Compliant end effector

Details Compliant end effector Compliant end effector

1999-12-22

2002-04-09

Browne, Lynne H. (Department: 3629)

Joints and connections

Interfitted members

Radially interposed shim or bushing

C403S370000

Reexamination Certificate

active

06368012

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates to a compliant end effector for an industrial robot, and more particularly to an end effector that compliantly holds an implement with a tool and allows the implement to pivot in any direction relative to a given reference line when the tool engages a workpiece in a robotic manufacturing application.
BACKGROUND OF THE INVENTION
Many industries use robotics to speed up manufacturing, improve product quality, reduce costs and provide a safer working environment for employees. Parts are robotically worked by securing them to a work station so that the part is located at specific coordinates. The robot is then programmed to move along predetermined paths of travel, and to rotate, twist and turn at prescribed points along these paths. For each path of travel, the robot is programmed to move between specific points. The path may be linear or have a defined arcuate shape, such as a circle, ellipse, parabola, or portion thereof. A tool secured to the robotic arm performs desired tasks on the part as the robot moves along its path of travel. The robot will follow the programmed path of travel without deviation. The robot performs the same tasks at the same places on each part passing through the work station. Such robotic work stations process parts quickly and manufacture them to relatively high degrees of tolerances.
A problem with robotic manufacturing is that the robots must be programmed to take into account every surface of the workpiece being machined. While the programming process can be fairly easy for parts with relatively simple shapes, programming becomes arduous for more complicated part shapes. Conventional robots and their end effectors do not compensate for small, intended deviations in an otherwise uniform surface of the workpiece. For example, a cast or molded part may have several flat surfaces with small abutments, recesses or screw holes for aligning or joining that part to another part. These abutments and the walls of the screw holes project outwardly from the otherwise uniform, linear or arcuate surface of the workpiece. The robot must be programmed to adjust for each of these deviations or the projection can be ground or cut out of the workpiece. The programmer must account for every intended deviation in the otherwise uniform surfaces of the workpiece, or risk producing a potentially defective product. Writing programs that take into account every intended deviation in a complex part shape is tedious, time consuming, expensive and prone to mistake. Several test runs may have to be performed before the program is ready for production.
Another problem with conventional robotic manufacturing is that the robot will not compensate for any misalignment of the workpiece at the work station. This is particularly problematic when manufacturing large, heavy workpieces, such as a vehicle transmission housing. Such workpieces are difficult to move into a precise orientation and coordinates. The robot will also not compensate for small imperfections in the workpiece, such as any warpage in its surface. The robot will gouge, undercut or miss the workpiece due to any such misalignment or imperfection.
A further problem with robotic manufacturing is that each change in path of the robotic arm can cause imprecision in the finished part due to the tolerances associated with the movements of the robotic arm. Complex parts with a variety of surfaces can be problematic because the robotic arm must travel along many different paths of travel. Small amounts of tolerance can accumulate to produce a significant imprecision in the workpiece.
A still further problem with robotics is that the robotic arms are designed to meet specific industry needs, and thus each arm has specific weight and torque capacities. Should the end effector and implement exceed these limits during use, the robotic arm can malfunction, operate inappropriately, wear out more quickly, or break down. The lighter the end effector and the more balanced it is when holding an implement, the heavier and more powerful the implement that can be held by the robotic arm. Accordingly, end effectors should have a compact, lightweight and balanced design.
A still further problem with robotics is designing an end effector suitable for compliantly matching a variety of differently shaped parts. The end effector must be able to compensate for deviation and misalignments that may arise along any horizontal, vertical, angled, or arcuate path of travel. Conventional end effectors may allow the tool to compliantly engage the workpiece when the end effector and tool are upright, but not when they are turned sideways or upside down. The end effector may become jammed or become to stiff or too lose when turned sideways or upside down. The amount of compliance may also deviate depending on the orientation of the end effector and tool. The workpiece will need to be repositioned one or more times to accommodate the limitation of the end effector. Such an end effector has a greatly reduced value in a commercial manufacturing operation.
A still further problem with conventional compliant end effectors is that it is difficult to easily and securely attach an implement to the end effector. Implements are frequently heavy, bulky and awkward to handle, which can result in misalignments. Workers can also bump or drop the implement, causing damage to the implement or injuring themselves or a coworker.
The present invention is intended to solve these and other problems.
BRIEF DESCRIPTION OF THE INVENTION
The present invention relates to a compliant end effector for securing an implement or tool such as a spindle to an arm of an industrial robot. The end effector has an internal passageway that extends completely through the end effector for receiving the spindle. The end effector includes a mounting assembly, a gripping assembly, a compliant assembly and a biasing assembly. The mounting assembly has a mounting bracket that rigidly secures the end effector to the robotic arm. The gripping assembly has a support bracket that rigidly secures the spindle to the end effector. The compliant assembly joins the mounting assembly to the gripping assembly, and includes an internal collar with two sets of opposed pivot pins that form first and second pivot axes. The biasing assembly includes a sponge rubber biasing ring with a number of uniformly spaced springs that combine to bias the spindle into a normal biased position. The spindle has a reference axis that forms a reference line for the end effector when in this biased position. The collar and biasing ring also have openings that form a part of the passageway. The two pivot axes allow the spindle to pivot in any direction relative to the reference line through 360° around the reference line. The biasing assembly includes a stiffness adjustment assembly that produces a pre-load condition to adjust the amount of force needed to pivot the spindle out of its normal biased position. As the robotic arm moves the implement along a uniform path of travel, the tool engages a workpiece with a substantially uniform surface or edge and cuts away unwanted burs or other unintended discontinuities from that surface or edge. However, the tool compliantly pivots relative to the reference line when the tool encounters a desired projection in the workpiece so that the tool does not gouge or cut away that desired projection.
One advantage of the present compliant end effector invention is its simplicity of use. The robotic arm does not need to be programmed to take into account every intended deviation in the surface of the workpiece. The end effector securely holds the spindle so that the tool will compliantly engage the workpiece as the spindle moves along a uniform path of travel. This dramatically reduces the amount of programming necessary to machine more complicated parts. Casted and molded parts with relatively uniform surfaces with a number of abutments, recesses and screw holes can be easily processed using a robotic arm with the present compliant end

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