Closed Cycle System

The closed cycle system uses feedback that part of the output signal is fed back into the input to reduce errors and improve stability.

Systems where the amount of output has no effect on the input into the control process are called open cycle control systems and this isCycle systems are precisely these open-ended feedback-free systems.

But the purpose of any electrical or electronic control system is to measure, monitor and control a process, and one way we can accurately control the process is to "feed" some of it back to monitor its output and compare the actual output. outputs the desired output to reduce the error, and if it fails, restores the output of the system to the original or requested response.

The amount of measured output is called a "feedback signal", and the type of control system that uses feedback signals to both self-control and adjust itself is called the Closed Cycle System.

The Closed Cycle Control System,also known as the feedback control system, is a control system that uses the concept of an open loop system as its forward path, but has one or more feedback loops (hence its name) or a path between it and its output.The reference to "feedback" simply means that part of the output is returned to the entry to form part of the system stimulation.

Closed cycle systems are designed to automatically obtain and maintain the desired output condition by comparing it to the actual situation.It does this by generating an error signal, which is the difference between the output and the reference input.In other words, a "closed loop system" is a fully automated control system in which the control action somehow depends on the output.

For example, consider our electric tumble dryer from previous Open Cycle System training.Let's say that we use a sensor or converter (input device) to continuously monitor the temperature or dryness of clothes and send a signal about dryness back to the controller as shown below.

Closed Cycle System

Closed Cycle System
Closed Cycle System

This sensor monitors (or removes) the actual dryness of clothes and compares it to the input reference.The error signal (error = required dryness – actual dryness) is strengthened by the controller, and the controller output makes the necessary correction to the heating system to reduce any errors.For example, if the clothes are too wet, the controller can increase the temperature or drying time.Similarly, if the clothes are almost dry, they can lower the temperature or stop the process so as not to overheat or burn the clothes, etc.

Then the closed cycle configuration is characterized by a feedback signal derived from the sensor in our garment drying system.The size and polarity of the resulting error signal will be directly related to the difference between the required dryness and actual dryness of the clothes.

In addition, since the closed loop has some information about a system output state (via the sensor), it is better equipped to address any system failures or changes in conditions that can reduce the ability to complete the desired task.

For example, as before, the dryer cover is opened and the heat is lost.This time the deviation in temperature is detected by the feedback sensor, and the controller corrects the error on its own to maintain a constant temperature within the limits of the preset value.Or it will probably stop the process and activate an alarm to notify the operator.

As we can see, in a closed cycle control system, the error signal (which can be the output signal itself or a function of the output signal), which is the difference between the input signal and the feedback signal, feeds into the controller. To reduce system failure and restore the output of the system to the desired value.In our case, dry clothes.Clearly, laundry is dry when the error is zero.

The term closed loop control refers to the use of a feedback control action at all times to reduce any errors in the system, and its "feedback", which distinguishes the fundamental differences between the open loop and the closed loop system. Therefore, the output depends on the feedback path, which can be done very accurately in general, and is more widely used within electronic control systems and circuits than feedback control, open loop or forward feed control.

Closed cycle systems have many advantages over open-cycle systems.The primary advantage of the closed-cycle feedback control system is the ability to reduce the sensitivity of the system to external parasites, for example, opening the dryer cover gives the system more robust control, since any change in the feedback signal will result in compensation.

We can then define the main features of Closed Cycle Control as follows:

  • Reduce errors by automatically adjusting system input.
  • To improve the stability of an unstable system.
  • To increase or decrease system sensitivity.
  • To increase the robustness against external factors in the process.
  • To produce reliable and repeatable performance.

While a good closed-loop system can have many advantages over an open-cycle control system, the main drawback is that it has a closedthe cycle system should be more complex by having one or more feedback paths.Also, if the controller's gain is very sensitive to changes in input commands or signals, it can become unstable and begin to oscilrge when the controller tries to overcorrect itself, and eventually something breaks down.Therefore, we need to "tell" the system how we want it to behave within some predefined limits.

Closed Cycle System Collection Points

For a closed-loop feedback system to regulate any control signal, it must first determine the error between the actual output and the desired output.This is achieved by using a collection point, also called a comparison element, between the feedback loop and the system input.These collection points compare a system setting point with the actual value and generate a positive or negative error signal that the controller also responds to.

Where: Error = Setting point – Actual point.

Closed Cycle System

The symbol used to represent a collection point in the closed-loop systems block diagram is the symbol of a circle with two diagonal lines, as shown.The collection point can add or remove signals using a Plus ( + ) symbol indicating that the device is a "sum" (used for positive feedback), in which case a Minus ( – ) indicates that the device is a "comparator" (used for negative feedback).

Collection Point Types

Closed Cycle System

Collection points can have multiple signals as aggregation or subtraction entries, but only one output, which is the algebraic sum of the entries.In addition, the arrows indicate the direction of the signals.Collection points can be cascaded together to allow more input variables to be collected at a specific point.

Closed Cycle System Transfer Function

The Transfer Function describes the behavior of any electrical or electronic control system as a mathematical relationship between system input and output.Also note that the ratio of the output of a particular device to the input represents its gain.Then we can accurately say that the output is always the transfer function of the system multi-input.Consider the closed loop system below.

Closed Cycle System
Closed Loop System Representation

Where: Block G represents open loop gains of the controller or system and is the forward path, and block H represents the gain of the sensor, converter or measuring system in the feedback path.

To find the transfer function of the closed loop system above, we must first calculate the output signal in εo input signal εi .For this we can easily write the equations of the given block diagram as follows.

Output = Output , Input = Input , Error = Error

Output from system equals Output = G x Error

Notice that the error signal is also the input of the forward supply block: G

The output from the collection point is equal to: Error = Entry – H x Output

If H = 1 (feedback):

The output from the collection point will be as follows: Error (εe) = Input – Output

To eliminate the term error:

Output equals: Output = G x (Input – H x Output)

Therefore: G x Input = Output + G x Y x Output

Rearranging the above gives us the closed loop transfer function of:

Closed Cycle System

The equation above for the transfer function of a closed cycle system shows a Plus ( + ) sign representing negative feedback on the denominator.With a positive feedback system, the denominator will have the minus ( – ) sign, and the equation is as follows: 1 – GH.

Closed Cycle System

In addition, as system stable state gain decreases G, the expression G/(1 + G) decreases much more slowly.In other words, the system is highly insensitive to changes in system gain represented by G, and this is one of the main advantages of a closed loop system.

Multiple Closed Cycle System

While our example above is a single-input, single-output closed loop system, the basic transfer function still applies to more complex multi-loop systems.Most practical feedback circuits have some kind of multi-cycle control, and for a multi-loop configuration, the transfer function between a controlled and manipulated variable depends on whether other feedback control loops are on or off.

Consider the multi-cycle system below.

Closed Cycle System

Cascading blocks such as G 1 and G 2, as well as the transfer function of the inner loop can be reduced, as shown.

Closed Cycle System

After further reduction of blocks, we get a final block diagram that resembles the previous single loop closed loop system.

Closed Cycle System

And the transfer function of this multi-loop system becomes:

Closed Cycle System

Then we can see that even complex multi-block or multi-loop block diagrams can be reduced to give a single block diagram with a single common system transfer function.

Closed Cycle Motor Control

So how can we use Closed Cycle Systems in Electronics?Consider our DC motor controller in previous open loop training.If we connected a speed measurement transducer such as a tachometer to the speed of the DC motor, we could detect its speed and send a signal to the amplifier proportional to the engine speed.The tachometer, also known as a tachometer generator, is a dc generator with a fixed magnet that gives a DC output voltage proportional to the speed of the engine.

Next, the position of the posiometer slider represents the input, εi, which is amplified by the amplifier (controller) to drive the DC motor , the output of the system at an N setting speed representing εo, and the tachometer T will be the closed loop.The difference between the input voltage setting and the feedback voltage level signals the error as shown.

Closed Cycle Motor Control

Closed Cycle System

Any external distortion of the closed-cycle engine control system, such as increased engine load, will make a difference at the actual engine speed and the ponciometer input adjustment point.

This difference will produce an error signal in which the controller will automatically respond by adjusting the engine speed.The controller then works to minimize the error signal with zero errors indicating the actual speed equal to the setting point.

Electronically, we can apply such a simple closed loop tachometer-feedback motor control circuit using a transactional amplifier (op-amp) for the controller, as shown.

Closed Cycle Motor Control Circuit

Closed Cycle System

This simple closed loop motor controller can be shown as a block diagram as shown.

Block Schema for Feedback Controller

Closed Cycle System

The closed-cycle engine controller is a common way to maintain a desired engine speed under changing load conditions by changing the average voltage applied from the controller to the input.The tachometer can be replaced by an optical encoder or Hall-effect type positional or rotary sensor.


We have seen that an electronic control system with one or more feedback paths is called the Closed Cycle System.Closed cycle control systems are also called "feedback control systems", which are very common in process control and electronic control systems.Feedback systems have some of the output signals that are "fed back" into the input for comparison with the desired setting point condition.The type of feedback signal can result in positive feedback or negative feedback.

In a closed-cycle system, a controller is used to compare the output of a system with the required condition and convert the error into a control action designed to reduce the error, the error, and restore the output of the system to the desired response.Then closed loop control systems use feedback to determine the actual input to the system and can have multiple feedback loops.

Closed cycle control systems have many advantages over open cycle systems.One advantage is that the use of feedback makes the system response relatively insensitive to external disturbances and internal changes in system parameters such as temperature.Therefore, it is possible to use relatively inaccurate and inexply expensive components to achieve accurate control of a particular process or plant.

However, system stability can be a big problem, especially in poorly designed closed loop systems, as it can try to overcorrect any failure that can cause the system to lose control and be released.

In the next lesson about Electronic Systems, we will look at different ways to add a collection point to the input of a system and different ways to feedback signals to it.