February 1955 Popular Electronics
[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular
Electronics was published from October 1954 through April 1985. All copyrights are hereby acknowledged. See all articles from
Early "servos" were nothing more than the electromechanical
equivalents of rubber band-powered escapement. Rather than energizing
a solenoid that would allow the rubber band to turn the control
arm, the pulse signal from the receiver would set a motor in
motion, and then limit switches would stop it once the predetermined
position was reached. They had a number of advantages over rubber-powered
escapements in that the power delivered to the control surface
was not diminished with every actuation (except from some negligible
energy drain from the batteries), they were able to deliver
a lot more power, and they took up less real estate inside the
fuselage. It was a first step toward today's proportional servos.
Compound Escapements & Servos
by E. L. Safford, Jr.
The more advanced the radio-control hobbyist becomes, the
more functions seeks to control on his model airplane, boat,
or racing car. To effect multiple control operations, complicated
escapements have been designed. Some of these will be explained
in this article.
The escapement illustrated in Fig. 1 was created in order
to obtain direct control over two physical functions using a
single-channel type receiver. In model aircraft for example,
it is used to control both rudder and elevator. One particular
advantage of this unit is that it only deflects the control
surfaces in the "on" signal positions and returns both to neutral
in the "off" signal positions. This is a safety feature of considerable
Fig. 1. A type 2 compound escapement in which
one standard two-finger escapement is used to actuate two separate
control loops via a cam.
Notice that this escapement consists of two two-finger escapement
arms and catch- points located on a single plate with a cam
mounted between them. The cam has a pin extension C. In the
drawing, a spring is shown holding catch-points 2 and 3 firmly
against the cam wheel causing 1 and 4 to rise up and intercept
fingers A and D. This is the starting neutral position.
If now, a signal is sent causing the standard two-finger
escapement to move finger F to position 6, the cam is caused
to rotate 90 degrees counterclockwise pushing catch-point 2
forward causing catch-point 1 to move down, releasing A. As
A rotates clockwise, finger B is intercepted by catch-point
2 and held. Loop N is deflected to the left.
Now assume that the signal ceases. The arm on the standard
escapement rotates another 90 degrees until G is intercepted
by 5. Through loop L, the cam is rotated another 90 degrees,
allowing catch-point 2 be pulled back by the spring and forcing
1 up to intercept B. Loop N is again at neutral.
Another "on" signal causes the cam to push catch-point 3
forward to intercept E, deflecting loop M to the right. An "off"
signal sends the cam back to the starting point, leaving E held
firmly by 4 and loop M again at neutral.
Tracing the action through a second evolution of the cam
will show that first loop N is deflected to the right, then
brought to neutral, then loop M is deflected to the left, and
brought back to neutral. It has taken two complete revolutions
of the cam to go through the complete cycle of operation for
Although it is necessary to go through the complete cycle
in order to effect any one desired control and return to the
original starting position, this is not necessarily a disadvantage
if the operation is done fast enough. Other surfaces may move,
but will not be held in a deflected position long enough to
affect the steering.
Another multiple escapement, this one sold by ECCO, is illustrated
in Fig. 2. The back cover has been removed in order to reveal
the special working parts. It, too, allows control over two
physical functions. It is a single, self-contained unit requiring
but a single rubber band for power.
The movement of the two output shafts is obtained through
the use of the two specially shaped cams, the two "Y" yokes,
and two sets of "four-fingers" which engage the armature catch-points.
The two catch-points are offset from each other so that only
one set of fingers contacts the "on" catch-point, the second
set contacting the "off." This is illustrated in Fig. 3.
If the outer cam of Fig. 2 were to rotate 90 degrees clockwise
because of rotation of the fingers, the protrusion of the cam
would force the left half of the "Y" yoke to the left. At the
same time the indented portion of the cam is moved opposite
the right half of the yoke, allowing motion to the left. Another
quarter revolution returns the "Y" to neutral. The third quarter
revolution moves the "Y" right, and the final 90 degrees returns
the "Y" to neutral and the cam to its original starting position.
This action has taken place in four steps.
Since there are two sets of "four-fingers," mounted as illustrated
in Fig. 3, there are actually eight catch positions instead
of just four. Thus, with the proper placement of the two cams,
the signal sequence is as shown in Table 1 .
Notice that the deflection of the yokes occur only in the
"on" signal positions. This again is a fail-safe feature.
Fig. 2. (left) Back view of an ECCO compound
escapement which uses two four-finger units and two cam wheels
to actuate two "Y" yokes, each with its own shaft. This allows
two control functions.
Fig. 3. (right) Simplified drawing of the
ECCO compound escapement showing how the four-finger units operate
separately with a single solenoid.
First, to remove any mystery that might be connected with
the word servo, this word comes from the Latin and means slave.
Since slaves were once used to do the hard physical work, the
word in modern electronic parlance has come to mean the unit
which furnishes physical power in a control system; in particular,
The De Bolt 2PN "Multi-Servo" shown in Fig. 4 is an electric
motor with associated reduction gears, switches, and anti-coast
device. It can be used in the same manner that the standard
two-finger escapement is used to move control surfaces. The
designation 2PN indicates its operation. This stands for "two
Fig. 4. Top and side views of a typical servo
unit for radio control. Motor is about 1" wide.
Examination of Fig. 4 will reveal the small brass terminal
on the face of the fiber disc to which the linkage to the control
surface is fastened. Notice also the two brass switches mounted
below and behind the output disc, and the small fiber pins on
the inside of the fiber disc which open the switches at the
Comparing this unit to escapements highlights the following:
1. No rubber bands are required.
2. Servos are small and lightweight (2 ounces).
3. They have fast response and are reliable.
4. They require conservative battery power (2 pen cells
or their equivalent).
5. They are failure-safe in case if receiver failure.
6 They consume no power in any control or neutral position.
7. They are rugged and compact.
8. They have a simple and definite control sequence. The
sequence is easy to remember and transmit using a standard pushbutton,
it is "off" for neutral; "on" for left; and "on-off-on" for
Fig. 5 shows a typical wiring setup for a servo. Notice the
fiber disc and the fiber shafts labeled N, R, and L. In the
position shown, neutral, shaft N is depressing switch 2, opening
its connection to D. The shaft does not open switch 1 since
this switch is located lower than switch 2 and shaft N is placed
nearer the center of the disc than L or R.
Fig. 5. Physical drawing indicating how a
servo may be used in a radio-control model airplane for proportional
control of the elevator. It can be used also for the rudder.
Let's trace through a signal sequence to see the operation.
First an "on" signal is transmitted. The armature of the receiver
relay F is pulled down to make contact with G. This lets power
flow from the battery through the relay to B of switch 1. B
is making contact with A, so the power goes through this switch
to one side of the motor. The second motor lead is permanently
fastened to the battery, so the motor runs, turning the fiber
disc in a clockwise direction until shaft R opens switch 1,
breaking the circuit and causing the motor to stop. As the fiber
disc rotates, it moves loop Z to the right, giving down elevator.
As long as the signal is sent, the loop will remain deflected,
but since the circuit to the motor is open, the servo consumes
Now the signal ceases. The receiver relay armature moves
from G to E. Power now flows through switch 2 to the motor,
causing it to run again. It runs until shaft N opens switch
2, opening the circuit. Loop Z is back to neutral.
To get up elevator, the sequence is "on-off-on," holding
the second "on" as long as this control position is desired.
It is important that the sequence be sent at a medium rate and
not too fast. The first "on" causes the motor to run as before,
but instead of stopping, the "off" signal comes and routes
power through switch 2 while switch 1 is being opened up by
the shaft R. The second "on" comes as L approaches switch 1,
and since this signal is held, the motor stops running when
L opens switch 1. The loop Z will now be right, giving up elevator.
When the signal stops, the motor again returns the loop to neutral.
While the illustrated action has concerned the elevator, it
should be mentioned that it works as well for rudders or steering
Posted July 25, 2011