Patent Publication Number: US-6911792-B2

Title: System and method for controlling movement

Description:
FIELD OF THE INVENTION 
   The invention relates to a system and method for controlling relative movement between two objects. 
   BACKGROUND OF THE INVENTION 
   In micro-electro-mechanical systems (MEMS), it is often necessary to effect very small, precise movements between objects through a range of motion. To do this, the objects must be closely controlled and monitored. Specifically, the relative positions of the objects must be precisely known, and the device producing movement between the objects (often referred to as a “micro-mover” or “micro-actuator”) must be capable of making very small and precise movements. 
   In some instances, in addition to the abilities to precisely know the relative positions of the objects and to effect precise movements of the objects, it is necessary or desirable to have a very high degree of control over the micro-actuator such that the objects may be accelerated and decelerated in a very smooth manner, while still effecting very small, precise movements with a very high resolution relative to the distance over which the objects are moved. 
   SUMMARY OF THE INVENTION 
   A system and method for controlling movement of a body is described herein. In one embodiment according to the invention, a system for controlling movement includes a position-based velocity profile, at least one mover, a data processor for calculating a next position of the at least one mover using data from the position-based velocity profile and passing a next position signal to the at least one mover, and an actuator for moving the mover to the next position. 
   In another embodiment according to the invention, a method for controlling the movement of a body comprises providing a current position and a target position for at least one mover, retrieving a desired mover velocity from a position-based velocity profile based on the current position and target position for the at least one mover, calculating a next position of the at least one mover using the desired mover velocity, passing a next position signal to the at least one mover, and moving the mover to the next position. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention are better understood with reference to the following drawings. The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts. 
       FIGS. 1-3  show different perspectives of a computer storage device with which the system and method for controlling relative movement between two objects according to an embodiment of the present invention. 
       FIG. 4  is a schematic depiction of an electrostatic drive for effecting movement between two objects. 
       FIG. 5  is a graph showing example position, velocity and acceleration profiles according to an embodiment of the invention. 
       FIG. 6  is a graph showing example velocity versus position data extracted from the velocity and position profiles of FIG.  5 . 
       FIG. 7  is a block diagram illustrating a motion profiler system and method according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   The present invention is directed to a system and method for controlling movement between objects (referred to herein as a “motion profiler” or “profiler”). The motion profiler embodiments described herein may be used in a variety of settings, but are particularly advantageous when used in very small computer storage devices and other MEMS systems. For purposes of illustration only, the motion profiler system and method described below will be discussed primarily in the context of an atomic resolution storage (ARS) device. 
     FIGS. 1 and 2 , respectively, show side and top cross section views of an atomic resolution storage device  100 , with which a motion profiler according to the invention may be used. Storage device  100  includes a number of field emitters, such as  102  and  104 , a storage medium  106  with a number of storage areas, such as  108 , and a micro-actuator  110 , which scans (moves) the storage medium  106  with respect to the field emitters  102 ,  104  or vise versa. Storage device  100  may be configured such that each storage area  108  is responsible for storing one bit or many bits of information. 
   Casing  120  typically is adapted to maintain storage medium  106  in a partial vacuum, such as at least 10 −5  torr. Each field emitter  102 ,  104  may correspond to one or more storage areas  108  provided on storage medium  106 . Where each field emitter  102 ,  104  is responsible for a number of storage areas  108 , storage device  100  typically is adapted to scan or otherwise effect relative movement between casing  120  (and thus, the field emitters ) and storage medium  106 . For example, micro-actuator  110  typically is adapted to scan storage medium  106  to different locations, such that each field emitter  102 ,  104  is positioned above different storage areas  108 . With such a configuration, micro-actuator  110  can be used to scan an array (typically two-dimensional) of field emitters over the storage medium  106 . Because storage medium  106  moves relative to casing  120 , it will at times be referred to herein as the “mover.” Correspondingly, casing  120  and various other components that are fixed relative to casing  120  (e.g., the field emitters) will at times be referred to herein as the “stator.” 
   The field emitters  102 ,  104  typically are configured to read and/or write information on the storage areas  108  via electron beams they produce. The field emitters may be provided as a two-dimensional array (e.g., 100 by 100 emitters), with an emitter pitch of 50 micrometers in both the X and the Y directions. Each emitter  102 ,  104  may access bits in tens of thousands to hundreds of millions of storage areas  108 . For example, the emitters may scan over (i.e., move relative to) a storage medium  106  that has a two-dimensional array of storage areas  108 , where the periodicity between adjacent storage areas  108  is anywhere from a fraction of nanometer to 100 or more nanometers, and where the operational range of the micro-actuator is 50 micrometers in both the X and Y directions. Also, the field emitters may be addressed simultaneously or in a multiplexed manner. Parallel addressing schemes may provide storage device  100  with significant performance enhancements in terms of access time and data rates. 
     FIG. 3  is a top view of an exemplary storage medium  106  depicting a two-dimensional array of storage areas  108  and a two-dimensional array of field emitters  102 ,  104 . External circuitry (not shown) is used to address the storage areas  108 . As indicated, it is often desirable to segment storage medium  106  into rows such as rows  140 ,  142 , where each row contains a plurality of storage areas, such as storage area  108 . Typically, each emitter is responsible for a number of rows, but is not responsible for the entire length of those rows. For example, as depicted in  FIG. 3 , emitter  102  is responsible for the storage areas within rows  140  through  142 , and within columns  144  to  146 . 
   The foregoing describes an exemplary storage device with which the motion profiler according to the present invention may be used. Other aspects of this type of storage device are disclosed in U.S. Pat. No. 5,557,596, the disclosure of which is incorporated herein by reference. 
   Micro-actuator  110  may be any of a variety of types of micro-actuators. One type of micro-actuator which may be used to produce relative movement between objects such as storage medium  106  and casing  120  is an electrostatic drive. By affixing or forming electrodes on storage medium  106  and casing  120 , and then applying voltages to the electrodes to generate electrostatic force, relative movement between storage medium  106  and casing  120  may be produced. By taking into account the details of the physical connection between storage medium  106  and casing  120 , the voltage and resulting electrostatic force may be manipulated to control the resulting movement between the objects. 
   One type of micro-actuator  110  suitable for use with the present invention is depicted in FIG.  4 . An electrostatic drive  150  includes a plurality of mover electrodes  152  secured to mover  154  (e.g., storage medium  106 ), a plurality of stator electrodes  156  secured to stator  158  (e.g., casing  120 ), and a driver  160 . Typically, as indicated in  FIG. 4 , both mover electrodes  152  and stator electrodes  156  are disposed in a linear configuration which is parallel to the motion axis (e.g., the X axis or the Y axis). Driver  160  (responding to a command control  170 ) causes voltages to arise at mover electrodes  152  and/or stator electrodes  156 , which results in application of electrostatic forces between mover  154  and stator  158 . Due to fringing of the electrostatic fields and the mechanical suspension used to couple mover  154  to stator  158 , the electrostatic forces cause mover  154  to move along the X and/or Y axis relative to stator  158 . Varying the voltages applied to electrodes  152 ,  156  produces changes the relative position of the mover  154  and stator  158 . Other aspects of this type of electrostatic micro-actuator are disclosed in U.S. patent application Ser. No. 10/043,971, filed Jan. 11, 2002, and commonly assigned herewith, the disclosure which is incorporated herein by reference. Upon reading and appreciating this disclosure, those skilled in the art will appreciate that similar systems may be capable of motion in multiple directions, including linear/axial motion and motion in curved directions or motions of other shapes. 
   The relative positions of mover  154  and stator  158  may be determined using a variety of different position sensing methods and systems. As one example, a calibrated position sensor based on a capacitance measurement can be used as a suitable high resolution encoder. Capacitive position sensors typically detect changes in position by measuring capacitance between two relative moving objects. The charge of the capacitor is measured and used to calculate a relative position between the two objects. Aspects of a suitable method and system for determining the relative positions of mover  154  and stator  158  are disclosed in U.S. patent application Ser. No. 10/100,204, filed Mar. 18, 2002, and commonly assigned herewith, the disclosure which is incorporated herein by reference. 
   To write and/or read data in storage areas  108  of storage medium  106 , the mover  154  must accelerate to a desired scan velocity (relative to stator  158 ), maintain that scan velocity during the data writing and/or reading process, and then decelerate to a stop. In some systems, the accelerate/scan/decelerate process may occur in approximately 2 milliseconds. If a constant scan velocity is not maintained during the writing and/or reading process, the periodicity between areas written to or read from on storage medium  106  will not be constant, thereby leading to increased error rates in the writing and/or reading process. For example, if field emitters  102 ,  104  are writing data at a fixed rate, and storage medium  106  is either accelerating or decelerating during the writing process, the points to which data are written will not be uniformly spaced. A later attempt to read that data may fail unless the acceleration/deceleration profile during the read process matches the acceleration/deceleration profile of the write process for that particular data. 
   Mover  154  is typically connected to stator  158  by resilient flexures (not shown) that permit mover  154  to move in an X-Y plane relative to stator  158 . The flexures provide very little or no mechanical motion damping of mover  154 . In addition, as noted above, casing  120  typically is adapted to maintain storage medium  106  in a partial vacuum. Thus, no or only very limited air-damping of the motion of mover  154  (storage medium  106 ) is available. Mover  154  therefore acts as an undamped spring-mass system. Because of the undamped condition of mover  154 , mover  154  is particularly vulnerable to vibration or “ringing” at a resonant frequency f r  of the device. Mover  154  may be treated as an undamped mechanical oscillator having a very high Q (on the order of 8000 or more), where Q is the “quality factor” of a system. A high Q indicates low damping, a narrow angular oscillation frequency Δω, and a long decay time. 
   Vibrations at the resonant frequency f r  introduce variability into the periodicity between adjacent storage areas  108 , and makes accurate writing and reading of data difficult. Resonant frequency f r  may vary from device to device, and is dependant on a number of variables, including the physical connection between mover  154  and stator  158 , the spring stiffness of flexures supporting mover  154  within stator  158 , the size and mass of mover  154 , materials used to form mover  154  and stator  158 , and manufacturing tolerances, to name a few. 
   The undamped condition of mover  154  allows harsh acceleration or deceleration of mover  154  to excite vibration at the resonant frequency f r  of the device. The motion profiler system and method described herein allows a commanded acceleration and deceleration profile of any shape as is required to minimize the excitation at the mechanical resonant frequency f r  and the resultant ringing after moving mover  154 . 
   Since the amount of space (distance) and time available to accelerate and decelerate the mover is limited, the acceleration and deceleration profile must be chosen to limit the energy input to mover  154  at the mechanical resonant frequency f r , yet allow rapid acceleration and deceleration to and from the desired scan velocity for accessing the data track (such as row  140  of storage areas  108 ). The implementation of the acceleration/deceleration profile permits movements of any length within the mover&#39;s operational range, even if the desired steady-state scan velocity is not achieved. 
   For smooth acceleration and deceleration of mover  154 , it is desirable to reduce and smooth the jerk of the acceleration, where jerk is the time derivative of the acceleration. In one embodiment according to the invention, the acceleration profile is sine-shaped. The time derivative of a sine-shaped acceleration profile produces a reduced and smooth jerk, and results in the least resonant ringing after accelerating or decelerating mover  154 . A sine-shaped acceleration profile has the benefit of easily derived integrals for velocity and position of mover  154 . However, acceleration profiles different than a sine-shaped profile may be used without departing from the invention. 
   For a sine-shaped acceleration profile, the equations defining the velocity, position, and acceleration for mover  154  versus time are provided below: 
   Where:
         X acc  is the distance allowed for acceleration; and   V scan  is the desired scan velocity,       

   Then:
 
The time allowed for acceleration to  V   scan  is  T   a =2 X   acc   /V   scan ;
 
The angular frequency  W   a   =πr/T   a ;
 
The peak acceleration  Acc   pk   =V   scan   W   a /2 =πV   scan   2 /4 X   acc ;
 
and
 
 velocity( t )= V   scan (1−cos( W   a   t ))/2;
 
position( t )=∫velocity( t )= V   scan ( t −sin( W   a   t ))/2  W   a ; and
 
acceleration( t )=∂velocity( t )/∂ t=V   scan   W   a sin( W   a   t )/2
 
   Using the provided equations,  FIG. 5  shows plots of the position, velocity, and acceleration profiles for the example case where the distance allowed for acceleration X acc  is 5.0 μm and the desired scan velocity V scan  is 30.0 μm/ms. For those conditions, the time T a  allowed for acceleration to the desired scan velocity is 0.333 ms. Of course, the motion profiler according to the present invention may be used with systems having distance and time constraints other than the example constraints, although the resultant position, velocity and acceleration profiles will differ accordingly. 
   Walking through the position curve of FIG.  5  and sampling the velocity curve, a velocity versus position curve or table may be extracted, as shown in FIG.  6 . Recalling from above that the distance allowed for acceleration X acc  to the desired scan velocity V scan  of 30.0 μm/ms is 5.0 μm, it can be seen in  FIG. 6  that the x-axis represents a distance of 5.0 μm that has been divided into 160 increments or index points, with each increment equaling 5.0/160 μm (approximately 0.03 μm). Of course, any different number of increments may be used, depending upon the desired or required resolution of the velocity versus position curve. The number of increments may be limited, for example, by the amount of memory available for storing the velocity versus position data. 
   By using position-based velocity profile data as shown in  FIG. 6 , any length of move by mover  154  is permitted. If mover  154  is moved to access a data track (such as row  140 ) to write or read data (a “scan move”), mover  154  must undergo a “full length” move (5.0 μm in the example) to reach the desired scan velocity (30.0 μm/ms in the example). In this instance, mover  154  will accelerate following the entire velocity versus position profile of  FIG. 6  to reach the scan velocity, maintain the scan velocity for the duration of time necessary to access the data track, and then decelerate to a stop by reversing the acceleration profile. However, if mover  154  is being moved to an adjacent data track (such as from row  140  to row  142 ) in preparation to write or read data (a “seek move”), mover  154  may be required to move less than the distance required to reach the desired scan velocity (i.e., shorter than a full length move). In this instance, mover  154  will accelerate following the velocity versus position profile of  FIG. 6  until one half the required move distance has been traversed, and then decelerate to a stop by reversing the acceleration profile. A seek move may also be longer than the distance required to reach the scan velocity (i.e., longer than a full length move). In this instance, mover  154  will accelerate following the velocity versus position profile of  FIG. 6  until the scan velocity is reached, maintain the scan velocity until mover  154  is within the “full length” (5.0 μm in the example) of the acceleration/deceleration profile of  FIG. 6 , and then decelerate to a stop by reversing the acceleration profile. By indexing the velocity versus position table with the value of the commanded position (that is, how far mover  154  must move from a given position), the desired end position can be reached with the correct acceleration and deceleration profile for any length of move without scaling the velocity versus position values for various move lengths, as would be required for a velocity versus time profile. As noted above, the deceleration profile is simply the reverse of the acceleration profile shown in FIG.  6 . 
   Referring to  FIG. 7 , a block diagram of a motion profiler system and method for controlling the movement of movers M 1  and M 2  according to the invention is illustrated. Movers M 1  and M 2  in  FIG. 7  may be individual movers  154 , or may alternately be a group of movers controlled in parallel. 
   Profiler  200  is a data processor, preferably a digital data processor, that is provided with a current position  202  and a target position  204  for at least one mover. Current position  202  is based upon the last commanded position of the selected mover. Target position  204  is based upon the position of the data track to be accessed. In one embodiment according to the invention, to accommodate the desired range and resolution of movement of the movers, target position  204  is a 19 bit digital word. Depending upon the required range and resolution of movement of the movers, target position  204  may be larger or smaller than a 19 bit digital word. 
   Using the current position  202  and target position  204  of mover M 1 , M 2 , profiler  200  indexes velocity table  210 . Specifically, the current position  202  and target position  204  are used to determine the “distance from start” on the acceleration portion of the move, or the “distance to go” on the deceleration portion of the move. Using either the “distance from start” or the “distance to go”, any of several well-known table look-up interpolation methods (such as straight-line interpolation or higher order polynomial-based interpolation) is used to index the velocity table  210  and provide the desired velocity Vel desired  to profiler  200 . Using the desired velocity retrieved from velocity table  210 , profiler  200  calculates the next commanded position (PosCmd next ) as the sum of the last commanded position (PosCmd last ) and the desired velocity Vel desired  times the control update period Ts, where Ts is the time interval at which profiler  200  updates. That is, PosCmd next =PosCmd last +Vel desired Ts. The next commanded position PosCmd next  is passed to the at least one selected mover and its associated driver (M 2  and M 2   driver  in the example of FIG.  7 ). In addition, the next commanded position PosCmd next  becomes the last commanded position PosCmd last  and is used to update the current position  202 . As discussed above with respect to target position  204 , in one embodiment according to the invention, to accommodate the desired range and resolution of movement of the movers, the next commanded position PosCmd next  is a 19 bit digital word. Depending upon the required range and resolution of movement of the movers, the next commanded position PosCmd next  may be larger or smaller than a 19 bit digital word. 
   In one embodiment according to the invention, to minimize the energy input to the movers&#39; mechanical resonance frequency f r , prior to being passed to the selected mover(s), the next commanded position PosCmd next  is passed through filter  220 . Filter  220  is preferably by a notch filter, and more preferably a digital notch filter. Filter  220  processes the next commanded position signal to further reduce or eliminate energy at the resonant frequency f r  of the mover. In one embodiment of the invention, if the memory available for storing velocity table  210  is adequately large, the characteristics of filter  220  may be integrated into the data of velocity table  210 . In another embodiment according to the invention, the resonant frequency f r  of individual movers is measured during their manufacturing process, such that filter  220  may be tuned to filter at the resonant frequency of each particular mover. 
   Using the next commanded position PosCmd next , drivers M 1   driver  and M 2   driver  move their associated movers M 1  and M 2 , respectively, to the next commanded position. Drivers M 1   driver  and M 2   driver  may be, for example, the electrostatic drive  150  disclosed in U.S. patent application Ser. No. 10/043,971 and referenced above. 
   In one embodiment according to the invention, a position sensing system may be coupled to micro-actuators  110  to provide a closed-loop feedback  230 . Closed-loop feedback  230  may be implemented to provide additional position and velocity control over individual movers, independent of the commanded position from profiler  200 . Closed-loop feedback  230  may be provided, for example, using the method and system for determining position of a body as disclosed in U.S. patent application Ser. No. 10/100,204 and referenced above.