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  Dual-speed drive mechanismUSPTO Application #: 20070182092Title: Dual-speed drive mechanism Abstract: A dual-speed printer drive mechanism is disclosed. The drive mechanism includes a drive motor, a drive roller for feeding a media sheet toward and through a printing area and a drive transmission for coupling the drive roller to the drive motor for turning the drive roller with different gear reduction ratios. The drive mechanism further includes a gear reduction ratio selector disposed in the drive transmission for selectively turning the drive roller at a first gear reduction ratio for feeding the media sheet to the printing area, and for selectively turning the drive roller at a second gear reduction ratio for feeding the media sheet with precision for image printing while in the printing area. (end of abstract) Agent: Hewlett-packard Company Intellectual Property Administration - Fort Collins, CO, US Inventors: Michael Nordlund, Manish Agarwal USPTO Applicaton #: 20070182092 - Class: 271270000 (USPTO) Related Patent Categories: Sheet Feeding Or Delivering, Feeding, By Means To Convey Sheet (e.g., From Pack To Operation), With Means To Vary Speed Of Conveyor Sheet The Patent Description & Claims data below is from USPTO Patent Application 20070182092. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] The present invention relates generally to motor drive mechanisms, and more particularly to printer drive mechanism having a dual speed drive mechanism. BACKGROUND OF THE INVENTION [0002] Conventional drive mechanisms of laser and inkjet printers, faxes, and copiers handle media sheets using a single gear reduction ratio. The gear reduction ratio chosen typically represents a compromise between high printing throughput and torque requirements of the printing device. A high gear reduction ratio provides high torque but requires the motor to operate at a high speed. This tends to result in loss of movement accuracy. A low reduction ratio provides accurate control at a slower speed. This increases the load on the motor and generally results in a large motor requirement. However, large motor has adverse effects on its highest speed achievable, size, and rotor inertia. Thus, a good balance is needed between high speed, high torque and high accuracy. [0003] Existing drive mechanisms utilize a closed loop feedback system that employs an optical disc encoder to track the rotational position of the motor of the drive mechanism. Although this is a cheap and simple approach, the limitation lies in the technical challenges of increasing the optical disc encoder resolution beyond 200 line per inch (lpi) at any given encoder disk diameter, in order to provide increased media sheet feeding accuracy. As an alternative to increasing the encoder resolution, the encoder disc may be correspondingly larger in diameter to increase the effective diameter ratio between the encoder disk and the paper drive roller. The larger the ratio, the higher the resolution at the paper drive roller, for the same given encoder disk. However, the enlargement of the encoder disc has the undesirable impact on the design and overall footprint of the printing device. For instance, a disk with 200 lpi and a 3:1 ratio to the paper drive roller provides a 600 lpi resolution at the paper drive roller. The same disk when used in a 4:1 ratio provides a 800 lpi resolution at the paper drive roller. [0004] A further limitation of existing drive mechanisms is associated with gear backlash. The accumulative error due to gear backlash may be reduced by reducing the number of gears in the transmission of the drive mechanism. However, this requires an increase in the torque handling ability of the motor, because of the larger reflected torque of the load on the motor. For instance, a load of 200 mNm when reflected onto a motor through a reduction ratio of 10:1 requires the motor to support 20 mNm, which is 200/10. However, when the ratio is increased to 20:1, the motor requires to support only 10 mNm. [0005] Other limitations of conventional drive mechanisms include gear stress and noise. Conventional spur gear systems have inherently high tooth stress due to the low number of teeth in mesh and the minimal area of force transfer between mating teeth. Additionally, more teeth in mesh and a large area of torque transfer result in increased operation noise. [0006] The ever-increasing demand for printing devices, such as inkjet printers, to provide high printing throughput in addition to providing high quality printing motivates the need to look for an alternative solution for the drive mechanisms of such printing devices. SUMMARY OF THE INVENTION [0007] The present invention is directed to a printer drive mechanism. The drive mechanism includes a drive motor, a drive roller for feeding a media sheet toward and through a printing area and a drive transmission for coupling the drive roller to the drive motor for turning the drive roller with different gear reduction ratios. The drive mechanism further includes a gear reduction ratio selector disposed in the drive transmission for selectively turning the drive roller at a first gear reduction ratio for feeding the media sheet to the printing area, and for selectively turning the drive roller at a second gear reduction ratio for feeding the media sheet with precision for image printing while in the printing area. BRIEF DESCRIPTION OF THE DRAWINGS [0008] FIG. 1 is a cross sectional diagram of a printer drive mechanism in accordance with an embodiment of the present invention; [0009] FIG. 2 is a cross sectional diagram illustrating in details a printer drive mechanism in accordance with an alternate embodiment of the present invention; and [0010] FIG. 3 illustrates step-by-step the process for advancing a media sheet in accordance with an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION [0011] FIG. 1 illustrates a drive mechanism for feeding paper and other print media sheets through a printer, and is referred to herein by the general reference number 100. Such a printer is representative of the many kinds of devices that use drive mechanisms. For example, some fax and copier machines are included in alternative embodiments. [0012] The drive mechanism 100 is mounted to a printer chassis and includes a motor 101 and a driver roller 103 for feeding media sheets towards and through a printing area for image printing. A drive transmission 105 connects the drive roller 103 to the motor 101 for turning the drive roller 103 at different speeds. A gear reduction ratio selector 107 is disposed in the drive transmission 105 for selecting the range of turning speeds of the drive roller 103. The drive roller 103 speed range can be selected at a first low gear reduction ratio for feeding the media sheets quickly to and out of the printing area and a second higher gear reduction ratio for feeding the media sheets with precision for image printing while in the printing area. The first and second range of speeds achieved through first and second gear reduction ratios are hereinafter referred to generally as fast and slow speeds, respectively. [0013] FIG. 2 illustrates in detail another drive mechanism embodiment of the present invention, referred to herein by the general reference number 200. The drive mechanism 200 includes a motor 201, a drive roller 203, and a drive roller shaft 205. A motor shaft 207 is coupled to and rotates together with the motor 201 for providing torque to other parts of the drive mechanism 200. In addition, an encoder disc 209 mounted to the motor shaft 207 works with an optical sensor 211 mounted on the chassis of the printer to detect rotational positions of the motor 201, which correspond to the rotational positions of the drive roller 203. The detected rotational positions of the motor will be fed back to a control mechanism (not shown) for monitoring rotation of the motor 201. [0014] Torque provided by the motor 201 is transmitted through one of a pair of transmission mechanisms 213 and 215 to the drive roller 203. In an embodiment, the pair of transmission mechanisms 213 and 215 include a low-reduction gear train 213 and a harmonic drive 215, respectively. [0015] The harmonic drive 215 assures high positional/rotational accuracy and provides a high-reduction ratio of, for example, 1/30-1/320. Therefore, when power is transmitted from the motor 201 through the harmonic drive 215 to the drive roller 203, the drive roller 203 rotates at a relatively low speed and provides a highly precise paper advance. The harmonic drive 215 includes a wave generator 217, a flexspline 219 and a circular spline 221 whereby the three components are mounted coaxially. [0016] The wave generator 217 is rotatable about the motor shaft 207 and is engagable with a clutch gear 223 for receiving torque therefrom. The wave generator 217 typically has bearings, such as ball bearings, built into the outer circumference of an elliptical cam. An inner raceway of bearings is fixed to the elliptical cam, and an outer raceway is elastically deformed by pressure applied by the bearings. The wave generator 217 is coupled to a clutch gear 223 via an input shaft 216, which is effectively one end of the motor shaft 207. [0017] The flexspline 219 includes a thin cup-shaped rim with teeth and is fixed to prevent absolute rotational motion. The flexspline 219 couples the wave generator 217 to the circular spline 221 for providing torque thereto as shown in FIG. 2. [0018] The circular spline 221 is typically a rigid ring and has a plurality of internal and external engaging teeth (not shown). The internal teeth of the circular spline 221 engage the external teeth of the flexspline 219 for receiving torque. The circular spline 221 is rigidly fixed to the drive roller shaft 205 for providing torque to the drive roller 203. [0019] In operation, the flexspline 219 is deflected by the wave generator 217 into an elliptical shape causing the teeth of the flexspline 219 to engage with those of the circular spline 221 at the major axis of the wave generator ellipse while the teeth across the minor axis of the wave generator ellipse are disengaged. When the wave generator 217 is rotated clockwise with the flexspline 219 fixed from rotating, the flexspline 219 is subjected to elastic deformation and its tooth engagement position moves by turning relative to the circular spline 221. For example, if the circular spline 221 has two more teeth than the flexspline 219, when the wave generator 217 rotates 180 degrees clockwise, the flexspline 219 tends to want to move counterclockwise by one tooth relative to the circular spline 221. However, since the flexspline 219 is restricted from rotating, the circular spline 221 is forced to rotate in a clockwise direction. Furthermore, when the wave generator 217 rotates one revolution clockwise (360 degrees), the flexspline 219 tends to want to move counterclockwise by two teeth relative to the circular spline 221 because, in this example, the flexspline 219 has two fewer teeth than the circular spline 221. Since the flexspline 219 is restricted from rotating, the circular spline 221 is forced to rotate in a clockwise direction, and in general terms, this movement is treated as output torque. Continue reading... Full patent description for Dual-speed drive mechanism Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Dual-speed drive mechanism patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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