The present invention relates to a control device as well as to a control method for a piezoelectric inertia motor.
In piezoelectric motors with an inertia drive, the tangential component of reciprocating motion generates a motion at a contact between a slider and a stator. In one direction of tangential motion, the stator element is slowly activated. During this activation period, the “stick phase” or “slow phase,” the inertial force acting upon the slider is smaller than the frictional force: the slider sticks to the contact surface of the stator and moves along with it. In the opposite direction of the tangential motion, the stator is deactivated faster relative to its initial position. During this time, the “slip phase” or “fast phase”, the inertial force acting upon the slider is greater than the frictional force so that the slider slides on the stator and lags behind the contact surface of the stator element. At the end of a cycle, or within a cycle of the stick and slip phase, the slider takes a microscopic step. The accumulation of these microscopic steps creates the macroscopic motion.
In an inertia motor with two actuators, two piezoelectric actuators or single-crystal multilayer actuators or bulk actuators in a stator element are driven by two inversely phased (“mirrored”) sawtooth-like signals. In such a structure, the expansion and contraction take place synchronously in opposite directions. As one actor expands, the other actor must contract (or shrink).
A signal applied to the piezoelectric element within a motor typically has a sawtooth shape. A typical idealized sawtooth waveform of a signal for an inertia motor is shown in
Correspondingly, during the fast phase or slip phase, the one piezoelectric actuator expands rapidly while the other contracts rapidly. Fast expansion or contraction is equivalent to fast charging or discharging of a capacitor. The capacitor there is the capacitance of a multi-layer actuator. In this disclosure, actuators are treated largely like capacitor elements used in filter components of drive circuits.
In
Sawtooth-shaped signal waveforms of the control signals for the actuators can have flattened sections between the slow and the fast phase or at the transition from the slow to the fast phase, respectively. This is shown in an idealized form in
Simple audio amplifiers cannot drive an inertia motor even if the frequency of the sawtooth signal is in the range of several hundred kHz to 20 kHz. The reason for this is that the fast phase of the sawtooth signal needs to be as short as possible in the 0.5 to 2 μs range, regardless of the operating frequency. When the operating frequency is at around 20 kHz, as is the case with standard audio amplifiers, it is not possible to actuate a fast phase of 0.5 μs. A drive should have at least 1 MHz bandwidth.
It is an object of the present invention to provide a method for an efficient drive of piezoelectric inertia motors. The efficiency presently refers to the electrical energy used for the drive and the possibility of miniaturizing drive circuits.
The object is satisfied according to the features of the invention described herein. Some advantageous embodiments are also described.
The invention is based on the idea of using an inductance and an actuator capacitance for actuating a piezoelectric actuator with a sawtooth-like, non-symmetrical voltage waveform. Resistive elements (resistors) are typically used with piezoelectric actuators. The use of inductances instead of resistive elements is made possible by the high-frequency operation of switching elements such as GaN (gallium nitride) transistors.
When inductive elements are used in series with a piezoelectric actuator, a voltage waveform typically has a sinusoidal shape. With the circuit topology proposed, non-symmetrical signal waveforms can be obtained by operating GaN transistors at very high frequencies, even when inductive elements are employed.
A particular approach of the present invention is the adaptation of the class D amplifier topology for the drive of piezoelectric inertia motors in which switching elements such as GaN transistors which can be operated in high frequencies with low on resistance are implemented.
The drive and control methods described in the present disclosure can be applied to piezoelectric inertia motors with two actuators as the drive source as well as with only a single actuator.
During the fast phase, a half-bridge high-frequency circuit of the H-bridge high-frequency circuit charges or discharges the capacitor of a piezoelectric actuator, or in the case of two piezoelectric actuators, two half-bridge (or H-bridge) high-frequency circuits charge and discharge the two capacitors of the actuators in parallel. The actuator capacitance and small inductances, which are operated in resonance, cause fast charging or discharging of the capacitor. The capacitors of the actuators charge according to a step response or discharge according to a natural response of an RLC (resistance-inductance-capacitance) series configuration. Slow charging and discharging takes place by applying high frequency pulse width modulation (PWM) signals, in the case of two actuators synchronous and in the direction opposite to the inputs of a single half-bridge.
Unlike purely resistive elements, the current passing through an inductance during charging or discharging times is stored therein as electrical energy instead of being completely dissipated as heat. This stored energy is used during the subsequent fast discharging or charging period. As a result, the current to be supplied by the source and therefore the dissipated energy in the inductance and in the actuator is reduced. Because GaN transistors can be operated as switching elements at high frequencies (1 to 40 MHz), small inductances suitable for the operation at high frequencies can be used with high efficiency. As a result, a heat sink can be eliminated, which enables the miniaturization of drive circuits.
A wireless control method for driving a piezoelectric inertia motor is furthermore provided. Electromagnetic energy is generated by driving a transmitting coil at a high frequency by switching elements such as GaN transistors. The electromagnetic energy is picked up by a receiving coil (or coils) and converted into current. This current is used to charge or discharge the capacitor(s) of the piezoelectric actuators (or the actuator) and thereby cause the expansion or contraction of the piezoelectric actuators.
According to a first aspect of the present invention, a control device for a piezoelectric inertia motor is provided, the control device comprising: a capacitive piezoelectric actuator, an inductance, a first switching element connecting the capacitive piezoelectric actuator via the inductance to a first potential, a second switching element connecting the capacitive piezoelectric actuator via the inductance to a second potential that differs from the first potential; and a control element which is suitable for repeatedly switching the first switching element and the second switching element with pulse width modulation in directions opposite to one another in a stick phase of the piezoelectric inertia motor, where, in the pulse width modulation, a time component of a first switching state of switching states ON and OFF increases relative to a time component of a second switching state and the pulse width modulation is filtered by the capacitive piezoelectric actuator and the inductance, and thereby carrying out a stepwise first charging operation of charging operations charging and discharging at the capacitive piezoelectric actuator, and reversing the time component of the first switching state and the time component of the second switching state at the beginning of a slip phase of the piezoelectric inertia motor, and thereby carrying out a second charging operation in the direction opposite to the first charging operation at the capacitive piezoelectric actuator.
According to a second aspect of the present invention, a control method for a piezoelectric inertia motor is provided, comprising, in a stick phase of the piezoelectric inertia motor, repeatedly switching in directions opposite to one another a first switching element connecting a capacitive piezoelectric actuator via an inductance to a first potential and a second switching element connecting the capacitive piezoelectric actuator via the inductance to a second potential, with pulse width modulation, where, in the pulse width modulation, a time component of a first switching state of switching states ON and OFF increases relative to a time component of a second switching state and the pulse width modulation is filtered through the capacitive piezoelectric actuator and the inductance, whereby a stepwise first charging operation of charging operations charging and discharging is carried out at the capacitive piezoelectric actuator, and reversing the time component of the first switching state and the time component of the second switching state at the beginning of a slip phase of the piezoelectric inertia motor, whereby a second charging operation in the direction opposite to the first charging operation is carried out at the capacitive piezoelectric actuator.
For example, a damped oscillating circuit containing the capacitive piezoelectric actuator and the inductance can exhibit an overshoot in the transition from the slip phase to the stick phase.
For example, the inductance represents a first inductance, the first switching element connects the capacitive piezoelectric actuator via the first inductance to the first potential, and the device comprises a second inductance, a third switching element connecting the capacitive piezoelectric actuator via the second inductance to the first potential, and a fourth switching element connecting the capacitive piezoelectric actuator via the second inductance to the second potential, where the control element is suitable in the slip phase for switching the third switching element during the first charging operation (of charging and discharging or charging in the direction and in a direction opposite to the direction of polarization of the capacitive piezoelectric actuator) equally to the first switching element and for switching the fourth switching element equally to the second switching element during the second charging operation.
The inductance can represent a first inductance, and the capacitive piezoelectric actuator which is connected via the first inductance to the first switching element and to the second switching element, and the control device can comprise: a third inductance, a fifth switching element connecting the capacitive piezoelectric actuator via the third inductance to the first potential, and a sixth switching element connecting the capacitive piezoelectric actuator via the third inductance to the second potential, where the control element is suitable for switching the fifth switching element equally to the second switching element and the sixth switching element equally to the first switching element.
For example, the inductance represents a receiving inductance, the control device contains a transmitting inductance, and the capacitive piezoelectric actuator is connected inductively via the receiving inductance and the transmitting inductance to the first switching element and the second switching element.
The control device can be configured to carry out the first charging operation and the second charging operation without contact via the transmitting inductance and the receiving inductance.
For example, the capacitive piezoelectric actuator represents a first capacitive piezoelectric actuator, the receiving inductance represents a first receiving inductance, and the control device contains a second receiving inductance and a second capacitive piezoelectric actuator which is connected inductively via the second receiving inductance and the transmitting inductance to the first switching element and the second switching element, and the first piezoelectric actuator and the second piezoelectric actuator are oriented in the opposite polarization direction.
The control device can comprise a transformer containing the transmitting inductance and the receiving inductance.
For example, the capacitive piezoelectric actuator represents a first capacitive piezoelectric actuator, and the control device comprises a second capacitive piezoelectric actuator connected in parallel or in series with the first capacitive piezoelectric actuator in the opposite polarization direction.
For example, the capacitive piezoelectric actuator represents a first capacitive piezoelectric actuator, the inductance represents a first inductance, the control device comprises a second capacitive piezoelectric actuator which is connected by a seventh switching element via the fourth inductance to the first potential and via an eighth switching element to the second potential, and the control element is suitable for switching the seventh switching element in the direction opposite to the first switching element and for switching the eighth switching element in the direction opposite to the second switching element.
For example, a frequency of the pulse width modulation is at least 1 MHz.
The frequency of the pulse width modulation can be higher by a factor of at least 30 than a charging frequency of the capacitive piezoelectric actuator.
For example, the control device comprises gallium nitride transistors as switching elements (the first to eighth switching elements).
The first and the second charging operation can comprise charging operations charging and discharging or charging in the polarization direction of the capacitive piezoelectric actuator and charging in the direction opposite to the polarization direction of the capacitive piezoelectric actuator.
Further details, advantages, and features of the invention shall arise from the following specification and the drawings to which reference is expressly made with regard to all details not described in the text, where:
A circuit topology according to an exemplary embodiment is shown in
A capacitive piezoelectric actuator 621 is connected via an inductance 631 by a first switching element 611 to a first potential 641 and by a second switching element 612 to a second potential 642.
By way of example, second potential 642 is referred to as ground (GRN). In general, however, it is sufficient for potentials 641 and 642 to differ from one another.
As shown in
Of the elements shown in
In the configuration shown in
In the slow phase or stick phase, charging or discharging is effected by an inductance, for example, inductance 631. For this purpose, switching elements 611 and 612 are actuated with a suitable pulse width modulation signal which causes switching elements 611 and 612 to open and close synchronously so that slow charging or discharging can take place. Switching elements 611 and 612 are repeatedly switched in directions opposite to one another, as shown schematically in
As can also be seen in
Averaged over the slow phase, an average time component of the first switching state can be greater than an average time component of the second switching state in order to reach the charge state to be obtained by the charging operation in the slow phase. On average, charging then outweighs discharging when the actuator is being charged in the slow phase, and discharging outweighs charging when the actuator is being discharged.
As can also be seen in
GaN transistors can be used as switching elements 611-614, 611′-614′. As can be understood from
In
Switching elements 613 and 614 can additionally be activated or switched during the fast discharging in the fast phase. In order to rapidly discharge capacitive actuator 621, switching elements 612 and 614 can be set together in the ON position for a short period of time (e.g. 0.1 to 6 μs). This is shown in
As described, the damped oscillating circuit, which comprises capacitive piezoelectric actuator 621 and inductance 631 and any equivalent resistance R and, in the case of a double half bridge, also inductance 632, exhibits overshooting in the transition from the slip phase to the stick phase. While the inertia motor then transitions from the slip phase to the next stick phase at the end of a cycle, in this transition from sliding to stick phase, the control signal which controls the charging and discharging of capacitive actuator 621 or actuators 621, 622 can have a transition phase which comprises the overshoot and damped oscillation.
Charging and discharging a second capacitive actuator 622, if present, occurs in an identical manner, while the signals are mirrored.
As can also be seen in
Furthermore, unlike what is shown in
Similarly, the switching elements are switched in the topology shown in
Switching elements 613 and 614 can be activated or switched, respectively, in parallel with switching elements 611 and 612 for fast charging of capacitive actuator 621 in the fast phase or slip phase to build up a voltage in the range of +Vin. In order to fast charge capacitive actuator 621, switching elements 611 and 613 are set to the ON state for a short period of time (0.1 to 6 μs). This is shown in
Even if the duration of the ON switching state for switching elements 611 and 613 is very short in the fast phase, charging the capacitive actuator behaves according to a step response or impulse response of an RLC circuit (taking into account an equivalent resistance R) for the reason that equivalent inductance L is also reduced due to the parallel connection of inductances 631 and 632 (L1//L2). The operating condition presently described with reference to
In synchronism with this, capacitive piezoelectric actuator 622 is actuated such that slow charging/expansion and fast discharging/contraction takes place in the former. In particular, the switching conditions are such that switching element 611 is actuated identically to switching element 612′, switching element 612 identically to switching element 611′, switching element 613 identically to switching element 614′, and switching element 614 identically to switching element 613′. Idealized voltage waveforms corresponding to the switching conditions described with reference to
For a further exemplary embodiment, a circuit topology is shown schematically in
As shown in
In addition, as shown in
If, in addition to actuator 621, a second actuator 622 is present for driving the piezoelectric motor, then, as shown in
In this configuration, instead of charging or discharging a capacitor, one can also speak of charging an actuator with a positive or negative potential (or bringing about a positive or negative voltage at the actuator). Charging a capacitive piezoelectric actuator can generate an electric field in the actuator. If, after charging, the electric field is oriented in the same direction as the polarization direction of the piezoelectric actuator, then the capacitance of the actuator (or the capacitor that the actuator acts as in the circuit) can be said to be “positively charged”. If, after charging, the electric field is charged in the direction opposite to the polarization direction of the piezoelectric actuator, then the capacitance of the actuator can be said to be negatively charged. While a positively charged or positively charging actuator expands, a negatively charged or negatively charging actuator contracts.
Capacitance or capacitor 621 can be charged slowly to a positive potential by simultaneously switching switching elements 1111 and 1112 via inductances 1131 and 1131′ with PWM signals, where switching elements 1111′ and 1112 during the slow phase, as previously described for switching elements 511 and 612 with reference to
In order to produce a slow contraction at actuator 621 in the stick phase and rapid extension or expansion in the slip phase, actuator 621 is slowly charged to a negative potential, as shown in
Similarly, a waveform of the charge signal that is mirrored with respect to actuator 621 is generated at second actuator 622. The capacitance of actuator 622 can be slowly charged to a positive potential with a PWM signal by switching elements 1211 and 1212′ and inductances 1231 and 1231. Once the potential at actuator 622 has reached a certain value, it is charged to a negative potential by a narrow pulse of switching elements 1211, 1214, 1211′ and 1213. Such actuation produces a slow expansion and fast contraction of actuator 622.
According to an embodiment shown in
In the case of two actuators 621 and 622 in the configuration shown in
Fast charging or discharging takes place at the end of each cycle of the PWM signal. For the fast phase or slip phase, the current path of the actuating signals is shown in
Although the slow rise and drop of the voltage at the capacitive actuators is shown to be linear in an idealized or simplified manner in the schematic voltage profiles shown in
In the configuration shown in
Accordingly, switching element 611′ is switched on for a short period of time and switching element 612′ is switched off. The (fast) charging of actuator 622 behaves according to a step response of an RLC circuit. Due to the small inductance and capacitance values of inductance 631′ or actuator 622, respectively, the actuator reaches an overshoot value within approx. 1 to 2 μs. After a small, heavily damped oscillation, the subsequent (slow) discharge period takes place.
The current path for the fast phase of the drive signals according to a simple half-bridge topology is also illustrated in
In a state where actuator 621 is connected via switching element 612 to capacitance Ca1, inductance 631 and resistor R to second potential 642, capacitive actuator 621 discharges. At the same time, capacitive actuator 622 is connected via resistor R as well as inductance 631′ and switching element 611′ to the first potential of the source voltage +Vin and charges under this condition.
As already mentioned, switching elements 611 and 612′ are ON or OFF at the same time. Switching elements 612 and 611′ are likewise switched ON or OFF at the same time. Resistance R in
A further exemplary embodiment is shown in
As in the case with the topology described with reference to
A further embodiment with a full bridge arrangement is shown in
In some further embodiments, capacitive piezoelectric actuator 621 or two actuators 621 and 622, respectively, is/are connected inductively via a receiving inductance and a transmitting inductance to two potentials 641 and 642. The transmitting inductance is connected via switching elements 1111 and 1111′ or 1112 and 1112′, respectively, to two potentials 641 and 642, and the receiving inductance is connected to at least one of two actuators 621 and 622. The transmitting inductance transmits electrical energy to the receiving inductance.
In one embodiment, the receiving inductance and the transmitting inductance are included in a transformer as an input coil and an output coil, respectively. A transformer element can increase or decrease the magnitude of the control signal for the actuators. As shown in Figure a transformer 2031 at the output portion of an H-bridge circuit topology is connected via switching elements 1111 and 1111′, and 1112 and 1112′, respectively, to first potential 641 and second potential 642, respectively. Although the output signal of a transformer is generally sinusoidal, sawtooth-like signals such as signals that approximate a sawtooth or flattened sawtooth can be generated at the piezoelectric actuators by switching at very high frequencies, as can be done, for example, by using GaN transistors as switching elements. The output coil or receiving coil of transformer 2031 also functions as an inductance that is connected to the capacitances of the piezoelectric actuators. As shown in
As shown in
Receiving coils 2131 and 2132 can absorb the wirelessly or contactlessly transmitted energy and convert it into a current that flows through them with a high signal frequency. Since the inductance of the receiving coil and the capacitance of the actuator each function as an RLC circuit, the voltage waveform or a voltage drop, respectively, at the actuator capacitance corresponds to a sawtooth-like signal.
In the embodiments illustrated in
The receiving coil absorbs the energy from the transmitting coil and supplies it to actuators 621 and 622. Actuators 621 and 622 can either be connected in series, as shown in
The present invention provides a control device for a piezoelectric inertia motor. In addition to one or more capacitive piezoelectric actuators and inductances and the switching elements that are interconnected according to the topologies described in this disclosure, this control device also comprises a control element that is suitable for controlling the switching elements of the control device in the stick phase and in the slip phase in order to generate at the actuator or actuators, respectively, the voltage signal waveforms which cause the charging operations in opposite directions in the stick phase and in the slip phase of the piezoelectric inertia motor and thus the expansion and contraction. This control element can be included in the control device, for example, in the form of an integrated circuit which generates the PWM signals as digital signals and/or a computer interface which receives the digital signals.
The measured voltage and current waveforms in the opposite direction are shown for multilayer actuator 622 with actuator capacitance Ca2 in
In
In order to generate the modified sawtooth waveforms, PWM signals are first generated in digital form with a high frequency (e.g. 0.5 to 5 MHz). The PWM signals are amplified by GaN transistor switching elements and amplified by the RLC circuit to obtain the final shape of modified sawtooth-like waveforms as voltage profiles for actuating and driving the piezoelectric actuators.
While the waveforms of the voltage profiles at the actuators in
In
As shown in
While one distinguishes between a stick phase (slow phase) and a slip phase (fast phase) when driving a piezoelectric inertia motor, the voltage signal at the charging and discharging actuators additionally has a transition phase or transition period at the transition from the slip phase to the stick phase, as already mentioned. This transition phase is characterized by the damped oscillation described that arises in the context of the step response or natural response of the RLC oscillating circuit, as shown in
The transition period from the slip phase to the stick phase is also shown in
The frequency (f2) of the pulse width modulation is advantageously at least 1 MHz. In addition, it is advantageously higher by a factor of 30 than a charging frequency of the capacitive piezoelectric actuator, i.e. the frequency of the voltage signal that corresponds to period T1 of the voltage signal. The effect of the PWM frequency on the slow phase (stick phase) of the sawtooth-like signal at the actuators is shown in
In all of the examples of the actuator voltage and PWM signal profiles shown in
As can be seen, the frequency or period T2 of the PWM signal controls the profile of the slow phase (stick phase) of the sawtooth-like voltage signal at the actuators. High-frequency operation or the high-frequency properties of the switching elements that generate or amplify the PWM signal (e.g. GaN transistors) play a role in generating an advantageous voltage profile at the capacitive actuators. If the PWM signal frequency f2 is not sufficiently high, e.g., lower than 1 MHz, then the waveforms in the profile of the slow phase (slow charging or discharging) will be disturbed as shown in
The natural response of an RLC oscillating circuit is calculated below. For this we assume that at t=0, the current flowing through inductance L, is equal to 0 and the voltage at the capacitor of the capacitance C is equal to V0. Then the equation
is fulfilled. Derivation results in
and the characteristic equation is
For the reason that experimentally observed voltage and current waveforms of the RLC circuits of capacitive piezoelectric actuator and inductance(s) exhibit a damped oscillation (ringing), it can be assumed that the system is a second-order system and is underdamped. This means that the characteristic equation has two complex conjugate roots s1,2:
s
1,2=−α±α2+ω02 (4)
where
is the damping factor,
is the resonant angular frequency and
ωd=ω02−α2 (7)
is the natural angular frequency or damping angular frequency. The parameters can be calculated from the initial conditions and the components of the circuit.
The half-bridge topology shown in
With equation (6), the resonance frequency f0 can be calculated as
The damping angular frequency (or natural angular frequency) co d or damping frequency/natural frequency fd can be experimentally read from the voltage or current waveform of
ωd=2π*fd=2π*166 kHz (9)
The symbol “*” presently denotes a scalar multiplication. With the measurement values presently illustrated, the current waveform should satisfy the following equation:
i(t)=B2*e−αt*Sin(2π*fd*t) (10)
From the measurement points on the current waveform marked in
R=α*2*L (11)
results in R=2.954Ω, which can be rounded to 3Ω.
With the parameters obtained by the above derivation, the current waveform satisfies the following equation:
i(t)_model=5*e−211000t*Sin(2*π*166000*t) (12)
As can be seen in the plots shown in
Measured voltage and current waveforms with associated configurations of RLC oscillating circuits with multilayer actuators with capacitances Ca1 and Ca2 during the fast phase (slip phase) are shown in
In summary, the present invention relates to a control device and a control method for a piezoelectric inertia motor. In the stick phase, a first switching element and a second switching element are switched in directions opposite to one another by pulse width modulation, where a time component of a first switching state of on and off increases relative to a time component of a second switching state of on and off the pulse width modulation is filtered by the capacitive piezoelectric actuator and an inductance, and a first charging operation is carried out, and the time components of the first switching state and the second switching state are reversed at the beginning of a slip phase, and, as a result, a second charging operation opposite to the first charging operation is carried out at the capacitive piezoelectric actuator. By storing electromagnetic energy in the inductance, the configuration provided allows for the reduction of energy dissipation as heat and can contribute to an energy-efficient drive for inertial motors.
Number | Date | Country | Kind |
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10 2020 132 640.8 | Dec 2020 | DE | national |
This application is a U.S. National Phase Application under 35 U.S.C. 371 of International Application No. PCT/EP2021/083293, filed on Nov. 29, 2021, which claims priority to German Patent Application No. 10 2020 132 640.8, filed on Dec. 8, 2020. The entire disclosures of the above applications are expressly incorporated by reference herein.
Filing Document | Filing Date | Country | Kind |
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PCT/EP2021/083293 | 11/29/2021 | WO |