Gimbal mount for a sensor

A gimbal mount for a sensor having an outer and inner gimbal mount to stabilize vibrations in a wide frequency band without having to statically balance the sensor. A direct drive is provided for at least one drive of an outer axis of rotation of the outer gimbal mount and an amplified piezo actuator is provided for at least one drive of an inner axis of rotation of the inner gimbal mount. The at least one outer axis of rotation is provided for vibration stabilization in a first range of the frequency band to be stabilized and the at least one inner axis of rotation stabilization is provided for vibration stabilization in a second range in the frequency band to be stabilized. The outer gimbal mount and the inner gimbal mount are embodied as mechanically rigid constructions which transmit vibrations in the frequency band to be stabilized essentially without damping.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of Austrian application A50306/2014, filed on Apr. 25, 2014, and PCT application PCT/EP2015/058897, filed on Apr. 24, 2015, and the content of both applications are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a gimbal mount for vibration stabilization in a frequency band to be stabilized for at least one sensor arranged on a sensor holder with an outer gimbal mount having a number of degrees of freedom and an inner gimbal mount arranged on the outer gimbal mount also having a number of degrees of freedom, wherein each degree of freedom is formed by an axis of rotation, and each axis of rotation is driven by a drive.

BACKGROUND

For mounting sensors, such as video cameras, photographic equipment, infrared sensors, etc., for example, on vehicles such as airplanes, helicopters, land vehicles or aquatic vessels, etc., gimbal mounts are often used. To do so, the sensor is mounted on the gimbal mount which is in turn fastened to a part of the vehicle. Gimbal mounts allow movement of the sensor about a number of axes, usually a roll axis, a yaw axis and a pitch axis. There may also be multiple roll, yaw and/or pitch axes. The existing axes are therefore structurally nested one inside the other to enable movement of the sensor about all existing axes. For active movement about the axes provided, individual axes or all axes may also be driven. This makes it possible to position and/or align the sensor that is mounted on the gimbal mount by means of active control of the axes in any desired manner (within the limits of movement). This is important in particular when the sensor must remain aligned at a certain point during the movement of the vehicle. The inner axes are often used for fine positioning and the outer axes are used for approximate positioning of the sensor. Such a gimbal mount is described in U.S. Pat. No. 7,561,784 B2, for example.

However, while the vehicle is moving, vibrations are also introduced into the gimbal mount. A sensor on a gimbal mount must be decoupled from these vibrations for proper functioning and/or the position of the sensor (and/or its alignment) with respect to the vibrations must be stabilized. In the case of a sensor supported on a gimbal mount and mounted on a helicopter, vibrations of only a few angle seconds would result in a significant deviation from the targeted position. For example, without such vibration stabilization, no stable images could be recorded with a video camera because of such vibrations. Therefore, a great deal of effort has been put into the development of systems for vibration stabilization of gimbal mounts.

To keep vibrations away from the sensor, gimbal mounts have already been proposed, in which an inner gimbal mount is supported in a vibration-decoupled support in an outer gimbal mount, as disclosed in U.S. Pat. No. 5,897,223 A, for example. In this case, an inner gimbal mount is disposed in a spring-mounted shell, wherein the spring-mounted shell is itself supported in the outer gimbal mount. The inner gimbal mount is fastened to a gimbal mount point in the shell. Due to the spring-mounted shell the inner gimbal mount is vibration decoupled from the outer gimbal mount. However, this means that the inner gimbal mount needs to be mounted on its mount in a statically accurately balanced way. Any change in or replacement of the sensor is thus no longer readily possible because it would then be necessary to first repeat the static balancing of the mount of the inner gimbal mount.

U.S. Pat. No. 7,812,507 B2 describes a gimbal-mounted camera in which the gimbal mount is driven by a piezoelectric motor. A piezoelectric motor is known to be a motor in which vibration of one or more piezoelectric elements is converted into a movement of a movable part, for example, a linear movement. In a piezoelectric motor a vibration of the piezoelectric element is thus converted into a continuous movement (either a linear movement or a rotational movement). In the case of U.S. Pat. No. 7,812,507 B2, the vibration of a three-dimensional piezoelectric unit is utilized to move a spherical gimbal-mounted camera mount by means of friction. This movement is also utilized to compensate for vibrations. Thus, here, there is direct stabilization of the camera mount with respect to outer vibrations. However, because of the limited torque that can be generated with such a piezoelectric motor, there can only be low payloads and they can only be moved with poor dynamics. Rapid equalizing movements such as those which would be necessary for stabilizing vibrations of a higher frequency, cannot be implemented in this way, in particular not for payloads of a greater weight.

SUMMARY

It is an object of the present invention to provide a gimbal mount for a sensor, which will make it possible to stabilize vibrations in a broad frequency band without having to statically balance the sensor and/or the sensor with the mount.

This object is achieved according to the invention in that a direct drive is provided for at least one drive of the outer axis of rotation of the outer gimbal mount and amplified piezo actuators are provided for at least one drive of the inner axis of rotation of the inner gimbal mount, wherein the at least one outer axis of rotation is provided for vibration stabilization in a first range of the frequency band to be stabilized, and the at least one inner rotation coordinate is provided for vibration stabilization in a second range of the frequency band to be stabilized, wherein the second range of the frequency band to be stabilized is situated at least partially above the first range of the frequency band to be stabilized, and wherein the outer gimbal mount and the inner gimbal mount are embodied as a mechanically rigid construction, which transmits vibrations in the frequency band to be stabilized essentially without any damping. This combination of a direct drive on an outer axis of rotation and an amplified piezo actuator on an inner axis of rotation as well as the mechanically rigid construction makes it possible to position the sensor highly dynamically even without static balancing and to stabilize it with respect to vibrations over a broad frequency band. This permits very flexible use of the gimbal mount because sensors can be replaced easily, since it is no longer necessary to balance them. Preferably all the axes of rotation of the outer gimbal mount are embodied with direct drives and all the axes of rotation of the inner gimbal mount are embodied with amplified piezo actuators.

Advantageous embodiments of the gimbal mount according to the invention are derived from the dependent claims and from the following description.

DETAILED DESCRIPTION

FIG. 1shows a helicopter as an example of vehicle1, on which a gimbal mount2is mounted, with a sensor3, for example in the form of a video camera, being mounted thereon. Of course several sensors3may also be mounted on the gimbal mount2. The sensor3is mounted on the vehicle by means of the gimbal mount2, so that it can rotate about a roll axis y, a pitch axis x and a yaw axis z. The sensor3can be aligned with a certain point P in space by pivoting it about the axes (within the possible limits of movement), as indicated inFIG. 1. The gimbal mount2serves, on the one hand, to accurately align the sensor3with the desired point P during the movement of the vehicle1and, on the other hand, serves to stabilize the sensor3with respect to any vibrations that might occur due to the movement of the vehicle1or due to the vehicle1itself, for example caused by its drive units. Stabilizing the sensor3means that movements of the sensor3caused by vibrations are compensated in a wide frequency band, in particular in the range from 1 Hz to 150 Hz, so that the alignment of the sensor3with the point P is not disturbed within a tolerance range. This requires high dynamics (acceleration rate) of the actuators of the gimbal mount2and a high angular accuracy of the actuators. Any required sensors, such as angle sensors on the axes, gyroscopes or acceleration sensors for detecting positions, speeds or accelerations, must of course also fulfill the accuracy requirements.

If a video camera with an HD image resolution of 1920×1080 pixels is used as sensor3, as an example, and if a maximum error of 1 pixel is required for a video recording of an object during movement of vehicle1at an object distance of 100 meters from the sensor3and an image diagonal of 5 meters, then an angle accuracy of <0.003° must be achieved. All of this must be ensured at high payloads, for example, video cameras including the lens can have a weight of 10 kg and the gimbal mount2can have an inherent weight of 10 mg, and high acceleration forces through the movement of vehicle1should be possible. These requirements of the gimbal mount2and its actuators are consequently challenging.

This precision is achieved by the design of the gimbal mount according to the invention, as will be described below with reference toFIGS. 2 and 3as examples.

The gimbal mount2according toFIG. 2is designed as a five-axis gimbal mount2with an outer gimbal mount30and inner gimbal mount31disposed on the outer gimbal mount30. Five axes of course means five degrees of freedom of the gimbal mount2, where each degree of freedom is formed by an axis of rotation.

A first outer axis of rotation11, here a yaw axis, is provided on a mounting part10, with which the gimbal mount2can be mounted on the vehicle1. To do so, a rigid holding frame13is mounted on the fastening part10, so that it can rotate about the first outer axis of rotation11. The holding frame13is rotated by means of a yaw direct drive12relative to the fastening part10.

In the case of a direct drive, the electric motor is connected directly to the driven part, in particular without an intermediate gear or other transmission means such as belts, etc. With a very compact design, direct drives are characterized by a very high specific energy density (available energy based on the mass of the direct drive) and a very high achievable accuracy with regard to the control of the angular position. The direct drive may thus be embodied in such a way that the stator of the electric motor is disposed on a stationary component and the rotor of the electric motor is disposed on a rotational shaft. In the exemplary embodiment shown inFIG. 2, the stator could thus be arranged on the mounting part10, and the rotor of the direct drive12could be mounted on a rotational shaft14, which is itself mounted to rotate and on which the holding frame13is mounted in a rotationally fixed manner. In this way, the holding frame13can be rotated directly by the electric motor of the direct drive12.

A second outer axis of rotation15, namely here a first pitch axis, is provided on the holding frame13. An outer rolling ring16is mounted on this axis of rotation15so that it can be pivoted about the second outer axis of rotation15. To do so, a first pitch shaft17, on which the outer rolling ring16is mounted in a rotationally fixed manner, is mounted in the holding frame13so that it can be rotated to form the second outer axis of rotation15. In addition, a pitch direct drive18, with which the outer rolling ring16can be rotated about the second outer axis of rotation15with respect to the holding frame13, is also arranged on the holding frame13.

The first outer axis of rotation11and the second outer axis of rotation15are preferably embodied as a shaft and/or a shaft journal and respective bearing, for example, in the form of a roller bearing.

An inner rolling ring19is mounted in the outer rolling ring16so that it can rotate about the roll axis y, for example, by means of a concentric roller bearing. To that end, outer rolling ring16and inner rolling ring19are advantageously nested concentrically one inside the other.

A rolling drive20is provided on the outer rolling ring16, for example, again a direct drive or a servo motor, which rotates the inner rolling ring19with respect to the outer rolling ring16. In the exemplary embodiment shown here, the rolling drive20drives a first pulley21, and the inner rolling ring19serves as a second pulley. A belt22is guided around the first and second pulleys. The inner rolling ring19has a radial step on its outer circumferential surface to form a running surface23for the belt22. However, the first pulley21may also be formed directly by the rotor of a direct drive.

However, the outer gimbal mount30may of course also be designed with fewer axis of rotations and/or with a different configuration of the axis of rotations. For example, an outer gimbal mount30, in which the roll axis y is arranged between the first outer axis of rotation11in the form of a yaw axis and the second outer axis of rotation15in the form of a pitch axis would be conceivable. In this case, the holding frame13could be embodied as an annular holding frame, which assumes the function of the outer rolling ring. The inner rolling ring19would then be arranged concentrically in the annular holding frame. The second outer axis of rotation15would be arranged between the inner rolling ring19and a connecting ring to the inner gimbal mount31. The inner gimbal mount31would then be arranged on the connecting ring. The rolling drive20would then also be arranged on the holding frame13.

However, only one axis of rotation, for example, only the second outer axis of rotation15may be provided in the outer gimbal mount30. In this case, for example, the holding frame13would be rigidly connected to the fastening part10.

The inner gimbal mount31is arranged on a part of the outer gimbal mount30in particular on the innermost movable part of the outer gimbal mount, as on the inner rolling ring19as in the exemplary embodiment shown, or also on the connecting ring with reversal of the roll axis y and the second outer axis of rotation15.

In the inner rolling ring19, a first inner axis of rotation24, in this case a second pitch axis, is provided. To this end an intermediate ring25is arranged on the inner rolling ring19so that it can pivot about the first inner axis of rotation24. The intermediate ring25is here connected to the inner rolling ring19by a pitch joint26.

A second inner axis of rotation27, here a second yaw axis, is provided on the intermediate ring25. To this end a sensor holder28is arranged on the intermediate ring25so that it can pivot about the second inner axis of rotation27. The sensor holder28here is connected to the intermediate ring25via a yaw joint29. A sensor3, such as a video camera with a lens, for example, is fastened in the sensor holder28.

The first inner axis of rotation24and the second inner axis of rotation27are preferably again embodied as a shaft and/or shaft journal and respective bearing, for example, in the form of a roller bearing.

The pivoting about the first inner axis of rotation24, here a pitch axis, and the second inner axis of rotation27, here a yaw axis, is achieved by amplified first and second piezo actuators34,35, as shown more clearly inFIG. 3. Amplified piezo actuators34,35are actuators which scale up the possible very small movements (elongations) of piezo actuators to large movements. In doing so, only the stroke of the piezoelectric element, which is installed in the amplified piezo actuators34,35, is converted into a larger stroke, usually by means of a mechanical transmission of the stroke. The stroke of the amplified piezo actuators34,35then follows the stroke of the installed piezoelectric elements. Amplified piezo actuators34,35are characterized by high precision, great force (torque), wide bandwidth and compact size for travel distances of up to 1 mm. Such amplified piezo actuators34,35are sufficiently well known and are available in various embodiments, which is why they will not be discussed further here.

In the exemplary embodiment shown inFIG. 3a pair of piezo actuators is provided in each case for pivoting about an axis of rotation, namely second piezo actuators34for the second inner axis of rotation27and first piezo actuators35for the first inner axis of rotation24. However, more or fewer piezo actuators34,35may of course also be provided per axis of rotation.

The first piezo actuators35for the first inner axis of rotation24are, on the one hand, arranged on the inner rolling ring19and, on the other hand, on the sensor holder28. To do so, first supports32are provided on the sensor holder28, with which the first piezo actuators35are connected with a joint. Actuating the first piezo actuator35(indicated by the double arrow) causes the sensor holder28to pivot about the first inner axis of rotation24. The two first piezo actuators34inFIG. 3must of course operate in opposition here.

The second piezo actuators34for the second inner axis of rotation27, on the one hand, are disposed on the inner rolling ring19and, on the other hand, on the sensor holder28. To do so, second supports33, with which the second piezo actuators34are connected via a joint are provided on the sensor holder28. The second supports33are arranged with an offset by a certain angle, preferably 90°, with respect to the first supports32. Actuating the second piezo actuator34(indicated by the double arrow), causes the sensor holder28to pivot about the second inner axis of rotation27. In the case of two second piezo actuators34, these would of course again have to operate in opposition to one another.

Because of the articulated connection of the first piezo actuator35and the second piezo actuator34on the respective supports32,33and also because of the small travel distances of up to 1 mm, it is possible in this way to pivot about both inner axis of rotations24,27, In addition, the second piezo actuator34and the first piezo actuator35may also be connected to the inner rolling ring19in an articulated connection.

In an alternative embodiment, the second piezo actuator34could not be attached to the inner rolling ring19but could instead be attached to the intermediate ring25.

The gimbal mount2shown in the figures is described as a five-axis mount. However, the gimbal mount2could of course also include more degrees of freedom or fewer degrees of freedom (axis of rotations). For example, it would be possible to omit the roll axis y, so that the outer rolling ring16and the rolling drive20and/or the pulleys21and the belts22could also be omitted. In this case, the inner rolling ring19would be pivotably arranged on the holding frame13via the second outer axis of rotation15.

In general, the gimbal mount2includes an outer gimbal mount30having a number of degrees of freedom, formed in the exemplary embodiment shown here by the mounting part10, the holding frame13and the outer and inner rolling rings16,19and the respective axis of rotations, bearings and drives, and an inner gimbal mount31arranged on a portion of the outer gimbal mount30, formed in the exemplary embodiment shown here by the intermediate ring25and the sensor holder28and the respective axes, bearings and drives. The outer gimbal mount30and the inner gimbal mount31are connected to one another via the inner rolling ring19. If no roll axis y is needed, then instead of the outer rolling ring16, the inner rolling ring19may be arranged directly on the holding frame13of the outer gimbal mount30.

If the inner gimbal mount31has only one degree of freedom, for example, only the first inner axis of rotation24or only the second inner axis of rotation27, the intermediate ring25may be omitted. In this case the sensor holder28would pivotably be connected to the inner rolling ring19via the respective axis of rotation24,27and the corresponding amplified piezo actuators34,35.

It is also conceivable to alter the sequence of the inner axis of rotations24,27, i.e. the second pitch axis and the second yaw axis, in the inner gimbal mount31so that, for example, the second inner axis of rotation27is provided between the inner rolling ring19and the intermediate ring25.

The gimbal mount2according to the invention makes it possible on the one hand to align the sensor3with high precision and on the other hand to stabilize the sensor3with respect to vibrations, which is also a prerequisite for high precision alignment of the sensor3. This is made possible first by the fact that the first and second axis of rotations11,15, i.e. the first yaw axis and the first pitch axis, can be actuated and controlled by the direct drives with a high energy density and high positioning accuracy. Thus the sensor3can be positioned in a highly dynamic manner within a certain frequency range. By that, control accuracies for frequencies of up to about 20 Hz as good as 0.01 degree are possible. Since the drive for the outer axis of rotations is provided by direct drives, the gimbal mount2is also capable of at least partially regulating out vibrations in a certain frequency range due to the first outer axis of rotation11and the second outer axis of rotation15. The remaining vibrations in this frequency range, i.e. 1 Hz to approx. 20 Hz, and vibrations in a higher frequency range, approx. 20 Hz to approx. 150 Hz. are regulated out by the inner axis of rotations24,27, i.e. the second yaw axis and the second pitch axis or in other words by their piezo actuators34,35.

To this end it is provided that the vibrations in this frequency range are introduced from the outer gimbal mount30into the inner gimbal mount31essentially without being dampened. Only when the vibrations (oscillations) in the frequency bands to be stabilized are introduced with the least possible damping can they be compensated by a highly dynamic control. If the damping were too high, for example due to a flexurally or torsionally soft construction, control interventions would also be dampened accordingly, which would prevent a highly dynamic control. The structural elements of the gimbal mount2may be considered as a low-pass filter in their vibration behavior. Vibrations with frequencies below a certain cutoff frequency are thus transmitted approximately without attenuation, whereas vibrations above the cutoff frequency undergo attenuation, which is to some extend substantial. To now be able to transmit the vibrations in the frequency bands to be stabilized with the least possible damping, the cutoff frequency of the gimbal mount2must be at least above the frequency range to be stabilized, preferably significantly above.

The structure of the gimbal mount2, consisting of mounting part10and the holding frame13and preferably also the outer and inner rolling rings16,19and the intermediate ring25as well as the bearing situated in between, is designed with sufficient mechanical rigidity, so that the vibrations in the frequency ranges to be stabilized are transmitted essentially without damping. This may be achieved, for example, by choosing suitable composite materials or through structural design of the holding frame13.

For the roll axis y, such precision is not usually required, which is why the requirements of the drive of the roll axis y are much lower and can be implemented with traditional drives.

FIG. 4shows the efficiency of the vibration stabilization of the gimbal mount2according to the invention. This shows the control efficiency R of the outer axis of rotations11,15, driven by direct drive, i.e. the first yaw axis and the first pitch axis, and the inner axis of rotations24,27, driven by the piezo actuators34,35, i.e. the second yaw axis and the second pitch axis. The control efficiency R here denotes the measure by which vibrations of a certain frequency can be compensated. A control efficiency R of 100% would mean that vibrations of a certain frequency can be fully compensated. In the low frequency range (to approx. 20 Hz), the direct drives regulate the input disturbances (vibrations) with a high efficiency curve40. The control efficiency R decreases with an increase in frequency f because of the moment of inertia. Above a certain frequency (in the range of >20 Hz), the control efficiency of the direct drives declines drastically because of its limited control bandwidth. In the high frequency range (above approx. 20 Hz), the amplified piezo actuators34,35regulate with a high control efficiency R—curve41. In the low frequency range, however, the control efficiency R of the amplified piezo actuators34,35is limited because of the limited travel distance (<1 mm). Consequently, due to the combination of direct drives on the outer axes and amplified piezo actuators34,35on the inner axes, it is possible to regulate the external disturbances (vibrations) out with a high precision over a wide frequency range (1 Hz to approx. 150 Hz). The gimbal mount2can thus be aligned and stabilized with a high precision in this frequency range.

Even higher frequencies (greater than approx. 150 Hz) can be filtered out by vibration decoupling at the mount between vehicle1and gimbal mount2. Special mounts, which are capable of decoupling such vibrations from the gimbal mount2, are known for this purpose.

For certain applications, it may be sufficient if at least one of the outer axes of rotation11,15, for example, the first pitch axis15, is driven with a direct drive18. Likewise, it may be sufficient if at least one of the inner axes of rotation24,27, for example, the second yaw axis, is driven with an amplified piezo actuator34.

The control concept is diagrammed schematically inFIG. 5. The gimbal mount2, which supports the sensor3, is shown here without restriction only as a biaxial gimbal mount, with an outer gimbal mount30having one outer axis of rotation15, for example the first pitch axis, and an inner gimbal mount31having one inner axis of rotation27, for example the second yaw axis. A pitch direct drive18is provided on the axis of rotations15,27, as described above, on the outer gimbal mount30, and an amplified piezo actuator34is provided on the inner gimbal mount31. Additional position sensors45,46, for example optoelectronic angle sensors, are installed on the axis of rotations15,27of the gimbal mount2in order to be able to determine the actual angular position of the axis of rotations15,27. In addition, other sensors48, for example gyroscopes or acceleration sensors may also be provided on the gimbal mount2. The position sensors45,46and the sensors48transfer their measured values to a control unit47, in which a suitable controller is implemented for controlling the axis of rotations15,27of the gimbal mount2. The controller in the control unit47controls the actuation of the drives of the axis of rotations15,27on the basis of given setpoint values S in order to align the sensor3accurately, on the one hand, and to keep it aligned and also to compensate vibrations in the gimbal mount2.