Enlarge this picture Figure 1 – Capacitor Model

Our universe is largely comprised of electrically charged particles. The ubiquitous nature of this phenomenon is the foundation for applications using capacitive sensing technology. While today there is not a standard name for this category of sensors, some common names are E-Field sensor, capacitance-to-digital-converter and capacitive sensor.

Enlarge this picture Fig. 2

The overall technology of capacitive sensing technology is the ability to sense changes to a generated electric field. The sensor detects the presence of objects entering into the field by measuring capacitance changes in the field. The equation and capacitive model in Fig. 1 demonstrate the capacitive operation of the sensor and illustrates how the sensor can be applied to not only recognize that an object has entered the field, but also sense the size, shape and even differentiate the material substance of the object. In the E-Field model, the strength of the electric field is:

• Proportional to the area of the electrode.
• Inversely proportional to the distance between objects.
• Proportional to the dielectric constant of the material between the electrodes.

By changing the variables, one sensor may be used to sense proximity, touch, size, shape and even differentiate the material inside the field.

Enlarge this picture Fig. 3 Gradient electrodes

Today, the capacitive sensing technology is finding application in many industrial and consumer products including large and small appliances. One of the most popular applications served today is touch panel controls for appliances where it eliminates the need for mechanical buttons, providing reduction in cost, sleek contemporary designs and better reliability with no wear or degradation in performance.

Capacitive sensing technology is extremely versatile and should not be limited to touch panel controls for user interface. For example, in a clothes washer, in addition to being able to handle the user interface requirements, this technology is able to replace other sensing technologies in water level measurement, soap content measurement, as well as auto-balancing.

Liquid level

Enlarge this picture Graph 1. Gradient electrode theory.

Today, most clothes washers utilize a metallic drum as the container for clothes and the water. In these machines, a water column usually made of vinyl tubing is used to measure the water level either with mechanical or solid-state pressure sensors.

Liquid level measurement is easily achieved using capacitive sensors. Some of the benefits of using non-contact capacitive sensors for liquid-level measurement in clothes washers include cost reduction, elimination of corrosion due to media exposure, better reliability, and the possibility of multi-functionality with a single device. The following proposed solutions utilize Freescale’s MC34940, which grounds all the electrodes while only one is selected. The systems described here show how that feature can be used to a designer’s advantage. An example of measuring liquid level in a water column is shown in Fig. 2. By placing vertical electrode strips across the water column, a vertical capacitor is formed between the walls of the water column. When no water is present, a single capacitor is formed as C1, while when water is introduced into the system, C1 is split into two capacitors with one being filled with a dielectric of 1 (air) while another one being filled with a dielectric of approximately 80 (water).

Enlarge this picture Graph 2. Gradient height measurement

A system can be easily developed to assign a liquid height for a given capacitance measurement. Unfortunately the system above will only work for applications where the liquid inside the container has a fixed dielectric constant, such as the water level in a coffee maker, etc. In a clothes washer, the problem is more complex, where the addition of soap changes the electrical properties of the water and consequently the dielectric constant. In addition, these changes occur dynamically, where the system may not know when the user will be adding soap and how much of that soap is being added.

In continuation of the previous method described, a more sophisticated method is proposed where a new geometry for the electrodes is explored, as shown in Fig. 3. With this electrode arrangement, different areas will be in contact with the liquid and therefore a unique ratio between these two areas can be extracted. Theoretically, this ratio will be directly related to the liquid level, while the absolute values of the area will provide information in regards to the dielectric of the liquid.

Graph 1 shows the change in area of the electrodes attached to E1 and E2 as the liquid level rises from below. By dividing the value from E1 by the value of E2, the following ratio is obtained.

Enlarge this picture Water level data

To validate this theory, an experiment was performed where the same container was filled with clean water and measurements were taken. After being emptied, a mixture of water and laundry detergent was used as liquid. The experiment was repeated. Graph 3 shows the ratio calculated by the fixture during the experiment.

With this method, in addition to providing the height of the liquid, the system can also monitor the absolute values of C1 and C2 to estimate the dielectric constant of the water and indirectly estimate the soap content of the water.

Auto-balancing

Fig. 4 Auto balancing

An unbalanced load during the high-speed, spin-cycle can cause a clothes washer to shake and, in some cases, to even walk across the floor if unrestrained. By taking advantage of the distance variable in the capacitor model equation, the appliance designer can use a capacitive sensor to construct a group of proximity sensors that can be used to auto-balance the load.

A simple four-electrode arrangement could be used, where the electrode pads are aligned in a circular pattern. This electrode ring would be place underneath the spinning drum. In order to obtain consistent results, this system requires that the drum be made of a conductive material, as well as being tied to the same electrical ground as the capacitive sensor. This way the drum will act as the grounding plate of the capacitors formed with the four electrodes.

Information of the spinning drum is obtained by looking at the values of E1, E2, E3 and E4. In a perfectly balanced drum, the distances between the drum and the four pads are going to be identical, and consequently their capacitance measurements are going to be same. If the load is unbalanced, the drum will lean towards on direction and consequently one side of it will be closer to one of the pads. (See Fig. 4.)

The benefits of a well-balanced load include faster spin rates that allow for a more efficient spin cycle when removing water from clothes. In addition, the benefits of auto balancing allow extended life for the transmission and bearings due to lower peak loads present on the motor.

Conclusion

As demonstrated in the previous examples, capacitive sensing technology can support multiple useful functions in a clothes washer application. Manufacturers can employ the technology to add functionality and differentiate their products in ways not previously possible or cost effective. Costs can be lowered by using a single capacitive sensor to perform multiple functions. Designers are afforded maximum flexibility and freedom to locate the control electronics remotely from the sensing area.

E-Field imaging technology, while relatively new, is cost effective for appliance applications and brings a high-value proposition to a multiplicity of sensing functions for appliances, while providing solutions for many traditional problems associated with solid-state sensors in appliance applications.

For more information email: jennifer.white@freescale.com