A coily magnetostrictive delay line arrangement for sensing applications E. Hristoforou a,* , D. Niarchos b , H. Chiriac c , Maria Neagu c a Laboratory of Physical Metallurgy, National Technical University of Athens, 9 Heroon Polytechniou, 15773 Athens, Greece b Institute of Materials Science, National Center for Scienti®c Research ``Demokritos'', 15310 Athens, Greece c National Institute for Researchand Development for Technical Physics, 47 Mangeron Blvd., 6600 Iasi 3, Romania Abstract A new magnetostrictive delay line MDL) arrangement is presented in this paper, using a magnetostrictive ribbon or wire wound around a straight conducting cylinder. Passing pulsed current through the straight conductor, the delay line is excited along the whole length of it, thus generating microstrains at the two ends of it, due to the magnetoelastic symmetry of the delay line in case of uniform ambient ®eld. Breaking that symmetry, an elastic signal appears, the modi®cation of which allows the use of the set-up as position, stress, and ®eld sensors. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Position sensors; Stress sensors; Field sensors; Magnetostrictive delay lines 1. Introduction The magnetostrictive delay line MDL) technique has been employed during the last decade as one of the sensing techniques based on magnetic effects [1±3]. Position, stress, and ®eld sensors have been developed, while magnetoelastic phenomena have been studied and analyzed due to this technique [4]. Position and displacement sensors based on MDLs exhibit a range of measurements from 10 mm up to several meters m) and a sensitivity of 100 ppm, after careful treatment of the sensing core and accurate signal condition- ing. Load cells and torque meters using the MDL principle have a variable range, dependent on the sensor housing and a sensitivity as good as 50 ppm. The ®eld sensors provide a span of a few Oe, with a poor sensitivity of 10 nT, providing thought ®eld distribution measurement ability. For some given applications, we needed to improve the resolution and, if possible, the uncertainty levels for our position sensors. The need for such further improvement of these sensors, led us to develop a new MDL arrangement according to which the magnetostrictive ribbon or wire is wound around a pulsed current conductor in the form of tube. As shown in Section 2, the arrangement can provide better levels of sensitivity in position and ®eld sensing. 2. The new arrangement The new set-up is presented in Fig. 1. According to it, a pulsed current generator is used to transmit current to a metallic tube [1] made of a low resistivity metal used as pulsed current conductor, preferably made of copper. Con- ductive spikes, housed in the metallic tube ends, are used to transmit pulsed current through the metallic tube connectors, to allow uniform distribution of the pulsed current density on the surface of the conducting tube. A magnetostrictive material [2] in the form of ribbon or wire is wound around the conducting tube forming a single-layer solenoid, paying attention in obtaining no stress along the length of it, forming a coily magnetostrictive delay line. Two receiving coils [3] are set at the two ends of this coily MDL, and their output is driven to a voltage holder [4] and then to an analog digital converter ADC), as well as to an oscillator and a delay time counter ODC). Finally, the outputs of the ADC and ODC are driven to a comparator and display unit. The two far-end pulses are propagating as elastic waves, and are detected by the receiving coils as two discrete pulsed voltage outputs, V o1 and V o2 , respectively. These voltages have a delay between them proportional to the wound length of the MDL. The ends of the delay line are terminated on the pulsed current conductor by means of welding. Provided that the magnetoelastic uniformity along the length of the MDL is broken for any reason, the above des- cribed symmetry is also broken, resulting in a propagating Sensors and Actuators A 91 2001) 91±94 * Corresponding author. 0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0924-424701)00515-5