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|The principle of capacitive
sensing is not a new one. The technique relies on the fact that any object in
free space has an electrical capacitance to earth. The value of this
capacitance depends on several factors, the principle ones being the object's
surface area, the distance to earth and the permitivity of the intervening
The range and sensitivity of an instrument depends on the ability of the
electronics to accurately measure extremely small values of capacitance. It is
in this area that much of Sensatech's research has been focused, resulting in
techniques which maximise noise rejection and minimise the unwanted effects of
Sensatech has developed specific and unique skills focused around areas
critical to the development and production of complex sensing solutions:
Sensor Head Design:
EIT Electrical impedance tomography:
types of electric field sensing:
Look or signal through optically opaque
Look around targets:
1 Sensor Head Design:
Sensing electrodes are designed using field modelling techniques to create
ideal sensing electrodes to match specific applications. Sense electrodes can
be optimised to retain very low parasitic capacitance which allows multiplexing
of multiple inputs from many electrodes (such as in an array) with minimum
signal degradation and the detection of very small changes in the target.
2 Array control:
Arrays of multiple sensor heads are multiplexed to allow Sensatech to
collect large sets of data and build images of a target object which can then
be analysed with proprietary embedded software. Because through careful
electrode design, parasitic capacitance remains very small, Sensatech are able
to measure down to 3 fempto farad total capacitance with 12 bit resolution and
3 atto farrad accuracy. Such small change is valuable for example in gap
measurement. This low parasitic capacitance also permits positioning sensor
electrodes at the end of co-ax cables separate from amplifier and electronics.
These remote sensor heads, sometimes multiples of separate heads, can then
operate in hazardous environments.
3 Software development:
Data sets generated by sensor arrays can be large and complex. Some arrays
contain 2000 odd electrodes all of which are scanned and the resulting data
once processed provides knowledge of the target object.
Software can be taught the characteristics of the particular sensor in order
to identify and reduce noise and interference. Often reference capacitors are
included in the PCB design to further calibrate sensor elements.
Sensatech analytical software can vary from simple to very complex depending
on the task requirements:
- Neural nets can be used to predict what a sensor should see if detecting
- Finite element analysis can split the target space into many sub spaces
each of which can be separately solved to produce an overall result,
- A sensitivity analysis database normally goes with each sensor to aid its
resolution of particular target families. In the example of the foot-gauge the
software copes with feet presented at widely varying angles and is programmed
to ignore targets that do not resemble human feet.
4 Amplifier design.
Low noise with high input impedence and low capacitance guard amplifiers
have been built with excellent temperature stability.
5 Electrical impedance tomography
Uses array generated data, then using back propogation techniques compares
and reshapes the permiability characteristics of the space until it meets the
data found by the sensor. This is the basis of X-RAY type tomography that
Sensatech carry out using electric fields.
6 Remote sensing:
15m away from the amplifier and signal electronics.
7 Basic types of electric field sensing:
- Standard electric field sensing: Transmit and receive: In this format a
transmitter produces a field and if the capacitance between the transmit and
receive plates is large the sensor detects this.
- Standard using an array: A more complex variant of the standard electric
field sensing model is to use an array of transmit and receive plates which
generates many fields. By AC driving the plates at a frequency for example less
than 1 Megahertz this provides a large data set. Using a swept frequency it is
even possible to determine chemical parameters of the target material or
object. This can be used for example for height independent soil water content.
- Potential sensing: Because Sensatech receiver plates have only a few fimto
farads of parasitic capacitance, if a voltage exists on an object, that voltage
will be creating a field and this can be sensed at a range.
can detect the potential created by a heart beat 1cm away from the surface of
For mine detector use Sensatech set up a voltage gradient in the
soil, with other electrodes and sense this gradient with a large matrix above
the soil. Any metal or plastic in the soil distorts the voltage gradient and
this distortion can be measured to determine what is inside the soil. Normally
such work is in conjunction with a normal transmit and receive sensing method.
- EIT Electrical Impedance Tomography: While EIT is normally carried out with
a transmitter and receiver plate in contact with the target object, using the
techniques outlined above, Sensatech do not require to contact or invade the
target object and can non invasively detect through other materials.