- 1 - Beyond Touch Panels: Appliance Solutions Using Electric Field Sensors Brad Stewart, Principal Applications Engineer Freescale Semiconductor, Inc. 2100 E. Elliot Road Tempe, AZ 85284 Phone: (480) 413-5292 FAX: (480) 413-5597 [email protected] www.freescale.com- 1 - ABSTRACT When an object comes in contact with an electric field (E-Field), certain properties of the object can be measured. For example, the difference between water and ice can be determined by observing the change in the dielectric constant. This information might be useful to improve performance of automatic ice makers or to increase the efficiency of the refrigeration defrost cycle. Changes in the dielectric constant of water might be useful to determine when a water filter needs changing, or if there are cleaning agents present. Using electric fields to detect the presence and makeup of the object has many unique applications for proximity detection and switch sensors as well. E-Field sensors can determine if refrigerator door seals are properly seated or detect when food has boiled over on an oven range top. E-Field sensors can measure speed and wobble of motors and drums, and water levels. Other potential applications include sensors for mechanical positioning, moisture detection, and safety systems. ELECTRIC FIELDS An electric field is a force vector (magnitude & direction) that’s present in any region where a charged object experiences an electrostatic force by other charged objects. Almost any object which is somewhat conductive and/or has a different dielectric constant than its surroundings can be sensed by its effect on the E-Field. Almost anything conductive can be part of an electric field sensor by being one side of a capacitor. Using multiple electrodes, one can determine the size and shape of an object. An electric field is suitable for detecting objects, fixed or in motion within the field. A basic block diagram of the sensor mechanism is shown in Figure 1. The effects of an external capacitive load reduces the AC signal and this results in a change of the detected signal. If the object that comes in contact with the E-Field is conductive (forms the other side of the capacitor plate), then current flows from the electrode through the external capacitor to a “virtual ground”, which is any point in space that has a different electric charge from the electrode. The amount of change is directly proportional to the “E-Field Capacitance”. The MC33794 E-Field sensor IC contains the necessary components to provide up to 11 E-Field sensor channels. An external microprocessor is used to select the desired channel and read the DC level using an analog-to-digital converter. The system measures the change in the amplitude of the AC signal in response to the external capacitive load applied to the field. This is quite different from other types of traditional capacitive sensing methods. Typically, these systems employ a loosely-coupled oscillator to the external electrode and rely on the electrode capacitance to change the oscillator frequency. The change in frequency is proportional to the external capacitance applied.- 2 - dAkC0ε=CkdEquation (1) Figure 2. Capacitor Model CAPACITOR BASICS Before getting to the applications portion of this paper, it is a good idea to cover the basic physics of a capacitor. With this knowledge, the designer will have a better feel for how to use E-Fields as a sensor. E-Field detection works on a simple concept: the object you are trying to detect forms a capacitor to some virtual ground. The diagram to the right (Figure 2) is a model of a parallel plate capacitor. The total capacitance, C is calculated as: where, A = area of the two plates d = distance between the plates k = dielectric constant of the material between the plates ε0 = permittivity of free space The dielectric constant, k, becomes important when we want to detect the presence of water or ice. The variable, d, becomes important if we wish to measure distances between objects, such as refrigerator doors, or detecting wobble in a spinning object. The area, A, improves our sensitivity—the larger the electrode is, the bigger the capacitance, and changes in capacitance are easier to detect. Note that the only dependence on temperature is with the dielectric material chosen for the application. Detector Low Pass Filter Voltage Level Proportional to 1/C (voltage divider) Drive level ~ 5 v p-p Load Resistor (22 K ohms) Sine Generator (120 KHz) Detected Signal Level Decreases with Increasing Capacitance Electrode (1 of 11) E-Field Capacitance between other electrodes and a virtual ground. Current proportional to C. Figure 1. Basic E-Field sensor block diagram.- 3 - Electrode Air, water, or ice dielectric. Ground Figure 3. Example Electrode Configuration Here’s a good example of exploiting the changes of the dielectric constant as a function of temperature. Consider the application of detecting the difference between water and ice. Place an electrode in a container as shown below in Figure 3. One side is connected to a virtual ground. The distance between them is fixed. This forms a basic capacitor. If the container is filled with air, (k=1) then we will measure a value CAIR. Now let’s fill the container with pure water, which has a dielectric constant of around 80. The result is a different capacitance of CWATER which is about 80 times larger according to Equation 1. The increased capacitance lowers the detected voltage. When we freeze the water, the dielectric again changes. As water becomes a crystal, the molecules have more space between them and the dielectric again changes, this time to around k= 5, and forms a capacitor CICE. (The dielectric constant for snow or frost is about 3.2.) In this case, we expect the voltage to rise back up, near the CAIR value. Note that all these values are different and are all directly proportional to the presence or state of the water. Referring back to Figure 1, as you alter the “E-Field Capacitance”, you change the voltage reading. Hence, here is a reliable way to determine if a container is filled with water, ice, frost, or air. HARDWARE SETUP Photo 1 shows the KIT33794DWBEVM evaluation module (EVM). Figure 4 is the block diagram of the EVM. This small MC33794 68HC908QY4 A/D InLevel4 /9 Electrodes / MAX232 Vcc (+5V)DB9 10-18 Volts In Figure 4.