Abstract: This article describes the development of a dedicated frequency converter for the squeegee unit of a flatbed screen printing machine based on DSP. It features a standard RS232C serial port for connection to the control panel and a CAN bus interface for connection to the central control PLC. It can perform all the functions required by the squeegee unit of the flatbed screen printing machine. 1 Introduction The control system of a flatbed screen printing machine used in textile printing and dyeing is complex. Its main drive—the back-and-forth motion of the guide belt—requires high positional accuracy and commonly uses servo control; its auxiliary drive—the squeegee control of the color—requires high dynamic performance and often uses frequency conversion speed regulation; its main controller often uses a programmable logic controller (PLC) with an LCD touchscreen human-machine interface. The squeegee control uses a dedicated frequency converter with ten speed levels. The number of squeegees and the squeegee stroke can be set via the panel, and it has RS-232 or RS-485 serial communication capabilities. A typical approach is to develop a dedicated frequency converter using an 8-bit microcontroller, employing ordinary V/F control. This converter has fewer functions but a simple structure and low cost. For example, the BUSH-5V and 7V flatbed printing machines manufactured in Switzerland, and the squeegee unit of the flatbed printing machines produced by Zhengzhou Textile Machinery Factory, adopt this solution. Since the first generation of DSPs was introduced to the market in the 1980s, processing speed and performance have continuously improved, while the price has dropped from hundreds of dollars to just a few dollars. Due to the increasing maturity, functionality, and price reduction of motion control-specific DSP chip technology, developing dedicated frequency converters for the squeegee unit of flatbed printing machines using them is not only technically feasible and functionally enhanced, but also economically viable. Our solution uses the TMS320LF2407 motion control DSP chip produced by TI. This chip not only facilitates the implementation of common V/F control, direct torque control, and vector control strategies, but also has a built-in CAN fieldbus controller and serial communication interface, enabling easy networking and panel connection, greatly improving the system's technical level and performance indicators. The inverter power module uses Mitsubishi's latest fifth-generation product, the PS21865, which can directly interface with the DSP without the need for opto-isolation or transformer isolation, greatly simplifying the power supply. Experimental results show that the developed dedicated frequency converter has a simple hardware structure, reliable performance, and uncomplicated software. 2. Dedicated Frequency Converter Scheme for the Scraper Printing Unit Figure 1 shows a simplified diagram of the dedicated frequency converter scheme for the scraper printing unit of a flatbed printing machine. It is called a dedicated frequency converter for two reasons: firstly, it has fewer functions compared to general-purpose frequency converters; secondly, it has some special requirements not found in general-purpose frequency converters. The main control chip uses the TMS320LF2407 motion control DSP manufactured by TI. One of its two PWM outputs (Group A) is used to directly drive the inverter module without opto-isolation. The input from the photoelectric encoder is directly sent to the QEP1/2 terminal of the DSP, providing the required speed or position feedback signal. The inverter module has short-circuit protection and control power undervoltage protection functions. Its fault output FO is directly sent to the power drive protection interrupt terminal PDPINT of the DSP, which makes its six PWM outputs high-impedance. The six drive input ports of the inverter module are internally connected with 2.5K pull-down resistors. High-efficiency drive logic is used. When the DSP’s six-channel PWM output is high, the corresponding switch of the inverter module is turned on; when the output is low or in a high impedance state, the corresponding switch of the inverter module is turned off. The DSP is connected to the control panel via the serial port SCI. The control panel can be used to set the squeegee speed, squeegee stroke, squeegee count, etc., set whether to run in single-machine mode or online mode, and can also start and stop the inverter individually. The CAN controller integrated in the DSP enables the inverter to have fieldbus communication function, and can form a fieldbus control network with many squeegee units and the central control PLC without adding any hardware. [align=center] Figure 1 Dedicated inverter for squeegee unit of flat screen printing machine[/align] 3 Features and protection of inverter module The inverter module adopts Mitsubishi’s fifth-generation IGBT integrated circuit, 5A-30A/600V dual-row direct-plug intelligent power module (DIP-IPM), which has the following features: (1) Integrated three-phase inverter circuit, using planar IGBT and CSTBT (Carrier Stored Trench-gate Bipolar Transistor) technology. (2) It has a built-in power bootstrap circuit and only requires a 15V single power supply for driving. Compared with the traditional four-channel isolated drive power supply, the requirements for the drive power supply are greatly simplified. (3) It has built-in control power supply undervoltage (UV) protection and main circuit short circuit (SC) protection functions. When the drive power supply voltage is lower than 12.5V, it automatically blocks the six-channel PWM drive input signals. (4) It uses an integrated high-voltage IC (HVIC), eliminating the need for opto-isolation and transformer isolation, and directly interfaces with the CPU, significantly simplifying the complexity of the drive circuit. (5) It uses highly active drive logic, eliminating the restriction on the order of drive power supply and drive input during power-on and power-off, giving the device the function of automatically preventing such faults. This point is often overlooked by designers, and therefore often causes damage. (6) It does not require level conversion and can directly interface with a 3V CPU or DSP. The TMS320LF2407 is a 3.3V DSP chip, which eliminates many troubles when interfacing with the inverter module. 4. CAN Bus Control Network (Figure 2) [align=center]Figure 2 CAN Bus Control Network of Flatbed Printing Machine[/align] The BUSH-5V flatbed printing machine uses RS-232 serial communication between the central controller and the printing units, which is slow and unreliable. Therefore, some critical operations still use the traditional method of direct wiring. The BUSH-7V uses RS-485 serial communication, which improves reliability. In our solution, the CAN bus is used to realize serial communication between the central control PLC and the frequency converters of each printing unit, broadcasting start and stop commands and monitoring the working status of each printing unit; the printing units can also communicate with each other, copy setting information, and simplify the repetitive setting of printing unit parameters. Given the high reliability of CAN, all control and status signals are sent through the bus, simplifying wiring and improving real-time performance. There are many articles discussing the CAN bus. The SJA1000 is a commonly used standalone CAN chip, and 8-bit microcontrollers with CAN controllers include the P8xC591. However, the CAN controller integrated in the TMS320LF2407 is quite unique. It has six mailboxes: two transmit mailboxes, two receive mailboxes, and two transmit/receive selectable mailboxes. Each transmit mailbox has an independent transmit identifier code, each receive mailbox has an independent receive acceptance code, and every two receive mailboxes share a receive mask code. This multi-mailbox arrangement, compared to the SJA1000's equivalent of only two mailboxes (one receive mailbox/one transmit mailbox), greatly facilitates users in constructing more complex networks and achieving more flexible communication. It also simplifies the writing of communication protocols. 5. Serial Communication Control Panel Each dedicated frequency converter for the scratch-off unit has a control panel. Through this panel, the scratch-off speed, number of scratches, and scratch-off stroke can be set. It can also be set for online/standalone operation and individual start/stop. Traditionally, the control panel simply has corresponding buttons installed, lacking a CPU, and the connection between the control panel and the scratch-off unit frequency converter is numerous. Our design employs a modern approach, utilizing serial communication to interconnect the control panel and the scratch-off unit inverter, as shown in Figure 1. The control panel contains a CPU (89C52). The serial communication interface standard used in this design is the EIA RS232C standard. While RS232C defines its own electrical standard, this standard does not meet the serial communication requirements of DSP chips, thus requiring level conversion. The MAX232 level conversion chip used in this design can perform bidirectional TTL/RS232 level conversion, as shown in Figure 3. Similarly, the control panel also needs to use MAX232 to perform bidirectional level conversion, transforming the TTL level of the 89C52 serial port to the RS232 level. Another advantage of using a standard serial interface is that it facilitates serial communication between the scratch-off unit inverter and a PC. With appropriate software, the inverter can also be set and monitored via a PC, resulting in a more user-friendly human-machine interface. Both of these methods have been implemented in our design. [align=center]Figure 3 Serial Communication Level Conversion Schematic[/align] 6 Results The developed dedicated frequency converter for the scratch-off unit can drive a 1.5kW asynchronous motor, using a common V/F control mode with a switching frequency of up to 5kHz. It offers ten speed settings, equivalent to an output frequency from 15Hz to 80Hz; the scratch-off stroke is measured using a photoelectric encoder, which outputs 200 pulses per revolution; the number of scratches can be set arbitrarily from 0 to 9. The scratch-off unit frequency converter has an RS232C standard serial port for communication with the control panel. The control panel includes an 89C52 microcontroller, a MAX232 level conversion chip, 12 buttons, and an LCD screen, forming a simple human-machine interface. The dedicated frequency converter for the scratch-off unit also has a CAN fieldbus interface, allowing communication with the central control unit PLC and other scratch-off unit frequency converters to form a field CAN control network. The central control unit uses a PLC from Bekalai, equipped with a CAN communication module. The system has up to a dozen or so scratch-off units, each responsible for scratching one color, all under the unified command of a central control unit (PLC). Laboratory use has proven that the dedicated frequency converter for the scratch-off units operates reliably and is easily controlled. Figure 4 shows the line current and line voltage waveforms when driving a 750-watt three-phase asynchronous motor under no-load conditions. The scheme described in this paper was supported by the Shaanxi Provincial Department of Education's industrialization project. [align=center] Figure 4 Experimental line current (top) and line voltage (bottom) waveforms of the frequency converter for the scratch-off unit[/align]