When operators use a measuring instrument and a control device in process control, they are using manual control. The measuring device indicates the pressure, flow, level, or temperature of the process variable. The control device may be a valve or a temperature adjustment that is manually operated to bring about changes in the variable to be controlled.
Manual control is simple when only one variable requires control, but when operators must control several variables, they have to coordinate many adjustments; consequently errors can occur. An example of complicated manual control is a Helicopter rising to hover:
The pilot lifts the collective lever – This changes rotor blade angle which increses rotor lift but slows down the engine/rotor speed, so he needs more fuel to the engine to increase engine/rotor speed.
This also causes engine torque to increase which causes the helicopter to rotate about the engine, so he needs to control this with the tail rotor.
All these actions cause decreases in engine / rotor speed which affect the lift of the main rotor, and the opposite when the pilot wants to descend.
This is why all helicopters have speed controllers or Governors to control the speed of the Main rotor by controlling fuel flow to the engine.
9.2 Open and Closed Loops
Control systems can be classified as open-loop or closed-loop There are many ways to control a process using mechanical, pneumatic, electrical, analog, and digital techniques. Regardless of the method used, the basic theory is similar.
A typical open-loop control system is shown in Figure 4. The process is controlled by setting the setpoint of the controller required. Since the only input to the controller is the set point, so process is controlled blindly. That is, the controller has no information about the status of the process or the need for any corrective action. Open-loop control systems are much cheaper and less complex than closed-loop systems. The inevitable result of their use, however, is poor process control.
Figure 4 Manual Loop
A simple example of an open loop or manual control is Figure 5, where steam is used to maintain the temperature of water close to the desired value.
Figure 5 Manual Control Water Heating System
Cold water enters at the left end of the heat exchanger, and heated water is discharged at the right. A manually operated valve is used to control the heating medium flow into the heater. If the water flow is constant, and heating medium flow is constant, the outlet temperature of the water will be constant. But, if the temperature of the incoming cold water changes, or if the flow of heated water increases or decreases, the outlet temperature will change from the desired value unless the heating medium valve was manually readjusted.
If the flow of cold water increased due to an increase in demand for hot water, or if the flow of the heating medium decreased, the result would be a decrease in the temperature of the water at the outlet.
This type of control system is called manual or open loop. There is no way to change the control valve opening except manually, because there is no feedback signal from the water outlet to automatically change the control valve. The operator must check the thermometer frequently to see if there is any error or deviation and to see whether the error is moving in an increasing or decreasing direction.
A manual, or open loop, process control system similar to this one needs constant attention. The desired value of the process variable is difficult to maintain. To overcome many control problems, automatic or closed loop control systems are used.
9.3 Open Loop (Manual Control)
Figure 6 The Open Loop
Figure 6 shows another example of the OPEN LOOP or MANUAL control using the same hot water principle. The process variable is temperature control. The indicator is a thermometer. The correcting unit is the gas control valve. The controller is the operator who uses his own judgment to keep the water temperature constant.
Manual control has its uses as it is cheap to install and maintain, and simple to operate. However, it is very seldom used in industry because:
- The operator must remain in position at all times.
- It cannot be used if the operator is placed in a dangerous area.
- The process changes faster than the operator can react.
- A mistake by the operator can have dangerous results.
These problems are avoided by using automatic control (closed loop). The job of the instrument technician is to make sure that this type of control operates correctly.
One type of closed-loop control system is one in which the output of a process has a ‘Signal Feedback’ to the controller. Within the controller the actual value of the process is compared to the set point, adjustments are made continuously by the controller and the final control element (control valve) until the difference between the set point and the process is eliminated.
Fig. 7 depicts a typical closed-loop control system. The output from the process sensor is converted by the transmitter and is feedback (hence the name feedback control) to the controller be added algebraically / mechanically to the set point. If there is an error, a signal to the controller causes it to take action to change the actual output to reduce the error to zero.
9.4 Closed Loop (Automatic Control)
Figure 7 Closed Loop System
A schematic diagram of an automatic control system is shown in Figure 7, and a practical diagram is shown in fig 8
Figure 8 Automatic Control System
Figure 9 Simple Automatic Control
Figures 8 & 9 show a simple automatic controller. The boiler now has the loop closed and no operator is required. The following items are added.
- Temperature transmitter (T.T). This measures (senses) the temperature of the water in the boiler and changes it to a standard signal.
- A signal line from the transmitter to the controller, The signal may be pneumatic, which is 3-15 psi or electrical/electronic 4-20 mA.
- A controller this keeps the temperature of the water at a desired temperature set by the operator (set point).
- Automatic control valve The controller adjusts the correcting unit (automatic control valve) using an output signal line similar to the input line from the transmitter. The controller may provide alarm signals to alert the operator if the system fails. It may also have the ability to shut off the gas if the water starts to boil.
- Automatic Process Control Principles
The manually controlled water heating system in Figure’s 6 has been converted into one that is automatically controlled in Figure 8. A sensing element, transmitter, and a controller are placed in the control loop between the heated water outlet and the final control element (control valve). This control valve position can be changed by varying the pneumatic signal.
During normal operation, the control valve permits the required amount of gas flow into the heater burner to increase the water to the set point temperature. If the final or discharge temperature of the water should drop, the temperature sensing element will sense this change and will produce a proportional change in output from the transmitter. The transmitter output, is compared with the set point signal in the controller. Since the temperature of the water has dropped, there will be an increased difference between the two signals.
The controller will respond to this error by opening the control valve further to increase the gas flow to the burners, the temperature of the water from the heater increases, the sensing element will sense this change and the transmitter output will increase. If this increases above the set point, the controller output will cause the control valve to reduce the gas flow , to reduce the heat energy into the water heater.
Comparing the open loop or manual control system with the automatic or closed loop system, the open loop system has no way of changing the position of the control valve because no intelligence about the process variable (temperature in this example) ever reaches the control valve to change its position. An operator must notice changes in temperature on the thermometer and adjust the flow of gas to the burner.
In the closed loop system, the value of the process variable is fed back to the controller constantly, so corrective action can be made more accurately and quickly to keep the process closer to the desired value.