from several individuals while performing four different physical activities. These included table tennis, soccer, Reserve Officers' Training Corps (ROTC) Training ...
Development of a Physiological Activity Monitoring Platform Stephen Sherbrook, Tolga Kaya Central Michigan University Mt. Pleasant, MI 48858 Email: {sherb1sc, kaya2t}@cmich.edu
1. Introduction Physiology is the scientific study of an organism’s vital functions. The need to study the physiological activity of the human body is growing and is vital to the advancement of understanding how the human body handles the stress of various situations. This is particularly true in the field of athletics and the military where the human body is deliberately pushed beyond its normal operation. Being able to study what external conditions the body is being subjected to at any moment is crucial to understanding why the human body behaves the way it does and could ultimately lead to the development of technologies to help manage the body under these conditions. The first area of interest is the temperature of the atmosphere surrounding the body. The ability to know the temperature of the atmosphere very close to the body and monitoring it in real time can be useful in determining how the body reacts to different temperatures. It can also be supportive in determining the rate at which the body radiates heat under different circumstances. The next area of interest is the relative humidity of the atmosphere surrounding the body. Again, the ability to know the humidity of the atmosphere surrounding the body is important in understanding how the body reacts to different humidity levels during different activities. Combining relative humidity with body heat allows for the body heat index (BHI) of a subject to be determined. This is extremely crucial since BHI determines what temperature the body actually feels and, moreover, how the body reacts to what it perceives [1]. The last area of interest to be monitored is the gravitational forces the body is experiencing in the horizontal and vertical directions. This allows for a three-dimensional view of what forces the body is subject to during various situations [2, 3, 7]. Knowing what forces the body is being subjected to at any moment could be very useful in determining what forces the body can handle before any damage is done. Combining these three sets of data, researchers can monitor the physiological activity of an individual. Using commercially made temperature and humidity sensors, a three-axis accelerometer to measure forces, and a wireless communication network, a physiological activity monitoring platform can be developed to monitor an individual in real time. This working model will serve as a research platform for a team of researchers designing an integrated circuit that encompasses all of these functions.
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2. Previous Work Fadiran, et al. [1], have conducted research concerning the assessment of an individual’s BHI. They used a commercially available temperature and relative humidity sensor and collected data from several individuals while performing four different physical activities. These included table tennis, soccer, Reserve Officers' Training Corps (ROTC) Training, and an outdoor run. To perform their tests, they placed the sensor on the test subject’s chest [1]. Their tests found that during physical activity, the subject’s BHI would increase as expected; however, once they started to sweat, their BHI would fluctuate as the sweat cooled the body. Vaz, et al. [8], have devised a temperature monitoring system that utilizes ultra high frequency (UHF) radio frequency identification (RFID) to transmit the data over moderate distances. This is important because it allows the sensor to operate without needing to be powered by batteries, greatly decreasing the area of the sensor. Pinet, et al. [9], proposed the use of optical fiber sensors (OFS) to measure vital functions like temperature. Advantages of OFS systems are their small size and immunity to electromagnetic interference that is emitted by other electronic devices. Chen, et al. [2], has also researched physiological activity monitoring platforms. The focus of their research was on using accelerometers and pedometers to monitor the different motions an individual experiences while performing physical activities. It was found that using an accelerometer as a pedometer was not only useful in counting the number of steps taken by an individual, but also in measuring the amount of force of each stride [2]. Therefore, the use of accelerometers to measure various types of motion can be useful in determining the energy expenditure of an individual [2]. Bonomi, et al. [3], and Bouten, et al. [7], have also performed extensive research on physiological activity monitoring using accelerometers. They proposed using multiple accelerometers placed on various parts of the body to attain a more detailed view of the subject’s motions. Not only could steps be counted, but posture could also be estimated. Using multiple sensors also opens up the possibility of being able to determine what kind of activity the subject is doing, such as walking, running, vacuuming, etc.[3] This kind of data could also greatly improve the accuracy of energy expenditure calculations. Using multiple accelerometers to monitor all of these different areas is quite limiting, however, due to the fact that separate sensors need to be placed in many different places on the body and likely need to be linked together. This creates a great hindrance to the subject’s motions. Najafi, et al. [4], proposed a new ambulatory measurement system using only one small kinematic sensor. This single sensor system could detect body posture and walking motion by placing the system on the subject’s chest. This type of system does not interfere with the subject’s activities due to its small size and placement. Safeer, et al. [6], and Kern, et al. [10], handled the problem of sensor interference to physical activity by placing many sensors on different parts of the body and weaving the wires connecting them into the fabric of the clothing being worn. Pook [5] expanded on this type of system and devised a completely wireless system that allows the subject to wear many sensors on different parts of the body without any hindrance to physical activity due to wires connecting the sensors. 2 Proceedings of the 2012 ASEE North Central Section Conference Copyright © 2012, American Society for Engineering Education
Based on the results of the previous works studied, our physiological activity monitoring platform will take commercially made sensors and integrate them onto a single small board that is approximately 2”x2”. This will ensure that the platform will not interfere with the subject’s activities by having various sensors placed on different parts of the body. To achieve such a small circuit size, surface mount components will be used wherever possible. An individual temperature sensor and an individual humidity sensor will allow researchers to not only collect temperature and humidity data but also combine the data to determine the subject’s BHI. A 3axis accelerometer will be used so that an accurate depiction of body position and the forces being experienced can be obtained. All of this data will be transmitted through a wireless transmission network so that data can be captured in real time, thus reducing the risk of not being able to collect any data if the platform is damaged during use. 3. System Design In order to simultaneously monitor all three of these sensors, a circuit will be developed combining all three of the sensors. The circuit will run on small (9.0x10.5mm), low-profile (3mm thick), 3 volt rechargeable batteries weighing 2.25 grams. To send the data through the wireless communication network, a series bit-stream must be generated. Since the information must be sent in series, the data from each sensor must be isolated from the others but also sent down the same path. Therefore, an 8-to-1 analog multiplexer (MUX) will be used to send the data from each sensor one at a time. Since data is only available from five sensors (the accelerometer outputs a separate signal for each x-,y-, and z-axis), the remaining three inputs of the MUX will be tied to the positive supply rail. This will allow researchers to distinguish when data from the sensors are being received and in what order. This analog data will be sent to a 10bit analog-to-digital (A/D) converter that is operating at ten times the frequency the multiplexer is switching at. Doing this will allow the A/D converter to convert each sensor’s analog signal to a 10-bit digital bit stream and output all 10-bits before the multiplexer switches to the next sensor. To achieve this, a three-bit counter will be constructed using an analog programmable timer and JK flip-flops. This timer will control the MUX’s select lines directly. In order to achieve a frequency at exactly ten times the clock frequency of the counter, a phase-locked-loop (PLL) will be set up that will connect the clock signal from the counter to the A/D converter and will boost it to exactly ten times the frequency. The digital signal will then be sent to the wireless communication network.
Sensors
8:1 MUX
A/D Converter
3-bit Counter
PLL
Transmitter
Receiver
Clock 3 Proceedings of the 2012 ASEE North Central Section Conference Copyright © 2012, American Society for Engineering Education
Figure 1: Block Diagram of the Physiological Activity Monitoring Platform
4. Circuit Components To measure temperature, the Analog Devices TMP35GT9Z will be used. The sensor is a simple three-terminal device that outputs a voltage proportional to temperature. At 25°C, the sensor will output a voltage of 250mV. The sensor has a linear relationship between temperature and voltage with a slope of 10mV/degree Celsius. The sensor can detect temperatures ranging from 10°C to 125°C. To measure relative humidity (RH), the Honeywell HIH-4000-001 will be used. The sensor is a simple three-terminal device that outputs a voltage proportional to the relative humidity. The sensor can determine the percent relative humidity on a scale of 0% to 100%. At 0% RH, the sensor outputs a voltage of about 500mV. As the RH increases, the voltage increases at about 20mV for every %RH increase. To measure the gravitational forces, the Analog Devices ADXL335Z evaluation board will be used. The accelerometer can measure changes in gravitational forces (g-forces) in the x-, y-, and z-directions. The device outputs a voltage proportional to the g-forces. For every unit change of g-force, the output voltage changes by 350mV for the x- and y-axes, and 550mV for the z-axis. Therefore, a three-dimensional view of the forces being experienced can be measured. To convert the analog information to a transmittable digital 10-bit serial data stream, the Texas Instruments TLC1549CP analog-to-digital converter will be used. This A/D converter converts analog data to digital data using switched-capacitor, successive approximation. In order to perform the conversion, the A/D converter must be supplied reference voltages to provide the input voltage range. In this case, the upper reference will be the positive supply rail (3V) and the lower reference will be the circuit’s ground (0V). Therefore, by using a 10-bit A/D converter with a 3V supply, a resolution of 3V/2^10 = 3V/1024 = 2.93mV is achieved. For an analog signal that is one half of the power supply (1.5V), the corresponding binary number will be 1000000000. By increasing the voltage to 1.50293V, according to the resolution, the A/D converter will output 1000000001, for example. Once the data has been converted to a serial bit-stream, it will then be sent to a wireless communication network. For this purpose, Digi XBee Pro RF modules will be used. These modules utilize Zigbee protocols to transmit data wirelessly over significant distances. Using the XBee Pro RF modules, an indoor range of 133 feet can be realized. Once the data has been transmitted to the receiver, researchers will be able to pull the digital bit-stream and send it to a digital display or save it to a memory device. 5. Results Each sensor will output its own unique analog voltage. The TMP35GT9Z, for example, will output 250mV at room temperature. As the temperature increases, the output voltage increases 4 Proceedings of the 2012 ASEE North Central Section Conference Copyright © 2012, American Society for Engineering Education
10mV/°C. The HIH-4000-001 outputs 500mV at 0% RH and increases at a rate of 20mV/%RH. The ADXL335Z will output about 1.5V at 0g and change at a rate of 0.3V/g. If the sensor’s environments are perfectly static, each sensor should steadily output each voltage listed above. Each sensor’s output will be tied to the input of an 8:1 MUX that will route each input sequentially to the output. The first three inputs of the MUX will be tied to the positive supply rail, so for the first three switching cycles, the output will be 3V. The MUX will then switch to the next input, which will be the temperature sensor, and should output about 250mV. Next, it will output the humidity sensor which is at about 1V. Finally, it will output the data from the accelerometer where each output should be between 1-2V at rest depending on the position it is resting at.
First three cycles at 3V
z-axis
Humidity y-axis Temperature x-axis
Figure 2: MUX Output Over a Complete Switching Cycle
This data will then be sent to the A/D converter. The A/D converter will take its input and convert it to a 10-bit serial bit-stream. Therefore, in order to get a full 10-bit representation of each sensor’s data, the A/D converter will operate at ten times the frequency that the MUX is switching through its inputs. Since the input will initially be at 3V, which is the full reference voltage, for the first three switching cycles of the MUX, the A/D converter will output three strings of 1111111111. This string of 1’s will be used to initialize how the data will be collected from the system and indicates that the sensor data, in the order of temperature, humidity, acceleration, will follow the third string of 1’s.
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First 3 cycles of 1’s
Sensor Data
Figure 3: Expected A/D Converter Output Over a Complete Cycle
Clock 1 Accelerometer Counter 8:1 MUX
A/D Converter Clock 2 (10x Frequency of Clock 1)
Temperature Humidity
Figure 4: Physiological Activity Monitoring Platform Test Circuit
6. Future Work Thus far, the circuit has been proven to work up until the wireless communication network. Accordingly, the wireless communication network using XBee Pro modules will need to be programmed and proven. Once the information is received by the receiver, the information will then be fed into a display so the data can be monitored in real time.
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Another feature of the platform that is desired is the ability to save the data directly on to a memory card before the information is sent to the wireless communication system. The proposed method is using microSD cards due to their small size and ability to retain data indefinitely so the card can be removed from the platform and loaded onto another device without any corruption of data that may be inevitable due to the wireless communication network. Lastly, in order to actually use the monitoring platform and perform tests on individuals, a printed circuit board will be designed and fabricated utilizing a micro-fabrication facility. This will allow for the circuit to only cover a very small area so that it does not hinder the test subject’s activities. 7. Relevance to Engineering Education This project was done as a research design project with the goal of providing a working model of a physiological activity monitoring platform to measure temperature, relative humidity, and acceleration using off-the-shelf components. This system will be crucial in guiding a research team in learning the specifications necessary to fabricate their own integrated multiple-sensor array that an individual will be able to wear without any hindrance to their activities. The work done on the wireless communication system will be crucial in providing a learning platform and insight into the customized circuit designs needed to transmit data wirelessly. The project will also aid in the development of a micro-fabrication facility. 8. Conclusions Developing a physiological activity monitoring platform made from commercial components will be a beneficial tool for monitoring the vital systems of individuals performing various activities. By integrating readily made commercial components and sensors, the research team will have a working model for how their integrated system should operate. This will provide them with the opportunity to study a functioning system and determine the specifications they require for the sensors they are fabricating. Based on the data collected, each sensor outputs the expected voltages based on their specifications. The resolution of the A/D converter provides accurate digital representations of the sensor’s outputs and allows for the use of a wireless communication system. Utilizing a wireless communication system will also be beneficial in providing insight into developing the customized circuits required for transmitting the data from the research team’s sensors. Integrating all of these systems using surface mount components onto a 2”x2” circuit board and using small, low-profile batteries to power the circuit, ensures that the platform will not interfere with the subject’s activities.
Bibliography
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[1] Fadiran, O., Kaya, T. Heat Index Characterization of the Human Body for a Wireless Track and Alert Sensor System (WTASS). Mount Pleasant: American Society for Engineering Education (ASEE) North Central and IlinoisIndiana Section Conference, 2011. [2] Chen C., Anton S., Helal A., A Brief Survey of Physical Activity Monitoring Devices. Technical Report MPCL08-09, 2008. [3] Bonomi1, A., Westerter, K. Advances in physical activity monitoring and lifestyle interventions in obesity: a review. International Journal of Obesity. 2010. [4] Najafi, B., Aminian, K., Paraschiv-Ionescu, A., Loew, F., Büla, C. Ambulatory System for Human Motion Analysis Using a Kinematic Sensor: Monitoring of Daily Physical Activity in the Elderly. IEEE Transactions on Biomedical Engineering, vol. 50, no. 6, June 2003. [5] Pook, M. A Wireless Sensor Network for Monitoring Physical Activity, Physiological Response, and Environmental Conditions. Thesis. Boise State University Graduate College, 2011. [6] Safeer, K., Gupta, P., Shakunthala, D., Sundersheshu, B., Padaki, V. Wireless Sensor Network for Wearable Physiological Monitoring. Journal Of Networks, vol. 3, no. 5, May 2008. [7] Bouten, C., et al. A triaxial accelerometer and portable data processing unit for the assessment of daily physical activity. IEEE Transactions on Biomedical Engineering, vol. 44, no. 3, March 1997. [8] Vaz, A., et al. Full Passive UHF Tag With a Temperature Sensor Suitable for Human Body Temperature Monitoring. IEEE Transactions on Circuits and Systems II: Express Briefs. [9] Pinet, E., et al. Health monitoring with optical fiber sensors: from human body to civil structures. Health Monitoring of Structural and Biological Systems (SSN10 Conference). 14th International Symposium, 18-22 March 2007. [10] Kern, N., et al. Multi-sensor Activity Context Detection for Wearable Computing. Lecture Notes in Computer Science, vol. 2875, 2003.
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