Which of the following statements regarding an electronic theodolite fitted with a graduated circle encoded in an incremental system is INCORRECT?
- AOne of the photodiodes is fixed and the other moves with the telescope.
- BTwo photodiodes are placed perpendicular to each other over the graduated circle.
- CThe graduated circle is fixed and it does not moveCorrect
- DThe graduated circle is made of glass.
Solution
Concept
In the incremental system, the glass graduated circle rotates with the telescope, and photodiodes read the graduations as they pass.
Step-by-step solution
1. In the incremental system, the circle is made of glass with fine radial graduations, so D is correct. 2. Light from an LED passes through the graduations and falls on photodiodes. 3. When the telescope turns, the circle moves, and the change in light intensity is counted to get the angle. 4. So the statement that the circle is fixed and does not move is incorrect.
Explanation
The electronic theodolite counts graduations as the glass circle moves past the photodiodes. If the circle did not move, no change in signal would occur and no angle could be measured. So statement C is the incorrect one.
Why the other options are wrong
A: One photodiode is fixed and one moves with the telescope; this is how the relative movement is sensed. B: The two photodiodes are placed in a set arrangement over the circle to sense the reading and the direction of rotation. D: The circle is made of glass so light can pass through its graduations.
Shortcut
Incremental system = counting increments, so the circle must move.
Common mistake
Mixing up the incremental system with the absolute (coded) system.
Correct answer: C) The graduated circle is fixed and it does not move