Objective

The objective of this project was to replace the existing pull system with a motor driven pull system with variable speed that can be selected and monitored.

Functional Requirements
 

• Pull speed range of 0 to 3 feet per second
• Labview software interface
• Printable plot of the actual pull speed
• Allow for addition of strain gage input

• Drive System (follow link for pictures)

A 12VDC gearmotor was selected as the motor. Its maximum rotational speed is 50 rpm. In order to get the desired pull speed range, a pulley system was designed that increased the rotational speed of the drive pulley.

A 6" diameter pulley is attached to the motor's output shaft and a 2.5" diameter pulley is connected to the 6" pulley by a belt. The 2.5" pulley is attached to a 3/8" diameter shaft. The shaft runs through a single row ball bearing and another 6" diameter pulley is attached as the drive pulley. The maximum rotational speed of the drive pulley is 120 RPM giving a maximum pull speed of 3.011 ft/s (Appendix A).

A proportional motor amplifier circuit was wired according to the schematic shown in Appendix B.
 

• Speed Control

An encoder is used to determine the actual pull speed. The 6" drive pulley has 24 equally spaced holes drilled 2" from the center. A light detector is used to detect the light from the emitter when they are aligned with a hole in the drive pulley. As the detector is triggered an output is generated that is input into a frequency to voltage converter circuit shown in Appendix B.  This circuit converts the detector pulse frequency to a voltage.  This voltage is input into Labview and is converted to an actual boat speed.  See Appendix C for experimental data and conversion equation.
 
• User Interface

To setup for a test run, the user will input a desired speed (0-3 ft/s) and Labview will show on screen what voltage to supply the motor.  Once the power supply is set to the correct voltage and the carriage is in the correct position, the motor is switched and the test begins.  The actual boat speed is displayed on a waveform chart, a dial speedometer, and a digital indicator.  The waveform chart allows the user to see when to take the speed reading.
 

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• Step 1: Load Labview 4.1 software.

• Step 2: Open c:\NationalInstruments\Labview\dragtank.vi file.

• Step 3: Click on the Continuous Run icon.

• Step 4: Input a desired boat speed and hit Enter.

• Step 5: Set motor supply voltage to voltage displayed on screen.
• Step 6: Pull back carriage and connect to boat.

• Step 7: Switch on motor to begin test run.

• Step 8: Record actual boat speed.
• Step 9: Once carriage has come to a stop click on Stop icon.

• Step 10: Print copy of the boat velocity waveform.

• Step 11: Repeat steps 4 - 10 for repetitive testing.