ASM - THE ADVANCED SAND LEVEL METER
Optical backscatterance principle provides a clear and easy interpretable sand-level measurement.
- The time series of measurement allows interpretation of dynamic changes in the sedimentation process.
- High resolution in the 1/2 inch range
- Long term measurements due to low power consumption and high memory capacity
- Tilt, pressure and temperature sensors incorporated to facilitate interpretation of co-factors that influence the data recorded.
Variance parameter recorded for all sensors so that the micro dynamics can be studied. Herewith even extending the range of useful sea floor level interpretations to mud and slit environments.
The idea to use optical backscatterance for sediment density measurements is known for a long time. It was shown that, given a sediment of known type, concentration measurements can be performed. In our application, the focus is on the sand level changes that are extracted by observing the kink in the sediment density graph (See ASMA).
The principle was studied by my partners (Argus) using standard embedded control hardware. Although results were promising, associated hardware problems like too little memory and too much power consumption could not be solved.
My task was to do the complete redesign of the electronics. I chose the Microchip PIC16C74 as the central processor. The processor incorporates many useful features for this design while consuming only minimal power, we lowered the consumption even further by using the processor's sleep mode.
Helpful features are: a high speed RISC core, a Real Time Clock, an SPI interface, a serial interface and many output ports to interface to memory and measurement hardware. The on chip 8 bit A/D converter is reserved for future use, we're using a Maxim 12 bit converter for higher accuracy.
This device integrates seamlessly through the SPI interface. We use between 64 and 128 standard reflexive opto couplers for the backscatterance measurement. To achieve optimal power efficiency and data quality the measurement time is minimal (15uS) while the IR-LED current is almost 1 amp. The line of sensors is clearly visible in the picture. The shown prototype was later revised to reduce the head diameter by a factor of 2 in order to improve stability in high current environments.
The people at Argus look back on many years of ocean measurement experience. They solved all the mechanical problems involved with sea floor measurements. Solved problems include: pressure resistance up to 70 psi (50m depth), strength against mechanical stress, stability of optical parameters, corrosion prevention, to name just a few.
The instrument is hermetically sealed, communication with a PC is made possible through an optical interface. For measurement results please take a look at the ASMA project.
THE ASMA PROJECT
The ASMA program is a C++ program running under 32 bit MS-Windows. It serves as a data analysis front end to the ASM sedimentation level meter. The program makes heavy use of the 32 bit features of Windows. Those include multithreading as well as virtual memory and demand paging. The graphics software uses OpenGL for the device independent rendering of the graphical output.
The image shows the outcome of a laboratory experiment in which the ASM is mounted in a tank filled with sand and water. The amplitude is the reflection of the material in front of the sensor while the color red is used do mark the variance of the reflection. The time axis is from the back to the front, one line is half an hour. The sensor number from high to low is from left to right one line being one sensor.
The image shows a large amplitude to the right which is from the high reflection of the sand surrounding the lower sensors of the instrument. At the beginning of the experiment we use a pump to entrain some of the sand. The entrained sand reflects part of the light, approximately half as much as the solid ground. The movement of the sediment particles is shown through the high red variance of the reflection.
Soon after switching off the pump the random movement ceases and resedimentation begins. It can be noticed, that after some latency resedimentation has an exponential time characteristic, being quicker farther away from the ground. In the middle of the experiment somebody knocks on the tank, leading to higher sand compression and the removal of some particles that stuck to the sensors.
The next image shows an outdoor experiment where the ASM is installed in shallow water. (See picture on the project page) The scaling is different here, the high resolution time axis is from front to back, one line is 10 seconds. The sensor axis is from right to left, again one line is one sensor.
The instrument is in the sand on the left up to sensor 39, in the water from there up to sensor 8 and in the air above, on the right. Sensors have a higher reflection in air due to surface effects of the lenses. The variance of the reflection is amplified here and again displayed in red.
In the beginning everything is calm, only very small surface waves around sensor 8 (red areas). Then a ship passes by, causing large water waves and sediment entrainment and movement (red). The waves even enter the ground and cause shifts of material there (left middle side of the image a little red were the high ground reflection is). After a little while the movement calms down and resedimentation leads to a lower material density in the water.
The information presented here is only a small fraction of the possibilities the ASM and the ASMA program offer. For further information please contact me or my partners at Argus.