NIGHTSIGHT Pan and Tilt: Design Analysis

After designing and prototyping several alternatives, we began the reflection phase of our design process. During this phase, each of the alternatives is weighted against the others and more rigorous testing is conducted. Upon conclusion of this phase, we are able to resolve clearly which design alternative is superior. Our confidence that the selected mechanism is the best design for our sponsor and the user is based largely on the strength of a well-defined and methodical design process.

8.1 Weighted Objective Analysis

A weighted objective analysis was conducted in order to decide which design was optimal. Each of the four design options were weighted against several parameters. The mirror design was not considered because we could not pursue it as a feasible option. These objectives were chosen from the constraints and the functional, physical, external, and environmental requirements. The objectives were weighted on how important they were to the final design. For example, cost and durability were felt to be more important to the final design then weight.

Minimize Cost 22
Maximize Durability 13
Maximize Weatherproof 12
Maximize Aesthetics 11
Minimize Height 10
Maximize Stability 7
Optimize Pan Angular Velocity 7
Optimize Tilt Angular Velocity 6
Minimize Power Required 5
Maximize Accuracy 4
Minimize Weight 3

The designs were scored 0 to 10 on how well they met a requirement or minimized a constraint. This eleven point scale proved to be very helpful. Because of the granuality, finding an adjective that defined a given design's performance was easy.

Totally Useless Solution 0
Inadequate Solution 1
Very Poor Solution 2
Poor Solution 3
Tolerable Solution 4
Adequate Solution 5
Satisfactory Solution 6
Good Solution 7
Very Good Solution 8
Excellent Solution 9
Ideal Solution 10

TABLE 16. Weighted Objective Analysis

The analysis shows that the Generic Design received the highest number of raw points but the Cradle Design received the highest weighted score. This indicates that the cradle design performs consistently better for the objectives that we consider to be the most important. The analysis was not the only tool we used to decide which design to pursue. We also used the data to decide which designs to prototype and test further. A design was selected only after conducting more rigorous tests.

8.2 Testing

Having used only rapid prototyping techniques to test each prototype during the Design Phase, we built a more realistic and robust prototype during the reflection phase. The goal for this prototype is that it meet the functional and physical requirements and be rugged enough to mount on a vehicle for testing.

TABLE 17.  Prototype Description
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Specification    Quantity     How                                      
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Field of Regard               The pan and tilt are driven by DC        
Pan:             Infinite     motors. Power is transmitted to the      
Tilt:            +/-22.5      tilt motor through a slip ring,          
                              allowing infinite rotation.              

Angular Voltaic               Torque from the pan motor is             
Pan:             48 deg/sec   transmitted to the pan base ring         
Tilt:            6 deg/sec    through a gear train that rotates the    
                              camera at 8 rpm. The tilt motor          
                              torque is also transmitted through       
                              a gear train that tilts the sight at 1   
                              rpm. The tilt gear train includes a      
                              worm gear that makes the tilt        
                              non-back-drivable.                        

Weight           8 lbs        The prototype is two pounds too          
                              heavy because of some of the             
                              materials used, like wood.               

Input Power      12 Volts DC  The unit receives power through          
                              an AC/DC converter that supplies         
                              12 volts at 1 amp.                       

Size                          The height and tilt base are within      
Height:          3.75 inches  requirements. The pan base               
Pan Base:        11 X 14 in   exceeds the 8 inches allowed             
Tilt Base:       11 inches    because we used an existing "cradle"         
                              to facilitate our prototyping       

Control                       Two toggles were used to manipulate           
                              the pan and tilt. The tilt was      
                              constrained between two limit            
                              switches which insured the motors        
                              turned off at the limit.The speed        
                              was constant, as opposed to the          
                              variable speed desired in the final      
                              product.                                 
                                                                       
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Our goal for the test was to determine how the prototype performed while the vehicle is in motion, patrolling, and when the vehicle is stationary, surveillance. In order to accomplish this, we mounted the prototype to the roof of a car, attached a VCR camera to it, and ran the cables to a monitor in the vehicle.

FIGURE 18. Prototype Test Run

Fully operational Cradle prototype seen mounted to the roof of a car

The lessons learned from this test were as follows:

TABLE 18.  Prototype Lessons Learned
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Observation                             Implication on Design                   
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Placement of interface: It is           The interface should be located to      
difficultto drive and manipulate        allow the user to see it and the road   
the Site simultaneously.                easily.                                 

Quiet drive is .                        Look at similar mechanism for Tilt.      
definitely an asset                     Look at using a cog belt for the pan.    
                                                                         
Given that the focal length             Interface and tilt face will not be     
of the Site is the same as the          weighted as heavily during future       
VCR camera, the user will not           analysis.                               
need to use the tilt mode much.                                                             

Definite need for feedback. User        This increases the need for a feed      
will look solely into TV, and will      back mechanism.                         
have no idea where the criminal                                                 
actually is on the ground.                                                      

Jerky motions while trying              Real need for variable speed during         
to track moving objects.                panning.                            

Very disorienting for the driver.       Graphic/symbolic feedback super         
                                        imposed on viewing screen as to         
                                        camera position. Position resetting     
                                        every time it passes a zero position    
                                        is a good idea.                         
                                                                                
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FIGURE 19. Prototype Drive Test

The Cradle Pan and Tilt was mounted to the roof a car with a video camcorder attached to the tilting platform. A monitor was connected to the live video feed to monitor the performance of the design. (This is a 1.5 megabyte MPEG file)

8.3 Future Areas of Emphasis

During the final quarter, we will optimize the design of the Cradle. The disadvantages will be minimized or eliminated. The mechanism will be weatherproofed either through the use of a rubber boot, a more sealable sliding mechanism, or another method to be determined. Friction surfaces will be minimized, components will be selected and other problems will be addressed. Finally, we will select the type of motors we will use. The two major options are Servos and Steppers.

FIGURE 20. Servo Motor System

FIGURE 21. Stepper Motor System