Systems for capacitive object detection

Abstract

The invention relates to a system (1) for capacitive object detection. In order to provide an apparatus for effectively using the circuit of an electric heating device for capacitive object detection, the invention provides a system comprising: an elongated conductive element (2) extending between a first terminal (2.1) and a second terminal (2.2), wherein the first terminal (2.1) is coupled to ground, for example via a first capacitive element (4) and is connectable to a power supply (5), and the second terminal (2.2) is connected to ground; and a detection circuit (10) connected to at least one detection node (20-27) on the conductive element (2) arranged between the first terminal (2.1) and the second terminal (2.2), wherein the detection circuit (10) is adapted to capacitively detect the presence of an object (50) in the vicinity of the conductive element (2) based on an impedance (Z) associated with the object (50).

Description

Technical Field

[0001] The present invention generally relates to systems for capacitive object detection. Background Art

[0002] In modern vehicles, it may be necessary to detect whether the driver has his hands on the steering wheel (e.g. in order to determine whether the driver is ready to carry out a steering action). Steering assistance may include active correction possibilities for the driver to be used in certain situations. For example, it may be provided that the steering assistance system is only activated when the driver has his hands on the steering wheel. In most countries, it is mandatory that the vehicle is under the control of the driver when moving, even if modern assistance systems are able to operate the vehicle safely in certain situations. In order to recognize whether at least one hand is placed on the steering wheel, several concepts have been developed. One method uses capacitive sensors, which detect the hands by their influence on the electric field generated by the sensor. Although these sensors are more reliable, they significantly increase the complexity of the steering wheel. In order to overcome this problem, attempts have been made to use existing circuits for steering wheel heating for capacitive detection.

[0003] In different applications, capacitive sensors are also used to detect the occupancy state of vehicle seats. Capacitive occupancy detection or classification systems generally comprise at least one antenna electrode, which is arranged in the seating surface of the seat. In operation, an oscillating electrical signal is applied to the at least one antenna electrode, which then emits an electric field into a spatial region close to the antenna. The influence of an object or a living being on the electric field is detected in order to determine the seat occupancy state. The idea of ​​using a seat heater structure as an antenna electrode in such a capacitive system has also been known for a long time.

[0004] However, using an electric heater as a capacitive sensing element requires electrical decoupling between the heating function and the sensing function. This electrical decoupling is achieved by additional discrete components (e.g., some switches, inductors, or common-mode coils). These additional decoupling components can be expensive and use a lot of space, especially if the maximum heating current is large (e.g., above 15A).

[0005] Purpose of the invention

[0006] It is therefore an object of the present invention to provide a circuit for efficiently using an electrical heating device for an apparatus for capacitive object detection.

[0007] This problem is solved by a system for capacitive object detection according to claim 1 . Summary of the invention

[0008] The invention provides a system for capacitive object detection.In general, different types of objects may be detected, in particular body parts of a user, such as a hand or a finger.

[0009] The system includes an elongated conductive element extending between a first terminal and a second terminal. The conductive element can be any type of conductor, such as a wire, a printed conductor path on a printed circuit board or on a flexible substrate, etc. The conductive element can be disposed along at least a portion of a component or device (e.g., a vehicle component such as a steering wheel, an instrument panel, or a seat). Typically, it is disposed beneath some insulating lining that forms a physical surface of the component that can be designed to be touched by a user.

[0010] The conductive element extends between a first terminal and a second terminal. Typically, but not necessarily, these terminals correspond to the physical endpoints of the conductive element. The first terminal is coupled to ground and can be connected to a power supply. The first terminal can be coupled to ground, for example, via a capacitive element (capacitor), but coupling via a dedicated capacitive element is not necessarily required. For example, if there is already an AC ground path through a parasitic element such as a power switch, a capacitive element is not required. In addition, if the heater is always powered, a dedicated capacitive element is not required. For other wired sensors that may not be heaters, a permanent AC ground connection can exist anyway. By coupling the first terminal to ground, it can also be said that the first terminal is connected to AC ground, that is, it has a well-defined AC potential. In addition, it can be connected to a power supply, which generally means that it is permanently connected to the power supply, or it is connected to the power supply via a switch so that an electrical connection can be established temporarily. The power supply can be a current source or a voltage source; typically, it can be a DC power supply. In particular, the conductive element can be a heating element, and the power supply can supply a heating current or voltage, respectively. The second terminal is connected to ground. In this context, it should be understood that the expression "connected to ground" means that the terminal is connected to AC ground, that is, it has an AC ground potential. When the conductive element is connected to a power source, current flows from the power source through the conductive element to the ground. Generally, each of the first terminal and the second terminal is a part of the conductive element, which is suitable for connecting to the power source or the ground, respectively. Each of the connections can be established via another conductor, such as a wire or a printed conductor path, which is detachably or non-detachably connected to the corresponding terminal.

[0011] The system also includes a detection circuit connected to at least one detection node, and the detection node is arranged on the conductive element between the first terminal and the second terminal. The detection unit is connected to at least one detection node, which means that it is electrically connected. However, the electrical connection does not necessarily have to be a DC connection, but can be an AC connection. For example, the coupling can be achieved via an electric field and / or a magnetic field. The detection circuit can, for example, include a power supply, a signal generator, an amplifier, an analog-to-digital converter, a transceiver, a processing unit, and other components required to implement the functions described below. It should be understood that the detection unit can be implemented at least in part by software.

[0012] The detection circuit is suitable for capacitively detecting the presence of an object near a conductive element based on an impedance associated with the object. If there is an object, the object is capacitively detected, which means that the object is detected based on the influence of the object on the electric field near the conductive element. The electric field can be generated by the charge in the conductive element, and / or the electric field can induce a charge in the conductive element. As will be explained below, the detection circuit can apply an electrical signal via at least one detection node, and / or the detection circuit can detect the electrical signal via at least one detection node. In particular, the detection circuit can be suitable for determining the impedance between at least one detection node and ground. The impedance in turn depends in part on the impedance of the conductive element and in part on the impedance associated with the object. The former impedance is usually mainly resistive and inductive, while the latter impedance is usually mainly capacitive. According to the present invention, the inductive impedance of the conductive element (or in other words, its inductance) is used to decouple the detection function of the detection circuit from the function of the power supply (e.g., heating function). The technician will understand that in the expression "decoupling" refers to the decoupling of the signal / measurement path from the power supply path. The inductance of the wires replaces the inductance of dedicated decoupling inductors or common mode chokes used in prior art systems.

[0013] Although the detection circuit is generally configured to detect the presence of an object, it is preferably configured to determine the position of the object. In other words, the detection unit can detect not only whether the object is present in the vicinity of the conductive element (e.g., on an electrically insulating surface below which the conductive element is disposed), but also in which part of the surface. For example, if the conductive element is disposed along the circumference of the steering wheel, the detection unit can be configured to determine the angular position of the driver's hand.

[0014] The detection circuit is configured to operate at least one detection node in a loading mode, in which the detection circuit applies a detection signal to at least one detection node, and detects an object based on a response signal at the detection node. The detection signal may be a voltage signal or a current signal. The detection circuit may include a dedicated power supply, which may be referred to as a detector power supply. The detector power supply may be a voltage source or a current source. Even if it is a voltage source, the detection signal may be a current signal, for example, if the detector power supply is connected to the detection node via a coupling element (such as an impedance device). The general principle of the loading mode is that when the detection signal is applied to the detection node, an (alternating) electric field is generated near the conductive element. If there is no object, the electric field generally extends between the conductive element and the ground (a vehicle component having a ground potential). If there is an object, this corresponds to connecting the conductive element to the ground via an (unknown) impedance caused by the object. Therefore, it can be said that operating the detection node in the loading mode corresponds to an impedance measurement, in which the unknown impedance of the object is connected in parallel to the impedance of the conductive element. The "branch point" of the parallel connection roughly corresponds to the part of the conductive element where the object is located. Therefore, if the part is close to the detection node, the effect of the object on the total impedance is more significant than if the part is close to the terminal.

[0015] Generally, the influence of the (mainly inductive) impedance of the conductive element may make it difficult to determine the impedance of the object with sufficient accuracy or reliability. However, there are several options for how to reduce or eliminate this influence. According to the present invention, the detection circuit is suitable for applying two different frequencies as detection signals to at least one detection node sequentially and / or simultaneously, and detecting the object based on the frequency-dependent response signal. In other words, the detection circuit applies a detection signal comprising at least two different frequencies. The different frequencies can be applied one after another, i.e. sequentially. Alternatively or in addition, they can be applied simultaneously, in which case the detection signal is a superposition of different frequencies. In both cases, the response signal is generally frequency-dependent. For example, if the detection signal is a voltage signal applied to the corresponding detection node, the response signal is the current flowing through the detection node. The relationship between the current amplitude and the voltage amplitude will be different for each frequency. Since the frequency dependence of the impedance of the conductive element is mainly inductive, it is different from the frequency dependence of the impedance of the object, which is mainly capacitive. Therefore, by considering the response signal for each of the two frequencies, the capacitive contribution can be isolated. If the detection signal includes two different frequencies f a and f b and the two corresponding measured complex admittances are called Y a and Y b , the corrected capacitance can be calculated as follows:

[0016]

[0017] It will be appreciated that the assumption here is that the impedance is purely inductive. For the case where there is also a resistive component, a curve fit of the RLC model to the measured RLC impedance over frequency is an option to determine the inductive and capacitive components.

[0018] According to the simplest embodiment of the present invention, the detection circuit is connected to only one detection node. Alternatively, the detection circuit can be connected to multiple detection nodes between the first terminal and the second terminal. In particular, the detection circuit can be connected to multiple detection nodes individually, while some detection nodes can be connected so that the detection circuit can only access them in common. Different detection nodes can be utilized in various ways, as will be explained below.

[0019] Preferably, the conductive element is arranged in a meandering manner between at least one detection node and at least one terminal. More specifically, the conductive element may include alternatingly oriented portions (i.e. portions that direct "forward" and "backward"), wherein the length of each portion is preferably much larger than the distance between two adjacent portions, for example, at least 3 times, at least 5 times or at least 10 times. This may also be referred to as a sequence of long, thin loops. In the case of such an arrangement, an object may be at least partially arranged near the portion closest to the detection node. As mentioned above, this has a relatively high impact on the generation of the total impedance, so that the object can be detected with increased reliability.

[0020] The total impedance depends partly on the impedance of the conductive element and partly on the (mainly capacitive) impedance between the conductive element and ground, which is affected by the presence of the object. If the impedance of the conductive element is known, it is convenient to evaluate the measurement. It can be determined by calculation or by calibration. In such an embodiment, the detection circuit is adapted to detect the presence of the object based on the known impedance of the conductive element. In other words, the detection circuit can refer to the impedance of the conductive element to eliminate its influence from the total impedance measurement result.

[0021] According to one embodiment, the detection circuit is configured to operate at least one detection node in a transmission mode and detect an object based on a received signal from at least one receiving unit. In the transmission mode, the detection circuit applies a transmission signal to at least one detection node. The detection circuit can apply a current signal or a voltage signal as a transmission signal. The transmission signal causes an electromagnetic field, which is received by at least one receiving unit and induces an electrical signal corresponding to the received signal. The received signal is affected by whether there is an object between the conductive element (or more specifically, the vicinity of the corresponding detection node) and the receiving unit, which can also be referred to as a receiving antenna, a receiving electrode, etc. The detection circuit can be connected to the receiving unit to detect the received signal. If the detection circuit is connected to multiple detection nodes, it can be configured to operate all nodes or only some of them (possibly only one) in the transmission mode.

[0022] According to another embodiment, the detection circuit is configured to operate at least one detection node in a receiving mode, in which the detection circuit detects an object based on a received signal from at least one detection node in response to a transmission signal applied to at least one transmitting unit. In other words, the transmission signal is applied to the transmitting unit, which is also called a transmitting antenna, a transmitting electrode, etc. The transmission signal causes an electromagnetic field, which is received by the conductive element and induces an electrical signal corresponding to the received signal at the corresponding detection node. The received signal depends on whether there is an object between the transmitting unit and the conductive element (or more specifically, in the vicinity of the corresponding detection node). The detection circuit can be connected to the transmitting unit so as to apply the transmission signal. The transmission signal can be a current signal or a voltage signal. If the detection circuit is connected to multiple detection nodes, it can be configured to operate all nodes or only some of them (possibly only one) in the receiving mode.

[0023] Measurement errors in the detected impedance can be reduced by resonant measurement. In one embodiment, the system includes a capacitive element that can be connected to ground and in parallel to at least one detection node via a switch. In other words, the capacitive element can be connected to ground or disconnected from ground by opening or closing the switch. When the switch is closed, the capacitive element is connected to ground and is connected in parallel to at least one detection node relative to a detector power supply of the detection circuit. In this embodiment, the detection circuit is configured to determine a first resonant frequency when the switch is open and a second resonant frequency when the switch is closed, and detect an object based on the first resonant frequency and the second resonant frequency. Each resonant frequency can be detected by scanning the frequency of the detector power supply and determining the frequency with the maximum response. For example, if the detector power supply is a current source, the resonant frequency frequency corresponds to the maximum voltage at the detection node. For example, if the first resonant frequency f when the switch is open ra corresponds to the unknown impedance connected in parallel to the impedance of the conductive element, and the second resonant frequency f rb The known capacitance C of the conductive element corresponding to the impedance connected in parallel to the unknown impedance and the conductive element ref , the corrected measured capacitance can be calculated as follows:

[0024]

[0025] There are several ways to reduce, if not eliminate, the effect of the impedance associated with the inductive element on the measurement. According to one such option, the detection circuit is configured to detect an object based on relative changes in the measured impedance. In other words, the total impedance, which depends at least in part on the conductive element, is ignored and only relative changes are considered. These changes, especially when considered on a short-term basis, can be associated with an object entering or leaving the vicinity of the conductive element.

[0026] According to one embodiment, the detection circuit is adapted to operate at least one detection node in an inactive mode, in which the potential at the detection node is maintained at the potential of the detection node operating in the loading mode or receiving mode, or is maintained at the AC ground. The detection node in the inactive mode can be respectively arranged between the detection nodes operating in the loading mode or receiving mode operation, or it can be arranged between one such detection node and the terminal. For example, if a single first detection node is operated in the loading mode, the second detection node and the third detection node operated in the inactive mode can be arranged between the first detection node and each terminal. If the potential of each of the second detection node and the third detection node is maintained at the potential of the first detection node, the potential on the terminal is shielded from the first detection node. In addition, if at least one detection node is operated in the transmitting mode and at least one detection node is operated in the receiving mode, at least one detection node can be operated in the inactive mode so that its potential is maintained at the potential of the "receiving" node. Operating the detection node in the inactive mode creates an inactive portion or area. In other words, the effective sensing area is reduced. Therefore, preferably, the detection node is only temporarily operated in the inactive mode, as will be further explained below.

[0027] According to one embodiment, the detection circuit is configured to operate at least two detection nodes in a loading mode or in a transmitting mode with different signals having different measurement frequencies, the different signals being orthogonal PRN (pseudo-random noise) sequences, or periodic signals modulated with orthogonal PRN sequences. In other words, at least one detection node is operated with one signal, and at least one detection node is operated with different signals. By applying such different signals, different active areas can be defined. Using different measurement frequencies (or more generally, separate spectrums) for different signals ensures that there is no interference between different active areas. The same effect can be achieved by using (paired) orthogonal pseudo-random noise sequences or periodic signals modulated with orthogonal pseudo-random noise sequences.

[0028] In particular, but not exclusively, if there is only one detection node, the transmitting unit or the receiving unit, respectively, must be outside the conductive element. However, if there are multiple detection nodes, one detection node can be used to transmit the signal, while the other detection node is used to receive the signal. In such an embodiment, the detection circuit is configured to operate at least one detection node in the transmission mode and to operate at least one detection node in the receiving mode. It should be noted that this embodiment can be combined with the above-mentioned embodiments, in which different detection nodes are operated with different signals in the transmission mode. Therefore, different coupling paths between different detection nodes can be used simultaneously. It should be understood that there are multiple possibilities here, such as operating multiple detection nodes with different signals in the transmission mode and / or operating multiple detection nodes in the receiving mode.

[0029] It is possible that the detection circuit always operates each detection node in a given operating mode. However, preferably, the detection circuit is configured to sequentially operate different detection nodes in a loading mode, a transmission mode, a receiving mode and / or an inactive mode. For example, if some detection nodes are operated in a transmission mode and other detection nodes are operated in a receiving mode, it may be useful to sequentially use different combinations of transmitting detection nodes and receiving detection nodes, which correspond to different coupling paths. In addition, if one or several detection nodes are operated in an inactive mode, this generally corresponds to an inactive portion or inactive region of a conductive element, in which an object cannot be effectively detected. This problem can be overcome if a specific detection node is only temporarily operated in an inactive mode. For example, the system may include 5 detection nodes, and the detection circuit may be configured to operate 2 detection nodes in a loading mode and 3 detection nodes in an inactive mode, while sequentially applying different combinations of "active" detection nodes (i.e., nodes in a loading mode). BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Further details and advantages of the present invention will become apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings, in which:

[0031] Figure 1 is a schematic diagram of a first embodiment of the system of the present invention for capacitive object detection;

[0032] Figure 2 is a schematic diagram of a second embodiment of the system of the present invention;

[0033] Figure 3 is a schematic diagram of a third embodiment of the system of the present invention;

[0034] Figure 4 is a schematic diagram of a fourth embodiment of the system of the present invention;

[0035] Figure 5 is a schematic diagram of a detection circuit for the system of the present invention;

[0036] Figure 6 is a schematic diagram of a fifth embodiment of the system of the present invention; and

[0037] Figure 7 Schematic diagram of a sixth embodiment of the system of the present invention. DETAILED DESCRIPTION

[0038] Figure 1 A first embodiment of the inventive system 1 for capacitive object detection is schematically shown. It comprises a heating conductor 2, which may be an elongated printed conductor path arranged along the surface of a heater 3. The heating conductor 2 may be, for example, a heater 3 for heating a steering wheel of a vehicle or a seat heater of a vehicle seat. The heating conductor 2 extends from a first terminal 2.1 to a second terminal 2.2. It is arranged in a meandering manner, with a plurality of alternatingly oriented conductor portions 2.3-2.6. The conductor portions 2.3-2.6 are long and arranged relatively close together, so that the length of each conductor portion 2.3-2.6 is at least 5 times the distance between two adjacent conductor portions 2.3-2.6. The first terminal 2.1 may be connected to a power supply 5 via a switch 6, which provides power to the heater 3. The switch 6 may be, for example, a semiconductor switch or any other suitable type. In this embodiment, the switch 6 is shown as a high-side switch, but for safety reasons, the switch 6 may also be a low-side switch, or one high-side switch and one low-side switch. In addition, the first terminal 2.1 is connected to ground via a first capacitor 4, which keeps the first terminal 2.1 at a defined AC potential. The second terminal 2 . 2 is connected to ground and is therefore also at a defined AC potential.

[0039] A detection circuit 10 adapted to detect the presence of an object 50 in the vicinity of the heating conductor 2 is connected to the first detection node 20. Figure 1In an embodiment of the present invention, the detection circuit 10 operates the first detection node 20 in a loading mode, in which the detection circuit 10 applies a detection signal (e.g., an AC voltage) to the detection node 20 and detects a response signal (e.g., an AC current) at the detection node 20, thereby measuring the complex impedance between the first detection node 20 and ground. The complex impedance between the first detection node 20 and ground can be considered as a parallel connection of an unknown impedance Z and the impedance of the heating conductor 2, which is mainly inductive. In this context, it should be noted that the impedance also includes the resistive one, however, this resistive part is actually irrelevant because it is always significantly lower than the impedance of the capacitance to be measured. The inductive part of the impedance of the heating conductor 2 decouples the terminals 2.1, 2.2 from the detection node 20. Due to the physical structure of the heating conductor 2, the sensing sensitivity depends on the position of the object 50 to be detected. The closer the object 50 is to the detection node 20, the more accurately its impedance Z can be measured. If the object 50 is closer to one of the terminals 2.1, 2.2, more measuring current will flow into the terminals 2.1, 2.2 and into the ground, so that the unknown impedance Z cannot be measured with high accuracy.

[0040] In order to at least partially compensate for this problem, the heating conductor 2 is arranged in a meandering manner so that the more sensitive parts of the heating conductor 2 are generally included in the area where the object 50 is located. Figure 1 As shown in , the object 50 can be reliably detected because the object 50 is located simultaneously on the more sensitive part (the second part 2.4 from the top) of the heating conductor 2 and the less sensitive part (the top part 2.3) of the heating conductor 2. Figure 1 and Figure 2 As shown in , the heating conductor 2 should comprise a plurality of long, thin loops with parallel portions 2.3-2.6 in the forward and reverse directions and preferably with a minimum distance between the parallel portions 2.3-2.6 between the first terminal 2.1 and the detection node 20 and between the detection node 20 and the second terminal 2.2, respectively.

[0041] Preferably, the detection signal applied by the detection circuit 10 has at least one frequency which is chosen such that the impedance of the inductance of the heating conductor 2 is at least a substantial part of the unknown barrier Z to be determined, for example at least 10% or at least 20%.

[0042] To facilitate determination of the impedance Z, the impedance of the heating conductor 2 is preferably known a priori, e.g. by calibration or by characterization, and then removed from the measured impedance at the detection node 20. Another preferred option is to evaluate only the change in the measured impedance, by which measurement errors can be minimized or eliminated.

[0043] Figure 3A second embodiment of the system 1 of the present invention is shown, which is Figure 1 A modification of the embodiment of the invention is shown. In this case, the heating conductor 2 is used as a transmitting unit in a coupled impedance measurement setup. The detection circuit 10 is not in a loading mode, but rather operates the detection node 20 in a transmitting mode, and the unknown impedance Z is not connected to ground, but to a receiving unit 30. The detection circuit 10 applies a transmitting signal to the detection node 20. This causes an electric field that is changed due to the presence of the object 50 and generates a receiving signal that can be detected via the receiving unit 30, which is also connected to the detection circuit 10.

[0044] Figure 4 A third embodiment of the system 1 of the present invention is shown, which is Figure 1 Another modification of the embodiment of . In this case, the heating conductor 2 is used as a receiving unit in a coupled impedance measurement setup. Here, the heating conductor 2 is used as a receiving unit in a coupled impedance measurement setup. The detection circuit 10 is not in a loading mode, but operates the detection node 20 in a receiving mode, and the unknown impedance Z is not connected to ground, but to the transmitting unit 40. In this embodiment, the transmitting unit 40 is also connected to the detection circuit 10, which applies a transmission signal to the transmitting unit 40. This causes an electric field whose electric field is changed due to the presence of the object 50, and generates a reception signal that can be detected via the detection node 20.

[0045] Use as Figure 3 and Figure 4 The advantage of the coupled impedance measurement arrangement shown in is that a substantial portion of the inductance of the heating conductor 2 is short-circuited by the detection circuit 10, thereby significantly reducing its effect on the measurement. It will also be appreciated that Figure 3 The receiving unit 30 in may be Figure 4 The heater in Figure 4 The transmitting unit 40 in may be Figure 3 The heater in the

[0046] In order to reduce the influence of the inductive impedance of the heating conductor 2 on the unknown impedance measured in the loading mode measurement, for example, when the detection signal is an AC voltage signal and the response signal is an AC current at the detection node 20, the two different frequencies f a and f b The measurements are performed at the two frequencies, which can be applied simultaneously as a superposition or sequentially. The complex admittance at the two corresponding measurements is Y a and Y b In this case, the corrected capacitance can be calculated as follows:

[0047]

[0048] Figure 5 Shows that it can be used Figure 1 . In this case, the detection circuit 10 can apply an AC current to a detection node 20, and can measure a voltage on the detection node 20. The detector power supply 13 (a voltage source in this case) generates an AC voltage, and the coupling element (an impedance 14 in this case) injects an AC current into the detection node 20. As an alternative to injecting current via the impedance 14, a current source can also be used as the detector power supply 13. In this case, the voltage on the detection node 20 is measured in order to determine the unknown impedance to be measured.

[0049] Figure 5 The detection circuit 10 in the embodiment further comprises a second capacitor 12 having a known capacitance, which can be connected to the detection node 20 via the second switch 11. This arrangement allows to reduce the measurement error. When the switch is closed, the capacitor 12 is connected to the ground and is connected in parallel to the detection node 20 with respect to the detector power supply 13. The detection circuit 10 is configured to determine the first resonant frequency f when the switch 11 is open. ra , and determines the second resonant frequency f when the switch 11 is closed rb The resonant frequency f can be detected by scanning the frequency of the detector power supply 13 and determining the frequency with the maximum response. ra 、f rb The corrected measured capacitance can be calculated as follows:

[0050]

[0051] Figure 6 A fifth embodiment of the inventive system 1 is shown, which represents another option for reducing the influence of the heating conductor 2 on the measurement result. The detection circuit 10 is connected to a second detection node 21 and a third detection node 22, which are arranged between the first detection node 20 and the first terminal 2.1 and the second terminal 2.2, respectively. The detection circuit 10 maintains the second detection node 21 and the third detection node 22 at substantially the same voltage as the first detection node 20, thereby shielding the AC ground potential of the terminals 2.1, 2.2 from the first detection node 20. It can also be said that the second detection node 21 and the third detection node 22 act as guard nodes or guard electrodes for the first detection node 20. The protection thus achieved eliminates the influence of the heater on the capacitive measurement as disclosed above. A disadvantage is that, due to the guarding portion of the heating conductor 2, the sensing area is smaller than that of the first detection node 20. Figure 1 The "unprotected" embodiment in is relatively smaller.

[0052] Figure 7 A sixth embodiment of the inventive system 1 is shown, corresponding to a multi-zone application. The detection nodes 23-27 are connected to the detection circuit 10. A number of sensing options are possible.

[0053] The detection circuit 10 can operate a first group including at least one detection node 23-27 in a loading mode, thereby defining (one or more) active regions, and by keeping a second group including at least one detection node 23-27 at the same potential as one detection node 23-27 from the first group or at an AC ground, the detection circuit 10 can operate a second group including at least one detection node 23-27 in an inactive mode, thereby defining an inactive region. Preferably, the detection circuit 10 sequentially operates the first and second groups of different detection nodes 23-27 in the loading mode and the inactive mode, respectively.

[0054] Furthermore, at least two detection nodes 23-27 may be operated in a loading mode at different frequencies, thereby defining at least two separate active sensing regions. Note that at least two detection nodes 23-27 may be operated in a loading mode with a detection signal, thereby defining at least two separate active sensing regions, the detection signal being an orthogonal PRN (pseudo-random noise) sequence or a periodic signal modulated with an orthogonal PRN sequence.

[0055] Furthermore, at least one detection node 23-27 may be operated in the transmit mode and at least one detection node 23-27 may be operated in the receive mode. Optionally, at least one detection node 23-27 may be connected to AC ground or the same potential as the detection node in the receive mode, thereby defining an active transmit region, an active receive region and optionally an inactive region, thereby allowing coupling measurements to be performed. Preferably, the detection circuit 10 sequentially operates different detection nodes 23-27 in the transmit mode and in the receive mode (and optionally in the inactive mode), respectively, thereby defining different coupling paths.

[0056] The detection circuit 10 can also operate at least two detection nodes 23-27 in the transmission mode at different transmission frequencies, thereby defining a multiple of the number of coupling paths. In addition to using different transmission frequencies, different transmission signals can also be used, which are orthogonal PRN (pseudo-random noise) sequences or periodic signals modulated with orthogonal PRN sequences.

[0057] Reference numerals list

[0058] 1 System for capacitive detection

[0059] 2 Heating conductor

[0060] 2.1, 2.2 terminals

[0061] Section 2.3-2.6

[0062] 3 Heater

[0063] 4.12 Capacitor

[0064] 5.13 Power supply

[0065] 6.11 Switch

[0066] 10 Detection Circuit

[0067] 14 Impedance

[0068] 20-27 Detection Node

[0069] 30 Receiving Unit

[0070] 40 Transmitter Units

[0071] 50 Objects

[0072] C ref capacitance

[0073] Z Impedance

Claims

1. A system (1) for capacitive object detection, comprising: - an elongated conductive element (2) extending between a first terminal (2.1) and a second terminal (2.2), wherein the first terminal (2.1) is coupled to ground via a first capacitive element (4) and can be connected to a power supply (5), and the second terminal (2.2) is connected to ground; and - a detection circuit (10) connected to at least one detection node (20-27) on the conductive element (2) arranged between the first terminal (2.1) and the second terminal (2.2); wherein the detection circuit (10) is adapted to capacitively detect the presence of the object (50) near the conductive element (2) based on an impedance (Z) associated with the object (50), the detection circuit (10) being configured to operate at least one detection node (20-27) in a loading mode, in which the detection circuit (10) applies a detection signal to the at least one detection node (20-27) and detects the object (50) based on a response signal at the detection node (20-27); The detection circuit (10) is configured to operate at least one detection node (20-27) in a transmission mode and to operate at least one detection node (20-27) in a reception mode, and to detect the object (50) based on a reception signal from at least one receiving unit (30), wherein in the transmission mode, the detection circuit (10) applies a transmission signal to the at least one detection node (20-27), and The detection circuit is adapted to sequentially and / or simultaneously apply two different frequencies as the detection signal to the at least one detection node (20-27), and detect the object (50) based on a frequency-dependent response signal.

2. The system according to claim 1, wherein: The detection circuit (10) is connected to a plurality of detection nodes (20-27) between the first terminal (2.1) and the second terminal (2.2).

3. The system according to claim 1, wherein: The conductive element (2) is arranged in a meandering manner between at least one detection node (20-27) and at least one terminal (2.1, 2.2).

4. The system according to claim 1, wherein: The detection circuit (10) is adapted to detect the presence of the object (50) based on a known impedance of the conductive element (2).

5. The system according to any one of claims 1 to 4, wherein: The system comprises a capacitive element (12) connectable to ground via a switch (11) and connected in parallel to the at least one detection node (20-27), and the detection circuit (10) is configured to determine a first resonant frequency when the switch (11) is open and a second resonant frequency when the switch (11) is closed, and detect the object (50) based on the first resonant frequency and the second resonant frequency.

6. The system according to any one of claims 1 to 4, wherein: The detection circuit (10) is configured to detect the object (50) based on a relative change in measured impedance.

7. The system according to any one of claims 1 to 4, wherein: The detection circuit (10) is adapted to operate at least one detection node (20-27) in an inactive mode, in which the potential at the detection node (20-27) is maintained at the potential of the detection node (20-27) operating in a loading mode or a receiving mode, or is maintained at an AC ground.

8. The system according to any one of claims 1 to 4, wherein: The detection circuit (10) is configured to operate at least two detection nodes (20-27) in a loading mode or in a transmitting mode with different signals having different measurement frequencies, wherein the different signals are orthogonal PRN sequences or periodic signals modulated with orthogonal PRN sequences.

9. The system according to any one of claims 1 to 4, wherein: The detection circuit (10) is configured to sequentially operate different detection nodes (20-27) in a loading mode, a transmitting mode, a receiving mode, and an inactive mode.

10. A system (1) for capacitive object detection, comprising: - an elongated conductive element (2) extending between a first terminal (2.1) and a second terminal (2.2), wherein the first terminal (2.1) is coupled to ground via a first capacitive element (4) and can be connected to a power supply (5), and the second terminal (2.2) is connected to ground; and - a detection circuit (10) connected to at least one detection node (20-27) on the conductive element (2) arranged between the first terminal (2.1) and the second terminal (2.2); wherein the detection circuit (10) is adapted to capacitively detect the presence of the object (50) near the conductive element (2) based on an impedance (Z) associated with the object (50), the detection circuit (10) being configured to operate at least one detection node (20-27) in a loading mode, in which the detection circuit (10) applies a detection signal to the at least one detection node (20-27) and detects the object (50) based on a response signal at the detection node (20-27); The detection circuit (10) is configured to operate at least one detection node (20-27) in a transmission mode and to operate at least one detection node (20-27) in a reception mode, wherein in the reception mode, the detection circuit (10) detects the object (50) based on a reception signal from the at least one detection node (20-27) in response to a transmission signal applied to at least one transmission unit (40), and The detection circuit is adapted to sequentially and / or simultaneously apply two different frequencies as the detection signal to the at least one detection node (20-27), and detect the object (50) based on a frequency-dependent response signal.

11. The system according to claim 10, wherein: The detection circuit is configured to sequentially operate different detection nodes in a loading mode, a transmitting mode, a receiving mode, and an inactive mode.

12. The system according to claim 10, wherein: The detection circuit is connected to a plurality of detection nodes between the first terminal and the second terminal.

13. The system according to any one of claims 10 to 12, wherein: The conductive element is disposed in a meandering manner between at least one detection node and at least one terminal.

14. The system according to any one of claims 10 to 12, wherein: The detection circuit is adapted to detect the presence of the object based on a known impedance of the conductive element.

15. The system of any one of claims 10-12, further comprising a capacitive element connectable to ground via a switch and in parallel to the at least one detection node, and the detection circuit is configured to determine a first resonant frequency when the switch is open and a second resonant frequency when the switch is closed, and detect the object based on the first resonant frequency and the second resonant frequency.

16. The system according to any one of claims 10 to 12, wherein: The detection circuit is configured to detect the object based on a relative change in measured impedance.

17. The system according to any one of claims 10 to 12, wherein: The detection circuit is adapted to operate at least one detection node in an inactive mode in which a potential at the detection node is maintained at a potential of a detection node operating in a loading mode or a receiving mode, or is maintained at an AC ground.

18. The system according to any one of claims 10 to 12, wherein: The detection circuit is configured to operate at least two detection nodes in a loading mode or in a transmitting mode with different signals having different measurement frequencies, the different signals being orthogonal PRN sequences or periodic signals modulated with orthogonal PRN sequences.

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