Brakes – Inertia of damping mass dissipates motion – Resiliently supported damping mass
Reexamination Certificate
2003-02-14
2004-10-26
Rodriguez, Pam (Department: 3683)
Brakes
Inertia of damping mass dissipates motion
Resiliently supported damping mass
C188S266100, C267S064280
Reexamination Certificate
active
06808051
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a vibration isolation and cancellation system and method, especially but not exclusively suited for high precision wafer-chip production and inspection equipment.
BACKGROUND ART
Products that make use of active vibration isolation and cancellation technology are available commercially, but their degree of effectiveness leaves room for improvement. Active vibration isolation and cancellation technology (also known as air mount technology) for IC production and inspection equipment needs to become more effective with the advancement of the production of chips that require ever-smaller features. Typically, actuators and sensors in an active isolation/cancellation system are not integrated. For instance, U.S. Pat. No. 6,286,644 to Wakui discloses and describes an active vibration isolator wherein in
FIG. 7
sensors ‘P
0
’ and air spring actuators ‘AS’ are separate elements.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an active vibration isolation/cancellation system that integrates an actuator and a sensor. The system provides a much improved vibration cancellation behavior. The system is useful for, amongst others, virtual all types of high precision production equipment, e.g., in an air-mount for chip production equipment, high precision microscopes and other high precision equipment. The application of the system, for instance, reduces an important barrier in the quest for chips with smaller features. The invention is based on a notion that an actuator and a sensor in an active vibration isolation/cancellation system can be integrated in such a way that many known performance limits (such as described by gain and phase relations from well known control theory) are removed to a degree where they are no longer a performance limitation.
It is another object of the invention to apply the proposed active vibration isolation/cancellation system in an absolute damper. By combining damper technology with air mount technology, a much improved air mount performance is achieved. The absolute type of (motion) damper of the invention typically but not exclusively comprises two parts that are displaceable relative to one another, at least in the direction to be damped. The first part of this type of damper, that may comprise an actuator coil and a complete sensor assembly, is connected to a first body (typically a mass to be damped). In case the sensor of this type of sensor is of a magnet and coil type, one part, e.g. the sensor-coil, is connected to the first body and the other part, e.g., the sensor-magnet of the invention, may be loosely connected to the first body with respect to motion that needs to be damped. The inventor proposes to attach the sensor-magnet to a reference mass that is supposedly not negatively affected by vibrations from other bodies. The reference mass can be realized by a floating mass (loosely connected to the first body). This floating mass acts in an absolute damper as an absolute reference. The second part of this type of damper, that may comprise an actuator magnet, is stiffly connected to a second body (typically a floor or reference without motion). The second body is only required with respect to the operation of the damper for providing a reaction force as reaction of the action force on the first body by the actuator. It is also possible to use an additional body in which to store the reaction forces. This will give at low frequencies a lower level of performance, but can be preferable in certain cases. In the descriptions above and below, the locations of coils and magnets could be exchanged without affecting the general idea of this invention. Whether or not to swap the locations of the coils and magnets is a discussion that depends on many factors and might change from one implementation to another. Typically however, it is preferred to mount the component with the lowest inertia to the body whose motion is to be damped or controlled
Various aspects of the invention are however also applicable for a relative damper and therefore not limited to the absolute damper. The relative type of (motion) damper typically comprises two parts that are displaceable relative to one another, at least in the direction to be damped. The first part of this type of damper, that may comprise an actuator coil and a sensor coil, is stiffly connected to a first body. The second part of this type of damper, that may comprise an actuator magnet and a sensor magnet, is stiffly connected to a second body.
In a preferred embodiment a Lorenz type coil as a sensor and another one as a actuator is used due to their close-to-ideal performance. By using a specific coil design a potential cross talk between the coils of the sensor and the actuator can be minimized to a level where it can be discarded. A damper provides an opposing force to velocity. Since Lorenz type of coils (also called voice coils) can sense velocities and can provide forces, they are appealing candidates as sensors and actuators. In practical implementations (such as in an active air mount) they are, for that and other reasons, frequently used. Other types of sensors and actuators can also be used. They might however require signal conditioning or other operations to make them applicable. An example of another type of sensor is a laser interferometer.
An important aspect of the invention lies in the observation that the sensor and the actuator of are preferably mounted in such a way that the combination (that is part of the damper) possesses certain relevant properties. One of the relevant properties is that the travel time for a mechanical signal caused by the actuator to the sensor is small. After the actuator induces a mechanical movement the sensor measures the mechanical movement. The sensor and the actuator combination of the invention have an acoustic delay, in a preferred embodiment, of far less than one millisecond (typically faster than 40 microseconds). Also the mass in the direct path between the sensor and actuator should be minimized while the stiffness should be maximized. All these prescriptions can be achieved by placing the sensor and actuator substantially close to each other. Having a limited travel time for the mechanical signal, without any substantial cross talk (between the actuator and the sensor) allows the damper to have a high gain feedback control loop without having any instability. Moreover the damper of the invention preferably, although not exclusively, has an electrical delay (that is between the sensor and the actuator) of less than one microsecond. That means that on detection by the sensor of a signal caused by a mechanical movement, a quick reaction is possible (that is commanding the actuator to generate a force).
It is yet another object of the invention to minimize crosstalk and interferences, in particular between the actuator and the sensor. The inventor found that crosstalk is reduced, amongst others, by using a magnetical type of actuator and a non-magnetical type of sensor. An example of a non-magnetical type of sensor is an optical one. An additional novel manner to achieve a minimum of unwanted cross talk is to place an actuator-coil and a sensor-coil perpendicular relative to each other. By doing so a magnetical field induced by the actuator-coil of the actuator will cause a minimum of induced current in a sensor-coil The sensor coil does not generate a field, so the cross talk concerns only in one way.
By having two instead of one sensor coil, and by using magnets of opposite polarityin the reference body, two sensors are made that give opposite signals when a motion is present, but they give an equal signal when an electrical or magnetical disturbance is present. By subtracting the two sensor signals, the measurement of the motion is amplified, and all common disturbances are cancelled. An equally effective method is to use identical magnet arrangements and an opposite coil winding direction.
The inventor found another way to reduce unwanted cross talk by using a shielding between the
Fortin Kevin
Koninklijke Philips Electronics , N.V.
Rodriguez Pam
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