How much do you know about the standard of touch screen HMI in the field of industrial automation?
In the field of industrial automation, the human-machine interface (HMI) is rapidly transitioning to touch screens. However, the factory has special requirements for touch screens, which must meet these requirements in order to obtain an excellent (safe to read) operating experience while increasing productivity and yield. These include three requirements of water resistance, noise immunity and advanced touch functions such as glove touch and/or proximity sensing.
In the past decade, the industrial automation market has experienced a change in the user interface. Nowadays, the interaction between people and automation equipment (now called "human-machine interface") is accomplished through a 3.5-10 inch touch screen instead of switches and levers.
Frost & Sullivan anticipates that much of the operational control in the factory will be accomplished through handheld devices that are wirelessly connected to the machine. With the rapid adoption of touch methods on the user interface, it has become inevitable to provide touch screens for handheld devices. In addition, considering the performance requirements, touch screens will use projected capacitive technology rather than resistive technology.
Nowadays, various factories are paying more and more attention to the quality of the user interface. Today's human-machine interface is no longer a background control interface, but a symbol of the machine and process. A poorly designed human-machine interface can cause input errors and delays, which can result in process errors and damage to equipment or products, and even cause personal injury to operators. In the worst case, the improper implementation of the touch screen will plague the operator. In contrast, a stable man-machine interface can increase productivity, increase production, and lead to higher profits.
The factory environment will bring unique challenges to the design of human-machine interfaces and touch screens. Three of these challenges include:
1) Waterproof: Prevent accidental touch caused by water, fingers passing through water droplets or wet fingers.
2) Immunity: Provide a seamless touch experience to prevent inadvertent touch under extreme interference pulses.
3) Advanced Touch Technology: Allows gloves to be operated and can detect fingers near the screen.
1. Waterproof
Water resistance is often overlooked, but it is important for a stable, reliable user interface. Many production environments have high humidity and operators may need to operate with water on their fingers or the screen. The touch screen must work smoothly and will not be mistakenly touched.
For the waterproofness of touch screens, a number of international standards have been detailed. For example, the IEC-60529 standard of the International Electrotechnical Commission (IEC) defines the degree of protection (IP class). Among them, a product can reach the highest level of IP-67, that is, it can work in the presence of a large amount of dust (dust level 6), and can be immersed in water for 1 meter (waterproof rating of 7) It will be damaged. Water resistance is a necessary condition in most industrial applications.
From a touchscreen controller's point of view, “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 touch screen from reacting unexpectedly to the presence of liquids.
b. After the liquid is wiped from the screen, smooth operation can continue.
2) Wet finger tracking:
a. When the liquid exists on the screen, the touch of the finger can be accurately sensed. Suitable for liquid film due to moisture, splashed liquid or multiple drops.
b. Touch the screen with a sweaty or greasy finger.
The projected capacitance senses the change in capacitance as the conductor steals charge from a metal (usually indium tin oxide (ITO)) wire grid. These grids are independent of each other and act as sensors when current passes through them. These metal lines are arranged as Tx (emission current position) and Rx (receiving current position), and a capacitance is formed between the Tx line and the Rx line.
There are two forms of projected capacitance.
(1) Self-capacitance sensing: Detects changes in the charge on the row and column (X+Y) of the sensor grid. The change in the charge of a particular row can be attributed to the charge variation of multiple columns, and self-capacitance sensing is suitable for single-touch applications.
(2) Mutual Capacitive Sensing: Detects the charge change (X*Y) at each interaction point on the grid. Therefore, it can accurately sense multi-touch. The mutual capacitance grid is shown on the right. Due to the difference in the scanning method, the self-capacitance grid cannot reliably track multiple fingers on the screen, and the mutual capacitance grid can.
The finger touch action manifests itself differently in self-capacitance and mutual-capacitance sensing modes. In the self-capacitance mode, a single touch will show an increase in current after the charge is transferred to the ground. In the mutual capacitance mode, the touch detection results in a decrease in the overall mutual capacitance between the two sensors at the intersection.
Water as a conductor enhances the marginal electric field between the proximity sensors and increases 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 replica electric field in the proximity sensor, effectively eliminating the marginal electric field generated between the proximity sensors. However, self-capacitance does not support multi-touch.
In a mutual-capacitance grid, water behaves in the same way, but it is perceived as an increase in charge, which is the opposite of the polarity of the finger-touch effect. In this way, when the water on the screen is wiped, it may be recorded by the sensor as a wrong finger touch.
The combination of self-capacitance sensing and mutual-capacitance sensing (as implemented in the Cypress TrueTouch controllers) provides a stable and reliable waterproof solution. It is also important to be able to switch between the Tx and Rx lines to accurately grasp the contour of the drop.
When the screen is covered with a film of water or large water droplets, it may produce effects similar to large objects such as the thumb or the palm (depending on the size of the water droplets/film). Need to use a special algorithm to accurately determine the position of the water body and track the movement of the finger.
2. Immunity
For touch screens, there are generally two sources of interference:
1) Direct coupling interference: This interference comes from the adjacent machines, high-voltage alternating current, and electronic ballasts of energy-saving lamps. These disturbances are present in the manufacturing plant and can be coupled into the human body and injected into the system by finger touch.
2) Common Mode Interference: This interference comes from inside the touch screen device (such as a power supply, poor quality charger) and is released to the ground by a finger.
Interference includes wideband and narrowband noise and usually has a higher amplitude. We see that the frequency of common-mode interference can reach up to 500kHz and the amplitude can reach 40Vpp.
In both cases, the user will see the situation of inadvertent contact; this includes reporting mis-coordinates or overloading the touch sensor (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 fills the receiving capacitor, thus missing the touch signal that should have been recorded at the intersection and affecting 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 types of interference. There are many ways to resist interference.
a) Increase Tx voltage: One of the most effective ways to increase SNR is to increase the signal voltage. This is a simple and effective SNR improvement. Some Cypress Semiconductor touchscreen controllers provide built-in 10VTx to increase SNR while avoiding additional material costs.
b) Frequency hopping: Rx channels can dynamically change frequencies to avoid interfering waves and their harmonics. In environments with strong interference, frequency hopping must be enabled and the touchscreen controller must have special algorithms built-in to constantly skip the interfering frequencies.
In addition to the above methods, there are many interference suppression techniques. Some of these new methods can effectively prevent channel saturation, while using the on-chip DSP windowing method to effectively restore the signal.
3. Advanced Touch Technology: Gloves and Proximity Sensors
For mobile phones, conductive gloves for detecting touch have existed in the market. In the factory, this solution is not effective because the operator may need to wear special gloves when operating other machines. It is inconvenient to require the operator to take off his gloves and operate the touch screen.
For the host CPU, it is no different to wear a glove touch and touch with your finger. Therefore, the sensitivity of the touch screen controller can be improved, and the recording threshold of the finger touch can be reduced. However, this may bring the following problems:
a) An unoccupied finger may be detected as a touch, which is not the user's intention.
b) Common mode interference may trigger a false touch.
c) Different gloves have different thicknesses and their performance will vary.
In addition to glove touch, the touch screen controller may need to sense the approaching finger within 24-30 mm. This requires triggering a liquid crystal display (LCD) startup event for the best user experience.
We can use various sensing methods, special algorithms, touch screen trimming, or comprehensive use of these methods to achieve various advanced touch functions.
With the wide use of touch screens in man-machine interfaces, the requirements for touch screen controllers must also be constantly changing for this market. Industrial users want their touch screens to work under a variety of interference conditions as well as conductive materials such as water and gloves. A touch screen design that meets these requirements ensures a good user experience and increases the productivity of workers, thereby increasing the overall output of the plant.
