Tag Archives: Touch Screens

Haptics: A sense of touch through the computer screen

Touch-screens are quite common in mobiles, smartphones and tablet computers nowadays. They allow us to control and command the computer according to our requirement. A sense of touch is something different; it is sensing an object displayed on your computer screen. Imagine seeing a rose on your computer screen and by reaching out with your fingers, feel the softness of its petals.

This amazing possibility is the result of a non-mechanical haptic interface that allows computer users to manipulate a three-dimensional object on the screen of a computer to receive tactile response from the surroundings of the object. In short, you can reach out to touch and feel the object displayed on your computer.

The haptic interface uses flotor, a device shaped like an inverted umbrella and it contains coils of wire. It also has a control handle, which the user moves to interact with powerful magnets underneath. Electronic circuitry transfers the motion on to the screen that responds when an object collides with anything in the virtual world.

Normally, you are able to see and hear the information displayed on your computer screen. Touching the image to feel its texture and movement was not an option until now. At Carnegie Mellon University of Pittsburgh, PA, Dr. Ralph Hollis is working on his specialization, haptics – the science and technology of touch.

The user has to grasp a handle inside a sphere that is attached to a desktop computer. The handle connects to an object that looks more like an upside-down umbrella and it is called a flotor. The flotor carries electrical current flowing through electrical coils. All this is immersed in a powerful magnetic field, created by several permanent magnets.

Normally, the magnetic field allows the handle to float freely inside the sphere. The user can move and rotate the handle to control the position of a 3-D object on the computer screen. As soon as the object touches something in the virtual world, the user can immediately feel it.

Although there are many other haptic devices around, the concept of grabbing a motorized arm to interact with the computer is unique. Dr. Hollis has used magnetic levitation to make a direct connection between the user’s hand and the software. The result is direct and immediate feedback.

Briefly, haptics brings touch within a digital environment. This may give you the feel of resistance when you use a joystick in a game or give you touch sensation when using gloves in a virtual reality environment. Using haptics, the user gets an instant feedback when an object collides with another and does not have to rely only on what he sees. You can estimate what it feels like when you have to walk when your foot has fallen asleep. Adding the feeling of touch to a virtual environment makes interactions more life-like and similar to the benefit you have when you have full feeling in your feet.

At present, haptics works only with objects that are magnetic by nature, that is, objects attracted by magnets.

How noise affects touch screens

Although not understood explicitly, touch-screens in devices are susceptible to noise. The offending noise sources may be both internal as well as external. Most common sources of noise affecting touch-screens are display and charger noise. Cheap chargers entering the market are inherently noisy, and this affects the functioning of touch-screens. In addition, as devices get thinner, display noise increases.

In addition, many other items of everyday use generate noise that may cause interference. This includes the AC mains, radio signals and the ballasts used for fluorescent lights. When noise is present, low-performance capacitive touch systems may distort the position reported and this may impact the overall system reliability and accuracy.

Injected noise causes large amounts of jitter (highly variable touch coordinates reported for a stationary finger), false touches reported even for no touch on the screen, non-recognition of a finger actually touching the screen and sometimes a complete lock up of the device. For example, noise may prevent you from being able to unlock your phone, since your finger touch is no longer reported or you dial wrong numbers because of jitter and false-touch reporting.

A user experience of touch interface quality is directly dependent on how well a touch-screen controller combats interference from noise. Poor touch performance when noise is present can make customers unhappy, resulting in an increase in returns. However, since noise may be of different types, touch-screen controllers must be able to detect, differentiate and combat noise, especially the two sources most problematic to users – chargers and displays.

The proliferation of Switch Mode Power Supply or SMPS type chargers has reduced the size, weight and cost of mobile chargers. However, this has also led to the market being flooded with chargers that prioritize cost over performance, using lower grade components and not using certain components that would assist in reducing common-mode noise.

High amplitude, high frequency, common-mode noise emanating from chargers is a major problem resulting in degradation of touch performance of capacitive touch-screen devices. Some manufacturers have addressed this problem of noisy chargers by providing limited functionality when a device is plugged into such a charger. Others may show a message on the screen that the charger is not supported when it is not the approved charger for the device. Online forums reveal customer dissatisfaction of touchscreen performance due to noisy chargers is quite prevalent.

Common-mode noise causes fluctuations of both, the power and ground supplies of the charger voltage, relative to earth ground, but keeping the same voltage differential between them. Such fluctuations affect the performance of the touchscreen only when the finger of the user touches the screen. Since the potential of a finger of the user is roughly the same as that of earth ground, and the charger’s ground and power lines are fluctuating relative to it, the resulting noise enters the touchscreen through the finger.

Manufacturers aggressively pursuing thinner form factors for touch-screen devices has led to displays coupling more noise into the touch-sensors because of their proximity. Earlier, touch-screens had an air-gap or a shield layer for protection. With devices getting thinner, such shields and air-gaps have disappeared and the touch sensor is now laminated directly atop the display.
This increases the capacitance, while the sensor electrodes are closer to the noise producing VCOM layer of the display, increasing the coupling.

What Are Proximity Sensors?

Those of you who use a mobile phone with a touch-screen may have wondered why items on the touch-screen do not trigger when you hold the phone to your ear while answering a call. Well, designers of mobile phones with touch-screen have built-in a feature that prevents a situation such as “My ear took that stupid picture, not me.” The savior in this situation is the tiny sensor placed close to the speaker of the phone, and this proximity sensor prevents touch-screen activity when anything comes very close to the speaker. That is what happens when your ear touches the screen as you are on a call, but does not generate any touch events.

So, what sort of proximity sensors do the phones use? Well, in most cases, it is an optical sensor or a light sensing device. The sensor senses the ambient light intensity and provides a “near” or “far” output. When nothing is covering the sensor, the ambient light falling on it makes it give out a “far” reading, and keeps the touch-screen active.

When you are on a call, your ear covers the sensor, obstructing the device to see ambient light. Its output changes to “near” and the phone ignores any activity from the touch-screen, until the sensor changes its state. Of course, the mobile phone considers more complications such as what happens when the ambient light falls very low, but we will discuss more on different types of proximity sensors instead.

Different types of proximity sensors detect nearby objects. Usually, the proximity sensor is used to activate an electrical circuit when an object either makes contact with it or comes within a certain distance of the sensor. The sensing mechanism differentiates the types of sensors and these can be Inductive, Capacitive, Acoustic, Piezoelectric and Infra-Red.

You may have seen doors that open automatically when you step up to them. When you are close to the door, the weight of your body changes the output of a piezoelectric sensor placed under the floor near the door triggering a mechanism to open the door.

Cars avoid bumping into walls while backing. The proximity sensor (a transmitter and sensor pair) used here works acoustically. A pair is fitted on the backside of the car. The transmitter generates a high frequency sound signal and the sensor measures the time difference of the signal bounced back from the wall. The time difference reduces as the car approaches the wall, telling the driver when to stop.

Computer screens inside ATM kiosks and the screen on your mobile are examples of capacitive proximity sensors. When you put a finger or a style on the screen, the device detects the change in the capacitance of the screen. The device measures the capacitance change in two directions, horizontal and vertical, or in x and y directions, to pinpoint the exact location of your finger and operate the function directly underneath.

When a security guard checks you out with a wand, or you walk through a metal detector door, the guard may ask you to remove your watch, coins from your pocket and in many cases, even your belt. The reason is the wand or the door has an inductive proximity sensor that will trigger in the presence of metals (mostly made of iron or steel).

Finally, the fire detector in your home or office is a classic example of a proximity sensor working on Infrared principles. Level of infrared activity beyond a threshold will trigger the alarm, and bring the fire brigade rushing.

How Does the Touch Screen on a Mobile Phone Work?

The mobile phone is an amazing piece of work. Earlier you had to press buttons, now you just touch the app on your screen and it comes to life. You can even pinch your pictures to zoom in on a detail or zoom out to see more of the scene. The movement of your finger in the screen causes the screen to scroll up, down, left or right.

The technology behind this wizardry is called the touch-screen. It is an extra transparent layer sitting on the actual liquid crystal display, the LCD screen of your mobile. This layer is sensitive to touch and can convert the touch into an electrical signal, which the computer inside the phone can understand.

Touch screens are mainly of three different types – Resistive, Capacitive and Infrared, depending on their method of detection of touch.

In a resistive touch-screen, there are multiple layers separated by thin spaces. When you apply pressure on the surface of the screen by a finger or a stylus, the outer layer is pushed into the inner layers and their resistance changes. A circuitry measuring the resistance tells the device where the user is touching the screen. Since the pressure of the finger or the stylus has to change the resistance of the screen by deforming it, the pressure required in resistive type touch-screens is much more than for capacitive type touch-screens.

Capacitive type touch-screens work on a principle different to that of the resistive touch-screens. Here the change measured is not in terms of resistance but of capacitance. A glass surface on the LCD senses the conductive properties of the skin on your fingertip when you touch it. Since the surface does not rely on pressure, the capacitive touch-screens are more responsive and they can respond to such gestures as swiping or pinching (multi-touch). Unlike the resistive type screens, the capacitive screen will only respond to touch by a finger and not to stylus or a gloved finger, and certainly not to fingers with long nails. The capacitive touch-screens are more expensive and can be found on high-end smartphones such as from Apple, HTC and Samsung.

As the screen grows larger, such as for TVs and other interactive displays such as in banking machines and for military applications, the resistive and capacitive type technologies for touch sensing quickly become less than adequate. It is more customary to use infrared touch screens here.

Instead of an overlay on the screen, infrared touch screens have a frame surrounding the display. The frame has light sources on one side and light detectors on the other. The light sources emit infrared rays across the screen in the form of an invisible optical grid. When any object touches the screen, the invisible beam is broken, and the corresponding light sensor shows a drop in the signal output.

Although the infrared touch-screens are the most accurate and responsive among the three types, they are expensive and have other disadvantages. The failure rate is high because diodes used for generating the infrared rays fail often.