Patent Description:
Energy harvesting is a process by means of which electricity is obtained from one or more external sources such as ambient electromagnetic fields, solar energy, thermal energy, wind energy, mechanic motion (e.g. vibration), etc., which are sometimes collectively referred to as "ambient energy". In this process, energy is captured (harvested) and stored for use in so-called passive systems, i.e. systems that do not have their own power supply. For example, the <CIT> describes a concept of power management for power constrained devices. This concept involves collecting RF energy emitted by other devices. Further reference is made to <CIT>, <CIT> Al, and <CIT>.

Energy harvesting circuits that convert ambient energy into electrical energy have created much interest in both the military and commercial sectors. Some systems, for example, convert motion, such as that of ocean waves, into electricity for autonomously operating oceanographic monitoring sensors. So-called "wearable electronics" which also do not have their own power supply, are another field in which energy harvesting is employed.

Energy harvesting circuits usually provide a rather small amount of power and are therefore only suitable for supplying low-power electronics. One very common application is in RFID and NFC tags (RFID = Radio-Frequency Identification, NFC = Near Field Communication), which obtain the power needed for operation from the electromagnetic field produced by an NFC-enabled device (e.g. a mobile phone). NFC is standardized in ISO/IEC <NUM> (Near Field Communication Interface and Protocol-<NUM>) and ISO/IEC <NUM> (Near Field Communication Interface and Protocol-<NUM>) and is therefore not discussed in greater detail herein.

As mentioned, usually only a relatively small amount of electrical power can be provided by means of energy harvesting. Therefore, known energy harvesting circuits are usually not capable of supplying sufficient energy for loads such as electromechanical actuators (e.g. electric motors), which consume significantly more power than the circuitry of, e.g., an RFID/NFC tag or the like. Collecting the required electric energy for such loads from ambient energy sources would, in many applications, consume an unreasonably large amount of time or would require huge buffer capacitors.

A system for operating an electromechanical lock is described herein. In accordance with one embodiment, as defined by the independent claim <NUM>, the system includes an energy harvesting circuit configured to collect ambient energy and to use the collected ambient energy to charge a buffer capacitor. The system further includes a control circuit configured to alternatingly connect and disconnect an electric motor, for moving a latch of the electromechanical lock, and the buffer capacitor such that a capacitor voltage provided by the buffer capacitor is applied to the electrical motor in a discharging phase, in which the electric motor is connected to the buffer capacitor and the capacitor voltage decreases, and that the electric motor (<NUM>) is disconnected from the buffer capacitor in a charging phase so that the capacitor voltage again increases. The durations of the charging and discharging phases are designed such that the capacitor voltage stays above a minimum supply voltage of the electric motor, wherein the control circuit is configured to alternatingly connect and disconnect the electrical load and the buffer capacitor until the end of the nth discharging phase (n being a predetermined number). Furthermore, a method for operating an electromechanical lock is described herein. In accordance with one embodiment, as defined by the independent claim <NUM>, the method includes collecting ambient energy using an energy harvesting circuit and using the collected ambient energy to charge a buffer capacitor. The method further includes alternatingly connecting and disconnecting an electric motor and the buffer capacitor, wherein a capacitor voltage provided by the buffer capacitor is applied to the electric motor in a discharging phase, in which the electric motor is connected to the buffer capacitor and the capacitor voltage decreases, and wherein the buffer capacitor is recharged in a charging phase, in which the electrical load is disconnected from the buffer capacitor in a charging phase in which the capacitor voltage again increases. Alternatingly connecting and disconnecting the electric motor and the buffer capacitor is performed until the end of the nth discharging phase (n being a predetermined number). The durations of the charging phase and the discharging phase are designed such that the capacitor voltage stays above a minimum supply voltage of the electric motor such that electric motor moves a latch of the electromechanical lock.

In the following detailed description, reference is made to the accompanying drawings. The drawings form a part of the description and illustrate examples of how the invention may be used and implemented. It is to be understood that the features of the various embodiments described herein may be combined with each other, unless specifically noted otherwise.

In the embodiments described herein, a passive system which operates using electric energy collected by means of energy harvesting is described, wherein the passive system includes an electrically controlled actuator such as, e.g., an electromechanical actuator (e.g. an electric motor). It should be noted that the electrically controlled actuator is merely an arbitrary example of an electric load that requires more electric power than the amount generally obtainable by means of energy harvesting. Furthermore, the energy harvesting circuit used in the embodiments described herein extracts energy from an electromagnetic field which is generated by a device, such as a mobile phone, that is enabled for near-field communication (NFC), which is a standard feature of most modern mobile telephones). It is noted that the concepts described herein may easily also be used in connection with energy harvesting circuits that collect energy from other ambient energy sources, as well, such as mechanical vibration, solar radiation, or the like.

<FIG> illustrates one example of the general structure of a passive system that includes an electromechanical actuator, such as an electric motor (see <FIG>, electric load <NUM>), that is supplied with energy by a buffer capacitor Cs that is charged by an energy harvesting circuit <NUM>. In the example of <FIG>, the energy harvesting circuit includes an antenna LR (which has a certain inductance) coupled in parallel to a capacitor CR to form an LC parallel resonant circuit. In NFC applications the resonant circuit is designed to have a resonance frequency of approximately <NUM> (which corresponds to <MAT>). However, the resonance frequency is a design parameter that may differ in various applications. The electromagnetic field that is generated, e.g., by a device capable of wireless communication (in particular an NFC-enabled device such as a mobile phone <NUM>) induces a voltage in the antenna LR which is rectified by a rectifier circuit <NUM> coupled to the resonant circuit, and the output voltage of the rectifier circuit <NUM> is applied to buffer capacitor Cs, which is charged by the energy harvesting circuit <NUM> when an NFC field is present.

The electric energy stored in the buffer capacitor Cs equals CsVs<NUM>/<NUM>, wherein Vs denotes the capacitor voltage and Cs also denotes the capacitance of the buffer capacitor. The average power that can be output by the energy harvesting circuit <NUM> may be rather low (in the low milliwatt range, e.g. <NUM> mW or less) and heavily depends on a-priori unknown parameters such as the distance between the NFC-enabled device <NUM>, the output power of the NFC-enabled device, etc. In one example, in which the energy harvesting circuit includes a small solar cell instead of the NFC antenna, one of the aforementioned unknown parameters is the current irradiation received by the solar cell.

In the system of <FIG>, the electric load <NUM> is an electric motor, wherein the mechanical output power (which equals output torque times angular velocity) - and thus the electric power consumption of the electric motor, may be significantly higher than the average power provided by the energy harvesting circuit <NUM>. Accordingly, a continuous operation of the electric load <NUM> is not possible. However, a continuous operation of the load <NUM> is not required in many applications and the energy needed to operate the load for a certain time can be stored in the buffer capacitor Cs.

The timing diagram in <FIG> illustrates an example of one operation cycle of the load <NUM> included in the system of <FIG>. For the following discussion it is assumed that the load <NUM> requires a minimum supply voltage VSTOP in order to be able to operate properly. In case of an electric motor, the rotor of the motor will stop as soon as the supply voltage Vs falls below the value VSTOP. Such an assumption (minimum supply voltage requirement) is valid for most electric loads in practical applications. In the example of <FIG>, the electric motor requires a minimum voltage VSTOP of approximately <NUM> V, whereas the energy harvesting circuit is capable of providing an open-loop supply voltage of approximately <NUM> V that is, the buffer capacitor Cs can be charged up to Vs=<NUM> V when the motor <NUM> is off (first idle phase, left part of <FIG>).

Once the motor is switched on, the capacitor voltage Vs will drop, while the rotor of the motor continues to rotate and to output mechanical power. In the present example, the desired rotation of the rotor (e.g. a <NUM>° rotation to move a mechanical latch of a lock) of the motor <NUM> must be completed before the capacitor voltage Vs drops below the threshold VSTOP because the rotor of the motor will stop rotating below that voltage threshold. It can be seen from <FIG> that a significant portion of the energy (namely CSVSTOP<NUM>/<NUM>) stored in the buffer capacitor Cs remains unused. For electric DC motors VSTOP may be, for example, between <NUM> V and <NUM> V.

The size of buffer capacitor Cs and the maximum capacitor voltage Vs must be chosen such that the load <NUM> (e.g. actuator, motor) is able to generate the desired output work W. A rough estimation neglecting losses yields W ≤ CSVS<NUM>/<NUM> - CSVSTOP<NUM>/<NUM>. The parameter Vs is usually limited by the energy harvesting method used, and the parameter VSTOP is usually given by the type of load used in the considered system. Consequently, to increase the output work, the buffer capacitor needs to be increased. Large capacitors, which may be in the range of a few mF in practical applications, naturally have correspondingly large dimensions, which may be unsuitable or undesired for many applications.

The timing diagrams of <FIG> illustrate a novel concept for controlling the load <NUM> (e.g. the electromechanical actuator) to efficiently use the energy stored in the buffer capacitor Cs and provided by the energy harvesting circuit <NUM>. Accordingly, the load/actuator is operated in an intermittent manner, so that the capacitor voltage Vs stays in an interval between VON and VOFF which is above the threshold voltage VSTOP (i.e. VSTOP<VOFF<VON). From a user's perspective, the actuator moves in a quasi-continuous manner because the phases, during which the actuator is switched-off to allow a recharging of the buffer capacitor Cs, are relatively short. In reality the actuator moves stepwise, wherein the number of steps may be freely configured. In practice the number of steps will be so high that the actuator will perform the desired movement or - in other words - will output the desired (mechanical) work W.

As illustrated in <FIG> (top diagram), the buffer capacitor Cs is charged as soon as the ambient power source becomes active (e.g. when the mobile device <NUM> generates an NFC field, see <FIG>) and the capacitor voltage Vs increases until it reaches the voltage level VON. As soon as the capacitor voltage Vs reaches the threshold VON (VS=VON), the load <NUM> (e.g. the electric motor) is switched on by the control circuit <NUM> (see also <FIG>). While the load <NUM> is active (switched on), the capacitor voltage Vs decreases, because the power consumption of the load <NUM> is higher than the power provided by the energy harvesting circuit <NUM>. As a consequence, the net charge stored in the capacitor Cs decreases and the capacitor voltage Vs falls.

As soon as the capacitor voltage Vs reaches VOFF (VS=VOFF) the load <NUM> is deactivated (switched off) by the control circuit <NUM>. Once the load <NUM> is off, the power consumption becomes substantially zero and the capacitor Cs can be charged by the energy harvesting circuit <NUM>. Thus, the net charge stored in the buffer capacitor Cs increases and the capacitor voltage Vs increases accordingly during this charging phase. As soon as the capacitor voltage Vs again reaches the threshold VON, the load <NUM> is again activated and the next discharging phase starts. The load current (actuator current) is illustrated in the bottom diagram of <FIG>. While this is not necessarily the case, in the embodiments described herein, the buffer capacitor Cs is continuously charged by the energy harvesting circuit <NUM> (provided that it is able to collect sufficient ambient energy) throughout the charging phase as well as the discharging phase. The capacitor voltage, nevertheless, decreases in the discharging phase, because the load <NUM> usually consumes - on average - more power than the energy harvesting circuit <NUM> is able to deliver, and, as a consequence, the net charge change of the charge stored in the buffer capacitor is negative during the discharging phase.

The timing diagram of <FIG> shows a part of the top diagram of <FIG> in greater detail. In the example of <FIG>, the duration of the charging phase (in which the load <NUM> is off) is denoted as time toff. Similarly, the duration of the discharging phase (in which the load <NUM> is on) is denoted as time ton. As mentioned above, switching on and off the load <NUM> (e.g. the electric motor) may be triggered by the capacitor voltage Vs reaching the threshold values VON and VOFF, respectively. However, it is noted that, for example, the discharging phase, in which the load <NUM> is on, may also be set to a fixed time interval ton. In this case, the load is not deactivated when the capacitor voltage Vs has fallen to the threshold VOFF but rather when a pre-determined time interval (with a length of ton) has elapsed since the activation of the load. As the current consumption of the load is usually (approximately) known, the time interval ton can be set such that the capacitor voltage Vs reliably remains above the minimum voltage VSTOP.

With the concept discussed above it is possible to decouple the desired work output of the load <NUM> (e.g. the desired angular displacement performed by the electric motor) from the size of the buffer capacitor Cs. Thus, the capacitor size can be significantly reduced, as well as the space required by the buffer capacitor and its associated costs. A smaller buffer capacitor will also reduce the initial charging time (see <FIG>, time t<NUM>), which may increase user satisfaction (as the user needs to wait less time before the actuator starts to move). Furthermore, the unused energy CS·VSTOP<NUM>/<NUM> (discussed above) is reduced when the buffer capacitor is reduced to lower capacitances. The efficiency of the overall system can thus be improved.

<FIG> illustrates an exemplary implementation of the control circuit <NUM> used to control the switching operation illustrated in <FIG> and <FIG>. In the present example, a transistor H-bridge is used to drive the load <NUM> (electric motor), wherein the transistor H-bridge is composed of the p-channel MOS transistors T<NUM> and T<NUM> and the n-channel MOS transistors T<NUM> and T<NUM>, wherein transistors T<NUM> and T<NUM> form a first half-bridge and transistors T<NUM> and T<NUM> form a second half-bridge and the load <NUM> is connected between the middle-taps of the halfbridges. Driving an electric load (and particularly a DC motor) with a transistor H-bridge is as such known and thus not further discussed herein. A control logic <NUM> is configured to generate the gate signals for the transistors T<NUM>, T<NUM>, T<NUM>, and T<NUM> based on the current level of the capacitor voltage Vs provided by the buffer capacitor Cs and further based on predetermined parameters such as, for example, voltage thresholds VON and/or VOFF, the maximum number cnt of switching cycles, the times ton and/or toff, the direction of the actuator movement, etc. The control circuit <NUM> may include a power management unit (PMU) <NUM> which may be coupled between the buffer capacitor Cs and the other components of the control circuit <NUM>. The PMU <NUM> is configured to provide a defined (e.g. regulated/stabilzed) supply voltage VD to the logic circuit <NUM> and the, optionally, also to the transistor H-bridge. Various concepts for generating a supply voltage VD from an (unregulated) input voltage Vs are as such known and thus not discussed herein in more detail. The PMU <NUM> may also be configured to split the input power received from the buffer capacitor Cs and distribute it between the control logic <NUM> and the load <NUM>.

The embodiments described herein and applications thereof are summarized below. It is understood that the following is not an exhaustive discussion of technical features of the embodiments but rather a summary of some aspects. One embodiment relates to a method for controlling an electrical load of a passive system. Accordingly, the method includes collecting ambient energy using an energy harvesting circuit and using the collected ambient energy to charge a buffer capacitor (see <FIG>, energy harvesting circuit <NUM> and capacitor Cs). The method further includes alternatingly connecting and disconnecting the electrical load and the buffer capacitor such that the capacitor voltage Vs provided by the buffer capacitor is applied to the electrical load during a discharging phase, in which the load is connected to the buffer capacitor (and the capacitor voltage Vs decreases), and that the buffer capacitor is recharged during a charging phase, in which the electrical load is disconnected from the buffer capacitor. The durations of the charging phase and the discharging phase are designed such that the capacitor voltage Vs stays above a minimum supply voltage VSTOP required by the electrical load. This alternating/intermittent operation is also visualized by the diagrams of <FIG>, wherein the top diagram of <FIG> is the same as in <FIG> and the bottom diagram of <FIG> illustrates the work output of the load (e.g. angular displacement times output torque in case the load being an electric motor). As shown in <FIG>, the work output increases in each discharging phase in which the load is active, whereas the work output does not increase during the charging phases as the load is inactive to let the capacitor recharge.

As mentioned, the electrical load may be, in one example, an electromechanical actuator such as an electric motor (e.g. a DC motor). Many energy harvesting concepts are known. In one specific example, the ambient energy is the energy of an electromagnetic field generated by an NFC-enabled device (see <FIG>, mobile phone <NUM>). Energy can be collected by the energy harvesting circuit once the NFC-enabled device is active in the proximity of the NFC antenna included in the energy harvesting circuit.

In one example, the durations of the charging and discharging phases are determined by voltage thresholds (see <FIG> and <FIG>, threshold levels VON, VOFF). Accordingly, alternatingly connecting and disconnecting the electrical load and the buffer capacitor is achieved by connecting the electrical load to the buffer capacitor (in order to apply the capacitor voltage to the electrical load), when the capacitor voltage Vs reaches an upper threshold voltage level (see <FIG>, VON), and disconnecting the electrical load from the buffer capacitor when the capacitor voltage Vs falls to a lower threshold voltage level (see <FIG>, VOFF). In an alternative embodiment, the duration of the discharging phase is a predetermined time (see <FIG>, ton). Setting the charging phase duration to a predetermined time toff would also be possible. The voltage threshold levels VON, VOFF and the durations ton, toff of charging and discharging phases are set such that the capacitor voltage Vs does not fall to or below the minimum supply voltage VSTOP of the load. It is understood that the parameters VON, VOFF, ton, and toff cannot be set independently from each other.

As illustrated in <FIG>, alternatingly connecting and disconnecting the electrical load and the buffer capacitor may be performed until the end of the nth discharging phase (wherein n is a predetermined integer number greater than one, n><NUM>) or, alternatively, until the work output of the electrical load reaches a desired target level (see <FIG>, Wend).

One example embodiment relates to a method for controlling an electromechanical lock. Accordingly, an electric motor or another electromechanical actuator is mechanically coupled to a latch of the lock and the method described above is used to charge the buffer capacitor and drive the electric motor to move the latch. Assuming that moving the latch requires a specific (constant) output torque of the electric motor, the steps in the bottom diagram of <FIG> may also be interpreted as angular displacement. As mentioned, the energy harvesting circuit may collect energy from an NFC field generated by an NFC-enabled device. In this example, the switching process (i.e. the alternatingly connecting and disconnecting load and buffer capacitor) may be initiated by receiving a respective command from the NFC-enabled device using Near-Field Communication. For this purpose, the logic circuit <NUM> (see <FIG>) may be configured to communicate with the NFC-enabled device using as such known Near-Field Communication techniques.

Another embodiment relates to a passive system including an electric load (e.g. an electromechanical actuator), and an energy harvesting circuit that is configured to collect ambient energy and to use the collected ambient energy to charge a buffer capacitor (see <FIG> and <FIG>). The system further includes a control circuit configured to alternatingly connect and disconnect an electrical load and the buffer capacitor such that a capacitor voltage provided by the buffer capacitor is applied to the electrical load in a discharging phase (in which the electrical load is connected to the buffer capacitor and the capacitor voltage decreases) and that the buffer capacitor is recharged in a charging phase, in which the electrical load is disconnected from the buffer capacitor. The durations of the charging phase and the discharging phase are designed such that the capacitor voltage stays above a minimum supply voltage of the electrical load (see <FIG>, minimum supply voltage VSTOP).

In one example the control circuit control circuit includes a transistor H-bridge. However, a single transistor or any other type of electronic switch may be sufficient, depending on the actual application. The control circuit may include a logic circuit (including driver circuitry) configured to generate the control signals for the transistor(s) used to connect and disconnect load and buffer capacitor. As mentioned, the control logic may also be capable of communicating with an NFC-enabled device using Near Field Communication.

Claim 1:
A system comprising:
an energy harvesting circuit (<NUM>) configured to collect ambient energy and to use the collected ambient energy to charge a buffer capacitor (Cs);
a control circuit (<NUM>) configured to alternatingly connect and disconnect an electric motor (<NUM>) for moving a latch of an electromechanical lock, and the buffer capacitor (Cs) such that a capacitor voltage (Vs) provided by the buffer capacitor (Cs) is applied to the electrical motor (<NUM>) in a discharging phase, in which the electric motor (<NUM>) is connected to the buffer capacitor (Cs) and the capacitor voltage (Vs) decreases, and that the electric motor (<NUM>) is disconnected from the buffer capacitor (Cs) in a charging phase so that the capacitor voltage again increases,
wherein the durations of the charging phase and the discharging phase are designed such that the capacitor voltage (Vs) stays above a minimum supply voltage (VSTOP) of the electrical motor (<NUM>), and
wherein the control circuit (<NUM>) is configured to alternatingly connect and disconnect the electrical motor (<NUM>) and the buffer capacitor (Cs) until the end of the nth discharging phase, n being a predetermined number.