FIELD OF THE INVENTION The present invention is directed to a conveyor system powered by linear motors, and more specifically, toward a conveyor system wherein power supplied along any length of the system can be controlled by changing the size and spacing of the linear motors.

BACKGROUND OF THE INVENTION A power and free conveyor system includes a moving power chain, a power track for guiding and enclosing the power chain, and a free track generally parallel to the power track and spaced apart therefrom. The free track supports a number of wheeled trolleys which travel therealong which in turn support carriers for carrying different objects. The trolleys include assemblies which extend toward the power track and which assemblies are engaged by protrusions called dogs extending from the power chain toward the free track to push the trolleys along the free track in a forward direction.

The assemblies are movable in the direction of the power track so that they can be engaged and disengaged from the power chain at proper times to cause the trolleys to move or stop, respectively. These assemblies and their moving parts tend to wear out and can introduce debris and contamination into the system as they move and wear as well as generate noise. Likewise, the moving parts in the trolley wheels wear, require maintenance and replacement, and can sometimes introduce contamination into the conveyor system and the objects being transported thereby.

A power chain is generally a rivetless chain comprising numerous links which require lubrication to minimize wear as the chain bends and the links rub against one another. A break at any one of the hundreds or thousands of links in a chain will shut down an entire conveyor system. Chains are also prone to jamming. The power chain and trolley assemblies represents a significant system cost, is difficult and time consuming to replace when worn out, and along with the lubricators and lubricants needed to maintain the chain and trolleys, is also a significant source of contamination of the materials being transported. In addition, chain links become thinner as they wear and chains tend to elongate as they age. This causes problems in maintaining a proper spacing between the trolleys and synchronizing different chains to allow transfers between conveyor lines. Therefore, complex take ups must be employed to remove any excess slack. Furthermore, the power chain is continuous and must travel in a loop. The free track, however, may only extend along one portion of the loop. Thus the length of the power chain must be about double the length of the free track to allow it to return to the beginning of the conveyor. Furthermore, the power chain must be enclose in the power track along its entire length. The return section of power chain and track therefore add considerably to the cost of a conveyor system even though the chain does no useful work along this portion of its path. Clearly it would be desirable to provide a conveyor system in which the driving force for the carriers on the free track could be provided by a mechanism other that a power chain, and wherein the number of moving parts which wear out and lead to contamination is reduced.

SUMMARY OF THE INVENTION The present invention addresses these and other problems by using linear motors, such as linear induction motors (LIM's) or linear synchronous motors (LSM's) to move conveyor carriers along a supporting track. The linear motors are arranged in proximity to the supporting track, and push the carriers from one linear motor toward the next. The linear motors can also be used to decelerate and stop carriers either alone or in combination with well known mechanical stops.

Different driving forces may be required along differ portions of a free track. For example, more force is required to move carriers up an incline and to move carriers holding heavy objects. The present system can readily accommodate such differing power requirements by using more linear motors of a proper size and length and spacing the motors more closely in areas where greater power is needed. In downhill sections of track, motors are used only for deceleration purposes or dispensed with entirely. This contrasts with prior chain-driven systems where the power chain was present even along downhill track portions where power was not needed and where_ jamming could occur if a trolley traveling faster than the power chain and overtook a preceding dog.

In several embodiments of the subject invention, the linear motors are modular and used together with a motor track having a slot path into which the motors can be plugged at almost any position. The slot path can also accommodate motors of various widths and lengths. Power and any necessary communications connections are provided along the entire length of the motor track so that a linear motor is ready to operate once it is plugged in. When different or additional motors are needed, the linear motors can be unplugged, rearranged, and returned to the track quickly and easily.

Each of the linear motors is preferably provided with sensors to sense the approach or presence of a carrier. The system is also provided with a central controller for monitoring the locations of the carriers as they travel throughout the system. When a carrier approaches a linear motor, the central controller will check and authorize the linear motor to move the carrier and then be energized to push the carrier forward toward the next motor. The carriers can also be brought to rest by controlling the motors in appropriate ways or by using mechanical stops as done in the prior art.

In a preferred embodiment, the system includes wheeled trolleys which travel along a free track. The trolleys are generally similar to standard trolleys, but instead of an apparatus for engaging a pusher dog on a power chain, each includes a reaction plate on its top surface. A motor supporting track positioned parallel to the free track includes a number of LIM's. When a sensor indicates that a trolley is approaching one of the LIM's the LIM is actuated to generate a force against the reaction plate and push the trolley along the track toward the next LIM.

LIM's are also used in a second embodiment, in this case to power pusher dogs similar to those which depend from prior art power chains. A plurality of LIM's are arranged along a power track positioned adjacent to a free track which supports the trolleys. Each of the trolleys is equipped with a projecting portion adapted to be engaged by the pusher dogs on the LIM's.

When a sensor on the LIM detects that a trolley is passing by the LIM, the LIM is actuated to slide its pusher dog forward, engage the projecting portion of the trolley and push the trolley toward another LIM. The pusher on the LIM then returns to its starting position to await the approach of the next trolley.

Because the LIM's can be controlled by a central controller, the trolleys do not require a moving apparatus for disengaging from a power track, thus further avoiding the use of moving elements which can wear and introduce contamination into the conveyor system.

In a third embodiment, all moving parts are eliminated and the carriers are propelled by LSM's. In this embodiment the carriers include magnetic plates which are both supported and moved in a forward direction by magnetic fields generated by the support track. By using LSM's built into a support track, the magnetic plates can be lifted off the support track and moved forward by the interaction between the magnets on the plates and the moving magnetic field generated by the LSM by using the retractable portion of the trolley.

The subject system eliminates the need for a power chain and the lubrication and maintenance which it requires. Because fewer or no moving parts are used, maintenance costs are reduced. Complicated drive systems having chain take-ups to accommodate changes in chain length are eliminated, and the system is substantially quieter than standard power and free systems. The subject system is adaptable for both overhead and inverted power and free systems.

The invention also includes a method of conveying loads using carriers. The carriers travel on a support track and are driven by a plurality of magnetically powered motors, either linear induction motors or linear synchronous motors. The motor operation is controlled to drive the carriers for load conveyance.

It is therefore a primary object of the present invention to provide a chainless power and free conveyor system; It is another object of the present invention to provide a power and free conveyor system in which magnetic force is used to move trolleys and carriers along a conveyor track; It is a further object of the present invention to provide a power and free conveyor system in which the power provided at any portion of the track can be varied; It is still another object of the present invention to provide a power and free conveyor system powered by modular linear motors which can be plugged into a power track in a wide variety of arrangements.

It is still a further object of the present invention to provide a conveyor system having few moving parts.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects will be better understood from a reading and understanding of the following detailed description of several preferred embodiments of the invention, together with the following drawings of which: Figure 1 is a perspective view, partly in section, of a first embodiment of a conveyor system having a motor track for holding a plurality of linear motors and a support track for supporting a plurality of trolleys ; Figure 2 is a plan view of the motor track shown in Figure 1 and one of the linear motors mounted thereon; Figure 3 is a perspective view of a linear motor and its connectors suitable for connection to the motor track shown Figure 1; Figure 4 is an elevational view of a section of conveyor track showing schematically the spacing of linear motors therealong; Figure 5 is a second embodiment of a conveyor system including a plurality of linear motors; Figure 6 is an elevational view, partly in section, of one of the linear motors of Figure 5; and, Figure 7 is a perspective view of a third embodiment of the subject invention wherein carriers are levitated and moved along a track by magnetic force.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing, wherein the showings are for purposes of illustrating several preferred embodiments of the subject invention only and not for purposes of limiting same, Figure 1 shows a conveyor system 10 having a motor track 12 upon which are mounted a plurality of linear motors 14, and a support track 16 for supporting trolleys 18 and a carrier 20 depending from a pair of the trolley 18. In this embodiment, linear motors 14 are preferably linear induction motors (LIM's). The trolleys 18 each include a reaction plate 22 formed in the top surface thereof which is separated from motors 14 by a small distance when the trolleys are properly positioned on support track 16. As one of the trolleys approaches one of the motors, a sensor 24 informs a central communication system of the trolley's approach.

If it is safe for the trolley to proceed past the given motor, a signal is sent to energize the motor which in turn generates eddy currents in the reaction plate in a well known manner to push the trolley along the track. The LIM's can also apply a breaking force to slow the trolleys when necessary; however, it is generally advisable to provide conventional mechanical stops (not shown) in critical portions of the track.

Figure 2 shows one surface of motor track 12 having a large number of slots 26 arranged in several rows. As seen in Figure 3, each of the linear motors 14 includes prongs 28 which fit securely within slots 26 to hold the linear motor in place and to connect the linear motor to power and communication lines (not shown) inside track 12. This arrangement allows varying numbers of linear motors to be placed on the track in order to provide the amount of power needed for a given application. It also makes it easy to replace motors in the event that one malfunctions.

Figure 4 shows a number of linear motors 14 arranged along a section of motor track 12 on which carriers will travel from left to right as viewed in the figure. The track includes a first horizontal portion 30, an incline portion 32, a second horizontal portion 34, a downhill portion 36 and a third horizontal portion 38. As will be appreciated from this showing, the linear motors 14 are spaced apart by a first distance in the horizontal portions 30,34, and 38, are more closely spaced along the incline portion 32 to provide additional force for moving carriers up this slope, and are provided only at the bottom section _ of downhill portion 36 to slow the carriers as they enter third horizontal portion 38. In this manner, only the necessary amount of power is supplie at any given section of the system, unlike prior art chain-driven systems where a constant power level was provided everywhere.

Figure 5 shows a second embodiment of a linear motor driven conveyor system in which a number of LIM's 40 are supported from the underside of a motor support track 42. Each of the LIM's 40 includes a pusher dog 44 adapted to travel between an upstream end 46 and a downstream end 48 of the LIM. As seen in Figure 6, the pusher dog 44 is attached to a reaction plate 50 constrained to follow a path 52 within the LIM housing 54. By properly controlling LIM 40 in a well known manner, reaction plate 50 and pusher dog 44 can be made to travel back and forth between upstream end 46 and downstream end 48. The system further includes a plurality of trolleys 56 supported for rolling motion along a free track 58. Each trolley includes a protrusion 60 extending in the direction of motor support track 42. Preferably, these protrusions are fixed with respect to the bodies of the trolleys; however they may comprise the movable assemblies that are found on standard trolleys for engaging and disengaging a power chain.

When used in connection with the subject invention, the movable assemblies would be fixed in place thus avoiding the problems associated with the use of moving parts.

Each LIM includes a sensor 62 which is located downstream of end 46 of LIM 40 at a position such that the leading end of a trolley 56 will be sensed by the sensor as protrusion 60 passes by pusher dog 44. When the leading edge of a trolley is sensed, a central controller determines whether the trolley should be advanced, and if authorization is obtained, the LIM is actuated to move pusher dog 44 toward downstream end 48, engaging protrusion 60 and pushing trolley 56 forward along the free track. As can be seen in Figures 5 and 6, upstream end 46 of LIM 40 is spaced away from free track 58 a greater distance than the downstream end 48 to hold pusher dog 44 out of the path of the trolleys 56 until such time as it is desired to have the pusher dog engage the trolley.

Figure 7 shows a third embodiment of the subject invention in which _ trolleys are eliminated and carriers 64 are moved along a support track 66 by magnetic levitation. Track 66 comprises a LSM for generating a moving magnetic field in a well known manner. Each carrier 64 is attached to a magnetic plate 68 by a support rod 70. When coils 72 generate a magnetic field having an opposite polarity to that of magnetic plates 68, plates 68 are repelled from track 66. By controlling the strength and location of the field produced by coils 72, the carriers can be moved as desired along the track.

The carriers can also be slowed or stopped by controlling the magnetic field produced by coils 72 in an appropriate manner.

While the drawings show the carriers being supported in an overhead power and free system, the carriers can be supported from below as in an inverted power and free system.

The present invention has been described in terms of several preferred embodiments, however obvious additions and modifications will become apparent to those skilled in the relevant art upon a reading and understanding of this application. For example, while the power track is located above the free track in the preferred embodiments, it could just as easily be located beneath the free track. These and all other obvious modifications are intended to be included within the scope of this application.