ENGR 499: Wireless ECG

Transcription

1 ENGR 499: Wireless ECG Introduction and Project History Michael Atkinson Patrick Cousineau James Hollinger Chris Rennie Brian Richter Our 499 project is to design and build the hardware and software for a wireless Electrocardiogram device, and to create a corresponding Android application. The envisioned end product is one that a person with known heart problems can use for twenty four hour ECG monitoring and feedback. This project is based on several 399 projects from fall 2012 one detailed the hardware, and another the software specifics for this device. We think that the hardware portion is going to be straightforward, and not large enough on its own to be a 499 project, while the software portion is complex enough that pieces can be worked on independently and in parallel. Electrocardiograms Electrocardiograms (ECGs) are used by medical professionals to monitor the heart of a patient. These devices usually operate with 12 leads connected to the patients skin in a prescribed pattern. An ECG can be used to detect abnormal cardiovascular symptoms, measure heart rates, and monitor heart diseases. The most common non medical application of an ECG is to measure a heart rate during a workout, however, the aim of this project is to prototype a device that could aid remote monitoring and feedback. The most important components of the ECG that the device needs to report include P wave, QRS interval, and the T wave. These waveforms have the inherent issue that the measurable signals have amplitudes in the range of 0.1mV to 10mV. Another issue with these signals is that the smallest time components last as little as 50ms(The PR segment), or 80ms (The entire QRS complex). This short time means that the sample speed for the signal needs to be significantly less than 25ms to ensure adequate sampling.

2 The device will not offer any medical opinions, but could valuable in monitoring for non continuous heart conditions and heart condition triggers that are not replicated in a hospital or clinical setting. As a result, the user would not need to continuously monitor the device, but could save and send abnormal conditions as they are encountered. This functionality has not yet been implemented, but is part of future plans. Hardware - building an ECG The original hardware proposal was to build the circuit using a series of amplifiers and filters. The figure below shows the proposed block diagram for the hardware components. Leads The current prototype has a three lead system. The prototype leads were made by soldering pennies onto a long wire to create a receiver for the small signal produced by the heart. Pennies were used due to their availability, low cost and high conductivity. Unfortunately, the magnitude of the signal produced by the heart is in the mV range, and is very susceptible to noise from motion, other body functions, and background noise. Two of the leads are connected across the heart to detect a signal, while the third is placed on the body to create a ground reference.

3 Amplifier The current design uses a single instrumentation amplifier with a high gain. An instrumentation amplifier was chosen because the heart signal has a floating reference. The model number of the chip used is the Texas Instruments INA126P. Using a total gain of around 1000, the signal will be in the range of 100mV to 1V. However, this signal will contain a large amount of noise that needs to be filtered out. The primary sources of noise will be high frequency noise and 60Hz power line interference due to our long leads. The diagram below shows the internal configuration of the instrumentation amplifier. High-Pass Filter The first stage in the filter network is a passive high pass filter with a 0.1Hz corner frequency. This filter removes the DC offset in our signal, and the corner frequency is set very low due to our signal components residing near 1Hz. The diagram below shows the designed high pass filter. Notch Filter The second stage is a 60Hz passive notch filter. This filter is crucial due to the leads used by the ECG. These function as antennas and pick up the 60Hz noise from power lines.

4 The diagram below shows the designed notch filter. This passive filter network may be replaced in the future with an active filter to increase the Q factor of the filter. Low-Pass Filter The low pass filter is designed with a corner frequency of 400Hz. This removes the unwanted high frequency noise from the waveform. As well, removing unnecessary high frequency portions of the signal will decrease signal bandwidth. A passive filter network was chosen as it is significantly easier to implement than an active filter. In the future, a second gain stage will directly precede this low pass filter. This is because we do not want to amplify the 60Hz noise as well. The diagram below shows the designed dual low pass filter. ADC The ADC is a critical component of the ECG module. It takes the received ECG signal from the electrode circuitry and digitizes it for transmission to the mobile phone application. At this stage of the project, we have chosen to use a Raspberry Pi microcomputer in order to fill the roles of ADC and bluetooth modules. Running a distribution of Debian Linux, the Raspberry Pi is capable of both the analog input and digital bluetooth transmission components of the ECG system. In the future, we would like to utilize an application specific microcontroller for this

5 portion of the project. For our prototyping stage, however, the Raspberry Pi is easy to use and implement. Bluetooth/Firmware Raspberry Pi At this stage of the project, we chose to utilize a Raspberry Pi linux based computer in order to handle the firmware side of the ECG. Utilizing a Python program, the Raspberry Pi will pair with and connect to the phone based android application using a USB bluetooth radio. RTOS vs non RTOS Standard operating systems focus on doing as much computation in the shortest time where real time operating systems put an emphasis on having a predictable response times. To ensure a predictable response time in the ECG it would be ideal to be using a real time operating system. For the prototype we are building we will be using a standard linux distribution. In light of this, the prototype ECG, running off of a Raspberry Pi, should meet any real time constraints that we have since the processor is overpowered for our application. We believe that the overhead from running a linux distribution should not significantly affect the sampling rate. Why not Arduino? We decided against using a widely available programmable microcontroller (such as Arduino) because of budgeting and complexity concerns. It creates a level of complexity when trying to send data over bluetooth. Using the Raspberry Pi with an attached usb module is trivial. The bluetooth library can be installed onto the Pi with a simple package installation. Software Progress The Android application portion of the project is progressing steadily, although we have had to deviate from the timeline. The timeline put major emphasis on the Bluetooth reading software component, as it blocked other tasks and components. However, we were not able to develop a hardware prototype as quickly as we planned, and thus had no device to transmit Bluetooth to test the Bluetooth server. Another false assumption we had made was that Android

6 would have a very simple Bluetooth wrapper library which would be as simple as Android s BluetoothAdapter object with which we had previous experience. Unfortunately, the Bluetooth connection involves setting up a listening Bluetooth server, and then servicing the connection and while this is not terribly difficult, it was more than we were expecting for this segment, and the increased complexity means more testing is required once we have a Bluetooth device to test. Bluetooth being blocked did not stop work on the whole application though: with some sample ECG data we created manually from a signal generator we were able to work on the signal charting component, making substantial progress. Deviation from original design As we put work into the software, it became apparent that the three application goals from the project s outset were large in and of themselves, let alone reading the Bluetooth data. We decided to focus on displaying the chart to the user and saving the data, and to not focus on the user notification/signal analysis portion. No group member is particularly well versed in signal analysis, or of ECG analysis, and testing of the feature would be involve simulating a heart attack from the Bluetooth data. This feature may be added in the future, but for now we will focus on just those two goals. We feel both goals are manageable: saving the data should be simple if Bluetooth data can be read properly, then it can be stored in a temporary data structure and saved in parallel. Displaying the chart gives the most feedback to a user, but is more difficult than saving: how real time should the display chart be? What amount of lag is acceptable? Should sensor data more than X seconds old be thrown out? What sampling rate should the hardware use to ensure the phone can process it in time? These questions will have to be addressed as the Bluetooth reading module is completed and we can test the app with an actual adjoining ECG sensor. Also, since this chart is the most user facing section of the application the UI will have to be carefully crafted to give the best look and feel possible. The toned down project design is reflected in the adjoining figure.

7 Figure 1.1 Android Application design without Notification Module Timeline Date March 1, Friday To be completed Last month: time to re evaluate goals/progress and set a new meeting

8 schedule to meet our needs. March 6, Wednesday Progress Meeting, ECS 124, 18:00 18:50 Progress Report #2 submitted to Supervisor, copied to Coordinator and TA March 11, Monday March 13, Wednesday March 15, Friday March 20, Wednesday March 25, Monday Software Finish data saving module Add a link from MainActivity to view data history. Some work done with Raspberry Pi and Bluetooth reading module Hardware Replace broken instrumentation amplifier and debug circuit Software Have Bluetooth reading module finished Hardware insert amplifier stage between notch and low pass filter stage Software Chart module should be displaying streaming Bluetooth data Hardware Duplicate amplification and filtering stages to ensure 5 leads Final Presentation Preliminary presentation design Software UI and overall look and feel should be acceptable quality. Layout/transitions should be finalized. Equipment Loan List for Project Demo to technical support staff Final Presentation Poster design completed March 27, Wednesday Final Presentation Poster printing (at ZAP?)

9 March 29, Friday April 5, Friday April 11, Thursday Poster Presentations starts at 16:00; Engineering Lab Wing Lobby; Setup starts at 13:00 Last day of classes, Final Report submitted to Supervisor (grade penalty may be applied to late submissions), project URL submitted to the Coordinator and TA Coordinator receives grades from Supervisors for Progress Reports and Final Report and Website evaluations from panel of judges