Design Considerations for Proximity Sensing Human Interfaces Using Infrared Sensors
Touchscreens are displays that can detect the presence and location of a touch. They enable users to interact directly with the device via the screen itself rather than with mechanical buttons or other indirect devices like a mouse. Many microcontrollers today incorporate embedded circuitry which enables them to be used for touchscreen control applications. The microcontroller itself can be used to establish thresholds, provide noise cancellation to minimize false triggers, and host firmware for many different types of touch inputs such as single touch, multi-finger touch, taps, etc.
To further improve human interface capabilities, a designer could add in a proximity sensor. A single proximity sensor could be used to detect the presence (or absence) of an object like a hand or the user’s body. This capability can be very useful for a variety of applications. For example, a computer monitor can use an embedded proximity detector to look for the presence of the user. If it detects that the user is not present, it can shut off the screen in order to save power and turn itself back on when it senses that the user has returned.
Another rapidly emerging human interface mechanism that is gaining prominence is motion detection. This motion-sensing capability refers to the ability of a system to specifically look for the motion of an object in order to perform a specific action. For example, a handset application may allow a user to page through a document simply by waving a hand over the handset. Adding a second proximity sensor to the design enables this type of one-dimensional motion capability. Through customized firmware, these two proximity sensors can be designed to interact very closely with the microcontroller and provide not just the presence of motion but also the direction the motion is taking place.
To understand the design considerations of a motion-sensing system, one must consider the differences between infrared (IR) light and visible light and explore how proximity and motion sensing systems can work with a single LED, and how motion sensing can be performed with multiple proximity measurements using multiple LEDs.
When we talk about “light,” we are normally referring to the visible light coming from the sun or from a light fixture. However, visible light accounts for only a small portion of the full spectrum. We define visible light as all the light that the human eye can detect. The typical human eye can detect light with wavelengths from about 380 to 750 nm. So what about invisible light with wavelengths that the human eye cannot detect, such as 850 nm light?
Infrared (IR) radiation is light with a wavelength between 750 nm and 100 µm. IR light has the same characteristics as visible light, such as reflectivity off surfaces, and it can be generated by a specialized light bulb or light emitting diode (LED). Since IR light is invisible to the human eye, we can use it to accomplish certain human interface tasks, such as proximity detection, without the need for any physical contact between the user and the system.
IR-based proximity sensing systems detect the presence of nearby objects and perform actions based on these readings. The applications for IR proximity detection are everywhere. Mobile phones, for example, use proximity sensing technology to detect whether you are holding the phone near your face while talking. If you are holding the phone against your ear, the phone will detect the presence of your head and automatically turn off the screen to conserve power. Other examples of proximity sensing systems include soap and water dispensers, which allow you to obtain water and soap in a sanitary, “touchless” way simply by placing your hands near the sensor, usually near the faucets or spigots. High-end vehicles also use a form of proximity detection in external collision-avoidance systems that alert drivers if they are drawing too close to other cars or other external objects. Some vehicles also use proximity detection inside the vehicle to detect the presence of passengers to adjust safety mechanisms such as airbags.
Proximity detection is performed by using an LED specifically designed to emit only IR light. In conjunction with the IR LED, a photodiode is used to sense the IR light emitted from the LED. If the IR LED and photodiode are positioned such that the sensor and emitter are facing the same direction, the photodiode will not detect any IR light unless an object in front of the LED is reflecting the light back to the photodiode. The intensity of the light reflected back onto the photodiode is inversely related to the distance of the reflecting object from the photodiode.
A single LED and photodiode combination can detect some motion, such as whether an object has moved closer to or farther from the photodiode. This is only one-dimensional detection. Consider a system with a layout similar to Figure 1. A single LED system would only use LED1 with the IR sensor.
Figure 1: Example of one-dimensional detection
Figure 2 illustrates the readings from a Silicon Labs Si1120 sensor with one IR LED during three hand gestures. The Y-axis in Figure 2 is directly related to the intensity of the IR light reflected onto the sensor, and the X-axis is time. The three hand gestures include a left-to-right swipe along the X-axis of the board in Figure 1, a swipe along the Y-axis from bottom-to-top, and a approach-retract motion along the Z-axis directly above the board from far-to-close and then from close-to-far. Figure 2 shows that a single LED system cannot distinguish between any of these gestures. Using a single LED, a system can only detect when something is moving closer or farther from the sensor from an unknown direction.
Figure 2: Performance analysis of single-LED system
Two-dimensional detection can be performed using two LEDs in different locations in concert with a single photodiode. After taking a measurement from LED1 and then quickly taking a measurement from LED2, this information can be used to calculate an object’s position within two dimensions. One dimension is closer to LED1 (left) or closer to LED2 (right), and the other dimension is closer or farther from the photodiode. Figure 3 illustrates the same three gestures from Figure 2 with the white line representing reads during LED1 active and the red line representing reads during LED2 active. During the left-to-right swipe motion, the white line rises before the red line. As the hand moves from left to right, it starts reflecting IR light from LED1 to the sensor before it starts reflecting IR light from LED2 to the sensor.
Figure 3: Performance analysis of multiple gestures
Three-dimensional movement can be detected using three LEDs and a single photodiode. With a third LED placed off of the line created by LED1 and LED2 such as in Figure 1, the LED measurements can be used to discern an x-axis between LED1 and LED2, a y-axis between LED1 and LED3, and a z-axis measuring an object’s distance from the photodiode and LEDs. Figure 4 shows the same measurements taken in Figures 2 and 3 with measurements taken during LED3 active represented by a blue line. A left-to-right hand swipe causes LED1 and LED3 measurements to rise before LED2 measurements because the hand goes above LED1 and LED3 at the same time. In the bottom-to-top swipe, LED3 rises before LED1 and LED2 because the hand first encounters IR light from LED3. In the approach-retract motion above the board, all three LED measurements are the same because the surface of the hand is reflecting all three LEDs equally at all times.
Figure 4: Performance analysis with LED3 active
When IR LEDs and an IR sensor are implemented in a product, the components usually are not externally mounted for cosmetic purposes. The end product requires at least one port, or transparent window, for IR light to shine through.
The IR LED shines out of the port and is reflected back into the port to the Si1120 sensor by an external object. The single-port configuration has a major drawback: The window will cause some internal reflection back to the Si1120, resulting in some level of feedback that raises all the levels of sensor output even when there is no external object in the detection range.
A dual-port design uses one window for the IR LED and another window for the sensor. With proper isolation between the LED and sensor, this design eliminates the internal reflection issue and thus provides much more sensitivity and range to the system.
The choice of which IR LED to use is an important design decision for any IR proximity sensing system. The IR LED’s viewing angle greatly affects the maximum detection distance and the area of detection for the system. The IR light coming out of an LED can be pictured with a conical shape, and the size of the apex of this cone where most of the energy of the LED is dissipated is called the viewing angle of the LED.
All LEDs have a specific viewing angle, and a narrow viewing angle means that the energy coming out of the LED is more concentrated and will shine farther than LEDs with wider viewing angles. This means that an IR system using an IR LED with a narrow viewing angle will have a longer range of detection at the expense of a narrower area of detection. Figure 5 illustrates the difference between narrow and wide viewing angle IR LEDs.
Figure 5: Comparison of narrow and wide viewing angles
When designing an IR system, it’s important to consider the characteristics of the object that the system is trying to detect. In addition to detecting hand movements, an IR proximity sensing system can be used to detect inanimate objects, such as a garage door (whether it is opened or closed). Larger objects of detection will make for longer distances of detection due to more IR light reflecting off the surface of the object. The object’s color is another consideration because IR light has the same qualities of visible light in that light-colored objects will reflect more light than dark-colored objects. The darker the object, the closer it will have to be to the IR system to be detected because less IR light will be reflected from the IR LED to the IR sensor.
Many electronic systems in consumer, industrial and automotive applications will benefit from a touchless method of feedback. IR proximity sensing provides an optimal method of providing feedback for systems that need to detect the presence of objects. Proximity sensing also can be used to detect motion, even hand gestures, in up to three dimensions, enabling sophisticated, intuitive human interfaces for next-generation electronics products.
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