This article discusses how to wake up touch devices such as tablet computers without touching the device. Instead, it uses basic gesture recognition and a novel proximity detection sensor. This article discusses the physical layout, speed limit, detection threshold, system integration, and human factors of the relevant design; the real-time routines of the software are given.
Sudden whim in the kitchen
If you use a touch device when cooking, you may notice that cooking according to the recipes listed on the device is not as simple as imagined. Technologists (such as scumbags) like to look at recipes on a tablet or smartphone when they walk into the kitchen. You might say, "Well, what's the difficulty?" Because the screen always turns on and consumes a lot of power, usually the handheld device will automatically go to sleep when it is not operated after 1, 2 minutes. Then, when you need to refer to the recipe, the device has entered the sleep state. In this case, you are faced with two options: either force the screen to remain on permanently; or turn on the device with a hand covered with food and leave a greasy stain on the screen. Of course, you can clean your hands every time you look at it, but repeatedly washing your hands and drying them is cumbersome and water-consuming.
I often ask myself: "How can I keep the screen on and not dirty the device?" In fact, there is a way to do two things with one stone, that is, turn on the display with one gesture (without touching the screen). Sounds complicated, doesn't it? Fortunately, it may be easier than it sounds.
Proximity detection sensor
Many touch screen devices, especially smartphones, have installed infrared (IR) proximity detection sensors inside. These sensors generally turn on / off the screen automatically during a call to avoid accidentally operating the phone's input interface. This sensor, coupled with smart software design, can realize the function of awakening the device with a gesture.
The basic design idea is: when the device enters the sleep state, the touch screen is turned off, the application processor is in a low-power mode, relying on the proximity detection sensor to "observe" background changes, and respond appropriately when the received signal is large enough. This is almost the same as the function of the proximity sensor to turn off the screen during a call. However, our application has a different interpretation of the data.
First record the count value of the sensor in the "normal" background, the value obtained at this time may be zero, but the actual design needs to consider the system offset (for example: scattering or crosstalk). Then set the obtained value as the detection threshold, when the received signal exceeds the threshold, trigger an interrupt or send a signal to the application processor to wake up the system and open the screen. Overall, this method is very simple and intuitive, and can be implemented using ambient light detectors and IR proximity detection sensors.
The solution described in this article uses the MAX44000, and the data reading interval for proximity detection can be set from 1.56ms to 100ms (in turn reading data with the ambient light detection sensor). Assuming that the maximum detection distance is 10cm and the radiation angle of the LED is ± 15 °, then the area that can be covered is about 22cm2 or the span is about 5.35cm, and only moving targets in this area can be captured. Thus, the fastest gesture that can be reliably detected at the slowest (ie, lowest power consumption) sampling speed is approximately 0.53 mps. Here, we also assume that the sensor only needs to collect a signal above the threshold to identify the target passing through the coverage area.
Hands up ...
In theory, the implementation of this program is very simple. When the device enters the sleep mode, the proximity detection sensor is set to the environmental scanning mode, and an interrupt signal is issued when a target is detected, indicating that a signal exceeding a preset threshold is captured. The status of the sensor can be polled through the I2C interface. Unfortunately, this method consumes too much power and exceeds the expectations of most users.
This is also the design focus of the proximity detection sensor. The MAX44000 sensor can get rid of the application processor's intervention in many ways, reducing processor load (reducing power consumption). Enable the MAX44000's internal proximity detection interrupt (register 0x01, bit 1), and write the wake-up threshold to the internal registers (0x0B and 0x0C). When the reading of the proximity detection sensor exceeds the threshold, the interrupt flag is set and the MAX44000 / INT pin is set to low level. When the application processor detects that this pin is driven low, it can wake up the device to exit low-power mode, turn on the screen, or complete other required actions.
... but not to be ignored
The actual application is often not as easy as theory, and the specific implementation of non-contact wake-up is not simply to detect signals above the threshold. In fact, the specific design needs to consider many factors.
Signal level and circuit layout
The most critical consideration should be the signal level that triggers the wake-up condition, and there is a trade-off between system response sensitivity and false alarm probability. If the threshold is too low, it is easy to detect the input (gesture work), but it will increase the probability of false alarms caused by transient noise or sudden conditions. Conversely, an excessively high detection threshold can reduce the probability of false positives to almost zero, but it can only detect very close targets, and even become unresponsive to any input (even if you shake your arm crazy).
The best way to solve this problem is: first reduce the system noise, which can be achieved by optical methods or rigorous circuit layout. The reduced noise floor helps reduce the probability of false alarms; second, choose the "average" detection distance (for example: 4cm To 5cm) and using the reference target to measure the signal, an 18% gray board is ideal, but if black glass is installed above the touch screen, such glass should also be used for measurement. The measured signal level can be used as the best setting threshold reference. Generally, you can follow the principle that the level is set at 8% to 15% of full scale, even if the level changes.
The proximity detection threshold register of the MAX44000 sensor can be set according to the above empirical data. Figure 1 shows the relationship between the signal strength and the distance. The 18% gray board is used, the drive current is 100mA, and there is no glass cover above the sensor. The blue line is the selectable wake-up threshold.
Figure 1. The relationship between the signal strength of the MAX44000 proximity detection sensor and the distance. It uses an 18% gray board, 100mA drive current, and no glass cover.
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