17-07-2012, 01:13 PM
dc motor speed control a fuzzy approach
speed control of D C motor using a fuzzy logic controller.doc (Size: 270 KB / Downloads: 40)
ABSTRACT:
This paper presents an insight into the speed control of D.C motor using a fuzzy logic controller to meet the desired speed. Fuzzy logic is one of the most successful applications of fuzzy set in which the variables are linguistic rather than numeric. A fuzzy logic controller (FLC) is based on a set of control rules (fuzzy rules) among linguistic variables. The personal computer provides the necessary flexibility in setting any speed profile with the use of fuzzy packages. Basically a D.C shunt motor has drooping speed-torque characteristics. The proposed fuzzy controller results in a better response compared to the basic fuzzy controller and normal response of D.C motor.
INTRODUCTION:
DC motor converts electrical energy into mechanical energy. DC motor is used in applications where wide speed ranges are required. DC shunt motor exhibits a drooping speed-torque characteristic. The speed of the DC motor is given by
Hence, the speed can be controlled by varying,
i) Flux/pole (). Field Control Method
ii) Voltage of armature circuit, by varying Ra, Armature Control Method.
FIELD CONTROL METHOD:
The speed is inversely proportional to flux. By decreasing the flux, the speed can be increased and vive versa. The flux of the DC motor is changed by changing
field current with help of a shunt field rheostat.
ARMATURE CONTROL METHOD:
This method is used when speeds below base speed are required. As supply voltage is normally constant, the voltage across the armature is varied by inserting a variable resistance in series with the armature circuit. As resistance is increased, armature voltage is decreased, thereby decreasing the armature speed.
FUZZY LOGIC CONTROL SYSTEM:
A fuzzy logic controller (FLC) is based on a set of control rules (fuzzy rules) among linguistic variables. Usually these rules are expression in the form of conditional statements (if x is small, then y is large). The execution of these rules is given by the compositional title. A basic architecture of FLC is shown in Fig 1. This structure consists of following four modules:
THE COMPLETE SPEED CONTROL SYSTEM:
The block diagram of the dc motor speed control system is as shown in Fig2. The inputs for the proposed fuzzy logic controller are the speed error (e) and the derivative of the speed error (de/dt). The output of the controller is the change in the motor voltage V. The linguistic variables for the input and output sets are Negative Large (NL), Negative Small (NS), ZERO, Positive Small (PS) and Positive Large (PL). The shapes of the fuzzy sets are isosceles triangles and trapezoid for large term sets (NL, PL) as shown in Fig 3. The Rule Base used in the design of the fuzzy logic controller is shown in Fig 4. The basic location of the singletons which are at the centre of gravity of the triangular sets are –3,-1,0,+1,+3 for NL, NS, ZERO, PS, and PL respectively.
PROPOSED FUZZY LOGIC CONTROLLER:
In most of the adaptive fuzzy controllers, attempt is made to change the Rule Base to make the system adaptive. In the scheme proposed, such a result is achieved by adjusting the defuzzifier as a function of the system response. Also it is possible to regulate the parameters of the time – domain response. The block diagram of the system with the proposed FLC is shown in Fig 5. A new functional block called the Error Interpreter is added to the basic system of Fig 2.The function of the block is to sense the error, identify its ranges.