Methods and Application Examples for Controlling Multiple Frequency Inverters Based on Modbus and Raspberry Pi
Zhang Shuangchi, Pan Zhiwei
R&D Center of King Fung (China) Machinery Industry Co., Ltd.
Abstract: This paper details the hardware setup, software configuration, and coding methods for using a Raspberry Pi to expand RS485/GPIO modules to control multiple frequency inverters, through examples. The unique advantages and application prospects of this method are highlighted.
Keywords: Modbus_tk, Raspberry Pi, Huichuan frequency converter
1. Introduction
The Raspberry Pi was initially developed for educational purposes. It only had a single 40-pin serial port (GPIO), which limited its use to experiments and simple engineering applications. Now, by expanding the GPIO with an RS485/GPIO module to enable Modbus communication, it can control frequency converters in real time, allowing for the development of practical engineering projects.
Figure 1 is a partial block diagram of a punch press group control system, which is described below.
2. System Configuration
Figure 1 Hardware Configuration Diagram
Hardware configuration:
Inverter: Huichuan MD380+MD380IO1[1], Raspberry Pi: 3B+RS485/GPIOShieldForRPiV3.0.
Software configuration:
Modify Raspberry Pi configuration: Disable Bluetooth and console, dedicate the serial port to Modbus communication. Operating system: Linux; Programming software: Python 3.4.2; Communication software: Modbus tk 0.5.4; Interface programming software: PyQt5.
3. Code compilation
3.1 Configure Modbus_tk
Configure the Modbus_RTU communication mode and set the communication parameters to 9600, 8N1. After reading the inverter parameters, the data needs to be stored in memory for subsequent processing; therefore, import the log file.
importserial
import modbus_tk
import modbus_tk.define sascst
frommodbus_tkimportmodbus_rtu
PORT="/dev/ttyAMA0"
logger=modbus_tk.utils.create_logger('console')
master=modbus_rtu.RtuMaster(serial.Serial(port=PORT,baudrate=9600,bytesize=8,
parity='N',stopbits=1,xonxoff=0))
master.set_timeout(0.5)
master.set_verbose(True)
logger.info("connected")
importlogging
3.2 Splitting Log Files
The inverter's operating data is stored in log files. Over time, these files grow increasingly large, eventually causing system crashes. Therefore, it's necessary to split the log files. Splitting means retaining some records and discarding the rest to reduce file size. There are two splitting methods: ① splitting by file size and ② splitting by time interval. This example uses method ②, splitting every 2 seconds, retaining a maximum of 5 files.
The following code reads 12 data points from the starting address 7000H of inverter #1 and stores them in a log file named "pzw" (the split log file and backup are less than 0.2MB), which is placed in the same folder as the program file.
logger.info(master.execute(1,cst.READ_HOLDING_REGISTERS,28672,12))
fromlogging.handlersimportTimedRotatingFileHandler
if __name__=='__main__':
logFilePath='pzw'
logger = logging.getLogger('')
logger.setLevel(logging.INFO)
handler=TimedRotatingFileHandler(logFilePath,when='s',interval=2,backupCount=5)
formatter=logging.Formatter('%(asctime)s-%(message)s')
handler.setFormatter(formatter)
logger.addHandler(handler)
3.3 Processing Log Files
To accurately obtain the parameters of the five frequency converters, the log file "pzw" needs to be analyzed:
2018-05-25 19:42:11,200-->1-3-112-0-0-12-95-15
2018-05-25 19:42:11,306-<-1-3-24-0-0-14-16-16-232-0-0-0-0-0-0-0-0-0-0-0-1-252-1-240-0-134-87-101
2018-05-25 19:42:11,395-(0,3600,4328,0,0,0,0,0,0,508,496,134)
2018-05-25 19:42:11,396-->2-3-112-0-0-12-95-60
2018-05-25 19:42:11,485-<-2-3-24-0-0-10-140-15-127-0-0-0-0-0-0-0-0-0-0-0-0-0-0-0-135-13-10
2018-05-25 19:42:11,527-(0,2700,3967,0,0,0,0,0,0,0,0,135)
2018-05-25 19:42:11,528-->3-3-112-0-0-12-94-237
2018-05-25 19:42:11,614-<-3-3-24-0-0-10-240-15-217-0-0-0-0-0-0-0-0-0-0-0-0-2-0-1-0-32-7-88
2018-05-25 19:42:11,655-(0,2800,4057,0,0,0,0,0,0,2,1,32)
2018-05-25 19:42:11,656-->4-3-112-0-0-12-95-90
2018-05-25 19:42:11,743-<-4-3-24-0-0-13-72-12-7-0-0-0-0-0-0-0-0-0-0-0-0-0-0-0-0-4-33-208-137
2018-05-25 19:42:11,834-(0,3400,3079,0,0,0,0,0,0,0,0,1057)
2018-05-25 19:42:11,835-->5-3-112-0-0-12-94-139
2018-05-25 19:42:11,923-<-5-3-24-0-0-10-200-11-252-0-0-0-0-0-0-0-0-0-0-0-0-0-0-1-4-33-182-98
2018-05-25 19:42:11,965-(0,2760,3068,0,0,0,0,0,0,0,1,1057)
Discoveries: ① After removing the date and time header from each record, the first character is the inverter's station number. ② Records containing inverter parameters have a significantly larger character count than other records. Based on these two characteristics, the following code was developed:
withopen('pzw')asfile_object:
lines = file_object.readlines() # Read the file line by line and store the results in the table lines.
n = len(lines) # Get the length of the table
for i in range(n):
lines[i]=(lines[i])[27:]# Extract the 27th item to the end of the table and save it to the table.
iflen(lines[i])>63and(lines[i])[0]=='1': # Check if the station number and whether it is data
abc1=lines[i] # Store in table abc1
a1=abc1[7:]# Extracts the 7th item to the end of table abc1 and saves it to table a1.
b1 = a1.split('-') # Splits the string into new tables using '-' as the delimiter.
u0_00_1=str(float(int(b1[0])*256+int(b1[1]))/100)#Inverter output frequency
self.l1_1.setText(u0_00_1+'Hz') # Label display
...
u0_11_1=str((int(b1[22])*256+int(b1[23]))/4)#pt100 Left bearing temperature
self.l7_1.setText(u0_11_1+'℃')
iflen(lines[i])>63and(lines[i])[0]=='2':
.................
iflen(lines[i])>63and(lines[i])[0]=='3':
.................
iflen(lines[i])>63and(lines[i])[0]=='4':
.................
iflen(lines[i])>63and(lines[i])[0]=='5':
...................................
The code above checks each of the n records according to characteristics ① and ②, and finally uses five if statements to retrieve all records in the file "pzw" that meet the conditions. The code fully demonstrates the powerful table processing capabilities of the Python language and is the core of the entire application.
3.4 Obtaining Inverter Parameter Values
The function of the code in the rectangle is to further process the records that meet conditions ① and ②: discard the first 7 characters (e.g. 1-3-24-) and the remaining 26 bytes are the inverter parameter values and check codes. The reading program can be compiled according to Table 1 [1].
Table 1:
3.5 Program Structure
Control signals such as “start”, “stop”, and “speed adjustment” are non-periodic commands, while reading inverter parameters and displaying data are periodic commands. Therefore, a main thread-sub-thread structure is adopted. Two timer modules QTimer[2] are defined, each with a timer of 2 seconds. During timer 1, the periodic reading command is executed, and during timer 2, the non-periodic command is executed. The process is shown in Figure 2, which constitutes a program execution process with a loop of about 2 seconds.
Figure 2 Control Flow
3.5 User Interface
Figure 3. Human-computer interface (partial view)
Figure 3 shows the operation interface. The output frequency of the inverter is set by the counter control QSpinBox[2]. You can easily set it by clicking the up and down arrows on the right or by directly entering numbers on the keyboard. The data in Figure 2 are: output frequency, running current, DI status, DO status, AI1, AI2, AI3.
4. Research and Development Experiences and Future Prospects
Read the inverter DI/AI signals (fault, flow, temperature, etc.), process them, obtain the operating status of the entire system equipment, display them on the interface, and then output switch signals or analog signals (open/close valve, start/stop pump, adjust valve opening, etc.) through DO/AO to control other equipment. Make full use of these ports, which is equivalent to adding a small PLC with: 50 DI, 25 DO, 15 AI and 10 AO. Due to space limitations, the specific usage will not be introduced in this article [1].
In the three-tier architecture of the Internet of Things (IoT), the PLC must rely on a gateway to connect to the external network. The Raspberry Pi integrates network functions (wired/wireless), essentially playing the dual role of controller and gateway.
Yeelink is currently the largest IoT cloud platform in China. It provides cloud services to the public for free. By developing related applications through the App interface provided by Yeelink, remote monitoring of products can be achieved.
Raspberry Pi is used in engineering projects, and it has great practical value and good development prospects in terms of both product cost and control capabilities.
References:
[1] Huichuan Technology: MD380 Series High-Performance Vector Inverter User Manual V1.4
[2] Wang Shuo and Sun Yangyang: PyQt5 Rapid Development and Practice, Electronic Industry Press, October 2017.
