HP 220 - Manual (Page 41)

5Ć9 Functional Overview C3187Ć90000
Stretching the Media
Overdrive
The overdrive roller stretches the media slightly as it is moved through the plotter. It thus
reduces bubbling and media cockle caused by media expansion due to the absorption of
waterĆbased ink into the media. Even though the geometry chosen for the plotters minimizes
the occurrence ofthis phenomena, it still could occur on very dense plots.
Operation
(Part illustrations ' page 10Ć25)
The overdrive roller is geared to the mediaĆdrive system and has a surface speed 10% greater
than that ofthe drive roller. The lower star wheels on the bail provide a normal force
between the media and the overdrive roller. The resulting friction force creates media
tension in the area between the upper pinchĆwheels and the overdrive roller (the print zone).
The overdrive roller is segmented into small rollers made ofpolyurethane. A clutch
(overdrive gear assembly) is geared to the speedĆratio gear (overdrive gear cluster) and drives
the overdrive roller in the forward direction only.
Moving the Carriage
CarriageĆDrive Mechanics
The Carriage (YĆaxis) drive consists ofa DC motor driving the carriage by means ofa dual,
kevlar belt system incorporating three pulleys and two tensioner assemblies.
Operation
(Part illustrations ' pages 10Ć19 and 10Ć21)
1 A gear pinion, pressed onto the carriageĆmotor shaft, drives a small belt tensioned with a
selfĆcompensating, springĆloaded idler.
2 The small belt drives a double pulley which transfers the motor drive to the main belt.
3 The main belt is attached to the carriage via frictionĆfit belt clamps. It is tensioned using
a selfĆcompensating, springĆloaded tensioner wedge and main idler. (Illustration ' page
10Ć15.) The belt moves the carriage along slider rods, which are part ofthe chassis
assembly. The carriage bearing system consists ofthree bushings.
4 Positional feedback is obtained using a 150ĆlineĆperĆinch linear encoder strip that is
spring loaded and passes through the carriage where the small slots are read by an
optical encoder. The feedback is used by the carriageĆmotor servoĆcontrol system.