In-line dynamic control monitoring of fluids for space systems

Powder

203

In-Line Dynamic Control Monitoring of Fluids for Space Systems*

C. A

RHODES,

L F_ STOWERS**,

College of Engineering.

University

L_ HAWKINS*** of South

Carolina.

and R. D_ BONNELL Columbia.

S. Car. 29208

(LISA.)

W. RAINES N-4.%%. Kennedy (Received

Space

September

Center.

Cape

Kennedy.

Florida

(USA.)

18. 1975)

SUMMARY

An electro-optical instrument is described which will measure particle size and concentration in hydraulic systems. The system utilizes a He-Ne laser source and a iinear array of 50 photodiode detectors_ Shadows are formed by the particles contained in the fluid as they pass through the laser beam and these shadows are magnified and projected onto the photodiode array. Particle size is determined by the number of photodiodes shadowed. The instrument operates c‘on-line” and is capable of counting approximately 10,000 particles/ set with the size range of 5 to 100 pm. The instrument will be interfaced with a computer for data collection, manipuiation, and displaying the results.

system. Particle sizes from 5 to 100 pm will be measured using a laser light source to form an image. The image, which consists of a shadow of the particle, is magnified and projected onto a linear array of 50 photodiodes. The size of the particle is determined by the number of photodiodes occulted_ Their typical residence time in the viewkg area is 10 ps and the anticipated spacing between particles is at least 80 ps. _Aperiod of approximately 2 minutes is required to produce a histogram giving the distribution of the contaminate particles by size. A computer is interfaced with the counting system for data collection, manipulation, and displaying the results. IL MECHANICAL

I. INTRODUCTION

A dynamic fluid contaminate level monitoring device is presently being developed to measure the contaminate level in high pressure hydraulic systems in the NASA Space Shuttle. The study is being performed under Kennedy Space Center NASA Contract NASIO-8354. The system being developed utilizes the image replication technique used to detect and size particles as the fluid flows through the test sections located within the hydraulic *Paper presented at the 7th Annual Fine Particle Society Conference. Philadelphia, August, 1975_ **Present address: Davis, California_

University of Caiifomia

***Present address: New Jersey.

Exxon

Corporation,

at Davis,

Morristown,

AND

FLUID

SYSTEMS

Although the system will ultimately be designed for space vehicle application, design simplifications were initially introduced by placing primary concern on the conceptual design. The following discussion expiains the low pressure system design. A similar design could be used at high pressures by redesigning only the windows and connecting bolts. Smaller windows should be used at high pressures in order to reduce stresses to a safe level.

Hydraulic fluid circulatesthrough the test

section where windows are placed so that the laser beam passing through the hydraulic fluid creates particle shadows. Shadows of the contaminate particles in the hydraulic fluid are counted as they flow through the viewing volume, shown in Fig. 1. The size of the pakicles is determined by the number of photodiodes occulted. The test section has a

Fig_ 1. Sketch showing viewing volume. and sampling area

volume.

sampling

thin flow channel, 02 cm, so that the particles will flow near the focal plane of the optical system and so that there is a high probability that no more than one particle is present in the viewing volume at any one time. However, the channel thic_kness is sufficiently large to prevent the hugest particles, 100 pm, from becoming lodged in the channel. Figure 2 shows the construction detaiIs of the test section, which was me-hined from two pieces of 304 stainless steel (21 X 8 X 6 cm)_ Stainless steel was chosen for its corrosion resistance. The hydraulic fluid enters and leaves through 0.635 cm drilled passages cormected to the low pressure test loop by polyethylene tubing. From the inlet passage, the hydraulic fluid enters the flow channel where the two viewing windows are located. The flow channel is 15.2 cIm long and has a rectangular cross-section (0.2 cm thick by 1.9 cm wide). The flow channel is located at the interface between two stainless steel blocks and is formed by machining slots 0.1 cm deep into each surface. An O-ring groove is machined on one of the surfaces surrounding the test section to prevent the leakage of hydraulic fluid. An O-ring seal was used rather than a flat . gasket so that the 0.2 cm flow channel thicknzss could be maintained more accurately when bolted together. Two fused quartz windows, one on each side of the 0.2 cm flow channel, allow the laser beam to pass through the hydraulic fluid. These windows consist of 2.54 cm diam. fused

k-

ro.s&~-q

Fig_ 2. Cross-sectional

drawing

of test section.

quartz disks, 0.635 cm thick. The windows are flush with the inside edges of the flow channel_ The 0.2 cm spacing between the windows is obtained by a (0.1 cm thick) flange on each side of the channel. The windows are held in place by a flanged sleeve inserted behind the windows and fastened to the base by three screws at the flange. An O-ring is used to prevent leakage of hydraulic fluid at the window-flange interface_ The test section has been operated satisfactorily in a low pressure test loop at approximately 50 psig.

IIL OPTICAL

SYSTEM

An electro-optical technique which utilizes a laser and an array of photodiodes was initially decided upon because of the success of Knollenberg [l] _ A sketch of the system is shown in Fig. 3. As can be seen, light passes through the sample fluid onto the array. Particles moving in the fluid transverse to the light source create shadows. These shadows are then magnified and focused on the light-sensitive photodiodes. Since the system is designed for a particular hydraulic fluid (MIL-H-5606B), light absorption measurements were made on the fluid using a spectrophotometer, and the results are shown in Fig. 4.1, is the incident intensity

205

Fig. 4. Absorption /MIL-H-5606).

spectra of the hydraulic

fluid

and I is the intensity measured after passing the light through a 1.0 cm thick sample of non-diluted hydraulic fluid. From this information the range of practical wavelengths falls between 0.60 and 1.10 pm since very little attenuation occurs in this region.

Therefore a Metrological Instrument, Inc., Model ML-930 He-Ne laser which has a power output cf 3.5 mW and a wavelength of 0.6328 pm was chosen as the light source. Its beam diameter, c, is 1 mm and the beam divergence is 0.8 m rad. Not only is the fluid transparent to radiation at tnis wavelength, but the photodiodes also have a near maximum sensitivity to radiant energy at the wavelength of the He-Ne laser, as seen in Fig. 5. Ordinary glass lenses are employed in the condenser and magnifier, and fused quartz windows are used for the optics without significant absorption losses. For example, the ratio of transmitted light to incident light for the 0.635 cm thickness of the fused quartz windows is greater than O-97 at a wavelength of 0.6328 pm. Measurements with a Hewlett-Packard radiant flux meter indicate that approximately 60% of the laser energy

206

reaches the photodiode array. A considerable portion of these losses are from reflections Fit the surfaces of the lenses and the optical windows in the test section. These losses can be greatly reduced by the use of coated lenses at a somewhat added expense. Coated lenses or a more powerful laser are not necessary, however, since tests indicate that a photodiode current of 1.5 microamperes results from a light intensity of 2.8 mW/cm’. This current was obtained with a reverse bias of 15 volts and is sufficient to overcome noise problems inherent in photodiodes. The magnification determines the number of photodiodes shadowed by particles of a particular size_ A quantization error occurs due to rounding the particle size to the nearest number of photodiodes. The smaller particles shadow the smallest number of photodiodes, and therefore the maximum quantization error occurs in sizing these particles_ With a threshold setting of 50% on the electronic counting circuit, the error of particle diameter measure_ment could be one photodiode- _A magnification of 80 X is used to size small particles, and consequently a 10 pm particle shadows 4 photodiodes and results in a quantization error of 2570, which appears acceptable_ The 80 X magnification cannot be used to size large particles, since a 100 pm particle will shadow 10 tunes as many photodiodes as the lo-pm particles or 40 photodiodes. Since the array has 50 elements with a total height of only 1.0 cm, the shadows of the 12-e particles extend aImost the complete photodiode array height, and, consequently, few large particles will shadow the photodiodes In order to avoid serious undersizing of shadows not completely within the array’s dimension,thetwo endelementsofthe array causethecountoftheparticleto

berejected

when occulted. Since the particle will not be counted if it shadows either of the end diodes, the sampling area decreases as the particIe size increases. A lower magnification would be desirable to increase the rate at which the large particles are counted. in order to obtain an acceptable quantization error for small particles and an acceptable rate of count for large particles, two different magnifications were chosen: an 80 X magnification for counting small particles in the range 10 - 50 Crrnand a 20 X magnification for

TABLE 1 Effectofmagnificationonthe diodes shadowed

numberofphoto-

Particlediameter

No. of photodiodesshadowed

Wn)

20x

50x

10 20 30 40 50 60 70 80 90 100

1 2 3 -:

4 8 12 16 20 24 28 32 36 -10

: 7 s 9 10

particles from 50 to 100 pm. Table 1 lists the number of photodiodes shadowed for these two magnifications_ In the schematic of the optical system shown in Fig_ 3, note that the laser beam is condensed before entering the test section, then the image of the shadows that are formed is magnified and projected onto the photodiode array. In order to maintain a parallel light source, a doublet lens system is used to condense the laser beam. The condenser reduces the diameter of the beam so that malsimum advantage can be made of the laser power. The beam diameter required is larger for the lower magnification, therefore the condenser was designed for the 20 X magnification- At this magnification a 0.5 mm diam. beam will fuiiy illuminate the photodiode array and wiZ increase the intensity by a factor of 4. The condensation is obtained by the llse of two simple convex lenses with the ratio of focal lengths equal to 2. By pIacing the lens with the larger focal length, f_L. near the laser and separating the lenses by the distance equal to the sum of their focal ler@hs, condensation of the beam remains collimated and no additional beam divergence is introduced_ The lenses presently being used have focal lengths of 30 mm and 15 mm respectively, separated by a distance of 45 mm. Because of possible errors due to edge diffraction and misalignment, it may be necessary to use a slightly larger beam to insure that the photodiode array is fully illuminated and that the beam intensity is as uniform as possible.

207

Magnification is required in order that the particle shadows occult several photodiodes. The magnifier magnifies the image in the object plane of the test section and projects this magnified image onto the photodiode array. There will be two magnifications, 20 X and 80 X _ These magnifications are obtained with a simpIe convex ocular having a focal length of 40 mm and a 12-5 focal length orthroscopic eyepiece_ The following thin lenses formulas were used to design the lens system: 1 1 -+7=s s m=-

1

(1)

f-l.

-S* (2)

s

is the image distance fr-3m the lens and s is the object distance. The focal length is represented by f-1. and m is the magnification. The negative sign in eqn. (I) means that the image is inverted_ Equations (1) and (2) assume an air media- Since the refractive index of the hydraulic fluid and quartz windows is approximately 1.5, these relations are only approximate. However, the exact location can easily be found experimentally. Knollenberg [I] used a threshold rejection technique to reject the count of particle shadows that are sufficiently out of focus. Threshold rejection requires the ability to tune the electronics. such that a photodiode

s’

AMSTCRGE

Fig_ 6_ Block

I--

Fig.

0.0’16scn

diagram

of the particle

detection

system

1

7. Dimensions

of the SA-50

photodiode

army

elements_

TABLE ThreshoId 40 50 60

2 setting

(‘?L)

C 1.88 2.63 4.13

will register only when the light intensity decreases to a preset fraction of background intensity. For esampie, a 40% threshold setting requires a reduction in intensity of 60% The maximum distance from the object plane at which particles will be counted is called the optical depth. Any particles farther from the object plane will not be counted since their shadows are not sufficientr ly dark. That is, the intensity is not reduced to the threshold setting of any photodiode. The optical depth, Do. is given by the following equation:

C is a constant which depends on the threshold setting. The above values for C have been determined by analysis of the Fraunhofer diffraction pattern (Table 2). Although the 40% threshold has the minimum depth of fteld, this setting was found to give the least amount of error in sizing. For example, it can be shown that the

208

diameter predicted with a 40% threshold would be approsimately 12 pm, compared with an actual diameter of 10 pm.

IV. SIGNAL

PROCESSING

SYSTEM

The signal processing system has been discussed in LIZearlier paper by Pettus et ale2], thus only a brief discussion will be given in this paper_ A functiorxl diagram of the signnl processing system is shown in Fig_ 6_ The q&em consists primarily of the photodiode array, the signal processing circuitry, particle sizing logic, and minicomputer display. The array was purchased from United Detector and consists of 50 photodiodes arranged linearly as shown in Fig. 7. The signal from each of the photodiodes is amplified_ When the amplified voltage reduces to a threshold level (normally 40% of its steady-state value), the signal is fed to a flip-flop. The information from the flip-flops of all diodes is processed following the transition of a particle to determme the particle size_ After processing,

the flip-flops are reset and are then ready for another particle. The computer system used in the project consists of a DEC PDPS/e minicomputer with 32 K words of memory, a dual TDSE DEC tape drive, two RK05 movable-head disk drives, two ASR-33 teletype terminals, one f-I-P 7260A card-reader, a Tektronix 4010 graphics terminal, a Tektronix 4610 hard copy unit, and a DEC FPP-12 floating point processor. The software provides for data collection manipuiation and display. A histogram of the particle size distribution will be displayed on the graphics terminal_

REFERENCES R. G. Knollenberg, Comparative liquid water content measurements of conventional instruments with an optical array spectrometer, J. Appl. Meteorol., 11 (1972) 501 - 507. R. 0. Pettus. A_ E. Rainsford and R_ D. Bonnell, In-line contamination monitoring of fluids for

space systems, Symp. March

Proc. ‘7th Annx

on System Theory, 20 - 21.1975.

Southeastern

Auburn

University,