Epson LQ-2550 Technical Manual page 56

Figure 2-7 shows the operation of the paper thichness detection mechanism.
When paper is not loaded, point A of the PT sensor lever pushes the platen using shaft B as a fulcrum.
At this time, point C of the PT sensor lever places pressure on the TCR (Touch Control Sensor) in the
PT sensor to indicate the thickness. Ta is used for this TCR reading. After paper is loaded, point A of
the paper sensor lever holds the paper against the platen. Tb is used for this TCR reading.
Platen
A
(a) When no paper is loaded:
The control circuit uses Tb - Ta as the paper thickness, and controls the platen gap mechanism so
as to provide the proper platen gap for the paper.
The control circuit adjusts printhead trigger pulses applied to the printhead coils according to the paper
width which is divided into three grades {Refer to Section 2.3.5.).
PT Sensor Lever
Figure 2-7. Paper Thickness Detection Operation
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(b) When paper is loaded:
2-9
Tb