Sensors in Washing Machines

Prufung . Technische Regel W294, DVGW, Bonn (1997). 

Yagi and S. Suzuki, Proc. Int. Workshop on Nitride Semiconductors, I PAP Conf. Series 1, 915-918 (2000). 

Displacement sensors are used in a very wide field of applications in research, development, quality inspection, automation, machinery and process control. Many physical parameters can be reduced to a displacement or change of distance, and can be measured with highest precision. Displacement sensors detect physical parameters like bending, deflection, deformation, diameter, eccentricity, elongation, gap, length, play, position, revolution, roundness, shift, stroke, thickness, tilt, tolerances, vibration, wear and width. 

There are two product categories of displacement sensing technology. Whenever the moving target is mechanically connected to a sensor, it is referred to as a contact displacement sensor. The physical principles adapted for contact displacement sensors are primarily  

If the object to be measured cannot be mechanically attached to the moving target, does not permit contact or any external force, a non-contact displacement sensor has to be used. The physical principles for non-contact displacement sensors are primarily  

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The use of pressure sensors for water level switches, or in a more sophisticated form also for foam content surveillance in washing machines and dryers. 

When microelectronics and solid state devices developed over the last five to four decades, the development of solid-state sensors followed suit, resulting in the introduction of NTC and PTC resistors to monitor temperature, and first Reed relais and inductive sensors to determine position and distance, or tachometers for rotational measurements in washing machines and dish washers over the past two decades. 

However, motor speed was also the first function in washing machines to be electronically controlled. The standard nowadays are controllable AC- or DC-mo-tors. Depending on the textile type or wash program, such drives can be used to achieve optimal wash speeds, reversing rhythms and activation times. Usually a tachogenerator on the motor is used as a speed sensor. Such a smaller modem motor, connected to a small electronic control unit, is shown on the right hand side of Fig. 3.2. 

The most common control functions in these early appliances are the control of temperature, pressure, position or distance. Mechanical sensing devices were introduced for these purposes, such as bimetal temperature switches or liquid expansion temperature switches for ovens, washing machines, dishwashers, refrigerators, etc. Electromechanical pressure switches and potentiometric level sensors have also been introduced quite early. 

Modem washing machines work with this type of pressure sensors with analogue output In such machines the software can be used to set any desired water level. 

Many Asian washing machines operate exclusively with cold water, although the cold-water temperature is not necessarily the 10-15 °C that can be expected in Europe. It may well be considerably higher. Therefore such machines are not fitted with a temperature sensor. Another machine type, particularly common in the USA, has a cold-water connection and an additional hot water connection but... 

By contrast, modern washing machines allow temperatures to be precisely controlled, virtually to the degree, through an NTC sensor. With appropriate monitoring, the set temperature can be maintained as long as required, as shown in Fig. 3.8. This means that even with low temperatures, the desired washing results can be ensured. 

It is known from laboratory tests that surface tension measurement can provide reliable information regarding existing detergent concentration. Work is being carried out in various institutes on such sensors for the commercial sector. However, for use in domestic washing machines, only sensors that are extremely inexpensive, maintenance-free and durable are suitable. How much of a breakthrough can be achieved here in the future remains to be seen. 

Since nearly all products for washing and dishwashing are complex mixtures of different ingredients, an in-situ access to the relevant data is of high interest This can be achieved by using on-line sensors in the specific applications. Washing and dishwashing machines equipped with on-line sensors will be discussed as an... 

Fig. 4.7 Typical behavior of a solid detergent product during the first 20 minutes in a commercially available washing machine. Relevant parameters (pH value, conductivity A, surface tension y, peroxide content ) were detected by on-line sensorics.

In some other washing machines that have recently entered the market, the wash load is detected by a weight sensor that measures the elongation of the spiral springs where the tub is suspended inside the washing machine. This is known as a load size indicator . Based on information from the detergent manufacturer, it tells the customer how much detergent to use. 

Low-Cost Acceleration Sensors in Automatic Washing Machines... 

However, very economical measurement of acceleration is possible when the washing machine is equipped with a high-resolution analog pressure sensor with microprocessor control. These pressure sensors come as a separate part or are integrated directly on to the PCB of the washing machine control. They work as differential pressure sensors and measure the static pressure via a hose in the lower part of the suds container, thus measuring the water level in the suds container (Fig. 5.57). 

Pressure sensors are being used in all water consuming household appliances (e.g. dish washers, washing machines), but also for filter (differential pressure) applications. Washing machines usually feature only one pressure sensor yielding at the same time ... 

Position/Displacement sensors can be used in active damping systems of industrial washing machines in order to compensate for vibrations coming from unbalanced drums /laundry. These can work much like a simple automotive shock absorber. 

The output of the sensor system delivers a digital signal- The sensor is supplied with 5VDC via a coded RAST 2.5 plug connection. The sensor consists of water resistant material and is therefore especially suitable for the use in household appliances Like dishwashers or washing machines. 

The benefits of optical fibres include their inmiunity to EMI. Optical sensors and couplers are often encapsulated in PC housings. Electronic controllers and their associated sensors are to be found in areas where electromechanical controllers formerly held sway as in white goods, for example, washing machines. 

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