DE29721213U1 - Circuit arrangement for a sensor element - Google Patents

Description

AHWENDONGSGSBXET AND STATE OF THE ART

The invention relates to a circuit arrangement for at least
a sensor element of at least one touch switch, in which a control signal is applied to the sensor element, which can be changed depending on the actuation state of the sensor element.

Touch switches are used in many electrical devices today, especially in household appliances.
By simply tapping the switch, which is usually a
10 metal surface, the user can initiate a desired switching operation.

In known touch switches that have a sensor element, a circuit is provided that applies a mostly high-frequency alternating voltage to the sensor element, which is changed when the touch switch is actuated. This change is detected and triggers a switching signal.
However, there are significant problems with interference that interferes with the signal. This interference is caused by
one from the power supply from the mains, since

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the increasing number of electrical devices has led to a sharp increase in so-called electrosmog. This means that numerous interference signals are superimposed on the mains voltage. In the same way, other devices, especially in the home, such as televisions, computers and mobile phones, emit interference signals, which together also interfere with the signal on the sensor element via the user.

For this reason, attempts have already been made to generate a stronger signal that is insensitive to interference by increasing the applied signal voltage from 5 V (the usual supply voltage for electronic circuits) to 20 to 30 V. This means that a second power supply is necessary, but this does not completely solve the problems.

TASK AND SOLUTION

The object of the invention is to create an improved circuit arrangement for a sensor element of a touch switch, which is not susceptible to interference signals and in particular does not require an additional power supply.

0 This task is solved by the control signal being a modulated, in particular a frequency-modulated, control signal. It preferably runs through a certain frequency range periodically, whereby interference signals, each of which has a certain fixed frequency, only occur very briefly synchronously with the control signal. Since the evaluation can be designed in such a way that it only recognizes changes in the signal level occurring over a longer period of time or a certain frequency range as switching state changes, interference signals with a fixed frequency play no role. In addition, interference signals usually only occur in certain

• it»

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frequency ranges, and when passing through a wide frequency range, these ranges are only briefly covered.

The circuit arrangement consists of at least one control part, one sensor part and one evaluation part, whereby in particular each sensor element has an associated sensor part, preferably a sensor circuit. This division makes it possible to combine several touch switches with only one control part and one evaluation part. With full functionality, the number of components and thus the effort and susceptibility to errors can be reduced.

Preferably, an operating frequency generated in the control part with a fixed frequency deviation can be periodically changed between a minimum value and a maximum value, whereby in particular the minimum value can be approx. 5 kHz and the maximum value approx. 50 kHz. The modulation frequency can advantageously be around 20 to 1000 Hz. The wide frequency range from 5 kHz to 50 kHz ensures that sufficient areas are also passed through in which no interference signals are present. In particular, low-frequency interference, for example harmonics of the mains frequency, is thus eliminated. The fixed modulation frequency ensures that the frequency range is passed through sufficiently quickly. The effort for this frequency generation is insignificantly greater than for a fixed output frequency.

The control signal is particularly preferably designed as a square-wave signal, in particular as a square-wave signal with a duty cycle of approximately 70% to 95% and a signal level of approximately 5 V. The high duty cycle ensures that the duration of the zero level is not too long, so that the DC voltage that is ultimately produced is sufficiently high.

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By using a signal level of 5 V, the operating voltage used for the electronic components can be used and no additional power supply is required. This reduces the effort, increases operational reliability and avoids interference from an additional power supply.

Advantageously, a component, in particular a supply capacitor, forms a first part of a voltage divider, in particular a capacitive voltage divider, with the sensor element forming the second part of the voltage divider. The touch switch is operated via the user's finger capacitance, and a capacitive voltage divider is therefore a good option. With this voltage divider, a change in one of the two capacitances, in this case the capacitance of the sensor, can be detected very easily by monitoring the partial voltages. The supply capacitor must have a constant capacitance.

In order to enable the control of several sensor parts with a single control part, the component that forms the first part of the voltage divider is contained in the sensor part. The sensor parts are connected to the control part via branch lines that can originate from a 5-pe-i-se node, so that the same control signal is present at all sensor parts.

Preferably, a filter, in particular an R-C filter, is connected upstream of the output of the sensor part. This filter converts the signal of the sensor element into a direct voltage that depends on the duty cycle, the signal level and the actuation state of the touch switch. Since the first two variables are fixed, an evaluation of the direct voltage can determine whether the touch switch has been actuated.

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detected. It is advantageous to have a filter in each sensor section to avoid interference. Such a filter can be set up without great effort and the resulting direct voltage can be evaluated using simple means.

The circuit arrangement advantageously contains switching means which are designed to short-circuit one phase of the control signal applied to the sensor element to a constant voltage, in particular the zero voltage. The zero phase of the control signal is preferably set to zero. As a result, interference signals which occur in this phase and which are particularly disruptive when superimposed on the zero phase are easily suppressed. The signal is therefore partially cleaned up.

In one embodiment of the invention, the switching means can contain a switch, in particular a switch with a semiconductor component. The semiconductor component can be, for example, a transistor or other fast-reacting components.

According to a further possible embodiment of the invention, the control part has a frequency generator—Z ³ Ur—for generating the frequency-modulated control signal with the operating frequency. The outlay for such a frequency generator is not significantly greater than for one with a fixed operating frequency, since such frequency generators are common in other areas of application.

If two inverting components are provided in series at any point in the signal path after the frequency generator and before the sensor element, for example before a supply capacitor, to control the switch, 0 where the simply inverted signal at the output of the first

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inverting component is led to the switch and forms its control, an automatic coupling of the switch and its function to the operating frequency is guaranteed. By connecting two inverters in series, the unchanged control signal is obtained again at the output of the second. The inverters work quickly and reliably and do not represent a source of error.

To evaluate several signals, the evaluation part has a signal multiplexer at its input with at least one input for connecting at least one output of a sensor part and an output to which a signal evaluation unit, in particular a microcontroller, is connected via a signal amplifier. This means that only one evaluation part is needed for all sensor elements used and the effort is reduced.

According to one possible embodiment of the invention, the circuit arrangement can contain at least one microcontroller, which is set up, among other things, to generate the control signal with the operating frequency for at least one sensor element. Such software generation provides an arbitrarily variable control signal, which enables a wide range of applications. In this way, an additional frequency generator can be saved, which saves costs and improves susceptibility to interference. The microcontroller can advantageously be used for other purposes besides frequency generation, such as for evaluating the sensor signal or completely different control tasks in a device belonging to the touch switch.

Preferably, in a design with a microcontroller 0, the control part and evaluation part of the circuit arrangement are contained in this. This further reduces the effort and

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enables precise evaluation of the signal applied to the sensor element. The microcontroller can also advantageously contain the switching means which are designed to short-circuit one phase of the control signal applied to the sensor element to a constant voltage, preferably zero voltage. This reduces the number of components required to a minimum, which also has a positive effect on the susceptibility to errors. An AD measurement is a suitable measuring method, whereby the AD conversion can be carried out using the dual slope method, the single slope method (with a comparator in the microcontroller) or using a separate AD converter. This is preferably contained in the microcontroller. However, AD conversion using the dual slope method is preferably used.

A particularly advantageous embodiment of the invention provides that the sensor element is a flexible, spatially variable and electrically conductive body and is intended for use in a touch switch on a hard glass or glass ceramic hob. It is preferably an electrically conductive foam. The use of such a sensor element for this purpose offers a number of advantages. The sensor element can have material properties that help suppress the occurrence of interference.

Advantageously, the body of the sensor element can have at least one opening in the direction of the contact surfaces, in which in particular a lighting element visible through a tempered glass or glass ceramic hob is contained. This can be, for example, an LED with which the exact position of the touch button can be displayed.

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These and other features emerge not only from the claims but also from the description and the drawings, whereby the individual features can be implemented individually or in combination in the form of sub-combinations in an embodiment of the invention and in other fields and can represent advantageous and protectable embodiments for which protection is claimed here. The division of the application into individual sections and subheadings does not limit the general validity of the statements made under these sections.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention is shown in the drawings and is explained in more detail below. In the drawings:

Fig. 1 a circuit arrangement consisting of

Control, sensor and evaluation part, with a sensor element attached to a glass ceramic hob,

Fig. 2 the circuit arrangement with a microcontroller, the control and evaluation part

contains,

Fig. 3 to 5 a commonly used sensor element

for installation on a glass ceramic hob and

Fig. 6 to 8 a novel sensor element according to a feature of the invention for installation on a

Glass ceramic hob.

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DESCRIPTION OF A PREFERRED EMBODIMENT

Fig. 1 shows a circuit arrangement 11 for a sensor element 12, which consists of a control part 13, a sensor part 14 and an evaluation part 15. The control part 13 contains a frequency generator 16, which generates a control signal 17 with an operating frequency, which is output at an output 18 of the frequency generator 16. The control signal 17 consists of a frequency-modulated square-wave signal. It is led to a first inverter 19 and from there to a second inverter 20. It then goes via a feed resistor 21 to a feed node 23. A first branch line 24 branches off from the feed node 23, to which a number of sensor parts 14 can be connected, but only one of which is shown. A connection also branches off from the first branch line 24 via a feed capacitor 22 to a signal node 25 in the sensor part 14.

Behind the first inverter 19 there is a branch 26 which leads to a switch node 27. A second branch line 28 branches off from the switch node 27, to which a plurality of switches 29 can be connected. Each sensor part 14 contains a switch 29 which is connected at one of its switching connections to the signal node 25 and at its other switching connection to the zero level of the circuit arrangement 11, which is connected to ground.

A connection goes from the signal node 25 to the connection of the sensor element 12, whereby the connection is made via a circuit board 38 on which the sensor element 12 is located, see Fig. 4. The structure of the sensor element 12 can be seen from Fig. 3. From the circuit board 38 comes a spring element 30, which must be conductive and preferably as a metallic spring.

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designed, extends upwards and carries a sensor plate 31, which is pressed against a tempered glass or glass ceramic hob 34 by the spring element 30. The sensor element 12 has a stray capacitance 3 3, which goes to ground.

The mechanical coupling of the sensor element 12 to the glass ceramic hob 34 creates an air gap capacitance 35, since the coupling is provided with an air gap 36 by a manufacturing-related nub 32 on the underside of the glass ceramic hob 34. This is shown in enlarged form in Fig. 5.

The capacitance of the glass ceramic hob 34 is represented by a glass capacitance 37. In Fig. 4 you can see a finger 39 that touches the glass ceramic hob 34 above the sensor element 12. This adds a serial finger capacitance 40 to the glass capacitance 3, which goes to ground.

A connection goes from the signal node 25 to a filter 41, which as an R-C filter consists of a serial filter resistor 42 and a filter capacitor 43 that goes to ground. A connection goes from the filter 41 to an output of the sensor part 14.

This output 44 of the sensor part 14 goes to one of several inputs 45 of a signal multiplexer 46. The signal multiplexer 46 is connected to a signal evaluation unit, in particular a microcontroller 48, via a signal amplifier 47. These three components form the evaluation part 15, to which as many sensor parts 14 can be connected as the signal multiplexer 46 has inputs. The microcontroller is connected with an output 49 to a part of a circuit (not shown) that implements the commands generated by the microcontroller, for example

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way of switching the power of an electric stove on or off.

Fig. 2 shows an embodiment with generation of the control signal in the microcontroller 48. The control signal 17 is sent to the sensor element 12 via the connection AO, a common series resistor and individual supply capacitors 22 and resistors 53. In this drawing, the sensor element 12 is shown as a variable capacitor, with the change in capacitance occurring by touching the touch switch. At the same time, the sensor elements 12 are connected to ground via the resistors 53 with further resistors 54 and capacitors 55.

The connections LO and Ll are each used to discharge the resistors 54 and capacitors 55, which form an RC element. The evaluation of the control signal 17 applied to a sensor element 12 takes place via connections of the connections 10 and Il to the capacitors 55 of the RC element.

0 As shown, the microcontroller 48 combines both the control part 13 and the evaluation part 15. The sensor part 14 is formed by the sensor elements 12 and the supply capacitors 22, the resistors 53 and 54 and the capacitors 55. Depending on the type of microcontroller 48 used, a different number of sensor parts 14 or sensor elements 12 can be operated.

An equivalent circuit diagram of a sensor element 12 made of an electrically conductive and flexible foam is shown in Fig. 6. It can be connected in parallel with an equivalent resistor 50 and a serial inductive

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resistor 51 and a parallel capacitance 52.

Fig. 7 shows a sensor element 12 mounted between the circuit board 38 and the glass ceramic hob 34, which consists of an electrically conductive and flexible foam. In Fig. 8 it can be seen that the sensor element 12 adapts so well to the dimpled underside of the glass ceramic hob 34 that the formation of an air gap is avoided.

FUNCTION

At this point, the version with frequency generator will be discussed first. The circuit is constructed according to the circuit diagram in Fig. 1, preferably using SMD technology and on the circuit board 38, which also forms the support and at the same time the connection for the sensor element 12. The use of a signal level of 5 V has the great advantage that only a 5 V power supply is required, since this is the operating voltage of the signal multiplexer 4 6 and the microcontroller 48.

0 The frequency generator 16 first generates a square wave signal with a signal level of 5 V, a duty cycle of approximately 70 to 95% and a frequency of 20 to 500 kHz. This square wave signal is frequency modulated with a modulation frequency of 20 to 1000 Hz, between corner values of typically 5 to 50 kHz. This produces the control signal 17.

The first inverter 19 is required to control the switch 2 9. When the control signal 17 returns to zero, it is inverted by the inverter 19 and set to 5 V, and

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This increase activates the switch 29 and closes the signal node 25 to ground. The signal at the signal node 25 is therefore always zero when the control signal 17 is at zero. Interference is thus suppressed during this time. The original output signal 17 is restored by the second inverter 20 behind the first. This goes via the feed resistor 21 and the branch line 24 to the feed capacitor 22.

In the non-actuated state, the sensor element 12 only has its stray capacitance 33, which together with the feed capacitor 22 forms a voltage divider for the output signal present at the feed capacitor. The feed capacitor 22 is preferably dimensioned so that half the voltage is present at the sensor element 12, i.e. the feed capacitance is approximately as large as the stray capacitance 33. When the touch switch is actuated, the air gap capacitance 35, the glass capacitance 37 and the finger capacitance 40, which are connected in series, are added in parallel to the stray capacitance 33. This parallel connection increases the total capacitance of the sensor element 12, the divider ratio of the total capacitance to the capacitance of the feed capacitor 22 changes, and the voltage at the sensor element and thus at the signal node 25 decreases. At the output of the filter 41 connected to the signal node 25, a direct voltage is produced which depends only on the signal level, the duty cycle, which is known and unchangeable, and on the sensor voltage, which depends on the actuation state of the touch switch.

This DC voltage is fed to a signal multiplexer 46, which selects one of several DC voltages from several 0 sensor circuits 14 and feeds it to a microcontroller 48 via a signal amplifier 47. The microcontroller 48 evaluates the DC voltage and detects whether the base

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switch was activated or not. The evaluation is preferably designed in such a way that it is evaluated over a certain period of time, corresponding to a frequency range covered in this period, for example by summing or integrating. Only when a change in the direct current occurs over this certain period of time should the evaluation report an activation. If an interference signal with a fixed frequency occurs, the direct current only changes for a moment, if at all, and this is not significant within the period of time provided for the evaluation. Depending on a detection, it issues commands at its output 49 to another circuit, for example the control circuit for a hob heater.

Furthermore, the equivalent resistor 50 with the supply capacitor 22 and the stray capacitance 33 forms a filter that dampens the coupling of interference signals via the finger 39 when the touch switch is actuated. In order to make the material of the sensor element 12 conductive, various materials, e.g. graphite, silver or the like, can be mixed in. If ferritic materials are mixed in instead or in addition, the inductive resistor 51 is created. This further improves the damping of interference signals. In this way, a particularly advantageous embodiment of the invention can be created with such a sensor element. It can be used equally in both versions for generating the control signal.

The design according to Fig. 2 controls the sensor element 12 according to the following scheme:

0 In the first step, the RC element consisting of the resistor 54 and the capacitor 55 is discharged via the LO terminal. In the second

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In step 55, the capacitor is charged via the LO connection until a logic high level is measured at the IO input. In practice, this is the case when the capacitor is charged to the general operating voltage of 5 volts.

In step 3, the control signal 17 is generated at the AO terminal for a certain time with the modulation or frequency modulation according to the invention and the capacitor 55 is thus discharged via the resistor 54. The control signal can be essentially the same as that of the version with a frequency generator. In this phase, the LO terminal is switched to high impedance before each change of the signal from low to high at AO and to low impedance against the zero voltage before the change from high to low at AO. In this way, in this version, grounding takes place during the zero phase of the control signal.

In step 4, the RC element consisting of the resistor 54 and the capacitor 55 is recharged via the LO connection. The time is measured until the signal at connection 10 changes to the high level. As mentioned above, the AD conversion of the signal at connection 10 or Il is preferably carried out using the dual slope method. The evaluation of the voltage applied to the sensor element can preferably be carried out dynamically, whereby a temperature drift of the signal level can be compensated.

Just as in the embodiment according to Fig. 1, the change in the control signal 17 is achieved here by touching the sensor element 12 fed by it. A resulting switching process can take place in exactly the same way.

Claims (15)

28 November 1997 JB/FR/sc Claims Circuit arrangement for a sensor element 1. Circuit arrangement for at least one sensor element (12) of at least one touch switch, in which a control signal (17) with an operating frequency is applied to the sensor element, which can be changed depending on the actuation state of the sensor element, wherein the control signal is a modulated, in particular a frequency-modulated, control signal. 2. Circuit arrangement according to claim 1, characterized in that the circuit arrangement (11) consists of at least one control part (13), one sensor part (14) and one evaluation part (15), wherein in particular each sensor element (12) has an associated sensor part, preferably a sensor circuit. 3. Circuit arrangement according to claim 1 or 2, characterized in that the operating frequency with a fixed frequency swing is preferably periodically variable between a minimum value and a maximum value, in particular the minimum value being approximately 5 kHz and the maximum value being approximately 50 kHz, the modulation frequency being in particular approximately 20 to 1000 Hz. 4* • I« ««If »•a · βt · c A32 001 4. Circuit arrangement according to one of the preceding claims, characterized in that the control signal (17) is a square-wave signal, in particular a square-wave signal with a duty cycle of approximately 70% to 95% and a signal level of approximately 5 V. 5. Circuit arrangement according to one of the preceding claims, characterized in that a component, in particular a supply capacitor (22), forms a first part of a voltage divider, in particular a capacitive voltage divider, the sensor element (12) forming the second part of the voltage divider, the component which forms the first part of the voltage divider being contained in the sensor part (14). 6. Circuit arrangement according to one of claims 2 to 5, characterized in that a filter, in particular an R-C filter (41), is connected upstream of the output (44) of the sensor part (14). 7. Circuit arrangement according to one of the preceding claims, characterized by switching means which are designed for short-circuiting one phase of the control signal (17) applied to the sensor element (12) to a constant voltage, in particular the zero voltage. 8. Circuit arrangement according to claim 7, characterized in that the switching means contain a switch (29), in particular a switch with a semiconductor component. A32 001 9. Circuit arrangement according to one of the preceding claims, characterized by a frequency generator (16) for generating the frequency-modulated control signal (17) with the operating frequency, wherein in particular the frequency generator is contained in the control part (13). 10. Circuit arrangement according to claim 9, characterized in that for controlling the switch (29) two inverting components (19, 20) are provided one after the other at any point in the signal path after the frequency generator (16) and before the sensor element (12), wherein the signal which is simply inverted after the first inverting component (19) is fed to switching means at the output of this component and forms the control thereof. 11. Circuit arrangement according to one of the preceding claims, characterized by a signal multiplexer (46) with at least one input (45) for connecting at least one output (44) of a sensor part (14) and an output to which a signal amplifier (47) a signal evaluation unit, in particular a microcontroller (48), is connected, wherein in particular the signal multiplexer is contained in the evaluation part (15). 12. Circuit arrangement according to one of claims 1 to 7, characterized by at least one microcontroller (48) which is set up to generate at least the control signal (17) with the operating frequency for at least one sensor element (12). A32 001 - 4 - 13. Circuit arrangement according to claim 12, characterized in that the control part (13) and the evaluation part (15) are contained in the microcontroller (48). 14. Circuit arrangement according to claim 12 or 13, characterized by switching means in a microcontroller, in particular the microcontroller (48) for generating the control signal (17), which are designed for short-circuiting one phase of the control signal applied to the sensor element (12) to a constant voltage, in particular the zero voltage. 15. Circuit arrangement according to one of the preceding claims, characterized in that the sensor element (12) is an inherently flexible, spatially variable and electrically conductive body and is intended for use in a touch switch of a tempered glass or glass ceramic hob (34).