Strain Gauge Bridge Completion Modules

The most common strain gauges used to quantify the state of stress on a test specimen’s surface, are uniaxial and rosette gauges.  For accurate measurements of stress and strain, these uniaxial and rosette gauges are independently connected as a Wheatstone bridge in a 3-wire quarter-bridge or half-bridge arrangement.

Today, most high-end data acquisition equipment manufacturers provide signal conditioning options for collecting data from single strain gauges.  Signal conditioning for strain gauges usually includes circuitry for bridge excitation voltage, quarter and half bridge completion arrangements, and shunt calibration.

Sometimes you may need to view or collect data from a single strain gauge using a device that only has analog voltage inputs and no strain signal conditioning.  This was precisely the case when I was working with the HX711 load cell/strain amplifier.  The HX711 requires a full bridge input so I instrumented my test specimens as such.  If I needed to use a single strain gauge with the HX711, I would have had to use an external bridge completion circuit.

What are your options for measuring single strain gauges with a device that only has voltage inputs or full bridge inputs?

Option 1: Buy a commercial off the shelf bridge completion modules.

The list below gives the specifications for some available bridge completion modules.  I plan on adding more completion modules to this list for future reference, so send me any additional completion options.

Strain Gauge Completion Modules
 
Manufacturer NI VPG Campbell Scientific
Model # NI 9926, NI 9945, NI 9944 MR1-10C-129, MR1-350-130, MR1-120-133 4WFBS1K, 4WFBS350, 4WFBS120
Description 3-Wire Quarter Bridge Completion 3-Wire Quarter Bridge Completion 3-Wire Quarter Bridge Completion 
Resistance 1000 Ω, 350 Ω, 120 Ω 1000 Ω, 350 Ω, 120 Ω 1000 Ω, 350 Ω, 120 Ω
Strain Gauge Connector Terminal Block Solder Tab Terminal Block
Device Connector RJ50 Solder Tab Pins and Lead Wire

Option 2: Build your own circuit.

If you are building a product or are in the strain business long term, building your own circuit may be a cost effective alternative to the potentially more expensive off the shelf bridge completion option.  I have built a few bridge completion circuits in the past.  Here is a list of things to keep in mind when designing a circuit.

  1. Use high precision, low resistance temperature coefficient resistors
  2. The voltage source used for bridge excitation should be from a stable source like a reference since the output of a Wheatstone bridge is inversely proportional to the excitation voltage Vout/Vex.
  3. Use remote sensing to compensate for errors in excitation voltage from long lead wires
  4. Amplification will increase measurement resolution and improve signal-to-noise ratio
  5. Filter data to remove external, high-frequency noise

For more information about bridge completion or our strain gauging services, contact Ryan Welker @ (844) 837-8797 x701.

Related Links

Strain Gauge Installations for Field Testing

iTestSystem Tip: Strain Gauge Rosette Analysis

Wireless Strain Measurements with iTestSystem, LabVIEW, and Arduino

Creating XLS files from TDMS Files

Precision testing generates precision data. Acquiring, accessing, and analyzing these data files allows engineers and scientists to lower product costs, engineer time-saving solutions, and even save lives. National Instruments (NI) created the Technical Data Management Streaming (TDMS) file format for high performance data streaming and retrieval. This binary file is structured for easier sorting and access of the complex data within the file. Before NI standardized on the TDMS data file engineers and scientists had to create their own high-performance file types. The TDMS file format is supported not only by iTestSystem, but also across all NI software.

WHAT HAPPENS WHEN YOU WANT TO SHARE THESE FILES WITH COLLEAGUES WHEN THEY DO NOT HAVE ITESTSYSTEM INSTALLED ON THEIR COMPUTERS?

In these instances, it is often easiest to export a TDMS file to a more commonly used file type like Microsoft Excel (XLSX).

NI provides a free program / plug-in that allows users to open TDMS files in Microsoft Excel. This means anyone can convert a TDMS file into an XLSX file and open it in Excel, and once there the file can be easily saved just like any other spreadsheet. Once saved as an Excel file, you can share, open, view and edit it just like any other XLSX file.

HOW DO I GET THE TDMS IMPORTER PROGRAM / PLUG-IN FOR MICROSOFT EXCEL?

If you have installed iTestSystem, the TDMS Importer program / plug-in for Microsoft Excel is already installed. If not, then you can download the software here: http://www.ni.com/example/27944/en/

Once you have downloaded the NITDMExcel program by following the link and using the built-in installation wizard, you can save TDMS files as XLSX Excel files. To open a TDMS file in Excel, you can simply double-click the TDMS file. If that doesn’t work, you will need to right-click, navigate to “Open With”, and find the NITDMExcel program wherever you installed it on your computer.

Figure1: Right click on a TDMS file to import into Excel.

Each time you open a TDMS file in Excel, it will create a new workbook. Each workbook will have multiple sheets, one for the file properties such as the name of the test, date the data was taken, and more. The other sheet(s) will contain the data points plotted as part of the test, which would normally be used to create graphs in TestView Plus.

Now you can save the Excel and share it just like any other XLS file!

Related Links

Auto-Zero Utility Update
Wireless Strain Measurements with iTestSystem
Strain Gauge Installation for Field Testing

For more information about sharing data, iTestSystem, or test equipment rental, contact Ryan Welker @ (844) 837-8797 x702.

Testing Services Team Annual Safety Training

I want to congratulate my colleague Ryan “RJ” Matthews on a job well done as he recently administered ITM’s annual safety training.  RJ did an excellent job presenting the material and addressing questions. His efforts will certainly help us stay focused on our vigilance toward workplace hazards.

For strain gauging or test services, contact Ryan Welker @ ryan.welker@itestsystem.com or ‪(844) 837-8797‬ x702

General Purpose Accelerometers for Rotating Machinery Vibration Measurements

Choosing an accelerometer for rotating machinery vibration measurements can be a daunting task since there are so many options available. This blog outlines the characteristics you should consider when choosing a piezoelectric single axis accelerometer for general purpose vibration measurements and presents some accelerometers to consider.

Characteristics of a General Purpose Accelerometer

When measuring vibration on rotating equipment such as motors, pumps, and generators, the most common measurement location(s) are on the shaft bearing housing(s) at the shaft centerline. At this location, typical vibration levels perpendicular to the shaft are < 100 g and the frequency range of interest is < 5000 Hz.  A general purpose single axis piezoelectric accelerometer with either a 10 mV/g or 100 mV/g sensitivity fits this criteria.

Other characteristics to consider are size, mounting options, cable connections, grounding, and cost. Several mounting options are available. They include magnetic bases, adhesive bases and stud mounts. The mounting option you choose affects the frequency range of your accelerometer measurements. The table below shows typical frequency limits for accelerometer mounting methods.

Mount Type Typical Frequency Limit
Magnet 2,000 Hz
Adhesive 5,000 Hz
Stud 6,000 Hz

5 General Purpose Accelerometers

The table below shows some examples of stud mounted general purpose piezoelectric accelerometers. These accelerometers all have a female 10-32 coaxial / microdot connector.  It is important to note that this is not a complete list of accelerometers and there are many options available from each manufacturer. I would encourage you to go to the websites linked in the table and see what’s available.

 
Manufacturer PCB Dytran BRÜEL & KJÆR Endevco Kistler
Model # 353B03 3055D1 4533-B 256HX -10 8702B500-M1
Sensitivity 10 mV/g 10 mV/g 9.8 mV/g 10 mV/g 10 mV/g
Frequency Range (±5%) 1 to 7000 Hz (±5%) 1 to 5000 Hz (±10%) 0.2 – 12800 Hz (±10%) 1 to 10000 Hz (±5%) 1-10000 Hz
Temperature Range -65 to +250 °F -67 to +250 °F –67 to +257 °F -67˚F to +257˚F -67˚F to +257˚F
Height 0.88 in 0.64 in 0.54 in 0.55 in 0.67 in
Weight 0.38 oz 0.35 oz 0.3 oz 0.14 oz 0.32 oz
Housing Material Titanium Titanium Titanium Titanium Titanium
Electrical Connector 10-32 Coaxial (side) 10-32 Coaxial (side) 10–32 Coaxial (side) 10–32 Coaxial (top) 10–32 Coaxial (side)
Mounting Thread 10-32 Female 10-32 Female 10-32 Female 10-32 Female 10-32 Female

For more information about collecting vibration data, accelerometers, iTestSystem, or test equipment rental, contact Mark Yeager @ (844) 837-8797 x701.

How Do I Collect Vibration Data with iTestSystem and a cDAQ?

Our test engineers collect vibration data on rotating machinery using four basic tools.  We use a Laptop computer with iTestSystem software to stream accelerometer and rotational/speed pulse sensor data from a National Instruments cDAQ equipped with vibration and voltage input modules.  The video above shows how to collect vibration data using iTestSystem and a cDAQ.

Vibration Test Equipment

Vibration measurements are usually derived by analyzing data collected from IEPE accelerometers mounted to the rotating machinery structures and components of interest with magnetic bases or epoxy and a rotational/speed pulse sensor.  Typical rotational/speed pulse sensors are magnetic pickups excited by gear teeth and keyways or optical sensors triggered by reflective tape adhered to the rotating machinery.

The most important part of the data collection process is choosing a sample rate.  If  you choose a sample rate that is too low, the data you have collected is useless.  According to the Nyquist Theorem data must be sampled at a rate that is at least 2X the highest frequency you wish to record.  2X the highest frequency is a minimum number.  Most test engineers like to sample from 2.5x to 10x higher than the highest frequency they wish to collect.

Typical general vibration measurements are sampled at 2kHz.  However, vibration data collected from accelerometers and gear teeth pulses which is used for phase and speed measurements, and bearing fault detection, and torsional vibration determination must be collected at much higher sample rates like 50kHz.

Related Links

For more information about collecting vibration data, iTestSystem, or test equipment rental, contact Ryan Welker @ (844) 837-8797 x702.

Wireless Strain Measurements with iTestSystem, LabVIEW, and Arduino

On a recent project, one of our engineers needed to measure structural strain at several locations on mobile lifting equipment while in operation. Since the strain measurements were distributed and mobile, a wireless internet of things (IOT) solution was required. This blog describes the steps and tools we used to integrate our solution into iTestSystem and LabVIEW™.

One of the wireless devices that we evaluated to monitor strain was a SparkFun Thing Plus (ESP32 WROOM) with a Load Cell Amplifier (HX711). The SparkFun Thing Plus uses the Espressif ESP32 Wi-Fi and Bluetooth MCU. It accepts a variety of digital interfaces including high-speed SPI, UART, I2S, and I2C. The HX711 load cell amplifier accepts four-wire Wheatstone bridges and outputs 24-bit data at either 10 Hz or 80 Hz. The digital signal from the HX711 was connected to a GPIO pin and clock pin on the SparkFun Thing Plus.   We used a Lithium Ion 2Ah battery to power both devices.

Figure 1: Wireless Strain Prototype Connected to a 4-Wire Bending Bridge

After making the connections and installing the device in a 3D printed case for mobility testing, our development engineer programmed this device using the Arduino IDE. Our wireless strain prototype was programmed to auto connect to a Wi-Fi network and output device ID, tag names, and data values via UDP or Webservice. We chose UDP because we only needed the latest strain/load values. Bundling the device ID with the strain data would allow iTestSystem to collect data from multiple devices. To test the wireless strain prototype and develop the UDP interface for iTestSystem, we modified the Simple UDP LabVIEW example vi.

Figure 2: Arduino IDE with Example Program

Next, we integrated the wireless strain prototype into iTestSystem by adding a new communication class into the existing iTestSystem IOT Communication utility. This new class allowed the utility to read the specific UDP data type associated with our prototype and output data to a shared variable. Shared variables can be logged to disk and analyzed with iTestSystem alongside other machine data.

Figure 3: Simple UDP LabVIEW™ Code

For more information about this application, iTestSystem, or our strain gauging services, contact Mark Yeager via email at mark.yeager@iTestSystem.com or phone @ 1.844.837.8797 x701.

ITM Winter 2020 Yeti Cooler Giveaway

Our engineering services department is sponsoring a Yeti Cooler Giveaway.  This giveaway runs from January 6th, 2020 to February 28th, 2020.  To register for this giveaway and view the official rules click the Register Now button below.

Contact Ryan Welker via email:ryan.welker@iTestSystem.com or phone: 1.844.TestSys x 702 for help with any structural testing, strain gauging, and industrial monitoring applications.

Happy Holidays 2019 from ITM

May your homes be filled with warmth and your hearts with joy.  ITM wishes you a happy holiday season and a new year full of prosperity and adventure!

iTestSystem Tip: Strain Gauge Rosette Analysis

When troubleshooting structural failures or validating FEA models through testing, strain gauge rosettes are used to find the full state of strain at areas of concern around the structure.  iTestSystem’s Rosette Analysis tool is used to calculate the principal strain, principal strain angle, shear strain, principal stress, and other values from strain gauge rosette data. This video shows how to use the Rosette analysis tool.

For questions about using the Rosette Analysis tool or other iTestSystem analysis tools contact Chase Petzinger.

Download your free version of iTestSystem today.

Another Successful Condition Monitoring System Installation

Last week our team successfully and safely installed another Boiler Monitoring System (BMS).  This system, a Sootblower Fouling Detection (SFD) system, monitors structural and vibration sensors that quantify the boiler’s response to sootblower operations. The SFD system analyzes the boiler response data and outputs Key Performance Indicators (KPIs) such as fouling level, sootblower efficiency, and sootblower health to automated boiler cleaning systems.

This boiler uses over 50 sootblowers located at different elevations to clean soot build-up from boiler steam tubes.  Since the vibration measurement locations are relatively far apart, the SFD system requires a distributed monitoring system consisting of several junction  boxes that monitor and process data for groups of sensors.  One team of engineers mounted the vibration sensors to the sootblowers and confirmed communication back to a local junction box containing the National Instruments condition monitoring hardware.  The other team installed the junction box panels and terminated the sensor cables.

After all the sensor installations and terminations were completed, each sensor’s location and calibration were verified.  While the sensor verification was being completed, one engineer worked with the mill IT department and the controls engineer to establish remote connection to the system and confirm communication with the mill’s automated cleaning system.

After commissioning the system and returning to our home base, our engineers are now monitoring the system through a VPN connection and assisting boiler operators with optimizing their cleaning process.

For more information about our boiler condition monitoring systems, contact Ryan Welker via email at ryan.welker@iTestSystem.com or phone @ 1.844.837.8797 x702