With all the recent excitement about capacitive sensing in portable media players, laptop PCs and mobile handsets, it is easy to forget that such interface technologies have been actively designed into major appliances applications for years. Significant improvements in sensing algorithms and control circuitry have expanded the suite of applications in which the technology can be implemented. Designers are seeing the value of capacitive sensing as a replacement for mechanical buttons and membrane switches, as well as discovering new, exciting applications such as touchscreens and proximity sensors.

A capacitive sensor is constructed of a conductive pad, the surrounding ground, and its connection to a controller. In most applications, the conductive pad is a large copper footprint and the surrounding ground is a poured fill. A native (parasitic) capacitance, CP, exists between these two objects. When a third conductive object, such as a human finger, is brought into proximity with the sensor, the capacitance of the system is increased by the capacitance of that object, CF. (See Fig. 1.)

There are several methods for detecting the increase in capacitance caused by the addition of CF.
Field Effect measurement uses a AC voltage divider between a sensor capacitor and a local reference capacitor. Finger detection is achieved by monitoring the change in voltage on this divider. Field Effect sensing is a highly sensitive technique and is robust with regard to environmental conditions. However, it is implemented with a single ASIC per sensor and does not provide analog function. 

Fig. 1. Illustration of capacitive sensing system.

Enlarge this picture Fig. 2. Schematic of capacitance sensing system.

Charge Transfer uses a switched capacitor circuit and a reference bus capacitance with repeated charge transfer steps from the smaller sensor capacitor to the larger bus capacitor. The voltage on the bus capacitor is proportional to the sensor capacitance. The capacitance can be determined by measuring the voltage after a fixed number of steps, or by counting the number of steps necessary to reach a threshold voltage. Charge transfer is a low-impedance sensing method, giving it good noise immunity, and is capable or supporting analog features in capacitive sensors. The direct connection to VDD requires a high-quality, dedicated voltage regulator for the sensor controller. 

A Relaxation Oscillator is a charge-time measurement where the charging ramp is determined by the current source (usually fixed) and the sensor capacitance value. Larger sensor capacitors yield longer ramp times, usually measured with a PWM and a timer. The relaxation oscillator is highly flexible and can be implemented in many standard microcontrollers. However, the high-impedance inputs can make it susceptible to noise sources without firmware or hardware modifications to filter out such interference.

Successive Approximation is a capacitance charging time measurement where start voltage is determined by successive approximation. The successive approximation method (patents applied for by Cypress Semiconductor), implemented with the PSoC device, uses a capacitance to voltage converter and single slope ADC. The capacitance measurement is achieved by converting the capacitance to a voltage, storing this voltage on a capacitor, and then measuring the stored voltage using an adjustable current source. This low-impedance technique has high immunity to interference and greater sensitivity and analog characteristics. The current source and the connection scheme allow for more tolerance in voltage regulator quality. (See Fig. 2.)

The capacitance-to-voltage converter is implemented with switched capacitor technology. The circuitry brings the sensor capacitor to a voltage relative to the capacitance of the sensor. The switched capacitor is clocked by the PSoC’s internal main oscillator.

The sensor capacitor is connected to the analog mux bus and is charged via a programmable current output digital-to-analog converter (iDAC) also connected to the bus. The charge on each bus is given by q=CV.  SW2 is opened and SW1 closed to bring the potential across CX to zero and reducing the charge on the bus by a value proportional to capacitance of the sensor capacitor. This (charge-discharge) is repeated so that the sensor capacitor is a current load on the bus.

With the switched capacitor circuit running, the iDAC uses a binary search to determine the value at which the voltage on the bus remains constant. This voltage is a factor of the switching frequency, the sensor capacitance and the iDAC value (current). The bus also functions as a bypass capacitor, stabilizing the resulting voltage. Additional capacitors can be added to the bus and affect performance and timing of the circuit. (See equation.)

The calculated iDAC value is then used to charge the bus again and the time required to take the bus from an initial voltage to the comparator threshold is measured. The initial voltage with no finger present and therefore the charge time is known. A finger on the sensor increases the value of CX, decreasing the initial voltage and increasing the charge time measurement. (See Fig. 3.)

Cypress Equation

Building a sensor

Fig. 3. A finger on the sensor increases charge time measurement.

Capacitive sensors have diverse forms and functions. They can use a variety of media, and their implementation ranges from simple to complex. Application requirements determine sensor construction and implementation details. 

Buttons and sliders are the most common. Buttons are large conductive pads connected to the controller. Capacitance is measured and compared against a series of thresholds.  Decisions can be made as digital outputs or with more analog characteristics for activation pressure or finger size. Sliders are linear or radial arrays of conductive pads.  Center of mass algorithms determine the position of activation to a resolution far greater than the number of pins used to sense. Most often, simple capacitive sensors such as buttons and sliders are deposited onto a printed circuit board using copper. Other substrates and deposition media such as silver ink can also be used. (See Fig. 4.)

Fig. 4. Center of mass algorithms determine the position of activation.

Dynamic user interfaces utilize buttons or activation regions that reconfigure in response to the display itself. These displays are moving the user experience forward by promoting more seamless and intuitive interaction. The construction of these systems is somewhat more complex that simple buttons or sliders. Projected-capacitance touchscreens use transparent conductive materials over a display. The conductive surface is deposited onto a substrate such as glass or PET film and connected to the control circuitry. The substrate is then adhered to the overlay between the overlay and the display.

The position of the activation is determined in the same way as a slider. Two sliders, one for each axis, are intertwined to provide complete coverage of the display area.  Activation is detected on both axes, and the position exported as x-data and y-data.   Because a projected-capacitance touchscreen is behind an overlay, it is protected from impact, flexion, and environmental factors that plague traditional resistive touchscreens.

Proximity sensors are essentially large buttons. The purpose of a proximity sensor is not to detect the exact position of a conductive object, but rather the presence. Since the device does not need to know exact position, the response time may be slower (3 ms to 4 ms vs. 250 us). The sensitivity of a proximity sensor is much greater; up to 30 cm can be achieved in a well constructed design. Since proximity sensors do not need to be associated with any display graphic, their placement on the device is more flexible. A copper ring around the outside of the control circuit board, or a wire behind the overlay, allow very basic, cost-effective construction of a proximity sensor. (See Fig. 5.)

Using the sensor

Fig. 5. Illustration of proximity sensor arrangements.

Usage of capacitive sensors is expanding. The described sensors have created new opportunities for designers to work with flexible, durable and elegant design elements.  Buttons are still used for basic menu navigation and activation. However, analog characteristics of buttons that are not expensive potentiometers is allowing easier and less expensive implementation of increased functionality and safety features.

One example is the LG LA-N131DR air cleaner developed by LG Electronics. The unit uses five capacitive sensors for front panel display menu navigation buttons. (See photos.) These buttons have allowed the designers to implement a seamless chassis design while still utilizing a feature-rich user interface. The capacitive buttons detect the presence of a human finger through 4 mm of glass. The control circuitry is located on the non-sensor side of a two-layer printed circuit board. LG uses the PSoC Mixed-Signal Array to control the sensors and output status to the main device processor.

Proximity sensors allow for reactive backlighting for nighttime operation. They also permit reactive safety features that require a larger activating element, such as an adult hand or metal pot to engage the range-top controls. Proximity sensors, buttons, sliders, and even touchscreens, can be controlled by a single processor when using PSoC. Firmware routines allow changes-in-state based on user inputs or host commands.

The PSoC Mixed Signal Array is a configurable array of digital and analog resources, flash memory and RAM, an 8-bit microcontroller, and several other features. These features allow PSoC to implement innovative capacitive sensing techniques in the CapSense portfolio. Designers can use the PSoC’s intuitive development environment to configure and reconfigure the device to meet design specifications and specification changes. In addition, new sensing technologies exhibit improved sensitivity and noise immunity, reduced power consumption, and increased update rate. 

For more information email: capsense@cypress.com