Gear transmission is a primary form of transmitting machine motion and power, widely used in machine tools, automobiles, aerospace, weaponry, and many other fields. Gear hobbing is the most important of all gear machining methods, with hobbing machines accounting for approximately 45% of all gear machining machine tools. The CNC transformation of hobbing machines has revolutionized machine tool structure and control, improving gear machining accuracy, expanding the machining range, achieving high automation and flexibility, and facilitating the use of new machining processes. 1. Open Modular CNC System for Hobbing Machines and PC-PMAC Strategy For a full-function CNC hobbing machine, all motion axes of the machine tool (hob head rotation A, hob rotation B, workpiece rotation C, axial feed Z, tangential feed Y, radial feed X) are CNC controlled. In software interpolation-based CNC hobbing systems, the tool spindle is generally controlled by a frequency converter, while other axes are directly driven by servo motors via CNC commands. The specific motion relationship between the tool and workpiece is determined based on the parameters of the gear being machined and the tool used (i.e., the so-called electronic gearbox). Its advantages include the fact that the spindle speed is entirely controlled by the CNC system software. Therefore, by developing appropriate software, non-circular gears and modified gears can be machined with high precision and speed using general-purpose cutting tools, achieving a machining accuracy far exceeding that of traditional mechanical template machining methods. With the rapid development of computer technology, developing CNC systems based on PCs allows full utilization of the numerous achievements (including software and hardware) brought about by the rapid development of standard computers. Furthermore, the standardization of PC hardware provides the most convenient way for system upgrades and maintenance. Specifically, given that the PC bus is an open bus, this system possesses openness, modularity, and embeddability. System manufacturers can extensively utilize commercially available software and hardware boards on the PC hardware platform and operating system to improve the functionality of the CNC system, shorten the development cycle, and reduce costs. Machine tool manufacturers and users can reset, modify, expand, and retrofit the CNC, and modularly integrate sensors, machining process monitoring, and other functions, ultimately constructing and recombining the most suitable CNC system functions and other control functions. Compared to other CNC machine tools, gear hobbing machines have relatively complex motions, hence their later development. Although full-function CNC systems have become dominant abroad, most are still variations of ordinary CNC systems and belong to closed systems of various companies. Moreover, there are very few open systems that truly reflect the characteristics of gear processing. Therefore, the development of modern gear hobbing machine CNC systems is very urgent, such as: being able to add or remove components according to the specific functional needs of the machine tool; and for the same CNC system, being able to flexibly control different objects such as gear hobbing machines, gear grinding machines, gear shaping machines, and gear processing units, or control different models of the same type of gear processing machine tools, through dynamic and static reconfiguration oriented to functions. At present, there are generally three ways to implement PC-based open CNC: (1) PC embedded CNC. The PC is used as the front-end interface of the traditional CNC. A special, open personal computer template is inserted into the traditional non-open CNC. The PC board and the CNC are connected through a dedicated bus, enabling the traditional CNC to realize some characteristics of a personal computer. In this mode, the CNC part is the same as the original CNC and performs real-time control; while the PC part performs non-real-time control. This form is mainly adopted by some large CNC controller manufacturers. Its advantages are that the prototype CNC can be used with almost no modification, and the data transmission is fast and the system response is fast. The disadvantages are that the PC cannot be used directly, the degree of openness is limited, and the cost is high, making it unsuitable for small-scale processing. This mode cannot be called a "PC-based" open CNC system in a strict sense. (2) CNC embedded in PC. The motion control board or the entire CNC unit is inserted into the expansion slot of the PC. The PC performs non-real-time processing, and the real-time control is undertaken by the CNC unit or motion control board. This structure allows the entire system to share the hardware resources of the PC and use its rich support software to directly connect with the network and CAD/CAM system. The software has strong universality, and the programming is flexible and low cost. For the structure of CNC unit inserted into PC, its openness is limited to the PC microcomputer part, and the professional CNC part is still in a closed state; while for the structure of motion control board inserted into PC, its openness depends on the openness of the motion control board. (3) Pure PC model. This refers to a fully software-based CNC system that utilizes a PC. All functions of the NC system, such as compilation, interpretation, interpolation, and PLC operation, are implemented by software modules, and the servo drives are controlled via an interface card installed in the PC's expansion slot. This type of system, leveraging existing operating platforms and with the support of application software, can achieve openness at all levels of CNC through appropriate organization, standardization, and development of the PC software. Its advantages include good openness, highly flexible programming, and strong software versatility. Disadvantages include difficulty in real-time processing on a general-purpose PC, difficulty in utilizing prototype CNC resources, and ensuring its reliability is a problem that requires further research. Gear transmission is a major form of transmitting machine motion and power, widely used in machine tools, automobiles, aerospace, weaponry, and many other fields. Gear hobbing is the most important of all gear machining methods, with hobbing machines accounting for approximately 45% of all gear machining machine tools. The CNC transformation of gear hobbing machines has revolutionized machine tool structure and control, improving gear machining accuracy, expanding machining range, achieving high automation and flexibility, and facilitating the use of new machining processes. 2. Open Modular Gear Hobbing Machine CNC System and PC-PMAC Strategy For a full-function CNC gear hobbing machine, all motion axes (hob head rotation A, hob rotation B, workpiece rotation C, axial feed Z, tangential feed Y, radial feed X) are CNC controlled. In software interpolation-based gear hobbing CNC systems, the tool spindle is generally controlled by a frequency converter, while other axes are directly driven by servo motors via CNC commands. The specific motion relationship between the tool and workpiece is determined based on the parameters of the gear being machined and the tool used (i.e., the so-called electronic gearbox). Its advantage is that the workpiece spindle speed is entirely controlled by the CNC system software. Therefore, by developing appropriate software, non-circular gears and modified gears can be machined with high precision and speed using general-purpose tools, achieving machining accuracy far exceeding that of traditional mechanical template machining methods. With the rapid development of computer technology, developing CNC systems based on PCs allows for full utilization of the numerous achievements (including software and hardware) brought about by the rapid development of standard computers. Furthermore, the standardization of PC hardware provides the most convenient way for system upgrades and maintenance. Specifically, given that the PC bus is an open bus, this system possesses openness, modularity, and embeddability. System manufacturers can extensively utilize commercially available software and hardware boards on the PC hardware platform and operating system to improve CNC system functionality, shorten development cycles, and reduce costs. Machine tool manufacturers and users can reconfigure, modify, expand, and retrofit CNC systems, and modularly integrate sensors, machining process monitoring, and other functions, ultimately constructing and recombining the most suitable CNC system functions and other control functions. Compared to other CNC machine tools, gear hobbing machines have relatively complex motions, hence their later development. Although full-function CNC systems currently dominate internationally, most are still variations of ordinary CNC systems and belong to closed systems owned by individual companies. Truly open systems that reflect the professional characteristics of gear machining are still rare. Therefore, the development of modern gear hobbing machine CNC systems is very urgent, such as: being able to add or reduce components according to the specific functional needs of the machine tool; for the same CNC system, different objects such as gear hobbing machines, gear grinding machines, gear shaping machines, and gear processing units can be flexibly controlled through functional dynamic and static reconfiguration, or different models of the same type of gear processing machine tools can be controlled. At present, there are generally three ways to implement PC-based open CNC: (1) PC embedded CNC. The PC is used as the front-end interface of the traditional CNC. A special, open personal computer template is inserted into the traditional non-open CNC. The PC board and the CNC are connected through a dedicated bus, so that the traditional CNC can realize some characteristics of a personal computer. In this mode, the CNC part is the same as the original CNC and performs real-time control; while the PC part performs non-real-time control. This form is mainly adopted by some large CNC controller manufacturers. Its advantages are that the prototype CNC can be used with almost no modification, and the data transmission is fast and the system response is fast. The disadvantages are that the PC cannot be used directly, the degree of openness is limited, and the cost is high, making it unsuitable for small-scale processing. This model cannot be strictly called a "PC-based" open CNC system. (2) CNC embedded in PC. The motion control board or the entire CNC unit is inserted into the expansion slot of the PC. The PC performs non-real-time processing, and the real-time control is undertaken by the CNC unit or motion control board. This structure allows the entire system to share the hardware resources of the PC and directly connect to the network and CAD/CAM system using its rich support software. The software is highly versatile and the programming is flexible and inexpensive. For the structure where the CNC unit is inserted into the PC, its openness is limited to the PC microcomputer part, and the professional CNC part is still in a closed state; while for the structure where the motion control board is inserted into the PC, its openness depends on the openness of the motion control board. (3) Pure PC model. That is, a CNC system that adopts the full software form of the PC. All functions of the NC system, such as compilation, interpretation, interpolation and PLC, are implemented by software modules and the servo drive is controlled through the interface card installed in the PC expansion slot. These systems, leveraging existing operating platforms and supported by application software, can achieve openness at various levels of CNC through appropriate organization, standardization, and development of PC software. Their advantages include good openness, highly flexible programming, and strong software versatility. Disadvantages include difficulty in real-time processing on general-purpose PCs, difficulty in utilizing prototype CNC resources, and the need for further research on ensuring reliability. Figure 1 shows a schematic diagram of the hardware structure of a PC + PMAC gear hobbing machine CNC system. The PMAC connects to each servo drive using connectors for easy connection. Radial (X) and axial (Z) movements can use semi-closed-loop control with encoder feedback (shown in Figure 1) or closed-loop control using gratings. Communication between the PMAC motion controller and the host computer employs two methods: bus communication, where the host searches for the PMAC at a specified address determined by the PMAC's jumper; and DPRAM-based data communication. The PMAC provides numerous automatic access functions for the DPRAM. These automatic functions periodically transfer real-time data between the PMAC and IPC. Furthermore, users can use the PMAC's M variable and the host's pointer variable to specify unused registers in the DPRAM to implement custom communication functions. Using DPRAM for data and command communication between the PC and PMAC has the advantage of speed. On one hand, when writing data to the PMAC, the dual-port RAM can quickly and repeatedly download position data or program information in real-time. On the other hand, when reading data from the PMAC, the DPRAM can quickly and repeatedly acquire system status information. For example, the status, position, speed, and following error data of an AC servo motor can be continuously updated and automatically written to the DPRAM by the PLC or PMAC. Since data access via DPRAM does not require sending commands and waiting for responses through the communication port, the response speed is much faster. Using DPRAM for automatic data access improves the system's response speed and processing accuracy, while also facilitating rapid communication between modules in the control system and the setting of address tables, reducing programming difficulty. The built-in PLC function of the PMAC system is realized through the input and output of the intelligent I/O interface. In the control system, the input signals sent to the PLC mainly include: signals from the control panel and machine tool control buttons and selector switches; signals from the limit switches and mechanical zero-point switches of each axis; machine tool electrical action, limit, and alarm signals; contactor and pneumatic switch contact signals in the power cabinet; and working status signals of each servo module. These signals are sent to the intelligent I/O interface after opto-isolation, effectively isolating the computer digital signal channel from the external process analog signal channel, greatly reducing interference from external factors and improving the reliability and stability of the entire system. The PLC output signals mainly include: indicator light signals; action signals from control relays, contactors, solenoid valves, etc.; and drive enable and speed enable signals for servo modules. 3. System Software Structure The software structure of the PC + PMAC gear hobbing machine CNC system is shown in Figure 2. The entire software system consists of a main control module and various functional modules. The main control module provides a user-friendly system interface, where various functional modules are accessed via menus. Due to the complexity of gear hobbing and the difficulty in calculating machining parameters, the main control module should display relevant parameters of the hob and the gear being machined, fixed gear machining cycles, coordinate positions of each axis during the hobbing process, and dynamic machining trajectories. Operators only need to input the number of teeth, height, and angle of the workpiece, select the appropriate machining cycle, and the CNC system can automatically generate the NC code for gear machining. Figure 2 shows a schematic diagram of the software structure of the PC + PMAC gear hobbing machine CNC system. The system's functional modules can be divided into two main categories: real-time control modules and non-real-time management modules. Real-time control modules control the current motion and actions of the machine tool, requiring millisecond-level or even higher time responses; non-real-time management modules do not have strict time response requirements. Non-real-time management modules include system initialization, system diagnostics, system communication, and NC program editing. These software modules can be implemented using the computer languages ​​and software tools provided by the PC and PMAC. Due to the low time response requirements, they are run by the PC. Real-time control modules include machining program interpretation, servo drive, motion interpolation, data acquisition, and PLC. The design of real-time control software should fully consider both software openness and the specialization of gear hobbing. Users can add functional modules according to specific requirements. PMAC provides excellent software development tools for these real-time control modules. The machining program interpretation module consists of G-code, M-code, and T-code interpreters. Existing PMAC interpreters can be edited and debugged in the PEW IN environment and downloaded to PMAC's fixed memory for automatic use during actual machining. Furthermore, parameters such as servo interrupt time and motor phase are set by the PEW IN execution program, thus achieving parameterization of the real-time control components. The interpolation module allows direct selection of linear interpolation, circular interpolation, and spline interpolation functions provided by PMAC. PMAC also provides a PVT (position-velocity-time) motion mode, which allows for direct and compact control of the trajectory graph. Users can select and combine these modes. Users can also define their own G, M, and T codes. Examples include G64 (generating motion function), G65 (helical gear hobbing), G66 (drum gear hobbing), and G67 (small taper gear hobbing) in gear hobbing. Since some gear hobbing processes involve very typical motion cycles that require multiple iterations, defining these fixed cycles with specific function codes can significantly reduce programming work. Examples include radial and axial rectangular up-milling and radial and axial rectangular down-milling. The servo drive module can select a PID position loop servo filter, notch filter, or extended filter, and set its control parameters; users can also customize their own servo algorithms to achieve personalized servo control. Because gear hobbing is an intermittent cutting process, not only do the cutting forces and hob speeds vary significantly during machining, but the workpiece and worktable also experience intermittent impact forces. Furthermore, the stress on the workpiece and worktable constantly changes during the hob's entry and exit. Conventional controller tuning methods are insufficient to achieve satisfactory machine tool performance. Therefore, in addition to online programmable digital PID tuning, the system should also incorporate speed feedforward and acceleration feedforward filtering. The PLC control module is used for the logic control of the machine tool system's switching quantities. Developers need to program the module according to their control panel requirements and machine tool control logic. While the motion program runs sequentially in the foreground, PMAC can run up to 32 asynchronous PLC programs in the background. The PLC program can monitor analog and digital inputs at extremely high sampling rates, command motion stop/start, and repeatedly scan the PLC program at 5-10ms or even higher cycle speeds. The PLC program is written using the command language provided by PMAC and can be run directly or compiled before execution. It is worth mentioning the dual-CPU communication program here, as its development requires significant effort and skill. Using the Pcomm32 dynamic link library and PTALKT control provided by DeltaTau is a wise choice. It encompasses all communication methods with PMAC, and categorizes and encapsulates its main functions, ultimately forming a user-friendly interface that frees users from the constraints of 32-bit driver libraries, allowing them to focus entirely on defining and developing their own CNC system applications. 4. Conclusion Based on the development trend of CNC systems and the characteristics of gear hobbing, among the three development strategies for open CNC systems, using a general-purpose industrial control computer as a foundation and employing the powerful motion controller PMAC to handle real-time tasks such as interpolation calculation, position control, and speed control is a relatively ideal development method for gear hobbing machine CNC systems. It can achieve openness at both the software management and real-time control levels, and has advantages such as high professionalism, flexible development, short development cycle, ease of technical implementation, and low cost.