Remote measurement of small displacements using a CD pickup head

F

W

RTH

Remote me~~ement of small displacement using a CD pickup head T.D. Rowsell engineering Cambridge Received

D~partmellt, Univ~rsi~ CBZ lPZ, UK

25 April

l!X34,

accepted

2 .Auugust

Keywords:

Remote measurement,

Med.

Plays., 1995, Vol. 1’7. 459-461,

Eng.

of Cambridge,

Trumpingt.on

Street,

1994

small displacements,

CD pickup

head

September

INTRODUCTION The recording and analysis of heart beats and other sounds produced naturally by the human fetus provides a valuable and non-invasive metric of fetal well-being. Vibrations initiated by fetal heart beats are heavily attenuated as they propagate through the intervening tissue to the surface of the maternal abdomen. Successful recording of these vibrations must allow for their microscopic amplitude and the high compliance of the maternal abdomen. Thus the ideal recording transducer presents no inertial load to the skin surface and there is no vibration attenuation* With the goal of designing an improved fetal sound ‘microphone’ an optical displacement measL~rement technique has been investigated. Using optical techniques in recording sub-millimetre displacements offers the possibility of precise measurements, insensitivity to electrical interference and the potential for remote sensing. The Compact Disc (CD) pickup head integrates a coherent light source, optics including an objective lens with built in actuator, and a photodiode array. This is within a compact package and available at comparatively low cost. As they are normally only available as a ‘spares item’ technical information is difficult to obtain. The purpose of this note is to describe a simple working system, easily constructed, to promote experimentation in using CD pickups for distance measurement. The interface circuit described is designed for a Sony pickup (KSSZlOA). However, only trivial modifications are required for it to be used with CD pickups from other manufacturers. A system in which a beam of light is directed at a mirror and reflected onto a photodiode provides a convenient technique for remote distance measure-

ment. When suitably configured, displacement of the mirror modulates the amplitude of the photodiode output. In this way relative displacement of the mirror is determined from the photodiode output. The same principle is utilised in compact disc (CD) players in order to maintain the CD pickup head at a fixed distance, (to within microns), above the CD surface. This principle is again used here to describe use of a CD pickup to measure small displacements of a reflecting surface. METHODS The principles of operation of a CD pickup head are documented in Baert et aP. A coherent light beam is projected onto a reflecting surface (in normal operation the CD). The reflected beam is collected in the objective lens and focused onto an array of photodiodes. The optics are arranged such that the distribution of light on t,he photodiode array is indicative of whether the system is correctly focused. When light is evenly distributed across all four photodiodes the system is focused. Two of four photodiodes become selectively illuminated when the system is out of focus. For example this may be caused by displacement of the reflecting surface towards the lens. Which two photodiodes are illuminated relates to the direction of the required focus correction. It follows that the differential in output between photodiode pairs, the focus error, gives the direction and mapitude of the correction needed to restore focus.

F&MY I gives a basic circuit to drive the Sony KSSZlOA CD pickup. The laser diode output is very temperature dependent. To compensate, CD players use a power supply circuit regulated via

Meawraent

using CD pickup

head: T.3. Roumll

Interface

KSS210A

WK ” PO Output

FQcyg+ 8

I

F’igure

1

Basic circuit

to interface

Sony KSS-2lOA

CD pickup

head

feedback from a monitor photodiode positioned in close proximity to the laser diode. For basic experimentation a simple current limited supply, (with no feedback) is sufficient. The lens actuator is a current-to-displacement transducer and enables focusing adjustments. The static transfer function has been experimentally verified by the author to be approximately 0.1 mA per micron displacement. The maximum range of displacement of the lens is approximately 1 mm. It is shown here being driven by a power operational amplifier. The CD pickup should be mounted on a fixed rig with the laser output directed downwards. This is to minim&e the risk of accidental exposure to laser light, and to ensure consistent operation of the lens actuator system. Displacements of a reflecting surface placed approximately 3 mm below the lens may then be recorded.

460

B

RESULTS Figure 2 shows the photodiode

outputs and focus error signal as the lens is moved through the point of focus (reflecting surface fixed). Equivalent results should, of course, be obtained by fixing the lens and displacing the reflecting surface. When the error signal is zero the system is focused. It is seen also from Figure 2 that the focus error signal may also be used to indicate displacements in the order of several tens of microns. Larger displacements may be estimated from the summed output of either or both pairs of photodiodes.

ACKNOWLEDGEMENT The research presented here was wholly by the British Heart Foundation under grant PG/93065.

funded project

Photodiode 4000

output vs. Lens displacement I ! I

3500 3000 1.;ii 2500; 2 2000f 3 15002 al 2 looou 2 g 500O-500 -

out of focwsignal

-100 900

Figure

2

Photodiode

outputs

versus

I 800

/ 900

I I 1000 1100 Lens displacement [urn]

I 1200

= A-B I 1300

1

lens displacement

REFERENCE 1. Baert L, Thuenissen L, Vergult and compact disc technology, Newnes.

G. (eds) Digital second edition

audio 1992.

461