Balanced Devices of Strain Measurements with Current Excitation
Ričardas Masiulionis, Rokas Kvedaras, VYGAUDAS KVEDARAS · 2013
The paper analyzes the proposed three circuits of strain measurement devices using high-output-current DACs. Operating principles and capabilities of these devices are analyzed. It has been shown that the proposed designs have an advantage of being relatively simple and flexible while ensuring high linearity and wide strain measurement range. The strain measurements are applied to the newly designed structures and constructions for carrying out tests in order to determine durability and resistance to applied forces, identify weaknesses, and to provide means for improvement or strengthening of these constructions. It is also important to monitor different statuses of the existing buildings. In this case it can be seen that in some time deformations reaches some critical values and gives possibility to predict construction accident, and to take appropriate actions to prevent it. Such strain measurements should be applied to number of specific points of the construction while these points can be distant one from another while measurement results should acquired to central data analysis and storage centers by using wired or wireless measurement equipment (1, 2). In this case measurement equipment should be organized in a way that there is number of measuring units that are situated in variuos places of the construction and acquiring strain measurements and after that sending the measurement information to the data analysis center. Therefore it is necessary to design simple and relatively cheap strain measurement units that are capable to operate under various external conditions. Strain gauges with balanced (1) and not balanced Wheatstone bridges (3) are widely used for this purpose. It is proposed to use bridge circuits with current excitation (4) as these have better resistance to external noise and automated balancing circuits with current balancing (5). Integrated circuits of DACs with current output are quite common in the market nowadays (e.g. MAX5548, MAX5550, AD5510, AD8810 and many others). These DACs are 10 to 16 bits resolution with capability to generate output currents of 20-30 mA. For some of these DACs output currents can be set by software and several DACs (up to 8) can be situated in one IC. New balancing strain measurement devices designed using DACs with current output are presented and analysed in this paper. 2. New circuits of balancing strain measurement devices A new structural design for monitoring strains of constructions using two current DACs (Fig. 1) is proposed. Two analogous current DACs are formed in one microchip MAX5550, using the same reference voltage, which allows to reduce environmental conditions on the monitoring results, as temperature changes almost equally affects both DACs' output currents and drift. This IC is designed to be used in balancing circuits in wide temperature range. The DACs' current ranges are determined before monitoring by the control code and it can be flexibly chosen for observable construction. Values of the output currents are also set by control code to any of 12 different sub-ranges starting from 1 up to 30 mA. Therefore it is possible to set the most suitable strain gauge excitation currents according to gauge specifications. Output currents of these DACs can differ one from another up to ± 2 %. Such