Making capacitive touch sensors water tolerant

In this Product How-To article , Mark Lee of Cypress uses the company’s CY8C21434 PSoC to explain the methods by which to integrate capacitive sensors into consumer white goods where the ability to operate reliably in wet environments is important.

Capacitive touch sensors are commonly found today in MP3 players and mobile phones. As this sensor technology expands into other product categories, new design challenges are encountered.

With electric ranges, dishwashers and other products in the white goods category, one of these new challenges is operation in a wet environment. This article shows how to design capacitive touch sensors that are water tolerant.

Water tolerant versus waterproof
A waterproof design implies system performance that is totally immune to the effects of water. For a water tolerant design, water levels encountered in normal operation do not interfere with sensor operation. Splatters and spills on the touch surface are tolerated, but total immersion is not. Water tolerance is a reasonable and cost effective solution for operation in a wet environment.

In a water tolerant design, only the touch of a finger produces a signal large enough to register as a “touch”. However, if a boiling pot overflows, and the touch surface is submerged in hot liquid, the water tolerant sensor will be challenged to operate normally. Through proper configuration of the sensor array, the submersion can be detected, and the system can be alerted that an abnormal event has occurred.

The safest response to such an event is to turn off the burner until the spill can be cleaned up. In contrast, a waterproof design will continue normal operation after the spill. To turn off the burner, the user of a waterproof system needs to touch the sensor through a coating of hot liquid.

If the liquid is too hot to touch, the burner stays on, and the pot keeps boiling, only making the situation worse. The water tolerant design leads to a system that turns itself off with a major spill. Comparing the two approaches for reacting to a spill of hot liquid, the water tolerant design is the safer and smarter choice.

Classifying the degree of surface wetness
In the following discussion, surface wetness is classified into three categories: Dry, Droplet, and Stream, as shown in Figure 1 below . When liquid is sprayed or splashed onto a dry surface, surface tension causes the liquid to bead up, forming droplets.

Figure 1. Cross section view of the three categories of surface wetness : a) Dry, b) Droplet, c) Stream.
A water tolerant design needs to operate normally when the surface is covered with droplets. For larger amounts of liquid, the droplets merge together and form a stream if set in motion, or a puddle if the surface is at a low point.

Special electrodes help in wet environments
Fingers are conductive, so they interact with the electric field that is set up around the touch sensors. Water is conductive, so it interacts with the same electric field when it is lands in the active sensing area.

This can lead to a report of a finger touch when water splashes onto the sensing surface even when no finger is present. Figure 2 below shows an example of drops of water producing the same signal level as a finger for a touch sensor that lacks any features for water tolerance.

Figure 2. Example of a finger touch and drops of water both producing a signal that crosses the finger detection threshold for a sensor with no water tolerance.
The Raw Count shown in the figure is the unfiltered output from the sensor. The Baseline is a continuously updated estimate of the average Raw Count level when a finger is not present. The Baseline provides a reference point for determining when a finger is present on the sensing surface.

Fingers and water interact in a similar, but not identical, way with electric fields. There is enough difference between the two to make possible techniques for discriminating between a touch and a spill.

On printed circuit boards and flex circuits, a practical level of water tolerance is achieved with the use of a shield electrode and guard sensor. These special electrodes add no material cost to the system since they are incorporated into the same circuit board layout as the touch sensors, as shown in Figure 3 below.


Figure 3. The shield electrode and guard sensor are added to the printed circuit board layout to add water tolerance to standard touch sensors.
The purpose of the shield electrode is to set up an electric field pattern around the touch sensors that helps attenuate the effects of water. The purpose of the guard sensor is to detect abnormally high liquid levels so the system can react appropriately.

The shield electrode
The shield electrode works by mirroring the voltage of the touch sensor on the shield. In practice, the shield electrode waveform only needs to approximate the shape and timing of the waveform on the touch sensor to be effective. In the CSD sensing method that runs on the PSoC shown in Figure 4 below , the shield is driven by internally switching the shield pin between VDD and ground.

Figure 4. Schematic of the shield electrode circuit implemented with the CSD sensing method that runs on a PSoC chip.
The switches in the shield circuit are driven by a two-phase clock. In the first phase, The sensor capacitor, Csensor , is charged up to VDD , and the terminals of the parasitic capacitance associtated with the shield are shorted together by switches SW1 and SW3 .

In the second phase, Csensor discharges into the capacitors Cmod and Cshield , and into the modulator. The average current flowing through switch SW4 sets the duty cycle of the modulator, which in turn sets the counter value of the CSD output.

Without the shield, switch SW1 and SW2 are not present, and the current flowing through switch SW4 is proportional to only to Csensor . Water and finger touches on the sensor increase the capacitance of Csensor .

The result is that water and fingers both increase sensor counts without a shield, as demonstrated in Figure 1. With the shield in place, the average current in switch SW4 is reduced since some of the charge in Csensor now makes its way into Cshield when SW2 is closed. With less current flowing into the modulator, the baseline level for sensor counts will be reduced.

Figure 5. Example of a finger touch producing a signal that crosses the finger detection threshold, while drops of water do not for a sensor with a shield electrode.
Water increases the capacitance of Cshield , which results in an even lower average current into the modulator. As shown in Figure 5 above , with the shield in place, water and fingers produce opposite responses in the sensor output. Fingers cause an increase in counts. Water causes a decrease in counts.

The guard sensor
The purpose of the guard sensor is to indicate that an abnormally large amount of water is on the sensing surface. The guard sensor is a special touch sensor electrode that surrounds the other touch sensors. When touched with a finger, the guard sensor indicates the presence of the finger.

What makes the guard sensor special is that it produces a much larger signal with stream than with a touch. To discriminate between a touch and a stream, the shield electrode is grounded when sensor counts are acquired from the guard sensor.

The counts for all the other sensors are acquired with the shield voltage tracking the voltage on the sensor electrode, as described in the previous section. Figure 6 below shows the result of using this technique.

Figure 6. A finger touches the dry surface, and then a stream of water activates the guard sensor.
A stream of water in this example produces twice the signal than a finger does. When the signal from the guard sensor crosses the threshold, the system is alerted that too much water is on the surface for normal operation. The system designer can then decide the appropriate action in respone to the spill.

Test circuit and Printed Circuit Board
The schematic of a water tolerant touch sensing system based on the CY8C21434 PSoC chip is shown in Figure 7, below . This design includes three touch sensors that are labeled SENS1, SENS2, and SENS3. The design also includes a shield electrode and a guard sensor.

Figure 7. Schematic of a water tolerant touch sensing system based on the CY8C21434 PSoC with three touch sensors, a shield electrode and a guard sensor.
The touch sensors, the shield and the guard sensor are all controlled by the PSoC. This microcontroller is also configured in firmware to drive a set of LEDs that indicate when a finger touch occurs.

Figure 8. The touch sensors, shield electrode and guard sensors are on the on the top layer of the printed circuit board, and all the components mounted on the bottom layer.
The ISSP/I2C port supports the dual functions of programming and I2C communication with a host computer. The CY8C21434 can support 20 sensor inputs when water tolerant features are enabled. Unused sensor inputs can either be programmed for additional I/O functions, or left unassigned. Figure 8 above shows the top view of the printed circuit board for this application.

Putting it all together
The final step in system design is assembly of the PCB with the chassis, and joining the PCB to the protective overlay. The overlay material is a 2mm-thick acrylic sheet that is joined to the PCB with a thin layer of nonconductive adhesive film. Figures 9, 10 and 11 below show the performance of the final assembly.

Figure 9. Dry surface: As a finger touches briefly on each touch sensor location, the sensor counts indicate contact with the finger, and the guard sensor shows a small amount of crosstalk.

Figure 9 above shows that the touch sensor and guard sensor response to a finger touch when the surface is dry. Figure 10 below shows what happens when the surface is covered with water droplets.

Figure 10. Wet surface covered by water droplets: Water droplet effects are visible in the sensor counts, but the peak change in counts caused by water is only 10% of the change caused by a finger.
With the shield in place, the finger response is around 10x the signal produced by water droplets. Setting the finger detection threshold above the signal produced by the droplets, only finger touches are seen by the system, while the droplet signal is lost in the noise. Figure 11 below shows that when a stream of water covers the surface, both the touch sensors and guard sensor produce a large signal.

Figure 11. Wet surface covered by water stream: With the surface is totally submerged in water, the guard sensor signal level is greater than each of the touch sensors, and 6x the level produced with droplets.
The guard sensor produces a 6x increase in signal with the stream of water compared to the crosstalk-induced signal level seen with water droplets and with a dry surface. This big increase in the signal level of the guard sensor makes it possible for the system to detect a big spill and react in a predetermined way.

Mark Lee is a Principal Applications Engineer at Cypress Semiconductor in Lynnwood, Washington. He has a PhD in Electrical Engineering from the University of Washington where his research topic was the computer-aided modeling of the dielectric properties of materials. He has been awarded several patents related to analog electronics. He can be reached at Mark.Lee@cypress.com .

References
[1] Application Note AN42851, “Proximity Detection in the Presence of Metal Objects”, Cypress Semiconductor
[2] Application Note AN2398, ” Capacitance Sensing – Waterproof Capacitance Sensing”, Cypress Semiconductor
[3] Application Note AN2292, “Capacitance Sensing – Layout Guidelines for PSoC® CapSense™”, Cypress Semiconductor

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