ECNG3032 Control and Instrumentation I

Transcription

1 sensor ECNG3032 Control and Instrumentation I Lecture 1 Temperature Sensors Sensors The sensor is the first element in the measurement system. Measurand Transducer Principle Excitation Signal Interface Circuit Output Generic Sensor Elements

2 Sensors Instrument designer interested in: Electrical form of sensor output V, I, C, L, f etc Sensor Performance Specifications Accuracy, drift etc Type of output Threshold: I, V, etc. Pulse Coded (digital): I, V, f etc. Analog: I, V, R, C, L, f etc Sensors Position detection sensors mostly produce: Threshold: I, V, etc. Pulse Coded (digital): I, V, f etc. Temperature sensors are excellent examples of analog output devices

3 Analog Temperature Sensors Resistance devices The resistance temperature detector RTD Semiconductor devices Thermistor Current devices IC temperature sensor Voltage (Thermoelectric) devices Thermocouples Others Resistance Thermometers Use the fact that the resistance of conductors varies with temperature For example a platinum resistance thermometer has a resistance of 100 at 0 C and a resistance of at 100 C The relationship between temperature and resistance is given by a power series of the form: R T R T T T 3,, and etc are temperature coefficients of resistance

4 Construction METAL SHEATH LEADS INSULATING FORMER RESISTANCE WIRE Actual Device Resistance Thermometers Typical metals used Platinum 100 Nickel 120 Copper 12 Nickel-Iron 604 R T Standardised Resistance values at 0 C R T T T US$ 1500/oz

5 Linearity of the metals used Preferred choice In spite of its cost Platinum is preferred for Lab and high grade industrial use because of its linearity. Lab grade device known as Standard Platinum Resistance Thermometer or SPRT Industrial grade as Platinum Resistance Thermometer or PRT Typical Non-linearity: +0.76% from 0 C and 200 C

6 Resistance Thermometers R 2 3 T T R T First order approximation = R ( 1+ ) R= R R T Temperature coefficient 0 1 T 0 T 1 R2 T = R0 ( 1 T1 ) - R0 (1 2) R= R T 0 = R R T R = 100% R T 0 0 Referred to 0 C Resistance Thermometers Temperature ranges C Platinum -200 to 850, = C Nickel -80 to 320, = C Copper -200 to 250, = C Nickel-Iron -200 to 260, = C Accuracies = ±0.75% FS up to 600 C. Accurate, sensitive, stable

7 Sources of error Tolerance built into the device: How good does the polynomial fit the real resistance curve Specified by international standards for tolerances For example Grade 1 SPRT is Grade2 PRT is 0.1 From 0 C and 200 C Sources of error Temperature gradients between the thermometer and the medium being sensed Good thermal contact essential Errors introduced in the path between the sensor and readout We will examine this in ECNG3032 Self heating Power dissipation is an issue since temp rise will change resistance Current flow needs to be kept to a minimum See Handout for details

8 Thermistors A resistance based temperature device Thermistors are temperature dependant semiconductor resistors and have, according to type, a negative (NTC); or positive (PTC) resistance/temperature coefficient (RTC). Manufactured from the oxides of the transition metals - manganese, cobalt, copper and nickel. Operating range of -200 C to C Thermistor Elements from Omega PFA=Perfluoroalkoxy

9 Thermistor probes NTC Thermistor temp characteristic Highly non-linear

10 Principle of operation In semiconductors, conductivity is due to electrons in the conduction band. When temperature increased some electrons promoted from valence band into conduction band conductivity increases. Number of electrons in conduction band depends on temperature Thermistors Characteristic very non-linear Can be approximated by R 1 1 ( ) 0 ( ) T T T R T0 e T = Temperature in K, T 0 = Reference temperature (0 C or 25 C) Manufacturer specific = Curve fitting constant (2000 to 5000) R typically tens of to M Sometimes combined with conditioning circuitry into one integrated circuit temperature sensor

11 + + - IC temperature sensors Example the Analog Devices AD590/2 A two-terminal integrated circuit temperature transducer Produces an output current proportional to absolute temperature Sensing elements are PN diodes in this case. For supply voltages between +4 V and +30 V the device acts as a high impedance, constant current regulator passing 1 µa/k AD590 1uA/K IC temperature sensors V1 9V Ammeter 500uA A Simple AD592 implementation circuit Ranges (nominal) 55 C to 150 C

12 IC temperature sensors Advantages Simple application setup High linearity Low voltage excitation Because of current output long leads not a problem. No bridge conditioning required As for example with RTDs