July 1968 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
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Capacitors have been used
as transducers
in one form or another since their electrical characteristics were first
discovered. An electrical transducer is a device that converts some form of
energy into an electrical signal. This transformation allows for the
measurement, detection, or communication of various physical phenomena. There
are various types of electrical transducers, each designed to convert specific
types of physical quantities into electrical signals. Some common types of
electrical transducers include: pressure, temperature, strain, light sensors,
accelerometers, microphones, ultrasonic, magnetic, gas, and position and
displacement. Back in the 1980s I worked as an electronics technician (pre-BSEE
degree) for a company in Vergennes, Vermont, called Simmonds Precision Products.
Their products were fuel measurement systems primarily for commercial and
military aircraft. The sensors / transducers were capacitive in nature,
consisting of coaxial tubes which stood vertically in the fuel tanks and used
the fuel as a dielectric to vary the capacitance according to its ratio relative
to air between the tubes. The transducers were positioned at strategic locations
within the tank to provide an accurate measure of fuel regardless of the
attitude and acceleration of the aircraft. Simmonds is now part of
Collins Aerospace, after having been bought in 1983 by
Hercules.
Capacitors as Transducers -They can be made to count, measure and gauge.
Fig. 3 - Edge-effect capacitance (top) is prevented by grounded
guard electrodes.
by J. Merino Y Coronado*
A capacitor is not just an electronic component wired into a circuit. Dozens
of kinds of unusual capacitors are used in labs and along production lines throughout
labs and industry. They count objects, gauge liquid levels or pressures, measure
vibration or angles of rotation. Capacitors of this type are classified as capacitive
transducers.
A transducer is a device with two input terminals and two output terminals. It
converts mechanical energy to electrical, or vice versa, according to a known law.
It is reversible, which means that both the input and output are interchangeable.
The cartridge in an inexpensive phonograph is a good example of a piezoelectric
transducer: the mechanical vibrations of the needle are transmitted to a piezoelectric
element (usually made of Rochelle salt or of a certain type of ceramic containing
titanates). The element generates electrical charges when twisted by the movement
of the needle. Conversely, if we put an alternating voltage across the cartridge,
the crystal will produce vibrations and sound, which is a form of mechanical energy.
Fig. 1 - Common capacitive transducer is the capacitor (condenser)
microphone.
Fig. 2 - Two forms of displacement-measuring capacitors: (a)
cylindrical; (b) plane. Moving element varies the capacitance by changing effective
plate area.
Fig. 4 - Variation of rotary meshing-plate variable capacitor
is used to generate audio tones ... useful in electronic organs.
Fig. 5 - This capacitive pendulum can measure very small tilts
or vibrations. Transducers are cylindrical type (Fig 2a).
Fig. 6 - This ac bridge is detecting device for capacitive "tiltmeter"
(Fig. 5). C1 and C2 are cylindrical transducers, changing voltage across A, B is
amplified.
The condenser microphone (Condenser is an old name for capacitor, which has persisted
in conjunction with microphones.) is a capacitance transducer that transforms the
mechanical energy of sound waves into electrical energy to operate an amplifier.
In its simplest form it consists of a thick, fixed, circular metallic plate in front
of which is a very thin conductive diaphragm separated from the plate by an insulating
ring a few thousandths of an inch thick (Fig. 1). The diaphragm moves back and forth
with the variations in air pressure caused by sound. As the distance between diaphragm
and plate varies, the electrical capacitance of the microphone varies proportionally.
A battery of usually 50 volts or more is connected to the plate and diaphragm
in series with resistor R. An increase in capacitance allows the microphone to admit
more electrons from the battery. A decrease forces some electrons from the mike
back to the battery.
As everyone knows, a flow of electrons is an electric current and this current
produces (Ohm's law) a variable voltage (E) across resistor R. This voltage is applied
to the grid of an amplifier tube or to a transistor.
The condenser microphone is reversible. If an alternating voltage is applied
between the diaphragm and plate in Fig. 1, the diaphragm will move accordingly,
pushed by variations in electrostatic force, and, if the signal voltage is audio,
a sound will be heard. This is the principle behind electrostatic loudspeakers.
It is possible to increase the capacitance of a capacitor in three ways. Two
are mechanical methods: increasing the area of the plates, or decreasing the distance
between them. The first is used in rotary variable-tuning capacitors with meshing
plates. The second is used in the compression-type of trimmer capacitor.
Figure 2 shows two forms of capacitance transducer widely used for industrial
and scientific purposes. In a we see a cylindrical configuration, in b, a flat or
plane transducer.
If one electrode is movable, both types produce a variation of capacitance linearly
proportional to the amplitude of the movement (arrows).
The capacitance of both transducers can be readily computed from the following
formulas:
For the cylindrical configuration we use:
C(pF)= 0.644KLllog (b/a)
in which L is the length in inches of the overlapping portions of the cylinders.
The constant K is the dielectric constant of the material between the cylinders.
The inner diameter of the outer cylinder is b, and a is the outer diameter of the
inner cylinder, which is also in inches.
For a capacitor made of two plane parallel plates the capacitance in picofarads
is computed by
C(pF) = 0.225 KA/D
where A is the area in square inches of the portion of the movable plate which
is actually over the fixed one, D is the distance in inches between the two plates,
and K is, as before, the dielectric constant of the material between the plates.
(For air it is very nearly 1.)
Plane parallel plates and cylindrical capacitors are much used in industry and
science to measure linear displacements.
To measure angles of rotation, variable capacitors of the type found in radios
are used, because their capacitance changes linearly with the angle, provided the
rotor plates are semicircular and measurements are made between 15% and 85% of the
rotation angle.
There is a third way to change the capacitance of a capacitor: to change the
dielectric.
Oil, for instance, has a dielectric constant of 25.8. For distilled water the
constant is 81.07. An air capacitor will have 25.8 times its capacitance when it
is in alcohol, or 81.07 times its capacitance when submerged in distilled water.
(Tap water cannot usually be used because the minerals dissolved in it make it too
conductive.)
Not all the capacitive transducers used for instrumentation purposes are built
in the simple form shown in Fig. 2. The electrostatic lines of force at the edges
of the plates or cylinders are not parallel, but bulge outward, producing stray
capacitances that make the actual capacitance greater than the computed value. This
edge effect or fringing must be kept to a minimum, since some transducers are made
to measure capacitance variations of the order of 0.2 pF. This is accomplished by
using a third, grounded electrode, called a guard ring or guard electrode. Fig.
3 shows the edge effect and how to eliminate it with a guard electrode for a capacitive
transducer using plane plates, one of which moves either up and down or in and out,
perpendicular to the plane of the paper. The arrangement is similar for cylindrical
capacitive transducers.
If the rotor shaft of a variable capacitor of the type used in ordinary radios
is coupled to a rotating piece of machinery, rotation angles from 0° to some
150° can be measured. The coupling can be made to multiply the angle of rotation
by a known factor - as by the use of gears, levers, or pulleys and strings.
The transducer itself may have many forms. In most cases it has a guard electrode.
Very small angles can be measured.
A capacitive transducer used to generate audio tones works on the same principle
as the condenser microphone. In Fig. 4, W is a wheel of Bakelite or any convenient
insulating material. This wheel has sectors of aluminum foil connected to the shaft,
which is grounded.
In front of these is a fixed metallic plate or screw. As the wheel rotates, the
capacitance between the conductive sectors and the fixed plate or screw varies and
so does the charge. An alternating voltage appears across resistor R, with a frequency
equal to the number of revolutions per second of the wheel multiplied by the number
of sectors. This frequency can be amplified and measured with a frequency meter,
and we have an electrostatic tachometer. The same principle is used to generate
musical tones in some types of electronic organs.
Measuring vibrations and tilting
Fig. 7 - Pressure sensitive capacitive transducer (similar to
condenser mike).
Fig. 8 - This device provides a continuous indication of liquid
level without moving parts. Nonconductive liquids vary the capacitance between two
electrodes.
Capacitive transducers are much used in instruments for measuring vibrations
caused by passing vehicles or heavy machinery, or tilting caused by heavy loads
in bridges or buildings. The principle of a tilt meter or vibration meter is shown
in Fig. 5. The bob of a pendulum carries a curved metal cylinder that moves in or
out of two lateral hollow cylinders mounted on porcelain or Bakelite insulators.
The two fixed cylinders and the grounded movable cylinder together make up a differential
capacitor.
If the pendulum support is tilted or otherwise set in motion, the capacitance
between the pendulum and one of the other plates increases, while it decreases by
the same amount be-tween the pendulum and the second fixed electrode. A simple ac
bridge can be used to measure and record such variations. In Fig. 6, C1 and C2 are
halves of the differential capacitor in a tilt or vibration meter. If R1 is adjusted
so that the bridge is not balanced when the pendulum is at rest, an ac voltage is
present between points A and B. This is called the carrier voltage, and it is amplitude-modulated
when the pendulum moves. The modulated waves are amplified and detected in the usual
way to be recorded or telemetered to another receiver.
Guard electrodes are a must in this type of instrument. In Fig. 5 they are not
shown for simplicity.
Some time ago I designed and built a very sensitive seismograph using this principle.
Capacitive pressure gauges
When the indicator or recording device is at some distance, or when the pressure
variations in pipelines carrying fluids are to be used for remote control, capacitance
transducers are second to none.
A pressure-sensitive transducer is shown in Fig. 7. It has a movable elastic
metal diaphragm, grounded for obvious reasons. An insulating ceramic or porcelain
sleeve holds a fixed conductive plate, strong enough to stand the full pipeline
pressure in case the diaphragm fails. It is made quite stiff, with a resonant frequency
of 10kHz or higher to reduce its response to vibrations or shock. The pressure gauge
is nothing but a condenser microphone working at high pressures.
Level measurement of liquids in tanks
For this a simple cylindrical capacitor is used. The inner electrode is a rod
or heavy metal cylinder, 1" or 2" in diameter. The outer electrode is a metal tube
with holes in it, grounded to the tank (Fig. 8), while the inner rod is insulated.
Nonconductive liquids cause the capacitance to increase according to their level
and dielectric constant. For conductive liquids such as water or saline solutions,
the inner rod is covered with plastic or any other type of convenient insulating
material.
A simple ac bridge measures capacitance variations, as described in Fig. 6. More
elaborate devices are often used in these applications, such as the trf receiver
that is shown in Fig. 9, to measure distance, rotation, pressure or liquid level
in a tank. A very stable rf oscillator is coupled to a resonant input circuit in
an rf amplifier. The capacitor of this tuned circuit is the capacitive transducer
(in series or in parallel with a fixed or adjustable capacitor used for calibration
or adjustment). A linear detector and power amplifier complete the instrument. Its
output is used to operate a recording instrument or another controlled device of
some kind.
Fig. 9 - Tuned radio frequency (TRF) method uses capacitive transducer
to generate amplitude, phase or frequency variation in rf carrier for detection,
amplifying.
Fig. 10 - In slope detection, carrier is tuned to point 0. Capacitance
changes may cause carrier shift to points A or B.
The resonance curve of trf receivers is very much like Fig. 10. The "receiver"
used to measure capacitance variations is never tuned to resonance, but to one side
of the curve where its slope may be considered linear, let us say to the point marked
0 in the figure. The corresponding point, 0', is the operating point. An increase
in capacitance from 0 to B displaces the operating point to B' and the output of
the detector decreases. Very rapid variations in capacitance can be measured by
this method.
A phase-sensitive detector (Fig. 11) is used with capacitive transducers in many
industrial and scientific instruments and applications.
What about using a superhet? This more complex receiver is used to measure very
small variations in capacitance. One grid of a mixer tube is fed by the output of
a crystal-controlled oscillator, while the other is fed by a variable-frequency
oscillator whose frequency is controlled by variations in capacitance caused by
a capacitive transducer.
In this way a variable i.f. is obtained. When both oscillators have the same
frequency, the i.f. difference is zero. As soon as the capacitance of the transducer
changes, the frequency of the variable oscillator shifts and an i.f. is produced
in the mixer. Differences in frequency of a few hertz are easily detected by ear
or by a frequency counter. R-E
Fig. 11 - Transducers C1 and C2 are used in phase-detector (discriminator)
circuits.
* Research physicist and professor of acoustics and electronics, Institute of
Geophysics and Polytechnic Institute of Mexico.
Posted August 30, 2023
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