Open Cycle System

The open cycle system does not monitor or measure the status of the output signal because there is no feedback

In the previous tutorial on Electronic Systems, we found that a system can be defined as a collection of subsystems that route or control an input signal to produce the desired output condition.

The function of any electronic system is to automatically regulate the output and keep it within the desired input value or "setting point" of the system.If the system input changes for any reason, the output of the system must respond accordingly and change itself to reflect the new input value.

Similarly, if something breaks the system output without any changes to the input value, the output should respond by reverting to the previous setting value.In the past, electrical control systems were basically manual or so-called Open Cycle System with little automatic control or feedback built in to regulate the process variable to maintain the desired output level or value.

For example, an electric tumble dryer.Depending on the amount of clothing or how wet they were, a user or operator would set a timer (controller) to run for 30 minutes, and at the end of 30 minutes the dryer would automatically stop and turn off, even if the laundry was wet or damp.

In this case, the control action is the manual operator, which evaluates the wetness of the clothes and adjusts the process (dryer) accordingly.

Therefore, in this example, the tumble dryer will be an open loop system as it does not monitor or measure the status of the output signal, which is drying of the laundry.Next, the accuracy of the drying process or the success of drying the laundry will depend on the experience of the user (operator).

However, if the user thinks that the original drying process will not be met, they can adjust or fine-tune the drying process of the system at any time by increasing or reducing the drying time of the timing controllers.For example, it can increase the timing controller to 40 minutes to prolong the drying process.

open loop system
Open Cycle Drying System

An Open loop system, also called a feedback-free system, is a type of continuous control system in which the input signal of the output has no effect on the control action.In other words, in an open-cycle control system, output is neither measured nor "fed back" for comparison with input.Therefore, an open loop system is expected to faithfully follow the input command or setting point, regardless of the final result.

Also, it does not know about the output state of an open loop system, so it cannot self-correct any errors it can make when the preset value slips, even if it results in deviations greater than the preset value.

Another drawback of open loop systems is that they are under-equipped to deal with disruptions or changes in conditions that can reduce their ability to complete the desired task.For example, the dryer cover is opened and the heat is lost.The timing checker continues for a full 30 minutes, but at the end of the drying process the laundry is not heated or dried.This is due to the fact that there is no information that feeds back to maintain a constant temperature.

open loop system

Next, we can see that open loop system errors can disrupt the drying process, and therefore a user (operator) requires extra auditing attention.The problem with this forward-looking control approach is that the user needs to look at the process temperature frequently and take any corrective control action when the drying process deviates from the desired clothing drying value.This type of manual open loop control, which reacts before an error occurs, is called Forward Feed Control.

The purpose of the forward-feed control, also known as predictive control, is to measure or predict any potential open loop distortions and compensate for them manually before the controlled variable deviates too far from the original setting point.For our simple example above, if the door of the dryer was open, it would be detected and closed, and the drying process would be allowed to continue.

open loop system

If applied correctly, if the user responds very quickly to the error situation (cover open), after 30 minutes the deviation from wet clothes to dry clothes will be minimal.However, if the system changes, for example, if the drop in the drying temperature is not noticeable during the 30-minute process, this forward feeding approach may not be entirely accurate.

Then we can define the basic features of an "Open Loop System" as follows:

  • There is no comparison between the actual and desired values.
  • An open loop system does not have the action of self-regulation or control over the output value.
  • Each input setting determines a fixed operating position for the controller.
  • Changes or distortions in external conditions do not cause a direct output change (unless the controller setting is changed manually).

Any open loop system can be represented as a serial multiple-digit block or a single block diagram with an input and output.The block diagram of an open loop system indicates that the signal path from input to output represents a linear path without a feedback loop, and for any control system type, input is given as εi and output is given as εo.

In general, we do not have to manipulate the open loop block diagram to calculate the actual transfer function.We can write the appropriate relationships or equations from each block diagram and then calculate the final transfer function from these equations, as shown.

Open Cycle System

open loop system

Therefore, the Transfer Function of each block:

open loop system

The general transfer function is given as follows:

open loop system

Then, the Open Loop Gain is simply given as follows:

open loop system

When G represents the Transfer Function of the system or subsysordial, it can be rewritten as follows: G(s) = εo(s)/εi(s)

Open cycle control systems are often used in processes that require events to be sorted with the help of "ON-OFF" signals.For example, a washing machine that requires the water to be first placed in the "ON" position and then to be put in the "OFF" position when it is full, and then the heater element to be turned on to heat the water, and then to be put in the "OFF" position at an appropriate temperature, and so on.

This type of "ON-OFF" open-loop control is suitable for systems where changes in load occur slowly and the process moves very slowly, requiring rare changes in control action by an operator.


We found that a controller can modify its inputs to achieve the desired effect on the output of a system.A type of control system in which the output has no effect on the control action of the input signal is called the Open loop system.

An "open loop system" is defined by the fact that the output signal or condition is neither measured nor "fed back" for comparison with the input signal or system setting point.Therefore, open loop systems are generally called "Systems without feedback".

Also, since an open loop system does not use feedback to determine whether the required output has been obtained, it "assumes" that the desired purpose of the entry was successful because it cannot correct any errors it can make and therefore cannot compensate for any errors.

Open Loop Motor Control

For example, suppose the DC motor controller is shown.The rotation speed of the motor will depend on the voltage provided by the pocinciometer to the amplifier (controller).The value of the input voltage may be proportional to the position of the pontiometer.

open loop system

If the posiometer is carried to the top of the resistance, the amplifier representing the full speed will be provided with maximum positive voltage.Likewise, if the poninciometer wiper is moved below the resistance, zero voltage will be provided, representing a very slow speed or stop.

Next, the position of the pontiometer slider represents the output of the system, the input raised by the amplifier (controller) to drive a DC motor (process) at an N setting speed representing εo.The engine will continue to rotate at a constant speed, determined by the position of the pocentrometer.

Since the signal path from input to exit is a direct path that does not form part of any cycle, the overall gain of the system will be the cascading values of individual gains from the potentiometer, amplifier, motor and load.It is clearly desirable that the output speed of the engine is the same as the position of the poisterometer, giving the overall gain of the system as a whole.

However, the individual gains of the posensiometer, amplifier and motor may change over time with changes in feed voltage or temperature, or the engine load may increase to represent external distortions in the open-cycle motor control system.

However, the user will eventually notice a change in system performance (change in engine speed) and can correct the ponciometer input signal by increasing or decreasing it accordingly to maintain the original or desired speed.

The advantages of this type of "open-cycle motor control" are that it is potentially inexpensive and simple to implement, and is ideal for use in well-defined systems if the relationship between input and output is direct and not affected by any external disturbances.Unfortunately, this type of open-cycle system is inadequate, since changes or corruptions in the system affect the speed of the engine.Then another form of control is required.

In the next tutorial on Electronic Systems, we will look at the effect of feeding part of the output signal back into the input so that system control is based on the difference between real and desired values.