Introduction to Control Technology
Introduction to Control Technology
Control technology covers all procedures and systems that allow you to automate machines, equipment, and manufacturing processes.
Control system: A set of components that act coordinately to achieve a government action within a process through direct or indirect manipulation of the magnitudes that intervene.
An automatic control system is a set of technical elements that aims to make a machine or process carry out their duties with minimal human intervention. (Human intervention is limited to entering orders in the operating system, ensuring the control system runs adequately through the phases of work.
Example: Operation of a timed light. The aim of this automation is to replace the manual operation of turning off the light.)
The technology applied to wired devices can be pneumatic, hydraulic, electrical, or electronic. It is carried out based on physical connections of the elements that constitute the control system (widely used in industry).
Disadvantages:
- It is inflexible to future changes.
- It occupies too much space.
- It does not allow complex control functions.
- Difficult to resolve faults.
- It is not very adaptable to changing roles.
Programmable technology does not present the drawbacks above and enjoys many advantages. It is very adaptable and can perform various control functions without altering the physical configuration; it only needs to change the control program.
Analog Systems
Work with continuous-type signals with a determined margin of variation. They often represent physical quantities of the process (temperature, pressure, speed) through a voltage or current proportional to its value (e.g., 0 to 10V).
Digital Systems
Work with discrete signals (binaries 0-1), which can represent only two levels (open-closed). They often represent logical variables.
Analog-Digital Hybrid Systems
The current control systems, with a high degree of complexity, use both analog and digital signal processing. The trend is towards a completely digital control unit because it contributes to greater calculation capacity and processing.
Open and Closed Loop Control Systems
Open-loop control systems are characterized by the fact that, once activated, the process runs for a predefined time, regardless of the outcome obtained. The control system does not monitor the outcome of the process exit.
Example: A timed light on a staircase. The timer is the device that controls the lights, and the staircase is what we want to control. The light turns off after a time determined by the timer. Therefore, it does not monitor whether someone has come up the stairs or not; it shuts off when the scheduled time elapses (this makes it open loop).
In closed-loop systems, the result of the process is analyzed and compared to a setpoint (value to be obtained at the exit) using a continuous process of feedback. The system checks the result and modifies the automatic response based on the setpoint.
Example: A thermostat or a timed lighting system with a closed loop. The latter would receive information about the presence of people on the stairs and keep the lights on until there is no one.
The transfer function is a mathematical expression, represented in a block, that relates the output variable to the input variable.
Components of Control Systems
Input Command Devices
- Binary elements (on/off, yes/no), e.g., switch, pushbutton.
- Numeric elements for entering numbers, e.g., calculator, keyboard.
Input Information Devices
They basically consist of sensors (devices that measure the output data and transmit it to the control unit. According to this information, the control system will act on the actuators).
Controller
An information processing system that analyzes the process and establishes how the system has to work.
Output Information Devices
Responsible for communication with the operator (employee), e.g., buzzer, siren, printers.
Actuators and Preactuators
Responsible for operating the process. Examples: Motors, cylinders (actuators); switches, variable speed drives (preactuators).
The Controller
The controller is responsible for developing the corrective signal to obtain the desired setpoint value (the setpoint helps avoid deviations) and ultimately make the necessary changes to the machine.
Basic Control Actions
- Proportional action (P)
- Integral action (I)
- Derivative action (D)
(P): The control signal is proportional to the error signal. The greater the error signal, the greater the control signal. The effect is instantaneous and strong, although it usually presents a steady-state deviation (offset).
(I): This type of controller emits a signal that varies with a speed proportional to the magnitude of the error. It reacts to an error signal that varies very rapidly to try to correct the deviation. The effect is slow and progressive but continues to act until the permanent deviation is eliminated. It is weak in the immediate moments of the appearance of a sudden deviation because it has a progressive effect. It may cause system instability. In practice, it is not used in its pure form but associated with a proportional (P) or proportional-derivative (PD) controller.
(D): Also called speed control, it generates a signal proportional to the speed of change of the error signal. If the error increases following a uniformly shaped ramp, the controller output is constant, with a value proportional to the slope of the error signal. It does not correct a constant error in the system; it is used to correct possible errors that may arise and anticipate them. It produces an effect of anticipation, achieving a faster dynamic response and a shorter response time.
(PID): The behavior of a PID controller corresponds to the superposition of these three actions. The proportional action acts on the total response. The aim of this type of control is to obtain all the benefits of the other three and overcome their disadvantages.
On/Off Control: The output of the controller takes only two states: disconnected and connected, or, which is the same, maximum and minimum output.
Transducers
Transducers are devices that transform a physical quantity into another physical quantity, usually an electrical signal, and have a certain structure:
- Sensor or sensing element: Converts variations in a physical magnitude into variations in an electrical or magnetic magnitude.
- Signal processing block: Its role is to filter, preamplify, and process the signal to adapt it to the next stage.
- Output stage: Includes amplifiers, relays, code converters, etc. – all those circuits that adapt the signal to the needs of external loads, usually a driver or a comparator.
Classification: They can be active or passive depending on whether they directly generate the captured signal or require external power to capture the signal. They can also be classified according to how they encode the measured magnitude: analog, digital, and on/off. Finally, they can be classified according to the physical quantity they capture:
- Position, end of travel, microswitch
- Proximity: optical, magnetic
- Linear displacement: laser, ultrasonic
- Linear or angular velocity: tachometric dynamo
- Temperature: thermostat, PTC, NTC
- Pressure: pressure switch, piezoelectric
Comparator
Also known as an error detector, it is the device responsible for comparing the reference value (setpoint) with the measured value of the output variable obtained by the feedback transducer. The result of this comparison is the operating error or deviation from the expected value of the output.
PLC or Programmable Logic Controller
An electronic machine designed for real-time environments and industrial automation, combining logic and sequential operations. It has a wide range of applications in industry thanks to its special design features: small size, simplicity of assembly and storage, and the possibility of modifying programs.
Features
- Ability to make changes without altering network connections
- Reduced space requirements
- Reduced installation labor costs
- Reduced project development time
- Possibility of controlling different machines with a single PLC
- Shorter commissioning time
- Reduced maintenance costs
- PLC reusability
Disadvantages
- Requires specialized personnel for programming and maintenance.
- The initial price may be higher than other technological options.
Structure
A control computer consisting of hardware independent of the controlled process and software containing the sequence of control operations to be carried out. Both the input signals and the output signals to the PLC are connected directly to the process through the connection terminals of the PLC.
A PLC consists of three fundamental parts: the central processing unit or CPU, memory, and input and output elements.
Grafcet
(Sequential Function Chart), standardized according to the international standard IEC 848, represents the succession of stages within a production cycle, separated by transitions or jump conditions between stages.