1. Field of the Invention
The present invention relates to vibration isolators.
2. Field of the Invention
The present invention relates to vibration isolators.
It is sometimes desirable to prevent relative movement between two surfaces. For example, integrated circuits are typically fabricated on a platform with photolithographic equipment. The location of directed light used to align and fabricate the integrated circuits must be very accurate.
The table is typically placed on the floor of a clean room. The floor may undergo vibrational movement that can be transferred to the table. The vibration may cause a displacement of the table which reduces the accuracy of the fabrication process.
Some tables incorporate vibration isolators to reduce or prevent the floor vibration from being transferred to the table. U.S. Pat. No. 5,000,415 issued to Sandercock and assigned to the assignee of the present invention, Newport Corp., discloses a vibration isolator assembly that actively isolates a load from a floor. The active isolator assembly includes a plurality of piezoelectric actuators that can vary the distance between the load and the floor surface to compensate for movement in the floor. For example, the floor may oscillate so that the floor surface moves toward the load and away from the load. When the floor moves toward the load the piezoelectric actuators contract so that the motion of the load relative to inertial space is reduced compared to that of the floor. Likewise, when the floor moves away from the load the actuators expand.
The active vibration isolator disclosed in the Sandercock patent includes a sensor that senses the movement of the floor and circuitry to provide a control loop to synchronize the contraction/expansion of the actuators with the movement in the floor. Sandercock also discloses the use of sensors which sense the velocity of the load to provide a feedback loop that is coupled to the feedforward loop.
The piezoelectric actuators and control loops are capable of isolating the load for relatively low frequencies. To roll off high frequencies, Sandercock employs an elastomeric mount that is interposed between the load and the actuators. The elastomeric mount includes an elastomer located between a pair of support plates. The elastomeric mount has a resonant frequency that varies with the weight of the load. The variation in the resonant frequency requires a calibration of the system during installation, or a reconfiguration, to compensate for a different weight of the load. The calibration or reconfiguration adds to the complexity of installing the table. It would be desirable to provide an elastomeric mount which has a resonant frequency that is relatively constant for a predetermined range of load weights to reduce the complexity of designing and installing the table.
The platform load may be large enough to buckle the elastomer within the mount. A buckled elastomer will produce inadmissibly large displacements and stresses that may cause a failure of the material and/or loss of vibration isolation. It is therefore desirable to design an elastomeric mount that has a relatively constant resonant frequency and will not buckle within the load limits of the isolator. In an article by Eugene I. Rivin, Shaped Elastomeric Components for Vibration Control Devices, Sound and Vibration, July 1999, pp 18-23, the author discusses varying the profile of the elastomer in a passive vibration isolator to obtain a resonant frequency that is relatively constant for a range of loads. Having a design iteration that varies the profile of the elastomer can be relatively expensive. It would be desirable to provide a design technique for a passive vibration isolator that allows the designer to obtain desired characteristics without varying the profile of the elastomer.
One embodiment of the present invention is a passive vibration isolator that contains a resilient element supported by a first support and a second support. The resilient element reacts with one or more of the supports so that the isolator has a natural frequency versus load curve. The curve has a first portion with a varying natural frequency and a second portion with a relatively constant frequency. The second support may have a profile so that a contact area of the support is approximately constant in the first portion of the curve and varies in the second portion of the curve.