What Is The Standard Of High Quality Touch Screen Man-Machine Interface ?

In the field of industrial automation, human-machine interface (HMI) is rapidly transitioning to touch screens. However, factories have special requirements for touch screens. The high quality touch screen must meet these requirements in order to obtain an excellent (safe to read) operating experience, while increasing productivity and output. These include three requirements for water resistance, noise immunity, and advanced touch functions (such as glove touch and/or proximity sensing).

In the past ten years, the industrial automation market has undergone a transformation regarding user interfaces. Now, the interaction between people and automation equipment (now called "human-machine interface") is done through a 3.5-10 inch touch screen instead of switches and levers.

The Use Of High quality Touch Screen Will Become Inevitable

In the future, many operations and controls in the factory will be completed by handheld devices that are wirelessly connected to the machine. With the rapid adoption of touch methods on user interfaces, it has become inevitable to equip handheld devices with touch screens. In addition, considering performance requirements, touch screens will use projected capacitive technology instead of resistive technology.

High Quality Touch Screen Can Increase Productivity

Now, factories are paying more and more attention to the quality of the user interface. Today's man-machine interface is no longer a background control interface. It is a symbol of machines and processes. A poorly designed human-machine interface can cause input errors and delays. This can cause process errors and damage to equipment or products. Even cause personal injury to the operator. In the worst case, improper implementation of the touch screen can bother the operator. On the contrary, a stable human-machine interface can increase productivity, increase output, and thus bring higher profits.

Three challenges of touch screen

The factory environment will bring unique challenges to the design of man-machine interfaces and touch screens.

Water resistance

Prevent accidental touch caused by water, fingers passing through water droplets, or wet fingers. We often overlook the water resistance . But it is very important for achieving a stable and reliable user interface. Many production environments have high humidity. And operators may need to operate with their fingers or screen wet. The touch screen must work smoothly without accidental touches.

Standards For Touch Screen Water Resistance

For the waterproofness of touch screens, a number of international standards have made detailed regulations. For example, the IEC-60529 standard of the International Electrotechnical Commission (IEC) defines the protection level (IP level). Among them, the highest level that a product can reach is IP-67. In other words, it can work in environments where there is a lot of dust (dust level 6). And it can be immerse in water for 1 meter (waterproof rating is 7) without damage. In most industrial applications, water resistance is a necessary condition.

Waterproof Requirements

From the perspective of touch screen controllers, "water resistance" (applicable to various forms of liquid or conductive particles on the screen) can be further subdivided into two requirements: waterproof and wet finger tracking.

1) Waterproof:
a. Prevent the high quality touch screen from accidentally reacting to the presence of liquid.
b. The smooth operation can be continued after the liquid is wiped off the screen.

2) Wet finger tracking:
a. It can accurately sense the touch of the finger when there is liquid on the screen. Suitable for liquid film, splashed liquid or multiple droplets caused by moisture.
b. Touch the screen with sweaty or greasy fingers.

The projected capacitance senses the change in capacitance when the conductor steals charge from the metal (usually indium tin oxide (ITO)) wire grid. These grids are independent of each other and function as a sensor when there is current passing through. These metal wires are arranged into Tx (transmitting current position) and Rx (receiving current position), and a capacitance is formed between the Tx wire and the Rx wire.

There Are Two Forms Of Projected Capacitance

(1) Self-capacitance sensing: Detect the charge changes on the rows and columns (X+Y) of the sensor grid. The charge change of a specific row can be attributed to the charge change of multiple columns, and self-capacitance sensing is suitable for single-touch applications.
(2) Mutual capacitance sensing: Detect the charge change (X*Y) at each interaction point of the grid. Therefore, it can accurately sense multi-touch.

The manifestation of finger touch action in self-capacitance and mutual-capacitance sensing modes is completely different. In the self-capacitance mode, a single touch will appear as an increase in current after the charge is transferred to the ground; and in the mutual capacitance mode, the detection result of the touch is that the overall mutual capacitance between the two sensors at the intersection decreases.

Water as a conductor will increase the marginal electric field between adjacent sensors and increase the capacitance. This may cause the touch screen to recognize water as a light-finger touch in self-capacitance mode. This can be solved by sensing the duplicate electric field in the adjacent sensor, thereby effectively eliminating the marginal electric field generated between the adjacent sensors. However, self-capacitance does not support multi-touch.

Mutual Capacitance And Self Capacitance

In the mutual-capacitance grid, the form of water is the same, but it is perceived as an increase in charge, and the polarity is opposite to the effect of finger touch. In this way, when wiping the water on the screen, it may be recorded by the sensor as an accidental finger touch.

The combination of self-capacitance sensing and mutual-capacitance sensing (as implemented in the Cypress TrueTouch controller) can provide a stable and reliable waterproof solution. It is also very important to be able to switch between the Tx and Rx lines to accurately grasp the contour of the droplet.

When the screen is with a layer of water film or large water droplets. The effect may be similar to that of large objects such as thumbs or palms (depending on the size of the water droplets/film). A special algorithm is required to accurately determine the position of the water body and track the movement of the finger.

Immunity

Immunity: Provide a seamless touch experience to prevent false touches under extreme interference pulses. For high quality touch screens, there are generally two sources of interference:
1) Direct coupling interference: This interference comes from neighboring machines, high-voltage alternating current and electronic ballasts of energy-saving lamps. These interferences all exist in the manufacturing plant and it will couple to the human body and injecte into the system through finger touch.
2) Common mode interference: This interference comes from the inside of the touch screen device (such as power supply, poor quality charger) and fingers will release it to the ground.

Interference includes broadband and narrowband noise, usually with high amplitude. We see that the frequency of common mode interference can reach up to 500kHz, and the amplitude can reach up to 40Vpp.

In both cases, users will see false touches. Including reporting the wrong touch coordinates or causing the touch sensor to overload. (The touch will appear as a long line extending along the Rx sensor). This will cause the pipeline to receive incorrect instructions and cause delays. In many cases, the interference pulse will fill the receiving capacitor. As a result, the touch signal that should have been recorded at the intersection is missed and affects the overall touch experience. A good signal-to-noise ratio (SNR) is one of the necessary conditions for the touch screen controller to resist various interferences.

Ways To Resist Interference

a) Increasing Tx voltage: One of the most effective ways to increase SNR is to increase the signal voltage. This is a simple and effective way to improve SNR. Some Cypress Semiconductor touch screen controllers will provide built-in 10VTx to improve SNR while avoiding additional material costs.
b) Frequency hopping: The Rx channel can dynamically change the frequency to avoid interference waves and their harmonics. In an environment with strong interference, it must enable frequency hopping . And the touch screen controller must have a built-in special algorithm to continuously skip the interference frequency.

In addition to the above methods, there are many interference suppression techniques. Some of these new methods can effectively prevent channel saturation, and at the same time use the windowing method implemented by the on-chip DSP to effectively restore the signal.

Advanced Touch Technology:

The advanced touch technology allows the operation of wearing gloves, and can detect the fingers close to the screen. For mobile phones, conductive gloves for detecting touch already exist on the market. In factories, this solution is not effective because operators may need to wear special gloves when operating other machines. It would be very inconvenient to require the operator to take off his gloves to operate the touch screen.

For the host CPU, there is no difference between wearing gloves and touching it lightly with your fingers. Therefore, it can improvde the sensitivity of the touch screen controller . And it can also reduce the recording threshold of finger touch .

Cause The Following Problems:

a) it will detect a floating finger as a touch, which is not the user's intention.
b) Common mode interference may trigger false touches.
c) The thickness of the glove is different, and its performance will be different.

In addition to glove touch, the touch screen controller may need to sense the approaching finger within a distance of 24-30 mm. This needs to trigger a liquid crystal display (LCD) startup event to get the best user experience.

We can use different sensing methods, special algorithms, touch screen fine-tuning or a combination of these methods to achieve various advanced touch functions.

With the widespread use of high quality touch screen in man-machine interfaces, the requirements for touch screen controllers will also be constantly changing specifically for this market. Industrial users expect their touch screens to work under various interference conditions and conductive materials such as water and gloves. A touch screen design that can meet these requirements can ensure a good user experience and increase worker productivity. Thereby increasing the overall output of the factory.

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