Control Systems

Electronic devices used in different machining industries or in our daily life must be functioning with a control system. Controlling means switching the device ON or OFF, running time, and managing temperature, lighting, and sound. Objects cannot use appropriately, instead of a suitable control system. For example, if a room air conditioner has no control system or the volume of any music system cannot control, it would be difficult to use them. Therefore, it is essential having a control system with an electrical or electronic device. The control system of any electrical or electronic device is mainly of two kinds, manual and automatic control.

 

Manual Control - Here, devices are manually controlled, which means machines can turn on or off or control manually, consistent with their demands. For example, lighting up a bulb through a switch, turning on a motor by a starter, and decreasing or increasing the fan speed with a regulator. 

 

Automatic Control - With an Automatic control system, the device is automatically controlling certain circumstances or conditions. During the Automatic control, a device turns on or off within a selected time frame or controls it, like temperature or intensity of light. However, the manual controller is consolidated with every automatic control system, and there is the co-existence of two control systems (Automatic and Manual) collectively. For example, a system may start manually and stop automatically after attaining a specific state, including three different control operations, managing automatic machines or devices.

 

1. Relays, Timers, and Contractors

2. Micro-Controller

3. Programmable Logic Controller or PLC

 

Relays, Timers, and Contractors

With helping of a relay, timer, and contactor, a machine can control automatically and manage small logical operations efficiently. However, there are some disadvantages to using such a control system. It is very inconvenient and time-consuming, to change any logical operations, as relays, timers, and contactors interconnect with numerous wires. Also, it is unsuitable for extended and complex logical operations. We find this type of control system only with small automated machines, where small logical operations are functional. The following picture describes a typical automatic control system, including a relay and timer.

 

Micro-Controller

A micro-controller is a Special Purpose computer managed for a specific purpose only. Only some limited and special instructions are stored inside a microcontroller and directed to operate a selective machine or control system. For example, washing machines, and microwave ovens are typically controlled with micro-controllers. Applying a micro-controller-based control system is convenient but has some disadvantages. Users can not change the logical operation sequence of an automatic control mechanism if required. It only operates according to the instructions written inside its memory. Using a microcontroller is inconvenient for complex control operations, working with distinct elements, and requires integrating into different methods or modifying the control sequence. The following picture shows a typical microcontroller found with simple machinery and automatic controlling mechanisms.

Programmable Logic Controller or PLC

Introducing of Programmable Logic Controller or PLC can solve technical hitches with the earlier two types of control mechanisms. PLC is a typical digital controller, controlling some output elements, depending on the status of some Input elements and the program written inside it. It makes functional or controls some automatic processes. Here, the operation process or sequence of operation can change easily within a short period, executing this type of control system versatile and easily acceptable. Different PLCs are employed extensively for controlling several automatic control mechanisms. The following picture shows different PLCs, found with small control mechanisms. 

A PLC widely admitted where the operational sequence compelled to change in an Automatic control system. Also, with multiple logical operations with simple or complex machines, a PLC can manage efficiently. The size of a PLC usually depends on input and output handling capability, program size, and memory capacity. Companies like SIEMENS, FANUC, ABB, DELTA, OMRON, BOSCH, MITSUBISHI, and TOSHIBA are some leading manufacturers of different model PLCs, as per requirement. There are some differences with electronic circuitry inside the PLC for a separate make, but the working principles are roughly the same. However, there are some differences in hardware configurations and programming. For example, with small PLCs, CPUs with Power supply and Input / Output ports embedded in a single compact module, and extended PLCs, all elements work as separate modules, interconnected with cables. There is also some specific type of PLC, designed by PLC manufacturers, as per the user’s requirement. With CNC machines, PLC is typically embedded inside a CNC controller and PLC CPU or input/output modules cannot identify separately from outside. Here, all the discussions are confined to a discrete PLC, working independently. 

 

PLC is primarily segmented into two different elements the hardware part designed and fabricated by the PLC manufacturer and the program part written by PLC users, employing special software. PLC program always addresses company-wise specific software, which cannot accomplish in other PLCs. A Ready-made PLC of different companies is available, and SIEMENS and FANUC make PLCs are the most popular ones commonly found in modern machining industries. The following picture shows the common elements of a prefabricated PLC.

 

Types of CNC Controller

Different types of CNC controllers are available in the market and can be categorized into three main segments based on functionality and the types of machines they are used with. These segments include microcontroller-based, PC-based, and industrial OEM controllers. The schematic diagram illustrates the different types of CNC controllers.

Microcontroller-based CNC controller:

A microcontroller is a circuit system based on a microprocessor that includes a CPU, internal memory, internal beat circuit, and I/O terminal. All these components are integrated into a single chip to maximize efficiency. G-code programming language is converted by a personal computer into the microcontroller programming language. The microcontroller interprets the data received from the computer and drives the stepper motors in a CNC machine. Microcontrollers such as Arduino and Raspberry Pi are commonly used in simple and open-loop CNC control systems with two to three axes. Microcontroller-based CNC systems find applications in CNC plotters and CNC routers. The following picture depicts a typical CNC system configuration with a microcontroller-based CNC controller.

 

PC-based CNC controller:

PC-based CNC machine tool control software and hardware toolkits allow users to create and customize their own controllers for 2 to 3-axis CNC machine tools using a standard personal computer. Popular software programs like Mach-3 & Mach-4, LinuxCNC, and Flashcut are installed on a computer, and stepper motors are driven by a motion control board or a motor driver unit. These boards can be connected to a computer using USB or Ethernet cables. PC-based CNC controllers are commonly used in applications such as CNC laser cutting, plasma cutting, and simple 2 to 3-axis CNC milling/turning. The following picture illustrates a typical CNC system configuration with a PC-based CNC controller.

Industrial OEM CNC controller:

This type of controller is a comprehensive CNC control system that can handle various levels of complexity, from simple point-to-point linear control to advanced algorithms with multiple axes of control. Servo motors are used for precise control, and motor position feedback is obtained through encoders or resolvers. Industrial communication protocols such as Attached Resource Computer Network (ARCNET), Controller Area Network Bus (CANbus), and Process Fieldbus (PROFIBUS) are utilized. Leading controller manufacturing companies like SIEMENS, FANUC, and HAAS develop their own controllers with numerous models. The modern machining industry extensively employs these controllers for machines such as CNC milling/turning, grinding, and 5-axis machining. The following picture shows a typical CNC system configuration with an OEM CNC controller.

Mode of operation

Running a CNC machine involves different modes or procedures, collectively known as its Mode of Operation. CNC machines can operate in various modes, and the following are some standard modes relevant to CNC machining processes:

• Manual or Jog mode
• Manual Data Input or MDI mode
• Automatic mode
• Reference mode
• Program Edit mode

The picture below displays two different mode selector switches found in CNC machining centers.

Manual or Jog mode:

In this mode, an axis can be manually moved in a positive or negative direction by pressing the corresponding switch buttons (e.g., X+, X-, Y+, Y-, Z+, Z-). The selection of the axis is done using an Axis selector switch, and the feed rate remains constant for axis movement. Manual or Jog mode can be further classified into Continuous, Incremental, and Hand-wheel modes.

Continuous Mode: In Continuous mode, the machine axis will continue to travel in a positive or negative direction as long as the "Jog button" switch is pressed. The CNC operator does not have complete control over the axis positioning, and it is impractical to stop the axis precisely at a predefined distance. This mode is typically used for random driving of an axis in a positive or negative direction.

 

Incremental Mode: In Incremental mode, an axis can be positioned at a specific distance with each command input. In a CNC machine, the axis movement is usually made in increments of 1, 10, 100, 1000, and 10000 microns (1 mm = 1000 microns). This means that each input command moves the axis by the defined distance intervals. For example, to move the X-axis in a positive direction by 10 microns, the operator selects the X-axis using the axis selector switch, sets the incremental mode switch to position '10', and presses the 'Jog+' button. Continuous pressing of the 'Jog+' button indicates a single-step movement. To change the direction of movement to negative, the operator can press the 'Jog-' switch after shifting the axis.

Hand-Wheel Mode: In Hand-Wheel mode, the axis is moved using a specific device called a Hand-wheel. This mode is usually used when an axis needs to be moved away from the operator panel, such as during a new work-offset setting. To move an axis, the operator selects the axis to be moved and rotates the hand-wheel knob clockwise or counterclockwise to move the axis in a positive or negative direction.

Manual Data Input Mode:

In this mode, the operator can enter commands to the controller in written form. Most programming-related instructions can be executed individually using this mode. Instructions such as spindle rotation, automatic tool change, and cutting coolant on or off can be performed separately. Axis movement commands are also possible, and sometimes small machining operations can be performed using this mode.

Automatic Mode:

In Automatic mode, the CNC machine runs continuously, following a selected program. The program is executed in a block-by-block sequence, where the subsequent block is not executed until the preceding block has finished successfully. The program execution can be continuous or step-by-step, using a control panel. It is also possible to manage the machine from any single block, following a part program, through the "Block search" option. The program execution can be interrupted and restarted with the "Optional stop" selection.

Reference Mode:

In Reference mode, all the axes are moved to the "Home" position, and the CNC machine calculates the measurements of the axis moves from the home position. Before proceeding with the referencing, there is no valid position information of the axes in the CNC controller, and it is not permissible to run a program in automatic mode until all the axes are referenced. Referencing the axes is necessary when using an Incremental encoder for position feedback. However, when using an Absolute Encoder, referencing the axes is not mandatory every time the CNC machine is switched on. A detailed discussion of the working principle of Absolute and Incremental encoders is provided in another chapter.

Program Edit Mode:

Program Edit mode is usually not considered a mode of operation for the CNC machine operating group. This mode is used to write a new program or edit an existing one. The CNC controller is set to this mode for such purposes. External devices like programming units, punch tape, and computers can interface with the CNC controller through a cable to load/unload programs, subprograms, machine data, and tool offsets. The loading/unloading process is performed through a RS 232 port on the CNC controller using a serial communication system. Currently, devices such as 'Compact Flashcards' or 'USB' support devices like 'Pen drives' are used to upload/download all necessary machine data with many CNC controllers.