Design Con 2015

Capacitive sensing--integrating multiple interfaces – Part II

Subbarao Lanka, Shruti H, Cypress Semiconductor -January 08, 2013

In Part 1 of this article series we looked at the basics of capacitive touch sensing and how to integrate feedback mechanisms. We also discussed how to overcome typical challenges faced during the integration. In Part II, we’ll see how capacitive sensing can be used in applications beyond touch sensing and discuss two classic applications, the obstacles faced, and solutions to overcome those challenges. In review:
  • Proximity Sensing: This technology is being widely adopted across many applications. We will briefly discuss the technology and the challenges faced during implementation.
  • Liquid Level Sensing: This feature is a step beyond the user interface, but it too can be implemented using capacitive sensing technology. We will explain the benefits of using the capacitive sensing method as well as how to overcome common issues with this method.


Proximity Sensing with Capacitive Sensing

As discussed earlier, capacitive touch sensing buttons detect a finger’s presence through overlays made of non-conductive materials such as acrylic and glass. In medical and industrial applications the buttons are operated using gloves. In such cases, the glove’s thickness adds to the thickness of the overlay, reducing the sensitivity of the buttons and is often the cause for the touch of a finger to go undetected. Similar issues include:

  • In such applications as digital photo frames, LCD monitors/TV’s, ID conscious products, and hidden intercoms, capacitive touch sensing buttons are hidden. When a human hand approaches the device, the front panel lights up with LEDs to provide an enhanced user interface.
  • In battery powered applications low power consumption is crucial. It is a challenging task to optimize both response time to a button touch and power consumption.
  • When you bring your cell phone near your ear for talking, you do not want the touchscreen to be activated when your face touches it. The touchscreen should be deactivated when the phone approaches your ear and reactivated when it is taken away.


The solution to all of these issues is proximity detection using capacitive sensing technology.

Capacitive sensing is by definition proximity detection. For standard buttons, the thickness of the overlay is the proximity setting. The sensor’s response is highest when a finger is touching the overlay. In true proximity sensing, no contact is required between the sensor overlay and the user's finger or hand. In this application, it is necessary to increase the sensitivity of the sensor over that typically required for buttons. Increased sensitivity is realized by acquiring data from the sensor for a greater time period. Longer acquisition times allow very small changes in capacitance that arise from more distant conductive objects to be magnified. Therefore, it is possible to detect conductive objects over greater distances while achieving the kind of update rate and response time required.

In battery powered applications, only the proximity sensor is scanned until the hand approaches. Once the proximity sensor is triggered the rest of the buttons are scanned for button touches. This algorithm saves power. One more advantage is that it hastens the response time for button touches as buttons are scanned at a fast rate as soon as the hand approach is detected.

Types of Proximity Sensor Implementation

Different applications have different proximity detection range requirements as well as restrictions on PCB layout. This has resulted in a variety of different proximity sensor patterns. The most commonly used are:

Gang Proximity Sensor

This is accomplished by combining multiple sensor pads together into one larger sensor using firmware to connect and disconnect the sensors from the internal analog multiplexer bus of the Soc. This method can be used when required proximity detection range is 1 or 2 cm.

Proximity Sensor Using Wire

This method gives a larger proximity detection range and is commonly used in applications where there are curved surfaces that cannot accommodate long PCBs.

 

Figure 1: Proximity Sensor Using Wire


Proximity Sensor Using PCB Trace

This version of a proximity sensor consists of a long trace around the perimeter of the user interface as shown in Figure 2. This method provides a larger proximity detection range than gang proximity sensing and is easier to mount than proximity sensors that use wires.

 

Figure 2: Proximity Sensor Using PCB Trace


Challenges of Proximity Sensing

High Sensor CP

To achieve a large proximity detection range, the proximity sensor loop size must be large. However, this long sensor trace has a high CP and will need to be scanned for a longer time for accurate touch detection. This results in increased power consumption and decreased response time. One way to reduce CP is to use a shield electrode as shown in Figure 3.

 

 Figure 3: Proximity Sensor Using PCB Trace and Shield Electrode


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