Novel opto-electronic and plastic optical fibre sensors.
The design and performance of a novel displacement sensor is investigated both theoretically
and experimentally. This is an optical, extrinsic and differential sensor based on the inverse
square law and is insensitive to source intensity variations. It can, in principle, be
implemented using only opto-electronic components or it can incorporate optical fibres to
allow for EMI free and remote operation. The sensor is implemented using Plastic Optical
Fibres (POF) as these offer considerable advantages over glass fibres or glass fibre bundles.
The sensor head consists of three POFs positioned side by side and displaced from each
other parallel to the axis of the sensor head by a separation X, (mm). The middle POF is
coupled to a red LED and emits light onto a flat target with the two outer fibres receiving
the reflected light from the target and guiding it to two silicon PIN photodiodes.
Theoretical investigations on the behaviour of the sensor are presented for ranges
between 0 mm and 100 mm, and for targets with different reflectivities. Non-linearities in
the form of a spike are shown to exist in the very short ranges resulting in a minimum
operational range of about 15 nun Beyond this minimum range the sensor response is linear
and depends on the reflectivity of the target, the accuracy of calibration between the two
detectors, any offset voltage present in any of the detectors, possible errors on the detected
signals and the X, separation which in principle can be used to scale the sensor.
Experimental results obtained confirm the long and linear operational range of the
sensor (between 15 mm and 90 mm for a mirror target and between 20 mm and 100 mm for
a matt white paper target). Likely variations in the source light intensity do not affect the
performance and accuracy of the sensor. Measurements performed with various X,
separations verify the scalability feature of the sensor in that by increasing X, one can
achieve longer operational ranges. Temperature variations up to 40 °c do not affect the
linearity of response. Effects arising from angular misalignment of the target and! or the ends
of the three POFs are also investigated and could be minimised by rotating the emitting
POF. Matt white paper is concluded as the preferred type of target since it offers a longer
linear operational range with less stringent alignment requirements as opposed to reflective
Operation of the sensor under ambient illumination conditions is demonstrated using
suitable electronic circuitry with filtering facilities. The result is a linear operational range
of 60 mm with 1 % accuracy with a matt white paper target.
An automated version of the sensor under software control is also demonstrated for
monitoring large amplitude (0.15 mm - 6 mm), single degree vibrations. The maximum
determined frequency of the vibrating surface is about 150 Hz and this is only limited by the
target displacement which is close to the resolution limits of this version of the sensor
This novel sensor offers considerable advantages over other sensors reported in the
literature. It is shown to offer a very long and linear operational range in excess of 100 mm,
with accuracy better than 1% and resolution better than 0.2 % of range, and currently this
performance is only limited by the electronic circuitry used. Overall, the proposed sensor
offers a superior sensor head arrangement and performance combination and its cost is
expected to be very low. Suggestions for improvements and other applications are offered.