Page 390 - The Mechatronics Handbook

The magnetostrictive transducer does not have the inherent compactness and ruggedness of ultrasonic
through air, but does achieve similarly large measurement lengths, up to several meters. Accuracy and
stability are excellent, far better than ultrasonic. Some misalignment or nonlinear motion is tolerated,
because the target magnet does not need to be in very close proximity to the magnetostrictive wire.

Laser Interferometry
Laser interferometers are capable of measuring incremental linear motions with resolution on the order
of nanometers. In an interferometer, collimated laser light passes through a beam-splitter, sending the
light energy on two different paths. One path is directly reflected to the detector, such as an optical
sensing array, giving a flight path of fixed length. The other path reflects back to the detector from a
retroreflector (mirror) attached to the target to be measured. The two beams constructively or destruc-
tively interfere with each other at the detector, creating a pattern of light and dark fringes. The interference
pattern can be interpreted to find the phase relationship between the two beams, which depends on the
relative lengths of the two paths, and therefore the distance to the moving target. As the target moves,
the pattern repeats when the length of the variable path changes by the wavelength of the laser. Thus the
laser interferometer is inherently an incremental measuring device.
Laser interferometers are easily the most expensive sensors discussed in this chapter. They also
have the highest resolution. Laser interferometers are very sensitive to mechanical misalignment and
vibrations.
More information about sensors may be found in Sensors magazine (http://www.sensorsmag.com/).

References
1. Histand, M. B. and Alciatore, D. G., Introduction to Mechatronics and Measurement Systems, McGraw-
Hill, Boston, MA, 1999.
2.Bolton, W., Mechatronics, 2nd edition, Addison Wesley Longman, New York, NY, 1999.
3.Horowitz, P. and Hill, W., The Art of Electronics, 2nd edition, Cambridge University Press, Cambridge,
UK, 1998.
4.Auslander, D. M. and Kempf, C. J., Mechatronics: Mechanical System Interfacing, Prentice-Hall, Upper
Saddle River, NJ, 1996.
5.Jones, J. L., Flynn, A. M., and Seiger, B. A., Mobile Robots: Inspiration to Implementation, 2nd edition,
A. K. Peters, Boston, MA, 1999.

19.2 Acceleration Sensors

Halit Eren

Acceleration relating to motion is an important section of kinematic quantities: position, velocity, accel-
eration, and jerk. Each one of these quantities has a linear relationship with the neighboring ones. That
is, all the kinematic quantities can be derived from a single quantity. For example, acceleration can be
obtained by differentiating the corresponding velocity or by integrating the jerk. Likewise, velocity can
be obtained by differentiating the position or by integrating the acceleration. In practice, only integration
is widely used since it provides better noise characteristics and attenuation.
There are two classes of acceleration measurements techniques: direct measurements by specific accel-
erometers and indirect measurements where velocity is differentiated. The applicability of these techniques
depends on the type of motion (rectilinear, angular, or curvilinear motion) or equilibrium centered
vibration. For rectilinear and curvilinear motions, the direct measurement accelerometers are preferred.
However, the angular acceleration is usually measured by indirect methods.

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