Automatic Control Systems: Techniques, Drivers, and Digital Advancements
Control Systems
A control system is a rational and systematic set of equipment and human intervention, allowing the operation of a controlled system. It seeks to:
- Eliminate the influence of disturbances
- Ensure the stability of the process
- Optimize the process
Basic Automatic Control or Regulation Techniques
These techniques are based on feedback (feedback or closed loop):
- Proportional-Derivative (does not act alone)
- Integral (does not act alone)
- Mixed: PI, PD, PID
- All or nothing
- Floating
- Proportional in time, etc.
Advanced Control Techniques
Based on the control strategy, these may use analog instruments for their implementation but require the treatment of digital information. Advanced control achieves more stability, and ties are more complex. Techniques include:
- Cascade
- Selective
- Feed Forward
- Ratio, and others
Drivers
Classic Analog Controller
Comprises a comparator and the controller itself. It has analog inputs and outputs (pneumatic or electric). The comparator receives two signals: the set point and the measure of the variable. This produces an analog signal representing the system error. This error signal enters the driver, and it produces a response proportional to the error by integrating or differentiating the response (output) that acts on a final element of control. Analog controllers on the market have a scale from 0 to 100 where the set point and the measure occurring in the system are set.
Digital Controllers
These use a microprocessor as a controller. The signal input, the parsing, and the output are all digital. Sometimes, it is necessary to connect analog and digital components using analog-digital converters (for input) and digital-analog converters (for output). The advantage of digital controllers is that the control action is performed by formulas included in their software. Another advantage is that many independent processes can be controlled via a multiplexer. Digital control provides greater accuracy.
A digital controller includes:
- A switch for manual or automatic mode
- A selector for reverse or direct action
- Programming capability using the control algorithm
The digital controller connects the microprocessor, main memory, and peripherals via the data bus, address bus, and control bus.
Delay is the time it takes the system to give the necessary response to a change in the process because the response is never immediate. It is the simplest way to control and send a signal from the controller to regulate or not the process, mainly taking into account if there is an error or not. In a chemical process, an oscillation in the regulation gives the final element the response to open or close the process, as this system was designed to control the variable (or reverse). With this driver, a small change gives a performance on the final element to be regulated. Summarizing, the controller is operating satisfactorily in an all-or-nothing manner if the process has a slow reaction rate and a minimum delay time. There is an all-or-nothing driver in two steps (on/off) to allow or not allow the passage of current, for example.
Floating Control
This is a variant of an all-or-nothing driver. It is a constant speed controller that moves the control element at a unique rate, regardless of the deviation. The advantage is that it compensates for load changes gradually, slowly displacing the valve position. The disadvantage is that it is not suitable for delays or rapid load changes. This form of control tends to produce oscillations in the control variable, but these can be minimized by properly choosing the speed of the valve to compensate for the characteristics of the process.