Heart Rate Monitor and Data Acquisition System Bill Leece Sofoklis Nikiforos TA: Ajay Patel June 22, 1999 Project Statement We selected this project because we are both interested in biomedical engineering and analog electronics. This project combines both areas of interests nicely. Secondly, since we both had hoped to take ECE 314/5, but didn’t have the time to do so during our studies at the U of I, we felt that this project would give us a good background on most of the material taught in these courses. Furthermore, in the fall Bill will be working for a company that makes pacemakers, so this background in biomedical circuits and sensors will prove to be very useful in that regard. Project Goals For this senior design project, we will build a heart rate monitor and interface it with a PC via DAQ board. We hope to gain a greater understanding of biomedical sensors and analog electronics as well as learning more about instrument-computer interface. We plan to use two different software packages for this project, simulating the analog circuits (for purposes of optimizing performance) with PSPICE, and writing the data acquisition programs using LabVIEW. Our heart rate monitor will use a sensor to detect a person’s heartbeat. This signal will then be sent to an ECG amplifier circuit. The output of the amplifier circuit will then be sent to a DAQ card in a PC. The DAQ will provide the interface between the real system and the virtual instruments created using LabVIEW. The LabVIEW programs will display the patient’s heartbeat on a virtual oscilloscope, as well as displaying the persons heart rate in beats per minute and overall caloric expenditure. If time permits, we will also build electronic circuits that will perform the same functions as the LabVIEW software. We will display the heart pulse on a real (as opposed to a virtual) oscilloscope as well as using LED displays to display heart rate. The purpose of actually building these circuits this is twofold. First, it would give us a chance to learn more about analog electronic circuits, perhaps giving us an opportunity to use the 555 timer circuit. Furthermore, it will provide an excellent demonstration of the value of LabVIEW because we suspect that it will be much more difficult to actually build circuits that calculate heart rate than “building” them with LabVIEW. Block Diagram LabVIEW NI-DAQ Software ECG Sensors Amplifier (Optional) Heart Rate Hardware Circuit Oscilloscope Heart rate monitor and data acquisition system Sensor Inputs The input to the monitor system is from a human via biopotential sensors. The sensor detects the patient’s heartbeat by transforming a physical signal from the body into an electrical signal. This signal is then sent to the ECG amplifier. ECG/ Instrument amplifier The purpose of the ECG amplifier is to take our sensor inputs and produce a meaningful signal. The ECG amplifier removes noise from the signal as well as amplifying it. The output of the ECG amplifier is then sent to the DAQ board. National Instruments Data Acquisition Board The DAQ board takes the analog input signal and sends it to the PC where the heart beat can be displayed on the computer using LabVIEW software. Furthermore, the DAQ can also digitize the input signal, which will prove to be useful for the calculation of heart rate. LabVIEW Software The last part in the monitor system is the output from a LabVIEW program to a computer monitor. The program will interface the NI-DAQ by taking the analog input signal and displaying it in real time on a LabVIEW strip chart. LabVIEW will also use a digitized version of the input signal to produce a value for heart rate in terms of beats per minute. Optional System The optional part of our project consists of making a hardware circuit that performs the same functions as the LabVIEW software. Displaying the heart pulse will be straightforward since the output of the ECG amplifier can be connected to an oscilloscope. In order to display the heart rate (in beats per minute) a complicated electronic circuit will have to be devised to keep tract of the heart rate and to display this information on LED’s. A very challanging part of this optional system would be to make it function properly while using only 9 V batteries. This would probably require more knowledge of low power electronics than we have. Once again, we will only do this part of the project if time permits. Required Performance The performance that we will require of the finished project is that the system is able to display Electrocardiograph data (e.g. heart pulse) on a strip chart in LabVIEW and that heart rate can be calculated and displayed in terms of beats per minute as well. In order to accomplish this goal, our sensor and ECG amplifier will have to be optimized to meet minimum performance parameters. Furthermore, there will have to be successful interfacing between the ECG amplifier and the computer via the DAQ board. Finally, the LabVIEW programs will have to function properly so that the data can be displayed in a meaningful manner. A few performance parameters are involved in the optimization of the ECG amplifier circuit. The first stage of our amplifier needs to have low noise and high input impedance. Optimization calls for minimizing the input bias current. We will be using the model 411 op amps instead of the more standard 741, since the 411 has lower input bias current. The common mode rejection ration must also be high for the first stage. PSPICE will be used to maximize the CMRR as well as obtaining the desired frequency response specified in Medical Instrumentation by John G. Webster. The gain after the difference amplifier will have to be approximately 1000 to 4000 . For the last stage of the amplifier, we need to optimize the frequency response of the bandpass filter by selecting appropriate resistor and capacitor values. The frequency response of the amplifier should have a high corner frequency of 120 Hz to minimize noise from the muscles. The low corner frequency should be set to 0.05 Hz to obtain the maximum signal. Once again, obtaining this particular frequency response will be accomplished by selecting appropriate resistor and capacitor values determined from PSPICE simulations. Detail of Test Procedures As discussed, we plan to maximize the amplifier performance by simulating the circuit with PSPICE. Our amplifier circuit contains a resistor pot that needs to be varied to maximize CMRR. We will make a plot of CMRR vs. Positive Differential Amplifier Resistor value (R13 in our circuit included at the end of this proposal) to see which value maximizes the CMRR. We will also make Bode plots of the frequency response to make sure that our amplifier has the frequency response that we desire. Our LabVIEW test procedure will consists of designing and testing subVIs to see if they function properly. Once the subVIs function properly, they can be assembled in a hierarchical nature into a high level VI that will measure the heart rate and display ECG data on a graph. Finally, if time permits, a way to document the projects performance is to compare values obtained via LabVIEW with those obtained with our hardware circuit that will be designed if time permits. Obviously, agreement between the software and hardware versions of the ECG and heart rate monitor would suggest proper performance in both. Timetable ________________________________________________________________________ June 17 Bill Leece, Sofoklis Nikiforos: Submit initial proposal. ________________________________________________________________________ June 19 Sofoklis Nikiforos: Study NI-DAQ manual. Bill Leece: Find LabVIEW manuals. ________________________________________________________________________ June 22- Bill Leece, Sofoklis Nikiforos: Analyze ECG Amplifier June 25 Sofoklis Nikiforos: Gather all hardware components. Bill Leece: Begin familiarizing with LabVIEW. ________________________________________________________________________ June 23 Bill Leece, Sofoklis Nikiforos: Submit written proposal. ________________________________________________________________________ June 27 -30 Bill Leece, Sofoklis Nikiforos: PSPICE Simulations. Build and refine the hardware. ________________________________________________________________________ June 28-30 Progress check. ________________________________________________________________________ July 1- Bill Leece, Sofoklis Nikiforos: Complete ECG amplifier; Begin July 12 LabVIEW interfacing. _______________________________________________________________________ July 13-20 Bill Leece, Sofoklis Nikiforos: Complete LabVIEW integration and begin testing. Test and compare different sensors ________________________________________________________________________ July 21-28 Bill Leece, Sofoklis Nikiforos: Final debugging and modifications. Add supplementary hardware and software features if time permits. ________________________________________________________________________ July 29 Demo. ________________________________________________________________________ July 30 Oral report. ________________________________________________________________________ August 2 Turn in final report Cost and Parts The only parts that we need to order are the disposable electrodes, HP13951C. These are disposable, cloth, solid gel, and snap electrodes, with a silver/silver chloride (Ag/AgCl) sensor, which can be repositioned. We estimate the costs of the case of electrodes to be about $45 based on market value price. We also plan to use different sensors (IR, piezoelectric) to determine which type of sensor is most readily able to detect a heart pulse. The other parts are already available in the lab or shop. Labor Typical EE entry salary $50,000/year * 1year/240 days * 1 day/8 hours = $26/hour $26/hour x 2.5 x 100 hours = $6,500 (per person) Total Labor $13,000 Parts HP13951C electrodes $45 Other sensors $55 411 and 741 Op amps $5 Resistors, capacitors $2 Personal Computer --- (available) LabVIEW Software --- (available) PSPICE Software --- (available) NI - DAQ --- (available) Total Parts $107 Total Costs $13,107 Tolerance Analysis and Test of Engineering Specifications For tolerance analysis and test of Engineering specifications, we will determine the range of values of the potentiometer (R13 in figure below), which is used to control the CMRR. Thus we will obtain high and low resistor values for our variable resistor such that the ECG amplifier circuit still operates in an acceptable manner. The design will have to be in the range of 47k 5k . We also will vary the resistors and capacitors responsible for the ECG frequency response to determine minimum and maximum cutoff frequencies that will still allow the ECG amplifier to function properly. At this time we really have no idea of what these values will be or of what their variances can be, but we will be able to determine this after a number of PSPICE simulations as well as physically varying these parameters in our actual ECG amplifier circuit.
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