Calibrating Translating 3-Axis Accelerometers Alex Vogel, Shahrokh Norouzi Sani, Brian J Roscoe, Zahiruddin M Alamgir, Xie Cai, Charles J Robinson Depts of Physics (AV), and of Electrical & Computer Engineering, and Center for Rehab Engr, Science & Technology Clarkson University Potsdam, United States robinson@clarkson.edu Abstract—A special test jig was constructed for the purpose of calibrating a 3-axis accelerometer. Test angles of 0, 6, 14.5, 19.5 and 30 degrees were set separately, in pairs, or in triplets for roll, pitch and yaw. Pitch and roll data are discussed here. Keywords- Accelerometry, Metrology I. INTRODUCTION Our SLIP-FALLS lab finds acceleration detection thres- holds to very short (1 to 16mm) anterior translations of stan- ding or sitting subjects, both blindfolded and not [1]. These small perturbations are used to determine the relative contri- butions of the visual, somatic and vestibular sensory systems to postural control. Subjects wear headphones that provide them with instructions, response feedback and 70 dB of masking noise. A tri-axial accelerometer is attached to the top of the headphones, but cannot be exactly aligned with the translational axis or with the gravity vector. Because of this the angle of the accelerometers with respect to world coor- dinates must be determined. The head can also move during our experiments (e.g., from sway), so a way is needed to cal- culate roll, pitch and yaw orientations of the accelerometers with respect to the platform or chair’s anterior acceleration. II. METHODS A test jig was constructed for this purpose. The jig con- sists of a Steelex M1076 - 4in. Rotary Table to set yaw, and two orthogonally mounted 4”X4”X2” Precision Angle Sine Plates (E-Bay: Discount-Machine) to set roll and pitch. The rotary table is attached to the platform, and the 2 orthogonal sine plates placed on top of the rotary table (Fig 1b). Three Endevco accelerometers (Microtron 7290A) are orthogonally mounted to a cube (Fig. 2) and the cube affixed on top of the second sine plate (Fig 1c) to take acceleration measurements and to determine how these measurements vary in roll, pitch, and yaw in different angles. An pneumatic solenoid (Mead Fluid Dynamics Space Saver SS112X0 75-FB) was connected to a 125 psi dried air source, and used to give an impulsive acceleration (~0.25G) to the SLIP Platform upon which the jig rested. Three sec of peri-impulse platform acceleration and position data, and those of the 3-axes of the test accelerometer, were digitized by an NI A/D card controlled by LabView® programming. The gravity component of Z-axis accelerometer was nulled. Test angles of 0, 6, 14.5, 19.5 and 30° were set separate- ly, in pairs, or in triplets for roll, pitch and yaw. We report here only the data for a simple pitch and a simple roll rota- tion, and for just the Anterior-Posterior (AP) and Right-Left (RL) planes. But we discuss the findings for all axes. Figure 1. (a) Attachment of three orthogonal accelerometers; (b) Assembly drawing showing attachment of accelerometer cube to a sine plate, and that plate to another sine plate, and that to the rotary table, and the entire structure to the platform; (c) the completed test jig. III. RESULTS AND DISCUSSION Differences are caused by the way the platform moves in the AP direction and which of the accelerometers is at an angle to that movement. If there is a change of acceleration in the direction that is being tilted at different angles, then the graph should vary either by the sine or cosine of those angles. During the move, the AP ACCEL was additive according to the sine of the angle (Fig. 2), and there was no change for the RL ACCEL. The roll response during move- ment shows the AP ACCEL tracks move acceleration only, whereas the RL does not change during the static phase. The black line is a reference for when all the angles are equal to zero, before and during platform movement. 2013 39th Annual Northeast Bioengineering Conference 978-0-7695-4964-4/13 $26.00 © 2013 IEEE DOI 10.1109/NEBEC.2013.60 180