In industrial applications, mechanical buttons on sensor nodes, industrial meters, and control panels are prone to dirt and grime accumulation that eventually cause their failure. Human-machine interface (HMI) systems that utilize capacitive touch mechanisms to replace mechanical buttons in industrial environments provide advantages such as sleeker designs, easy to clean and less prone to mechanical failure. The most recent capacitive touch technology goes farther in addressing some of the toughest challenges in industrial HMI.
Noise immunity challenges
Motors, relays and switches operating in an industrial environment inject significant noise into the power line. These noise sources can false trigger a device as the fluctuations in the measurement signal cross the detection threshold. Pictures below show how capacitive measurements are affected in the presence of noise.
With unique features such as multi-frequency scanning and processing, spread spectrum modulation, zero crossing detection built in hardware; buttons built with capacitive technology can overcome the effects of noise in the system. Below a picture shows a noisy signal processed with the aforementioned techniques
Thick overlay and glove challenges
In order to protect electronics from hazardous and dirty environments, industrial panels often have a thick glass or plastic overlay. It is usually not possible to co-locate a mechanical button without drilling through the overlay making it more prone to fail. Industrial environments also often require machine operation while wearing gloves. While this is less of a problem with mechanical buttons, this can be a major issue with capacitive touch solutions.
Capacitive buttons can work through up to 60 mm thick glass and 25 mm thick plastic overlays allowing for use in sterile as well as hazardous environments while keeping the overlay material intact.
Capacitive buttons rely on change in capacitance to understand a command, when a grounded object— human hand— comes into the electric-field of a touch sensor that detection is triggered. Wearing industrial gloves adds a layer of insulating material (dielectric) between a person’s hands and the electrode thereby leading to unreliable detection.
Capacitive touch can also be made to work with gloves by using metal overlays. Rather than measuring the change in capacitance due to a hand coming into the e-field, It measures the change in capacitance due to deflection of a grounded metal overlay. This allows for the technology to support glove friendly designs.
Low-power operation challenges
Several field transmitters in industrial systems are powered by current loop, which offer a very limited power budget. With power consumption <4 uA while maintaining ability to detect up to four buttons, capacitive based technology can offer industrial customers the ability to program and monitor devices on-site with an extremely low-power budget.
Solutions
Developing with MSP430™ microcontroller (MCU) CapTIvate™ technology can help prevent many of these challenges. CapTIvate touch MCU not only offers a compact easy to maintain alternative to mechanical buttons in an industrial HMI system, it also solves many of the problems in existing capacitive touch based solutions such as robustness under noisy environments, IEC certified solutions making it glove friendly and low power.
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