Wearable ECG SoC for Wireless Body Area Networks: Implementation with Fuzzy Decision Making Chip
Summary (3 min read)
1. Introduction
- According to World Health Organization (WHO), cardiovascular and modern human behavior-related diseases are the major cause of mortality worldwide.
- Wearable sensors/electrodes (deployment in accordance with the clinical application) collect the physiological signals for monitoring the patient’s health status.
- To aid low cost, ultra-low power design is essential for developing wearable devices.
2. Prior Art
- Wireless Body Area Network (WBAN) is the fundamental component of a wireless ECG monitoring system.
- WBAN allows the integration of various other components, like intelligent systems, miniaturized components, low-power sensor nodes, etc.
- For terrestrial and space applications, physiological parameters of the astronauts in space should be monitored continuously.
- An integrated wireless ECG SoC for WBSN applications is proposed in [6], which comprises a two-channel ECG front-end, an 8-bit SAR-ADC, a simple micro- controller, a SRAM memory, and RF-transceivers.
- This increases the need for low cost and easy to use wearable wireless ECG sensors with integrated decision making to alert personnel.
3.1 System Overview
- The proposed health care architecture includes two parts: (a) Main unit and (b) Remote unit, as shown in Fig.2.
- The remote unit (personal gateway) can be a mobile phone or a personal computer with an USB interface.
- The main unit records the ECG from wearable textile electrodes and wirelessly transfers the data to a remote unit.
- The Fuzzy Decision Making (FDM) chip (3×3 fuzzy controller; nine rules are accessible) takes decisions when necessary.
- ZigBee protocol is chosen as a wireless communication protocol (TI CC2420) to provide reasonable power consumption and adequate data rate.
A. ECG Analog Front-end Amplifier
- The ECG front-end amplifier is mainly responsible for noise suppression, signal conditioning, and amplification, which comprises two phases as shown in Fig.3, namely, low noise AFG with band pass function and a programmable gain amplifier (PGA) to amplify the acquired ECG signals (from textile electrodes), with amplitude in a few millivolts, adopting a flip-over-capacitor technique.
- The Low noise amplifier not only acts as a preamplifier, but also acts as a band pass filter function with bandwidth between 0.3 and 100 Hertz.
- In the AFG design, two switches (S1 and S2) are integrated to settle down quicker when power is applied, due to the large resistance by the pseudo-resistors.
B. Analog to Digital Converter (ADC)
- Successive Approximation Register (SAR) ADC is chosen for this WBSN application because of its moderate accuracy and low power overhead.
- Fig. 4 depicts the architecture of the SAR ADC, adopted from literature.
- The analog ECG output is driven directly by the preceding buffer stage, without the need of an additional hold amplifier, sampled through a bootstrapped switch and held in the capacitive 12 bit DAC, and is then used by open-loop Sample/Hold.
- The reason for open-loop Sample/Hold is to obtain low power, low cost, fast settling, and less offset error.
- An on-chip crystal oscillator is used to drive the logic and timing sequence for achieving low power consumption and low jitter.
C. Heart Rate Calculation and QRS Detection
- The morphological filter [8] is adopted to reduce the noise artifacts present in the ECG data and to detect\estimate the QRS complex details and R-R intervals.
- The filter comprises a pair of Opening and Closing operations, using dilation and erosion operators, which suppress peaks and valleys.
- The current threshold is updated regularly when a new Rpeak is identified.
- By counting the number of clocks between R peaks using a binary counter, R-R interval is measured.
- A parallel-to-serial converter is integrated with the wearable system for transmitting the HR variable through the SPI interface.
D. System Control Unit and SPI Interface
- System Control Unit (SCU) is solely responsible for generating the interface control signals, based on the host or main controller commands for all the blocks in ECG on-chip.
- In-order to interface the chip with various host CPUs, the System Control Unit uses an asynchronous FIFO with 8 Kb buffers.
- Data from the ADC and QRS block is continuously written into the FIFO at the sampling frequency of 256Hz.
- Based on the FIFO status, FIFO write/read controllers generate many status signals, which are “full”, “nearly full”, “empty”, and “nearly empty.”.
- A microcontroller is employed externally to communicate with the proposed wearable device via a duplex SPI communication interface.
4. Results and Discussion
- The wearable ECG sensor node system fits perfectly on a shirt.
- The main unit provides a versatile framework for incorporating sensing, monitoring, and information processing devices.
- The inference performance test is done, based on physical activity under various conditions.
- The abnormal ECG signal is measured and stored in the fuzzy inference engine.
- A wearable smart shirt transfers the physiological ECG signals over a wireless sensor network at the test.
4.1 ECG Acquisition
- To ensure comfort, the clothing is designed from a knitted conductive textile fabric for reducing flammability.
- The conductive textile fabric is realized from a blended yarn of the composite containing silver nanoparticles, which provide electrical conductivity of the yarn and the resultant knitted fabric.
- The content of silver nanoparticles provides corrosion resistance of textile electrodes, antibacterial and anti-allergic properties, and mechanical and electrical stability when exposed to sweat.
- The designed conductive textile fabrics are circular in shape, with dimensions 5 × 5 cm. Fig.12 shows the wearable electrodes, which comprise a conductive fabric electrode pair and the wearable sensor node system placed on the wearer’s chest placement.
- To provide a sufficient potential difference, the electrodes are positioned 100 mm apart.
4.2 Performance Evaluation
- The status is continuously sent to the remote unit every 2 minutes or preset time in the controller.
- When the signal is sensed, the system detects the status, and if abnormal, an alert signal is transmitted.
- Therefore, the proposed system can make decisions, based on the acquired ECG data.
- Number of data sets used for testing Number of data sets correctly classified Number of data sets wrongly classified Accuracy (%) Drowsiness 89 82 7 92 Sleep Onset 102 98 4 96 Normal 213 213 0 100 Fig.14 depicts the designed graphical user interface for the proposed architecture.
- Designed Graphical User Interface for testing and measurements.
5. Concluding remarks
- A wireless ECG on a chip with an integrated Fuzzy Decision making system is proposed for real-time ECG health monitoring.
- The proposed wearable device is small, user-friendly, has a long battery life, and is capable of wirelessly transmitting ECG data continuously to a remote station for detailed diagnosis.
- The FDM chip is integrated with ECG on Chip to take the decisions for alerting the patients when necessary.
- The designed FDM responds immediately when anomalies are found in ECG data.
- The proposed device has already been tested with a reference high-quality measurement system for verification of accuracy and showed that the accuracy of the proposed device is good enough, and the variation in key ECG parameters obtained from the proposed device and the reference device is acceptable for clinical usage.
6. References
- Manikandan Pandiyan, Geetha Mani, Jovitha Jerome, Natarajan Sivaraman.
- Design of an analog CMOS fuzzy logic controller chip.
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"Wearable ECG SoC for Wireless Body ..." refers methods in this paper
...In fuzzy interface, three basic circuits are used: a ramp generator (RG) circuit [8], a minimum circuit, and a fuzzy complementary circuit....
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...A novel defuzzifier is used [8,10] in which the center of the area is...
[...]
...The ECG samples are loaded serially into the shift register and then added/subtracted (for dilation/erosion respectively) with the structure element g(x) [8]....
[...]
...The morphological filter [8] is adopted to reduce the noise artifacts present in the ECG data and to detect\estimate the QRS complex details and R-R intervals....
[...]
...Level shifter circuit (LSC) is used to compensate offset voltage [8]....
[...]
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Frequently Asked Questions (24)
Q2. What is the basic circuit used in a fuzzy interface?
In fuzzy interface, three basic circuits are used: a ramp generator (RG) circuit [8], a minimum circuit, and a fuzzy complementary circuit.
Q3. What is the content of silver nanoparticles?
The content of silver nanoparticles provides corrosion resistance of textile electrodes, antibacterial and anti-allergic properties, and mechanical and electrical stability when exposed to sweat.
Q4. What is the main cause of mortality worldwide?
According to World Health Organization (WHO), cardiovascular and modern human behavior-related diseases are the major cause of mortality worldwide.
Q5. What are the frequency bands used to determine the power density of the conductive textile fabric?
Power Spectral Density of HR variations is calculated, and the three frequency bands, such as Very low frequencies (VLF: 0-0.04 Hz), Low frequencies (LF: 0.04-0.15 Hz), and High frequencies (HF: 0.15-0.5 Hz), have been utilized.
Q6. What is the purpose of the ZigBee protocol?
ZigBee protocol is chosen as a wireless communication protocol (TI CC2420) to provide reasonable power consumption and adequate data rate.
Q7. What is the purpose of the prototype model?
The prototype model is designed for patients, regarding comfort and ease of use, thus, not affecting regular activities of patients.
Q8. What is the responsibility of the SCU?
System Control Unit (SCU) is solely responsible for generating the interface control signals, based on the host or main controller commands for all the blocks in ECG on-chip.
Q9. What is the effect of the defuzzifier on the pectoral muscles?
Rectangles of electrically conductive textile fabric in knitted design were stitched on the position of the pectoral muscles [13].
Q10. What is the purpose of developing a wireless body area network?
Developing SoC for Wireless Body Area Network applications intends to carry healthcare monitoring closer from clinical intervention to domiciles.
Q11. What is the main idea of the defuzzifier?
The main idea is based on parallel conductances 𝑔\\, stating implicitly that the output voltage of the defuzzifier circuit is the average value of the inputs.
Q12. What is the proposed integrated wireless ECG SoC for WSBN applications?
An integrated wireless ECG SoC for WBSN applications is proposed in [6], which comprises a two-channel ECG front-end, an 8-bit SAR-ADC, a simple micro-controller, a SRAM memory, and RF-transceivers.
Q13. What is the main challenge of the wearable ECG monitoring system?
Thisprimarily requires the support of innovative sensor technologies, especially Wireless Body Sensor Networks (WBSN), formed with various wearable biomedical sensors.
Q14. What is the architecture of the controller in Fig. 6?
The controller architecture in Fig. 6 is constructed with CMOS components, such as Membership function generator (MFG), MIN circuits and a defuzzifier (D blocks) circuit.
Q15. What is the design of the user interface for the proposed system?
Designed Graphical User Interface for testing and measurementsA wireless ECG on a chip with an integrated Fuzzy Decision making system is proposed for real-time ECG health monitoring.
Q16. What are the requirements of a Fuzzy Classifier chip?
a Fuzzy Classifier chip meets the critical requirements of medical applications: no delay in response, reliable, high-safety, and low cost.
Q17. What is the purpose of the ECG front-end amplifier?
The ECG front-end amplifier is mainly responsible for noise suppression, signal conditioning, and amplification, which comprises two phases as shown in Fig.3, namely, low noise AFG with band pass function and a programmable gain amplifier (PGA) to amplify the acquired ECG signals (from textile electrodes), with amplitude in a few millivolts, adopting a flip-over-capacitor technique.
Q18. What is the purpose of the fuzzy logic engine?
The features extracted from HRV and PSD are used to feed the fuzzy logic engine that computes epoch-by-epoch (30 or 60 seconds per period) inferences.
Q19. What is the structure element used to add/subtract the ECG samples?
The ECG samples are loaded serially into the shift register and then added/subtracted (for dilation/erosion respectively) with the structure element g(x) [8].
Q20. How is the impact of wandering baseline drift eliminated?
The impacts of wandering baseline drift are eradicated by subtracting the mean result of operations (opening and closing) with the original input.
Q21. What is the main reason for the need for a wearable system for continuous monitoring?
In applying measurement of physiological signals for continuous monitoring, patients usually cannot carry a bulky instrument, which restricts their mobility and makes them uncomfortable, with so many electrodes and cables attached to their bodies.
Q22. What is the definition of an electrocardiogram?
Electrocardiogram (ECG) embodies the cardiovascular condition, therefore, is considered one of the most important human physiological signals.
Q23. What is the need for wearable wireless ECG sensors?
This increases the need for low cost and easy to use wearable wireless ECG sensors with integrated decision making to alert personnel.
Q24. What are the main advantages of wearable technology?
the combination of SoC concepts, wearable technology, Wireless Sensor Network (WSN), and research in artificial intelligence produce novel approaches, resulting in better health care services.