Optical heart rate measurement at the earbud: Page 2 of 6

November 18, 2019 // By Christoph Kämmerer
heart rate
Advancements in sensor technology have transformed how and where people diagnose their vitals and health. Portable, noninvasive measurement techniques permit fast and simple measurements that can be performed while we go about our daily lives. But although this diagnostic technology has become very popular in the fitness industry, there were limits to its accuracy that we have only recently overcome.

The components of the measuring system are as follows. The ADPD144RI chip from Analog Devices is used as an analog front end, which additionally integrates the photodiode and the LEDs. The measurement is supported by a triple-axis accelerometer, which is used not only for the recognition of step patterns and motions but also for artifact removal. The ADXL362 model was used in the present example. The entire process is controlled by the ADuCM3029 microcontroller, which serves as an interface for the various sensors and contains the algorithm.

Fig. 1: Test system with an integrated optical sensor
and accelerometer with scale shown for comparison.

Figure 1 shows the test system, which houses both the optical sensor and the accelerometer in regular earbuds. Care was taken to limit the ADC sampling rate to 100 Hz and minimize the LED intensity to keep the power consumption as low as possible.

For system characterization, five different scenarios were considered for different movement patterns. Only the optical signal was used for evaluation. This allows for the evaluation of what scenarios pulse measurement inaccuracies appear in and when accelerometer data is required for increasing the accuracy of the pulse measurement.

The scenarios cover the following movement sequences:

  • Standing still

  • Standing still and chewing

  • Working at a desk

  • Walking

  • Running and jumping

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