The invention is directed to a sensor system which could be used for determining whether an automobile seat is occupied, more particularly for example to control an automotive airbag safety system.

It is now quite common for automotive companies to provide automobiles with both a standard locking system and a dead-locking system. The standard locking system is of conventional design and would involve power door locks that are responsive to activation from within the vehicle at a central location, typically accessible by the driver, or at each of the individual doors and from the outside of the vehicle by use of a key, either conventional or electronic. This locking system results in the doors being locked but they can still be opened from the inside of the vehicle. The additional and newly incorporated dead-lock feature is incorporated for anti-theft protection and provides that the doors will be prevented from being opened without the use of a "key". The "key" may either be a conventional key, an electronic code or an infra-red source as is now common in remote locking devices.

While the dead-lock feature performs adequately as an anti-theft device, the system is not completely user friendly in that it is possible that the system would be activated with an occupant still in the vehicle. If this were to occur, it is not possible for the occupant to exit the vehicle, as even from the inside, without the "key", once activated, the dead-lock feature will maintain all of the doors in a locked condition. Especially with the use of the infra-red locking devices and where operators are not fully cognisant of the systems within the vehicle, it is possible that someone may become accidentally trapped within the vehicle. It would be especially advantageous to provide a sensor system that would sense when a person is in the vehicle and would prevent activation of the dead¬ lock inhibitor, whereby the locking system might instead default to the standard locking condition where the

occupant could unlock the door and exit the vehicle.

In addition, automotive companies now commonly provide so-called safety restraint systems (airbags) in passenger automobiles. It is typical for one airbag to be located within the steering wheel on the driver's side, while another is located within the front dash on the passenger's side. Other locations for airbags may also be used. During an auto crash, the airbags are ignited and activated to prevent the occupants from incurring harm. While these systems have increased the safety within the automobiles, they are yet to be perfected, in so far as sensing capabilities are concerned. Said differently, the airbags always fire regardless of whether or not someone occupies the seats and independent of the physical characteristics of the occupant. Due to the high expense of recharging the airbags, reinstalling the bags in their proper location and possible injury, there presently exists a need within the automotive market for a sensing system to determine whether the seat is occupied. The aforementioned needs may be addressed by providing a sensor for determining if the seats of a vehicle are occupied and then controlling the described systems, or others, based upon the occupancy of the vehicle. United States Patent 5,164,709 teaches a seat occupancy device and an analysis circuit which emits a signal when the seat is occupied by providing a lateral force sensitive cable (such as piezoelectric cable) embedded in the area of the seat surface. The device provides a plurality of signal changes occurring continuously during seat occupancy while the analysis circuit registers the changes. While this device represents a significant improvement over simple on-off switch sensing for seat occupancy, a problem still exits in that the device only is taught to sense occupancy by a load and not whether the load represents an animate object, such as a human being.

The present invention describes a novel sensor system

which could be especially useful in determining the presence of a human occupant in a seat (typically a car passenger seat) through the use of a novel device. A sensor system for determining occupancy of a place or seat where the system is adapted to distinguish between animate and inanimate loads occupying the seat, the sensor system comprising a movement transducer coupled to the seat and configured to emit a first output signal in response to movement of the load upon the seat; and a control system for receiving the first output signal and discerning a component of the first output signal characteristic of animate movement. The device may be enhanced in such a way as to allow a self-test to be preformed periodically.

While a piezoelectric sensor cable responds strongly to the event of a person sitting down on the seat, it is anticipated that the more useful information content of the signals from the cable comprise the much smaller signals from slight bodily movement, muscular impulses and other physiological signs such as pulse and respiration. It is these smaller signals that enable distinguishing whether an alive being is present upon the seat or an inanimate object, such as a box of books, is resting thereupon. These signals are reasonably easy to detect when the vehicle is stationary making such a sensor particularly useful for the deadlock inhibitor application described above.

However, these small signals generated by the occupancy sensor are all found to be substantially incoherent when the vehicle is in motion due to the associated vehicle induced vibration that is also detected. In the referenced Prior Art teaching, it is primarily these signals that are sensed once the vehicle is in motion to determine occupancy. In the present invention, it is desirable to further distinguish between animate and inanimate objects even while the vehicle is moving. Therefore, it is necessary to somehow filter out the movement induced vibrations. This may be accomplished by further inserting into the system an accelerometer(s)

and enhanced through a force transducer(s) . In this way, comparison of the signals from the accelerometer(s) and from the piezoelectric cable sensor(s) may be compared/interpreted to deduce whether the load (if any) borne by the seat is animate or inanimate. The force transducer could provide the magnitude of the load, thereby providing additional useful information.

By way of example, the force transducer may operate on a principle whereby an ultrasonic pulse is used to measure the thickness of a compressible compliant elastomeric layer, and the accelerometer may use one or several cantilever beams, with each surface of the beam(s) bearing a piezoelectric layer such that one layer may be used to detect vibration and the other used to excite vibration. By this means, the combined sensor may be used to detect weight applied in compression, with overall acceleration of the structure being detected and corresponding correction applied. It may even be desirable to combine the two into a single device. Combined force transducers and accelerometers are known, and used in the field of vibration monitoring to measure the mechanical impedance of a structure.

US Patent No. 4,964,302 discloses a force transducer comprising a base, an ultrasonic transducer capable of launching and receiving fast acoustic signals, an elastomeric layer, and a reflector plate. The time of flight of an ultrasonic pulse is used to determine the compression of the elastomeric layer which in turn yields an estimate of the applied force. It is a characteristic feature of this device that, when operating, a train of pulses is always present, with the width of each pulse representing the distance between the piezoelectric layer and the reflector plate. If a malfunction occurs, then the pulse is either absent, or has a width which lies outside normal operating limits, and so the malfunction is readily detected. The device of the present invention further differs from the known art in that each component is designed to allow self-test, and preferably supplies an

output of digital form which may be read easily by a micro-controller or similar interface.

The following component descriptions are included by way of example only and should not exclude the possibility of other constructions offering the same or similar functionality.

In the device of the present invention, an ultrasonic force transducer as described above can take the form of an annular piezoelectric film deposited on an annular washer, with a second annular washer sandwiching the piezoelectric film as well as an annular shaped elastomeric member. Preferably a fastener can be positioned between the two washers to retain them n the expanded position, but where they are allowed to contract. Also preferably the washers are the ground electrodes.

In a further embodiment of the present invention, an ultrasonic force transducer as described above is formed using an annular printed circuit board as a carrier for the piezoelectric transducer, preferably formed using thin PVDF (polyvinylidene fluoride) or copolymer (P(VDF)TrFE) film. A cavity above the PCB/film structure is filled with compliant elastomer such as silicone rubber. An upper cap of annular form acts both as a thrust plate (for exerting compressive force on the elastomer) , and as a reflector for the ultrasonic signals. A cavity below the PCB allows the mounting of a metallic beam structure, forming the accelerometer device as described above. The accelerometer comprises at least one beam, angled so as to respond to vertical acceleration through the sensor. A further embodiment allows for another sensing axis in a lateral direction, such that forward acceleration is detected independently. The required electronic circuitry for operation of the force sensor and the accelerometer may be formed separately and mounted in chip form on appropriate surfaces of the PCB, or alternatively may be combined into a single structure and similarly mounted to the PCB. It is anticipated that the chips be wire-bonded to the PCB, commonly described as "chip-on-board" process.

In the preferred embodiment, the housing for the above outlined structure comprises a metal lower body and metal top cap, such that the entire assembly is well shielded from electromagnetic disturbance. In a further embodiment, the outer metal housing may be connected separately to the chassis of the vehicle, isolated from the sensor ground line.

It is advantageous if the ultrasonic force sensor as described above allows provision of an additional propagation path of fixed length, or known additional length beyond the moving path length, such that the response of the compliant elastomeric layer can be compensated for the effects of temperature. Such compensation may be applied within the sensor drive circuitry if the output signal comprises the difference in flight-time between the moving path and the reference path. By way of example, the reflecting top plate may include a recess of known depth, covering an area of one- half that of the total reflecting area, such that two echoes arrive back from this reflector: one at the time corresponding to (2h/c) , and another at a time corresponding to (2(h+dh)/c), where h is the spacing between the piezo film sensor and the nearer portion of the top plate, dh is the depth of the recess, and c is the speed of sound in the elastomer.

Combined force and acceleration sensor may conveniently be installed as part of the seat mounting arrangement, such that a known proportion of the seat load is borne by the structure. In the preferred embodiment, the signals presented by both force transducer and accelerometer are in the form of digital signals, for example with a pulse width coding relating to amplitude. It is anticipated that force, acceleration in "Z-axis" and (where optionally detected) acceleration in "Y-axis" signals are brought out independently, to allow flexibility in the further interpretation and processing of the information.

Signals generated by the force transducer allow

approximate measurement of the total load borne by the seat. Further compensation for vehicle vibration (using signals from the accelerometer(s) allow more accurate weighing of the load, such that a weight threshold may be applied to the occupancy decision algorithm.

Finally, in order to assure the occupancy mat or cable is intact, a novel self-test feature for such piezoelectric devices is presented. The piezoelectric cable may be terminated at its far end with a capacitance, where the value of this added capacitance is similar or greater than the sensor cable, such that, by periodically measuring total capacitance, the integrity of the cable may be checked.

The preferred embodiment of the invention will now be described by way of reference to the following drawing figures, where:

Figure 1 shows a representational view of a seat occupancy sensor system according to the present invention; Figure 2 shows a schematic view of the seat occupancy system of Figure 1;

Figure 3 shows an electrical circuit incorporated into the system of Figure 1;

Figure 4 shows a cross sectional view of the first embodiment of a transducer of the system of Figure 1;

Figure 5 shows piezoelectric film deposited on the lower washer of the transducer of Figure 4;

Figure 6 shows a representation of the electrical characteristics of the force transducer of Figure 4; Figure 7 shows the upper and lower cover of a second embodiment of the transducer of Figure 4;

Figure 8 shows the lower housing part being installed with a printed circuit board carrying the piezoelectric film and an elastomeric member of the second embodiment of Figure 7;

Figure 9 shows the complete assembly of the second embodiment;

Figure 10 shows a further embodiment of a transducer

including an accelerometer mounted to the lower side of the printed circuit board; and

Figure 11 shows the time response characteristics of the embodiment shown in Figure 10. ith reference now to Figure 1 and Figure 2, a representational and schematic view is shown comprising a seat 2 having an occupancy sensor 4 of piezoelectric cable 5 and a combination force transducer and acceleration transducer 6 affixed to the seat 2 by an anchor point 8. The signals from the occupancy sensor 4 and force transducing accelerometer 6 are brought into an electronic control module (ECM) 10 which, by simple means of counting and timing of digital events, discriminates the various conditions of occupancy of the seat (vacant, animate occupant, inner load, etc.). Another sensor such as a transponder 12 can also be included, which could be used to detect if a child's car seat is present, and if so, in what direction it is facing. The ECM 10 would then control the firing of the airbag. In principle, the piezo mat 4 is comprised of a length of piezo cable 5, approximately two meters in length, which is wound into the sheet or mat 4 that could fit into the seat 2. The far end 14 of the cable 5 is connected into a charge amplifier 15 as shown in Figure 3. The opposite end 16 can be terminated with an extra capacitive load 18. If the cable 5 has a capacitance of approximately 800 Picofarads/metre which is normal, two metres would yield 1600 Picofarads. This may result in difficulty detecting significant changes in capacitance. By putting a passive capacitor 18 at the far end (across the cable) , an overall capacitance can be found, and if even the last inch of cable 5 is lost, the capacitive termination 18 would also be lost, and therefore the whole capacitance would drop by a huge factor. By providing the capacitive termination 18, this allows the verification that the cable 5 has conductivity all the way along the lines of the cable 5, and that there is no defect somewhere in the cable 5.

In order to provide self-test capability for such a length of piezoelectric cable 5, the aforedescribed solution is to incorporate a load capacitance 18 at the end of the cable 5, whereby severing or shorting of the cable 5 along its length is detected electrically. Typical piezoelectric cable shown a capacitance of 800pF/metre. Based on this figure, the total sensor capacitance is determined and a load capacitance 18, desirably at least double that of the sensor 5, can be added to the end 16 of the sensor 5. This combined capacitance can then be used in an electrical circuit of Figure 3 in order to monitor the capacitance and detect sensor responses. The capacitance presence can also be detected by an oscillator operating at a frequency determined in part by the capacitance. By arranging a voltage dividing network of capacitive reactances the fault conditions of short circuited or severed cables would create distinctly different oscillator frequencies which could be filtered to create logic signals for "gating" the final output. Furthermore, a DC bias may be incorporated to detect resistance across the cable such as if contamination of a splice or termination were to be occurring. All of which is in circuitry incorporating logic that reverts to a state detecting a piezo event. In the one embodiment of Figure 1, the cable 5 or mat 4 is actually put into the seat 2 itself, close to the surface, as shown in Figure 1. The cable 5 or mat 4 may also be disposed in other locations, such as beneath the seat 2. When somebody or something sits, large voltage signals are given off by the cable 5 or mat 4 initially, but that is not the interesting signal. Rather what is being looked for is a certain frequency band that is quite characteristic of human life. If a box is placed on this cable mat 4, an initial signal is given as the box is placed. A band pass filter 20 should also be included possibly set to 0.5 Hz to 5 Hz concentrated about 1 cycle per second. That is typical of an animate object being present and where a pulse occurs and also tends to

coincide with muscular twitching. This is something that looks quite easy to detect. The only difficulty is when you have a lot of vehicle vibration present as well, you want to make sure that the vibration your picking up is not just the box bouncing on the seat.

In the simpler system of utilizing a seat occupancy sensor system for applications where the vehicle is intended to be stationary at the time of sensing and therefore vehicle vibration would not induce a response from an inanimate object, such as in the locking application described above, will now be further described with reference to Figures 1-3. As described above the mat 4 could be directly incorporated into an automobile seat 2. The seat 2 would be any one of the seats in the vehicle. The system would incorporate suitable electronics (for example, buffer, bandpass filter, event detector or counter), such as shown in Figure 3, for isolating the desired signals as described above. In this application, the ECM 10 would then take on the function of controlling various aspects of the vehicle, such as the locking systems to inhibit the external deadlock activation if required. It is important to note that the ECM 10 may take on other control functions such as individual seat heaters and may control one or more functions. In the more complex sensor system for use with a moving vehicle it is necessary to distinguish between the movement induced vibrations and true seat occupancy. The accelerometer/force sensor 6 is especially useful in this regard and will now be described. With respect first to Figure 4, the load sensor 6 is shown at 30 comprised of upper and lower washer members in the form of annular shaped disks, 32 and 34 respectively where a fastener assembly 36 is inserted through the centre of the washer members 32 and 34. A piezoelectric film element 38 is bonded to the lower washer 34, as shown in Figures 1 and 2, and has a silver ink electrode 40 printed on the upper surface of the film only. The film is bonded with a very thin layer of epoxy resin or similar adhesive to the lower

washer, whereby the lower washer 34 serves as a ground electrode with electrical coupling through the fastener member 36 to the upper washer 32. An annular shaped elastomeric member 42 is positioned above the piezoelectric film 10 and is also sandwiched by the upper and lower washer members 32,34.

The piezoelectric film element 38 is used to launch an acoustic pulse into the elastomer 42 whereby a much larger proportion of the energy propagates into the elastomer than the steel washer 34, since the acoustic impedance of the elastomer 14 is much more closely matched to the piezo polymer 38. The time delay between transmission of this pulse, and the arrival time at the film 38 after the pulse reflects from the upper washer 32, allows the thickness of the elastomer 14 to be measured.

The lock nut 44 of the fastener 8 arrangement prevents any loosening of fastener under load, so that the device 30 works only in compression and not for extension.

It is anticipated that further bracketry be included (not shown) so that the force between two load bearing members (anchoring structure 8) may be measured. As shown in Figures 4 and 5, an extension portion 46 of the piezoelectric film 38, together with the silver ink electrode, acts as the signal electrode 48, whereas the upper and lower washers 32,34 act as the ground electrodes 50 electrically, see Figure 6.

With reference now to Figures 7-9, the second embodiment of the load sensor identified as 60 will be described with greater detail. As shown in Figure 7, the upper and lower washers 32,34 have been replaced by an upper cover 62 and a lower housing portion 64. Both of these members however are preferably of annular shape, the lower housing member 64 including a piezoelectric film 66 positioned on a printed circuit board 68 with an annular elastomeric ring 70 being positioned above the piezoelectric film 66. As shown in Figure 9, the assembly is completed by the placement of the upper cover 62 over the assembly of Figure 8, and although not specifically

shown, it is anticipated that a fastener member, similar to that shown in Figure 4, would retain the two cover members 62,64 together. As shown best in Figure 7, the lower housing member 64 has a cup shaped cross-sectional recess 72 defining an upwardly directed shoulder at 74, upon which the printed circuit board 68 may rest (Figure 8) . This defines a recess 76 beneath the printed circuit board 68 within the cup shaped recess 72. As shown in Figure 10, an accelerometer 78 can be mounted to the under side of the printed circuit board 68 to reside in the recess 76.

The printed circuit board 68 includes a copper track on the top thereof forming a signal electrode, with the piezo film 66 bonded thereto with no metallization underneath, such that the film 66 sits over the copper. The copper then effectively becomes the active electrode for the film 66. The top of the film 66 would be fully metallized and connected to ground.

The double sided circuit board 68 is significant, as it allows a shielded sensor just by using standard patterning of copper tracks on a circuit board 68. Both sides of the circuit board 68 carry copper tracks of different patterns. The upper side of the piezo film 66 is fully metallized and just connected to the ground. The signal electrode which is a copper track between the circuit board 68 and the piezo film 66 is driven on outsides of the receiver signal electrode.

When a pulse is then applied onto that copper track on the board, it causes the piezo film 66 to expand or contract and it launches an acoustic signal into that elastomer 70 which will actually propagates down into the circuit board 68 material itself but most of it is absorbed or dissipated. So most of the energy travels up into the elastomer 70. The acoustic signal hits the top plate 62 which will be metallic and the top plate 62 acts like a mirror, the elastomer 70 carries the signal back down and then it arrives back at the same piezo film element 66 that it was sent on. By which time the

electronics have switched into receive mode and is ready to catch the echo. So actually pulse echo detection is being carried out with exactly the same piece of piezo film. So the time of flight of the signal going up hitting the reflector 62 and coming back eventually gives the distance that the sound pulse has travelled and therefore the compression of the elastomer 70 can be calculated which is inversely portional to the force F. It is anticipated that this assembly would be mounted in the seat mounting point 8 that carries a proportion of the load of the seat 2, and the occupant is thereby weighed.

By providing a recess 80 in the top plate 62, as shown in Figure 10, or a shaped reflector so that there are two distinct paths, a short path (h) and a long path (h+dh) , as long as there is a known difference apart that allows the two paths (h, h+dh) electronically to be read independently, and therefore to compensate for any change in the speed of sound in that elastomer as temperature varies, e.g. from -40 to +70 ^* C this would mean some change in the speed of sound.

The underside of the circuit board 68 would carry a miniature accelerometer 78 possibly with its own custom chip as well. So the principle of this type of device is that a beam therein basically flexes as it undergoes vertical acceleration and piezo film is applied to at least one surface and picks up the bending of that beam, the output of which is then a measure of acceleration. It is also possible to put piezo film on both sides of that beam and to do the self diagnostics, by putting in a short test pulse onto the underside. The pulse causes the beam to deflect and that deflection can then be picked up by the piezo film on the opposite side which would generate an output. If there is film on both sides of the beam and if a 4 Volt logical pulse is applied to one side of the beam and listen, using the film on the other side of the receiver then one can actually verify that the beam does indeed deflect when we apply the 5 Volt pulse.

The beam is mounted as a cantilever, so that it is

clamped on one end and the other end is free. As the whole assembly is vertically accelerated upwards, then the beam deflects under its own inertia resulting from its own self mass and the film bonded onto the beam. The beam must be tiny (1mm) wide and a couple of mm's long, possibly (3mm) , but it does flex and, as the film is highly sensitive, it detects flexing or deflection. The film itself would see a stretching as beam flexes. The resulting electrical signal is proportional to the acceleration. The force transducer 32 on its own can weight the occupant but only if the vehicle was stationary, so the force transducer 32 with the accelerometer, can compensate with the vertical acceleration.

In summary, the piezo cable sensor detects displacement of the load thereupon. The physiological life signs of the occupant, against the counter example that there is a box on the seat must be separated to determine true occupancy. The sensor system is then further enhanced so that if you were driving over a cobblestone street, where you would then get signals that look rather similar to what you get from a person under different conditions, by incorporating an accelerometer into the system the information therefrom would be used to further qualify the output of the cable sensor. In other words, if the output signals appear to look like human vibration, but there is also a lot of vehicle vibration and they are almost matched in coherence (is a very useful digital function which actually acts like the similarity between the signals but it is measured as a function of frequency) , this phenomena may be recognized as basically the signals from a passive load which tend to be coherent with the vehicle vibration i.e. when the vehicle moves up the load on the cable would almost be dynamic force. However, when an animate object is on the mat the signals generated (i.e. the pulsing of the cable) is in response to both the vehicle vibration and the occupant shifting about that can be resolved into a signal that tends to be very incoherent with the vehicle acceleration. A force sensor may also be

provided as part of the system whereby the seat occupant's weight is determined which could then further be fed to the ECM in order to control various aspects of the automobile. The accelerometer functions then partly to compensate for both the force sensors and the cable sensors for the dynamics of vehicle motion.