C1.4 - A Wireless Sensor for Ingot Mold Temperature Monitoring

Abstract

In order to avoid ruptures of a metal strand in continuous casting machines a matrix of numerous temperature sensors is applied to monitor the temperature profile of the strand. The need for frequent reconstruction and maintenance of the ingot mold leads to efforts with respect to the cabling of the numerous sensors. In this paper a prototype of a completely wireless thermoelectrically powered temperature sensor is introduced. The solution includes the energy-harvesting and sensor electronics and a robust sensor design. The approach of the dimensioning is described and the relevant energy harvesting components are characterized in the trial. Measurements show that the developed thermoelectric powered sensor can transmit every second up to 28 bits of user data in addition to a 32 bit unique sensor ID if a temperature difference of 16 °C between the sensors hot surface and the ambient air is given. Introduction A rupture of a metal strand is one of the most severe disruptions that can happen during steel production. To avoid this accident, it is essential to measure the temperature profile of the metal strand in order to identify the development of cracks [1]. For this task a matrix of up to 120 spatially distributed temperature sensors is used. The setup of a typical continuous casting plant is depicted in Fig. 1. The sensor matrix is located at the inner side of the ingot mold that is oriented towards the molten metal strand. The cabling of the numerous sensors causes notable problems with respect to installation efforts and reliability. In addition, if the ingot mold needs to be re-arranged, the required adjustment of the cabling leads to long and thus costly downtimes. To overcome this problem, a concept for a wireless sensor system has been developed. It is proposed that a thermoelectric generator (TEG) utilizes the process heat of the ingot mold as an energy source. The sensor is screwed into the hot mold wall, so that the absorbed heat can pass through the TEG mounted in the interior of the sensor to the ambient air. The temperature is measured with a sensing element in a probe that is pressed against the inner side of the ingot mold. The extreme high humidity, pollution by settled dust, the potential mechanical forces, and the need for an easy installation and handling were considered in the sensor design. As usually the electronic part of the energy harvesters consists of an energy converter, an energy conditioner, and an energy storage (see Fig. 2). The storage supplies a microcontroller that manages the supply and the data conversion for the transducer and the RF transmitter module. A simple low power transmission protocol with a low probability of collision is used for the radio transmission. thermoelectric generator (energy converter) DC/DC converter (energy conditioner) capacitor (energy storage) microcontroller RF transmitter resistance (sensing element) transducer STM 110 Fig. 2: Schematic of the sensor electronics tundish with liquid steel water-cooled ingot mold extension bolt with counternut extension bolt with wireless sensor soft steel strand exemplary temperature measuring point C 1.4 Fig. 1: Soft steel strand S E N S O R + T E S T C o n f e r e n c e s 2 0 1 1 S E N S O R P r o c e e d i n g s 3 8 1 Design of the Thermoelectric Energy Converter For a suitable choice of the energy converter several Seebeck elements were investigated. Their parameters were determined experimentally. The results are summarized in Tab. 1. To ensure efficient energy conversion the modules need to have a high Seebeck coefficient, a low internal resistance, and a low thermal conductivity. Since all parameters rise with the number of thermocouples it is possible to increase the thermoelectric efficiency [2] by optimizing the material composition of the used thermocouples. A preferred material composition is characterized by a highest possible figure of merit [2], as in the example of the TEG 1. Other manufacturers rely on temperature resistant and mechanical robust modules such as the TEG 2. However, because of the used polyimide insulation between the thermocouples in order to stabilize the structure, these modules are attended by a loss of the thermoelectric efficiency. Also the TEG 3 does not has a high figure of merit, but considering its small surface an excellent power factor [2]. The TEG 3 is an extremely miniaturized module, based on a novel production technology. Depending on the restrictions of the application (for example weak heat source, high temperature or low space) the type of the TEG should be selected. The bold parameters in the table show the main highlight of each TEG type. Tab. 1: Measured parameters of tested Seebeck elements description TEG 1 TEG 2 TEG 3 manufacturer Eureca HI-Z Micropelt model TEG1-30-30-2.1 HZ-2 MPG-D751 maximum temperature 150 °C 250 °C 200 °C area 900 mm2 841 mm2 13.9 mm2 height 3.6 mm 5.1 mm 1.1 mm number of thermocouples 128 98 540 Seebeck coefficient 34.1 mV/K 18.3 mV/K 99 mV/K internal resistance 6.5 Ohm 3.4 Ohm 524.6 Ohm thermal conductivity 0.23 W/K 0.33 W/K 0.15 W/K figure of merit 779 1/K 302 1/K 124 1/K power factor 4.97 μW/(K2·cm2) 2.93 μW/(K2·cm2) 32.68 μW/(K2·cm2) When choosing the appropriate energy converter, the thermal sensor assembly and the environmental conditions must be considered as well. This includes the absorption and dissipation of the heat, as well as the arrangement and mounting of the TEG. The hot side has to provide enough heat and the cold side has to release enough heat to produce a sufficiently large and durable temperature difference for the TEG. A simplified arrangement with an infinite heat source and a heat sink with a finite energy transfer are shown in Fig. 3. With the hot side temperature and the ambient temperature the temperature difference at the TEG can be calculated by (1) Fig. 3: Parallel array of thermoelectric generators S E N S O R + T E S T C o n f e r e n c e s 2 0 1 1 S E N S O R P r o c e e d i n g s 3 8 2 where represents the thermal conductivity of the heat sink and the thermal conductivity of a single TEG [3]. The losses of the thermal coupling and other thermal bridges are neglected in this approach. The number of the TEGs reduces the effective temperature difference due to the additional heat flows with the result that the heat sink heats up to the temperature . Assuming identical modules with a Seebeck coefficient the thermoelectric voltage at each TEG is given by