For reasons such as aesthetics, maintenance, cost, and hygiene, touch
screen technology is beginning to penetrate into the medical, industrial, and
automotive markets beyond the consumer market. With the advent of touch screens,
a number of touch technologies have emerged, such as capacitive, resistive,
inductive, surface acoustic wave and infrared touch technologies. Each design
technique has its own pros and cons. Capacitive touch screens are based on
electrode designs on printed circuit boards and are popular among users for
their touch keys, sliders and scroll wheels. The easy touch functionality adds a
lot to the user experience. Surface acoustic wave touch technology is based on
sound waves and is found in display designs that require transparency, such as
entertainment parks and high-traffic indoor environments. Infrared touch
technology is based on the light interruption method and is mainly used for
low-resolution ultra-large screens. Inductive touch screen technology is
primarily used on panels made of plastic, aluminum or stainless steel, or panels
that will be exposed to liquids. Among them, resistive touch screen technology
is the most cost-competitive and can be easily integrated into embedded designs.
This technology is mainly used to design touch screens with panel sizes no
larger than 19 inches. Support for finger touch detection and stylus detection
expands the application scope of resistive touch technology in consumer
electronics (see Figure 1).
Figure 1: Finger and stylus detection makes resistive touch screens better
to use
This article will mainly discuss the characteristics of resistive touch
screen technology, issues that should be paid attention to during the design
process, and potential application areas. Understand electronic touch sensor
design and controller selection requirements
Because resistive touch screens are now readily available and prices have
dropped over time, the technology is becoming more widely used. In order to
select the best touch screen technology, application designers must deeply
consider the application requirements. Resistive touch screen technology
requires only a simple printed circuit board design, unlike capacitive and
inductive touch screen technologies, which require electrodes or coil etching on
the printed circuit board. Because the touch screen overlays directly on the
display, it saves PCB space required for mechanical switches or capacitive touch
key electrodes. It is not recommended to use resistive touch screens in harsh
environments, such as mining areas or construction sites with frequent
explosions or excessive dust. Small amounts of breakage on resistive touch
screens can affect touch accuracy and linearity.
How resistive touch screens work
1. A resistive touch screen is a transparent glass plate covered with a
touch-responsive film.
2. The resistive touch screen panel consists of two resistive layers
(indium tin oxide) with a thin separation layer in the middle.
3. The two film layers of the resistive touch screen form a resistor
network, which acts as a voltage dividing circuit for the touch position
detection function.
4. The touch screen will cause a voltage change on the voltage divider
composed of a resistor network. This voltage is used to determine the contact
position of the touch screen.
5. The touch screen controller (TSC) converts the captured analog voltage
signal into a digital touch coordinate signal. Built-in analog-to-digital
conversion channel acts as a voltmeter for measuring analog voltage.
6. After touching the screen, the touch controller, which functions as a
voltmeter, first applies the voltage gradient VDD at the X+ point and the ground
voltage GND at the X- point. Then, the analog voltage on the Y-axis resistor is
detected, converted into a numerical value, and the X coordinate is calculated
using an analog-to-digital converter (Figure 2). In this case, the Y-axis
becomes the induction line. Likewise, the Y coordinate can be measured by
applying a voltage gradient at the Y+ and Y- points.
7. Some touch controllers also support touch pressure measurement, that is,
Z-axis measurement. When measuring the Z-axis coordinate, voltage gradients are
applied to the Y+ and X-axes.
Figure 2: Resistive touch screen: X coordinate measurement:
Resistive touch comes in two main forms: software haptic solutions and
dedicated touchscreen controller chips.
In software haptic solutions, the microcontroller must be responsible for
all touch detection and coordinate calculation tasks. The microcontroller-based
software algorithm uses the internal microcontroller to perform touch position
voltage measurement and perform touch detection functions and coordinate
processing functions.
In the dedicated touch screen controller, the controller initiates an
interrupt request to detect the touch event to the system host (microcontroller)
and outputs digital data representing the touch coordinates. Then the main
processor (MCU) reads the digital data and executes the operation commands
expected by the customer.
The design method based on MCU calculation parameters requires the main
processor to be very fast, only in this way can frequent touch operations be
managed. This is not a very reliable design for fast touch detection
applications. Because there is no data averaging and touch detection delay
functions, the detection accuracy of this type of design is relatively low. A
dedicated touch screen controller chip with data sampling, measurement
averaging, touch detection delay configuration and digital touch coordinate
calculation functions is a true touch screen controller. These chips are easy to
integrate into product designs and offer higher performance.
Resistive touch screen classification
According to the number of sensing lines on the touch screen, resistive
touch screens can be further divided into three categories: 4-wire, 5-wire and
8-wire. The strip electrodes of the 4-wire touch screen are installed on two
different resistive layers (X+, X- on the same layer, Y+, Y- on another
resistive layer). 5-wire touch screens only have circular electrodes (X+, X-,
Y+, and Y-) on the bottom layer. The top layer is used to measure voltage during
touch, and the voltage gradient is only applied to the bottom layer.
The working principle of 8-wire touch screen is similar to that of 4-wire
touch screen. Just add a reference voltage line to each line, so the final total
number of lines reaches 8. The newly added 4 lines are used to provide reference
voltages to the original 4 lines. The 8-wire touch screen uses the measurement
principle of a proportional measurement analog-to-digital converter.
Because of their low cost and simple touch sensing algorithm, 4-wire touch
screens are widely used in low-end consumer electronics products. 5-wire and
8-wire touch screens are mainly used in expensive high-end medical equipment and
important industrial controllers.
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