In a world that is increasingly focused on capacitive touch solutions, it is easy to forget your roots. Though it is obvious that handset innovators like Apple made projective capacitive touch screen control cool and brought it into view of the average consumer, it is not the only way to control your touch screen application. In fact, there are numerous other methods - both legacy and cutting edge - in the market today. One newer idea takes and merges benefits of traditional resistive touch and projective capacitive touch to create a low-power, low-latency, and low-cost system with multi-touch capabilities that is not constrained by screen size.
First, let’s cover the basics on how traditional resistive and projective capacitive touch screen control works before we unveil the new hybrid. Single resistive touch solutions are comprised of two Indium Tin Oxide (ITO) layers spaced apart by microdots (Figure 1).
Figure 1: Typical LCD setup of a single resistive touch solution.
Think of the touch screen as just a simple grid governed by two linear axes, x and y. A finger (gloved or uncovered, or any semi-pointy object) pushes down on the top layer of the touch screen, forcing it into contact with the bottom layer; this physical connection creates a closed circuit. This newly formed circuit connects a voltage reference (typically Vcc) through the ITO layers, which act as a series of resistors and create a voltage differential less than the supplied voltage reference. That lesser voltage is then fed into an analog-to-digital converter (ADC) and translated into a specific region on the touch screen, yielding one half of the Cartesian coordinates useful to an applications processor. The same process is then repeated in the other direction to get the last coordinate. Once the full set is deciphered, it is then passed on via the designer’s serial communication of choice.
Capacitive touch is a bit more complicated. There are two options for capacitive touch solutions: surface and projective. Surface capacitive touch is good for single-touch applications and really is outside of the scope of this discussion. Projective capacitive touch is used in multi-touch applications and involves a layering of safety glass and ITO sensor layers on top of an LCD display, as shown in Figure 2.
Figure 2: Typical LCD setup of a projective capacitive touch solution.
Unlike with resistive touch solutions, the ITO in this case is no longer a large blanketed panel, but rather two specifically tessellated patterns that will interlock to completely cover the LCD display. Each layer comprises half of this checkerboard style, creating a series of rows and columns. These rows and columns each have their own conductive properties relative to ground, also known as self capacitance, and between each point of row-column intersection, known as mutual capacitance.
Self and mutual capacitance each play an important role in finding individual touch points in a multi-touch system. In general, the touch screen controller is constantly scanning all the columns and looking for a change in self capacitance. Once one is detected, the rows are then scanned to look for changes in mutual capacitance and to resolve any ghost points that may occur due to aliasing intrinsically found in self capacitive multi-touch systems. Once the points of contact are detected, they are then passed on to the applications processor via serial communication. The key is that there is a constant scanning of the columns and eventually the rows.
Resistive multi-touch combines all these key aspects mentioned into one solution. Imagine the x and y-axes of the single multi-touch solution, but what if instead of one axis, you had multiple ones (Figure 3)?
Figure 3: The lone x-y axis of a single multi-touch solution transformed into a 3x3 array of x and y axes for resistive multi-touch solutions.
For example, start with a single x and y-axis or regular 4 wire sensor of a single touch resistive controller. Then, through post-processing etching you can create the number of rows and columns in the array to suit the desired application, creating the multi sensor array.
If you chose a 3x5 array, you will have a network of 15 individual “mini-touch” areas working together to detect up to 15 simultaneous touch points. A controller like this will retain all benefits of detection through resistive technologies, but also apply the scanning columns and rows concept of projective capacitive touch control. See Figure 4 for an example array showcasing the x and y axes.
Figure 4: 3x5 resistive multi-touch arrays and one X row (LEFT), and one y column (RIGHT)
This system is an innovative way to implement multiple touch detection as well as gesturing, such as intuitive pinch-and-pull zoom commands, or edge rotation commands. The TSC2020 from TI implements all of these features, making it a powerful resistive multi-touch solution. Additionally, the average power consumption of projective capacitive controllers is around 7 or 8mW, whereas the TSC2020 consumes 300µW (that’s micro, people!) Not to mention that a typical projective capacitive controller is constrained by the pitch of each individual ITO sensor. With multi-touch resistive solutions, you can have each mini-touch screen be as big as you like; your only “constraint” is the size of the ADC that the value is being fed to. So, when you have 12 bits, that’s over 416,000,000 discernable touch points within just one touch area! This is ideal for any stylus-based application that wants to incorporate character recognition into the mix. Those two benefits alone would drive a lot of value into any application looking for a lower cost solution, but then there is the final cost of the controllers themselves; in general, projective capacitive touch controllers tend to be around double the cost of a resistive controller, if you’re being conservative with your estimate.
There are a few downsides, of course, such as proximity sensing. Proximity sensing is the ability to detect an object near the screen without touching it, and since resistive touch detection is pressure-based, contact is required. However, through optical implementation, proximity detection is still a cost-effective alternative. Additionally, the screen on a resistive touch solution is usually less transparent than a projective capacitive screen because you are using two full panels of ITO. Differences, though, can be as little as 10% in terms of clarity, which is a livable compromise for all the other system benefits you receive from resistive touch solutions.
One final potential downside for multi-touch resistive touch screen control is more of a design consideration: screen size. In theory, the multi-touch array can be scaled to any size necessary, but if it is scaled too large, such that two touch points fall into one “mini-touch” area, that zone will result in the same function as two touch points on a single touch controller, creating an average value between the two. To circumvent this phenomenon, simply make sure your object creating the touch points is large enough not to allow two points of contact per area.
Resistive multi-touch solutions combine the benefits of both projective capacitive and resistive touch screen control methodologies. With its power saving abilities, as well as its virtually infinite position resolution, resistive multi-touch controllers are a viable and effective option for any touch screen design.
For more information, please visit the links below:
• Find more information on the TSC2020 touch screen controller: www.ti.com/tsc2020-ca
• Explore TI’s full touch screen controller portfolio: www.ti.com/touchscreencontroller-ca
Eric Siegel is a product marketing engineer for touch screen controllers and haptics drivers at Texas Instruments. He has an M.S. in Electrical Engineering from the University of Florida, and has also worked as a test engineer for digital signal processors and in marketing for microcontroller solutions. In his spare time, Eric is a huge movie buff and also practices mixed martial arts. Eric can be reached at email@example.com.
Posted by Janine E. Mooney, Associate Editor