Computerized Numerical Control – What is CNC actually?

Numerical controls, also known as NC controls, are devices for the targeted control of machines. They convert the coded data provided to the NC control on a data carrier into corresponding control commands and the resulting work and movement sequences. The introduction of the computer opened up new possibilities for improving NC controls and advancing them by integrating the computer into CNC controls. Both of these systems are examined in more detail in this article. In addition, it covers PLC control, which is also an approach to controlling machines and industrial equipment and is often used in combination with (C)NC controls.

How does an NC control work?

The first NC controls were realized in the 1970s by installing hard-wired components. There was a customized solution for each application. The NC control reads out the control commands, which are previously entered as code via a data carrier, and the control then converts these control commands into work or movement sequences. Adaptation to various products is relatively easily possible within the scope of the machine’s capabilities and available parameters, which is why NC controls are used primarily in machine tools.

The disadvantage of NC controls is that they are limited in terms of their memory capacity and usable control commands. Therefore, pure NC controls are almost no longer used. Rather, they are used together with computers in CNC controls.

Program Structure

DIN 66025 defines the following program structure for an NC control:

• The first line starts with a %character followed by the program name.
• The additional lines are started with N and a consecutive number, ideally in steps of ten.
• The second parameter is a command initiated by the letter G.
• Path information is then provided by specifying values for X, Y, Z, U, V, and W. If a circular movement is to occur, values for I, J and K are added.
• Other options are: T functions for the tool selection, S functions for the spindle speed and F or M functions for the feed.
• There is always an M function at the end of the program. This resets the program.

Example:

%MSM

N10 G00 T32

N20 G01 X-10 Y0 Z-10

N30 M20

It is important to note that the commands remain active until they are replaced by new commands:

For example, if you enter coordinates for the X, Y and Z axis in line 2 and should this alignment also remain the same in the future, it does not have to be repeated in the subsequent lines. The new coordinate is only entered when the saved orientation is to be changed.

Advancement to the CNC Control

With the integration of computers, it is possible to use them for controlling machines directly. CNC controls enable a significantly greater flexibility than NC controls alone. CAD or CAM software can easily be adapt processing parameters without having to change the hardware of the control unit itself.

Principles of CNC Control

A program is executed via a computer or microcontroller for the CNC control. The desired signals are then sent via an electrical circuit to the machine control system and implemented there. Typically, a CNC control consists of the following components:

• Drive system: Consists of motors and control electronics. The drive system serves the movement of the CNC axes. Step motors, servo motors, etc. Drives are used.
• Memory: The memory contains the G-code (control program) and other information for operating the CNC machine.
• CPU: The Central Processing Unit processes commands and controls movements and functions.
• Input and output interface: The interfaces enable communication between CNC control and sensors and other devices or systems.
• Control panel: The control panel is the human-machine interface. This allows the user to set parameters, monitor the process or execute programs.

During the physical implementation of the commands to the machines, various components are used such as ball screw drives, actuators, etc. linear guides, motors, encoders and tool holders. You can also find these in a variety of versions at MISUMI.

Types of CNC Controls

Control concepts can be divided into point-controlled, track-controlled and path-controlled.

Point-controlled refers to the control of a specific point or an individual machine positioning, e.g. for drilling or punching. The tool is moved exactly to a workpiece position, where the tool then starts with processing. The positioning is done from point-to-point. A flexible influence on e.g. the travel speed to the new position itself is not possible.

As opposed to point control, track controls allow the control of one axis at a time in terms of velocity and position. This means that axial or paraxial distances can also be traveled. The movements are limited to the left, right, front side or rear.

Path-controlled means that the machine moves several axes simultaneously in order to follow a path with the tool. Feed control is generally used for path control. Depending on the system, the paths can be any straight lines running anywhere in space or curves and circles.

Another differentiation option is the number of controlled axes. Possible variants here are the 3-, 4- or 5-axes control. The more complex the workpiece to be processed, the more axes are used. Basically, the X-axis, Y-axis and Z-axis are always controlled. Rotary axes can still be added as the fourth and fifth axis. Five axes enable processing of complex spatial shapes.

Advantages and Disadvantages of CNC Control

CNC controls are very precise and enable the realization of complex manufacturing processes. This enables mass production and reduces human labor. At the same time, however, more qualified personnel is needed. The high acquisition costs also result in high initial expenses. However, these should be quickly offset by long-term process optimization and increasing production figures.

PLC - The Programmable Logic Controller

The PLC is another approach for controlling machines and industrial systems. However, PLCs are not only used to control movement, but they are primarily used to monitor and control industrial processes. They perform complex logic operations and control digital and analog inputs and outputs.

The minimal design of a PLC control always consists of an input unit, a processing unit and an output unit, the so-called EVA principle. In addition, there are status indicators, a storage medium and a power supply. Therefore, the components are similar to those of a CNC control. The inputs such as sensors and scanners communicate with the outputs such as motors and lights via a CPU. The functions can be realized via various modules. For this purpose, so-called logic modules are used.

Mode of Operation and Logic Functions of a PLC

The PLC performs fundamentally the following functions: data collection, data processing, decision-making and actuator control. For example, data is obtained from sensors that monitor the system status during data collection.

Parameters can be: temperature, position information, pressure, etc. These data form the basis for the next steps: Is the ambient temperature too high? Was there a drop in pressure?

Data processing then compares values or performs logical operations to decide ultimately what actions need to be performed. The decision made, e.g. changing machine parameters, is now implemented by actuating the actuators. The PLC sends control signals to the actuators (e.g. motors, valves, etc.), which then implement the desired action.

Logic functions that are used and link the inputs together can be:

• AND operation: If both input values ​​are true, “true” is signaled and the function is executed. Example: a door should be automatically secured with an active security alarm after closing it. The alarm function can only be activated if the door is locked and the security alarm is activated.
• OR operation: It signals “true”, if at least one input value is true. Example: I have a door can be opened with a key card or a PIN code. Both options open the door.
• XOR operation: stands for “Exclusive OR” (only OR). It signals “true” when exactly one input value is true. It is suitable for comparing multiple inputs. Example: Pre-run/return switching of a cable winch with two buttons. If no button is pressed, there is no signal (motor off). If the pre-run or return button is pressed, the signal for pre-run or return is forwarded. If both buttons are pressed, there is no signal (motor off).
• NON operation: It reverses the input value. Example: The alarm system of a door should only be active if the door is closed.

The logical linking of the input and output variables is shown in a so-called function plan: Here, all inputs and outputs, function blocks as well as the connections and directions are visualized in a type of circuit diagram. The function plan supports the design, implementation and analysis of PLC control systems.

Advantages and Possible Applications of PLC Control

Many industrial applications can benefit from the use of PLC control. In automation, they are used to control machines, automate production lines, and increase efficiency. They are also ideal for process control due to the logic functions that can be linked as desired.

The advantages of PLC control include:

• changes and corrections are easy to make without modifications
• errors can be quickly corrected, since the circuit can be tested directly on the programming device
• signal curves can be observed

interaction of different control modes

For more complex application examples, PLC and (C)NC controls can be used together, e.g. to perform the following tasks:

• exchange of data and communication
• Parent Control
• Safety and Monitoring

PLC controls are, for example, very flexible and can be easily adapted. CNC controls in turn ensure a high degree of precision and are optimized for specific processing tasks. Together, a precise and simultaneously flexible system is created for various industrial applications. Combining process (PLC) and machine (CNC) control creates a seamless coordination for efficient manufacturing.

Application Example

NC controls and PLC controls can be used, for example, to regulate the temperature. For example, a bi-metal strip can be connected to a PLC and a boiler. When the correct temperature is reached, the circuit closes and the PLC receives the signal that the boiler can now be turned off. If the temperature drops, the boiler will be turned on again in the same way.