Open loop control systems

Grafiche Bronca

As we have seen in the previous article on electronic systems, a system can be defined as a set of subsystems that control an input signal to produce the desired output. Open loop control systems are one of the possible types to achieve this.

The function of any electronic system is to automatically adjust the output (automatic control) and keep it within the desired input value or “set point”. If system input changes for any reason, the system output must respond accordingly and change to reflect the new input value.

Similarly, if something disturbs the system output intervenes without any change to the input value, the system must respond by returning the output to the previous set value. In the past, electrical control systems were essentially manual (called open loop systems) with very few built-in control or automatic feedback functions to adjust the process variable to maintain the desired output level or value.

For example, an electric dryer. Depending on the amount of clothes or how wet they are, a user or operator will set a timer (controller) say to 30 minutes and at the end of the 30 minutes the dryer will stop automatically and turn off even if the clothes are still wet or damp.

In this case, the control action is the operator who manually assesses the humidity of the clothes and sets the process (the dryer) accordingly.

In this example, therefore, the dryer would be an open loop system as it does not monitor or measure the condition of the output signal, which is the dryness of the clothes. It follows that the accuracy of the drying process or the success of drying clothes will depend on the user (operator) experience.

However, the user can adjust or fine-ure the drying process of the system at any time by increasing or decreasing the drying time of the timing controllers, if he believes that the original drying process is not satisfactory. For example, he can increase the timer to 40 minutes to prolong the drying process. Consider the following open-loop block scheme.

So an open loop system, also known as a no feedback system, is a type of continuous control system where the output has no influence or effect on the input signal control action. In other words, in an open loop control system, the output is neither measured nor “reintroduced” for comparison with the input. Therefore, it is expected (blindly!) that an open loop system will faithfully follow its input command or set point regardless of the final result.

In addition, an open loop system is not aware of the output drlla condition, so it cannot self-correct any errors it might make when the preset value drifts, even if this involves large deviations from the preset value.

Another disadvantage of open loop systems is that they are poorly equipped to handle disturbances or changes in operating conditions that can reduce its ability to complete the desired task. For example, the door of the dryer opens and the heat is lost. The time controller continues independently for all 30 minutes, but clothes are not heated or dried at the end of the drying process. This is because no information is provided to maintain a constant temperature.

It is then understood that errors in the open loop system can disturb the drying process and therefore require additional supervision attention from a user (operator). The problem with this early control approach is that the user should frequently check the process temperature and take any corrective control action whenever the process deviates from the desired value of drying clothes. This type of manual open loop control that reacts before an error actually occurs is called Feed forward Control.

The goal of the Feed forward control, also known as predictive control, is to measure or predict any open loop disturbances and compensate them manually before the controlled variable deviates too far from the original setpoint. So, for our simple example above, if the dryer door were open, it would be detected and closed allowing the drying process to continue.

If applied correctly, the deviation of the drying level of the clothes from the desired one at the end of the 30 minutes would be minimal if the user had responded to the error situation (open door) very quickly. However, this feed forward approach may not be completely accurate if the system changes: for example, if the decrease in drying temperature was not noticed during the 30 minutes of the process.

Then, we can define the main characteristics of an “open loop system” as follows:

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

Open-loop systems and transfer function

Any open loop system can be represented as multiple cascade blocks in series or as a single block with an input and output. The block diagram of an open loop system shows that the signal path from input to output represents a linear path with no feedback loop. The entrance is given the designation θi and the output θo.

In general, it is not necessary to manipulate the open loop block scheme to calculate its actual transfer function. We can simply write the relationships or equations of each block, and then calculate the final transfer function from these equations as shown.

The transfer functions of each block are:

G_{1}=\frac{\theta _{1}}{\theta _{i}}, \; G_{2}=\frac{\theta _{2}}{\theta _{1}},\; G_{3}=\frac{\theta _{o}}{\theta _{2}}

The overall transfer function is therefore:

G=G_{1}\times G_{2}\times G_{3}=\frac{\theta _{o}(s)}{\theta _{i}(s)}

Open loop control systems are often used for processes that require event sequencing with the help of “ON-OFF” signals. For example, a washing machine for water intake requires a switch to “ON” and therefore when the level is reached requires a switch to “OFF”; then the heating element will be turned on on “ON” to heat the water and once an adequate temperature is reached it will be switched to “OFF”, and so on.

This type of open loop control “ON-OFF” is suitable for systems where load changes occur slowly and the process acts very slowly, requiring infrequent changes to the control action by an operator.

Summary on open loop control systems

We have seen that a controller can manipulate its inputs to achieve the desired effect on the output of a system. A type of control system in which the output has no influence or effect on the input signal control action is called an open loop system.

An “open loop system” is defined by the fact that the output signal is neither measured nor “reintroduced” for comparison with the input signal or system set point. Therefore, open loop systems are commonly referred to as “no feedback systems”.

In addition, since an open circuit system does not use feedback to determine if the required output has been achieved, it “assumes” that the desired goal of the input has been successful because it is unable to correct any errors it may make, and therefore cannot compensate for any disturbances outside the system.

Open loop motor control

So, for example, we assume that the DC motor controller is as depicted. The speed of rotation of the motor will depend on the voltage provided to the amplifier (the controller) by the power meter. The value of the input voltage may be proportional to the position of the potenziometer.

If the potentiometer is moved to the higher resistance value, the maximum positive voltage will be provided to the amplifier and then the maximum speed to the engine. Similarly, if the potentiometer is moved to the lower resistance, zero voltage will be provided which represents a very slow speed or engine stop.

Thus the position of the potenziometric cursor represents the input, θi which is amplified by the amplifier (controller) to drive the DC motor (process) at a set point N speed representing the output, θo of the system. The motor will continue to rotate at a fixed speed determined by the position of the power meter.

Since the signal path from the input to the output is a direct path that has no loop, the overall G gain of the system will be given by the product of the cascade values of the individual gains of the potentiometer, amplifier, motor and load. It is clearly desirable that the output speed of the motor is identical to the position of the potentiometer, giving the overall gain of the system as a unit.

However, individual potentiometer, amplifier, and motor gains may vary over time with changes in voltage or temperature, or engine load may increase, and these represent disturbances outside the open loop motor control system.

The user will eventually become aware of the change in system performance (change in engine speed) and can correct it by increasing or decreasing the input signal of the potentiometer in order to maintain the original or desired speed.

The advantage of this type of “open loop motor control” is that it is potentially economical and simple to implement, making it ideal for use in well-defined systems where the ratio of input to output is direct and not affected by any external disturbance. Unfortunately this type of system is inadequate when variations or disturbances occur in the system that affect the speed of the engine. In such cases, another form of control is required.

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