ENGR 40M Project 4b: Displaying ECG waveforms. Lab is due Monday June 4, 11:59pm

Transcription

1 ENGR 40M Project 4b: Displaying ECG waveforms Lab is due Monday June 4, 11:59pm 1 Introduction Last week, you built a circuit that amplified a small 1Hz simulated heartbeat signal. In this week s you will use this circuit to display your actual ECG. Of course to be safe, you can t connect your circuit to anything that connects directly to wall power. This means you must not use an oscilloscope, when you are measuring your heartbeat. Instead you will use your Arduino s analog input to digitize the signal, and then use a program called Processing on your laptop to display it on your screen. By completing this lab, you will: Learn to use the Analog input of the Arduino if you haven t already. Learn how to plot data from your Arduino on your screen using Processing. 2 Safety Since you will be connecting the ECG to yourself during this lab, we will stress safety and introduce each of the isolation schemes you will implement in this project. For this class we will use triple current limits, and double isolation to guarantee your safety while working with ECGs. isolator 100 kω data circuit only NO! 100 kω battery The first and most important rule is that you should never be connected, through any devices, to the wall outlet. This includes computers, function generators, or oscilloscopes that may be connected to the wall electrical outlet. Strict adherence to this rule will guarantee that you will not be hurt. This means computers should only run on batteries and all other equipment should not be connected to your circuit when when the ECG is connected to the body. You must not use the oscilloscope when you are going to measure your heartbeat! The second method of protection will come from a USB isolation board built using Analog Devices icoupler technology. This board isolates the Arduino based circuit from the computer and guarantees that you will not be directly connected to power supply of the laptop. Please always use the USB decoupler whenever you need to connect to the Arduino to a computer for this experiment. Finally, current-limiting resistors will be used to limit the maximum amount of current going through your body even if you are accidentally connected the Arduino to a high voltage. These are 100kΩ resistors in series with each cable that connects to your body. Please ensure that these current-limiting resistors are implemented.

2 3 New Parts and Equipment There are very few new devices that we introduce in this lab. We are mostly using the circuit that you built in the previous lab. The main new device you will encounter in this lab is the USB isolator chip from Analog Devices (ADUM3160). This USB isolator chip acts as a communication relay for USB devices but separates power domains, protecting us from the battery of the computer. For this lab, please make sure that you use this adapter board whenever you need to connect the Arduino to your computer. Please do not remove the these boards from lab. The isolation board is shown in the figure below. For this lab, you will also be using electrodes like the ones shown on the above. This will be attached to your body and can measure your heartbeat. You will be using two of these, connected to the instrumentation amplifier inputs. 4 Prelab P1: Before anything else, because of safety concerns, we want to use the solar-powered charger as your your power supply to your Arduino. Verify that yours still works. If it doesn t, please leave it in the sun so it has lots of charge in it or bring a backup battery pack with a usb connection (i.e. a portable phone charger). 4.1 Reading and Viewing a Signal To visualize the analog waveform on your computer, we will use a software called Processing. Processing is a simple programming environment very similar to the Arduino programming environment. You will first program the Arduino to send values to the computer via the serial connection. Then, you will slightly modify an example Processing code to plot a sequence of numbers on the screen. P2: Follow the instructions below. 1. Go to and download the software. Open Processing. The interface should look very similar to the Arduino development interface. 2. From the class lab website, please download ecg.ino. This is the simple code we need to run on the Arduino. Make sure you understand how it works. 3. Think about the following questions: Given that the input to the analog voltage is converted to a 10-bit number whose maximum value represents 5V, what value would represent 2.5V? If I want to slow down the transfer rate of data, what do I change? 4. Set the input pin to the analog pin you are using to read the output of the ecg op-amp. void setup () { // initialize the serial communication : Serial. begin (9600); 2

3 void loop () { // send the value of analog input 0: Serial. println ( analogread ( inputpin )); // wait a bit for the analog - to - digital converter to stabilize // after the last reading : delay (2); 5. From the class lab website you should next download ecg.pde, which is the processing code that will display the data from the Arduino. Once you have the code you should load it into processing and look at it. Now look at the Processing section of the code. The first few lines set up the variables that the program will use to run. You will need to change SERIAL PORT to the right number for your Arduino. If set to -1, it will print out the list of ports, which makes it easy to find the right port. The ports are printed in the status bar under the main screen and look like /dev/tty.usbmodemfd1331 and /dev/cu.usbmodemfd1331. You should connect to the tty port that corresponds to your Arduino. If you re not sure which port that is, try unplugging your USB cable to see which port goes away. It is often the first port (port 0), but you can run it with -1 first to double check. Like the Arduino code, Processing also has a setup routine that runs once to setup the rest of the code. In addition to setting up the serial link, it also created the graphics window with the size(x,y) command. This command creates a graphics window that is x pixels wide and y pixel tall. You should change these numbers to make sure the window fits on your computer screen. // TODO r e p l a c e t h i s with the a p p r o p r i a t e s e r i a l port index. You can run t h i s // code with SERIAL PORT = 1 to get a l i s t o f a v a i l a b l e s e r i a l p o r t s. s t a t i c f i n a l int SERIAL_PORT = 1; Serial myport ; // The s e r i a l port int xpos = 1 ; // H o r i z o n t a l p o s i t i o n o f the graph f l o a t heightold = 0 ; // The old v e r t i c a l p o s i t i o n so I can draw a l i n e void setup ( ) { // Set the window s i z e // TODO: Adjust t h i s to f i t your computer s c r e e n size (1200, ) ; i f ( SERIAL_PORT == 1) { println ( P l e a s e s e t SERIAL PORT to an a p p r o p r i a t e value i n the code. ) ; The main draw loop is shown below. Since we are drawing lines that are one pixel wide, we can erase the previous line (written on the previous scan line by simply drawing a vertical black line. This gives a screen display that updates each position as it passes it instead of clearing the screen at the end of the scan. stroke(r, g, b) Sets the color of the line (entering one number makes all the colors that number), and line(x0, y0, x1, y1) draws a line from x0, y0, to x1, y1. The only tricky part is that in processing 0,0 is the top left part of the screen, so you need to plot height-y to get a graph that looks correct. So the first line call draws a black line to erase the previous drawing, and then it draws the next segment. If you would prefer a display that blanks when the scan is through, you should comment out the code that draws this black line, and uncomment the command that sets the background to black when the scan line finishes. f l o a t outheight = map ( output, 0, 1024, 0, height ) ; // e r a s e the next p o s i t i o n to be w r i t t e n stroke ( 0 ) ; line ( xpos, height, xpos, 0 ) ; 3

4 // Draw the l i n e the numbers s p e c i f y the RGB c o l o r, pick one you l i k e. // Remember that p r o c e s s i n g draws t h i n g s upside down. stroke (255, 255, ) ; line ( xpos 1, height 1 heightold, xpos, height 1 outheight ) ; heightold = outheight ; // I f at the edge o f the screen, go back to the beginning i f ( xpos >= width ) { xpos = 1 ; // Not zero, s i n c e I the l i n e s t a r t s at x 1 // background ( 0 ) ; // Clear the s c r e e n f o r the next scan else { xpos++; You do not have to turn in anything for prelab this week, but it is essential that you complete it before lab. 5 Lab Steps Once you get in lab the first thing you should do is to test that your ECG circuit still works. Using the voltage divider you created in the last lab, connect your circuit to the function generator to create a 0.4mVpp, 10mV offset, 1Hz signal, and feed that signal into your instrumentation amplifier. View the output of the entire circuit using the oscilloscope to make sure it is still working. If something doesn t look right, you should check to make sure that you have power connected to your parts, and then check to see if the instrumentation amplifier is working. From the output of the IA, you should be able to decide whether you need to debug the IA circuit or the op-amp. If you are lucky, everything will still work and you can go on to the next step. 5.1 Testing your Viewing Code Now you should place your Arduino on the proto-board that contains your circuit. Connect your circuit s 5V and Gnd connections to the Arduino, so the Arduino powers your circuit (if you have not done this already), and then connect your op-amp output to the analog input you chose in your Arduino code. Now, attach this output to the Arduino input pin in your code. In your Arduino code, if you leave the delay to be every 2ms, and your screen is greater the 1000 pixels wide, you will get a plot that is about two seconds of data. You should see several cycles of a sinewave centered on your screen. Leave your front-end circuit connected to the Arduino. L1: Take a screenshot of the output on your screen. 5.2 Testing the Isolation Board Now that you have everything working, your next step is to connect the USB isolator between your Arduino and your laptop computer. These isolation boards should have three USB connectors that are labeled on the board. It shouldn t be possible to connect them up backwards, but please double check just to be sure. Remember that since the Arduino is isolated you will need to power it from a different power supply. That is where the solar chargers come in. Once you have connected your Arduino to your laptop through the isolator boards, and connected your solar chargers to supply the power to the Arduino, please repeat the previous experiment, and make sure you are still getting the output waveform you got before. If this doesn t work, you should ask your master maker to help you. 4

5 5.3 Viewing your Heartbeat Now, you are ready to view your heartbeat! You will need: The circuit you just got working above, with the isolation board and solar charger 2 1MΩ resistors 2 100kΩ resistors 2 Foam electrodes to connect to your body. 1. Disconnect the function generator from your circuit, and remove the voltage divider from the input of the instrumentation amp. Make sure there is no lab equipment connected to your circuit (no power supply, lab DMM, oscilloscope, etc.), and turn all machines on the bench off for safety. Your breadboard & Arduino should be powered by your solar charger battery and should only be connected to your computer through the USB isolator board. The computer should not be connected to the wall (unplug your laptop). Once you have checked all these steps, you may proceed. 2. Without connecting anything to yourself, use your handheld DMM to check voltages on important nodes. You should have 5V at Vdd, 0 at GND and 2.5V at IA REF. 3. Run your Processing code to display the output from your circuit (which is not connected to anything right now). You should see a flat line somewhere in the middle of the window. Don t be surprised if the value moves around a little bit. There is noise in the circuit. 4. Add a 100kΩ resistor at each input pin of the IA. These resistors will be in series with the leads connected to your body and will serve as current limiting resistors. Also add a 1MΩ resistor between each of the IA inputs and Vref (2.5V). 5. Now, connect test leads to yourself. Using banana cables and alligator clips, attach two electrodes to your left and right inner wrists and use these as the inputs to your IA (through the 100kΩ resistors). Be careful not to short anything. At this point you should see an approximately 1Hz signal that is your heartbeat. If you want to see a more impressive signal, you can put the two electrodes closer to your heart. L2: Take a screenshot of the output on your screen. 6. OPTIONAL: You may notice that there is still a lot of noise. If you have time, you can implement a simple moving average filter in your Processing code. The basic idea is to use a sliding window and plot the average of N values instead of just the Nth value. Given some input sequence x, the filtered sequence y is given by y n = 1 N (x n + x n x n (N 1) ) L3: Take a screenshot of your the now less noisy output on your screen. 6 Reflection Instead of the usual reflection, please fill out a post-course survey. We appreciate honest opinions we ll use them to inform how we teach the course in future. Responses will be anonymous. The survey will be sent via during week 9. 5

6 7 Congratulations! You now have successfully built your first ECG. Through this experience, you learned how to build a notso-simple analog circuit block to amplify a very weak signal into something measurable with an Arduino. You also learned how to use Processing to display a simple real time waveform on your computer screen. This data acquisition pipeline will be useful not only for measure ECGs but the measurement of any analog signals. Finally, you learned how to read datasheets for ICs, which is a valuable skill for designing any future electronic projects. More importantly, this your last E40M lab, so congratulations for a great quarter of making and breaking things! Go forth, young Maker. 6