Printhead-To-Paper Distance Control (Pdc) Mechanism; Mechanism Design - Digital Equipment LB12 Field Support Manual

2.1.6

Printhead-to-paper distance control (PDC) mechanism

Mechanism design

2-12
A wire spring, fitted under the carriage, pushes the printhead towards the print bar. The
PDC stepper motor, with its cam wheel, can pull the printhead away from the print bar to a
number of defined positions by means of the tie rod connected to the cam wheel.
A sensor arm with a nose wheel is fitted to the front of the printhead. The slide shown in the
figure decides how far the nose wheel can be pushed in. When the slide is in its left
position, called the document position , the nose wheel can extend 0.3—1.2mm in front of
the nose. With the slide in the correct position, called the book position , the nose wheel can
extend 0.5—1.2mm. A sensor indicates the position of the nose wheel in relation to the
printhead nose. The sensor comprises a permanent magnet, fitted to the end of the sensor
arm, and a Hall element. The output voltage from this Hall element indirectly controls the
turning of the PDC motor and hence the position of the cam wheel and the printhead. The
sensor is called the distance sensor.
Figure 2-15 Printhead-to-paper distance mechanism in document position
LB12/LB15 Field Support Manual 9710