Kinematic Couplings In Robotics ME 250 Precision Machine Design San Jose State University Spring 2003 Kevin Zhang K.C. Jones

Overview Motivation of Research Applications in Robotics Types of Couplings High-load Tool Mountings Case Studies Results

Motivation of Research Apply kinematic design principles to high-load robotic application in order to Improve repeatability and accuracy in mounting different tools on the robotic arm. Improve repeatability and accuracy in positioning robots on the manufacturing floor. Eliminate need for on-floor calibration of precision robotic tools with each tool change or floor layout change.

Applications in Robotics Tool Mounting Robot Positioning

Types of Couplings SurfaceSurface/LineLinePoint Stiffness Repeatability

High-load Tool Mountings Assume:2000 N total force 304 stainless mounts Yield Stress: 276 MPa 6 Point Contacts  25.4 mm balls Contact Pressure: 1.63 GPa 6 Line Contacts  25.4 mm cylinders 25.4 mm long Contact Pressure: 46.4 MPa

Option 1: Ball/Canoe Groove Exact constraint design create contacts at single points or lines, resulting in high contact stress. Canoe/ball design place a section of a sphere of diameter 1 meter on a small block. Stiffness and load capability are 100 times better than a 1” ball.

Option 1: Ball/Canoe Groove 15mm Diameter Canoe ball surface after eleven base mountings.

Option 1: Ball/Canoe Groove Expensive canoe features on permanent structure and cheaper groves on the “disposable” wrist. Predicted laboratory repeatability in the microns Robot Wrist Robot Base

Option 2: Three Pins

Option 2: Three Pin In-plane preload “Fp” and contact reaction force (F1, F2, F3) against three pin interface.

Option 2: Three Pin Interface plates fitted with three pin couplings

Results Based on the measurement results, the overall accuracy benefits of precision machine canoe ball setup is negligible over the simple three-pin interface. Canoe ball ($1000-$3000) X more expensive than the simple pin

Summary When interfaces are standardized across manufacturer, the kinematic interface becomes a simple handshake. Quick-change factory interface improve flexibility during production Reducing uncertainty involved in the repair/replace decision Replacement and maintenance process are faster and more efficient.

References [1]Hart, J. “Design and Analysis of Kinematic Couplings for Modular Machine and Instrumentation Structures,” S.M. Thesis, MIT, [2]Willoughby, P. “Kinematic Alignment of Precision Robotic Elements in Factory Environments,” S.M. Thesis, MIT, [3]Line Hertzian Contact Calculator. (n.d.) Retrieved March 14, 2003, from tacts.HTML tacts.HTML

Hertzian Stress