Control Systems 101

[Jenette Bregantini]

A control system manages, commands, directs, or regulates the behavior of other devices or systems using control loops. It can range from a single home heating controller using a thermostat controlling a domestic boiler to large industrial control systems which are used for controlling processes or machines. The control systems are designed via control engineering process.

For continuously modulated control, a feedback controller is used to automatically control a process or operation. The control system compares the value or status of the process variable (PV) being controlled with the desired value or setpoint (SP), and applies the difference as a control signal to bring the process variable output of the plant to the same value as the setpoint.

In open-loop control, the control action from the controller is independent of the "process output" (or "controlled process variable"). A good example of this is a central heating boiler controlled only by a timer, so that heat is applied for a constant time, regardless of the temperature of the building. The control action is the switching on/off of the boiler, but the controlled variable should be the building temperature, but is not because this is open-loop control of the boiler, which does not give closed-loop control of the temperature.

In closed loop control, the control action from the controller is dependent on the process output. In the case of the boiler analogy this would include a thermostat to monitor the building temperature, and thereby feed back a signal to ensure the controller maintains the building at the temperature set on the thermostat. A closed loop controller therefore has a feedback loop which ensures the controller exerts a control action to give a process output the same as the "reference input" or "set point". For this reason, closed loop controllers are also called feedback controllers.[1]

The definition of a closed loop control system according to the British Standards Institution is "a control system possessing monitoring feedback, the deviation signal formed as a result of this feedback being used to control the action of a final control element in such a way as to tend to reduce the deviation to zero."[2]

A closed-loop controller or feedback controller is a control loop which incorporates feedback, in contrast to an open-loop controller or non-feedback controller.A closed-loop controller uses feedback to control states or outputs of a dynamical system. Its name comes from the information path in the system: process inputs (e.g., voltage applied to an electric motor) have an effect on the process outputs (e.g., speed or torque of the motor), which is measured with sensors and processed by the controller; the result (the control signal) is "fed back" as input to the process, closing the loop.[4]

In the case of linear feedback systems, a control loop including sensors, control algorithms, and actuators is arranged in an attempt to regulate a variable at a setpoint (SP). An everyday example is the cruise control on a road vehicle; where external influences such as hills would cause speed changes, and the driver has the ability to alter the desired set speed. The PID algorithm in the controller restores the actual speed to the desired speed in an optimum way, with minimal delay or overshoot, by controlling the power output of the vehicle's engine.Control systems that include some sensing of the results they are trying to achieve are making use of feedback and can adapt to varying circumstances to some extent. Open-loop control systems do not make use of feedback, and run only in pre-arranged ways.

In some systems, closed-loop and open-loop control are used simultaneously. In such systems, the open-loop control is termed feedforward and serves to further improve reference tracking performance.

Logic control systems for industrial and commercial machinery were historically implemented by interconnected electrical relays and cam timers using ladder logic. Today, most such systems are constructed with microcontrollers or more specialized programmable logic controllers (PLCs). The notation of ladder logic is still in use as a programming method for PLCs.[6]

Logic controllers may respond to switches and sensors and can cause the machinery to start and stop various operations through the use of actuators. Logic controllers are used to sequence mechanical operations in many applications. Examples include elevators, washing machines and other systems with interrelated operations. An automatic sequential control system may trigger a series of mechanical actuators in the correct sequence to perform a task. For example, various electric and pneumatic transducers may fold and glue a cardboard box, fill it with the product and then seal it in an automatic packaging machine.

The rules of the system are written in natural language and translated into fuzzy logic. For example, the design for a furnace would start with: "If the temperature is too high, reduce the fuel to the furnace. If the temperature is too low, increase the fuel to the furnace."

Measurements from the real world (such as the temperature of a furnace) are fuzzified and logic is calculated arithmetic, as opposed to Boolean logic, and the outputs are de-fuzzified to control equipment.

When a robust fuzzy design is reduced to a single, quick calculation, it begins to resemble a conventional feedback loop solution and it might appear that the fuzzy design was unnecessary. However, the fuzzy logic paradigm may provide scalability for large control systems where conventional methods become unwieldy or costly to derive.[citation needed]

The range of control system implementation is from compact controllers often with dedicated software for a particular machine or device, to distributed control systems for industrial process control for a large physical plant.

Logic systems and feedback controllers are usually implemented with programmable logic controllers. The Broadly Reconfigurable and Expandable Automation Device (BREAD) is a recent framework that provides many open source hardware devices which can be connected to create more complex data acquisition and control systems.[8]

Control systems are a central part of production and distribution in many industries. Automation technology plays a big role in these systems. The types of control loops that regulate these processes include industrial control systems, such as supervisory control and data acquisition, systems and distributed control systems.

These control systems operate with human input. The control action is independent of the output. In household use, a washing machine is an example of an open-loop system because someone needs to make selections among the settings for it runs. A time-based traffic light system is an industrial example of an open-loop control system, where traffic engineers must decide the timing for the stop, go and caution lights.

These systems can be actively managed or set to operate autonomously. They use feedback signals from the system to provide automatic control and maintain specific settings or a desired state without human intervention. Some control loops can be switched between closed and open modes. When open, a switchable loop is manually controlled; when closed, it can be fully automated.

A thermostat is an example of a closed-loop system. It turns a heating system on and off based on signals it receives from sensors that monitor air temperature. Temperature control is a particularly important part of maintaining a proper data center environment.

In Figure 2, the technician manages a system that can be remotely controlled. The technician regularly sends input signals to the device, and it sends output signals via a feedback loop and a sensor that monitors the device. When the sensor receives an error signal from the device, it sends an alert message over the feedback loop to the technician, who then sends instructions to the device as needed to counter the negative feedback.

The control loops that make up the overall system generally include a sensor, a controller and a final control element. The sensor reads the process variable or a related process control measurement. The controller receives the signal from the sensor and forwards it to the instrumentation, the remote terminal units and the final control elements. There, the process variable is adjusted to be kept constant at the chosen set point.

Closed-loop control systems are widely used in many applications. They are effective in controlling externally located devices, providing dependable and readily available output data while also withstanding external disruptions.

However, control systems are complex, and require training and documentation for optimum operation and to achieve the desired output. Malfunctions to remote sensors can provide inaccurate data on system performance, possibly resulting in unnecessary system changes. Their complexity also means they aren't necessarily ready to use out of the box and might require programming and other prelaunch activities before using.

Ensuring safety and security is our goal at Control Systems, Inc. We provide an array of solutions for a multitude of facilities that include fire protection, fire suppression, fire sprinklers, electronic security, access control, video management, audio/visual, surveillance and more.

We cater to our clients with care and efficiency that transcends beyond their expectations, which is why we are leading the industry in customer satisfaction. We are pioneering the industry through an innovative and comprehensive approach that is simply unmatched by any of our competitors through the combination of knowledge and experience that you can only find at Control Systems, Inc. Our offices cover all over Ohio, Colorado, Texas, Florida and Alabama.

We are a licensed systems integration contractor offering a full spectrum of services including: sales, engineering, installation, service and maintenance. We are also a systems integration company representing a majority of the leading fire alarm and electronic security.

c80f0f1006