There’s one touch technology that’s most suited for your design. Understanding all the tradeoffs makes that choice easier.
By: Gary L. Barrett, Touch International | April 25, 2012
Deciding on the right touch technology can be challenging for even the seasoned technology guru. With over 1200 touch-related patents in existence, it is easy to become confused about which touch technology to choose to integrate into a new product.
Whether it is the credit card terminal at the grocery store, the ticket kiosk at the local movie theater or the mobile phone in your pocket, the use of touch is making its way into virtually every environment and is here to stay. It may be augmented by motion and voice, but until we develop a direct telepathic link to the machine, touch will be present.
When choosing a touch screen, it’s important to first carefully evaluate the needs of your product and the environment of the display. Once your key requirements have been identified, it’s easier to weigh the advantages and disadvantages of each technology to find the touch screen that’s right for your application.
In the simplest of terms, touch is defined as “to come into or be in contact with something.” An example of this is when a hand, finger, stylus, etc., comes in close proximity or contact with an add-on touch sensor (most common) or the display itself. Some touch technologies don’t actually require the user to make contact with a surface, but are, nevertheless, still considered touch technology.
When a touch is initiated, a Cartesian coordinate (X, Y, and sometimes Z) is generated within the touch controller. When the touch system is married to the operating system, such as Windows or Linux, the X, Y coordinates are converted to mouse moves (up, down, right, and left “mickeys”). The difference between a mouse and a touch screen is the difference between relative and absolute. A mouse moves the cursor on the screen up and down, right and left, and it doesn’t matter where the mouse is on the desk. A touch screen moves the cursor to a place under the touch point—this the absolute mode. To move the cursor to the touch point, the software driver looks into the video RAM to see where the cursor is on the screen. It then looks at the coordinates from the touch point, and issues the number of mickeys needed to move the cursor from where it is to where it should be.
As the name implies, multi-touch refers to the ability to simultaneously register more than one touch at one time. One common multi-touch feature is the ability to use gestures. The most common gestures are the same features that you’ll find on your smart phone or tablet: pinch, expand, and rotate. When using multi-touch, a driver is written to recognize specific gesturing functions and relay them to the software to act upon.
There is a bit of a debate within the touch community about redefining the term multi-touch. For a system to be classified as multi-touch, the coordinates must be absolute, meaning that there must be two or more specific coordinates. All of the technologies covered here, excluding surface capacitive and analog resistive, are capable of doing gestures (this is controller dependent); however gestures don’t require true multi-touch capabilities, as they can rely on ghosting. Ghosting occurs when a replica of the transmitted image, offset in position, is super-imposed on top of the main image. The most commonly used multi-touch capable technologies are projected capacitive, multi-touch analog resistive and optical touch.
Until the recent introduction of gestures, touch was introduced whenever it was found that it could complete a task 3X faster than using a keyboard (point of sale terminal in a restaurant), or where the device could instruct the casual user on a complicated device (medical instrument), or where pen input for writing was needed (PDA). Gestures have added the first new use for touch in several years.
Once you begin taking steps to put a touch product into production, it’s important to consider the factors below to identify which touch technologies are the best fit for your application, as well as those to avoid:
Answering these questions will better prepare you to identify touch technologies that fit your requirements. It is important to note that there is no perfect touch solution. While the goal of every manufacturer is to produce indestructible touch screens that last forever, have perfect optical clarity, are immune to interference, and cost nothing, the reality is that each technology has strengths and weaknesses, so it’s important to explore all options and make an educated choice based on your application.
There are many touch technologies in the market. However many have proven to be niche products, too expensive, not reliable, or difficult to produce. The top touch technologies are projected capacitance, analog resistive, surface capacitive, infrared (IR), and surface acoustic wave (SAW). However, projected capacitive and analog resistive make up 90% of the total market. Other touch technologies include optical touch, multi-touch analog resistive (AMR or MARS), dispersive signal (DST or bending wave), in-cell/on-cell, and acoustic pulse recognition (APR). By analyzing the pros and cons of each, which we do here, you’ll be able to narrow your options by matching up your requirements with the technologies’ capabilities and limitations.
Since the release of the iPhone in 2007, the demand for projected capacitive (also called p-cap, pro cap, or PCT) has seen steady growth, especially in mobile devices, and is now the most popular technology for today’s touch products.
In the short time since the introduction of projected capacitive touch screens in the iPhone, myriad construction methods have been developed. All projected capacitive touch screen designs have two key features in common—the sensing mechanism (ITO layer) that lies behind the touch surface and no moving parts.
Mutual capacitance is now the more common projected capacitive approach and makes use of the fact that most conductive objects can hold a charge if they are very close together. If another conductive object, such as a finger, bridges the gap, the charge field is interrupted and detected by the MCU.
Projected capacitive touch screens are “scanned,” meaning that most of these touch screens consist of a matrix of rows and columns that are read one by one to get a reading or count. To get an exact coordinate, the results from several row/column intersections are read and the counts are used to triangulate the exact touch location. Figure 1 shows the rows and columns made by the ITO and a very basic stack-up design with the ITO layer protected by glass on both sides.
1. The sensing mechanism, or ITO layer, holds the rows and columns of the touch display.
For 20 years, analog resistive reigned supreme, but fell to the number two spot in 2010 when it was unseated by projected capacitive. At one point, there were more analog resistive touch screens manufactured in one day, than all other touch technologies combined in one year. Even though projected capacitive has replaced it as the most popular technology, resistive still has an important place in today’s market and shouldn’t be overlooked.
Five types of resistive technology are defined by the signal lines and include 3-, 4-, 5-, 6-, and 8-wire touch screens. Today, we most commonly build 4-wire (lowest cost) and 5-wire (most durable). Generally, all resistive touch screens consist of a glass layer with an ITO conductive coating on top and a polyester top sheet, with a conductive coating on the bottom. The conductive surfaces are held apart by “spacer dots,” usually glass beads silk-screened onto the coated glass. In the case of 5-wire, a toggled voltage is applied to the four corners of the glass layer, and when a person presses on the top sheet, its conductive side comes in contact with the glass’ conductive side, effectively closing a circuit (known as pressure sensing). The voltage at the point of contact is read from a wire connected to the top sheet, which is the fifth-wire.
2. The image represents a basic illustration of how analog resistive technology works.
For 4-wire, the glass layer has two conducive buss bars on either side of glass layers, and on the top, two conductive traces perpendicular to the two below. A positive voltage and opposite side ground are put first on the ITO glass layer, which has a uniform resistance between the buss bars. The voltage is read at the point where the top layer pushes to the bottom, passed through an analog-to-digital converter to get the X coordinate. Then the voltage is applied to the top layer and read back by the bottom layer to get the Y coordinate. This toggling is repeated more than 100 times per second to get very fast, high-resolution coordinates.
The durability issue comes from the flexing and rubbing of the ITO, which breaks or wears out the ITO, which is only a few angstroms thick. In addition, the multiple air gaps are reflective, which reduces the light transmission and degrades the image.
Surface capacitive (s-cap or surface cap) is a declining technology and is now mostly used in legacy gaming and amusement machines. One positive aspect of surface capacitive is that its optics are as good as projected capacitive and the sensor dimensions can be larger. For example, Touch International offers a 42-in. surface capacitive sensor for a gaming table application. The information kiosk business, including ticketing systems, is shared equally with IR touch.
Unfortunately the drawbacks of this technology outweigh its benefits. This is mainly due to its inability to take heavy use, lack of true multi-touch capability, limited suppliers, and design flexibility limitations due to manufacturing methods used.
The surface capacitive touch screen has a conductive coating on the front surface with wires connected to each corner. A small voltage is applied to each corner. The operation relies on the human body’s capacitance. When the screen is touched, a small current flows to the point of input, causing a voltage drop which is sensed at the corners. The reason this technology wears out with heavy use is because the conductive coating is on the outer area of the glass. As the screen is continually touched, this coating eventually wears off, causing the screen to stop working.
IR touch is one of the oldest touch technologies and remains a survivor. Today, it can be found in public access kiosks, point-of-sale terminals, and big displays.
Recent improvements to IR sensing technology have introduced true multi-touch. However, this application requires a lot of computing power and sophisticated software.
IR uses a pcb frame around the display’s perimeter. Two sides contain closely spaced IR LEDs; the opposing two sides have matching photo transistors. The LEDs are turned on in sequence and the signal is read from the photo transistor to the matching transistor. If no signal is read, then that indicates a blocked IR beam, meaning a touch. No actual touch "screen" is required for operation. However, a plate of glass is generally used to protect the underlying display and to provide anti-glare properties. Problems, mostly solved, include sunlight making the IR pulse unreadable, or objects such as gum, water, bugs, and accumulated dust blocking the beams.
The first to be able to compete with the original big three (analog resistive, IR, and surface capacitive), surface acoustic wave (SAW) offers superior optics and durability.
The demand for SAW technology is declining, mostly because it offers few advantages over projected capacitive. The main reasons are that it doesn’t last forever and isn’t multi-touch capable. It’s also important to note that this technology is always sold as a kit (touch screen plus electronic controller) as there are few vendors offering this technology.
A SAW touch screen consists of a piece of glass with sound wave reflectors deposited along all four edges. Two emitting transducers are mounted in two corners and receivers are mounted in the opposing two corners. A sound wave travels parallel to the glass’ borders. As it encounters the sound wave reflectors, some of the wave passes through to the next sound wave reflector, and some of it is reflected across the touch screen. On the opposite side, the wave is passed through the sound wave reflectors to the receivers. The receivers can detect a drop in the sound wave’s amplitude when a sound absorbing material (such as a finger) comes in contact with the glass. Note that SAW won’t work with a hard-tipped stylus because it uses the absorption of sound waves to detect touch. Hence, SAW requires a soft-tipped stylus or finger input.
Optical touch technology uses two or more cameras mounted over the display’s surface, usually in the corners, and its sophisticated software performs the touch recognition. This technology is primarily used for displays 32 in. and up, and especially for digital signage. Today, most large format displays use either IR or optical, which share most advantages and disadvantages.
Optical touch had the potential to reduce cost over IR systems (which require hundreds of pairs of IR emitters and receivers). But this was only true if one could assume that there would be enough ambient light to illuminate the probe so the cameras could see it. While most optical touch suppliers make this assumption today, it means low-light applications may not work. Also, difficulty in integrating the frame often causes people to purchase an integrated display which carries all of the overhead in a complete unit.
If your application requires a large display or something for digital signage, optical touch and IR are the most common choices. If the display is 32 in. or smaller, optical touch won’t be the best option.
Optical touch uses optical sensors (usually two, but can be more) mounted in the bezel or on the glass’ surface that track the movement of objects close to the surface by detecting the interruption of IR light. The light is emitted in a plane across the screen’s surface and can be either active (IR LED) or passive (special reflective surfaces).
At the heart of the system is a printed-circuit controller board that receives signals from the sensors. Its software then compensates for optical distortions and triangulates the position of the touching object.
Multi-touch analog resistive, also called MARS, MAR, or AMR, is the new way to achieve true multi-touch for applications that need the ability to use any probe, glove, or non-body activation. This technology is similar to replacing a standard analog resistive touch surface with a multitude of tiny, finger-tip size, touch panels on one touch surface. Each of these panels will report analog output, so writing on the touch surface results in the same high-resolution ink you’d expect from a single-touch analog resistive touch surface. Because this technology is a cross between a digital and analog sensor, the result is more accurate over time than a traditional analog resistive system which can suffer from drift in the touch coordinate. In addition, this technology lets you solve the palm rejection problem by only looking at input from the area where writing is involved.
Many applications can benefit from a multi-touch pressure sensitive solution, such as two pilots with flight gloves simultaneously entering information on the same display. Traditional applications that use writing or typing as opposed to a graphic user input (GUI) will benefit from this technology. However, compared to projected capacitive, the display image isn’t as nice and the sensor won’t have the same life expectancy.
The construction of MARS is similar to that of regular resistive, and is essentially a 4-wire resistive sensor cut into many small 4-wire touch screens. Unlike traditional resistive, however, MARS consists of an X-Y grid that’s scanned. The advantage of this technology is that it provides multi-touch abilities, while still allowing for input from any pointing device because it’s pressure sensitive.
Introduced by 3M a few years ago, dispersive signal technology (DST) is used in large format displays for digital signage. While overall performance is good, DST will stay in the shadows of optical and IR because it doesn’t scale to sizes over 50 in., is difficult to integrate, and doesn’t detect input if the finger or probe isn’t continuously moving.
DST is a simple technology to build and consists of plain glass with one transducer in each corner. When a touch occurs, mechanical energy (bending waves) is radiated out from the touch location and detected by the sensors.
Twenty-five years ago, it was believed that spoken inputs would replace touch. Today, however, building touch into the LCD is considered to be the killer of touch screen companies. Although some on-cell LCDs have been used, this isn’t a technology that you could choose. Samsung and Microsoft have jointly developed a table size in-cell system and the Apple TV is rumored to be incorporating this technology.
On-cell simply means that the transparent conductors used to make a separate projected capacitive touch panel are instead incorporated into the LCD layers, essentially adding a projected capacitive sensor to the LCD’s layers. In-cell touch means that something in the LCD pixel is touch sensitive, usually photo-sensitive, though it could be self-capacitance type projected capacitive. This is done by scanning the pixels and reading the input back through the drive multiplexer. Thus, the multiplexer both changes (drives) the color pixel and then reads the touch input on the pixel (receives).
Acoustic pulse recognition (APR) was introduced a few years ago by Tyco International's Elo division. It’s similar to 3M’s DST technology and shares most of the same advantages and limitations. Unlike DST, APR comes in small sizes as well as large format.
Like DST, APR is a simple technology to build and consists of plain glass with one transducer in each corner. When a touch occurs, mechanical energy (bending waves) is radiated out from the touch location and detected by the sensors. APR determines the exact touch coordinates by generating a unique sound for each position on the glass. Similar to DST, after the initial touch, a motionless finger can’t be detected. However, for the same reason, touch recognition isn’t disrupted by any resting objects, making it a good choice for palm rejection.
There are more than 1200 patents for different touch technologies. Theose described here are the most common. Others, including force sensing, banana bar, cyclops touch, fiber optic, water bottle, and GAW, have made appearances from time-to-time, but haven’t had the staying power to compete with the more entrenched technologies.
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