philba said:
I'm sorry nigel, you are playing with words. you could build an absolute position scheme but it would be complex, expensive and, as dkn says, have resolution drawbacks. using quad feedback in conjunction with a starting point can give you absolute positioning. how do you think CNC stuff works? Yes, it is "relative" but from a starting point. With an extra line for a reference sensor, you can gain absolute positioning at a very reasonable cost.
"in conjunction with a starting point" is by definition relative positioning, there's no 'playing with words' involved - it's either relative or absolute, you can't say a relative postion is absolute because you specify a starting point.
The CNC machine you mention uses relative positioning, and will presumably move itself to a zero point before it can move anywhere else - so it moves 'relative' to it's zero point. Incidently, floppy disk drives used this same technique.
For examples of positioning:
Relative - move 25 degrees left from where you are now.
Absolute - move 25 degrees east of south.
The CNC machine can do either (to some extent), but to simulate an absolute position it will have to move to it's reference point first, or be 100% certain of where it currently is, regardless of power cuts or anything else.
However, likewise, an absolute positioning system like a servo will need to maintain a record of where it is, in order to move relative to it's current position - you can't tell it to go 25 degrees left, you need to work out where it is now, add the 25 degrees left to it, then tell it to go to the resulting position.
Really we need to know what the application is, and what it requires? - presumably as a servo was originally specified?, absolute positioning is required?.