Faculty of Engineering


Monitoring bed sediment transport in rivers with the smart pebble


This project aims to record and understand the 'life’ of a pebble on a river bed, as it is picked up and moved from one part of a river to another.   The granular materials (sands and gravels) comprising the beds of rivers are typically in motion during periods of high river flow and stationary at other times. A crucial aspect related to movement of these particles is the cause of the initial motion which lifts them up in water. This process is often called 'entrainment'.

Monitoring bed sediment transport in rivers with the smart pebble

Particle entrainment is determined by many forces such as the velocity and turbulence of the water flow. Each particle is subject to hydrodynamic forces such as lift and drag, and the weight of the pebble is also a critical factor. To understand particle entrainment processes we need to understand the stability and movement of individual particles. 

At present there is no device available which can measure the hydrodynamic forces on a bed sediment particle when it is moving. The aim of the present work is to develop a ‘smart pebble’ which can record the hydrodynamic forces acting on it leading to its entrainment.

The smart pebble is a complete, battery powered unit equipped with two dual-axis accelerometers and three angular rate sensors, a microcontroller, non-volatile external memory and a user interface capable of communication with a PC. Inertial navigation system (INS) concepts are used as the basis of this work. An INS is a system which can determine its exact location in three-dimensional space, relative to a known starting location.

 

Key focus areas/issues


Design considerations for the smart pebble include:

  • Packaging, which determines the size and weight of the pebble.
  • Powering, to provide data logging for a sufficient time period with minimal energy usage.
  • Relative physical placement of the sensors to ensure the centre of gravity is at the centre of the sphere.
  • Signal conditioning systems.
  • Providing a bare minimum digital subsystem.
  • A compact and simple data communication interface.

Current major developments


cl-smart-pebble-size1

A 40mm diameter ‘smart pebble’ is designed, developed and successfully tested inside the water flume.

Calibration is a critical process in inertial navigation system applications as it significantly affects the final performance of the device. The smart pebble calibration process was done in two independent steps

  1. Individual sensor calibration (accelerometers and gyroscopes).
  2. Calibration of the whole smart pebble system.
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The smart pebble has been tested using a shake table and in a flume to evaluate its final performance. This picture shows the shake table setup.

cl-table-pebble-chart1
The picture shows a comparison between the shake table and smart pebble output.
cl-flume-side-and-top-view1
In the flume, motion of the ‘smart particle’ performed by a Particle Image Velocimetry (PIV) camera is used as a reference measurement to compare the outputs of the smart pebble.

 

The following graphs show the comparison between acceleration obtained from PIV camera and ‘smart' pebble output.

cl-smart-pebble-camera-x1
cl-smart-pebble-camera-y1

Key people


  • Bruce Melville
    Civil and Environmental Engineering
  • Zoran Salcic
    Electrical and Computer Engineering
  • Akshya Swain
    Electrical and Computer Engineering
  • Ehad Akeila,
    PhD candidate, Electrical and Computer Engineering
  • Ambuj Dwivedi,
    Research Fellow , DHI-NTU center, Department of Civil Engineering, Nanyang Technological University, Singapore

Contact


Zoran Salcic (for smart pebble mechanism)
Email: z.salcic@auckland.ac.nz 
Phone: 09 373 7599 extn 87802

Bruce Melville (for smart pebble and sediment transport),
Email: b.melville@auckland.ac.nz
Phone: 09 373 7599 extn 88165

Related publications


Ambuj Dwivedi, 2010. Mechanics of sediment entrainment, Doctoral Thesis, The University of Auckland, New Zealand.

Akeila, E. Salcic, Z., and Swain, A., 2010. Smart pebble for monitoring river-bed sediment transport, Sensors Journal, IEEE, vol. 10, pp 1705-1717.

Kularatna, N., Mc Dowall, J., Melville, B., Kularatna-Abeywardana, D., Hu, A.P., Dwivedi, A., 2010. Low-cost autonomous 3-D monitoring systems for hydraulic engineering environments and applications with limited accuracy requirements, IEEE Sensors Journal, vol. 10 (2), art. no. 5361425,pp331-339

Dwivedi, A., Melville, B., Akeila, E., Salcic, Z., 2008. Design of 'Smart' pebble for sediment entrainment study, WIT Transactions on Engineering Sciences, vol. 59, pp. 341-350.

Akeila, E. Salcic, Z. and Swain, A., 2008. Direct gravity estimation and compensation in strapdown INS applications, in 3rd International Conference on Sensing Technology, pp. 218-223.

Akeila, E. Salcic, Z. and Swain, A., 2008. Implementation, Calibration and Testing of GFINS Models Based on Six-Accelerometer Cube, in IEEE Region 10 Conference, Hyderabad, India, 2008.

Akeila, E., Salcic, Z., Kularatna, N., Melville,B. and Dwivedi, A., 2007. Testing and calibration of smart pebble for river bed sediment transport monitoring, Proceedings of IEEE Sensors, art. no. 4388624, pp. 1201-1204.

Kularatna, N., Melville, B., Akeila, E., Kularatna, D., 2006. Implementation aspects and offline digital signal processing of a smart pebble for river bed sediment transport monitoring, Proceedings of IEEE Sensors, art. no. 4178811, pp. 1093-1098.

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