GB2459848A

GB2459848A - Self-commutating asynchronous sequential reading system for chain of sensors

Info

Publication number: GB2459848A
Application number: GB0808187A
Authority: GB
Inventors: Cassim Ladha; Karim Ladha
Original assignee: CASCOM Ltd
Current assignee: CASCOM Ltd
Priority date: 2008-05-06
Filing date: 2008-05-06
Publication date: 2009-11-11
Anticipated expiration: 2028-05-06
Also published as: GB2459848B; GB0808187D0

Abstract

A system is described for asynchronously sequentially sending sensor measurements down a common output signal wire in a time multiplexed manner. The primary embodiment describes an array of capacitive pressure sensors C1a, C1b to C1n and C1(n+1). Each sensor unit in the chain comprises an RC circuit and a schmitt trigger which is used to compare the voltage across the pressure sensing capacitor to a threshold. The value of the capacitance effects the time delay before the input to the Schmitt trigger reaches the voltage threshold required to turn on the output of that Schmitt trigger. This converts the capacitance value to a variable time delayed voltage step. When the output of the Schmitt trigger goes high current flows through the next force sensing capacitor to ground. This current flow is detected using a bipolar transistor connected to the output line. When the output of the preceding Schmitt trigger goes high the next pressure sensing capacitor in the chain begins charging. The result is a series of low voltage pulses on the output line (see fig. 4). The time between each pulse represents the capacitance of each sensor stage. The time delays can be read by a microcontroller. The principle could be applied to other sensor types such as inductive, resistive, switch based, etc.

Description

Patent Application of CASCOM Ltd for Self-Commutating Asynchronous Sequential Reading System

of which the following is a specification:

FIELD OF THE INVENTION

The present invention pertains generally to electronic sensing technology, and more particularly to method of reading large sensor arrays.

BACKGROUND OF THE INVENTION

As technology becomes increasingly integrated with our world, electronic sensors have found many new applications. Large arrays of sensors can diversity to measurement applications and can allow hi-definition capture of a subject image. The main problem with implementing large sensor arrays is the supporting electronic hardware. Supporting hardware has traditionally needed complex interfac-ing circuits for the extrapolation of meaningful data. These circuits are often bulky, require complex management and are costly to manufacture. This invention details a novel, self-commutating, asyn-chronous method for sequentially reading large banks of sensors.

SUMMARY OF THE INVENTION

The object of this invention is an arrangement of passive and active electronic components to facilitate reading large sensor arrays.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which: FIG. 1 is a perspective view of a typical capacitive type sensor consisting of two parallel plates and a compressible dielectric material. The capacitance between the two plates varies as the dielectric is compressed; FIG. 2 is a schematic diagram used to instigate a delay proportional to the product of a variable capacitance and a resistance. Such a variable capacitance may be derived through an arrangement as shown in FIG 1; FIG. 3 is a schematic diagram of an array capacitive sensors organised in a self-commutating asynchronous sequential manor; FIG. 4 is a timing diagram of the output from FIG 3; While the patent invention shall now be described with reference to the preferred embodiments shown in the drawings, it should be understood that the intention is not to limit the invention only to the particular embodiments shown, but rather to cover all alterations, modifications and equivalent arrangements possible within the scope of appended claims.

Throughout this discussion which follows it should be understood that the terms "resistor", "transistor", "capacitor", "inductor" and "Schmitt trigger" are used in the functional sense and not exclusively with reference to specific solid state components, mechanical equivalents, discrete components or semiconductors.

DETAILED DESCRIPTION OF THE DRAWINGS FIG 1

The capacitive pressure sensor can be summarised as a parallel plate capacitor with a variable gap. A compressible dielectric is placed inside this gap and the sensor becomes a pressure sensitive capacitor (this is illustrated in FIG 1). As a force is applied to the plates, the dielectric compresses and the capacitance increases according to C = where A is the area of the plate, d is the separation distance and e is the permittivity of the dielectric. A relation can be then found to convert the measured capacitance to the applied force through modelling and or experimentation. FIG 2

To convert a capacitance to a time domain signal, an RC circuit and a comparator can be used.

The RC circuit creates a delay that is proportional to the capacitance and can be measured using a microcontroller. The delay associated with the capacitor sensor shown in FIG 1 when applied to FIG 2 may be calculated as follows: VcVi(ie) -+ t=RCln() (1) FIG 3 As an enhancement to the FIG 2, a Schmitt trigger is introduced to devolve any noise surrounding the measurements during pulse transition. This step allows multiple sensors to be read sequentially in a self commutating manner. In order to detect the transition of each logic gate, a capacitor is placed between the output and ground of each gate so that a low to high transition causes current to flow from the supply to ground. A current detection circuit consisting of a bipolar transistor and two resistors is used to detect the current spikes and allow them to be detected by the microcontroller.

The microcontroller measures the delay between pulses and hence detects the capacitance of each sensor. A microcontroller can be used to control the sensor power and initiate the reading process.

The final gate has a termination resistor chosen to draw enough current to bias the transistor on which provides confirmation that the read was successful after the microcontroller detects the final falling edge. FIG 4

The diagram in FIG 4 depicts a rising edge on the input, which is buffered by the first Schmitt trigger in FIG 2. The resulting voltage on the first RC network causes a voltage ramp on the input to the second logic gate. When this voltage reaches l', thresho'd the second logic gate output transitions from low to high. This results in a voltage on the second RC network which causes a further rising voltage on the third logic gate. The time taken for the rising voltage edge to reach the Schmitt trigger threshold is proportional to the product of the resistor and capacitor in the sensor network of FIG 3; as is equation 1. During the transition of each logic gate the corresponding transistor is temporarily biased into saturation by current flowing through C2 and the transistor's base emitter P-N junction. The transistor quickly desaturates due to the bias provided by R2. The total time during which the transistor is saturated is determined by C2 R2 and the current provided by the Schmitt trigger. While the transistor is saturated the output is held low. This whole process sequentially repeats through each sensor device in the array resulting in a series of negative pulses on the output which may be captured by a mircocontroller.

ENHANCEMENTS

The principle behind this invention can be encapsulated into an application specific integrated circuit (ASIC). Such an ASIC could embody single or multiple sub-circuits. When very large quantities of the invention are embodied on a strained semi-conductor it would be possible to have an application that reads high-definition data over the area of the implementation.

BENEFITS OF THE INVENTION

The benefits of this invention include, but are not limited to, significant advancement on current interface circuitry. Although the given description references a capacitive type sensor circuit, the underlying principle of the invention can be applied to many applications including: arrays or banks of switches; inductive networks and resistive networks. Furthermore, the invention could be used to capture video or audio type signals for encoding, transmission, or re-transmission.

OTHER EMBODIMENTS

From the foregoing description it will thus be evident that the present invention provides a design for reading large sensor arrays. As various changes can be made in the above embodiments and operating methods without departing from the spirit or scope of the following claims, it is intended that all matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense.

Variations or modifications to the design and construction of this invention, within the scope of the appended claims, may occur to those skilled in the art upon reviewing the disclosure herein (especially to those using computer aided design systems). Such variations or modifications, if within the spirit of this invention, are intended to be encompassed within the scope of any claims to patent protection issuing upon this invention.

REFERENCES

* Palasagaram J.N. Rarnadoss R. MEMS Capacitive Pressure Sensor Array fabricated Using Printed Circuit Processing Techniques, 2005 IEEE 0-7803-9252-3/05 * Patel A. Webster J.G. Tompkins W.J. Wertsch J.J. Electronic Circuits for Capacitive Pressure Sensors, 1989 IEEE Engineering in Medicine and Biology, CH2770-6/89/0000-1437 * Rey P. Charrvet P. DelayeM.T. Hassan S.A. A High Density Capacitive Sensor Array For Fingerprint Sensor Applications, 1997 IEEE 0-7803-3829-4/97 * Sander C.S. Knutti J.W. Meindl J.D. A Monolithic Capacitive Pressure Sensor with Pulse-Period Output, 1980, IEEE Transactions on electron devices Vol. ed 27, No.5 * Sergio M. et a! A Textile Based Capacitive Pressure Sensor, 2002 IEEE 0-7803-7454-1/02 * Sherman D. Measure Resistance and Capacitance without an A/D, 1993 Philips Semiconduc-tors Application Note AN449 * Tekscan Inc. "Flexible, Tactile Sensor for measuring foot pressure distributions and for gas-kets". Patent Number PCT/US1990/007326, Filed 11-12-1989 * Electric Field Sensing Device. European Patent EP1815258

Claims

CLAIMSThe embodiments of the invention in which I claim an exclusive property or privilege are defined as follows: 1. A self-communtating asynchronous sequential reading system comprises of a chain of individ-ual sensor units capable of passing sensor information in time domain signals down a signal wire and stating the next reading signal down the chain automatically. The term chain implies one or more electronic components or arrangement of electronic components. The term sensor units is used in a broad sense to encompass capacitive, inductive, resistive, switch based, or in fact any other device that would be suitable for reading in the style described by this invention.The claim of self commutating feature, refers to the ability of the invention to configure the next sensor in the said chain for reading in the same fasion as described in 1.Amendments to the claims have been filed as follows:CLAIMSThe embodiments of the invention in which I claim an exclusive property or privilege are defined as follows: 1. The invention comprises of a series wired concatenation of at least 2 electronic subcircuits that convert the values from electrical sensors, wired to the subcircuits, into time encoded pulses that are passed through the subcircuits to a decoding circuit connected to the the assembly, particularly at the end.
2. A circuit as described in claim 1, wherein an output of each subcircuit is connected to au input of another subcircuit, particularly an adjacent subcircuit, such that it causes that sub circuit to output its time encoded pulse once the first sub circuit has finished outputting its pulse.
3. A circuit as describes in 1 and 2, wherein the resulting concatenation and wiring of adjacent subcircuits cause the subcircuits to output their time encoded sensor values before triggering the : * adjacent subcircuit resulting in sequential commutation along the chain of subcircuits.
4. A circuit as described in claims 1,2 and 3, wherein the resulting sequence of time encoded pulses are passed along a single common wire connected to each sub circuit and a decoding circuit.
5. A circuit as decribed in claim 4, wherein the common signalling line is embodied as a logical ****** * wired OR' circuit. I...

*
6. A circuit as described in claim 4, wherein the electrical sensor element is a capacitive pressure, * displacement, position or proximity sensor.

* "a
7. A circuit as described in claim 4 and 6, wherein the sensors are arranged in an array to provide **** a pressure sensitive area or surface.
8. A circuit as described in claim 4 and 6, wherein the sensors are arranged in an array to produce an electronic keypad or keyboard.
9. A circuit as described in claim 4 and 6, wherein the sensors are arranged to measure a pressure distribution on a surface.
10. A circuit as described in figure 4.