Apr
28
The secret of load detection in washing machines
For commercially available washing machines today, a typical load is up to 8kg. However, depending on the washing cycle, the maximum load is often not exploited. However, the automatic washing programmes, including information on the correct amount of washing powder to use, are based on the maximum load. If the maximum load is not fully used, too much water and detergent will be used.
A flexible solution is to integrate automatic load detection in the washing machine. The correct quantities of water and detergent are then calculated from this information.
The strength of the oscillations of the tub must also be monitored as too high an oscillation can cause the drum to collide with the housing.
Due to the sophisticated use of modern sensor equipment, both these requirements can be met by using one sensor. Various concepts currently on the market provide a solution for this. The Micro-Epsilon approach is very different but offers very promising potential. It solves both requirements using a new measuring method.
Inductive magnetic distance measurement
Each washing machine is fitted with integrated dampers, which counteract the depression of the tub during loading. The oscillations are also reduced during the spinning cycle. If the damper movement is measured by a displacement sensor, the load and drum deflection can be determined from this. Micro-Epsilon measures this movement using a new method of displacement measurement. The sensors have the name “mainSENSOR” which stands for “magneto-inductive sensor”.
The underlying functioning principle of the mainSENSOR uses eddy current and can be described as follows: a coil supplied with alternating current results in a primary magnetic field. According to the Maxwell formula, this magnetic field induces eddy currents in the electrically conductive material arranged opposite the coil. There is a special film in the field of the eddy current sensor, which attenuates the eddy current sensor depending on the strength of the magnetic field. Using this method, a linear relationship between the distance to the permanent magnet and the output signal can be established. As printed coils can be used for mainSENSORs, the required manufacturing method does not depend on semiconductor processes.
Dipl.-Ing. Thomas Birchinger, Product Manager of the mainSENSORs describes the benefits of the product: “The sensor has an extremely high basic sensitivity, which enables a very simple and consequently low cost evaluation circuit. With an eddy current sensor as the technological core, both very fast and very high resolution versions of the sensor can therefore be produced”.
Compared to Hall Effect sensors, significantly larger measuring ranges (currently 60mm has been achieved) are possible. A pulse width modulated signal is available as an output signal, which can be easily read by a microcontroller using a time measurement. There are numerous application possibilities using this measuring method. For industrial applications, the manufacturer currently offers a version of the sensor with an M30 stainless steel housing and a 40mm measuring range. A model with a plastic housing can also be used in high volume OEM applications. The sensor can be modified depending on the individual requirements of OEM customers.
Displacement measurement on the damper
This sensor was developed as part of the framework of an OEM customer project. A permanent magnet is integrated on the piston rod in the damper. The movement is measured by using an MDS sensor installed on the outside of the daFor fast mounting and safe handling, the MDS sensor has an injection moulded housing that can be quickly clipped onto the damper. The PWM signal present at the signal output is supplied to the washing machine’s controller, from which conclusions can be drawn regarding load status and oscillations during the spinning cycle. Dipl.-Ing. Thomas Birchinger, Product Manager, describes the application: “The specific benefits of using this measuring principle are illustrated particularly well in this application. On the one hand, the loading can be measured to a very high resolution, while on the other hand, the movement of the tub during spinning can be measured with high dynamics”.
In a second variant, the magnet is attached directly to the tub. Here, the sensor measures the movement of the container directly.
In both cases, the displacement between the magnet and the sensor board is measured.
Result
A new patented method has now been developed for this project. Due to the high dynamics and large measuring range, this low cost sensor is used both for load detection as well as for monitoring the oscillations during the spinning cycle . This non-contact method results in a longer service life, since it is wear-free and maintenance-free. Using the MDS sensor does not require any major re-design of existing washing machine components. Therefore, added value is created for the washing machine in a simple way, which can be quickly communicated to the end customers.
