Experimental analysis of an integrated optics vibrometer
Nicola Paone and Lorenzo Scalise
a)
Dipartimento di Meccanica, Universita
`
degli Studi di Ancona, Ancona, Italy
͑Received 5 June 2001; accepted for publication 3 December 2001͒
A multipoint vibration sensor realized in integrated optics is presented. The sensor is composed of
four optical heads that have been conceived for the simultaneous measurement of vibrations ͑in
terms of displacement͒ in several points of a vibrating mechanical system with the aim of an
industrial diagnostic. The system presented here is realized on small silicon chips ͑28 mmϫ28 mm͒
using integrated optic ͑IO͒ technology, each chip realizing a dual Michelson interferometer. The
conditioning and acquisition system for the signal, based on an A/D conversion board is reported
together with a brief description of the optical circuit. Experimental tests carried out on the sensor
have provided maximum velocity and maximum operating frequency of the sensing system.
Frequency response function tests carried out with a commercial single-point laser Doppler
displacement sensor are reported in the present article together with the laboratory setup and test
results. The network of vibration sensors in integrated optics, the object of the study, have proven
to be capable of measuring the vibration displacement of varnished metal surfaces optically
untreated with good accuracy ͑1% standard uncertainty on the instantaneous amplitude͒ in a
frequency range 0–200 Hz with surface velocities up to 100 mm/s. Limitation of the data processing
unit have been discussed for vibration frequencies higher then 200 Hz. © 2002 American Institute
of Physics. ͓DOI: 10.1063/1.1445861͔
I. INTRODUCTION
The constantly increasing performances of optical mea-
surement techniques based on interference phenomena have
made instruments based on such phenomena of interest for
use in research laboratories, in industrial environments for
on-line applications, as well as for industrial line monitoring
and control of the production process.
1,2
For this reason, the
necessity to design and develop such instruments is becom-
ing increasingly urgent. Dimension reductions as well as cost
lowering are the main objectives of the research in this field.
The possibility offered by integrated optics ͑IO͒ to realize
complex optical functions in small chips becomes extremely
attractive for optical communications and for measurement
in scientific fields. Advantages provided by the use of IO
technology in sensor design may be summarized as
3–5
͑1͒ full integration between the electronic and optical part of
the sensor in reduced dimensions;
͑2͒ low single cost for industrial production;
͑3͒ mechanical stability of the sensor and absence of optical
adjustments and alignments; and
͑4͒ remote single source supply for multiple sensors.
The last three points make the sensor extremely attractive. In
fact, at present, the main limits for an optical sensor based on
interference effects are: sensibility of all discrete optical
heads to mechanical and thermal inputs present in the indus-
trial environment and high cost for each single sensing sys-
tem.
The technology employed for the fabrication of the mul-
tipoint vibration sensor is based on the realization of a guid-
ing layer of phosphorous doped silica embedded in two
layers.
3
The system is composed of a single control unit and
four sensor heads. It realizes a simple sensors network for
the simultaneous vibration measurement of multiple points
on a vibrating target. This is extremely important for optical
sensors devoted to vibration assessment, because multipoint
vibration measurement can provide important information
for modal analysis as shown in Refs. 6 and 7 and on-line
monitoring of the working status of complex systems.
Each of the sensor heads works as dual Michelson inter-
ferometer and is realized in IO. The development of such a
sensor on integrated optics represents a large effort toward
the miniaturization of interferometric devices designed for
vibrational purposes. The sensor system represents one of the
first examples of an integrated optics sensor specifically de-
voted to vibration measurement. In the opinion of the au-
thors, this could allow an introduction of such sensors and
systems into a large field of industrial applications.
The multipoint vibration measurement system reported
here has been designed and realized with the aim of devel-
oping a system for the on-line quality control of household
appliances, specifically of washing machines. The quality
control system is based on the measurement of the vibra-
tional signals coming from the machines and on their pro-
cessing by a neural network based software able to discern
the presence of mechanical defects and consequently to clas-
sify the machine under test as ‘‘not defected’’ or as ‘‘de-
fected,’’ classifying the defect on board.
8
The use of the IO
sensors allows one to reduce the time request necessary in
the positioning of the sensors and consequently to dramati-
cally reduce the testing time for each machine. In Ref. 8 a
95% of correct mechanical fault detection has been obtained
a͒
Electronic mail: lorenzo@mehp1.unian.it
REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 73, NUMBER 5 MAY 2002
21800034-6748/2002/73(5)/2180/5/$19.00 © 2002 American Institute of Physics