WO2011115488A1

WO2011115488A1 - Active vibration isolation system, arrangement and method

Info

Publication number: WO2011115488A1
Application number: PCT/NL2011/050185
Authority: WO
Inventors: Jeroen De Kort; Duncan Denie; Johannes Gerhardus Hendrikus Schabbink; Servaas Louis Bank; Bernhard Michael Bakker
Original assignee: Mecal Applied Mechanics B.V.
Priority date: 2010-03-17
Filing date: 2011-03-17
Publication date: 2011-09-22
Also published as: NL2004415C2

Abstract

Active vibration isolation system, arrangement and method Abstract An active vibration isolation system combines a passive vibration isolation system with and motion control to minimize motion of a table. Actuators between the floor and the suspended table are used to counter table vibrations measured with motion sensors attached to the table. A first motion sensor measures a position of a reference mass. A second sensor with a higher 1st eigen frequency measures the position of another reference mass. By using sensor fusion vibration reduction can be obtained over a larger frequency range than would be possible with a single sensor using extreme optimization of sensor dynamics. Furthermore, a vibration isolation arrangement and method are proposed.

Description

Title: Active vibration isolation system, arrangement and method FIELD OF THE INVENTION

The invention relates to an active vibration isolation system, to an arrangement and to a method for vibration isolation, in which a motion sensor measures position of the reference mass (instead of velocity or acceleration) to reduce sensor noise, a second sensor has a higher 1^st Eigen frequency and said system uses Sensor Fusion instead of extreme optimization of sensor dynamics. BACKGROUND OF THE INVENTION

The high-tech industry constantly requires improvement of the accuracy of mechatronic equipment. Along this improvement, elimination of floor vibrations is becoming more and more important.

In environments where systems with high accuracy are placed, vibrations with amplitude at micrometer level are present. These vibrations - mainly at low frequencies around 1 Hz- limit the performance of a mechatronic equipment.

Systems for vibration isolation are known. For example, Patent Specification US 2007/0235276 Al discloses an active vibration isolation system which detects structural vibration of the mass to be isolated via a sensor, in particular eigenfrecuency vibration, and is taken into account of active control. Vibration isolation systems such as these can be used in widely differing fields of engineering, in particular in the field of semiconductor manufacture and for high-resolution imaging systems such as MRI

installations and electron beam microscopes.

Today's passive vibration isolation technology is not sufficient to reduce disturbances caused by low frequency vibrations because they are optimized for high frequencies and not sufficient to reduce disturbances caused by low frequency vibrations. Passive vibration isolation technology that is optimized for low frequencies lacks stability.

As manufacturing precision increases, the requirements for vibration isolation systems such as these also become more stringent.

Vibration isolation systems have therefore been developed which have active control via actuators in addition to mechanical decoupling of the mass to be isolated, for example by means of air bearings. In vibration isolation systems such as these, position changes of the mass to be isolated, in particular vibration, are detected via sensors. The position change is counteracted via actuators.

Electronic control systems make vibration isolation systems such as these even able to predict position changes caused by disturbance influences and to counteract a possible position change before it occurs.

Known vibration isolation systems such as these have been found to have the disadvantage that the mass to be isolated is regarded as a rigid body and it is not possible to take account of vibration originating from the body, such as natural-frequency vibration of isolated masses, in the control of the vibration isolation system. It has not been possible to include structural vibration such as this with previous control models. In fact, it has been found that any attempt to take account of structural vibration in the drive for the actuators of an active vibration isolation system leads to it not being possible to reduce the structural vibration, and in many cases it is even increased.

In known active vibration isolation systems, the mass to be isolated is therefore normally regarded as a rigid body and is masked out for control purposes, that is to say the vibration of the mass to be isolated itself is ignored in the calculation of compensation signals.

The basic vibration problem is depicted in

Fig. 1. The figure shows a table (1) connected to the floor via a machine frame (2) with stiffness cbase. Vibrations of the table (1) can be caused by floor vibrations, via the machine frame (2), and by disturbance forces acting directly on the table (1).

The amplitude and frequency of floor vibrations are determined by vibration sources (such as adjacent machines, traffic, etc) and by the dynamic properties of the building (stiffness, mass, eigenfrequencies). The disturbance forces normally are caused by processes and moving stages mounted on the table (1).

On the table (1), an accurate component (3), for instance a lens or a sensor, can be mounted. The position error between this accurate component (3) and the table (1) is depicted as Δ (see Figure 1). This position error determines the machine accuracy.

Normally the mass of the accurate components on the table is much smaller than the mass of the table (meq « mtable). In that case relation between utable and position error Δ is stated by (eq. 1):

This means that the error Δ is proportional to table movement utable. The challenge of vibration reduction therefore is to keep the table still despite the disturbance forces and floor vibrations. The effect of floor vibrations (ufloor) on movement of the table (utable), in other words the actual isolation of the table, is described by the transmissibility function (eq. 2):

In this equation,□ is the frequency of the disturbance and cbase and dbase the stiffness and damping of the table supports.

The sensitivity of the table for disturbance forces acting on the table (Fd) is described by the compliance function (eq. 3) ^U table _ _

^Fd - m_tabk ^■ ω² + d_base ^■ jo)+ c_base (cq.3)

The optimum solution, leading to minimization of table movement, therefore is the design with minimal transmissibility (i.e. maximum isolation of floor vibrations) and minimal compliance (i.e. minimum sensitivity to disturbance forces). An important value governing both transmissibility and compliance is the eigenfrequency of the table with respect to the floor (eq.4).

²^ V ^mtable This frequency forms in effect a cut-off frequency for the transfer of floor vibrations and disturbance forces to motion of the table: for frequencies higher than fn the table becomes increasingly less sensitive to disturbance forces and floor vibrations.

However, decreasing the stiffness between floor and table will increase the sensitivity to disturbance forces below fn. This means that in order to reduce the effect of disturbance forces, the most effective way is to decrease fn by increasing the mass of the table instead of decreasing the stiffness of the support.

Equations 2 and 3 show that around the frequency fn both compliance and transmissibility become very high for low values of dbase. This means that a high damping is needed to prevent amplification of vibrations around the eigenfrequency fn. However, for high frequencies the compliance and transmissibility increase with dbase, this means that low damping is needed for optimal isolation at high frequencies.

Passive isolation Passive isolation means that no active components, such as sensors and actuators, are used. To minimize both compliance and transmissibility, these systems normally consist of a heavy table, as a basis for the accurate process, supported with reasonably weak supports with high damping.

This means: a high support stiffness cbase and low eigenfrequency fn, the latter resulting from a high mass mtable. In practice these numbers are limited. Firstly the allowed mass of the table is limited for practicality.

Secondly, in order to function properly, any application must be robust enough to handle a certain degree of disturbance forces, which means that the stiffness of the table supports can not be decreased to far. In practice, stable supports with a frequency lower than about 2 to 3 [Hz] are very difficult to design without active components.

As an example: a 100 [kg] table at 2 [Hz]. A small force of 10 [N] (= 1 % of the gravity force acting on the table) will already induce a displacement larger than 0.6 [mm].

In practice this means that the table is very sensitive to drift and 'feels' very unstable to the user.

High damping is needed to prevent the amplification of floor vibrations and disturbance forces at frequencies close to the eigenfrequency of the table on its supports. However, too much damping will reduce the vibration isolation performance at high frequencies.

Overall, passive vibration isolation is a very straightforward technology and can be quite effective to solve vibration problems for

frequencies higher than 3 to 5 [Hz] when a modest reduction of amplitude is sufficient.

Active isolation

If a reduction of floor vibration amplitude with more than a factor 2 at 5 [Hz] is needed, passive isolation will be insufficient. In that case active vibration isolation can be used. This means that sensors and actuators are used to measure and counter vibrations. This can be combined both with high stiffness supports and with isolating supports. Today, various solutions for active vibration isolation (or reduction) are based on three concepts as discussed below.

A) Piezo solutions

In cases where passive isolation provides too little vibration reduction, active systems are used. In these systems electronic components are used to measure and suppress vibrations.

In the piezo solution as shown in Figure 2, motion sensors (4) are used to measure vibrations of the table (1). The table is mounted on (stiff) piezo actuators (6) that are used to counter these vibrations. A motion controller (7) interprets the motion sensor output and calculates the optimal counter forces.

The error Δ can also be decreased by minimizing the mass meq and maximizing the stiffness ceq of accurate objects mounted on the table. This is an important part of the design process of the accurate machine itself and is not part of the vibration reduction problem and therefore outside the scope of this application.

In most piezo solutions there is no passive isolation whatsoever. Therefore the compliance is determined by the high stiffness of the piezo support (6). This delivers a very low compliance compared to other solutions.

Because there is no passive isolation, the reduction of floor vibrations is provided only in the frequency range where the controller (7) is active. The lower boundary of this range is determined by the resolution of the sensors (4), the higher boundary by the controller bandwidth.

A high controller bandwidth therefore is essential. To enable this, sensors (4) must be used with good performance at high frequencies (>100 Hz), such as accelerometers or specific geophones. The penalty is that in practice, at low frequencies, i.e. below 5 to 10 [Hz], this cannot be combined with low sensor noise and a good resolution. The effect is that it is very difficult to reduce vibrations at low frequencies using a piezo solution. In some cases the sensor noise exceeds measured floor vibrations, inducing the controller (7) to increase the table vibration level rather than reducing it.

In some commercially available configurations, passive isolation is added between the piezo mounts (6) and the supported table (1). In effect a table (1) with passive isolation is mounted on top of the piezo mounts (6). In this way the isolation performance of the passive system is enhanced, but the compliance is limited by the passive system.

Note that because of the stiff connection between table (1) and floor (8) the controller bandwidth is limited by floor dynamics. This means that a in piezo solutions a very rigid floor (8) is needed.

B) Separate reference mass

In this solution, a reference mass (9) is suspended separate from the isolated table (1). Sensors (4) measure the displacements of the isolated table (1) relative to the reference mass (9). The controller (7) forces the table (1) to copy the movements (or lack thereof) of the reference mass (9) using a force produced by actuators (11) (see Figure 3).

The basis of the concept is that the reference mass (9) is suspended with passive isolation systems with very low stiffness. This means that the transmissibility of floor vibrations to the reference mass (9) is very low. The major disadvantage of passive isolation, namely the sensitivity to disturbance forces, is not a problem because the process forces act on the table (1), not on the reference mass (9).

Note that amplification of floor vibrations to motion of the reference mass (9) will occur around its natural frequency. This will be directly transferred by the controller (7) to motion of the table (1). Considerable damping of the reference mass suspension is therefore essential.

In this design the table (1) is supported with a 'classic' passive isolation system at a reasonably low frequency. The compliance of the passive isolation system (10) can be greatly enhanced between 0 [Hz] and the controller bandwidth. For higher frequencies the characteristics of the passive table supports (10) apply. This means that with this concept, passive isolation can be applied in situations with large disturbance forces acting on the table (1).

An important aspect of this solution is that vibration isolation is impossible below the natural frequency of the reference mass (9) on its suspension (e.g. a 2.5 [Hz] support). Below this frequency, the amplitude and phase with which the reference mass (9) moves is (almost) equal to the floor vibrations. Therefore the controller (7) will force the table (1) to copy floor vibrations.

Inertial control

The basis of the inertial control configuration is a table (1) suspended by passive isolation systems (10). A motion controller (7) is used to minimize motion of the table (1): actuators (11) between the floor (8) and the suspended table (1) are used to counter table vibrations measured with the motion sensors (4) attached to the table (see Figure 4).

Compared to passive isolation the compliance and transmissibility can be greatly reduced in the frequency range where the controller (7) is active. The lower boundary of this range is determined by the resolution and noise level of the sensors (4): better resolution and lower noise means this boundary can be close to 0 [Hz], but never at 0 [Hz], as no motion exists at 0 [Hz]. The higher boundary of this range is determined by the controller bandwidth. The controller (7) will delete the amplification of floor vibrations around the eigenfrequency of the suspension (10). Damping of the passive suspension (10) is therefore not necessary. For frequencies higher than the controller bandwidth the vibration isolation is determined by the passive isolation system, with low damping, and therefore still considerable.

The major disadvantage is that systems based on inertial control are sensitive to disturbance forces with low frequency content. For static forces (0 [Hz]) active leveling can be added, but between the leveling frequency range and the lowest frequency at which the controller is active, the compliance is equal to the compliance of the passive system, which normally is quite low.

Furthermore, in solutions that are based on inertial control, such as in one preferred embodiment of the present invention, the problem of 'tilt-to- horizontal-coupling' occurs. If the table tilts the gravity force acting on the reference mass will cause it to move (see Figure 6) .Theoretically it is impossible for the sensor to distinguish this effect from acceleration. This means that the controller will misinterpret tilt as a horizontal motion, and will try to correct for it by accelerating the table in opposite direction. At low frequencies the tilting effect will become dominant over actual horizontal motion. The tilting effect will therefore limit the performance of the controller below the natural frequency of the sensor.

In order to be effective for reduction of vibrations at low frequencies, the tilt-to-horizontal-coupling problem needs to be solved.

The object of the invention is to solve these problems associated with vibration isolation by way of inertial control.

SUMMARY OF THE INVENTION

The object of the invention is achieved by an active vibration isolation system and by a method for vibration isolation as claimed in the appended independent apparatus and method claims. Preferred embodiments and developments of the invention are specified in the respective dependent claims.

An active vibration isolation system is accordingly provided which comprises at least one mass to be isolated, which is mounted on vibration isolation systems, in particular air bearings.

The sensors in the active vibration isolation system of apparatus claim 1 may be configured such that at least one of the sensors is fixed to the intermediate mass element in all directions except the tilting direction, and at least one of said sensors is attached to the supporting surface with a fixation that is rigid only in the tilting direction. By configuring the sensors in that way, the tilt-to-horizontal-coupling problem referred to above can be solved.

The invention is furthermore based on the object of providing a system and a method for vibration isolation in which the isolation effect is further improved.

In particular, the object of the invention is to provide an active vibration isolation to reduce structural vibration of the mass to be isolated (Sensor Tilt), reducing of said sensor noise by measuring of the position of said mass instead of velocity or acceleration, in particular natural-frequency vibration, can also be taken into account such that it is even possible to reduce vibration caused by the structure of the supported mass by using sensor fusion of at least 2 sensors instead of extreme optimization of sensor dynamics.

The first eigenfrequency of the sensor needs to be low (about 2 [Hz]) to enable sufficient movement of the reference mass to be measured correctly. However, the 2nd and higher eigenfrequencies will induce motion of reference mass in unwanted directions. This movement is detected by the sensor and will interfere with the feedback motion controller used for active isolation and can cause instability of the motion controller. Normally this limits the maximum controller bandwidth to about ¼ of the higher eigenfrequency of the sensor. Because of this, the mechanical design of the sensor is optimized to maximize second and higher modes. However, in practice it is extremely difficult to realize a second mode more than 100 times higher than the first mode. For a sensor with a first eigenfrequency at 2 [Hz], the second eigenfrequency can be optimized to about 160 to 200 [Hz], which limits the controller bandwidth, and therefore the effective range of the active vibration isolation system, to 40 [Hz] .

To enable higher controller bandwidths, and therefore lower compliance and transmissibility over a greater frequency range, the system according to the invention includes sensor fusion. In this concept, a second sensor (position, velocity or acceleration) is aligned with the position sensor. This second sensor corresponds with a reference mass at a much higher frequency. For low frequencies the controller uses information from the position based motion sensor, for higher frequencies the information from the second sensor is used. This results in a good resolution in a very large frequency band without problematic higher sensor modes.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an illustration showing the basic vibration problem.

Fig.2 shows a piezo-based active vibration isolation according to prior art.

Fig. 3 shows an active vibration isolation based on separate reference mass according to prior art.

Fig. 4 shows a prior art example of an active vibration isolation based on inertial control.

Fig. 5 shows a 1- dimensional representation of the basics of an embodiment of the invention. Fig. 6 is an illustration of the tilt-to-horizontal-coupling problem in inertial control systems using geophones or other reference mass based motion sensors.

Fig. 7 shows a solution for the tilt-to-horizontal-coupling provided by an embodiment of the invention.

Fig. 8 illustrates another embodiment of the invention

DETAILED DESCRIPTION Examples of embodiments of the invention are now described with reference to the figures.

The vibration isolation technology applied in the invention is an enhanced version of the inertial control concept (Fig. 4). In existing inertial control systems the motion of the table (1) is measured with geophones or accelerometers. In the present concept, motion of the table (1) is determined by measuring position changes between the table (1) and a reference mass (9) that is suspended on the table (1). This technology yields a significant advantage.

By measuring position instead of velocity, the noise levels and resolution of the motion sensors (4) at low frequencies are greatly improved. This decreases the lower boundary of the frequency range in which the controller (7) is active to about 0.2 [Hz] compared to around 1 [Hz] for Inertial Control systems based upon velocity sensors. As a result the vibration isolation system is capable of a vibration reduction of -30 dB at 1 [Hz], in six Degrees Of Freedom (DOF).

Also, the low frequency band where the system is sensitive to disturbance forces is minimized to small band around 0.2 [Hz], which greatly reduces the impact of the major disadvantage of Inertial Control systems described above. Figure 5 shows a 1- dimensional representation of the basics of an embodiment of the invention. The technology can be used for isolation in 3 or 6 directions. The figure shows the passive vibration isolation (10), provided by weak springs supporting the table (1). These can be air mounts or mechanical springs. The supports have very low damping «1 %.

In order to be effective for reduction of vibrations at low frequencies, the tilt-to-horizontal-coupling problem illustrated in Fig. 6 needs to be solved. The embodiment of the invention shown in Fig. 7 solves that problem. The sensor, including the reference mass, is fixed to the table in all directions except the tilting rotation. The sensor is attached to the floor with a fixation that is rigid only in the tilting direction and very compliant for the other five degrees of freedom.

In an embodiment of the invention according to Fig. 8, a position sensor (4) with low 1st eigenfrequency is used for the lower frequency range and a 2nd sensor (5) with a higher 1st eigenfrequency is used for the higher frequency range. By using a sensor fusion technique, reduction of vibration is obtained over a large frequency range.

For the purposes of the invention, an active vibration isolation system is any vibration isolation system in which regular or irregular position changes of the mass to be isolated are actively counteracted via actuators, irrespective of whether the actuators that are provided for this purpose act on the mass to be isolated or on the bearings of the vibration isolation system. A mass to be isolated also includes a holding device for components or

assemblies, for example a vibration- isolated table on which components and assemblies can be arranged such that they are isolated from vibration.

The active vibration isolation system comprises at least two sensors in order to detect position changes, in particular vibration of the mass to be isolated. Said sensors need not necessarily detect movements in all spatial directions. Depending on the purpose, it is possible to use sensors which detect movements in two, four or six degrees of freedom.

Any position change, in particular vibration of the mass to be isolated, is counteracted via an actuator which, in one preferred embodiment of the invention, is arranged in or on the bearings of the vibration isolation system. The actuator or actuators is or are in this case driven by a control device which evaluates the signals from the sensors and uses them to calculate correction signals for driving the actuators.

The sensor or sensors for detection of the position changes of the mass to be isolated are in this case preferably located close to the bearings in order not to be influenced by structural vibration of the mass to be isolated, in particular of the table of a vibration isolation system.

The inventors have found that at least one further sensor which is preferably arranged on the mass to be isolated or detects vibration of the mass to be isolated detects structural vibration, includes this in the calculations for the control system, thus making it possible to take this into account. The additional sensor is in this case preferably arranged at a point at a distance from the bearings.

It is therefore possible by a comparison of the sensor or of the sensors, which is or are located in the vicinity of the bearings to determine which movement components are caused by structural vibration of the mass to be isolated.

For the purpose of the invention, structural vibration is any vibration or movement component which results from the isolated mass not being an ideal rigid body.

The further sensor is in this case preferably located at least 10 cm, and particularly preferably at least 15 cm, from a bearing. The vibration isolation system preferably comprises a holder for supporting an object to be isolated, in particular a table. Structural vibration of the holder or of the table can be detected via the at least one further sensor.

Alternatively or in combination, a sensor is provided which detects structural vibration of the object to be isolated. It is therefore possible to take account of structural vibration of the entire mass to be isolated.

In one preferred embodiment of the invention, a compensation signal, which represents the structural vibration of the mass to be isolated, is calculated via the control device which drives the actuator or actuators, and is added to the other compensation signals for active vibration isolation.

Depending on the complexity of the system and the nature of the information about the various degrees of freedom of the vibration it is possible, as provided in one development of the invention, to include an additional transformation in the calculation, in which even further system conditions, in particular the further compensation signals for active vibration isolation, are taken into account for control of the actuators.

Sensors which operate without making contact are preferably used as sensors, in particular capacitive, inductive and/or optical sensors.

Depending on the purpose, the further sensor is also designed to detect structural vibration, in order to detect position changes in at least two, preferably four and particularly preferably six degrees of freedom.

The detection of structural vibration is preferably optimized by the further sensor being as far away as possible from the bearings of the vibration isolation system. For this purpose, it is preferably arranged symmetrically with respect to opposite bearing pairs, and essentially at the same distance from them.

The invention makes it possible to provide an active vibration isolation system which has damping of more than 3 dB, preferably of more than 5 dB and particularly preferably of more than 10 dB even at a low excitation frequency of 5 Hz. Damping of more than 10 dB, preferably of more than 20 dB and particularly preferably of more than 25 dB is possible even at an excitation frequency of 15 Hz.

At the same time, the active vibration isolation system according to the invention allows the provision of systems with a very high load capacity, in particular a load capacity of more than 1000 N. preferably of more than 5000 N. and particularly preferably of more than 10 000 N.

In one preferred embodiment of the invention, in order to avoid the introduction of further disturbance vibration, actuators act on the mass to be isolated without making contact. Electrostatic or magnetic actuators are provided, in particular, for this purpose.

Furthermore, the invention covers a method for vibration isolation in which a mass to be isolated is mounted on vibration isolation systems and position changes, in particular vibration of the mass to be isolated, are detected via at least one sensor and any position change and/or vibration of the mass to be isolated is counteracted via at least one actuator.

Structural vibration of the mass to be isolated is detected via a further sensor, and is taken into account in the drive for the at least one actuator. In this case, each bearing preferably has one associated actuator. The detection process preferably covers not only vibration of the holder but also structural vibration of the object to be isolated, in particular of the components and assemblies arranged on the vibration isolation system.

The invention provides a platform for vibration-isolated mounting, which is equipped with a system for vibration isolation. One particularly advantageous feature of the system is that no further hardware need be implemented in the system for the actuators. The system can therefore also easily be retrofitted to existing vibration isolation installations.

Various embodiments and features of the invention described above may be combined to benefit from the combined advantages.

Claims

1. An active vibration isolation system comprising:

an intermediate mass element placed in a space between a position for an object to be isolated from vibration and a supporting surface;

a first elastic member having one end fixed to the intermediate mass element and the other end to be fixed to the object and to exert spring action on the intermediate mass element and the object;

a solid element placed between the intermediate mass element and the supporting surface and, with respect to an acting direction along the length thereof the length varying with a variation in voltage or magnetic field generated in the solid element, having one end fixed to the supporting surface or having the other end fixed to the intermediate mass element;

a second elastic member placed between the intermediate mass element and the solid element or between the solid element and the supporting surface and exerting spring action on the intermediate mass element, directly or indirectly through the solid element;

a first sensor, fixed relative to the supporting surface, for detecting a vibratory state of the object with respect to the acting direction;

a second sensor, fixed relative to the supporting surface, for detecting a vibratory state of the intermediate mass element with respect to the acting direction;

a controller connected to, for receiving signal input from, the first sensor and the second sensor, and for outputting to the power input portion a signal which causes the variation in voltage or magnetic field such that the intermediate mass element vibrates to cancel out the vibrations of the object in the acting direction; and

means for sensor fusion.

2. An active vibration isolation system as claimed in claim 1

characterized in that at least one of said first and second sensor has a higher 1st eigenfrequency than said other sensor.

3. An active vibration isolation system as claimed in claim 1 or 2, wherein each of said first and second sensors is an acceleration sensor or a velocity sensor.

4. An active vibration isolation system as claimed in claim 1 to 3, characterized in that said sensors measure position of a reference mass to reduce sensor noise.

5. An active vibration isolation system as claimed in any claim, wherein the solid element is formed of a piezoelectric element of which the length varies with the variation in voltage.

6. An active vibration isolation system as claimed in claim 1, wherein the solid element is formed of a magneto- striction element of which the length varies with the variation in magnetic field.

7. An active vibration isolation system as claimed in claim 6, wherein each of the first and second sensors is an acceleration sensor or a velocity sensor.

8. An active vibration isolation system as claimed in claim 1-7, the controller including a main feedback system by which a signal from the first sensor is fed back to the power input portion through a first controlled element and a second controlled element, and a local feedback system by which a signal from the second sensor is fed back to the power input portion through the second controlled element, and wherein the local feedback system causes the solid element to generate an operating force which cancels out the vibrations of the intermediate mass element and the main feedback system causes the solid element to generate an operating force which causes the intermediate mass element to vibrate so as to cancel out the vibrations of the object in the acting direction.

9. An active vibration isolation system as claimed in claim 1-8

characterized in that at least one of said sensors includes the reference mass.

10. An Arrangement for vibration isolation systems as claimed in any of said claims.

11. Method for reducing transmission of vibration said method comprising:

an intermediate mass element placed in a space between an object to be isolated from vibration and a supporting surface;

a first elastic member having one end fixed to the intermediate mass element and the other end fixed to the object and exerting spring action on the intermediate mass element and the object;

at least one of said sensors is fixed to the intermediate mass element in all directions except the tilting direction;

at least one of said sensors is attached to the floor with a fixation that is rigid only in the tilting direction; a controller connected to, for receiving signal input from, the first sensor and the second sensor, and for outputting to the power input portion a signal which causes the variation in voltage or magnetic field such that the intermediate mass element vibrates to cancel out the vibrations of the object in the acting direction; and

means for sensor fusion providing sensor fusion.

12. Method for reducing transmission of vibration of a first entity to a second entity, said method comprising:

providing a spring assembly which includes resilient member, an upper securement member and a lower securement member;

engaging with said spring assembly a feedback loop system, said engaging including:

establishing at least one collocation of a sensor with a corresponding vibratory actuator so that said sensor and said corresponding said vibratory actuator are each coupled with said lower securement member at

approximately the same location, and so that said sensor senses and said corresponding said vibratory actuator actuates in approximately the same direction and in approximately the same locality of said lower securement member;

connecting each said sensor and each said vibratory actuator with a processor/controller so that, for each said collocation, said sensor generates a sensor signal representative of the vibration of said locality, said processor- controller generates a control signal representative of said sensor signal, and said vibratory actuator generates a vibratory force representative of said control signal; and

providing power for said feedback loop system; and

mounting said first entity with respect to said second entity, said mounting including fastening said first entity with respect to said upper securement member and fastening said second entity with respect to said lower securement member; whereby, in series, said spring assembly effects passive reduction of said vibration at a first plurality of frequencies, then said feedback loop system effects active reduction of said vibration at a second plurality of frequencies, wherein at least one frequency among said second plurality of frequencies is not among said first plurality of frequencies.

13. Method for reducing transmission of vibration as in claim 12, wherein said establishing includes establishing at least two said collocations, wherein said feedback loop system includes at least two feedback loop subsystems, and wherein said connecting includes corresponding each said collocation to a separate said feedback loop subsystem