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What are some examples of control methods that can be implemented using a PLC program?

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Control methods are techniques that allow a system to achieve a desired output or behavior by adjusting its inputs or parameters. A programmable logic controller (PLC) is a device that can execute a control program to manipulate the inputs and outputs of a system based on predefined logic, timers, counters, and other functions. PLCs are widely used in industrial automation, manufacturing, and process control applications. In this article, we will explore some examples of control methods that can be implemented using a PLC program.

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Jose Carlos Villajulca

Especialista en Ejecucion y Gestion de Proyectos de Automatizacion Industrial. Fundador y Consultor en…
Paolo Cavallanti

Advanced Services PM @ Omron | Driving innovation in industrial ecosystems
Luis Rodriguez L.

Industrial Automation Geek | PLC | Drives | SCADA

1 On-off control

On-off control is the simplest form of control, where the output is either fully on or fully off depending on the input. For example, a PLC can control a heater by turning it on when the temperature sensor detects a value below a setpoint, and turning it off when the temperature reaches or exceeds the setpoint. On-off control is easy to implement and suitable for systems that do not require precise regulation or fast response. However, it can cause oscillations, overshoots, and wear and tear of the output devices.

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Jose Carlos Villajulca

Especialista en Ejecucion y Gestion de Proyectos de Automatizacion Industrial. Fundador y Consultor en InstrumentacionyControl.NET & MyAutomationClass.com
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Un ejemplo comun es el control de un calentador, donde normalmente poner setpoints alto y bajo y el control ON OFF activa y desactiva cuando el valor de temperatura sale fuera de los rango Altos y Bajos. No desestimemos el control ON/OFF de hecho es multiples controladores PID en la Industria, que por el requerimiento estricto, simplemente pueden funcionar como ON/OFF. Lo simple es mejor.

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2 Proportional control

Proportional control is a type of feedback control, where the output is proportional to the error between the input and the setpoint. For example, a PLC can control a motor speed by varying the voltage applied to it based on the difference between the actual speed and the desired speed. Proportional control can improve the accuracy and stability of the system compared to on-off control, but it can also introduce steady-state error and offset.

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3 PID control

PID control is an extension of proportional control, where the output is also influenced by the integral and derivative of the error. The integral term reduces the steady-state error by accumulating the past errors and adjusting the output accordingly. The derivative term anticipates the future errors by measuring the rate of change of the error and damping the output accordingly. For example, a PLC can control a temperature process by applying a PID algorithm to adjust the heater power based on the error, its integral, and its derivative. PID control can achieve high performance and robustness for many systems, but it requires careful tuning of the parameters and may cause instability or oscillations if not properly designed.

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Jose Carlos Villajulca

Especialista en Ejecucion y Gestion de Proyectos de Automatizacion Industrial. Fundador y Consultor en InstrumentacionyControl.NET & MyAutomationClass.com
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Aqui dejo dos comentarios que me parecen relevantes: 1. Nunca cambies de Controlador PID a estrategias de control avanzadas (cascada, feedforward, etc) sin agotar las caraceteristicas y herramientas propias del PID industrial (como configuracion de ganancia programada, ramps, limites, etc). Estoy seguro que muchas estrategias avanzadas, se podrias reemplazar con un solo PID bien sintonizado (ha ocurrido mucho en mi esperiencia). 2. Salvo sea muy importante, relevante, rentable y necesario, migra a otros tipos de controladores mas haya de los PID y Estrategias de Control PID. El esfuerzo de mantener controladores PID y su sintonia (con el conocimiento correcto) es simple y muy facil de transferir conocimiento entre colegas nuevos.

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Paolo Cavallanti

Advanced Services PM @ Omron | Driving innovation in industrial ecosystems
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In my opinion PID algorithm is very useful in temperature control, especially in fast processes such as packaging machines. Using PLCs with advanced PID-2loop controls allow machines to rise in performances.

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4 Fuzzy logic control

Fuzzy logic control is a type of control that uses fuzzy sets and rules to handle uncertainty and nonlinearity in the system. Fuzzy sets are collections of values that have different degrees of membership, such as low, medium, or high. Fuzzy rules are conditional statements that relate the fuzzy inputs to the fuzzy outputs, such as if the temperature is low then the heater is high. For example, a PLC can control a fan speed by using fuzzy logic to infer the optimal output from the inputs of temperature and humidity. Fuzzy logic control can cope with complex and dynamic systems that are difficult to model or analyze with conventional methods, but it requires a lot of knowledge and experience to define the fuzzy sets and rules.

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5 State machine control

State machine control is a type of control that uses a finite number of states and transitions to describe the behavior and logic of the system. Each state represents a condition or mode of the system, and each transition defines the criteria and actions for moving from one state to another. For example, a PLC can control a traffic light by using a state machine to switch between the red, yellow, and green lights according to the timer and the sensor inputs. State machine control can simplify the control program and make it easier to understand and modify, but it can also become complex and cumbersome if the number of states and transitions is large.

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Ravi Pillay

Principal Engineer at First Quantum Minerals (Cobre Panama)
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State machine control and sequence control serve as the foundational principles for batch control, which represents another extensively employed method within Programmable Logic Controllers. In industries such as chemical manufacturing or food & beverage processing, PLCs manage the ingredients mixing in batches. This involves controlling valves, mixers, heaters, pumps , and other equipment to ensure the correct sequence, timing, and conditions for each batch.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Luis Rodriguez L.

Industrial Automation Geek | PLC | Drives | SCADA
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El control PID, aunque antiguo, es el más usado para PLCs debido a que los sistemas modernos, como el control difuso, robusto, espacio de estados, etc. Utiliza funciones matemáticas más complejas lo cual conlleva a usar recursos que muchos controladores no tienen, o tienen reservados para otras funciones del sistema de control. Esto ya se relega a las IPCs o similares.

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Ravi Pillay

Principal Engineer at First Quantum Minerals (Cobre Panama)
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Based on extensive work in this sector I would add Motion control. PLC's can effectively control movements of machines such as motors and actuators, using precise commands for speed, position, torque, and acceleration. This is critical in applications such as robotics, conveyor systems, and CNC machines. In addition from experience Safety Interlock Systems. PLCs can monitor conditions and execute emergency shutdown procedures or activate alarms when unsafe thresholds reached or unsafe conditions are detected. Especially important for functional safety compliance.

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Giampierre Poma Monago

Project Manager | MPM | PMP? | PSM? | IT/OT | Industria 4.0 | Docente de automatización industrial | Transformación Digital
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Hay que tener claro que no siempre hay que buscar aplicar una técnica de control compleja, si el sistema se puede estabilizar con un control on-off no es necesario intentar aplicar un control PID. Recordemos que debemos priorizar el rendimiento del PLC para mantener el ciclo de procesamiento lo más corto posible. Inclusive habrán ocasiones en las que hay que usar una combinación de todas estas técnicas de control para lograr estabilizar el sistema en todo su rango de funcionamiento. Es importante entender que en la mayoría de casos en el control de procesos "menos es más".

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What are some examples of control methods that can be implemented using a PLC program?

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1 On-off control

2 Proportional control

3 PID control

4 Fuzzy logic control

5 State machine control

6 Here’s what else to consider

Control Engineering

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