Registers – Coded record sensors – Particular sensor structure
Reexamination Certificate
2000-11-02
2001-12-11
Pitts, Harold I. (Department: 2876)
Registers
Coded record sensors
Particular sensor structure
C235S462450
Reexamination Certificate
active
06328216
ABSTRACT:
FIELD OF THE INVENTION
The field of the present invention relates to dithering assemblies or more particularly, a resonantly driven dithering mirror assembly employing feedback and employing travel stops for scanning an illumination beam for a barcode scanner.
BACKGROUND
In applications requiring rapid scanning of an illumination beam, such as barcode scanning, one method commonly employed for rapidly and repetitively scanning the illumination beam across a scanned region is mirror dithering. Dithering, i.e. rapid rotational oscillation of an illumination beam steering mirror about an axis substantially parallel to the mirror face, causes the illumination beam to move rapidly back and forth generating a scan line. When this scan line illuminates a barcode, the resulting time dependent signal due to detected light scattered and/or reflected from the bars and spaces of the barcode is decoded to extract the information encoded therein. To be used in such scanning applications, the dithering motor generating the mirror motion must be stable and typically employs some sort of feedback between the motor and the motion of the mirror. Particularly for handheld scanning applications, the dithering assembly should be light, compact, reliable, and consume minimum power while producing sufficiently large amplitude motion for scanning. Scanners are typically constructed with a feedback control circuit to actively adjust the length of the scan line so as to remain substantially constant.
Previous dithering assemblies have typically comprised a pair of magnets and a pair of magnetic coils.
FIG. 1
illustrates a dithering assembly
100
comprising a mirror/magnet assembly
110
, drive coil
106
, feedback coil
108
, bending member
112
, and mounting member
114
. The mirror/magnet assembly
110
comprises mirror
102
, mirror bracket
103
, and drive magnet
104
and feedback magnet
105
. The drive coil
106
, feedback coil
108
and mounting member
114
may be part of or mounted within a housing (not shown) for dithering assembly
100
. The bracket
103
holds mirror
102
and is connected to mounting member
114
by bending member
112
, which may comprise a thin, flat sheet of flexible material which acts as a bendable spring. Bending of member
112
results in rotation of mirror/magnet assembly
110
about an axis substantially parallel to mirror
102
, perpendicular to the plane of FIG.
1
.
It has been generally thought to be advantageous to locate the axis, i.e., the center of rotation (COR), coincident with the center of gravity (COG) of mirror/magnet assembly
110
. To protect the ditherer in the presence of severe mechanical shock, a means to constrain the motion of the center axis of rotation may be employed which is convenient in that, at this point, there is no lateral motion (i.e., motion in the plane perpendicular to the COR axis). For example, such means may comprise a moving pin, whose axis is the same as the COR axis, rotating in a stationary hole. The pin does not touch the inside of the hole in normal operation, since this touching would dampen the motion of the ditherer and reduce efficiency. Since there is no lateral motion of the pin—it merely rotates about its axis—the required clearance inside the stationary hole need only be sufficient to accommodate process and temperature variations. Under shock, the pin functions to restrain movement of the COR. If the COR and the COG are the same, and movement of that point is constrained, then shock along any rectilinear axis will only translate the pin to the inside of the hole—no rotation will occur. Thus, no additional shock constraint features are necessary. The present inventors have recognized that if the COR and the COG are not coincident, rotational motion need be restrained in the normal dithering direction.
The dithering assembly
100
comprises an oscillating structure which has a resonant frequency determined by the effective spring constant of bending member
112
and the effective mass of the mirror/magnet assembly
110
and any components attached thereto. The motion of mirror/magnet assembly
110
is driven by passing an oscillating drive current through drive coil
106
thereby generating an oscillating magnetic driving force on drive magnet
104
. The maximum amplitude of dithering motion of the mirror
102
occurs when the drive current oscillates at the resonant frequency of dithering assembly
100
, i.e., when the dithering assembly
100
is driven resonantly. It is important to drive the dithering assembly
100
resonantly to obtain the maximum dithering amplitude with minimum drive power consumption. It is also important that the position and length of the resulting scan line remain constant.
Even when feedback is employed to keep the drive frequency matched to the resonant frequency, there still can be considerable variation in the amplitude and position of the resulting dithering motion. These amplitude variations may result from a variety of manufacturing and operational variables which may be difficult to control, including but not limited to the precise mass of mirror/magnet assembly
110
and any components attached thereto, the precise dimensions and force constant of bending member
112
, the temperature, wear of the dithering assembly, and/or the spatial orientation of the moving drive magnet with respect to the drive coil. Since the amplitude of the dithering motion determines the position and length of the scan line produced by the dithering assembly, and since it is important for the position and length of the scan line to be constant for proper operation of the barcode scanner, the amplitude variations of the dithering motion must be minimized. Such amplitude variations may be minimized by using position feedback to control the amplitude of the drive force. However, such feedback necessitates additional sensing and control electronics, and adds to the overall power consumption, cost, and/or complexity of the barcode scanner. Furthermore, optimization of such a feedback system for proper operation may depend on the same variables which cause the amplitude fluctuations in the first place.
FIG. 2
illustrates typical waveforms for position, velocity, and drive force for a resonantly driven dithering assembly. Position waveform
152
and velocity waveform
154
are substantially sinusoidal, with a phase shift of 90 degrees between the position and the velocity. For a dithering assembly driven at its resonant frequency, velocity waveform
154
will be in phase with drive force waveform
156
. Drive force waveform
156
is shown as a square wave in
FIG. 2
, but may also comprise a substantially sinusoidal waveform.
This feedback has been accomplished in previous dithering assemblies by velocity feedback. Feedback coil
108
experiences an oscillating magnetic field due to feedback magnet
105
, which is attached to bracket
103
. The electrical potential developed across feedback coil
108
varies directly with time derivative of the magnetic flux at feedback coil
108
, and hence with the velocity of feedback magnet
105
and dithering mirror
102
. The zero crossings of the feedback potential, which occur when the mirror velocity is zero, are used to trigger switching of the polarity of the drive current in drive coil
106
, thereby reversing the drive force exerted on drive magnet
104
and mirror
102
. In this manner, the switching frequency of the drive force is always locked to the frequency of the dithering motion of dithering assembly
100
and the drive force is in phase with the velocity as required for a resonantly driven system.
There are several weaknesses with the feedback scheme described above. The electrical potentials developed across feedback coil
108
are typically quite small, on the order of a few millivolts. These signals must be amplified for use as a feedback signal, and the resulting feedback signal is quite noisy. There may be significant cross talk between the drive magnetic fields and feedback coil
108
because the drive coil
106
and drive magnet
104
are ne
Arends Thomas C.
Colley James E.
O'Donnell Patrick M.
Ring James W.
Lyon & Lyon LLP
Pitts Harold I.
PSC Scanning Inc.
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