In the field of industrial automation, Human-Machine Interfaces (HMIs) are rapidly transitioning to touchscreens. However, factories have specific requirements for touchscreens to achieve a superior (safely readable) user experience while improving productivity and output. These requirements include waterproofing, interference immunity, and advanced touch functionality such as glove touch and/or proximity sensing.
According to an IMS Research market research report, the industrial automation market is projected to grow from $159 billion in 2012 to $200 billion in 2015, as more countries look to drive economic growth through manufacturing.
Over the past decade, the industrial automation market has undergone a transformation in user interfaces. Now, interactions between people and automated equipment (now known as "human-machine interfaces") are accomplished through 3.5- to 10-inch touchscreens, rather than switches and levers.
Frost & Sullivan anticipates that much of the operational control in factories will be accomplished through handheld devices that are wirelessly connected to machines in the future. With the rapid adoption of touchscreens in user interfaces, equipping handheld devices with touchscreens has become essential. Furthermore, considering performance requirements, touchscreens will use projected capacitive technology instead of resistive technology.
Today, factories are increasingly focused on the quality of user interfaces. Human-machine interfaces (HMIs) are no longer just back-end control screens; they are symbols of machines and processes. Poorly designed HMIs can lead to input errors and delays, resulting in process failures, equipment or product damage, and even personal injury to operators. In the worst-case scenario, improperly implemented touchscreens can confuse operators. Conversely, stable HMIs can improve productivity, increase output, and ultimately lead to higher profits.
The factory environment presents unique challenges for the design of human-machine interfaces and touchscreens. Three major challenges include:
1) Waterproof: Prevents accidental touch caused by water, fingers passing through water droplets or wet fingers.
2) Anti-interference: Provides a seamless touch experience and prevents accidental touches under extreme interference pulses.
3) Advanced touch technology: Allows operation while wearing gloves and can detect fingers near the screen.
1. Water resistance
Water resistance is often overlooked, but it is crucial for achieving a stable and reliable user interface. Many production environments are highly humid, and operators may need to work with wet fingers or the screen. The touchscreen must function smoothly and prevent accidental touches.
Several international standards have detailed regulations regarding the waterproofing of touchscreens. For example, the International Electrotechnical Commission (IEC) standard IEC-60529 defines the protection rating (IP rating). The highest rating a product can achieve is IP-67, meaning it can operate in environments with high levels of dust (dust rating 6) and can be submerged in water up to 1 meter deep (waterproof rating 7) without damage. Water resistance is a necessary requirement in most industrial applications.
From the perspective of touchscreen controllers, "water resistance" (applicable to various forms of liquids or conductive particles on the screen) can be further subdivided into two requirements: water resistance and wet finger tracking functionality.
1) Waterproof:
a. To prevent the touchscreen from reacting unexpectedly to the presence of liquid.
b. You can continue to operate smoothly after wiping the liquid off the screen.
2) Wet finger tracking:
a. Accurately detects finger touch even when liquid is present on the screen. Suitable for liquid films, splashes, or multiple droplets caused by moisture.
b. Touch the screen with sweaty or oily fingers.
Projected capacitance senses changes in capacitance as a conductor steals charge from a grid of metal (typically indium tin oxide (ITO)) wires. These grids are independent of each other and act as sensors when current flows through them. These metal wires are arranged as Tx (current-emitting positions) and Rx (current-receiving positions), and capacitance is formed between the Tx and Rx wires.
Projected capacitance exists in two forms.
(1) Self-capacitance sensing: detects changes in charge on rows and columns (X+Y) of the sensor grid. A change in charge in a particular row can be attributed to changes in charge in multiple columns. Self-capacitance sensing is suitable for single-point touch applications.
(2) Mutual capacitance sensing: Detects the charge change (X*Y) at each interaction point of the grid. Therefore, it can accurately sense multi-touch. See Figure 1.
Figure 1: The self-capacitance sensor grid is shown on the left, and the mutual capacitance grid is shown on the right. Due to the difference in scanning methods, the self-capacitance grid cannot reliably track multiple fingers on the screen, while the mutual capacitance grid can.
Finger touch actions behave very differently in self-capacitance and mutual capacitance sensing modes. In self-capacitance mode, a single touch will result in an increase in current after the charge is transferred to the ground; while in mutual capacitance mode, the touch detection result is a decrease in the overall mutual capacitance between the two sensors at the intersection.
Water, as a conductor, enhances the marginal electric field between proximity sensors and increases capacitance. This can cause a touchscreen to recognize water as a light finger touch in self-capacitance mode. This can be addressed by sensing a replicated electric field in the proximity sensors, effectively eliminating the marginal electric field generated between them. However, self-capacitance does not support multi-touch.
In a mutual capacitance grid, water behaves in the same way, but is perceived as an increase in charge, with the polarity opposite to that of a finger touch. Thus, wiping water off the screen might be recorded by the sensor as a mis-touch.
The combination of self-capacitance sensing and mutual capacitance sensing (as implemented in the Cypress TrueTouch controller) provides a stable and reliable waterproof solution. The ability to switch between Tx and Rx lines to accurately determine the water droplet profile is also crucial.
When a film or large droplet of water is placed on the screen, the effect can be similar to that of a large object such as a thumb or palm (depending on the size of the droplet/film). Special algorithms are needed to precisely determine the position of the water and track finger movements.
2. Interference immunity
For touchscreens, there are generally two sources of interference:
1) Direct coupling interference: This type of interference originates from nearby machines, high-voltage AC power, and the electronic ballasts of energy-saving lamps. These interferences exist in the manufacturing plant and can couple into the human body and be injected into the system via finger touch.
2) Common-mode interference: This interference originates from inside the touchscreen device (such as the power supply or a poor-quality charger) and is released to the ground through the fingers.
Interference includes both broadband and narrowband noise, typically with high amplitude. We observed that common-mode interference can reach frequencies as high as 500 kHz and amplitudes as high as 40 Vpp. (See Figure 2)
Figure 2: Interference diagram of narrowband and broadband chargers
In both cases, users will see accidental touches; this includes reporting the coordinates of the accidental touch or overloading the touch sensor (the touch will appear as a long line extending along the Rx sensor). This causes the pipeline to receive incorrect instructions and causes delays. In many cases, interference pulses can fill the receiving capacitor, thus missing the touch signal that should have been recorded at that intersection and affecting the overall touch experience. A good signal-to-noise ratio (SNR) is one of the necessary conditions for a touchscreen controller to resist various interferences.
Interference can be resisted in various ways.
a) Increasing the Tx voltage: One of the most effective ways to improve SNR is to increase the signal voltage. This is a simple and effective way to improve SNR. Some Cypress Semiconductor touchscreen controllers offer a built-in 10VTx to improve SNR while avoiding additional material costs.
b) Frequency hopping: The Rx channel can dynamically change its frequency to avoid interference waves and their harmonics. In environments with strong interference, frequency hopping must be enabled, and the touchscreen controller must have a built-in special algorithm to continuously skip interference frequencies.
Besides the methods mentioned above, there are many other interference suppression techniques. Some of these newer methods can effectively prevent channel saturation, while others utilize windowing techniques implemented on-chip DSPs to effectively recover the signal.
3. Advanced touch technology: gloves and proximity sensing
For mobile phones, conductive gloves for touch detection are already available on the market. However, this solution is not effective in factories because operators may need to wear special gloves when operating other machines. Requiring operators to remove their gloves before operating the touchscreen would be inconvenient.
For the host CPU, there is no difference between touching the screen with gloves on and lightly touching it with a finger. Therefore, the sensitivity of the touchscreen controller can be increased, and the threshold for recording finger touches can be lowered. However, this may lead to the following problems:
a) A suspended finger may be detected as a touch, even if it is not the user's intention.
b) Common-mode interference may trigger accidental touches.
c) The performance of gloves varies depending on their thickness.
In addition to glove-based touch control, touchscreen controllers may need to detect approaching fingers within a distance of 24-30 millimeters. This requires triggering an LCD startup event for optimal user experience.
We can use different sensing methods, special algorithms, touchscreen fine-tuning, or a combination of these methods to achieve various advanced touch functions.
With the widespread use of touchscreens in human-machine interfaces, the requirements for touchscreen controllers are constantly evolving to meet the specific needs of this market. Industrial users expect their touchscreens to operate under various interference conditions and in the presence of conductive materials such as water and gloves. Touchscreen designs that meet these requirements can ensure a good user experience and improve worker productivity, thereby increasing overall factory output.
