WO2003000004A2

WO2003000004A2 - Autonomous intermittent-pulse-train motion control system

Info

Publication number: WO2003000004A2
Application number: PCT/US2002/017671
Authority: WO
Inventors: Zoltan A. Kemeny
Original assignee: True Gravity Enterprises, Inc.
Priority date: 2001-06-22
Filing date: 2002-06-04
Publication date: 2003-01-03
Also published as: WO2003000004A3; AU2002326305A1

Abstract

A mass damper (14) is attached to a machine tool (12) to suppress the vibrations and oscillations induced by the machine tool. A mass (18) receives the vibrations through a spring (16). A shaft (20) is connected to the mass. An inductive coil (22) is wrapped around the shaft. A control circuit (30) provides a pulse train drive signal to the coil to impose a magnetic force on the mass opposing the vibration. A second coil (24) may be wrapped around the shaft for sensing movement of the shaft and providing a sense signal to the control circuit to activate the pulse train drive signal. A sensor (34) may be locally mounted to the shaft to sense movement and provide a sense signal to the control circuit to activate the pulse train drive signal.

Description

CLAIM TO DOMESTIC PRIORITY

[0001] The present non-provisional patent application claims priority to provisional application serial no. 60/300,285, entitled "Autonomous Intermittent-Pulse-Train Motion Control System", filed on June 22, 2001, by Zoltan A. Kemeny .

FIELD OF THE INVENTION

[0002] The present invention relates in general to mechanical vibration damping and mass dampers and, more particularly, to a pulse train driven actuator for a mass damper.

BACKGROUND OF THE INVENTION

[0003] Many types of manufacturing equipment and tools exhibit or experience vibrations and oscillations during operation. The vibrating and oscillating action is a natural side-effect of motor operation, rotating bearings and shafts, oscillating and reciprocating parts, impacting sources, and other apparatus undergoing physical movement. The vibrations and oscillations induced by some manufacturing equipment can cause an undesirable and detrimental action either on the same equipment or on other equipment. This is especially true in environments that require very precise processing, e.g. masking, cutting, shaping, application and removal of material, and other manufacturing steps that involve precise operations and extremely small dimensions.

[0004] If, for example, a manufacturing step involves applying material or making a cut, and the manufacturing equipment is experiencing vibrations or oscillations, then the result of the operation will likely be at least partially in error. The path or width of the cut, or the area where material is supposed to be applied, or the area where material is not supposed to be applied, will be off in relation to the nature, frequency, magnitude, and direction of the undesired vibrations or oscillations. Any vibration in a precision machine tool often leads to defects and imperfections in the work piece and thereby reduces production yield. [0005] The vibration problem exists as well in the manufacture of semiconductors because of the large number of motors, rotating shafts, and mechanical movement underway in the same environment with high precision machine tools and precise processes that must be accurately performed on extremely small dimensions. One of the semiconductor processes that is known to induce vibration is chemical mechanical polishing (CMP) which involves polishing a semiconductor wafer and uses a combination of chemical and mechanical steps. The mechanical portion uses a motor rotating a shaft which is attached to a polishing pad. A semiconductor wafer is brought in contact with the rotating polishing pad to planarize or smooth out the surface of the wafer. The chemical portion involves application of a slurry chemistry under the polishing pad to help dissolve or etch away surface materials. There are a number of different options to the CMP process depending on the type of semiconductor material and desired precision of the process. The CMP process may vary with the rotating speed of the polishing pad, pressure of the polishing pad against the wafer, and rate of application and type of slurry chemistry. [0006] The three-dimensional physical movement of the CMP operations are known to develop vibrations having high frequency or low frequency components. The nature, frequency, magnitude, and direction of the vibration is dependent on the speed, movement, and physical action of the CMP tool, which in turn is dependent upon the CMP process being performed. The vibration can be transmitted through the CMP support table or base, and through the factory floor, to other vibration sensitive machines and tools which are processing semiconductor wafers. Thus, the vibrations can cause errors in the same or other manufacturing processes and defects to the semiconductor wafer.

[0007] One solution known in the prior art involves the use of a broad band, passive mass damper to suppress or cancel the vibrations and oscillations. A broad band mass damper is disclosed in U.S. Patent 6,364,077 entitled, "Conservative Broadband Passive Mass Damper." The mass damper is a mechanical device which comprises a bob or moveable mass supported by springs and a plurality of ball-in-recess assemblies. A base of the mass damper is rigidly attached to a source of vibration, e.g. CMP tool or other manufacturing equipment. The vibration generated by the manufacturing equipment is transmitted to the mass damper and induces an opposite phase vibration or movement in the bob which suppresses or cancels the vibration source. The characteristics and bandwidth of the mass damper, e.g. mass of the bob, spring constant, and center frequency, are fixed to match the manufacturing process which is causing the vibration, e.g. rotation speed and downward force of the polishing pad, and rate of application and type of slurry. To handle a broader range of vibrating frequencies sometimes a plurality of passive mass dampers are used each tuned to a particular frequency band. [0008] Other prior art solutions have focused on active mass damping systems. An active mass damping system typically uses a feedback system with motion sensors to monitor the amplitude and frequency of the vibration and generate a continuous cancellation or compensation force which over time reduces or eliminates the vibration. The motion sensor senses acceleration of the source of vibration. The cancellation motion may be generated by an electro-hydraulic, piezo- electronic, or electro-dynamic action as an opposing force or acceleration proportional to the acceleration of the source of vibration. The active mass damping system is often highly complex, prohibitively expensive, and a potential source of equipment downtime for repairs and maintenance. [0009] A need exists for a mass damper system with an active control system having a simple, efficient, and cost effective design.

BRIEF DESCRIPTION OF THE DRAWINGS

[00010] FIG. 1 illustrates a mass damper having an actuator operating with a control circuit;

FIG. 2 illustrates an alternative embodiment of the mass damper having an actuator with locally mounted sensor; FIG. 3 is a block diagram of the control circuit of

FIG. 2; and

FIG. 4 is a block diagram of an alterative embodiment of the control circuit of FIG. 2.

DETAILED DESCRIPTION OF THE DRAWINGS

[00011] A mass damper system 10 is shown in FIG. 1. Machine tool 12 represents a portion of a unit of manufacturing equipment or tool which induces, exhibits, or otherwise experiences vibrations, oscillations, or shaking during its operation. The vibrating and oscillating action is a common side-effect of motor operation, rotating bearings and shafts, oscillating and reciprocating parts, impacting sources, and other apparatus undergoing physical motion. Machine tool 12 represents equipment that can be found in virtually any manufacturing environment.

[00012] For example, in the manufacture of semiconductors, machine tool 12 can be a CMP tool which is involved in the polishing a semiconductor wafer and uses a combination of chemical and mechanical steps. The mechanical portion uses a motor rotating a shaft which is attached to a polishing pad. A semiconductor wafer is brought in contact with the rotating polishing pad to planarize or smooth out the surface of the wafer. The chemical portion involves application of a slurry chemistry under the polishing pad to help dissolve or etch away surface materials. There are a number of different options to the CMP process depending on the type of semiconductor material and desired precision of the process. The CMP process may vary with the rotating speed of the polishing pad, pressure of the polishing pad against the wafer, and rate of application and type of slurry chemistry. [00013] The three-dimensional physical movement of the CMP tool is known to develop vibrations having high frequency or low frequency components. The nature, frequency, magnitude, and direction of the vibration is dependent on the speed and motion of the CMP tool, which in turn is dependent upon the CMP process being performed. The vibrations can be transmitted through the CMP support table or base, and through the factory floor, to other vibration-sensitive, precision machines and tools. The vibrations and oscillations induced by machine tool 12 can cause an undesirable and detrimental action, especially in environments like the manufacture of semiconductors that involve precise operations and/or extremely small dimensions. Any vibration transmitted to a precision machine tool can lead to errors in the manufacturing process and defects and imperfections in the semiconductor wafer which reduces production yield.

[00014] In mass damper system 10, mass damper 14 is attached to machine tool 12 to counteract, suppress, reduce, cancel, and/or neutralize the vibrations and oscillations effects induced by machine tool 12. Mass damper 14 counteracts the vibration of machine tool 12 though an action of passive mass resonance. Mass damper 14 includes a bob or moveable mass 18 and a support mechanism for bob 18, shown in part as spring 16. Bob 18 is moveably attached within a housing (not shown) by spring 16 and other support mechanisms, e.g. coil springs, crest-to-crest springs, stack of belleville or disk springs, pre-loaded mechanical springs, rollers, bearings, pendulum, and sliders, attached to other major surface (s) of bob 18. Spring 16 has one end attached to a surface of machine tool 12 and a second end attached to a surface of bob 18. Spring 16 is attached at both ends with screws, weld, adhesive, and any other attachment mechanism which provides a strong yet pliable connection. Bob 18 is sufficiently large in comparison to the mass of machine tool 12 with a mass magnitude about 2.0-20.0 percent of the mass magnitude of machine tool 12. Bob 18 is made of steel, stainless, steel, or other material providing the desired mass for the application, and can be cubical, cylindrical, spherical, conical, or other shape or dimension (s) compatible with the support mechanism.

[00015] An actuator 19 is rigidly attached to the housing with screws, weld, adhesive, and any other attachment mechanism which provides a strong, rigid contact. Actuator 19 includes shaft 20, which is made of iron, steel, or other suitable magnetic material. Shaft 20 is rigidly attached to a surface of bob 18. Actuator 19 further includes inductive coils 22, 24, 26, and 28, each wrapped around shaft 20 and coupled to control circuit 30. An acceleration of shaft 20 to the right through the center of the windings of coil 22 induces a sense current I₂₂ to flow in coil 22 and into control circuit 30 as shown in FIG. 1. Coil 22 senses the motion of shaft 20 to the right and provides a representative current to control circuit 30. Control circuit 30 causes a drive current I₂₄ to flow through coil 24 in response to sense current I₂₂- Drive current I₂₄ induces a magnetic force which is imposed on shaft 20 to oppose the rightward movement of shaft 20 as sensed by coil 22. Conversely, an acceleration of shaft 20 to the left through the center of the windings of coil 26 induces a sense current I₂₆ to flow in coil 26 and into control circuit 30 as shown in FIG. 1. Coil 26 senses the motion of shaft 20 to the left and provides a representative current to control circuit 30. Control circuit 30 causes a drive current I₂₈ to flow through coil 28 in response to sense current I₂₆- Drive current I₂₈ induces a magnetic force which is imposed on shaft 20 to oppose the leftward movement of shaft 20 as sensed by coil 26. [00016] Mass damper 14 operates to conserve kinetic energy. The vibration of machine tool 12 is transmitted through the support mechanism, e.g. spring 16, and causes motion in bob 18. The motion of bob 18 has a function x (t) =ax ' ' +bx' +cx, where x¹' is acceleration, a is the coefficient of acceleration, x' is velocity, b is the coefficient of velocity, x is displacement, and c is the coefficient of displacement. The motion of bob 18 is equally transmitted to shaft 20 by the rigid connection.

[00017] Assume a vibration in machine tool 12 is transmitted through spring 16 and causes bob 18 and shaft 20 to move to the right at a given point in time, i.e. a positive acceleration, velocity, and displacement. As described above, coil 22 senses the acceleration of shaft 20 and provides sense current I₂₂ to control circuit 30. Control circuit 30 integrates the acceleration once for velocity and once again to get a displacement signal. Control circuit 30 converts the displacement to a drive current I₂₄ having a pulse train waveform and a frequency higher than the natural frequency of mass damper 14, which is f_n = (k/m) ^{1 2}/2π, where k is the spring constant of spring 16 and m is the mass of bob 18. [00018] There are a number of design techniques to convert the displacement signal to a pulse train drive current. For example, the displacement signal may be compared to a threshold using a comparator. A pulse train switches between high and low, on and off, and is applied to one side of a transmission gate. When the displacement signal exceeds the threshold, then the comparator generates an enable signal which enables a transmission gate to pass the pulse train as drive current I₂₄. Alternatively, the enable signal can be multiplied by the signal levels of the pulse train drive signal. If the enable signal is a value zero, then the pulse train drive signal is zero. If the enable signal is a value one, then the pulse train drive signal switches between high and low according to its frequency and waveform. Other embodiments of control circuit 30 can also activate the pulse train drive signal when there is a displacement of shaft 20. [00019] The drive current I₂ flowing through coil 24 induces a magnetic force which is imposed on shaft 20 to oppose its displacement to the right. If mass damper 14 is properly tuned, i.e. the vibrating center frequency of machine tool 12 falls within the bandwidth of the frequency response of mass damper 14, then the opposing force on shaft 20 is transmitted to bob 18 which counteracts, suppresses, reduces, cancels, and/or neutralizes the vibration and oscillation effects induced by machine tool 12.

[00020] Next assume a vibration in machine tool 12 is transmitted through spring 16 and causes bob 18 and shaft 20 to move to the left at a given point in time, i.e. a negative acceleration, velocity, and displacement. As described above, coil 26 senses the acceleration of shaft 20 and provides sense current I₂₆ to control circuit 30. Control circuit 30 integrates the acceleration once for velocity and once again to get a displacement signal. Control circuit 30 converts the displacement to a drive current I₂₈ having a pulse train waveform. The drive current I₂₈ flowing through coil 28 induces a magnetic force which is imposed on shaft 20 to oppose its displacement to the left. The opposing force on shaft 20 is transmitted to bob 18 which counteracts, suppresses, reduces, cancels, and/or neutralizes the vibration and oscillation effects induced by machine tool 12. [00021] The pulse train waveform of drive currents I₂₆ and I₂₈ offer a number of advantages. The pulse train provides an instantaneous change in velocity of shaft 20 by the principal of momentum exchange. At each pulse, an incremental change in velocity is applied to shaft 20 and bob 18 to counteract the displacement induced by the vibration of machine tool 12. The pulse train waveform provides repeated incremental changes in velocity according to the frequency of the waveform and the length of time that shaft 20 is experiencing a positive or negative displacement. The effective counteracting force is predominately a function of the frequency of the pulse train signal. The magnitude of the opposing force can be small as compared to a continuous force, i.e. force over time = mass times acceleration according to Newton's second law. A smaller force which is repeated applied involves smaller drive currents I₂₄ and I₂₈ and a simpler, efficient, and most cost effective design for control circuit 30.

[00022] In another embodiment of actuator 19, coils 22 and 26 can be combined into one coil to sense positive and negative acceleration. Coils 24 and 28 can be combined into one coil to receive drive currents I₂₄ and I₂₈ flowing in opposite directions through the same coil. Control circuit 30 can be readily adapted to receive bi-directional sensing and provide bi-directional drive currents. In yet another embodiment of actuator 19, the function of coils 22, 24, 26, and 28 can all be combined into one coil to sense positive and negative acceleration and receive drive currents I₂₄ and I₂₈. Again, control circuit 30 can be readily adapted to receive bi-directional sensing and provide bi-directional drive currents since the measurement of the acceleration occurs at a different frequency than the driving current. The driving current pulse train has a very high frequency components at its transition and DC frequency components during its high and low portion of the cycle. The measurement of acceleration frequency falls in between.

[00023] In FIG. 2, an alternative embodiment of the mass damper system is shown. Components having a similar function are given the same reference numbers used in FIG. 1. Actuator 32 includes a sensor 34 locally mounted to shaft 20 or bob 18. Sensor 34 may be an accelerometer to measure a positive or negative acceleration of shaft 20. The acceleration signal representative of the positive or negative acceleration of shaft 20 is routed to control circuit 36. Control circuit 36 provides drive currents I₂₄ and I₂₈ having a pulse train waveform to flow through coils 24 and 28 which impose a magnetic force and oppose the motion of shaft 20 induced by the vibration of machine tool 12 as described above. [00024] In another embodiment of actuator 32, coils 24 and 28 can be combined into one coil receiving drive currents I₂₄ and I₂₈ flowing in opposite directions through the same coil. Control circuit 30 can be readily adapted to provide bidirectional drive currents. [00025] Further detail of control circuit 36 is shown in FIG. 3. The acceleration signal from accelerometer 34 is integrated by integrator 38 to get a velocity signal. The velocity signal is applied to a non-inverting input of comparator 40 and to an inverting input of comparator 42. The inverting input of comparator 40 receives a reference signal V_REFI • The non-inverting input of comparator 42 receives a reference signal V_REF2. The output of comparator 40 is coupled to a first input of NAND gate 44. The output of comparator 42 is coupled to a first input of NAND gate 46. A pulse train or square wave signal from square wave signal generator 48 is applied to second inputs of NAND gates 44 and 46. There are a number of designs that will function for NAND gates 44 and 46. For example, NAND gates 44 and 46 can be implemented with an input stage including two transistors stacked as a totem pole. The output signal of the comparator drives the control terminal of one transistor and the pulse train signal is applied to the control terminal of the other transistor. Both signals must be high to complete the signal path through the totem pole input stage and pass the pulse train signal.

Alternatively, NAND gates 44 and 46 can be implemented as a single bipolar transistor where the output signal of the comparator drives the base of the bipolar transistor and the pulse train signal is applied through a resistor to the collector of the bipolar transistor. If the output signal of the comparator is high, then the bipolar transistor turns on and shunts the pulse train signal to ground. If the output signal of the comparator is low, then the bipolar transistor turns off and allows the pulse train signal to pass. The output of NAND gate 44 is coupled to driver circuit 50 which provides the drive current I₂₄. The output of NAND gate 46 is coupled to driver circuit 52 which provides the drive current

I₂8 • [00026] If the velocity signal is greater than reference signal V_REFι, then the output signal of comparator 40 is a logic one and NAND gate 44 passes the pulse train signal to driver circuit 50 which generates the pulse train drive current I₂ . If the velocity signal is less than reference signal V_REF2, then the output signal of comparator 42 is logic one and NAND gate 46 passes the pulse train signal to driver circuit 52 which generates the pulse train drive current I₂₈. [00027] Turning to FIG. 4, sensor 34 is shown as an infrared sensor comprising a light emitting diode (LED) mounted to shaft 20 and two or more infrared phototransistors, or a photo detector strip, mounted on a support member, e.g. the housing, which is stationary with respect to shaft 20. The infrared phototransistors are mounted close together, e.g. less than 4.0 millimeters to 4.0 centimeters apart, in a line along the direction of movement of shaft 20. Infrared detectors 54 and 56 measure a differential positive or negative displacement of shaft 20. The differential displacement signal from infrared detectors 54 and 56 are applied to differential amplifier 58. The output signal of differential amplifier 58 is differentiated by differentiator 60 to get a velocity signal. The velocity signal is applied to a non-inverting input of comparator 62 and to an inverting input of comparator 64. The inverting input of comparator 62 receives a reference signal V_REF1. The non-inverting input of comparator 64 receives a reference signal V_REF2. The output of comparator 62 is coupled to a first input of NAND gate 68. The output of comparator 64 is coupled to a first input of NAND gate 70. A pulse train or square wave signal from square wave signal generator 72 is applied to second inputs of NAND gates 68 and 70. The output of NAND gate 68 is coupled to driver circuit 74 which provides the drive current I₂₄. The output of NAND gate 70 is coupled to driver circuit 76 which provides the drive current I₂₈. [00028] If the velocity signal is greater than reference signal V_REFι, then the output signal of comparator 62 is a logic one and NAND gate 68 passes the pulse train signal to driver circuit 72 which generates the pulse train drive current I₂₄. If the velocity signal is less than reference signal V_REF2, then the output signal of comparator 64 is logic one and NAND gate 70 passes the pulse train signal to driver circuit 76 which generates the pulse train drive current I₂₈.

[00029] The active mass damper is also applicable to optics, fluidonics, photonics, and other gas and liquid flow systems and fields of applied science where an undesired vibration or oscillation phenomenon occurs and needs to be damped. [00030] In summary, a mass damper is attached to a machine tool to suppress the vibrations and oscillations induced by the machine tool. A mass receives the vibrations through a spring. A shaft is connected to the mass, and an inductive coil is wrapped around the shaft. A control circuit provides a pulse train drive signal to the coil to impose a magnetic force on the shaft and on the mass opposing the vibration. A second coil may be wrapped around the shaft for sensing movement of the shaft and providing a sense signal to the control circuit to activate the pulse train drive signal, or a sensor is locally mounted to the shaft ^"to sense movement and provide a sense signal to the control circuit to activate the pulse train drive signal. The pulse train drive signal provides repeated incremental changes in velocity according to the frequency of the waveform and the length of time that the shaft is experiencing a positive or negative displacement. The pulse train waveform provides for smaller drive currents and a simpler, efficient, and most cost effective control circuit .

[00031] Although the present invention has been described with respect to preferred embodiments, any person skilled in the art will recognize that changes may be made in form and detail, and equivalents may be substituted for elements of the invention without departing from the spirit and scope of the invention. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed for carrying out this invention, but will include all embodiments falling within the scope of the appended claims.

Claims

CLAIMSWhat is claimed is:

1. A mass damper for suppressing a vibration, comprising: a mass adapted to receive the vibration; an actuator coupled to the mass; and a control circuit providing a pulse train drive signal to the actuator to impose a force on the mass opposing the vibration.

2. The mass damper of claim 1, wherein the actuator includes : a shaft coupled to the mass; and a first coil wrapped around the shaft and coupled for receiving the pulse train drive signal for imposing a magnetic force opposing the vibration.

3. The mass damper of claim 2, further including a second coil wrapped around the shaft for sensing movement of the shaft and providing a sense signal to the control circuit to activate the pulse train drive signal.

4. The mass damper of claim 2, further including a sensor for sensing movement of the shaft and providing a sense signal to the control circuit to activate the pulse train drive signal .

5. The mass damper of claim 4, wherein the control circuit includes : a differentiator having an input coupled for receiving the sense signal; a first comparator having a first input coupled to an output of the differentiator and a second input coupled for receiving a first reference signal; a second comparator having a first input coupled to the output of the differentiator and a second input coupled for receiving a second reference signal; a signal generator; a first gate having a first input coupled to an output of the first comparator and a second input coupled to an output of the signal generator; a second gate having a first input coupled to an output of the second comparator and a second input coupled to the output of the signal generator; a first driver circuit having an input coupled to an output of the first gate and an output for providing a first drive signal; and a second driver circuit having an input coupled to an output of the second gate and an output for providing a second drive signal.

6. The mass damper of claim 4, wherein the control circuit includes : an integrator having an input coupled for receiving the sense signal; a first comparator having a first input coupled to an output of the integrator and a second input coupled for receiving a first reference signal; a second comparator having a first input coupled to the output of the integrator and a second input coupled for receiving a second reference signal; a signal generator; a first gate having a first input coupled to an output of the first comparator and a second input coupled to an output of the signal generator; a second gate having a first input coupled to an output of the second comparator and a second input coupled to the output of the signal generator; a first driver circuit having an input coupled to an output of the first gate and an output for providing a first drive signal; and a second driver circuit having an input coupled to an output of the second gate and an output for providing a second drive signal.

7. The mass damper of claim 1, further including a support mechanism for supporting the mass.

8. The mass damper of claim 7, wherein the support mechanism includes a spring having a first end coupled to receive the vibration and a second end coupled to the mass.

9. A mass damper, comprising: a mass adapted for receiving a vibration; a shaft coupled to the mass; and a coil wrapped around the shaft and coupled for receiving a pulse train drive signal to impose a magnetic force on the shaft opposing the vibration.

10. The mass damper of claim 9, further including a control circuit providing the pulse train drive signal to the coil to impose the magnetic force on the shaft opposing the vibration.

11. The mass damper of claim 9, further including a sensor for sensing movement of the shaft and providing a sense signal to the control circuit to activate the pulse train drive signal.

12. The mass damper of claim 11, wherein the control circuit includes : a differentiator having an input coupled for receiving the sense signal; a first comparator having a first input coupled to an output of the differentiator and a second input coupled for receiving a first reference signal; a second comparator having a first input coupled to the output of the differentiator and a second input coupled for receiving a second reference signal; a signal generator; a first gate having a first input coupled to an output of the first comparator and a second input coupled to an output of the signal generator; a second gate having a first input coupled to an output of the second comparator and a second input coupled to the output of the signal generator; a first driver circuit having an input coupled to an output of the first gate and an output for providing a first drive signal; and a second driver circuit having an input coupled to an output of the second gate and an output for providing a second drive signal.

13. The mass damper of claim 11, wherein the control circuit includes : an integrator having an input coupled for receiving the sense signal; a first comparator having a first input coupled to an output of the integrator and a second input coupled for receiving a first reference signal; a second comparator having a first input coupled to the output of the integrator and a second input coupled for receiving a second reference signal; a signal generator; a first gate having a first input coupled to an output of the first comparator and a second input coupled to an output of the signal generator; a second gate having a first input coupled to an output of the second comparator and a second input coupled to the output of the signal generator; a first driver circuit having an input coupled to an output of the first gate and an output for providing a first drive signal; and a second driver circuit having an input coupled to an output of the second gate and an output for providing a second drive signal.

14. The mass damper of claim 9, further including a support mechanism for supporting the mass.

15. The mass damper of claim 14, wherein the support mechanism includes a spring having a first end coupled to receive the vibration and a second end coupled to the mass.

16. A method of damping a vibration, comprising: providing a mass adapted to receive the vibration; providing an actuator coupled to the mass; and activating a pulse train drive signal to the actuator to impose a force on the mass opposing the vibration.

17. The method of claim 16, wherein the step of providing an actuator includes: providing a shaft coupled to the mass; and providing a coil wrapped around the shaft and coupled for receiving the pulse train drive signal for imposing a magnetic force opposing the vibration.

18. The method of claim 17, further including: providing a sensor for sensing movement of the shaft; and providing a sense signal to activate the pulse train drive signal.

19. The method of claim 16, further including providing a spring having a first end coupled to receive the vibration and a second end coupled to the mass.

20. A method of suppressing a vibration, comprising: inducing the vibration in a mass which is coupled to a shaft; and activating a pulse train drive signal to a coil wrapped around the shaft to impose a magnetic force on the mass opposing the vibration.

21. The method of claim 20, further including: providing a sensor for sensing movement of the shaft; and providing a sense signal to activate the pulse train drive signal.