WO2021239317A1

WO2021239317A1 - Drive device, robot arm and method for torque measurement

Info

Publication number: WO2021239317A1
Application number: PCT/EP2021/059632
Authority: WO
Inventors: Dominik Bergmann; Matthias Gerlich; Siraj Issani; Takeru NEMOTO; Simit Pradhan
Original assignee: Siemens Aktiengesellschaft
Priority date: 2020-05-27
Filing date: 2021-04-14
Publication date: 2021-12-02

Abstract

A drive device (1) is provided, comprising - a drive motor (5) with a drive shaft (7), which is rotatable with respect to a central axis (A), - a base mount (20), - a transmission unit (50) with a fixed element (51), an input element (52) and an output element (53), wherein the input element (52) of the transmission unit (50) is rotationally coupled with the drive shaft (7) of the drive motor (5) - and at least one solid-state actuator (60) with a first end (60a) that is fixedly connected to the base mount (20) and a second end (60b) that is fixedly connected to the fixed element (51) of the transmission unit (50). Furthermore, a robot arm (80) with such a drive device (1) is provided and a method for measuring a torque within such a drive device (1).

Description

Drive device, robot arm and method for torque measurement

The present invention relates to a drive device, in particu lar a drive device comprising a drive motor, a base mount and a transmission unit that is rotationally coupled to the drive motor. The invention further relates to a robot arm with such a drive device and a method for measuring an internal torque within such a drive device.

From the prior art, drive devices for robotic joints are known, in which an electrical drive motor drives the rotation of a drive shaft. This drive shaft is in turn coupled to an input element of a transmission unit. In addition to the in put element, the transmission unit usually has a fixed ele ment and an output element, where the fixed element is fixed to a base mount of the joint and the output element is used to transfer a rotational output motion of the overall drive device. The transmission unit can for example be a strain wave gearing or an epicyclic gearing. In common robotic joints, the transmission ratio is selected such that the ro tation speed of the output element is lower than the rotation speed of the input element and therefore also the drive shaft of the drive motor. This reduction of rotation speed is often significant, for example the factor can be around 100.

The amplitude of the rotational motion at the output there fore depends on the amplitude of the motion of the drive mo tor and the transmission factor. On the other hand, the fre quency response at the output characterizes the dynamics of the device. The maximum response frequency is the highest frequency of changes in the angular position that can be transferred from an initial stimulus to the output motion.

For typical robotic drive devices, this maximum response fre quency is limited to about 1 kHz. Several reasons exist for this limitation. One reason is that an electric drive motor has a relatively large electrical time constant, mostly due to the inductance of the motor. This hinders a sudden change in the motor current and therefore also a sudden change in the rotational speed of the drive shaft and, correspondingly, the output element. A second reason for the frequency limita tion is that the mechanical transmission unit often has a relatively low stiffness. Many mechanical transmission units are based on interlocking teeth. For robotic drives, these interlocking teeth are often relatively small and relatively soft compared to other parts of the drive train. This allows a deformation of the teeth, in other words a deformation of one or more elements of the transmission unit in the tangen tial direction and therefore a time lag in the transfer of a high-frequency change at the input. The combination of these effects leads to a limitation in the response frequency of the drive device and therefore limits the frequency of a change that can be generated in the torque and in the rota tional motion at the output.

It is therefore a goal of the present invention to provide a drive device, which overcomes the above-mentioned problems In particular, a drive device shall be provided, which allows the generation of a predetermined high-frequency change of the motion at the output. A further goal of the invention is to provide a robot arm with such an improved high-frequency response for the rotational motion of the respective robotic joint (s). Still a further goal of the invention is to provide a method for measuring an internal torque of such a drive de vice.

These objects of the invention are achieved by claim 1 with respect to the drive device, by claim 14 with respect to the robot arm and by claim 15 with respect to the method.

The drive device according to the invention comprises a drive motor with a drive shaft, which is rotatable with respect to a central axis A. It further comprises a base mount and a transmission unit with a fixed element, an input element and an output element. The input element of the transmission unit is rotationally coupled with the drive shaft of the drive mo tor. The drive device further comprises at least one solid state actuator with a first end and a second end, where the first end is fixedly connected to the base mount and the sec ond end is fixedly connected to the fixed element of the transmission unit.

In the context of this disclosure, the terms "axial", "radi al" and "tangential" are generally defined with respect to the central axis A. The drive motor can more particularly be an electrical motor. This electrical motor can comprise a ro tor and a stator.

By "rotationally coupled" it is meant that the input element of the transmission unit and the drive shaft of the drive mo tor are connected in a torque transmitting fashion, in par ticular via a fixed connection. The base mount serves to me chanically couple the drive device with an outer mechanical ground (also called mechanical reference). This mechanical ground serves as a reference point for the relevant drive ap plication. It does not need to be an absolute mechanical ground, but instead it can be a local mechanical reference point. For example, the reference can be a base of a robot arm or a local base of an individual robotic joint. It can also be a base of another industrial drive, the chassis of a vehicle or any other mechanical reference point. In any case, the base mount serves as an inner mechanical reference point of the drive device, relative to which an output element is rotated. In this way, it serves for supporting all torques that are generated by the drive device.

The "fixed element" of the transmission unit is not entirely fixed with respect to the mechanical reference point of the base mount. It is called "fixed element" here because it moves much less than the (normally fast) rotating input ele ment and also much less than the (normally slowly) rotating output element of the transmission unit. And it corresponds to the (really) fixed element in a conventional drive device. However, it has a small range of motion with respect to the base mount which is enabled by the range of motion of the at least one solid-state actuator. In other words, there are one or more such actuators, and the base mount and the fixed ele ment are mechanically connected via these actuators (or actu ator).

In general, every transmission unit can be driven with two inputs and one output, in which case it is usually referred to as a differential transmission unit. In this (generally known) configuration, the ports are often called "main in put", "main output" and "control input". In this sense, the drive device according to the present invention comprises a differential transmission unit, where the so-called "fixed element" is actuated by the solid-state actuator(s) and therefore serves as the "control input".

In case there are several solid-state actuators, they can be mechanically connected in parallel with each other, but as a group they are connected in series with the base mount and the fixed element, being arranged between these two elements. In order to allow the relative rotational motion between the fixed element and the base mount, a rotational bearing is fa vorably provided between these two elements.

An important advantage of the drive device according to the present invention is that the additional solid-state actuator allows the introduction of an additional component of the mo tion at the output element. The rotational motion of the out put element which is for the most part initiated by the drive motor can therefore be "supplemented" with a second motion component. The first motion component of the output element, which is due to the drive motor, can advantageously be a rel atively large-amplitude, but low-frequency motion component. On the other hand, the second motion component of the output element, which is due to the additional solid-state actuator, can advantageously be a relatively smaller-amplitude, but higher-frequency motion component than the first motion com- ponent. In this fashion, a higher-frequency motion can be added to a lower-frequency baseline motion that is effected by the drive motor. Therefore, the overall high-frequency re sponse of the motion of the output element can therefore be improved by the features of the invention. In particular, this can be achieved without changing the inductance of the motor and/or the stiffness of the transmission unit. Also, the motion range of the one or more solid-state actuators can be relatively small compared to the motion range of the out put rotation due to the drive motor, because for many appli cations the necessary high-frequency part can be a small cor rective motion that rides on top of a baseline motion with lower dynamics. For example, the solid-state actuator can be used to "impress" a high-frequency, but low-amplitude ripple on an otherwise slowly changing curve. For many applications, in particular in robotics or other industrial drive trains this favorably enhances the dynamics of the available output motion profile.

An additional aspect of the present invention is that the above-mentioned low stiffness of the transmission unit is usually responsible for the first (or lowest) eigenfrequency of the conventional drive systems. In this lowest eigenfre- quency, the highest amount of energy is stored when the sys tem oscillates. A further advantage of the invention is that these system oscillations can be reduced, for example by damping them quickly via the additional motion component of the solid-state actuator(s). In a similar way, the invention can serve to compensate oscillations from external forces.

The robot arm according to the present invention comprises one or more robotic joints, at least one of which comprises a drive device according to the present invention. The ad vantages of the robot arm correspond to the above-mentioned advantages of the drive device.

The method according to the present invention is a method for measuring an internal torque within the drive device. This torque is derived from an electrical signal generated by the at least one solid-state actuator. This method uses the ad vantage of the drive device that the additional solid-state actuator not only allows a higher-frequency output motion, but it also allows the measurement of the torque that acts between the base mount and the fixed element of the transmis sion unit. Such a torque can also be referred to as a "trans mission support torque". With the described drive device, this internal torque can be derived from an electrical signal from the solid-state actuator, as the mechanical strain of such an actuator typically also changes its electrical char acteristics. For example, the internal torque can be derived from the voltage across one or more piezo elements in this position. A particular advantage of this embodiment is that additional elements for the measurement of an internal torque become unnecessary. In particular, an additional torque meas uring wheel with deformable spokes is not necessary in this case. This allows the measurement of an internal torque in a highly integrated drive device with a simple mechanical set up.

Advantageous configurations and embodiments of the invention follow from the claims dependent on claim 1 as well as the following description. The advantageous features of the drive device, the robot arm and the method for measuring the torque can generally be combined.

According to a preferred embodiment, the at least one solid- state actuator is configured such that it acts as an addi tional direct drive for the output element. In other words, the transmission factor of the transmission unit does not ap ply to the additional motion caused by the at least one solid state actuator. Instead, its (or their) range of motion is transferred to the output element with a transmission factor of 1. In this way, a relatively small range of motion of the at least one solid-state actuator is not reduced by the transmission ratio, but it is applied in full at the output element. This is particularly favorable, when the maximum range of motion of the solid-state actor(s) is small, as is often the case with this type of actor.

According to a preferred embodiment, the transmission unit is a strain wave gearing. This type of gearing is also sometimes referred to as a harmonic gearing or harmonic drive gearing. In particular, the fixed element of the transmission unit can be the circular spline of the strain wave gearing. The input element can be the wave generator and the output element can be the flex spline of the strain wave gearing. Such a strain wave gearing particularly allows the realization of a drive device with a relatively large transmission ratio. This way, the rotational speed of the output element can for example be reduced by a factor of about 100 or so, compared to the input element. Alternatively, there are other possible and favora ble embodiments of the transmission unit, for example it can be configured as an epicyclic gearing, also referred to as a planetary gearing. Alternatively, a strain wave gearing with any different assignment of the main components as the "input element", "output element" and "fixed element" can generally be used.

According to a particularly favorable embodiment, the at least one solid-state actuator is electrically connected to a read-out unit that is configured to derive a value for an in ternal torque of the drive device from an electrical signal of the solid-state actuator. In a typical solid-state actua tor, e.g. a piezo actuator, an electrical signal is converted to a mechanical motion and vice versa. In other words, a me chanical strain on the actuator can be related to the magni tude of an electrical signal. For example, a piezoelectric actuator can always be used as a piezoelectric sensor as well, because the voltage across the piezo element depends on the mechanical strain on the element. Since the at least one solid-state actuator is positioned between the base mount of the drive device and the fixed element of the transmission unit, the strain across the actuator is representative of the torque that acts between these two elements. The internal torque measured at this position is the support torque of the transmission unit. Obtaining a measure of this support torque is desirable for closely monitoring the system state of the drive device. In particular, the system state of a robotic joint can be monitored very precisely by using this infor mation. This is particularly useful for a robotic joint that is used in a collaborative working environment (e.g. in a so- called Cobot), where a failure state or another dangerous state needs to be recognized immediately in order to trigger suitable safety measure for protecting the human co-workers.

In a particularly advantageous implementation of this embodi ment, the drive device is free of any additional torque meas uring unit. In particular, it is free of any torque measuring wheel with deformable cross beams or spokes. In other words, in such an embodiment, the one or more solid-state actuators together with their read-out unit form the only torque meas uring unit in the system. The space for an additional torque measuring wheel can therefore be saved, and the drive device can be made particularly compact.

According to a preferred embodiment, the at least one solid- state actuator has a maximum range of travel between 5 pm and 100 pm, more particularly between 20 pm and 60 pm. Such a relatively small range of travel can easily be reached with typical solid-state actuators. In particular, piezo actuators generally have a high force capability, but rather low strokes with are typically in the quoted range. This small motion is directly translated to a corresponding rotational motion at the output element. In particular, this translation into an angular displacement purely results from the geomet rical arrangement (i.e. the angle) of the one or more solid- state actors, and an additional transmission factor does not apply. In this way, a small additional rotational motion is caused at the output element. This additional motion is char acterized by the high-frequency response of the solid-state actuator, as described above. Even though the range of motion is much smaller than the range of motion typically allowed by the (primary) drive motor, this configuration can still greatly enhance the high-frequency response for small ampli tude motions at the output.

It is generally preferred that the at least one solid-state actuator has a linear range of motion. In the case of several solid-state actuators, each one of them has such a linear range of motion. The motion axis of each of these linear mo tions is preferably oriented with at least a directional com ponent that is (locally) tangential with respect to the cen tral axis. In an even more preferred embodiment, the motion axis is oriented with a predominant component in the (local) tangential direction. Particularly, the motion axis of each actuator can be essentially oriented along the (local) tan gential direction. The tangential component of the linear mo tion direction allows the translation of the linear motion of the actuator(s) into an additional rotational motion of the output element.

Therefore, according to a particularly preferred embodiment, the drive device is configured such that the linear motions of all existing solid-state actuators together result in a rotational motion of the fixed element of the transmission unit relative to the base mount. This additional rotational motion of the "fixed element" (which is not completely fixed, as explained above) relative to the base mount (which acts as a mechanical reference) directly translates to a correspond ing additional rotational motion of the output element, be cause the output element is rotated by the drive motor rela tive to the fixed element.

In a first favorable embodiment, there exists a plurality of solid-state actuators which are mechanically connected in parallel with each other. In other words, they are each in serted mechanically between the fixed element and the base mount, but between different nodes. In this first embodiment, the resulting total range of motion of the additional motion due to the solid-state actuators directly follows from the angular orientations of the actuators. In particular, the in dividual ranges of motion of the single actuators are not added up. The plurality of actuators only leads to an in crease of the stiffness and force of the additional motion component. This first alternative embodiment is therefore particularly preferred for robotics applications.

In an alternative second embodiment, there exists a plurality of solid-state actuators which are mechanically connected in series with each other. In other words, they form a chain of actuators, where the whole chain is inserted at the same node between the fixed element and the base mount. In this second embodiment, the resulting total range of motion of the addi tional motion due to the solid-state actuators is obtained as the sum of the individual contributions of each actuator. In other words, their individual ranges of motion are added up. This second embodiment is particularly favorable, if the range of motion of one solid-state actuator is too small for the desired high-frequency motion component. For example, a drive device with a stacked piezo actuator that is formed as a series arrangement of several single piezo elements can be regarded as an example of this second embodiment.

In a third embodiment, there exists a plurality of solid- state actuators, where some of them are mechanically connect ed in series with each other and then a plurality of those "series chains" are connected in parallel with one another.

In other words, this third alternative embodiment is a favor able combination of the first and second embodiments, which combines the advantages of both.

According to another generally favorable embodiment, the rel ative rotational motion that is added due to the one or more solid-state actuators has a maximum range of travel of 5 milliradian or less. More particularly this additional angu lar motion can be between 0.1 mrad and 5 mrad, even more par ticularly between 0.1 mrad and 2 mrad. Such an additional an gular motion can for example be achieved from a solid-state actuator positioned at a radius of a few 10 of microns (e.g. 50 pm) and a linear maximum range of motion in the above- mentioned range.

In a preferred embodiment, the at least one solid-state actu ator is a piezo actuator. Piezoelectric actuators are readily available and have many advantages for uses as motion actua tors, for example their high precision, high stiffness and high force. Their main drawback for many applications is a generally low range of motion. In the context of the present invention, this can be tolerated, because only a small- amplitude additional motion is required.

In a particularly preferred variant, the at least one piezo actuator is a stacked piezo actuator. A stacked piezo actua tor is a series arrangement of a stack of several individual piezo elements, e.g. an integrated stack in the form of a multi-layer system. Such a stacked piezo actuator is particu larly advantageous for achieving a higher range of motion than what is possible with an individual piezo element. The length of such a stacked piezo actuator in the dimension of the linear range of motion can for example be between 5 mm and 100 mm. Typically, the ratio between the length and the maximum range of motion in this direction is around 1000:1.

However, the invention is not limited to piezo actuators as solid-state actuators. In particular, the above-mentioned ad vantages also apply to other types of solid-state actuators. For example, the solid-state actuator can alternatively be a magnetostrictive actuator, an electostrictive actuator and/or a shape-memory actuator.

Generally, the drive device can be configured such that the at least one solid-state actuator can cause additional rota tional motions of the fixed element with both directions of rotation. In other words, the linear motions of the solid- state actuators can be actuated back and forth, so that a clockwise additional rotation component and a counter- clockwise additional rotation component both become possible. In order to achieve this motion in both directions, the sol id-state actuator(s) can for example be configured such that for each actuator both a contraction and an expansion can be achieved by the corresponding actuation signal.

In a particularly favorable configuration, the drive device comprises an even number of solid-state actuators, which are arranged in pairs. Particularly, each of these pairs can be arranged in a symmetrical butterfly configuration, i.e. front-to-front or back-to-back. In order to enhance the sym metry also on an electrical level, the pair can be configured such that the corresponding ends of the two actuators in each pair (in particular with the same polarity) face towards each other. In a particularly favorable configuration, each such pair of actuators can be fixedly connected to a common cen tral element. This common central element can form a connect ing element, either for a connection to the fixed element of the transmission unit or for a connection to the base mount. Preferably, the other one of these elements is then connected to the two opposite ends of the pair of actuators.

The main advantage of this paired configuration is that for a given rotational direction (either clockwise or counter clockwise), one actuator of the pair is contracted and the other one is expanded relative to its reference geometry. The reference geometry can favorably be a pre-compressed state of each solid-state actuator in the pair. If the rotation direc tion is to be reversed, only the assignment of contract ing/expanding actuators needs to be reversed. If a symmet rical pair configuration is used, a big advantage is that the range of motion, the force and the stiffness for both rota tional directions is always the same. For example, if the em ployed type of actuator can generate a higher expansion force than the corresponding contraction force, then there is ad vantageously always one actuator in the pair to generate the high expansion force, and vice versa. In other words, one ac tuator can always expand and another one can always contract for each desired motional direction. Therefore, a high force can always be generated, irrespective of the rotational di rection and irrespective of whether a pushing or a pulling can be more easily generated by the given type of actuator.

For example, with piezo actuators, an expansion motion can be generated relatively easily and with high force. It is there fore particularly preferred that the actuator pair is a sym metrical pair of two pre-compressed piezo actuators, where the active expansion of one actuator can result in a clock wise rotation and the active expansion of the other one actu ator can result in a counter-clockwise rotation. When one pi ezo actuator is actively expanded, the other more passive one is compressed even further as a result. In an extreme embodi ment of this, only the expanding actor of the pair can actu ate the motion and the other one can contract as a response to this in a completely passive way. However, in an alterna tive and usually more favorable embodiment, the expanding ac tuator is the main driver of the motion but the other con tracting one can help to support this contraction by a corre sponding electrical driving signal on this second actuator as well. In this case, the first actuator is still the main driver of the motion (as the expansion of a piezo actuator can be driven with a much higher force), but the second actu ator also takes part in driving it by supporting the contrac tion.

In a favorable configuration of this embodiment, these pairs of actuators can be evenly distributed over the circumference of the transmission unit. In other words, the actuator pairs can be distributed in a circular, ring-shaped, symmetrical arrangement. In particular, the pairs can be arranged mechan ically in parallel with each other. This way, the ranges of motions of the individual pairs are not added, but a high stiffness and high actuation force can be achieved.

The pairs of actuators that are distributed around the ring can be attached to their respective common central elements via their second ends. In this embodiment, the central ele ment conveys the mechanical connection of the actuator pair to the fixed element of the transmission unit. The resulting ring-shaped arrangement of actuator pairs and their common central elements can be offset from the fixed element of the transmission unit, for example in the axial direction. Alter natively or additionally, this ring-shaped arrangement can be offset from the fixed element in the radial direction. In any case, there are two ring-shaped structures (the fixed element and the arrangement of actuators), which are mechanically connected to each other in a series of connection points or nodes, which are also distributed over a circular ring.

The total number of solid-state actuators can generally be an even number, preferably an even number between 2 and 12. Two actuators is the minimum number with which a symmetrical pair can be built, as described above. With 12 actuators, espe cially when grouped into six such pairs, a particularly sym metrical distribution with high stiffness and high force can be obtained. The favorable range of even numbers between 2 and 12 therefore provides a reasonable compromise between symmetry, stiffness and complexity.

In a particularly favorable embodiment, the drive device is configured as a drive device for a robotic joint. The ad vantages of the present invention are particularly effective in such a robotic drive device. However, the advantages also apply for many other applications, for example other indus trial drives, drives in automated guided vehicles and in oth er electric vehicles. In all these applications, the in creased high-frequency response of the drive device can pro vide an advantage.

Further advantages and details of the invention may be found in the exemplary embodiments as described below and in the drawings, in which: Figure 1 shows a schematic longitudinal section of a conven tional drive device

Figure 2 shows a similar schematic representation of a drive device according to a first embodiment of the in vention,

Figure 3 shows a partial cross-sectional view of a drive de vice according to a second embodiment, Figure 4 shows a partial schematic longitudinal section of the drive device of Figure 3, Figure 5 shows an exemplary curve for the rotational motion of a drive device over time and

Figure 6 shows a schematic perspective view of a robot arm according to an embodiment of the invention.

In these figures, the same and corresponding elements share the same reference numerals.

Figure 1 shows a schematic longitudinal section of a conven tional drive device 1, cut along the central axis A. The drive device 1 can for example be a drive device for a robot ic joint. It comprises a drive motor 5, a base mount 20, a transmission unit 50 and an output shaft 40. It can be a highly integrated drive device, where the above-mentioned components and optional other components are integrated into a compact housing. The optional components can for example be a brake, a control unit and/or one or more sensor elements.

The drive motor can for example be an electromagnetic motor, comprising a rotor and a stator. The stator can be rigidly connected to the base mount 20, which functions as a mechani cal reference and is only schematically indicated by the usu al symbol for the mechanical ground. This symbol is used in several places, but these can for example all refer to one overall base mount which the other elements are attached to in different regions of the device. The rotor of the drive motor drives a drive shaft 7 which is rigidly coupled to an input element 52 of the transmission unit 50. This input ele ment 52 rotates relatively fast, and this rotation is then transmitted to a relatively slower motion of the output ele ment 53 of the transmission unit. This output element 53 can be rigidly coupled to an output shaft 40 or another output body. For the sake of clarity, the bearings and the interdig- itations by teeth between the individual elements 51, 52, and 53 of the transmission unit are not shown in the Figures.

The transmission unit 50 can for example be a strain wave gearing, where the input element 52 is the wave generator, the output element 53 is the flex spline 53 and the fixed el ement 51 is the circular spline. The fixed element 51 func tions to support the transmission unit against the base mount 20. In this conventional drive device, there is a rigid con nection between the fixed element 51 and the base mount 20. Therefore, the fixed element 51 is a true fixed mechanical reference for the transmission unit.

A drawback of this conventional drive device is that the high-frequency response of the output rotational motion is limited by the slow response of the motor motion to an abrupt change in a control signal and also by the low stiffness of the transmission unit. Therefore, the profile of the output motion has a limited high-frequency capability. The amplitude of the rotational output motion can be rather high, but high- frequency components such as ripples or sharp edges of a de sired motion profile are smoothed out in the actual response.

Figure 2 shows a similar schematic representation of a drive device 1 according to a first embodiment of the present in vention. This schematic is not a longitudinal section in the strict sense; therefore the central axis A is left out. How ever, the arrangement of the drive motor 5, the drive shaft 7, the transmission unit 50 and the output shaft 40 is analo gous to the drive device of Figure 1. This is also true for other optional elements that are not shown. The main differ ence is that the connection between the fixed element 51 of the transmission unit 50 and the base mount 20 is not a rigid connection. Instead, there is at least one solid-state actua- tor 60 in between these two elements, which allows a small relative motion. The fixed element 51 of the transmission unit can be a circular element and the base holder can also comprise a circular structure which surrounds the circular fixed element 51. In between these circular structures, there can be a plurality of such solid-state actuators 60, which can be arranged mechanically in parallel with each other so that they connect the elements 51 and 20 at different nodes. As an example, the schematic representation of Figure 2 shows two such solid-state actuators 60 between the elements 51 and 20. The first end 60a of the actuator is rigidly connected to the base mount 20, and the second end 60b of the actuator is rigidly connected to the "fixed element" 51. Even though the solid-state actuators 60 allow a small relative movement, the element 51 is still called the fixed element in the context of this invention, because it is the part of the transmission unit with the least possible motion and it corresponds to the element 51 of Figure 1, which is completely fixed at the base mount for the conventional drive device. It is worth noting however, that in spite of using this term, the "fixed ele ment" 51 is not entirely fixed for the embodiments of the in vention.

The one or more solid-state actuators 60 between the base holder 20 and the fixed element 51 allow a small-amplitude motion of this fixed element. Each actuator 60 allows a line ar motion, which can in turn lead to a relative rotational motion of the elements 51 and 20. This will become more ap parent from Figure 3 below. It is therefore worth noting that Figure 2 is not a real sectional view, as it does not proper ly show the orientation of the actuators 60 with respect to the central axis or the other components. It only indicates the way that the main mechanical elements are interconnected. As an exemplary representation, the bottom part of Figure 2 also shows a read-out unit 70, together with its electrical connections to the bottom actuator 60. Of course, each of the present actuators can be connected to such a read-out unit (either commonly or separately). In addition to this read-out unit 70, there can also be a control unit, or the functional ity of a control unit can be integrated with the depicted read-out unit. In any case, the control unit can be config ured to provide the actuator with an electrical signal in or der to cause a desired relative motion. On the other hand, the read-out unit can be configured to read out an electrical signal from the actuator, which is a function of the mechani cal strain on the actuator. In this fashion, a measure of an internal torque acting between the two elements 20 and 51, can easily be obtained from this signal.

Figure 3 shows a partial view of a drive device according to a second embodiment of the invention. Overall, the configura tion of the drive device can be analogous to the described configuration of the first embodiment in Figure 2. The par tial view of Figure 3 shows a cross section of a pair of sol id-state actuators 61 and 62, which are connected to a common central element 65. The cross section is perpendicular to the central axis A, but the circular spline 51 which lies behind the sectional plane is also shown by dotted lines. The first ends 61a and 62a of the two actuators are rigidly connected to the base mount 20, which is again only roughly represented by the mechanical ground symbol. The second ends 61b and 62b of the two actuators are connected to the common central ele ment 65. In this embodiment, each of the two actuators is a stacked piezo actuator with a length (or stack height) 1.

Each actuator allows a small linear range of motion which is denoted as dmax. The central element 65 of the actuator pair is rigidly connected to the circular spline 51 of the trans mission unit, which lies behind the sectional plane. The cen ter of this connection is denoted as the connection point p. The direction x of the linear motion of the two actuators is tangential with respect to the central axis A, when viewed in relation to the local connection point p. Due to this tangen tial orientation, a joint linear motion of the two actuators is translated to a relative rotational motion between the el ements 51 and 20, in other words to a change in the relative angular position a. The linear motion ranges dmax therefore translate to an angular motion range by the sine relation.

In order to produce a clockwise rotation of element 51 rela tive to base mount 20, the first actuator 61 needs to be ex panded and the second actuator 62 needs to be simultaneously contracted. Most advantageously, the corresponding motions of the two actuators can be effected simultaneously by the cor responding electrical signals. However, in principle, it would be sufficient if the rotational motion would be driven only by a contraction or only by an expansion, depending on which relative motion is easier to generate in the actual given type of solid-state actor. Correspondingly, a rotation in the counter-clockwise direction can be achieved by revers ing the directions of the linear motions of the two actuators in the pair.

Figure 3 only shows one actuator pair 61, 62 to visualize the working principle. In other embodiments, a plurality of such pairs can be distributed around the circumference of the drive device at several such connection points p between the fixed element 51 and the base mount 20. In particular, these actuator pairs can be arranged mechanically in parallel with each other, so that the force and stiffness from the individ ual actors is added up. In this fashion, an additional rota tional motion component with a particularly high-frequency response can be achieved with the plurality of solid-state actuators.

Figure 4 shows a partial schematic longitudinal section of the drive device of Figure 3. Only a few of the key elements are shown: The output element 53 (flex spline) and the fixed element 51 (circular spline) of the transmission unit. The corresponding input element is left out for simplicity. The fixed element 51 is surrounded by a bearing 55 to allow for the additional rotational motion of this "semi-fixed" element relative to the outer base mount that is not shown in the drawing. The fixed element 51 is rigidly connected to two ex- emplary central elements 65. They belong to two exemplary ac tuator pairs, which are not shown because they lie behind and in front of the cross sectional plane (tangentially offset from the top and bottom positions, as was explained in the context of Figure 3). Figure 4 mainly serves to show how the central elements 65 (together with the corresponding actua tors 61,62) are offset from the circular spline 51 in an axi al direction. However, in an alternative embodiment, the ac tuator pairs and their central elements could also be offset from the circular spline in a radial direction or in both, axial and radial, directions.

Figure 5 shows an exemplary curve for the rotational motion of the output element 53 of a drive device over time t. The rotational motion is represented by an angular position a.

In the left part of the figure, a large-amplitude but low- frequency motion profile 75 is shown. This slowly rising mo tion profile can be achieved with the drive devices of the current invention and also with the conventional drive de vice. On the other hand, the right part of the figure con tains a section with a fast, high-frequency ripple 76 and a section with a sharp edge 77. Both these high-frequency sig nals have a rather low amplitude and are overlaid on the slow and high-amplitude baseline. These high-frequency response profiles can be favorably "imprinted" on the low-frequency baseline by the additional motion from the solid-state actua tor(s). They could not be obtained with the conventional drive device.

Figure 6 shows a schematic perspective view of a robot arm 80 according to a further embodiment of the present invention. This robot arm 80 comprises seven robotic joints J1 to J7, where each joint allows a rotational motion with respect to a corresponding rotational axis A1 to A7. Therefore, it is a robot arm with seven rotational degrees of freedom. The "in nermost" joint J1 is connected to the arm base 81. The "outermost" joint J7 can carry a tool mount. Each of the in dividual joints has a local mechanical reference point for the local rotational motion, which is given by the base mount for each joint. For example, the base mount of joint J3 is labelled 20. In this robot arm 80, at least one of the robotic joints com prises a drive device 1 according to the present invention. Particularly advantageously a plurality of joints and possi bly even all the joints are configured in this fashion. For the robot arm of Figure 6, at least the joint J3 has a drive device according to the present invention, which therefore allows a high-frequency motion component for the rotation around axis A3, which is achieved by at least the one solid- state actuator.

Claims

Patent Claims

1. Drive device (1), comprising a drive motor (5) with a drive shaft (7), which is rotata ble with respect to a central axis (A), a base mount (20), a transmission unit (50) with a fixed element (51), an in put element (52) and an output element (53), wherein the input element (52) of the transmission unit (50) is rota- tionally coupled with the drive shaft (7) of the drive mo tor (5) and at least one solid-state actuator (60) with a first end (60a) that is fixedly connected to the base mount (20) and a second end (60b) that is fixedly connected to the fixed element (51) of the transmission unit (50).

2. Drive device (1) according to claim 1, wherein the trans mission unit (50) is a strain wave gearing.

3. Drive device (1) according to claim 2, where the strain wave gearing comprises a wave generator as the input element (52), a flex spline as the output element (53) and a circular spline as the fixed element (51).

4. Drive device (1) according to any one of claims 1 to 3, wherein the at least one solid-state actuator (60) is elec trically connected to a read-out unit (70) that is configured to derive a value for an internal torque of the drive device (1) from an electrical signal of the solid-state actuator (60).

5. Drive device (1) according to any one of claims 1 to 4, wherein the at least one solid-state actuator (60) has a max imum range of travel (dmax) between 5 pm to 100 pm.

6. Drive device (1) according to any one of claims 1 to 5, wherein the at least one solid-state actuator (60) has a lin ear range of motion, with a motion axis (x) that is oriented in a tangential direction with respect to the central axis (A).

7. Drive device (1) according to claim 6 where the linear mo tion of all existing solid-state actuators (60) results in a rotational motion of the fixed element (51) of the transmis sion unit (50) relative to the base mount (20).

8. Drive device (1) according to claim 7, where the relative rotational motion has a maximum angular range of travel be tween 0.1 and 2 milliradian.

9. Drive device (1) according to any one of claims 1 to 8, wherein the at least one solid-state actuator (60) is a piezo actuator.

10. Drive device (1) according to any one of claims 1 to 9, comprising an even number of solid-state actuators (61,62), which are arranged in pairs, each pair of actuators (61,62) being fixedly connected to a common central element (65).

11. Drive device (1) according to any one of claim 10, where the pairs of actuators (61,62) are evenly distributed over the circumference of the transmission unit (50).

12. Drive device (1) according to any one of claims 1 to 11, wherein the number of solid-state actuators (61,62) is an even number between 2 and 12.

13. Drive device (1) according to any one of claims 1 to 12, which is a drive device for a robotic joint (J1-J7).

14. Robot arm (80), comprising one or more robotic joints (J1-J7), at least one of the robotic joints (J3) comprising a drive device (1) according to any one of claims 1 to 13.

15. Method for measuring a torque within a drive device (1) according to any one of claims 1 to 13, where the torque is derived from an electrical signal generated by the at least one solid state actuator (60).