Designing and implementing an optical heart rate monitoring (HRM) system (also known as photoplethysmography, or PPG) is a complex class of projects involving multiple fields. Design elements include ergonomics, signal processing and filtering, optical and mechanical design, low noise signal receiving circuitry, and low noise current pulse generators. Silicon Labs (also known as "core technology") recently released a technical article on the theme of "System Integration Considerations for Optical Heart Rate Sensing Design", which details the principles, key considerations, and integrated devices of optical design. The demand also introduces our latest high-precision, high-integration, low-power HRM module solution. Please watch the full technical article. Heart rate monitoring is gradually becoming the standard for wearable electronics Wearable electronics manufacturers are continually adding heart rate monitoring to their healthcare and fitness products, and integration is driving the cost of sensors in heart rate monitoring applications. Many heart rate detection sensors currently include discrete components such as analog front ends (AFEs), photocell detectors, and light-emitting diodes in their highly integrated modules. These modules support a more convenient implementation that reduces cost and complexity when adding heart rate monitoring to wearables. The appearance of wearable products is also changing. While the chest strap has effectively served the health and fitness market for many years, heart rate monitoring is now entering a variety of wrist-worn products. Technological advances in optical sensing technology and high-performance, low-power processors have led to the wearing of this shape for many designs; the precision of heart rate detection algorithms has reached a new level, allowing It is accepted by products that are worn with a wrist. Other new wearable sensing applications are also emerging in appearance and application scenarios, such as headbands, sports and fitness apparel, and earplugs. However, the most important application scenarios for wearable physiological indicators monitoring will also be collected on the wrist. Integration, precision is the key to heart rate monitoring system design No two heart rate monitoring applications are identical, and system developers must consider many design compromises: product comfort, sensing accuracy, system cost, power consumption, sunlight effects, handling many skin types, motion effects, development Time and physical size, etc. All of these design factors affect system integration, whether it is a highly integrated module solution or an architecture that integrates more discrete components. Figure 1 shows the basic method of measuring heart rate signals, which relies on heart rate pressure waves that are optically extracted from human tissue. Figure 1 illustrates the pathway of light transmission after it enters the skin: capillary dilation and contraction motion caused by heart rate pressure waves modulate the light signal injected into the body tissue by the green LED. The received signal is greatly attenuated by the passage of the skin, which is received by a photocell and sent to the electronics subsystem for processing. The amplitude modulated signal from the pulse is detected (filtered out motion noise), analyzed and displayed. A basic method of heart rate monitoring system design uses a custom microcontroller (MCU) that is used to control the pulse signal generation of an external LED driver while simultaneously reading the current output data on a separate photocell. . It is known that the current output of a photocell must be converted to a voltage and output to an analog to digital conversion (A/D) unit. The schematic illustrates the basic principles of such a system. It is known that a current to voltage converter produces a voltage equal to VREF when the photocell current is zero, and the voltage will drop as the current increases. Current pulses used in heart rate systems are typically between 2 mA and 300 mA, depending on the skin color being measured and the desired signal must offset sunlight. The attenuation of infrared radiation in sunlight through the skin tissue is small, which is different from the light emitted by the desired green LED, so it can flood the green light unless the green light used is very strong or an expensive one is added. Infrared blocking filter. Under normal circumstances, the intensity of green LED light that enters the skin is 0.1 to 3 times the intensity of sunlight. Due to the severe attenuation of human tissue, the signal to the photocell is very weak and requires only enough current to achieve a reasonable 70-100dB signal-to-noise ratio (SNR) for even a perfect, noise-free operational amplifier and The case of the A/D converter eliminates noise. The reason for this noise cancellation is that in the data reading of 25 times per second, each photocell receives a certain amount of electrons. Photocells used in the design range in size from 0.1 mm2 to 7 mm2. However, when it is larger than 1 mm, an indeterminate return value occurs due to the influence of sunlight. In an optoelectronic heart rate system design, the difficult and costly functional modules are: fast, high current drive voltage to current converters for LEDs, current to voltage converters for photocells, and running algorithms and An MCU that processes sequence signals. A very low current that can be set to as low as 2mA, while still producing very narrow optical pulses as short as 10μs (75 - 100 dB SNR) 300mA LED driver is also a very expensive module implemented with separate devices . The narrow light pulse as shown in Figure 2 supports a system that tolerates motion and sunlight. A typical method is a fast light test of 25 times per second at a sampling rate, one measurement with the LED off. The completion is done, and the other time is done with the LED on. The calculated difference eliminates the effects of ambient light, and it is important to give the desired raw optical signal measurements that are insensitive to the flickering background light. Very short pulses of light provide a stronger pulse of light and can be brighter than sunlight, which may be one of the current solutions and the PPG signal carrier is not corrupted by sunlight signals. If the sunlight signal is greater than the PPG carrier, although it can be eliminated by subtraction, the arm movement will bring about difficult to eliminate noise. So the system uses a low current LED driver and a large area of ​​photocell to compensate for motion and sunlight noise. Many of the desired heart rate detection sensing functions can be implemented with just one device through prior design and integration. Most of the circuit's circuitry is integrated into a single chip, enabling a relatively small 3 mm x 3 mm package, which even integrates the photocell itself. Highly integrated, small size, low power, built-in algorithm Si118x HRM module Figure 3 illustrates the circuit principle using Silicon Labs' Si118x optical sensor as an example. Based on this device, the heart rate detection design can be implemented relatively conveniently. Engineers only need to focus on the optical part of the design, including the optical unit between the various parts of the board and how the system fits the skin. A high-performance heart rate detection solution can be implemented that is not a miniaturized or high-efficiency solution that some designers expect. To achieve a smaller solution, the LED core and control chip must be integrated into the same package, integrating all the necessary functions, including the optical unit and the lens that improves the LED output. Figure 4 illustrates this more integrated solution based on Silicon Labs' Si117x optical sensor. This heart rate detection design does not require additional LEDs because the LEDs and photocells are integrated inside the module, and the module can be mounted directly underneath the optical interface, ie inside the back cover of a wearable product such as a smart watch. . This advantageous approach makes the distance between the LED and the photocell shorter than the discrete device design. This shorter distance supports very low power operation because of the lower light loss when penetrating the skin. Integrating multiple LEDs also solves the problem of light leakage between the LED and the photocell, with the result that the designer does not have to add photoresist isolation on the printed circuit board. The result of not taking this approach is to treat the insulation with a plastic or foam insert and a special copper layer on the printed circuit board. Of course, there is also a part of the heart rate detection design that developers don't have to care about: heart rate detection algorithms. This software unit attached to the main processor is very complicated, because the signal deterioration occurs when the user exercises or performs exercise. End-user movements usually produce their own signals, which can deceive true heart rate signals and are sometimes mistakenly recognized as heart rate rhythms. If a developer of a wearable product does not have the resources to develop an algorithm, a third-party vendor can provide the software in a software-licensed transaction. Silicon Labs also offers heart rate algorithms for its Si117x/8x optical sensors that can be run on most processors after compilation. Of course, it is ultimately up to the designer to determine how much integration is needed in a variety of heart rate detection applications. Designers can use a highly integrated modular solution and purchase algorithm authorization to streamline the design process and speed time to market. For developers who have a deep understanding of optical sensing and have plenty of time and resources, you can choose to use separate components (sensors, photocells, lenses, etc.) and your own system integration, and even develop yourself. Heart rate detection algorithm. Car Phone Holder,Mobile Holder For Cars,Mobile Phone Holder For Dashboard,Mobile Phone Holder For Car Dashboard Ningbo Luke Automotive Supplies Ltd. , https://www.car-phone-holder.com