Load Sensors Based On The Magnetostrictive Delay Line Technique D. Bargiotas, S. Voliotis, V. Karagiannis, C. Manassis Department of Electrical Engineering, TEI of Chalkis, Psahna, Euboea 34400, Greece Abstract - In this paper we present results on load sensors based on the magnetostrictive delay technique. The sensors translate the displacement of soft magnetic plate, hard magnetic disk and aluminium disk into force with respect to three different magnetostrictive delay line set-ups. The range of displacement determines the range of measurable load. Stainless steel springs have been used as active core supports. The best results both in sensitivity and repeatability have been obtained using soft magnetic plate as active core. I. Introduction Sensors and transducers have an increasing interest because of their importance in many technological applications [1, 2]. Mass sensors are divided into three main categories: the load cells, the pressure sensors and the torque meters; the flow meters and mass flow meters are a derivative sensing application. All these devices can be based on position or strain sensors detecting indirectly the applied stress. The most classical example is the strain gauge. However, there are magnetic materials and corresponding arrangements, which can detect directly the applied stress and therefore the load, pressure and torque on them, using the magnetization change caused by the stress- induced anisotropy. Load cells measuring tensile stress directly are mainly based on inductive arrangements using as ferromagnetic core, a material sensitive to tensile stress. Such a core is usually a positive magnetostrictive material. The permeability decreases dramatically with stress, so that the output of the coil decreases correspondingly. Accelerometers can also be based on such arrangements. A number of load sensors, mainly based on inductive arrangements, have been presented in the past [3–9]. The MDL technique [1, 10-16] has also been involved in direct stress measurement. Typical values of sensitivity and uncertainty of these devices are 10-100 ppm and 100-300 ppm respectively, with an average cost of 1 kEuro/sensor. Pressure gauges have also been proposed based mainly on thin film arrangements, using the piezomagnetic effect. Today’s load sensors are based on miniaturized elements fabricated by lithography techniques, thus allowing better performance and a drastic reduction in their cost. Recently, the MDL technique has been used for some interesting applications of load and derivative size measurements, with sensitivity better than the strain gauge arrangement. In this paper we present results on load sensors based on displacement sensors presented in the past [17-20]. II. Soft magnetic plate setup The first of the sensors presented in this paper is shown in Figure 1. A soft magnetic plate (1) supported by a stainless steel spring (2) is used as the active core (AC) of the sensor. This plate is placed close to the crossing point (PO) of the MDL (3) and a pulsed current conductor (PCC, 4). A receiving coil (RC, 5) is used to detect any acoustic pulse generated along the MDL as a voltage pulse. Pulsed current transmitted orthogonal to the MDL through the PCC causes a pulsed magnetic field along the axis of the magnetically soft ribbon, resulting in a mechanical strain at the region of the PCC-MDL crossing point PO due to the magnetostriction effect. So, an acoustic pulse occurs propagating in both directions, which is detected by the receiving coil RC around the MDL due to the inverse magnetostriction effect in the form of a pulse. The peak of this pulse V o , is the output of the sensor. If the core AC is absent, the magnetic flux density in the MDL is maximized. By placing a load (6) on the magnetic plate, the spring is elongated and the core AC approaches the MDL, causing the magnetic flux den- sity in the MDL to decrease due to the magnetic coupling between core and MDL. From the geometry, one can arrive at the following conclusion: the closer the core AC is to the MDL (i.e. the largest the load placed on the magnetic plate), the less magnetic flux in the delay line exists, resulting in a decrease of Vo. The displacement of the active core AC, and therefore the load placed on the AC, could be calculated with respect to the detected output V o . It was experimentally observed that the presence of the core AC results in a change of the caused acoustic pulse for MDL-AC distance less than 2 mm. Sensor response is presented in Figure 2. The results illustrated were produced using Fe 74 Co 2 Si 8 B 16 ribbon for active core construction, Fe 78 Si 7 B 15 ribbon for the MDL and a stainless spring with k=150 N/mm. Sensor sensitivity varies between 0.13 and 0.28 mV/N and is higher in the region 30 – 75 N and lower for regions 0 – 30