A Serpentine Robot Designed for Efficient Rectilinear Motion A Thesis Submitted to the Faculty of Drexel University by Richard Anthony Primerano in partial fulfillment of the requirements for the degree of Master of Science September 2008 ii © Copyright 2008 Richard A. Primerano. All Rights Reserved. iii Table of Contents List of Figures..................................................................................................................... v List of Acronyms .............................................................................................................. vii List of Acronyms .............................................................................................................. vii Abstract............................................................................................................................viii Introduction......................................................................................................................... 1 1 Background.................................................................................................................. 2 2 Mechanical Design...................................................................................................... 5 2.1 Overview......................................................................................................... 5 2.2 Frame .............................................................................................................. 8 2.2.1 Design Options/Future Considerations................................................... 9 2.3 Cam/U-Joint Device......................................................................................10 2.3.1 Kinematics of the U-Joint and CV Joint............................................... 11 2.4 Foot Mechanism............................................................................................15 2.4.1 Design Options/Future Considerations................................................. 16 2.5 Servo Mechanism..........................................................................................16 2.5.1 Design Options/Future Considerations................................................. 22 3 Electrical Design........................................................................................................23 3.1 Segment Processor Board ............................................................................. 25 3.2 Drive Motor Processor Board ....................................................................... 26 3.2.1 Design Options/Future Considerations................................................. 26 3.3 Communication Bus......................................................................................27 3.4 Control Topology.......................................................................................... 29 3.4.1 Instruction Set....................................................................................... 30 3.5 Sensors .......................................................................................................... 32 3.5.1 Tactile ................................................................................................... 33 3.5.2 Motor Current....................................................................................... 40 3.5.3 Heading/Orientation.............................................................................. 43 3.5.4 Object Detection Sensors...................................................................... 44 3.5.5 Special Purpose Sensors ....................................................................... 45 3.6 Power and Communications ......................................................................... 46 4 Software..................................................................................................................... 46 4.1 Background Processes.................................................................................. 47 4.1.1 Stability Monitoring.............................................................................. 47 4.1.2 Ground Hugging................................................................................... 48 4.1.3 Motor Torque Limit .............................................................................. 49 4.2 Foreground Processes................................................................................... 49 4.2.1 Kinematic Equation Calculations ......................................................... 49 5 Specifications and Performance ................................................................................ 50 5.1 Mechanical Specifications............................................................................ 50 5.2 Electrical Specifications................................................................................ 51 5.3 Test Results................................................................................................... 51 5.3.1 Motion in the Plane............................................................................... 52 5.3.2 Cantilevering/Gap Crossing.................................................................. 53 6 Conclusion................................................................................................................. 54 iv Appendix A – Mechanical Components........................................................................... 57 Appendix B – Instruction Set............................................................................................ 61 v List of Figures Figure 1: Computer Rendering of Robotic Snake............................................................... 6 Figure 2: Zoomed in look at three adjacent ribs ................................................................. 6 Figure 3: Components on a typical rib................................................................................ 7 Figure 4: Snake Processor Rib............................................................................................ 8 Figure 5: Cam/U-Joint Device.......................................................................................... 10 Figure 6: Illustrated U-Joint and Assembled Snake U-Joint ............................................ 11 Figure 7: U-Joint Test Setup............................................................................................. 12 Figure 8: U-Joint analysis results...................................................................................... 13 Figure 9: Constant Velocity Joint ..................................................................................... 14 Figure 10: Constant Velocity Joint Analysis .................................................................... 14 Figure 11: Foot Mechanism Detail ................................................................................... 15 Figure 12: DC brush motor............................................................................................... 17 Figure 13: Servomechanism block diagram ..................................................................... 17 Figure 14: Servo Gear Train ............................................................................................. 18 Figure 15: Spool Illustration ............................................................................................. 19 Figure 16: Control cable attachment points on a typical segment.................................... 20 Figure 17: Control Cable Rigging..................................................................................... 21 Figure 18: Geometric relationship between cable length and joint angle......................... 21 Figure 19: Push-pull rod design........................................................................................ 23 Figure 20: High-level electrical system view ................................................................... 24 Figure 21: Segment processor PCB .................................................................................. 25 Figure 22: PCB block diagram ......................................................................................... 26 Figure 23: Drive motor PCB............................................................................................. 26 Figure 24: Control Network Topology ............................................................................. 29 Figure 25: Robotic Snake Instruction Set ......................................................................... 31 Figure 26: Force Measurement and Foot Bracket Location ............................................. 33 Figure 27: Strain Gauge Image and Schematic [18]......................................................... 35 Figure 28: Finite Element Analysis of Foot Bracket (6 lb. applied)................................. 35 Figure 29: Quarter Bridge Configuration ......................................................................... 36 Figure 30: Half Bridge Configuration............................................................................... 37 Figure 31: Full Bridge Configuration ............................................................................... 38 Figure 32: Wheatstone Bridge .......................................................................................... 39 Figure 33: Typical Current Sense Amp Application [14]................................................. 41 Figure 34: Block diagram of Allegro Hall Effect current sensor [15].............................. 43 Figure 35: Sharp GP2D120 sensor [22]...........................................................................