After Class: What is a Servomechanism?
April 1959 Popular Electronics
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.
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After Class: What is a Servomechanism?
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 in operation.
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.
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.
The ABC's of Servos.
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 these orders.
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.
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.
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.
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.
Fig. 4. Wiring and installation of modified bi-metallic element in a homemade experimental thermostatically
controlled incubator or crystal (oscillator) oven.
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.
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.
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.
Posted December 4, 2013