3.10. Digitisers.   

A digitiser accurately measures the position of a point within a 2-D area and encodes the measurement into x,y coordinates. It is normally a flat square surface 1 foot square ( and upwards to 4ft x 5ft). The positioning device is either a pen like object or a flat cursor box or mouse with a cross hair. The cursor box will have one or more buttons for selection. By moving the cursor box or mouse to the appropriate places, the cursor on the graphics screen can be made to follow.

 e.g. Picture could be put on the flat surface and its shape traced out

-> information stored in computer and graphed on screen.

(See Figure 3.14.)

3.10.1. Electro magnetic   

Flat surface is electro magnetically active and pen or cursor box detects where it is via timing signals.

     


    
            Fig. 3.11 : An Electro Magnetic digitiser

 May be 10,000 * 10,000 lines (criss crossed) on digitising area.


If 10" x 10"

      10

->  -----  =  0.001" resolution

    10000


Simplistic view:  knows which pulse in x direction and which in y

direction -> x,y coordinate.


Picks up pulse and uses that to work out which x,y lines were pulsed.


 It would not be practical to have 10,000 x 10,000 crossed lines since

this would be 20,000 wires in total!

A microprocessor is used to determine the coordinates from information

supplied from a fewer number of crossed lines.

3.10.2. Touch Sensitive   

Flat surface is sensitive and pen or finger is detected.

     


    
             Fig. 3.12 : A Touch sensitive digitiser

 Use sharp pen or stylus to pin point x, y coordinate wanted.

Pressure causes two resistive surfaces to meet -> electrical current

flow depends on resistance -> depends  on x,y coordinate.

3.10.3. Sonic   

Flat surface is acoustically sensitive.

     


    
                  Fig. 3.13 : A Sonic digitiser

:   Sound impulses caused by pen are picked up by two

          microphones to determine x,y coordinate.

    Note:  No hands in way.

     


    
            Fig. 3.14 : Sublime example of Digitising

3.10.4. Light Detector Digitisers   

Low resolution (finger size) grid.

     


    
              Fig. 3.15 : Light Detector Digitiser

Matrix of light source and light detectors.

Used mainly for VDU for pointing operations.

     Place finger on screen -> No light -> position in terms

    of rows and columns.

    Very low resolution.