SPECIFICATION: AUTOMATED PROJECTION SCREEN
1. The screen is made from a single sheet of white plastic-coated cloth material 3 m wide (side-to-side) by 3 m long (top to bottom) and 3 mm thick. The density of the material is 1,500 kg/m3.
2. The top edge of the screen is attached to a cylindrical rod (the winding rod) which is rotated by a motor to wind the screen up and down. The diameter of the rod is 1.6 cm. When fully wound the screen is wrapped around the rod. A second cylindrical rod is glued along the bottom edge of the screen to act as a tension weight to keep the screen from creasing. The mass of the rod used to tension the screen is 5 kg.
3. You may ignore the design of the mechanical components used to mount the motor and the winding rod to the wall. You may assume that the mechanical loss in the bearings supporting the winding rod and the moment of inertia of the rod are negligible.
4. The screen is controlled using two push-buttons on a small control box located on the wall next to the screen at a height convenient for humans to reach.
5. The buttons on the control box are mounted one above the other, the upper- most button is labelled UP and the lower-most button is labelled DOWN
6. When the DOWN button is pressed, if the screen is not already fully down, it is unwound by the motor until it reaches its fully-down position.
7. When the UP button is pressed, if the screen is not already fully up, it is wound by the motor until it reaches its fully-up position.
8. Neither the UP nor the DOWN button have any effect if pressed while the screen is moving.
9. The screen should change from its fully-up position to fully-down position, and from its fully down position to its fully up position in 20 seconds +/- 2 seconds.
10. If a power cut occurs while the screen is moving, when power is restored the screen will remain stationary and one of the buttons must be pressed to resume operation.
11. The motor stops automatically when the screen reaches either the fully up or fully down position
12. The design of the system should prioritise low cost and reliability.