Types of control system
As per the realization of the controllers these are divided as Pneumatic, Electronic and Digital controllers.
In earlier days all the process controllers (P, PI, PID) are pneumatic type where there is no so much advancement or progress in electronic components. The advantage of pneumatic controllers is its ruggedness, and major limitation is the slow response. This controllers are designed using mechanical components which operates according to air/liquid pressure. Due to this mechanical components these controllers are strong and insensitive to the temperatures in plant. However the response of mechanical components are slow compared to electronic components so these controller slower in response. These pneumatic controllers will acts on the difference between the air/liquid pressure of measured signal and set point signal. The best example of pneumatic/hydraulic controller is Hydraulic Governor which is placed to control the steam turbine parameters.
The simple scheme of implementation of Proportional controller is shown in figure. Here the set point and measured values are converted as the hydraulic pressure signals and applied to one end of the force beam. The force beam moves up/down according to the differential pressure of measured and set values. If the set point is high then the left end of the force beam moves down and the right end of the force beam moves up. the movement ratio of left and right end of beam can be controlled by the adjustable weight which can be called as proportional gain. Due to this movement of right end of the beam the flapper which is connected to it also moves upward towards fixed flapper. this reduces the nozzle gap between the fixed and moving flappers and hence reduces the oil flow through the nozzle. This in-turn increases the output control oil pressure and flow which is used drive the control valve actuator.
Electronic or Hardwired controller:
The later advancements in electronic components prompted the use of electronic components in control systems. The introduction of this electronic control systems enhanced the performance of the control much faster compared to mechanical systems. Due to the small size of electronic components the overall size of the controller became very small. However the system response is faster in this control system but it also has a disadvantage that it is very sensitive to temperature or any internal faults in components. There will be some internal and external noise because of electronic components and care has to be taken to filter the noise which otherwise affects overall response of the controller.
Nowadays the electronic controllers with advanced technology which are offering with very less noise with low sensitivity for the internal faults. A simple schematic of electronic PID controller is shown in below figure.
The PID controller is designed using basic electronic components like resistor, capacitor and op-amp comparator. The PID controller is a combination of Proportional gain, Integral component (Low-pass filter) and differential component (High Pass filter).
Output = Proportional gain * error + Low pass filter (error) + high pass filter (error).
The input comparator calculates the error between input set point and actual measured value. This error is parallelly applied to the gain amplifier, low pass filter and high pass filter. The parallel output is combined at the output node and given to the actuator to drive the actuator.
The biggest disadvantage in hardwired controller is that it is not flexible for changing the logic of the controller. That is if the PI controller is designed to control the liquid level in the tank, suppose with same controller should be used to control the temperature with PD design then the whole circuit should be redesigned for this operation. Hence it is not suitable in design and testing environment.
In the earlier days the controllers using digital systems are used in power plant controls because of their flexibility. The complex control logic can be replaced by the programming instructions and the digital computer executes these instructions and provide an output to the controller. Nowadays the advanced controllers are embedded with Human Machine Interface (HMI) based control screens for the operator to change the controller parameters dynamically and to test the controller before putting it to actual place. The control logic’s pertaining to the similar components like circuit breakers can be clubbed and embedded into digital system so that usage of excess hardware for each component can be reduced. This creates a lot of difference in the view of plant cost.
The above figure shows the simple schematic digital controller for a liquid level of tank. The measured level is connected to digital computer controller through I/O devices which converters the level value into corresponding digital signal. The digital computer scans this level signal at specific intervals and compared with the set-point provided by the operator from HMI control panel. Hence the error signal will be processed by the control instructions stored in controller memory and provides an output control signal to the final actuating device through the I/O device. The I/O device converts the digital output signal into analog output signal which is suitable for actuation of final element. In the same controller we can embed the pump starting logic if the level is low by increasing I/O cards and by writing some extra programing instructions..