# Development of a Small CNC Machining Center for Physical Implementation and a Digital Twin
## 2 Literature Review  
CNC machines use feed drive systems realized in the form of closed-loop motion control systems and use PID (proportional-integral-derivative) controllers as common control strategies. The majority of control systems in use today, including motion control systems, use the PID controller due to its simple structure, ease of implementation, and the existing knowledge base of PID control over a long time [1; 2; 3]. A model reference adaptive control (MRAC) based on PID control was proposed in [4], which was also successfully validated experimentally. However, experimental validation was carried out by studying the tracking error on a single axis, without studying the contouring regime (simultaneous displacements on two axes). The work in [5] presented a new method for tuning a nonlinear PID (NLPID) controller using a particle swarm optimization based on a generalized opposition-based learning (G-PSO) algorithm. The results showed an improvement in the system's step response (reduced settling time and suppression of overshoot) but did not test the system's behavior in the fast single-axis positioning regime or in the simultaneous multi-axis motion (contouring) regime. The work presented in [6] proposed a fuzzy PID controller based on a sparse fuzzy rule base (S-FPID). It was demonstrated by means of simulation and experiments that the proposed controller can effectively control the position of a nonlinear CNC feed drive. Also in this case, the validation of the proposed method was based only on the study of the step response, without taking into account either the one-axis positioning regime or the two-axis simultaneous displacement regime with working feed-forward algorithms (contouring regime). Open-architecture CNC systems are built modularly, with each module fulfilling specific tasks for CNC equipment [7]. The open nature of these systems gives the user the possibility to intervene on component modules, either hardware or software, which can lead to performance improvements [8; 9]. Open-architecture CNC systems are used not only in machine tools but also in industrial robots [10]. The study in [11] presented the development of a three-axis CNC machining center with open architecture. The motion control systems were based on a master-slave combination between Raspberry Pi 3B and Arduino MEGA 2560 boards. The implementation of linear and circular interpolation algorithms was reported, but the concrete implementation of the motion control was not specified, nor were experimental results related to the system performance presented. In [12], the development of a laser engraving system with open architecture was presented. The motion control was realized based on Arduino MEGA boards (ArduinoS.R.L., Monza, Italy). The study and improvement of the system behavior were based on the step response, without taking into account the positioning or contouring work regimes. Another implementation of the open architecture of a CNC system, also a laser engraver, was presented in [13]. Motion control was implemented using Arduino boards, but this time, open loop control with stepper motors was used. Another approach showing the design and development of a writing machine in the form of a CNC equipment using Arduino boards, stepper motors for the X- and Y-axes, and a servo motor for the Z-axis was presented in [14]. The performance of the machine was demonstrated experimentally, but it should be kept in mind that due to the purpose of the machine, writing and drawing, the technological resisting forces had negligible values. For machining processes run on CNC equipment, digital twins technology can be used for intelligent monitoring and analysis by mirroring the actual machining process in a virtual environment [15; 16; 17; 18; 19]. A study that used digital twins technology to synchronize the load exerted on the main spindle and NC code data and to uniformize the machining load by analyzing the stored data was presented in [20]. Digital twins technology is also used to simulate machining processes on CNC machines.