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In this project some form of area scanner was needed with control over the azimuth and elevation of the transmitter was required. Galvanometers were initially researched however it was decided that these would be too expensive to implement in this project. A possible solution could be to mount mirrors on the end of the servo shaft in a similar arrangement to a scanning galvanometer. This idea was not researched further, as it was chosen to develop a pan and tilt head.
Commercial pan and tilt heads were researched however it was found that generally these were either all too big or too expensive, it was therefore decided to build a custom one for this project. Designs for pan and tilt units are easy to come by on the internet as there is a wealth of tutorials and guides from people using them to control webcams remotely[xviii].
The most important factors when building a pan & tilt unit, are the size, the weight and the travel. The unit in this project is required to be relatively small and easy to fabricate using sheet metal and basic fittings. The weights of the moving parts are also important as they dictate factors such as speed and response time. The unit in this project is required to be reasonably fast. The travel is dictated by the motors chosen and determines how far in each direction the unit can ‘see’.
Pan and tilt heads require two motors, one to drive the pan unit and another to drive the tilt unit. DC motors are not suitable as they rotate continuously. Stepper motors were initially researched however the implementation of steppers is harder due to the drive electronics required. Servo motors were therefore chosen to drive the pan and tilt head due to the low cost and easy availability within the department.
The chosen design had a servo motor mounted within the base for driving the pan unit. The servo for driving the tilt unit was mounted sideways within the pan unit.
Figure 18 - Pan/Tilt Unit design
Servos were chosen as they provide a great method of positional control since they use a closed loop feedback system. What was needed was a method of moving the pan and tilt unit to a specific position and keeping it there.
The servos chosen were two Futuba S148 servos, which were easy to source from the electrical engineering departments stores. These offer a travel of 90˚, not as much as some which can offer travel of up to. These servos are analogue feedback controlled servos, meaning they are less accurate than digital servos. The S148 also uses a nylon gearbox[xix] which is a plastic. There is a small chance that eventually the plastic gears could wear out through usage or start ‘slipping’.
Another type of servo investigated was the digital servo which is much more accurate than the analogue counterparts[xx]. Digital servos have a small microcontroller inside, instead of an analogue feedback circuit. It was also discovered it is possible to obtain servos which have metal gears, meaning the chance of wear is much less.
Table 1 - Pan/Tilt Specification
Note these specifications exclude a 10mm block of MDF mounted on the bottom, used to attach a tripod mount to. They also exclude anything mounted on the unit.
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