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issue: August 2004 APPLIANCE Magazine

Engineering Control Panels
Designing a Non-Contact Appliance Control Panel

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by Ron DeLong, senior systems and applications engineer, Freescale Semiconductor, and Catherine Booth, digital design engineer, Networking & Communications Systems Group, Freescale Semiconductor, Engineering Rotational Program (ERP)

The design of the human-machine interface (HMI) can strongly impact the perception of an appliance.

The creation of control panels that support an increasing array of features and layouts has become a key issue for appliance makers, which requires interfacing new, more complex systems with the consumer interface. Up until now, choices have typically been confined to mechanical switches or electronic keyboard-type products connected to low-end microcontrollers.

However, recently developed electric field (E-field) imaging circuit technology simplifies the design of low-contact force touchpad sensors and offers several advantages for appliances. The E-Field sensor can allow immediate reconfiguration of the user interface in support of new models, implementation of flat panels for cleanliness, and elimination of all moving and mechanical products in the user interface. The use of an E-Field sensing IC provides a switch design that avoids wear-out, contact bounce, corrosion, and arcing as well as providing a high-tech image to an appliance.

Background on E-Field Sensing

An E-Field imaging circuit offers designers an alternative to mechanical pushbuttons for control panel applications. The integrated circuit (IC) contains the circuitry necessary to generate a low-level E-Field and measure the field loading caused by objects moving into or out of the field. The IC can be used to implement non-contact sensing, proximity detection, and three-dimensional E-Field imaging, and it integrates support for a microcontroller and up to nine electrodes. The electrodes can be used independently to determine the size or location of an object in a weak electric field.

The E-Field IC also contains circuitry to support two references, a shield driver to reduce cable capacitance circuit, and a +5.0-V regulator to power external circuitry [1]. As shown in the application circuit (see Figure 1), additional capability provided in the design includes the following: ISO 9141 physical layer interface (ISO TX and RX), lamp driver output, Watchdog (WD IN), Power-On Reset timer, and high-purity sine wave generator tunable with external resistor.

With the E-Field IC, membrane switches and resistive touchpads may be replaced with an array of touchpads consisting of conductive electrodes embedded beneath an insulating surface. Since the circuitry is able to detect touch and proximity through the insulating surface without direct electrical contact to the electrode metal, problems of wear, contamination, and corrosion are eliminated. This capability is also important for sophisticated touch-control applications, including a user-interface panel that is sensitive to different degrees of proximity—enabling the system to go from standby to active mode as a finger approaches the panel (see Figure 1).

Figure 1: Basic E-Field application circuit showing the E-Field IC, MCU interface, and connection to electrodes.

CLICK to see larger graphic.

Creating the Electric Field

An electric field is created by the oscillator circuitry within the E-Field IC that generates a high-purity, low-frequency, 5-V, peak-to-peak sine wave [2]. This frequency is tunable by an external resistor and is optimized for 120 kHz. The a.c. signal is fed through an internal 22-k resistor to a multiplexer that directs the signal to the selected electrode or reference pin or to an internal measurement node. Unselected electrodes are automatically connected to the circuit ground by the IC.

The “unused” electrodes can act as the return path to create the electric field current, since the current must follow a complete path out of the electrode pin and back to the common ground of the IC GND pin. An E-Field will cause a current to flow between the active electrode and any object with an electrical path to the IC ground, including unselected electrodes.

The current flowing between the electrode and its surrounding grounds produces a voltage drop across the internal resistance and results in a voltage change at the pin. An on-board detector in the IC converts the a.c. signal to a d.c. level. The d.c. voltage is multiplied, offset, and sent to the level pin of the IC.

To simplify the evaluation of the capability of the E-Field IC, a reference design in the form of an evaluation board has been developed. As shown in Figure 2, the board includes a printed wiring board and an E-Field IC, a pre-programmed 8-bit microcontroller (MCU), a communication IC, and other passive components on a printed circuit board. The 8-bit MCU converts the analog signal from the E-Field IC into an 8-bit byte. The MCU then transmits this value to a computer via its COM port. A Windows®-type software program is included in the kit that allows the measured A-to-D value to be displayed in real-time along with its corresponding analog bar graph. The program allows the scanning of all electrodes, references, and internal nodes of the E-Field IC or any combination of them (see Figure 2).

Figure 2: The E-Field IC evaluation board requires only an external power source to connect up to nine electrodes, whose operation can be observed though the communication (COM) port. CLICK to see larger graphic.

Designing a Touch-Control Panel

The E-Field IC is able to detect anything that is either conductive or has different dielectric properties than the electrodes’ surroundings. Human beings are well-suited for E-Field imaging because the human body is composed mainly of water that has a high dielectric constant and contains ionic matter, which gives it good conductivity. The body also provides good electrical coupling to earth ground that can be connected to the ground return of the IC. When a finger is brought close to a metal electrode, an electrical path is formed, producing a change in electric field current that is detected by the E-Field IC and translated to a different output voltage.

The size of the electrically conductive electrodes must be taken into account for any design. The larger the electrode, the more range and sensitivity will be obtained. As the electrode size is increased, its susceptibility to interference, electrical noise, and “stray” electric-field paths in its surroundings also increases. However, the area of the touchpad only needs to accommodate the contact area of the finger. This limits its useable size. Therefore, the distance or spacing factor will play a significant role in how the electrode should be laid out.

The E-Field IC works best when the total capacitance between an electrode and ground or another electrode is approximately 50 picofarad (pF) when the finger is in the “activate” range. The total system capacitance should be below 100 pF and preferably below 75 pF for best sensitivity. This includes the IC pin, PWB trace, wire, and any other stray capacitance. Large electrodes should be used when distances are great, and small electrodes when distances are small.

The placement of ground is important. As mentioned earlier, electric field currents can exist between the active electrode and any grounded object. By intertwining the electrode with ground, the essential ground source needed to create an E-Field is directly accessible to the electrode. This path is less variable than the path through a body and earth and provides a more predictable and less noisy path.

Figure 3: Intertwined electrode and ground designs and dimensions.

CLICK to see larger graphic.

Evaluating Electrode Designs

To investigate how variables in ground can impact the E-Field measurement, the ground effect was tested with a two-electrode design, a spiral, and an interdigitated layout (See Figure 3). In addition, the width of the ground electrode intertwined with a signal electrode was varied to determine how much the ground area affects the readings. One electrode ground test configuration was designed with the ground having the same width as the electrode, and another with the ground electrode thinner than the signal electrode. A touchpad with an area large enough to accommodate a typical finger was designed in a square shape with a length and width of 0.6 in. The dimensions and layout of the electrodes are shown in Figure 3. A 4.5-mm (0.0045-in) thick vinyl film was used as an insulator over the patterns. Subsequent testing determined that the layout with the narrower ground electrode provided a slightly greater amount of difference in comparison to the design with ground having the same width (see Figure 3).

Other Uses for E-Field Sensing in Appliances

By simplifying the design of E-Field sensing, a number of other applications may be considered. In addition to touch controls, the E-Field IC is able to provide additional functionality for appliances, especially those appliances in which the person contacting the appliance can provide additional data, such as force, or where protection is required in order to prevent a safety hazard if the person drops or lets go of the appliance.

When using the E-Field IC for other functions, adding touch control to the same object may maximize its use—if all of the electrodes are not used to obtain the other functions.

References

[1] Motorola Data Sheet MC33794, “Electric Field Imaging Device.”

[2] Motorola Application Note AN1985/D, “Touch Panel Applications Using the MC33794 E-Field IC.”

This is an edited version of the paper that was originally delivered at the 55th Annual International Appliance Technical Conference (IATC), held March 29-31, 2004, in Lexington, KY, U.S.

About the Authors

Ron DeLong is a senior systems and applications engineer for Freescale Semiconductor’s Analog Products Division of the Transportation & Standard Products Group, Semiconductor Products Sector (SPS). Mr. DeLong was involved in reducing to practice an integrated circuit (IC) to image an occupant in a vehicle and determine their size and proximity to an airbag. The IC is now in volume production and has proven to be an effective method of preventing airbag injuries to children and small adults. He is actively involved in expanding this technology into more types of applications.

Catherine Booth is digital design engineer, Networking & Communications Systems Group for Freescale Semiconductor’s Engineering Rotational Program (ERP), a program that allows young engineers to explore different disciplines in the engineering field. Through the ERP program, she was able to experience applications and designs in the fields of sensors, analog, and digital. She received her bachelor of science degree in Electrical Engineering from Arizona State University.