Programmation d'instrumentation avec Python
Cet article de base de connaissances présente la programmation d'un produit d'alimentation programmable Magna-Power avec le langage de programmation Python. Python est un langage de programmation populaire, reconnu pour sa simplicité, la lisibilité de son code et l'absence de compilation spéciale requise. La facilité d'utilisation de Python et sa courbe d'apprentissage rapide en font un excellent langage pour créer des programmes de contrôle, de mesure et même de création de graphiques pour l'instrumentation programmable. De plus, le support étendu de Magna-Power pour les commandes standard pour l'instrumentation programmable (SCPI) signifie que les produits de l'entreprise peuvent être facilement contrôlés en Python avec des commandes simples et intuitives.
Les produits Magna-Power prennent en charge une variété d'interfaces de communication différentes, notamment : RS-232, TCP/IP Ethernet, USB, RS-485 et IEEE-488 GPIB. Malgré ces différentes interfaces, les commandes SCPI sont identiques pour une série de produits donnée. Les commandes SCPI sont documentées dans le manuel d'utilisation de la série de produits correspondante. Lors de la création d'un programme Python, la seule différence entre les interfaces réside dans les paramètres de connexion à l'appareil.
USB, série ou RS-485, qui utilisera pySerial pour créer une connexion série à l'instrument :
import serial
conn = serial.Serial(port='COM4', baudrate=115200)The serial baud rate for MagnaLOAD products is 115200, while the serial baud rate for MagnaDC products is 19200. The port location is defined by your operating system. In Windows, this port can be found in the Device Manager.
Subsequent sending and receiving of commands over serial connection will be as follows:
conn.write('*IDN?\n'.encode())
print(conn.readline())Les paramètres de connexion TCP/IP Ethernet seront les suivants :
import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('192.168.0.86', 50505))L'envoi et la réception ultérieurs de commandes via la connexion TCP/IP Ethernet seront les suivants :
s.sendall('*IDN?\n'.encode())
print(s.recv(4096))La connexion IEEE-488 GPIB nécessitera PyVISA, avec une connexion comme suit :
import visa
rm = visa.ResourceManager()
inst = rm.open_resource('GPIB0::12::INSTR')L'envoi et la réception ultérieurs de commandes via la connexion IEEE-488 GPIB seront les suivants :
print(inst.query("*IDN?"))The following examples provide more in-depth example Python programs using a MagnaLOAD DC electronic load. Programming a MagnaDC programmable DC power supply in Python will be almost identical, with subtle changes to the SCPI commands as documented in the respective product series’ user manual.
The following basic example creates a TCP/IP Ethernet connection, sends some initialization commands, enables the DC input, raises the current level to 5 Adc, waits 20 seconds, then shuts down.
# Import time: Time access and conversions to allow for pausing
# Import socket: Low-level networking interface to allow for socket programming
import time, socket
# Create socket object s with (host, port)
# Define host as hostname in Internet domain
# Define socket type as stream, allowing a port number to be defined
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# Connect to product's IP address address at default 50505 socket
s.connect(('192.168.0.86', 50505))
# Send SCPI command requesting the product to identify itself
s.sendall('*IDN?\n'.encode())
# Receive the product's response and display it in the terminal
print(s.recv(4096))
# Send SCPI command to configure the MagnaDC for local control
s.sendall('CONF:SOUR 0\n'.encode())
# Send SCPI command to set the DC output current to 0 Adc before enabling DC input
s.sendall('CURR 0\n'.encode())
# Send SCPI command to enable the MagnaDC power supply output
s.sendall('OUTP:START\n'.encode())
# Send SCPI command to set the DC input current to 5 Adc
s.sendall('CURR 5\n'.encode())
# Wait 20 seconds
time.sleep(20)
# Send SCPI command to disable the DC output
s.sendall('OUTP:STOP\n'.encode())
# Close the communication channel to the product
s.close()L'exemple suivant crée une connexion série au produit, identifie le produit concerné, puis envoie une séquence de commandes de courant avec 20 secondes entre chaque niveau de courant. Ce type de programme peut être étendu pour parcourir également des valeurs de tension, de puissance et de résistance.
# Import pySerial, which encapsulates the serial port access
# Import time: Time access and conversions to allow for pausing
import serial, time
# Create serial connection object with default baudrate for MagnaLOADs
conn = serial.Serial(port='COM4', baudrate=115200)
# Send SCPI command requesting the product to identify itself
conn.write('*IDN?\n'.encode())
# Receive the product's response and display it in the terminal
print conn.readline()
# Create array of current set points
currSetPoints = [50, 100, 150, 250]
# Send SCPI command to configure the MagnaDC power supply for local control
s.sendall('CONF:SOUR 0\n'.encode())
# Send SCPI command to enable the MagnaDC power supply
conn.write('OUTP:START\n'.encode())
# For each entry in currSetPoints array
# Print static text and current set point to the terminal
# Send the new set point to the MagnaDC power supply
# Wait 20 seconds
for currSetpoint in currSetPoints:
print 'Setting Current to %s A' % currSetpoint
conn.write('CURR {0}\n'.format(currSetpoint).encode())
time.sleep(20)
# Send SCPI command to disable the MagnaDC power supply output
conn.write('OUTP:STOP\n'.encode())
# Close the communication channel to the product
conn.close()In the final in-depth example, a MagnaLOAD is programmed to discharge a battery using set points and times read from a comma-separate value (.csv) file, measure the DC input using the product’s high accuracy measurement commands, and then provide a plot of the measured data versus time. This program could be further expanded to generate a PDF test report, integrating the measured data, plots, as well as information from other instruments.
# Import plotting library Matplotlib
# Import .csv parser
# Import pySerial, which encapsulates the access for the serial port
# Import time: Time access and conversions to allow for pausing
# Import numpy for mathemetical manipulation of arrays
import matplotlib.pyplot as plt
import csv, serial, time
import numpy as np
# Create serial connection object with default baudrate for xGen products
conn = serial.Serial(port='COM8', baudrate=115200)
# Create an empty numpy data array 999 rows, 4 columns
outputSamples = np.empty([999, 3])
iSample = 0
# Send SCPI command to configure the MagnaDC for local control
s.sendall('CONF:SOUR 0\n'.encode())
# Send SCPI command to enable the MagnaDC power supply output
conn.write('OUTP:START\n'.encode())
# Open data stream to .csv file. .csv has two columns:
# Column 1: Current set point in amperes
# Column 2: Time in seconds
# The first row is headers
with open('example_profile.csv', 'r') as csvfile:
# Read the .csv file using comma (,) as the delimeter
dataset = csv.reader(csvfile, delimiter=',')
# Skip the header row
next(dataset)
# Split the dataset up to rows
rows = list(dataset)
# Create an empty array for measurements, the same length as .csv row numbers
inputSamples = np.empty([len(rows), 2], dtype=float)
# Record the time that the measurements began
testStartTime = time.time()
# Run for loop while there are still rows of data left
for idx, data in enumerate(rows):
# Store row data to array
inputSamples[idx] = [data[0], data[1]]
# Send SCPI command to MagnaDC power supply to current set point at present row
conn.write('CURR {0}\n'.format(data[0]).encode())
# Determine how long the MagnaDC power supply should stay at this current set point
stopTime = testStartTime + int(data[1])
# Run while loop while there is still time left
while time.time() < stopTime:
# Send SCPI command to MagnaDC Power supply to measure all DC output variables
conn.write('MEAS:ALL?\n')
# Get the MagnaDC power supply's response and split it up into its respective variables
[curr, volt, pwr] = (conn.readline()).split(',')
# Round measurements and store them to array, along with with time
outputSamples[iSample] = ([round(float(curr), 2), round(float(volt), 2), round(time.time() - testStartTime, 2)])
iSample += 1
time.sleep(0.5)
# Send SCPI command to disable Magna DC power supply output
conn.write('OUTP:STOP\n'.encode())
# Create a plot series of current vs. time
plt.subplot(2, 1, 1)
plt.plot(outputSamples[0:iSample, 2], outputSamples[0:iSample, 0], 'r--')
plt.ylabel('Output Current(A)')
plt.title('I-V Profile')
# Create a plot series of voltage vs. time
plt.subplot(2, 1, 2)
plt.plot(outputSamples[0:iSample, 2], outputSamples[0:iSample, 1], 'b--')
plt.xlabel('Time (s)')
plt.ylabel('Output Voltage(V)')
# Show the plot
plt.show()