Achieving Top Performance in Capacitive Touchscreens with Simulation

To make a phone call, compose a text message, or even to beat the next level of an Angry Birds™ game, we rely on being able to pick up our smartphone and interact with it without a second thought. No matter the size of our fingers, whether or not we have recently applied hand cream, or if the phone is resting on a flat surface, the touchscreen responds seamlessly, bringing just one more thieving farm animal to justice.

Optimizing touchscreen design with COMSOL Multiphysics

Engineers at Cypress Semiconductor, the lead­ing supplier of smartphone touchscreen technologies and touch-sensing solutions, are hard at work mak­ing this possible, ensuring that touchscreen applications perform flawlessly under a variety of conditions. “And it’s not just about smartphones,” says Peter Vavaroutsos, a member of the touchscreen mod­eling group at Cypress. “Our technologies are used in smartphones, mp3 devices, laptops, automotive environments, industrial applications, home appliances, and more. For each of these uses, a different design is needed.”

Capacitive touchscreens are by far the most commonly used method of touch sensing in the elec­tronics industry, and consist of varying layers of transparent lenses, substrates, adhesives, and indium-tin-oxide (ITO) electrodes. Together, these elements are known as touchscreen panels (TSPs) or stack-ups. Depending on the type of product in which they will be used, each stack-up and electrode pattern is customized for its intended environment and use. A stack-up contains an LCD layer, followed by a sub­strate, a pattern of horizontally and vertically aligned dia­mond-shaped ITO electrodes, and finally an optically clear adhesive layer that bonds the glass cover onto the screen.

 At Cypress, multiphysics simulation and simula­tion apps have emerged as key tools for ensuring effective product development, allowing design­ers to predict and opti­mize the behavior of numerous designs with­out needing to build mul­tiple physical prototypes.

AN ENGINEERING RULE OF THUMB

As a rule of thumb, touch­screens must track fin­ger or stylus positions with high accuracy. This means that at any point in time, a touchscreen must not only be able to determine that it is being touched by an object of variable size, but also where, for how long, and whether the “touch object” is moving in a cer­tain direction. To achieve this, a capacitive sensor is composed of a pattern of horizontally and vertically connected ITO electrodes, where a touch object is sensed at the grid intersec­tion. When a finger or stylus touches the screen’s surface, it distorts the electrostatic field and causes a measur­able change in the coupling capacitance between the transmitting and receiv­ing electrodes.

Depending on where and how the touchscreen will be used, the stack-up components are configured in a variety of ways. “The design of a touch­ screen stack-up for the automotive industry is very different than one used in, say, a laptop,” says Vavaroutsos. “My job at Cypress is to design dif­ferent stack-ups for dif­ferent consumer products, taking into account such things as how interactions between a horizontally mounted GPS, for example, will differ from a smart­phone, which can be held and interacted with in a myriad of different ways.”

Cypress R&D engineers create multiple electrostatic simulations for a par­ticular device geometry and for many different parameters, what the team refers to as a “design box”.

“Our findings from a specific design box are then used by our sales engineers and customer support team so that they can optimize cer­tain design specifications in order to meet a customer’s individual needs,” explains Vavaroutsos.

Using the COMSOL Multiphysics® simula­tion software, R&D engi­neers at Cypress perform analyses to determine the electrical performance of the ITO pattern, includ­ing measuring the change in mutual capacitance between electrodes when a stylus or finger is pres­ent. In one example, floating poten­tial boundary conditions were used in the electro­static model, a feature that is instrumental in allow­ing Cypress engineers to simulate the boundaries of touch objects and any elec­tric shielding or electrodes that are not currently being excited. Because these objects are affected by an externally applied electric field, they will be at a con­stant but unknown elec­tric potential and therefore are represented as surfaces over which a charge can freely redistribute itself.

“Since the screen can be interacted with in so many different ways, in order to optimize a stack-up for use in a certain device or product, we have to run numerous electrostatic simulations in order to test different touch object posi­tions,” says Vavaroutsos. “We try to minimize effects such as when you get water on your screen and it doesn’t work as well, or when you put your phone down on the table and the screen responds poorly. Simulation has been a very valuable tool for ensuring that our product responds effectively over a range of different environments and conditions, since we can single out certain fac­tors and determine how to most effectively opti­mize performance. ”

Because COMSOL® software can be run on unlimited multiple cores and using cluster and cloud computing with no limit to the number of compute nodes, Cypress engineers are able to quickly run many simu­lations with virtually no limits on the size of the design boxes analyzed. “We can reduce the num­ber of assumptions we have to employ and accu­rately model capacitive touchscreens by captur­ing changes between active electrodes in great detail while working with real­istic geometry and materials,” says Vavaroutsos.

Within a single design box, Cypress engineers might test different cover lens thicknesses, alter the permittivity of various lay­ers, or change pattern parameters. Depending on the application area, a sin­gle touchscreen may be designed to have more than one electrode layer, or have different layers in a different order. For example, a design box might include a range for cover lens thick­nesses from 0.5 millimeters to 1.5 millimeters. The R&D team at Cypress will model a variety of different parameter ranges in order to precisely understand a cer­tain design, but anything outside the modeled range will remain unknown.

TOUCHSCREEN SIMULATION APPS FOR CUSTOMIZED DESIGNS

In order to extend the usability of their mod­els, Cypress engineers are using the Application Builder in COMSOL Multiphysics® to create simulation apps based on their models. “In order to com­municate more effectively with our customer support teams, we’ve started using the Application Builder to build simplified user inter­faces over our models,” says Vavaroutsos. “Before we started using simulation apps, any time a customer wanted a design that was slightly outside of the design box, we’d have to be involved again to run simulations for minor parameter changes. A lot of times, a sales engineer might try to run the simulations themselves, even though they had little expe­rience using the COMSOL® software. Not only would we have to check the simula­tions, but they also took up a seat on the software as well.”

For instance, in one app, the user can change design parameters ranging from the finger location to the thickness of the different layers in the sensor. The app then generates a report detailing the capacitance matrix, an integral piece of information for capacitive sensor design. The app can also show the elec­tric field distribution in the sensor and a drop-down list can be used to select a solution corresponding to the excitation of different sensor traces.

Cypress is also using the COMSOL Server™ license to share their simula­tion apps with colleagues around the world, which allows anyone to access simulation apps using either a Windows®-based client or a web browser. “We’re finding that letting our support teams have access to multiphysics simulation results is hugely helpful. We can control the parameters that the app user has access to so that we know the apps are delivering accurate results, while also letting our sup­port engineers experiment with thousands of differ­ent design options with­out the need to involve an R&D engineer—or use a seat up on our COMSOL Multiphysics® license.”

TEST-DRIVING AUTOMOTIVE TOUCHSCREEN DESIGNS

In addition to touchscreens for consumer products, Cypress also cre­ates touchscreen designs for use in the automotive industry. For these applica­tions, engineers experiment with different designs in response to certain automo­bile requirements.

“In the automotive group, our designs are more cus­tomer driven and are often created on a case-by-case basis for a specific product or customer,” says Nathan Thomas, an R&D engineer working in the automo­tive group at Cypress. “Our design boxes are irregu­larly shaped, and we do more simulations that are customer-specific. For example, an automotive company might use touchscreens for different appli­cations such as in the cen­ter console, in rear seat entertainment systems, or in overhead entertainment systems, all of which will need their own models.”

Instead of creating a new model for each and every instance, the automotive group is now using apps to let field engineers test new designs that would other­wise have been outside of the design box. The apps can be used to explore spe­cial requests from customers who are interested in how varying a parameter will affect end perfor­mance. “For cases such as these, we’ve been using the Application Builder to cre­ate simulation apps that our field engineers can apply directly without having to go through us to create the simulation for them. While it’s still a new technology, I can foresee simulation apps becoming the primary tool used by our field engineers.”

POWERING UP

Whether it be smartphone designs, auto­motive applications, or other industrial processes, Cypress R&D engineers can create simulation apps that allow other support engineers to experiment with designs that would otherwise have required the expertise of an R&D engineer. Through the use of simulation, Cypress engi­neers are delivering more customizable designs faster than ever before.

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