Open-loop and closed-loop (feedback) control

Fundamentally, there are two types of control loop;

1.     open loop control

2.     Closed loop feedback control.

In open loop control, the control action from the controller is independent of the "process output" (or "controlled process variable").

A good example of this is a central heating boiler controlled only by a timer, so that heat is applied for a constant time, regardless of the temperature of the building. (The control action is the switching on/off of the boiler. The process output is the building temperature).

In closed-loop control, the control action from the controller is dependent on the process output. In the case of the boiler analogy, this would include a thermostat to monitor the building temperature, and thereby feedback a signal to ensure the controller maintains the building at the temperature set on the thermostat.

A closed loop controller, therefore, has a feedback loop which ensures the controller exerts a control action to give a process output the same as the "Reference input" or "set point". For this reason, closed-loop controllers are also called feedback controllers.

The definition of a closed loop control system according to the British Standard Institution is 'a control system possessing monitoring feedback, the deviation signal formed as a result of this feedback being used to control the action of a final control element in such a way as to tend to reduce the deviation to zero.

Likewise, a Feedback Control System is a system which tends to maintain a prescribed relationship of one system variable to another by comparing functions of these variables and using the difference as a means of control.

The advanced type of automation that revolutionized manufacturing, aircraft, communications, and other industries, is feedback control, which is usually continuous and involves taking measurements using a sensor and making calculated adjustments to keep the measured variable within a set range. The theoretical basis of closed-loop automation is control theory.

Difference between discrete signals and analog signals:-

Digital Signal:-

Discrete (digital) signals behave as binary switches, yielding simply an ON or OFF signal (1 or 0, True or False, respectively).

Examples of digital signals:-Push buttons, limit switches, and photoelectric sensors are examples of devices providing a discrete signal.

Discrete signals are sent using either voltage or current, where a specific range is designated as ON and another as OFF.

For example, a PLC might use 24 V DC I/O, with values above 22 V DC representing ON, values below 2VDC representing OFF, and intermediate values undefined. Initially, PLCs had only digital I/O.

Analog Signal:-

Analog signals are like volume controls, with a range of values between zero and full-scale.

These are typically interpreted as integer values (counts) by the PLC, with various ranges of accuracy depending on the device and the number of bits available to store the data.

As PLCs typically use 16-bit signed binary processors, the integer values are limited between -32,768 and +32,767.

Examples of analog signal:-Pressure, temperature, flow, and weight are often represented by analog signals.

Analog signals can use voltage or current with a magnitude proportional to the value of the process signal. For example, an analog 0 to 10 V or 4-20 mA input would be converted into an integer value of 0 to 32767.

Current inputs are less sensitive to electrical noise (e.g. from welders or electric motor starts) than voltage inputs.

Sequential Control of Two Cylinders Using Pneumatic Logic

Problem Description:

The industry uses many control circuits. In many industrial control systems, for instance, there can be many more than a pair of pneumatically actuated operators that must perform their functions in a specific sequence. Such a process is called sequential control. Here, our goal is to show how to control two double-acting pneumatic actuators in a predetermined sequence.

Hence, control valves, limit valves, and pneumatic logic will be employed in the control of two pneumatic cylinders that are in parallel and undergo two-action movements. In this way, we understand the importance of sequence control to understand pneumatic logic.

Diagram:

Condition 1: A+ (Cylinder A Extends)

When the start push button is pressed, compressed air enters at the pilot port of 5/2 valve in cylinder A.

The spool will shift, allowing air to enter the left chamber of Cylinder A.

The air inside is exhausted through port R/S.

Consequently, Cylinder A will move forward (A+).

              

 Condition 2: B+ (Cylinder B Extends)

The limit valve A1 is actuated when Cylinder A reaches its fully forward position.

This sends a pneumatic signal to the pilot port of Cylinder B's 5/2 valve.

The spool moves and air enters the front side of Cylinder B.

Thus, Cylinder B extends (B+).

Condition 3: A– (Cylinder Retracts)

Thus, when cylinder B attains its forward position, limit valve B1 is actuated.

This signal is fed back to pilot port of Cylinder A’s valve.

When this vale detects motion, it switches position.

Air now enters through the rod side of Cylinder A and exhausts from the other side.

Therefore, Cylinder A retracts (A−).

 

Condition 4: B− (Cylinder B Retracts)

When Cylinder A fully retracts, the limit valve A0 gets actuated.

This signal causes Cylinder B's valve to go to its return position.

Air enters on the rod side of Cylinder B and exhausts at the forward side.

Finally, Cylinder B retracts (B−).

 

Solution:

We could understand from the figure the use of pneumatic logic to find the sequential action of two double-acting cylinders. This in turn controls cylinder 'A' and cylinder 'B' in a fixed sequence by the use of limit valves and directional control valves.

 

Under the first condition, upon pressing the start push button, compressed air is fed to the directional control valve of cylinder A. For this reason, cylinder A moves in a forward direction, which is represented as A+.

When cylinder A extends to its full position, it actuates a limit valve. The limit valve sends a pneumatic signal to the control valve of cylinder B. This allows the cylinder B to extend, which is represented as B+.

 

When cylinder B arrives at the forward position, another limit valve is triggered. The signal shifts the position of the directional control valve of cylinder A, and cylinder A begins to retract-at position A−.

 

When it is fully retracted, cylinder A turns on another limit valve. The signal goes to the control valve of cylinder B. Cylinder B retracts back to the initial position denoted as B−.

 

In this manner, the entire sequence A+ B+ A− B− is realized employing exclusively pneumatic devices.

List of Inputs

1 -Start Push Button

2-  Limit Switch of Cylinder A (Forward)

3-  Limit Switch of Cylinder B (Forward) 4– Limit Switch of Cylinder A (Backward)

 

Program Description:

·       In the first step, when the start push button is pressed, the output is activated, and cylinder A extends.

·       When cylinder A reaches the forward position, the limit switch labeled is activated, so the output labeled activates cylinder B.

·       When cylinder B is at forward position, limit switch will turn ON, and cylinder A will retract.

·       Also, when cylinder A is in backward position, limit switch is ON and cylinder B retracts.

·       Thus, the whole process of A+ B+ A- B- occurs smoothly.