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Tham khảo tài liệu 'robotics 2 e part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 200 Feedback Sensors 5.4 Temperature Sensors We consider three groups of phenomena used in the design of temperature sensors 1. Electrical phenomena 2. Thermal expansion 3. Optical phenomena. 1. Electrical sensors are based mainly on thermoresistors and thermocouples. In addition an electroacoustic method based on the dependence of the speed of sound upon gas temperature and a thermonoise method are in use. Figure 5.39 shows a bridge layout including ratiometers Ro and Ro1. Permanent resistors Rp R2 and R3 form three bridge arms while thermoresistor Rt serves as the fourth arm. Resistors R4 are used for precise tuning of the circuit. Resistor rt compensates for the effects of temperature changes on the circuit except for in the thermoresistor . This kind of device is very accurate for instance at about 1000 C the error is less than 0.0l c for a platinum thermoresistor. Figure 5.40 shows a possible layout for a thermocouple with compensation for its cold arm. In a thermocouple two wires made of two different metals or alloys whose ends are welded together are used. When placed in environments with different temperatures the weld points create an electromotive force E which is described well enough by the expression E a Tỵ-T2 . 5.28 Here Tỵ and T2 are temperatures of the environments at the two ends of the thermocouple and a is a coefficient that depends on the specific properties of the materials the device is made of. In Figure 5.40 point 1 is the hot end of the thermocouple temperature T and point 2 is the cold one. As implied by Expression 5.28 the EMF depends on the difference between the temperatures. However automatization usually requires measuring the absolute value of Tp Therefore a compensation element is inserted between points a-a in the design. This compensator is a bridge in which one arm is thermoresistor Rt. The circuit is tuned so as to create voltage AV which is added to or subtracted from that created by the thermocouple. The accuracy is about .