DE29500749U1 - Seat sensor and device with a seat sensor and an evaluation unit - Google Patents

Description

Seat sensor and device comprising a seat sensor and a

Evaluation unit

The invention relates to a seat sensor for detecting the free/occupied state of a seat, in particular a motor vehicle seat, and a device with at least one seat sensor and an evaluation unit for measuring and evaluating the output signal of the seat sensor.

Different seat sensors for detecting the free/occupied status of a seat are known from the state of the art. Seat sensors commonly used include mechanical pressure sensors, switches, infrared sensors or electrical sensors, such as capacitive sensors.

For example, a capacitive sensor is known in which two metal strips, such as aluminum strips, are arranged one above the other in the seat cushion in such a way that when the seat is occupied, the distance between the metal strips is reduced, thereby increasing the capacitance of the capacitor formed by the metal strips as capacitor electrodes. This capacitive sensor is therefore a signal converter that converts the relative displacement of the capacitor electrodes achieved by occupying the seat into a change in the capacitance of the capacitor, which can then be detected and evaluated using an appropriately designed electrical evaluation unit. However, this capacitive sensor as a seat sensor is sensitive to interference. On the one hand, it is difficult to arrange the metal strips in the seat cushion in such a way that a clearly definable change in the distance between the metal strips is achieved when the seat is occupied. On the other hand, with this sensor, only the metal strips are arranged in the seat cushion, so that the capacitor formed by these must be connected to the evaluation unit via a relatively long connecting cable, which further worsens the interference sensitivity of the device constructed from the seat sensor and the evaluation unit.

The invention solves the problem of creating a seat sensor with which the free/occupied state of the seat provided with the sensor can be reliably and largely trouble-free detected, and which also has a simple structure and can be easily installed in a seat.

Furthermore, the invention is based on the object of creating a device for detecting, measuring and evaluating the free/occupied status of at least one seat and of designing it in such a way that it can work reliably and largely without disruption, can operate a variety of electrical devices connected to it, possibly with different input parameters, is flexible in its functions and has the simplest possible structure.

This is achieved according to the invention with regard to the seat sensor in that it has an oscillator circuit with a single-core sensor line connected to it and arranged in the seat, the stray capacitance of which, when the seat is occupied, serves as the oscillation frequency-determining capacitance for the oscillator circuit, a load capacitor connected to the output of the oscillator circuit, and a two-core connecting cable connected to the oscillator circuit, via which a supply voltage can be applied to the oscillator circuit, and to which a means for measuring the supply current conducted via the connecting cable can be connected as an output signal of the seat sensor.

According to the invention, a seat sensor based on capacitance change is created, which, however, has only a single, single-core sensor line, the stray capacitance of which is changed by the seat in which the sensor line is arranged being occupied or released. This capacitance change is already incorporated in the oscillator circuit of the seat sensor, which is preferably also arranged in the seat.

The oscillation frequency is converted into a change in the oscillation frequency by connecting the sensor line to the corresponding frequency-determining input of the oscillator circuit. The oscillation frequency of the oscillator circuit therefore changes depending on the free/occupied state of the seat provided with the sensor. The load capacitor connected to the output of the oscillator circuit is charged and discharged in each oscillation period. The energy required for this charging and discharging of the load capacitor is proportional to the oscillation frequency of the oscillator circuit.

It follows from this that the current consumption of the seat sensor, which is essentially determined - i.e. apart from negligible losses of the oscillator circuit - by the respective energy requirement of the load capacitor, is proportional to the oscillation frequency of the oscillator circuit. Accordingly, the seat sensor according to the invention acts as a whole as a signal converter, which converts the change in capacitance of the sensor line achieved by occupying or releasing it into a change in the supply current of the seat sensor. The output signal of the seat sensor according to the invention is therefore the supply current flowing in the connecting cable, via which the oscillator circuit is supplied with a supply voltage. Therefore, the free/occupied state of the seat can be determined by measuring the current consumption of the seat sensor.

Although the seat sensor according to the invention with the oscillator circuit, a frequency-determining resistor and the load capacitor can be constructed from any known electrical components, for the purpose of miniaturization it is preferred according to the invention that the seat sensor is constructed using SMD technology and a CMOS inverter is used as the oscillator.

The seat sensor according to the invention with the sensor cable can be installed in any place in the seat, provided that it is ensured that the stray capacitance of the sensor cable is changed by occupying or releasing the seat provided with the sensor. In order to meet this requirement

In order to comply with the requirements in any case and to be able to mount the seat sensor and lay the connecting cable as simply as possible, it is preferred according to the invention that the seat sensor is arranged in the seat cushion, preferably below the backrest, and that the sensor cable is laid in the seat cushion along the seat surface.

The length of the sensor cable of the seat sensor according to the invention should be such that an optimal change in capacitance is achieved when the seat is occupied or released. Tests have shown that this is achieved with a sensor cable length of around 30 cm.

The object underlying the invention with regard to the device is achieved according to the invention in that it has at least one seat sensor according to the invention described above, a DC voltage supply unit connected to the connection cable of the seat sensor for supplying the seat sensor, and an evaluation unit to which the seat sensor is connected with its connection cable, the evaluation unit having a signal converter for converting the output signal of the seat sensor into a digital signal, a central processing unit connected to the signal converter for processing the digital signal output by the signal converter, a non-volatile memory assigned to the central processing unit for storing a reference signal which corresponds to the output signal of the seat sensor when the seat is free, and a switching output unit connected to the central processing unit with a switching output for outputting a switching signal when at least one seat provided with a seat sensor is occupied.

The proposal to equip the evaluation unit with a central processing unit (CPU) enables flexible monitoring and processing of the output signals emitted by the seat sensor(s) connected to the evaluation unit.

In the device according to the invention, the seat sensor is supplied with a stabilized direct voltage via its connecting cable from the direct voltage supply unit. The supply current flowing in the connecting cable of the seat sensor, which is the output signal of the seat sensor according to the invention, is converted with the help of the signal converter into a digital signal that can be processed by the central processing unit.

A reference signal for each seat sensor is stored in the non-volatile memory assigned to the central processing unit, which corresponds to the output signal of the respective seat sensor when the seat assigned to it is free. This memory is preferably implemented in the form of an EEPROM, which makes it possible for a new reference signal to be stored in the memory for this seat sensor in the event of a drift in the current consumption of a seat sensor, which can be caused, for example, by a change in temperature or a displacement of the sensor line of the seat sensor.

Although the device according to the invention is already functional with a single seat sensor, it is preferred according to the invention to design the device in such a way that up to four, eight or sixteen sensors can be connected to the evaluation unit. This makes it possible to monitor a larger number of seats if required.

In certain applications, it is sufficient for the switching output unit of the evaluation unit to have a single switching output, via which a switching signal is emitted when at least one seat provided with a seat sensor is occupied. However, in order to ensure that the device according to the invention is compatible with a variety of electrical devices with different electrical input parameters, it is preferred according to the invention that the switching output unit has two switching outputs, which are connected to ground or a positive voltage, e.g.

+5 V. This means that when at least one seat equipped with a seat sensor is occupied, the central processing unit outputs a switching signal with L level and a switching signal with H level via the switching output unit.

In a preferred embodiment of the device according to the invention, the evaluation unit also has a manually activatable means for delaying the activation or deactivation of the respective switching signal by a predetermined time. This inventive proposal ensures that insignificant temporary changes in the free/occupied state of the seat sensed by the seat sensor, which can be attributed to a change in the sitting position, for example, are detected by the evaluation unit, but do not cause any changes in the switching signals. This means that switching the switching outputs back and forth can be avoided, which can be disruptive in certain applications. This manually activatable means can be implemented, for example, in the form of a switch or a jumper in the evaluation unit.

In the device according to the invention, the switching output unit is designed in such a way that a switching signal is generated at the switching output(s) - regardless of the number of seats each equipped with a seat sensor - even if only one seat equipped with a seat sensor is occupied. In certain applications, however, it can be advantageous to be able to obtain information about the free/occupied status of all seats equipped with a seat sensor. According to the invention, it is therefore preferred that the evaluation unit has a serial interface connected to the central processing unit, via which signals can be output in succession in accordance with the sensed free/occupied status of the seats equipped with a seat sensor.

As already mentioned, the evaluation unit is preferably designed

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designed so that the reference signal, which corresponds to the output signal of the respective seat sensor when the assigned seat is free , can be re-stored in the memory assigned to the central processing unit if it changes during operation of the device. However, in order to enable a calibration of the seat sensors installed and connected to the evaluation unit to be initiated manually, in particular after the installation of the entire device or a new or additional seat sensor, it is preferred according to the invention that the evaluation unit has a calibration button assigned to the central processing unit. By pressing this calibration button, the output signal of each seat sensor can be called up via the signal converter in the central processing unit and stored as a reference signal in the non-volatile memory.

The seat sensor according to the invention and the device according to the invention with this sensor and the evaluation unit can be used to monitor the free/occupied status of one or more seats. For example, the device according to the invention can be used to control a display that shows, for example, the seats in a hall that are already occupied. However, the device according to the invention is preferably used in a motor vehicle. It can be used, for example, to control a taximeter in a taxi or a so-called shared taxi, whereby the taximeter can be switched on or off using the switching signal output to the switching output of the evaluation unit when a single seat in the vehicle is occupied and released. It is also possible to register the switching signal to record the free/occupied status of the taxi. With the help of the serial interface, a protocol can be created for the free/occupied status of the individual seats in the vehicle in these application examples.

The invention is explained further below with reference to the drawing. In the drawing:

Figure 1 is a block diagram explaining an embodiment of the device according to the invention, and

Figure 2 shows a circuit diagram of the seat sensor according to the invention that can be connected to the evaluation unit of the device shown in Figure 1.

Figure 1 shows a block diagram of the device according to the invention with four seat sensors S1, S2, S3 and S4, an evaluation unit B and a DC voltage supply unit A arranged therein. This device can, for example, monitor the free/occupied state of the passenger seat and the three rear seats of a taxi for the purpose of registering the free/occupied state of the taxi and/or controlling a taximeter to switch it on and off and to create a protocol about the free/occupied state of the individual seats.

The seat sensors Sl, S2, S3 and S4 are all designed the same and, as can be seen from Figure 2, each have a CMOS inverter 1 as an oscillator. The oscillation frequency of the CMOS inverter 1 is determined by the resistance value of a resistor R, which is preferably implemented in the form of a series connection of a resistor and an NTC resistor for temperature compensation, and the stray capacitance of a single-core sensor line SL. The sensor line SL is exposed at one end and is connected at its other end to the corresponding frequency-determining input of the CMOS inverter 1. A load capacitor C _L is connected to the output of the CMOS inverter and is charged and discharged during each oscillation period of the CMOS inverter. Figure also shows a two-core connecting cable 2 which is connected to the supply voltage connections of the CMOS inverter I.

The seat sensors Sl, S2, S3 and S4 are constructed using SMD technology and are arranged in the seat cushion of each seat of the taxi below the backrest of this seat. The sensor lines SL are approximately 30 cm long and

are also laid in the seat cushion along the seat surface of the seat.

When a passenger approaches a seat sensor, the stray capacitance of the sensor line SL, which acts as the frequency-determining capacitance of the CMOS inverter I, is changed, whereby the oscillation frequency of the CMOS inverter I is also changed. Since the energy required by the load capacitor C _L to charge and discharge it is proportional to the oscillation frequency of the CMOS inverter 1, the change in the stray capacitance of the sensor line SL also changes the current consumption of the seat sensor. Therefore, the free/occupied state of the seat provided with the seat sensor can be detected by measuring the supply current flowing in the connecting cable 2.

As can be seen from Figure 1, the connecting cable 2 of each seat sensor Sl, S2, S3 and S4 is connected to the DC voltage supply unit A via a plug connection (not shown) for the purpose of easy installation, which in turn is connected to the car battery and from which a stabilized voltage suitable for supplying the seat sensors Sl, S2, S3 and S4 is generated. The DC voltage supply unit A also supplies the electronic components of the evaluation unit B with the required DC voltage.

The evaluation unit has an analog-digital converter 3 as a signal converter, which is connected with its inputs to the respective ground line of the connecting cable 2 of the seat sensors Sl, S2, S3 and S4. In the A/D converter 3, the supply current flowing in the respective connecting cable 2, which is the output signal of the seat sensors Sl, S2, S3 and S4, is converted into a voltage signal using a capacitor suitably inserted into the ground line of the connecting cable 2, which in turn is converted into a digital signal proportional to the supply current using a comparator unit. The A/D converter 3 also has a discharge circuit for discharging this in the respective

capacitor arranged on the ground line.

The signal outputs of the A/D converter 3 are connected to the inputs of a central processing unit (CPU) 4, which forms the core of the evaluation unit B and processes the digital signals output by the A/D converter and proportional to the output signal of the respective seat sensors. An EEPROM 5 is connected to the central processing unit 4 as a memory, in which a reference signal is stored for each seat sensor Sl, S2, S3, S4 connected to the evaluation unit B, which corresponds to the output signal of the seat sensor Sl, S2, S3 and S4 when the vehicle seat to which this seat sensor is assigned is in the free state.

The central processing unit 4 is also assigned a switching output unit 6, a means 9 for delaying the activation and deactivation of the switching signal, a serial interface 7 and a calibration button 8.

The switching output unit 6 is connected to an output of the central processing unit 4 and has two switching outputs 6a and 6b, one of which switches to ground and the other to +5 V if a switching signal is sent from the central processing unit 4 to the switching output unit 6.

The means 9 for delaying the switching signal on or off is implemented using jumpers. These jumpers enable the delaying the switching signal on or off to be switched on and off manually by changing the jumpers depending on the application. The delay time is e.g. 8 seconds. When the delay is switched on, the switching signal is delayed and sent to the switching outputs 6a, 6b, which prevents the switching outputs 6a and 6b from switching back and forth due to insignificant temporary changes in the free/busy state.

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condition of the respective vehicle seat can be prevented.

Signals corresponding to the free/occupied status of the vehicle seats equipped with the seat sensors Sl, S2, S3 and S4 can be output cyclically via the serial interface 7.

In a specific embodiment of the device according to the invention, the comparator unit and the discharge circuit of the A/D converter 3 of the evaluation unit B are implemented with an integrated circuit from digital technology (HC245), to which eight seat sensors can be connected. Only two such ICs are therefore required to detect a maximum of sixteen sensors. The central processing unit is an 87C51 CPU, and the serial interface is a standard interface (RS 232 C).

When installing the device in a taxi, the seat sensors Sl, S2, S3 and S4 are arranged with the sensor lines SL in the seat cushions of the taxi seats and are connected with their connecting cables 2 via plug connections to the evaluation unit B and the DC voltage supply unit A. This is connected with its inputs to the car battery and the taxi meter is connected to one of the switching outputs 6a, 6b of the device. Furthermore, the output 7a of the serial interface 7 of the device is connected to the taxi meter or a suitable electrical device for creating a protocol about the free/occupied status of the individual taxi seats. The jumpers are preferably set in such a way that the delay in the switch signal is switched on.

When the device is put into operation and cyclically during operation, the central processing unit 4 checks whether the adjustment button 8 has been pressed. Pressing the adjustment button 8 triggers an adjustment process in which the output signals of the seat sensors S1, S2, S3 and S4 are read in one after the other by the CPU 4 via the A/D converter 3, processed and stored as reference values in the EEPROM 5.

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stored. Since these reference values are used in the further processing as values that are assigned to the free status of the taxi seats, the comparison button to start the comparison process may only be pressed if the taxi seats are unoccupied.

During operation of the device according to the invention, the output signals of the seat sensors S1, S2, S3 and S4 are read in one after the other by the CPU 4 and compared with the assigned reference values stored in the EEPROM. If, based on this comparison, the CPU 4 determines that at least one monitored seat is occupied, a switching signal is generated by the CPU 4 and, if the free/occupied status of this seat does not change within the delay time, is sent via the switching output unit 6 to the taximeter to switch it on (and/or to a unit for registering the switching signal). The switching off of the taximeter is controlled by the CPU 4 via the switching output unit 6 when all taxi seats are released - also only after the delay time has elapsed. During operation of the device, signals are also output from the CPU 4 via the serial interface 7, e.g. to the taximeter, with the help of which a protocol can be created according to the free/occupied status of the individual taxi seats.

During operation, new reference values for one or the other sensor Sl, S2, S3 and S4 can also be stored in the EEPROM 5 if the CPU 4 determines that the output signal of this sensor is changed, e.g. due to a change in temperature or a displacement of the sensor line SL. This allows a drift in the current consumption of this seat sensor Sl, S2, S3, S4 to be compensated for by the evaluation unit B of the device according to the invention.

Claims (11)

13 ' Protection claims 1. Seat sensor (Sl, S2, S3, S4) for detecting the free/occupied state of a seat, in particular a motor vehicle seat, with an oscillator circuit (1) with a single-core sensor line (SL) connected to it and arranged in the seat, the stray capacitance of which, which changes when the seat is occupied, serves as the oscillation frequency-determining capacitance for the oscillator circuit (1), a load capacitor (Cl) connected to the output of the oscillator circuit, and a two-core connecting cable (2) connected to the oscillator circuit, via which a supply voltage can be applied to the oscillator circuit (1), and to which a means for measuring the supply current conducted via the connecting cable (2) can be connected as an output signal of the seat sensor (Sl, S2, S3, S4). 2. Seat sensor according to claim 1, wherein the seat sensor (S1, S2, S3, S4) is constructed using SMD technology and the oscillator circuit (1) has a CMOS inverter. 3. Seat sensor according to claim 1 or 2, wherein the seat sensor (S1, S2, S3, S4) is arranged in the seat cushion and the sensor line (SL) is laid along the seat surface of the seat. 4. Seat sensor according to one of claims 1 to 3, wherein the sensor line (SL) is approximately 30 cm long. 5. Device for detecting, measuring and evaluating the free/occupied state of at least one seat, in particular a motor vehicle seat, with a seat sensor (Sl, S2, S3, S4) assigned to the seat according to one of claims 1 to 4, a direct current supply unit (A) connected to the connecting cable (2) of the seat sensor (Sl, S2, S3, S4) and an evaluation unit (B) to which the seat sensor (Sl, S2, S3, S4) is connected with its connecting cable (2), wherein the evaluation unit (B) has a signal converter (3) for converting the Output signal of the seat sensor (Sl, S2, S3, S4) into a digital signal, a central processing unit (4) connected to the signal converter (3) for processing the digital signal output by the signal converter (3), a non-volatile memory (5) assigned to the central processing unit (4) for storing a reference signal which corresponds to the output signal of the seat sensor (Sl, S2, S3, S4) when the seat is free, and a switching output unit (6) connected to the central processing unit (4) with a switching output for outputting a switching signal when at least one seat provided with a seat sensor (Sl, S2, S3, S4) is occupied. 6. Device according to claim 5, wherein a maximum of four, eight or sixteen seat sensors (S1, S2, S3, S4) can be connected to the evaluation unit (B), each of which is assigned to a seat. 7. Device according to claim 5 or 6, wherein the Switching output unit (6) has two switching outputs (6a, 6b) for outputting a switching signal with L level or a switching signal with �EEgr; level when the at least one seat provided with a seat sensor (S1, S2, S3, S4) is occupied. 8. Device according to one of claims 5 to 7, wherein the evaluation unit (B) has a manually activatable means (9) for delaying the activation or deactivation of the switching signal by a predetermined period of time. 9. Device according to one of claims 5 to 8, wherein a serial interface (7) is connected to the central processing unit (4), via which signals corresponding to the free/occupied state of the seats provided with a seat sensor (S1, S2, S3, S4) can be output sequentially. 10. Device according to one of claims 5 to 9, wherein the central processing unit (4) has a calibration key (8) 15 is assigned, by the actuation of which the output signal of each sensor (S1, S2, S3, S4) can be called up via the signal converter (3) into the central processing unit (4) and can be stored as a reference signal one after the other in the non-volatile memory (5). 11. Motor vehicle with a device for detecting, measuring and evaluating the free/occupied state of at least one motor vehicle seat according to one of claims 5 to 10.