Robotic arm driving mechanism

A robotic arm assembly is provided comprising a driving mechanism including a rotor which is rotatable relative to a stator. The rotor is coaxially rotatable about a columnar shaft having a substantially hollow central longitudinal axis. A first arm having proximal and distal pivotal ends is rigidly mounted on the rotor such that the first arm is drivable to rotate by the driving mechanism. A proximal pivotal end of a second arm having proximal and distal pivotal ends is pivotally connected to the proximal pivotal end of the first arm. A first timing pulley is mounted coaxially onto the columnar shaft, and a second timing pulley which is spaced from the first pulley is drivable to rotate together with rotation of the first arm. A timing belt connects the first and second timing pulleys and is operative to rotate the second arm relative to the first arm as the first arm is driven to rotate by the driving mechanism, such that the distal pivotal ends of the first and second arms are configured to always lie along a substantially straight line.

FIELD OF THE INVENTION

The present invention relates to a robotic arm, and in particular to a robotic arm structure which is operable for positioning an end-effector of the robotic arm to perform various functions.

BACKGROUND AND PRIOR ART

Conventional Selective Compliant Articulated/Assembly Robot Arms (SCARA) in the market are driven through transmission systems, such as gear trains, belts-and-pulleys, chains and linkages, which are used to transmit power from the drive motors to move a load carried by an end-effector attached to the robotic arms. The motors are typically located at the base of the robot structure, and the robotic arms are moved by the transmission systems linking the drive motor to the arms.

Generally, robotic arms exhibit angular (θ), vertical (Z) and radial (R) motions in a cylindrical coordinate system. Angular motion refers to rotation of the robotic arm about a vertical axis at the position which the arm is pivoted to the robot body. Vertical or Z motion comprises vertical elevation of the robotic arm with reference to a base of the robotic system. Radial or R motion denotes extension or retraction motion of the robotic arm resulting in a straight-line motion of the end-effector which is typically attached to a distal end of the robotic arm. The rotational motion of a radial motor may be converted into linear motion of the end-effector to attain such radial motion.

U.S. Pat. No. 5,064,340 entitled “Precision Arm Mechanism” discloses an arrangement of a pulley and transmission belt system to achieve radial motion of an end-effector in a robotic arm. An arm structure of the robotic arm includes either two or three links pivotally connected to one another with an end-effector at the distal end of the distal link. Straight line movement of the pivoting mounting place of the end-effector is provided from a rotating drive wheel coaxial with the pivoting of the proximal end of the proximal link. A drive wheel arranged coaxially with the pivot of the proximal end of the proximal link causes the first and second links to pivot about their pivot axis. This pivoting causes the first link to pivot about its proximal end and causes the end-effector to pivot in the two link version and causes the second and third links to pivot about their pivot axis in the three link version.

As such, the power from a motor driver is not directly transmitted to a first pivot axis pivoted to a base of the robotic arm to a proximal link, but is transmitted to a second pivot axis before it is transmitted back to the first pivot axis to drive the proximal link. A similar transmission of power takes place from the second pivot axis to the third pivot axis which brings about straight line movement of the pivoting mounting place of the end-effector. A gearing housing may be located intermediate the ends of the proximal link to provide a gearing adjustment. The interconnected links of the robotic arm in this prior art are driven through a relatively complex timing belt transmission mechanism which makes the structure of the robotic arm complex. Therefore, accuracy and reliability may suffer.

U.S. Pat. No. 6,709,521 entitled “Transfer Apparatus And Accommodating Apparatus For Semiconductor Process, And Semiconductor Processing System” discloses a simpler arrangement of a pulley and transmission belt system comprising a first motor stacked on top of a second motor. A main transmission which is part of the main driving mechanism for rotating and extending/retracting a robotic arm connects the first motor and first to third links of the apparatus. The main transmission comprises a pair of gear pulleys disposed at the proximal and distal ends of each of the links. A timing belt extends between each pair of the gear pulleys and coaxial shafts comprised in each link.

The first link is driven directly by the first motor to rotate when the motor turns a shaft which connects the motor to the first link. The main driving mechanism transmits this rotational driving force to the third link or end-effector through an elaborate arrangement of gear pulleys and timing belts. One shortcoming of this design is that the housing of the first motor is completely enclosed and this apparatus does not ensure that electrical and service lines that drive the motors do not affect the production environment. External electrical cabling and wiring to the motors may inhibit rotation of the robotic arm about a 360° angle and may introduce foreign matter or unwanted obstructions caused by the exposed wiring. Furthermore, the first link is joined to the first motor by a relatively long and slender shaft that is coupled to a rotor of the first motor. This may cause compliance errors resulting from the bending of the shaft when the robotic arm is being driven such that the degree of rotation of the first motor's rotor at one end of the shaft may not be exactly the same as the degree of rotation of the first link at the other end of the shaft.

It is therefore desirable to devise a robotic arm structure that is able to achieve the required radial motion by implementing a simple pulley and belt transmission mechanism using only one motor to achieve radial motion, which also offers a substantially uninhibited rotational range as well as compactness.

SUMMARY OF THE INVENTION

It is thus an object of this invention to seek to provide an improved driving mechanism for a robotic arm structure which is operative to generate radial motion in the robotic arm and rotary motion about an axis. It is another object of the invention to seek to provide a more compact robotic arm structure as compared to the aforesaid prior art and which is capable of achieving increased accuracy and reliability.

Accordingly, the invention provides a robotic arm assembly comprising: a driving mechanism including a rotor which is rotatable about a stator, the rotor being coaxially rotatable relative to a columnar shaft having a substantially hollow central longitudinal axis; a first arm having proximal and distal pivotal ends rigidly mounted on the rotor such that the first arm is drivable to rotate by the driving mechanism; a second arm having proximal and distal pivotal ends, the proximal pivotal end of the second arm being pivotally connected to the proximal pivotal end of the first arm; a first timing pulley mounted coaxially onto the columnar shaft; a second timing pulley spaced from the first timing pulley which is drivable to rotate together with rotation of the first arm; and a timing belt connecting the first and second timing pulleys that is operative to rotate the second arm relative to the first arm as the first arm is driven to rotate by the driving mechanism, such that the distal pivotal ends of the first and second arms are configured to always lie along a substantially straight line.

It would be convenient hereinafter to describe the invention in greater detail by reference to the accompanying drawings which illustrate preferred embodiments of the invention. The particularity of the drawings and the related description is not to be understood as superseding the generality of the broad identification of the invention as defined by the claims.

FIG. 1is a perspective view of a robotic arm assembly10having a base12and a plurality of interconnected links or arms14,16,18according to the first preferred embodiment of the present invention. Top covers of the arms14,16,18have been partially removed to show respective shoulder, elbow and wrist pivots20,22,24connecting the links. The robotic arm assembly10has a relatively stationary base12which is fixedly mounted onto a stationary support surface (not shown) and about which the plurality of arms14,16,18rotate.

The plurality of arms14,16,18are rotatably coupled with respect to one another via the pivots20,22,24. The first arm, which in this embodiment is a shoulder arm14, has proximal and distal pivotal ends and is supported at its distal pivotal end at the shoulder pivot20and is rotatable about the shoulder pivot20. The second arm, which in this embodiment is an elbow arm16, also has proximal and distal pivotal ends and is rotatable about the elbow pivot22at its proximal pivotal end where it is connected to the proximal pivotal end of the shoulder arm14. The third arm in the form of an end-effector18is rotatable about the wrist pivot24at the distal pivotal end of the elbow arm16. The distance between the shoulder and elbow pivots20,22at the proximal and distal ends of the shoulder arm14is preferably substantially equal to the distance between the elbow and the wrist pivots22,24at the proximal and distal ends of the elbow arm16. The shoulder arm14and the elbow arm16are also preferably of substantially equal length.

The end-effector18is located furthest away from the base12and is preferably mounted with a processing tool. Examples of processing tools that may be mounted to the end-effector18are a wafer holder or a pick-and-place tool, which may even be configured to follow a different axis of motion from the shoulder and elbow arms14,16, if necessary.

FIG. 2is a top view of the robotic arm assembly10ofFIG. 1illustrating radial motion of the end-effector18along an x-axis. Radial or R motion of the end-effector18refers to the end-effector travelling along a substantially straight line that passes through a center of the shoulder pivot20connecting the base12and the shoulder arm14. In this example, the straight line is parallel to the x-axis.

To realize R motion, the relationship between the angles formed by the shoulder arm14, elbow arm16and x-axis is preferably as follows:
Δθ=Δβ/2
Δα=−Δβ/2
where Δθ denotes the change in the angle between the elbow arm16and the end-effector18which is aligned with the x-axis; Δβ denotes the change in the angle between the shoulder arm14and the elbow arm16; and Δα denotes the change in the angle between the shoulder arm14or the elbow arm16and the x-axis. Generally, the ratio of the relative rotational speed of the elbow arm16with respect to the shoulder arm is thus 2:1, such that the distal pivotal end of the shoulder arm14at the shoulder pivot20and the distal pivotal end of the elbow arm16at the wrist pivot24are configured to always lie along a substantially straight line (the x-axis in this example).

FIG. 3is a sectional side view of the robotic arm assembly10ofFIG. 1showing the components of the arms14,16,18and the base12of the assembly10. A motor assembly54located at the base12of the robotic arm assembly10comprises a first direct drive motor assembly or driving mechanism mounted onto a second direct drive motor assembly. The first direct drive motor assembly, such as R motor28, includes an R motor rotor38that is rotatable relative to an R motor stator40, which surrounds the R motor rotor38. The R motor rotor38is also rotatable relative to a columnar shaft or coupling shaft30, which is relatively stationary. A first timing pulley42is mounted at the distal pivotal end of the shoulder arm14on top of and coaxial with the coupling shaft30. The R motor28generates R motion of the end-effector18that is observable as linear motion of the end-effector18towards or away from the base12. The end-effector18is indirectly coupled to the R motor28via the elbow arm16and the shoulder arm14.

The second direct drive motor assembly or universal motor located at the base12of the robotic arm assembly10, such as T motor26, is located co-axially with the R motor28and generates universal rotary motion of the robotic arm assembly10. In the first preferred embodiment, the R motor28and coupling shaft30are mounted onto the T motor26. The T motor26comprises a T motor rotor34which is rotatable relative to a T motor stator32. The T motor rotor34is mounted onto a universal columnar shaft or T motor shaft36, which has a substantially hollow central longitudinal axis. The T motor rotor34is mounted directly onto the side walls of the T motor shaft36and is surrounded by the T motor stator32. The T motor26rotates the T motor shaft36about an S axis of the shoulder arm14to generate universal rotational motion of the R motor28, coupling shaft30, shoulder arm14, elbow arm16and the end-effector18concurrently.

The coupling shaft30is substantially hollow along its central longitudinal axis, which makes it lightweight and allows electrical and service lines49such as electrical cables and wirings to be routed to the robotic arm assembly10within the hollow center of the coupling shaft30. This approach is superior to connecting the electrical cables and wirings outside the robotic arm assembly10as in the prior art which may obstruct the free rotary motion of the robotic arm assembly10and which may also introduce foreign matter or even interfere with the production environment. Moreover, there is a further benefit that the length of the coupling shaft30can be made almost the same as the length of the R motor28. Not only would this lead to a more compact design, it also makes the shaft30more rigid and significantly avoid compliance issues when the robotic arms are being driven. Greater accuracy and precision may thus be achieved. The hollow central longitudinal axis of the coupling shaft30also reduces the weight of the assembly.

The R motor28rotates the R motor rotor38, and the shoulder arm14mounted on the R motor rotor38at the shoulder pivot20about the S axis to generate R motion such that the shoulder arm14, elbow arm16and the end-effector18are all made to rotate relative to one another by the transmission system. Vertical or Z motion of the robotic arm is possible by elevating or lowering the shoulder arm14, the elbow arm16and the end-effector18along the S axis using a Z motor (not shown).

The shoulder arm14is directly driven by the R motor28. The elbow arm16is driven by the R motor28to rotate relative to the shoulder arm14through a first timing belt44such that the angle between the shoulder arm14and the elbow arm16is always maintained at twice that of the angle between the shoulder arm14or the elbow arm16and the substantially straight line (x-axis). The end-effector18is driven by the R motor28through the first timing belt44as well as a second timing belt52such that the angle between the elbow arm16and the end-effector18aligned along the x-axis is maintained to be the same as the angle between the shoulder arm14and the x-axis.

The actuation of the R motion is now described in greater detail. The R motor rotor38rotates and turns the adjoining shoulder arm14which rotates the first timing belt44that is wound around the first timing pulley42and a second timing pulley46spaced from the first timing pulley42, which are all housed in the shoulder arm14. The first timing belt44is operative to rotate the elbow arm16at the elbow pivot22about an E axis relative to the shoulder arm14. This in turn rotates a second timing belt52that is wound around third and fourth timing pulleys48,50. The third and fourth timing pulleys48,50are located at respective proximal and distal ends of the elbow arm16, wherein the third pulley48is coaxial with the second timing pulley46.

The elbow arm16rotates relative to the end-effector18about a W axis at a wrist of the robotic arm assembly10as the fourth timing pulley50turns the end-effector18relative to the elbow arm16at the wrist pivot24. The result of this transmission process is that the rotational action of the R motor28is ultimately transmitted to the end-effector18and the robotic arm extends or retracts along a substantially straight line, which is parallel to the x-axis in this example. A simple transmission is thus realized by a train of belt and pulley systems using a single R motor28located at the base12of the robotic arm structure.

In this manner, R motion of the robotic arm assembly10is easily achievable using only a single R motor and a simple arrangement of transmission belts and pulleys. It should be appreciated that the axes of motion of the three arms14,16,18lie on planes that are preferably substantially parallel to one another.

FIG. 4is a sectional side view of a robotic arm assembly10′ according to the second preferred embodiment of the present invention showing the components of the arms14,16,18and the base12of the assembly10′. The universal motor, or T motor26, is located at the base12on which the shoulder arm14is mounted and the driving mechanism, such as an R motor28′, is located coaxially with the E axis at the elbow pivot22. In this embodiment, the first arm is the elbow arm16and the second arm is the shoulder arm14, and they are connected at the elbow pivot22located at the proximal pivotal ends of the shoulder arm14and the elbow arm16. The base12is located at the distal pivotal end of the second arm or shoulder arm14. The R motor28′ comprises an R motor rotor38′ and an R motor stator40′. The R motor rotor38′ is rotatable relative to a columnar shaft or coupling shaft39′, which is relatively stationary. The coupling shaft39′ is fixedly coupled to the shoulder arm14.

The rotation of the R motor28′ drives the elbow arm16which is mounted on the R motor rotor38′ directly as the R motor rotor38′ rotates about the E axis. The rotational motion of the R motor28′ also drives a first timing pulley46′ on R motor rotor38′ which rotates a first timing belt44′ that is wound around the first timing pulley46′ and a second timing pulley42′. In this embodiment, the first timing pulley46′ is located along the E axis and is coaxial with the R motor28′. The first timing pulley46′ and second timing pulley42′, and the first timing belt44′, are all housed in the second arm or shoulder arm14. The first timing belt44′ rotates the second timing pulley42′ at the shoulder pivot20about the S axis of the shoulder arm14which then rotates the shoulder arm14relative to the base12.

As the R motor28′ drives the first arm or elbow arm16, a third pulley50′ on the end-effector18rotates a second timing belt52′, which is also wound around a fourth timing pulley48′. The fourth timing pulley48′ is mounted onto and is coaxial with the coupling shaft39′. The third timing pulley50′ and fourth timing pulley48′ are located at respective distal and proximal pivotal ends of the elbow arm16. The end-effector18turns relative to the elbow arm16about the W axis as the third timing pulley50′ turns the end-effector18at the wrist pivot24. In this way, the transmission process permits the rotational action of the R motor28′ located at the elbow pivot22to be ultimately transmitted to the end-effector18and the shoulder arm14such that the end-effector18of the robotic arm10′ extends or retracts along a straight line.

The second direct drive motor assembly or T motor26located at the base12of the robotic arm assembly10′ generates universal rotary motion of the robotic arm assembly10′. In the second preferred embodiment, the T motor26comprises a T motor rotor34which is rotatable relative to a T motor stator32. The T motor rotor34is mounted onto a universal columnar shaft or T motor shaft36, which has a substantially hollow central longitudinal axis. The T motor rotor34is mounted directly onto the side walls of the T motor shaft36and is surrounded by the T motor stator32. The T motor26rotates the T motor shaft36about the S axis of the shoulder arm14which is mounted on the T motor26to generate universal rotational motion of the shoulder arm14, R motor28′, elbow arm16and the end-effector18concurrently. Electrical and service lines49may be housed within the central longitudinal axis of the T motor shaft36.

While the elbow arm16is directly driven by the R motor28′, the end-effector18is driven by the R motor28′ through the second timing belt52′ such that the angle between the elbow arm16and the end-effector18aligned along the x-axis is half that of the angle between the shoulder arm14and the elbow arm16. The shoulder arm14is driven by the R motor28′ through the first timing belt44′ such that the angle between the shoulder arm14and the x-axis is also half of the angle between the shoulder arm14and the elbow arm16. The result of this transmission process is that the rotational action of the R motor28′ at the E axis is ultimately transmitted to the end-effector18and shoulder arm14using a simple transmission system realized by a train of belts and pulleys system provided by a single R motor28′ located at the E axis of the robotic arm structure.

Another advantage of this embodiment is that the R motor28′ is located closer to the end-effector18than in the first preferred embodiment, which results in increased accuracy and controllability of the end-effector18.

FIG. 5is an exploded side view of the motor assembly54of the robotic arm assembly10according to the first preferred embodiment of the invention showing the main components of the T motor26and the R motor28. The R motor28is coupled to the T motor26by the coupling shaft30such that rotation of the T motor26turns the R motor28to bring about universal T motion of the coupling shaft30, shoulder arm16and the other arms of the robotic arm assembly10about the S axis.

A T motor housing29houses the T motor stator32, T motor rotor34and T motor shaft36. It also houses other parts of the T motor26, including a T motor bearing31, a T motor bearing flange35and a T rotary motor encoder50. The T rotary motor encoder50helps to determine a rotary position of the robotic arm assembly10. An R motor housing39houses the R motor rotor38and the R motor stator40. It also houses an R motor bearing41, an R motor bearing flange45and an R rotary motor encoder52.

The T and R rotary optical encoders50,52of the T and R motors26,28are positional measurement apparatus. The T rotary optical encoder50comprises a T motor encoder head37and a T motor encoder scale33. The R rotary optical encoder52comprises an R motor encoder head47and a T motor encoder scale43. The encoders may be located next to the motor shaft36and the coupling shaft30. An encoder scale defines angular positions whereas an encoder head ascertains an angular disposition of the robotic arm. The encoder scales have an annular track disposed around a surface concentric with the longitudinal S-axis at the shoulder pivot20. The encoder head may be supported by an encoder mount and disposed at a relatively short distance from the encoder scale to read the encoder scale and optically sense the angular disposition of the rotating shafts36,30that are actuating the robotic arm and therefore ascertain the rotary disposition of the shoulder arm14. Although a rotary optical encoder is herein described, it should be appreciated that other types of positional measurement apparatus, such as a magnetic encoder, may be used with the present invention.

The T motor shaft36is preferably in the form of a hollow cylindrical sleeve mounted with the T motor rotor34of the T motor drive assembly26. The T motor rotor34is surrounded by the T-motor stator32, which coaxially receives the T motor rotor34and T motor shaft36. The R motor28is mounted on top of the T motor shaft36via the coupling shaft30. As described above, the coupling shaft30also comprises a hollow cylindrical sleeve with one end coupled to the timing pulley42which turns the timing belt44. The hollow centers of the T motor shaft36and coupling shaft30allow the respective shafts36,30to contain and enclose electrical or service lines49for the robotic arm assembly10so that they are not exposed to the production environment. As the electrical wiring is contained within the hollow shafts, another advantage is that T rotation of the robotic arm throughout a 360° angle is also possible without interference by exposed electrical or service lines.

The T motor bearing31and the R motor bearing41, which may both be in the form of cross-roller bearings, rotatably support the turning T motor shaft36and R motor rotor38respectively. The bearings31,41may be supported and kept in position by the T motor bearing flange35and the R motor bearing flange45respectively located immediately above the bearings31,41. The bearing flanges35,45may each include an inner flange portion for supporting a movable inner portion of the bearings31,41against the motor shaft36and the coupling shaft30, and an outer flange portion for supporting an outer portion of the bearings31,41.

It should be appreciated that the preferred embodiments of the present invention provides a transmission mechanism with a relatively simple construction which adopts a single direct drive assembly to bring about radial motion of the end-effector18. Furthermore, the direct connection of the timing pulley42to the coupling shaft30increases the reliability of the rotating action of the timing pulley42since there is increased correspondence between rotation of the first timing pulley42and the rotation of the second timing pulley46. Having only one single driving mechanism to drive R motion is also more cost-effective than using multiple direct drive assemblies. In the second embodiment, locating the driving mechanism between the shoulder arm14and the elbow arm16locates the radial motor28′ closer to the end-effector18which improves the accuracy of the driving mechanism. Overall, there is significant improvement in the compactness and controllability of the robotic arm assemblies10,10′ according to the preferred embodiments of the invention.