Pulse Sensor Individual Progress Report

Transcription

1 Pulse Sensor Individual Progress Report TA: Kevin Chen ECE 445 March 31, 2015 Name: Ying Wang NETID: ywang360

2 I. Overview 1. Objective This project intends to realize a device that can read the human pulse rate from a fingertip. The pulse rate in the unit of beats per minute (BPM) and a calculated Target Heart Rate will be displayed by a 16 by 2 LCD display. According to the National Institute of Health, the average resting heart rate for children 10 years and older, and adults (including seniors) is beats per minute. Well-trained athletes is beats per minute. A real-time plot of pulse signal versus time will be displayed on a laptop screen. 2. Modules and Module Input/Output Requirements There are four modules in this design: Power Module, Sensor Module, Microcontroller, and Display Module. Figure I-1 shows a block diagram of their relations Power Module Figure I-1 Modules and module relation The required power supply for this device is DC 5V. In order to obtain a constant and stable 5V output, a regulator circuit is designed. The regulator circuit will take an input voltage of 9V battery and output a constant 5V voltage. For details and simulation results please see the module design part below Sensor Module The sensor is in direct contact with a human finger (input). It first contains a sensor that can sense the blood circulation and convert it to electrical signals. This will be followed by a circuit for amplification and noise reduction. The output of the sensor in this design will be a real-time

3 analog voltage signal that swings within a reasonable range between 0 V and Vcc = 5 V. The Sensor Module will be placed on a PCB board for the final device Microcontroller The Microcontroller (will be Arduino Uno in this design) takes the output of the Sensor Module. The output of Sensor Module will first subject to A/D conversion. Through Microcontroller programming, the digital signal is processed, the pulse rate (BPM) should be calculated then sent to the output ports of the Microcontroller and a calculated result of Target Heart Rate in beats per minute will also be sent to the output port of the microcontroller. The output signal needs to be compatible with the input requirement of a 16 by 2 LCD display unit. The Microcontroller also needs to handle the communication with a PC via a serial port in order to send the real-time pulse signal to PC for display. A free software called Processing 1 can be used to handle the display for data coming from a serial port. 2.4 Display Module Display Module takes the output of Microcontroller. It will contains two components. The first is an on-board display unit which shows the Pulse Rate and Target Heart Rate. The second is a window opened on a PC screen that can do real-time display of pulse signal transported from the serial port. This is also to confirm the device has received reasonable signal. II. Module Design 1. Power Module The regulator circuit will take an input voltage of 9V battery and output a constant 5V voltage. Schematics: using a zener diode D1, resistor R1 and transistor Q1 to form a voltage regulator, R2 is the load resistor. V1 Q1 V3 V Qbreakn V 9Vdc VOFF = 0 VAMPL = 0.2 FREQ = 100 AC = V1 V2 R1 100 V2 D1 Zener Voltage = 5.6 R

4 Figure II-0 voltage regulator circuit (5V) Simulation Result: Q1 is using 2N3055 mode, other parameters are shown in the figure above. The vibration of the input voltage is modeled by a sine signal. The green line is the total input voltage and the red is the output voltage of the regulator, which is fixed at ~ 4.9 V. Tolerance Analysis: We can calculate the output voltage to be: V3 = ((V1-0.6)/R1 + 5/ZZT)*(beta + 1)/(1/R2 + (beta+1)/r1 + (beta+1)/zzt), where ZZT = 11 ohm from the Zener model and beta = 130. The load resistance is estimated to be 10 ohm, leading to a current of 0.5 A. Assuming V3 = 5 ± 0.4 V, this lead to R1 should be larger than ~80 ohm. The minimum current passing R1 should be 0.5 ma/(beta+1) + Is = A. This lead to the maximum R1 allowed to be about 700 ohm. 2. Sensor Module The sensor I will use to convert blood pulse into electrical signal is a pair of infrared (IR) emitter and detector. The emitter and detector will be put side by side facing upwards. When a fingertip is place on top of both, IR light will be reflected by the finger and the blood circulation inside the

5 finger can cause a periodic change of IR intensity reflected, therefore, a period current response in the detector. The response is rather weak and could be noisy. Shown below by Figure II-1 is the original design from makezine.com 2. This design has some problems and cannot satisfy the requirement stated above. I will do two modifications. Figure II-1 Original sensor module design 2-1. Load Resistors of Emitter and Detector The first modification is rather trivial. I need to adjust the value of load resistors (shown in Figure II-2) for the emitter and detector to ensure they work fine and more importantly to make sure a reasonable DC component of the detector output (not too close to ground and not too close to Vcc). The recommended forward voltage of the emitter from the datasheet is 1.2 V with a forward current of 20 ma. This leads to a load resistance R1 = (5 V 1.2 V) / 20 ma = 190 Ω This happens to be not very different from 220 Ω in the original design so I will just keep R1 = 220 Ω. 2 Makezine.com/projects/ir-pulse-sensor

6 Figure II-2 Sensor composed by an IR emitter and detector. In order to get a reasonable R2, I use a voltmeter (from the toolbox) to measure Vo in Figure II-2. It was found that when I use R2 = 8.2 kω, Vo has a reasonable range between ~2 V to ~3 V when a finger is placed on the sensor Re-design the circuit after detector The circuit in Figure II-1 will not work properly mainly because of two reasons. 1. The first OPAMP, in my opinion, is for noise reduction. But a quick analysis of its transfer function reveals that it is band reject filter that reject the frequency from ~1.6 Hz to ~194 Hz. This doesn t make sense to me because there are still useful signal above 1.6 Hz(for example, pulse above 96). 2. The second OPAMP is to amplify the signal however, it can be calculated that the gain at f = 1 Hz is about 100, which is possibly too large that could result in cut-off (clipping) of the real signal. This is indeed observed as will be shown in the Test and Verification Section. Before design my own circuit, I first look at the real-time output Vo of the detector (with a fingertip on the sensor), which will serve as the input of the following circuit. Figure II-3 shows the Vo versus time recorded by Arduino Uno and displayed on my laptop by Processing. Although the signal shown here is pretty clean, sometimes noisy signal was seen. Details of how the test was done will be provided in the Test and Verification Section. Basically, by using Arduino and Processing, the laptop can be used as an oscilloscope. As we can see the output voltage has a DC level of ~ 2 V a peak-to-peak voltage is ~ 0.13 V. This tells me that I need to design the amplifier gain to be ~10 to 20. A gain of 100 is definitely too high!

7 Figure II-3 Pulse signal measured from the output V o of the detector with a finger on. No load was added to V o. Directly after the IR detector output, I plan to first add a low-pass filter to filter out the highfrequency noise. I choose to use the Sallen-Key structure 3. C2 0.13Vac 2Vdc Load of IR detector V1 R2 8.2k R3 Open Circtui output of IR detector R4 C1 U1 + - OUT OPAMP output to amplif ier Figure II-4 Butterworth low-pass filter using Sallen-Key architecture Figure II-4 shows a low-pass filter using Sallen-Key architecture. The output resistance of the IR detector is just R2 = 8.2 kω. It is decided for now that any signal with f > ~20 Hz should be suppressed. Guided by the design rule in the footnote document, we assume R4 = R, R2+R3 = mr, C1 = C, and C2 = nc. The cut-off frequency fc = 1/(2πRC(mn) 0.5 ) and for Butterworth filter Q = (mn) 0.5 /(m+1) = So in order to meet fc = 20 Hz, the following design can be calculated: n m C1 C2 R3 R uf 0.66 uf 11.4 kω 50 kω 3

8 The next stage will be an amplifier that has a gain of ~ 10 at f = 1 Hz. In addition, the DC component should not be amplified, only the AC component needs to be. So the amplifier needs to have a high-pass feature which is reject the very low frequency signal. It is decided here the low-frequency roll-off frequency to be 0.1 Hz. The circuit can be realized as in Figure II-5. output of f ilter C3 U2 + OUT go to Arduino A0 R5 - R6 OPAMP R7 Figure II-5 amplifier circuit with a high-pass design to only amplify AC component The transfer function of Figure II-5 is H = R7 + R6 s R5 C3 R7 1 + s R5 C3 To realize a gain of 10, R6 can be 9 kω and R7 can be 1 kω. The low-frequency roll-off fc = 1/(2πR5C3) = 0.1 Hz. So it can be designed that R5 = 160 kω and C3 = 100 uf. The total Sensor Module design should connect Figure II-3, II-4 and II-5 together Re-design the circuit after filter After further testing of the pulse sensor circuit, I find that the current gain 10 is too big. So I modified the gain to 4 and redesigned the amplifier part of the circuit as following: output of f ilter C3 U2 + 5V R51 - R6 OUT OPAMP go to Arduino A0 R52 R53 R71 R72

9 C3 = 100 uf, R51=160 kω, R52 = R53 = 9 kω, R6 = 9 kω, R71=1.8 kω, R72=1 kω, now the gain is (R6+R71+R72)/(R71+R72) = 11.8/2.8 = 4.2 The testing result could be seen in the following testing part. 3. Microcontroller Module I am going to use Arduino Uno in this design. The Microcontroller takes the output of the Sensor Module, which is periodic analog voltage signal Arduino Uno Inputs and Outputs The inputs and outputs of an Arduino Uno is shown below as Figure II-6. Figure II-6 Input and Output ports of Arduino Uno Arduino Uno has 6 analog inputs A0 to A5 (inputs between 0 V - 5 V), the analog signal is sampled and digitalized and then output to the serial port on the top right of Figure II-6. The digitalization has 1024 levels thus one sample is a number between 0 and Each output data consists of 5 characters in total: a 4-digit decimal number, represented by four characters, and a new line character. Each character uses 10 bits, 8 of which are for the actual character and the other 2 are for other use. The bit rate of the serial port is 9600 Hz by default. The sampling rate therefore can be calculated by 9600/5/10 = 192 samples/sec. As have been mentioned before the serial port can be read by a PC using Processing. Processing can also draw a dynamically updated graph each time a sample arrives. About the output for the 7-segment displays, three displays will be used because a pulse rate could not exceed 999 BPM. I am planning to use the 12 of the 14 digital I/O ports, denoted here as O0, O1,, O11 to output the binary coded decimal (BCD) signal. O0, O1, O2 and O3 will be used for the lowest decimal digit. O4, O5, O6, O7 for the second decimal digital. O8, O9, O10, and O11 for the third.

10 The inputs and output connections have also been shown in Figure II Obtaining Pulse Rate by Programming Here is the basic algorithm I am going to use to get the pulse rate (BPM). Imaging a pulse signal that looks like what is show in Figure II-3, First, the digital signal might be subject to an averaging to make it very smooth Second, I am planning to record all the local maxima including their time stamps T[i] and values V[i] with certain time interval, say, 2 seconds. See Figure II-7 below for the flow chart. Figure II-7 Flow chart for recording all local maxima with a time interval of 2 seconds. Next, we can scan V[i] and discard those maxima that are apparently smaller than others. Only keep the V[i] that are the largest and within certain variation range. Then the pulse rate can be calculated by the two arrays T[i], i = 1 to m: m BPM = 60 i=1 (T[i] T[i 1])/(m 1) Finally, the BPM number, which should be a 3-digit integer here, needs to have each of the three digits into a BCD number and then output to O0 to O11, which can be read by Display Module.

11 This is rather simple and can be illustrated by Figure II-8 below. After this the program should go to calculate the averaged pulse rate of the next 2 seconds. Figure II-8 convert BPM to output that can be read by Display module Target Heart Rate Range Calculation Formula: (220-age)*50% (220-age)*80%. This formula is concluded from the data information on American Heart Association website : Rates_UCM_434341_Article.jsp 4. Display Module For the display module I will use a 16 by 2 LCD display (LCM by-2) for displaying the pulse rate and Target Heart Rate. III. Test and Verification 1. Test/verification plan Requirement Verification Points Power Supply Output a stable 5V(+/-0.4V) voltage to the rest of circuit and within +/-0.2A of needed current Put a voltmeter across the output of regulator circuit, the voltage should be within 5V+-0.4V. The components of the circuit should all work properly without any short or 15 Pulse Sensor The pulse sensor should output an accurate periodic analog signal to microcontroller. The periodic overload. The output of the pulse sensor circuit could be verified by using a free software called Processing It could sketch the output of the pulse sensor 40

12 wave should correspond to the pulse of the testing people within 5 beats per minute. Microcontroller The Microcontroller takes the output of the Sensor Module and calculate the beats per minute and output to the display module. The beats per minute should be accurate within 5 beats/min Display Module Takes the output of the microcontroller and convert it to seven segment display circuit synchronously to a PC screen. Then we can count the pulse per minute based on the waveform and compare it with the result of iphone App Heart Rate The calculated beats per minute (binary value) can be displayed by processing then we can compare the output of the microcontroller with the waveform generated by pulse module. To be compared with the binary value number displayed by processing Schedule March 1 st week March 2 nd week March 3 rd week March 4 th week April 1 st week April 2 nd week April 3 rd week April 4 th week Test the Sensor Module output, modify circuit design if necessary. Should be a periodic signal that swings within a reasonable range. This can be verified by using Arduino and Processing to do realtime monitoring. Simulate Power supply module, Microcontroller programming and test the LED display. Both can be verified by the real-time signal recorded by Arduino and Processing. Finalize circuit design and start to design PCB board Prepare for mock Demo, finalize PCB board Test PCB board Demonstration Write final report 3. Initial Test Results 3-1. Test setup using Arduino and Processing.

13 Figure III-1 Circuit test setup, using Arduino and Processing as an Oscilloscope. Figure III-1 shows the test setup used for the Sensor Module. How to connect Arduino has been explained previously. The code for creating such dynamic figure using Processing can be found on the Arduino website 4. Basically, whenever the serial port receives a number, the code will update the figure with a new date point. The voltage output of the IR detector has been shown by Figure II-3. Here I show in Figure III- 2 the results of Sensor Module output using the original and problematic design. Figure III-2 Output of the original design 4

14 As can be seen here, the signal is very noisy. In addition, when compare Figure III-2 to Figure II-3, we can see the output of the original design is significantly distorted, which I believe is due to that the gain is too high. The new design has been described in the text above. 4. Further Testing Results After modifying the pulse sensor circuit and change the gain to 5, I did the testing and the result is as following: The above figure is clean but some peaks got clipped off because the gain is still a little bit too high. So I modified the circuit again so the gain is reduced to 4. I did the testing again as following:

15 The above figure is showing the pulse rate with a light press of finger on the IR emitter and detector. The following figure is showing the pulse rate with a harder press of finger on the IR emitter and detector. As could be seen from the two figures, the IR emitter and detector are sensitive to the pressure of the finger, the harder press of the finger gives a better and cleaner waveform.

16 IV. Cost Analysis Labor Name Hourly Rate Total hours invested Total = Hourly Rate x 2.5 x Total Hours Invested Ying Wang $ $41250 Parts List Part name Numbers Total cost ($) IR emitter 1 2 IR detector 1 2 Arduino Uno OPAMP LM LCM by-2 1 Free 5.6 V Zener diode N3005 NPN transistor V battery V battery clip pin solder tail strip socket Resistors, Capacitors and Free LEDs Total Total cost: $41298 V. Safety The safety of this pulse sensor is one of the most important aspect when designing this device. This pulse sensor will be powered by 9V battery so it is safe for people to use. The IR emitter and detector may contain Arsenide which could be hazardous if broken. They are safe under normal conditions. Children should not use this device without an adult s guidance. It is also important to note that there are many factors that could affect people s pulse, like emotions, temperature, body position, body size and medication use. This device is not a substitute for professional diagnostics. If your pulse is very high or if you have frequent episodes of unexplained fast heart rates, especially if they cause you to feel weak or dizzy or faint, tell your doctor, who can decide if it s an emergency. VI. Ethics

17 This pulse sensor is designed to improve the quality of people s life. The pulse rate is an important indicator of people s health condition. So measuring the pulse rate accurately and anytime they want at home or outside satisfies people s needs and benefit them in many ways. VII Work to be done So far I have performed the pulse sensor module testing, power module simulation and prepared the PCB board. Further work need to be done on power module testing, Arduino programming and display in the next few weeks.