Touchscreens, large and small

The new paradigm where fingers are the primary tools for interacting with computers

BY BINAY BAJAJ
Director of Touch Marketing
Atmel
www.Atmel.com

Smartphones and tablet computers are omnipresent in today’s society used by both businesses and consumers. First introduced on smaller devices several years ago, touchscreen-based devices are rapidly moving to larger formats as well. For manufacturers of computing and electronics devices, this new market is more than just the latest consumer craze. It may well represent a fundamental shift in the way people interact with information and computing hardware. The growth has unleashed a flurry of activity among device manufacturers, who are actively porting touchscreen technologies to large-format hardware.

However, moving from small screens and simple touch-enabled applications to a new paradigm where hands and fingers are the primary tools for interacting with computers is not going to be straightforward. Manufacturers must rethink the way touchscreens will be used and address new and more demanding requirements. Most important, the move to larger screen sizes has made a multiple touch capability essential. While a few finger strokes on today’s 5-in. screens are sufficient, the minimum requirement for 12- and 40-in. devices is at least ten. These devices often have multiple users on one device.

Resistive controls

Resistive controls have a flexible top layer, an insulating spacer, and a lower substrate. Graphics are applied to the top layer upper surface, and a conductive pattern of silver or carbon conductive ink is printed onto the lower surface. A matching conductive pattern is printed onto the lower substrate. The conductive layers are pressed together through holes in the spacer to create a contact. To create tactile feedback, metal or plastic domes placed beneath the overlay and embossing on the top layer can be used to guide users’ fingers to the “sweet spot” of each switch. However, membrane switches have a number of disadvantages. First, they are not true touch switches. A force and physical travel are needed to make a contact.

Resistive touchscreen technology

In the same manner, a resistive touchscreen comprises several layers, the most important of which are two thin, electrically conductive layers separated by a narrow gap. Pressing down on a point on the panel’s outer surface causes the two metallic layers to contact. This changes the current through a voltage divider and resulting change in voltage is sent to the controller for processing.

Resistive touchscreens have been favored since they are lower cost and had excellent stylus capability that found many supporters, particularly for Asian character-based applications. However, resistive technology could not support multi-touch. Also, the multiple layers required impaired optical qualities. The display had poor visibility in sunlight due to reflections and display brightness was attenuated. Requiring an outer flexible layer that came in contact with the stylus also meant it was prone to scratching and ingress of moisture and dust.

Projected capacitance technology

Projected capacitive field-based technology has quickly won support from users because it can have a solid glossy outer surface that looks great and is completely sealed from dust and moisture. It works by measuring small changes in capacitance when an object (such as a finger) approaches or touches the screen surface. However, all capacitive touchscreens are not created equal. Choices in the capacitive-to-digital conversion (CDC) technique and the spatial arrangement of the electrodes that collect the charge determine the overall performance.

Two basic options for arranging and measuring capacitance changes on a touchscreen are self-capacitance and mutual-capacitance. Most early capacitive touchscreens relied on self-capacitance, which looks at an entire row or column of electrodes for capacitive change. This approach is fine for one-touch or simple two-touch interactions, but has serious limitations for more advanced applications. The system detects touches at two x and two y coordinates, but has no way to know which x goes with which y. This leads to “ghost” positions. Alternatively, mutual-capacitance touchscreens use transmit and receive electrodes arranged as an orthogonal matrix, letting them measure where a row and column of electrodes intersect and each touch has a specific pair of x,y coordinates (see Fig. 1 ).

Fig. 1: Self-capacitance versus mutual-capacitance touch methods.

The underlying capacitive-to-digital conversion technique also affects performance. Usually the receive lines are held at zero potential during the charge acquisition process, and only the charge between the specific transmitter x and receiver y electrodes touched by the user is transferred. This has the key advantage of high immunity to noise and parasitic effects. This immunity allows for addition system design flexibility; for example the sensor IC can be placed either immediately adjacent to the sensor, or further away on the main board.

Capacitive sensor design

The density of x,y nodes on the touchscreen, or the electrode pitch, largely determines the touchscreen resolution and accuracy. Naturally, different applications have different resolution requirements. But today’s multi-touch applications, which need to interpret fine-scale touch movements such as stretching and pinching fingertips, require high resolutions to uniquely identify several adjacent touches.

Typically, touchscreens need a row and column electrode pitch of approximately 5 mm or less. This allows it to properly track fingertip movements, support stylus input, and with proper firmware, reject unintended touches. When the electrode pitch is between 3 to 5 mm, the touchscreen becomes capable of supporting input with a stylus with a finer tip to support a broader range of applications.

At the core of any successful touch sensor system is the underlying driver IC and software technology. The chip should have high integration, small footprint, and near zero power consumption, plus the flexibility to support a broad range of sensor designs.

Delivering true multitouch

Users of the Apple iPhone and other contemporary devices will be familiar with today’s multi-touch gestures, typically pinching or stretching of two fingers. With a larger screen it becomes possible to envision much more complex multi-touch gestures. Imagine painting and music applications for young students, for example, that involve gesturing with all 10 fingers and thumbs. New tablet-based games pit two or more users against each other on the same screen. However as large-format touch computing evolves, application developers will want the flexibility to take full advantage of new kinds of touchscreen interactions. Device manufacturers don’t want to stand in their way and certainly don’t want to build a device that can’t support the next hot application.

As large-format touch applications begin using 4, 5, and 10 touches, it’s important to consider how the controller chip will use this information to create a better user experience. For example, the ability to track incidental touches around the edge of a screen, and classify them as “suppressed,” is even more important on a large-format device than on a small one.

Just as a mobile phone’s touchscreen needs to be able to recognize when a user is holding the phone or resting the screen against her cheek, so a larger-format system must account for the different ways that users will hold and use the device for example, resting a hand on the edge of the screen when using a stylus, or resting both palms when using a virtual keyboard. And it’s not enough to simply identify and suppress incidental touches; the device must track them so that they remain suppressed even if they stray into the active region. The more touches that a controller can unambiguously resolve, classify, and track at once, the more intuitive and accurate the user experience can be.

Factors to consider

When designing a touchscreen application, engineers need to consider a number of factors. The first is usually the required accuracy, the fidelity with which the touchscreen reports the user’s finger or stylus location. An accurate touchscreen should report touch position better than ±1 mm.

Hand in hand with accuracy is linearity, which measures how “straight” a line drawn across the screen is. Linearity depends on sound screen pattern design, and should also be accurate within ±1 mm. Our fingertips can only be brought together so far before they may be interpreted as a single touch, so finger separation is key.

The resolution of the screen also needs attention the smallest detectable amount of finger or stylus motion. It is important to reduce the resolution to the fraction of a mm level for a number of reasons, chief among them being the enabling of the stylus based handwriting and drawing.

One of the most important user evaluations of a touchscreen-based device is the response time. For basic touch gestures such as tapping, the device should register the input and provide feedback to the user in less than 100 ms. Factoring in various system latencies, that typically means that touchscreens need to report a first-qualified touch position in less than 15 ms. And, applications such as handwriting recognition require even faster response.

Signal-to-noise ratio also impacts the user experience of the screen. This refers to the touchscreen’s ability to discriminate between the capacitive signal arising from real touches and that arising from noise. Capacitive touchscreen controllers measure very small changes in the row-to-column coupling capacitance; how those measurements are performed has a strong influence on the controller’s susceptibility to noise.

Large-format touchscreens are especially challenging in this regard, as one of the most significant noise generators is the LCD itself. As touchscreens get larger and support more simultaneous touches—and more complex interactive content—achieving top performance in all of these categories becomes even more important. ■