The iPhone was the breakthrough product that introduced multi-touch—the ability to manipulate a touch screen interface with multiple fingers at once—to the average consumer. Along with the popularity of the iPhone came the realization that this new technology could make a user interface more flexible and more intuitive than previously possible. As such, MindTribe has seen a surge in companies looking to incorporate multi-touch interfaces into their products.
While the tools needed to implement a multi-touch interface are increasing in availability, they are still not established enough to be in the hands of every company’s engineers or contract manufacturers, and product technologies and offerings are rapidly evolving from week to week.
Some of our clients see the addition of multi-touch as an avenue to differentiating themselves, some see a means of creating new user experiences, while others seek insight in determining whether multi-touch is feasible for their product.
The rush for multi-touch is on. To help our clients quickly get an intuitive feel for the possibilities and limitations of multi-touch interfaces, we built a mobile reference platform to enable quick and easy experimentation. The product of this effort, a handheld demo unit, will serve as an anchor to future client discussions on the technology.
In addition to creating a reference platform for our clients, it was also a good opportunity for us to refresh our knowledge on the state of the whole landscape of the multi-touch industry, from the vendors involved to the range of technologies available. To that end, here’s a brief rundown of some of the most common approaches to incorporate multi-touch into a product:
Resistive
Resistive touch screens have long been the low cost option for single-touch interfaces, commonly found in smart phones, GPS navigators and Chumbys. With some custom sensor hardware and a lot of software IP, a few multi-touch enabled resistive touch screens (such as those from Stantum) have become available.
These modules are targeting the handheld device market with sizes between two and five inches. This puts them in direct competition with the capacitive multi-touch sensors in the next section.
Projected Capacitive
There are two different flavors of capacitive touch: Surface Capacitive and Projected Capacitive. The details of how these two technologies function could fill an entire blog post, so they will not be covered here. There are, however, key differences that can have a big impact on the design of a product. Finding a good breakdown of these differences proved difficult in our searches, so they will be listed here for those who want to know:
1) The electrodes of a Surface Capacitive sensor must be directly touched by a finger in order to work. As such, they must be on the top-most layer or surface of the touch panel and cannot be covered. The electrodes of a Projected Capacitive sensor, on the other hand, actually sense the proximity of a finger and are able to sense through thin materials such as the hardened Oleo-phobic glass of the iPhone. In essence, the ability to sense is projected onto the top layer of the panel.
2) Surface Capacitive sensors are simpler than the projected capacitive type and are much cheaper and more common as a result.
3) The difference that is most relevant to this blog is that Surface Capacitive sensors cannot be used for multi-touch, at least not in current and common implementations. This should not imply that every projected capacitive sensor is capable of multi-touch, however, only that you should start your search for a sensor at a company which already offers the Projected Capacitive variety.
What makes a Projected Capacitive sensor a multi-touch sensor lies in the layout of its sense electrodes. This pattern is determined by the IC that drives the sensor and currently there are three major players in this space: Cypress Semiconductor, ATMEL and Synaptics. These vendors all have their own electrode patterns associated with their products, so it is important to specify your controller IC to the sensor supplier. Fortunately, sensor variety has been growing along with demand and these compatibility issues should become less of a hurdle.
Projected Capacitive is currently the leader in the handheld multi-touch market with sizes that range from two to ten inches. New products, such as PCs that support multi-touch, have been pushing out the bounds of how large of a screen projected capacitive can support.
Camera-Based
This technology is at the heart of some of the first multi-touch devices and has gained a lot of attention as the core of Microsoft’s Surface product. This approach uses a camera located behind the screen to see where the user’s fingers are touching. There are a number of techniques that use infra-red light and different surface materials to try to get the best touch resolution. With all necessary equipment available to the average consumer, camera-based multi-touch has taken root in the hobbyist community. Open source software packages, such as those from NUIGroup, provide DIY-ers with all of the information they need for a homemade multi-touch setup.
Since the camera needs to be set back from the touch surface a ways, these multi-touch setups must have some depth and are well suited for large applications. Camera-based multi-touch systems have a very wide range of sizes and are well suited to multi-user applications where big touch surfaces are a must.
MindTribe’s Reference Platform
There were a number of pressures that lead us to choose the handheld form factor for our multi-touch demo. Primarily, a small device is most directly relevant to the needs of our clients, whose products are generally smaller than a breadbox. As I outlined above, the technology needed to make a six foot touch screen has nothing in common with what goes into a smart phone. Second, camera approaches aside, the best supported screen size in multi-touch is 3.5 inches. In fact, projected capacitive glass manufacturers such as TPK, Touch International and Wintek have pre-engineered touch sensors available at 3.5 inches. This way, the development costs have been covered and there is no barrier to customers who want this commonly requested size.
MindTribe Mobile Multi-Touch Reference Platform
Another boon to the development of this demonstration platform was a smart phone development platform that we came across. It allowed us to tackle the multi-touch interface without having to first spend the time needed to build up the rest of the system. This device runs Windows CE, comes with a LCD, battery and a slew of interfaces that make it easy to add functionality. We removed the GSM cell module and replaced it with a custom board that held the multi-touch controller, which reports any touch information to the phone’s processor over an I2C interface. The simplicity of this board is a testament to the complexity of the controller IC, in this case a Cypress part (we’re working on additional platforms to showcase the technology of other manufacturers). These controller chips are designed to do all of the heavy lifting when it comes to driving the sensor while still fitting into a tight space.
Speaking of fitting things into tight spaces, we packaged the stack of touch glass, LCD, processor board and battery into a custom enclosure. One of our mechanical engineers was looking for an excuse to try a new rapid prototyping technique called DDM, a variant of FDM. We figured that we could kill two birds with one stone with this project and check this method out while we were developing the multi-touch demo. Take a look at the resulting enclosure in the picture of the assembled demo. Consequently, you can learn more about many available rapid prototyping methods in Troy’s June blog post.
With some quickly written software loaded to show off the multi-touch functionality we were finished with the demo. Overall we found that implementing a projected capacitive multi-touch interface is a straightforward process. There were fewer electrical noise issues than we had feared and, after a few hiccups and slow downs, the I2C interface is working to send the coordinates of our fingertips through our software and onto the screen. While the performance of this demo is decent, it is not as fast as some PC-based demos that we have seen. It goes to show that a lot of optimization work goes into each device that makes it to market. When a touch screen isn’t tracking in real time, it’s apparent. With multi-touch, there is even more data thrown into the mix and good software ensures that the human-machine interaction remains seamless.
Multi-Touch Performance Demonstration Application
With the increasing availability and simplicity of the components needed to add multi-touch to a new product, we expect to see this interface in more and more products. The technology has grown mature enough that product developers no longer need to view it as a risky feature so long as its limitations are well understood.
Tags: capacitive, DDM, demo, multi, multi-touch, projected, resistive, surface, touch