1. Introduction
Frequency converters are widely used in AC drive technology in industrial control field. Traditional frequency converter control is completed by the operation panel, or it can be achieved through local control terminals (including logic input and analog input). However, this usually leads to problems such as increased hardware cost, insufficient number of logic input ports, unstable analog transmission, insufficient given accuracy and more wiring. With the development of network technology and the higher requirements for the automation of production in the field, the controller and frequency converter are more inclined to communicate through the field real-time bus to realize the remote control and monitoring of the frequency converter. This method not only reduces the cost of system integration and maintenance and greatly reduces the number of wiring, but also achieves high accuracy and high stability of speed given. At the same time, the host computer can control and monitor the frequency converter in real time [3].
2. Introduction to Frequency Converters
A frequency converter is a device that uses the switching action of power semiconductor devices to achieve voltage and frequency conversion. Its main task is to convert constant voltage and constant frequency AC power into variable voltage and variable frequency AC power to meet the needs of AC motor frequency conversion speed regulation. This paper selects the flux vector Altivar71 series frequency converter produced by Schneider Electric, which is mainly composed of three parts: AC-DC rectifier circuit, DC filter circuit, and DC-AC inverter circuit. It adopts the AC-DC-AC method to realize the conversion of power frequency power to power of other frequencies. First, the power frequency AC power is converted into DC power through the three-phase rectifier bridge rectifier circuit; in the intermediate DC link, it is filtered, stored, and buffered for reactive power; finally, the DC power is converted into AC power with variable frequency and voltage through the inverter [4]. Figure 1 shows the basic structure of the AC-DC-AC variable frequency converter.
Figure 1. Composition diagram of AC-DC-AC transformer frequency converter
The control method of the frequency converter refers to the coordination relationship between parameters such as frequency, voltage, magnetic flux and electromagnetic torque. The two most commonly used control methods are vector control and U/f control. U/f control is an open-loop speed control method. The control circuit is simple and does not require a speed sensor. The load can be a general standard asynchronous motor. Therefore, this control method is economical and versatile, and is currently the most widely used control method for frequency converters [4]. According to the dynamic mathematical model of the AC motor, the stator current of the AC motor is decomposed into torque component current and magnetic field component current by means of coordinate transformation, and then controlled separately to obtain the vector control method.
Schneider Electric's series of frequency converters belong to the flux vector control category. Their terminal blocks have two analog inputs, one analog output, six logic inputs, one safety input, two relay outputs, and one 24V input terminal. For communication, they come standard with Modbus and CANopen communication protocol interfaces. Various communication cards support other mainstream communication protocols on the market, such as Fipio, Ethernet, Modbus Plus, Profibus DP, DeviceNet, Uni-Telway, and INTERBUS. By selecting an appropriate communication interface card for protocol conversion, the Altivar71 frequency converter can be used in almost all industrial communication environments.
3. DeviceNet bus
DeviceNet is a fieldbus developed by Rockwell Automation in the United States, based on CANbus technology, used to achieve low-cost, high-performance network interconnection at the device layer. DeviceNet is one of the most widely used fieldbuses in the world, with significant advantages in industrial automation. It can connect to various industrial control products such as frequency converters, robots, and PLCs. It is suitable for communication networks between field devices (such as drives, switches, I/O, and HMIs) and PLCs, with a transmission rate of 125-500 kbt/s, a maximum transmission distance of 500m, and a maximum number of nodes of 63.
The DeviceNet bus follows the Open Systems Interconnection (OSI) model, which is a layered ISO standard for network communication. It defines all the necessary functions of the network protocol, from physical implementation to protocol and methods for implementing communication control and data transmission on the network [5]. The DeviceNet network uses a backbone/branch topology, with two pairs of twisted pairs transmitting power and signals respectively, and thin and thick cables used for branches and backbones respectively. The network length varies depending on the cable length and speed.
In actual production, many motors use frequency converters and DeviceNet networks for variable frequency speed control. A DeviceNet communication card enables the frequency converter to convert from Modbus to DeviceNet protocol, connecting the ATV71 to the DeviceNet network. Through an integrated display terminal or graphic display terminal, users can access and modify various communication functions of the frequency converter.
4. Inverter configuration and address allocation
4.1 Inverter Configuration
To enable communication between the Rockwell ControlLogix 5555 and the Schneider ATV71 frequency converter via the DeviceNet network, the converter's communication settings must first be configured. All devices connected to the DeviceNet network must have a unique node address and adhere to a uniform communication rate. The ATV71 frequency converter's communication card has an 8-bit DIP switch. The first six DIP bits, weighted according to their respective bit values, are used to set the frequency converter node address; the last two bits specify the communication rate of that node. For example, setting node 21 as the frequency converter address and the communication rate to 125 kbit/s, the configuration switch settings are shown in Table 1.
Table 1 Inverter Configuration Switch Settings
Changes to the DIP switch configuration on the frequency converter will take effect upon the next power-on. By connecting to the DeviceNet network via the DeviceNet open five-pin connector, the node can be scanned and identified by a scanner.
Since the database of the DeviceNet network only contains equipment information of AB Automation products, and the ATV71 series frequency converter is a product of Schneider Electric of France, although the relevant frequency converter nodes can be scanned in the software, the ATV71 frequency converter cannot be identified; and since RSNetWorx organizes the network through EDS files, the frequency converter and the corresponding EDS file must be associated and configured first [4]. Generally, EDS files can be downloaded from the official website of the equipment manufacturer, but due to product updates and other reasons, sometimes the corresponding EDS file cannot be found. By setting the communication mode and data size of the input and output of the specified frequency converter in RSNetWorx, we can manually create the EDS file. Since the output of the DeviceNet card only supports polling mode, the polling communication mode is set, with 8 bytes of input and 8 bytes of output. The specific settings are shown in Figure 2.
Figure 2. ATV71 Inverter Input/Output Communication Mode Selection
After the Schneider ATV71 frequency converter is identified by the scanner, the first step is to configure the ATV71 network, and then configure the converter's input/output components via a graphical terminal. Different control components will result in different input/output channels for the frequency converter, and the meaning of each byte in each channel will also differ.
There are three available configurations for the Schneider ATV71 inverter: first, control via a communication scanner; second, control via an Open Device Network Suppliers Association (ADS) AC inverter configuration file; and third, control based on an Allen-Bradley inverter configuration file [4]. Since the control method via the communication scanner is the best way to interact with the inverter’s built-in controller card, the first method is chosen for this system. Method 1 allows customization of the input/output interface with a specific PLC; it maximizes the number of communications by defining 4 variables and 8 bytes of input/output parameters; and it also allows the use of all available configurations of the ATV71 inverter. When configuring using the communication scanner, the communication scanner output component 100 and the communication scanner input component 101 should be selected. The variables exchanged by these two components can be defined as a maximum of 4 parameters. The specific input/output variables can be configured in the graphical terminal. Table 2 shows the control parameters of the input component 101 selected in the graphical terminal [1].
Table 2 Parameter settings for input component 101
The corresponding bytes of different parameter logical addresses contain different information. Status word 8603 represents the status information read by the controller from the frequency converter. The type is bit memory, and each bit represents a specific meaning. Output speed 8603 is a two-byte integer, a read-only parameter, and outputs the motor speed value in units of 1. Motor current 3204 is a two-byte unsigned integer. The output motor current signal is in units of 1. It needs to be processed in the controller [4]. Table 3 gives the specific meaning of each bit of status word 8603.
Table 3. Meaning of each bit in status word 8603
In the RSNetWorx network configuration, devices registered with EDS files are identified. In the inverter parameter tab, the PollProdPath and PollConsPath parameters are set to input component 101 and output component 100, respectively. After setting the inverter's input and output components, the ATV71 inverter information needs to be mapped to the DNB scan adapter memory. In the Rockwell programming software RSlogix5000, the Schneider ATV71 inverter is addressed by calling the mapped address in the 1756-DNB module.
4.2 DeviceNet Network Address Allocation
RSNetWorx for DeviceNet is software used to configure input/output devices on a device network and create scan lists. Configuration information and scan lists are stored in the DeviceNet communication adapter. This section uses the configuration of an ATV71 frequency converter as an example to detail the address mapping process of the DeviceNet network.
First, search for the corresponding scanner in the RSNetWorx software, and associate the EDS file downloaded from the official website or manually created with the corresponding frequency converter so that the RSNetWorx configuration software can organize the network. The specific association method is shown in Figure 3:
Figure 3. Inverter EDS file association method
After configuring the EDS file, open the properties dialog box of the 1756-DNB module, select the "Scanlist" tab, and the devices under "AvailableDevices" are all available devices in the current network. Select the modules that need to be configured in the project, and add them to the "Scanlist" list using the ">" button, and remove them using the "<" button. If you need to add all devices under the "AvailableDevices" list to the "Scanlist" list, you can add them all at once using the ">>" symbol, and similarly, you can remove all devices from the "Scanlist" list at once using the "<<" symbol. After editing and configuring the module's input and output addresses, click Apply to map the module addresses to the corresponding scanner memory.
The type of connection established between the scanner and the device depends on the device. The scanner sequentially selects the first communication connection method supported by the device, based on the following methods: status change, cyclic refresh, strobe, and polling. The ATV71 inverter scanner supports all of the above communication methods, and the parameter settings for the ATV71 module are shown in Figure 4.
Figure 4. ATV71 Inverter Parameter Settings
In Figure 4, the "AutomaponAdd" option is checked, indicating that the module's address will be automatically mapped in the scanner. This may result in wasted adapter memory addresses. When there are many devices in the network, we usually choose to manually map the address to better organize the communication adapter memory. First, the ATV71 module is mapped into the DNB scanner, and then the corresponding input and output can be called in the program. In the right figure of Figure 5, 6 is the input word address of the ATV71 module manually mapped in the DNB module, and 16 represents the ATV71 module starting from the 16th bit of the 6th word address in the DNB module, which is the data address of the ATV71. 1 represents the 1st slot of the 1756-DNB module in the ControlLogix framework; I represents that the address corresponds to the input data; Data[] indicates that the data organization method in the DNB is in the form of an array; in the program, it means that the 8 32-bit double words are stored starting from the address Local:1:I.Data[0].
Figure 5. Example of ATV71 module mapping
Similarly, the output address of the ATV71 frequency converter is mostly configured manually. Select the corresponding frequency converter, then click the Advanced option. In the pop-up dialog box, assign the starting byte and starting bit of the output address. After checking, click ApplyMapping to complete the address setting. The frequency converter network address configuration is now complete.
5. Conclusion
This configuration system has been put into operation on the factory production line. Since its actual operation, the host computer has provided timely and accurate control over the frequency converter, good monitoring of the motor status, stable and reliable operation, and simple and convenient use and maintenance. All indicators have met the design requirements, improving the company's level of automation.
