1. Design Overview: Active Transmitting RF tag (2.45 GHz) Bracelet Antenna Array + Accelerometer Three antennas measure received signal strength from transmitter Reference table from testing determines linear distance Angle of accelerometer and distance used to calculate height Transmitter: The transmitter is implemented with an Atmel ATAVRRZRAVEN development board that continuously broadcasts a 2.4GHz radio signal. Receiver Array: The receiver array is a bracelet containing 3 antennas that are 60 degrees apart from one another and an Adafruit Industries ADXL335 3-axis accelerometer. Each antenna is connected to an Analog Devices AD8313 Logarithmic Amplifier that measures the signal strength of the radio frequency obtained by the antenna and outputs a corresponding DC voltage. DESIGN OVERVIEW GOAL l RF-Based Blood Pressure Monitor Location Sensor Students: Jordan Lippitt, Mic Johnson, Ming Dai, Pat Benike, and Thai Pham Advisors: Professor Paul Ruden and Larry Beaty (IEEE Phoenix Project) The University of Minnesota Chronobiology Center has found that through continuous monitoring of blood pressure, Vascular Variability Disorders (VVDs) can be detected and treated early before they result in events such as heart attack or stroke. Members of the Twin Cities IEEE Phoenix Project are currently in the process of developing a wrist- worn blood pressure monitor (BPM) that can be worn continuously over the course of a week to monitor the cyclical nature of one’s blood pressure. The major drawback of current wrist-worn BPMs is that they require users to keep their wrist level with their heart at all times in order to generate accurate readings. This is not practical because it becomes nearly impossible for the user to comfortably wear the BPM continuously over the course of a week. Our design combines the functionality of radio frequency technology with an accelerometer to calculate the orientation of the wearer's wrist with respect to their heart, and the linear distance between them. The height between the heart and the wrist can be calculated with these variables and then applied to adjust the incorrect measurements recorded by the BMP. Chronobiology is the study of biological rhythms. Blood pressure has a 24 hour rhythm. When measured every 30 minutes for 7 days straight, vascular variability disorders can be detected. Vascular Variability Disorders can predict morbidity events like a strike or heart attack. They can only be detected by continuously monitoring blood pressure. The goal of the Phoenix Project is to improve on the design of current wrist-worn blood pressure monitors (Figure 1) to make them affordable, robust, and able to take accurate blood pressure measurements every 30 minutes for a week. Wrist-worn blood pressure monitors that are held at a different height than the heart will give an inaccurate measurement. Fictitious example of continuous monitoring shown below. Continuous monitoring allows for the detection of Vascular Variability Disorders (VVD’s): Experiment Setup/Results: The RF transceiver, embedded in a development board, is configured to transmit an RF signal continuously. The board is placed in parallel with the receiver, and the signal strength received by the antenna at specific distance is recorded to establish a reference table. Accomplishments: Determine distance accurately when the arm was within 40 centimeters of the active tag Determine orientation of the wrist with respect to the heart Future Recommendations/Tasks: Use phase of arrival rather than received signal strength to determine distance Replace the active tag with a passive one to reduce design cost Scale down the size of the system Increase number of antennas Improve accuracy of the measurements Acknowledgements: Professor Robert Sainati Professor Anand Gopinath Larry Beaty Figure 1: Picture of current wrist-cuff blood pressure monitors. BPM is accurate if at the same height as the heart BPM is inaccurate if not at the same height as the heart To design a sensor that uses radio frequency technology to calculate the difference in height between one’s wrist and his or her heart. INTRODUCTION l THE PHOENIX PROJECT l MEASURING DISTANCE l TEST RESULTS l FINDINGS l