Programación de instrumentación con Python
Este artículo de base de conocimiento presenta la programación de un producto de potencia programable de Magna-Power con el lenguaje de programación Python. Python es un lenguaje de programación popular, conocido por su simplicidad, legibilidad de código y por no requerir ninguna compilación especial. La facilidad de uso y la rápida curva de aprendizaje de Python lo convierten en un excelente lenguaje para crear programas que controlen, realicen mediciones e incluso generen gráficos para instrumentación programable. Además, el amplio soporte de Magna-Power para los Comandos Estándar para Instrumentación Programable (SCPI) significa que los productos de la empresa pueden controlarse fácilmente en Python con comandos simples e intuitivos.
Los productos de Magna-Power admiten una variedad de interfaces de comunicación diferentes, incluyendo: RS-232, TCP/IP Ethernet, USB, RS-485 e IEEE-488 GPIB. A pesar de estas diferentes interfaces, los comandos SCPI son idénticos para una serie de productos en particular. Los comandos SCPI están documentados en el manual de usuario de la serie de productos correspondiente. Al crear un programa en Python, la única diferencia entre interfaces serán los ajustes para la conexión del dispositivo.
USB, Serial o RS-485, que utilizarán pySerial para crear una conexión serial al instrumento:
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())Los ajustes de conexión TCP/IP Ethernet serán los siguientes:
import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('192.168.0.86', 50505))El envío y recepción posterior de comandos a través de la conexión TCP/IP Ethernet será de la siguiente manera:
s.sendall('*IDN?\n'.encode())
print(s.recv(4096))La conexión IEEE-488 GPIB requerirá PyVISA, con la conexión de la siguiente manera:
import visa
rm = visa.ResourceManager()
inst = rm.open_resource('GPIB0::12::INSTR')El envío y recepción posterior de comandos a través de la conexión IEEE-488 GPIB será de la siguiente manera:
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()El siguiente ejemplo crea una conexión serial al producto, determina qué producto es y luego envía una secuencia de comandos de corriente con 20 segundos entre cada nivel de corriente. Este tipo de programa puede ampliarse para recorrer valores de voltaje, potencia y resistencia también.
# 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()