Transducers
This pedantic sounding word refers to product components which
can save you money and increase the performance and salability
of your products. A simpler word for the same thing would be
TRANSLATORS, but you and I can't change it now!
Transducers are information devices rather than power devices.
There are many ways that physical phenomena are converted to
electrical power, but the word "transducer" is used only for
conversion for information puroses, i.e. for display or control.
You are familiar with the transducer outputs on the instrument
panel of your car: gasoline level, engine temperature, speed, oil
pressure. Other transducers are strain gages, thermocouples, and
pressure gages.
This essay explains the common transducers and their uses and
benefits.
THE BASIC IDEA
ISOMETER INSTRUMENTS measure by comparing the unknown
quantity with known quantities of the same kind. For example, a
yardstick measure length, a graduated beaker measures volume, a
spring scale measures force, a beam balance measures mass, a
potentiometer measures voltage, a Wheatstone bridge measures
resistance, and a clock measures time.
In many transducers the unknown is first converted to an
intermediate quantity such as a displacement and that
intermediate quantity is then converted to the output quantity,
such as electric voltage. The output quantity is often then
digitized.
Why bother? Transducers are used instead of isometer instruments
when
- they are smaller and more rugged,
- the measurement must be transmitted through a distance,
- the measurement is part of a feedback control system,
- the required range, speed, accuracy, or frequency response of
an isometer instrument is inadequate,
- a human readable display is wanted.
TRANSDUCER ELEMENTS
DISPLACEMENT
- changes electrical impedance,
- generates electrical voltage,
- changes pneumatic resistance,
- displaces an indicating needle over a scale,
- displaces a recording pen,
- changes the tension (and natural frequency) of a vibrating
wire,
- precesses a gyroscope,
- displaces a mass and spring by accelleration,
- counts marks on a scale, optical or magnetic,
- changes light in magnitude or position,
- counts optical or RF interference fringes,
- times the return signal following a transmitted signal
(ultrasonic signal from a reflector on a sound conducting rod,
radar(RF), sonar(acoustic), lidar(light)),
- moves a pointer over a dial via displacement magnifying
gears.
ELECTRICAL IMPEDANCE
is changed by
- stretching a resistor in a strain gage,
- compressing carbon granules,
- changing the distance or the dielectric between the plates of
a capacitor,
- sliding the wiper of a potentiometer over the resistance
element,
- moving the coil or core of a transformer or inductor,
- changing the illumination of a photocell.
ELECTRICAL VOLTAGE
is generated by
- motion between a coil and a magnetic field,
- bending of a piezo-electric crystal,
- illumination of a photocell.
PNEUMATIC RESISTANCE
is changed by
changing the orifice through which air flows.
LIGHT
changes the conductivity of, or voltage generated in,
photoelectric cells.
Color is analyzed by prisms and photoelectric cells.
HOW PARAMETERS ARE MEASURED
PRESSURE
bends an elastic element such as a diaphragm, Bourdon
tube, bellows, or spring backed piston. The resulting
displacement, in turn,
- moves a needle over a scale,
- changes an electrical impedance,
- changes a pneumatic resistance.
SOUND PRESSURE
with a frequency range of 20 to 20,000 Hertz, bends a diaphragm
which either
- changes an electrical impedance, or
- generates an electric voltage.
TEMPERATURE
- expands solids, liquids, or gases whose dimensions
or pressures are inputs to displacement transducers.
- Temperature also directly generates voltage in thermocouples
and thermistors.
STRAIN
- stretches and shrinks resistors. Their resistances change
and the change is measured electrically. Many transducers first
convert their sensed quantity into strain, by bending a diaphragm
for example, and then measuring that strain.
- stretches a vibrating string whose natural frequency is
measured electrically.
LINEAR DISPLACEMENTS
from micro-inches to many feet directly operate
- electric voltage elements
- interferometers
- impedance elements
- digital counters of marks on a scale
- digital encoders
- timers of signals transmitted and reflected
ANGULAR DISPLACEMENTS
operate similar elements to those of
linear displacements. They also precess gyroscopes.
Some effects are proportional to the quantity while some effects
are proportional to the rate of change, or the rate of change of
the rate of change, of the quantity. For example, an
accelerometer responds to the rate of change of the rate of
change (velocity) of position.
Many other quantities and effects are measured. Examples:
- Viscosity is measured by its damping of a vibrating reed.
- Reflected radio waves measure the position of airplanes and
satellites (RADAR).
- Reflected sound waves measure the position of underwater
objects (SONAR) and ultrasonic targets in both humans and non-
biological structures. Sonar and radar may be augmented by
transponders on the measured objects.
- Reflected light waves measure surveying distances (LIDAR).
- Medical imaging systems using several types of radiation show
details of the body's interior.
- Reagent color indicates chemical quantities.
TRANSDUCER OUTPUTS
VISUAL INDICATORS
- Panel meters
- Computer screens
- Plotters
- Printers
COMPUTER MEMORIES
FEEDBACK AMPLIFIERS
This has been a "once over lightly" introduction to a very large
art.
Other essays in this series deal with motors and generators,
electro-mechanical actuators, amplifiers, and other devices.
For a full treatment, please see my book:
"Understanding Electro-Mechanical Engineering" published by IEEE
Press. It is included by McGraw-Hill in their Electronic
Engineers' Book Club. Click here.
My other books are:
"Designing Cost-Efficient Mechanisms" published by SAE Press.
This is thebook which introduced and explains Minimum Constraint
Design. Click
here.
"Real-World Engineering" Published by IEEE Press. How to be a
successful engineer. Click here.
Questions? Telephone me at 619-224-3494 or e-mail
ljkamm@ljkamm.com. No charge!
Lawrence Kamm, Consulting Electro-Mechanical
Engineer
e-mail:ljkamm@ljkamm.com
Consulting
electro-mechanical engineering.
Resume and index of writings.
