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Capacitive sensor introduces intriguing new capabilities in fluid level sensing

AS8579 sensor can accurately measure the level of more than one liquid in a container. If you care how well a washing machine cleans clothes, take note!

Detecting the level of a fluid in a closed container is an important function, and one which end users tend to take for granted. But if the system for measuring the amount of fuel left in a car’s tank were not accurate, drivers would soon start to care about it. After all, no-one likes to be left stranded, far from a filling station, after their car unexpectedly runs out of fuel. 
So it is good to be able to report on an improvement to the capacitive sensing method which can be used to measure the amount of fuel in a fuel tank, of water in a washing-machine tub, or of just about any liquid in any kind of container. And the improved fluid level-sensing technology in the new AS8579 capacitive sensor from ams is blessed with some clever capabilities which make it better than the existing products for fluid level-sensing in use today. 
 

Novel multi-level sensing, for better clothes washing results

Capacitive sensing is a widely used technology in fluid level sensing. It works on the principle of capacitance: charge accumulates in the gap between two sheets of metal, or ‘electrodes’, when a voltage is applied to one electrode. The amount of charge which accumulates depends on a property, the ‘permittivity’, of the material between the sheets. 

The permittivity of air, for instance, is different from that of gasoline. This means that a capacitive sensor can detect the change in capacitance when gasoline is drained from a tank, and the gap between electrodes attached on either side of the tank becomes filled with air rather than with fuel. It is a simple matter for a processor chip to convert this change in capacitance to a measurement of the level of fuel in the tank. 

At the heart of the AS8579’s measurement system is a type of capacitive sensing technology called I/Q demodulation. This method measures the resistive as well as the capacitive element of a system’s impedance. The effect of this is that, unlike other methods for capacitive sensing, it works reliably even in difficult conditions, and is sensitive even to small changes in capacitance. This means that it can measure small changes in the level of fluid such as gasoline in a tank. 

Just as impressive is an additional, and unusual, capability of the AS8579: to distinguish between the levels of multiple different fluids in the same tank. 

The product designers at ams have cleverly provided pins on the AS8579 for as many as 10 sensor inputs, so it can be connected to ten electrode strips attached to a fluid container, concurrently providing ten separate fluid level measurements. Imagine how this can be used in a washing machine, for instance: we know that water, air, and foam all have different permittivity. During the washing cycle, as the tub fills with detergent-rich water and spinning agitates the water, air is squeezed from the tub. A series of horizontal electrode strips affixed one above the other on the side of the tub, and each connected to a single AS8579 chip, can be used to detect the fill level of water (providing one capacitance value), the level of the foam on top (providing a different capacitance value), and the amount of free space left at the top, since the air will produce a third capacitance value. 

In a washing machine, this allows the control system to implement sensitive protection functions to ensure that the tub does not become overfilled with foam. There are countless other applications in home appliances and industrial equipment for the multi-level sensing capability which can be implemented with just an AS8579 sensor and a basic microcontroller. 

The AS8579 is also ideal when the application calls for very accurate measurement of a single fluid level: in this case, a single electrode is applied vertically to the tank or container, so that the AS8579 can measure the minute changes in capacitance as fluid rises and falls, covering or exposing the electrode.  
 

Ready for use in automotive, industrial and consumer designs

The AS8579 fluid level sensor is available now for use in consumer and industrial products as well as in automotive applications such as the fuel tank. It is fully automotive qualified, and offers multiple on-chip diagnostic functions, ensuring support for the automotive ISO 26262 functional safety standard up to ASIL Grade B. Operating at one of four selectable driver-output frequencies – 45.45kHz, 71.43kHz, 100kHz or 125kHz – the AS8579 offers high immunity to electromagnetic interference. It is rated for operation over a temperature range of -40°C to 125°C. 

Designers can start developing with the AS8579 fluid level sensor immediately using its dedicated evaluation kit, the AS8579-TS_EK_DB

For more technical information or for sample requests, go to AS8579 capacitive sensor.