Abstract:
The invention provides a hoist drive system for controlling the position of a hoist platform in a palletizer. In an embodiment of the invention, a hoist drive system comprises two electrical motors with dual output shafts. One motor may have an encoder for taking measurements relating to the rotation of the motor. A common shaft with a spring set safety brake ties the motors together. The two motors may be flux vector motors controlled with input from the encoder. The motors drive two gearboxes, thereby enabling vertical displacement of a hoist platform coupled to the drive system.

Description:
FIELD OF INVENTION 
   The present invention relates to hoisting, and more particularly to an electrical hoist drive system for a palletizer. 
   DESCRIPTION OF RELATED ART 
   Many goods are packaged in shipping containers such as boxes and stacked on pallets for transportation from a manufacturer to a user or distributor. Palletizing is automatically stacking goods onto a pallet typically constructed of wood. Goods are stacked onto pallets as layers separated by paperboard slipsheets. The layers of goods and the slipsheets on which the layers rest are secured on the pallet by banding, plastic sheet wrap, or by other conventional methods. The pallet facilitates the transportation of a significant number of goods from a manufacturer through distribution and ultimately to the end user of the goods. 
   Generally, manual loading of pallets is slow, requires strenuous physical labor, and is relatively cost inefficient. A number of different types of palletizing machines (herein referred to as palletizers) have been developed that quickly perform the function of loading and/or unloading pallets. Examples of palletizers are described in U.S. Pat. Nos. 2,774,489, 3,780,884; 3,844,422; 3,954,190; 4,058,225; 4,197,046; 4,214,848; 4,557,656; 4,861,226; 5,395,209; 5,868,549; 5,961,275; 6,164,900; and 6,371,720; each of which is incorporated herein by reference in its entirety. 
     FIG. 1  illustrates a conventional in-line palletizer  100  comprising many features found in the prior art. Generally, palletizer  100  includes an object infeed section A, a hoist section B, and an object outfeed section C. In order to simplify the explanation of the construction and operation of a conventional palletizer, the figure has been simplified by deleting many of the intricate component parts of the palletizer, examples of which are disclosed in the above-listed patents, which enable a palletizer to operate in a manner as described herein. Although palletizer  100  is shown and described as palletizing objects such as case goods, it should be understood that “objects” is intended to mean any type of goods that can be transported in pallet loads. 
   The sections of palletizer  100  are supported by a framework  110 . At the object infeed section A, framework  110  supports one end of a supply conveyor  120 , which can be any type of conventional supply conveyor such as a belt or roller conveyor employed to transport case goods  125  onto a forming conveyor  130 . Forming conveyor  130  typically comprises one or more case turners, mini stops, and side pushers, and like devices to orient multiple cases  125  into a layer to be loaded onto a pallet  127  in hoist section B. After a layer of cases  125  has been formed, the layer of cases is placed onto a stripper or apron  140 , which draws the layer into proper place for loading. The layer of cases is compressed on all four sides by a set of flight bars  142  as apron  140  is stripped from beneath, squarely depositing the layer onto an empty pallet  127  positioned under by a hoist platform  150 . After the layer has been placed onto hoist platform  150 , hoist platform  150  is lowered by a height approximately equal to the height of one layer of cases  125  plus suitable clearance needed to position another layer of cases  125  on top if necessary. When another layer of cases  125  is ready for loading by apron  140 , hoist platform  150  lifts up until the top of the previous layer is just below the apron  140 . Apron  140  then opens placing the new layer on top of the previous layer. This process is repeated until a full pallet load is completed, at which time hoist platform  150  travels down to feed the full load to an outfeed conveyor  160  at outfeed section C. Once the full load has cleared hoist platform  150 , an empty pallet  127  is conveyed via a pallet input conveyor  170  to hoist platform  150  and the entire loading process is repeated. 
   Several sensors (not shown) are mounted at key positions within palletizer  100  to detect the movement of the component parts described herein as well as the movement of empty pallets, layers of objects being loaded onto the pallets, and loaded pallets. The information acquired by these sensors is monitored by a control unit (not shown), which controls the systematic operation of the palletizer components. Various different types of conventional sensors can be employed. The control of palletizer  100  implemented by control unit via the sensors is apparent to one of ordinary skill in the art. 
     FIG. 2  illustrates a conventional hoist  200  for vertically displacing hoist platform  150  (not shown) in hoist section B of palletizer  100 . Hoist  200  comprises a motor  210 , which is mounted to frame  110 . A rotatable shaft  220  is affixed to frame  110  by one or more supports  222 . Motor  210  is connected to shaft  220  by a drive belt  230 . At least one mechanical brake  224  is provided on shaft  230  to control rotational movement. Dual gearboxes  240  each with a coupling  242  is provided to convert rotational motion of shaft  220  into a vertical displacement of hoist platform  150 . In operation, motor  210  under the direction of a control unit (not shown) drives shaft  220  via drive belt  230 , which in turn raises or lowers hoist platform  150  via gearboxes  240 . Brake  224  positions hoist shaft  220  into proper place by slowing and halting the movement of shaft  220 . One primary drawback of such a system is the development of imprecision in positioning hoist platform  150 , a problem which tends to increase over the lifetime of the mechanical parts, e.g., brake  224  and drive belt  230 . For example, belts wear, slip, and break, and mechanical brakes wear and slip; thereby causing positioning error leading to poor hoist platform control. 
   SUMMARY OF THE INVENTION 
   The present invention overcomes these and other deficiencies of the related art by providing a brakeless flux vector hoist drive system employing dual motors linked together via a shaft. Particularly, the inventive concept eliminates the need for drive belts and a mechanical brake in the positioning of a hoist platform. 
   The present invention discloses the use of multiple motors and gearboxes, which are mechanically linked via a shaft for positive drive of a hoist or elevator platform. An encoder is disposed on the shaft or within a motor to provide rotational measurement and feedback used for positioning control of the hoist platform. Positioning is controlled through operation of the dual motors themselves and not a brake. Nevertheless, a mechanical brake can be incorporated for safety in the event that power is lost to the motors. 
   In an embodiment of the invention, a hoist drive system comprises two electrical motors with dual outputs. A common shaft with a spring set safety brake links the motors together. For example, as one motor rotates clockwise, the other motor rotates counter-clockwise. One motor comprises an encoder for providing rotational measurement and feedback. A flux vector motor controller with input from the encoder controls the operation of the two motors. The two motors drive two hollow bore gearboxes, which in turn vertically displace a hoist platform. 
   In another embodiment of the invention, a hoist system comprises a first motor, an encoder, and a control unit. The first motor comprises an output axle connected to an input of a gearbox that is in-turn coupled to a hoist platform. The hoist platform is vertically displaced upward or downward by operation of the motor. The control unit stops the motor based on input provided by the encoder. A second motor can be provided with a shaft connecting the first motor to the second motor. The shaft is rotatably driven by the first and second motors. The first and second motors are preferably direct drive motors, stoppage of which positions the platform into a static position. 
   An advantage of the invention is that mechanical brakes and drive belts have been eliminated in the positioning of a hoist platform in a palletizer. Accordingly, hoist positioning and control problems are minimized, if not entirely eliminated, thereby leading to improve performance and cost savings. 
   The foregoing, and other features and advantages of the invention, will be apparent from the following, more particular description of the preferred embodiments of the invention, the accompanying drawings, and the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, the objects and advantages thereof, reference is now made to the following descriptions taken in connection with the accompanying drawings in which: 
       FIG. 1  illustrates a conventional in-line palletizer; 
       FIG. 2  illustrates a conventional hoist system for vertically displaying a hoist platform; 
       FIG. 3  illustrates a hoist drive system according to an embodiment of the invention; and 
       FIG. 4  illustrates a side view of the hoist drive system depicted in FIG.  3 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention and their advantages may be understood by referring to  FIGS. 3-4 , wherein like reference numerals refer to like elements, and are described in the context of an electrical hoist drive system for a palletizer. Nevertheless, the inventive concept can be adapted for systems other than palletizers, which require vertical hoisting of a platform, lift, or other load bearing structure. 
     FIG. 3  illustrates a hoist drive system  300  according to an embodiment of the invention. Hoist drive system  300  is affixed to a palletizer frame  305  as shown. Particularly, hoist drive system  300  comprises a first motor  310  and a second motor  320  linked by a shaft  330 . Although not absolutely required, shaft  330  is preferably connected to or supported by frame  305  via one or more bearing systems  332 , which constrain shaft  330  from moving in a vertical or horizontal direction relative to frame  305 . In a preferred embodiment, motors  310  and  320  are flux vector drive motors having dual outputs on opposite ends. For example, motor  310  comprises a shaft output  312  on one end and a motor output  314  on another end. Likewise, motor  320  comprises a shaft output  322  and a motor output  324  disposed along a motor axle. Outputs  312  and  314 , or outputs  322  and  324  can be integrated along a single motor axle. Shaft outputs  312  and  322  respectively couple motors  310  and  320  to shaft  330 . Shaft outputs  312  and  322  can be any type of conventional shaft coupling, the identification and implementation of which is apparent to one of ordinary skill in the art. 
   Motor outputs  314  and  324  are respectively linked to gearboxes  340  and  350  to enable the conversion of the rotational motion of shaft  330  into a vertical motion of a hoist platform  360 . Particularly, gearbox  340  or  350  comprises an input (not shown), preferably a c-faced input, which is respectively connected to motor output  314  or  324 , thereby directly coupling motors  310  and  320  to respective gearboxes  340  and  350 . Gearboxes  340  and  350  each comprises a gearbox output  342  or  352 . Gearbox outputs  342  or  352  preferably comprise a number of sprocket wheels, which are coupled to hoist platform  360  via links  344  or  354 . In a preferred embodiment, links  344  and  354  are metal chains designed to mesh with the sprockets of gearbox outputs  342  or  352  to prevent slippage. Links  344  and  354  are connected to platform  360  via conventional means, the identification and implementation of which is apparent to one of ordinary skill in the art. For example, each end of link  344  or  354  is respectively fastened to a top side or bottom side of platform  360  as shown. Guides  346  and  356  are disposed on a bottom portion of frame  305  to facilitate proper travel of links  344  and  354  and provide stability to hoist platform  360 . Other types of conventional coupling systems can be substituted to connect gearboxes  340  and  350  to platform  360 , e.g., a pulley or belt system, the implementation of which is apparent to one of ordinary skill in the art. 
   One or more encoders  334  are disposed on shaft  330  to provide rotational measurements and feedback to a control unit (not shown). In an alternative embodiment of the invention, encoder  334  is built into either one or both of motors  310  and  320 , thereby alleviating the need for affixing an encoder directly onto shaft  330 . Encoder  334  outputs a total number of revolutions or an angular frequency that shaft  330  undergoes for input to the control unit, which adjusts the operation of motors  310  and  320  to position hoist platform  360 . Because the vertical position of hoist platform  360  is directly dependent on the rotation of shaft  330  based on the known gear ratio of gearboxes  340  and  350 , the control unit can determine hoist platform&#39;s  360  change in vertical position from the rotational changes of shaft  330 . As such, the control unit can control the operation of motors  310  and  320  to accurately position hoist platform  360  as desired. For example, the control unit instructs motors  310  and  320  to respectively rotate clock or counter clockwise at a given speed. In an embodiment of the invention, the control unit can be programmed to enable motors  310  and  320  to accelerate and decelerate for soft starts and stops of platform  360 . 
   Brake  370  is preferably provided on shaft  330  as a safety precaution in case power is lost to motors  310  and  320 ; thereby preventing sudden or dramatic vertical displacement of platform  360 . In a preferred embodiment, brake  370  is an air spring brake, the implementation of which is apparent to one of ordinary skill in the art. Brake  370  is only used for safety precautions and not for positioning platform  360  during normal operation. In an another embodiment of the invention, brake  370  can be implemented in one or both of motors  310  and  320 , or within one or both of gearboxes  340  and  350 . 
   Gearboxes  340  and  350  facilitate the conversion of shaft  330  rotation into a rotational motion having a rotational axis not parallel, and preferably perpendicular, to the rotational axis of shaft  330 . For example, shifting the axis of rotation ninety (90) degrees enables all four corners of a square or rectangular hoist platform  360  to be connected to hoist system  330 . Such a feature is shown in  FIG. 4 , which shows a side view of hoist system and particularly illustrates gearbox  340  and gearbox output  342 . In a preferred embodiment of the invention, gearbox output  342  comprises a shaft  402  extending from gearbox  340 . Shaft  402  is preferably connected to or supported by frame  305  via one or more bearing systems  404 , which constrains movement in a vertical or horizontal direction relative to frame  305 . Gearbox output  342  comprises one or more sprocket wheels  406  disposed on shaft  402 , which are coupled to hoist platform  360  via links  344 . As shown, gearbox  340  lifts two corners of platform  360 . Likewise, gearbox  350  (not shown) lifts the two other corners (not shown) of platform  360 . Gearboxes  340  and  350 , each having its own input motor  310  or  320  linked together by shaft  330 , keep the four corners of platform  360  level. 
   In a related embodiment of the invention, a hoist drive system comprises a single motor instead of dual motors. In this embodiment, the single motor comprises dual outputs each connected to inputs of gearboxes  340  and  350 . For example, each end of a motor axle extending through the motor is connected to a shaft input of gearboxes  340  and  350 . In operation, the single motor directly drives gearboxes  340  and  350 , and hence hoist platform  360 , control of which is implemented as described in the above embodiments. 
   With no mechanical slip in the drive system, an empty pallet can be risen by platform  360  at a desired speed. Likewise, a pallet with a number of layers of goods can be hoisted or lowered at any desired speed. In operation, the hoist is stopped and held in position by the control unit directing the motors to halt and hold at zero speed. To change the position of hoist platform  360  to a preprogrammed vertical position, the control unit instructs the motors to run at a programmed speed and acceleration. Accordingly, the motors accelerate up to speed for a specific programmed point or time. A stop command with a zero speed is sent by the control unit, which commands the motors to decelerate to zero speed and hold that load/hoist in position. Positioning of hoist platform  360  is accomplished without the use of mechanical brakes or belts. The motors and control unit position the hoist platform  360  in the same position each time. 
   Although the invention has been particularly shown and described with reference to several preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims.