Sensors and Actuators Sensors Physics

Transcription

1 Sensors and ctuators Sensors Physics Sander Stuijk Department of Electrical Engineering Electronic Systems

2 2 THERMOELECTRIC SENSORS (Chapter 3.9, 16.4)

3 3 Thermoelectric effect thermoelectric effect is the direct conversion between a temperature difference and electric voltage electric voltage (emf) thermoelectric effect temperature difference thermoelectric effect is reversible sensors based on this effect are passive sensors thermoelectric effect at atomic scale temperature gradient causes charged carriers to move from hot to cold side causes thermally induced current

4 4 Thermoelectric effect measurement device is part of the circuit measurement device should use different material compared to metal bar (otherwise currents cancel each other) thermoelectric effect occurs at junction (Peltier effect) Q P I T thermoelectric effect occurs in bar (Thomson effect) Q P I T combination of both effects leads to Seebeck effect V T Seebeck potential is small copper-constantan has Seebeck potential of 41μV/K

5 5 Thermocouple assemblies thermocouple assembly consists of sensing element (junction) protective tube (ceramic or metallic) insulated thermocouple wires (isolate sensor) terminations (connections)

6 6 Thermocouple types Junction materials (+/-) Sensitivity (at 25 C) (μv/ C) Full-range output (mv) Error ( C) Temperature range ( C) Type Copper / constantan ± to 400 T Iron / constantan ± to 760 J Chromel / alumel ± to 1260 K Chromel / constantan Silicon / aluminium ±1.0 0 to 982 E ± to 150 many types have a very low sensitivity silicon is special small temperature range larger sensitivity error due to temperature dependence of Seebeck coefficient

7 7 Transfer function of a thermocouple

8 8 Transfer function of a thermocouple Seebeck potential can be approximated by 2 2 C ( T T ) C ( T ) T T ) C C ( T ) V T2 ( T2 C 1, C 2 constants depending on material Seebeck potential depends on temperature difference absolute temperature error reduced by selecting a thermocouple with small C 2 C μV/K 2 for a T-type thermocouple Seebeck coefficient not dependent on nature of thermocouple (e.g. welded, pressed) depends only on the temperature and materials at the junction

9 9 Thermocouple example J-type thermocouple one junction at 0 C and other at 45 C sensitivity 51.7μV/K what is the open circuit voltage? V = 51.7μV/ C (45 C - 0 C) = 2.3 mv example J-type thermocouple circuit with one junction at 10 C generates 5mV output voltage sensitivity 51.7μV/K what is the temperature at the measuring junction? using specified sensitivity we find T = 107 C (or T = -87 C if we assume that polarity is reversed)

10 10 Thermocouple Thermocouple versus RTD (+) broader measurement range (-270 C to 3000 C) (+) robust, simple to use (+) no self-heating problem (usable in gas measurement) (+) accepts long connection wires (-) lower sensitivity (μv/ C) (-) known reference temperature needed

11 11 Thermoelectric laws temperature difference between two thermocouple junctions leads to a voltage between the open ends of the circuit junctions of different materials lead to different sensitivity voltage is proportional to the temperature difference reference junction must be at known, constant temperature sensing ( hot junction ) reference ( cold junction ) operation of thermocouples governed by three thermoelectric laws thermoelectric laws simplify the analysis of thermocouple circuits useful as practical systems may contain more than just two junctions

12 12 Thermoelectric laws law of homogeneous circuits: thermoelectric current cannot be maintained in a homogeneous circuit by heat alone implications junctions of different materials must be used single conductor will never cause a Seebeck potential intermediate temperatures do not influence Seebeck potential T 1 T 1 T 1 V 0 = V 0 = 0 T 3 T 6 T 4 T 5 V 0 = V 0 T 2 T 2 T 2

13 13 Thermoelectric laws law of intermediate metals: algebraic sum of all Seebeck potentials in a circuit composed by several different materials remains zero when the whole circuit is at a uniform temperature implication materials may be connected in the circuit without affecting the output of the circuit as long as the new junctions are kept at the same temperature

14 14 Thermoelectric laws law of intermediate metals: algebraic sum of all Seebeck potentials in a circuit composed by several different materials remains zero when the whole circuit is at a uniform temperature corollary thermal relation between metals and C follows from the relation between and and the relation between and C implication not all material combinations need to be known use single material as reference

15 15 Thermoelectric laws law of intermediate temperatures: two junctions at T 1 and T 2 produce Seebeck voltage V 2 two junction at T 2 and T 3 produce Seebeck voltage V 1 then temperatures T 1 and T 3 will produce V 3 =V 1 +V 2 implications calibration can be done at reference temperature T 2 and device can used with different reference temperature T 3

16 16 Thermocouples in series thermocouple laws allow analysis of circuits like T 1 T 2 C V 0 C output voltage multiple of single thermocouple V o T T T T T T T T1 circuit know as thermopile amplifies output voltage without use of operational amplifier

17 17 Thermocouples in parallel thermocouple laws allow analysis of circuits like T 1 T 2 T 4 C V 0 T 3 C output voltage proportional to the average temperature V o T T T T T T3 3

18 18 Thermoelectric laws example simulate reference temperature of 0 C three types of wires (,, C) combined into thermocouples with two intermediate temperatures T 1 and T 2 and temperature T at measurement junction T 1 T 2 T a C T V 0 C what relation must T 1 and T 2 have such that the output voltage only depends on T?

19 19 Thermoelectric laws example simulate reference temperature of 0 C three thermocouples formed T 1 T 2 T a C T V 0 C V 1 = α (T 1 -T) V 2 = α (T 2 -T 1 ) V 3 = α CC (T a -T 2 )

20 20 Thermoelectric laws example simulate reference temperature of 0 C three thermocouples formed V 1 = α (T 1 -T) V 2 = α (T 2 -T 1 ) V 3 = α CC (T a -T 2 ) output voltage V o is equal to V o = V 1 + V 2 + V 3 (law of intermediate metals) it holds α = -α α cc = 0 (law of homogeneous circuits) output voltage V o is thus equal to V o = α (T 1 -T) - α (T 2 -T 1 ) = α (2T 1 - T 2 + T)

21 21 Thermoelectric laws example simulate reference temperature of 0 C output voltage V o is thus equal to V o = α (T 1 -T) - α (T 2 -T 1 ) = α (2T 1 - T 2 + T) requirement: V o only depends on T requirement fulfilled when 2T 1 = T 2 circuit simulates references temperature of 0 C through controlled temperatures T 1 and T 2

22 22 Thermocouple circuit reference junction must be kept at constant, known temperature water-ice bath or boiling water requires frequent maintenance one of the wires must be very long T a V 0 T=0 C melting ice law of intermediate metals solves wiring problem fixed reference still needed T a T=0 C C C V 0 T=0 C

23 23 Thermocouple circuit reference junction at ambient temperature circuit directly compensates for temperature fluctuations Fe Cu Cu T R 2 R 3 V 0 Cn Cu V R Cu T a R T R 1 R 4 T a law of intermediate metals solves wiring problem C fixed reference still needed T a T=0 C C V 0 T=0 C

24 24 Thermocouple output equivalent electrical circuit for a thermocouple T x T r R p internal resistance of the thermocouple e h, e c model Seebeck potential of hot and cold junction V p Seebeck potential at terminals of the thermocouple thermocouple requires cold junction at reference temperature