Build A Touch-Sensor Solution For Wet Environments

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 Cypress Semiconductor’s PSoC chip, the shield is driven by internally switching the shield pin between V_DD and ground (Fig. 4).

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 associated with the shield are shorted together by switches SW₁ and SW₃.

In the second phase, C_SENSOR discharges into the capacitors C_MOD and C_SHIELD, as well as into the modulator. The average current flowing through switch SW₄ sets the duty cycle of the modulator, which in turn will set the counter value of the CSD output.

Without the shield, switches SW₁ and SW₂ aren’t present, and the current flowing through switch SW₄ is proportional to only to C_SENSOR. Water and finger touches on the sensor increase the capacitance of C_SENSOR. 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 C_SENSOR now makes its way into C_SHIELD when SW₂ is closed. With less current flowing into the modulator, the baseline level for sensor counts will be reduced. Water increases the capacitance of C_SHIELD, which results in an even lower average current into the modulator. It’s interesting to note that with the shield in place, water and fingers produce opposite responses in the sensor output. Fingers cause an increase in counts, while water causes a decrease in counts (Fig. 5).

THE GUARD SENSOR
The guard sensor indicates 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 sensor indicates the presence of the finger.

What makes the guard sensor special is that it produces a much larger signal with a stream than with a touch. To discriminate between the two, the shield electrode is grounded when sensor counts are acquired from the guard sensor. The counts for all other sensors are acquired with the shield voltage tracking the voltage on the sensor electrode, as described in the previous section.

Figure 6 shows the result of using this technique. A stream of water produces twice the signal than that from a finger. 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 designer can then decide the appropriate action in response to the spill.

TEST CIRCUIT AND PCB
One example of a water-tolerant touchsensing system is based on Cypress Semiconductor’s CY8C21434 PSoC chip (Fig. 7).² This design includes three touch sensors that are labeled SENS1, SENS2, and SENS3. In addition, the design includes 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. 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 shows the top view of the PCB for this application. The board layout follows the guidelines for CapSense PCBs, which are described in an application note from Cypress.³

PUTTING IT ALL TOGETHER
The final step in system design is assembly of the PCB with the chassis and adhering the PCB to the protective overlay. The overlay material is a 2-mm thick acrylic sheet joined to the PCB with a thin layer of nonconductive adhesive film. When tested, performance figures showed that when the surface is covered with water droplets, and with the shield in place, finger response is around 10 times 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.

Testing also showed that when a stream of water covers the surface, both the touch sensors and guard sensor produce a large signal. The guard sensor produces a sixfold increase in signal with a stream of water compared to a crosstalk-induced signal level seen with water droplets and a dry surface. This big increase in the guard sensor’s signal level makes it possible for the system to detect a big spill and react in a predetermined way.

REFERENCE
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