Abstract:
An isolation system includes a housing that contains an electromagnet, a sensor and a controller. The housing is mounted underneath a table top of an isolation table (or other structure requiring isolation from a second body), and target elements, such as permanent magnets, are associated with the frame of the table in proximity to the housing. The sensor provides an input signal representative of motion of the table to the controller, which generates a control signal for the electromagnet to produce a force acting on the target element. The housing can be attached to the table top with magnets, which facilitate installation. The housing provides a unit that is simple to install, is unobtrusive, and advantageous for retrofitting existing isolation tables.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     The invention is directed to devices for isolating one body from vibration transmitted by another body. In particular, the invention provides a low cost, self-contained, active isolation system that can be readily mounted to an isolation table, or similar structure, to control vibrational disturbances. 
     Electromagnet actuators are especially well suited for generating forces in situations where the relative displacements are low. Electromagnets are low cost and inexpensive device to operate, which is an additional advantage. Known systems using electromagnets in vibration isolation tables are unduly complicated and lack ease of installation or adjustment. Further, known systems are not readily adaptable for retrofit into an existing isolation table or other structure. 
     The invention overcomes the deficiencies in the art to provide an isolation system having at least one electromagnet actuator to generate force for canceling vibration in an isolated body (e.g., the table top) transmitted by a supporting body (e.g., the table frame or legs). According to the invention, the system comprises a device having an electromagnet, a motion sensor, and a controller all contained within a single housing that is easily mounted to the isolated body. One or more of the devices can be mounted to the isolated body depending on what modes of vibration are of concern. 
     Housing the components in a single container or housing facilitates mounting the device to an isolation table, and reduces installation cost and complexity. The housing makes the devices less conspicuous and less intrusive for the user. According to the invention, the housing can be mounted to the table top (which are typically made of a ferrous metal) with permanent magnets, which provides instant attachment and the ability to easily remove and reposition the housing, if desired. Other attachment means, such as adhesives or screw fasteners, could be used as alternatives. 
     The invention is advantageously installed on an isolation table to act in parallel with a spring system supporting the table on a frame or legs. Such spring systems are typically pneumatic, but the invention can work with other types of spring supports. 
     According to a preferred embodiment, the sensor is an accelerometer which produces a signal representative of acceleration in the table top. The signal is processed by the controller, which generates a control signal for the electromagnet to cause it to produce a force for canceling the vibration. A preferred control mode is to integrate the acceleration signal to obtain a velocity, and then to feedback a sum of the proportioned acceleration and velocity. Alternatively, another motion sensor could be used with appropriate control. For example, a geophone, velocimeter, or a displacement sensor could be used to sense motion of the table top. 
     According to one aspect of the invention, a target element is included for the electromagnet to act against with a vibration-canceling force. According to a preferred embodiment, the target element is a permanent magnet. The target element is attached to the supporting frame or legs and the housing is mounted to the table top in proximity to the magnet so that a gap is established between the electromagnet and permanent magnet. The permanent magnet produces a bias that permits the electromagnet to generate a push or a pull force on the permanent magnet, making the vibration canceling forces more effective. Isolation tables typically have horizontal support rails near the table top. For vertical vibration, the target element is conveniently mounted to the rail. For systems without rails between the legs, brackets can be mounted on the legs to provide for a target element. For horizontal vibration, the target element can be mounted to a table leg or bracketry. The housing is appropriately oriented so that the electromagnet is positioned to generate a force acting on the target element across a gap. 
     According to the invention, the target element alternatively may be a magnetically sensitive element, such as a ferrous metallic member, for which the electromagnet will produce an attractive force. Accordingly, a bias can be applied to the electromagnet by applying a DC input, or by installing a permanent magnet in the electromagnet, so that both attractive and repulsive forces can be generated for effective vibration canceling. 
     According to another aspect of the invention, the controller includes input and output signal processing. The input processing includes means for trimming any DC bias that might be present thereby adjusting for any DC drift that may result from processing of the input signal. Moreover, at least one, and preferably both, the input and output signal processing may include active gain switching means for switching between a first low gain value to a second higher gain value, preferably by means of a programmable gain amplifier. This provides an adequate strength signal to the control method and/or the actuator for a large range of transmitted vibration levels. Furthermore, one or both of the input and output processing may include means for monitoring the signals for system health monitoring. 
     The devices according to the invention can include a single electromagnet for single direction vibration control. Alternatively, the device housing can contain two electromagnets relatively oriented for damping in different directions. Sensors are included in the housing for measuring movement in each direction. In addition, target elements are associated with each electromagnet and may, when separate elements, be attached to the table frame at appropriate locations. 
     A table top is subject to various vibrational disturbances that can cause different movements of the table top, including simple linear vertical vibration, roll movement, pitch movement, longitudinal and lateral horizontal movement, and yaw movement. A system according to the invention includes an isolation table provided with a plurality of devices for controlling these different modes of vibration. 
     According to one embodiment, a table top is provided with three damping devices for controlling all of the vertical based movements: linear vertical, pitch, and roll vibrations. The devices each include a single electromagnet and associated target permanent magnet. One device is positioned at each of the front two legs of the table, and one device is positioned along the back edge of the table top midway between the back legs. The devices act independently at the various locations and produce damping forces to control the three mentioned vibration movements. The relative spacing and positioning of the active damping units about the table top optimizes the control forces required. 
     A three-device system advantageously provides vertical vibration control with a minimum number of devices. Alternatively, a table can be provided with four damping devices, one positioned at each table leg, for more robust damping control, i.e., less control force required for each device. 
     An isolation table can be provided with two devices for control of the two horizontal modes of vibration. The devices are mounted to the table top at adjacent legs with the target elements mounted to, or integral with, each of the legs. Preferably, to position the devices as unobtrusively as possible, the devices are positioned at the back legs. The devices are appropriately aligned to produce a diagonal force vector which passes through the Center of Gravity (CG) of the table. In this way, the longitudinal and lateral movements can be controlled. 
     According to another embodiment, the devices each include vertically and horizontally oriented electromagnets contained in the single housing, and an isolation table is configured for control of both vertical and horizontal vibration in all six modes. Advantageously, control of all six modes can be achieved with three devices mounted to the table, one each at the front legs and a third at the back edge midway between the back legs. Target elements are positioned on each of the front legs and on the support rails adjacent the front legs. Additionally, a target element is positioned on the rear support rail for vertical control, and a bracket mounted to the rear rail extends vertically upward to support an additional target element for horizontal control by the rear positioned device. 
     Other configurations are also possible, for example, using multiple damping devices for more robust vibration control. In addition, if a particular form of vibration or movement is a concern, for example, because of the equipment carried on the isolation table, the devices can be positioned accordingly. As one example, for a robot oriented lengthwise on an isolation table, multiple devices can be mounted to the table and oriented for producing horizontal canceling force directed in the longitudinal direction. 
     Another advantage of the system according to the invention is that having all of the active components in a single housing permits the simple retrofit of existing isolation tables or other structures. In addition, when magnet mountings are used, the damping in an isolation table can be easily adjusted, or even adapted temporarily for a particular use, by adding devices to the table or changing the position or orientation of the devices on the table. 
     The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become better understood by reference to the following description in conjunction with the appended drawings, in which: 
     FIG. 1 is a side view of a damping device for single mode damping in accordance with the invention mounted to an isolation table; 
     FIG. 2 a  is a schematic diagram of an exemplary feedback control loop for the damping devices according to the invention; 
     FIG. 2 b  is a schematic diagram of another embodiment of feedback control loop for the damping devices according to the invention; 
     FIG. 2 c  is a schematic diagram of yet another embodiment of feedback control loop for the damping devices according to the invention; 
     FIG. 3 is a side view of a damping device according to another embodiment mounted to an isolation table; 
     FIG. 4 is a side view of a damping device mounted to an isolation table on which a support bracket is mounted to support a target element; 
     FIG. 5 is a side view of a damping device for two mode damping mounted to an isolation table; 
     FIG. 6 is a perspective view illustrating a system according to the invention including an isolation table including damping devices arranged for vertical damping; 
     FIG. 7 is a schematic diagram of the system of FIG. 6; 
     FIG. 8 is a schematic diagram of an alternative system for horizontal damping; 
     FIG. 9 is a schematic diagram of an alternative system for both vertical and horizontal damping; 
     FIG. 10 is a schematic diagram of an alternative embodiment of the system of FIG. 9; 
     FIG. 11 is a schematic diagram of a system for increased front to back vibration control; 
     FIG. 12 is a schematic diagram of a system for increased side to side vibration control; 
     FIG. 13 is a schematic diagram of an alternate embodiment of the system shown in FIG. 7; 
     FIG. 14 is a schematic diagram of a system with minimal bracketry; 
     FIGS. 15 a  and  15   b  are block diagrams illustrating the components of the device and the controller therefor; an 
     FIGS. 16 and 17 are block diagrams illustrating the components of the input and output processing. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows an isolation table  10  including a damping device  20  in accordance with a first embodiment of the invention. As will become apparent to those skilled in the art, the device according to the invention isolates a second member from vibration transmitted from a first member by producing forces to cancel the vibration in the second member. The invention is particularly suited for isolation tables, as is described herein, but has application to other apparatuses, for example, engine mounts, rotating equipment, and turbomachinery, and the description is meant to be illustrative rather than limiting. 
     The isolation table  10  includes a table top  12  supported by a frame  11  having a plurality of legs  14  (only one leg is shown in FIG.  1 ). Adjacent legs of the table frame  11  are preferably interconnected by horizontal rails  18  to provide rigidity to the frame. The rail is usually omitted from the front edge of the table  10  to provide room for an operator&#39;s legs to extend under the table. A spring system  16  is mounted between the leg  14  and the table top  12  to isolate the table top from the legs. Typically, the spring system  16  is a pneumatic isolator with a low spring stiffness. A problem in undamped spring systems is an inability to handle vibration occurring at or near the resonance frequency of the system. Moreover, they may amplify low frequency vibrations. The present invention, used in parallel with a spring system, will cancel vibration at or around the resonance frequency and/or at low frequencies. 
     The damping device  20  in FIG. 1 includes a container  22  mounted to the underside of the table top  12 , and a target element  24  associated with and mounted to the frame  11  in proximity to the container and below the electromagnet  30 . The container  22  and target element  24  are thus easily installed and are positioned conveniently out of direct view and out of the way of an operator using the table. The container  22  houses the active components of the device, including a force-generating electromagnet  30 , as a self-contained unit. The container  22  is a rigid box that provides an interior space to house the active components, described below. The container can be formed as an aluminum casting, as a folded or stamped sheet metal box, or other suitable structure. 
     The active components of the damper device unit  20  include an electromagnet  30 , a motion sensor  32 , and a controller  34 . The electromagnet  30  includes a pole piece  27  and a coil  23  and is of the type manufactured by Magnetic Products, Inc. of Highland Mich. Specifically, model RE-12121 rated at maximum pull load of 35 lbf (156.8 N) was found to be particularly effective in the device  20 . The motion sensor  32  senses movement of the table top  12  and sends an input signal representative of the motion to the controller  34 . Preferably, the motion sensor is an accelerometer manufactured by Oceana Sensors Technologies, Inc. of Virginia Beach, Va. Model number A8000-01 with 1 Volt/g sensitivity was found to be particularly effective. The controller processes the input signal, and generates and sends a control signal to the electromagnet  30  to produce a magnetic force. A power cord  36  provides electric power to the active components in the container  22 . It should be understood that the arrangement of the components within the container is exemplary and the locations thereof may vary. 
     The container  22  can be mounted to the table top  12  by any suitable means, including, for example, mechanical fasteners (screws, bolts, or rivets), adhesives, and double-sided tapes. When the table top  12  is made of a ferrous metal, it is particularly convenient to mount the container  12  with one or more mounting magnets  26 , which permit both instant mounting without preparation of the table, and the ability to easily remove or change the position of the container  22 , if necessary. FIG. 1 illustrates the mounting magnets  26  outside the container  12 ; however, it is also possible to position the magnets  26  inside the container for a cleaner appearance. 
     The container  22  is mounted to the table top  12  with a gap  28  between the electromagnet  30  and the target element  24 , which in this case is mounted to the rail  18 . As seen in FIG. 1, the electromagnet  30  is positioned in opposition to the target element  24  across the gap  28 . The force produced by the electromagnet  30  acts on the target element  24  to counteract the disturbance movement sensed by the sensor  32 , and thus cancel vibration in the table top  12 . The width of the gap  28  should be as small as possible without the container  22  and target element  24  coming into contact for optimum force effectiveness. 
     The vibration disturbances transmitted from the floor to the table top  12  are typically on the order of 0.2 to 20 microns (0.000008 to 0.0008 inch) for isolation tables. Due to pneumatic table resonances, unacceptably high vibration levels are transmitted to the table top  12 . However, small movements of the table top require only a low force produced by the electromagnet for vibration damping. A force in the range of 0.5 to 1.0 lbf has been found to be effective when the devices are used with the Model VH, VW, and LW isolation tables manufactured by Newport Corporation, for example. In a system integrated with tables of this type, the gap  28  is preferably between about 0.05 inches (1.27 mm) and 0.45 inches (11.4 mm), and a gap of about 0.125 inches (3.18 mm) has been found to be quite effective and allows some sway space to avoid hard contact between the active damping units and the frame  11 . Preferably, the gap  28  is made as small as practicable whilst still accommodating static motions due to loss or shut off of pneumatic pressure to the air springs and/or transient loads applied to the table top  12 . The actuator  30  may protrude through the container  22  slightly, as shown in FIG. 4, in an effort to minimize the gap  28  between the actuator and target element  24 . Moreover, the table top  12  may be raised or lowered using its adjusting screws to set the appropriate gap  28 . 
     According to a preferred embodiment of the invention, the sensor  32  is a piezoelectric accelerometer to sense acceleration of the table top  12 . Alternatively, other motion sensing devices could be used, for example; a geophone, velocimeter, or other sensing device. The particular sensor selected will effect and require adjustments to the control algorithm, as will be understood by those skilled in the art. 
     For example, as illustrated in FIGS. 2 a  and  2   c,  a feedback control method based on the acceleration and velocity may be used. The acceleration (e.g. vertical) of the table top  12  is sensed by the sensor  32  and an input signal representing acceleration in line  33  is transmitted to the controller  34 . Within the controller, the input signal is amplified and conditioned in block  29   a  as will be described in more detail later herein. This highly amplified and conditioned signal is provided to an integrator  35  where an integration is performed to obtain a signal representative of velocity of the table. The representative velocity signal in line  41  and the representative acceleration signal in line  45  are then again scaled (amplified or attenuated) in first and second gain blocks  37 ,  38  to produce gain-adjusted and relatively weighted representative acceleration and velocity signals in lines  43  and  44 , respectively. The gains  37 ,  38  are selected based upon experience with the system and are preferably hard coded into the micro-controller. 
     A summation of the gain-adjusted signals  43 ,  44  is performed at summing junction  39 . Following the summation, the summed signal is again amplified and conditioned in block  29   b  thereby producing a control signal  40  which is provided to the electromagnet actuator  30 . The electromagnet  30  produces a control force F to cancel the movement of the table top  12 . Combined acceleration and velocity control provides better high frequency vibration control than velocity feedback alone (described with reference to FIG. 2 c ) and broadens the effective range of control. 
     The FIG. 2 b  embodiment differs from the FIG. 2 a  in that an additional compensator  31  is provided. The compensator  31  is a low frequency lead compensator which causes a phase lead at low frequencies. More specifically, the compensator provides a maximum of 15 degrees phase lead centered at 0.5 Hz. This compensator  31  improves performance of the device  20  by compensating for the phase lag caused by the sensor  32  and input signal conditioning and amplification  29   a.  As shown, the block diagram of FIG. 2 a  depicts vertical vibration control. However, it should be understood that the present invention and the control described herein are equally useful for controlling horizontal vibrations of the table 12. Moreover, when used in multiples, multiple modes may be controlled, as described later herein. The device  20  according to the invention is controlled for providing damping at the resonance frequency of the table system  10 , which is typically between about 1 to 4 Hz. Preferably, the device  20  acts at frequencies between about the 0.2 to 10 Hz range. The aforementioned control method used is very stable, such that any transient loads applied to the table top  12  will be effectively damped. 
     A simpler control method is shown in FIG. 2 c.  In this method, the acceleration of the table top  12  is sensed by the accelerometer sensor  32 . This signal representative of acceleration in line  33  is amplified and conditioned in block  29   a  and then integrated in integrator block  35  to produce a signal representative of velocity of the table top  12  in line  41 . This signal in line  41  is appropriately amplified or attenuated in gain block  38 . Again the gain value is determined based upon knowledge of, and experience with, the system. The gain multiplied signal  44  is then amplified and conditioned again in block  29   b  and then passed on as a control signal  40  to the electromagnet actuator  30 . Velocity feedback provides for good control of the resonances of the table top. Notably, if a velocimeter or geophone is used as the sensor, the integrator  35  would be deleted. 
     The damping device  20  in FIG. 1 is arranged to provide a vertically directed damping force; that is, the electromagnet  30  is oriented to act across the vertically-aligned gap  28  between the container  22  and the target element  24 . The device  20  may be positioned for damping horizontally directed vibration by simply positioning the target element  24  on a vertical surface so that a horizontally-aligned gap is provided, and orienting the container  22  so that the electromagnet  30  is in opposition to the target element  24  across the gap  28 . 
     In addition, according to a preferred embodiment, the target element  24  is a permanent magnet, which provides a continuous bias to the electromagnet  30 . The bias permits operation of the electromagnet  30  for generating both pushing and pulling forces relative to the table frame  18 , which facilitates canceling vibration movements. An additional advantage of a permanent magnet as the target element  24  is ability to instantly attach it to the table frame  11  when the frame is formed of a ferrous metal. 
     Alternatively, the target element  24  could be a magnetically sensitive mass, that is, a mass that the electromagnet force will attract or it could be a ferrous leg, rail or other portion of the frame  11 . In such an embodiment, a bias can be applied to the electromagnet by supplying a DC input. Of course, this is less desirable than using a permanent magnet because of the higher power requirements. 
     FIG. 3 illustrates an alternative embodiment in which a disc-shaped permanent magnet  25  is disposed in the electromagnet  31  to provide the bias. The target element  24  is a magnetically permeable pole piece that can magnetically react with the magnet and electromagnet force fields. Alternatively, if the rail  18  is formed of a material having sufficiently high magnetic permeability, the target element  24  can be eliminated, and the rail  18  can act as the target element. This alternative provides the advantage that all of the components of the device are contained in the container  22 , which further simplifies installation. 
     FIG. 4 illustrates a feature of the invention for isolation tables not having horizontal ails, or where the horizontal rail is not sufficiently close to the table top  12  to be suitable for holding the target element  24 . A bracket  42  is mounted to the leg  14  of the frame  11  to support the target element  24  in proximity to the table top  12 . The bracket  42  provides a surface on which the target element  24  is mounted. Preferably, the bracket  40  is mounted by mounting magnets  26 , similar to those used for mounting the container, which allow for ease of positioning and installation of the bracket. FIG. 4 also illustrates another alternative for the target element  24 , that is, a target element consisting of both a permanent magnet  25  mounted on a pole piece  27 . The combination of a permanent magnet with the pole piece increases the effective area on which the electromagnet field can act. 
     FIG. 5 illustrates an embodiment of the device  20   a  adapted for damping vibrations in both the vertical and horizontal directions. The container  22  includes two sensors  32 V,  32 H: a first  32 V oriented for sensing vertically directed vibration, and a second  32 H oriented for sensing horizontally directed vibration (arrows shown indicate sensing direction). Two electromagnets  30 V,  30 H are provided: a first  30 V positioned to produce a vertical force, and the second  30 H positioned for producing a horizontal force. Accordingly, two target elements  24 H,  24 L are mounted to the frame  11 , one on the table leg  14  to interact with the horizontal force electromagnet  30 H, and the second on the rail  18  to interact with the vertical force electromagnet  30 V. The single controller  34  is adapted to receive the input signals from the sensors  32 V,  32 H and to produce control signals for the two electromagnets  30 V,  30 H. Control for the individual directions is accomplished via the control method described with reference to FIG. 2 a.    
     FIG. 6 illustrates in perspective view an isolation table 10 including a reference coordinate frame to illustrate the types of vibration movements that could be experienced by the table top  12 . Disturbances transmitted to the table top  12  can result in simple vertical vibration along the Z axis, that is, simple up and down motion. If the vertical disturbance is unequal in amplitude or in time across the table top  12 , rocking motions may occur, for example, a pitch movement in the YZ plane, a roll movement in the XZ, or a motion combining pitch and roll. 
     In addition, a disturbance could result in vibration of the table top  12  in the horizontal or XY plane, including simple linear movement on the X or the Y axis, or a yaw movement in the XY plane. Various ways to mount the devices  20 ,  20   a  according to the system of the invention to cancel these disturbances are described below. 
     The most common are the vertical movements, the simple linear vertical and the pitch and roll vibrations. The system shown in FIGS. 6 and 7 is arranged to cancel any of these three vertically directed vibrations. For example, FIG. 7 shows a bottom view of the table 10. The table 10 includes three mutually-spaced, vertically-acting active damping unit devices  20  as described in FIGS. 1,  3  or  4 . One device  20  is mounted to the table top  12  adjacent to each of the two front legs  14   a,    14   b.  An accompanying target element (not shown), preferably a permanent magnet, is mounted to, or integral with, the side rails (not shown) in proximity to each of the damping devices  20 , as described in connection with FIGS. 1,  3  or  4 . A third damping device  20  is positioned midway between the two rear legs  14   c,    14   d  with a target element mounted adjacent on, or integral with, the rear rail (also not shown). The devices  20  act independently, that is, the controllers of the respective devices are not connected for coordinated action, and thus, each device  20  acts to cancel vibration that is sensed at its individual mounting location. The power cable  36  shown in the FIG. 7 is run in the manner shown to keep wiring from the power supply located on the floor the table to a minimum. This reduces the propensity for vibration to be imparted to the table 12 through multiple electrical cords. 
     Again referring to FIG. 7, it can be understood that the three devices  20  will act to cancel linear vertical vibration (Z axis movement-into and out of the paper). Pitch movement in the YZ plane is handled by cumulative action of the front damping devices  20  and the rear damping device  20  acting on opposite front and rear edges of the table top. Roll movement in the XZ plane is handled by cumulative action of the damping devices  20  at the opposite sides of the table at the front legs  14   a  and  14   b.  The illustrated arrangement requires a minimal number of damping devices to cancel vibration of three degrees of freedom. Of course, additional damping devices could be installed if more robust damping is required, for example, one damping device could be positioned at each leg. 
     FIG. 8 illustrates an arrangement useful for damping only the horizontal vibration disturbances, the X axis and Y axis linear vibrations. The damping devices  20  are positioned adjacent to each of the rear legs  14   c,    14   d,  and are oriented for providing a horizontal force directed against the respective leg. The damping devices are also oriented diagonally with respect to the axes, which allows the damping devices to act with a force vector through the center of gravity CG without creating a torque about the Z axis. The associated target elements (not shown) are mounted on, or integral with, the legs. Alternatively, additional devices could be mounted to the table top for increased damping or for controlling yaw motions. 
     According to another aspect of the invention, an isolation table 10 can be provided with damping devices  20   a  for damping in both the horizontal and vertical directions, and thus act to cancel all six modes of vibration movement in the table top. FIG. 9, for example, illustrates an embodiment using a minimum number of damping devices  20   a  to achieve damping of all six modes. The damping devices  20   a  each include two electromagnets mounted in the container  22  as described in connection with FIG.  5 . The damping devices  20   a  are placed adjacent to the front legs  14   a  and  14   b  and preferably midway between the rear legs  14   c  and  14   d.  Two target elements  24  are associated with each damping device  20   a.  At the front side, the target elements are positioned on, or integral with, the rails (not shown) for vertical damping and on the legs  14   a,    14   b  for horizontal damping along the X axis. The rear damping device  20   a  is mounted to the table top  12  with a first associated target element installed on, or integral with, the rear rail (not shown). A bracket  40  is mounted to the rail (not shown) and extends upward to support a second target element  24  for horizontal damping along the Y axis. 
     The arrangement in FIG. 9 relies on the two devices  20   a  at the front to cancel horizontal vibration in the Y axis and a single damping device  20   a  at the rear of the table to cancel horizontal vibration along the X axis. FIG. 10 illustrates an arrangement in which two damping devices  20   a  are positioned at the rear legs  14   c,    14   d  for improved damping along the X axis. Likewise, the front damping devices  20   a  are oriented for damping along the Y axis in conjunction with target elements included on, or integral with, the legs  14   a,    14   b.  Additional target elements installed on, or integral with, the rear and side frame rails (not shown) act with the front rear damping devices  20   a  to cancel vertical vibration along the Z axis. Thus, in the arrangement shown in FIG. 10, the two damping devices  20   a  at the front of the table act to cancel vertical and Y axis horizontal vibration, and the two damping devices at the rear of the table act on vertical and X axis vibrations. The cumulative action of the damping devices  20   a  acts on the pitch, roll and yaw movements, as previously explained as well as vibrations along the X, Y and Z axes. 
     FIGS. 11 and 12 illustrate arrangements that are useful where a particular form of horizontal vibration is of concern. FIG. 11 shows an arrangement for strong damping along the Y axis. The damping devices  20  are positioned so that the horizontally-oriented electromagnets act along the Y axis. This arrangement is useful for devices, such as robots, that are supported on the table top  12  across the width, and which accordingly produce strong vibrations along the Y axis. 
     FIG. 12 shows an arrangement useful for excess X axis vibration, for example, for a robot or similar apparatus supported across the length of the table top  12 . The damping devices  20  are accordingly positioned so that the horizontally-oriented electromagnets act along the X axis. In the arrangements shown in FIGS. 11 and 12, the damping devices  20  can also be equipped for both horizontal and vertical damping using the device illustrated in FIG.  5 . 
     FIGS. 13 and 14 shows several other arrangements useful for multi-mode vibration control. In FIG. 13, the units  20 ,  20   a  may be horizontally acting alone or multi-axis devices as shown in FIG.  5 . This FIG. 13 embodiment is an alternate embodiment to that shown in FIG.  7 . The arrangement in FIG. 13 can damp vibration along the X and Y axes as well as yaw about the Z axis. If the units are of the type described in FIG. 5, then vertical motion along the Z axis, pitch motion in the YZ plane and roll motion in the XZ plane may also be controlled. This arrangement is particularly well suited for controlling Y axis vibrations. Notably, when the system does not include vertical damping, the units preferably act directly upon the legs  14   a,    14   b  and  14   c  of the table 10, i.e., the target element is the respective leg. Alternatively, similar target elements as shown in FIGS. 1,  3 , or  4  may be mounted to the legs. FIG. 14 illustrates a system with a slightly different configuration as compared to FIG.  13 . In general, the system of FIG. 14 can damp X, Y, Z, pitch, roll and yaw motions, if the units  20   a  as shown in FIG. 5 are utilized. If units  20   a  are utilized, this embodiment eliminates the brackets required for the embodiment of FIG.  9 . This system is generally most effective at damping X axis vibration. 
     FIGS. 15 a-   17  illustrate details of the damping device  20 , focusing on the electronic hardware aspects. The sensor  32 , preferably the accelerometer described above, provides a signal representative of the acceleration. The current source  21  provides the current to operate sensor  32  thereby providing the input signal to the input conditioning and amplifying block  29   a.  The input conditioning and amplifying block  29   a  includes a signal conditioner  46  and may also include an optional Programmable Gain Amplifier  52   a.  The signal conditioner  46 , as best shown in FIG. 16, includes a gain and filtering block  48  which includes a high pass filter, a low pass filter, a capacitor (about 10 microfarad) and operational amplifier. Effectively, the gain and filtering block  48  provides an output signal which has been multiplied by a gain, most preferably a gain of five (5), and having signals outside of the desired operational range removed. In the case of the isolation table, it is desired to remove signals outside the range of between about 0.04 Hz and 90 Hz. This filtered and amplified signal is then provided to a gain block  50  which further multiplies the signal by an additional gain value. Preferably, for the isolation table, the additional gain value is set to about fifty (50). 
     The output of gain block  50  is preferably provided to means for actively switching between a first and second gain value. In this case, the active switching means is comprised of an optional Programmable Gain Amplifier (PGA)  52  which is actively switchable between gains of a low and a high value. By way of example, and not to be considered limiting, the low value is preferably about 3.52 and the high value is preferably between about 30-60, but may be as high as 250. The gain value selected is dependent upon the magnitude of the incoming signal to the input A/D  58   a  as a function of its total dynamic range. Preferably, the input D/A is integral within the micro-controller  54  and samples the signal provided from the optional input PGA  52   a  at about 1000 Hz. It is desirable to utilize the Programmable Gain Amplifier (PGA)  52  in cases where the magnitude of the signal present is unknown or changeable through broad ranges. If the average power of the signal is above a threshold, then the gain is set to the first setting, whereas, if the average power is below the threshold, then the gain is set to a second higher value. This ensures that a signal of appropriate signal strength is provided to the micro-controller  54 . A similar PGA is preferably provided on the output processing portion of the device  20  as has been described for the input processing portion and is switched according to the similar, but opposite, logic and at the same time, i.e., if input PGA  52   a  has gained up the signal, then output PGA  52   b  will gain it back down by preferably the same amount. Notably, if the PGA is not used, then the gain value of gain block  50  would need to be set higher, say about 176, such that a total system input gain of about 880 is achieved. Such high gains, of 750 or greater, are desired for achieving good system performance when subjected to base disturbances of between about 2-10 microns and when using a 8-bit microcontroller. 
     According to another aspect of the invention, means for adjusting the DC bias of the input signal is provided. As shown in FIG. 16, the microcontroller  54  monitors the nominal DC level of the signal provided at the input D/A  58   a  and outputs a value to the Active Trim Digital to Analog Converter (DAC)  65  which resultantly provides a DC voltage which either adds or subtracts from the DC voltage outputted from the gain and filtering block  48  at the junction  64 . This active trim function maintains the incoming signal voltage at a nominal value, for example at about 2.5 volts. This maintains the DC voltage centered within the voltage range thereby allowing maximum use of the dynamic range. 
     The input and output processing portions may also include monitoring means useful for detecting failures of the sensor  32  and/or actuator  30  within the device  20 . The input monitoring means comprises extracting the signal at junction  67  representative of the raw accelerometer signal. The signal is provided via line  68   a  to the monitoring A/D  70   a  in the micro-controller  54 . The signal is then tested to determine if it falls within a preset range. If it does, it is indicative of a properly operating sensor  32 . 
     Within the micro-controller  54 , which is preferably a Motorola 68HC705P6ACDW, a control method is executed, such as any of the methods described with reference to FIGS. 2 a-   2   c.  An output signal is provided from the control D/A  58   b  to the optional output PGA  52   b  at a sample frequency of about 200 Hz. The controller preferably includes multi-rate sampling as described in co-pending application U.S. Ser. No. 09/053,478 to Hamo et al. entitled “Dynamic System Controller” which is hereby incorporated by reference herein. The signal from the PGA  52   b  is then provided to a bridge amplifier  56  which includes multiple power amps  60   a,    60   b  positioned on either side of the electromagnet  30  and a current control block  62  for providing feedback control to ensure that the control signal to the amps  60   a,    60   b  is in actuality what is being commanded. The control signal from the PGA  52   b  drives power amp  60   a  directly. Contrarily, by way of resistive inverter  63  which inverts the control signal, power amp  60   b  is driven in the opposite direction thereby creating an alternating current scenario across the windings of the electromagnet actuator  30 . Preferably, feedback means are utilized to measure the current supplied through the power amps  60   a,    60   b  and then feedback a signal which is additive with the control signal to appropriately adjust same. Feedback signals proportional to the current supplied to each power amp  60   a,    60   b  are fed to the current control block  62  via feedback lines  61   a,    61   b.  These signals are then combined to produce an output at line  68   b.  This output is provided at junction  69  and is summed with the control signal thereat. This constantly corrects the current i through the electromagnet such that it is approximately equal to the controlled (commanded) current. The signal in  68   b  is also provided to the A/D converter  70   b  within the micro-controller  54  to provide the means for monitoring the current to the electromagnet  30 . If the voltage value present is within a predetermined tolerance of the control signal commanded, then the electromagnet  30  and bridge power amplifier  56  are operating properly. If the signal is unchanging, then it is indicative of a short or other circuit dysfunction. 
     In FIG. 15 b,  the operations taking place within the micro-controller  54  are illustrated. The filtered and amplified input signal is provided from the PGA  52   a  to the input A/D  58   a  which converts the analog signal to digital form at the sample rate aforementioned. A decimation filter  72  is provided which is a low pass filter, preferably first order, having a break frequency of less than 90 Hz, and more preferably about 40 Hz. An additional high pass filter  74  having a break frequency of about 0.125 Hz is provided for further rejecting low frequency noise. The signal is then down sampled in down sampling block  73  at 200 Hz by sampling approximately every 5 th  sample exiting from the decimation filter  72 . The output of the down sampling block  73  is then integrated in integrator  35  in a first path and multiplied by a first Gain  1  in block  38 . Similarly, the a second path is provided where the down sampled signal is multiplied by a second Gain  2  in block  37 . The signal in the first and second paths are then summed at summing junction  39  and then provided to the control D/A  58   b  for transmittal to the actuator  30 . It should be noted that the amplify and condition block  29   a  in FIGS. 2 a-   2   c  comprises the signal conditioning block  46 , the PGA  52   a,  the A/D  58   a,  decimation filter  72 , HP filter  74  and down sampling  73 . Likewise, the Amplify and condition block  29   b  comprises the control D/A  58   b,  the output PGA  52   b  and the bridge power amp  56 . The bridge power amp  56  includes the power amps  60   a,    60   b  and current control  62 . 
     The invention has been described in terms of preferred principles and structure; however, the particular examples given are meant to be illustrative and not limiting. Substitutions and equivalents as will occur to those skilled in the art are included within the scope of the invention as defined by the following claims. For example, the number and arrangement of the units may be changes to effectively damp the dominant motions in the system. In addition, although described with reference to an isolation table, the invention can be used for any structure for isolating one member from another, such as in engine or machine mounts structures.