Applications Of Weak Capacitance/Conductance Measurement Techniques In Process Industry

There are large numbers of parameters which should be measured in process industries. However, the related signals are often too weak to be measured by conventional methods. Therefore, the study on the weak signal measurement technology becomes increasingly popular. As many parameters can be obtained indirectly by measuring their corresponding capacitance or conductance value, to study weak capacitance or conductance measurement techniques is of great significance.This dissertation makes attempts to propose applications of weak capacitance and conductance measurement techniques in process industries. The main works are shown as follows:1. A weak capacitance measurement system based on the principle of charge transfer has been built. It is stray-immune and simple. The signals, which are used to control the switches of the charge transfer circuit, are produced by a specified circuit. And the control timing is implemented totally by hardware and no software is needed. The voltage value related to its capacitance value can be output directly by the measurement circuit. In addition, different frequencies can be selected by switches in order to meet requirements of different applications. Here, the measurement system proposed is applied to the column chromatography process of Tripterygium wilfordii multi-glycoside. Experimental results show that there is certain relationship between the capacitance value of the measured solution and the concentration of triptolide or tripdiolide, so it is possible that the process can be monitored on-line by analyzing the measured capacitance values. Practical results show that the concentration of triptolide is controlled effectively, and the product of tripdiolide is increased to 28.5‰ from 27.5‰.2. Study of measuring weak conductance based on the principle of the bi-directional current pulse technique is carried out. The excitation source is similar to a square wave signal. During each half cycle, the magnitude of the signal is the same while the polarity is opposite. So polarization effects on the electrodes are eliminated. Moreover, drive electrodes seem to be excited by direct current (DC) in each half cycle and DC voltage signal can be measured directly on the sense electrodes. So no complicated and time-consuming signal conditioning circuits, such as filters, are needed, and it can be used for high-speed measurements. This technique adopts four electrodes. It can be used to measure not only the conductivity of one certain point, but also those of multiple points. Here, the technique mentioned above is applied to the study of an electrical resistance tomography (ERT) system with a 16-electrode sensor. ERT technique produces cross-sectional slice images depicting spatial variations in conductivity based on sets of boundary measurements. Experimental results show that ERT technique based on the principle proposed is effective. Tests show that the online imaging speed can be 20 frames per second while the off-linedata capture rate without imaging can be about 80 frames per second at the baud rate of 115.2Kbps. Moreover, the stability and resolution of the system is also improved.