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# Moving and Maneavering

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on 27 September 2014

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#### Transcript of Moving and Maneavering

DAY 9
MOVING AND MANEUVERING
Newton's Laws
Producing Thrust
Thruster Placement
First Law
An object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force.
Second Law
The vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration vector a of the object.
Third Law
When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.
F = m.a
First Law
For a vehicle to maintain
constant speed
and
direction
, all of the forces acting on it must be
in balance
, so their effects cancel.
Second Law
For a vehicle to
change speed
and/or
direction
, there must be an
imbalance
in the forces acting on it, so the net force is not zero.
Third Law
To push a vehicle in one direction, you must push something else equally hard in the opposite direction.
FORCES ON UNDERWATER VEHICLES
Drag
Weight
Buoyancy
Thrust
Lift
Estimating Thrust Requirement
Empirical Approach
Theoretical Approach
Viscous Drag
Pressure Drag.
Reynolds Number.
Cd : Drag Coeff.
A: Frontal Area
V: Relative Speed Through the Fluid
How to determine the drag coefficient for our ROV in SolidWorks ?
How to calculate the Surface Area using SolidWorks ?
Nearly finished vehicle.
The need for: Spring Scale, long string, a stopwatch, tape measure.
Access to a swimming pool where the vehicle can move straight for 15m.
The Process
Tricks
String length = 5m and pull horizontally.
Check if the spring scale measures horizontally accurately.
Give 5m before measuring speed.
Measure 2 above and 2 below the max. speed.
Propulsion Options
Propellers
Impeller-driven water jets
Reciprocating fish-like tails
Magnetohydrodynamic drives (MHD)
Gas generator
Magnus effect drives
Electric Thrusters
Electric Motors
Propellers
DC Brushed Motors
The least expensive.
The most common
DC Brushless Motors
Better performance
Need complex driver circuit
More expensive
Gear Motors
Gear box attached to motor shaft
Increase the torque without drawing more current
Reduce speed
Stepper Motors
Used for precision control of motor shaft rotation angle
Servo Motors
Precise angle control
Commonly, 0-180 deg. only
Has built-in circuit
The Parts
Variations
Types
Diameter
Pitch
Material
Handedness
Method to attach prop hub to shaft
Ducted props or not
How it works ..
Screws !!
Foils
Forward and backward thrust
Impeller-driven Water Jets
Kort Nozzles
Hubless Propellers
1. Choosing a Motor
2.Waterproofing a Motor
3.Attaching the Thruster
Provide strong support that won't flex or vibrate when the thruster is at full power underwater.
Minimize blockage of the thruster water jet in forward and backward.
Minimize drag.
Resistant to snapping.
Allow some adjustment of position and angle.
Allow for easy removal and reattachment of the thruster.
4.Selecting and Attaching a Propeller
5.Matching Propeller to Motor
Too large or too small !
Optimal DC Motor Speed.
Questions
How powerful (required force) does each motor has to be ?
How much voltage and current can you realistically and safely provide for them ?
How much can you afford to pay (especially since you'll need 3 of 4 motors) ?
Do you want to use brushed or brushless motors ?
Do you need a gear motor ?
How will you waterproof the motors ?
How will you attach the propeller to the motor shaft ?
How important is it to be able to replace the motors quickly and easily ?
Electrical Specifications
Under optimal conditions: 1N per 3W
Usually, homemade thrusters produce 1/2 or 1/3 this value
Look for 12V motors which produce 4N for 3A
Commercial thruster produce from 44.4N - 133.4N
Brushed VS Brushless
Gearing and Gear Motors
Gear motors as thrusters
Motors are more efficient when spinning at high speed not high torque.
Gear Ratio
Planetary Gear Motors
Trolling Motors
Bilge Pump Motor
Small Commercial ROV Thruster
Cordless Drill DC Motor
Cordless Drill Gearbox
RC Cars Brushless Motor
Sealing The Shaft !
Oil Compensation
Brushed VS Brushless Again !
O-Ring
U-Cup
Oil Seal
Mechanical Sealing
Magnetic Coupler
Connecting Prop to Motor Shaft
Ducted Prop
Can be: Section of PVC Pipe.
Work best when ?
Little space between the blade tip and the duct.
Must be exactly centered.
How to connect the duct to the thruster or to the ROV ?
The wrong diameter -_-
Couplings
Alignment
Flexible Couplers
Connection
Tight friction fit
Cotter Pin
Bolt
Thruster Test Jig
Interpreting the Results
Record
Voltage
,
Current
,
Thrust
and calculate the
Power
in a table.
For each propeller, do the test for 4 different voltages (i.e 3, 6, 9 and 12V).
Plot the results.
Notice the thrust at lower powers.
Notice the power at the maximum thrust and if it exceeds the rated power of the motor.
Choose the propeller that gives you the maximum thrust within the range of the motor's rated power.
Example.
THE DANGERS !!
Splattering water can make the wires get wet and make shorts.
Wear Eye Protection because the mechanical connections are temporary and can fly at the face !
Too big props can get the motor extremely hot and burn wires, skin and make fire.
If you think you find good combination, test it in water for at least 5 minutes to make sure nothing overheats.
Design a test jig mechanism that can be used to measure both pushes and pulls at low cost and with easy construction steps. The best design will be fabricated and utilized.
Three-Thruster Arrangement
Four-Thruster Arrangement
More Elaborate Thruster Arrangement
Checklist
Obstacles to flow
Streamlining
Longer shafts
Unwanted Turning Moments
Thrust Asymmetry
Prop Torque
Counter rotating props
Power Consumption
Thruster Spacing for fast turns.
Impact of thrusters on circuits
Vertical Thruster
Horizontal Thrusters
Vectored Thrusters