Research On The Design And Application Of Micro-controller Based PID Controllers

Control engineering is one of the most important part of modern day industries. Its main objective is to avoid disturbances and ensure the desired output in industrial processes. One of the generic control strategies that is widely used in industrial control is the Proportional Integral and Derivative(PID) control algorithm. The PID controller has been deemed by some experts to represent the ultimate in control of continuous processes for which a specific mathematical description(transfer function) can be written. The PID controller in all its ability to eliminate steady-state errors and anticipate the future through integral action and derivative action is a simple implementation of the feedback principle. More than 95 percent of the control loops in process control are of PID type; a greater number of those loops are actually PI control. PID controllers come in many different forms. They can be implemented as stand-alone systems in boxes for one or a few loops or as distributed systems for process control. Even systems which are as diverse as atomic force microscopes, cars cruise control systems, or CD and DVD players contain PID controllers. Because they are sufficient for many control problems with benign process dynamics and modest performance requirements, they are found in large quantities in almost all industries.For many years the PID controllers have been implemented in analog format. After their introduction, these electronic controllers gradually excelled in performance over their mechanical predecessors, both in terms of performance, speed and cost. As result single loop analog controllers have enjoyed a steady popularity and they have reached the highest level of sophistication due to the rapid advances made in the industry. But the use of these analog PID controllers however always presented difficulties in changing the controller parameters whenever there are changes in plant dynamics due to changes in the operating conditions. This scenario always required the engineer to change the hardware in order to change the controller parameters to match with the new operating conditions. During that time digital computing had not yet advanced and it was very slow and costly compared to the situation today. There was little software available and machine code was used to program the required solution to engineering problems. During the past 3 decades the situation has been drastically changing. Rapid progress made in microelectronics and microcontrollers in recent years has made it possible to apply modern control technology in the control of industrial processes. The cost of digital computing decreased drastically and its speed of operation has increased astronomically and this has seen the gradual replacement of the analog computer with the digital computer.In line with the recent developments made in solid state technology, the ability to redesign the controller by changing the software(rather than hardware) is now an important feature of digital control against analog control. Hence, in this project we consider the research and study of the design and application of a micro-controller based PID controller which enables the parameters of the controller to be changed without any hardware changes by merely reprogramming the microcontroller. Firstly the history and theory of the PID controller as a feedback control system was studied in detail. The mathematical modeling of the PID controller was also carried out. Once a relevant control law is established the next step is to design the PID controller. This process, known as tuning, involves the finding of the proper values of the PID parameters, Kp, Ti and Td. Various tuning methods, both traditional and modern, were studied and a suitable tuning method was selected for our simulations and experiments. In order to use the microcontroller to implement the PID control strategy various aspects of the digital implementation strategy like sampling, discretization, and quantization were looked into. Various methods of discretization were studied and a suitable discretization method was chosen in order to digitalise our PID control law.In order to demonstrate the effectiveness of the proposed microcontroller based PID controller a dc-dc converter system was chosen for the practical implementation. The phenomenal advancement in technology has given rise to modern electronic systems that require reliable, high quality, efficient, small and lightweight power supply system. This has inspired the wide use of dc-dc systems in many industrial and electrical systems. The main function of a DC-DC converter is to convert a fluctuating DC input voltage into a regulated DC output voltage and supply it to a variable-load resistance. DC-DC converters are mostly applied in applications such as computers, television receivers, communication devices, medical instrumentation, battery chargers and many other devices that require regulated DC power. DC-DC converters are also used for DC motor speed control applications to provide regulated variable DC voltage. The dcdc converters can be found as step down(buck) or step up(boost) converters. In this project the buck converter system was chosen for our analysis and demonstration of the microcontroller based PID controller. In order for a buck converter to maintain a constant voltage output, it employs a feedback or closed loop system which continuously monitors the output voltage and takes corrective action whenever the output voltage shifts away from the desired value. What this corrective action does is to change the duty cycle of the signal driving the MOSFET. Our proposed micro-controller based PID controller was going to be that feedback loop which adjusts the duty cycle of the MOSFET to maintain a constant output voltage on our converter.In order to design a controller that best fits the job, a detailed knowledge of the converter‘s dynamic behaviour, transient response as well as the small signal characteristics is needed. However, this cannot be fully obtained from the physical model of the converter. As a result mathematical modeling was developed to provide a way to obtain the much needed information by representing the system as time independent systems, which can be defined by a set of differential equations that are capable of representing the circuit waveforms. Therefore, mathematical modeling is one of the most widely used approaches in designing controllers that can be applied to switched converters. Many techniques have been developed to come up with mathematical models for switched converters and these include current injected approach, circuit averaging and state space averaging and whichever method is used they all give the same end result. However in this project the state space averaging technique was used to derive a model for a pulse width modulated(PWM) buck converter operating in continuous conduction mode(CCM). The mathematical models for open loop, closed loop and digital controller were used for simulation with MATLAB/Simulink to obtain results for analysis. From the research the following major points were obtained. 1. Various tuning methods, both traditional and modern, to find the proper values of the PID parameters, Kp, Ti and Td, were studied in detail. Even though a number of modern methods has been developed, research shows that none has replaced the Z-N method in terms of simplicity and application as a first go to tuning method. Therefore there is still need to develop other tuning methods that are simpler and more effective than the traditional Z-N method in order to match with the major developments made in modern day control engineering techniques.2. In order to properly implement a continuous-time PID control law in a microcontroller, it is important to get an approximation of the derivative and the integral terms that appear in the control law. Therefore various methods of discretization were studied in detail and from the research it was observed that. The backward difference technique is the best method to discretize the derivative term because it produces good approximations for all values of Td and it is easier to compute than the other techniques.3. In this project a microcontroller based PID controller was designed and implemented on a buck converter system to converter a fluctuating 12 V input voltage into a 5-V constant voltage output. A 5V constant output voltage with a 2 seconds settling time was successfully obtained for a buck converter operating in CCM mode. From the simulation results obtained, it was also observed that the PID based closed loop buck converter outperforms the open loop controlled buck converter. It clearly showed shorter overshoots, shorter rise times and faster settling time than the open loop model. Therefore the PID controller in the closed loop controlled buck converter resulted in improved transient and steady state performance of the buck converter than the open loop controlled buck converter. The digital model gave almost the same transient and steady state waveforms as the PID closed loop model except for the slightly reduced overshoot voltage during start up and this was due to the quantization of the error signal.4. For the practical experiment the PIC16F84 micro-controller was used to implement the digital PID controller because of its robustness, simplicity and low cost. The results obtained matched those obtained from the simulation. From the comparison of the results obtained from simulations and practical experiments, this research shows that a microcontroller based PID controller enables the parameters of the controller to be changed without any hardware changes by merely reprogramming the microcontroller. It was also observed that microcontroller based PID controllers are more resilient to the effects of noise and disturbances and they require fewer components than their analog counterparts.