Optimize power for user interfaces through wake-on-approach using capacitive proximity sensing – Part 2
Proximity Sensors Basics
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 realized 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.
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 4, or we can have two groups of three ganged sensors each.
Each proximity sensor or two groups of ganged sensors are scanned alternately. When a hand approaches, it is expected to trigger one of the two sensors, so only when one of the two sensors is triggered the system is activated. In contrast, when a human body approaches, both the sensors are expected to get triggered. Thus, the approach of human body can be distinguished from a human hand approach. We can vary the number of proximity sensors and use interpolation to precisely distinguish between the types of objects to be detected. While this approach does increase power consumption a bit when compared to having one sensor, it allows catering to specific application requirements.
Distance at which the object needs to be detected
Distance is another critical parameter which poses problems in proximity sensor designs. It is generally hard to achieve detection at greater distances with robust performance. Guidelines have to be followed in designing the hardware as hardware is crucial for proximity sensing performance.
Considerations to achieve good detection range
As mentioned before, often achieving good detection range is a challenge. However, there are ways to improve the detection range based on the requirements in your application. One important point to keep in mind while designing a system with proximity sensors is that the proximity sensor must meet minimum SNR criterion. Making sure that the SNR is above the minimum ensures that proximity sensing, which is inherently very sensitive and hence vulnerable to small increase in noise, works robustly in noisy environments.
Driven Shield
A shield electrode surrounds touch sensors such that it sets up an electric field pattern around the sensors to reduce coupling between the sensor electrode and ground and hence reduce the parasitic capacitance of the sensor. Lower parasitic capacitance means the sensor can be tuned to be more sensitive thus be capable of detecting at greater distances.
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. By the virtue of shield signal characteristic with respect to that of the sensor, most of the electric field lines from the sensor get repelled from the shield signal surrounding the sensor and are directed away from the PCB. Thus when a conducting object approaches, large number of electric field lines get coupled with the conducting object. Thus more capacitance is added. This means larger distance can be achieved.
Refer to the figure below for a typical layout with shield:
Driving the shield, a signal with frequency in MHz always in a design can pose problems in applications with tight specifications on EMI/EMC. So, it is advisable to
- Drive the shield only when sensors are being scanned
- Reduce the size of the shield pattern
The shield electrode pattern should surround the sensor pad and exposed traces, and spread no further than 1 cm from these features. Spreading the shield electrode beyond 1 cm has negligible effect on system performance.
Place an external series resistor on the shield electrode which forms a low-pass RC filter that can dampen RF noise amplitude.
Software Filters
Intelligent software filters can be designed to reduce the noise and hence improve the performance of proximity sensors. However, it’s often a tradeoff between the response time and the noise improvement. Filtering the noise out by acquiring large number of samples improves the SNR but increases the response time as well which may not be desirable. Refer to the code examples available at Cypress’ website to become familiar with typical software filters used in capacitive sensing applications.
If the proximity sensor is designed to detect more than the distance that is desired, it is recommended to reduce the sensitivity, which avoids false detection and potentially reduce the power consumption.
Directionality
Directionality is very important for wake-on-approach proximity sensors. System designers must ensure that the proximity sensor/s is/are placed such that it detects the object in the direction of approach. For example, consider the example of a wireless mouse shown in the figure below. Human hand is expected to approach the device right above mouse. So, it is very important to place the proximity sensor at the top of the mouse in which direction the hand is expected to approach. If the proximity sensor is placed elsewhere which fails to detect the hand approaching from top, the design would fail.
In Part II, we cover the basics of proximity sensors and hardware implementation details of wake-on-approach proximity sensors. In Part III, we will cover different types of proximity sensor hardware designs, common challenges faced while designing proximity sensors, firmware implementation of wake-on-approach proximity sensors and consideration in designing wake-on-approach proximity sensors.
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