIO CONTROL
article resulted from a workshop hosted by a member of our R/C model club,
the 495th R/C Squadron. I was asked to make a presentation “in
simple terms” on the workings of our radio systems. The modelers attending
this workshop comprised some newcomers to the hobby as well as some
“seasoned” members that were interested in learning how these unique
electronic marvels work.
have limited the article scope by not covering historical development of
R/C, recent state-of the art advancements, or extensive component level
transmitter is a generator of an alternating voltage.
Transmission of this signal through space occurs when connected to a
matching antenna. A quarter-wavelength antenna at R/C frequencies is about
3.4 feet long; at 60 Hz power line frequency, nearly 800 miles. One Hz is
one cycle per second.
signal, by itself, cannot send information except to indicate there is a
radiation source and it is coming from some point (a beacon). This
unchanging signal at radio frequencies is the RF carrier.
order to transmit information, some characteristic of the transmitted signal
must change; this is modulation. Modulation can be in the form of on-off
switching (keying) of the signal or changing its frequency; see figure 1.
|Figure 1. Two types of
Modulation for R/C
the modulation scheme is either amplitude (AM) or frequency (FM), the method
of transmitting control information is the same; i.e. modulating pulses that
determine control information causes either an amplitude or a frequency
change in the RF carrier. AM and FM are the types of modulation
commonly used in model radio control.
that we read in descriptions of radio control such as PPM (pulse position
modulation) and PCM (pulse code modulation) are not really methods of
modulation but methods of transmitting the control signals; PPM and PCM
transmitters use amplitude or frequency modulation.
correctly stated in these terms is the “P” for pulse. All control
information is sent in the form of either discrete on-off of the RF
amplitude in the case of AM or a step shift of frequency in the case of FM.
first multi-channel digital R/C systems, and most of today’s, utilize a
series of pulses to define the controlling signals for each servo channel.
Figure 2 depicts a typical train of pulses from a five-channel transmitter;
the total number of pulses in the train is one more than the number of
|Figure 2. Typical PPM
Transmitter Pulse Train
train of pulses is followed by a longer period; this is to identify which
pulse is the start of the first channel and is called the synchronization
pause or sync-pause for short.
systems, each pulse has a length in time of about a quarter millisecond (mS).
Spacing between the start of each pulse can vary between one and two mS and
defines the pulse width sent to each servo channel. Movement of the
transmitter’s controls change the position in time of the transmitted pulses
and result in change of the respective servo control pulses. Since the
pulse positions change to effect servo control, this is commonly called PPM
or pulse position modulation.
particular settings depicted in figure 2 show channels 1,2, and 4 at
neutral, channel 3 at maximum and channel 5 at minimum pulse widths. This
may be a snapshot in time where the positions of aileron, elevator and
rudder are at neutral, the throttle is at maximum and the landing gear or
flaps are at minimum.
particular control function that each channel controls is determined by the
transmitter manufacturer and varies among several brands of radios.
pulse train repeats at about 50 times per second. Figure 3a is a simplified
block diagram of a typical R/C transmitter. Most of today’s transmitters
use potentiometers, a resistor with a moveable tap point, attached to each
channel’s control. These vary the voltage applied to the encoder’s input
for each channel. The encoder converts these voltages into time delays that
change the position of each pulse in the train commensurate with the
controlled channel. The encoder’s signal output drives the modulator (AM or
FM) and the modulated signal with encoding is amplified and applied to the
antenna for transmission.
the simplest to the most sophisticated, so-called computer, transmitters,
the block diagram of figure 3a still applies; the difference is in the
encoder’s complexity. In computer radios, processing of the control inputs
allow many combinations of servo travel adjustment and neutral position
settings as well as various mixing options between the controls. Many of
these radios also have setup memories for saving several model
transmitters have encoders that convert the control inputs to a binary
number that is sent to the modulator as a series of pulses. This binary
number has all the information needed to define each of the servo channel’s
properties. The receiver decodes this information and sends the appropriate
servo pulse width to each servo.
3b shows how a “Buddy Box” trainer function operates. The trainer control
box has its encoder output connected through a cable and to a switch in the
master transmitter. When the instructor holds the trainer switch on, the
students encoder output connects to the master transmitter’s modulator thus
controlling the model. Upon release of the trainer switch, the instructor
|Figure 3. R/C Transmitter Block
from the transmitter excite the receiver’s input through the receiver’s
antenna. Receivers have means of providing frequency selectivity in order
to reject signals that are in adjacent bands. This selectivity is typically
in the form of resonant circuits tuned to the received frequency.
achieve high receiver gain without oscillation due to feedback from the
later amplifier stages, a superheterodyne receiver technique is used. This
type receiver uses a local oscillator to generate a signal that mixes with
the incoming signal to produce sum and difference frequencies between the
incoming and local oscillator frequencies. Figure 4a shows a block diagram
of a single conversion receiver. For our R/C receivers, resonant crystals
control the local oscillator frequency. A tuned circuit that follows the
mixer selects the lower frequency output of the mixer; this is called the
intermediate frequency (IF) and is typically 455 kHz (0.455 MHz).
|Figure 4a. Single Conversion
important advantage of the surperheterodyne receiver is improved
selectivity. Since the bandwidth of a tuned circuit relates to its center
frequency, use of a lower intermediate frequency provides an inherent
narrow-banding proportional to the ratio of the IF to the received
disadvantage of such a low IF is that the receiver may detect what is called
the image frequency that is located exactly the intermediate frequency on
the other side of the local oscillator frequency. For example, if the local
oscillator is 71.545 MHz and the IF is 0.455 MHz, frequencies of 72.000 and
71.090 will be detected unless a selective circuit suppresses the unwanted
signal. Single conversion receivers must have very selective receive
frequency circuits to reduce the image frequency response.
conversion receivers correct the problem of response to image frequencies.
Figure 4b shows a block diagram of a dual conversion receiver. In these
receivers, two local oscillators and two mixers are used. The first mixer
uses a local oscillator frequency to provide an IF of typically 10.7 MHz.
Since the image frequency of this mixer is 10.7 MHz away from the local
oscillator, the input selective circuits are much more effective in reducing
image frequency response. A second local oscillator and mixer follow the
first mixer; this typically provides a 455 kHz IF with its’ inherent narrow
|Figure 4b. Dual Conversion
Following the last IF stage, a detector (either AM or FM) converts the
modulation to a signal that replicates the transmitter’s encoder output to
the modulator previously described.
decoder circuit in the receiver follows the detector. This is typically an
integrated circuit microcontroller specifically designed to convert the
detected receiver signal into the individual signal pulses for each servo
are the muscle of our systems and perform a function similar to how a human
pilot moves the control surfaces in a full-sized aircraft. Each servo
control channel of the system provides dedicated control of specific
aircraft axes of movement or other function such as throttle (speed)
Mechanics of servos are not complex and comprise a DC electric motor, gear
reduction, and either a rotary arm or linear output.
such, operating the servo motor would cause the output to travel to its
mechanical stop. The motor would continue to draw current until
disconnected. Reversing the motor’s polarity would cause output arm motion
in the opposite direction to its other mechanical stop.
servo receives an input signal pulse width that can vary from one to two
milliseconds; a pulse width of 1.5 milliseconds is the servo’s neutral
position. These values will change to some degree by the transmitter’s
settings of servo travel end points and neutral trim.
Proportional control of the servo’s output requires the following:
A signal indicating the objective output arm position
Some indication of the actual output arm position
A method of determining the error between 1 and 2 and applying a
voltage to the motor of the proper polarity to reduce the error.
5 shows a simplified block diagram of a typical servo.
|Figure 5. Servo block Diagram
receiver’s signal input to the servo is a pulse that typically varies
between one and two milliseconds. This satisfies the first requirement in
servo’s output arm connects to a potentiometer that supplies a voltage to a
voltage controlled pulse generator; as the output arm changes position, the
generator’s is output pulse width also changes. This pulse width is an
indication of the servo arm’s position; this satisfies the requirement in 2)
comparator circuit compares pulses from the receiver and the voltage
controlled pulse generator and provides either a positive or a negative
output depending on whether the signal pulse width is larger or smaller than
the position generator’s output pulse. Application of this error voltage to
the motor driver circuit causes the motor to turn in a direction that will
minimize the error. When both pulse widths are the same, the motor receives
no voltage and the output arm position has reached the position indicated by
the receiver servo signal’s pulse width.
comparator circuit also implements a “deadband” function that prevents servo
jitter and hunting. This is a range over which differences between the
input and reference signals will not cause motor operation. When the signal
differences exceed this “deadband” range, drive to the motor occurs.
sophisticated servo circuits, using a modern microcontroller, eliminate the
voltage controlled pulse generator and replace it with an analog to digital
function. Measurement of the input signal’s pulse width, compared with the
potentiometer voltage value determines the motor control signal polarity.
“digital’ servos also have an advantage in providing a true step input to
the motor; it is either fully on or off. With older design “analog” servos
described above, the comparator output is a short pulse. This pulse repeats
approximately every 20 milliseconds and is especially short at small
differences between the two pulses. This represents a small duty cycle (the
percentage of on-time); the comparator output therefore employs a pulse
stretcher that increases the duty cycle to the motor. However, at small
deviations between signal and reference pulses, the pulse stretcher cannot
maintain the required drive voltage and the motor receives pulses of voltage
that can be heard as buzzing when the servo is driving a load. The result
is that at small deviations from the objective, the servo cannot maintain
its holding force and the servo arm will deflect from its objective position
until sufficient current to the motor occurs.
with digital comparators provide full current to the motor whenever the
input signal is outside the deadband range thereby providing more accurate
positioning under mechanical load.
Today’s R/C radios have
reached a degree of sophistication and reliability undreamed of in the
beginning days of radio control modeling. They have become the modeler’s
“Plug-and-Play” component. Hopefully, for many, this article has provided
some insight into how R/C systems work.
Posted: Dec. 18, 2003.