Pen qualification and verification in a graphics plotter

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Generally speaking, the invention is incorporated in a pen plotter comprising a pen for producing markings such as graphical representations on a medium, for example on a sheet of paper.
The pen is held on a main carriage by a z-axis carriage which can be lowered such that the tip of the pen contacts the medium in order to produce markings thereon.
The pen comprises an ink reservoir containing ink of a certain color.
The pen is held on the z-axis carriage in a way so that it can easily be replaced by another pen, for example if a different color for the graphical representation is desired or if the ink reservoir in the pen is empty.
The replacement of the pen can be done manually by a user, but it is preferred to perform the replacement automatically undercomputer control.
Seeveral replacement pens and additional pens of different colors can be arranged in a pen carousel (not shown) from which they can be transported to the z-axis carriage and inserted there.
For producing two-dimensional plots on a sheet of paper, the paper is moved in a first direction (x-direction) and the main carriage is moved in a second direction (y-direction) orthogonal thereto.
The driving of the paper can be accomplished by means of a grit wheel and pinch wheel assembly between which the paper is moved.
By appropriate movement of the paper (either in the positive or negative X-direction) and coordinated related movement of the main carriage (either in the positive or negative Y-direction), any desired graphical representation can be made on the paper sheet.
Of course, the invention is not limited to any particular type of plotter, but can be utilized in any plotter configuration where line vectors are drawn on media by pens.
For example, another type of plotter which may utilized the invention holds the paper stationary while the main carriage is movable in the X- and Y-directions so that the pen can be, placed on any desired point of a media sheet.
A sensing system intermittently monitors the quality of the pen on the media by scanning across a point on a line, with the point overlying a white reference strip in the platen and illuminated by light emitted from a light source such as an LED emitting a beam of green light