Patent Application
20140093048
European Patent Application No. EP 12 183 680.3, with a filing date of Sep. 10, 2012, is incorporated herein by reference.
The invention generally relates to vibration damping means. More particularly, the invention relates to an active vibration damping of a vibratory system using an absorber mass that is moved by a drive.
Vibration damping means which are not attached between two portions but only to one vibrating portion of a device are referred to as a vibration absorber, or briefly as an absorber, or a pendulum-type absorber. Due to a stretchable or compressible connection between a mass of the absorber and the vibrating portion, the mass of the absorber follows the movements of the portion with a certain delay. Due to the stretching and/or compression, energy is withdrawn from the vibration movement, and in this way a damping effect is obtained.
Typically, the mass including the deformable connection, i.e. stretchable and/or compressible and/or twistable connection, has a natural frequency. The latter is usually set to a specific proportion of the vibration frequency to be damped. At this optimized frequency the mass will be deflected widely, so that correspondingly large forces and correspondingly large energies are transferred from the vibrating portion to the absorber mass.
From DE 197 25 770 A1 a device for damping vibrations is known which comprises a vibration absorber. The vibration absorber includes a mass resiliently coupled to the resilient system, the mass being coupled to the resilient system through a controllable actuator which is variable in length in the operating direction of the vibration absorber. This is intended to dampen vibrations in an enlarged frequency range as compared to a passive vibration damping system. Similar principles are also disclosed in DE 10 2006 056 757 A1 and DE 10 2010 002 262 A1, for damping vibrations of buildings.
The absorbers described in DE 6 941 4628 T2 and in EP 6 183 80 B1 are based on another mechanism of action. Here, the absorbers are attached to pendulum-like structures and virtually constitute a pendulum-type absorber. Specifically, a vibration absorber for cable car gondolas is provided, with a movable damping mass member attached to the pendulum structure of the gondola above the center of gravity thereof. EP 6 183 80 B1 furthermore gives an overview of industrial applications of absorbers which are likewise considered for the invention.
DE 10 2006 053 232 B4 discloses an absorber with two directions of action. This absorber is intended to damp a vibration of a conduit, for example, and may also be operated actively. The active absorber comprises at least two actuators acting in different movement directions, and a fastening device. The actuators are coupled to the fastening device via leaf springs. Additionally, the two actuators are coupled with each other by a connecting member.
DE 10 2010 039 977 A1 discloses a piston-shaped passive absorber which retains a balancing mass within a piston so as to be movable at a defined friction.
The invention aims to provide for dampening in particular low-frequency vibrating structures which include more than one mass-spring element or which exhibit vibration characteristics that can be described by a plurality of coupled mass-spring elements, in a manner so that the vibration amplitude thereof is significantly reduced. One application field of the invention are mobile or stationary X-ray scanners, for example.
For this purpose, an active vibration absorber is provided for damping vibrations of a cantilevered portion of a support structure, wherein the vibration absorber comprises a mass which is coupled to a fastening means without using a spring and typically mechanically, through a drive that is controlled by a control device of the vibration absorber, which fastening means serves to fasten the drive to a support structure to be damped, so that upon a movement of the mass relative to the fastening means an inertial force caused by this movement is directly transmitted via the drive to the fastening means. The control device comprises a motion sensor, the control device being adapted to control the drive in function of the signals from the motion sensor and thereby to dampen the vibrations of the support structure.
In other words, on the one hand the absorber mass is coupled to a first portion of the drive without using a spring, or rigidly, on the other hand a second portion of the drive, which is movable relative to the first portion, is coupled to the fastening means without using a spring, so that the only degree of freedom that remains for the movement of the absorber mass relative to the support structure to be damped and/or to the fastening means is a movement of the drive.
Accordingly, the invention also provides a support structure with active absorption, the support structure being retained at least at one end and having a cantilevered portion to which a vibration absorber is attached, in particular a vibration absorber as described above, wherein the vibration absorber comprises at least one mass which is movably mounted to be moveable at least along one direction by means of at least one drive, wherein the mass is coupled to the cantilevered portion of the support structure through the drive, so that upon a movement of the mass the drive directly exerts a force to the portion of the support structure, without interposition of a spring member.
Accordingly, the absorber forces are not generated by a combination of damping and inertia, but only by an acceleration of the absorber mass which is actively controlled by a control loop in a manner so that the movement detected by the motion sensor is damped by opposing forces.
So a direct, rigid connection via the drive is established herein with the system to be controlled, such as an arm having a cantilevered end, without using a common absorber spring. This eliminates the adaption of spring stiffness with respect to the vibratory system to be controlled. The inertia forces generated by the movement of the absorber mass are therefore directly transferred via the drive to the support structure to be damped. Similar to a swing where simultaneous swinging of one's legs leads to a strengthening of the rocking motion, a swinging of one's legs and body in the opposite direction would cause the rocking motion to come to a halt. This principle is also exploited by the invention, which enables to dampen a plurality of vibrational modes of different frequency.
It will be apparent to a person skilled in the art, how a spring-free coupling can be established between the absorber mass and the support structure to be damped via the drive. Although a rigid connection may also have associated therewith a spring constant, the latter will be much higher for the coupling of the absorbing mass than the spring constants of the support structure to be damped. Accordingly, the lowest frequency of the natural vibration of the system of absorber mass and coupling thereof will be much higher. Generally, for the purposes of the invention, the spring-less coupling may be designed such that the lowest natural vibration mode of the system of absorber mass and coupling with the support structure or with the fastening means is at least four times, preferably at least eight times as high as the lowest frequency of the natural vibration of the support structure. The invention is in particular intended for damping low-frequency vibrations. Therefore, the coupling of the absorber mass through the drive to the fastening means for achieving a spring-less connection preferably has a significantly higher frequency of the lowest frequency natural vibration mode, which is preferably at least 10 Hz, more preferably at least 20 Hz.
Preferred motion sensors are velocity sensors, such as a geophone. However, an acceleration sensor is likewise conceivable as a motion sensor. A geophone is particularly suitable because it exhibits a high signal-to-noise ratio at low frequencies.
The absorber according to the invention is in particular intended to be mounted to a vibratory system with multiple degrees of freedom, which will be referred to as an “arm” below, for the sake of simplification and by way of example, in order to dampen the entire system in its vibrations. The absorber preferably comprises a linear drive as a drive which enables to move the mass in an axial direction, and a housing, and a fastening means by which the damper can be mounted to the end of the arm. The motion sensor, preferably a velocity sensor, is arranged at one of the non-movable parts of the absorber and is capable of sensing the movements of the arm's end and to supply it to a control loop of the control device.
This control loop will then supply a control signal to the linear drive in a manner so that the resulting movement (or acceleration which results in the movement) exerts a counter force on the system so that the overall movement of the arm is minimized.
The invention will now be described in more detail by exemplary embodiments and with reference to the accompanying drawings. In the drawings, the same reference numerals refer to the same or to equivalent elements. In the drawings:
One embodiment of a vibratory system which represents one of the application cases thereof is shown in
The X-ray scanner 12 of this exemplary embodiment comprises a container 5 which houses the measuring equipment for an X-ray scan. An arm 3 is mounted to container 5, and X-ray detectors 13 are attached to the arm. Detectors 13 are arranged at the side of arm 3 which faces the container, and therefore are hidden by the arm in the view of
An active absorber 1 according to the invention is rigidly mounted by a fastening means, not shown, to the free or non-retained end 31 of arm 3. For a mobile use of this application, the container 5 may be placed onto a trailer of a semitrailer truck. This permits X-ray scanning of a stationary object, such as a truck with cargo, by driving therealong, while the truck or more generally the object to be checked passes through the intermediate space 7 between the container 5 and the end 31 of arm 3. In contrast to stationary installations where the cabin has generally to be occupied because the truck drives through the stationary X-ray scanner and therefore the truck cabin must not be scanned, mobile X-ray scanners may also check the cabin, because the latter does not have to be occupied.
While being moved by the semitrailer truck, the arm 3 is subjected to excitation at its base due to unevenness of the ground. This causes movements of the container 5 and hence of the arm 3. The vibrations of the arm 3 in turn may become so strong that the X-ray detectors at the inner side thereof will swing out of the X-ray path from the measurement container 5 and so will not produce any picture.
Therefore, without being limited to the example shown in
As can be seen from
Such an arm 3 may be modeled, in terms of control technology, as a two-mass oscillator comprising masses m1 and m2, as illustrated in
Mass m2, in turn, has attached thereto a mass m1, via a spring 36 having a spring constant D12, the movement of masses m1, m2 relative to each other being damped by an attenuator 34 having a damping constant r12.
It can now be shown that by effecting control at the outer free end of the system, i.e. at m1 and accordingly at the end 31 in the example shown in
Although according to the invention in this case the absorber is coupled to the arm without the use of an absorber spring, the equivalent system diagram of
However, the system cannot be compared with a standard absorber, because D12 and r12 are inaccessible to parameterization since they are system properties.
Rather, it is now largely the task of a control scheme to eliminate the mismatch of the natural frequencies in the sense of a standard absorber.
Useful for the invention herein are velocity feedback control techniques which have also been known for active vibration isolation systems. According to one embodiment of the invention, the sensor signal of a velocity meter such as a geophone which is proportional to the movement of m1, is looped back through a controller which is preferably implemented digitally and allows for control loops that are parameterizable in wide ranges.
This control signal may then be fed to an amplifier for the drive, i.e. a linear motor in the example shown in
Depending on the configuration, the amplifier may use this signal as a target signal for a motor current or for a translational speed. The use directly as a motor current is advantageous, since the velocity of the arm is to be directly counteracted by a force. Since the current supplied to the linear motor is proportional to the acceleration thereof, this would be achieved immediately.
On the other hand, linear drives exhibit a non-linear characteristic for small amplitudes, which is caused by the breakaway torque of the mechanical components at the start of a movement. Therefore such a signal is not a reliable basis.
Instead, a phase shift of 90 degrees is accounted for and the control signal is intended to determine the target velocity. This allows an amplifier of the linear motor, or more generally of the drive, to take on the task to drive the current to a level, in every moment, so that the target speed is really achieved. A non-linearity is thus avoided.
Therefore, according to a further embodiment, the vibration absorber comprises an amplifier or a driver for providing the supply current for the drive, wherein the control device is adapted to supply a signal to the amplifier as an output signal which corresponds to the target velocity of the drive.
The so-called open-loop transfer function of a two-mass oscillator as shown above in
First of all it can be seen that the amplitude of the transfer function has two maxima. The first peak 40 of the illustrated example is at a frequency of about 0.5 Hz, the second, higher frequency peak is at about 2.5 Hz. These maxima correspond to the vibration modes of arm 3 mentioned above. The maximum at 0.5 Hz is caused by the low frequency mode at which the entire arm swings around its base. The natural mode at 2.5 Hz corresponds to the oscillation of the vertical end portion of the arm around its upper suspension point.
As can be further seen from
However, it has proven to be very effective to use two control loops or two controllers for control purposes. Specifically, according to one embodiment of the invention without any limitation to the specific exemplary embodiments described herein the control device comprises at least two control loops, the control loops being adapted to generate control signals for different vibration frequencies.
Specifically, in one embodiment of the invention the absorber according to the invention, which is only shown herein by way of example, employs two control loops with the same input signal from the motion sensor, such as preferably a geophone, and first controls the higher frequency portion in one control loop, then the lower frequency portion.
According to still another embodiment of the invention, for control purposes the open loop or controlling of a first control loop is adapted so that the amplitude at 0.5 Hz is lowered and the phase is appropriately shifted such that the frequency of this oscillation is not affected.
Finally,
Without being limited to the exemplary embodiment shown, the second control loop according to one embodiment of the invention comprises a band-pass. More generally, therefore, at least one of a plurality of control loops of the control device may comprise a band-pass.
As will be apparent from a comparison of
In the exemplary embodiment illustrated in
In the two figures, this can be seen from the fact that in a range around this frequency the phase and amplitude are altered, whereas vibrations at more distant frequencies, such as above 10 Hz or below 0.2 Hz are not affected or controlled. Therefore, according to one embodiment of the invention, an inventive support structure is provided with an active absorber, in which the control device comprises at least two control loops which are adapted for controlling vibrations in different frequency ranges, wherein the support structure has at least two vibration modes, and wherein at least one of the control loops is implemented as a band-pass and effects control in a frequency range which includes one of the vibration modes.
The example shows that even with an unfavorable position of the frequencies and phases, the digital control loops which are adjustable over a wide range enable to always find a parameter set which effectively damps the movements.
It is generally advantageous for the absorber to be attached to a point of the support structure at which only one vibration mode occurs, for selectively damping only this one. Instead, a point is carefully chosen, at which at least two vibration modes or natural oscillations superimpose, such as the outer point of arm 3 in this case.
Furthermore, generally, the invention is especially suitable for damping low-frequency vibrations. Preferably, the absorber is adapted for damping vibrations below 50 Hz, preferably below 20 Hz, more preferably below 10 Hz. With respect to the plurality of vibration modes mentioned above this means that the vibration modes of the support structure are preferably in the aforementioned frequency ranges.
From the embodiments described above by way of example only it will also be apparent that in contrast to known vibration absorbers a parameter of the absorber will not be matched to a system only once, but preferably any excitation of the system will be prevented by means of a wideband feedback control loop. This enable to respond to disturbances induced by excitation at the base as well as to disturbances induced externally. Moreover, because of the active control it is superior in terms of respond time over any system operating through friction/damping loss.
Thus, the design of the active absorber according to the invention may be restricted to an adaptation of the movable mass 15, the necessary accelerations, and the travel distance of the absorber mass 15 to the respective vibratory system.
An example of parameter dependency of a vibratory support structure is illustrated by the 3D diagram of
Preferably a velocity sensor whose signal is proportional to the movement of the arm is used as a motion sensor for feedback control. The signal from this motion sensor is digitized by an analog-to-digital converter 23 of the control device 22, and is supplied in parallel to controllers 261, 262, . . . , 26N, which in turn may comprise cascade filters. According to one exemplary embodiment, up to 8 different cascade filters may be used on the same input signal. It has been found that for the application example of an arm 3 as shown in
More generally, control device 22 may comprise any cascade of digital filters which may be selectable and/or parameterizable in the field.
According to one exemplary embodiment, the low-frequency mode (0.5 Hz) as shown in
The individual output signals of the parallel configurations are digitally summed by an adder 25 and are supplied to a digital-to-analog converter 24 which provides the control signal for the actuator or drive 20.
Control device 22 is preferably implemented digitally, as is specifically the case in the example of
Also, the parallel connection of individual controllers 261, . . . , 26N as shown in
In the exemplary embodiment shown in
For example,
The processing and feedback control may be easily effected separately for each drive 20, 200 and the associated motion sensor 19, 190. Accordingly, two independently operating control devices 22 could be provided instead.
In the illustrated example, the vibrations are damped in a plurality of directions using a shared absorber mass 15. However, it is likewise possible to provide separate absorber masses 15. Furthermore, it is also possible for the input signals to be detected by a single motion sensor 19 which is sensitive in several spatial directions. The signals from such a motion sensor 19 may then be disassembled into components of individual spatial directions and may be processed separately by control device 22.
Therefore, for damping orthogonal vibration modes, one embodiment of the invention without any limitation to the example shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 12183680.3 | Sep 2012 | EP | regional |
