Load Cells, LVDTs and Thermocouples Introduction Load cells are utilized in nearly every electronic weighing system while LVDTs are used to measure the displacement of a moving object. Thermocouples have a wide variety of applications both in the laboratory and commercial environment. Understanding how these sensors operate, you will better be able to comprehend the systems in which they are used. Objective Learn to calibrate and use load cells, LVDT s and thermocouples To use load cells, LVDTs and thermocouples to acquire data To process and interpret recorded data Theory Load Cell In contemporary control applications, weighing systems are used in both static and dynamic applications. Some systems are technologically advanced, interfacing with computers for database integration and using micro-processor based techniques to proportion material inputs and feed rates. To send the weight information to computers, signal conditioners are utilized to permit direct communication from the load cell via conversion of the load cell s analog signal to a digital signal. A load cell is classified as a transducer. This device converts force or weight into an electrical signal. The most common type of load cell in use today is the strain based electronic load cell. This type of sensor uses a strain gauge to measure the strain on a known member within the cell. The strain is then used to calculate the applied load. A strain gauge is a thin wire mounted on a piece of film. The gauge is cemented to the surface of the strain element. The type of strain gauge, mounting procedure, and materials used all have a measurable effect on overall performance of the load cell. As the surface to which the gage is attached becomes strained, the wires stretch or compress changing their resistance proportional to the applied load. One or more strain gauges are used in the making of a load cell. Figure 1 shows an image of a strain gauge. Typically, these types of sensors use many strain gauges wired together into a Wheatstone bridge. The Wheatstone bridge was developed by English physicist Sir Charles Wheatstone in 1842. The bridge measures the change in resistance of the strain gages and converts it into a voltage. The change in resistance is linearly proportional to the change in strain. The schematic of the Wheatstone bridge can be seen in Figure 2. Figure 1: Strain gauge
R1 R3 Vs ΔV R2 R4 Figure 2: Wheatstone Bridge The Wheatstone bridge can be used to eliminate unwanted effects such as thermal, bending, and torsion. The layout of the Wheatstone bridge depends on the type of load cell. The load cell can measure force in many ways including bending beam and shear beam methods. Load cells can also be classified by their shape such as canister, S-type, beam, shear and button. Each type of load cell has different advantages and disadvantages as well as varying ranges and accuracies. Choosing a load cell depends on the application and the environment that the load cell will be performing in. Figure 3 shows different types of load cell designs. Figure 3: Various types of load cells LVDT s A Linear Variable Differential Transformer (LVDT) is a type of displacement transducer. It measures the displacement of a mechanical moving object in actual applications ranging from jet engines to robotics. For example, hydraulics and mechanical assemblies utilize LVDT s.
Figure 4: Schematics of an LVDT Figure 4 depicts a transformer with a primary winding and two secondary windings connected in opposition with a moveable core. The dots at each transformer winding indicate the polarity of the induced voltage. The movable core of an LVDT is part of a shaft that extends out of the LVDT and attaches to any moveable object. As the object moves, causing the shaft or core to move within the LVDT, the LVDT accurately measures the displacement of the object. The excitation provided to an LVDT is usually a sine wave measuring several volts RMS and is typically between 1 khz and 20 khz. The output of an LVDT is based upon the relative displacement of the magnetic core. When the magnetic core is centered, with respect to the two secondary windings, the output summation of both secondary windings is zero. As the core moves toward one of the secondary windings, the net summation output increases in amplitude and produces a non-zero differential AC voltage output. The phase of the summation signal will be in phase with primary or 180 degrees out of phase with the primary, depending on which secondary winding the core moves towards. LVDT s have a given range. This range is given as a plus or minus displacement that corresponds to a plus or minus excitation voltage. This means that for every increment of displacement is a given increment of voltage. Each LVDT comes with a calibration certificate that shows how linear the LVDT is and what range it is capable of operating at. Thermocouples German physicist Thomas Seebeck discovered that if two ends of metal were at different temperatures an electric current would flow through it. This is known as the Seebeck effect or thermoelectric effect. Seebeck also discovered that if two different metals were connected in a loop and each junction was at a different temperature, an electric current would flow. This is the theory behind thermocouples. One junction (cold junction) of a thermocouple is held at a known temperature while the other end is the measuring junction (hot junction). The thermocouple measures the difference in the two temperatures which can be converted into the actual temperature of the hot junction based on the known temperature of the cold junction.
There are various types of thermocouples based on the two metals used. Table 1 shows some common types of thermocouples and their corresponding temperature ranges. Different thermocouples can be used depending on the environment and cost considerations. This lab will be using a type J thermocouple. The cold junction is connected to a National Instruments module designed specifically for thermocouples. This module has a built in thermistor to measure the cold junction of the thermocouple. The module then can convert the volts measured by the module into a hot junction temperature. The type of thermocouple used must be configured into the module in order to read temperature. This lab will be reading the millivolt output. The thermocouple will then be calibrated using a probe thermometer for calibration. Figure 5 is an image of a thermocouple. It is simply two wires connected together and incased in insulation. Figure 5: Thermocouple Table 1: Thermocouple types and temperature ranges Type Material Normal Range, C J Iron-constantan -190 to 760 T Copper-constantan -200 to 37 K Chromel-alumel -190 to 1260 E Chromel-constantan -100 to 1260 S 90% platinum + 10% rhodium-platinum 0 to 1482 R 87% platinum + 13% rhodium-platinum 0 to 1482 Procedure Thermocouple 1. Open and run the Thermocouple.vi 2. Fill one Styrofoam cup with hot water and add ice cubes and the probe thermometer 3. Enter the temperature measured by the thermometer into the Actual Temperature control and press the Record Data button. This will record the output voltage and the actual temperature to a text file. 4. Wait one minute and record another data point. Repeat this for a total of 5 data points. 5. Empty the Styrofoam cup and fill it with room temperature water. Add ice cubes and the probe thermometer.
6. Repeat steps 3 and 4 until 5 more data points have been recorded. The program will close and ask for a name and location to save the data file. LVDT 1. Open and run the LVDT.vi 2. Set the micrometer to 0 inches 3. Enter a value of 0 for the actual displacement. The actual displacement is a control used to record the actual displacement of the LVDT tip relative to its 0 displacement. This corresponds to the magnetic core centered between the two secondary coils. 4. Make sure the Offset Null control is set to zero 5. Enter a value of zero in the Actual Displacement control. Press the Record Data button to record the current displacement along with the LVDT displacement. 6. Move the micrometer 0.1 inches. Enter the current displacement of the micrometer into the Actual Displacement control. Press the Record Data button. 7. Repeat step 6 until the micrometer reads 1 inch. Once the micrometer reaches 1 inch repeat the step 6 moving backwards from 1 inch to 0 at 0.1 inch increments. 8. Press the Save and Quit button to save the data file. The program will close and ask for a name and location to save the data file. 9. Check to make sure the data file is correct. There should be a text file with two columns. The first column is the actual displacement recorded from the micrometer and the second is the LVDT displacement readout.
10. Repeat the previous procedure but apply an offset to zero the displacement readout of the LVDT. This means that the Displacement indicator should read zero when the micrometer is at zero. This will in effect give a 1 inch compressive range. Record and save the data to a file using the previous procedure. Load Cell 1. Open and run the LoadCell.vi 2. Make sure the Actual Force reads zero 3. Press the Record Data button to take the first data point. 4. Apply one 2.5 lb weight to the load cell 5. Enter the value for the actual weight in the Actual Force control. Press the Record Data to take the next data point. 6. Repeat steps 4 and 5 until all the weights have been applied to the load cell. Then repeat the procedure moving backwards from the maximum weight. 7. Once all data points have been recorded press the Save and Quit button. The program will close and ask for a name and location to save the data file.
Pre-Lab Preparation Read through the theory section for this experiment to understand the principles of load cells, LVDT s and thermocouples. Read through the procedures section for each sensor. Workstation Details A laptop computer with National Instruments LabVIEW software NI cdaq-9174 chassis NI 9237 load cell module with load cell 20 2.5 lb weights NI 9215 with BNC LVDT module with LVDT, micrometer and stand NI 9211 thermocouple module with J type thermocouple Control company probe thermometer Styrofoam cup and ice ± 15 volt DC power supply Lab Report You should submit a lab report. Your lab report should include but is not limited to the following information:
Thermocouple Use Excel to create a table and plot of the calibration data. How many millivolts per degree does your thermocouple read? What are thermocouple tables? How does the slope of your calibration compare to that shown for a J type thermocouple in a thermocouple table? Are they different and if so what might be the reason for this? What are the advantages of using a thermocouple over other types of measurement devices? What are the materials used in the J type thermocouple? What is the range? LVDT Use Excel to create a table and plot for each set of data. What is the range of the LVDT? What is the purpose of having an offset for the LVDT? An experiment requires you to record a displacement that could be 0.2 inches in compression and up to 0.8 inches in tension. How can the offset of the front panel be used to change the output reading of the LVDT to correspond to this range? In what types of situations might an offset want to be used in an experiment? Load Cell Use Excel to create a table and plot of the data How linear is your data? Is there a good correlation between the load cell readout and the actual weight applied? Is there a hysteresis? What could cause this? The current load cell is able to measure 2.5 lb increments with relatively good accuracy. This means the sensitivity of the load cell is good enough to recognize a change in weight of 2.5 lb with little error. Could this be done with a load cell with a 20000 lb rating? Why should the range of the load cell be considered when performing a test? Can the same load be used to measure the weight of a car and the weight of a role of coins?