Microelectromechanical technology (MEMS) accelerometers and gyroscopes have been widely used in navigation and gaming applications, and another highlight of such sensors is health care . The current popular application is a diagnostic device that measures the performance of a patient's heart. The function is to measure the electrocardiogram, which requires a series of electrodes to be in contact with the human body.
In a heart rate monitor or exercise device, one can obtain a complex ECG vector map, a typical electrocardiogram, based on the amplitude and timing in the P-QRS-T peak information in the electrocardiogram, or just the timing of the R peak. As shown in Figure 1.
Figure 1: P-QRS-T waveform in ECG
An electrocardiogram can give a lot of information, such as a heart failure or illness, a recovery state of the body, or a physical and psychological stress state. However, the ECG does not measure the pumping function and capacity of the heart very well. In addition, when the ECG is measured, so many electrodes are clamped on the human body, which invisibly exert pressure on the measured person, which also affects the accuracy of the measurement. Fortunately, there are other ways to measure the function of the heart, such as ultrasound meters and projected heart beats. Mechanical projection heart beats typically lag 30--40 mS.
By projecting the heart beat graph, we can derive the mechanical motion of the heart by measuring the force and acceleration of the chest, or measuring the blood pumping activity of the heart by a remote heart beat graph. With the heartbeat technique, measurement can be performed in daily life without any device, and it is sufficient to measure in the uniaxial direction of the human body, because this is the main direction of blood flow.
Projected heart beats provide a global basis for medical predictions, such as detecting physical or mental stress, or early detection of coronary heart disease. As shown in the amplitude of the signal in Figure 2, the stroke volume and its variability of the heart can be measured. From the time series, the overall function of the heart and heart rate and its variability can be seen. Stroke variability makes it easy to monitor breathing and heart rate variability to estimate recovery status and patient stress status. The amplitudes of I and IJ waves in Figure 2 can be used to assess certain diseases, such as aortic valve disease, coronary artery disease, and analysis of life expectancy.
Figure 2: Projected heart beat waveform
When measuring the projected heartbeat data with an accelerometer, the main challenge is the very low level of the acceleration signal relative to the noise of the sensor itself and the environment, as well as the effects of frequency response and mechanical resonance. Measuring the projected heart beat data from the bed can measure sleep activity, physical condition and rehabilitation status, so that no electrodes are required to be installed on the patient's body, which will not affect the patient's comfort or interfere with sleep. This type of measurement can be used to monitor people's sleep problems caused by the body, as well as to monitor athletes to optimize training and avoid overtraining.
In a recent project design, Murata engineers analyzed the patient's activity in the hospital bed and separated the heartbeat data. The heartbeat data is shown in Figure 3. From this, the resonance of the bed can be seen to affect the signal. And decay with time. The reliable measurement signal is in the 10mg range, which requires the resolution of the acceleration sensor to be particularly high. One of the sensors suitable for this application is Murata's SCA61T, which has a noise density of about 14μg/√Hz.
Figure 3: Raw data of heartbeat waveforms acquired using Murata accelerometer SCA61T
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