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Innovation in Teaching & Learning in Undergraduate Lab Experimentation in Engineering Education

Introduction (LabVIEW Based Identification of a DC Motor Transfer Function and Simulink-based Triangle Response)


Objectives: Students will learn (1) how to use dataflow programming to model a DC motor to achieve various design specifications and (2) simulate the unit step response using Simulink 

Prelab:  DC motor is one of the most commonly used actuating devices for control systems. Visit the 258 web site at http://eecs.vanderbilt.edu/courses/eecs258/ and investigate the characteristics of the DC motor. 

INLAB: Students will use the National Instruments LabVIEW to model  the Quanser QNET DC Motor and to experiment with PID (Proportional Integral Derivative) controller parameter tuning using Simulink.

REQUIREMENTS: You will be required to learn the virtual instrumentation approach using LabView and dataflow programming and another control system analysis tool Simulink to run the experiment and achieve the  following design specifications:

                        1. Getting the transfer function of a DC motor

                        2. Marginal Stability (Square wave)

                        3. Best tracking of a triangle wave (~0 steady-state error, <4% overshoot, and <.4sec. settling time).  Also, observe the effect of a disturbance (load) on the motor when Ki has a very low value.

Introduction to Instruments

Personal Computer (PC)

Computer receives the input signal from the terminal board and output the control signal according to the Requirement. System identification and PID control system is designed using LabVIEW on the computer which is related to the hardware, specifically the PCI card inside the computer.

Read about the MATLAB guideline

National Instruments ELVIS II

NI ELVIS is a LabVIEW-based platform designed to provide an integrated and flexible environment that enhances measurement, design, and prototyping in an educational laboratory. NI ELVIS integrates LabVIEW software, a multifunction data acquisition board, and the NI ELVIS Workstation to build a suite of virtual instruments. visit http://www.ni.com/academic

2. Components

DC Motor

The NI ELVIS II platform incorporates a DC motor. This is a high efficiency, low inductance motor with a small rotor inductance.

The figure below shows an electrical diagram of a simple DC motor. Notice that the DC voltage is applied directly to the field winding and the brushes. The armature and the field are both shown as a coil of wire. In later diagrams, a field resistor will be added in series with the field to control the motor speed.

When voltage is applied to the motor, current begins to flow through the field coil from the negative terminal to the positive terminal. This sets up a strong magnetic field in the field winding. Current also begins to flow through the brushes into a commutator segment and then through an armature coil. The current continues to flow through the coil back to the brush that is attached to other end of the coil and returns to the DC power source. The current flowing in the armature coil sets up a strong magnetic field in the armature.

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The magnetic field in the armature and field coil causes the armature to begin to rotate. This occurs by the unlike magnetic poles attracting each other and the like magnetic poles repelling each other. As the armature begins to rotate, the commutator segments will also begin to move under the brushes. As an individual commutator segment moves under the brush connected to positive voltage, it will become positive, and when it moves under a brush connected to negative voltage it will become negative. In this way, the commutator segments continually change polarity from positive to negative. Since the commutator segments are connected to the ends of the wires that make up the field winding in the armature, it causes the magnetic field in the armature to change polarity continually from north pole to south pole. The commutator segments and brushes are aligned in such a way that the switch in polarity of the armature coincides with the location of the armature's magnetic field and the field winding's magnetic field. The switching action is timed so that the armature will not lock up magnetically with the field. Instead the magnetic fields tend to build on each other and provide additional torque to keep the motor shaft rotating.

When the voltage is de-energized to the motor, the magnetic fields in the armature and the field winding will quickly diminish and the armature shaft's speed will begin to drop to zero. If voltage is applied to the motor again, the magnetic fields will strengthen and the armature will begin to rotate again.

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Encoder

The DC motor has an optical encoder installed that measures the angular position of the load shaft. The encoder in the system has a resolution of 8192 counts per revolution in quadrature mode(2042 lines per revolution). Remark that incremental encoders measure the relative angle of the shaft.

The position signal generated by the encoder can be directly connected to the data-acquisition device using a standard 5-pin DIN cable.

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In this lab, you will be required to implement LabVIEW based system to get the transfer function model of a DC motor and control a DC motor using PID control law in simulation


After you have completed the Perspective and Resources, you may proceed to "Research and Revise" stage.

<Research and Revise> <Return Home>

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