A Novel Adaptive Spectrum Noise Cancellation Approach for Enhancing Heartbeat Rate Monitoring in a Wearable Device

This paper presents a novel approach, Adaptive Spectrum Noise Cancellation (ASNC), for motion artifacts removal in Photoplethysmography (PPG) signals measured by an optical biosensor to obtain clean PPG waveforms for heartbeat rate calculation. One challenge faced by this optical sensing method is the inevitable noise induced by movement when the user is in motion, especially when the motion frequency is very close to the target heartbeat rate. The proposed ASNC utilizes the onboard accelerometer and gyroscope sensors to detect and remove the artifacts adaptively, thus obtaining accurate heartbeat rate measurement while in motion. The ASNC algorithm makes use of a commonly accepted spectrum analysis approaches in medical digital signal processing, discrete cosine transform, to carry out frequency domain analysis. Results obtained by the proposed ASNC have been compared to the classic algorithms, the adaptive threshold peak detection and adaptive noise cancellation. The mean (standard deviation) absolute error and mean relative error of heartbeat rate calculated by ASNC is 0.33 (0.57) beats·min-1 and 0.65%, by adaptive threshold peak detection algorithm is 2.29 (2.21) beats·min-1 and 8.38%, by adaptive noise cancellation algorithm is 1.70 (1.50) beats·min-1 and 2.02%. While all algorithms performed well with both simulated PPG data and clean PPG data collected from our Verity device in situations free of motion artifacts, ASNC provided better accuracy when motion artifacts increase, especially when motion frequency is very close to the heartbeat rate

A low-voltage CMOS-compatible time-domain photodetector, device & front end electronics

During the last decades, the usage of silicon photodetectors, both as stand-alone sensor or integrated in arrays, grew tremendously. They are now found in almost any application and any market range, from leisure products to high-end scientific apparatuses, including, among others, industrial, automotive, and medical equipment. The impressive growth in photodetector applications is closely linked to the development of CMOS technology, which now offers inexpensive and efficient analog and digi-tal signal processing capabilities. Detectors are often integrated with their respective front end and application-specific digital circuit on the same silicon die, forming complete systems on chip. In some cases the detector itself is not on the same chip but often part of the same package. However, this trend of co-integration of analog front end and digital circuits complicates the design of the analog part. The ever-decreasing supply voltage and the smaller transistors in advanced processes (which are driven by the development of digital cir-cuits) negatively impact the performance of the analog structures and complicates their design. For photodetector systems, the effect most importantly translates into a degradation of dynamic range and signal-to-noise ratio. One way to circumvent the problem of low supply voltages is to shift the operation from voltage domain to time domain. By doing so, the signal is no longer constrained by the supply rails and analog amplification is avoided. The signal takes the form of a time-based modulation, such as pulse-width modulation or pulse-frequency modulation. Another advantage is that the output signal of a time-domain photodetection system is directly interfaceable with digital circuits. In this work, a new type of CMOS-compatible photodetector displaying intrinsic light-to-time conversion is proposed. Its physical structure consists of a MOS gate interleaved with a PN junction. The MOS structure is acting as a photogate. The depletion region shrinks when photogenerated carriers fill the potential well. At some point, the anode of the PN structure is de-isolated from the rest of the detector and triggers a positive-feedback effect that leads to a very steep current increase through the PN-junction. This translates into a signal of very high amplitude and independent from light-intensity, which can be almost directly interfaced with digital circuits. This simplifies the front end circuit compared to photodiode-based systems. The physical behavior of the device is analyzed with the help of TCAD simulations and simple behavioral and shot-noise models are proposed. The device has been co-integrated with its driver and front end circuit in a standard CMOS process and its characteristics have been measured with a custom-made measurement system. The effect of bias parameters on the performance of the sensor are also analyzed. The limitations of the device are discussed, the most important ones being dark current and linearity. Techno-logical solutions, such as the implementation of the detector on Silicon-on-Insulator technology, are proposed to overcome the limitations. Finally, some application demonstrators have been realized. Other applications that could benefit from the detector are suggested, such as digital applications taking advantage of the latching behavior of the device, and a Photoplethysmography (PPG) system that uses a PLL-based control loop to minimize the emitting LED-current

Evaluation of Wearable Optical Heart Rate Monitoring Sensors

Heart rate monitoring provides valuable information about an individual’s physiological condition. The information obtained from heart rate monitoring can be used for a wide range of purposes such as clinical diagnostics, assessment of the efficiency of training for sports and fitness, or of sleep quality and stress levels in wellbeing applications. Other useful parameters for describing a person’s fitness, such as maximal oxygen uptake and energy expenditure, can also be estimated using heart rate measurement. The traditional ‘gold standard’ for heart rate monitoring is the electrocardiograph, but nowadays there are a number of alternative methods too. Of these, optical sensors provide a relatively simple, lowcost and unobtrusive technology for monitoring heart rate and they are widely accepted by users. There are many factors affecting the measurement of optical signals that have an effect on the accuracy of heart rate estimation. However, there is a lack of standardized and unified methodology for comparing the accuracy of optical heart rate sensors to the ‘gold standard’ methods of measuring heart rate. The widespread use of optical sensors for different purposes has led to a pressing need for a common objective methodology for the evaluation of how accurate these sensors are. This thesis presents a methodology for the objective evaluation of optical heart-rate sensors. The methodology is applied in evaluation studies of four commercially available optical sensors. These evaluations were carried out during both controlled and non-controlled sporting and daily life activities. In addition, evaluation of beat detection accuracy was carried out in non-controlled sleep conditions. The accuracy of wrist-worn optical heart-rate sensors in estimating of maximal oxygen uptake during submaximal exercise and energy expenditure during maximal exercise using heart rate as input parameter were also evaluated. The accuracy of a semi-continuous heart rate estimation algorithm designed to reduce power consumption for long-term monitoring was also evaluated in various conditions. The main findings show that optical heart-rate sensors may be highly accurate during rhythmic sports activities, such as jogging, running, and cycling, including ramp-up running during maximal exercise testing. During non-rhythmic activities, such as intermittent hand movements, the sensors’ accuracy depends on where they are worn. During sleep and motionless conditions, the optical heart-rate sensors’ estimates for beat detection and inter-beat interval showed less than one percent inaccuracy against the values obtained using standard measurement techniques. The sensors were also sufficiently accurate at measuring the interbeat intervals to be used for calculating the heart rate variability parameters. The estimation accuracy of the fitness parameters derived from measured heart rate can be described as follows. An assessment of the maximal oxygen uptake estimation during a sub-maximal outdoor exercise had a precision close to a sport laboratory measurement. The energy expenditure estimation during a maximal exercise was more accurate during higher intensity of exercise above aerobic threshold but the accuracy decreased at lower intensity of exercise below the aerobic threshold, in comparison with the standardized reference measurement. The semi-continuous algorithm was nearly as accurate as continuous heart-rate detection, and there was a significant reduction in the power consumption of the optical chain components up to eighty percent. The results obtained from these studies show that, under certain conditions, optical sensors may be similarly accurate in measuring heart rate as the ‘gold standard’ methods and they can be relied on to monitor heart rate for various purposes during sport, everyday activities, or sleep