When a manufacturing organization uses computers to control its industrial processes, it is possible to obtain benefits of all kinds in all the processes involved controlled. Plant efficiency is increased, as well as productivity and capacity, product quality repeatability is ensured, material usage and energy costs are optimized, plant operation safety is enhanced and, as a result, the profitability of the organization is increased.
From the technological point of view, computer process control represented a great advance, which has allowed the implementation of novel and complex control systems in the industrial field. Initially, computer process control was limited to imitating, with digital technology to analog controls, but given the capabilities of computers it was possible to achieve total control of a manufacturing plant, making feasible what we call integrated manufacturing systems today, where the concepts of process control and production management are mixed.
The application of computers to industrial process control begins in the fifties. Its first appearance was in the so-called process industries, which, being very difficult to control, required a lot of personnel and new methods to implement it; therefore, the quality of control depended above all on the experience and decisions of such personnel.
In the seventies, the application of computer control took a quantitative and qualitative leap with the mass introduction of microprocessors and integrated circuits. These extraordinary electronic components reduced the cost of computers and increased their processing capacity exponentially, both in quantity and in response time.
The impact of computer process control has been very important in the evolution of industrial work, given that its field of application in industrial or manufacturing organizations has been very deep and wide. It has impacted, since its appearance, the control of hard processes, facilitating, optimizing and streamlining control actions, as well as the soft processes of the industry, through examples such as plant production scheduling, analysis and design of manufacturing processes, control of production machines, individually and in groups or manufacturing cells.
In this topic you will review the fundamentals of computer control from the point of view of hard processes and you will study some examples of computer programs that can perform the programming of the production of a plant making use of simulation techniques.
4.1 Fundamentals of computer control
Components of a typical computer control system
Computer process control systems, in most cases, must be capable of operating in real time, which is defined as the response time necessary so that there is no degradation or malfunction of the system.
The programs of a control system can be of three types:
The ideal example of sequential control is the Programmable Logic Controller (PLC); this has been used for years in endless industrial applications. Its main characteristic is that it follows a sequence of instructions that become control actions, equally sequenced, which can be conditioned in different ways with information that the PLC receives internally or externally (Groover, 2018).
The internal conditions can be scheduling rules: drums, clocks and calendars; the external conditions are information received from the plant usually through sensors or other information collection devices compatible with some PLC communication protocol. These devices are also used to control processes in which a high level of intelligence of the controller device is not required and the process is highly predictable; for example: the operation of mechanical and repetitive type machinery, such as die-cutting presses, conveyors, open-closed type valves, among others.
PLCs use input signals and output signals that can be interpreted as binary (they are present, or they are absent). PLCs are programmed by means of human-machine interfaces, which can be interfaces dedicated to communication, specialized programmers, screens, keyboards, switches, etc. Or, they can be of more general purpose, such as PC or a laptop.
Among the activities that a computer control system performs include:
The objectives that a control system pursues are the following:
In addition to sequential control, there is feedback control, which is based on the existence of a control loop. The control loop output is fed back into the system so that the control system control knows the state of some plant process and can make corrections to the relevant process to keep the output continuously controlled within a previously specified range.
Feedback control is part of the direct digital control loop family, other components of which are inferential control, feedforward control and adaptive control (Figure 1).
Figure 1. Representation of a feedback control system.
The feedback control family can control a plant or process continuously, that is, the input and output signals it handles cease to be binary only to handle signals of the continuous type. Its programming is no longer merely sequential but is based on complex control algorithms. The brain of this family of controls is a computer capable of processing the continuous signals it receives, making decisions based on the control algorithms and sending responses (output signals) of a continuous type.
The feature of the system is the ability to compare the state of the plant (state of the monitored and controlled variables) with a programmed state called set-point and its performance as a result of the result of the comparison made. Examples of the type of processes controlled by this family are all those that involve continuous and non-repetitive variables (processes involving temperature, pressure, acidity, concentrations, resistivity, conductivity, position, speed, voltage, current, among others.).
The signals from the plant are received by means of information collecting devices of the continuous variables involved in the system and compatible with the communication protocols of the controller. The output or control signals are sent to the actuators to be managed from the controller and must also be compatible with the communication protocols of these actuators.
The human-machine interfaces of the feedback control family are those traditionally used by any computer, although they can also be devices dedicated to the visual monitoring of the variables or change of their set-points.
Inferential control is that in which the response of the variable to be controlled is not measured directly, instead it is inferred from the measurement of other signals that have a known relationship with the variable to be controlled.
In advance control certain signals that are present before the process to be controlled are measured in advance, thus making decisions in advance of the occurrence of the process, thus shortening the time required for the controlled process to be carried out.
Adaptive control seeks to ensure that the variable response is always the same regardless of what happens in the process, the system continuously monitors the process parameters and self-adjusts so that the process response is always the same.
Another type of computer process control is supervisory control. The idea behind it is that the system is only responsible for displaying the value of the relevant signal and allows the change of its reference value (set-point), but it is not directly responsible for its control, which is housed in a separate system.
There is also the so-called hierarchical control system. In this type of systems, several control systems are linked in a pyramidal structure, where the main feature is that the lower hierarchy control systems are those that are in direct contact with the controlled variables, only have control over them and have a very fast response time. The higher the hierarchy of a system, the greater its range of control, so its response time is slower to act on the systems it controls and the calculations it must perform are more complex and its response is slower.
Distributed control systems are those systems in which the control functions are distributed among several computers connected in a network, providing redundancy to the system, if one fails, the others will back it up.
SCADA type control system
Computer-aided Production Planning and Control (PP&C)
PP&C systems are computer programs in a graphical environment for decision support in scheduling and interactive control of production in a short-term period. Interactive programming takes advantage of the relationship between man and computer, the ability to recognize patterns, adaptation and decision making on the one hand, and the ability to store and process large amounts of information and display it graphically on the other hand.
Interactive systems make possible the resolution of conflicting objectives of a manufacturing system, in that same direction goes the hybrid approach of automatic and interactive programming, with the support of techniques such as process simulation, artificial intelligence, among others.
PP&Cs are characterized by being information systems for the production floor, capable of coordinating other production systems around the broadcast programming. The concept of PP&C is related to the hierarchical decentralization of planning, programming and production control activities that prioritize human intervention.
The basic inputs for a computer-aided visual production scheduling and control system are the following: production orders (with quantities, start and end dates), availability of resources, tooling and machine loading status, process specifications such as routes and times related to setup or manufacturing (Figure X).
Figure 2. Representation of a PP&C system.
The basic components of this type of programs are the following:
PP&C has also been successfully applied to other areas besides manufacturing. It can be found to be related to aircraft preventive maintenance scheduling, hospital operating room scheduling and construction sites.
Representative examples of tools for computer-aided production planning and control simulation are PROMODEL Siemens Tecnomatix Plan Simulation.
4.2 PROMODEL simulation software
The simulation of a system allows us to observe parameters and variables in a certain time and thereby gather valuable information about its behavior. This technique is used to estimate system performance measures, and its application has been extended to several spheres: industrial problems, queuing systems, communication networks, equipment investment evaluation, inventory reduction, material handling, plant layout, chemical processes, area under curve station, evaluation of integrals, social problems, pricing, business, economics, and so on.
Among the concepts that make up a simulation tool we can mention the following:
PROMODEL
For manufacturing systems simulation, one of the iconic tools that has been used for a long time in both academic and professional environments is the ProModel software. According to its own description on its web portal, it can be classified as a general-purpose, open architecture system simulation program.
Its main features include the ability to simulate Just in Time processes (JIT), Theory of Constraints, PUSH systems, PULL systems, logistics elements, customer service, service models, among others (Promodel, n.d.).
Among the advantages presented by this software we can mention:
A screenshot of the ProModel user interface is shown in Figure 3:
Figure 3. ProModel user interface.
Source: Promodel. (2021). Fourth Edition of Simulation Using ProModel Released.
Retrieved from https://promodel.files.wordpress.com/2017/10/dock-screen-shot.png. For educational purposes only.
To learn more about ProModel and its recent integration with AutoCAD software, check out the following video:
Autodesk Inventor. (2021, July 26). ProModel AutoCAD edition 2022 Demo [Video file]. Retrieved from https://www.youtube.com/watch?v=a8L8WYxeIO4
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when accessing to them, you must accept their terms and conditions.
4.3 Siemens Tecnomatix Plan Simulation Software
Tecnomatix Plan Simulation belongs to the Siemens PLM family, which is a suite which integrates several simulation tools. Plan Simulation focuses on the work and optimization of manufacturing systems and processes such as assembly lines, molding areas, manufacturing cells, among others.
Plan Simulation allows you to analyze and optimize the flow of materials through the plant, as well as the resources used in the manufacturing process. This type of software, by means of simulation, allows to analyze the costs of different strategies that can be implemented in order to select the most economical and efficient one. Likewise, it integrates among its functionalities the simulation of other strategies, such as JIT systems (Just in time) or Kanban systems.
Plan Simulation allows you to create digital models of logistics systems, such as assembly lines. In turn, it allows you to explore them and optimize each of the stations involved in the manufacturing processes. To do so, it models and executes different scenarios analyzing and comparing the results, which is very beneficial before purchasing expensive equipment. One of its most relevant features is that it allows introducing cycle times, waiting times, set up times, dead times, as well as statistical models that describe the system behavior.
Among the sales offered by Siemens Tecnomatix Plan Simulation we can mention:
A screenshot of the Plan Simulation user interface is shown in Figure 4:
Figure 4. Siemens Tecnomatix Plan Simulation user interface.
An engineer who is familiar with control technologies, their methodology and related techniques is indispensable for operating an industrial plant. At the same time, it is highly desirable that the engineer is trained in the activities of the control processes that take place in the manufacturing system in which he/she is working, so that he/she can communicate smoothly with the rest of the experts in the different areas that make up the plant.
Among the activities performed by the control engineer we can mention the following: the definition of the control strategy that needs to be applied so that the system requirements are covered, determining the details of the control system to be implemented (relevant system variables, their treatment and handling), specifying the equipment needed (controllers and the type of interconnection), performing the adjustment of all selected devices, defining the necessary sequential control protocols for the operation, and specifying and implementing the supervisory control that the manufacturing process needs.
Control engineering is a wide field of development for a professional who has the necessary knowledge to progress, since practically all manufacturing industries will need their services on more than one occasion.
You have finished reviewing the introductory topics of manufacturing, planning and control. In the following topics you will study group technology, whose concepts are very interesting from a practical point of view, I invite you to continue with enthusiasm this learning process.
Make sure that you:
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To learn about Production Planning and Control, check out the following video:
To learn about plant simulation, check out the following video:
To learn more about implementation of a computer-aided process control system, we recommend reading:
To learn more about Opcenter APS program for production scheduling and control, we recommend reading:
To learn more about PROMODEL program for production scheduling and control, visit its Official Website:
The student will identify the elements that make up a simple manufacturing system and perform the simulation of this using the Siemens Tecnomatix Plan Simulation software.
To simulate a manufacturing process using the Siemens Tecnomatix Plan Simulation software.
Description:
By means of an information search and classification, the student will distinguish and describe the main concepts on the fundamentals of process control.
Instructions: