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The role of PLC in industrial control, test and measurement

Posted by: EDM Machining 2021-05-24 Comments Off on The role of PLC in industrial control, test and measurement

Programmable logic controller (PLC) is an industrial computer

Monitoring and control of industrial automation applications

Perform tasks related to test and measurement operations

The copyright belongs to the author. For commercial reprint, please contact the author for authorization. For non-commercial reprint, please indicate the source. PLC receives data from sensors and input devices, processes data to make logic based decisions, and outputs control commands to mechanical or electrical systems. They are embedded systems that combine computer processors and memory with input / output (IO) devices, very similar to their competing hard line relay based logic and PC based logic.

In terms of physical form, today’s PLC can be anything from a very simple computer in the form of an integrated chip (IC) to a large rack mounted collection of controller subcomponents housed in multiple chassis. Simple PLC based on microcontroller or system on chip (SOC) PLC can be very reliable, and can run under a very moderate power input. On the contrary, the most complex PLC blurs the boundaries between PLC and general-purpose computer for real-time industrial control… Although the former still emphasizes reliability and real-time performance.

Initially, PLC was designed to directly replace hardwired control logic based on relay and drum sequencer. These early PLCs only need to perform basic operations by converting input to output. Any machine tasks that require PID control are outsourced to the connected analog electronic devices. Now, PID control and even more complex operation has become a standard part of PLC instruction set.

In fact, with the passage of time, the expected functions of PLCs have proliferated, so today, many PLCs are very complex and can perform complex and adaptive routines. The continuous improvement of semiconductor chip power and the reduction of size (due to Moore’s law) have made the small controller achieve unprecedented intelligence. This trend continues with the integration of motion control, vision system and communication protocol. At the other end of the PLC size range, some programmable automation controllers (PACS) integrate PLC with PC to replace PLC and proprietary control system (running proprietary programming language) of some applications. Nowadays, more and more PLC are integrated into human machine interface (HMI).

Industrial digital environment in which PLC operates

Today’s industrial automation relies on machine feedback and operational data as well as complex interconnections between digital devices

Control digital devices.

Run advanced functions, such as those related to iiot connectivity and machine reconfigurability.

Human decisions can be made on a variety of machines and operating conditions.

Improve overall productivity and workpiece quality.

Such automated installations include different information systems for storing, processing, and providing this data.

MRP system provides production planning, planning, finance and inventory control. In contrast, the historian system stores time series data from sensors and instruments for graphical rendering to help operators and management systems understand and process automation trends. Statistical process control (SPC) is a historical application.

The human machine interface (HMI) is a machine control Panel (or module wirelessly connected to a mobile device) that allows the operator to view data and issue commands. Closely related to the function of HMI is supervisory control and data acquisition (SCADA) system, which can real-time control and monitor the interaction between automation machine and its HMI. Using SCADA, HMI can control multiple machines… And Display data related to multiple devices.

Manufacturing execution system (MES) includes functions such as operation planning and data collection. In some ways, it can be seen as being between and overlapping with MRP and SCADA.

ERP system integrates MRP, MES, PLM and CRM information system related to manufacturing. ERP system can be an integrated software suite dealing with all these functions, or a core ERP system with special application program interfaces of multiple suppliers. Usually, only top management can interact with ERP, and most people in a given organization will interact with one of the component systems fed into it.

PLC usually runs at a lower level than these information systems. They communicate with machines, motors and sensors. They can also interact with the above information levels, send data to history recorder or SCADA, or receive control input from SCADA or HMI. In more and more cases, more complex PLC can also perform SCADA and history recorder functions, and even HMI functions.

Please note that PLCs are not only involved in automation: they are also used to control test bench (product development) and laboratory measurement tasks.

As mentioned above, automation usually focuses on diagnosis and requires deterministic real-time operation from PLC to achieve true effectiveness.

On the contrary, the PLC used in the measurement task pays more attention to the fast and accurate implementation of measurement collection and other forms of data collection.

For machine automation tasks, PLC relies on real-time processing, in which the delay between input and response output is in milliseconds. In addition to the simplest PLC functions, all other functions require the use of real-time operating system (RTOS). Although many PLCs still use proprietary operating systems, there is a growing interest in open standard operating systems.

For test and measurement tasks, PLC relies on real-time processing, in which the delay between the measurement of field equipment and its acquisition is measured in milliseconds. Gone are the days when engineers had no choice but to use interface converters and transmission channel systems. Intelligent devices and I / O components now provide advanced and simplified signal collection via digital and analog inputs.

Today, engineers can also choose more options based on standardized interfaces and cross manufacturer compatibility of components that can be used as interoperable components.

Only I / O components with integrated PLC function are considered. They are compatible with configurable HMIS running Windows or Linux operating systems with Ethernet connections, but lack simple recalibration options or analog I / O for field devices that generate low voltage analog signals. Such I / O components can also be used with PLCs that are set up to collect data from remote I / O devices… And directly from sensors through their own onboard I / O.

Figure 2: T7 multi function data acquisition equipment (DAQ) includes Ethernet, USB, WiFi and MODBUS connection, which can be used with various field devices, industrial HMI and PLC. Modbus / TCP connectivity, in particular, provides controllability through a variety of third-party software and hardware options for openness and flexibility – thus providing vendor neutral data collection options and automation applications for industrial system architects and R & D engineers( Photo source: labjack)

Of course, PLC is not the only choice for machine automation or test and measurement. As all industrial controls become more complex, some vendors are beginning to differentiate certain hardware from programmable automation controllers (PACS) to represent enhanced functionality, in many cases with multiple processors on a single hardware. In fact, the complexity of PLC is getting higher and higher – therefore, there is no unchangeable rule for when to perform certain hardware components of PLC functions. Most PACS integrate PLC and PC, and can be used as complex automation systems with multiple PC based applications, as well as HMI and historian. An obvious difference is that PAC has a more open architecture than traditional controls, so it is easier for developers to use PAC.

Another option today is modular PLC. These are composed of modules that perform different functions. All PLCs must include a CPU module that contains the processor and memory for the operating system and programs. There may be a separate power supply module and other input / output (I / O) modules. The PLC may include digital and analog I / O modules. Network communication may require another module.

PLC can be integrated with all modules in a cabinet, and can also be modularized. Integrated PLCs are more compact, while modular PLCs are more versatile, usually allowing multiple modules to be easily connected together by directly inserting into each other or using a common rack as the bus. Address the module according to its position on the bus. Although the physical support aspect of the rack may conform to standards such as DIN, the data bus is usually proprietary to the PLC manufacturer.

The role of PLC in the Internet of things

With the growing interest in industry 4.0 (also known as iiot), industrial users increasingly want to use internet protocol to connect their industrial controllers to the company network. This means using transmission control protocol (TCP) and Internet Protocol (IP) or only TCP / IP for communication. However, the trend of iiot involves not only the use of Internet protocols, but also machine learning and big data. As the function of PLC becomes more and more powerful (more advanced control makes PLC function a function), more and more host functions (such as visual system) will appear. The Internet connection also allows engineers (through the system PLC) to use cloud based algorithms to process very large data sets (also known as big data) for machine learning.

In practical application, EtherCAT, which is used to control automation technology, performs well in the function of iiot PLC. This is a communication protocol based on Ethernet, which is suitable for real-time control applications with cycle time less than 0.1ms. It is the fastest industrial Ethernet technology with nanosecond precision synchronization capability. Another important advantage is the flexibility of the EtherCAT network topology that works without the use of network hubs and switches. Devices can be simply linked together in a ring, line, star or tree structure. PROFINET is a competitive standard for providing similar functions.


The current trend is that more and more complex data collection and industrial control will continue. This means that PLCs for industrial automation, testing and measurement will be more and more like PACS and integrated with SCADA and computers. Internet protocol and open standards (such as EtherCAT) have been steadily adopted in PLC communication. This connectivity, in turn, will stimulate more use of industry 4.0 technologies, such as big data analysis and machine learning, partly thanks to the allocation of the required processing power and memory to:

Cloud based computing

Edge devices capable of data processing

In addition to these trends, more traditional PLCs are still needed to perform relatively simple tests and measurements as well as control functions with maximum reliability and energy efficiency.