Sensing and Control

Turbidity Sensor

One of the most important parameters that requires monitoring in a wash process is turbidity, a measure of the dirt, food or other particles suspended in the solution. Industrial grade turbidimeters are used, for example, at water treatment plants to assess water quality in the treatment cycle. These meters are very precise and very expensive.

Current technology to measure turbidity depends on optical techniques, where water or other fluids pass through a tube or vessel and a beam of light is transmitted through a cross section of the vessel. As the photons that make up a beam of light pass through the liquid being tested, some are reflected by the particles suspended in the solution while others pass through unimpeded. Two optical detectors-one positioned head on to the light source, the other at an angle of 90° to the light source-measure the transmitted and scattered light photons respectively. The dirtier the water, the less light gets through and the more it is scattered. The turbidity of the water is determined by analysis of the ratio of the scattered light signal divided by the transmitted light signal.

Like conventional sensors of this type, the original HTC conceptual prototype had a flow-through design. However, in the dishwasher, the flow-through sensing was corrupted by air bubbles in the circulated water. The solution was to change the configuration of the sensor to that of a probe placed into the pump reservoir and immersed in the water circulating in the machine. The same optical arrangement is maintained but in an open probe design.

A visible light-emitting diode (LED) is used for a light source. Two photodiodes serve as optical detectors. Light on the detectors creates a current flow through the diodes which is converted to a digital representation by a delta-sigma analog-to-digital technique directly into the on-board microprocessor. A drive control circuit is used to eliminate adjustments to calibrate the LED intensity from sensor to sensor. This allows the sensor to use unsorted LEDs, allowing for lower cost parts.

The development of the new turbidity meter required considerable design effort. Problems we encountered included uneven distribution of the LED intensity over the illuminated spot, the use of lensed versus non-lensed systems, and the design of an aperture system to control stray light and spot size.

^{Figure 2. Like conventional turbidity sensors, the HTC prototype had a flow-through design. Light from an LED passes through the vessel and is impressed on two photoreceptor diodes that monitor transmitted and scattered light. The ratio of these two signals provides a measure of turbidity.}

^{Figure 4. Turbidity sensor data are plotted in an expanded view to show more detail of the main wash and two rinse cycles beginning at 20 minutes.}