Mounting a position sensor equipped planar armature, in operative juxtaposition to a stator, by a spring colonnade basket of flexure columns, provides a low-friction precision X-Y-Theta positioner which has computer control of operations with feedback of actual position for closed loop servo operation. The spring colonnade basket provides freedom of motion in the X, Y and Theta (rotation about an axis mutually perpendicular to X and Y) dimensions without static friction present in sliding or rolling motions. The armature, which carries a gripper or other end effector and is free to move only within its own plane, is positioned by a motor made up of the armature and a stator mounted on a support platform. The stator is made up of a pair of U-shaped permanent magnets, each having U-shaped pole pieces mounted orthogonally to each other, at 45 degree angles to the axis of the magnet, so that each electromagnet controls motive power in both X and Y dimensions. Imbalances in the vector sums of the motive power of the electromagnets produces torque which provides Theta motion. Position sensors are mounted on the armature and stator. Position sensing signals are fed back to a computer for use in operations including repositioning The computer can vary the compliance of the flexure columns by adjusting the motor control currents as a function of a compliance variable.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to electromagnetic precision positioners operable to 
provide precision positioning capability to a robot end effector, and more 
particularly relates to an electromagnetically powered X-Y-Theta 
positioner device, comprising an armature platform supported for X-Y-Theta 
motion by a spring colonnade basket in juxtaposition to its stator, which 
positions operates without static friction, with controlled power and 
compliance with precise control of position, and with positional feedback 
for closed loop servo operation. 
2. Description of Related Art 
The need for precision positioning is well known in a number of mechanical 
and electronic arts. Close tolerance machining, for example, requires that 
the tool be positioned exactly. Various mechanisms for accomplishing such 
positioning, and various techniques for getting the most out of such 
mechanisms, are known. 
A known mechanism for accomplishing precision positioning within a plane is 
the use of a linear motor to drive and hold a device in a particular 
position. Such linear motors, as well as electromechanical stepping 
motors, hydraulic actuators, and other mechanisms are known for use in 
precision positioning. 
A typical solution to the need for high precision motion, in a plane of 
scanning for a robot end effector, is to provide a pair of linear 
actuators mechanically connected in tandem so that the Y-actuator is 
physically carried at the end of the X-actuator. A disadvantage of this 
approach is that the Y and X stages are mechanically in series, the Y 
stage moves the payload, but the X stage must move both the Y stage and 
the payload. Symmetry is broken, and in critical applications the control 
strategies for the X stage and the Y stage must be different for the 
different dynamics involved. Theta motion (rotation about an axis 
orthogonal to X and Y) may require still another tandem stage. 
RELATED PRIOR ART PATENTS 
U.S. Pat. No. 3,457,482, Sawyer, Magnetic Positioning Device, July 22, 
1969, shows a head incorporating two parallel sets of magnets along each 
of two perpendicular axes, with driving circuitry for selectively 
energizing the magnet coils. 
U.S. Pat. No. 3,735,231, Sawyer, Linear Magnetic Drive System, May 22, 
1973, shows an orthogonal linear motor mechanism, for moving a head about 
a platen, with a servo loop for precision control of position. 
U.S. Pat. No. 3,867,676, Chai et al, Variable Reluctance Linear Stepper 
Motor, Feb. 18, 1975, shows a variable reluctance linear stepping motor 
with a special set of windings in series aiding relationship. 
U.S. Pat. No. 3,935,486, Nagashima, Finely Adjustable Table Assembly Jan. 
27, 1976, shows an electromechanically adjustable table assembly in which 
the table is suspended by oval, flat springs and positioned 
electromagnetically. 
U.S. Pat. No. 4,286,197, Eberhard et al, Two-Coordinate Positioning Device, 
Aug. 25, 1981, shows a two-coordinate tool positioning device having 
orthogonal linear motors. 
U.S. Pat. No. 4,455,512, Cornwell et al, June 19, 1984, System for Linear 
Motor Control, shows a feedback system for table positioning. 
U.S. Pat. No. Re 27,289, Sawyer, Magnetic Positioning Device, Feb. 15, 
1972, shows a precision positioner having two U-shaped soft iron pole 
pieces, each wound with copper wire, biased with fields produced by a 
permanent magnet. In the absence of coil current, magnetic flux links the 
two pole pieces, symmetrically dividing between the left and right legs of 
each pole piece, forming a closed path through the air gaps and soft iron 
armature. Induced fields in the pole pieces add or subtract from the bias 
fields and provide positioning impetus to an armature. The Sawyer motor 
shows a two-dimensional linear positioner based on permanent magnet flux 
steering, using a pair of magnets arrayed asymmetrically or achieve within 
a single step limited two-dimensional linear motion over a restricted 
displacement. 
Application Note 197-2, "Laser and Optics," 5501A Laser Transducer, 
Hewlett-Packard Corp., 1501 Page Mill Road, Palo Alto, Calif., 1980, pp. 
32-33, 41-43, shows a technique for sensing table position optically. 
Copending U.S. patent Ser. No. 563,334, filed Dec. 20, 1983, Hollis, 
Precision X-Y Positioner, shows a friction-free X-Y positioner similar in 
some respects to the X-Y positioning capability of the X-Y-Theta 
positioner of this patent application, of which Hollis is a co-inventor. 
DISADVANTAGES OF THE PRIOR ART 
In the prior art, fine positioners tend to require sliding motion planar 
mechanisms, or wheeled carriages, to allow the multidimensional motions 
required to position the end effector as desired. Such fine positioners 
tend to be heavy because of the multiple coils and associated magnet 
armatures. This heaviness causes the fine positioners to be slow in 
response, subject to wear problems, and in need of frequent lubrication, 
adjustment and repairs. 
Even a cantilevered fully-suspended platform positioner such as described 
by Nagashima does not resolve all problems of excess mass, and 
susceptability to oscillation. Nagashima's response to the oscillation 
problem, damping in silicon oil, slows the response and makes it difficult 
to operate in attitudes other than horizontal. 
In the prior art fine positioners of the linear motor type, the position of 
the armature may be estimated as a function of the current, but the actual 
position of the armature cannot accurately be determined under conditions 
of varying load and varying dynamics. 
Particularly in the context of tool positioning for machining, the need 
persists for a strong, durable precision positioning device which does not 
suffer the wear and clearance problems associated with frictional sliding 
and wheeled carriages, and which provides actual position feedback. 
SUMMARY OF THE INVENTION 
In the electromagnetic precision positioner according to the invention, an 
armature is mounted for free movement in X, Y and Theta directions in a 
reference plane. The armature is supported in its own plane by a spring 
colonnade basket of flexure columns which permit limited free motion of 
the armature in X-Y-Theta directions. The armature is arranged to carry a 
gripper, chuck, cutting tool, or other end effector, and is free to move 
within the reference plane to the desired X-Y-Theta coordinates. A stator 
positioned close to the armature provides motive power by means of a pair 
of U-shaped magnets, each having coil-equipped U-shaped pole pieces 
mounted orthogonally to each other and at 45 degree angles to the axis of 
the magnet. The armature and stator together form the positioner motor. 
Electrical currents applied to the coils provides motive power to move the 
armature to the desired X-Y-Theta coordinates, still within the armature 
plane. There is an inconsequential Z-component which can be ignored or 
compensated. Electromotive force differentials applied to the armature 
oppose any forces tending to move the armature from the desired X-Y-Theta 
coordinates. Position and angle sensors provide feedback to a computer 
which calculates position and angle error statements to control motor 
control circuits driving the positioner motor. Compliance, the resistance 
to repositioning force, may be controlled by the computer by varying the 
response of the motor control circuits to instantaneous position and angle 
error statements. 
An object of the invention is to provide X-Y-Theta precision positioning 
for the end effector of a robot, without encountering the problem of 
static friction which is inherent in sliding planar mechanisms and wheeled 
carriages or in oscillation damping mechanisms. 
Another object is to provide a robust precision X-Y-Theta positioner which 
is easy to manufacture and to service. 
Another feature of the invention is its provision of variable compliance 
capability in a precision X-Y-Theta positioner. 
An advantage of the invention is that it permits X-Y-Theta positioning of a 
tool with precision in the sub-micron and fractional-degree ranges. 
Another advantage of the invention is that it permits the quick and easy 
X-Y-Theta positioning of a tool without the loss of precision required to 
overcome static friction inherent in sliding or wheeled devices. 
The foregoing and other objects, features and advantages of the invention 
will be apparent from the more particular description of the preferred 
embodiments of the invention, as illustrated in the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows the precision X-Y-Theta positioner in abbreviated detail for 
ease of description. Support post 1 and bracket 2 may be mounted on a 
movable gross positioning device or may be solidly mounted to a table. 
Solidly mounted at the top of support post 1 is stator 3, which defines a 
reference plane from which motions may be defined. The precision X-Y-Theta 
positioner may be inverted from the attitude illustrated in FIG. 1, or 
used in other attitudes with gravity compensation. 
Stator 3 includes electromagnets and may be considered as stator means. The 
stator means includes suitable electrical connections and mechanical 
support structure as is well known in the art. The stator means carries 
position sensors and rests inside a spring colonnade basket of flexure 
columns (4, 5 6 shown) which support armature 7 in its own plane parallel 
to the reference plane. Armature 7 may carry a chuck or other fixture 8, 
which fixture is suspended, protected against any significant Z-motion 
normal to the base plane, but allowed friction-free X-Y-Theta motion to 
positions determined by currents in coils (not shown) on magnet 9 pole 
pieces 10 of the stator means. The armature, of magnetically soft 
material, is configured for weight minimization and flux maximization, 
with several teeth arranged for flux coupling to the electromagnets in the 
stator means, as is known in the art, and may be considered as armature 
means. The instantaneous position of the armature means is sensed and 
signaled by position sensors 11. The position sensors may be lateral 
effect cells, quad cells or other position sensors exhibiting proper 
qualities of precision, cost and mass, and may be considered as position 
sensing means. 
The spring colonnade basket, which serves as armature support means, is 
balanced so as to provide a friction-free suspension to the armature in 
operative juxtaposition to the stator. The flexure columns (4, 5, 6 shown) 
may be of piano wire material or other material capable of flexing in 
X-Y-Theta dimensions while maintaining longitudinal stability. 
FIGS. 2 and 3 illustrate one of the two positioner motors. The motors are 
positioned so as to share a common stator and a common armature. Each 
motor is made up of armature 7 and stator 3. Permanent magnet 9, with its 
pole pieces 10S and 10N, operates with armature 7 to provide motive force 
in both the X and Y dimensions at the same time. Theta torque is derived 
from imbalances in X drives, or from imbalances in Y drives, from the two 
motors. 
Stator 3, made up of two permanent magnets 9 with their pole pieces 10S and 
10N and associated windings 30S and 30N, provides flux differentials at 
teeth (7-1, 7-2, 7-3, 7-4 and 7-5 shown) of armature 7, providing motive 
power in both the X and Y dimensions at the same time. Pole pieces 10S and 
10N are set orthogonal to each other, and at 45 degree angles to the axis 
of their permanent magnet 9. Armature 7 is positioned appropriately for 
the positioning of pole pieces 10S and 10N so that vector X-Y motions may 
be impressed on armature 7 with respect to the fixed platform base and 
magnet 9. 
FIGS. 4-7 diagram various types of motions within the repertoire of the 
precision X-Y-Theta positioner. There are at least two electromagnets so 
as to provide magnetomotive forces balanced or imbalanced selectively for 
pure translational (X-Y) motions, for rotational (Theta) motions, or for 
composite vector translational-rotational (X-Y-Theta) motions. 
FIG. 4 shows a representative pure translation (plus x) motion. FIG. 5 
shows a representative pure translation (plus y) motion. FIG. 6 shows a 
representative pure rotation (plus Theta) motion. FIG. 7 shows a 
representative composite vector translation-rotation (minus x, plus y, 
minus Theta) motion. 
FIG. 8 diagrams the electronic relationships of the armature, the magnet 
pole pieces, the computer and the position sensor. Position sensors 11 
serve as the position sensors for the armature, providing unbalanced 
voltage signals related to the instantaneous position of the armature with 
respect to related reference beacon lamps mounted on the stator at 
corresponding reference points. The computer accepts these position 
signals, calculates position error statements required to move from 
instantaneous position to desired position, and sends motion control 
currents back to motor magnet coils 12, 13, 14 and 15 (which are wound on 
pole pieces 10) as required to cause movement of armature 7 to the desired 
position and orientation. Reference point beacon lamps 16, provided with 
current via circuitry 17, provide positioning information with respect to 
the reference points of position sensors 11. The position sensors 11 in 
the preferred embodiment are a pair of lateral effect cells, located in 
the stator along diameter d at distances d/2 from the center of the 
stator, directly opposite their related beacon lamps. As the light which 
may be infra-red) moves between the electrodes of the cells, the cells 
provide unbalanced location voltage signals to the computer 18, via 
circuitry including representative amplifier 22, multiplexer 23 and 
analog-to-digital converter 24. Computer 18, which has been provided with 
the coordinates of the desired new location, calculates the position and 
angle error, and provides a position and angle error statement to motor 
control means made up of digital-to-analog converters, including 
representative D/A converter 20 and circuitry including representative 
amplifier 21, which provides motion control currents. This completes the 
servo loop. Motion of the armature is accordingly provided, and a new 
instantaneous position and orientation is signaled by position sensors 11. 
This new instantaneous position and orientation signal is carried back to 
the computer for dynamic updating of the position and angle error 
statement. Relatively simple programming changes to computer 18 can be 
made to provide not only desired motions to armature 7, but also to 
control the compliance (flexure column resistance to repositioning of the 
armature 7). The flexure columns provide a nominal compliance; by 
programming in a ratio change to the digital values provided to the 
analog-to-digital converter, the nominal compliance may be altered from 
minimum resistance to repositioning to maximum resistance to 
repositioning. Compliance may also be programmed to vary as a function of 
position (for example, higher near the limit of travel) or as a complex 
variable or logarithmic function. 
DESCRIPTION OF HOW THE MOTOR WORKS 
The motor differs from the well known linear positioners, such as that of 
Sawyer, in that a single magnet, with one pole piece normal to the other, 
provides both X-motion and Y-motion. Theta motion is derived from 
imbalances between X motions and Y-motions from different motors; these 
imbalances provide Theta torque for rotational motions. Controlled 
imbalances result in composite X-Y-Theta motion vectors. 
Referring to FIG. 3, a permanent magnet 9 (which can be Alnico or 
rare-earth or other magnetic material) provides a bias field. Magnetic 
flux passes through magnetically soft pole pieces 10S and 10N, dividing 
equally through the two legs of each pole piece. Several teeth provided in 
the pole pieces and armature serve to increase the available force. In the 
absence of current in the coils 30N and 30S, the armature assumes a 
neutral position, supported by the flexure columns. Coils 30N are wound 
with opposite sense on the two legs of the pole piece 10N; similarly for 
30S and 10S. Current flowing through the coils 30N induces a magnetic flux 
which adds to and subtracts from the permanent magnet flux in the two legs 
of pole piece 10N, causing the armature to be attracted to the right or 
left, depending on the sign of the current. The armature is free to move 
by slightly deforming the flexure columns; the small restoring force 
related to energy stored during previous flexing of the flexure columns is 
predictable and can generally be compensated for or ignored. Thus a motion 
is generated whose value is proportional to the algebraic value of current 
flowing in the coils. The same relationship holds for pole piece 10S and 
coils 30S, except the armature motion is in and out of the plane of the 
paper in the view shown. The teeth on the north and south pole pieces are 
arranged orthogonal to each other, so motions due to coils 30S and 30N are 
independent. The provision of plural motors allows for generation of 
torques to produce rotational (Theta) motions about an axis mutually 
perpendicular to the X and Y axes. 
LATERAL EFFECT CELLS 
Lateral effect cells (position sensing photodiodes) are well-known in the 
art, and are commercially available. The lateral effect cell is 
essentially a planar photodiode which reacts to a beacon light to provide 
electrical signals as a function of the location of the spot of light from 
the beacon. In the preferred embodiment, the beacon lights are lamps 16 
mounted on the stator. The lateral effect cell has a square active 
surface; the position of the small spot of light on the surface of each 
lateral effect cell is determined by measuring the generated photocurrents 
in four electrodes arranged on the periphery of the square active surface 
of the lateral effect cell. If we denote the two primary measuring 
directions as X and Y, then electrodes X1 and X2, whose principal axes are 
perpendicular to the X measuring direction, are used to measure the X 
position of the light spot; similarly for Y. The X position is given by 
dividing the difference in the X1 and X2 currents by their sum. The Y 
position is given by dividing the difference in the Y1 and Y2 currents by 
their sum. These calculations are performed by analog circuits, digital 
circuits, by software, or by a combination of these methods, as is well 
known in the art. 
In the preferred embodiment, two lateral-effect cells are used as position 
sensors to determine position and orientation of the armature. Each of the 
two lateral effect cells is located at some radial distance d/2 on a 
diameter at opposite sides of the stator. 
Let x,y denote the local coordinate system on one cell, and x',y' denote 
the local coordinate system of the second cell a distance d away. The x,y 
and x',y' frames are unrotated with respect to each other; i.e., x is 
parallel to x' and similarly y is parallel to y'. 
Let X,Y denote a coordinate system unrotated with respect to x,y and x',y' 
and lying midway (d/2) between the two cells. Required is the position and 
orientation of an imaginary mark in the center of the armature (home 
position coincident with X=0, Y=0, Theta=0). 
From geometrical considerations, 
##EQU1## 
FOUR MODES OF OPERATION OF THE PRECISION X-Y-THETA POSITIONER 
AS A FINE POSITIONING DEVICE 
The fine positioning device may be used to execute fine X-Y-Theta motions, 
which motions are sub-micron in X and Y precision and sub-degree in Theta 
precision, from a total range of the order of 2 millimeters and several 
degrees. Motion is rapid and controlled by the feedback loop involving the 
built-in lateral effect cell position sensors, but could be provided by 
separate, external sensors which sense the work environment directly. The 
ability to execute fine motions has many applications in science and 
engineering. 
AS A VARIABLE COMPLIANCE DEVICE 
The precision X-Y-Theta positioner, when operating in a "regulator mode", 
attempts to maintain its commanded position. Any external forces tending 
to displace the device will be met by restoring forces generated by the 
servo controller. By varying the closed loop gain parameter (in the 
preferred embodiment this is done simply by changing coefficients in the 
computer control program) the compliance or stiffness of the device is 
varied. The natural compliance of the device in open-loop mode is 
determined by the spring constants of the flexure columns. In closed-loop 
mode, the compliance is programmed, and may range from much greater than 
to much less than the natural compliance, as well as equal to the natural 
compliance simulating open-loop mode. This property is extremely useful 
for tasks involving the fitting together of two or more mating parts, as 
in a robot assembly operation. 
AS A VARIABLE FORCING DEVICE 
In some applications, it is desirable to exert known forces on a workpiece, 
as in pull-testing of electrical connector pins. In many cases, negligible 
motion occurs during the action of the force. The precision X-Y-Theta 
positioner is capable of exerting programmed translational forces and 
torques, since the force exerted on the armature by the pole pieces is 
proportional to the current flowing through the coils, and the forces on 
the flexure columns of the colonnade are linearly related to displacement 
from equilibrium. 
AS A MEASURING DEVICE 
By virtue of its built-in position sensors, the precision X-Y-Theta 
positioner can be used as a passive measuring device, in a mode where the 
coil drive currents are disabled. Applications such as parts profiling can 
be accomplished by sensing the relationship between a mechanical probe or 
stylus attached to the movable armature and the fixed part of the device. 
In this mode, attached to an external coarse positioner, the size and 
shape of parts can be determined. 
COMBINATION MODES AND APPLICATIONS 
Since the precision X-Y-Theta positioner incorporates digital control, 
operation can be switched between the various modes described above as may 
be necessary to perform a given task. Typical tasks for the precision 
X-Y-Theta positioner are semiconductor mask alignment, chip probing and 
chip placement; tape and disk head assembly; interpolators for stepping 
motors; scanning microscopy; and laboratory investigations. 
FIGS. 9 and 10 illustrate details of the preferred embodiment of the 
precision X-Y-Theta positioner. FIG. 10 shows a colonnade of flexure 
columns (including 4, 5 and 6), which support armature 7. The armature is 
configured for light weight and to present itself to the motor magnets. 
Armature 7 carries reference point beacon lamps 16. Support post 1 and 
bracket 2 form a substantial base upon which the X-Y-Theta positioner is 
assembled. Stator 3 is made up of electromagnets and suitable supporting 
structure. Flexure columns 4 support armature 7 in operable juxtaposition 
with stator 3. Stator 3 includes electromagnets 9, with associated coils 
(12 shown) wound about pole pieces 10. Position sensors 11 operate with 
associated beacon lamps 16. Flexure columns (4, 5 and 6 shown) support 
armature 7 and provide for armature motion in its own plane by flexing in 
X-Y-Theta directions. 
Motor magnet pole pieces (10S shown) provide magnetic flux to teeth (7-1 
shown) on armature 7 to provide power for positioning. Lateral effect 
cells 11 operate with beacon lamps 16 at reference points on the stator 
and armature and provide position signals from which position error 
statements are derived by the computer. 
Adjustment may be required to provide appropriate clearance between 
armature and stator. Adjustment mechanism 25, made up of a leadscrew and 
capstan nut, provides for manual adjustment, by capstan bar in the capstan 
nut, so as to locate the stator with proper clearance to the armature. 
Adjustment mechanism 25 operates by raising or lowering stator support 
platforms 27 and 28, which are in fixed relationship to one another in 
their function of supporting the electromagnets of the stator, and which 
slide on support rods 29. Support rods 29 do not flex as do flexure 
columns such as 4, 5, and 6. 
These and other alternatives will be apparent to those skilled in the 
positioner art, without departing from the spirit and scope of the 
invention as pointed out in the following claims.