April 1959 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Servomechanisms are all around us in the form
of electronically actuated controls for our cars, stepper motors in our ink jet printers, overhead garage
door openers, and anywhere else you can identify where a combination of electricity and mechanics operates
with some form of positional sensing and feedback. The author of this article in one instance declares
a control circuit with a human operator as part of the operation as being "open loop," but I contend
that the human element is part of the loop and therefore constitutes a component in the effectively
"closed" loop, albeit not strictly a pure electromechanical system. Those of us who operate
radio-controlled model airplanes, boats, cars, etc., are very familiar with servos
for moving control surfaces as commanded by the transmitter's joystick position.
After Class: What is a Servomechanism?
By Harvey Pollack
Before the age of electronics, machinery was controlled directly by the hands of the operator-hands
which would shift a gear, pull a lever, or apply a brake. Today's machinery is controlled by the push
of a button or the twist of a knob, and many measurements and decisions are made automatically by electronic
circuits. Wherever electronics and machinery work together we are apt to find some kind of servomechanism
An automobile driver is the human counterpart of a servomechanism - he watches and controls a machine.
As he steers his car along a winding highway, he constantly makes small corrections on his steering
wheel to keep the car on the road. His eyes measure distances, his brain makes simple decisions, and
his arm muscles exert corrective pressures on the steering wheel. A servomechanism performs the same
function, but it is faster, more sensitive, does not make mistakes in judgment, and operates continuously.
The definition of a servomechanism recommended by the Feedback Control Committee of the American
Institute of Electrical Engineers is: "A feedback control system in which the controlled variable is
mechanical position." Let's examine, in practical terms, what that means.
The ABC's of
Consider our automobile driver again. From his reactions we can determine the
requirements of a machine that could capably replace him.
Before the start of the drive, he accepts
the fact that the road divider must always remain from six to fifteen inches from his front left wheel
if he is to navigate the highway turns and straightaways safely. During the trip, he must be aware of
the separation that actually exists during every instant of time. Then he must compare the actual separation
between his wheels and the white line with the desired separation that was initially stipulated. This
might be called the error in the car's path at that instant. Once the error is determined, he must then
dictate a corrective order to his arm muscles so that they can apply a force in the proper direction
to eliminate the error.
If we now analyze these steps, we can state the ABC's of servos. A servo
must: (A) Accept instructions that tell it what should be done; (B) Be aware of the actual conditions
that exist at every instant; (C) Compare what is being done with what should be done; (D) Dictate orders
that will correct the error noted by this comparison; and (E) Energize some mechanism that can follow
Consider the antenna rotator servo shown in Fig. 1. This is referred to as an "open-loop" servo system
because a human operator is required as one of the links in the ABC chain. The knob at the TV set is
secured to a disc in contact with all the contact points save the one that happens to be in line with
the notch cut in the disc. A permanent spring contact is made to the disc as shown.
Fig. 1. Example of an "open-loop" type servomechanism. It serves practically as an
antenna rotating mechanism for TV installations. "Open-loop" refers to the need for a human control
element in the servomechanism system.
Suppose the TV viewer decides that he would like to rotate his antenna from position 2 to position
1. He turns the knob to position 1, bringing the notch in line with contact point 1 at the same time;
but when he does this, contact point 2 touches the disc and feeds electrical energy to the motor through
commutator segment 2. Each time the commutator arrives at a new segment, the power flows uninterruptedly
to the motor until it reaches segment 1, when the circuit is again opened and the motor stops, leaving
the mast in the desired position.
Note that the human operator must dictate the necessary instructions to the servo by rotating the
positional switch to the desired setting. In a "closed-loop" system, a human operator is totally unnecessary.
Imagine that the course of a ship is to be due west and that its gyro-compass has been set for this
direction. Along comes a gust of wind or an ocean current that tends to swing the prow of the ship to
the north. The gyro-compass, of course, continues to point to the west as the boat turns under it, thereby
producing an error angle between itself and the boat's axis.
If the compass is coupled to an electrical generator of a suitable type which can feed an error signal
proportional to the error angle to a rudder motor at the stern, then the rudder will swing over to an
extent that will just correct the deviation from the proper course.
Fig. 2. A course control system as used in ocean-going vessels is a typical
application of a "closed-loop" type of servomechanism. Once the course has been set, no further human
guidance or readjustment is needed for operation.
The error angle represents the comparison between what is actually being done and what should be
done. Corrective orders are dictated by means of an electrical signal that varies with the amount and
direction of the error; this order signal then energizes a rudder control motor which makes the necessary
correction in course.
When you set the thermostat of your oil-burner, you
have issued instructions that it keep the house at, say, 70°F. The bi-metallic strip inside the thermostat
retains "awareness" of existing temperatures by bending toward an electrical contact as the house cools.
When the contacts finally close as the temperature goes below 70°F, the thermostat issues a corrective
signal in the form of a current to the relay of the oil burner.
If you would like to experiment with thermostatic control to get the "feel" of the ABC's of servos,
you can pick up an old fluorescent starter and use it as the base for a thermostatically controlled
chick incubator or a transmitter crystal oven. Pry up the four sheet-metal fingers that secure the disc
base to the metal casing and lift out the whole inside structure. If a capacitor is present (some starters
omit the capacitor), clip it out by cutting its leads close to the disc base.
Wrap a single layer
of cloth around the glass tube, place it between the jaws of your vise, and apply pressure gradually
until the glass just cracks. Be careful not to damage the bi-metallic assembly that is now exposed.
The concave portion of the curved strip is brass and the convex section iron. Since brass has a higher
coefficient of linear expansion than iron, this bar will tend to straighten when heated, i.e., it is
normally off and makes contact when its temperature rises.
To reverse this action, bend the free vertical bar as shown in Fig. 3 so that the inner brass face
of the strip barely touches the bent bar at room temperature. The temperature at which contact will
be broken will then depend upon the extent to which the strip presses on the vertical bar. This can
easily be adjusted experimentally by further bending either to the right or left.
Fig. 3. Modification required to convert a fluorescent lamp starter into a
sensing element of a thermostatic control circuit. "Before " view (left) shows internal assembly of
starter after protective glass is broken. "After" view (right) illustrates modification made to the
contact to enable use of the starter as a thermostatic switch.
An incubator or oven thermostat may be set up easily using the circuit of Fig. 4. The "heater" is
a 25-watt incandescent lamp blackened with candle-black or sprayed lightly with flat black lacquer.
The enclosure in which it is placed should be fairly well insulated so that it retains its heat.
Fig. 4. Wiring and installation of modified bi-metallic element in a homemade
experimental thermostatically controlled incubator or crystal (oscillator) oven.
Posted December 4, 2013