Simultaneously acquire data and update graph using python and matplotlib

Question

I'm writing a code in Python to communicate to an ultrasonic distance meter over the serial port, to do the following:
-Every 0.1 seconds, send a command to the sensor to make a distance measurement, and register the response of the sensor
-Display a plot of all the distance measurements from the last 5 seconds

Here's my code:

import serial
import numpy as np
import time
from matplotlib import pyplot as plt

tagnr=2#Tag number of the sensor that we're pinging  
samplingRate=.1#Sampling Rate in seconds  
graphbuf=50.#Buffer length in samples of logger graph  

!#Initialize logger graph  
gdists=np.zeros(graphbuf)  
ax1=plt.axes()  

!#Main loop  
nsr=time.time()#Next sample request  
try:  
    while True:  
        statreq(tagnr)#Send status request to sensor over serial port  
        temp,dist=statread(tagnr)#Read reply from sensor over serial port  
        gdists=np.concatenate((gdists[1:],np.array([dist])))  
        print gdists  
        nsr=nsr+samplingRate  
        while time.time()<nsr:  
            pass  

finally:  
    ser.close()#Close serial port  
    print 'Serial port closed.'

Right now, my code can acquire an array of the last 50 measurements, but I don't know how to display these in a graph at the same time (I usually plot my graphs using Matplotlib). Should I use threading? Or use an animated graph using pyGTK or pyQt4? I was also thinking of using pygame? My timing mechanism is not very optimal either but I'm thinking it's pretty accurate.

Paul · Accepted Answer · 2011-03-31 21:45:34Z

1

matplotlib has animated plots that allow the data to be updated while the plot is displayed: take a look at this page.

Your code might look like this:

import serial
import numpy as np
import time
from matplotlib import pyplot as plt

plt.ion() # set plot to animated

tagnr=2#Tag number of the sensor that we're pinging  
samplingRate=.1#Sampling Rate in seconds  
graphbuf=50.#Buffer length in samples of logger graph  

!#Initialize logger graph  
gdists=np.zeros(graphbuf)  
ax1=plt.axes()  

# make plot
line, = plt.plot(gdists)

!#Main loop  
nsr=time.time()#Next sample request  
try:  
    while True:  
        statreq(tagnr)#Send status request to sensor over serial port  
        temp,dist=statread(tagnr)#Read reply from sensor over serial port  
        gdists=np.concatenate((gdists[1:],np.array([dist])))  
        print gdists

        line.set_ydata(gdists)  # update the data
        plt.draw() # update the plot

        nsr=nsr+samplingRate  
        while time.time()<nsr:  
            pass  

finally:  
    ser.close()#Close serial port  
    print 'Serial port closed.'

Just some advice (possibly bad): I would personally use time.sleep in a way that frees up some processor without losing accuracy. I would also put some error type on your try/except block. And I think np.roll is better/faster than concatenate.

answered Mar 31, 2011 at 21:45

Paul

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1 Comment

Thijs Lanckriet Over a year ago

Thanks for your comment, but I still couldn't get the plot to update. In the mean time, I found a complete python code to log and display serial datafrom Eli Bendersky at link, which does pretty much exactly what I wanted.

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