Student Name: Atia Syed Center Name: Network for Earth Quake Engineering Simulation Host Institute: SUNY at Buffalo Faculty Advisor: Dr. Reinhorn, Mark Pitman Title of Presentation: Development of In-House Accelerometer Dynamic Calibration System Abstract An in-house accelerometer dynamic calibration system has been developed for the SEESL at University of Buffalo during the summer 2007. The system will be used by laboratory personnel to verify the accuracy of the calibrated sensors over the frequency range of interest to researchers conducting experiments at SEESL. Motion sensors constitute a vital component of the apparatus used in Earthquake Engineering experimentation. While several different types of motion sensors are available, one the most commonly used is the accelerometer. The accelerometer is a class of instruments used to measure shock and vibration motion, producing an electrical output signal related to the applied motion. Accurate accelerometer calibration provides physical meaning to this electrical output and it is a prerequisite for quality motion measurements. NEES at Buffalo has been using the “turn over” static calibration method to calibrate the piezoresistive accelerometers being used in the Earthquake Engineering experimentation. The newly developed system would provide SEESL with an in-house method of dynamically calibrating the accelerometers. The dynamic calibration system uses an air-bearing shaker (APS Dynamics, Inc. Model # 113-AB) to test the frequency response of accelerometers over a frequency range of 0-50 Hz, in comparison to a reference standard PCB accelerometer (PCB, Part # 482A23) with an absolute deviation of 0.1% up to 500 Hz. A National Instruments Data Acquisition System using NI Hardware and LabView 8.0 software has been used to drive the shaker, as well as to acquire data from the reference standard and tested accelerometers, and finally to generate the frequency response function comparing the tested accelerometers vs. the standard. LabView is programmed to print the calibration certificate of the tested equipment using the software DIA-Dem. The in-house accelerometer dynamic calibration system will facilitate an easy-to-use, accurate and much more reliable calibration process.

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2. Methods and Materials The air bearing shaker (Model # 113-AB) used to develop the dynamic calibration system was connected to an air Supply (Condenser) using air filter/regulator (Part # 5003), air Tubing and a low pressure switch (Part # 502) which turns off the system if the pressure drops below 34 Psi. A power amplifier (Modal Shop, Part # 2050E05) is connected to the system to amplify the signal. In addition a current monitor, interconnecting cables and BNC cables are used. LabVIEW 7.0 (National Instruments) is used to generate a sine wave to excite the air bearing shaker as well as data acquisition. Accelerometer 301M26 (PCB, Part # 482A23) by PCB Piezotronics is used as a gold standard (Reference) with 0.01% accuracy over a frequency range of 0 to 1000 Hz. (The operating temperature is 71 °F or 22 °C with a relative humidity of 54%; see appendix A). The block diagram for system installation is indicated; Fig (2.1).

Current Monitor

LabView 7.0 Signal Source

Air Supply

Air Filter/ Regulator

5003

Inlet

Input Output

Air Tubing

8114-6A

8114-4A

Interconnect Cable

0081-4A/2C Model 113-AB Shaker

Interconnect Cable

Model 502 Low pressure

Switch

Amplifier

Air Tubing

Outlet

Figure 2.1: The block diagram indicating the apparatus to set up for the dynamic accelerometer calibration system at SEESL

Proceedings of the 2007 Earthquake Engineering Symposium for Young Researchers

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The apparatus set up is indicated in figure 2.2

3. Experimental Procedure Lab View 7.0 (National Instruments) was used for shaker excitation as well as data acquisition. The lab view program can be divided into three different phases. 3a. Lab View Program: The Lab View program developed for this system can be divided into three different phases. 1) The excitation of shaker with a desired frequency range. 2) Collecting data from the tested and reference standard accelerometers and processing it to generate frequency response graphs. 3) Presenting results and printing calibration certificates. The Lab View is programmed to read the frequency from a word file. The word file encodes desired frequency range of 0-50 Hz with an increment of 1 between 1-10 Hz and 2 between 12-30 Hz. The increment is set to be 4 Hz between 34 and 50 Hz. The system therefore is reading 25 different frequency values. The time delay (Fig 3.1) is adjustable and determines the time duration / frequency. (System reads a particular frequency for this duration of time). The time delay is set to be 5sec/frequency for testing and evaluation.

Figure 2.2: The dynamic accelerometer calibration system.

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Generates sine wave according to the

defined frequency

Send signals to the shaker

While Loop shuts down the program

automatically at the end of the file

Time Delay

Frequency

Phase 1 of the program comprises of a signal simulator and a DAQ assistant (Fig 3.1). The signal simulator receives the frequency as an input form the word file, and generates a sine wave, which is then sent out to the shaker using the DAQ assistant voltage out put. A while loop (Fig 3.1) is used as a link between the word file and the Lab View program, which shuts down the program as the word file ends. Phase 2 of the program is used to acquire data. DAQ assistant 2 receives signals from both the tested equipment and the reference (gold standard). A notch filter is used to cut off line noise. These signals are then split into reference, test equipment 1 and test equipment 2. Another VI (Fig 3.2), allows the user to define the amplitude, the amplitude is set to be 1g (9.8m/s2) for testing purposes. Amplitude could be changed using a knob at the front panel (Fig 3.3).

Figure 3.1: Phase 1: this part of the program is used to excite the air bearing shaker with a desired frequency red form a word file, the signal simulator generates a sine wave, which is then sent out to the shaker.

Proceedings of the 2007 Earthquake Engineering Symposium for Young Researchers

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Receives signals from

accelerometers

Filters for line noise

Split signals into standard and test

If and Else statement, filters for 0/0 ∞

Process Signals to generate

Frequency Response

output

Amplitude

Each signal is processed separately; the RMS value of each signal is calculated. The ratio of the tested accelerometer RMS value and the gold standard (reference) RMS determines the calibration factor for the tested accelerometer. Log of the ratio is calculated and displayed. An “if or Else” statement is used (Fig 3.2) to avoid 0/0 ∞ (when the two sine signals cross 0). The final graph displays the ratio over the tested frequency range, on a dB scale (Fig 4.2). The processed signals from the block diagram could be viewed on the front panel of the Lab View program. Several user defined variables could also be controlled from the front panel (Fig 3.3).

Figure 3.2: Phase 2: this part of the program receives signals from the tested accelerometers and the gold standard (reference), the signals are filtered, processed to generate frequency response and sent to the front panel

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The front panel (Fig 3.3) has two adjustment knobs for time delay and amplitude respectively. The end of file is indicated on the front panel using an LED. Current shaker frequency is also indicated. A scroll bar at the front panel (Fig 3.3) allows the user to adjust the calibration factor until the ratio is exactly one. The calibration factor for each tested accelerometer could then be red from the front panel directly. The frequency response graphs generated for each tested accelerometer could be viewed on the front panel (Fig 3.3) or exported to either Lab View report generator or DIA-Dem to print the calibration certificate for each of the tested equipments.

Duration/frequency Adjustment Knob

Shaker Frequency Indicator

Amplitude Adjustment Knob

End of File Indicator

Frequency Response Graphs

Figure 3.3: Front Panel: The front panel of the program has time delay (duration/frequency) and amplitude adjustment knobs. The shaker frequency (Hz) is displayed on an indicator. As the program approaches to an end a green LED (end of file indicator) flashes to indicate that the program has reached to an end. The frequency responses of tested equipments can also be seen on the front panel.

Calibration Factor

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4. Results and Recommendations The SEESL personnel would use the In-house dynamic calibration system, developed during the summer for the years to come. It would replace the “turn over” calibration method used in SEESL before. The turn over calibration method only gives two data points (0g and -2g), the dynamic calibration system on the other hand is programmed to read a user defined frequency range and produce the frequency response of the calibrated accelerometer. It is therefore much easier and convenient to detect errors during a seismic experiment on the shake table. Results from two sample calibrations (Sensotec’s Model JTF Accelerometers; screw mount) performed are shown below; Fig 4.2, 4.3.

The frequency response graph generated for the first tested accelerometer (Sensotec’s Model JTF Accelerometers) indicates 0.1 % accuracy up to 50 Hz.

Figure 4.1: The accelerometer used for the test calibrations

Figure 4.2: The frequency response graph of tested accelerometer 1 indicates 0.1% accuracy over a frequency range of 0-50 Hz with a few deviations at frequencies below 10 Hz

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The frequency response curve of the second tested accelerometer (Sensotec’s Model JTF Accelerometers) is shown below (Fig 4.3)

The dynamic accelerometer calibration system would make the metrology practices at SEESL much easier and accurate. However the system could be further improved for ease of use and accuracy. The system is currently programmed to calibrate two accelerometers simultaneously; the program should be changed, such that the user is allowed to determine the number of accelerometers to be calibrated. The Lab View program currently presents the results on a dB scale, the program should be expanded such that the calibration certificate present the result in both dB and linear scale. The Lab View program was not connected to DIA-Dem (National Instruments) due to installation problems, however it is recommended to export the data to DIA-Dem, which allows to print the calibration certificates directly from the program. DIA-Dem also provides the option to include the institution logo, the technician name and current date.

Figure 4.3: The frequency response of Sensotec’s Model JTF Accelerometers over a frequency range of 0-50 Hz. The curve represents 0.1% accuracy with slight deviation at 30 Hz.

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5. References [1] Ashley, K. L.; Analog Electronics with LabView, Prentice Hall PTR; 2002 [2] Bishop, R.; LabView 8 Student Edition, National Instruments; 2007 [3] Dosch, J.; Air Bearing Shaker for Precision Calibration of Accelerometers, IMAC 2006 [4] Finnie I., Lim B. Y.; A simple model to illustrate some aspects of vibration excitation by delayed terms, Experimental Mechanics; 2007 [5] Johnson G. W.; LabView Graphical Programming : Practical Applications in Instrumentation and Control, Mcgraw-Hill; 1997 [6] Matthew C.; Calibration of Accelerometer Output for Adults, Medicine & Science in Sports & Exercise. 37(11) Supplement:S512-S522; 2005. [7] Mihura B. LabView for Data Acquisition, Prentice Hall PTR; 2001 [8] Mohiuddin T., Bitter R., Nawrocki M.; LabView Advanced Programming Techniques, CRC Press; 2007 [9] Peres M., Sill R. D,; A New Solution for Shock and Vibration Calibration of Accelerometers, retrieved from: http://www.modalshop.com/techlibrary/New%20Solution%20for%20Shock%20and%20Vibration%20Calibration%20of%20Accelerometers.pdf on August 3rd , 07 [10] Sill, Robert D., Kim, Sang H., Accelerometer Shock Sensitivity Calibration Using a Pneumatic Exciter, 77th Shock and Vibration Symposium, November 2006, Monterey, CA, USA.

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Appendix A

Proceedings of the 2007 Earthquake Engineering Symposium for Young Researchers

paper 33 10 Seattle, Washington | August 8-12, 2007