Elements of an Industrial OEM CNC controller

Different Industrial OEM CNC controllers are available in the market with varying basic hardware configurations, but their fundamental working principles are generally the same. Some controllers are specifically designed to work only with a particular CNC system. Companies like SIEMENS, FANUC, MITSUBISHI, YASKAWA, HEIDENHAIN, and REXROTH manufacture popular controllers, each with different models. Among these companies, FANUC and SIEMENS have the highest-selling controllers and are market leaders in the machinery industry. Depending on the design and type of controller, sometimes the PLC and motor driver units function together as a standard enclosed arrangement, while other times they operate as separate components that coordinate with an external controller. The following is a list of accessories incorporated into an advanced CNC system:

1. Central Processing Unit (CPU)
2. Servo Control Unit
3. Motor Driver Unit
4. Operator Panel
5. Machine Control Panel
6. Programmable Logic Controller (PLC)

1. Central Processing Unit (CPU)

The central processing unit (CPU) is considered the heart and brain of a controller as it controls and supervises a complete mechatronics system. It encodes explicit program information stored in the controller memory and decodes it to obtain signals for specific position and velocity movements of the axes. Since it manages the position and velocity of the moving elements (such as axis movements of a machine), it maintains complete control. If it does not match the programmed values, the CPU immediately takes corrective measures. It also checks basic security features of the CNC system, taking remedial action as needed. When the situation exceeds the CPU's limitations, it decides to shut down the machine or system.

2. Servo Control Unit

The input command signals for position and velocity control of movements are generated by the CPU and considered as signals for the servo control unit. The servo control unit generates suitable signals based on each movement command and sends them directly to the servo drive or amplifier unit. The servo drive or amplifier then generates the voltage necessary to rotate the servo motors. The servo control unit also receives the actual position signal from position feedback devices like encoders or linear scales that are integrated with the servo motors. Finally, the feedback signal is sent to the CPU unit for further processing. In summary, the servo control unit facilitates dual communication between the moving elements and the CPU through a command and feedback system. A separate motor driver unit always follows the axes movements, and the displacement depends on the feedback system.

3. Motor Driver Unit 

A motor driver unit is incorporated into a complete CNC system to operate separately controlled mechanisms. Typically, stepper and servo motors are used for controlled movements in a CNC system. A stepper motor driver unit drives the stepper motor, while a servo drive or amplifier rotates the servo motor. In a stepper motor, the motor driver unit generates distinct voltage pulses that turn the motor shaft at specific angles, providing the required torque. A servo amplifier typically provides pulse-width-modulated (PWM) voltage to rotate the servo motor shaft. The working principles of a servo amplifier and stepper motor driver will be discussed later. The following picture shows a stepper motor driver unit and a servo amplifier.

4. Operator Panel

The operator panel serves as a two-way communication system between the operator, the controller, and the machine. Most operator panels consist of two main parts: a video display unit and a keyboard. The video display unit shows the present situation and status of the machine or system, while the keyboard is used for writing programs into the machine and issuing commands. Sometimes, an operator panel may not have a video display unit. The following are two operator panels frequently used with CNC machines in the machining industry, manufactured by SIEMENS and FANUC.

5. Machine Control Panel 

The machine tool and CNC controller are directly connected to the machine operator through a Machine Control Panel (MCP). During a machining operation, the MCP controls the cutting speed or axis feed as well as the spindle speed. It is also used to run the machine in different operational modes like Jog, Incremental, and Automatic modes. The MCP controls the execution of a running program, allowing it to stop or be executed step by step. The following picture shows MCP units manufactured by SIEMENS and FANUC, commonly found in multiple CNC machines.

6. Programmable Logic Controller (PLC)

PLCs are currently used in various advanced mechatronics systems, including the machine tool industry, elevators, process control, and traffic light control. PLCs are typically manufactured by specialized companies and can be customized according to specific requirements with standard hardware. They replace complex relay logic circuits and offer greater versatility and flexibility in circuit design for modern mechatronics systems. In an advanced CNC controller, the PLC is embedded and cannot be identified independently from the outside. However, PLCs can also function as standalone units, coordinating with the CNC controller as a separate device. The supervision and sequence of operation of a complex mechanism are performed by a PLC through programs written inside it. The following picture shows some standalone PLCs coordinating with the CNC controller.