28-01-2013, 12:23 PM
Design and Implementation of PID Controller for a Two Quadrant Chopper Fed DC Motor Drive
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DC MOTOR
Generally, DC motors are used in a huge number of industrial applications. In particular, separately excited DC motors have many applications. In order to control the speed of a dc motor at different required torque levels it is necessary to adjust the voltage applied to motor for any particular constant voltage the motor speed is determined by the torque requirement and top speed is reached under minimum torque requirement condition. The electric drive system used in industrial applications is increasingly required to meet higher performance and reliability requirement. The dc motor is an attractive piece of equipment in many industrial application requiring variable speed and load characteristic due to its ease of speed controllability. So we are going to control DC motor in open loop & closed loop using chopper with PID controller.
DC CHOPPER
A chopper is a static power electronic device that converts fixed dc input voltage to a variable dc output voltage. A Chopper may be considered as dc equivalent of an ac transformer since they behave in an identical manner. As chopper involves one stage conversion, these are more efficient. Choppers are now being used all over the world for rapid transit systems. These are also used in trolley cars, marine hoist, forklift trucks and mine haulers. The future electric automobiles are likely to use choppers for their speed control and braking. Chopper systems offer smooth control, high efficiency, faster response and regeneration facility. The power semiconductor devices used for a chopper circuit can be force commutated Thyristor, power BJT, MOSFET and IGBT,GTO based chopper are also used. These devices are generally represented by a switch. When the switch is off, no current can flow. Current flows through the load when switch is “on”. The power semiconductor devices have on-state voltage drop of 0.5V to 2.5V across them. For the sake of simplicity, this voltage drop across these devices is generally neglected. As mentioned above, a chopper is dc equivalent to an ac transformer, have continuously variable turn’s ratio. Like a transformer, a chopper can be used to step down or step up the fixed dc input voltage.
PRINCIPLE OF CHOPPER OPERATION
A chopper is a high speed “on" or “off” semiconductor switch. It connects source to load and load and disconnect the load from source at a fast speed. In this manner, a chopped load voltage as shown in Fig. is obtained from a constant dc supply of magnitude Vs. For the sake of highlighting the principle of chopper operation, the circuitry used for controlling the on, off periods is not shown. During the period Ton, chopper is on and load voltage is equal to source voltage Vs. During the period Toff, chopper is off, load voltage is zero.
MOSFET AS CHOPPER:
Unlike the bipolar junction transistor (BJT), the MOSFET device belongs to the Unipolar Device family, since it uses only the majority carriers in conduction. The development of the metal oxide semiconductor technology for microelectronic circuits opened the way for developing the Power Metal Oxide Semiconductor Field Effect Transistor (MOSFET).fig 1.3.1 shows N-channel enhancement-type MOSFET. It is the fastest power switching device with switching frequency more than 1 MHz, with voltage power ratings up to 1000V and current rating as high as 300 A.
MODELING OF SEPARATELY EXCITED DC MOTOR
This DC motor system is a separately excited DC motor, which is often used to the speed controlling and the position adjustment. This project focuses on the study of DC motor linear speed control, therefore, the separately excited DC motor is adopted. Make use of the armature voltage control method to control the DC motor velocity, the armature voltage controls the distinguishing feature of method as the flux fixed, is also a field current fixedly. The control equivalent circuit of the DC motor by the armature voltage control method is shown in Figure 3.2.1.
PID CONTROLLER TUNING:
"Tuning" a control loop is the adjustment of its control parameters (gain/proportional band, integral gain/reset, derivative gain/rate) to the optimum values for the desired control response. The optimum behavior on a process change or set point change varies depending on the application. Some processes must not allow an overshoot of the process variable from the set point. Other processes must minimize the energy expended in reaching a new set point. Generally stability of response is required and the Process must not oscillate for any combination of process conditions and set points. Tuning of loops is made more complicated by the response time of the process; it may take minutes or hours for a set point change to produce a stable effect. Some processes have a degree of non-linearity and so parameters that work well at full-load conditions don't work when the process is starting up from no-load. This section describes some traditional manual methods for loop tuning.
There are several methods for tuning a PID loop. The choice of method will depend largely on whether or not the loop can be taken "offline" for tuning, and the response speed of the system. If the system can be taken offline, the best tuning method often involves subjecting the system to a step change in input, measuring the output as a function of time, and using this response to determine the control parameters.
If the system must remain online, one tuning method is to first set the I and D values to zero. Increase the P until the output of the loop oscillates. Then increase I until oscillation stops. Finally, increase D until the loop is acceptably quick to reach its reference. A fast PID loop tuning usually overshoots slightly to reach the set point more quickly; however, some systems cannot accept overshoot.
CONCLUSION
The speed of a dc motor has been successfully controlled by using Chopper as a converter and Proportional-Integral-Derivative type Speed and Current controller based on closed loop system model. Initially a simplified closed loop model for speed control of DC motor is considered and requirement of current controller is studied. Then a generalized modeling of dc motor is done. After that a complete layout of DC drive system is obtained. Then designing of current and speed controller is done. The optimization of speed control loop is achieved through Modulus Hugging approach. A DC motor specification is taken and corresponding parameters are found out from derived design approach. The simulation results under varying reference speed and varying load are also studied and analyzed. The model shows good results under all conditions employed during simulation.
A hardware setup for the same is designed and tested for open loop and close loop control system. For implementation of close loop control system the PID controller gains are tuned by simulation and fixed gains have been implemented. The hardware setup gives the desirable output in two of the quadrants (i.e. forward and reverse motoring)