Balanced Constant Current Excitation For Dynamic Strain Measurements

Balanced Constant Current Excitation for Dynamic Strain Measurements Douglas R. Firth Alan R. Szary Precision Filters, Inc. Ithaca, New York (607) 277-3550 Balanced constant current excitation uses a pair of matched constant current sources to excite a single active strain gage. The technique provides enhanced immunity from electrostatic noise pickup and may be employed for dynamic strain measurement applications that require only two wires to the transducer. In this paper, the properties of balanced constant current excitation will be examined and compared against traditional techniques using the Wheatstone bridge or single-ended constant current excitation. A method to verify transducer health and cabling will also presented. INTRODUCTION There are various cabling and hookup schemes used for strain gage measurements ranging from a simple two-wire connection, to as many as eleven wires depending on the desired AC and DC accuracy. Strain gage measurements that minimize hookup wiring are often desired on tests of rotating machinery, low mass/low inertia test models, or simply to reduce cabling cost on large channel count systems. Also, cable connections to the test article may be limited by slip ring contacts. A special class of strain gage testing is the dynamic (AC only) measurement of single active strain gages. Electrostatic Noise Source REXT EN Exc CNH CNL CNSH RG REXT Signal Conditioner Interface eIN Figure 1: Two-Wire Wheatstone Bridge Connection The Wheatstone bridge is the most commonly used methodology for strain gage measurements. Figure 1 shows a two-wire Wheatstone bridge connection, which may be used for single-gage dynamic strain measurements. Zero errors are caused by drift of the extension wire lead resistance, REXT, however they are removed by the AC coupled differential amplifier an