When Michael Faraday introduced the concept of an electric field, little did he realize how far science would run with the idea. Today, engineers are using electric fields to sense the presence of other objects without relying on physical contact. Referred to as e-field sensors or capacitance sensors, they are becoming more and more prevalent in a wide range of inexpensive and long-lasting applications. When you take a closer look at how they work, you quickly see why their popularity is growing.

Capacitance Sensors in Theory

Capacitance sensors exhibit a change in capacitance in response to a change in physical stimulus. Certain capacitance sensors measure the change by generating an electric field and measuring the attenuations suffered by this field. Unlike inductive sensors, which can detect only metallic objects, a capacitance sensor can detect anything that is either conductive or has dielectric properties different from the sensor's electrodes' surroundings. Coincidentally, this makes human beings very good candidates for e-field imaging, because, being mostly water, we have a high dielectric constant and we contain ionic matter, which makes us good electric conductors.

Oscillator circuitry in the sensor integrated circuit (IC), such as the Freescale MC34940 e-field imaging device, generates a high-purity, low-frequency 5 V sine wave, tunable by an external 39 kΩ resistor. This AC signal is fed to a multiplexer that directs the signal to a selected electrode (the MC34940 supports seven) or reference pin, or to an internal measurement node. The IC automatically connects the unselected nodes to the circuit ground, and these act as the return path needed to create the electric field current.

Figure 1. The Freescale MC34940 e-field imager design

When an object is brought close to a metal electrode—for instance, a finger from our highly dielectric and conductive human subject—an electric path is formed, producing a change in the electric field current. The sensor measures the AC impedance of the generated e-field and translates that measurement into a DC output voltage (Figure 1). An external microcontroller can then process this information to perform any number of functions, such as those that are associated with a touch pad control panel. Capacitive touch sensing is favored among designers due to increased reliability (no moving parts), greater design freedom, and a more contemporary look.

This method of measuring the AC impedance provides a more accurate reading compared to other methods, such as measuring the period or frequency of an RC oscillator. Also, by generating a pure sine wave rather than a square wave, signal interference is much less of a problem.

Capacitance Sensors at Work

Capacitance sensors can be found in a wide variety of industrial and consumer products: PC peripherals, health care patient monitors, refrigeration frost sensors, point-of-sale terminals, and garage door safety sensors.

Some of the most popular and easy-to-spot applications are touch screen and touch panel solutions. When developing a touch panel solution, keep three important considerations in mind:

• The touch pad electrode design and layout
• The different dielectric materials for the surface of the panel
• The effect on e-field measurements of various environmental conditions