Abstract:
A system and method for reducing reaction forces induced in a machine frame by an oscillating tool employs a counterforce assembly which is driven to move along linear slides mounted to the machine frame. The counterforce assembly is driven with a drive signal derived from two signals: a first signal which is proportional to the acceleration of the tool, and a second signal which is directly proportional to the velocity of the oscillating tool. By properly-adjusting the acceleration and velocity components of the drive signal, the magnitude of the reaction forces induced in the machine frame by the oscillating tool can be substantially reduced. The counterforce assembly preferably includes a centering means which prevents it from moving to either end of the slide. An accelerometer is preferably mounted to the machine frame to sense its vibration, with the accelerometer output used to adjust the counterforce assembly&#39;s drive electronics to reduce vibration to a minimum.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to the field of oscillating tools, and particularly to systems and methods for reducing the reaction forces induced in a machine frame by such tools.  
           [0003]    2. Description of the Related Art  
           [0004]    Many workpieces are machined by means of an oscillating tool which is mounted to a machine frame. The motion of the tool gives rise to reaction forces, which induce vibrations in the machine frame. These vibrations can be coupled back into the tool and adversely affect the surface of the workpiece.  
           [0005]    These reaction forces can be particularly troublesome when machining workpieces which require smooth surfaces, such as spectacle lenses. The reaction forces induced while lathing or surface generating such lenses can produce surface artifacts which result in surface aberrations, or discontinuities, which must then be removed by secondary processes such as fining and polishing. These additional process steps, which must be performed on every workpiece produced, are both costly and time-consuming.  
           [0006]    Many approaches have been taken to reduce reaction forces of this sort. For example, U.S. Pat. No. 5,959,427 to Watson describes the application of “reaction cancellation forces” which are applied through the center of gravity of a moving stage assembly. As described in Watson at column 3, lines 58-61, the system is arranged to apply a net reaction cancellation force which is “equal in magnitude to the mass of the stage multiplied by the acceleration of the stage in the stage travel direction.” 
           [0007]    Unfortunately, the approach described in Watson may be unacceptable in some circumstances. For example, when tool motion is produced using coils driven with magnetic fields, eddy currents are generated which result in drag forces. These drag forces can also be coupled to the machine frame and result in unwanted artifacts on the machined surface. These eddy current drag forces are proportional to the velocity of the coil in the magnetic field; Watson&#39;s system does nothing to counter these forces.  
         SUMMARY OF THE INVENTION  
         [0008]    A system and method are presented for reducing reaction forces induced in a machine frame by an oscillating tool, which overcomes the problems noted above. The invention counters forces which arise from both the acceleration and the velocity of the oscillating tool.  
           [0009]    Reaction forces are reduced by means of a counterforce assembly, which includes a weight mounted on one or more linear slides, which are in turn supported by low friction bearings mounted to the machine frame which supports the oscillating tool. The weight moves along an axis coaxial to that of the tool. The weight is driven with a driving means responsive to a drive signal which is derived from two signals: a first signal which is proportional to the acceleration of the oscillating tool, and a second signal which is directly proportional to the velocity of the tool. By properly adjusting the acceleration and velocity components of the drive signal, the magnitude of the reaction forces induced in the machine frame by the oscillating tool can be substantially reduced.  
           [0010]    The counterforce assembly preferably includes a centering means which prevents the assembly from moving to either end of the linear slides. An accelerometer is preferably mounted to the machine frame to sense its vibration, with the accelerometer output used to adjust the counterforce assembly&#39;s drive electronics to reduce vibration to a minimum.  
           [0011]    Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 a  is a plan view of a system for reducing reaction forces in accordance with the present invention.  
         [0013]    [0013]FIG. 1 b  is a side elevation view of the system shown in FIG. 1 a.    
         [0014]    [0014]FIG. 2 is a block diagram of drive circuitry suitable for use with a system for reducing reaction forces in accordance with the present invention.  
         [0015]    [0015]FIG. 3 is a schematic diagram of a preamp circuit suitable for use with a system for reducing reaction forces in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    A system for reducing reaction forces induced in a machine frame by an oscillating tool is illustrated in FIGS. 1 a  (plan view) and  1   b  (side elevation view). A machine frame  10  supports a plate  12  to which a housing  14  is mounted. The housing contains the armature  16  of a voice coil assembly  17 , to which a tool  18  is mounted. Drive circuitry (not shown) is used to drive armature  16 , which in turn causes tool  18  to oscillate in accordance with the armature&#39;s drive signal. Both armature  16  and tool  18  are driven to oscillate; these two components are referred to herein as “tool assembly  19 ”.  
         [0017]    A workpiece (not shown) is shaped by being brought into contact with the oscillating tool. For example, by bringing a spectacle lens blank into contact with the oscillating tool while rotating the blank in synchronization with the oscillation, tool  18  is used to machine the lens blank to provide a particular correction.  
         [0018]    As noted above, the oscillation of the tool results in reaction forces being induced in the machine frame, which can be transmitted back to the tool and workpiece to cause unwanted artifacts to appear on the workpiece surface. The reaction force F r  at the machine frame  10  is equal to, and opposite in direction, to the force F d  required to drive armature  16 . Drive force F d  is given by m·a, where m is the mass and a is the acceleration of tool assembly  19 .  
         [0019]    To reduce the magnitude of the reaction force F r , a counterforce assembly is used. The counterforce assembly includes a weight  20  which is preferably affixed to a pair of linear slides  22 , which are in turn supported by low friction bearings  24 . Weight  20  is also coupled to a driving means  26 . Driving means  26  moves weight  20  back and forth along slides  22  in response to a drive signal.  
         [0020]    In the preferred embodiment, weight  20  and slides  22  move as a unit between bearings  24 . Note that, alternatively, linear slide(s)  22  might be mounted directly to machine frame  10 , with weight  20  coupled to the slides by means of one or more low friction bearings, such that weight  20  moves along a static slide. Also note that weight  20  might be affixed to a single linear slide, or to more than two linear slides.  
         [0021]    The counterforce assembly is arranged such that weight  20  is oscillated at the same frequency as tool assembly  19 , along an axis  28  which is coaxial to that along which tool assembly  19  oscillates. To reduce the reaction force induced in machine frame  10  by tool assembly  19 , weight  20  is oscillated along axis  28  to produce a force equal and opposite to F d . That is, if F d  is given by: 
           F   d   =m   1   ·a   1 , 
         [0022]    where m 1  and a 1  are the mass and acceleration of tool assembly  18 , weight  20  is driven to produce an equal and opposite counterforce F c  given by: 
           F   c =−( m   1   ·a   1 )= m   2   ·a   2 , 
         [0023]    where m 2  and a 2  are the mass and acceleration of weight  20  and any components that are coupled to and thus move with weight  20 —referred to herein as “weight assembly  29 ”.  
         [0024]    Driving means  26  is preferably a voice coil assembly  30 , similar to the voice coil assembly  17  which drives tool  18 , such that weight assembly  29  comprises weight  20  and an armature  31 . When weight  20  is mounted on linear slides  22  that move between low friction bearings, weight assembly  29  includes slides  22 . When so arranged, the drive force F d  applied to tool assembly  19  is equal to the vector cross product of the current I 1  applied to voice coil  17  and the magnetic flux in housing  14 . This flux is typically provided with permanent magnets; as a result, the flux is essentially constant and F d  is directly proportional to the instant applied current I 1 . Since F d  is directly proportional to I 1 , the force necessary to reduce reaction force F r  can be obtained by electronically scaling and inverting I 1 , and applying the result to voice coil assembly  30 . As an alternative to inverting I 1 , voice coil assembly  30  can simply be wound in the opposite direction from that of voice coil assembly  17 .  
         [0025]    If the masses of tool assembly  19  and weight assembly  29  are equal, a system as described above—i.e., with voice coil assembly  30  driven with a signal derived from I 1 —can effectively reduce reaction force-induced vibration of machine frame  10 . However, it is difficult to provide exactly equal masses and identically performing voice coil systems for the tool and weight assemblies. The present invention overcomes this by providing an electronic means of controlling and adjusting the counterforce assembly, which also enables the masses of the respective assemblies to be different. Thus, the mass of weight assembly  29  can be greater than that of tool assembly  19 , which would result in the distance weight  20  needs to travel in order to counter the reaction forces being less than that of tool armature  16 . For example, if the masses of tool assembly  19  and weight assembly  29  are 4 lb. and 20 lb., respectively, weight assembly  29  need move only ⅕ the distance displaced by the tool. The reduced travel distance enables the system to be more compact, and also results in a reduced velocity for weight assembly  29 , which tends to reduce the dynamic performance requirements for the counterforce assembly.  
         [0026]    This gives rise to a possible source of error: because the distance traveled by tool assembly  19  is greater than that of weight assembly  29 , its velocity is also greater. In devices that utilize coils being driven in magnetic fields, eddy currents are generated which result in drag forces, which can couple to the machine frame and result in additional vibration. These eddy current drag forces are proportional to the velocity of the coil in the magnetic field. The drag force F v  due to eddy currents is given by: 
           F   v   =k·V   1 , 
         [0027]    where V 1  is the velocity of the coil and k is a constant which is a function of the system mechanics. This means that the total force F T  applied to the tool&#39;s voice coil is given by: 
         
       F 
       T 
       =F 
       d 
       +F 
       v 
       =m 
       1 
       ·a 
       1 
       +k·V 
       1 
     
         [0028]    With the masses of tool assembly  19  and weight assembly  29  being different, their respective velocities—and thus their respective drag forces—will also be different.  
         [0029]    Tool assembly voice coil current I 1  must provide all the forces needed to drive tool assembly  19  in a desired manner. Thus, I 1  includes a component necessary to obtain a particular tool assembly acceleration, as well as a component necessary to overcome the velocity-related eddy current drag forces. Therefore, if the signal used to drive weight assembly voice coil  30  is derived from I 1 , the resulting drive signal will be unable to fully counter the vibration attributable to the tool assembly&#39;s eddy current drag forces, due to the unequal velocities of the two masses.  
         [0030]    The invention overcomes this problem by recognizing this source of error, and adjusting the motion of weight assembly voice coil  30  to compensate for the error. This compensation is accomplished electronically; a block diagram of such a system is shown in FIG. 2. As noted above, tool assembly  19  is preferably driven with a voice coil assembly  17 . The voice coil is driven with a current I 1  generated by a control circuit  52 . Circuit  52  includes a processor  54  which produces current I 1  in response to a COMMAND input which represents a desired tool position, and a velocity input which varies with the velocity of tool assembly  19 . Tool velocity is preferably determined by means of a position encoder  56  which produces an output  58  that varies with the position of voice coil assembly  17 , and a tachometer circuit  60  which monitors the position output with respect to time to generate an output signal  61  which varies with tool velocity. Current I 1  is preferably amplified with amplifier  62  to produce the drive signal for voice coil assembly  17 .  
         [0031]    The present system also includes the weight&#39;s voice coil assembly  30 , which is driven with a signal  64  provided by a “reaction force reduction” (RFR) preamplifier  66 ; signal  64  is preferably amplified with an amplifier  65  to produce the drive signal for voice coil  30 . In order to produce a drive signal which can counter the reaction forces induced by tool assembly  19 , RFE preamp  66  receives current I 1  at an input, and is arranged to vary signal  64  with I 1 . However, as noted above, when the tool assembly and weight assembly masses are different, a drive signal derived solely from I 1  cannot completely counter vibration attributable to the tool assembly&#39;s eddy current drag forces. Countering these drag forces requires that preamp  66  also receive velocity signal  61  at a second input, and deriving drive signal  64  from both I 1  and velocity signal  61 . When velocity signal  61  and current I 1  are combined in the proper proportions, both the reaction force-induced vibration of machine frame  10 , and the vibration that arises due to the different velocities of the tool and weight assemblies is substantially reduced. This is achieved when the reaction force reduction system is adjusted such that the oscillation of weight assembly  29  produces a counterforce Fc which is equal to −F T .  
         [0032]    The force created by the oscillation of tool assembly  19  is primarily related to its acceleration, and is proportional to sinωt, where ω is the oscillation frequency. The force necessary to compensate the tool&#39;s eddy current drag forces is related to tool velocity—the integral of acceleration—and is thus proportional to cosωt.  
         [0033]    Current I 1  also includes a component that drives oscillating tool assembly  18  into and away from the workpiece as it oscillates. This component appears in the output  64  of RFR preamp  66 , and can cause weight  20  to move until it hits one of bearings  24 . This is preferably avoided by keeping the weight centered between the bearings. There are several ways in which the weight can be kept centered. For example, springs could be placed between weight  20  and bearings  24  to keep the weight nominally centered. However, the spring resistance may adversely affect the magnitude of the vibration reduction provided by the oscillating weight. The weight is preferably kept centered with the use of a position encoder  70  which produces a signal  72  that varies with the weight&#39;s position. Signal  72  is provided to RFR preamp  66 , which maintains the weight&#39;s nominal position at a target location. The target position is preferably maintained at a low servo bandwidth so that the higher vibration frequencies reduced by the system are essentially unaffected. To avoid degrading the performance of the reaction force reduction system, position encoder  70  preferably senses position without contacting weight  20 , as with a linear variable differential transformer (LVDT).  
         [0034]    The low friction bearings  24  supporting linear slides  22  are preferably air bearings.  
         [0035]    One possible implementation of RFR preamp  66  is shown in FIG. 3. A differential signal derived from current I 1  is received at a pair of differential inputs  80 , and a differential signal derived from velocity signal  61  is received at a pair of differential inputs  82 . Inputs  80  and  82  are fed to respective operational amplifiers  84  and  86 , which buffer and provide gain for their inputs. The op amp outputs are fed to respective potentiometers  88  and  90 , and the pot taps are connected to a summing node  92  via respective resistors. The summing node is buffered with an amplifier  94 , which produces the output  64  of the RFR preamp. In this way, the I 1  and velocity inputs are combined to provide the drive signal delivered to voice coil assembly  30 . As is discussed in more detail below, potentiometers  88  and  90  are adjusted as needed to reduce the vibration of machine frame  10 .  
         [0036]    As noted above, the system preferably includes a means of keeping weight  20  centered, with the preferred means being a position encoder  70  which produces a position signal  72 . When so implemented, RFR preamp is arranged to receive position signal  72  at an input  96 . The position signal is preferably buffered with an amplifier  98 , and then provided to a proportional-integral-differential (PID) control circuit  100 . Circuit  100  also receives a signal  102  representing the desired target position, and produces an output  104  which is summed into summing node  92 , preferably via a filter  106 , and thereby coupled into drive signal  64 . PID circuit varies output  104  as necessary to keep weight  20  near the target position.  
         [0037]    Note that the RFR preamp implementation shown in FIG. 3 is merely exemplary. Many other circuits could be designed which combine the I 1 , velocity, and position inputs to produce a suitable drive signal.  
         [0038]    Referring back to FIGS. 1 a  and  1   b , the system is preferably calibrated to achieve a minimum vibration level with the use of an accelerometer  110 , which is preferably mounted near tool assembly  19 . Accelerometer  110  is typically excited with an interface circuit (not shown), which also receives and amplifies the accelerometer&#39;s output. In practice, tool assembly  19  is commanded to oscillate, and the output of accelerometer  110 —which varies with the vibration level of machine frame  10 —is monitored. RFR preamp potentiometers  88  and  90  are then adjusted as necessary to reduce the magnitude of the accelerometer output—and thus the machine frame vibration—as low as possible.  
         [0039]    When properly calibrated, vibration reductions of up to 99% (when compared with a similar oscillating tool system which lacks a reaction force reduction system) are achievable. When employed on a spectacle lens machining system, for example, the number and severity of vibration-induced artifacts on the lens surface is substantially reduced, and the need to perform secondary processes such as fining and polishing may be eliminated.  
         [0040]    If calibrated at one oscillation frequency, the reaction force reduction system should be effective at other oscillation frequencies as well. This is because the primary forces—acceleration and velocity—increase and decrease nearly linearly with frequency.  
         [0041]    While particular embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.