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1、An Ultra-Wearable, Wireless, Low Power ECG Monitoring SystemChulsung Park and Pai H. Chou University of California,Irvine, CA 92697-2625 USAEmail: chulsung, phchouuci.eduYing Bai, Robert Matthews, and Andrew Hibbs Quantum Applied Science & Research (QUASAR),Inc.5764 Pacific Center Blvd., Suite 107 S
2、an Diego, CA 92121, USAAbstractWearableel ectrocar diograph(ECG) monitoring systemstoday useelectrod esthatrequireskin preparationin advance,andrequire pastes or gels to make electrical contact to the skin. Moreover, they are not suitable for subjects at high levels of activity due to high noise spi
3、kes that can appear in the data. To address these problems, a new class of miniature, ultra low noise, capacitive sensor that does not require direct contact to the skin, and has comparable performance to gold standard ECG electrodes, has been developed. This paper presents a description and evaluat
4、ion of a wireless version of a system based on these innovative ECG sensors. We use a wearable and ultra low power wireless sensor node called Eco. Experimental results show that the wireless interface will add minimal size and weight to the system while providing reliable, untethered operation.I. I
5、NTRODUCTIONElectrocardiograph (ECG) is one of the most widely used biomedical sensing procedures to date. The heartbeat is the definitive indicator for a wide range of physiological conditions. Although ECG instruments were quite bulky, miniaturization in recent years has opened up brand new applica
6、tions by enabling wearable versions to collect data in scenarios that were not possible before.A. Current ECG sensorsMany wearable ECG systemshave been proposed to date. Virtually all of them use some form of electrodes that must make electrical contact with the subjects skin surface. This necessita
7、tes the use of sticky pads, pastes or gel. While this method works for stationary patients, it suffers from several problems. First, the material used to construct the electrode or the paste could cause skin irritation and discomfort, especially if the subject is performing rigorous physical exercis
8、e and may be sweating. Another problem is that, during motion, the electrodes may become become loose, breaking electrical contact and causing high noise spikes in the data. Paste/gel-free resistive contact ECG sensors have been developed. Many of them still suffer from similar noise levels to “wet”
9、 electrodes, and the contact can still cause irritation problems as well as being more sensitive to motion.B. Insulated BioelectrodesRecent breakthroughs have been made in the form of insulated bioelectrodes (IBEs). They can measure the electric potential on the skin without resistive electrical con
10、tact and with very low capacitive coupling. This has been made possible by a combination of circuit design and the use of a new, low dielectric material. These IBEs enable through-clothing measurements, and results over 40 subjects have shown them to be capable of over 99% correlation with gold stan
11、dard conventional electrodes.Fig. 1. QUASARs ECG Sensor and Eco shown with a dime coin for scaleC. Wireless Enabled IBEsTo realize the full potentials of these IBEs, they need to be built in a miniature, wearable form factor and be wirelessly enabled. To accomplish this, we are integrating an existi
12、ng wireless sensor node platform with the IBE. These IBEs can be very sensitive to outside noise sources and adding a digital wireless interface could introduce noise in different forms. Therefore, an important consideration in this case is to test the data quality in such a setup.II. BACKGROUNDSeve
13、ralwireless ECG monitoring systems have been proposed.Allofthemuseconventional“wet”ECGsensors. For data sampling and wireless transmission, they use either existing standard wireless interfaces or general-purpose wireless sensor nodes. This combination results in many system-level drawbacks such as
14、big form factor, low transmission speed, short battery lifetime, and lack of wearability. In this section, we first review the previous works and discuss their shortcomings. Next, we demonstrate the design goals of our system to resolve the discussed problems.A. Related WorkThe most recent work 1 is
15、 a system which uses the Tmote Sky platform 7 and a 3-lead system based on conventional ECG sensors. Tmote has an 802.15.4 radio interface (for Zigbee) at 250 Kbps and is controlled by the MSP430F1611 microcontroller. The authorsdesignedaninterfaceboardbetweenTmoteandECGsensors. Also, they use a PDA
16、 for data collection and Wi-Fi or GPRS for the host interface. This system samples one ECG sensor at 1 KHz (16-bit resolution). However, this system is too obtrusive to be worn,considering its sensor node is larger than 66(L) 32(W) 15(H) 3mm . Also, its conventional ECG sensors must be attached dire
17、ctly onto the skin, which is another main obstacle towards wearability.Other work done at Harvard University 2 used the MICA series and Telos platforms for their first prototypes. In order to make more integrated systems, they developed their own mote platform called Pluto. Pluto is functionally ide
18、ntical to Tmote Sky . It 3 measures 57(L) 36(W) 16(H) mm , which is still as large as Tmote Sky. It also uses standard ECG sensors.Researchers at Imperial College developed their own wireless sensor node, called the BSN node. It measures 28(L) 37(W) 3 12(H) mm (w/ a sensor board and w/o a battery).
19、They used this platform to design a wireless ECG monitoring system . This system also used the 802.15.4 radio and conventional ECG sensors. Most systems use the 802.15.4 radio, even though it was originally developed for event-detection applications rather than real-time monitoring ones. There are t
20、wo exceptions . The first one uses a CC1050 transceiver (at 76.8 Kbps max), which is similar to MICA2s transceiver. The other one uses a Bluetooth interface (721Kbps max). Neither radio interface was originally designed for realtime monitoring applications.B. Design GoalsTo address the problems desc
21、ried above, we are designing a new ECG monitoring system. There are four main design goals: Ultra-Wearability, High Throughput, Low Power, Universal Connectivity. Ultra-Wearability Wearability is the most crucial issue in designing a wireless ECG monitoring system. However, to the best of our knowle
22、dge, none of the existing miniature sensing systems can be considered truly wearable in the strict sense, not just because they are still bulky but also because conventional ECG sensors can cause skin irritation. Therefore, we are using QUASARs innovative ECG sensor and an ultra-compact wireless sen
23、sor node specially designed or wearable applications. High Throughput The other design goal is to achieve high network throughput, which is necessary for a low latency/realtime monitoring system. We decided to use a 1 Mbps proprietary radio instead of 802.15.4. Although in-sensor processing can redu
24、ce bandwidth demand, our chosen faster radio with a simpler Media Access Control (MAC) is actually more energy efficient, as discussed next. Low Power Low power consumption is another highly important design goal. Low power consumption contributes not only to prolonged lifetime, but also to system m
25、iniaturization, because the size of a battery occupies more than 50% of system volume. It is well known that the most power hungry component in a wireless monitoring system is the wireless transceiver. Therefore, we carefully chose a very low power transceiver that consumes less than 10 mA in transm
26、ission mode (1 Mbps, 0 dBm) and 22 mA in receiving mode.Universal Connectivity Universal connectivity means that the nodes should able to connect to virtually any computer on one of its communication interfaces. This versatility is necessary for applications that not only collect and record or repla
27、y data, but also integrate them with actuators and other infrastructures. Thus, wedesigned our system to be able to transmit data via most common communication interfaces including USB, Ethernet, and Wi-Fi.III. SYSTEM DESIGNIn order to achieve the design goals described in the previous section, we a
28、re developing a new ECG monitoring system that takes advantage of QUASARs ECG sensors 11 and Eco wireless sensor nodes 12. The QUASAR sensor is a wearable, tiny, low-power ECGsensingdevice,andEcoisanultra-compact,low-powerwireless sensor node. Fig. 1 (a) and (b) show the QUASAR ECG sensor and Eco wi
29、th a US dime coin for reference of scale. They are similar in size, power, and are well matched in terms of data rate. This section first shows various system architectures for an ECG monitoring system that integratesthesetwotechnologies.Next,weexaminethespecifications of the QUASAR ECG sensor, Eco
30、node,andthe base station in detail. Fig. 2. System Architecture of ECG Monitoring SystemA. System ArchitecturesOur ECG monitoring system can be functionally divided into four subsystems: ECG Sensors, Data Sampling, Wireless Transmission, and Host Interface. ECG signals are first digitized by ADCs an
31、dtransmitted wirelessly to a base station that interfaces with a host computer via USB, Fast Ethernet, or 802.11b. We propose three different system architectures as shown in Fig. 2. The first architecture Fig. 2(a) consists of multiple ECG sensors, a data-sampling module, an Eco node, and a base st
32、ation. In this architecture, the system has a separate data-sampling module, which contains a microcontroller unit (MCU) and ADCs as shown in Fig. 1(c). All signals from ECG sensors are first sampled and buffered in this module. Then, data are fed to Eco via SPI and transmitted wirelessly to the bas
33、e station. This architecture uses two MCUs to distribute workload. The MCU in the data-sampling module is dedicated to sampling signals. The other MCU in the Eco node handles wireless data transmission. By adopting this two-MCU architecture,we can achieve very accurate data sampling (low jitter) as
34、well as high communication throughput and low latency 13. In addition, the system can have separate high resolution ADCs and increase monitoring granularity without changing the main MCU. However, this architecture imposes extra cost and volume for the data-sampling module, and it also needs a data
35、transmission protocol between the two MCUs. Fig. 3. (a) QUASAR ECG sensor worn by subject, (b) Data Comparison:Green trace is from QUASAR sensor, Blue trace is from conventional electrodeThe second architecture Fig. 2(b) uses the Eco for both datasampling and wireless transmission. All the ECG signa
36、ls are directly fed to the ADC channels on the Eco node. The ADC built into the main MCU on the Eco node digitizes and transmits data according to its timing requirements. In this architecture, one Eco has to run all sensing and communication tasks by itself. Therefore, systemperformance is degraded
37、 as the number of ECG sensors increases. Another system issue is that the number of ECG sensors that can be monitored simultaneously is limited by the number of Ecos ADC channels (8 channels in this case). Multiplexing also means that the samples on multiple channels cannot be taken precisely at the
38、 same time. However, this is a very simple and low cost design, which is suitable when the system is equipped with only two or three ECG sensors. The third architecture Fig. 2(c) uses one Eco node for each ECG sensor. This architecture is similar to the second one in the sense that the Eco node perf
39、orms both sampling and communication tasks, except that an Eco node serves only one ECG sensor in this case. Therefore, we can achieve higher throughput and lower latency as well as lower jitter. This is also a more reliable and distributed architecture than the other two. However, we need a sophist
40、icated MAC protocol to coordinate the wireless traffic among several Ecos. The cost will increase in proportion to the number of monitoring points.B. ECG SensorQUASARs sensor (Fig. 3) is a compact ECG sensor that does not require skin preparation, gels, or adhesives. It includes not only a sensing d
41、evice, but also signal conditioning circuitry such as lownoise amplifiers and voltage reference chips. Its output signal range is adjustable from differential (-4.5 V to 4.5 V) to single-ended (0 V to 4.5 V). It measures only 15 mm (in diameter) 3.8 mm (in height) and weighs 5 g. Also, it consumes o
42、nly 1 mW active power on average. These features enable our monitoring system to betruly wearable. This sensor measures ECG signals using capacitively coupled electrodes that do not require ohmic contact. As shown in Fig. 4, QUASARs sensors have at least equivalent and often superior signal quality
43、and artifact rejection compared to the standard wet/resistive ECG sensors.Fig. 4. Eco Dimensions: 13(L) 11(W) 7(H) mmFig. 5 Eco PCB: (a) Top View, (b) Bottom View C. Eco Wireless Sensor Node Eco is an ultra-compact and low power wireless sensor node developed by the coauthors at UC Irvine. It measur
44、es only 13 mm(L) 11 mm(W) 7 mm(H) and weighs 2 grams (Fig. 5). Also, it consumes less than 10 mA in transmission mode (0 dBm) and 22 mA in receiving mode. Its maximum data rate and RF range are 1 Mbps and 10 m, respectively. Considering its small form factor and low power consumption, Eco is very su
45、itable for real-time biomedical signal monitoring applications, which require relatively high throughput, low latency, and high wearability.Fig. 6 shows photos of the Eco hardware. Eco uses Nordic VLSIs nRF24E1, a 2.4 GHz RF transceiver with an embedded 8051-compatible MCU (DW8051). The MCU has a 51
46、2-byte ROM for a bootstrap loader, a 4 KB RAM for the user program, SPI (3-wire), RS-232, and a 9-channel ADC. The ADC is software-configurable for 612 bits of resolution. A 32 KB serial (SPI) EEPROM stores the application program. The nRF24E1s 2.4 GHz transceiver uses a GFSK modulation scheme with
47、125 frequency channels that are1 MHz apart. The transmission output power is also software-configurable for four different levels: -20 dBm, -10 dBm, -5dBm, and 0 dBm. The RainSun chip antenna (AN9520) measures 9.5 mm(H) 1.5 mm(W) 1 mm(H) and has a maximum gain of 1.5 dBi.Fig. 6. Base Stations for EC
48、G monitoring SystemIn addition, Eco has a 3-axial acceleration sensor, Hitachi-Metals H34C, which measures acceleration from -3gto +3 g and temperature from 0 75?C. Eco also has a light sensor (S1087). Ecos power subsystem includes a regulator (LTC3410), battery protection circuitry, and a custom 40
49、 mAh rechargeable Li-Polymer battery. LTC3410 is an adjustable boost regulator whose output voltage is set to 2.7 V. Its maximum output current is 300 mA, and its average efficiency over a Li-Polymer batterys output voltage range (3.0 V 4.2 V) is higher than 90% at Ecos maximum operating current of 30 mA.Eco has a flexible-PCB type expansion port that has 16 pins. This expansion port includes tw
