April 1959 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 Popular Electronics.
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.
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 in 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.
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.
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