Mobile high-rigidity robotic system with compact end-effector

Abstract

The present invention relates to an apparatus to improve the rigidity of robotic systems by disclosing the introduction of prismatic joints seen on larger scale machines to the kinematic chain of industrial robots, which allows for improved overall rigidity and thus positioning accuracy. The prismatic joints are introduced at the robot base, which have the greatest contribution to TCP error. The invention provides a means to improve positional accuracy passively, thus offering a potential cost reduction, in addition to being compatible with existing methods for a combined effect. Additionally, the invention relates to an apparatus to allow for more compact robotic end-effectors and part access. Specifically with regards to airframe drilling, which requires machine clamp-up and normalization on the part. It introduces a combination of non-contact sensors, rotary asymmetric nosepiece and thin-walled, 3D printed nosetip, which allows for clamp-up on the part, normalization and drilling much closer to vertical objects than the current art, thus enabling the use of automation in areas only currently assisted by traditional manual methods such as drill jigs.

Description

[0001] MOBILE HIGH-RIGIDITY ROBOTIC SYSTEM WITH COMPACT ENDEFFECTOR FIELD OF THE INVENTION

[0002]

[0001] This invention relates to a mobile robotic system, for processing large workpieces, in particular airframe parts and assemblies, in particular hard-to-reach areas, but not restricted to these areas and maximizing the mobile robotic system access areas, by providing a system based on a high-rigidity industrial robot of relatively compact dimensions, with a relatively compact end-effector, which can be self-driven and which can be self-guided.

[0003] BACKGROUND OF THE INVENTION

[0004]

[0002] The manufacturing of aircraft structures typically requires machines to have a large work envelope (part lengths on the order of 20 m or longer are not uncommon). Conversely it is often required that the positioning accuracy of the machine tool center point (TCP) be within ±0.3 mm or lower throughout this envelope. As such controlling the sources of positioning error is critical for machine performance. The lack of structural stiffness and drivetrain compliance are two major contributors to positioning error.

[0005]

[0003] For parts with unrestricted access on both sides, which are nearly flat (e. g. wing panels) or with constant cross-section (e. g. fuselage panels), large scale, stiff machines with gantry or c-frame structures are typically employed. While delivering unrivaled performance, especially in terms of cycle time, these machines are capital intensive not only due to the equipment itself but also required supporting elements such as dedicated foundations.

[0004] Accordingly, for complete assemblies such as fuselage sections, wings and control surfaces where backside access is not commonly required (or possible), systems based on industrial robots have been employed for at least twenty years with lower implementation costs. Most often these systems are based on a specific type of industrial robot, articulated robots, which are widely available as commercial-off-the-shelf and are characterized by six revolute joints in series.

[0006]

[0005] Overall, industrial robots with open chain kinematics inherently lack stiffness due to being essentially a beam with an unsupported end. Besides its own weight, a robot supports a load at its free end (the endeffector weight in addition to process forces, e. g. cutting forces). For articulated robots in particular, these forces are typically resolved through gearboxes at each revolute joint. Drivetrain compliance and backlash contribute greatly to positioning error, especially for the three base joints where link lengths have a larger influence on the TCP positioning accuracy.

[0007] STATE OF THE ART

[0008]

[0006] A method to improve TCP accuracy on articulate robots based on secondary feedback is known by US 8, 989, 898 B2. Alternative configurations of articulated robots also exist: an inverted slider- crank configuration, in which the revolute joints are kept, but power is transmitted via linear elements is known by US 11, 220, 001 B2.

[0009]

[0007] Compared to large scale machines, industrial robots per se have relatively limited reach (typically 2-4 m), which may not be sufficient to process larger aircraft parts. Alternatives to extend the work envelope of robots include the use of external axes (i. e. rails, both horizontal and vertical), and robots mounted to mobile platforms. Mobile robotic platforms for airframe manufacturing have been published in the literature for at least a decade. More recently, document US 12, 053, 880 B2 describes a mobile platform with a custom SCARA manipulator as a means to improve robot rigidity for aircraft manufacturing.

[0010]

[0008] Conversely, the number of machines that can work on a given part simultaneously is limited by the machine size. The term "high-density robotic system" was coined in document US 11, 584, 503 B2, describing an apparatus that can be positioned in front of the part while allowing enough space for other machines of the same kind to also be positioned on the same part.

[0011]

[0009] Part access is also affected by end-effector size. Automated airframe drilling in general is limited to exposed surfaces free from surrounding obstacles. Obstacles can restrict end-effector access entirely (e. g. the inside of a landing gear bay) which can be observed in Figure 1. In airframe drilling, which requires machine clamp-up and normalization on the part, features extending normal to the work surface can also limit nosepiece access as can be seen in Figure 2, due to insufficient edge margin (distance between the hole center line and the vertical obstacle). In such cases, manual solutions such as drilling templates are largely used, preventing such parts from benefitting from automation improvements in quality, productivity and cost.

[0012]

[0010] Comprehensively, manufacturing systems based on collaborative robots (cobots) are also known. Document WO 2021 / 148709 Al describes a drill / fasten mobile system based on a cobot. Compared to industrial robots however, cobots inherently lack stiffness and positioning accuracy due to power limitations imposed by widely adopted safety standards. For airframe drilling applications for instance, cobot-based systems typically require a guiding and anchoring feature on the part to provide an adequate reaction force to drill thrust. As such they do not compete with industrial robots for the purposes of the present invention.

[0013]

[0011] Accordingly, it is desirable to have a machine that can simultaneously: be based on an industrial robot in order to reduce costs; feature a mobile platform in order to overcome the limited reach of a robot; improve on rotary joints as a means to increase the robot rigidity and therefore the TCP positioning accuracy compared to traditional articulated robots; have compact overall dimensions in order to be employed in higher-density configurations and therefore improve productivity; have compact end-effector dimensions in order to enter restricted access areas and— therefore increase the adoption of automation and its benefits.

[0014] BRIEF DESCRIPTION OF THE INVENTION

[0015]

[0012] The present invention relates to an apparatus to improve the rigidity of robotic systems. The invention discloses the introduction of prismatic joints seen on larger scale machines to the kinematic chain of industrial robots, which allows for improved overall rigidity and thus positioning accuracy. The prismatic joints are introduced at the robot base, which have the greatest contribution to TCP error. Thus, the invention provides a means to improve positional accuracy passively, thus offering a potential cost reduction, in addition to being compatible with existing methods for a combined effect. This part of the invention is applicable to industrial processes in general, with particular relevance to machining operations and airframe drilling.

[0016]

[0013] Additionally, the invention relates to an apparatus to allow for more compact robotic end-effectors and part access. Specifically with regards to airframe drilling, which requires machine clamp-up and normalization on the part, it introduces a combination of non-contact sensors, a rotary asymmetric nosepiece and a thin-walled, customized nosetip that can be 3D printed, which allows for clamp-up on the part, normalization and drilling much closer to vertical objects than the current art, thus enabling the use of automation in areas only currently assisted by traditional manual methods such as drill j igs.

[0017] BRIEF DESCRIPTION OF THE DRAWINGS

[0018]

[0014] To assist in identifying the main characteristics of the present invention, each figure is briefly described as follows:

[0019]

[0015] Figure 1 shows an end-effector access from the state of the art.

[0020]

[0016] Figure 2 shows a nosepiece access from the state of the art.

[0021]

[0017] Figure 3 shows a robot kinematic configuration.

[0022]

[0018] Figures 4A and 4B show linear rail arrangements.

[0023]

[0019] Figure 5 shows a robot kinematic configuration with platform.

[0020] Figure 6 shows a compact end-effector according to the present invention.

[0024]

[0021] Figure 7 shows an asymmetric nosetip according to the present invention.

[0025]

[0022] Figure 8 shows a normality range finders and rotary nosepiece.

[0026]

[0023] Figure 9 shows an overview of the mobile platform, robot and end-effector.

[0027] DETAILED DESCRIPTION OF THE INVENTION

[0028]

[0024] Figure 3 presents a diagram of a six-axis industrial robot wherein joints 1 and 3, whose linear movement is represented by the arrows identified with dl and d3, are provided in the prismatic shape, as opposed to revolute joints 2, 4, 5 and 6 whose revolution movement is represented by the arrows identified with θ2, θ4, θ5 and θ6 traditionally used in articulated robots, and in particular articulated robots employed in airframe manufacturing. This arrangement allows for all moment loads at each of these joints to be reacted by linear motion components, in contrast with revolute joints in traditional articulated robots where the torque around the axis of motion is solved through the gearbox. In the preferred embodiment, linear rails are used and assembled with preload, eliminating any gaps between the rail and bearing car.

[0029]

[0025] Furthermore, in this arrangement the motion path of the joint actuator (drivetrain) coincides with the motion path of the joint and is thus more efficient (as opposed to slider-crank mechanisms where the actuator motion is a component of the joint motion), in addition to allowing for a more compact mechanism packaging. In the preferred embodiment, the linear motion on each prismatic joint 1 and 3 is transmitted from the servomotor to the corresponding robot link via a ball screw assembled with preload, which eliminates gaps between the screw and ball nut and thus backlash. The servomotor may be attached to the ball screw directly with a coupling or via a belt drive (should mounting constrains require it), or if a further reduction ratio (in addition to the ball screw ratio) is needed. In the preferred embodiment, the belt drive pulley ratio and ball screw ratio are chosen such as to eliminate any need for a gearbox in the prismatic joint drivetrain, which allows for backlash to be eliminated.

[0030]

[0026] As such the kinematic arrangement shown in Figure 3 eliminates backlash and makes for a more rigid setup if compared to a revolute joint without secondary feedback. A more rigid joint, in combination with no backlash results in a more positionally accurate joint, which in turn results in a more positionally accurate TCP.

[0031]

[0027] The choice of which joints are made prismatic is strategic. This choice concerns joints 1, 2 and 3, since they are the basis joints. These joints have the most influence on the TCP positioning accuracy as their error is augmented by the largest links in the robot. Joints 4, 5 and 6 are kept in a traditional spherical wrist arrangement.

[0032]

[0028] In the preferred embodiment joint 1 is prismatic while joint 2 is revolute as can be seen in Figure 3. Such arrangement allows for the first robot link to be supported along the range of motion of joint 1, reducing its deflection under load and thus its contribution to the TCP positioning error. Making joint 3 the revolute joint is impractical as this would severely limit the work envelope of the robot. In an alternative embodiment, joint 1 could be made revolute and joint 2 could be made prismatic while still preserving the spirit of the invention.

[0033]

[0029] Furthermore, in the preferred embodiment, for joint 1 as shown in Figure 4A the linear rail 7 is fixed while the bearing cars 8 move with the sliding assembly 9. In an alternative embodiment showed in Figure 4B, the bearing cars 10 are fixed and the rails 11 move with the sliding assembly 12.

[0034]

[0030] In terms of rigidity and moment loads around joint 2, by having a single revolute joint at the base, this invention presents a clear advantage over SCARA configurations, which have three revolute joints in parallel.

[0035]

[0031] In another embodiment represented in Figure 5, a mobile platform 13 is added to the base of the robot to enhance its working envelope. The mobile platform (13) can be configured with idling or driven wheels 25a and 25b and is capable of any combination of translation and / or rotation movement on a floor.

[0036]

[0032] A multitude of wheel types and arrangements can be employed, including but not limited to idling casters, driven wheels in differential configuration, driven wheels in steerable configuration, driven mecanuum wheels, driven omni-directional wheels.

[0037]

[0033] Figure 6 shows an overview of a compact endeffector that is mounted to the free-end of the robotic mechanism, wherein the element 14 is a rotary nosepiece assembly. With a rotary nosepiece assembly, an asymmetric nosetip can be used as can be seen in Figure 7. An asymmetric nosetip allows for the chip extraction duct 16, which is crucial for hole quality and is typically bulky, to be placed on one side, while the opposite side 17 is kept as thin as possible. By rotating the thin side against the obstacle, a lower edge margin can be obtained in drilling. In the preferred embodiment the nosetip is built with additive manufacturing methods, such as the walls can be as thin as possible.

[0038]

[0034] Normality range finders 15 are mounted to the compact end-effector as seen in Figure 6. Figure 8 shows how the normality range finders 18 can be mounted such that their beams 19 can be combined to measure the normality of the nosetip in relation to the part surface 20. Three range finders are needed at a minimum. In a preferred embodiment, four range finders are used to account for line-of-sight issues.

[0039]

[0035] Figure 9 depicts the preferred embodiment of the entire system. The mobile platform 21 sits on wheels 22, holding robot 23 with the end-effector 24. In this embodiment there are four driven mecanuum wheels which allow the platform to be self-driven. In this embodiment sensors 25 allow the mobile platform to be self-guided. The sensors 25 can be lidars, area scanners, range finders, magnetic tape readers, vision cameras.

Claims

CLAIMS1. A mobile high-rigidity robotic system CHARACTERIZED IN THAT comprising:a serial kinematic, open chain robotic mechanism, with at least three joints (1), (2) and (3), wherein at least two of the initial three joints (1), (2) and (3) are prismatic;wherein the sliding motion of each said prismatic joint is guided by a linear motion component;wherein the actuation of each said prismatic joint is powered by a motor coupled to a linear drive component; wherein the direction of said actuation coincides with the direction of said sliding motion;wherein an end-effector is mounted to the free-end of said robotic mechanism.

2. The system of claim 1, CHARACTERIZED IN THAT said robot has six axes, with the final three axes being rotary and configured in a spherical wrist kinematic arrangement.

3. The system of claim 1, CHARACTERIZED IN THAT the motor may be attached to the ball screw directly with a coupling or via a belt drive.

4. The system of claim 1, CHARACTERIZED IN THAT joints (1) and (3) are prismatic while joint (2) is revolute.

5. The system of claim 1, CHARACTERIZED IN THAT joint (1) is revolute while joints (2) and (3) are prismatic.

6. The system of claim 1, CHARACTERIZED IN THAT the linear rail (7 ) is fixed for joint (1) while the bearing cars (8 ) move with the sliding assembly ( 9).

7. The system of claim 1, CHARACTERIZED IN THAT the bearing cars (10) are fixed and the rails (11) move with the sliding assembly (12).

8. The system of claim 1, CHARACTERIZED IN THAT a mobile platform (13) capable of any combination of translation and / or rotation movement on a floor is added to the base of the robot.

9. The system of claim 1, CHARACTERIZED IN THAT said mobile platform can be configured with idling or driven wheels 25a and 25b.

10. The system of claim 1, CHARACTERIZED IN THAT comprises a navigation sensor.

11. The system of claim 1, CHARACTERIZED IN THAT the part to be processed is an aircraft assembly.

12. A compact end-effector for airframe drilling, CHARACTERIZED IN THAT comprising:a rotary nosepiece which includes an asymmetric nosetip with a chip extraction channel, wherein said nosepiece has a motor associated therewith for controlled movement of the nosepiece rotation;a non-contact surface normality measurement system with more than two sensors capable of measuring linear distance, wherein all sensors are pointed towards the same area of interest;a part clamping axis with force feedback;a spindle axis and spindle feed axis.

13. The compact end-effector of claim 12, CHARACTERIZED IN THAT comprises a vision camera for part referencing.

14. The compact end-effector of claim 12, CHARACTERIZED IN THAT comprises a hole measurement tool.

15. The compact end-effector of claim 12, CHARACTERIZED IN THAT comprises a tool shuttle axis.

16. compact end-effector of claim 12, CHARACTERIZED IN THAT at least three range finders are mounted in the compact end-effector.

17. compact end-effector of claim 12, CHARACTERIZED IN THAT four range finders are mounted in the compact endeffector.

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