Functional Levels of a manufacturing control operation

SCADA system has the facility to handle different levels in the manufacturing plant. 

Level 0:-Level 0 contains the field devices in the plant such as flow and temperature sensors, and final control elements, such as control valves, final control elements.

Level 1:- Level 1 contains the controller’s industrialized input/output (I/O) modules, and their associated distributed electronic processors.

Level 2:-Level 2 contains the supervisory computers, which collect information from processor nodes on the system, and provide the operator control screens.

Level 3:- Level 3 is the production control level, which does not directly control the process, but is concerned with monitoring production and targets.

Level 4:- Level 4 is the production scheduling level.

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.