ICSES 2008 INTERNATIONAL CONFERENCE ON SIGNALS AND ELECTRONIC SYSTEMS, SEPT. 14-17, 2008 KRAKÓW, POLAND
Implementation of PSD Sensor for Measurement of Vibrations G. Beziuk, A. Grobelny, J. Witkowski, Institute of Telecommunication, Teleinformatics and Acoustics, Wroclaw University of Technology, Wyb.Wyspianskiego 27, 50-370 Wroclaw, Poland, e-mail:
[email protected] Abstract— The presented in the paper optical method of measuring vibrations and oscillation has many benefits. They are: high measuring speed and high sensitivity of the method. Another benefit is the possibility of the non-contact measurement, when both laser and the gauge are both not mounted to the oscillating element. Changing the configuration of the optical system enables an increase in the resolution of the measurement. Furthermore, it is important to emphasize the ease of selecting the laser beam’s course in many nontypical measuring conditions.
I. INTRODUCTION The oscillation (vibration) of machines, devices and even building structures is one of the basic problems connected with their utilization. Oscillation measurements are taken in order to state their own oscillatory frequency, form, and source. The purpose of these is usually to diagnose the technical condition or utilization and construction parameters of the tested objects [1,2]. Decreasing the vibration level slows down the wear and tear of parts, decreases their negative effects on the environment, and in the extreme case allows to prevent accidents (for example: in case of buildings). Different types of sensing devices are used to measure oscillation [3]. One can separate them into two groups: relative and absolute gauges. Absolute gauges include piezoelectric accelerometers and electro-dynamic transducers with a seismic mass. Relative gauges are eddy-current gauges, as well as capacitive proximity sensors. Inteferometic systems are another group of sensing devices, which allow not only to measure the vibrations, but also to visualize the shapes of oscillation. In comparison to the earlier mentioned systems, the interferometric systems are very expensive. Authors suggest using an optical method, PSD gauge (Position Sensitive Detector) [4], to measure relative
B. Marek Institute of Production Engineering and Automation, Wroclaw University of Technology, Wyb.Wyspianskiego 27, 50-370 Wroclaw, Poland, e-mail:
[email protected]
oscillation. A PSD consists of a monolithic PIN photodiode with a uniform resistance in two dimensions (one dimension PSD are possible as well). PSD’s have many advantages when compared to discrete element detectors such as high position resolution, fast response speed, and simple operating circuits. Position data is independent of the size of detector’s light spot - it shows the spot’s center of gravity. Furthermore, PSD can be used for non-contact distance measurement, laser beam alignment and optical tracking of an object. The usage of PSD gauge to measure vibrations as described in this paper is not a typical use for these sensing devices. Measurement relies on the observation of the laser beam’s deviation on the active surface of the gauge. Information about the spot’s position is possible to obtain through processing of the four current signals Ix1, Ix2, Iy1 i Iy2 that are available at the exit of the gauge. Tensional signals, which are equivalent to the coordinates x and y of the laser spot’s position’s center of gravity, are obtained through the realization of the following equations:
(I + I y1 ) − (I x1 + I y 2 ) U x = R x 2 I x1 + I y1 + I x 2 + I y 2 (I x 2 + I y 2 ) − (I x1 + I y1 ) U y = R I x1 + I y1 + I x 2 + I y 2
(1)
Processing of the current signals from the PSD gauge’s exits to the corresponding coordinates of the spot’s tension signals can be done both in the digital and analog techniques. II. OSCILLATION MEASUREMENTS Building of the system to measure mechanical oscillation and, at the same time, a mean to lead a laser beam, depends on many factors such as: the placement of the oscillating element, a possibility to mount the gauge
or the source of laser beam to the oscillating element, etc. In Fig.1, 2, 3 only the three simplest examples of the positioning of the laser and gauge are shown.
VIBRATING MASCHIN WITH LASER
PSD
Fig.1. The idea of oscillatory measurement with the help of PSD sensor.
VIBRATING MIRROR ON A MASHINE
Fig.2. Non-contact measurement with the PSD gauge.
ANGLE φ
LASER
ANGLE 2φ
ROTATIN G MIRROR ON A
PSD Fig.3. Oscillation measurement of the rotating system with the help of PSD gauge.
The first system (Fig.1), on the positioning of the separate machine elements, to be measured. Relative
Second example (Fig.2) shows a possibility of measuring oscillation of the element when only a mirror that reflects laser beam is attached to the vibrating element. Laser itself and the gauge are placed away from the machine. This example shows that the proposed measurement method enables non-contact measurement. Example from the Fig.3 shows a method of non-contact measurement of the rotating system. A mirror is placed on the rotary axis, and tilted to the surface, which is perpendicular to the rotary axis. In the case of lack of oscillation, laser beam reflected from the mirror outlines a circle on the PSD gauge. If the machine begins to oscillate, the outlined curve diverges from the circle. In this case, the difference between the received curve and the circle is the measure of oscillation of the rotating system. III. EXAMPLES OF RESULTS
PS
LASER
(laser’s and PSD gauge’s) are registered on the X and Y axis, which are perpendicular to the direction of laser beam’s incidence.
the most obvious one, relies laser and the gauge to the whose relative oscillation are oscillation of both systems
Proper analog circuit used by the authors, which computes the equation (1) is depicted in Fig.4. The system contains 16 operating amplifiers and two analog voltage multipliers. Current signals from the gauge are processed in the current-tension converter to the tension signals. In order to eliminate noise interferences, converters’ stream is limited to the frequency, which equals to the limit frequency of PSD gauges. Then, the signals in the tension form are passed to the further part of the system, composed of analog adders, differential amplifiers, and voltage dividers, which compute equation (1). Knowing the tension values of Ux and Uy, and the measurements of the used PSD gauge, one can compute, with the micrometer precision, the position of the light laser’s spots on the active part of the gauge [1]. Exemplary results were made for the lathe using system illustrated in the Fig.3. As measuring tools, a semiconductor, singlemode, 1 mW laser and PSD gauge type S1880 (Hamamatsu) were used. Laser and gauge were placed about 1 meter away from the mirror and angled 10 degrees to the system’s axis. Angle of inclination of the mirror to the surface perpendicular to the axis of rotation was about φ=0.2deg. Fig.5 presents graphs of deviation of receiving beam on the axis X, Y (Fig.5 a and b). From these graphs, it is difficult to distinguish the changes resulting from the system’s oscillation. One can see oscillation much more clearly on the graph from the picture 5c, where a trajectory of the laser beam moving on the surface of the gauge was drawn. Here the measurement of the system’s oscillation is the deviation of the trajectory from the ideal circle. .
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Fig.4. Scheme of signal processing for the PSD gauge.
a)
φx[deg] 0.2
0.2
b)
-0.2 0 φy[deg] 0.2 -0.2
-0.2 -1 -0.8 -0.6 -0.4 -0.2 0
0.2 0.4 0.6 0.8 1 t[s] φ[deg]
-0.2
0
Fig.5. Signals X and Y of the measuring system from the Fig.3 (a) and trajectory XY of laser’s radiance on the gauge’s surface (b).
0.2
φ[deg] .0100
.22
Φ[deg]
.22
.0075 0.03
.0050
.0025 0.01 0 0
0 10 20 30 40 50 60 70 80 90 100 [Hz] Fig.6. Amplitude spectrum of X signal of the measuring system of the lathe.
100 150
200 250
300
350 [Hz]
Fig.8. Amplitude spectrum of X signal of the measuring system of the drill machine.
Additionally, Fig.6 shows a Fourier transform of the exiting signals, from which one can read frequency of the turns of machine’s spindle (about 10 Hz) as well as vibrations with different frequencies-most of all with harmonics of rotary frequency. Graphs were made using a φ value for mirror’s angle inclination (Fig.3).
IV. CONCLUSIONS The above presented method of measuring vibrations and oscillation has many benefits. Among others, benefits include: high measuring speed (up to about 3kHz) and high sensitivity of the method (resolution of 1 um of spot’s movement on the surface of PSD gauge). Changing the configuration of the optical system enables an increase in the resolution of the measurement.
0.2
Yet another benefit is the possibility of the non-contact measurement, when both laser and the gauge are not mounted to the oscillating element. Furthermore, it is important to emphasize the ease of selecting the laser beam’s course in many non-typical measuring conditions.
0
-0.2
φ[deg]
50
-0.2
0
0.2
REFERENCES [1]
Fig.7. Trajectory XY of laser’s radiance on the gauge’s surface of the drill machine. In Fig.7 a trajectory of the beam spot on the PSD sensor measured for poor grill machine In the same conditions as for the lathe. Fig.8 shows the spectrum of the X signal. As seen the vibrations in this case are much greater and the spectrum is much richer.
[2] [3] [4]
Benaroya, Haym Mechanical vibration : analysis, uncertainties, and control; Prentice Hall, 2004 A. Shabana Ahmed. Theory of vibration : an introduction, Spriger, 1996 FESTO Sensors for handling and processing technology, proximity sensors. Textbook FP 1110; http://www.festo.com/didactic PSD (Position Sensitive Detector) is an optoelectronic position sensor utilizing photodiode surface resistance. http://jp.hamamatsu.com/products/sensorssd/pd123/index_en.html
