Power supply circuit, associated actuators, and method for powering a load

Abstract

A power supply circuit for powering a load, which may be an SMA element or a piezoelectric element, may use short high voltage pulses to achieve rapid heating of the load, and in order to comply with functional requirements, the SMA or piezoelectric element should not be powered by directly connecting it to an electrical line at a relatively high voltage. A method for producing a power supply circuit according to the present disclosure, comprising: an actuator comprising at least one load comprising at least one smart material selected between a piezoelectric device and a shape memory alloy (SMA) element; and the steps of: connecting an AC / DC voltage converter of the power supply circuit to the AC mains by closing an input switch to charge a tank capacitor of the converter and simultaneously opening an output switch to disconnect the load from the tank capacitor; disconnecting the AC / DC voltage converter of the power supply circuit from the AC mains by opening the input switch and at the same time closing the output switch to power the load by discharging the tank capacitor; A method of powering a load is also disclosed, comprising:

Description

[Technical Field]

[0001] The present disclosure relates generally to electric actuators, and more particularly to a power supply circuit adapted to power a shape memory alloy (SMA) element, a power supply circuit and an associated actuator comprising a load comprising at least one smart material selected between a piezoelectric device and a shape memory alloy (SMA) element, and a method of powering the load. [Background technology]

[0002] Shape memory alloys (also known as SMAs) are materials that "remember" their original shape and return to that shape when heated. Thermal activation of elements made from SMAs, such as wires made from SMAs, can be driven by electric current via Joule heating using a power supply circuit configured to force current through the wire. Shape memory alloys' inherent property of regaining shape upon heating, such as SMA wires and springs produced by SAES Getters SpA under the trademark SmartFlex®, can be effectively packaged into compact, lightweight, powerful, and silent actuators, replacing alternative technologies based on wax, DC motors, and electric motors. Their unmatched energy density, degree of integration, and simplicity of design make shape memory alloys the new industry standard for linear and rotary actuators.

[0003] In order to have a fast response of the SMA actuator, it is important to heat the SMA element quickly by using an appropriate power supply circuit configured to force a current across the SMA element.

[0004] An example of a high-speed SMA actuator is disclosed in document US2016 / 0186730 and is shown schematically in Figure 1. The SMA element is powered by a drive unit 31 when the output MOSFET is closed, and the current flowing through it is provided by a tank capacitor C1 that is charged by a power supply unit 2. This conventional SMA actuator has a simple structure but is specifically designed to work with a low-voltage power supply unit 2 that injects a controlled DC current of typically 5 amperes. The power supply voltage is adjusted depending on the electrical resistance of the SMA actuator when it is switched on. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] US2016 / 0186730 [Non-patent literature]

[0006] [Non-Patent Document 1] “cumulative current magnetizing method for a capacitor-discharged impulse magnetizer”, Pai et al., 2017, International Journal of circuit theory and applications Summary of the Invention [Problem to be solved by the invention]

[0007] Controlling an SMA or piezoelectric element with this conventional actuator results in a slow actuation response due to the long time required to warm up the SMA or piezoelectric element, resulting in a large amount of heat being lost to the environment, which results in low efficiency.

[0008] According to the present disclosure, a power supply circuit for powering a load, which may be an SMA element or a piezoelectric element, may use short, high-voltage pulses to achieve fast heating of the load; in order to comply with functional requirements and avoid element damage related to overheating and / or burning, the SMA or piezoelectric element should not be powered by directly connecting it to an electrical line at a relatively high voltage. In this regard, solutions such as those described in the article "cumulative current magnetizing method for a capacitor-discharged impulse magnetizer" by Pai et al., published in the "International Journal of circuit theory and applications" in 2017, differ because they increase the output voltage relative to the mains input, leading to damage to the voltage-responsive actuator element, and do not include any safety element that can discharge the capacitor if the load or electrical connections are damaged. [Means for solving the problem]

[0009] The above requirements are met by means of a power supply circuit as defined in claim 1.

[0010] Also disclosed is an actuator comprising a power supply circuit of the present disclosure and at least one load comprising at least one smart material selected between a piezoelectric element and a shape memory alloy (SMA) element, as well as a method of powering the load, comprising the steps of: · Connecting the AC / DC voltage converter of the power supply circuit to the AC mains by closing the input switch to charge the tank capacitor of the converter, and simultaneously opening the output switch to disconnect the load from the tank capacitor. The input switch is opened to disconnect the AC / DC voltage converter of the power supply circuit from the AC mains, while the output switch is closed to power the load by discharging the tank capacitor.

[0011] Preferred embodiments are defined in the appended claims. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 shows a conventional SMA actuator operating at low DC voltage. [Figure 2a] FIG. 1 illustrates one embodiment of an SMA actuator according to the present disclosure having a single input switch. [Figure 2b] FIG. 10 illustrates another embodiment of an SMA actuator according to the present disclosure having two input switches. [Figure 3] 4 is an exemplary time graph of the voltage on the tank capacitor during a charging phase. [Figure 4] 4 is an exemplary time graph of the voltage on the tank capacitor during a discharge phase and of the corresponding voltage pulse delivered to the powered load. DETAILED DESCRIPTION OF THE INVENTION

[0013] An exemplary embodiment of a power supply circuit adapted to power a load using short high voltage pulses and without directly connecting the load to an electrical line at a relatively high voltage when powered is shown in Figures 2a and 2b.

[0014] The power supply circuit comprises two mains current paths, labelled with the letters A and B, leading from the AC input terminals of the power supply circuit and to DC output terminals to which a powered load 1 and an output switch 2 are connected. Within electrical path B are AC-DC voltage converters 3, 5, 6, 8 connected between the AC input terminals and the DC output terminals, the AC-DC voltage converters 3, 5, 6, 8 being configured to receive an AC mains voltage at the AC input terminals for generating a corresponding rectified DC voltage at the DC output terminals, and the AC-DC voltage converters 3, 5, 6, 8 comprising a tank capacitor 3 operatively connected across the DC output terminals for holding the rectified DC voltage.

[0015] Generally, the AC-DC voltage converter may be of any type. According to an optional embodiment shown in Figures 2a and 2b, the AC-DC voltage converter may comprise a full-wave diode bridge rectifier 6 operatively connected between the AC input terminals and the DC output terminals to generate a rectified DC voltage as a rectified replica of the AC mains voltage, which is made available to the powered load 1. According to one embodiment, the tank capacitor 3 has a capacitance value comprised between 10 µF and 470 µF.

[0016] 2a and 2b, the AC-DC voltage converter may also include an input capacitor 8 operatively connected between the full-wave diode bridge rectifier 6 and the AC input terminals to limit the AC current absorbed by the full-wave diode bridge rectifier 6. According to one embodiment, the input capacitor 8 may have a capacitance value comprised between 0.1 μF and 47 μF.

[0017] The auxiliary resistor 5 is electrically connected in parallel with the tank capacitor 3 to discharge the tank capacitor 3 when the load cannot be powered because the load is faulty or its electrical connections are damaged. The auxiliary resistor 5 may have a resistance value suitable to not significantly discharge the tank capacitor 3 when the electrical connections to the load 1 and the load 1 itself are functional, but to safely discharge the tank capacitor 3 in the event of a fault in the load 1. According to one embodiment, the auxiliary resistor 5 has a resistance value comprised between 150 and 500 KΩ.

[0018] In the other current path A, from which electrical paths A and B leave, there is a switching relay 7 having an excitation coil connected between the AC input terminals and configured to be powered whenever an AC voltage is applied to the AC input terminals, and having at least one internal switch controlled by the excitation coil and configured to connect an intermediate terminal T of the power supply circuit to one of the two DC output terminals when the excitation coil is not powered.

[0019] The two current paths A and B are either both energized simultaneously or both disconnected by an input switch 9 configured to connect / disconnect at least one of the AC input terminals to the AC mains. When the input switch 9 is closed, the AC-DC converter is energized, so that the tank capacitor 3 is charged with the rectified DC voltage, and at the same time, the excitation coil of the switching relay 7 pulls the switch of the relay 7 to keep the intermediate terminal T disconnected. When the input switch 9 is open, the AC-DC converter is not energized, so that the tank capacitor 3 is disconnected from the mains terminal 10 and the switch of the relay 7 is released.

[0020] The power supply circuit also includes at least one drive unit 4 connected directly between one of the DC output terminals and a respective intermediate terminal T of the power supply circuit. The drive unit 4 is configured to generate a control signal that is logically active when at least one respective intermediate terminal T is not disconnected, and this control signal is provided to a control terminal of a respective at least one output switch 2 that is configured to be turned on when the control signal is logically active. In practice, when the input switch 9 is open, the AC-DC converter is disconnected from the AC mains terminal 10, and simultaneously the switch of the relay 7 connects the intermediate terminal T in a current path between the two DC output terminals of the power supply circuit. In this configuration, the drive unit 4 generates a logically active control signal, and the output switch 2 connects the respective load 1 to be powered through the DC output terminal, allowing the charge stored in the tank capacitor 3 to flow across the load 1.

[0021] When the input switch 9 is closed, the tank capacitor 3 is charged by the AC mains voltage, for example with a time profile as shown by way of example in Fig. 3. During this charging phase, the load 1 is disconnected from the tank capacitor 3 and therefore from the AC mains terminal 10. When the input switch 9 is open, the tank capacitor 3 is disconnected by the AC mains terminal 10 and the output switch 2 connects the load 1 to the DC output terminals, and the load 1 is therefore supplied with a voltage pulse, as shown, purely by way of example, in the time diagram of Fig. 4. Thus, using the power supply circuit defined in claim 1, the load 1 is supplied with a discharge pulse starting from a relatively high voltage, but the load 1 is not directly connected to the AC mains terminal 10 at all, and it is therefore easy to meet safety requirements for an electrical load 1 that cannot be powered by a direct connection to a relatively high voltage line.

[0022] According to one embodiment, the AC mains terminals 10 are at a voltage provided between 220 and 240 VAC.

[0023] In general, the drive units 4 may be any circuit block capable of generating a logically active control signal when the respective intermediate terminal T is not disconnected. According to an optional embodiment shown in Figures 2a and 2b, at least one drive unit 4 is a resistive voltage divider connected directly between the DC output terminal and the respective intermediate terminal T, and the control signal is a voltage made available at the intermediate terminal of the resistive voltage divider. By way of example only, the resistive voltage divider 4 comprises a first resistor 4a having a value comprised between 5 and 10 KΩ, sharing a connection node with the output switch 2, and a second resistor 4b having a value comprised between 400 and 450 KΩ, connected directly to the intermediate node T.

[0024] According to an optional embodiment, a drive unit 4, and by way of example a resistive voltage divider 4 shown in Figures 2a and 2b, is connected from one side to the negative DC output terminal of the tank capacitor 3 and from the other side to the intermediate terminal T.

[0025] The output switches 2 may be any controlled switch that is closed when the control signal is active and open when the control signal is logically inactive. By way of example only, at least one output switch 2 may be a thyristor operatively connected to be turned on when the control signal is logically active.

[0026] The power supply circuit of the present disclosure may be configured to power a plurality of loads 1 by providing a plurality of drive units 4 directly connected between the DC output terminal and a respective plurality of intermediate terminals T; by providing a plurality of output switches 2 configured to be turned on when a respective plurality of control signals are logically active and configured to connect a respective plurality of loads 1 to be powered by the DC output terminal; and by providing a switching relay 7 including a plurality of internal switches all controlled by the same excitation coil and configured to connect the respective plurality of intermediate terminals T to the DC output terminal when the excitation coil is not powered; all of the multiple drive units 4 are configured to generate a respective plurality of control signals that are all logically active when the respective intermediate terminals T are not disconnected.

[0027] The power supply circuit of the present disclosure may be used to realize an actuator for powering one or more loads 1, the loads 1 comprising at least one smart material chosen between a piezoelectric device and a shape memory alloy (SMA) element.

[0028] According to one embodiment, the load 1 comprises at least one SMA wire, which may be made from a Ni-Ti alloy, for example of the type comprising one or more elements selected from Hf, Pt, Cu, Nb.

[0029] According to one embodiment, the SMA wire has a diameter comprised between 76 and 350 μm.

[0030] Using the power supply circuit of the present disclosure, it is possible to realize an actuator suitable for activating a piezoelectric or SMA element at a time interval comprised between 1 and 100 ms, preferably between 5 and 25 ms. [Explanation of symbols]

[0031] 1. Load 2 Low voltage power supply unit, output switch 3 Tank Capacitors 4 drive unit, resistive voltage divider 4a First Resistance 4b The Second Resistance 5 Auxiliary resistor 6 Full-wave diode bridge rectifier 7 Switching Relays 8 Input Capacitor 9 Input Switch 10 AC mains terminal 31 Drive unit

Claims

1. A power supply circuit, an AC-DC voltage converter having AC input terminals and DC output terminals, the AC-DC voltage converter configured to receive an AC mains voltage at the AC input terminals to generate a corresponding rectified DC voltage at the DC output terminals, the AC-DC voltage converter comprising: a tank capacitor operatively connected across the DC output terminals to hold the rectified DC voltage; an input capacitor operatively connected between the full-wave diode bridge rectifier and one of the AC input terminals to limit AC current absorbed by the full-wave diode bridge rectifier; and a resistor electrically connected in parallel with the tank capacitor to discharge the tank capacitor when the load cannot be powered because the load is faulty or its electrical connections are damaged; at least one input switch configured to connect / disconnect at least one of said AC input terminals to the AC mains; at least one drive unit directly connected between a first DC output terminal of the DC output terminals and at least one respective intermediate terminal of the power supply circuit, the at least one drive unit configured to generate a control signal that is logically active when the at least one respective intermediate terminal is not disconnected; at least one output switch configured to be turned on when the control signal is logically active and configured to connect a respective at least one load to be powered by the DC output terminals allowing charge stored in the tank capacitor to flow across the load; a switching relay having an excitation coil connected between the AC input terminals and configured to be powered whenever an AC voltage is applied to the AC input terminals, and having at least one internal switch controlled by the excitation coil and configured to connect the at least one respective intermediate terminal to a second DC output terminal of the DC output terminals when the excitation coil is not powered; Equipped with 10. A power supply circuit, wherein the at least one drive unit is a resistive voltage divider connected directly between the first DC output terminal and the at least one respective intermediate terminal, and the control signal is made available at an intermediate terminal of the resistive voltage divider.

2. 2. The power supply circuit of claim 1, wherein the full-wave diode bridge rectifier is operatively connected between the AC input terminals and the DC output terminals to generate the rectified DC voltage as a rectified replica of the AC mains voltage.

3. 2. The power supply circuit of claim 1, wherein the at least one output switch is a thyristor operatively connected to be turned on when the control signal is logically active.

4. a plurality of drive units directly connected between the first DC output terminal and a respective plurality of intermediate terminals, the drive units configured to generate a respective plurality of control signals that are all logically active when the respective plurality of intermediate terminals are not disconnected; a plurality of output switches configured to be turned on when the respective plurality of control signals are logically active and configured to connect a respective plurality of loads to be powered by the DC output terminals; the switching relay comprising a plurality of internal switches all controlled by the excitation coil and configured to connect the respective plurality of intermediate terminals to the second DC output terminal when the excitation coil is not powered; 2. The power supply circuit according to claim 1.

5. A power supply circuit according to claim 1; at least one load comprising at least one smart material selected between a piezoelectric device and a shape memory alloy element; An actuator comprising:

6. 1. A method of powering a load from an AC mains, comprising: installing the power supply circuit of claim 1; connecting a load to the output switch of the power supply circuit; connecting the AC / DC voltage converter of the power supply circuit to the AC mains by closing the input switch to charge the tank capacitor operatively connected across the DC output terminals of the power supply circuit, and simultaneously opening the output switch to disconnect the load from the tank capacitor; disconnecting the AC / DC voltage converter of the power supply circuit from the AC mains by opening the input switch and simultaneously closing the output switch to power the load by discharging the tank capacitor; A method for providing the above.

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