It is a method by which a "process" of manufacture can be controlled automatically and continuously, with regular and consistent results. Process control defines the overall system, its components, and their respective capabilities. Process control can have the following names.
Control (batch) continuous.
Closed loop control.
Control of pump. Level control.
Thermal control zone. Automatic Control.
There are the following advantages:
The ability to manufacture a product with repeatable accuracy.
The most effective and efficient use of plant facilities.
Allows the operator to engage in more productive jobs requiring greater skill.
Reduced and avoid boring tasks which workers are exposed to hazardous operations.
Increased productivity, less waste.
Open loop control (without feedback)
Is the name given to a control system that senses its own output and therefore does not make corrections in the process. No feedback control system that allows it to regulate the process.
Closed loop control (with feedback)
It offers users the ability to program a particular operation is performed so regularly and consistently. A control system that has been properly prepared will do so regardless of almost all the influences (disturbances) external.
The control Proportional-Integral-Derivative (PID) is to maintain regularity specific process and compensate for external shocks.
Block diagram of a feedback system
Control systems are usually represented by a series of interconnected blocks. The blocks represent system-specific functions. See Figure 2-16.
All feedback system can be divided into four basic operations:
1. Measurement of the controlled variable.
The controlled variable may be temperature, velocity, thickness, water pressure, etc. As a measurement using a sensor, and the measurement obtained is then converted into a signal compatible with the control inputs, usually voltage (0-10V) or current (4-20mA). This signal represents the controlled variable.
2. Determination of the error.
This is done in the comparison section.
Error = Vref - VMED (2-6)
3. The error signal
Is then used by the control to change the torque or engine speed.
4. The controlled variable
Is used then the torque or engine speed to reduce the error signal driving the control so that the real value of the controlled variable approaches the reference value (Vref). Importantly, feedback control systems are driven by the error, ie there must be an error before the system try to make the amendment in question.
Definition of "P" (Proportional Gain)
Amplification is applied to the error signal of the process and will result in a specific control output.
The proportional gain is defined as:
Where: Aout = Out of control
KP = Proportional gain
E = process error signal
Equation (2-7) can be interpreted as:
The amplitude of the control output is a function of process error, multiplied by the proportional gain.
For a given magnitude of error, the larger the proportional gain, the greater the output.
For a given value of the proportional gain, the greater the error, the greater the output.
See Figure 2-17 to clarify the definition of proportional gain.
Definition of "I" (Integral Gain)
The integral gain (as well as the proportional gain) is a signal amplification process error, but depends on time.
A steady-state error is maintained over a long period of time is known as a deviation (offset or imbalance). The integral gain compensates for the deviation or error in the long term.
The integral gain is defined as:
Where:
Aout = out of control.
E = Error signal of the process ..
At = change in time.
The control output (Aout) is equal to the integral gain (Ki), multiplied by the accumulated error during a time interval t.
The long-term error accumulates over time and the integral gain to compensate and reduce the error term.
In general, if you used a process only proportional control, the output would never control the controlled variable is exactly equal to the reference value. There would always be a small amount of error. Integral feature detects this deviation and corrects long-term control output to reduce the effect of such diversion
Definition of "D" (Gain Differential)
The differential element is proportional to the change in the error rate of the process. The differential gain is provided to reduce the overshoot (overshoot or overshoot) of process control during sudden disturbances of great magnitude. The differential element responds only during transient conditions.
The differential gain is defined as:
Where:
KD: Differential Gain
AE / At: Change in process error signal divided by the change in weather.
The interpretation of equation (2-9) is:
- Upon a major change in process error for a fixed period of time, the differential term will have a big effect on the control output.
- A small change in process error for a fixed period of time will exert less effect on the output of the control.
In most applications the differential gain is rarely used. If necessary, it should be used carefully, since it could cause instability.
Definition of "PID" (Proportional, Integral, Derivative)
It is the sum total of the three elements of profit, and can be expressed as follows:
One can interpret the above equation:
Proportional gain is a gain of steady state and is always active.
Integral gain will be active only to deviations from long-term errors. Not active in the control loop when the errors are short-lived.
The differential gain will be active only to deviations from transient errors, short-term. Not active in the control loop when the errors are long lasting.
