Team SAGE

Mechanical System

Design Goal:

The primary motivation behind this project was a building a devise that could slowly pan through a field of view, facilitating the capture of time laps or panoramic photographs. To accomplish this we set out to design a robust camera mount with a large degree of freedom. Key requirements were precise pointing abilities, sufficient size to accommodate larger SLR and DV cameras, a wide range of rotational freedom, and robust design that could securely and stability hold heavy cameras and stand up to extensive use.

Materials Selection:

The pan tilt mechanism is controlled by two HS-785 servos. These particular servos were used because of their high torque output and 1296° of rotational freedom. Both of these servos were connected to the relevant axis of rotation by a chain and sprocket drive. This particular combination was selected because the servos provide both the high torque necessary to deal with a heavy camera and the degree of freedom necessary to provide sufficient coverage. The chain and sprocket were used because of the freedom of servo placement they afford us in the design process.

The camera mount was built out of aluminum bar stock and standard fasteners. Aluminum was an effective choice because it was readily available, easily machined, and most importantly stiff and lightweight. Standard fasteners and a bolt together design were utilized because they provided a simple strong connection and also allowed for easy disassembly and modification.

Design:

Design was carried out using SolidWorks CAD software. A complete 3D model, pictured below, and set of detailed drawings available here were completed prior any manufacturing. By modeling our system prior to building it we avoided any serious design oversights that would have required remaking major components. Drawings were also completed to facilitate the actual manufacturing process.

Camera Pan Tilt Assembly

Key features in this design include the sliding servo brackets, hollow main pivot, removable shoe, and compatibility with existing photography equipment. The sliding servo brackets allowed us to quickly tension or remove the drive chains and made precise servo placement or chain length unnecessary. The hollow main pivot prove to be particularly important as a conduit for routing the servo cables from the servos to the PIC controller. Any cabling outside of the base would have suffered due to the cables wrapping around the base when it rotated but this configuration allowed the cables to twist and didn't hamper rotation. The removable shoe, which dropped in and was secured with two twist tabs, allowed for quick camera changes if each camera was mounted on a compatible shoe. Lastly, the base and shoe were both designed to bolt onto existing tripods and cameras respectively, allowing for use with existing equipment.

Fabrication:

The majority of the mechanism was fabricated using a two axis milling machine. This allowed for precise hole placement that guaranteed good alignment between individual components. While not necessary, a water jet cutter and CNC mill were also used.

Results:

Camera Pan Tilt Assembly

After completing fabrication, a few design failures were evident. Primary among these was the slop in both the drive train and main pivot. The pivot suffered from slop due to the cheap bearing we used. To correct this problem we plan on revising the design to incorporate two stacked bearings of higher quality. Also, the chain and sprocket suffered from the slop due to a lose chain and inherent imprecise connecting a chain and sprocket provide. Lastly, while the selected servos preformed admirably providing a extremely wide range of motion, because their signal resolution was the same as traditional servos but they were capable of 7 times as many rotations, the resolution of the position sensor was lower than desirable resulting in somewhat abrupt changes in position. To address these two failures in the drive train, we plan on replacing some of the drive train with gears instead of the chain and sprocket as well as directly mounting position sensors to the relevant axels. Directly measuring the angular position eliminates the loss of resolution due to slop in the drive train directly increasing the pointing accuracy of our instrument.