Trends in Electromechanical
Transduction
I1ene J. Busch-Vishniac
Aerospace
Mechanical Engineering Department, The Universiy of Texas Austin, R 78712 and
and Mechanical Engineering Department, Boston University, 110 Cummington St., Boston, MA 02215
Abstract: Electromechanical transducers are devices which monitor the state of a system or impose a state on a system, and
which have one mechanical port and one electrical port. Examples include strain gages and motors. This article examines
trends in electromechanical transduction, including the fising pervasiveness of sensors and actuators, the increasing demands
placed upon them, the expansion of applications arenas, the push for miniaturization, the growing concern wi[h actuators,
and the escalating pace of change.
~TRODUCTION
Although most naturally occurring events are primarily either mechanical or chemical in nature, it is common to
convert information into an electrical form in order to take advantage of the speed and linearity of electrical systems.
The devices that accomplish this energy conversion from one form to another are referred to as transducers.
Electromechanical
transducers are devices in which one port is associated with electrical energy and one with
mechanical energy. Examples include accelerometers, strain gages, loudspeakers, and motors.
The largest breakdown generally divides transducers into two
There are many ways to categorize transducers.
classes which depend on broad function. Those transducers used to monitor the state of a system, ideally without
affecting that state, are sensors. Those transducers which impose a state on a system, ideally without regard to the
system load, are actuators.
The 1970’s and 1980’s brought dramatic changes in electronics and in signal processing techniques, but only
As a result, transducers are often the least reliable and most
modest changes in electromechanical
transducers.
expensive elements in measurement and control systems. For this reason, there is a growing emphasis on the field of
transduction, and significant changes are beginning to emerge.
This article examines the current trends in electromechanical transduction and some of their repercussions.
These
trends include the rising pervasiveness
of transducers, the increasingly stringent demands on transducers,
an
expansion of applications arenas, a push for miniaturization of sensors and actuators, a growing focus on actuators,
and a reduction in the time between discovery of new materials and their introduction into transducers.
PERVASWENESS
In the last few decades, the decreasing cost of electronics has prompted its adoption into products of all sorts. The
growth of electronics in consumer products has been driven by two phenomena: the perception that low technology
(nonelectronic)
devices are not as good as high technology (electronic) devices, and the push for products with
intelligence. Examples of low technology devices whose market is being overtaken by high technology counterparts
include office equipment, such as staplers and pencil sharpeners, and kitchen appliances, such as juice squeezers. In
many cases this leads to the introduction of electromechanical
sensors and actuators.
The growing market for
intelligent products (i.e. products which include a decision-making process) is prompted by the desires to automate
some functions that are performed by people and to add functions that humans cannot perform.
Examples of
common intelligent products that perfom jobs humans are not able to do are smoke detectors, automobile airbags,
and clothing dryers with autodry cycles, In each case sensors aid the decision-making process.
Because of the move to high technology devices the growth in sensor and actuator markets has been rapid and is
predicted to continue on its current pace through the turn of lhe century. The sensor market alone rose to a $5 billion
industry by 1990 [ 1] with projections for a $13 billion worldwide market by the year 2000 (a compound annual
growth of 8%). The bulk of this market is divided pretty evenly between the US, Japan, and Europe, with about
14% of the market share going to the rest of the world [ 1].
DEMANDS
ARE ON THE RISE
In product design it is well accepted that performance demands always increase over time. ~his is sometimes
called the “new and improved” syndrome.) In electromechanical transduction, the recent trend in product demands
reflects a dichotomy of purpose and is leading to goals that are, for the most part, mutually exclusive.
995
On the one hand, there are proponents of modularity who see sensors and actuators as components of a larger
system, and seek to create as few of these components as are necessary to ensure operation of any useful system
created by assembling components.
This suggests a need for great flexibility in transducer performance - wide
ranges of operation, adjustable sensitivities, standardized shapes, etc.
On the other hand, the demands on transducers for performance within any particular system in terms of quality,
reliability, and cost are becoming progressively more stringent. This has spawned mechatronics,
a view that the
design process ought to include models of sensors, actuators, and the controlling electronics as well as the main
physical plant. The premise of mechatronics is that it is only through consideration of the mechanics and the
electronics in an integrated fashion that one can produce an electromechanical
product that performs as desired.
Taken to its natural limit, this suggests that there is an optimum transducer for a given set of system performance
characteristics.
In the extreme this leads to custom transducers for each application, the opposite of the few broadapplication transducers used in a modular view.
While there is merit in both views of transducers, there are compelling reasons to favor mechatronics.
These are
well presented by Whitney [2] who argues that the modular approach to electronic system design will never apply to
mechanical systems. Among his reasons: mechanical components typically perform multiple functions rather than
one, and they perform differently in a system than they do in isolation (which is not the case for electrical
components).
NEW APPLICATIONS
ARENAS
A great deal of the expansion of the electromechanical
transducer industry has come about because of new
applications emerging, especially for sensors. In 1990 approximately half of the sensor market was for automotive
applications including position, temperature, emission, pressure, and acceleration sensors [1]. Another third of the
market was for industrial applications, and the rest was dominated by biomedical applications.
In these statistics,
consumer products other than cars accounted for only about one percent of the sensor market.
It is clear that the sensor and actuator market is changing its focus both in application and scale. Examples of
With medical costs rising rapidly, the impetus for
application shifts are in health care and manufacturing.
improvements in patient monitoring and early diagnosis prompt more biomedical applications.
This is particularly
affecting home products and creating more intelligent devices. An example of this trend is the bathroom scale,
which is now being outfitted with sensors to aid in determination of balance problems.
In manufacturing
applications,
the most successful industrial control systems have focused on continuous
processes that rely on low speed monitoring of chemical characteristics using electrochemical transducers. In recent
years attention has shifted toward unit (or discrete) manufacturing processes and operations performed by specific
machines. In discrete processes one normally monitors mechanical variables and might need to sense or actuate on
short time scales. This shift has been accompanied by a move to faster and more precise industrial sensors.
Another industrial change relates to scales of operation, A growing segment of industrial fabrication facilities is
concerned with very small dimensions.
For instance, the microelectronics
industry is constantly pushing toward
smaller feature sizes (and is already at about the 0.1 pm level). Small scale features have created a market for
sensors with higher sensitivity, and actuators with greater positioning accuracy and resolution. These characteristics
normally are achieved by compromising performance in another area, such as range.
M~IATURIZATION
A clear trend in electromechanical
transduction is the push for extreme miniaturization of sensors and actuators.
Further, miniaturization is taken to be synonymous with use of microelectronic fabrication techniques for sensors
and actuators. The logic driving the production of silicon sensors and actuators deserves careful consideration.
Ideally monitoring a system state without affecting it suggests devices that have a small footprint, i.e. that use low
amounts of power and are small in size. For sensors then, the push for extreme miniaturization has some compelling
logic. A remaining question is whether miniaturization must lead to silicon sensors fabricated using microelectronic
approaches. In general, the arguments that have been put forward for solid state sensors focus on three issues: 1).
one can achieve better dimensional control, 2). electronics can be integrated wilh the sensors, and 3). costs can be
cut through bulk production. However, it is not clear that these issues resolve the miniature sensor question in favor
of silicon-based approaches.
It is true that VLSI fabrication techniques offer dimensional control at a level (roughly 0.1 pm) that is difficult if
not impossible to achieve by conventional methods. However, the range of dimensions offered is small. In
conventional processes it is common to have features which are smaller than largest part dimensions by a factor of
105. Using microelectronics
approaches, the same ratio is limited to between 103 and 104.
996
circuit~ is twoThe abili(y to integrate electronics with a sensor is another misleading issue. Microelectronic
sensors and actuators
dimensional, existing on the surface of a silicon substrate. By contrast, electromechanical
require a greater dimensionality, and hence different manufacturing techniques have been developed. Unfortunately,
the fabrication techniques for transducer structures and for electronics are not wholly compatible. Thus, integrating
the two on a single chip requires making compromises
in each component
that might not be acceptable.
Additionally, yields in the two processes are dissimilar, so integration of electronics with the transducer means that a
significant amount of functioning electronics are likely to be trashed.
While mass production of microelectronic circuits have led to great reductions in their cost, it is not clear that the
same savings would be available for sensors. Consider, for instance, one of the most written about solid state
The solid state microphone must compete with the miniature electret
sensors so far -- the silicon microphone,
microphone made by conventional fabrication processes. Estimates of the worldwide annual production of electret
microphones are about 800,000,000 devices. Sizes down to a couple of millimeters in diameter and costs as low as
$5 including amplification are commercially available. Against this backdrop, solid state microphones generally
offer poorer performance in terms of signal to noise ratio and frequency response (see Scheeper et al. [3] for a good
review of performance characteristics), and yet it is not clear that they can be made price competitive.
Ideal electromechanical
actuators seek to impose a state on a system independent of the system load suggesting a
device with a large footprint: large size and high power. Thus, miniaturization
is not logical unless there are
constraints compelling one [o seek small sized actuators. Given this, the case for solid state actuators is made by
arguing in favor of large numbers of small actuators put into arrays. Historically, the question of single large
actuators versus arrays of smaller actuators has been called the staging problem. The classic example is the problem
of the analog watch which could either use a separate motor to turn each of the hands, or a single motor with
gearing. In general, which approach will prevail ought to be compelled by the specific application. Of course, even
for those cases favoring multiple small actuators, it is not clear that fabrication in silicon presents advantages to
miniaturization using conventional approaches.
In addition to the problems cited for miniaturized sensors, miniaturized actuators have the additional problem
associated with delivery of significant power. Standard options available on macroscopic scales, such as pneumatic
and hydraulic power or combustion of a fuel, cease to be logical on very small scales.
Also, the use of
microelectronic
fabrication approaches to produce transducers requires a transition from conventional
design
essentially independent of fabrication considerations,
to design in which fabrication options are the dominant
constraints.
To date, the limited microelectronic
manufacturing
options are severely restricting the miniature
transducer types.
FOCUS ON ACTUATORS
The task of imposing a state on a system is generally more difficult than simply monitoring that system state.
Hence, it is typically more of a challenge to design and build an actuator than a sensor. As a consequence, the
progress in electromechanical
sensors has outstripped that in actuators in the last decades.
For instance, consider
electric motors, a very common actuator for industrial applications.
Of the various \ypes of electric motors
available, the squirrel cage induction mo[or is the most common found in industry. Because of its dominance in
industry one would expect the motivation for improvements in squirrel cage motors to be substantial, However, the
current specifications (starting current of 480- 900%, starting torque of 100- 300Y0, efficiency of 80- 92%) match
those given in texts in the 1950s (see for instance, Siskind [4]).
There is a general recognition that improvements in electromechanical actuators have been few and far between.
As demands for enhanced system performance have increased, there has been a growing focus on actuators with
aims of speed, efficiency, size, reliability, and cost improvements.
Unfortunately, significant improvements have
not materialized to date, a fact that is Iikely to fuel an even greater intensity of research on actuators.
RAP~
CHANGE
One last trend is that the pace of change is dramatically escalating. Consider, for example, the discovery of the
piezoelectric effect in quartz. Jacques and Pierre Curie reported this discovery in 1880 [5] but it was not until 1921
that Langevin [6] produced the first patent for a transducer using the piezoelectric effect in quartz. By contrast, let
us consider the modern creation of TerfenolR, a rare earth magnetostrictive material. In magnetostrictive materials,
the magnetic permeability is a strong function of the mass density, thus coupling mechanical and magnetic fields.
The key US patent on TerfenolR was granted to Savage, Clark, and McMasters in 1981 [7]. Excluding the inventors
listed in the materials patent, the first US patent to mention Terfenol in a device was filed in 1984 [8], just three
years after the materials patent issued.
997
There are a few obvious reasons for the rapid change in the field of transduction: technology itself is improving
rapidly, free information is accessible nearly instantaneously via the worldwide web, and the amount of money at
stake is enormous. In general, the effects of rapid change on the field of transduction are the same as those seen in
all sectors of industry: design times are growing shorter, there is a rise of small and medium-sized companies to
address niche needs, and there is a growing variety in the transducers that are commercially available.
PREDICTIONS
FOR THE FU~RE
Based on the current trends in transduction it is possible to make predictions about the future. First, there is no
apparent slowing of the demand for intelligent products so there will be a continued expansion of the markets for
sensors and particularly for those that are coupled with electronic controls.
Second, the current tension between proponents of modularity and those of mechatronics
will certainly be
resolved, probably with mutually exclusive applications arenas for each. It is easy to imagine that high volume
products with noncritical specifications for sensors and actuators (e.g. toys) will move toward standard transducers,
sold as OEM (other equipment manufacturers) products which are purchased in bulk.
On the other hand, the
demands on high end products will continue to rise. As these demands are already “pressing the envelope” of what
is feasible, it is quite likely that meeting them will require a move toward more custom devices.
Third, the market for sensors and actuators is very likely to shift focus to match the changing world economy.
This suggests a growing concentration
on biomedical applications and on consumer products other than the
automobile.
Fourth, eventually there must be a more logical view toward the role of size scales. The current push for
miniaturization
of sensors will undoubtedly continue but, it will be a small subset of the currently investigated
actuator applications
which ultimately will be seen to be improvements
over alternative, more conventional
approaches. Rules for when the staging problem is best solved by one actuator and when it is best solved by many,
smal 1act uators w i]1emerge.
Fifth, it is clear that progress in electromechanical
actuation is stalled and that this will soon bc viewed as a
critical situation. This branch of transduction is certainly ripe for major technological advances, although it is not
clear at [his point what form they will take, Given that the history of transduction is steeped in exploitation of new
materials, one might anticipate advances achieved through collaborative efforts of materials scientists and transducer
designers.
Finally, given the pervasiveness of information and the market forces pushing for new sensors and actuators, there
will be an diminishing delay from the time new technologies and materials are discovered to the production of
sensors and actuators incorporating them
REFERENCES
1, Ristic, L. (cd), Sensor Technology and Devices, Boston: Artech House, 1994, ch. 1.
2. Whitney, D. E., “Why Mechanical Design Cannot be like VLSI Design,” Res. in Engr. Design 10, 125-138 (1996),
3. Scheeper, P. R., van der Donk, A. G. H., Olthius, W., and Bergveld, P., “Fabrication of Silicon Condenser Microphones Using
Single Wafer Technology,” IEEE J. Microeiectromech. Syst. 1, 147-154 (1992).
4. Siskind, C. S., Electrical Machines: Direct and Alternating Current, New York: McGraw-Hill, 1959.
5. Curie, P. J. and Curie, J., “Crystal Physics - Development by Pressure of Polar Electricity in Hemihedral Crystals with
inclined Faces,” (in French), Acad. Sci. (Paris) C. R. Hebd. Seances 91, 294 (1880). A translation appeans in Lindsay, R. B.
(cd), Acous[ics:Historical and fhilo~ophical Development, Stroudsburg, PA: Dowden Hutchinson Ross, 1973.
6, Langevin, P., “Improvements Relating to the Emission and Reception of Submarine Waves,” British Patent 145,691, Jul. 28,
1921.
7. Savage, H. T., Clark, A. E., and McMasters, 0. D. , “Rare Earth-Iron Magnetostrictive Materials and Devices Using These
Materials,” U.S. Patent 4308474, Dec. 29, 1981,
8. Hasselmark, E. D., Waters, J. P., and Wisner, G. R., “Magnetostrictive Actua[or with Feedback Compensation,” U. S. Paten[
4585978, Apr. 29, 1986.
998