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Instrumentation and control Project file GROUP MEMBERS: Farrukh Ali khan -1909 Sufian Abrar - 1934 BATCH: 19 (2007-2008) TH SUBMITTED TO: “ENGR. Abdul wAhAb” Abstract The world cannot rely on the fact that temperature measurements using devices are easily operated manually. The industrial process cannot continue with manual temperature measurements by human because in industrial process temperatures are so high that cannot be measured by human. It requires a vast technology for industrial process. The aim of this project is to demonstrate how high temperatures measuring devices are design and how these devices work in the industrial process. TEMPERATURE MEASUREMENT USING ?? The course incharge of instrumentation and control decides to give an object to his students in which the every group of students design a circuit related to his subject and the students gave a full demonstration about the circuit which they design to the course incharge After this our group decided to design a circuit which is totally uncommon to others circuits After the couple of days we decided to design a circuit related to thermo couple because it is the most important topic in our course There was much much problem about the designation of circuit because we only knew the theory not thermo couple practically. Before designing thermocouple we should have to know about its function, its types, its application and much more. Thermocouples A thermocouple is formed when two electrical conductors composed of dissimilar materials, come in contact with each other. Thermocouples are able to generate voltages through a process called the thermoelectric effect. The ability to generate voltage as a function of temperature makes them suitable as a temperature measuring device. The underlying principle behind thermocouples, the thermoelectric effect, was discovered by Thomas Johann Seebec. The thermoelectric effect, sometimes called the Seebec effect, occurs when a thermal gradient is applied to a conductor and produces an Electromotive Force generated across the gradient (See Figure 1). The magnitude of the Electromotive Force is a function of the temperature differential applied, and the property of the material being used. (Each material type has its own unique temperature characteristics.) Figure 1 In Figure 1, if the temperature T1 is greater that T2 then the Electromotive force causes a potential to develop at T1 as compared to T2. The reverse is true if T2 is greater than T1. While interesting, this is hardly useful, since measuring the voltage at T1 and T2 requires additional conductors (e.g., voltmeter probes) which pick up not only the voltage at T1 and T2 but also the temperature at T1 and T2. The resultant thermal gradients in these new conductors counteract the Electromotive force and so the voltage reading is always zero. A second conductor of a different type can be introduced to create a circuit for current to flow. In Figure 2, two conductors of differing materials are joined at each end. (The connection points between the conductors are thermocouples, also called junctions, or thermocouple junctions). Figure 2 Since the two conductors are of differing materials, they have different Electromotive forces for any given temperature gradient applied. Remember the Electromotive Force is dependent on the conductor material (each material has unique thermoelectric properties and so can produce greater or lesser Electromotive Forces). In Figure 2, with differing materials and a temperature gradient between T1 and T2, the Electromotive Force will cause a current will flow around the loop. Assuming Material A generates a stronger EMF for a positive thermal gradient between T1 and T2, current will flow around the loop in a counterclockwise fashion (A wins over B). Conversely, if T2 is hotter than T1 the current flows in a clockwise fashion (A wins over B again, but in the opposite direction). The magnitude of current is a function of the temperature difference and controlled by the resistance of the conductors. (i.e., higher current will be generated given heaver wire, or higher temperature gradient. So the temperature differential between T1 and T2 can be determined by knowing the materials and their properties, the conductor resistances and by measuring the current. Needless to say the circuit is difficult to use and a more complex circuit is necessary if the Seebec Effect is to be realized into a useful tool. Before launching into discussions on more complex circuits, a point of interest is in order. Any two dissimilar conductors can be used for the circuit in Figure 2 and will create electromotive forces characteristic of those two conductors. Over many years, thermocouple manufacturers have painstakingly tested and measured the characteristics of various material pairs and developed a thorough understanding of the properties for these pairs. This has resulted in a standardization of a number of useful wire pairs with well known characteristics. Figure 3 depicts a practical circuit for measuring the temperature difference between T1 and T2. The circuit introduces a third conductor, Copper (CU), and three thermocouples instead of just two. A meaningful Temperature/Voltage measurement on the Copper wires depends entirely upon holding the same temperature on two junctions at T2. With these two thermocouples at the same temperature, the Copper material’s Electromotive Force contribution cancel away, and the circuit can be treated as having just two material (similar to the circuit in Figure 2). Figure 3 Furthermore, since the voltmeter at Vt is high impedance the wire resistance is no longer a factor. This circuit is quite functional and is quite repeatable given any particular T1 and T2. Unfortunately the voltage produced at Vt is only a function of the difference between T1 and T2 and not an absolute measurement of the T1 temperature. Historically, this has been solved by holding T2 at a precisely known temperature. Commonly the junctions at T2 were submersed into an ice bath, as is shown in Figure 4. Holding temperature T2 to exactly 32°F finally allows the temperature at T1 to be accurately calculated from the voltage read at Vt. As a side note, the practice of holding T2 in an ice bath earned it the name the cold junction. And as one would expect, T1 is referred to as the hot junction. (The term cold junction is still used even though ice baths are hardly ever used today.) Figure 4 Earlier we discussed that various material pairs had been painstakingly tested and their thermoelectric properties evaluated. Through this process, standard pairs have been established. As it turns out, regardless of the thermocouple type, the voltage produced is nonlinear with respect to the temperature at T1, and T1 cannot be accurately determined by a simple slope and offset conversion from Vt. The relationship between the voltage at Vt and temperature can be described fairly accurately as a polynomial equation of the form: Where the order of the polynomial (n) is between 5 and 8, depending on the thermocouple type. The coefficient values (a0...an) have been calculated for each thermocouple type and are readily available for reference. Calculations such as there are somewhat difficult to implement and a simpler approach is available. Fortunately, data tables have been produces that very accurately define the relationship between voltage and temperature for each thermocouple type. These data tables have excellent resolution with voltage values for each degree of temperature over the whole working temperature range of the thermocouple. As expected these data tables are referenced to an ice bath (32°F or 0°C). Standard thermocouple types are identified by their letter code, which define the materials being used. Common thermocouple types include: Type B, R and S Platinum / Rhodium Stable, lower sensitivity, good at high temperature Type C Tungsten / Rhenium Used at extremely high temperatures Type E Chromel / Constantan High output, suitable for cryogenic used Type J Iron / Constantan Limited range, medium sensitivity, problematic due to corrosion Type K Chromel / Alumel General purpose, wide range, inexpensive, Type N Nicrosil / Nisil High Temperature, good stability Type T Copper / Constantan Non-magnetic (good for measurements in strong magnetic fields Table 1 Looking at any of the thermocouple tables, the zero point for volts is at 32F (or 0C) with positive voltages above freezing and negative below. This makes sense since the thermocouple tables were developed with T2 in an ice bath, 32F. (i.e., no gradient, no voltage) While the ice bath approach produces accurate results, it is difficult to use and limited in practicality for most applications. A better approach is shown in Figure 5 where an absolute temperature sensor is used to measure the temperature of T2. While the temperature T1 may be at an extreme temperature (e.g., inside a furnace, or cryogenic chamber) T2 is likely to be in a rather fairly benign temperature environment where an accurate absolute temperature sensor can be used. A typical silicon sensor, such as Analog Devices AD590, has a fairly wide working temperature range of -55°C to 150°C which supports most environments for T2. Figure 5 To implement this approach, and use the zero-reference thermocouple tables, the voltage read at Vt must be corrected for the temperature at T2. One way of thinking about the voltage at Vt is that it has two contributors, Thermocouple T1 and Thermocouple T2. The zero-reference table must be used twice to perform the compensation. To compensate, read the absolute temperature sensor and then using the zero reference table determine the voltage contribution of T2. Subtract this value from the voltage reading Vt. And next use the zero-reference table again to determine the temperature at T1. Then we take referance from our course incharge who suggests us to use the k type of thermo couple in our circuit which is easy to understand and assembled in the circuit Datasheet of Thermo couple Thermocouples are the most popular temperature sensors. They are cheap, interchangeable, have standard connectors and can measure a wide range of temperatures. The main limitation is accuracy, system errors of less than 1°C can be difficult to achieve. Then from our research about the circuit from the internet, library and other resources we first compile a simple circuit which is: The above circuit is the thermo couple amplifier In this circuit the Operational Amplifier is used which amplify the voltage generation produce by thermo couple Also we use the IC LM35 which is also a temperature sensor integerated circuit After few days we compile a circuit which named as *THERMO COUPLE PREAMP* The schematic of simple circuit is: Now let's create a practical circuit using this popular sensor. Because the actual voltage is small, you need a lot of gain to make this signal big enough to pass to an ADC. The other tricky bit is that the measurement depends on the temperature both at point of interest (measure junction) AND the where the thermocouple connects to your circuit (reference junction). Because the reference junction will change with room temperature, you need to compensate for these changes (cold-end compensation). Armed with a thermocouple, a local temp sensor and a summing amplifier you can design a practical preamplifier. As a project goal, let's design a circuit for a k-Type thermocouple that produces a 0 to 100 mV output for a 0 to 100 deg C temperature change - effectively, an output of 1 mV / C. Calculation SUMMING AMPLIFIER What circuit can juggle the multiple functions required by the thermocouple preamp? The summing amplifier is perfect for accomplishing these three functions. 1. Amplify the thermocouple signal. 2. Subtract the effect of the reference junction at room temperature. 3. Add an offset adjustment. The summing amplifier is nice because you can adjust each input independently using input resistors R1, R2 and R3. Vo = (-RF/R1) · V1 + (-RF/R2) · V2 + (-RF/R3) · V3 Let's design a circuit to accomplish these three functions above using three simple steps. STEP 1. THERMOCOUPLE GAIN How much gain is needed to generate a 100mV for 100 deg C temp change? First, find how much voltage Vtc the thermocouple produces. Although it's somewhat nonlinear over a large temp range, calculate the average temperature coefficient for a k-Type thermocouple over 100 deg C. dV/dT = [Vtc(100) - Vtc(0)] / [100 deg C - 0 deg C ] = [4.096mV - 0 mV ] / 100 deg C = 40.96 uV / deg C Given this coefficient and a target output of dVo/dT = 1 mV / deg C, we calculate the gain K1 needed. K1 = (dVo/dT) / (dVtc/dT) = (1mV / C) / (52.69uV / C) = 24.414 V/V All that remains is finding R1 to achieve the gain. First choose RF to be 100k (you can always choose a different value if the other resistors are not practical or available.) Then, calculate R1 R1 = RF / K1 = 100k / 24.414 = 4096 ohms. Remember the summing amp is inverting! To get a positive output, connect the neg side of the thermocouple to the amplifier input. STEP 2. COLD-END COMPENSATION A major thorn in the side of a thermocouple measurement is that they don't measure the absolute temperature of the junction. However, they do measure the temperature difference between the measure junction Tjunc and the reference junction Tref. So what happens if Tjunc = 100 C and Tref (room temperature) increases from 22 C to 23 C. The difference changed from 100 C - 22C = 78 deg C to only 100 C - 23 C = 77 deg C! Accordingly, the thermocouple voltage changed proportionately by -1 deg C even though the measure junction did not change! Correspondingly, the preamp's output will change by -1 mV. To compensate for this decease in voltage, you need to add a voltage to the output that's proportional to +1 deg C = +1 mV. To accomplish this feat, simply place a local temperature sensor close to the reference junction. Then, use the summing amp to get +1mV output for every deg C of local temp change. For this design, we've chosen an LM335 IC. This sensor produces a voltage change of +10mV / deg C. (Actually the sensor operates in Kelvin, 0 deg C = 273 K, generating an output at 22 C of Vsensor = (273 + 22) C * 10mV/C = 2.95 V.) So how much gain do we need to transform 10mV / C into 1 mV/deg C? K2 = (dVo/dT) / (dVsensor/dT) = (+1mV / C) / (+10 mV/ C) = 0.1 V/V Then, calculate R2 R2 = RF / K2 = 100k / 0.1 = 1000k ohms. Again, because the summing amp is inverting and we need a +1mV/C at the output, connect the sensor's negative terminal to V2. The upshot of this is V2 = -10mV/C and Vo2 = +1mV/C. The only challenge left is subtracting the sensor offset of 2.95V. STEP 3. ADD OFFSET VOLTAGE For the final step, we need to subtract the large offset of the local temp (-2.95V) and accurately adjust the output for the correct temperature. This fairly easy step involves picking a specific measurement and reference temperature, then using our summing amp equation to find the offset voltage Voff required at V3. (Voff is typically implemented with a potentiometer.) Our grand equation looks like Vo = (-RF/R1) · V1 + (-RF/R2) · V2 + (-RF/R3) · V3 (-RF/R1) · Vtc + (-RF/R2) · Vsensor + (-RF/R3) · Voff All that remains is choosing R3 and rearranging the equation to find Voff. Voff = ( Vo + (RF/R1)·Vtc + (RF/R2)·Vsensor ) / ( -RF/R3 ) To make our calculation easier, let's choose the Tjunc = 22 and Tref = 22 C. Why is this easier? With the Tjunc == Tref, the thermocouple output is zero, Vtc = 0 mV. Let's substitute what we know and choose R3 = 1000k. Given our design goal of 1mV/C, we expect 22mV at the output. 22mV = (-100k/4096) · 0mV + (-100k/1000k) · -2.95V + (-100k/1000k) · Voff = (-100k/1000k) · -2.95V + (-100k/1000k) · Voff Voff = ( Vo + (RF/R2) · Vsensor ) / (-RF/R3) = ( 22mV + (100k/1000k) · -2.95V ) / (-100/1000k) = +2.73 V Question: can you easily generate Voff with an available reference voltage and a potentiomemter? Imagine for this design we have a Vref = +5V and a 5k ohm pot connected from 5V to ground - piece of cake! The wiper of the pot can be adjusted from 0 to 5V. However, if your Voff was greater than Vref, you could choose a different R3 such that Voff < Vref. SPICE CIRCUIT Here's the SPICE circuit of the thermocouple preamp. To simulate the measure junction temperature Tjunc at V(10) and reference junction temperatures Tref at V(11), we'll use two voltage sources where V really represents T in deg C. The Piece-Wise Linear (PWL) sources will linearly ramp the temperatures over a 100ms interval. V_TJUNC 10 0 PWL(0MS 0 100MS 100) V_TREF 11 0 PWL(0MS 22 100MS 22) Initially, we'll sweep Tjunc from 0 to 100 deg C while keeping Tref fixed at 22 deg C. These voltages (temperatures) feed the thermocouple defined by a subcircuit . The negative terminal connects to R1. X_TC1 10 11 0 1 TC_J_1 The cold-end compensation is achieved by a local temperature sensor (LM335 IC) modeled simply as a Voltage Controlled Voltage Source (VCVS) at V(11). An equation describes it's V vs. T behavior of +10 mV / K where 273 K = 0 deg C. Similar to the thermocouple, the negative terminal connects to R2. E_VSENSOR 0 2 VALUE = { 2.73 + V(11)*0.010 } A simple voltage source creates the offset VOFF 3 0 DC 2.73 Finally, our op amp SPICE model is a simple one capturing it's open-loop gain and gain- bandwidth product. XOP1 0 4 5 OPAMP1 SPICE SIMULATION Let's see if our calculations actually mean something. We'll simulate operation from 0 to 100 deg C with the reference junction held at 22 C (room temperature). CIRCUIT INSIGHT Run a simulation of TC_PREAMP1.CIR. Plot Tjunc at V(10) and Tref at V(11). Open a second window and plot the thermocouple output at V(1). Open a third window and plot the preamp output at V(5). Does the output match the expected 0 to 100 mV? The preamp's output should be within a mV of the target value. Actually, because we calculated Voff at conditions Tjunc = Tref = 22 deg C, the output should be spot on at this temperature. Move your cursor to around 22 deg C. How accurate is the output in mV? Because the thermocouple is a non-linear device, (requiring an 8th order polynomial to approximate it's output), the accuracy falls off as you move away from the 22 deg C adjustment point. Finally, let's put our cold-end compensation to the test! Keep Tjunc fixed at 100 deg C and sweep Tref (room temperature) from 10 to 40 deg C. How well does the local temp sensor and circuitry keep the measurement steady? Rerun the simulation and check V(5). Did the output hover near 100 mV as the room temperature swung through it's expected range? Seeing the cold-end compensation was the big payoff for me. You can see the thermocouple V(1) changing with room temperature while the local sensor (open a new window and plot V(2) ) was moving in the opposite direction to cancel the effect! THE REAL DEAL This was a marathon topic - high fives for staying the course! Just a few closing notes. To get a highly accurate reading over a large temperature swing, many systems use calibration and software to linearize the highly non-linear output of a thermocouple. These T vs. V equations are typically published with the V vs. T equations used to create the SPICE model. The gain resistor R1=5269 will not likely be a standard resistor value. But you can always use a potentiometer or a parallel/series combination to get very close. Also, you'll find the local sensor for cold-end compensation comes in many forms - IC sensor, thermistor, diode, Vbe junction. Which ever is used, simply choose the polarity and apply a gain such that it's output cancels thermocouples change due to local junction temperature change. In a real circuit, the local sensor LM335 needs a bias resistor not shown in the simulation. We didn't need one because our simplified SPICE model used a voltage source defined by an equation. A friend of mine designed and bread-boarded a preamp circuit for the K thermocouple. He added a gain adjust (potentiometer) for R2 as well as one for the offset adjust voltage Voff. For calibration, he used two handy temperature references inside every house - a container of ice cubes and a pot of boiling water. Thank you Mike for the picture of the home lab. SPICE FILE TC_PREAMP.CIR - THERMOCOUPLE PREAMPLIFIER * * T MEASURE JUNCTION (deg C) V_TJUNC 10 0 PWL(0MS 0 100MS 100) RD10 10 0 1MEG * * T REFERENCE JUNCTION (deg C) V_TREF 11 0 PWL(0MS 22 100MS 22) RD11 11 0 1MEG * * J THERMOCOUPLE (V) X_TC1 10 11 0 1 TC_J_1 R1 1 4 5269 * * COLD JUNC - IC TEMP SENSOR (10MV/ DEG K) E_VSENSOR 0 2 VALUE = { 2.73 + V(11)*0.010 } R2 2 4 1000k * * OFFSET VOLTAGE VOFF 3 0 DC 2.73 R3 3 4 1000K * * FEEDBACK R RF 4 5 100K * * OPAMP XOP1 0 4 5 OPAMP1 * * *** J THERMOCOUPLE SUBCIRCUIT ************************************ * T_JUNC(C) - 1 T_REF(C) - 2 V_TC+(V) - 3 V_TC-(V) - 4 * .SUBCKT TC_J_1 1 2 3 4 * * USE POLYNOMIAL EQN TO FIND V VS. T (REF TO 0 DEGC) AT BOTH JUNC AND MEAS TEMPS. * V = B1*T + B2*T^2 + ... * * V_TJ - JUNCTION EMF (MV) VS. TEMP (C) E_TJ 5 0 VALUE = { + 0.503811878150E-01*V(1) + 0.304758369300E-04*V(1)**2 + + -0.856810657200E-07*V(1)**3 + 0.132281952950E-09*V(1)**4 + + -0.170529583370E-12*V(1)**5 + 0.209480906970E-15*V(1)**6 + + -0.125383953360E-18*V(1)**7 + 0.156317256970E-22*V(1)**8 } * * V_TR - REFERENCE EMF (MV) VS. TEMP (C) E_TR 6 0 VALUE = { + 0.503811878150E-01*V(2) + 0.304758369300E-04*V(2)**2 + + -0.856810657200E-07*V(2)**3 + 0.132281952950E-09*V(2)**4 + + -0.170529583370E-12*V(2)**5 + 0.209480906970E-15*V(2)**6 + + -0.125383953360E-18*V(2)**7 + 0.156317256970E-22*V(2)**8 } + * THERMOCOUPLE OUTPUT: * V_TC = V_TJ - V_TR * DIVIDE BY 1000 TO CONVERT MV TO V * E_TC 3 4 VALUE = { (V(5)-V(6))/1000 } * .ENDS ****************************************************************** * * OPAMP MACRO MODEL, SINGLE-POLE * connections: non-inverting input * | inverting input * | | output * | | | .SUBCKT OPAMP1 1 2 6 * INPUT IMPEDANCE RIN 1 2 10MEG * GAIN BW PRODUCT = 10MHZ * DC GAIN (100K) AND POLE 1 (100HZ) EGAIN 3 0 1 2 100K RP1 3 4 1K CP1 4 0 1.5915UF * OUTPUT BUFFER AND RESISTANCE EBUFFER 5 0 4 0 1 ROUT 5 6 10 .ENDS **************************************************************** * * ANALYSIS .TRAN 1MS 100MS * VIEW RESULTS .PROBE .END Taking digital output When the circuit is fully operated by resulting analog output we want to take output of it is in digital way But the main problem is to convert analog signal into digital signal and for this problem what should we have to use? After some research we use ICL7107 which is high performance, low power A/D converter For digital output we use 31/2 digital segment which gives digital output The data sheet for ICL7107 is : From more research we also compile the analog to digital converter circuit: Figure-ICL7107 Test circuit and typical application with LED display components selected for 200mv full scale The components selected for it is The final circuit which we design by hard struggle is LM35 THERMOCOUPLE ICL7107 LM324 Figure- Digital temperature measurement using thermocouple, the main components in above figure are shown in small text boxes- Advantages The circuit is so cheap to design The design is assembled easy for any industrial purpose All reading can br known easily Disadvantages The whole circuit cannot be placed to the high energy area because except thermocouple the all components are so delicate or cruicial to high temperature Acknowledgement I would like to express deepest appreciation to our course incharge ENGR. ABDUL WAHAB, who has the attitude and the substance of a genius: he continually and convincingly conveyed a spirit of adventure in regard to guide us in every step when we tired of thinking so much about project. Without his guidence and persistent help this dissertation would not have been possible. I would like to thanks my group leader, FARRUKH ALI KHAN (1909), who played a vital role for designing this project.