This invention comprises a method for closed-loop motion control of an ultrasonic motor according to claims1to10.

Ultrasonic motors (UM) with one actuator or several actuators made of a piezoelectric or electrostrictive or magnetostrictive material are replacing the classical drive elements like electromagnetic drives due to their specific advantages. In this context, UM can achieve much higher positioning accuracies in sub-nanometer range due to a missing backlash. Furthermore, UM can be designed and constructed with fewer parts than electromagnetic systems, such that UM are compacter. Besides, UM consume less energy, and their efficiency is not changing drastically with dimensions. With UM, a direct actuation without any requirement of mechanical system like gear assembly is possible. In addition to the aforementioned advantages of UM over electromagnetic drives, UM are highly dynamic drives.

In general, motion with UM is generated through friction interaction between a vibrator or actuator (stator) and a body or element, respectively, to be driven (rotor or slider). There is always a nonlinearity or a dead zone, respectively, which is a threshold of voltage level to be exceeded in order to create a first motion of the rotor or slider, i.e. a breakaway force or torque has to be overcome. Contact friction which is existent between the contacting surfaces of the stator and the rotor is a highly nonlinear phenomenon. Additionally, there is no direction symmetry with respect to said dead zone, and the position of the boundaries of the dead zone is unpredictable, which causes a nonlinearity and hysteresis with respect to voltage-speed characteristic. The difference between static and dynamic friction properties of mating surfaces even worsen the aforementioned nonlinearities.

The voltage for overcoming the breakaway force can be compensated with offset values in the linear control mechanism. There are various control concepts to get high precision trajectory following in precision stages. But still, UM have not been able to be driven precisely with small motion at velocities in the mixed friction region where friction values change from static to dynamic or vice versa. Precise smooth motion especially at low velocities is in particular demanded at biomedical devices, robotic arms, medical operations and microscopy stages. Noiseless actuation during the operation is a prerequisite for the above mentioned applications. This nonlinear characteristic, especially at low velocity drive, is always associated not only with discontinuous stick-slip movement with high positioning errors, but also unpleasant audible squeak and noise. Acoustic sound is especially magnified when frequency at low velocity matches to the natural frequency of the piezomotor systems with mass, spring and damper.

US 2006250047 A1 discloses an open-loop low speed driving method for a piezoelectric motor having two drive electrodes or terminals, respectively. Here, a first vibration mode is generated by exciting the two active terminals with two signals that are in phase, and a second vibration mode is generated by exciting the two active terminals with two signals that are in reverse phase. In order to obtain a slow movement, both terminals are excited at the same time with voltage signals being in phase and having same frequency, thereby changing the magnitude of the current going to each terminal.

For a piezoelectric element operating at or near a resonance frequency, the magnitude of the vibration generated on the piezoelectric element is proportional to the current going to the element. By altering current passing through each terminal, vibration amplitude of each side and thus direction of the oblique motion at the friction tip can be adjusted. The direction of the oblique motion is changing proportionally with motor speed. Depending on the demand, if a slow speed is needed, the current passing to one terminal is selected with a small difference compared to the other terminal, and the generated direction of the oblique motion is close to 90 degrees. If a fast speed is necessary, the angle of the oblique motion is selected to be around 45 degrees to the slider element, which means that the current passing to one terminal is selected with a large difference compared to the other terminal. However, having a vibratory motion that is almost perpendicular to the moving slider element decreases the motor performance such as push pull force or torque, and causes mechanical vibration resulting in unpleasant audible noise.

A method for closed-loop motion control of a piezoelectric motor with two drive electrodes being fed by periodic control voltages from two separate power sources is known from U.S. Pat. No. 6,747,391 B1 where two vibration modes are applied to a piezoelectric actuator in order to get higher performance and better controllability. Here, a single source square wave input is loaded to the piezoelectric actuator in order to catch two resonance modes from harmonics simultaneously. However, the accuracy of said piezoelectric motor especially at low velocities is poor.

Therefore, the task of this invention is to provide a method for closed-loop motion control of an ultrasonic motor with an improved positioning accuracy and insignificant noise generation at low velocities.

Said task is solved with the method according to claim1, with the following sub-claims describing at least appropriate embodiments of the invention.

The inventive method for closed-loop motion control with position feedback is intended for an ultrasonic motor comprising at least one actuator made of piezoelectric or electrostrictive or magnetostrictive material with a friction means or friction portion, with at least one excitation electrode and with at least one common electrode, and with the ultrasonic motor in addition comprising an element to be driven. The ultrasonic motor further comprises a controller and at least one electrical generator for generating at least a first excitation voltage U1and a second excitation voltage U2to be simultaneously applied to the electrodes of the actuator and thereby generating vibration of the actuator, wherein the friction means of the actuator, due to its vibration, intermittently comes into contact with the element to be driven and thereby generating a driving force on the element to be driven.

The inventive method for such an ultrasonic motor comprises the step of providing said at least two excitation voltages U1and U2generated by the at least one electrical generator with different frequencies, the frequency of U1corresponding to a first resonance frequency of the actuator and the frequency of U2corresponding to a second resonance frequency of the actuator, and with the frequency difference between U1and U2deviating from a servo sampling frequency of the controller by 5 kHz at the most, and applying said at least two excitation voltages U1and U2to the electrodes of the actuator simultaneously or at the same time, respectively.

Known closed-loop motion control of UM generally operates at a constant servo sampling frequency which is the number of servo cycles done in one second. After reading feedback from an encoder (i.e. position feedback), the controller reacts to the new position of the UM by recalculating and adjusting the drive signal or voltage according to the commanded position using control algorithms like Proportional Integration Derivative (PID) at the beginning of each servo cycle or servo sampling period, respectively. The UM with adjusted drive signal responds to this position change. The controller is ready to receive the next feedback, and the position control cycle repeats.

UM normally operate at resonance frequencies in the range of 40 kHz to 500 kHz, and typical motion controllers used for UM have servo sampling frequencies of 10 kHz to 20 kHz, resulting in 3 to 10 oscillations of the actuator and its friction means at the operating frequency in one single servo sampling period of the motion controller. For example, it is assumed that the first oscillation within one single servo sampling period exceeds the static friction force and makes the rotor or slider to be moved. Once the slider starts to move, the subsequent oscillations in the same servo sampling period generate further movement of the slider. Here, it is highly possible that the amount of motion at the end of the servo sampling period is (excessively) higher than the commanded position. The amount of motion is unpredictable due to the friction nonlinearities. This problem especially at low velocities even severely affects positioning accuracy of the slider.

According to the invention, the UM is operated by a method using at least two excitation voltages U1and U2generated by at least one electrical generator in order to excite at least two eigenfrequency-modes at two different resonance frequencies of the actuator. The exited two resonance frequencies of the actuator resulting in the vibrations of the actuator to be modulated, and a mixed oscillation of the actuator or the friction means attached to it, respectively, is obtained. The resulting movement of the friction means—which is responsible for the motion or movement of the element to be driven with which the friction means intermittently comes into contact—has two trigonometric components with changing amplitudes and frequencies. Therefore, direction of movement of the friction means varies with respect to time. The aforementioned particular oscillations suppress the nonlinearities of friction at the contact surfaces of the actuator or its friction means, respectively, and the element to be driven.

Since the frequency difference of the excitation voltages U1and U2is similar to the servo sampling frequency value, i.e. the frequency difference between U1and U2deviates from a servo sampling frequency of the controller by 5 kHz at the most, acceleration of the UM just over the breakaway friction level can be controlled within one closed-loop controlled sampling time (servo loop or servo cycle) completely.

As the inventive method utilizes at least two resonance modes at two different exciting frequencies which are similar to the sampling frequency of the servo loop or servo cycle, the motion of the UM especially in the low velocity region can be controlled in one single servo cycle. Movement of the actuator or the friction means, respectively, can be started and finished in one single servo cycle. While at least one of the resonance modes is responsible for high performance actuation, the at least one other mode compensates the friction nonlinearities. As a result, positioning accuracy of the UM is considerably enhanced at slow velocities. The difference between two eigenfrequencies which are dependent on the geometry, stiffness and the mass of piezoelectric plates is not in the audible range (i.e. higher than 15 kHz).

Due to the abovementioned fact that movements can be started and finished in one single servo cycle, controllability of the UM is simplified and enhanced. Since voltage fluctuation from one servo cycle to the subsequent one is reduced, less adaptive parameters are needed for a wide range of velocities.

The inventive method furthermore reduces the value of mean acceleration in one single servo cycle which is the main reason for friction induced vibration and noise with driving methods according to the prior art.

Not only simple trajectories, but also complex patterns of position and velocity profiles such as circles, arcs and ellipses can be followed with minimum tracking and contour errors by applying the inventive driving method. On the other hand, high precision scan patterns with dynamic step-settling characteristics with linear control algorithm like PID can be performed as well.

It can be advantageous if the at least two alternating excitation voltages U1and U2have a sinusoidal, rectangular or a triangular waveform, such that the resulting movement of the friction means has two sinusoidal trigonometric components with different amplitudes and frequencies. In case of a sinusoidal waveform, it can be advantageous if the at least two alternating excitation voltages U1and U2can be expressed by U1=A1sin (ω1t+ϕ1) and U2=A2sin (ω2t+ϕ2), with A1and A2being amplitudes, ω1and ω2being angular frequencies and ϕ1and ϕ2being phase angles of voltage signals.

It can also be advantageous if the frequency difference between the excitation voltages is in a range between 15 kHz and 25 kHz.

Furthermore, it can be advantageous if the at least one actuator is a rectangular piezoelectric plate. Here, it may be preferable that the actuator has at least two excitation electrodes on one of its main surfaces, and at least one common electrode on the other of its main surfaces, wherein the first excitation voltage U1is applied to at least one of the excitation electrodes and the second excitation voltage U2is applied to at least one of the other excitation electrodes, and with the common electrode being grounded.

However, it may also be preferable that the actuator has at least two excitation electrodes on one of its main surfaces, and at least one common electrode on the other of its main surfaces, wherein the first excitation voltage U1is applied to at least one of the excitation electrodes and the second excitation voltage U2is applied to at least one of the other excitation electrodes, and with the at least one common electrode being applied with a third excitation voltage U3with U3having a phase difference of 180° with respect to the first excitation voltage U1or to the second excitation voltage U2.

The preceding expression ‘main surface’ of the rectangular piezoelectric plate describes the surfaces of the plate having the largest dimensions, i.e. largest area.

It can be beneficial that the ultrasonic motor comprises at least two actuators where the at least two actuators are combined by at least one coupling element. Here, it may prove useful that the first excitation voltage U1is applied to one of the two actuators, and with the second excitation voltage U2being applied to the other actuator. It may also prove useful that the at least two actuators are combined or connected by two coupling elements, where one of the coupling elements is built by the friction means.

It can be beneficial that the voltage of the second excitation signal is lower than the voltage of the first excitation signal. It is observed that the voltage value of second excitation signal is inversely dependent on motor velocity. Therefore, voltage level of second electrode can be adjusted to motor velocity.

