Active vibration attenuation

An apparatus and method are provided for attenuating vibration when driving an object. The apparatus includes a driving device for generating a driving force to drive the object and an attenuation actuator mounted in reciprocal relationship with the driving device for generating an attenuation force substantially concurrently with the generation of the driving force. The attenuation force is generated so as to oppose and at least partially cancel a reaction force exerted by the object in response to the driving force.

FIELD OF THE INVENTION

The invention relates to the attenuation or reduction of vibration caused when moving a mass with a motor due to a reaction force in response to the driving force. Such vibration attenuation is achievable by attempting to cancel or reduce the effects of such reaction force concurrently with the generation of a driving force to move the mass.

BACKGROUND AND PRIOR ART

According to Newton's third law of motion, for every action, there is an equal and opposite reaction. In practical application in an industrial machine, when a motor is activated to move a mass or object, the object exerts a reaction force on the motor equal to the force generated to the object by the motor. This reaction force acts on the motor as well as on parts connected to the motor. As a result, the reaction force received by the motor, if sufficiently large, causes vibration or shock to the machine.

Vibration and shock are undesirable and will affect machine performance, especially in the semiconductor industry where speed and precision are important. An example is when creating wire bonds to make electrical connections between electrical contacts on a semiconductor die and a leadframe using a wire bonder machine. A motor has to move an object, namely a bond head and attached ultrasonic transducer of the machine, between different predetermined electrical contacts on the die and the leadframe. To achieve higher throughput, the motor has to move the bond head at a high speed. However, reaction forces created by the driving movement acts on the motor and motor platform, causing vibration of the same. Thus, precision may be sacrificed at the expense of speed if such vibration is not attenuated. Conversely, to achieve higher precision, the bond head has to be driven at a lower speed.

A conventional solution is to separate a body of a driving motor from its platform or base. A vibration insulator is inserted between the motor body and its base. In this way, vibration forces from the motor body to the rest of the machine can be dampened by the vibration insulator. However, since the body of the motor is no longer rigidly fixed, there is a need for more critical control requirements and complicated control algorithms.

There have also been attempts in other industries to attempt to cancel vibrational forces. An example of this is described in U.S. Pat. No. 5,231,336 for “Actuator for Active Vibration Control”. An electromagnetic actuator is arranged so that a moving element undergoes reciprocal motion in response to an electrical input signal. The moving element is decoupled from any mechanical load in one embodiment so as to produce reaction forces which may be used to cancel vibrational forces. However, a drawback of the device described therein is that it requires a vibration sensor to sense the vibrations of a vibrating surface before the vibrations are processed by a processing unit to produce a suitable feedback. The processing unit generates a driving signal to electromagnetic coils to impart motion to an armature in such a manner as to produce reaction forces to reduce the sensed vibrations. This means that vibrations need to be sensed first, then measured, before a counteracting force is generated to reduce the vibrations. This solution is not sufficiently responsive where vibration forces are continually changing, and there is not sufficient time to sense an amount of vibration before seeking to counteract such vibration.

In the light of the above disadvantages, it would be desirable to develop a solution wherein the effects of vibrational forces are reduced in an efficient and substantially instantaneous manner while having a motor's main body fixed to the rest of the machine for stability.

SUMMARY OF THE INVENTION

It is therefore an objective of the invention to seek to provide an apparatus and method for vibration attenuation such that the effects of reaction forces caused by an object being moved are reduced substantially concurrently with the generation of such reaction or vibrational forces.

According to a first aspect of the invention, there is provided an apparatus for attenuating vibration when driving an object, comprising: driving means for generating a driving force to drive the object and an attenuation actuator mounted in reciprocal relationship with the driving means for generating an attenuation force substantially concurrently with the generation of the said driving force; wherein the attenuation force opposes and at least partially cancels a reaction force exerted by the object in response to the driving force.

According to a second aspect of the invention, there is provided a method for attenuating vibration when driving an object, comprising the steps of: generating a driving force to drive the object; and generating an attenuation force substantially concurrently with the generation of the driving force, such that the attenuation force opposes and at least partially cancels a reaction force exerted by the object in response to the driving force.

It will be convenient to hereinafter describe the invention in greater detail by reference to the accompanying drawings, which illustrate one embodiment of the invention. The particularity of the drawings and the related description is not to be understood as superseding the generality of the broad identification of the invention as defined by the claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is an isometric view of an X-Y table10of a wire bonder machine incorporating attenuation actuators24,26according to a preferred embodiment of the invention. For simplicity of illustration, the rest of the wire bonder machine is not illustrated. The X-Y table10comprises generally of an X-Y table base12, on which is mounted an X-Y stage14of the X-Y table. A wire bonder bond head16is mounted onto the X-Y stage14. An ultrasonic transducer18for ultrasonically forming wire bonds is mounted on one side of the bond head16. Movement of the X-Y stage14in the X direction or axis and Y direction or axis carries the bond head16and ultrasonic transducer18to bonding positions on a semiconductor die and leadframe (not shown), whereby wire bonding is performed.

An X motor20drives the X-Y stage14and bond head16in the X direction whereas a Y motor22drives X-Y stage14and bond head16in the Y direction. As the bond head16is driven in either direction by the respective motors20,22, it generates reaction forces in directions opposite to driving forces from the motors. Consequently, the reaction forces are transmitted through the respective motors20,22to the X-Y table base12, and the whole X-Y table starts to vibrate. Such vibration may also be transmitted to other parts of the wire bonder machine that are connected to the X-Y table10. The drawbacks of vibration on the wire bonding process have already been mentioned above. One way to reduce the effects of vibration is to strengthen the rigidity of the X-Y table base12, but this is not ideal. It would be desirable to seek to at least partially cancel the effects of the reaction forces to limit vibration of the X-Y table10.

According to the first embodiment of the invention, an X attenuation actuator24is attached to the X motor20and a Y attenuation actuator26is attached to the Y motor22. The function of the attenuation actuators24,26is to generate a force that opposes the reaction force acting on each of the respective motors20,22, that is, in a direction that is opposite to the reaction force. Ideally, the value of the attenuation force is substantially equal to the reaction force, but if that is not possible due to design requirements, the value of the attenuation force may be a portion of the reaction force instead. It is possible to generate an attenuation force that is substantially equal to the reaction force because the reaction force is equal in value to the driving force from the respective motors20,22, which is known or determinable. The attenuation actuator24,26, is thus designed to produce a force that is essentially equal to the driving force, but mechanically decoupled from the driving force.

FIG. 2is an isometric view of the X-Y table10incorporating attenuation actuators24,26according to another preferred embodiment of the invention. In this embodiment, the X attenuation motor24is attached to the X-Y table base12instead of to the X motor20. This layout may be more desirable or necessary if there is insufficient space next to the X motor20for attachment of the X attenuation motor24, or because of other design considerations. For comparison, the Y attenuation actuator26is still attached to the Y motor26. Mechanically, it is more desirable to locate the attenuation motor24,26in alignment with the driving force of the motors20,22, since a resulting turning moment may otherwise be present if the force of the attenuation actuator24and the reaction force are not aligned, as will be explained in more detail below. Nevertheless, the effect of the reaction force is still significantly reduced notwithstanding the non-alignment. This second embodiment illustrates that the attenuation actuator24,26may be attached at any position in relation to the X-Y table10, as long as it is capable of producing an attenuation force equal to and in an opposite direction to a reaction force created by a driving motion of the motors20,22, and this force can be transmitted to the components affected by the reaction force.

FIG. 3is a side view of the X-Y table10looking from direction A of FIG.2. This view exclusively illustrates the forces acting in the X direction, but forces acting in the Y direction follow a similar principle. A driving force F is generated by the X motor20, which creates a reaction force, F′. The X attenuation actuator24produces a driving force N, which in turn generates a reaction or cancellation force N′ in a direction opposite to that of the reaction force, F′. N′ is preferably equal and opposite to F′, therefore in essence serving to cancel the effect of the reaction force.

Referring toFIG. 3, the following is a more detailed description of the various forces. The driving force required to drive a mass, such as the bond head16, in the X direction is F. Therefore, the reaction force acting on the X motor20is F′. The attenuation force required to cancel the effect of the reaction force F′ is N′. Since the values of these forces are equivalent, F=F′=N=N′.

Assuming that the driving force N of the attenuation actuator is generated by a linear motor, N=SI, where S is the force constant of the attenuation actuator24,26, and I is the current flow through the linear motor of the attenuation actuator24,26. As the driving force, F, is known or can be determined, the attenuation force N′ can be generated accordingly by providing an appropriate current flow I through its motor.

Furthermore, a turning moment is created by the reaction force, which is equivalent to F′×L1. With the addition of the attenuation actuator24,26, the countering turning moment provided by the attenuation force is N′×L2. In the event, since F′=N′, the value of the resultant turning moment is F′×(L1−L2). The initial turning moment is hence reduced as compared to when there is no attenuation force. It would be appreciated that the smaller the difference between L1and L2, the lower would be the turning moment. The turning moment may thus be almost eliminated if the attenuation actuator24,26is attached to the respective driving motor20,22as in FIG.1.

FIG. 4is a cross-sectional view of an attenuation actuator24,26that may be used with the preferred embodiments of the invention. The actuator24,26has a mounting plate30for attaching the actuator24,26to a mounting surface, such as a driving motor20,22or an X-Y table base12. Preferably, the mounting plate30is attached to the surface using screws (not shown) although other means of attachment are possible. Extending from the mounting plate30is a shaft32with a cavity at its center to house a compression spring44and an extension spring46. A moving element34is connected by the compression spring44and extension spring46to the mounting plate30and shaft32. The compression spring44and extension spring46help to ensure that the moving element34is biased at an equilibrium position substantially at the center of the shaft32so as to have enough travel distance on either side of the shaft32depending on the direction of a force generated.

The moving element34substantially covers the shaft32, and is slidably located along the shaft32. Bushings36help to reduce friction due to the sliding motion. The moving element34further incorporates a magnet40whereas the shaft incorporates a coil42. The magnet40and coil42form an electromagnetic linear motor, wherein the magnet is a movable component. Other configurations are possible, such as a moving coil linear motor. When current is passed through the coil42within a field of magnetic flux generated by the magnet40, an electromagnetic force, N, is created which tends to drive the magnet40and moving element34in the direction of such force. A reaction force N′ is created that is equal and opposite to electromagnetic force N, and N′ will serve to counter or cancel the reaction force F′ due to the driving force F of a driving motor20,22(see FIG.3). After discontinuance of the electromagnetic force N, the moving element is moved back to its equilibrium position by the compression spring44and extension spring46.

The attenuation actuator24,26can be introduced as an add-on mechanism for a machine, or built as an integral part of a motor to be mounted onto the machine. Although the attenuation actuator24,26is shown to comprise a linear motor by way of example, it should be appreciated that other types of motors may be used if they are capable of generating a controlled force in an opposite direction to a reaction force. Some examples are pneumatic linear drivers or motors, or rotational electromagnetic motors with linear elements, such as a ball screw. The reaction force attenuation principles herein can also be implemented in other types of machines, with suitable modifications if necessary. The various embodiments of the invention make it possible to cancel the effects of reaction forces significantly. Since the body of the driving motor is rigidly fixed to the machine, the need for feedback control is also obviated.

Another advantage of such apparatus and method is that a machine base need not be as strong as otherwise required because vibration forces are reduced. Furthermore, precision of a machine can be improved as explained above.