This patent application is a national phase filing under section 371 of PCT/EP2018/056729, filed Mar. 16, 2018, which claims the priority of German patent application 102017106188.6, filed Mar. 22, 2017, each of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The invention relates to driver circuits for analyzing and controlling a piezoelectric device. The piezoelectric device, connected with a button or as part of a button, may supply a user with haptic feedback. The application further relates to such a button and to a method for operating such a button with haptic feedback.

BACKGROUND

Buttons with short travel support the trend towards miniaturization. The shorter a button's travel is, the less the button contributes to the structural height or thickness of an electrical device in which the button is installed.

As travel diminishes, so the haptic impression made on actuation of the button also changes. To make the usual impression on a user when a button is actuated, it is possible to provide the button with an actuator which is connected mechanically with the surface of the button and effects movement of the button's surface. This also makes it possible to indicate to the user that actuation of the button has indeed taken place, e.g., in the case of a relatively smooth surface of uniform feel.

Circuits which are provided to respond to user input are known, for example, from U.S. Pat. No. 8,605,053 B2.

Sensor circuits with a piezoelectric device are known from European Patent No. EP 3,018,824 A1.

Electrical circuits with piezoelectric elements are likewise known from European Patent No. EP 3,065,029 A1 and from European Patent No. EP 2,600,224 A1 as well as from European Patent No. EP 2,472,365 B1.

SUMMARY OF THE INVENTION

Embodiments provide buttons with haptic feedback which are capable of imparting additional information, use less energy and are less susceptible to defects. For this purpose, novel circuits are needed for initiating activation states and for controlling such switches, as are methods for controlling the mechanical elements of the switches.

Embodiments provide a driver circuit. The driver circuit comprises a signal port with a first terminal and a second terminal. The circuit additionally has a first node and a second node. Moreover, the driver circuit has a comparator with an inverting input, a non-inverting input and an output. Furthermore, the driver circuit has an operational amplifier with an inverting input, a non-inverting input and an output. The first terminal is electrically conductively connected with the inverting input of the operational amplifier. The second terminal is electrically conductively connected with the non-inverting input of the comparator. The inverting input of the comparator is electrically conductively connected with the output of the operational amplifier. The first node is electrically conductively connected with the output of the operational amplifier. The inverting input of the comparator is electrically conductively connected with the inverting input of the operational amplifier. The second node is electrically conductively connected with the non-inverting input of the operational amplifier.

The signal port is intended to be electrically conductively connected with a piezoelectric device. To this end, the first terminal of the signal port may be connected with a first electrode of the piezoelectric device. The second terminal of the signal port may be connected with a second electrode of the piezoelectric device.

The piezoelectric device may have a stacked structure with a main body consisting of a piezoelectric material and patterned electrode surfaces in metallization layers, with layers of the piezoelectric material and layers of the electrode surfaces each being arranged alternately. The electrode surfaces are here each electrically conductively connected with one of two external electrodes. At the top and/or bottom thereof, the stack may have a “roof”. Depending on the electrical voltage applied to the external electrodes of the piezoelectric device, the stack of layers has a different height and a different width. That is to say, if the voltage applied is varied, a change in length is induced, which is amplified by the roof in the manner of a lever.

If the piezoelectric device experiences a pressure in the stack direction (due to the button being pressed, on the inside of which the layer stack is mounted), the change in voltage induced at the electrodes by an associated change in length of the layer stack is a measure of the force with which the button is being pressed. If the piezoelectric device experiences a pressure via the roof, the device is extended. The change in voltage induced by the associated extension of the stack is a measure of the force with which the button is being pressed.

The travel in the event of active or passive actuation of the button may in this case be of the order of magnitude of a few 100 μm, e.g., at most a few 100 μm, and is thus relatively short for button travel.

The induced change in voltage, which is a measure of the force acting on the button, may be read out. In response thereto, a voltage pulse or a voltage signal may in turn be output to the piezoelectric element in order to effect mechanical movement of the button. Such haptic feedback informs the user that he or she has successfully actuated the button.

The above-stated driver circuit has the advantage of managing on the one hand with relatively few circuit components. At the same time, the indicated driver circuit allows the pressure acting on the button to be read out and driver-side activation of the piezoelectric device to bring about haptic feedback. The change in voltage applied to the piezoelectric device may thus simultaneously be both read out and open- or closed-loop controlled.

The relatively small number of circuit components which may be interconnected in the driver circuit reduces the risk of driver circuit failure. The service life of the driver circuit is thus increased.

Furthermore, the driver circuit makes it possible for electrical energy provided to activate the piezoelectric device to be reutilized. Energy consumption is thereby reduced compared with circuits known hitherto and which for instance occur at driver end stages with linear controllers.

The driver circuit makes it possible to control closed-loop control) or regulate (open-loop control) the piezoelectric device with regard to a signal shape, i.e., the voltage characteristic over time, transmitted to the device, and with regard to the magnitude of the output voltage to the device in such a way that the device reliably gives a feedback signal as soon as an output voltage is generated by the piezoelectric device as a result of pressure. The signal shape is freely configurable subject to compliance with given edge times.

A voltage signal generated on pressing or releasing the piezoelectric device or a change in voltage is fed to the operational amplifier and processed appropriately thereby, so that the signal can be analyzed, e.g., by a microcontroller.

The operational amplifier further processes a specified signal (target specification) of a microcontroller in such a way that it may be fed to the comparator.

For signals from the operational amplifier to a microcontroller, the operational amplifier may be connected via an analog/digital converter (ADC). For signals from the microcontroller to the operational amplifier, the microcontroller or an output of the microcontroller may be connected via a digital/analog converter (DAC).

An electrical signal, e.g., a current signal, may be measured at the piezoelectric device, e.g., via a low-ohmic resistor, and fed to the comparator. This signal is compared with the signal output of the operational amplifier. If the component current to the piezoelectric device and thus the voltage at the piezoelectric device is fallen below, countermeasures are taken via the comparator, which may control a further circuit element, e.g., a half-bridge. The half-bridge may in this case control further circuit elements, e.g., transistors. In this way, it is possible to increase the voltage applied to the piezoelectric device, if it is too low relative to a setpoint value. Conversely, the voltage may be reduced if it is too low relative to a currently set comparison value.

The operational amplifier may, to this end, have a feedback loop.

It is possible for a signal to be output at the piezoelectric device if pressing of a button is detected. Alternatively or in addition, pressure on the piezoelectric device may also switch a free output at the microcontroller.

It is thus possible for the driver circuit additionally to comprise a half-bridge. Moreover, the driver circuit may have a third node, a fourth node and a fifth node. It is moreover possible for the driver circuit to have a first transistor and a second transistor, a first diode and a second diode and a capacitor. The half-bridge is electrically conductively connected with the first terminal, with the first transistor, with the second transistor and with the output of the comparator and with the fourth node and with the fifth node. The first diode is electrically conductively connected with the first terminal and the capacitor. The second diode is electrically conductively connected with the first terminal. The first transistor is electrically conductively connected with the first terminal and the third node. The second transistor is electrically conductively connected with the first terminal and the third node.

The third node may constitute a supply terminal of the driver circuit or be electrically conductively connected with a supply circuit for the driver circuit. The capacitor serves to store electrical energy temporarily. The first transistor and the second transistor or the first diode and the second diode serve to control the signal port, to which the piezoelectric device may be connected. If the component current of the piezoelectric device and thus the voltage at the piezoelectric device is fallen below, the half-bridge, which in turn controls the first transistor and the second transistor, is controlled via the comparator. If the first transistor is closed and the second transistor open, then voltage is supplied if the voltage at the piezoelectric device is too low. If the first transistor is open and the second transistor closed, then the signal port is supplied with electrical power, e.g., if the voltage at the signal port is not too low. The energy supply to the signal port SP may be passed via a coil, to smooth the current or voltage characteristics.

The coil may in this case be interconnected in series between the half-bridge and the first terminal of the signal port.

Inclusion of the half-bridge results in a switching controller, which is controlled by the comparator and via which the piezoelectric device or the signal port is supplied with a desired voltage. The switching controller also ensures that, on discharge of the piezoelectric device, i.e., in the event of a falling voltage (falling edge) present at the signal port, the energy is fed to the capacitor. This energy is removed again during the next charging cycle of the piezoelectric device (rising edge: increasing voltage at the signal port).

It is possible for the driver circuit additionally to comprise a microcontroller, an A/D converter, a D/A converter, a voltage converter and the piezoelectric device. The first node is electrically conductively connected with the microcontroller via the A/D converter. The second node is electrically conductively connected with the microcontroller via the D/A converter. The fourth node is electrically conductively connected with the microcontroller. The fifth node is electrically conductively connected with the microcontroller. The third node is electrically conductively connected with a supply terminal via the voltage converter. The signal port is electrically conductively connected with the piezoelectric device.

One object of the operational amplifier is to process a specified signal (target specification) from the microcontroller, provided via the D/A converter, in such a way that the signal may be fed to the comparator, which regulates the half-bridge of the switching controller. The signal from the D/A converter predetermines the signal shape for the output signal to the signal port. At the same time, the voltage is measured at the signal port and fed to the operational amplifier via a high-ohmic resistance divider. In this way, in a feedback loop the specified signal is compared with the actual voltage at the signal port and optionally corrected.

It is possible that the piezoelectric device is part of a switch with haptic feedback. In this case, pressing of the button may be registered by analysis of a voltage present at the piezoelectric device. The haptic feedback may be produced by applying a voltage to the piezoelectric device. Alternatively or in addition, one or more outputs of the microcontroller may be switched.

It is accordingly possible for the driver circuit additionally to have a voltage divider between the signal port and the comparator and/or a voltage divider between the signal port and the operational amplifier.

Activation voltages for piezoelectric devices in the form of piezoelectric actuators may be of the order of magnitude of around at most a few wo volts, e.g., 100 volts. Such voltages might destroy electrical devices such as comparators, operational amplifiers, half-bridges and transistors. Voltage dividers, e.g., in the form of high-ohmic resistance dividers, between the signal port and the sensitive devices reduce the voltage level for the sensitive devices. While the signal from the D/A converter predetermines the signal shape for the output signal at the signal port, the voltage is measured at the signal port and therefore fed via the high-ohmic resistance divider to the operational amplifier in a feedback loop. The resistance divider thus ensures that the voltage at the operational amplifier and subsequently at the microcontroller is reduced to a safe level, e.g., of 5 volts or less, to protect the low-voltage part of the circuit.

The circuit may have a low-ohmic resistor at the signal port. By way of the low-ohmic resistor, the current signal at the signal port is measured and fed to the comparator.

The advantage of such a driver circuit lies in the fact that control of the signal port is performed by means of a switching controller. This ensures that only the momentarily required energy is fed to the signal port during the rising edge. On discharge of the device connected to the signal port (falling edge), the transistors of the switching controller are switched such that the charging energy is fed to a capacitor and is available again during the next charging cycle. In contrast to conventional circuit designs, surplus energy is no longer dissipated. The efficiency of the driver circuit is improved and the components of the driver circuit do not have to be designed for relatively high currents and electrical powers. Moreover, on discharge, the electrical charge is not diverted to ground in order to bring about steep edges. The discharge edge is thus regulated. Short-circuiting of the signal port is prevented, in order to achieve better electromagnetic compatibility.

It is thus possible for a voltage at the signal port to be adjustable by switching controller. The voltage to be adjusted by the switching controller may in this case be a signal with amplitude varying over time. Both on actuation of the switch and on release of the switch, it is thus possible for any desired movement pattern, e.g., vibration pattern, to be generated by the piezoelectric device. The signal shape may in this case be stored in the microcontroller.

It is accordingly possible for the comparator to be embodied by an operational amplifier without feedback. Such comparators may be produced with relatively little design effort and satisfy the requirements of the driver circuit.

It is possible for the comparator to be produced with adjustable hysteresis.

It is possible for the voltage supply of at least one operational amplifier to be implemented with a series regulator. The microcontroller and the comparator may also be embodied by the voltage supply with the series regulator or an additional voltage supply with an additional series regulator. Series regulators in principle ensure good signal smoothness.

To generate the supply voltage of the signal port, a “step up” converter or a charging pump may be used. The supply voltage for the entire driver circuit may amount to 12 volts and be made available, for example, from the vehicle electrical system of a motor vehicle. The supply voltage in this case generates the voltage level of 100 volts or more for the signal port.

It is possible for the driver circuit to comprise a high-ohmic voltage divider, via which the voltage present at the signal port is fed to the operational amplifier.

It is possible for the signal port to be controlled by the comparator via a switching controller and supplied with a predetermined voltage, wherein the predetermined voltage is stored in a microcontroller.

It is possible for the driver circuit to comprise a coil, which is electrically conductively connected to the first terminal and which smooths the electrical signal to the signal port.

It is possible for the signal port to be activatable by triggering one or more digital inputs of a microcontroller.

It is possible in this respect for said triggering to be initiated by a touch sensor.

The touch sensor may in this respect be part of the driver circuit. The touch sensor may in this respect also be part of an external circuit environment with which the driver circuit is electrically conductively connected.

It is possible for the driver circuit to comprise a resistive element, which is interconnected between the inverting input of the comparator and the inverting input of the operational amplifier. Furthermore, the driver circuit may comprise a series interconnection with a resistive element and a capacitive element. This series interconnection may be interconnected in parallel with the resistive element and thus likewise between the inverting input of the comparator and the inverting input of the operational amplifier. It is possible in this respect for the capacitive element to be interconnected between the comparator and the resistive element of the series interconnection.

The resistive element of the series interconnection may have a resistance of from 80 kΩ to 160 kΩ, for example, 120 kΩ. The resistive element interconnected in parallel with the series interconnection may have a resistance of from 180 kΩ to 260 kΩ, e.g., 220 kΩ. The capacitive element of the series interconnection may have a capacitance of from 5 nF to 15 nF, for example, 10 nF.

Provision of the series interconnection or provision of the parallel connection consisting of the resistive element and the series interconnection improves the control behavior and thus the signal quality of the signal which can be provided at the signal port SP.

A further resistive element, for example, with a resistance of 3 kΩ, may be interconnected between the inverting input of the comparator and the series interconnection.

Embodiments provide a button. A button with haptic feedback comprises a piezoelectric device and a driver circuit. The driver circuit may be configured, for example, as indicated above. The driver circuit is provided to register a change in voltage induced by the piezoelectric device and to apply a voltage to the piezoelectric device on pressing and/or release of the button.

It is additionally possible for an output to be switched at the microcontroller on pressing and/or release of the button or of the piezoelectric device.

Embodiments provide a method for operating a button. A method for operating a button with haptic feedback comprises the steps of: —registering a change in voltage induced by a piezoelectric device, —activating the piezoelectric device by application of a voltage, if the voltage induced by the piezoelectric device rises or falls.

In addition or as an alternative to activation, it is possible for an output of a microcontroller to be switched on registration of a change in voltage at the piezoelectric device.

It is possible, with the method, for some of the energy to be recovered on activation of the piezoelectric device.

It is possible for registration of the change in voltage to include analysis of a change over time in a voltage at the electrical device and/or analysis of a voltage gradient.

It is possible for the method to comprise one or more of the following steps on initialization of a driver circuit by a microcontroller: ascertaining, —whether a component is connected to a signal port, —the magnitude of the capacitance of a circuit component connected to a signal port, and/or—the magnitude of the resonant frequency of a circuit component connected to a signal port.

The method may accordingly be carried out with at least partial use of the above-described driver circuit. The circuit component may accordingly be a piezoelectric device with a capacitance and a resonant frequency.

Ascertainment may proceed using a signal of constant or variable frequency at the signal port. The shape of the signal, e.g., a square wave signal, may be stored in the microcontroller. In order to determine the resonant frequency, the frequency-dependent current flow to the signal port may be determined. If the resonant frequency is determined by measuring or by reading out a stored value in the microcontroller, the signal port may be controlled with an adjusted, e.g., an optimized, frequency.

One of the above-stated electrically conductive connections between different circuit components of the driver circuit may in this respect be a direct electrical interconnection or an interconnection via series-interconnected further circuit elements such as, for example, capacitors, coils or ohmic resistors. For instance, the comparator and the operational amplifier are interconnected electrically conductively with the signal port. Due to the markedly different voltage levels of the operating voltages of these components, it is preferable to integrate voltage dividers into the electrical connection in order to obtain different voltage levels for the different devices.

