WO1996034722A1

WO1996034722A1 - Two axis drive arm

Info

Publication number: WO1996034722A1
Application number: PCT/US1996/006189
Authority: WO
Inventors: Genco Genov; Dimitre Todorov; Gou-Kin Cui; Michael Milev
Original assignee: Genmark Automation, Inc.
Priority date: 1995-05-02
Filing date: 1996-05-02
Publication date: 1996-11-07

Abstract

An arm structure (10) has n links (12, 14) and an end effector (16). Each link and the end effector rotates about parallel axes. A radial drive shaft (24) rotates a pulley (40) at the first axis (21) which communicates with additional pulleys at the other axes via belts. A rotary drive shaft (80) about the first axis drivingly rotates the first link. Radial and rotary drive sensors sense the rotational positions of the drive shafts in electrical signal format. A computer computes the locus of the end effector from the electrical signal and controls the drive motors such that the end effector moves linearly.

Description

TWO AXIS DRIVE ARM

Technical Field The present invention relates to a precision arm mechanism which will extend and retract in a straight line and is suitable for positioning various objects such as semiconductor wafers, cassettes holding such wafers, computer hard discs, and the like for processing and/or use.

Background Of Invention

The use of robot arms for positioning and placing objects is well known. Generally, the arms have Z, R and θ motion in a conventional cylindrical coordinate system. The capability of providing straight line motion is very important in the processing of semiconductor wafers so as to allow them to be very accurately positioned at a work station where processing steps take place. The R or straight line movement of the end effector or mechanical hand at the end of the arm has been accomplished in a number of manners.

As one example, telescoping arms have been utilized for this purpose. In such a structure one slidable member fits within another thus allowing linear extension of the arm.

More commonly, two link arms with equal length links have been utilized for this purpose. The links are connected to each other so that distal end of the first link is pivotally attached to the proximal end of the second link. The links utilize belt drives which are provided for coordinately rotating the second link to the first link to provide a rotation ratio, i₂. of 2/1, and to provide a rotation ratio, i₃₂ of 1/2 between the end effector and the distal link of the robotic arm. When i₂. is equal to 2/1 and i₃₂ is equal to 1/2, the result is that i₃₁, the rotation ratio of the end effector relative to the first link, is equal to 2/1 x 1/2 or unity and straight line motion results. In the case of 3 link arms, such as those shown in U.S. Patent 5,064,340, the rotation ratio between the third and second links is 1/1 and other ratios are as just discussed above. In this situation i₂₁ is equal to 2/1, i₃₂ is equal to 1/1 and i₄₃ is equal to 1/2. This assures that i₄. is equal to unity and straight line motion results.

In the two link arm the connection between the rotation ratio and the torque at the pivots in the links is i₂., which is equal 2/1, is equal T,/T₂ (assuming for simplicity of understanding 100% efficiency of the gearing, e.g., belt and pulley, arrangement), and i₃₂ is equal to 1/2, which is equal to T₃/T₂, what results is that i₃₁ is equal to T,,/T₃ or unity. This requires that T₁ equals T₃ wherein T_1# T₂ and T₃ represent, respectively, the torques of the respective pivots. Accordingly, the torque of the end effector pivot is equal to that of the driver. Usually two independent motors are used, one for radial motion of the end effector and the other for θ motion. When the torque of the end effector is not enough to provide needed stability and accuracy of the object being positioned, it is necessary to increase the motor power. This problem is particularly troublesome when the arm length is relatively long and/or the object which must be moved has a relatively large weight.

United Kingdom Patent Application GB 2193482A, published February 10, 1988 discloses a wafer handling arm which includes two unequal length links with the distal end of one link being pivotally attached to the proximal end of the other link, with the hand being integral with the distal end of the distal link and which utilizes a belt drive which is fixed to a cam to attain nearly straight line motion.

It is also known to utilize an isosceles triangle type linkage wherein two equal length links are pivoted together and a mechanical hand is pivoted to the distal end of the distal link. Pulleys and belts are utilized in such a manner that the angle between the two links changes at twice the rate as do the angles that each of the links makes with a line connecting the points about which their other ends are pivoted. This linkage provides drive directly from a motor shaft to the proximal end portion of the proximal link. A belt about a stationary pulley coaxial with the motor shaft passes about a pulley at the point of pivoting of the two links to one another. Another pulley and belt arrangement provides pivoting of another pulley where the second link is pivotally connected to the mechanical hand.

In another apparatus a pair of isosceles triangle type linkages face one another and the mechanical hand is pivotally attached to the distal ends of both of the distal links. The proximal ends of each of the proximal links is driven in an opposite direction of rotation by a single rotating motor shaft, generally through use of appropriate gearing. What results is a frogs leg type of motion with each isosceles triangle type linkage serving as means for controlling the other such linkage in such a manner that the angles between the two links of each of the isosceles triangle linkages changes at twice the rate as do the angles that each of the links makes with a line connecting the points about which their other ends are pivoted.

In U.S. Patent 5,064,340, which is incorporated herein in its entirety by reference, an arm structure is disclosed comprising first, second and third longitudinally extending links each having proximal and distal end portions. The second longitudinally extending link is twice the effective length of the first link. The proximal end portion of the second link is pivotally mounted to the distal end portion of the first link about a first pivot axis. The proximal end portion of the third link is rotatably mounted about a third pivot axis to the distal end portion of the second link. An end effector is pivotally mounted to the distal end portion of the third link for rotation about a fourth pivot axis. Means is provided for coordinately rotating the first link, the second link, the third link and the end effector at a rotation ratio of the first axis to the second axis to the third axis to the fourth axis of 1:2:2:1. Again, the torque of the end effector pivot equals that of the driver.

There is a problem which is common to all of the above described radial positioning arms. This problem is that the arms must sit idly by while a workpiece is being worked upon. For example, a semiconductor wafer is picked up from a loading cassette by an end effector located at the end of the arm. The wafer is moved to a processing station and deposited. The arm moves away and sits idly by until processing at the station is completed. Once the process is completed the single end effector must move into the processing chamber, pick up the processed wafer and retract, rotate to the receiving cassette, place the processed wafer in the receiving cassette, rotate to the loading cassette, move in and pick up another wafer, retract, rotate back to the processing chamber, place the wafer and retract again to wait for the process to be finished. This is a total of eleven movements, and the time these movements take limits the throughput, i.e., the number of workpieces (e.g., wafers) which can be processed in a given time.

U.S. Patent 5,007,784, which is incorporated herein in its entirety by reference, has addressed the above problem by providing a robotic arm which comprises an end effector structure which has a central portion and two substantially oppositely extending hands each capable of picking up a workpiece. The central portion is centrally pivotally mounted to the distal end portion of a distalmost of the links. The links, end effector structure and static structure are constructed to allow the robotic arm to reverse across the pivot axis of the proximal end portion of the proximalmost of the links. Radial drive means drives the links in a manner such that the pivot axis of the central portion of the end effector structure moves only substantially linearly radially along a straight line passing through and perpendicular to the pivot axis of the proximal end portion of the proximalmost of the links and to the pivot axis of the central portion of the end effector structure. The end effector structure is maintained at a selected angle to the line. Rotational drive means is also present. While this solution to the problem has worked generally quite well it is somewhat limited due to arm length constraints, particularly the constraints that

1) the proximalmost arm must be at least a minimum length as dictated by the requirements that the end effector reaches as far from its axis as is necessary in some semiconductor processing application along with

2) the required 1:2:1 or 1:2:2:1 geometric length relation between the links to provide straight line movement. As a result, situations exist wherein the dual end effector cannot fully efficiently reach positions which are particularly close to the axis of the arm. A solution to this problem would be highly desirable.

Another and very important problem which exists with present day robotic arm mechanisms is that they can only follow a straight line path or an arcuate path in the R, θ plane from one point another. Accordingly, if there is an object which protrudes into that plane of operation of the arm as may occur in a semiconductor processing operation, an inefficient path must be followed to anything hidden behind or shadowed by that object, namely, a straight line path must be followed to beyond the object and then radial motion must be imparted to move the end effector of the arm to the desired work station. The ability to follow a curved path would be desirable in that it would allow faster operation of the robotic arm mechanism.

Another serious problem with current day robotic arms is that when they are carrying relatively heavy loads, such as cassettes of wafers and/or are highly extended, they can exert only a relatively small amount of torque, namely the torque generated by the θ motion motor alone which is normally transmitted to provide the rotary motion via an axis which is located at the distal end of the first link. This creates a serious problem in properly positioning objects, particularly when the robotic arm is fully extended. A solution to this problem, for example of the nature of providing higher torque without going to significantly larger motors, would likewise be highly desirable.

Disclosure of Invention

The present invention is directed to overcoming one or more of the problems set forth above.

An arm structure constitutes one embodiment of the invention. The arm structure comprises n longitudinally extending links having respective proximal and distal end portions where n is 2 or is a larger integer. The proximal end portion of each of the links is pivotally mounted to rotate about a respective n th axes. An end effector is pivotally mounted to rotate about an n+1 st axis located at the distal end portion of the n th link. The pivotal axes of the links and of the end effector are all parallel. A radial drive rotatable shaft has a driven end portion and a driving end portion and extends along a first of the axes. Radial drive motor means serve for rotating the driven end portion of the radial drive shaft about the first of the axes. The driving end portion of the radial drive shaft is located adjacent the proximal end portion of the first link but not in contact therewith. A rotary drive rotatable shaft has a driven end portion and a driving end portion and extends along the first axis. Rotary drive motor means serves for rotating the driven end portion of the rotary drive shaft about the first of the axes. The driving end portion of the rotary drive shaft is located adjacent the proximal end portion of the first link and is in rotational driving contact therewith, n belt means are present with each being rotated by a respective one of n corresponding pulleys. A first of the pulleys is rotated by the driving end portion of the radial drive shaft. The pulleys each have a respective effective diameter, D_n. A quantity, K, is defined as equal to 2(D₂/D,,) - 1 wherein the subscripts 2 and 1 refer, respectively, to the second and first of the effective diameters. Each succeeding belt mean except for the n th is arranged to rotate the next successive link about its respective axis. The n th is arranged to rotate the end effector about the n+1 st axis. Radial and rotary drive sensor means each measure a quantity indicative of the rotational position of the respective radial and rotary drive shafts and generate electronic signals representative of such rotational positions. Means communicate the electronic signals to electronic computing means. The electronic computer means computes the locus of the end effector from the rotational positions of the radial and rotary drive shafts. The computer means controls the drive motors such that the number of revolutions of the radial drive shaft, n_R, divided by the number of revolutions of the rotary drive shaft, n_θ, is equal to -K so as to position the end effector to follow a desired path and to arrive at a desired locus.

In accordance with another embodiment of the invention a method is set forth of controlling an arm structure. The arm structure comprises n longitudinally extending links each having respective proximal and distal end portions where n is 2 or is a larger integer. The proximal end portions of the links are pivotally mounted to rotate about respective n th axes. An end effector is pivotally mounted to rotate about an n+1 st axis located at the distal end portion of the n th link. The pivotal axes are parallel. A radial drive rotatable shaft and a rotary drive shaft each have a driven and driving end portion. They extend along a first of the axes. Radial and rotary drive motor means serves for rotating the driven end portions of the drive shafts about the first axis. The driving end portion of the radial drive shaft is located adjacent the proximal end portion of the first link but is not in contact therewith. n belt means, are each rotated by a respective one of n corresponding pulleys. A first of the pulleys is rotated by the driving end portion of the radial drive shaft. The pulleys each having a respective effective diameter, D_n. A quantity, K, is defined as equal to 2(0₂/0.,) - 1 wherein the subscripts 2 and 1 refer, respectively, to the second and first of the effective diameters. Each succeeding belt mean except for the n th is arranged to rotate the next successive link about its respective axis. The n th belt means is arranged to rotate the end effector about the n+1 st axis. The method comprises connecting the rotary drive shaft to rotationally drivingly contact the proximal end portion of the first link. Radial and rotary drive sensor means measure, respectively, quantities indicative of the rotational positions of the radial and rotary drive shafts and generate electronic signals representative of such positions. The electronic signals are communicated to electronic computing means. The electronic computer means computes the locus of the end effector from the rotational positions of the radial and rotary drive shafts and controls the drive motors such that the number of revolutions of the radial drive shaft, n_R, divided by the number of revolutions of the rotary drive shaft, n_θ, is equal to -K so as to position the end effector to follow a desired path and to arrive at a desired locus.

The present invention provides a robotic arm structure and control method useful for a number of things, particularly for positioning semiconductor wafers for processing. The system provides higher torque operation for rotary motion, when desired, than do prior art systems whereby either smaller motors may be utilized or larger objects may be more readily moved from one position to another. It also provides design parameters which can be utilized by engineers which are considerably less restrictive than those with prior art robotic arms. This has a number of practical advantages. The arm structure and method are adapted to be used with dual end effectors and to allow a chuck for wafer prealignment to be positioned in more versatile locations than is possible with prior art arm structures and control methods therefor.

Brief Description of the Drawings

The invention will be better understood by reference to the figures of the drawings wherein like numbers denote like parts throughout and wherein:

Figure 1 illustrates, schematically, an apparatus within the ambit of the present invention;

Figure 2 illustrates, in side partial sectional view, an apparatus in accordance with an embodiment of the present invention and operable in accordance with the method of the present invention;

Figure 3 illustrates, in block diagram form, operation of a sensor/computer system useful in the practice of the present invention.

Detailed Description of Invention

The present invention provides a unique use of the radial and rotary drive motors in combination whereby significantly increased torque can result. Furthermore, the motion of the arm can be controlled via mechanisms which includes measuring the rotational rate (and thereby positions) of the radial and rotary drive shafts and utilizing a computer to control both quantities such that the end effector of the robotic arm follows a desired path. There are a number of unique features to the invention and is capable of straight line motion. One of these is that the rotary drive motor directly rotates the proximal link of the arm about the common axis of the rotary and radial drive shafts. This is suitably accomplished through a gear. This, in combination with certain physical requirements which are set forth in detail below in mathematical form, allows very precise and efficient utilization of the robotic arm.

For a better understanding of the invention it should be noted that the terms "belt", "belt means", "pulley" and "pulley means" are, at times, referred to as gearing. It should further be understood that the terms "belt" and "belt means" are used broadly to include toothed and untoothed constructions, chains, fabric belts, woven belts and the like. They may be constructed of any suitable material, natural or synthetic, organic, inorganic, polymeric, composite or metallic. Likewise the terms "pulley" and "pulley means" are used broadly to include toothed and untoothed constructions, constructions which positively engage with respective belts or which engage only frictionally with such belts. They too may be constructed of any suitable material, natural or synthetic, organic, inorganic, polymeric, composite or metallic. With this in mind the following detailed discussion of the invention will be set forth.

Referring to Figure 1, a robotic arm assembly 10 is illustrated which comprises links 12 and 14 and end effector 16. A radial drive motor 18 drives a radial drive shaft 20 having a driven end 22 and a driving end 24. The radial drive motor 18, via a shaft 26 generally, in practice, first motivates a gear train 28 which, via a shaft 30 and an appropriate coupling such as a belt 32, drives the driven end 22 of the radial drive shaft 20.

In the particular embodiment shown in Figure 1 there are n longitudinally extending links with n being equal to 2. These links are the links 12 and 14. The link 12 has a proximal end portion 12p and a distal end portion 12d. Similarly, the second link 14 has a proximal end portion 14p and a distal end portion 14d. It should be noted that n can be larger than 2; in fact it can be as large as desired. For example, there can be three such links or more. If it has three links it can be generally as shown in Figure 2 and in U.S. Patent No. 5,064,340. When there are three or more such links there are, however, certain limitations on the sizes of the links in order to provide straight line motion as will be discussed in detail below. Each of the links 12, 14, --n, has it proximal end portion pivotally mounted to rotate about a respective n th axis. Thus, the first link 12 has its proximal end portion 12p pivotally mounted about a first axis 21, the second link 14 has its pivotal end portions 14p pivotally mounted to rotate about a second axis 23, etc. The end effector 16 is pivotally mounted to rotate about an n+1 st axis, in the embodiment of Figure 1 about a third axis 25 which is located at the distal end portion of the preceding link (in the embodiment illustrated in Figure 1 at the distal end portion 14d of the second link 14) . As will be noted the axes of the links and of the end effector are all parallel to one another.

In the embodiment illustrated the radial drive rotatable shaft 20 extends along the first axis 21. The proximal end portion 14p of the second link 14 is pivotally mounted to rotate about the second axis 23 and a portion 16r of the end effector 16 is mounted to rotate about the third axis 25 which is located at the distal end portion 14d of the second link 14.

Belt means, in the embodiment illustrated the belts 36 and 39, are each rotated by respective ones of n corresponding pulleys 40, 49. A first of the pulleys, namely the pulley 40, is rotated by the driving end portion 24 of the radial drive shaft 20. The pulleys each have a respective effective diameter, D_n, and the diameters define a quantity, K, which is equal to 2(D₂/D-) - 1 wherein D₂ and D. refer, respectively to the second and first of the effective diameters. It should be noted that the effective diameter of each link is defined as the diameter which defines the gearing ratio portion provided by or at such pulley. Each succeeding belt-means except for the n th is arranged to rotate the next successive link about its respective axis. The n th belt means is arranged to rotate the end effector 16 about the n+1 st axis 25.

The operation of the radial drive components are substantially the same as those described in U.S. Patent No. 5,064,340. It should be noted that the effective length of each link is defined as the distance between a first pivot axis where the proximal end of that link is pivotally mounted and a second pivot axis where the proximal end portion of the next link is pivotally mounted. The radial drive rotatable shaft 20 rotates relative to a relatively static structure 44. The driving end portion 24 of the radial drive rotatable shaft 20 motivates the pulley-drive wheel 40 which rotates therewith. The drive wheel 40 is coaxial with the first axis 21. The first link 12 is pivotally mounted to the relatively static structure 44 at a bearing structure 46.

A post 48 is mounted to the distal end portion 12d of the first link 12 along the second pivot axis 23. The post 48 has the second effectively cylindrical surface 49 on it with the second effectively cylindrical surface 49 being cylindrical about the second pivot axis 23. The second link 14 is pivotally mounted relative to the post 48 by bearings 50, whereby the second link 14 is rotatable at its proximal end portion 14p about the second pivot axis 23. The second link 14 has the first pulley surface 38 aligned opposite the first pulley surface 40 and it has the third pulley surface 49 aligned opposite a fourth pulley surface 52 located about the third axis 25 whereat the end effector 16 is rotatably mounted via bearings 54 about a post 56 mounted to the distal end portion 14d of the second link 14. Belts 36 and 39 serve to impart the needed rotation about the axes 23 and 25. If desired, gearing can be provided in the first link 12 between the first axis 21 and the second axis 23. Such is shown in previously mentioned U.S. Patent No. 5,064,340.

Figure 2, which is not as complete as Figure

1 but is more structural, does not show an end effector rotating about the third axis 25 but, instead, shows a third link 60 rotating about the third axis 25. Also shown is an end effector 16' which may be mounted about a fourth axis 27 in the manner shown in Figure 1. It will be noted that the effective length of the third link 60 is not equal to the effective length of the first link 12 or the second link 14. There is, however, a necessary relationship between the effective lengths of the links 14 and 60 to provide straight line motion as will be discussed below.

Referring again to Figure 1 operation of the rotary or θ drive will be explained. Rotary drive motor 62 drives a shaft 64 which provides power to a gear box 66. An output shaft 68 from the gear box 66 drives a cylindrical surface 70 which in turn drives a cylindrical surface 72 to provide rotation about the axis 21. Bearings 74 allow this rotation without affecting rotation of the radial drive shaft 20. The cylindrical surface 72 drives a driven end portion 76 of a rotary drive shaft 78 which also has a driving end portion 80. The rotary drive shaft 78 is rotatably mounted by the bearings 46 relative to the stationary structure 44. The driving end portion 80 of the rotary drive shaft 78 is located adjacent the proximal end portion 12p of the first link 12 and is, importantly, in rotational direct driving contact therewith, for example via an appropriate weldment 84. Figure 2 shows the same type of rotational driving contact but accomplished via use of bolts shown at 90. In the particular embodiment illustrated there is a direct physical connection then between the rotary drive rotatable shaft 78 and the proximal end portion 12p of the first link 12. Since the driving end 80 of the rotary drive shaft 78 is in rotational driving contact with the proximal end portion 12p of the first link 12, it will be clear that the first link 12, and along with it all subsequent links and the end effector 16, will thereby be rotated along with the rotary drive shaft 78.

Referring to the end effector 16 shown in Figure 1, it will be noted that it is illustrated as being of single end operation. That is, it extends only in one direction from the post 56. However, it is contemplated that a dual end effector of the nature shown in U.S. Patent No. 5,007,784 can be readily utilized in the manner described in such patent and, indeed, the present invention is particularly useful with such an embodiment since it allows the double end effector to be utilized more efficiently, particularly in the neighborhood of the first axis 21. Figure 2 illustrates the use of a dual end effector 16' .

In accordance with the present invention, radial .drive sensor means 91, in practice an incremental photo encoder, is provided for measuring a quantity indicative of the rotational position about the axis 21 of the radial drive shaft 20 and for generating an electronic signal representative of the rotational position of the radial drive shaft 20. For convenience and accuracy the quantity actually measured which is indicative of the rotational position of the radial drive shaft 20 is the rotational position of an extension of the shaft 26, i.e., the rotational position of the radial drive motor 18. Since the two quantities are proportional to one another the resulting electronic signal is indicative of the desired quantity, namely, the rotational position of the radial drive shaft 20. The photo encoder can be in the nature of a light source and a light sensor aligned to receive light from the light source when the light path is not blocked off. The light beam from the light source is directed parallel to the rotating shaft which has a generally circular disc mounted on it between the light source and the light sensor and extending into the light beam. The disc has alternating transparent and opaque areas and the number of pulses generated at the light sensor during rotation of the motor is indicative of the speed of the motor and thereby of the rotational position of the shaft. Means represented by line 92 is present for communicating the electronic signal representative of the rotational position of the radial drive shaft 20 to electronic computing means 96. Similarly, rotary drive sensor means 98 is provided for measuring a quantity indicative of the rotational position of the rotary drive shaft 78 and for generating an electronic signal representative of the rotational position of the rotary drive shaft 78 about the first axis 21. In practice, the quantity is measured at the shaft 64. Means, represented by line 100 is provided for communicating the electronic signal representative of the rotational position of the rotary drive shaft 78 to the electronic computing means 96.

The electronic computer means 96 (See principally Figure 3) includes locus computing means 102 for computing the locus of the end effector 16 from the sensed rotational positions of the radial drive shaft 20 and the rotary drive shaft 78 using the geometric relations dictated by the specific lengths of the various links and the diameters of the various pulleys. The electronic computing means 96 further includes drive controlling means 104 for controlling the radial drive motor 18 and the rotary drive motor 62 such that the number of revolutions of the rotary drive shaft 20, n_R, divided by the number of revolutions of the radial drive shaft 78, n_θ, is equal to -K so as to position the end effector 16 to follow a straight line path and to arrive at a desired locus. The control signal is represented by lines 106, 108 to the radial drive motor 18 and rotary drive motor 62, respectively.

In accordance with an embodiment of the present invention, if n is 3 or greater, all links except for the first link 12, and the last link n, are equal in effective length, L, and a ratio equal to the effective length, L, of the n-1 th link, minus the effective length, K_nL of the n th link divided by the effective length, K-L of the first link, as represented by the equation:

(L-K_nL)/K.L = 1

to have* the capability of providing straight line motion. The arm structure 10 as defined above provides synchronized rotation of both drive shafts 20 and 78 through control of the respective motors 18 and 62 either directly or via the gear boxes 28 and 66. To be able to accomplish straight line motion or arc motion, as desired, which is an attribute of the present invention, certain geometric relationships must be maintained which can be expressed mathematically. In practice, of course, this is accomplished by using a computer controller and appropriate sensors. The following table has been set forth to define various mathematical symbols which are utilized in the computer control system to maintain the required geometric relations to accomplish motion in the manner desired and to provide increased torque, if desired, as will be explained in the following, for the overall arm structure 10.

Mathematical Symbol Definition

M(R) = motor for R - motion of end effector Mθ = motor for θ - motion of end effector

tion tion motion (rotary drive shaft

n_R = n₁ - rev/min of the driver pulley 40 from the first link 12 belt drive n₂ = rev/min of the driven pulley 38 from the first link 12 belt drive n₃ = rev/min of the first pulley 49 from the second link 14 belt drive relatively θ axis ₄ = rev/min of the driven pulley 52 from the second link 14 belt drive (i.e. end effector 16) i, _k = rotation ratio of link j to link k D_n' = diameter of pulley n T. = torque on link i In Figure 2 there are four pulleys which operate respectively about the axes 21, 23, 25 and 27. It will be apparent that i₂ , is equal n₂/n. (if the first link is in static condition) which in turn is equal to D,,/D₂. Furthermore, i₃₂ is equal to n₄ divided by n₃ which in turn is equal to D₃ divided by D₄ giving an overall value of 1/2. Expressed mathematically:

i_2<1 n₂/n. = D./D-,; and

In such an instance the torque of the end effector 16 can be increased. For example, let i, , which is equal to n₂ divided by n_1# be less than 2, i.e., let D. be less than twice D₂. Expressed mathematically this requires that:

^L2,1 = Ωm/n.. < 2 (or D, < 2D₂)

This will provide increased torque but will violate the straight line motion requirement. For that reason it is essential that the overall radial motion be provided with synchronized rotation of both the radial drive shaft 20 and the rotary drive shaft 78. If

then the relation between the revolutions of the motors must be as follows:

where :

(3) K = 2/i₂ - 1 = 2n,/n₂ -1 = 2D₂/D -1.

In order to assure pure θ motion of the arm it is necessary that:

By way of example, if the torque about the radial (R) axis 20 is T_R and

(5) i₂ - = 1/2 and i₃₂ = 1/2, then i₃. = 1/4

the torque of the end effector (due to the reciprocal relationship between rotation ratio between the links and torque) will be:

(6) T,/T_B = 4, or T, = 4T, R'

This means that the torque on the end effector 16 is four times greater compared to the case where i₂₁ = 2. From equations 1 and 3 it follows that:

(7) K = 3 and n^/n^ = -3i_GΘ/i_GR, and

(8) if i_M = i_GR then n_MR/n_MΘ = -3.

The above calculations ignore any effect of the coefficient of efficiency of the gearing.

The present invention also provides a method for controlling an arm structure 10 which comprises n longitudinally extending links each having respective proximal and distal end portions when n is 2 or a larger integer. In accordance with the method the rotary drive shaft 78 is connected to rotationally drivingly contact the proximal end portion 12p of the first link 12. A quantity indicative of the rotational position of the radial drive shaft is measured. An electronic signal representative of such rotational position is generated. Similarly, a quantity indicative of the rotational position of the rotary drive shaft is measured and an electronic signal representative thereof is generated. The electronic signals representative of the rotational positions of the radial and rotary drive shaft are communicated to electronic computer means. The electronic computer means computes the locus of the end effector 10 from the electronic signals representative of the rotational positions of the radial and rotary drive shafts. It also controls the radial and rotary drive motors such that the number of revolutions of the radial drive shaft, n_R, divided by the number of revolutions of the rotary drive shaft, ri_g, is equal to -K so as to position the end effector 10 to follow a straight line path and to arrive at a desired locus.

The single mathematical relationship which must be satisfied for straight line movement is significantly less rigorous of a constrain on the design engineer than the relationships required by the prior art wherein in a two link arm the first and second link must be of equal size and in a three link arm the links must be in the ratio of 1:2:1 if straight line motion is to result. In the present instance all that is necessary is that all links except for the first and last link must be equal in effective length, L, and that a ratio equal to the effective length, L, of the n-1 th link minus the effective length, K_nL, of the n th link divided by the effective length, K^L, of the first link, as represented by the equation:

(L-K_nL)/K-L = 1.

It will be apparent that this provides less restrictive design parameters for the engineer.

This can be useful in a number of situations. For example, silicon wafers are generally in the nature of circular disks which have one or more flats along their periphery. A flat is what results if one were to take a circular disk and snip off an edge along a cord. It is essential in the processing of semiconductor wafers that the flats be aligned in a particular direction relative to processing machinery. As a result, it is common to provide a rotatable chuck on which the wafer is positioned with sensing means so that the flat can be detected as the chuck is rotated and then aligned pointing in a particular direction. The chuck/sensing means arrangement is at times incorporated as part of the overall arm structure. By being able to control the length of the first link of a robotic arm structure to be as long as the engineer desires, i.e., to remove the restriction that it be so directly related to the length of all of the other links (other than the final link) , allows a chuck/sensor system to be positioned closer to the center of rotation of the arm structure 10 if such is desirable. As another example, a dual end effector can replace the end effector 16 as shown at 16' in Figure 2. The ability to make the first link longer or shorter, as desired by the engineer, allows such a dual end effector to be used more efficiently. Industrial Applicability

The present invention provides a robotic arm structure 10 useful for a number of things, particularly for positioning semiconductor wafers for processing. The system provides higher torque operation for rotary motion then do prior art systems whereby either smaller motors may be utilized or larger objects may be more readily moved from one position to another. It also provides design parameters which can be utilized by engineers which are considerably less restrictive than those with prior art robotic arms. This has a number of practical advantages.

While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification, and this application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth, and as fall within the scope of the invention and the limits of the appended claims.

Claims

That which is claimed is:

1. An arm structure, comprising:

n longitudinally extending links each having a respective proximal end portion and a respective distal end portion where n is 2 or is a larger integer, the proximal end portion of each of the links being pivotally mounted to rotate about a respective n th axis;

an end effector pivotally mounted to rotate about an n+1 st axis located at the distal end portion of the n th link, the pivotal axes of the links and of the end effector being parallel to one another;

a radial drive rotatable shaft having a driven end portion and a driving end portion and extending along a first of the axes;

radial drive motor means for rotating the driven end portion of the radial drive shaft about the first of the axes, the driving end portion of the radial drive shaft being located adjacent the proximal end portion of the first link but not in contact therewith;

a rotary drive rotatable shaft having a driven end portion and a driving end portion and extending along said first axis;

rotary drive motor means for rotating the driven end portion of the rotary drive shaft about the first of the axes, the driving end portion of the rotary drive shaft being located adjacent the proximal end portion of the first link and in rotational driving contact therewith;

n belt means, each being rotated by a respective one of n corresponding pulleys a first of the pulleys being rotated by the driving end portion of the radial drive shaft, the pulleys each having a respective effective diameter, D_n, a quantity, K, being defined as equal to 2(D₂/D.) - 1 wherein the subscripts 2 and 1 refer, respectively, to the second and first of the effective diameters, each succeeding belt mean except for the n th being arranged to rotate the next successive link about its respective axis, the n th being arranged to rotate the end effector about the n+1 st axis;

radial drive sensor means for measuring a quantity indicative of the rotational position of the radial drive shaft and for generating an electronic signal representative of the rotational position of the radial drive shaft;

rotary drive sensor means for measuring a quantity indicative of the rotational position of the rotary drive shaft and for generating an electronic signal representative of the rotational position of the rotary drive shaft;

electronic computing means;

means for communicating the electronic signal representative of the rotational position of the radial drive shaft to the electronic computing means; means for . communicating the electronic signal representative of the rotational position of the rotary drive shaft to the electronic computing means;

wherein the electronic computer means includes means for computing the locus of the end effector from the rotational positions of the radial and rotary drive shafts and means for controlling the radial drive motor and the rotary drive motor such that the number of revolutions of the radial drive shaft, Hm, divided by the number of revolutions of the rotary drive shaft, n_θ, is equal to -K so as to position the end effector to follow a straight line path and to arrive at a desired locus.

2. An arm structure as set forth in claim 1, wherein n is 3 or greater, all links except for the first and last link are equal in effective length, L, and a ratio equal to the effective length, L, of the n- 1 th link minus the effective length, K_nL, of the n th link divided by the effective length, K-L, of the first link, as represented by the equation:

(L-K_nL)/K₁L = 1.

3. A method of controlling an arm structure which comprises n longitudinally extending links each having a respective proximal end portion and a respective distal end portion where n is 2 or is a larger integer, the proximal end portion of each of the links being pivotally mounted to rotate about a respective n th axis; an end effector pivotally mounted to rotate about an n+1 st axis located at the distal end portion of the n th link, the pivotal axes of the links and of the end effector being parallel to one another; a radial drive rotatable shaft having a driven end portion and a driving end portion and extending along a first of the axes; radial drive motor means for rotating the driven end portion of the radial drive shaft about the first of the axes, the driving end portion of the radial drive shaft being located adjacent the proximal end portion of the first link but not in contact therewith; a rotary drive rotatable shaft having a driven end portion and a driving end portion and extending along said first axis; rotary drive motor means for rotating the driven end portion of the rotary drive shaft about the first of the axes, the driving end portion of the rotary drive shaft being located adjacent the proximal end portion of the first link; n belt means, each being rotated by a respective one of n corresponding pulleys, a first of the pulleys being rotated by the driving end portion of the radial drive shaft, the pulleys each having a respective effective diameter, D_n, a quantity, K, being defined as equal to 2(D₂/D.) - 1 wherein the subscripts 2 and 1 refer, respectively, to the second and first of the effective diameters, each succeeding belt mean except for the n th being arranged to rotate the next successive link about its respective axis, the n th being arranged to rotate the end effector about the n+1 st axis; comprising:

connecting the rotary drive shaft to rotationally drivingly contact the proximal end portion of the first link;

measuring a quantity indicative of the rotational position of the radial drive shaft; generating an electronic signal representative of the rotational position of the radial drive shaft;

measuring a quantity indicative of the rotational position of the rotary drive shaft;

generating an electronic signal representative of the rotational position of the rotary drive shaft;

computing the locus of the end effector from the electronic signals representative of the rotational positions of the radial and rotary drive shafts; and

controlling the radial and rotary drive motors such that the number of revolutions of the radial drive shaft, n_R, divided by the number of revolutions of the rotary drive shaft, n_θ, is equal to -K so as to move the end effector along a straight line path and to arrive at a desired locus.

4. A method as set forth in claim 3, wherein n is 3 or greater, all links except for the first and last link are equal in effective length, L, and a ratio equal to the effective length, L, of the n-1 th link minus the effective length, K_nL, of the n th link divided by the effective length, I^L, of the first link, is represented by the equation:

(L-i - /K.L = 1.