How to quickly implement a reliable touch screen
Author: Steven Keeping
Compared with keyboards and mice, people are increasingly using touch screens as human-machine interfaces (HMI) to program, configure and control consumer, enterprise and industrial systems. Touch screens are more intuitive, faster, and use a single integrated interface to replace multiple input devices. In addition, it can provide greater convenience for people with disabilities and can be made into very compact sizes.
The wide range of applications for touch screens means they must be rugged, can be operated with bare fingers or with gloves, while also being cost-effective. Resistive touch screens can meet these requirements, but designers need to be able to get to market quickly with an off-the-shelf solution that includes a touch screen paired with an appropriate controller. They must also understand the difference between four-wire and five-wire resistive touch screen interfaces.
This article provides a brief introduction to resistive touch screens. It then uses NKK Switches' touch screens and controllers as examples to illustrate how to use these products for design.
How resistive touch screens work
A resistive touch screen is a separate component that covers a flat panel display. The touch screen, used in conjunction with the controller, allows users to interact with displayed symbols by touching specific areas. Touchscreens can detect the precise location of a finger or stylus touch point. The application software then determines what further actions to take on the screen based on that location.
Because resistive touch screens are inexpensive, rugged and can be operated with bare fingers, gloves or a stylus, they are suitable for a variety of consumer, retail, enterprise, industrial and medical applications. The technology uses a deformable plastic film with a conductive material such as indium tin oxide (ITO) on the back. The back of the touch screen is made of glass or acrylic substrate, and there is also a layer of ITO on the front.
Non-conductive isolation points separate the plastic film from the glass or acrylic substrate. When the plastic film is pressed with a finger or stylus with a force of one to two Newtons (N), the film contacts the substrate, effectively closing the switch in the localized pressure area. A controller board with a four- or five-wire connector can determine the position of the closed switch, and the software will react accordingly (Figure 1).
Figure 1: Resistive touch screens work by bringing two conductive surfaces into contact with each other. (Image source: NKK Switches)
Resistive touch screens are widely used in applications that require low cost, ruggedness, and operation with gloves or a non-conductive stylus. Such products are often able to withstand millions or even tens of millions of touches without failure. Additionally, resistive touch screens can be manufactured to be waterproof and resistant to chemical splashes.
The difference between four-wire and five-wire touch screen configurations
A four-wire touch screen has two electrodes on the bottom plate and two electrodes on the top plate. The electrodes on the base plate extend along the Y-axis, allowing them to measure resistance along the X-axis. Likewise, edge electrodes on the top plate extend along the X-axis, allowing them to measure resistance along the Y-axis (Figure 2).
Figure 2: A four-wire resistive touch screen has two edge electrodes on the bottom and top plates. These two pairs of electrodes are perpendicular to each other and thus determine the XY position of the contact. (Image source: NKK Switches)
At the finger contact, the bottom layer effectively separates the top layer into two series resistors. The top layer also divides the voltage on the bottom layer at the contacts. With proper biasing, each board can act as a voltage divider, where the output voltage represents the contact coordinates.
In a five-wire system, the top plate has four edge electrodes that serve as voltage sensing nodes. The four corners of the base plate form electrodes that generate voltage gradients in the X and Y directions. Measurements in the X and Y directions were obtained using different bias configurations (Figure 3).
Figure 3: A five-wire resistive touch screen uses four corner electrodes on the bottom plate to create voltage gradients in the X and Y directions, and two pairs of edge electrodes on the top plate to sense voltage. (Image source: NKK Switches)
In a five-wire structure, only the base plate is movable. This means that even if the top plate is damaged, the touch screen will still work. In contrast, both boards of a four-wire touch screen are active; damage to the top plate can cause the touch screen to malfunction. Five-wire touchscreens tend to be more durable, but at the cost of a more complex design and higher cost.
Commercial resistive touch screen solutions
To minimize complexity and speed time to market, NKK provides proven commercial solutions for touch screens and accompanying controllers. Designers still have the option of purchasing a touchscreen from NKK and using it with another vendor's or their own controller.
NKK's FT series is a model for resistive touch screens. The series is available in a variety of screen sizes from 5.7 inches to 15.6 inches (diagonal), in four- and five-wire configurations, and with a touch activation force of 1.4 N (Table 1). Both versions have a flex circuit tail end that connects to the controller board.
Table 1: Comparing four-wire and five-wire resistive touch screens, it can be seen that the five-wire resistive touch screen lasts longer (measured in clicks). (Image source: NKK Switches)
The FTAS00-5.7AS-4A is a 5.7-inch four-wire resistive touchscreen with 1 mA at 5 VDC, XY resistance of 250 Ω to 850 Ω, 1.5% linearity, and 10 MΩ insulation resistance. The expected life of the touch screen is 50,000 writes or 1 million clicks.
The FTAS00-10.4A-5 is a 10.4-inch five-wire resistive touchscreen with 1 mA current at 5.5 VDC, XY resistance of 20 Ω to 80 Ω, 2% linearity, and 10 MΩ insulation resistance. Expected service life is 50,000 writes or 10 million clicks.
Whether it is a four-wire or five-wire touch screen product, NKK provides controllers with RS232C or USB interfaces. The controller board comes with device driver software compatible with Windows 7, 8, and 10. FTCS04C and FTCU04B are RS232C and USB interface controller boards for NKK four-wire touch screens respectively, while FTCS05B and FTCU05B are similar products for five-wire touch screens.
Getting Started with Resistive Touch Screens
The design process for four-wire and five-wire touch screens is similar. The core of the RS232C and USB four-wire controller board is the FTCSU548 controller chip. This 48-pin LFQFP IC features an asynchronous serial interface and a full-speed USB 2.0 interface. The device operates from a 3.3 V to 5 V supply for RS232C or a 5 V supply for USB, has a rated output current of 170 mA, an operating frequency of 16 MHz, and an analog-to-digital converter (ADC) resolution is 10 bits. The chip has built-in calibration capabilities.
When the touch screen is pressed, the controller IC uses the analog voltage value detected by the ADC to determine the coordinates and forwards the coordinates to the host through the RS232C or USB interface (Figure 4).
Figure 4: FTCSU548 controller IC (IC1) mounted on the FTCU04B (Four-Wire USB) controller board. CN1 (left) is the connector at the end of the touch screen's four-wire flex circuit. (Image source: NKK Switches)
The four-wire flexible circuit end of the touch screen is connected to the controller board through CN1. The controller board is connected to the host PC via CN4. The CN4 USB interface also supplies power to the controller board. The host computer will run the device driver and touch screen application software (Figure 5).
Figure 5: Typical four-wire USB controller board and host PC configuration. (Image source: NKK Switches)
Resistive touch screens require calibration during installation. The FTCSU548 controller IC has built-in calibration functionality. First, the controller IC must be set to "source data mode" to perform calibration. The PC then displays a reference point (P1) on the touch screen, the operator presses the point with the stylus and the ADC voltage information is sent to the PC via the controller board. This process is then repeated at the second point (P2) elsewhere on the touch screen. The physical coordinates of P1 and P2 are sent to the PC in the form of 8-byte numbers. Next, set the touch screen to "Calibration Data Mode" and the application software will use the voltage and coordinate readings of the two known points, plus the built-in "0,0" reference point, to calibrate all other points within the Calibration Data Mode area. Coordinate interpolation (Figure 6).
Figure 6: Because resistance changes as touch screens age, calibration is required during initial configuration and periodically thereafter. (Image source: NKK Switches)
The screen's resistance changes as the device ages, requiring repeated calibration throughout its lifetime.
Most importantly, the display frame must be grounded to prevent electromagnetic interference (EMI). In addition, the initial contact resistance of the finger may also cause "jitter". To prevent jitter, a built-in delay function can be used to allow the voltage to settle before the system calculates coordinates.
Designers must also be careful not to instruct users in the software to tap two touch screen areas simultaneously. The technology cannot distinguish between two separate contacts and defaults to the midpoint between the two points. Finally, when using the stylus to draw a line on the screen, a breakpoint appears above the isolation point in the screen that separates the two layers. Designers should ensure that the application software fills these gaps.
Resistive touch screens are suitable as HMIs in applications that require low cost, ruggedness, and operation with bare or gloved hands or with a non-conductive stylus. To simplify implementation, NKK's commercial solutions include touch screen overlays, controller boards with dedicated controller ICs, and device driver software.
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