Infrared proximity sensing human-machine interface design elements

Touchscreens are a type of display device that can detect the presence and location of touch, allowing users to interact with the device directly through the device screen, rather than mechanical buttons or other indirect devices such as mice. Many microcontrollers today have embedded circuitry that allows them to be used for touchscreen control. Microcontrollers can be used to set thresholds, provide noise cancellation to minimize false triggers, and implement host firmware that supports a variety of different types of touch input, such as single-touch, multi-touch, and tapping.

To further improve the performance of the human-machine interface, designers can add proximity sensors. A single proximity sensor can be used to detect the presence of an object, such as a hand or the user's body. This capability is very useful in many applications. For example, a computer monitor can use an embedded proximity detector to sense the presence of a user. When it detects the user's absence, it can turn off the screen to save power and turn it back on when it senses the user's return.

Another human-machine interface technology that is rapidly gaining popularity is motion detection. This motion sensing capability refers to the ability of a system to recognize the movement of an object in order to perform a specific function. For example, a mobile phone application may allow the user to turn pages in a document by shaking the phone. Adding another proximity sensor to the design gives the device the ability to detect motion in one dimension. Through custom firmware, two proximity sensors work closely with the microprocessor to provide not only the ability to detect the presence of motion, but also the direction from which the motion is occurring.

To understand the theoretical basis of motion sensing system design, it is necessary to understand the difference between infrared (IR) and visible light, explore how proximity and motion sensing systems operate with a single LED, and how motion sensing works when multiple LEDs are used for multi-proximity measurement.

When we talk about "light", we usually mean visible light from the sun or lamps, however, visible light only occupies a small part of the light spectrum. We define visible light as all light that can be detected by the human eye, which is generally 380-750 nm. So what about non-visible light that cannot be detected by the human eye, such as light with a wavelength of 850 nm?

Infrared ( IR ) radiation has wavelengths of 100um -750mn. IR light has the same properties as visible light, such as reflectivity, and it can be generated by special bulbs or light emitting diodes. Because the human eye cannot see IR light, it can be used to complete some special human-machine interface tasks, such as proximity detection, without requiring the user to have any direct contact with the system.

IR proximity sensing systems are able to detect the presence of nearby objects and react based on the results of the detection. The applications of IR proximity detection are everywhere. For example, mobile phones can use proximity sensing technology to detect whether the phone is close to the face during a call. When you hold the phone to your ear, the phone will detect the presence of your head and automatically turn off the screen to save power. Other examples of proximity sensing systems include soap dispensers and water dispensers, where you can place your hand near the sensor (usually near the soap dispenser tube or faucet) to obtain soap or water in a "non-contact" and hygienic way. In high-end cars, exterior collision avoidance systems also use proximity detection to alert the driver when the car is too close to other cars or objects. Some vehicles can also use in-car proximity sensing systems to detect the presence of passengers and adjust safety devices (such as airbags). Proximity detection

is achieved using specially designed IR LEDs. The counterpart to IR LEDs is a photodiode, which is generally used to detect the IR light emitted by the LED. When the IR LED and photodiode are placed in the same direction, the photodiode will not detect any IR light unless an object is in front of the LED and reflects light back to the photodiode. The intensity of light reflected back to the photodiode is inversely related to the distance of the object from the photodiode.

A single LED combined with a photodiode can detect some actions, such as whether an object is close to or away from the photodiode, which is only a one-dimensional spatial detection. Assume a system with the layout shown in Figure 1. The single LED system only uses LED1 and the IR sensor.

Figure 1: Action detection in one dimension

Figure 2 shows the output values ​​of the Silicon Labs Si1120 sensor after sensing the IR LED during three gestures. The Y-axis is the reflected IR light intensity and the X-axis is time. The three gestures include sliding from left to right along the X-axis of Figure 1, sliding from bottom to top along the Y-axis, and reciprocating along the Z-axis from far to near and then from near to far. Figure 2 shows that a single LED system cannot distinguish these gestures. Using a single LED, the system can only detect that an object is approaching or moving away from the sensor, but cannot determine its direction.

Figure 2: Single LED system performance analysis

Two-dimensional spatial detection consists of two LEDs at different locations and a single photodiode. A measurement is taken from LED1, then another measurement is quickly taken from LED2, and the two measurements are used to calculate the position of the object in two dimensions. One dimension is close to LED1 (left) or close to LED2 (right), and the other dimension is close to or away from the photodiode. Figure 3 shows the same three gestures as Figure 2, where the white line represents the data read from LED1 and the red line represents the data read from LED2. During the left-to-right swipe, the white line rises, followed by the red line. As the hand slides from left to right, LED1 reflects IR light to the sensor, followed by LED2.

Figure 3: Gesture performance analysis in two-dimensional space

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The three-dimensional motion detection consists of three LEDs and a single photodiode. LED3 is not in the same straight line as LED1 and LED2. As shown in Figure 1, the line between LED1 and LED2 can be regarded as the X-axis, the line between LED1 and LED3 can be regarded as the Y-axis, and the line from the photodiode and LED to the object to be measured can be regarded as the Z-axis. Figure 4 shows the same measurement process as Figures 2 and 3, where the blue line represents the measurement data of LED3. When the hand slides from left to right, because the hand passes over LED1 and LED3 at the same time, the LED1 and LED3 data lines rise at the same time, followed by the LED2 data line. When the hand slides from bottom to top, because the hand first encounters the IR light from LED3, the LED3 data line rises, followed by LED1 and LED2. When reciprocating, because the hand reflects the same amount of LED light throughout the process, the three LED measurement values ​​are the same.

Figure 4: Analysis of motion performance in three-dimensional space after adding LED3

When IR LEDs and IR sensors are applied to products, these components are usually not placed outside for decorative purposes. The end product needs at least an opening or transparent window to let the IR light pass through.

The IR LED shines out of the window, reflects off external objects, and enters the Si1120 sensor through the window. The main disadvantage of a single window configuration is that the window will cause some light to be internally reflected back to the Si1120, and a large amount of reflected light may cause the sensor to output even when there are no external objects in the detection range.

A dual window design uses one window for the IR LED and the other for the sensor. By providing proper isolation between the LED and the sensor, the design eliminates the problem of internal reflections and provides the system with better sensitivity and detection range.

The choice of IR LED is a very important decision for the design of an IR proximity sensing system. The viewing angle of the IR LED has a great impact on the maximum detection distance and range. The IR light emitted from the LED forms a cone, and the top angle of the cone (where most of the LED energy is output) is called the LED viewing angle.

All LEDs have a specific viewing angle, and a narrow viewing angle LED means that the energy emitted is more concentrated and shines farther than a wide viewing angle LED. This means that using a narrow viewing angle IR LED will result in a longer detection range in a narrow detection area. Figure 5 illustrates the difference between narrow and wide viewing angle IR LEDs.

Figure 5: Difference between narrow and wide viewing angle IR LEDs

When designing an IR system, the characteristics of the objects being detected in the system are also important to consider. In addition to detecting gestures, IR proximity sensing systems can also be used to detect inanimate objects such as garage doors (open or closed). When detecting larger objects, the detection distance will be greater because more IR light is reflected. The color of the object is another factor to consider because IR light has the same characteristics as visible light, and light objects reflect more light than dark objects. The darker the object, the closer it must be to the IR system because only a small amount of IR light from the IR LED is reflected to the IR sensor.

Many electronic systems in consumer, industrial and automotive applications benefit from contactless reflection. IR proximity sensing provides an optimal method for systems that need to detect the presence of an object. Proximity sensing can also be used to detect motion in up to three dimensions, and even gestures, making the human-machine interface of the next generation of electronic products more advanced and intuitive.