传感器 第二章.ppt

* * CHAPTER 2 BRIDGE CIRCUITS 1.INTRODUCTION This section discusses the fundamental concepts of bridge circuits. Resistive elements are some of the most common sensors. They are inexpensive to manufacture and relatively easy to interface with signal conditioning circuits. Resistive elements can be made sensitive to temperature, and light.Using these basic elements, many complex physical phenomena can be measured. Sensor elements’ resistances can range from less 100Ω to several hundred kΩ. RESISTANCE OF POPULAR SENSORS Strain Gages 120Ω ,350Ω 3500Ω Weigh-Scale load Cells 350Ω -3500Ω Pressure Sensors 350Ω -3500Ω Relative Humidity 100kΩ –10MΩ Resistance Temperature Devices(RTDs) 100Ω,1000Ω Thermistors 100Ω-10MΩ Resistive sensors produce small percentage changes in resistance in response to a change in a physical variable such as temperature or force.Platinum RTDs have a temperature coefficient of about 0.385%/oC. Thus, in order to accurately resolve temperature to 1oC, the measurement accuracy must be much better than 0.385Ωfor a 100Ω RTD . One technique for measuring resistance is to force a constant current through the resistive sensor and measure the voltage output. VOUT=I(R+△R) R+△R 2.Bridge Configurations Bridges offer an attractive alternative for measuring small resistance changes accurately.The basic Wheatstone bridge(actually developed by S.H. Christie in 1833) is shown below. It consists of four resistors connected to form a quadrilateral. The detector measures the difference between the outputs of two voltage dividers connected across the excitation. A bridge measures resistance indirectly by comparison with a similar resistance. The two principle ways of operating a bridge are as detector or as a device that reads a difference directly as voltage THE WHEATSTONE BRIDGE V