Industrial Touch Screen HMI

The resistive touchscreen consists of a flexible top layer made of Polyethylene (PET) and a rigid bottom layer made of glass. Both the layers are coated with a conducting compound called Indium Tin Oxide (ITO) and then spaced with spacers. While the monitor is operational, an electric current flows between the two layers. When a touch is made, the flexible screen presses down and touches the bottom layer. A change in electrical current is hence detected and the coordinates of the point of touch is calculated by the controller and parsed into readable signals for the operating system to react accordingly.

These systems transmit only 75% of light from the monitor. The resistive touchscreen is further divided into 4-, 5-, 6-, 7- and 8-wired resistive touchscreen. While the constructive design of all these modules is similar there is a major distinction in each of its method to determine the coordinates of touch.

The Four-wire Resistive Touchscreen uses both the layers to calculate the axes information of the touch. Touch measurement in the 4-wire is a 2 step process. The x-coordinate of the touch point is calculated by creating a voltage gradient on the flexible layer, and the y-coordinate is determined by creating a voltage gradient along the bottom layer.

The Eight-wire Resistive Touchscreen is simply a variation of the 4-wire one with the addition of 4 sense wires, two for each layer. The sensing points aid in reducing the environmental drift to increase the stability of the system. The 8-wire systems are employed in sizes of 10.4” or larger where the drift can be significant.

The Five-wire Resistive Touchscreen do not uses the coversheet (flexible layer) in determining the touch coordinate. All the position sensing is employed on the stable glass layer. In this design, one wire goes to the coversheet and four wires are deployed to the four corners of the bottom sheet. The coversheet only acts as a voltage measuring probe. The functioning of the touchscreen remains unscathed even with changes in the uniformity of the conductive coating over the coversheet.

The Six and Seven wire resistive touchscreen is also a variant to the 5 and 4 wire technology respectively. In the 6-wire resistive touchscreen an extra ground layer is added behind the glass plate which is said to improve system's performance. While, the seven–wire variant has two sense lines on the bottom plate. However, these technologies are as prevalent as their counterparts.

The Capacitive Touchscreen Technology is the most popular and durable touchscreen technology used all over the world at most. It consists of a glass panel coated with a capacitive (conductive) material Indium Tin Oxide (ITO). The capacitive systems transmit almost 90% of light from the monitor.

Surface-Capacitive screens, in this technique only one side of the insulator is coated with a conducting layer. While the monitor is operational, a uniform electrostatic field is formed over the conductive layer. Whenever, a human finger touches the screen, conduction of electric charges occurs over the uncoated layer which results in the formation of a dynamic capacitor. The computer or the controller then detects the position of touch by measuring the change in capacitance at the four corners of the screen.

In the Projected-Capacitive Touchscreen Technology, the conductive ITO layer is etched to form a grid of multiple horizontal and vertical electrodes. It involves sensing along both the X and Y axis using clearly etched ITO pattern.

The Surface Acoustic Wave Touchscreen technology contains two transducers (transmitting and receiving) placed along the X-axis and Y-axis of the monitor's glass plate along with some reflectors. The waves propagate across the glass and are reflected back to the sensors. When the screen is touched, the waves are absorbed and a touch is detected at that point. These reflectors reflect all electrical signals sent from one transducer to another. This technology provides excellent throughput and image clarity.

In the Infrared Touchscreen Technology, an array of X- and Y- axes are fitted with pairs of IR Leds and photo detectors. The photo detectors detect any change in the pattern of light emitted by the Leds whenever the user touches the monitor/screen.