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Open-loop Versus Closed-loop Control Systems

Open-loop Versus Closed-loop Control Systems

The main difference between closed-loop and open-loop control systems is how or if the system handles feedback. Although they are both a major part of electro-mechanical systems, they each provide a beneficial method for motor control when used in the right application.

The great thing about both open and closed-loop systems is that, between the two types of systems, you can essentially design a machine to complete most any process, for as long as you need the process to run.

Still, the basic feedback loop of the control system is what sets open and closed-loop systems apart, even though many systems share very similar components (drives, motors, controllers, etc.).

The Basic Breakdown of a Control System

Control systems typically have three basic parts: a method of input, a process of amplification, and an output.

The method of input is generally a signal that is derived from a source such as temperature, velocity, force, torque, or position. The input can be derived from many other physical or environmental events as well. The input that is registered by the control system then activates a process of amplification (or gains) that changes the input signal into some form of output.

While this is the most elementary breakdown of a control system, more complex systems can feature multiple sources of input, gain scheduling and modeling, complex control loops, input and output signal filters, and more. As a result it is not uncommon for these complex control systems to have a variety of output signals.

The Open-loop Control System

In an open-loop system, topology is similar to the typical control system (as described above – input, amplification, output); an input starts the system resulting in a desired output. Once the task is started, the task typically continues to run until completion. It is a one-way system that has no feedback to alter the machine’s operation.

An example of this type of system is a timer-based toaster. Bread is put into the toaster, a timer is set, and a lever is pushed down acting as a switch to start the process. The toaster coils heat and stay heated for the time set on the timer, then the toast pops up, coils turn off, and the process ends.

The Closed-loop Control System

Closed-loop systems add some type of feedback that allows the control system to make changes to its processes. The input, feedback, and output are constantly monitored and compared. The output is updated, often at defined a periodic rate. The amplification phase essentially runs over and over again to produce a constantly changing output.

Two very common examples of closed loop systems people use frequently are temperature control systems (house thermostat) and cruise control systems (in vehicles). Both rely on feedback and a closed-loop system to make automatic adjustments without input from a user, other than creating a set point.

When the temperature of a room changes, the actual temperature is fed back into the closed-loop system and compared to the set point temperature, and the controller then controls the mechanisms and processes that manage the output (hot or cold air generation and flow).

Similarly, in a vehicle cruise control system, the feedback input is the actual velocity of the vehicle. After comparison to the desired set point velocity, amplification controls the rate of change in velocity command (acceleration) to make the vehicle travel smoothly and consistently at the set point speed.

Want to Learn More?

Open and closed-loop systems are the backbone of machine automation, just one of the control systems understood by the experts at AMMC. We have deployed control systems using many types of feedback including position, speed, torque, pressure, temperature, force and more, contact us today to put our knowledge to work for you.