Topics being covered :-
2.Types of touchscreens
7.Comparison between various touchscreens
Do you know what a touch screens is?
It is a human machine interface (HMI). HMI, as its name indicates, is an interface or device that allows communication between machine and its user.
A touch screen is also this type of HMI that uses images for communications between a user and electronic device. Unlike mouse and keyboard, touch screen allows a user to interact with electronic device by directly touching images displayed on the screen.
Touch screen itself is just a transparent switch that detects touched locations. How the device reacts to your touches is controlled by software. Various input devices can be made with combination use of touch screen, display and software.
1. Switch and buttons are not physically required. Device makers can make and modify various input interfaces creatively by software.
2.With multi-touch function, various operations/inputs (eg: zoom-in/zoom-out, rotation) are possible.
3.Because a user operates an electronic device by directly touching the images on the display he is seeing, the operation will be intuitive, thus anyone can operate it from first use.
4.The whole unit is space-saving because display and input space are integrated. There is a lot of flexibility in design.
5.Unlike keyboard or physical switch, there will be no dirt, dust, and moisture getting into the spaces between buttons. Thus, it is easy for maintenance.
1.Since a display is directly touched, the display may get dirty, then become less-visible. A direct touch may also cause scratches on the screen that may cause malfunction in some cases.
2.Unlike push-button and mouse, users will not feel “click” when a user input a touch screen, thus operation may become clumsy sometimes. However, there are certain touch screens that provide “click feeling” when touched.
4 Wire Touchscreen
They are all constructed similarly in layers-a back layer such as glass with a uniform resistive coating plus a polyester coversheet,
with the layers separated by tiny insulating dots. When the screen is touched, it pushes the conductive coating on the coversheet against the coating on the glass, making electrical contact. (fig 2)
Analog 4-wire resistive is the most basic sensing method of resistive technology.
Top and bottom transparent conductive sheets that have uniform resistance value, are facing each other with a gap.
When the top sheet is touched, the touched point of the top sheet yields, and contact the bottom sheet.
The contacted point is conducted, and its location is sensed.
The sensing process (refer fig 1 for understanding)
1. Voltage is imposed between the electrode X1 and X2 on the top sheet. Then, there will be formed on the top sheet an equipotential distribution that is parallel to the X1 and X2.
2.Assuming that the electrode X1 is 0V (Ground), then impose 5V to the electrode X2. If the central point between the right and left electrodes is touched, 2.5V would be measured by the bottom sheet. If a point 1/5 away from X1 (4/5 away from X2) is touched, 1V would be measured by the bottom sheet. The X coordinate point is detected in this way.
3. After detecting the coordinate point in X direction, Voltage is imposed between the electrode Y1 and Y2 on the bottom sheet. The point between Y1 and Y2 (Y coordinate) would be measured by the top sheet, just in the same manner as X coordinate in the step 1-2.
4.As the set of step 1 ? 3 is repeated, the X and Y coordinate points of touched locations keeps being detected.
In analog 4-wire resistive technology, one sheet has equipotential distribution in X direction, and the other sheet has equipotential distribution in Y direction.
The two sheets measure each others’ voltages. In analog 5-wire resistive technology on the other hand, one sheet (bottom sheet) has equipotential distribution in both X and Y directions. Voltage of the bottom sheet is measured by the top sheet.
The sensing process (Refer fig3 for understanding)
1. 5V is imposed on the electrodes A and C. The other side (electrode B and D) will be 0V (Ground). In this way, there will be formed an equipotential distribution parallel to AC and BD.
2. Voltage of a touched point will be measured by the top sheet. If the central point between AC and BD electrodes is touched, 2.5V would be measured by the top sheet. If a point 1/5 away from AC (4/5 away from BD) is touched, 1V would be measured by the top sheet. Touched coordinate point in X direction will be measured in this way.
3. After detecting the coordinate point in X direction, 5V is imposed on the electrodes A/B. Then, C/D will be 0V (Ground). The point between AB and CD (Y direction) would be measured by the top sheet, in the same manner as X direction.
4. As the set of step 1 ? 3 is repeated, the touched points in X and Y direction keeps being detected.
An analog 5-wire resistive sensor was regarded as more durable than analog 4-wire resistive sensor.Since,there is no equipotential
distribution formed on the top sheet in analog 5-wire resistive.
So, the top sheet did not need to be good at resistance uniformity and environmental durability in this sensing method.
8 Wire touchscreen
The structure and sensing method of 8-wire resistive technology is same as 4-wire resistive technology.
Two transparent conductive sheets are facing. One sheet has electrodes on right and left sides, and the other sheet has electrodes on top and bottom sides.
Voltage is imposed on the sheet with electrodes on right and left sides, then a touched point in X direction is detected as the voltage is measured by the other sheet.
Then, voltage is imposed on the sheet with electrodes on top and bottom sides, then a touched point in Y direction is detected as the voltage is measured by the other sheet.
You would be thinking it same as 4 wire touchscreen??
Yes,you are right but there are additional wirings connected to each electrode in analog 8-wire resistive technology. Each electrode is added with one more wiring. These added wirings work as supplemental electrodes and measure voltage on each electrodes, then feedback the information to the controller.
Analog 4-wire resistive technology required calibration not only at the beginning but regularly because touch detecting points are gradually going off the alignment due to change of resistance value on wiring and/or connector parts with the passage of time. In analog 8-wire resistive technology, supplemental electrodes measure the voltage on each electrode automatically, and feedback the measured values to the controller. The voltage detected at touching is transformed into locational information in the relative ratio with the feedback voltage. In this way, the effects of voltage change at electrodes can be cancelled, thus recalibration would not be required.
There are two types in capacitive technology that are surface capacitive and projected capacitive.
Surface capacitive is employed for applications such as ATM and factory automation, but demand is limited.
As projected capacitive was developed and started to be employed for smartphone, the number of capacitive touch screen has been increasing dramatically.
Transparent conductive coating is on the base glass sheet, and glass protective coating is placed over it. Electrodes are placed on the four corners.
The same phase voltage is imposed to the electrodes on the four corners, then a uniform electric field will be forming over the panel. When a finger touches on the panel, electrical current will flow from the four corners through the finger. Ratio of the electrical current flowing from the four corners will be measured to detect the touched point. The measured current value will be inversely proportional to the distance between the touched point and the four corners.
There are various structures in projected capacitive technology. “One sheet piled-up structure” has X and Y electrodes piled on one sheet. “One sheet two-sided structure” has X and Y electrodes on its surface and backside of one sheet respectively. “Two-sheets-laminated structure” consists of two sheets facing each other with electrodes in between.
In the two-sheets-laminated structure of projected capacitive, X electrodes are forming on one glass, and Y electrodes are forming on another glass. The two glass sheets are laminated in the way that two electrode sides are facing. The X and Y electrodes are intersecting in matrix.
There are two types of sensing methods in projected capacitive technology. They are GRID type and wire sensing type. GRID type will be introduced here.
Human body is conductive since it contains a lot of water. When a finger comes close to the patterning of X and Y electrodes, a capacitance coupling will occur between the finger and the electrodes. The capacitance coupling makes the electrostatic capacitance between the X and Y electrodes change. The touch sensor detects touched points as it checks where on the electrode lines the electrostatic capacitance changed.
Projected capacitive technology make use of conductivity of human body. Then, you can imagine that projected capacitive can detect only fingers just like surface capacitive.
Projected capacitive can detect gloved finger as well.
Surface acoustic wave technology makes use of ultrasonic waves for detection. It is often abbreviated to SAW.
Let’s look at the structure of the touch screen first.
Take a look at the fig10.
Surface acoustic wave touch screen consists of one glass sheet with transmitting transducers, receiving transducers, and reflectors. Transmitting transducers generate ultrasonic waves that travel over the panel surface. The ultrasonic waves are reflected by the reflectors and received by the receiving transducers.
The ultrasonic waves traveling over the panel surface are surface acoustic waves.
1.If contacted by a soft material, SAW will get absorbed by it. Surface acoustic wave technology makes use of this nature of SAW.SAWs are sent out from the transmitting transducers, and traveling along the edge of panel. The reflectors located on the edge of the panel change directions of the SAWs at the angle of 90 degrees, thus the SAWs travel over the panel. Once the SAWs reached the other side of the panel, their directions get changed again by the reflectors located on the other side, and travel toward the receiving transducers. Once the SAWs are received by the receiving transducers, they will be converted into electric signals.
2.If it is touched by a finger, the SAWs will be absorbed by it and do not reach to the receiving transducer. Then, the sensor detects where the SAWs were absorbed?
3.There are routes on which the SAWs travel from the transmitting transducers to the receiving transducers. Each route has its own distance. If one of the routes is touched by a finger, the pulse will be absorbed, and the SAW on the route will not be received by the receiving transducers. Thus, the sensor will recognize which route was touched, and locate the touched point.