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  • Engineering Magic: Touch through Thick Glass with LEDs and LCD

    • SSZTAZ4 August   2016 MSP430FR2633

       

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Technical Article

Engineering Magic: Touch through Thick Glass with LEDs and LCD

Giovanni Campanella

Not impressed with the magic trick revealed in the previous post? Then check out this one: We kept working on our touch-through-glass application and managed to improve it. Using an MSP430™ microcontroller (MCU) with CapTIvate™ technology (the MSP430FR2633), we were able to control light-emitting diodes (LEDs) and a liquid crystal display (LCD) upon button presses, beyond just sensing the buttons as we did with the Low-Power Touch Through Glass Reference Design (TIDA-00343). Figure 1 is a block diagram of the Touch Through Glass with LCD Reference Design (TIDA-00494).

GUID-5E105330-1A33-4F9A-AC22-45145556D0CC-low.jpg Figure 1 Block diagram for the LCD reference design (TIDA-00494)

If you’ll recall from my previous post, process plant operators need to interact daily with a keypad for programming functions. The keypad is encapsulated in an explosive-proof screw-on metallic enclosure with thick glass windows, with local readouts through an LCD (see Figure 2).

GUID-433299AA-8DF6-443A-B4AF-43C1C603F0DF-low.jpg Figure 2 TIDA-00494 Touch through Glass reference design

Factory automation and process control designers struggle to find ways for operators to interact with these explosion-proof field transmitters without having to open the enclosures, which can be time-consuming.

Let’s think about already existing technologies that could solve this issue:

  • Infrared technology detects button presses through light changes when a finger approaches the sensor. However, this technology is unreliable in industrial environments. If the glass is caked with oil or dust, a spot on the button area could cause a false touch detection.
  • A magnetic solution would entail having a magnetic switch, which closes when it detects a strong magnetic field nearby. But the user would have to have a magnetic pen on hand at all times to generate the magnetic field.

Capacitive technology solves these problems; in addition, the number of buttons does not affect the cost of the application, since adding buttons means adding only extra copper to the PCB.

We have proved the robustness of CapTIvate™ technology by performing many tests, including touch with bare fingers and touch with gloves. We have also tried to recreate false-touch situations. In each test, we took 1,000 samples while pressing a button and 1,000 samples without performing any action. Then we calculated the mean and standard deviation of each event to estimate the signal-to-noise ratio (SNR) and crosstalk among the buttons.

To ensure that the probability of a button being touched is equal to 99.73%, the SNR must be greater than 9.5dB. To ensure that a button press does not influence the nearby button, the crosstalk should be as well greater than 9.5dB. The test results meet this threshold; see Figure 3.

GUID-FB0B6EB4-0BF7-4669-8531-C8FC54EBB6DF-low.jpg Figure 3 Touch & crosstalk tests

Capacitive technology has its own limitations (which I mentioned in my previous post) related to the air gap needed between the glass and the PCB, which should be as small as possible.

The reference design can detect a touch event with or without gloves, even though the electrode board has an air gap of as much as 2mm from the 10mm-thick glass and gives feedback through LEDs and an LCD with only one chip.

Additional Resources:

  • Check out High-Temperature Touch through Glass Reference Design (TIDA-00464).
  • Read “Capacitive touch in industrial HMI” blog post on Industrial Strength.
  • Order a Sharp Memory LCD BoosterPack plug-in module.
  • Find out more about factory automation and process control.
  • Watch our demo video to see the reference design in action:

Touch Through Glass with Sharp LCD Reference Design

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