Origin detector in image scanner

An apparatus according to the present invention includes a stepping motor (5) and a motor drive circuit (4) for first moving a lamp unit to hit the unit against a reference object opposite to an origin and then moving the unit toward the origin, a CPU (3) for measuring the distance through which the lamp unit is moved from the reference object toward the origin, and the CPU (3) for controlling the lamp unit so that the lamp unit stops at the origin according to the distance measured from the reference object.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an origin detector for a lamp unit in an 
image scanner. 
2. Description of the Related Art 
The image scanner needs an origin (reference position) for a lamp unit to 
hold a reading start position. Thus conventional image scanners use a 
photosensor, a reed switch, or the like to detect a reference position. In 
the arrangement of a conventional image sensor in FIG. 3B, an output 
signal from a sensor 1 is linked with an interrupt terminal 2 also serving 
as an I/O port of a CPU 3, which performs interrupt operation by detecting 
the timing of turn-on and turn-off of the sensor at a rising edge and a 
falling edge. 
Referring now to FIG. 5C, the arrangement of a conventional scanner is 
described below. Numeral 7 indicates a lamp unit, which is arranged to be 
moved along a shaft 12 by a motor 11. Numeral 6 designates a home position 
sensor for detecting an origin for the lamp unit. Numerals 8, 9, and 10 
indicate a mirror unit, a CCD for reading an image, and a lens, 
respectively. Numerals 13 and 14 designate belts, and numerals 15 and 16 
represent pulleys. 
Referring now to the flowchart in FIG. 2 and the arrangement of the scanner 
in FIGS. 5C and 5D, a conventional controlling method using a sensor is 
described below. 
Sensor condition is checked immediately after the power is turned on 
(S201). If the sensor is off (the lamp unit is not in the position of the 
sensor) as shown in FIG. 5C, the lamp unit moves back toward the sensor 
(S202 and S203) until the sensor turns on (the lamp unit is in the 
position of the sensor). After the sensor turns on, the lamp unit further 
moves back a few steps to make sure that the sensor is on (S204). Then the 
lamp unit begins to move forward (S205) and stops (S207) at such a 
position that the sensor turns off (S206), that is, an origin. 
If the sensor is on as shown in FIG. 5D immediately after the power is 
turned on (S201), the lamp unit moves forward and stops at such a position 
that the sensor turns off, that is, an origin. This allows the previous 
operations (S202 to S204) to be omitted. 
Such a position that the sensor is switched off is specified as an origin, 
since the range in which the sensor turns on cannot be neglected in 
relation with variations in the reading start position particularly if a 
reed switch is used. 
A scanner using a conventional origin detector is disclosed in Japanese 
Patent Application Laying open (KOKAI) No. 4-329531. 
Conventional origin detectors are expensive because they use a sensor. What 
is worse, conventional origin detectors need space for a sensor and 
cables, so that the detectors are prevented from being reduced in size. In 
addition, problems with assembly may arise because of installation 
variations. The present invention is made to solve the problems and 
provide an origin detector enabling a small inexpensive image scanner to 
be created. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an origin 
detector which requires no sensor and thus is inexpensive and enables an 
image scanner to be small and inexpensive. 
A further object of the invention is to provide an origin detector which 
has no problem with assembly due to installation variations. 
These and other objects of the invention can be achieved by an apparatus 
comprising means for moving the lamp unit toward an origin after hitting 
the lamp unit against a reference object opposite to the origin, means for 
measuring the distance through which the lamp unit is moved from the 
reference object toward the origin, and means for controlling the lamp 
unit so that the unit stops at the origin according to the distance 
measured. Because the position of the origin is found by measuring the 
distance traveled from the reference object by the lamp unit, the 
apparatus eliminates the need for a sensor for detecting the position of 
the origin, thus reducing an image sensor in cost and size. 
The means for moving the lamp unit may be a stepping motor, in that case 
the lamp unit is hit against the reference object at a speed which is in a 
through range and outside a resonance range. Hitting the lamp unit at such 
a speed reduces noise produced while the lamp unit is in motion and after 
the unit hits against the reference unit, since torque is small. 
The distance traveled by the lamp unit toward the reference object may be 
set more than a predetermined maximum travel to hit the lamp unit against 
the reference object without failure. Since the lamp unit is moved the 
predetermined maximum travel, it can securely be hit against the reference 
object. 
After the stepping motor steps out when the lamp unit hits against the 
reference object, the stepping motor is operated a few steps at a 
frequency less than a self-starting frequency to smoothly transfer the 
operation of the stepping motor to a slow-up table. Moving the stepping 
motor several steps at a frequency less than the self-starting frequency 
after the motor steps out allows the operation of the stepping motor to be 
smoothly transferred to the slow-up table. 
Further objects and advantages of the present invention will be apparent 
from the following description of the preferred embodiments of the 
invention as illustrated in the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3A shows the arrangement of an embodiment of the present invention. 
Unlike the conventional apparatus in FIG. 3B, the embodiment needs no 
sensor. 
Referring now to the flowchart of FIG. 1, the operation diagram of FIG. 4, 
and the scanner arrangement of FIGS. 5A and 5B, the effects of the present 
invention are described below. 
Because the lamp unit 7 is at any position when the power is turned on, the 
CPU cannot locate the position. As shown in FIG. 5A, the lamp unit makes 
the stepping motor step out by moving forward to hit a contact 17 thereof 
against a cabinet 18 opposite to the origin, thereby detecting the 
reference position. The origin is calculated backward from the reference 
position. In this case, the stepping motor must be force operated a 
certain number of steps to make the motor step out without failure. The 
distance through which the motor is force moved is the maximum travel L in 
FIG. 4. Assuming that the feed per motor step is 800 dpi, the number of 
steps required to move the lamp unit the maximum travel L is represented 
by the equation L.times.800/25.4, where the unit for L is the millimeter, 
and one inch is 25.4 mm. 
The lamp unit may be hit against a cabinet nearer to the origin. Since the 
lamp unit is usually near the origin, however, the stepping motor 
unfavorably steps out for a long time when it is operated steps 
corresponding to the maximum travel L. 
Before the lamp unit is hit against a cabinet, the stepping motor is slowed 
up from 500 to 3000 pps to reduce noise while the motor steps out (S101). 
The lamp unit is likely to bounce back when hitting against the cabinet, 
because the unit is at a high speed. The amount of the bounce appears to 
be almost negligible, since the lamp unit hits against the cabinet. 
To operate the stepping motor at a high speed, the speed thereof must be 
transferred from the self-starting range to the through range. The 
self-starting range is a range that allows the stepping motor to be 
started or stopped at a certain pulse rate with reference to load. On the 
other hand, the through range is a range that allows the stepping motor to 
respond without becoming asynchronous when the pulse rate or load torque 
is gradually increased beyond the self-starting range. The curve PULL-IN, 
one of the motor torque curves in FIG. 6, represents the maximum 
self-starting frequency, that is, the maximum pulse rate in the 
self-starting range, and the curve PULL-OUT, the other of the motor torque 
curves, represents the maximum response frequency, that is, the maximum 
pulse rate in the through range. The graphs in FIG. 6, plotted for the 
same motor and motor driver, with the current per phase kept constant, 
vary with conditions. The graphs give the maximum speed corresponding to a 
necessary torque. For example, the stepping motor can be started at 1200 
pps if a torque of 200 g-cm or less is required, while the stepping motor 
must be started at 400 pps or less if a torque a little less than 300 g-cm 
is required. In other words, when the stepping motor is started, the 
torque of the motor must be set less than indicated by the curve PULL-IN. 
The maximum speed after slow-up must be set less than indicated by the 
curve PULL-OUT. 
The resonance range is a range where if the frequency of vibration due to 
motor rotation is equal to or near the natural frequency of the housing 
containing the stepping motor, the housing resonates, so that the 
vibration thereof sharply becomes violent. The speed of the stepping motor 
must therefore be set outside the resonance range. 
After step-out, the actual phase condition differs from the regular phase 
condition. To make the actual phase condition agree with the regular phase 
condition, in-phase excitation is performed for about 0.6 second using two 
phases (S103 and S104). Rotating the stepping motor through an angle 
equivalent to a 5-mm step at 50 pps below the self-starting frequency 
immediately after start-up (S105) and linking the operation of the 
stepping motor with the slow-up table allow the stepping motor to be 
smoothly operated (S106). 
As shown in FIG. 7, to control the stepping motor, four patterns are 
switched to each other for 2-phase excitation, and eight patterns are 
switched to each other for 1-2-phase excitation. During normal operation, 
the actual phase condition always agrees with the regular phase condition 
controlled by the CPU. When the motor steps out, however, the actual phase 
condition disagrees with the regular phase condition. The probability that 
the regular phase condition disagrees with the actual phase condition is 
7/8 and 3/4 for 1-2-phase excitation and 2-phase excitation, respectively. 
The number of steps through which the stepping motor is operated at 50 pps 
below the self-starting frequency must be at least seven steps or more and 
three steps or more for 1-2-phase excitation and 2-phase excitation, 
respectively. 
After the distance between a reference position 18 for the contact and the 
origin, which distance is specified in steps (S107), is traveled, the lamp 
unit is returned to the origin using the stepping motor, and the position 
of the origin is constantly controlled until the power is turned off. The 
sequence described above allows the origin to be kept in position. 
An origin detector according to the present invention enables image 
scanners to be reduced in cost and size, because the detector uses no 
sensor. The origin detector also has the effect of reducing 
electromagnetic interference (EMI) noise, since cables from the sensor, 
through which noise is conveyed, are eliminated together with the sensor 
itself. 
An origin detector according to the present invention reduces noise which 
is produced when a lamp unit hits against a reference object and when a 
stepping motor steps out. 
An origin detector according to the present invention allows an origin to 
be detected with a lamp unit at any position when the power is turned on. 
An origin detector according to the present invention causes the operation 
of the stepping motor to be smoothly linked with a slow-up table without 
step-out. 
Many widely different embodiments of the present invention may be 
constructed without departing from the spirit and scope of the present 
invention. It should be understood that the present invention is not 
limited to the specific embodiments described in the specification, except 
as defined in the appended claims.