Simple Electromechanical Transducers

The most common transducer is the potentiometer, which converts mechanical movement into a proportionate electrical signal. We use them every day on radios and cassette recorders for volume, tone, and balance controls. They are also the simplest and cheapest devices for transducer projects.

ill. 2-1. A dual potentiometer is commonly used as a two-dimensional control or position indicator. This device is often referred to as a joystick.

Stick position (and trim position) signals devised from the transducers (potentiometers) are then encoded in digital form for transmission. At the other end of the system the signal is detected by the receiver, and the signal information is extracted to feed to the servos. The latter operates on a closed-loop circuit, where the actual mechanical output movement is measured, fed back into the receiver circuit, and compared with the signal information. If there is any difference, and error-signal position is present that works to correct the output movement until the error signal is reduced to zero. This servo movement faithfully duplicates the command signal. The device that detects the amount of movement and generates any necessary error signal is another potentiometer coupled to the servo control movements.

CLOSED-LOOP CIRCUIT

A closed-loop circuit that demonstrates the use of feedback, or which can equally well be used as a project requiring proportional control response is shown in ill. 2-2. For simplicity this circuit uses a relay (which could be replaced by an equivalent solid-state switching circuit). The command input signal is devised from one potentiometer (the control potentiometer). The servo is a dc electric motor, the output of which is mechanically linked to a second Potentiometer (the feedback potentiometer). As connected, the servo motor will drive in either direction, according to whether the relay armature is pulled in or drops out. With the armature midway between the two contacts the motor is switched off. This is a typical proportional setup.

ill. 2-2. A simple closed-loop electromechanical circuit using potentiometers as transducers.

If we assume, for the sake of example, that the relay pulls in at a little over 10 mA and drops out at a little under 10 mA, then a 10-mA input signal to the servo circuit will maintain a “null” condition (armature midway and motor switched off). If now the input signal is increased to 15 mA by movement of the input control potentiometer, resistance in the circuit is decreased, so the relay will pull in and drive the servo in one direction. At the same time the servo will drive the feedback potentiometer to increase the value of its resistance until such a point where the increase in resistance exactly balances the reduction in resistance given by movement of the control potentiometer. At this point the relay current will have fallen to 2 mA again, giving “null” conditions and switching off the motor.

In more general terms, any variation of input signal, causing the relay armature to close the motor circuit and drive in one direction or the other, is progressively compensated by movement of the feedback potentiometer movement (driven by the servo) until null conditions are established again when the motor stops. Thus, the feedback control (potentiometer) ensures proportional movement of the servo output drive relative to the actual change in input signal. Any greater or lesser movement leaves unbalanced resistance and an “error signal” remaining in the circuit to continue driving the servo until the “null” condition is reached.

In a practical feedback circuit the input signal variation is provided by the receiver response to proportional signals rather than direct movement of a control potentiometer, with the value of the feedback potentiometer chosen accordingly. Exactly the same working principle applies, however, although it may be necessary to provide some means of preventing overrun of the motor so as to avoid momentary “hunting” of the servo about the null point. This can be done by providing a damping signal ( e.g., by 0 the motor brush to the free end of the feedback potentiometer) or mechanical or dynamic braking across the serVom0t0 itself.

Note that this is an analog system, which is the simplest to construct for general applications. For more precise proportional control, digital working is used (as on all model radio-control sets).

LINEAR OR ROTARY INPUTS

Another convenient feature of a potentiometer as a transducer is that it’s available in two different geometric forms: circular with a rotary wiper, or linear with a sliding wiper. The former is easily related to rotary input movements; e.g., the movement is coupled to the potentiometer spindle to give it a proportionate amount of rotation. With a slide-type potentiometer any push-pull (linear) input movement can be coupled directly to the slider.

Of course, only a simple form of crank linkage is needed to couple a linear (push-pull) movement to a rotary (circular) potentiometer or a rotary input movement to a slide-type potentiometer. However, the resulting mechanical movement of the potentiometer wiper won’t then follow exactly the input movements. Only rotary-to-rotary or linear-to-linear couplings will give this.

TYPES OF POTENTIOMETERS

Besides having the basic geometric forms (circular or linear), potentiometers can also have different electrical characteristics. Thus, the electrical signal resistance may be strictly proportional to wiper/slider movement (true linear response) or nonlinear in various degrees through to a logarithm response where increasing movement produces more and more signal.

In addition, there are various forms of combinations used for potentiometers, some better than others for particular applications. Potentiometers are also produced specifically as transducers, too, rather than simple variable resistance controls, with particular attention given in their design to reducing wiper/slider movement friction, reducing wear, and minimizing electrical noise.

POTENTIOMETERS AS COMPUTERS

Before leaving simple applications of potentiometers as control and feedback transducers, an interesting project is to build an elementary analog computer using two potentiometers (with a third potentiometer employed as a means of calibrating the circuit). The circuit involved is very simple ( ill. 2-3).

ill. 2-3. An analog computer using two control potentiometers (A, B) and one calibration potentiometer (C).

The three potentiometers are mounted on a suitable panel and wired to a microammeter and a battery, as shown. If we select a value of 25 kΩ for potentiometers A and B and using a 0-500-μA meter, the circuit will work off a 1.5-V battery. (If you want to use a 0.1-μA or 0.5-μA meter, adjust the battery voltage upwards accordingly.)

The third potentiometer is now used to calibrate the circuit. Potentiometers A and B are turned to maximum resistance, and the “adjust” potentiometer is turned until the meter reads exactly 100 μA. A suitable value for this adjust potentiometer is 1 to 5 kOhm. Once set up it should not need further adjustment.

With the setup showing 100 μA, we now mark the pointer position for potentiometer A as 100 and for potentiometer B as 0. This corresponds to the meter reading (100) equaling A plus B (100 + 0 = 100).

potentiometer A is now adjusted until the meter reading advances to 200 1 and this position is marked as 200 on the potentiometer scale. Repeat the process to obtain calibrated positions for potentiometer A at 300, 400, and 500 A. The potentiometer is then returned to its original (100) position.

Leaving potentiometer A in this position, we now turn potentiometer B until 200 is indicated on the meter scale. The pointer position at this setting is marked 100 for potentiometer B. This corresponds to the 100 position on A plus the 100 position on B = 200 on the meter (meter reading = A + B). Repeat the process for meter readings of 300, 400, and 500 to obtain calibration points for potentiometer B of 200, 300, and 400, respectively; that is, with potentiometer A at its 100 position the calibration value of the potentiometer B setting will always be 100 less than the meter reading.

The analog computer is now calibrated for working. Any settings of potentiometers A and B will be read on the meter as A + B. Unlike a digital computer, which can only count, the analog computer will give the sum over an infinite range of variations within the limits of the scale values.

This basic design lends itself to development in a number of ways. Different values of potentiometers and meter scales can be selected to extend the range of A + B so that they can be read or adapted to other units. The meter scale, e.g., can be replaced by another pasted-on scale. Thus, in the elementary model described, a new scale could be calibrated for the meter, starting at zero for minimum reading (instead of 100). Further, if required, the meter scale could be calibrated to required positions (calibration values) on the two potentiometers.

The computer also need not necessarily be confined to arithmetic working. It’s only necessary to render a particular problem as an analogy to the basic relationship between current flow and variable resistance that the analog computer presents to make the computer read in directly the type of information required.

To extend the principle further, a more advanced analog Computer could be designed, which incorporates a number of fixed resistors rather than potentiometers, in the form of a plugboard. Each resistor value represents a different condition value or analogous data state, that is, where the variation of one factor is analogous to the effect on current (the related or analogous factor) when resistance is varied.

Further extension, in simple analog working, is also possible by varying voltage. Because...

current = voltage / resistance

...varying the voltage is equivalent to multiplying or dividing, as far as the current value is concerned. This considerably extends the scope both for arithmetic and analogous working. This can be done by a variable voltage supply (e.g., a tapped battery in a simple unit) for multiplication or by dropping resistors for division.

The analog computer is a very simple device, but it’s surprising what it can be made to perform. It’s largely a matter of individual ingenuity as to how far it can be adapted or programmed to give the solutions required. Even with very simple designs such as the one described here, its scope is enormous. The elementary analog computer is also attractive because it’s neither expensive not complicated to make (although programming an elaborate pegboard or fixed resistor values can be a lengthy process), and in most cases it can be checked and calibrated directly.

Although what we have dealt with is basically an instrument circuit, it can equally well be applied to time transducer working as well, where the two potentiometers respond to mechanical in put signals.

DIAPHRAGMS and BELLOWS

Diaphragms and bellows can be called all-mechanical transducers capable of transforming one type of energy (pressure) into a mechanical movement proportional to the applied pressure. Another way of looking at it’s that they can convert some form of energy or force that can't be seen (pressure) into another form that can be seen (mechanical movement). “Seeing” in this case can be made much easier, and the system much more sensitive, by limiting to a mechanical system that amplifies the original movement of the transducer. A common example is the recording barograph with a pen on the end of an arm traversing a paper scale. A simple lever system multiplies the movement of the aneroid capsule as it expands or contracts under changing pressure so that the free movement is very much greater than the aneroid movement ( ill. 2-4).

ill. 2-4. Mechanical movement can be enhanced by the use of a simple lever.

The main limitation of diaphragms and bellows as transducers is that they are only capable of producing mechanical movement response. There is no way they will generate an electrical signal on their own unless they are coupled to another type of transducer that directly converts mechanical movement into an electric signal. This, indeed, is the most common form of pressure transducer capable of giving a direct electrical signal output. Pressure transducers are very common and are described in detail in a later section.

ELECTRIC MOTORS AS TRANSDUCERS

The electric motor is an energy converter. Supply it with electricity, and it rotates to provide a power output via its shaft. The efficiency with which it does this can vary considerably. With the very small toy-type dc electric motor, overall efficiency may be 20 percent or less. With industrial ac motors, especially large ones, the overall efficiency is usually well over 90 percent.

The energy-conversion principle of an electric motor also works the other way round. Rotate it, and it will generate electricity. Used this way, mechanical power input is converted to electric power, which is the way electric motors are used as transducers.

Normally a simple dc motor would be the choice for a transducer because this will produce a dc output signal that is simplest to process. It can be used directly to give a readout in a simple meter, e.g.,. Also, the efficiency of energy conversion is not necessarily important. The main thing is that the output signal should be linear with respect to input variations.

This is the case over the majority of the working range of a simple low-voltage dc motor where the current consumption (with a constant supply voltage) varies linearly with speed. The faster the motor is allowed to run the lower the current it needs ( ill. 2-5). The actual speed at which it will run depends on the braking effect of any load applied to it.

ill. 2-5. For constant input voltage the relative current required is proportional to the load resistance and inversely proportional to the speed.

It follows that if the motor is driven so that it produces rather than consumes electricity—that is, it’s operated as a generator— the current it generates in a fixed (electrical) load circuit varies linearly with the speed at which it’s being driven ( ill. 2-6). The actual current at any point on this characteristic graph is then a measure of speed revolutions per minute (rpm).

ill. 2-6. When a motor is used as a generator, the current through a given resistance increases in proportion to the shaft speed.

ELECTRIC TACHOMETERS

The electric tachometer uses a motor as a transducer. The rotor spindle is coupled to the shaft whose speed is to be measured and connected by two wires to a milliameter and a calibrating resistor ( ill. 2-7). The latter can be a fixed value if the dynamic characteristics of the rotor are known. The resistor motor is then calculated to give maximum meter current at the maximum speed to be indicated. If the dynamic characteristics are not known, a normal resistor can be used instead to operate the circuit.

ill. 2-7. A simple tachometer.

This type of transducer is very simple to set up and has maximum advantages over mechanical revolution-counting systems. A major advantage is that it can be worked remote from the Point of measurement without involving mechanical cable drives.

It also has no unknown or variable losses (it is simply calibrated against its own output), and it’s generally reliable over the life of the motor brushes.

The main point in designing a revolution counter of this type is to use the optimum working range of the motor chosen. Typically small dc motors are designed to have free-running speeds of up to 10,000 rpm. An optional operating range, over which linear characteristics should hold, would then be from about 1,000 RPM to 10,000 rpm. At low speeds its characteristics may be increasingly nonlinear.

If the maximum shaft speed to be measured is, say, only 2500 rpm the signals obtained may not be as linear as we would like. This could be overcome by calibrating the meter scale accordingly. But a better answer is to use a geared-up drive for the transducer (motor). e.g., a geared-up 4:1 2500-rpm shaft speed now produces a 10,000-rpm motor speed. Down at the other end of the range, a 500-rpm shaft speed rule produces a 2000-rpm motor speed. A more accurate, or at least a more easily calibrated, revolution counter should result.

PORTABLE ELECTRIC TACHOMETER

A portable electric tachometer is easily made by mounting motor, meter, and resistor together on a suitable panel, shaped to be easily held without obscuring the meter. The elaboration of a step-up drive gearbox is largely unwarranted in this case, so the motor used should be chosen to give a reasonable sign of current output over the driven revolution range it’s designed for. For a less sensitive meter, a simple solid-state amplifier could be incorporated in the circuit.

This is one of those projects where cut-and-try is the best design approach. Start with the electric motor, see what sort of signal level it produces when driven at various speeds, and then finalize the circuit accordingly.

SIMPLE PROBES

The use of probes is another basic method of producing a resistance signal in an indicating or secondary circuit. The principle involved is that a physical change of state in the vicinity of the probe produces a change in signal level in the circuit. Such probes don’t work as true transducers but as detectors or sensors.

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