Calibration Activity

In order to trust the measurements from our sensors and to get physical quantities in meaningful units (rarely are volts meaningful), we need good calibration data on the instrument. Ideally the response of the sensor is linear across its designed measurement range or the response is easily characterized. Minimal hysteresis effects are also desirable. In this activity you will perform a calibration on a transducer. This is a real exercise that we must perform on many transducers in the laboratory on an annual or semi-annual basis.

Tasks

  1. Hookup the transducer to the power supply and voltmeter as shown below or as advised by the instructor. In this lab we will be using a direct current displacement transducer (DCDT) for the transducer and a vertical height gage as the transfer standard. Setup the height gage and DCDT to perform the calibration. Make sure that all components are aligned to reduce any error during the calibration. Take a photo of your setup and include it in the lab report. (4 pts.)


  2. Measure the output of the power supply. In this case, the power supply should be set to +/- 15 V. What are the actual output values? (2 pts.)


  3. What are the maximum and minimum voltages you would expect to be able to measure as outputs from this transducer? (2 pts.)


  4. Move the transducer until you are at one end of the output range. Move the transducer core in 1 mm steps using the vertical height gage and record the output voltage at each step until you reach the other end of the output range. You could record the output continuously on a computer in the laboratory. In this case, at a random time, look up and write down the number displayed on the voltmeter at that instant. Repeat this three times and use the average value of those readings as the measurement at that displacement. Once you have reached the maximum range, do back down the range in the same 1 mm steps repeating your measurements. This will help us understand if there are any hysteresis effects in the transducer. Present your raw and averaged data in a data table. (10 pts.)


  1. Make a plot of displacement vs. output voltage for the transducer. Is the output of the transducer linear over the entire range? Are there any hysteresis effects observed in the data? If so, what could have caused them? (5 pts.)


  2. Fit a line to the linear portion of this data and explain the meaning of the slope and intercept of this line. What can we say about readings outside of the linear portion of the transducer’s range? Would we use them as experimental observations? (5 pts.)


  3. Apply your calibration factor to the transducer sample data to provide a relative displacement time-series. The data file contains two columns of data separated by commas. The first column is relative time in seconds, the second is the output of the transducer in volts Assume the transducer starts at “0 mm” displacement and produce a plot of the displacement time series. (6 pts.)


    DCDT data file


  4. What calibration biases could be introduced in our setup? (2 pts.)


  5. What other factors could influence the output of the transducer that we are not considering in this calibration? (2 pts.)