This application is based upon and claims priority from U.S. Provisional Patent Application No. 61/395,609 filed on May 13, 2010 and U.S. Provisional Patent Application No. 61/395,984 filed on May 19, 2010.

BACGROUND OF THE INVENTION

The present invention is directed to a platform leveling system for maintaining at least two opposite sides of a moveable platform level during use. In one particular embodiment of the invention, the platform leveling system is used to maintain the platform of an aircraft cargo loader level during lifting and lowering of the platform.

One type of prior art aircraft cargo loader comprises a loading platform which is supported on a frame by a pair of scissors. In these types of cargo loaders, the platform is lifted and lowered relative to the frame by a pair of hydraulic lift cylinders. Each cylinder comprises a cylinder body which is connected to the frame and a piston rod which is movably supported in the cylinder body. A roller is connected to the top of each piston rod and a lift chain having a first end connected to the platform and a second end connected to the frame is trained over the roller. Thus, the height of the platform is directly related to the height of the piston rods. Consequently, when the piston rods are extended the platform is raised relative to the frame, and when the piston rods are retracted the platform is lowered relative to the frame.

During operation of the cargo loader, it is important that the platform remain level at all times as cargo is being lifted and lowered. However, in certain prior art cargo loaders in which the lift cylinders are positioned on the left and right sides of the platform, maintaining the platform level in certain situations, such as when the load is positioned off center, may be difficult. This is due to the fact that in these prior art cargo loaders, the cylinders are connected in parallel to their source of hydraulic fluid and therefore exert an equal force on the left and right sides of the platform. As a result, when the cargo is positioned off-center, which results in the weight of the load being distributed unequally between the cylinders, the cylinders will not be able to maintain the piston rods at the same height. SUMMARY OF THE INVENTION

In accordance with the present invention, these and other limitations in the prior art are addressed by providing an aircraft cargo loader which comprises a frame; a platform which is supported for vertical movement relative to the frame, the platform comprising at least first and second opposite sides; first and second hydraulic cylinders for lifting and lowering the first and second sides relative to the frame; at least one sensor for generating signals representative of the height of the first side relative to the height of the second side; and a controller for controlling the flow of hydraulic fluid to and from the first and second cylinders. In operation of the cargo loader, the controller controls the flow of hydraulic fluid to or from at least one of the first and second cylinders in response to the signals generated by the at least one sensor in order to maintain the first and second sides level.

In accordance with one embodiment of the invention, in operation of the cargo loader an operator directs the lifting and lowering of the first cylinder and the controller controls the flow of hydraulic fluid to or from the second cylinder in order to maintain the second side level with the first side.

In accordance with another embodiment of the invention, the cargo loader may comprise a first valve for controlling the flow of hydraulic fluid to and from the first cylinder and a second valve for controlling the flow of hydraulic fluid to and from the second cylinder, the first and second valves being controlled by the controller. In this embodiment, each of the first and second valves may comprise a four-way three-position valve.

In accordance with another embodiment of the invention, the cargo loader may comprises a third valve which is connected between the first and second cylinders and which in the event of a failure of the at least one sensor opens to allow hydraulic fluid to flow between the cylinders.

In accordance with another embodiment of the invention, the at least one sensor comprises a first sensor for generating a first signal representative of the height of the first side relative to the frame and a second sensor for generating a second signal representative of the height of the second side relative to the frame. In this embodiment, each of the first and second sensors may comprise a magnetostrictive linear position (MLP) sensor. In addition, where each of the first and second cylinders comprises a piston rod which is connected to a piston that is slidably received in a cylinder body, each MLP sensor may comprise a sensor head which is fixed in position relative to the cylinder body; a position magnet which is fixed in position relative to the piston rod; and a sensor pipe which extends from the sensor head through the position magnet. In this embodiment, the position of the position magnet relative to the sensor head is representative of the height of the corresponding first or second side relative to the frame. Furthermore, where the cylinder body comprises a cylinder barrel having an end which is closed by a cap, the sensor head may be mounted to the cap, the position magnet may be mounted to the piston, and the sensor pipe may extend axially through the position magnet and the piston rod.

In accordance with another embodiment of the invention, during operation of the cargo loader the controller determines the actual speed at which the platform is being lifted or lowered based on at least one of the first and second signals. In this embodiment, the controller may control the flow of hydraulic fluid to or from the first and second cylinders in order to maintain the actual speed at which the platform is being lifted and lowered approximately equal to a desired lift or lower speed. Also, the controller may control the flow of hydraulic fluid to or from the first and second cylinders in order to adjust the actual speed at which the platform is being lifted and lowered depending on the height of at least one of the first and second sides relative to the frame.

The present invention also provides a novel method for maintaining the platform of an aircraft cargo loader level in use, the platform being supported for vertical movement relative to a frame and having first and second opposite sides which are raised and lowered by corresponding first and second hydraulic cylinders. The method comprises the steps of communicating hydraulic fluid to or from the first and second cylinders in order to lift or lower the platform; generating signals representative of the height of the first side relative to the height of the second side; and controlling the flow of hydraulic fluid to or from at least one of the first and second cylinders in response to the signals in order to maintain the first and second sides at approximately the same height during lifting and lowering of the platform. In accordance with one embodiment of the invention, the flow of hydraulic fluid to the first cylinder is manually controlled and the flow of hydraulic fluid to the second cylinder is automatically controlled in response to the signals in order to maintain the height of the second side approximately the same as the height of the first side during lifting and lowering of the platform.

In accordance with another embodiment of the invention, the step of generating signals representative of the height of the first side relative to the height of the second side comprises generating a first signal representative of the height of the first side relative to the frame and generating a second signal representative of the height of the second side relative to the frame.

In accordance with another embodiment of the invention, the method comprises the step of determining the actual speed at which the platform is being lifted or lowered based on at least one of the first and second signals. In this embodiment, the method may also comprise the step of controlling the flow of hydraulic fluid to or from the first and second cylinders in order to maintain the actual speed at which the platform is being lifted and lowered approximately equal to a desired lift or lower speed. Also, the method may comprise the step of controlling the flow of hydraulic fluid to or from the first and second cylinders in order to adjust the actual speed at which the platform is being lifted and lowered depending on the height of at least one of the first and second sides relative to the frame.

These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numbers may be used to denote similar components in the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a perspective view of an exemplary cargo loader comprising a first embodiment of the platform leveling system of the present invention;

Figure 2 is a partial cross sectional view of the lift cylinder component of the platform leveling system shown in Figure 1 ;

Figure 2A is a partial cross sectional view of an alternative embodiment of the lift cylinder component of the platform leveling system shown in Figure 1 ;

Figure 3 is a schematic drawing of the hydraulic circuit of the platform leveling system shown in Figure 1 ; and Figure 4 is a perspective view of an exemplary cargo loader comprising two alternative embodiments of the platform leveling system of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The platform leveling system of the present invention has wide application to a variety of equipment, such as cargo loaders, conveyors and lift tables. The invention is particularly useful in aircraft cargo loaders which comprise a cargo platform that can be raised and lowered relative to a vehicle frame by two or more hydraulic lift cylinders.

One example of an aircraft cargo loader with which the platform leveling system of the present invention may be used is shown in Figure 1 . The cargo loader of this embodiment, generally 10, includes a rear platform 12 which is supported by a pair of rear scissors 14 on a cargo loader frame 16. In this exemplary embodiment, the rear platform 12 is raised and lowered relative to the frame 16 by left and right hydraulic lift cylinders 18 and 20.

Each cylinder 18, 20 comprises a cylinder body 22 which is connected to the frame 16 and a piston rod 24 which is movably supported in the cylinder body in a conventional manner. A roller 26 is connected to the top of each piston rod 24, and a lift chain 28 having a first end connected to the platform 12 and a second end connected to the frame 16 is trained over the roller. Thus, the height of the platform 12 is directly related to the height of the piston rods 24.

Consequently, when the piston rods 24 are extended the platform 12 is raised relative to the frame 16, and when the piston rods are retracted the platform is lowered relative to the frame. These basic details of the cargo loader 10 described so far are conventional and well know in the art.

During operation of the cargo loader 10, it is important that the platform 12 remain level at all times as cargo is being lifted and lowered. However, in certain prior art cargo loaders in which the lift cylinders are positioned on the left and right sides of the platform, maintaining the platform level in certain situations, such as when the load is positioned off center, may be difficult. This is due to the fact that in these prior art cargo loaders, the cylinders are connected in parallel to their source of hydraulic fluid and therefore exert an equal force on the left and right sides of the platform. As a result, when the cargo is positioned off-center, which results in the weight of the load being distributed unequally between the cylinders, the cylinders will not be able to maintain the piston rods at the same height.

In accordance with the present invention, the platform leveling system maintains the piston rods 24 at the same height when the cylinders 18, 20 are activated to raise and lower the platform 12. Thus, the platform 12 will remain level at all times during operation of the cargo loader 10.

In one exemplary embodiment of the present invention, the platform leveling system maintains the piston rods 24 at the same height by individually controlling the left and right cylinders 18, 20 in response to signals representative of the height of the left and right sides of the platform 2 relative to the frame 16. These left and right side position signals may be provided by any of a number of conventional sensors, such as linear or rotary encoders which are operatively connected between the platform 12 and the frame 16 adjacent each of the cylinders 18, 20. However, in the embodiment of the invention illustrated in Figure 1 , each of the left and right side position signals is generated by a respective magnetostrictive linear position sensor which is embedded in a corresponding cylinder 18, 20.

Referring to Figure 2, an example of a magnetostrictive linear position (MLP) sensor, which is indicated generally by reference number 30, is shown installed in the cylinder 18 (only the bottom portion of which is shown). The cylinder 20 is identical to the cylinder 18 and therefore does not need to be described separately. In this embodiment of the cylinder 18, the piston rod 24 is connected to a piston 32 which is slidably received within a cylinder barrel 34 that forms a portion of the cylinder body 22 shown in Figure 1 : The end of the barrel 34 is closed by a cap 36 to which an end piece 38 is secured, such as by bolts 40. The barrel 34 is connected to a source of hydraulic fluid through a port 42 which extends through the cap 36.

A suitable MLP sensor 30 for use in the present invention is the

Temposonics® M-Series Model MH MLP sensor manufactured by MTS Systems Corporation of Cary, North Carolina. As shown in Figure 2, the MLP sensor 30 includes a sensor head 44 which is fixed in position relative to the cylinder body 22, a ring-shaped position magnet 46 which is fixed in position relative to the piston rod 24, and a sensor pipe 48 which is connected to the sensor head and extends through the position magnet and the piston rod 24 to approximately the opposite end of the cylinder body. The sensor head 44 may be mounted in a corresponding bore in the cap 36 and retained in position by the end piece 38 and/or a retainer screw 50. In an alternative embodiment of the invention which is shown in Figure 2A, the end piece 38 is omitted and the sensor head 44 is simply mounted in a corresponding bore in the cap 36 and retained in position by the retainer screw 50. The position magnet 46 may be mounted in a

corresponding aperture in the piston 32 and retained in position by suitable means, such as an adhesive.

The MLP sensor 30 uses a time-based magnetostrictive position sensing principle to determine the distance of the position magnet 46 relative to the sensor head 44. In operation, the sensor head 44 generates a current pulse which travels along a waveguide located in the sensor tube 48. This current pulse generates a first magnetic field which interacts with a second magnetic field generated by the position magnet 46. The momentary interaction of these two magnetic fields produces a strain pulse which travels back along the waveguide and is detected by the sensor head 44. The sensor head 44 measures the elapsed time between the generation of the current pulse and the detection of the strain pulse and uses this elapsed time to calculate the distance between the sensor head and the position magnet 46.

Since the sensor head 44 is fixed in position relative to the cylinder body

22 and the cylinder body is fixed in position relative to the frame 16, and since the position magnet 46 is fixed in position relative to the piston rod 24 and the height of the piston rod is directly related to the height of the platform 12, the distance between the sensor head 44 and the position magnet 46 is indicative of the height of the platform 12 relative to the frame 16. Thus, the output of the MLP sensor 30 in the left cylinder 18 is a measure of the height of the left side of the platform 12 relative to the frame 16 and can therefore be used as the left side position signal. Similarly the output of the MLP sensor 30 in the right cylinder 20 is a measure of the height of the right side of the platform 12 relative to the frame 16 and can therefore be used as the right side position signal. In this regard, the MPL sensors 30 may be calibrated by simply setting the left and right side position signals equal when the platform is verified by an external measuring device to be level.

In accordance with the present invention, the platform leveling system includes means responsive to the left and right side position signals for adjusting the hydraulic flow to or from at least one of the cylinders 18, 20 in order to maintain the left and right sides of the platform 12 at the same height. In an exemplary embodiment of the invention, one of the cylinders, say cylinder 18, is designated a master cylinder and moves as directed by the operator, and the other cylinder, in this case cylinder 20, is designated a slave cylinder, and the flow adjusting means adjusts the hydraulic flow to or from the slave cylinder in order to maintain the height of the right side of the platform 12 equal to the height of the left side of the platform.

Referring to Figure 3, the flow adjusting means in accordance with one embodiment of the invention comprises a suitable controller, such as a

programmable logic controller (PLC) 52. The PLC 52 controls two solenoid valves 54, 56. Each solenoid valve 54, 56 may be a four-way three-position valve which in a first position connects a corresponding cylinder 18, 20 with a source of hydraulic fluid, such as a pump 58, in a second position connects the, cylinder with a sump 60, and in a third position disconnects the cylinder from both the pump and the sump. The first or master valve 54 is positioned in a first hydraulic line 62 extending between the pump 58 and the master cylinder 18. ~, The second or slave valve 56 is positioned in a second hydraulic line 64 which extends between the pump 58 and the slave cylinder 20. For purposes which will be described later, the cylinders 18 and 20 are connected by a third hydraulic line 66 which is normally closed by a by-pass valve 68.

In operation of the platform leveling system, the PLC 52 receives the left and right side position signals from the MLP sensors 30 and compares these signals to determine if a difference exists between the height of the left and right sides of the platform 12. If a difference exists, the PLC 52 applies this difference as an error signal to the slave valve 56 in order to adjust the flow of hydraulic fluid to or from the slave cylinder 20 to thereby bring the height of the right side of the platform level with that of the left side of the platform. For example, if the difference between the left and right side position signals indicates that the right side of the platform is lower than the left side of the platform, the PLC 52 will open the slave valve 56 an amount sufficient to raise the right side to the level of the left side. Similarly, if the difference between the left and right side position signals indicates that the right side of the platform is higher than the left side of the platform, the PLC 52 will close the slave valve 56 an amount sufficient to lower the right side to the level of the left side.

This same operation can be performed to maintain the left and right sides of the platform 12 level during raising and lowering of the platform. In normal operation of the cargo loader 10, the platform 12 is raised by moving the valves 54, 56 to their first position in order to open the lines 62, 64 connecting the pump 58 to the cylinders 18, 20. The platform 12 may then be lowered by moving the valves 54, 56 to their second position in order to connect the cylinders 18, 20 to the sump 60. The amount the valves 54, 56 are opened in each of these positions determines the lift and lower speed of the platform 12.

Thus, during raising or lowering of the platform 12, the PLC 52 will compare the left and right side position signals and, if a difference between these signals exists, apply the difference to the slave valve 56 in order to bring the right side to the level of the left side. For example, if during raising of the platform 12 the right side is lower than the left side, the PLC 52 will open the slave valve 56 an additional amount sufficient to bring the right side level with the left side as the platform is moving. Similarly, if during lowering of the platform 12 the right side is lower than the left side, the PLC 52 will close the slave valve 56 an additional amount sufficient to bring the right side level with the left side as the platform is moving.

In accordance with another aspect of the invention, the platform leveling system may be used to control the lift and lower speed of the platform 12. In many prior art cargo loaders, the lift and lower speed is adjusted by varying the amount the valves 54, 56 are opened. However, due to differences in loads, components and viscosity of the hydraulic fluid, the speed corresponding to a particular position of the valves will often fluctuate. The platform leveling system addresses this problem by determining the actual lift and lower speed from the derivative of the left and right side position signals. The PLC 52 then adjusts the valves 54, 56 in order to maintain a desired speed.

In a similar manner, the platform leveling system of the present invention may be used to automatically adjust the speed of the platform 12 as it is being raised or lowered. For example, the platform leveling system may adjust the valves 54, 56 to slow down the platform 12 as it is nears its fully lowered or fully raised position and to speed up the platform in between these positions. In addition, the platform leveling system may be used to enable or disable certain functions when the platform 12 reaches a predetermined height.

Referring now to Figure 4, an exemplary cargo loader 10 comprising two alternative embodiments of the platform leveling systems of the present invention is shown. In one of these embodiments, the platform leveling system comprises two level sensors 70, each of which is mounted on a corresponding scissor 14 at approximately the same longitudinal distance from the frame 16. In this embodiment, each level sensor 70 provides a corresponding left or right side position signal which the PLC 52 uses to control the valves 54, 56 in the manner described above in order to maintain the left and right sides of the platform 12 level. In the other embodiment, the platform leveling system comprises a single level sensor 72 mounted under approximately the center of the platform 12. In this embodiment the level sensor 72 provides both the left and right side position signals which the PLC 52 uses to control the valves 54, 56 in the manner described above in order to maintain the left and right sides of the platform 12 level. The level sensors 70, 72 may be, e.g. , mercury switches, inclinometers or accelerometers.

In another embodiment of the invention which is not shown in the drawings, the platform leveling system is used with a cargo loader having four lift cylinders, each of which is located near a corresponding corner of the platform. In this embodiment, the platform leveling system employs two sensors or sets of sensors, one for detecting the difference in height between the left and right sides of the platform and one for detecting the difference in height between the front and back of the platform. The first set of sensors may comprise two MLP sensors 30 mounted as described above, and the second set of sensors may comprise a single tilt sensor 72 mounted in approximately the center of the platform. In this embodiment, one of the cylinders is designated as a master cylinder, the remaining cylinders are designated as slave cylinders, and the PLC controls the valves for each of the slave cylinders in a manner described above in order to maintain the left side level with the right side and the front level with the back.

Referring again to Figure 3, the platform leveling system of the present invention may also include means for disabling the system in the event of, e.g. , a failure of the system. In order to disable the platform leveling system, the by- pass valve 68 connecting the cylinders 18 and 20 is opened. This will equalize the pressure in the cylinders 18, 20 and cause the cargo loader 10 to operate in a manner similar to the prior art cargo loader described above.

It should be recognized that, while the present invention has been described in relation to the preferred embodiments thereof, those skilled in the art may develop a wide variation of structural and operational details without departing from the principles of the invention. Therefore, the appended claims are to be construed to cover all equivalents falling within the true scope and spirit of the invention.