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Optimize power for user interfaces through wake-on-approach using capacitive proximity sensing – Part 3

Optimize power for user interfaces through wake-on-approach using capacitive proximity sensing – Part 3

Technology News |
By eeNews Europe



In Part II, we covered basics of proximity sensor and hardware implementation details of wake-on-approach proximity sensor. In Part III, we will cover different types of proximity sensor hardware designs, common challenges faced while designing and implementing proximity sensor, firmware implementation of wake-on-approach proximity sensors and some consideration in designing wake-on-approach proximity sensors.
 
Proximity sensor using wire

A wire is used as a proximity sensor. This method gives greater proximity detecting range and can be used when there are curved surfaces or where a longer PCB cannot be placed in the application.

Proximity sensor using trace on PCB

A long PCB trace can form a proximity sensor. The trace can be a straight line, or it can surround the perimeter of a system‘s use interface, as shown in figure below. This method is appropriate for mass production, but it is not as sensitive as a wire sensor.

 

Gang proximity sensor

In an application where the size of the PCB is limited, placing a dedicated proximity sensor would be difficult. Also, the application does not call for large distance detection. In such applications, all the buttons are ganged and scanned as single sensor; this method can be used when required distance is smaller compared to what we can achieve with wire or PCB trace.

So far we have seen how to design proximity sensors from the design requirement standpoint. Now let us see what are the common challenges faced while designing a proximity sensor.

 
Higher sensor parasitic capacitance (Cp)

The higher the proximity distance, higher should be the trace length on the PCB to achieve it. Higher trace length results in higher Cp.  In general, it will take more time to scan sensors with higher Cp and this result in higher power consumption and slow response time.

This problem can be solved using shield electrode, which was discussed earlier. Shield electrode reduces Cp of sensor.

 
Noise

In general, the proximity sensor is tuned for high sensitivity to get maximum range. Any small increase in noise can cause a high impact on SNR (Signal to Noise Ratio). It is recommended to use software filters to overcome this.

 
Metal Object

In the real world, the important factor that impacts the proximity performance is the presence of the metal object close to the proximity sensor.

 

 
The figure above illustrates the effect of metal object on the proximity range. On the Left side of the figure there is no metal object close to sensor, so all the electric field lines move in to air as shown, hence the detection range is high. With external casing (ground) or metal object or even ground introduced on the PCB for noise immunity purposes, the proximity detection is reduced. This is because, due to presence of metal object, some of the electric field lines converge towards metal objects

This problem can be solved using a shield electrode.

Firmware implementation of wake-on-approach proximity sensors

In order to implement the scan-sleep-scan-sleep technique, a timer/counter is required. Some of modern capacitive touch sensing devices come with peripherals like timer/counters along with sensing engine. This timer can be used to control the rate of sensors scan and to wake up the device from low power sleep mode.

In designs with wake-on-approach proximity sensors, only proximity sensor is scanned as long as there is no hand approach and hence no activation of the sensor. When the proximity sensor gets activated, all sensors are scanned.

Below is code that shows how to implement wake-on-approach proximity sensing.

 

 
As we saw above, little structural and flow change in the firmware is sufficient to implement wake-on-approach proximity sensors.

 
Considerations in designing wake-on-approach proximity sensors:

Early detection buys more time for device readiness. The detection range depends on the application as well as the capability of the device/design. Once the hand is detected, the device must get back to active mode. In the process of transitioning from low power mode to active, the device needs to restart some of its block which requires some time. So, the farther the distance, at which the hand is detected, larger is the time between the hand approach and the touch within which the device can prepare itself for transitioning to active mode. This essentially means the response time for first touch is improved.

 
Levels of power optimization with wake-on-approach proximity sensors

By the virtue of proximity sensors being able to detect conductive objects at large distances, the analog nature of the output broadens the scope of usage of proximity sensors. Thus more than one distinguishing level can be detected using proximity sensors. This method is more advanced in the sense that it retains user friendliness and yet saves the power of whole system by switching on blocks only when they are required to be functioning.

Take an example of a laptop, when a human hand is approaching the keyboard. At a certain distance, the hand is detected and the backlight of the keys gets turned on and the CPU begins scanning the keys. At this point, the monitor lights up with only low intensity. However, when the hand is on the keypad, the monitor lights up with higher intensity enabling user for better visibility.  The 2 levels of actions are taken based on the proximity of the hand to the proximity sensors. When the hand is detected at a set distance, the keypad gets activated and the monitor lights up with low intensity. When the hand comes nearer, the monitor glows with higher intensity. This means that instead of lighting the monitor up with light intensity when the hand is detected at the first level where it is more power consuming, the monitor is lit up with low intensity first. This gives better power consumption savings.   

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