"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.
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.
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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.
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 merit.
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
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
Fiq. 1. A type 2 compound escapement in which one standard two-finger
escapement is used to actuate two separate control loops via
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
Fig. 3. (right) Simplified drawing of the
ECCO compound escapement showing how the four-finger units operate
separately with a single solenoid.
Fiq. 4. Top and side views of a typical servo unit for radio
control. Motor is about 1" wide.
Fiq. 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.
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 both loops.
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.
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, motors.
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 positions-one neutral."
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 proper time.
Comparing this unit to
escapements highlights the following:
1. No rubber
bands are required.
2. Servos are small and lightweight
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
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 right.
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.
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 no power.
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 wheels.