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Distinguishing signal from noise

Capacitive touchscreens are constructed using an orthogonal grid of transmitter and receiver (or ‘drive’ and ‘sense’) electrodes which are usually formed on either side of a transparent substrate and behind a transparent touch surface such as glass.

In the steady state, the capacitance between transmitter and receiver is more or less uniform across the surface. When a finger (or other conductive object) approaches the surface, there is a small change in capacitance at that point. Measurement of the resulting change at the affected nodes allows a finger position to be determined.

The magnitude of the change is referred to as the touch delta. In an ideal system, the threshold at which a touch event is reliably confirmed by the system is set as close to the background measurement level as possible.

However, in real-world applications, two competing factors can affect the ability to set a low touch threshold:

  • the size of the signal generated by the user’s finger or other input device such as a conductive stylus.
  • electrical noise in the system which contaminates the signal. The problem for touchscreen system design is that, in touching the screen’s surface, the user couples noise into the electrodes.

Sources of ambient noise can include electro-magnetic emissions from devices such as radios and light fixtures. The display itself can also be a source of significant noise which may be coupled to the sensor’s electrodes.

In the touchscreen of a consumer device adequate system performance is assured because the signal is rarely compromised in conditions of normal use.

In automotive and industrial touchscreens, the operating conditions can be more difficult – and calls for a touch-sensing system with the ability to distinguish signal from noise.

The ability to reliably sense a small touch delta matters much more in industrial and automotive applications than in consumer devices. This is because of the way touchscreens are used and the operating environments in which they are used tend to reduce the signal generated by the user’s finger, therefore reducing the touch delta.

These factors include: the wearing of gloves; thick protective cover material and the use of sculpted and contoured surfaces to enable users of the display UI to position their fingers more accurately; and the use of a hover or proximity function.

These features have been difficult to implement because all entail the detection of a much smaller touch delta than would normally be the case – a finger hovering in the air a few millimetres from the surface of the screen draws around a thousand times less change in capacitance than when it comes into contact with the screen.

While the size of the touch signal is much reduced – the amplitude of noise is often greater than in consumer applications, and can occur over a broader range of frequencies.

Industrial touchscreens are often used in proximity to motors and other high-voltage equipment that generate vast amounts of EMI.

When the touch delta signal is smaller and the noise is greater, requirements for high reliability are at risk of being compromised: this must be avoided, so that valid touch events are always detected, and noise-induced false touch events are rejected.

The benefit of a high SNR

Repurposed consumer touchscreen controller ICs can struggle to meet these requirements. This is because they typically have a signal-to-noise ratio (SNR) of 50-55dB, which is enough to provide reliable touch detection in a mobile handset touchscreen, but which is inadequate in the high noise/small signal environment of automotive or industrial touchscreens.

The obvious solution for systems based on this kind of touchscreen controller is to boost the amplitude of the signal by raising the drive voltage supplied to the sensing elements transmitter electrodes: in general, the higher the drive voltage, the greater the touch delta. Touchscreen controller manufacturers have consequently modified their devices to operate at drive voltages above the 3V level commonly used in consumer devices, up to much higher levels of 30V or even as much as 40V for use in automotive and industrial applications.

Increasing the drive voltage, however, has two profound drawbacks. First, it generates much higher radiated emissions, which can undermine EMC compliance efforts. The harmful effect of the touchscreen emissions is amplified because conventional drive signals often have square waveforms that generate complex harmonic content. For typical capacitive touchscreen elements, the measurements are usually made at a frequency between 50kHz and 500kHz. The complex emissions from a 30-40V touchscreen driver at these frequencies are strong enough to badly impair the audio quality of media systems in the vehicle. In medical systems, high emissions can lead to interference with the sensitive instruments which are used in a healthcare setting.

The second drawback of a 30-40V drive circuit is that it can accelerate a familiar process of corrosion of traces at the edge of the display assembly. The metals (especially silver or copper) commonly used here can corrode or migrate when exposed to high temperatures and high humidity and in the presence of a voltage difference from that of a neighbouring conductor. At a pulsed drive voltage of 30V, the local field strength is a huge 3MV/m during the pulse and 0V/m when inactive while the gap between traces is as small as 10µm.

Over time, electro-migration can form small conductive dendrites between traces which eventually short the touch sensor channels and cause the touchscreen to fail prematurely (Figure 1). This is true for sensors that are fabricated on glass or plastic substrates. Copper traces are prone to the same effect, but at a slower rate than silver.

The dielectric materials in the sensor stack, such as the adhesives, are also subject to damage from the high electric field strengths and can begin to degrade optically as microscopic bubbles form, reducing the lifetime of the product.

Figure 1: Silver dendrites between display traces at 18V drive after a standard test cycle (504h, 60°C, 90% RH)

Low, neutral drive voltage

A much better way to solve the problem of detecting small signals in a high noise environment is to maintain a low drive voltage but to increase the touchscreen controller’s SNR. The touch-sensing performance achievable with a high-SNR touchscreen controller has recently been demonstrated in a reference design system, from TouchNetix, intended for deployment in industrial or automotive applications.

The demonstration touchscreen design is based on the aXiom AX310 touchscreen controller, which has an SNR of 80dB.

In the aXiom family of controllers, TouchNetix has made use of sophisticated narrowband transmission techniques little used previously in touchscreen systems. Allied to noise mitigation technology and a sophisticated DSP engine, the aXiom controller’s sensing architecture and analogue front end are so sensitive that they can recover the carrier even when it is thousands of times smaller than the interference.

The aXiom chip’s high sensitivity enables it to operate with a sinusoidal drive waveform at a low drive voltage of 2.5Vpk-pk with an overall neutral bias. This generates extremely low levels of electromagnetic emissions, enabling OEMs to easily meet CISPR25 Level 3 specifications for radiated emissions without implementing expensive noise counter-measures. The low drive voltage also dramatically reduces the risk of early touchscreen failure due to materials degradation in hot and humid environments.

Because of the high SNR, even with this very low drive voltage, excellent touchscreen performance is possible: the TouchNetix reference design demonstrates the ability to reliably detect finger touches through acrylic overlay more than 10mm thick, and even through screen assemblies with a small air gap.

Hover capability is equally impressive: the AX310 can detect a pre-contact target above the screen surface at a distance of more than 80mm. In contrast to existing methods for hover detection, the aXiom system requires no extra electrodes, and no extra edge margin is occupied around the outside edge of the touch panel.

The availability of the aXiom high-SNR touchscreen controller opens up new possibilities in display UI design, adding huge value to the touchscreen in vehicles and industrial equipment. Sculpted overlays and hover and zoom functions make the touchscreen much easier to use and, combined with low emissions and minimal rates of materials degradation, is transforming the OEM’s approach to modern touchscreen design.

Author details: Chris Ard is Managing Director, TouchNetix

Author
Chris Ard

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