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Abstract
This project aims the control of a DC motor using LabVIEW based on obstacle detection by ultrasonic sensor. This can be used in a number of applications like Adaptive Cruise Control, Automated wheelchairs, Robotics etc.An ultrasonic sensor is useful for the obstacle detection and range measurement even under the conditions of poor lighting and transparent objects. The control action is developed by using National Instrument’s LabVIEW software and data acquisition board.
Keywords— Ultrasonic sensor, Motor control, LabVIEW, PWM
I. INTRODUCTION
Speed control of DC motor based on obstacle detection is widely used in fields like Robotics, Autonomous navigation of vehicles. The most common method is optical sensors. Even though they are cheaper and faster in response time they have non linear characteristics and depend on the reflectance properties of the obstacle. In an unknown environment the errors in distance measurement is more for optical sensors. The time of flight (ToF) is the most accurate method.
In this paper an open loop DC motor control system is developed using National Instruments DAQ board and LabVIEW software package. It reduces the hardware complexity of the system by making efficient use of programming with the help of VIs(Virtual Instruments) which are GUI based applications that can be programmed on LabVIEW. Pulse Width Modulation technique is used to control the motor speed.
II. SYSTEM ARCHITECTURE
Controlling of a DC motor is often achieved by speed feedback using a tachometer. It is possible to control the motor without speed feedback. In this project an open loop control system drives the motor at a speed proportional to the input control voltage. Figure 1 illustrates overall system architecture.
A. Ultrasonic sensor
An ultrasonic sensor from Microsense Technologies is used for distance measurement. Both transmitter and receiver are enclosed in the same module and operated synchronously. The sensor is excited using 12V direct current supply. It gives a voltage output proportional to the distance of the object from the sensor
B. DAQ card and Analog input/output modules
The analog output voltage from the sensor is given to the computer using DAQ card. This is achieved using the National Instruments cDAQ and analog input module NI9201.The input signal is scanned, buffered, conditioned and then sampled by a single 12 bit ADC.
C. Controller
An open loop control action is implemented using LabVIEW. The VI contains the PID controller and driver circuit to control the motor. It will update the speed corresponding to the distance of the obstacle from the sensor. The variation in the speed of the motor is achieved using PWM. The duty cycle of the output signal to the motor is varied according to the input signal.
Duty cycle=(ON time/Time period)*100
D. Motor Driver
The output signal from the controller is taken out using cDAQ and Analog output module NI 9263.The digital signal is converted into analog signal and is given to the motor through a power transistor SL100.The power transistor acts as a current driver between the output module and motor. The transistor is used in switch configuration wherein the on-off pulses from the output module are used to switch the transistor between saturated(on) and non-saturated(off) states and thereby driving the motor only during the on state of the pulses.
III. EXPERIMENTATION AND RESULTS
The motor control was based on ultrasonic sensor measures and LabVIEW VI. Two advantages of ultrasonic sensors over IR sensors are their ability to measure distance irrespective of the surface of the obstacle and greater range.The VI for motor control is shown in the figure.
In the above VI motor control action is implemented using a square wave generator and by varying its duty cycle in correspondence with the varying input amplitude from the ultrasonic sensor. The signal acquired from the sensor through the cDAQ module is fed into the VI using DAQ assistant which gives an output of the amplitude of the DC signal. PID control action is implemented by feeding the signal into the integrator and differentiator blocks and multiplying it with a proportional gain of 10. The outputs of all the blocks are summed up by using combiner units. The combined signal is fed to the duty cycle %(0 to 100) input of the simulate signal block which generates a pulse width modulated square wave. This signal is then fed to the analog output module using DAQ assistances that interfaces the cDAQ(hardware layer) with LabVIEW(application layer).
The waveform graphs 1 & 2 are used to monitor the input and output waveforms of the VI. It can be seen that as the amplitude of the input signal changes the duty cycle % of the output square wave changes.