Industrial harsh environment system design encounters numerous challenges as the system is anticipated to function consistently in water, dust, mist, ice, humidity changes, moisture, high/low temperatures, at high speeds, and with gloved hands. The beauty is that capacitive touchscreen applications are designed to effectively endure each of these challenging elements.
Capacitive Liquid and Water Tolerance Considerations
Water tolerance and liquid tolerance is a crucial requirement for industrial capacitive touchscreen monitor systems operating in industrial and commercial environments with liquid exposure (water, bodily fluids, chemicals, etc.).
Water/liquid tolerance safeguards two important considerations:
- The capacity to sense and ignore water or liquids to avert a false touch signal
- The capacity to perceive human touch when water or liquids are present.
This requires special attention to design since a simple commercial capacitive touch screen display system might register a false touch if water or liquids are present. Essentially, there are two methods to tackle liquid-tolerant design requirements.
- Implement a shield electrode as this ensures the dependable detection of touch ON / OFF in the presence of liquid or water droplets.
- Implement a guard sensor that detects the flow of liquids or water and disables additional system sensors in order to prevent false touches.
Capacitive Shield Electrodes
The driven-shield signal is an isolated version of the sensor-switching signal. As such, the driven-shield signal has an amplitude, frequency, and phase that matches the sensor-switching signal.
This buffer delivers sufficient current for the driven-shield signal in order to drive the high parasitic capacitance of the hatch fill on the PCB. As the hatch fill near the sensor is linked to the driven-shield signal, this is called the shield electrode. When the shield electrode is powered with voltage just like the sensor-switching signal, then the capacitance supplied by a single drop of liquid when it contacts the touch surface will be nullified.
Capacitive Guard Sensors
Shield electrodes are efficient ways to manage small water droplets, but the effect of the shield electrodes are reduced or completely nulled by streams of water.
A guard sensor is used in scenarios involving larger quantities of liquids. In capacitive technology, a guard sensor is a dummy sensor of copper trace amidst the actual system sensors. The copper trace dummy sensor senses larger water or liquid flows and sends this data to the CPU.
When a stream of water is detected, the firmware logic in the CPU restricts the actual system sensors in order to avert registering a false touch. The major setback involved in the process is that the touch of a finger will not be sensed while the water stream exists.
Capacitive Resistance to Extreme Temperatures
Industrial capacitive touch screen monitor designs must function in harsh environments with extreme temperatures. Capacitive sensing circuits tend to be sensitive to environmental factors including temperature, and humidity changes. The ‘raw count’, which is the digital equivalent of input capacitance, typically differs due to shifts in environmental conditions. At times this may trigger false touch data.
In order to ensure reliable operation, effective compensation for environmental conditions is required. Generally speaking, compensation is applied in both hardware and firmware. The hardware compensation is what automatically calibrates sensing circuits by changing the input current depending upon temperature changes in order to maintain the output raw count constant.
On the other hand, firmware compensation manages any run-time temperature changes. A sensor’s raw count value might fluctuate slowly if there are environmental changes like extreme temperatures or humidity. Consequently, the raw count value is low-pass filtered in firmware so as to create a new count value which is referred to as a baseline that tracks and compensates for any gradual changes in the raw count value.
This renders the baseline less sensitive to any sudden changes in the raw count that can be caused by a registered touch. Hence, the baseline value offers a reference level for calculating the signals. Reliable operations of capacitive touch sensors in harsh conditions are ensured by these techniques.
faytech North America is one of the leading manufacturers of commercial capacitive monitors. This technology can be applied in a broad spectrum of standard-use and harsh environment applications.
If you have any questions or concerns about using capacitive displays in your industry or upgrading existing displays, then contact us today. Our capacitive technology experts can help you make the best decision based upon your industry, use case scenarios and budget.