Alternate method of sensing paper entry

A system and method detect the location of a workpiece as it passes from a feeding mechanism to a receiving pair of rollers. The system generally consists of a feeding mechanism having a pair of feeding rollers, a pair of receiving rollers, a motor control system for applying a fixed torque to the receiving rollers causing the receiving rollers to rotate, and a tachometer for monitoring the revolution speed of the receiving rollers. The motor control system for revolving the rollers consists of a motor, motor controller and a microprocessor to program the motor speed or torque. A decrease in the revolution speed of the receiving rollers indicates the workpiece has been received by the receiving rollers. An increase in the revolution speed of the receiving rollers subsequent to the decrease in speed indicates the workpiece has been released from the feeding mechanism.

FIELD OF THE INVENTION 
This invention relates in general to a system and method for detecting the 
location of a workpiece as it is passed from a feeding mechanism to 
receiving rollers, more specifically, to detecting the location of the 
workpiece by sensing the speed of the receiving rollers. 
BACKGROUND OF THE INVENTION 
The prior art method of sensing paper in a printer is to use mechanical 
flags in the paper path. Paper passing along the paper path causes the 
flags to move, triggering a signal. The signal indicates the presence of 
the paper. One drawback to the use of these flags occurs when the flags 
are used to monitor paper entering or exiting rollers. The mechanical 
flags must be located some distance away from the rollers so that neither 
the flags nor the rollers interfere with the other's operation. The flags 
therefore cannot be used to determine the precise time at which the paper 
enters or exits the rollers. 
An inversion process is an example of when a more accurate determination of 
this time is important. The inversion process is used to discharge a 
printed page with the printed side down. Discharging pages with the 
printed side down allows a sequence of pages to be printed and discharged 
into a stack with the first page of the sequence being the bottom page of 
the stack. 
Inverting a page must be performed quickly as another page may be following 
closely behind the first printed page. In order to take full advantage of 
the time allowed for inverting a page, it is desirable to start the 
inverting process as soon as possible. Providing a means for precisely 
detecting the exit time of a page from a roller allows the inverting 
process to begin as soon as the page leaves the rollers immediately 
preceding the inverter. 
Accordingly, given the foregoing background relating to sensing paper entry 
and exit with respect to rollers, an object of the present invention is to 
provide a system and method for detecting more precisely when a workpiece 
is received by or released from a pair of rollers. 
SUMMARY OF THE INVENTION 
According to principles of the present invention in a preferred embodiment, 
a system and method provide a simple, cost effective and timely method of 
detecting the location of a workpiece, e.g., paper sheets, with respect to 
rollers used to process the workpiece. A fixed torque is applied to at 
least one receiving roller of a pair of receiving rollers causing the 
receiving roller to rotate. The fixed torque is set so that the peripheral 
speed of the receiving roller, when the workpiece is not between the pair 
of receiving rollers, is greater than the linear speed of the workpiece. 
The linear speed of the workpiece is determined by the speed of the 
feeding mechanism. The rotation speed of the at least one receiving roller 
is monitored. As the slower moving workpiece is received between the pair 
of receiving rollers, the peripheral speed (and the rotational speed) of 
the at least one receiving roller is slowed to the linear speed of the 
workpiece. As the workpiece is released from the feeding mechanism, the 
rotational speed of the at least one receiving roller increases. Thus, 
changes in the rotational speed of the receiving roller indicate the 
location of a workpiece with respect to a feeding mechanism and a pair of 
receiving rollers. 
According to further principles of the present invention in a preferred 
embodiment, when no workpiece is between the pair of receiving rollers, 
the revolution speed, and consequently the peripheral speed, of the 
receiving rollers is fixed. The revolution speed of the receiving rollers 
is fixed such that the current required to maintain the fixed revolution 
speed without the workpiece is less than the current required to produce 
the fixed torque in the receiving rollers. As the workpiece is received by 
the receiving rollers, the current increases to maintain the fixed 
revolution speed. The current increases only up to the current required to 
produce the fixed torque. As the workpiece continues to be received by the 
workpiece, the peripheral speed of the receiving rollers decreases to the 
linear speed of the workpiece. As the workpiece is being released from the 
feeding mechanism, the decrease in resistance allows the revolution speed 
of the receiving rollers to increase. When the rollers again reach the 
revolution speed at which the receiving rollers are fixed with no 
workpiece, the current decreases to again maintain the fixed revolution 
speed. 
Other objects, advantages, and capabilities of the present invention will 
become more apparent as the description proceeds.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 represents a preferred embodiment of the present invention system 10 
as incorporated into a printer system. Dc motor 15 rotates receiving 
rollers 20 by applying a fixed torque to receiving rollers 20. 
Alternatively, only one of receiving rollers 20 is rotated by dc motor 15. 
Feeding rollers 25 are rotated at a constant speed by a separate drive 
motor (not shown). Paper 30 is fed from feeding rollers 25 to receiving 
rollers 20. Feeding rollers 25 are spaced from receiving rollers 20 so 
that paper 30 is received by receiving rollers 20 before paper 30 is 
released from feeding rollers 25. 
Alternatively, feeding rollers 25 are any feeding mechanism which will 
allow paper 30 to be received by receiving rollers 20 before paper 30 is 
released from the feeding mechanism. 
Tachometer means 35 monitors the revolution speed of dc motor 15. The 
revolution speed of receiving rollers 20 is determined from the revolution 
speed of dc motor 15. Alternatively, tachometer means 35 monitors the 
revolution speed of receiving rollers 20. The peripheral speed of 
receiving rollers 20 is determined from the size and revolution speed of 
receiving rollers 20. Tachometer means 35 is any means by which the 
revolution speed of dc motor 15 or receiving rollers 20 is determined. 
Examples of tachometer means 35 include a tachometer and back EMF 
measurement. 
In a preferred embodiment, dc motor 15 is a brushless dc motor. Brushless 
dc motors have a stationary armature and a rotating field structure. 
Permanent magnets provide magnetic flux for the field. Dc current to the 
armature is commutated with transistors rather than with the brushes and 
commutator bars of conventional dc motors. The transistors are located 
within dc motor controller 40. Armatures of dc brushless motors typically 
contain 2 to 6 coils, whereas conventional dc motor armatures have from 10 
to 50. Brushless motors have fewer coils because either two or four 
transistors are required to commutate each motor coil. This arrangement 
becomes increasingly costly and inefficient as the number of windings 
increases. 
The transistors controlling each winding of a dc brushless motor are turned 
on and off at specific rotor angles. The transistors provide current 
pulses to the armature windings that are similar to those provided by a 
commutator. The switching sequence is arranged to produce a rotating 
magnetic flux in the air gap that stays at a fixed angle to the flux 
produced by the permanent magnets on the rotor. Torque produced by the 
brushless dc motor is directly proportional to armature current, which in 
turn is controlled by the transistors in dc motor controller 40. 
In a preferred embodiment, the rotational speed of dc motor 15 or receiving 
rollers 20 is sensed by tachometer means 35 and transmitted back to 
microprocessor 45 via dc motor controller 40 at conductors 50 and 55. The 
torque of motor 15 is controlled by dc motor controller 40 which in turn 
is directed by microprocessor 45. Microprocessor 45 directs the torque by 
requesting a specific current from dc motor controller 40 to dc motor 15. 
Dc motor controller 40 controls the torque by providing a specific output 
power 60 to the rotating field of dc motor 15. 
In a further preferred embodiment of the present invention, when no 
workpiece is between the pair of receiving rollers, the revolution speed, 
and consequently the peripheral speed, of the receiving rollers is fixed. 
The revolution speed of the receiving rollers is fixed such that the 
current required to maintain the fixed revolution speed without the 
workpiece is less than the current required to produce the fixed torque in 
the receiving rollers. As the workpiece is received by the receiving 
rollers, the current increases to maintain the fixed revolution speed. The 
current increases only up to the current required to produce the fixed 
torque. As the workpiece continues to be received by the workpiece, the 
peripheral speed of the receiving rollers decreases to the linear speed of 
the workpiece. As the workpiece is being released from the feeding 
mechanism, the decrease in resistance allows the revolution speed of the 
receiving rollers to increase. When the rollers again reach the revolution 
speed at which the receiving rollers are fixed with no workpiece, the 
current decreases to again maintain the fixed revolution speed. 
FIG. 2, illustrates an alternate embodiment of the present invention where 
dc motor 15 and dc motor controller 40 are replaced by stepper motor 65 
and stepper motor controller 70. The primary characteristic of a stepper 
motor is its ability to rotate a prescribed small angle (step) in response 
to each control pulse applied to its windings. Below about 200 pulses per 
second, the motor rotates in discrete steps in synchrony with the pulses; 
at higher frequencies, the motor skews without stopping between pulses. 
Although motors are available for step angles of 90 to 180.degree., the 
common step is 1.8.degree.. Stepper motors are categorized as 
permanent-magnet (PM) rotor, variable reluctance (VR), or hybrid (PM-VR). 
The rotor of the PM aligns itself with the energized stator poles and the 
rotor turns until the poles are aligned at each step. In the present 
application, control pulse or frequency can be utilized between 0 and 1000 
Hz to control torque. Stepper motor 65 and stepper motor controller 70 
include an encoder and synchronizer to simulate the current-torque 
response of a brushless dc motor. In a preferred embodiment, the torque of 
stepper motor 65 is controlled by stepper motor controller 70 which in 
turn is directed by microprocessor 45. Torque is controlled by the output 
frequency at 75 to stepper motor 65. 
FIG. 3 represents a graph of the peripheral speed of receiving rollers 20 
versus time. The fixed torque applied to receiving rollers 20 is selected 
so that the resulting peripheral speed 80 of receiving rollers 20 is 
greater than peripheral speed 85 of feeding rollers 25, when no paper 30 
is in system 10. A typical peripheral speed 80 of receiving rollers 20 is 
300 mm/sec. 
Paper 30 is fed from feeding rollers 25 and is received by receiving 
rollers 20 at time T1. As paper 30 is received by receiving rollers 20, 
the peripheral speed of receiving rollers 20 drops at time T2 to the 
peripheral speed 85 of feeding rollers 25. The peripheral speed of 
receiving rollers 20 drops because paper 30 moves at the constant speed 85 
of feeding rollers 25. The decrease in the peripheral speed of receiving 
rollers 20 indicates that paper 30 has been received by receiving rollers 
20. Preferably, tachometer means 35 transmits a signal indicative of the 
revolution speed of receiving rollers 20 to microprocessor 45. 
Microprocessor 45 analyzes the signal and detects the change in speed of 
receiving rollers 20. 
As paper 30 remains in contact with both receiving rollers 20 and feeding 
roller 25, the peripheral speed of receiving rollers 20 remains the same 
as the peripheral speed 85 of feeding rollers 25. In a preferred 
embodiment of the present invention the torque applied to receiving 
rollers 20 is reduced after time T2 in order to prevent paper slipping and 
tearing. In the preferred embodiment, microprocessor 45 receives a signal 
from tachometer means 35 indicative of the rotational speed of receiving 
rollers 20. When a change in the rotational speed is indicated, 
microprocessor 45 instructs motor controller 40 to reduce the torque 
applied to receiving rollers 20. 
At time T3, the trailing edge of paper 30 is released from feeding rollers 
25. Since feeding rollers 25 no longer limit the speed of paper 30, the 
peripheral speed of receiving rollers 20 increases due to the fixed torque 
applied to receiving rollers 20. The increase in the peripheral speed of 
receiving rollers 20 indicates paper 30 has been released from feeding 
rollers 25. 
One use for the present invention is in an inverting process wherein paper 
30 is inverted as it exits a printer so that it exits with the printed 
side facing down. FIG. 3 further illustrates a typical peripheral speed 
versus time plot for an example where receiving rollers 20 are part of an 
inverting apparatus. The increase in speed at time T3 triggers 
microprocessor 45 to accelerate receiving rollers 20 to nominally 640 
mm/sec as shown at T4 and at the same time actuate the inverting process 
that is completed between times T4 and T6. The process requires reversal 
of receiving rollers 20 at T5 to effect the paper inversion. This process 
must be achieved quickly as another page of paper may follow the first 
page of paper 30. To achieve the inverting process quickly, the peripheral 
speed of receiving rollers 20 is increased as soon as possible after paper 
30 is released from feeding rollers 25. The present invention detects the 
release of paper 30 from feeding rollers 25 as feeding rollers 25 are 
releasing paper 30. 
Additionally, the length of workpiece 30 is determined from a measured time 
between points T1 and T3. Multiplying the measured time by the linear 
speed of workpiece 30 produces the workpiece length. The workpiece length 
is useful in determining errors in a conveying system and for computing a 
speed profile for the inverting process. A speed profile is used to 
determine the speed at which a sheet of paper must be inverted in order to 
complete the inverting process before the next sheet of paper appears. 
Although the present invention has been described with reference to a 
printer system, the present invention is alternatively implemented in any 
system where a workpiece is received by receiving rollers before being 
released from a feeding mechanism. Furthermore, paper 30 is any workpiece 
such as cardboard, metal, string, or other linear product that can be 
progressed by rollers.