Sensors

Sensors are a critical component of physical modeling. They produce information that is used to quantify and understand physical phenomena. The centrifuge facility at Rensselaer Polytechnic Institute uses many types of sensors and instrumentation. The following sections introduce traditional and advanced sensors, with the term advanced describing those which require separate data acquisition hardware or software.

Traditional Sensors
Advanced Sensors
Tactile Sensor Video
Bender Elements

Traditional Sensors

The geotechnical centrifuge facility at Rensselaer maintains an inventory of sensors and instrumentation that are available for use. These include:

  • Accelerometers
  • Laser Displacement Transducers
  • Linear Variable Differential Transformers (LVDTs)
  • Pore Pressure Transducers
  • Strain Gauges (120 Ω and 350 Ω Bridge Completion)

Detailed information regarding the sensor inventory may be found at NEEScentral. In addition to the sensors maintained in inventory, the data acquisition hardware supports the following:

  • Load Cells
  • String Potentiometers
  • Thermocouples with Cold Junction Compensation
  • Voltage or Resistance based Instrumentation

Specifications


 

Accelerometers
Manufacturer PCB Piezotronics
Model 353B17
Connector BNC
Conditioner SCXI-1531
Laser Displacement Sensors
Manufacturer Keyence
Model LB-11 (Sensor) LB-70 (Conditioning Unit)
Connector RJ12
Conditioner SCXI-1102c


LVDTs
Manufacturer Schaevitz (now Measurement Specitalties)
Model MHR Series
Connector RJ12
Conditioner SCXI-1540 with SCXI-1315 Accessory
Pore Pressure Transducers
Manufacturer GE Druck, Keller
Model PDCR 81-3478
2MI-81840
Connector RJ12
Conditioner SCXI-1520 with SCXI-1314 Accessory

Strain Gauges
Manufacturer Vishay
Model CEA 13 032UW 120 and EP 08 125AC 350
Connector RJ12
Conditioner SCXI-1520 with SCXI-1314 Accessory

Advanced Sensors

Traditional sensors produce a one-dimensional data stream. Advanced sensors are those which perform outside the specification of the traditional data acquisition hardware. These include

  • Tactile Pressure Sensors
  • Bender Elements

Tactile Pressure Sensors

Each tactile pressure sensor is composed of approximately 2000 sensing elements (sensels). They are a resistance based technology and can be calibrated over a wide range of pressures. Due to the two-dimensional nature of the data stream, the sensors require separate data acquisition hardware and software. The hardware, called a “handle”, is standardized to accept many sizes and sensor models.

The tactile pressure sensors, data acquisition hardware, software, and calibration device are produced by Tekscan. Two types of sensor are available for use: high-resolution and high-speed. High-resolution sensors contain nearly 2000 sensing elements and a maximum sampling rate of 275 Hz. High-speed sensors have 280 sensels and a maximum sampling rate of 1400 Hz. Models used in the Rensselaer centrifuge include:

Tactile Pressure Sensors

Bender Elements


Bender elements are piezoelectric transducers that are capable of generating and detecting mechanical waves in soil. They may be used to determine the small-strain shear modulus, shear wave velocity, and compression wave velocity of a soil media. They are composed of a ceramic (Navy Type II Lead Zirconate Titanate) that exhibits piezoelectric properties. Under electrical excitation, the material will either physically expand or contract, depending on the sign of the input. Conversely, it will produce an electrical response when physically strained. The facility uses custom hardware and software in order to facilitate rapid collection of dense instrumentation arrays. Bender Elements Data Acquisition (BEDAQ) is a software suite that allows researchers to rapidly collect, verify, and analyze data from piezoelectric transducers. en in LabVIEW and interfaces with several National Instruments modules in addition to custom-made components. It includes several features to facilitate data collection and analysis. These include:

  • High channel capacity: software supports up to 16 receivers and 16 transmitters for high-density tomography applications.
  • Auto-Save: when enabled, data is automatically saved based on the experimental parameters and time of test.
  • Save/Load Settings: settings can be saved and recalled in future testing.
  • Signal Stacking: improves signal/noise ratio using a smart stacking algorithm that amplifies the real signal while reducing random and cyclical noise.
  • Digital Filtering: feature allows for digital filtering of data when signal stacking is not sufficient (rare).
  • Signal Generator: user-selectable signal generator for the transmitters produces a single pulse of a sine or square wave.
  • Data Viewer: recall and view data quickly and easily while utilizing powerful graph manipulation tools.

The following video showcases the software’s stacking algorithm. The sensors used in the video are setup to produce noisy signals via electromagnetic interference. The software is able to reduce the ambient noise and produce a true visible signal.

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