Assuming at this point that we now understand how signals are modified at their transducer's output and how they are demodulated or received at the receiving station or location, we must now understand how these data can be put to practical use.
Transducers may be put to work performing either or all of the following duties: (1) measurement, (2) display, (3) recording, and (4) control. Each of these functions is discussed below.
2-4.1 - Measurement
The simplest and easiest way to receive transducer data signals is merely to hook up a recording or display instrument such as a voltmeter, oscilloscope, or ammeter to the transducer's output and record the readings. Obviously, the instruments used must be capable of responding adequately to the output's signal quantity. If you do use this method, it will be necessary to have some sort of calibration curve available to convert
the instrument's readings into the proper measurand amounts. Many times this calibration information is supplied by the transducer's manufacturer, but in many other cases you will have to generate the calibration information yourself.
If this turns out to be the case, and chances are pretty good that it will be, you must then be able to calibrate the transducer using some reliably known standard for accuracy comparison purposes.
Figure 2.26 Conversion graph for converting frequency to flowrate based on data in Figure 2-25.
Figure 2-25 shows a typical calibration chart for a transducer. This particular chart is for a transducer that responds to fluid flow in a pipe. The transducer transmits a variable frequency that varies directly (but not proportionally in this case) to the fluid flowing through it. The volumetric flow rate can then be interpreted by reading the transducer's output using a frequency counter since the calibration curve makes these data available. These data are plotted as a curve in Figure 2-26.
Figure 2.27
Typical analog display on a measuring device.
(Reproduced with the permission of
Omega Engineering, Inc.)
2-4.2 - Display
In portable or hand-carried instruments that require the use of transducers, some sort of direct readout display is usually built into the instrument, making hand conversion of transducer output data unnecessary. This display can be either an analog meter (Figure 2-27) or a digital readout display such as the one shown in Figure 2-28. In either case the display usually is not an integral part of the transducer itself but rather is housed in a separate unit, although there is no firm rule governing this.
Figure 2-28
Typical digital display on a measuring device.
(Reproduced with the permission of
Omega Engineering, Inc.)
2-4.3 - Recording
Transducers may also have their outputs directed to the graphics display of a computer for direct readout or they may even drive the output of a chart recording system. In the case of the computer, appropriate software is needed to interpret the transducers' data and to make any appropriate conversions and preparations necessary for display purposes.
Figure 2-29 Temperature controller used for controlling temperature inside a liquid-filled vat.
2-4.4 - Control
In many instances the transducer signals may be coupled directly into a system for control purposes. Transducers are used extensively for process control applications where a measurand is sensed, a control signal is generated, and some controlling action is taken on the measurand to control its behavior. A good example is a temperature controller used for monitoring and controlling the temperature of a liquid-filled vat similar to the one shown in Figure 2-29. The sensing transducer would be housed inside the vat. The sensor would then send its temperature signal to a comparator circuit, where the voltage data would be compared to the desired set-point controller temperature (i.e., the thermostat) voltage. The difference between these two voltages would then result in an error signal that would, after proper amplification, drive either the vat's heating unit or refrigeration unit in a proportional manner. The greater the error signal's positive voltage value, the hotter the heating unit would become; the greater the error signal's negative voltage value, the more rapidly the vat would be cooled by its refrigeration unit.
Up to this point in our exploration of transducers we have discussed, in very general terms, the physics needed to understand sensors. We have discussed the various measurands, the methods used for evaluating transducers, and the means to transmit the generated data along with how these data are received and used. We have completed our treatment of the transducer as a black box. We are now ready to delve into these boxes to see what is taking place and how they manage to do what they do.
We start this investigation beginning with Chapter 3.
Review Questions
2-1. Explain 4- to 20-mA analog standard data transmission.
2-2. Explain the difference between PWM and PAM.
2-3. Why is AM not used for data transmission?
2-4. Why are FM transmissions not as prone to electrical noise interference as AM transmissions?
2-5. Why can an AM demodulator not detect a PWM signal? (Hint: Can an AM detector "hear" a CW signal? Why or why not?)
2-6. Explain in detail how a PFM signal is demodulated. Use a block diagram if necessary.
2-7. What advantage is there in using a Gray code for a rotary encoder versus using an 8-4-2-1 BCD code?
2-8. Explain what is meant by the term parity checking.
2-9. What is a voltage-controlled oscillator? Give an example of how one is used.
2-10. What is a flash converter? What are their advantages?
Problems
2-11. A particular transducer, when used in conjunction with a certain power supply, produces an output current between 5 and 50 mA. The transducer has a range of 0 to 150 psi. What is the pressure being measured for a current reading of 38.5 mA from this transducer?
2-12. A transducer purchased from a catalog is capable of indicating length measurements between 10 and 150 mm. If its output range is 1 to 4 V dc, what sort of voltage scale resolution is needed (i.e., what is the smallest voltage increment that has to be read) in order to read to the nearest millimetre?
2-13. Determine the sensitivity of the sensing device whose response curve is depicted in Figure 2-2.
2-14. Draw a complete flow diagram of a PPM demodulator that uses a PWM demodulation scheme. (Hint: refer to Figure 2-10.) Explain the functions of each block that you decide to include for each circuit portion, using waveforms where necessary,
2-15. Refer to Table 2-4. Determine the odd-parity check values for each of the ten 8-4-2-1 BCD codes listed. Explain how an odd- or even-parity check system can be devised to check the validity of binary data emitting from a transducer. Would this system be foolproof? Explain your reasons.
References
Carstens, James R. Automatic Control Systems and Components, Englewood Cliffs, NJ: Prentice-Hall, 1990.
Duncan, Frank R. Electronic Communications Systems, Boston: Breton, 1987.
Leach, Donald P., and Albert Paul Malvino
Digital Principles and Applications, New York: McGraw-Hill, 1981.
Miller, Gary M. Modern Electronic Communication, Englewood Cliffs, NJ:
Prentice-Hall, 1988.
ARRL. The Radio Amateur's Handbook, 60th ed. Newington, CT:
American Radio Relay League, 1983.
Vergers, Charles A. Handbook of Electrical Noise: Measurement and Technology, Blue Ridge Summit, PA: TAB Books, 1979.
Chapter 3 - Sonic Sensors
Chapter Objectives
1. To understand how sound is quantified.
2. To understand how microphones work.
3. To study sonic detection methods.
3-1 - Introduction
A general review of the physics of sound was given in Section 1-5.3. We will review additional properties of sound as we begin discussing specific sensors.
Sonic sensing, as it is referred to in this book, refers to any type of sensor that has been specifically designed to respond to sound, whether that sound is sub-audible (i.e., below about 16 Hz, which is the lower limit of the unimpaired human ear), audible (between 16 Hz and 20 kHz, which is considered the hearing range of the unimpaired human ear), or super-audible (above 20 kHz). The term ultrasonic has in recent years replaced the expression "super-audible." Sound is comprised of periodically spaced fronts of modified atmospheric pressure, each front being either slightly above or below the existing ambient atmospheric pressure. These fronts can easily be detected using a flexible diaphragm or other free-moving member of fairly broad surface area designed so that it is free to vibrate. The approaching wavefronts then impinge on this diaphragm, causing movement or vibration to take place. As it vibrates, however, some method has to be employed to sense the diaphragm's movements as it responds to these fronts. It is these methods of detection that create new categories of sound-sensing microphone transducer.
In general, then, we can state that the purpose of the microphone transducer is to convert sound to an electrical signal that is proportional to that sound's intensity. This type of sensor is used primarily for the use of audio reproduction applications and is used extensively in the radio, recording, and television industries.
Another category of sonic detection is one that is used not so much for the detection of the sound measurand but rather for the measurement of distance and motion in water.
This category of sound detection is called sonar detection. Sonar is an acronym meaning sound navigation and ranging. Sonar was developed and used extensively during World War II for the purpose of detecting enemy submarines.
3-2 - Commonly Sensed Measurands
The measurands most often sensed using sonic sensors are the following: (1) sound, (2) position (i.e., distance), (3) motion (i.e., velocity), and (4) flow (usually liquids).
Sound, the primary measurand, is detected by using what is commonly referred to as a microphone. It is the transducer with which most of us are most familiar. The other three secondary measurands are detected by using some form of sonar device. It has been only in the last few years that much research and development has been done in the area of sonar detection, especially in the areas of motion and flow detection. We investigate next all four of these areas, beginning with the detection of sound.
3-3 - How Sound Intensity is Measured
Before getting into the sensing methods used for sound detection, we must determine how sound levels are measured. To begin with, sound intensity is usually expressed in
decibels by making a comparison to a standard reference intensity. The amount or magnitude of this intensity is stated as a sound pressure level given in decibels and defined as
0
dB = 20 log p p
(3-1)
where dB = sound intensity expressed in decibels (dB)
p = sound pressure front created by a sound source (expressed in the same units of pressure as the reference source: micro bars, lb/in2, dyn/cm2, etc.)
p0 = sound pressure created by reference source, usually stated as 0.0002 μbar or 0.0002 dyn/cm2.
Example 3-1
It was found that a power hand tool generated a sound pressure of 0.030 μbar.
Convert this measurement into decibels by comparing the sound to a standard sound source.
Solution: Using the standard sound source value of 0.0002 μbar and eq. (3-1), we obtain
This means that a sound pressure of 0.0300 μbar generates a sound with a loudness value of 43.5 dB, compared to a standard sound source of 0.0002 μbar.
The standard reference pressure most often used for making sound comparisons is 0.0002 dyn/cm2 (or 0.02 μbar). This amount of pressure change is just barely detectable by the average human ear and is equivalent to 0 dB as produced by a 1000-Hz signal. In other words, a pressure change of this magnitude will just produce an audible-level change in hearing. Now that we have defined sound intensity, let's look at the variety of ways that it is used to detect sound.
3-4 - Audio Reproduction: The Variable-Resistance Microphone
The variable-resistance microphone, often referred to as a carbon microphone, is pictured in Figure 3-1. The carbon microphone is probably the oldest method of sound detection in use today. The method was developed and perfected by Thomas Edison sometime before 1900.