Photoelectronic displacement sensor with nanometre resolution

Photoelectronic d i s p l a c e m e n t sensor w i t h n a n o m e t r e resolution N. Ikawa, S. Shimada and H. M o r o o k a * The design of an ultraprecision displacement sensor with sub-nanometre resolution is proposed for the use of ultraprecision technologies. The sensor is based on the highly sensitive detection of power change at a specific point in light reflected from an object surface when the surface is subject to a small displacement. The sensor consists of a single light source, the reflective object and reference surfaces, optical fibre bundles for transmission of the illuminating and the reflected light, and the photodiode set-up. In operation, the object and the reference surfaces are illuminated by light transmitted through the optical fibre bundle from the single light source. The reflected light from both surfaces is then transmitted through the optical fibre bundle to two individual but equivalent photodiodes. By using a differential amplifier, the diodes give a highly sensitive displacement signal which is included in the total signal of the reflected light. The sensor proposed has some distinctive features in its performance, such as non-contact measurement, a high resolution of 0.5 nm, a wide working range of about 30pro within 5% linearity, and the sufficient stability of I nm in 20 s for specific research purposes.

Keywords: optical techniques, displaced measurement, non-contact measurement

Ultraprecision production techniques have been extensively used in a variety of recent advanced industries. Surface roughness and dimensional accuracy of the products in some cases have been attained in the order of 10 nm and 100 nm respectively. To evaluate quantitatively performances of machines and tools used in these techniques and then control them for a specific process, development of non-contact high sensitivity displacement sensors are of essential importance. Among the sensors for these purposes, laser interferometric, capacitive, eddy-current and photoelectronic types are well known for their high sensitivity. However, the resolution of these sensors is not always high enough and may be only 20 to 10 nm at the maximum. Considering this situation, the authors have proposed a design for, and presented preliminary experimental results on, a non-contact reflective photoelectronic displacement sensor which can attain nanometre resolution using a differential method 1. This paper, then, deals with the detailed design of the sensor, which has been improved in terms of sensitivity and stability.

Principle and design Fig 1 shows the principle of a conventional photoelectronic displacement sensor, the optical * Department of Precision Engineering, OsakaUniversity, 2- I Yakada-Oka, Suita, Osaka565, Japan

PRECISION ENGINEERING

cantilever 2. The sensor consists of the light source, the photosensor and a pair of optical fibres as a probe. The object surface is illuminated at a specific distance Y0 from the end of the illuminating fibre. The reflected light is incident to the receiving fibre and is transmitted to a photosensor through the fibre. The power detected at the photosensor depends on the distance between the object and the probe, as shown in Fig 1 (b). The larger change in the power of the reflected light for displacement Ay in the range (I) shows the possibility of highly sensitive detection of small displacements. However, it is difficult to detect the small displacement signal with this conventional system, because the power change in the light transmitted by the fibre is so small that the displacement signal is hidden in a large dc component corresponding to the offset distance Y0. Fig 2 shows a schematic view of the improved design of the sensor proposed by the authors. The differential set-up which can compensate for the dc component is employed to improve the signal-tonoise ratio. In this design, through the specially formed optical fibre bundle, the object and reference surfaces are illuminated by the same light source, and the reflected light from both surfaces is transmitted to their equivalent photodiodes (1) and (2) respectively. The difference between the photocurrents obtained from photodiodes (1) and (2) is converted to an output voltage by the differential amplifier as shown in Fig 2. Thus, the amplifier gives the signal corresponding to displacement only.

0141-6359/87/020079-04/$03.00 © 1987 Butterworth & Co (Publishers) Ltd

79

Ikawa, Shimada and Morooka --photoelectronic displacement sensor with nanometre resolution Object surface J

Offset

Illuminating fibre

Illuminating fibre

•4 l - -

I ,-.ll~ ....

Photosensor

Receiving- - - I ~ o

Displacement

(z~y)

fibre

utput -,-( photocurrenf)

,undle

Water i

a

Prnox

;] filter

/ ~ , ~ , ~ ' - " ~ Low sensitivityregion

r'(rr) High sensitivity region ]= o ,o~.

,

A/ r oo.et

b

Distance between probe and object

Fig 1 Principle of a conventional photoelectronic displacement sensor (a) optical cantilever principle (b) change in power of reflected light corresponding to the distance between the probe and the object. Po = dc component, &P = variable component corresponding to displacement, Yo = offset Each fibre bundle consists of 1200 optical fibres of 25 #m diameter for transmitting larger power. To avoid electrical noise problems batteries are used for the power source. As the light source, an ordinary 50 W tungsten lamp is used. The metal case of the lamp and the condenser lenses are water-cooled

Si - PIN ~ photodiode (2)

Reference ~ surface

I'l Reference

rl %

lOOK

fibre bun~lengsten

rout measurement)

I'1

(12V)

0 ~

Measuring hJ fib~ bundle Displacement Itl I ~ ,L, Si-PIN _

_"

<> ,o )

=T- photodiode (i) o.~ -

22

O~ @rout

( for peak power detection)

Fig 2 Schematic diagram of the reflective photoelectronic displacement sensor with sub-nanometre resolution 80

Fig 3 Schematic view of the light source set-up

and thermally stabilized to prevent the fluctuation in the incident angle and the focus, see Fig 3. Resolution

and frequency response

The sensitivity of the sensor is calibrated by measuring the displacement of an aluminium spattered silicon plate fixed either onto the piezoelement driven by a rectangular pulse and/or onto the cantilever, which can be deformed by a small weight, see Fig 4. The displacement of the piezoelement is calibrated by Talystep, Fig 5 shows examples of the displacement time-chart measured on the plaza-element, which has a displacement of 0.5 nm. From the results shown, the resolution of the sensor is considered to be greater than 0.5 nm. A resolution of the order of 0.1 nm is attainable with a low pass filter. Although the output voltage, due to a certain displacement, varies depending on the power of the illuminating light and the reflectance of the object's surface, the value relative to that of the maximum reflection (Pmax in Fig 1 (b)) remains unchanged. Thus the sensitivity, for the specified object surface and light source, can be obtained by comparing the output for the maximum reflection from the specified set-up with that of the standard set-up for which the sensitivity is calibrated. In principle, the frequency response of the sensor is determined by the characteristics of the photodiode and the amplifier. The cut-off frequency APRIL 1987 VOL 9 NO 2

Ikawa, Shimada and Morooka--photoelectronic displacement sensor with nanometre resolution

_

~

Sensorprobe

• ~Yo

/ -I

Reflecting plate

I

L --

I oscillator

Linearity

a

~

r

. . . .

Sensorprobe

-l--

I Ay

!::+<: ,:i~?::

:+~ :::::, :::::

String Cantilever

(~

b

ratio can be obtained by employing a low pass filter, so reducing the high frequency noise contained in a specific signal. In the present set-up, one nanometre resolution can be obtained without any low pass filter. As shown in Fig 5, using the low pass filter with 10 Hz cut-off, the sensor is capable of measuring a sub-nanometre displacement when its rate is slow.

-Weight

Fig 4 Sensitivity calibration of the sensor by (a) displacement of the piezo-e/ement, and (b) elastic deformation of a cantilever. Yo = offset, Ay = variable component corresponding to displacement

The short range linearity of the sensor is measured by the method shown in Fig 4. The sensor shows a sufficient linearity--less than 0.5% over 7 # m - when the distance between the sensor probe and the object surface is from 13 #m to 20/~m, see Fig 6. The wider the range, the worse the linearity: in the range over 10/~m the linearity is about 1%. To estimate the working range of the sensor, long range linearity is measured by a fine positioning mechanism using a pair of parallel leaf springs as shown in Fig 7. A linearity range of less than 5 % can be obtained over 30 #m, as shown in Fig 8. Then, the practical working range is considered to be about 30 #m. The stability of the sensor (drift) is a problem in practical applications. Fig 9 shows the fluctuation in the output signal for nominally no displacement between the sensor probe and~object surface (both firmly fixed on the anti-vibration base). The sensor is considered to have a stability of 1 nm in 20 s which can be sufficient for a specific research purpose. The drift of the sensor is mainly affected by the thermal deformation of the light source set-up by the heat generated from the tungsten lamp.

of the amplifier in the present design is set at about 1.6 kHz which is much lower than that of the photodiode. In practical applications, however, the upper cut-off frequency of the sensor is set such that the desired resolution or higher signal-to-noise

]0.2 mV

]0.2 mV

and stability

0.5

g o

without LPF•

with 2kHz LPF~* ]0.2 mY

a

withI kHz LPF*

]o.z v

=)

with 500 Hz LPF*

Fig 5 Resolution of the sensor. (a) Outputs corresponding to 10 Hz, 0.5nm rectangular displacement at various cut-off frequencies, (b) output corresponding to 1Hz, 0.5nm rectangular displacement (with 10 Hz LPF) (*LPF = low pass filter). PRECISION ENGINEERING

~O.4

o.]

IO

I

15

I

20

Distancebetweensensorprobeendobjectsurface,pm

1

25

Fig 6 Short range linearity of the sensor, (* relative value to that of maximum reflection) 81

Ikawa, Shimada and Morooka--photoelectronic displacement sensor with nanometre resolution r 1 ~ I

Mount for sensor probe

Micrometerhead for ./adjusting probe position I 7

,% 1.0

I

.................................................. III ~, ;I II ......

5% lineority

..ran, micrometer

heed

/

0.~

•

2

.=>_ "6

Fig 7 Calibration set-up for larger displacements

Conclusion An improved design of the non-contact reflective photoelectronic displacement sensor with subnanometre resolution is proposed. The sensor is based on the highly sensitive detection of power change at a specific point in light reflected from the object surface, when the surface is subject to a small displacement. By applying the differential principle to the conventional design to improve signal-to-noise ratio and stabilize thermal drift, the sensor proposed has achieved good performance. It has distinctive features in its performance, such as sub-nanometre resolution, the high linearity of 0.5% over the range of 7/Jm, a wide working range of about 30/Lm within the 5% linearity, and the stability of 1 nm in 20 s which is sufficient for specific research purposes.

Acknowledgements The authors express their sincere thanks to the Yamazaki Foundation for its financial support. They are also indebted to Messrs S. Matsubara and H. Hirano for their cooperation in the experiments. Special thanks are due to Messrs T. Suwaki and C.

82

0

20 40 60 Distance between sensor probe and object surface, pm

Fig 8 Working range of the sensor

I nm

~

5s

Fig 9 Drift of the sensor Sato, Olympus Optical Co Ltd for their cooperation in the development of the optical fibre bundle.

References 1 Ikawa N,, Shimada S, M o r o o k a H. and Matsubara S. Preprints, Jpn Sac. Precis. Eng., .Tokyo, 25-27 March 1984, 917. (to be published in J. JSPE, in Japanese) 2 Cook R. O. and Harem C. W. App/. OpL, 1979, 18, 19, 3230

APRIL 1987 VOL 9 NO 2