Abstract: The core controlled object of a motion control system is the axis. Using an object-oriented approach, the class structure, state transition diagram, and data flow diagram of the axis object are established. Based on this model, axis control parameters can be set, motion control library functions can be planned, and control algorithms and schemes can be formulated.
Keywords: Motion control axis modeling
0. Introduction
Axis is the core controlled object of a motion control system. In application systems, there are many types of axes, such as the coordinate axes and spindles of CNC machine tools, and the tension control axes of winding machines; all of these are objects of motion control. Motion control systems are developing towards openness, reconfigurability, and networking, and the standardization of control systems and controlled objects is key to achieving this development.
Object-oriented modeling is a mature modeling technique. By using object-oriented technology to abstract the structure, behavior, and function of axes, and comprehensively describing the attributes and operations of axes, we can lay the foundation for the standardization of axis control objects.
Liu Quanbin, Chen Hu and others summarized the common properties of various controlled axes, established an object-oriented model of the axis using Rational Rose, and implemented the motion control system [1]; the authors also studied the UML modeling method of automatic machine tools and carried out similar modeling of machine tool control axes [2]. The technical committee of the international organization PLCopen made a detailed description of the behavior model of the axis under the framework of the IEC61131 standard [3]. These works have positive significance for the openness of motion control system and the reusability of control program. However, the current research work is mostly focused on the class structure and behavior model of the axis, and the functional model of the axis is rarely studied and the model is not complete; and this kind of research work is mostly aimed at object-oriented programming, and its class structure design is often too complex. The motion control system on the MCU and DSP hardware platform does not currently support C++. The purpose of the axis object modeling in this paper is to serve the parameter setting of the axis control system, the planning of motion control library functions and the design of control scheme.
The object-oriented model mainly includes the class structure model, behavior model and function model[4]. The class structure model mainly describes the attributes and operations of objects, defines the association between object instances, and facilitates the reusability of the control program; the behavior model describes the state and state transition, that is, the relationship between input and response and the interaction between objects, which facilitates the standardization of the control program; the function model mainly describes the input data of the system and the data transformation form, which is the basis for the design and scheduling of control algorithm schemes.
1. Class structure model of axes
In motion control systems, the properties of axes are the most complex. Reference [1] defines the class object of the axis as a complex inheritance structure. Although it conforms to the object-oriented design method, its state switching takes up system time (the experimental test delay is 0.2% of the sampling period, and the worst is 5%). The motion control system is a strong real-time system, and state switching occurs frequently. The key is that most motion control platforms use MCU and DSP, which do not yet have C++ support. This object-oriented model has no practical significance for MCU and DSP type motion control systems. The axis control class proposed in this paper adopts a concise structure to define the properties and operations of the axis class, and focuses on the setting and analysis of the attribute parameters of the class.
In summary, axes have five categories of attributes and more than 40 parameters. Due to space limitations, only the classification of axis attribute parameters is listed in Table 1: Table 1 Classification of Axis Attribute Parameters
Axis operations are the core of motion control. The library functions of a motion control system mainly consist of axis operation functions. These operation functions are extracted from the axis's state transition diagram (described later). In the class structure, they are manifested as modifications and updates to axis attribute parameters. Axis operations can be divided into four categories, totaling 16 functions, as shown in Table 2:
Table 2 Axis Operation Functions
In motion control systems, in addition to shaft classes, there are also logic controllers, sensors, and other classes. These classes can be combined to form the final controlled object manipulator class, and their class relationships are shown in Figure 1.
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Figure 1. Relationships between axis control classes and other classes.
A manipulator can consist of multiple axes, sensors, and a logic controller instance. Manipulator control actually becomes the control of the axes and the logic controller. 2. Axis Behavioral Model
Following the principle of openness, the PLCopen Technical Committee defined eight axis states: unavailable, stationary, stopped process, error-stopped, return to reference point, continuous motion, synchronous motion, and discrete motion. Continuous motion refers to speed and torque control modes without position requirements; discrete motion refers to motion with trajectory and position requirements, such as linear or circular motion; synchronous motion refers to electronic gear and electronic cam motion modes. The state transitions are shown in Figure 2.
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Figure 2. Axis control state transition diagram
Axis state transition diagrams are important tools for planning and evaluating motion control functions. For example, whether a continuous motion state can be directly transitioned to a discrete motion state, whether a stopping process state needs to be inserted in between, and what impact the establishment of this control function has on other axis states can all be planned and evaluated using axis state transition diagrams, considering state transitions, triggering events, transition conditions, and initiating events.
3. Shaft Functional Model
The functional model of an axis is primarily a data flow diagram. A data flow diagram consists of three elements: interface, data processing, and data storage. The interface specifies the source and destination of the data, the processing specifies the data processing algorithm, and the data storage specifies the data storage buffer.
The axis control calculation of a motion control system includes two main parts: interpolation calculation and servo calculation. Interpolation calculation provides the target position for each sampling period, while servo calculation enables the axis to track the target position. In actual execution, the motion control command must first be decoded and preprocessed. For high-precision position control, notch filtering is also performed after interpolation to eliminate certain frequency components and reduce excitation factors. If speed control is included in the interpolation calculation process, the axis control processing includes four parts: control command decoding, interpolation calculation, notch filtering, and tracking control. Its data flow diagram is shown in Figure 3.
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Figure 3. Data flow diagram for axis control.
As can be seen from the axis control data flow diagram, the data flow is serial. At each data processing point, different control strategies will select different processing algorithms. Among them, interpolation, notch filtering, and tracking operations will consume more system resources and have longer delays. In CPU resource configuration or allocation, it should be ensured that these data processing operations work serially.
In addition, the arrangement of data storage involves the performance of the system and the corresponding hardware structure. For example, in the NURBS interpolation and PID+velocity feedforward+acceleration feedforward control algorithms, since NURBS interpolation takes a lot of time, on the TMS320F2812DSP, the delay of PID+velocity feedforward+acceleration feedforward is about 1/4 of the delay of NURBS interpolation[5]. By adopting the dual DSP pipeline processing mode, this serial process is changed to a parallel process, with one DSP performing NURBS interpolation calculation and another DSP performing PID control. A dual-port RAM is arranged as a data buffer, and the sampling period is greatly shortened. The data form in the data flow diagram is relatively simple, as shown in Table 4:
Table 4 Data Format of Axis Control Data Flow Diagram
All buffers in the diagram are first-in, first-out (FIFO) queues.
The data processing procedure for the flow graph is shown in Table 5:
Table 5 Data Processing Procedure
4. Application of Axis Object Model
This model is primarily applied to the design of motion control systems based on MCUs and DSPs. It employs a concise object class structure with multiple parameters and operations, which facilitates understanding the controlled object and setting control parameters. The state transition diagram defines the axis's state and, under what conditions and by what events, triggers the axis to transition from one state to another. Analyzing and summarizing these triggering events provides the basis for designing motion control library functions. The axis control process can be viewed as a data transformation process, from control command input to control signal generation. The data flow diagram expresses the flow and addition of data, serving as the foundation for determining the control scheme and designing the control algorithm.
5. Conclusion
Axis control is the core of motion control systems. Using an object-oriented approach, a concise axis object class structure and state transition diagram are established, and data flow diagrams from structured analysis are referenced. This provides a useful reference for setting control parameters, planning motion control library functions, and formulating control schemes and algorithms in motion control system design.
The author's innovation lies in establishing a concise axis object analysis model by adopting an object-oriented model combined with a structured model's data flow graph. This model provides a basis for axis state expression, axis motion control function analysis, and function operation implementation.
References
[1] Liu Quanbin, Chen Hu, et al. An open controller model for axis motion control. Manufacturing Automation. 2005.4(27),75-78.
[2] Liu Ning. Research on automatic machine tool analysis and modeling methods for control system design. Modular Machine Tools and Automated Machining Technology. 2006.4.27-29 [3] PLCopen. Technical Specification PLCopen-Technical Committee 2 –Task Force Function blocks for motion control Version 1.1. http://www.PLCopen.org.
[4] Wang Jianjun. UML Modeling: Case Analysis. Microcomputer Information. 2002.15.66-68.
[5] Ning Liu, Gao Wang, et al. "Research on a Real-time Interpolator DSPs Based for NURBS", in Proceeding of the 6th World Congress on Control and Automation.June 21-23, 2006. Dalian.China.8206-8210.
