Force Sensing in SB2’s Wings Anna-Katrina Shedletsky BDML Friday, August 24, 2007

Goal StickyBot should be able to adapt its gait in response to the environment. Design and fabricate a compliant shoulder which: Allows for force sensing Does not prevent SB from climbing Is consistent and robust Implement wing force sensors Install and calibrate hall effect sensors Define mathematical relationships between the hall effect sensors and the forces applied by the feet. My Goal

Design Reduced tendency of IE-20A flexture to delaminate with smaller scalloping. Increased robustness and ensured proper placement of three-prong shoulder with notches in the wings. Varied stiffness of compliant joint by fabricating both 4mm and 3mm thick versions. Beginning with prototypes in the lab and Sal’s three-prong design, I developed a compliant shoulder component which: Compliant ShoulderWing Assembly

Manufacturing

Installation Wing components installed on SB2

Flexture Performance Designed an apparatus to compare the k-values of four different flextures: 4mm thick 3mm thick (first cure) 3mm thick (softer cure) Rubber bands (previous design)

Flexture Comparison k=3.8 E-4 Nm/deg k=3.29 E-4 Nm/deg k=1.74 E-4 Nm/deg k=0.43 E-4 Nm/deg Torsional Spring k = F r 

Hall Effect Sensors Magnets Sensor A hall effect sensor changes its output based on proximity to a magnetic field. The flexture allows the sensor to move between the magnets. The sensor returns a value from 1 to 256. Through empirical tests, this value can be correlated to the actual force of the foot (both pushing and pulling).

Sensor Calibration Method Place one of SB2’s feet on digital scale Zero force sensors and tare scale when feet are in home position Use StickyBot_GUI to move the wing servo from +30 to -50 At each position, take three snapshots and record force on the scale Average sets of three, plot against scale reading, and fit curves to the data Goal: Find a mathematical relationship between the A2D clicks of the sensor reading and the actual force applied by the foot in Newtons.

Test Cycle Verification Pushing Down Pulling Up As there is no significant difference between pushing down and pulling up, a model for data in one direction should hold in the opposite direction.

Other Effects in the Data Drift Hysteresis Drift and hysteresis are effects that will need to be mitigated in software.

Statistics The relationships between the force sensor and the force applied at each foot took on a parabolic shape. Near the origin, the relationship is nearly linear; for forces larger than.4 N, a 2 nd order relationship is a better model.

Test Climb with Wing Sensors An initial climbing test with the force sensors suggests maximum forces of no more than.13 N in either direction. In this range, linear models are good. Error in the force sensors and hysteresis near the origin may provide an argument to amplify the signal to  20 clicks. Time Step A2D Clicks Wing Sensors During ClimbMagnified Forces During Stride

Future Work Begin integration of calibration data into SB software. Continue investigation of force sensors while climbing to determine if a linear model is accurate enough for the range of forces observed. Investigate bi-directional versus uni-directional compliance in the shoulder joint. Continue towards adaptable climbing goal.