Paper feed sensing mechanism for printer

Both the operative position of a movable platen and the presence of paper to be advanced past the platen in a line at a time printer are affirmatively sensed by a single mechanism. A cantilevered spring arm extending from a fixed mount includes a terminal switch sensor at its free end in proximity to the paper. At an intermediate region along its length the spring arm traverses adjacent to but spaced apart from the platen, and an adjustable set screw on the arm is disposed for contact with the platen. When the platen is in the operative position, behind the print hammers, and the printer is loaded with paper, the spring arm biases the actuator into contact with the paper, transferring the switch contact to a given position and thereby indicating that both needed prerequisites for printer operation are satisfied. If the platen is disengaged from its normal position, as for paper loading, the spring arm is concurrently moved away from the paper and the switch actuator freed to transfer the switch to its opposite contact. In the absence of paper the switch actuator is also freed regardless of platen position. The length of travel of the spring arm is substantially greater than that of the switch actuator and the spring arm has a greater spring force than the switch actuator arm, insuring both forceful operation of the switch despite variations in paper thickness, and easy vernier adjustment of the trip point of the switch.

BACKGROUND OF THE INVENTION 
A typical data processing printer operates on a line-at-a-time basis, using 
forceful impacting of printing elements on multiple paper sheets. The 
printing elements may be front mounted, as with a dot matrix printer, in 
which case a platen or bed mechanism of some kind is used to provide a 
base against which the printing elements may react. The platen is 
advantageously movable away from the paper, for freedom in loading paper 
and for adjustment for different paper thicknesses. Such a mechanism is 
shown in recently issued U.S. Pat. No. 3,941,051 entitled "Printer System" 
and assigned to the assignee of the present invention. A variety of other 
printer mechanisms in which different structures are separable to permit 
paper insertion have also been employed. In general it is considered 
highly desirable to affirmatively sense both the operative position of the 
platen or equivalent structure, and the presence of paper in the system. 
Operation without paper would damage the printing elements and platen, 
while operation with paper in but the platen withdrawn would result in 
loss of data. The platen must, however, be open (spaced apart from the 
paper path) for easy paper loading. 
The "paper out" condition is generally sensed in such printers by the 
actuator arm of a small electromechanical switch, such as the widely used 
"Microswitch" (a trademark of Honeywell, Inc.). In most prior art 
structures the actuator arm of the switch is easily depressed and presents 
no barrier to paper loading. However, once the paper is inserted it cannot 
be adjusted sideways, and if improperly aligned can only be withdrawn and 
reinserted. The switch must also be unresponsive to normally encountered 
variations in the paper, such as are introduced by bulges, fold lines, and 
perforations in fan fold paper. These variations, however, can exceed in 
distance the short operating travel of the switch actuator. Thus with 
existing systems the actuator must be precisely positioned and accurately 
set, but can still provide erroneous paper out indications because of 
paper variations. The platen open status normally requires a second 
sensor, which may be another switch or may be a contact arm spaced to 
engage the withdrawn platen. 
SUMMARY OF THE INVENTION 
A sensing mechanism in accordance with the invention disposes a 
cantilevered spring from a fixed base to have a free end adjacent the 
printing medium, and to be responsive at an intermediate region to the 
position of a movable platen. Substantial movement of the platen away from 
the paper automatically disengages a switch at the free end of the spring 
from the paper, causing the switch to transfer contacts even if paper is 
present. If paper is out, the same switch also transfers contacts 
regardless of the position of the platen. 
In a specific example of a sensing system in accordance with the invention 
the ratio between the displacement of the end of the cantilevered leaf 
spring and the travel of the switch actuator is made significantly large. 
The spring force of the leaf spring exceeds that of the actuator arm and 
excessive variations in paper thickness or characteristics are thus 
compensated for by the spring action of the cantilever mechanism. A large 
effective throw is thus provided for the switch, making paper sensing 
uncritical, while a vernier adjustment of the spring arm relative to the 
platen can be made to insure precise indication of excessive displacement 
of the platen relative to the paper. The paper bridges a paper guide 
gridwork which provides a non-frictional reactive force against the 
actuator arm, but through which the actuator arm can move when no paper is 
present.

DETAILED DESCRIPTION OF THE INVENTION 
Only those parts of a printer system that pertain to a sensing mechanism in 
accordance with the invention are shown in the simplified fragmentary view 
of FIG. 1, inasmuch as reference may be made to the above-identified U.S. 
Pat. No. 3,941,051 for further details. It should be expressly understood, 
however, that the present invention may be applicable to any such systems 
in which a movable impact member is disposed adjacent the printing medium 
and both paper out and platen out (or its equivalent) conditions are to be 
detected. 
In the system, the printing medium comprises one to a plurality of paper 
webs 10, the number being variable at the user's option. The paper 10 
moves upwardly along a predetermined path from a supply (not shown) to a 
takeup or hopper (not shown) and in doing so passes a printing position or 
line along which (in this example) are disposed a number of print hammers 
12 on the front side of the paper 10. In the above-identified patent the 
lateral reciprocation of the bank of hammers 12 and the manner in which 
the hammers are actuated are fully described, so it suffices here to note 
that characters, symbols or plots are printed in dot matrix patterns and 
that other printers can provide like functions. Typically, an inked ribbon 
(omitted for clarity) is fed between the print hammers 12 and the first or 
original paper 10, and the remainder of the paper copies comprise pressure 
responsive sheets, such as carbon paper or paper with micro-encapsulated 
ink. The paper 10 may be of the continuous fan fold type, and in any event 
can be subject to considerable variation in thickness, such as at 
wrinkles, fold lines and perforations. 
The print hammers 12 impact ultimately against an adjustable platen 14 
positioned on the back side of the paper 10 adjacent the paper path. The 
platen 14 is advantageously a hard-surfaced cylindrical member mounted 
parallel to the printing line on an eccentric shaft 16 and movable a minor 
amount toward or away from the paper path by pivoting of a control handle 
18. Minor adjustment within a given range of control handle positions 
permits variation of platen 14 position for paper thickness; a further 
displacement (effected by a further pivoting of the control handle 18) is 
also used in which the platen 14 is shifted back away from the paper path 
to permit paper loading. The further displacement is comparatively major 
by comparison to paper thickness adjustments and creates a condition in 
which there is loss of data if printing is attempted. 
The paper 10 is advanced past the printing position by conventional paper 
tractors 22, and held firm thereat by a paper ironer 24 comprising a thin 
plate on the front side exerting uniform drag to keep the paper 10 in 
tension and also to iron out air bubbles and imperfections by urging the 
paper 10 against the platen 14. Below the platen 14 and on both sides of 
the paper 10, open gridworks of guide rods 26, 27 forming diverging entry 
and exit portions provide non-frictional planar support and guiding of the 
paper 10. The above-defined structure, apart from the guide rods 26, 27 is 
described in the previously mentioned patent. 
A sensing mechanism 30 for monitoring both paper out and platen out 
conditions is mounted on a fixed base 31 on the back side of the paper 10. 
Typically, the sensing mechanism 30 is disposed adjacent one margin of the 
printer (usually the left margin) in order to print narrow forms inserted 
with an edge aligned with the margin. A cantilevered leaf spring 32 
coupled to the base 31 extends past and adjacent to the platen 14, at an 
angle approximately normal to its direction of displacement relative to 
the paper. A switch bracket 34 is coupled to the leaf spring 32 at or 
adjacent to its free end, and a small electromechanical switch 36 (such as 
a Microswitch) is coupled to the bracket in a position such that the 
extending actuator arm 38 of the switch 36 intercepts the paper path when 
the platen 14 is in the operative position. Although the switch 36 may be 
said to be "opened" and "closed", it is advantageous to use a double throw 
type of switch to complete a circuit in either position of actuation, so 
that the switch functions to effect contact transfer when the actuator arm 
is moved adequately in either direction. A roller contact 40 at the 
terminus of the actuator arm 38 bears against the paper 10 in one of the 
openings of the gridwork formed by the paper guide rods 26, 27. 
The extent of travel of the actuator arm 38 is inherently limited, being 
dependent upon the chosen switch type in any event but here approximately 
0.025 inch at the contact roller 40, and roughly half that at the interior 
bearing point 41 which the actuator arm 38 depresses to complete the 
internal circuit. At an intermediate region along the length of the 
cantilevered leaf spring, however, a vernier set screw 44 is threaded into 
the spring 32 along an axis approximately axial with the direction of 
movement of the platen 14 relative to the paper 10. A follower tip 45 on 
the end of the set screw 44 engages the platen surface when the platen is 
in the out position, and a locknut 46 on the set screw provides locking in 
any axial position. For convenience of adjustment, the front end of the 
set screw 44 may include means (not shown in the Figures.) such as lands 
or a fixed head for axial variation of set screw position. 
In the operation of the sensing mechanism 30, assume that the platen 14 is 
in the out position, permitting free insertion of the paper 10. The platen 
14 thus urges the set screw 44 and the leaf spring 32 back away from the 
paper path, thereby also removing the contact roller 40 on the switch 
actuator arm 38 from the path of the paper. The greater distance of the 
free end and switch 36 from the pivot point, in comparison to the spacing 
of the platen 14 from the pivot point, multiples the displacement at the 
free end in comparison to changes in set screw 44 and platen 14 position. 
Although variations in paper thickness for which minor platen adjustments 
may be made are typically of the order of 0.015 inch, the platen 14 is 
displaced much more (e.g. 0.500 inch) for loading paper. With the paper 10 
in the operative position, therefore, as shown in FIG. 3, the vernier set 
screw 44 may be adjusted so as to permit the switch 36 to transfer 
contacts reliably in response to platen 14 position. This adjustment is 
made by threading the screw 44 so that the switch 36 becomes open when the 
platen 14 is opened beyond the point where proper printing can occur. 
With the platen 14 open for paper loading, as seen in FIG. 2, the switch 36 
is responsively displaced a correspondingly greater amount from the paper 
path, minimizing chances of interference by or damage to the actuator arm 
38. The paper 10 may also be moved freely sideways once inserted. No 
adjustments are needed for the paper out condition (position of the 
actuator arm shown in phantom in FIG. 3), because of the dual action of 
the cantilevered spring 32 and the actuator arm 38. With both paper loaded 
and platen closed, both the travel of the switch actuator and the spring 
deflection act to press the actuator into the paper, increasing the 
effective actuator throw to about 0.500 inch instead of actuator travel 
alone of the order of 0.025 inch or less. The cantilevered spring 32 is 
selected to have a higher spring force than the switch actuator arm 38, so 
that for all variations in paper thickness the actuator is fully depressed 
against its stop and the variations are absorbed by the large travel 
available in the long spring 32. Thus there is affirmative sensing of both 
the platen out and paper out conditions with a single switch, without 
interference with paper loading. When no paper 10 is present but the 
platen 14 is in operative position, as shown in FIG. 3 in phantom, the 
cantilevered spring 32 straightens, inserting the switch 36 and actuator 
arm 38 some distance through the guide rods 26, 27 networks, as also seen 
in FIG. 4. This frees the actuator arm 38 to effect contact transfer 
within the switch 36, indicating the paper out status. 
Although various forms and alternatives have been described, it will be 
appreciated that the invention encompasses all modifications and 
variations falling within the scope of the appended claims.