Patent ID: 12221015

DETAILED DESCRIPTION

FIG.1shows a first embodiment of an inventive sensor arrangement1which could be used for hand detection on a steering wheel or occupancy detection on the vehicle seat. A detection device10is connected to an electrode arrangement20which comprises a sensor electrode21and a guard electrode22. The guard electrode22is a heating element for that is connected through via a decoupling circuit60and two switch MOSFETs to a heating power source2(e.g. a battery of the vehicle) supplying a first potential V1and a second potential, in this case ground. A sensor circuit11of the detection device10is connected via a first capacitor13to the sensor electrode21. In a detection mode of the sensor arrangement1, the sensor circuit11applies a periodic detection signal to the sensor electrode21which gives rise to an electric field between the sensor electrode21and ground. A complex impedance3between the sensor electrode21and ground is influenced by the presence of an object100, which can therefore be capacitive leading to by the sensor circuit11. A guard driver12is connected through a second capacitor14to the guard electrode22. During detection mode, the guard driver12is supposed to apply a periodic guard signal that is identical to the detection signal so that the sensor electrode21and the guard electrode22always have the same potential. However, if the electrodes21,22have different potentials, and isolation capacitance23between them can considerably affect the capacitive measurement. Any disturbance of the periodic signal on the guard electrode22will lead to a (periodic) electrical potential between the sensor electrode21and guard electrode22and thus to a parasitic periodic current through the isolation capacitance23. This would cause an error in the determination of the unknown impedance3.

The heating element4is connected to the heating power source2and ground, respectively, through a high-side switch MOSFET30and low-side switch MOSFET40. A gate controller50controls the switch MOSFETs30,40to connect the heating element4to the power supply2and the ground during heating mode and to disconnect the heating element4from power supply2and ground during detection mode.

The decoupling circuit60comprises a decoupling MOSFET70, also controlled by the gate controller50and placed in the appropriate direction between the high-side switch30and the heating element4. The appropriate direction means that a body diode72of the decoupling MOSFET70has an opposite for what direction with respect to the body diode is32,42of the high-side switch30and the low-side switch40. During detection mode, the high-side switch30and the MOSFET70are switched off. A first node61between the high-side switch MOSFET30and the decoupling MOSFET70is then DC decoupled from the power supply2and the heating element4as the DC impedances of the MOSFETs30,70are very high. The first node61is AC grounded by a third capacitor64, preventing power supply transient voltages to be coupled to the heating element4through the output capacitances31,71of the high-side switch MOSFET30and the decoupling MOSFET70. A third potential V2of the first node61is fixed by a first DC voltage source62and a first resistor63.

Similarly, a fourth potential V3of a second node65, between the heater element4and the low-side switch MOSFET40, is provided by a second DC voltage source66and a second resistor67. The voltage across the decoupling MOSFET70is thus constant and less sensitive to the variations of the power supply2. This protects the heating element4, i.e. the guard electrode22, from an effect of dynamic capacitive load change due to the sensitivity of the output capacitance71(i.e. CSD) to voltage variations across the decoupling MOSFET70. The DC voltage across an input capacitance73(i.e. CGS) of the decoupling MOSFET70is also fixed by the gate controller50and the impact of the loading of the input capacitance73is limited by a third resistor68disposed between the gate controller50and the gate of the decoupling MOSFET70. Typically, output and input capacitances of the switch MOSFETs30,40present values comprised between several hundreds of pF and several nF. The DC biasing of the second node65also limits the changes of an output capacitance41of the low-side switch MOSFET40.

As a result of the placement of the decoupling MOSFET70, its body diode72prevents power supply2negative voltage transients from reaching the heating element4through intrinsic body diodes32,42of the high-side and low-side switch MOSFETs30,40. This prevents an overload of the guard driver12or a disturbance of the periodic guard signal applied to the heating element4. Resistances63,67and DC voltage sources62,66provide DC potentials V2, V3to node61,65to reverse bias the body diodes42,72of the low-side switch MOSFET40and the decoupling MOSFET70so that they cannot be conductive during detection mode.

During heating, the decoupling MOSFET70is switched ON by the gate controller50and the heating power loss is limited due to the small impedance (i.e. RDSON) of the decoupling MOSFET70.

InFIG.1, the decoupling MOSFET70is represented as a P-channel MOSFET. However, an N-channel MOSFET, with its source connected to the node61and its drain connected to the heating element4, could also be used. In this case, the gate controller50would have to be slightly more complex to switch OFF the MOSFET70during a negative power supply event, i.e. a temporal inversion of the polarity of the first potential V1that could e.g. occur as part of a characteristic noise in a vehicle power network. On the other hand, the use of an N-channel MOSFET leads to a lower output capacitance71between the heating element4and the first node61, because for similar current characteristics, N-channel MOSFETs are smaller in size than P-channel MOSFETs.

FIG.2shows a second embodiment of an inventive sensor arrangement1which is largely identical to the first embodiment and will insofar not be explained again. Here, the heating element4is used as the sensor electrode21during detection mode. Although not shown inFIG.2, the sensor arrangement1could also comprise a guard electrode22connected to a guard driver12. In this configuration, the parasitic output capacitances31,41,71of the MOSFETs30,40,70introduce a relatively large parasitic capacitance offset into the measurement of the unknown impedance3.

Preferably, this configuration can e.g. be for seat occupancy detection when the classification of object100can be performed by the detection of a relatively fast variation of the unknown impedance3with, for instance, an appropriate adaptative baselining of the measurement of the impedance3(i.e. one could use an adaptive algorithm with base-lining feature that evaluates the capacitive measurement values and determines different classes that shall be discriminated by the capacitive measurement). In this case, the detection should not be sensitive to relatively slow parasitic impedance variation that are related to temperature effect. The decoupling MOSFET70could be either a P-channel or N-channel MOSFET as already explained.

FIG.3shows a third embodiment of an inventive sensor arrangement1, where the decoupling MOSFET70is an N-channel MOSFET with its body diode72having the same forward direction as the body diodes32,42of the switch MOSFETs30,40. As it is not providing a reverse voltage protection, this configuration will be preferably used in a system which is not sensitive to negative power supply transients or in a system where a reverse voltage protection is already available upfront of the heating power supply2. This configuration is offering similar decoupling of the high-side part of the heating element circuit from power supply2. The decoupling MOSFET70could be either a P-channel or N-channel MOSFET.