Open loop control system

Types Of Open Loop Control Systems

An open loop control system is a type of system where the output does not influence the control action. The system's assumption is based on predefined conditions and not on the measured output. Open-loop control systems are the simplest type of control system, and system designers can imagine the control system behavior without measurement or feedback. These systems are very useful when only rough control is adequate. Generally speaking, the application of open loop control avoids the need for feedback measurement systems. Based on the applications and mechanisms, open loop systems can be categorized into the following types:

• Sequential control system

  A sequential control system works in a pre-defined sequence. The temperature in the sequential control system must be regulated for a particular sequence in which the product must be cooked. The system then automatically proceeds to the packing stage, after which it must be cooled. All of these processes must occur in the right sequence, which is why the system is sequential.

• Time-based control system

  The time-based control system works on time-based parameters. The open loop control system time-based example is a clock ring on-off control system. The system can control switching on and off of a device based on a pre-determined time. This type of control system is often found in devices like toaster ovens and air conditioners.

• Magnitude-based Control System

  A magnitude-based control system works on the pre-defined magnitude of the system that must be maintained. Systems based on magnitude are decanter systems and level control systems. If the water in the tank reaches a specified level, the control system will turn the motor on to open the outlet valve. The motor will turn off when the water level decreases to the required amount.

• Position Control System

  Position-based systems allow control over the position of the mechanism. These systems include robotics, automotive, and aerospace applications. The position control system uses a PID( proportional-integral-derivative) controller.

Functions and features of open loop control systems

Functions:

• Monitoring: Some open loop systems actively monitor how the process is changing. Checking things like pressure, temperature, or flow rate as they change is part of monitoring. This helps make sure everything is working well even without feedback control.
• Adjusting: Open loop systems can make small adjustments to keep values as close to set points as possible, called fine-tuning or adjusting. While they don't use feedback, basic control systems can still perform some automatic fine-tuning of output based on direct measurements.
• Record Keeping: Some systems keep records over time of measurements and output. Keeping a log of values over time helps understand how the system performs and can be useful later to check for issues if something goes wrong.
• Alarms: They can also have alarms that signal when something is out of the normal range. These alarms help operators react quickly if a measurement shows a problem.

Features:

• Simplistic design: Open loop control systems are easier to build and less expensive.
• None-feedback mechanism: Open loop systems do not receive feedback from the output. Therefore, they may sometimes produce an error that the system cannot correct.
• Less precise: Precision may be an issue because the system cannot adjust according to the output.
• Faster operation: Open loop control systems respond quickly since they do not process feedback.
• Simpler tracing: Troubleshooting and maintaining open loop control systems are easier because of their simple construction and lack of feedback processing.

Applications of open loop control systems

The open loop control system finds relevance in various industries.

• An aircraft elevator. An aircraft elevator operates under the open loop control system. The pilot gives orders, and the elevators respond by moving the tail section of the airplane up and down. The elevators control the flight by increasing or decreasing the elevation; thus, it is referred to as an elevator.
• Automated food preparation. Open loop control systems are widely employed in automated food preparation equipment such as coffee makers, fryers, and ovens. These devices receive user input in the form of time, temperature, or cooking settings, and then they carry out the specified tasks without monitoring the actual cooking conditions. For instance, a convection oven with an open-loop control system provides even heat distribution based on the set temperature and cooking time. However, it lacks sensors to adjust cooking conditions autonomously.
• Irrigation systems. Irrigation systems utilize an open-loop control system where water is distributed to crops without feedback on soil moisture levels or weather conditions. Timers and controllers in sprinkler systems or drip irrigations give instructions on when and how long to irrigate the plants. However, the system does not adjust based on the actual irrigation needs of the plants.
• Manufacturing processes. Several manufacturing processes such as integrated circuit manufacturing, paper production, and metal forming all make good use of the open loop control system. Here, the first step is user-initiated, which involves setting parameters like speed, pressure, or temperature. Then, the equipment operates according to these settings without feedback to monitor the actual production conditions.
• Automated laundry machines, car washing, and dishwashers with open loop control systems are mainly dependent on user input to determine the washing cycle, water level, and detergent dosage. The machines then carry out the programmed sequences without adjusting based on the actual soil level or load size.
• Power generation. The open loop control system is widely used in power generation turbins, windmills, and hydroelectric dams. User input sets the operating conditions, such as speed and load, but the system does not have feedback to adjust based on the actual power generation conditions.

How to choose open loop control systems

• Understanding the Process:

  Knowing the exact task or workflow to be controlled is vital. It involves recognizing the influential factors, desired output, and system's boundaries. An appropriate external controller can be selected after understanding the process and its requirements.

• Defining Objectives:

  Specific process goals like minimizing travel time or maximizing output must be stated to gauge performance and efficiency.

• Evaluating System Dynamics:

  The system's internal adaptation must be examined to determine order (1st, 2nd), reaction speed, and elasticity.

• Reviewing Input/Output Relationship:

  The correlation between variables using mathematical modeling or simulation must be understood for accurate estimation of external control.

• Determining Influential Factors:

  All factors impacting the process must be identified and classified as primary or secondary to comprehend system behavior.

• Assessing Real-Time Requirements:

  The time it takes to make control decisions and execute them is essential in many processes.

• Evaluating System Stability and Robustness:

  Resistance to changes in parameters or external disturbances is essential in many open-loop systems.

• Controlling System Costs:

  Budgeting and understanding costs throughout the system's life cycle is vital, extending to maintenance and training.

• Deciding Controller Type:

  Controllers can be fixed or variable, manual or automatic, based on the above factors. Simple mechanical controllers may suffice for stable systems, while complex ones might require computers.

• Ensuring Compatibility:

  The chosen controller must seamlessly integrate with existing infrastructure, minimizing adaptations and restrictions.

Q & A

Q1. How does open loop control work?

A1. Open-loop systems use a control signal that the system output does not influence. The output in an open-loop system has no effect on the input or procedure. Because the second variable is not fed back to the first, the two variables do not interact.

Q2. Where are open loop control systems used?

A2. Some common applications of an open-loop control system include vehicle cruise control, washing machines, and dehydrators.

Q3. What is the difference between an open and closed loop system?

A3. An open-loop system is a one-way system that does not use feedback, while a closed-loop control system uses feedback to compare the actual output to the desired output. This allows the closed-loop system to continuously adjust and improve its performance.

Q4. What are the advantages of an open loop control system?

A4. These systems are generally less expensive, simpler, and easier to maintain than closed-loop control systems. Because they are easier to operate, open-loop control systems also have quicker responses.

Q5. What are the disadvantages of an open loop control system?

A5. Open-loop control systems handle only unavoidable disturbances and are less accurate and reliable than closed-loop systems. If there are any changes in the working conditions, these systems are unable to correct them.