Ever since Apple launched the iPhone in 2007, touchscreens have invariably become the choice feature for modern handheld design. Touchscreens optimize handheld applications by leveraging the entire display area for both input and output—the display is the interface. With innovative applications and intuitive user interfaces driving differentiation in handsets through the touchscreen, the stakes are high to implement a successful touch experience.
As touchscreen technology proliferates among handheld devices, software designers and product engineers face new sets of challenges as well as opportunities. Developing devices requires design teams to fully comprehend the complex ecosystem of touch, which involves multiple system, mechanical, electrical, and user experience factors.
To meet evolving user requirements and the need to differentiate through unique industrial design and user interfaces, OEMs are migrating to clear-sensing technology. Until about 2006, the resistive-touch panel (Fig. 1) was the only option for handheld devices. Resistive, which requires the force of a finger to activate controls on the screen, was an attractive solution for designers —relatively simple electronics, few systems design issues, an established supply chain, and a good price point.
Though simple to design-in, resistive solutions are not ideal from an end-user perspective. Poor display optics, poor durability, constrained industrial design options, and insufficient gesture performance have limited the widespread popularity of resistive touchscreen devices.
THE MOVE TO CAPACITIVE TOUCH
To address these challenges, many new high-profile mobile phones employ projected capacitive-touch technology. Capacitive touch was originally developed as an extension of the same sensing technology used in PC laptop computer TouchPads. Now, however, it is gaining wide adoption thanks to superior optics, accuracy, gestures, responsiveness, and multi-finger capability.
For sensor implementation, capacitive and resistive touchscreens both use a clear conductor, indium tin oxide (ITO), deposited onto polyester film and/or glass. Beyond the common materials, a capacitive touchscreen differs radically from resistive.
Foremost, a capacitive touchscreen (Fig. 2) is completely solid state. A user’s finger position is computed by detecting changes in an electric field within the touchscreen sensing region. The sensors project the field lines and touchscreen-electrode-array capacitance changes measure the changes in the field, hence the name “projected capacitive sensor.”
The solid-state nature of a capacitive sensor holds a number of key advantages over resistive technology. First, solid state means no moving parts; thus, capacitive sensors do not wear out over time. Furthermore, field lines can be projected through non-conductive materials. As a result, a capacitive sensor can be mounted underneath a lens or device casing. Under-lens mounting greatly improves durability as well as increases industrial design flexibility.
Cosmetically, a capacitive sensor offers superior optics because its construction does not require the sensor to move, nor is there an air gap. Both conditions can severely distort the image quality of the underlying display.
Functionally, a capacitive sensor detects a user’s finger without pressure, essentially eliminating the stiction required to move the finger. This dramatically improves the usability of capacitive touchscreens for gestural input involving finger movement on the touchscreen surface (e.g., flicking for scrolling, drag-and-drop).
Finally, projected capacitive touchscreens can detect multiple locations that may contain disturbed field lines. As such, these touchscreens are able to report multiple finger touches.
While it is indisputable that capacitive touchscreens enhance the end-user experience, the designer experience is often fraught with the unfamiliarity of certain underlying design-in challenges. Most significant among them is the inherent difficulty in detecting a user’s finger(s) through capacitance, as well as the active and analog nature of the projected capacitive-sensing implementation.
These challenges occur regardless of the specific variations in projected capacitive-sensing schemes offered by various suppliers. Accordingly, a basic understanding of capacitance-based finger sensing and a discussion of some key design decisions can help smooth the transition from resistive to capacitive sensing.
Magnitude of finger “signal”: Before digging into the design issues that impact capacitive-sensing performance, it is worth considering the scope of the finger-detection problem in an ideal situation. In terms of input, a user's finger typically contributes less than 1 pF to system capacitance. Depending on the system’s design, the background or baseline capacitance of each sensor electrode can vary from 5 pF to over 30 pF.
This suggests that designers should attempt to minimize the overall electrode capacitance to ensure an adequate "signal" upon a user's touch. However, a number of other factors influence the overall electrode capacitance, so minimization is rarely possible.
The implication is that even in an ideal situation, the finger "signal" can be quite small in comparison to the electrode capacitances. The small finger signal is further complicated by the typical need to minimize power consumption in the capacitive-sensing system. Raising power consumption could improve the situation, but typical capacitive touchscreens are constrained to operate at less than 3 mW.
While all capacitive-touchscreen sensor controllers are designed with this in mind, the user's finger generally contributes little to on the overall capacitance measurement. Therefore, designers must pay extremely close attention to the overall device design to optimize capacitive-touchscreen performance.
Finger detection: Another systems-level issue is the seemingly inconsequential matter of finger detection. That is, the instance when the sensor actually reports a finger is in contact with the “touch surface.” Note that the touch surface is not necessarily the sensor itself, since capacitive sensors can be mounted under a lens or device casing. Unlike a resistive sensor, or even a mechanical button, capacitive-sensing systems do not have a simple mechanism to determine contact.
Because a user’s finger will already alter the electric field lines before it comes into contact with the touch surface, reliable algorithms to differentiate between a finger “hovering” and a finger “touching” become especially critical. Unless “hover” is employed as a specific feature, it must typically be suppressed so that the device does not initiate any host actions prior to the finger touching.
In addition, finger detection is further confounded by the inherent variations in finger capacitance values. These not only vary from user to user, but also can vary for a given user depending on which finger is used or how much finger pressure is applied. Put another way, the touch system needs to reliably detect a small finger touching the surface, but reject a large palm hovering. Though such finger-detection algorithms are found in proprietary algorithms contained in the capacitive-touchscreen supplier’s IC, overall device design can seriously affect their performance.
Mechanical design affects systems design: Another issue is that a capacitive touchscreen actively senses the user’s finger with a sensitive analog electric circuit, which is a similar problem encountered in antenna design, albeit at a relatively low broadcast/sensing frequency. Therefore, the touchscreen’s mechanical design is a critical factor in the system’s overall performance.
This places great importance on freezing the mechanical design at an early phase of device development, because any late changes in the device mechanicals can (and will) profoundly alter performance. For device users, less-reliable finger sensing could take the form of false positives and/or false negatives for finger detection, reduced accuracy of finger-location reporting, or other unpredictable behavior.
Stated another way, changing the system’s mechanical design after a capacitive sensor is crafted and tuned essentially requires the a redesign of the capacitive sensor, regardless of whether the mechanical design directly changes the capacitive sensor’s mechanics. Accordingly, the capacitive sensor design also must be considered and finalized early in the design process.
Mechanical design considerations: Current capacitive-touchscreen systems come on either a laminated multi-layer polyester (PET) substrate or glass substrate (Fig. 2, again). PET-based sensors historically offer a slight cost advantage compared to glass-based sensors. In addition, PET sensors allow for more options in terms of cover lens material (can be laminated to either plastic-type acrylic or polycarbonate as well as glass) and it is possible to make them thinner. On the other hand, glass sensors offer improved optics, slightly larger viewing/active areas (per given sensor size), and better tolerance to higher operating temperatures.
Although the two sensor types offer comparable performance, the choice is extremely critical in determining the overall design and manufacturing path. Therefore, the decision must be made early in the design phase.
One direct consequence of substrate selection on mechanical design is the cover-lens design. PET sensors are usually laminated to plastic lenses (glass sensors can only be laminated to glass lenses). Although a projected capacitive touchscreen is designed to be laminated under a lens and sense a finger through this lens, the lens’ thickness impacts the sensing performance. Typically, thickness can be no more than about 1 mm.
Furthermore, no air gaps can exist anywhere between the sensor and the surface touched by the user. Ideally, lens thickness is uniform throughout the touch surface to minimize variations in sensing thickness.
Another critical mechanical design consideration is the routing and placement of conductive material near the touchscreen. Ideally, any conductive material is located at least 1 mm away from the touch sensor, including the trace routing at the border. This material should be stationary since moving conductive material essentially acts like a finger input. Also, conductive traces with noisy digital signals near the capacitive sensor should be shielded to minimize the disturbance of the capacitive sensor’s analog electrodes.
Finally, conductive material should be placed with regard to the capacitive sensor’s ability to withstand electrostatic discharge (ESD) events. While a capacitive sensor is typically protected by a cover lens and surrounded by device casing, such sensors can still be susceptible to ESD events through small gaps in the device case (e.g., between the main casing and the lens). It is recommended that the capacitive sensor be surrounded with multiple low-resistance paths to the device ground to deflect excess charge introduced by ESD events.
Display considerations: The display, which is found in most handheld devices, merits specific discussion. A capacitive touchscreen is typically located directly above a display, thus presenting proximity and EMI emission challenges.
From a purely cosmetic/optical perspective, the optimal design would laminate a capacitive sensor directly to the display surface. It reduces parallax as well as minimizes the air interfaces that introduce optical distortion. From a practical standpoint, though, this is infrequently done due to manufacturing issues. Recall that the capacitive sensor must be mounted to a lens, so the lamination of such a lens-sensor-display system would be a challenge mechanically, and re-working such a system would prove costly. Finally, laminating the capacitive sensor to the display introduces significant noise issues, due to their direct coupling.
Fortunately, certain display types offer much less EMI emission in the range of frequencies that affect a capacitive sensor. For liquid-crystal displays, dot-inversion or dc VCOM style drive schemes offer the best performance with capacitive sensors. Active-matrix organic LED (AMOLED) displays also offer significantly lower EMI.
Nonetheless, the most popular display continues to be line-inversion drive LCDs. With such displays, it is critical for designers to work closely with capacitive-sensing suppliers to characterize the EMI performance, since the selection of the LCD can affect certain mechanical design parameters, such as the minimum allowable distance between the sensor and display surface.
Solid capacitive sensing turns into good user experience: As mentioned, designing a capacitive touchscreen presents more challenges when compared its resistive counterpart. Not paying close attention to the underlying details of the capacitive touchscreen design can greatly diminish overall usability and user experience.
Still, even if properly functioning, a capacitive touchscreen offers little user feedback. Often, the user does not know if a finger input has activated the device until the device responds either though the display graphics or some alternative feedback mechanism. Accordingly, beyond optimizing the capacitive touchscreen design, overall design of the user interface in terms of visual feedback from the display graphics as well as other mechanisms, like haptics, play a significant role in the overall user experience.
To that end, the mobile handset market now offers numerous devices with capacitive touchscreens that offer excellent illustrations of good user interface design. Of course, it is likely these devices provided the motivation for considering capacitive touchscreens in your next design.
Beyond the usability and design benefits of capacitive sensing, the decreasing costs of capacitive touchscreens now make them competitive with premium resistive solutions. Furthermore, solutions are now available from a number of providers.
As this article highlights, the success of any capacitive touchscreen design depends heavily on a deep understanding of the mechanical and systems requirements of both the capacitive touchscreen and the overall device. Suppliers that are most in tune with their capacitive-sensing technology, and have the most experience partnering with their customers, will provide the best guidance in helping designers navigate through the development process of their capacitive touchscreen-enabled device.