OPM Sensors

Due to inexact and expensive measurement techniques the lab developed a new system: Optical Pressure Measurement (OPM) sensor.

The main components of the OPM sensor are the Contact Image Sensor (CIS) Module and the ATMega32 microcontroller. The CIS is similar to those you would find in a fax machine or a flat bed scanner.

The Contact Image Sensor (left), front and back, while to the right the microcontroller is displayed front and back.

The OPM’s are mounted on square shaped standpipes. On the back of the standpipes a reflective label is mounted.  The CIS Module is able to take images of the water column.
The CIS Module has it own light source and measures the amount of light that get reflected by the scanned object. The microcontroller drives the sensor and receives the gray-scaled images. These images are 1728 pixels high and only one pixel wide.

Using an algorithm the microcontroller can filter the image and calculate the water depth. Because the density of water is close to that of the standpipe the pixels are brighter (due to less refraction) where water is present.

At the water surface there is a drastic increase of the brightness. To find the water depth, the micro controller calculates the derivative, and returns the position of the smallest value. The resolution of the CIS modules is 200 dpi. All  OPM’s are connected to one RS485 bus, which is connected via a RS485/RS232 Bridge to the PC. The PC sends broadcast messages to all OPM’s, this way all sensors measure at the exact same time. A setup of 16 sensors is able to measure with 16 mps (measurements per second). A special communication cable for RS485 bus and switching to an USB/RS485 bridge would increase the number of mps dramatically.