INDUSTRY AUTOMATIC CONTROL FUNDAMENTALS
The basic idea of a feedback control loop is most easily understood by imagining what an operator would have to do if automatic control did not exist.
Figure I shows a common application of automatic control found in many industrial plants, a heat exchanger which uses steam to heat cold water. In manual operation, the amount of steam entering the heat exchanger depends on the air pressure to the valve which is set on the manual regulator. To control the temperature manually, the operator would watch the indicated temperature, and by comparing it with the desired temperature, be would open or close the valve to admit more or less steam . When the temperature had reached the desired value, the operator would simply hold that output to the valve to keep the temperature constant. Under automatic control, the temperature controller performs the same function. The measurement signal to the controller from the temperature transmitter is continuously compared to the set point signal entered into the controller. Based on a comparison of the signals, the automatic controller can tell whether the measurement signal is above or below the setpoint and move the valve accordingly until the measurement (temperature) comes to its final value.
This simple feedback control loop serves to illustrate the four major
elements of any feedback control loop
Measurement must be made to indicate the current value of the variable controlled by the loop. Common measurements used in industry include flow rate, pressure, level, temperature, analytical measurements such as pH, ORP and conductivity and many others particular to specific industries.
For every process there must be some final actuator, which regulates the supply of energy or material to the process and changes the measurement signal. Most often this is some kind of valve, but it might also be a belt or motor speed, louver position, etc.
The kinds of processes found in industrial plants are as varied as the materials they produce. They range from the simple and commonplace, such as loops to control flow rate, to the large and complex such as distillation columns in the petro-chemical industry.
The automatic controller
The last element of the loop is the automatic controller. Its job is to
control the measurement. To "control" means to keep the measurement
within acceptable limits. In this article, the mechanisms inside the automatic
controller will not be considered. Therefore, the principles to be discussed can
be applied equally well to both pneumatic and electronic controllers and to the
controllers from any manufacturer. All automatic controllers use the same
general responses, although the internal mechanisms and the definitions given
for these responses may be slightly different from one manufacturer to another.
One basic concept is that for the automatic feedback control to exist, the automatic control loop must be closed. This means that information must be continuously passed around the loop. The controller must be able to move the valve, the valve must be able to affect the measurement, and the measurement signal must be reported to the controller. If this path is broken at any point, the loop is said to be open. As soon as the loop is opened, as for example, when the automatic controller is placed on manual, the automatic unit in the controller is no longer able to move the valve. Thus signals from the controller in response to changing-measurement conditions do not affect the valve and automatic control does not exist.
CONTROLLING THE PROCESS
In performing the control function, the automatic controller uses the difference between the set point and measurement signals to develop the output signal to the valve. The accuracy and responsiveness of these signals is a basic limitation on the ability of the controller to correctly control the measurement. If the transmitter does not send an accurate signal, or if there is a lag in the measurement signal, the ability of the controller to manipulate the process will be degraded. At the same time, the controller must receive an accurate set point signal. In controllers using pneumatic or electronic set point signals generated within the controller, miscalibration of the set point transmitter will necessarily result in the automatic control unit in the controller bringing the measurement to the wrong value. The ability of the controller to accurately position the valve is yet another limitation. If there is friction in the valve, the controller may not be able to move the valve to a specific stem position to produce a specific flow and this will show up as a difference between measurement and set point. Repeated attempts to exactly position the valve may lead to hunting in the valve and in the measurement. or, if the controller is only able to move the valve very slowly, the ability of the controller to control the process will he degraded. One way to improve the response of control valves is to use a valve positioner, which acts as a feedback controller to position the valve at the exact position corresponding the controller output signal. Positioners, however, should be avoided in favor of volume boosters on fast responding loops such as flow arid liquid pressure.
To control the process, the change in output from the controller must be in such a direction as to oppose any change in the measurement value.
Figure 3 shows a direct connected valve to control level in a tank at midscale. As the level in the tank rises, the float acts to reduce the flow rate coming in thus, the higher the liquid level the more the flow will be shut off. In the same way, as the level falls, the float will open up the valve to add more liquid to the tank. The response of this system is shown graphically.
As the level goes from 0% to 100%, the valve goes from fully open to fully closed. The function of an automatic controller is to produce this kind of opposing response over varying ranges; in addition, other responses are available to more efficiently control the process.
Other topics : Stepping motors , widely used in industry , basic theory .