AU5975598A - Infra-red sensing device - Google Patents

Description

INFRA-RED SENSING DEVICE

This invention relates to an infra-red sensing device. The invention has particular but not exclusive application in an infra-red touch-switch system.

In one aspect the invention provides an infra-red sensing device having an element transparent to infra-red radiation, infra-red radiation means at one side of the element for directing infra- red radiation on a path through the element, and infra-red detector means at said one side, positioned to detect infra-red radiation passing from the infra-red radiation means through the element and reflected by an object at the other side of the element back through the element to the infra-red detector means, wherein the infra-red radiation means and the infra-red detector means Figures 4A, 4B and 4C such that there is defined a sensing zone within which an object must be positioned to enable detection thereof, which detection zone is positioned at said other side of the element but so as to be at least predominately away from a surface of the element at said other side of the element. Preferably, the infra-red sensing means has an optical filter which in use substantially blocks visible light from detection by the infra-red sensing means.

In one form of the invention, the infra-red radiation means and the infra-red detector means are arranged at opposite sides to a normal to the surface of the element and such that the infra-red radiation means in use predominately directs said infra-red radiation therefrom at a first angle relative to said normal to said surface which is substantially greater than zero degrees. For example, said angle may be in the range 45 to 75 degrees, such as 60 degrees. The space within which the infra-red radiation from the infra-red radiation means is projected, and the space within which infra-red radiation may be detected by the infra-red detector means, are preferably arranged such that areas on the surface of the element remote from the infra-red radiation means and infra-red detector means, and defined at the intersection of that surface with said spaces, are arranged to touch but not intersect each other. The detector means may be arranged to in use predominately detect radiation passing thereto at a second angle relative to said normal to the surface which second angle is substantially 90° .

In a second aspect, the invention provides an infra-red sensing device having an element transparent to infra-red radiation, infra-red radiation means at one side of the element for directing infra-red radiation on a path through the element, and infra-red detector means at said one side, positioned to detect infra-red passing from the infra-red radiation means through the element and reflected by an object at the other side of the element back through the element to the infra-red detector means, including switch means and control means coupled to the infra-red detector means and the infra-red radiation means, said control means in use causing said infra-red radiation means to emit infra-red radiation for a first time interval and t o not emit infra-red radiation for a second time interval, and said control means being coupled to said infra-red detector means to be in use to actuate the switch, under the condition that the difference between levels of infra-red radiation detected by the infra-red detector means during the first and second time intervals falls within a predetermined range. The first time period may follow the second time period, but that is not essential; the second time period may precede the first time period

In a fourth aspect, the invention provides an infra-red sensing device having an element transparent to infra-red radiation, infra-red radiation means at one side of the element for directing infra-red radiation on a path through the element, and infra-red detector means at said one side, positioned to detect infra-red passing from the infra-red radiation means through the element and reflected by an object at the other side of the element back through the element to the infra-red detector means, including switch means and control means coupled to the infra-red detector means and responsive to said detection of infra-red radiation by the infra-red detector means to actuate the switch, wherein said control means includes an active load for the infra-red detector means, which load is arranged to present a substantially constant output from the infra-red detector means when the infra-red radiation incident on the infra-red detector means changes slowly over a substantial range, but such that the output changes substantially when the incident infra-red radiation changes more rapidly. The load may be formed by a FET.

In another aspect the invention provides an infra-red sensing device having an element transparent to infra-red radiation, infra-red radiation means at one side of the element for directing infra-red radiation on a path through the element, and infra-red detector means at said one side, positioned to detect infra-red passing from the infra-red radiation means through the element and reflected by an object at the other side of the element back through the element to the infra-red detector means, including switch means and control means coupled to the detector means and responsive to said detection of infra-red radiation by the infra-red detector means to actuate the switch means if infra red radiation reflected from an object in a detection region and detected by the detector means is indicative of approach of an object to the device followed within a predetermined time by retreat of the object.

In another aspect the invention provides an infra-red sensing device having an element transparent to infra-red radiation, infra-red radiation means at one side of the element for directing infra-red radiation on a path through the element, and infra-red detector means at said one side, positioned to detect infra-red passing from the infra-red radiation means through the element and reflected by an object at the other side of the element back through the element to the infra-red detector means, including switch means and control means coupled to the detector means and responsive to said detection of infra-red radiation by the infra-red detector means to actuate the switch means if infra red radiation reflected from an object in a detection region and detected by the detector means is indicative of approach of an object to the device.

In another aspect the invention provides an infra-red sensing device having an element transparent to infra-red radiation, infra-red radiation means at one side of the element for directing infra-red radiation on a path through the element, and infra-red detector means at said one side, positioned to detect infra-red passing from the infra-red radiation means through the element and reflected by an object at the other side of the element back through the element to the infra-red detector means, including switch means and control means coupled to the detector means and responsive to said detection of infra-red radiation by the infra-red detector means to actuate the switch means if infra red radiation reflected from an object in a detection region and detected by the detector means is indicative of retreat of the object.

In another aspect, the invention provides an infra-red sensing device having infra-red radiation means and infra-red detector means positioned to detect infra-red radiation from the infra-red radiation means and reflected to the infra-red detector means, including switch means and control means coupled to the infra-red detector means and responsive to said detection of infra-red radiation by the infra-red detector means to actuate the switch, wherein said control means includes an active load for the infra-red detector means, which load is arranged to present a substantially constant output from the infra-red detector means when the infra-red radiation incident on the infra-red detector means changes slowly over a substantial range, but such that the output changes substantially when the incident infra-red radiation changes more rapidly.

In another aspect, the invention provides an infra-red sensing device having infra-red radiation means and infra-red detector means positioned to detect infra-red radiation from the infra-red radiation means and reflected to the infra-red detector means, including switch means and control means coupled to the detector means and responsive to said detection of infra-red radiation by the infra-red detector means to actuate the switch means if infra red radiation reflected from an object in a detection region and detected by the detector means is indicative of approach of an object to the device followed within a predetermined time by retreat of the object.

In another aspect, the invention provides an infra-red sensing device having infra-red radiation means and infra-red detector means positioned to detect infra-red radiation from the infra-red radiation means and reflected to the infra-red detector means, including switch means and control means coupled to the detector means and responsive to said detection of infra-red radiation by the infra-red detector means to actuate the switch means if infra red radiation reflected from an object in a detection region and detected by the detector means is indicative of approach of an object to the device.

In another aspect, the invention provides an infra-red sensing device having infra-red radiation means and infra-red detector means positioned to detect infra-red radiation from the infra-red radiation means and reflected to the infra-red detector means, including switch means and control means coupled to the detector means and responsive to said detection of infra-red radiation by the infra-red detector means to actuate the switch means if infra red radiation reflected from an object in a detection region and detected by the detector means is indicative of retreat of the object.

The invention also provides an infra-red touch-switch device including an infra-red sensing device as described in any of the above-described forms of the invention, coupled for changing the state of a switch when infra-red radiation is detected by the infra-red detector means

In another aspect the invention provides a method of detecting an object by use of an infra-red sensing device as described in any of the above-described forms of the invention.

The invention is further described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a block diagram of an infra-red sensing device constructed in accordance with the invention;

Figure 2 is a timing diagram illustrating timing waveforms occurring in use of the device of Figure 1 ;

Figure 3 is a diagram illustrating how Figures 3A, 3B and 3C fit together to show a circuit diagram of components forming part of the device of Figure 1; Figure 4 is a diagram illustrating how Figures 4A, 4B and 4C fit together to show a circuit diagram of further components forming part of the device of Figure 1 ;

Figure 5 is a circuit diagram of an analog level comparator forming part of the device of Figure 1 ;

Figure 6 is a circuit diagram of a detection timing and storage unit forming part of the device of Figure 1 ; and

Figure 7 is a diagram illustrating the physical arrangement of an infra-red radiation source and associated infra-red detector in accordance with the invention;

Figure 8 is a further timing diagram illustrating timing waveforms occurring in use of the device of Figure 1 ;

Figure 9 is a graphical representation of changes in change on integrating comparators occurring during use of the infra-red sensing device of Figure 1 ; and

Figures 10 and 11 are scrap views showing possible modifications to the circuit of the infra- red sensing device of Figure 1.

Referring first to Figure 1, the infra-red sensing device 10 shown is in the nature of a so- called "touch switch", having an array of switches actuable by a user placing his or her fingertip adjacent corresponding marked touch zones on a touch panel. Thus, the device includes a scanned emitter array 12 comprising infra-red radiation devices effective to provide separate infra-red beams, and an array 14 of photodetectors, the latter having an associated auto-bias network 16.

The physical arrangement of infra-red radiation sources 18 within array 12, and of associated detectors 20 within the array 14 is shown in Figure 7. In particular, each source 18 is arranged behind the touch panel, in this case a glass panel 22, transparent to infra-red light. Each source 18 emits infra-red radiation in a beam 25 so as to pass through the panel 22. The detectors 20 are disposed so as normally to receive no, or relatively little radiation from the sources 18, but each associated source/detector pair is disposed such as to define, on and/or 5 above the panel 22 a respective sensing volume 24. When an object such as a person's fingertip is positioned in a sensing volume, radiation from the respective source is reflected back through the panel 22 to the associated detector 18, thereby causing a change in the electrical signal condition in the device 10. The beam of infra-red radiation from the source 18 is, as shown, arranged to be at an angle θ to the panel 22, such as at 60° thereto. The 10 angle of the axis of the reception cone for the detectors 20 may, as shown, be about 90° to the panel 22. These angles however may be chosen as desired consistent with achieving the desired arrangement of the sensing volume 24 as next described.

In a particular arrangement, the sources 18 may have an emission cone angle for the beam 15 25 of the order of 20° and the detectors 20 may have a reception cone angle of 20° . These are, as illustrated, arranged such that the sensing volume 24 represented by the intersection of the beam 25 and the reception cone 27 is predominantly positioned above the outer surface of the panel 22. In particular, areas on the outer surface of the panel 22 defined at the intersection thereof with the beam 25 and reception cone 27 are arranged to be touching but 20 not overlapping each other. In this way, reflection from surface contaminants may be substantially reduced.

The sources 14 are scanned for repetitive sequential energisation. The auto-bias network 16 is arranged to provide proper operating conditions for the detectors 18.

25

Output from the detectors 18 is passed to an analogue multiplexer 20. Output from this is amplified in amplifier 30, de-multiplexed in 1 :2 multiplexer 32, and passed to a store and instrumentation amplifier 34. Output from the latter is passed to separate long and short term averagers 36, 38, which produce respective output signals designated "Y" and "Z" .

30 Signals Y and Z are passed to processing circuit 50. In this, at steps 52, 54 signals Y and Z are analysed to detect whether they represent approach or retreat of an object towards a detector 20. Resultant outputs are provided through de-multiplexers 56, 58, and represent the de-multiplexed signal conditions for all the source/sensor pairs in the arrays 12, 14. Output from de-multiplexer 56 is analysed at step 60 to determine if the approach and retreat signals occur within a predetermined time window. The retreat detect signal is used directly in this step, the depart signal however being first analysed at steps 64, 66 to determine if minimum timeouts prevail.

The processing circuit is arranged for executing a reset thereof (step 68 illustrated).

The diagram of Figure 1 also shows clock 70 having a master clock 200 and a seven stage binary counter 72 and which generates various timing signals next described.

The clock and timing signals are generated components of clock 70 as shown in Figures 3A, 3B and 3C. The principal signals so generated are illustrated in Figures 2 and 8. Thus clock signals are generated by the clock 200, comprising an inverter 202 with feedback resistor 204 and a capacitor 206 coupling the input to negative supply rail 96. The clock frequency in this instance is approximately at a frequency of 6.4 kHz. The output signal, CLK appears at terminal TP2 shown and is also applied through a resistor 207 to the input of an inverter 208, which input is connected to the negative supply rail via a capacitor 210. The time constant of the RC circuit comprised of resistor 296 and capacitor 210 is chosen to provide at the output of invertor 208 an inverted clock signal NOT CLK-D which is delayed by a predetermined time interval. Output from the inverter 208 is applied to a further inverter 212 to produce at the output TP4 thereof a clock signal CLK-D which is a delayed version of the original clock signal. The latter signal is applied to the counter 72, which has outputs 1 , 2, 4, 8, 16 and 32, producing at these clock signals of frequency relative to the applied clock signal frequency which are respectively, 1:2, 1:4, 1:8, 1: 16, 1:32 and 1:64. These signals are shown at 504, 506, 508, 510, and 512 respectively in Figure 2. Various of these are applied to three 4-16 de-multiplexers 220, 222, and 224 as shown in Figures 3A, 3B and 3C. The output from terminal 32 of the counter 72 is applied via a series circuit comprising an inverter 226, resistor 228 and a further inverter 230 to reset the counter each time thirty-two incoming CLK-D pulses have been counted. In this example there are eight infra-red sources, and detectors and events in relation to each are controlled with reference to successive periods 5 of four cycles the CLK-D signal, a cycle of events in relation to each being repeated each 32 cycles of the CLK-D signal beginning with each resetting of the counter 72.

De-multiplexer 220. It produces at outputs 1 , 3, 5,...16 thereof signals 518 of the form shown in Figure 8. Thus for each 32 cycle period of the CLK-D each signal 518 is a pulse 10 518a of duration equal to two cycles of the CLK-D signal. The signals 518 at the mentioned outputs of de-multiplexer 220 are similar form, but delayed by successive time intervals corresponding to the twice the duration of the pulses 518a.

Two other timing signals, READ-A and READ-B are generated from the signals 518, for use 15 by the 1:2 de-multiplexer 32. READ-A is generated by applying all signals 518 to a gate 230 , so that the READ-A signal, as appearing at the input of the gate 230 is a signal of half the frequency of the CLK-D signal. The READ-B signal is the inverse of this, being obtained at the output of an inverter 232 which has its input connected to the output of gate 230.

20 A strobe signal STROBE- 1 for use in the detection timing and storage units 60 is generated by the de-multiplexers 222, 224. These together provide, at outputs labelled 0, 1, 2, 3, 4, ...31 in Figures 3A, 3B and 3C, signals 516, the first twenty five of which, as occurring in each 32 CLK-D cycle time interval, are shown in Figure 2. Thus, each signal 516 comprises an inverted single pulse of pulse width twice that of the CLK-D signal, the pulses at the

25 outputs 0, 1, 2, 3, 4, ... 31 being successively displaced by twice the pulse width of the CLK- D signal. The outputs 3, 7, 11 , ... , 31 are coupled each to an input of a respective one of eight gates 228 which gates also receive the NOT CLK-D signal. The gates produce at outputs thereof respective pulses each of duration equal to the pulse width of the CLK-D signal, successively time-displaced by intervals equal to eight CLK-D cycles, and appearing within

30 the respective 8 such cycles at the time of the eighth positive pulse thereof. The signals generated at the outputs of gates 228 are referred to herein as STROBE-2. The STROBE- 1 AND STROBE-2 signals are shown in Figure 8.

During start-up, it is desirable to inhibit generation of output from counter 72, and the circuitry includes a capacitor 240 connected to the negative supply rail 96. The capacitor is connected to a resistor 244 and to a diode 242, the latter being connected to the junction between inverters 226, 230, and the resistor 244 being connected to the positive supply rail 94. On start up, the capacitor 242 charges through resistor 244, so that the diode 242 is forwardly biased until the capacitor charges to level which occurs after a time interval from start up has passed. During this time period, the input to inverter 230 is held down to disable counter operation.

Resetting of operation may be effected by application of a SYS RESET signal to the reset terminal of counter 72. Also shown in Figures 3A, 3B and 3C is an inverter 246, which generates a NOTSYS RESET signal being the inverse of the SYS RESET signal, and which may be used to control operation of the circuit.

The eight infra-red sources 18 are shown as LEDs LED0, LED1 , ... , LED7 in Figures 3A, 3B and 3C. Resistors 98 couple anodes of these to the positive supply rail 94, and the cathodes are coupled to emitters of respective transistors 100. The collectors of the transistors 100 are coupled to the negative supply rail 96, with the bases being connected to the 1, 3, 5, ... , 17 outputs of de-multiplexer 220, so that the transistors are turned on sequentially and thus place the LEDs across the DC supply sequentially during each 32 cycle interval of the clock signal CLK-D, for periods corresponding to the two first clock cycles of the four clock cycles allocated to each source 18.

Referring now to Figures 4A, 4B and 4C, cathodes of the eight infra-red detectors 20 are connected from positive rail 94 and the anodes to the drains of respective FETs 250. The sources of the FETs are connected to negative supply rail 96 and the gates thereof are coupled to the junction between respective pairs of resistors 252, 254 connected, respectively from the negative supply rail 96 to the drain of the respective. FET. Capacitors 256 connect from the respective FET gates to the negative supply rail 96. The auto-bias circuits previously mentioned are formed by the respective resistors 252, 254 and capacitors 256. These circuits tend to maintain the voltage across the FETs constant, so that bright light does not cause the FETs to saturate. Thus, the signal condition of the FETs is largely independent of the ambient light level. The capacitors 256 also cause the FETs to present a high impedance to fast changing signals. On incidence of light on a detector 20, particularly reflected light from the associated source 18, the voltage at the gate of the associated FET rises until switch on of the FET occurs. The capacitors 256 however bypass fast changing signals which will then not cause switching.

The analog multiplexer 26 is shown in Figures 4A, 4B and 4C. This is effective to cyclically sample the condition of each FET within repetitive 32 cycle intervals of the CLK-D clock signal, sampling being for the intervals corresponding to the four CLK-D cycles allocated to each detector 20. The resultant samples are then multiplexed by the multiplexer 26. The multiplexed signals at the output of the multiplexer are applied to the amplifier 30 which comprises an operational amplifier 260 having its non-inverting input connected to the output of multiplexer 26, and its non-inverting input connected to ground via a resistor 262 and also to the movable contact of a potentiometer 268 connected from the output of the operational amplifier to the resistor 262. Thus the gain of the operational amplifier is variable, by operation of potentiometer 268. Output from the amplifier 30, output "X" in Figures 4A, 4B and 4C, is taken via a resistor 266 coupled to the output of operational amplifier 168, the output "X" itself being coupled to ground via a resistor 264.

Output "X" is applied to the 1:2 de-multiplexer 32, shown in Figures 4A, 4B and 4C as including two MOS switches 270, 272 which in parallel receive the signal "X" . Switches 270, 272 respectively receive the READ-A and READ-B signals. Thus, for each detector 20 in sequence, over the four CLK-D cycles allocated to that detector and its FET, the switch 270 gates there through a signal component in signal "X" for the first two such CLK-D cycles only, that signal component thus representing the condition of the particular FET under the condition where the associated source 18 is on and thus generating infra-red radiation. For the second two such CLK-D cycles allocated to that detector 20 and FET, the switch 272 gates therethrough a signal component representing the condition of the particular FET under the condition where the associated source 18 is off, and thus not generating infra-red radiation. Thus, sequentially for each detector 20, the signals at the outputs of the switches 270, 272 represent the state of the respective detector 20 when possibly receiving reflected infra red radiation from its associated source together with ambient light, and when receiving ambient light only. Capacitors 274, 276 connected to the outputs of the respective switches 270, 272 are effective to temporarily hold the output states in accordance with the "X" signal components, successively for each detector and associated FET, and to provide resistance to noise in the incoming signals.

The outputs from the switches 270, 272, representing the output of the 1:2 de-multiplexer 32, are applied to the instrumentation amplifier 34. Generally, this produces at its output a signal corresponding to the difference between its two inputs. The subtraction process so effected leads to effective cancellation of signal arising from ambient light. A high degree of noise rejection results from this differencing technique using two closely spaced samples. The outputs from switches 270, 272 are the non-inverting inputs of two operational amplifiers 278, 280, forming part of store and instrument amplifier 34. Resistors 282, 284, 286 are connected in that order in series from the output of operational amplifier 278 to the output of operational amplifier 280. The latter outputs are also connected to respective non-inverting and inverting inputs of an operational amplifier 288, via respective resistors 290, 292. The non-inverting and inverting inputs of operational amplifier 288 are also connected to ground via respective resistors 294, 296. Operational amplifier 288 delivers at its output a repetitive signal, successive components of which represent the difference between the signals from the operational amplifiers for each detector 20 and associated FET, for the conditions where the associated source 18 is on when it is off. This output represents the output from the instrumentation amplifier 34.

The output from instrumentation amplifier 34 is applied to the long and short term averagers 36, 38 via respective resistors 298, 300. The long term averager 36 comprises eight capacitors 302 connected commonly at respective first terminals thereof to output from operational amplifier 288, via resistor 298. The second terminals of the capacitors 302 are connected to respective terminals of an eight channel multiplexer 304 forming part of long term averager 36. Long term averager 36 receives clock pulses from counter 72 and in accordance with these is effective during each 32 cycle CLK-D interval to successively ground the otherwise floating second terminals of the capacitors 302, at times when the output from operational amplifier 288 is respectively representative of corresponding ones of the eight difference signal components thereof. Thus each capacitor 302 is cumulatively charged or discharged in accordance with the period of time for which the respective difference signal component is, over repeated 32 cycle CLK-D intervals, at a high level, indicative of receipt of reflected light by the respective detector, or the period of time for which the respective difference signal component is, over repeated 32 cycle CLK-D intervals, at a low level and indicative of no receipt of reflected light by the respective detector. This effect is illustrated in Figure 9 where plot 400 illustrates the rise in voltage at one capacitor 302 which occurs when over successive 32 cycle CLK-D intervals, there is reflected infra-red radiation detected by the associated detector 20 and the subsequent fall in such voltage which occurs when, over subsequent successive 32 cycle CLK-D intervals, there is no reflected infra-red radiation detected by the associated detector 20. That is to say, the voltage rises at a rate determined by the time constant of the RC circuit comprised of the capacitor 302 and the resistor 298, until a value representing the magnitude of the relevant signal components applied and then decays to zero at a rate again determined by the time constant of the RC circuit comprised of the capacitor 302 and the resistor 298.

The short term averager 38 comprises eight capacitors 306 connected commonly at respective first terminals to output from operational amplifier 288, via resistor 298. The second terminals of the capacitors 306 are connected to respective terminals of an eight channel multiplexer 308 forming part of short term averager 36. Short term averager 38 receives clock pulses from counter 72 and in accordance with these is effective during each 32 cycle CLK-D interval to successively ground the otherwise floating second terminals of the capacitors 306, at times when the output from operational amplifier 288 is respectively representative of corresponding ones of the eight difference signal components thereof. This grounding is, in this arrangement, for substantially the whole of the four CLK-D cycles allocated to each detector, but may be for less than this such as for the last two of each four cycles. Each capacitor 306 is cumulatively charged or discharged in accordance with the period of time for which the respective difference signal component is, over repeated 32 cycle CLK-D intervals, at a high level, indicative of receipt of reflected light by the respective detector, or the period of time for which the respective difference signal component is, over repeated 32 cycle CLK-D intervals, at a low level and indicative of no receipt of reflected light by the respective detector. The operation of the short term averager 38 is thus similar to that of long term averager 36, the plot 402 in Figure 10 illustrating the rise in voltage at one capacitor 306, being that associated with the detector 20 to which plot 400 applies, which occurs when over the same successive 32 cycle CLK-D intervals as for plot 400 there is reflected infra-red radiation detected by the associated detector 20, and the subsequent fall in such voltage which occurs when, over the same subsequent successive 32 cycle CLK-D intervals, there is no reflected infra-red radiation detected by the associated detector 20. The values of the capacitors 306 are however less than the values of capacitors 302, so that the time constant of the RC circuit represented by that capacitor 306 and the resistor 300 through which it charges is smaller than that of the RC circuit represented by the capacitor 302 and resistor 298. Thus, the voltage at the illustrated capacitor 306 rises at a higher rate than that at the capacitor illustrated 302, during the initial intervals at which infra-red radiation is detected, and falls more rapidly when no such radiation is detected.

The signal from the long and short term averages is shown as "Y" and "Z" respectively in Figures 4A, 4B and 4C. It should be observed that the signals "Y" and "Z" will over each 32 cycle CLK-D interval assume successively for preselected sub-intervals corresponding to the intervals for which the capacitors 302, 306 are grounded via the multiplexers 304, 308, values successively representing the charge state of each successive capacitor 302, 306, the unearthed capacitors being left with floating second terminals. In the usual case where only one switch actuation is to be effected by a user, by causing a finger to approach and the depart from a single detector 20, each 32 cycle CLK-D interval for the respective "Y" and "Z" signals will have present only a single pulse the magnitude of which varies over successive such intervals as in Figure 9.

5 The signals "Y" and "Z" are applied to an analog level comparator 48, illustrated in Figure 5. The "Y" signal is applied to the non inverting input of an operational amplifier 620, the inverting input being coupled to the output thereof via a resistor 622. Output from the operational amplifier 622 is connected to the junction between two resistors 624, 626. Resistor 624 couples to the collector of a transistor 628, the base of which transistor is

10 connected to the base of another transistor 630. The collectors of both transistors 628, 630 are connected to positive rail 94, and the collector of transistor 630 is connected to ground via a resistor 632. Resistor 626 couples to the collector of a transistor 634, the base of which transistor is connected to the base of further transistor 636. The collectors of both transistors 632, 634 are connected to positive rail 94, and the collector of transistor 634 is connected to

15 ground via a resistor 638. The pairs of transistors 628, 630 and 634, 636 form respective current mirrors, and are effective via resistors 624, 626, to provide, at the collectors of transistors 628, 634, signal voltages which are equal to the output from comparator 620 plus a small fixed predetermined offset voltage, and also representative of the input "Y" signal plus that offset. These voltage signals are coupled to inputs of respective comparators 640,

20 642. Comparator 640 has its inverting input so connected, and comparator 642 its non- inverting input.

The non-inverting input of comparator 640 is connected to its output via feedback resistor 646, and its output is also connected to positive supply rail 94 via a resistor 644. The 25 inverting input of comparator 642 is connected to its output via feedback resistor 648, and its output is also connected to positive supply rail 94 via a resistor 650.

The "Z" signal is applied to the non inverting input of an operational amplifier 660, the inverting input being coupled to the output thereof via a resistor 662, and to ground via a

30 resistor 665. Output from the operational amplifier 660 is connected via a resister 666 to both the non-inverting input of comparator 640 and the inverting input of comparator 642.

Comparator 640 operates as comparator, providing an APPROACH output which is indicative of the signal condition that the input "Z" signal is magnitude greater than the sum of the " Y" signal plus the aforementioned offset. Comparator 642 operates as comparator, providing a RETREAT output which is indicative of the signal condition that the input "Z" signal is magnitude less than the sum of the "Y" signal plus the aforementioned offset. These outputs, which appear as output signals for the comparator 600, will be high at the time in each 32 cycle CLK-D interval where, for the detector in question, the relevant signal conditions are sensed at the comparators formed by the comparators 640, 642.

The determination that a person has by finger action in the detection region for a detector 20 made a switch actuating gesture is finally determined, on the basis of the APPROACH and RETREAT signals, by the detection device 85, which includes eight detection timing and storage units 88 as shown in Figure 6. The units 88 are similar and the following description of one unit is applicable to each.

The illustrated unit 88 includes three flip-flops 540, 542, 544, with associated resest circuits 552, 554, 556. The reset circuits each comprise resistors 564, 568, 670 and diode 562 and capacitor 566. The reset circuits are arranged to reset the respective flip-flops after predetermined periods following a change of output state, whereby the flip-flops function as mono stables.

One input of the flip-flop 540 is connected to receive the STROBE- 1 signal at one input thereof, the input D thereof, the eight units 88 being coupled to receive respective ones of these. The APPROACH signal is applied to the other input, input C, of the flip-flop 550 via an inverter 550 and is commonly applied in this way to all of the units 88. If the input D is high at the time the STROBE-2 signal is applied to flip-flop 540, indicating that an approach to the detector associated with the unit 88 has been detected, the output of flip-flop 540 is latched until reset by reset circuit 552. A reset time of 20 mS has been found satisfactory for this purpose. The Q output of flip-flip 540 is connected to one input of flip-flop 542. On resetting of the Q output at resetting of flip-flop 540, the flip-flop 542 is latched for an appropriate period, such as 1 second, established by the circuit values of the components of reset circuit 554. This one second interval represents a time window within which a RETREAT signal must be detected by unit 88 for valid switch actuation to occur.

The Q output of the flip-flop 542 is connected to one input of an AND gate 580, the other input of which is connected receive the STROBE-2 signal for that unit 88, and the output of gate 580 is connected to the D input of flip-flop 544. The C input of flip-flop 544 is connected to receive the RETREAT signal. If, at the time the output of gate 580 goes high (pursuant to the output of flip-flop 542 being latched and the STROBE-2 signal for that unit 88 being present) the RETREAT signal is present, the flip-flop 544 is latched, for a time period established by the values of the components of the reset circuit 556. The output of the flip-flop 556 is thus held high for an appropriate period which may be 20 mS. This output is coupled to the base of a transistor 582 and operates to turn the transistor on to pass current through a LED 584 to provide visual indication that switching has occurred. A sound device may also be operated, and other LEDS or other devices (not shown) may also be switched or actuated as desired.

If a RETREAT signal does not appear at the D input of the flip-flop 544 within the allowed time window provided by flip-flop 542, no actuation occurs. In the event that the flip flop 544 is set, the flip-flops 540, 542 are also reset, the Q of flip-flop 544 being connected by a diode 586 to two further diodes 588, 590 which couple to the reset terminals of the flip-flops 540, 542. Also the junction between the diode 586 and diodes 588, 590 is commonly coupled to the corresponding junctions of each of the units 88, so that each of these is likewise reset, when the flip-flop 544 of any one of the units is set.

Various modifications may be made to the described device. In particular, while an implementation using discrete circuit components has been described, the invention may of course be implemented using a suitably computer techniques, such as a microprocessor or a microcontroller.

The touch-sensing circuitry may be simplified by using a common FET load for all photodetectors, rather than discrete FET loads as described. Alternatively, the FET load may 5 be replaced by a single operational amplifier transimpedance circuit as shown in Figure 10. This has a high pass frequency characteristic. The circuit components, may in this case be chosen to produce low gain at about 100-120 Hz. In this realisation, the operational amplifier 260 is reconfigured as shown, and the multiplexer 26 is not then needed. Also, in this case, the READ-A and READ-B signals should be interchanged, while the operational amplifiers 10 620 may be configured to provide some gain, by provision of resistors from ground to each inverting input.

The described circuit uses multiplexed drive for the sensors. This avoids the need for analogue multiplexing of the relevant amplifier input. In a microprocessor implementation, 15 the multiplexing could be achieved by synchronising an A-D converter, in the microprocessor input, with a multiplexed LED drive.

In the described circuit, the output has an average value of zero. The modified circuit of Figure 11 provides DC referencing of the detector signals. Pulses corresponding to detections 0 are ground-referenced and positive-going out of the second amplifier 261 shown.

The arrangement of the sources and detectors to provide a detection zone as described, has the advantage that light reflected from the surface of the glass plate, such as from dirt or grease will generally not pass to the detector, providing good immunity from surface 25 contamination.

In the described arrangement, the illuminating radiation for the detectors is pulsed, so that reflected light to the detectors is likewise pulsed. The load seen by the detectors is the drain of a shunt (MOSFET shunt) biased with the described resistors and capacitor. The load

30 presents low impedance to slowly changing signals as might occur due to ambient light sources at 100 or 120 Hz or lower. The FET also maintains a substantially constant voltage across the infra-red sensors. On the other hand, due to the action of capacitive by-pass an the gate of the MOSFET, the active load presented has high impedance at the higher pulse frequency of the illumination. Consequently, a relatively large sensing signal is generated when the illuminating pulse is present and there is an object in the detection zone.

Also, in the described embodiment, measurement of the reflected light level as seen by the detector circuit, is taken during the time for the illumination pulse, and another separate sequential measurement taken immediately after the illumination pulse. These measurements are then combined to give the difference between them. This differencing technique substantially reduces the effects of extraneous noise voltages and common mode voltages due, for example, to low frequency ambient light.

The described system is configured to react to changing signal levels rather than simply to absolute signal levels. In order for a detection to occur, a reflector must approach the detector at above a certain minimum rate and must similarly leave at above a certain minimum rate. That is to say, APPROACH and RETREAT signal are generated provided the reflection satisfies these rate conditions. In this embodiment, it is a further requirement that the retreat signal must occur within a certain time- window after the occurrence of the approach signal for a switch actuation signal to be generated. That is, the retreat detection is gated such that a signal occurring too soon after the approach signal, or too late after the approach signal, is ignored. These times may be set to encompass the typical times seen with human operation of the switch. Minimum time might typically be 10 millisecond and maximum time typically 1.25 seconds. This gating reduces the likelihood of false detections due to trigger sources such as insects passing through the detection zone, or detection's due to cooking fat splashes, where the device is used in cookers or the like.

Also in the described embodiment, sensing of changes in signal levels is done through the accumulation or integration of repeated samples of these signals at fixed regular intervals.

In an exemplary device constructed as described, eight detector stations were scanned sequentially using a master clock frequency of 6.4 kHz with a dwell time at each switch of 0.625 millisecond. Since there were eight switches, a complete scan occupied 5 milliseconds, corresponding to a scan frequency of 200 Hz. The scan frequency is not critical, provided it allows a sufficient number of samples to be taken at each switch station so that a quasi continuous signal can be generated within the minimum reaction time desired for each switch. This minimum time is typically around 20 millisecond. In the described case, four scans occur within this 20 millisecond minimum time. The sampling frequency should generally be sufficiently high to allow adequate filter differentiation between the pulsed signal and low frequency noise signals. However, adoption of very high sampling frequencies may prevent the use of common low cost amplifiers and digital circuits.

As described, four clock cycles are used per switch to facilitate the timing of suitable illumination and strobe signals although of course, any suitable system for may be adopted for sequential reading, at each switch station, of the photodetector signal amplifier output, with illumination "on" and illumination "off'. The manner whereby these readings are variously processed together, temporarily stored and integrated, to permit further signal processing, has already been described in detail.

Generally, the described device uses the sequential differential sampling technique in which measurement of the signal with the LED drive on and the quiescent signal with the LED drive off is done in a, preferably closely spaced, sequence. The subtraction of one of these signals from the other thus produces a signal that is substantially solely related to the amount of reflected light. This measurement technique, effectively tends to reject signals due to ambient light or electrical leakage, or due to out of band electrical interference. The differential measurement technique makes the measuring system particularly robust in terms of resistance to noise and adds another layer of resistance to ambient light.

The technique of spatially separating the illuminating and return acceptance zone at the top glass surface in order to reduce or eliminate the negative effects of surface contaminates, has been found to be particularly effective in use.

The invention contemplates the use of specially shaped illumination patterns and receiving corridors. The aim of such shaping is to minimise the height of the active volume above the glass plate and to expand the effective active touch area at the plate surface. The reduction of the height of the sensing volume whilst increasing the active area over the switch, is helpful in achieving reliable operation. An example of a configuration aimed at achieving this is the use of a fan-like transmitted beam with partially interleaved but separate receiving corridors with the aim of coriftning the sensing volume above the glass surface to a lower profile wider area pattern.

A wider distribution of sensing zone will permit reflection of the illuminating beam from finger regions curving back from the contact point. This reduces the criticality of the sensing system which is preferably designed such that it cannot receive light reflected from an object actually on the surface.

While the glass panel 22 as described may generally be quite transparent to infra-red radiation, it is possible to have a diffuse (eg frosted) surface on the glass plate so that fingerprints and dirt are not obvious. A highly reflective or rriirror-like surface accentuates the visibility of fingerprints and similar marks, which may be at least aesthetically undesirable. In any event, of course, the panel need not necessarily be formed of glass, but may be formed of any suitable material, such as plastics,

The described arrangement has been advanced merely by way of explanation and many modifications may be made thereto without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (27)

CLAIMS:

1. An infra-red sensing device having an element transparent to infra-red radiation, infra-red radiation means at one side of the element for directing infra-red radiation on a path through the element, and infra-red detector means at said one side, positioned to detect infra-red radiation passing from the infra-red radiation means through the element and reflected by an object at the other side of the element back through the element to the infra-red detector means, wherein the infra-red radiation means and the infra-red detector means are arranged such that there is defined a sensing zone within which an object must be positioned to enable detection thereof, which detection zone is positioned at said other side of the element but so as to be at least predominately away from a surface of the element at said other side of the element.

2. An infra-red sensing device as claimed in claim 1 , wherein the infra-red sensing means has an optical filter which in use substantially blocks visible light from detection by the infra-red sensing means.

3 .An infra-red sensing device as claimed in claim 1 or claim 2, wherein the radiation means and the infra-red detector means are arranged at opposite sides to a normal to the surface of the element and such that the infra-red radiation means in use predominately directs said infra-red radiation therefrom at a first angle relative to said normal to said surface which is substantially greater than zero degrees.

4. An infra-red sensing device as claimed in claim in the range 45 to 75 degrees.

5. An infra-red sensing device as claimed in claim 4, wherein said angle is substantially 60 degrees.

6. An infra-red sensing device as claimed in any preceding claim wherein the space within which the infra-red radiation from the infra-red radiation means is projected, and the space within which infra-red radiation may be detected by the infra-red detector means, are arranged such that areas on the surface of the element remote from the infra-red radiation means and infra-red detector means, and defined at the intersection of that surface with said spaces, are arranged to touch but not intersect each other.

7. An infra-red sensing device as claimed in any preceding claim, wherein the detector means is arranged to in use predominately detect radiation passing thereto at a second angle relative to said normal to the surface which second angle is substantially 90┬░ .

8. An infra-red sensing device having an element transparent to infra-red radiation, infra-red radiation means at one side of the element for directing infra-red radiation on a path through the element, and infra-red detector means at said one side, positioned to detect infra-red passing from the infra-red radiation means through the element and reflected by an object at the other side of the element back through the element to the infra-red detector means, including switch means and control means coupled to the infra-red detector means and the infra-red radiation means, said control means in use causing said infra-red radiation means to emit infra-red radiation for a first time interval and to not emit infra-red radiation for a second time interval, and said control means being coupled to said infra-red detector means to be in use to actuate the switch, under the condition that the difference between levels of infra-red radiation detected by the infra-red detector means during the first and second time intervals falls within a predetermined range.

9. An infra-red sensing device as claimed in claim 8, wherein the first time period follows the second time period.

10. An infra-red sensing device as claimed in claim 9, wherein the second time period precedes the first time period.

11. An infra-red sensing device having an element transparent to infra-red radiation, infra- red radiation means at one side of the element for directing infra-red radiation on a path through the element, and infra-red detector means at said one side, positioned to detect infrared passing from the infra-red radiation means through the element and reflected by an object at the other side of the element back through the element to the infra-red detector means, including switch means and control means coupled to the infra-red detector means and responsive to said detection of infra-red radiation by the infra-red detector means to actuate the switch, wherein said control means includes an active load for the infra-red detector means, which load is arranged to present a substantially constant output from the infra-red detector means when the infra-red radiation incident on the infra-red detector means changes slowly over a substantial range, but such that the output changes substantially when the incident infra-red radiation changes more rapidly.

12. An infra-red sensing device as claimed in claim 11, wherein the load is formed by a FET.

13. An infra-red sensing device as claimed in claim 11 or claim 12 wherein there are a plurality of said infra-red detector means, and said load is common to these.

14. An infra-red sensing device having an element transparent to infra-red radiation, infrared radiation means at one side of the element for directing infra-red radiation on a path through the element, and infra-red detector means at said one side, positioned to detect infra- red passing from the infra-red radiation means through the element and reflected by an object at the other side of the element back through the element to the infra-red detector means, including switch means and control means coupled to the detector means and responsive to said detection of infra-red radiation by the infra-red detector means to actuate the switch means if infra red radiation reflected from an object in a detection region and detected by the detector means is indicative of approach of an object to the device followed within a predetermined time by retreat of the object.

15. An infra-red sensing device having an element transparent to infra-red radiation, infrared radiation means at one side of the element for directing infra-red radiation on a path through the element, and infra-red detector means at said one side, positioned to detect infra- red passing from the infra-red radiation means through the element and reflected by an object at the other side of the element back through the element to the infra-red detector means, including switch means and control means coupled to the detector means and responsive to said detection of infra-red radiation by the infra-red detector means to actuate the switch means if infra red radiation reflected from an object in a detection region and detected by the detector means is indicative of approach of an object to the device.

16. An infra-red sensing device having an element transparent to infra-red radiation, infrared radiation means at one side of the element for directing infra-red radiation on a path through the element, and infra-red detector means at said one side, positioned to detect infrared passing from the infra-red radiation means through the element and reflected by an object at the other side of the element back through the element to the infra-red detector means, including switch means and control means coupled to the detector means and responsive to said detection of infra-red radiation by the infra-red detector means to actuate the switch means if infra red radiation reflected from an object in a detection region and detected by the detector means is indicative of retreat of the object.

17. An infra-red sensing device having infra-red radiation means and infra-red detector means positioned to detect infra-red radiation from the infra-red radiation means and reflected to the infra-red detector means, including switch means and control means coupled to the infrared detector means and the infra-red radiation means, said control means in use causing said infra-red radiation means to emit infra-red radiation for a first time interval and to not emit infra-red radiation for a second time interval, and said control means being coupled to said infra-red detector means to be in use to actuate the switch, under the condition that the difference between levels of infra-red radiation detected by the infra-red detector means during the first and second time intervals falls within a predetermined range.

18. An infra-red sensing device as claimed in claim 17, wherein the first time period follows the second time period.

19. An infra-red sensing device as claimed in claim 18, wherein the second time period precedes the first time period.

20. An infra-red sensing device having infra-red radiation means and infra-red detector means positioned to detect infra-red radiation from the infra-red radiation means and reflected to the infra-red detector means, including switch means and control means coupled to the infrared detector means and responsive to said detection of infra-red radiation by the infra-red detector means to actuate the switch, wherein said control means includes an active load for the infra-red detector means, which load is arranged to present a substantially constant output from the infra-red detector means when the infra-red radiation incident on the infra-red detector means changes slowly over a substantial range, but such that the output changes substantially when the incident infra-red radiation changes more rapidly.

21. An infra-red sensing device as claimed in claim 20, wherein the load is formed by a FET.

22. An infra-red sensing device as claimed in claim 20 or claim 21 wherein there are a plurality of said infra-red detector means, and said load is common to these.

23. An infra-red sensing device having infra-red radiation means and infra-red detector means positioned to detect infra-red radiation from the infra-red radiation means and reflected to the infra-red detector means, including switch means and control means coupled to the detector means and responsive to said detection of infra-red radiation by the infra-red detector means to actuate the switch means if infra red radiation reflected from an object in a detection region and detected by the detector means is indicative of approach of an object to the device followed within a predetermined time by retreat of the object.

24. An infra-red sensing device having infra-red radiation means and infra-red detector means positioned to detect infra-red radiation from the infra-red radiation means and reflected to the infra-red detector means, including switch means and control means coupled to the detector means and responsive to said detection of infra-red radiation by the infra-red detector means to actuate the switch means if infra red radiation reflected from an object in a detection region and detected by the detector means is indicative of approach of an object to the device.

25. An infra-red sensing device having infra-red radiation means and infra-red detector means 5 positioned to detect infra-red radiation from the infra-red radiation means and reflected to the infra-red detector means, including switch means and control means coupled to the detector means and responsive to said detection of infra-red radiation by the infra-red detector means to actuate the switch means if infra red radiation reflected from an object in a detection region and detected by the detector means is indicative of retreat of the object. 10

26. An infra-red touch-switch device including an infra-red sensing device as claimed in any preceding claim, coupled for changing the state of a switch when infra-red radiation is detected by the infra-red detector means

15 27. A method of detecting an object by use of an infra-red sensing device as described in any preceding claim.