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Optimise UI power using wake-on-approach (Part 2)

Posted: 15 Jan 2014     Print Version  Bookmark and Share

Keywords:Proximity sensors  wake-on-approach  UI design  shield electrode  SNR 

Proximity sensors sense the proximity of a conductive object. For many applications, we are particularly interested in sensing human hand/fingers. There are different technologies to implement proximity sensors, including capacitive, inductive, infrared, etc. Capacitive sensing is a cost-effective yet robust technology.

Capacitive sensing is by definition proximity detection. For standard buttons, the thickness of the overlay is the proximity setting. The sensor response is highest when a finger touches the overlay on top of the sensor. 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 required for buttons. Increased sensitivity is realised by acquiring data from the sensor for a greater time. Longer acquisition times allow very small changes in capacitance that arise from more distant conductive objects to be magnified. Obviously, when the acquisition time is increased for such applications, the update rate is slower. However, proximity detection applications require that sensors only detect presence, not fine, rapid movements. Therefore, it is possible to detect conductive objects over greater distances while achieving the kind of update rate and response time that proximity sensing requires.

Figure 1: Design having one proximity sensor as wake-on-approach sensor.

Figure 2: Design having two proximity sensor as wake-on-approach sensor.

Hardware and firmware Implementation of wake-on-approach proximity sensors
Designing a sensor for proximity sensing is never a easy task whereas firmware implementation is fairly straightforward. Let us now discuss the hardware and firmware aspects of wake-on-approach proximity sensors.

Hardware implementation
One needs to be careful while laying out the hardware for the wake-on-approach proximity sensors, which is an inherent tricky portion of a UI design. The following aspects have to be considered while designing hardware for wake-on-approach proximity sensors:
 • The size of the object to be detected – i.e., do you want to detect a human finger or the whole hand
 • The distance at which the object needs to be detected
 • Directionality

The size of the object to be detected
The proximity sensor must be designed specifically, depending on whether a human hand needs to be detected or human finger. For example if a human finger needs to be detected, the sensor should be more sensitive than when it has to detect a human hand.

Let us take an example of TV; If the UI is in a front panel, only a finger or a hand approaching the UI to touch the buttons should be detected but not a human body when it comes close to the UI. This can be achieved by having more than one proximity sensor and using intelligent placement.

For example, in a six-button application, we can have two proximity sensors placed next to each other horizontally as shown in the figure 2, or we can have two groups of three ganged sensors each.

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