NONCONTACT SPEED MEASUREMENT ON RUNNING THREAD USING SPATIAL FILTER

by H. Morikawa, M. Nakazawa and T. Hayashi

N e w sensors with advanced func- tions are of maor interest to the textile industry. In partic- ular, sensors for noncontact measurement that are able to measure the speed of moving textile materials are required. This article is devoted to speed measurements based on the spatial filter, which is a sensor for dealing with spatial information such as pattern recognition.

The spatial filtering method for noncontact speed mea- surement was proposed by A. Kobayashi.'The basic con- cept of spatial filtering is to observe the natural irregularity of a moving object through an optical system and a set of parallel slits. This works as a kind of narrow-band-pass spatial filter that selects a particular spatial frequency component from the irregularity. When the object moves, a narrow-band random signal with a center frequency is proportional to the running speed of the object. The speed of the object is then determined from the center frequency of output signal.

The purpose of this paper is to study, experimentally, the problems encountered when the spatial-filtering method is applied to the noncontact speed measurement of running thread on which distinguishable marks or small reflectors cannot be attached. A new measuring technique is pro- posed for threads with no optical irregularitysuch as nylon.

As a result of examining data for measuring the speed of running threads, experimental results are in good w e e - ment with calculated results; the error in estimating a center frequency, which is proportional to the speed, is about 2.0 percent. This study shows a good possibility that the spatial-filtering method can be widely applied to the noncontact speed measurement of textile machinery.

Dr. H. Mor&awa is Associate Professor and Dr. M. Nakazawa is Professor at Shinshu University, Ueda-shi, Japan, and I: Hayashiis with the Olympus Optical CO., LTD., ha-shi, Japan.

/ Photo Detector 4 Condenser - Lens

I I Spatial Filter

Moving Object

f(x,y)

Uniform Light Flux

Fig. 1-Schematic model of spatial filtering

PRINCIPLE OF SPATIAL FILTERING

A schematic model of spatial filtering is shown in Fqj. 1. When an object is moving along the x coordinate in its negative direction at a constant speed, V; and the uniform light flux irradiates to the object, an optical spatial pattern with random intense distribution q x , y ) is obtained because of different transmissivity of the object. The spatial Nter is a spatial-weighting function h( x,y) and the integration is realized by the condenser lens collecting the hght flux.

The output is expressed as

whereX, L are the spatial filter dimensions in the x,y direc- tions, respectively, and no = V, + C,, YO = Ca. CI,Ca are constants of relative position.

The spatial power-spectral density function W ( p ) is defined as,

m w.4 = j-, W p , v ) IH(p,v)l2dv (2)

where L X

H(F,Y) = j, du j, dx h ( x , y ) exp { - j 2 r ( p x + vy)} (3)

H ( p , v ) is a Fourier transform of the spatial filter in a spa- tial domain. @ ( p , Y ) is a power spectral density off ( x , y ) and p , Y are spatial frequencies concerning the Cartesian coordinatex,yrespectively. I H ( p , v)l ineq(2) represents the filtering effect. If h ( x , y ) is constant in y direction, eq (2) is approximated by

* ( p ) * L * ( p , O ) IH(p)12 (4)

(5) X

~ ( p ) = j, h ( x ) exp ( - j 2 r p x ) d x

It is clear from eq (4) that the spatial filter has a filtering effect corresponding to the form of IH(p)l in the object spectral @ ( p , O ) .

Considering the effect of velocity V; the power-spectral density function Q ( f ) of output in a time domain is expressed as

(6) 1 f n(f) = -* (-) v v

Ifthefunction IH(p)l 2ineq(4)canselectonlythespatial frequencyp = w,thefunction * ( p ) hasasharppeakspec- trum at c = PO, except 9 ( ~ , 0 ) = 0.

The function n(f) has also a sharp peak at the temporal frequency f = V,. Therefore the output obtained is a narrow-band sinusoidal signal whose center frequencyf,is proportional to the speed K It is therefore possible to measure the object speed by measuring the frequency.

Different i a I construct i on

o f parallel s l i t

Spat ial frequency p

Flg. 2-Spatial frequency characteristics of I H (c) 1 1

In the case of the optical spatial pattern with magnifica- tion m the output frequency fo is expressed as

fo = mVM (7)

To realize a speed measurement, it is necessary to con- struct the spatial filter so that the function I H(p)l a selects only the desired spatial frequency PO.

The spatial filter formed by differential construction 01

parallel slits exhibits a sharp peak at PO = 1/P (shown in Fig. 2). Substituting value of PO into eq (7), the following equation can be obtained

fo = mVIP (8)

where Pis a pitch of spatial filter.

P=1.2 mm W=0.53 mm L=12.7 mm N =46

Silicon photo - cell Output reed(1)

/ \ \ A \ . '. I I

,

I \ I/ T \ Output reed(2)

Ceramic base board

Fig. 3-Schematic outlook of spatial filter detector

MEASURING SYSTEM CONS'IXUCTION

A spatial-filter detector has two functions: to perform as a spatial filter and a signal detector. The spatial-filter detector is a silicon solar cell whose surface pattern is designed as shown in Fig. 3. The solar cell forms the differ- ential construction of a parallel-slit array which is similar to a comb in shape. The speed-measuring system is shown in Fig. 4. A thread as an experimental material driven by a D/C servo-motor is illuminated by a constant light flux provided from a tungsten halogen lamp and the image of the thread is focused on the spatial-filter detector through an optical system. The output of the spatial-filter detector,

D.C.motor Spatial filter pu1’X detector

D . C

Load resistors

which is the difference between thevoltages across the load resistors, is amplified by a differential amplifier. The output signal, which is a narrqw-band random signal, is processed by a microcomputer through an A/D converter. After the frequency of the signal is analyzed by programmed FFT, a center frequency which is proportional to the speed of the thread is estimated. Then the speed of the running thread is determined from the center frequency by eq (8). The out- put signal of the spatial-filter detector and the results of the frequency analysis are plotted with an XY plotter.

Materials used in this experiment are as follows: threads with optical irregularity as shown in Fig. 6 such as Cotton thread A (4 0.48 mm); Cotton thread B (4 1.65 mm); Wool yarn (4 2.82 mm); and threads with no optical irregularity such as a Nylon thread (4 0.15 mm).

Cotton thread A

Cotton thread B

Woolen yarn

Fig. 5- Threads with optical irregularity

EXPERIMENTAL RESULTS AND DISCUSSION

Sample wave forms of both an output signal and apower spectral-density function in the case of measuring the speed of a running thread are shown in Fig. 6. The ordinates

A/D converter

computer

aDn display plotter Printer Fig. 4-Scheme of speed-measuring system

Cotton thread A (a) Output signal

-1 Ti me

(b) Power spectrum 1 1 ,

L a,

0 3 a

- 1 41 msec

Center frequency foz1.761 kHz

0 2 4 6 Frequency f (kHz)

Fig. 6- Wave forms of both an output signal from the spatial filter and a power spectrum

of both graphs indicate normalized values. The output sig- nal (a) from the spatial-filter detector is a narrow-band random sinusoidal-like signal of which amplitude and phase change at random. The spectral distribution (b) shows a sharp peak spectrum whose frequency is a center frequency. A result of the speed measurement of the run- ning threads is shown in Fig. 7. The experimental result shows good agreement with theoretical results. It may be considered that the nap of the materials provides an opti- cal irregularity, and that there is no effect of thread size. The speed-measurement error in estimating the center fre- quency is about 2.0 percent for cotton threads and a wool yarn. The cause of the error can be considered to be the lateral motion of a thread.

2.0- n

2 Y W

2 1 . 5 - >r v

k 0.5 aJ

V

m 4 .4 Cotton thread

0 A(90.48mm) 0 B (6 1.65mm) A Woolen yarn

(82.82 mm) / B

0 0.5 1 .o 1.5 Speed of running thread V(m/s)

Fig. 7- Relationship between the speed of running thread V and the center frequency f,

On the other hand, the speed of threads with no optical irregularity, such as nylon threads, could not be directly measured using the system just described. However, a new measuring technique is proposed. It is found that the speed measurement can be obtained by spraying the nylon thread with a substance which gives the thread no change in mechanical quality, but does provide optical irregularity. The effect of this marking should dissipate after measure- ment. This is a kind of marking technique which is generally used in temporary operations. The experimental results for nylon threads sprayed with water and ethyl alochol are shown in Fig. 8. Photographs in Fig. 8 are taken through a microscope of 30 magnification. As is evident from the photograph of the nylon thread sprayed with water, water drops distributed on the surface of the nylon thread can be observed and work as the optical irregularity. The center frequency can be estimated from the spectral distribution.

In the case of the nylon thread sprayed with ethyl alco- hol, the magnitude of the output signal is very small com- pared with that of noise. The peak value of the available power spectrum for the nylon thread sprayed with ethyl alcohol is one-tenth of that for the nylon thread sprayed with water. Therefore, it is difficult to slect a center fre- quency. That is why a film of ethyl alcohol formed on the surface of the nylon thread does not work as an optical irregularity. As a result of the nylon thread sprayed with water as shown in Fig. 9, the experimental result approxi- mately agrees with the theoretical result and the error is about 6.6 percent.

CONCLUSIONS

The concept of spatial filtering is described, and noncon- tact speed measurement of textile materials using the spatial-filtering method is studied.

c c 3 3 ao no c c 3 3 0- 1 0- 1

1 z 3 n 0

0

1 -

t 3 n

f o =1587 kHz

0

2 4 6 8 1 0 2 4 6 8 1 0

Frequency f (kHz 1 Frequency f (kHz)

Fig. S(a)- Wave forms of both an output signal from the spatial filter detector and a power spectrum for a nylon thread sprayed with water

FM. S(b)- Wave forms of both an output signal from the spatial filter detector and a power spectrum for a nylon thread sprayed with ethyl alcohol

n 3.0 N I Y W

0 .c

>.2. c z Q 3 U Q L .c L 1.0 Q C Q u c,

0

m 4 . 4 Experimental J - Theoretical

t'. r

1.0 2.0 3.0 Speed of running nylon thread

V ( m l s )

Fig. 9- Relationship between the speed of a running nylon thread V and the center frequency f,

The experimental results are summarized as follows: (1) the error in estimating a center frequency which is propor- tional to the speed is about 2.0 percent for threads which exhibit an optical irregularity, and (2) the spatial-filtering method is also applicable to the speed measurement of textile materials whose optical irregularity cannot be observed when the materials are sprayed with a substance like water, which gives the materials no change in quality.

It is hoped that the spatial-filtering method is widely applied to the noncontact speed measurement of textile mlac hinery.

REFERENCES

1. Naito, M., Ohkami, Y; and Kobayashi, A., "Non-Contact Speed Measurement Using Spatial Filter," J S.I.C.E. (Japan), 7-1 I , 761 - 772 (1 968). 2. Kobayashi A, "Spatial Filter and Its Application [l]," J of f3I.C.E. (Japm), 194,40941 7(1980). 3. Kobayashh A, "Spatial Filter and its Application [Z],,"J.SI.C.E. (Japan), 19-6,671-680 (1980) 4. Kobayashi A and Naito, M., 'Narrow-band-pass OpticalSpatial Filter for Measurement, "S.I.C.E. (Japan), 5-2,142-149 (1969). 6. Ator, J. 2, uImage-veloci@Sensing with Pardel-SIit Reticles,nJ. of OpticalSoc. o fher i ca , 63-12 1416-1422 ( 1 M ) .

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