Background Lift assemblies are useful for providing workers or machines with access to elevated installations or other generally inaccessible work areas. Some lift assemblies include wheel-sets that facilitate movement between different work locations. Examples include trailerable and self-propelled lift assemblies. Personnel lifts are one type of lift assembly, and usually include a work platform mounted to one end of an extendible boom or other type of extensible assembly.

On conventional personnel lifts using extendable booms, the extendable boom is often pivotably connected to a boom chassis. A hydraulic lift cylinder connected between the extendible boom and the boom chassis can be used to pivot the extendible boom between horizontal and vertical positions relative to the boom chassis. The work platform can be maneuvered to a desired location by pivoting the extendable boom to an appropriate angular or vertical position and extending the extendible boom as required.

During operation of conventional personnel lifts, it is possible to maneuver the extendable boom so that the work platform is positioned outside of the foundation provided by the wheel-sets or other supporting structure. Positioning the work platform outside of this foundation can introduce a destabilizing load-moment into the personnel lift. As the weight on the work platform and the extension of the boom approach their limits, this destabilizing load-moment may exceed the counter- balancing capability of the personnel lift and cause substantial instability of the personnel lift.

There have been prior attempts to develop a personnel lift that cannot be made unstable by overextending the boom or overloading the work platform. One such approach involves pivotally mounting one end of the boom chassis to a support structure and suspending the other end from springs. This arrangement allows the

entire boom chassis to rotate in proportion to the load-moment. The springs are calibrated so that the boom chassis will rotate toward the support structure and bottom-out against the support structure if the load-moment becomes excessive. This bottoming-out of the boom chassis provides an indication that the extendable boom is approaching an unstable configuration, and that further extension of the boom should be avoided. The additional support structure called for by this approach results in an undesirably heavy and awkward personnel lift. In addition, suspending the boom chassis from springs can result in substantial motion of the extendable boom at its distal end, thus creating unfavorable movement of the work platform.

Another approach used with extendable boom lift assemblies employs microprocessors to continually measure two pieces of information in real-time during lift operation: boom extension and boom angle. The boom extension and boom angle are used to determine the maximum load that can be stably carried on the work platform at any given time, and this load is displayed in real-time for the lift operator to view. If the operator happens to know the current load in the work platform, then the boom extension and boom angle can accordingly be limited to only those positions which can stabley accommodate the current load. In addition to requiring two pieces of information to determine the stable limits of lift operation, this approach also has the drawback of requiring the operator to know or determine the weight of persons and material in the work platform before every use. This approach also introduces the added complexity of a computerized control system.

As the foregoing illustrates, conventional extendable boom lift assemblies that have attempted to overcome the problem of tipping under extreme load and extension conditions often demonstrate the unfavorable characteristics of excessive weight, excessive platform motion, or excessive complexity. Therefore, a simple and robust apparatus capable of limiting a personnel lift to only stable maneuvers would be desirable.

Summary A load-moment sensing apparatus usable with a lift assembly is provided. In one embodiment of the invention, the load-moment sensing apparatus is usable with a lift assembly having an extendable boom coupled to a boom chassis at a rotation point. A lift cylinder is operatively coupled to the extendable boom for pivoting the extendable boom relative to the boom chassis about the rotation point.

The load-moment sensing apparatus of this embodiment includes a pivot member pivotally mounted to the boom chassis at a first pivot point and operably connected to the lift cylinder at a second pivot point offset from the first pivot point. In one aspect of this embodiment, the first pivot point, the second pivot point, and the rotation point are all positioned on the same line. A bearing portion is fixedly attached to the pivot member and spaced apart from the first pivot point. A mechanical stop configured to resist pivoting motion of the pivot member is fixedly attached to the boom chassis adjacent to the bearing portion defining a space therebetween. In another aspect of this embodiment, a deformable member is fixedly positioned in the space between the mechanical stop and the bearing portion. A movement detector is positioned adjacent to the bearing portion to detect a selected pivoting movement of the pivot member.

In another embodiment of the invention, a load-moment on the lift assembly produces a linear reaction on the deformable member which compresses the deformable member against the mechanical stop. In one aspect of this embodiment, the deformable member is a compressible member that is shaped and sized such that when the load-moment approaches a predetermined magnitude, the compressible member will compress enough to allow the bearing portion to activate the movement detector. The activated movement detector sends a signal to a lift control system limiting further motion of the extendable boom to only those motions that will accordingly reduce the load-moment on the lift assembly. As a result, the load-moment on the lift assembly is limited to the predetermined magnitude.

In another embodiment of the invention, the deformable member is a cylindrical bearing pin fixedly attached to the pivot member and spaced apart from the first pivot point. The cylindrical bearing pin is mated to a mechanical stop on the boom chassis that comprises a first support hole for supporting a first end of the bearing pin and a second support hole for supporting a second end of the bearing pin. In this embodiment, the movement detector is a strain gauge operably attached to the cylindrical bearing pin and configured to detect a movement of the pivot member by detecting a deflection of the bearing pin, the deflection of the bearing pin being proportional to the load-moment on the lift assembly. The load-moment on the lift assembly is limited to a predetermined magnitude in this embodiment by a signal that is sent by the strain gauge to the lift control system when the deflection of the

bearing pin indicates that the load-moment has reached the predetermined" magnitude.

Brief Description of the Drawings Figure 1 is a partial cut-away isometric view of a mobile personnel lift having a load-moment sensing apparatus in accordance with an embodiment of the invention.

Figure2 is an enlarged partial cut-away isometric view of the load- moment sensing apparatus of Figure 1 in accordance with an embodiment of the invention.

Figure 3 is a partial cross-sectional view taken substantially along the line 3-3 in Figure 2 with the load-moment sensing apparatus in an unloaded position.

Figure 4A is a partial cross-sectional view of the load-moment sensing apparatus similar to Figure 3 with the load-moment sensing apparatus sensing a substantial load-moment.

Figure 4B is a partial cross-sectional view of the load-moment sensing apparatus similar to Figure 3 with the load-moment sensing apparatus sensing a substantial reverse load-moment.

Figure 5 is an enlarged partial cut-away isometric view of a load- moment sensing apparatus in accordance with an alternate embodiment of the invention.

Detailed Description In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. The present disclosure describes load-moment sensing apparatuses useable with personnel lifts. Many specific details of certain embodiments of the invention are set forth in the following description and in Figures 1 through 5 to provide a thorough understanding of these embodiments. One skilled in the relevant art will understand, however, that the present invention may have additional embodiments, or that the invention may be practiced without several of the details described below. In other instances, well-know structures associated with personnel lifts, such as lift assemblies and hydraulic systems, have not been shown or described in detail to avoid unnecessarily obscuring the description of the embodiments of the invention.

Figure 1 is a partial cut-away isometric view of a mobile personnel lift 100 having a load-moment sensing apparatus 110 in accordance with an embodiment of the invention. The mobile personnel lift 100 includes a work platform 120 operatively coupled to an extendable boom 130. The extendable boom 130 is extendable and retractable along its longitudinal axes. The extendable boom 130 is connected at its distal end to the work platform 120 and is pivotally coupled at its other proximal end to a boom chassis 140 at a rotation point 132. While the lift assembly of the illustrated embodiment is a personnel lift, in other embodiments the work platform 120 can be replaced by other types of structures or machines that require elevation to selected positions. In addition, while the lift assembly of the illustrated embodiment comprises an extendable boom, in other embodiments the lift assembly can comprise other types of extensible apparatuses capable of supporting and extendably positioning the work platform 120 relative to the boom chassis 140.

Thus, the present invention is not limited to personnel lifts with extendable booms, but applies to other lift assemblies capable of supporting and extendably positioning persons, structures, or machinery.

The boom chassis 140 when viewed from the perspective shown in Figure 1 has a left side member 141 spaced apart from a right side member 143 defining a space therebetween to accommodate the proximal end of the extendable boom 130. A lift cylinder 150 is pivotally coupled at one end to the extendable boom 130 and pivotally coupled at the other end to a pivot member 210 of the load-moment sensing apparatus 110. The pivot member 210 is pivotally coupled to the boom chassis 140 at a first pivot point 212, and is pivotally coupled to the lift cylinder 150 at a second pivot point 216. In one aspect of this embodiment, the first pivot point 212, the second pivot point 216, and the rotation point 132 are aligned on a line 190. As will be explained in further detail below, by aligning these three points, the load- moment sensing apparatus 110 of the present invention is able to directly measure the load-moment on the personnel lift 100, and limit boom movement accordingly. In other embodiments, if these three points are not aligned, the load-moment sensing apparatus 110 may not directly measure the load-moment, however, it can still be utilized to maintain stability of the personnel lift 100 in accordance with the present invention.

The extendable boom 130 is depicted in Figure 1 in a substantially horizontal, stowed position. Pivoting of the extendable boom 130 about the rotation point 132 to a more vertical position is accomplished by extending the lift cylinder 150. Similarly, pivoting the extendable boom 130 downward from a more vertical position is accordingly accomplished by retracting the lift cylinder 150.

In one aspect of this embodiment, the boom chassis 140 is fixedly attached to a turntable 170 rotatably mounted to a trailer bed 181 or other selected frame structure. Rotation of the turntable 170 and boom chassis 140 as a unit rotates the extendable boom 130 in a horizontal plane about a turn point 172 with respect to the bed 181. In the illustrated embodiment, the trailer bed 181 includes fore and aft wheel-sets 180 and 182, respectively, to provide mobility to the mobile personnel lift 100. The fore and aft wheel-sets 180 and 182 are spaced apart by a longitudinal wheel-base 184 and a transverse wheel-track 186, and define a foundation that supports the mobile personnel lift 100 during operation. In an alternate embodiment, the trailer can include outriggers if needed to expand the support foundation.

In the illustrated embodiment, the personnel lift 100 includes a lift control system 160 configured to control the operation of the extendable boom 130.

In one aspect of this embodiment, the lift control system 160 is a hydraulic system that controls extension and retraction of the extendable boom 130 with a first hydraulic valve 162, vertical rotation of the lift assembly about the rotation point 132 with a second hydraulic valve 164, and horizontal rotation of the lift assembly and chassis on the turntable 170 with a third hydraulic valve 166. The hydraulic valves 162,164, and 166 are operatively coupled to the load-moment sensing apparatus 110 and to manual controls which a lift operator can use to position the work platform 120 as desired. In other embodiments, the lift control system 160 can be other types of control systems capable of controlling the operation of the extendable boom 130.

For example, in one alternate embodiment the lift control system 160 can be an electrical control system. In another embodiment, the lift control system can be a pneumatic control system. And in yet other embodiments, other suitable control systems capable of extending, retracting, pivoting, and rotating the extendable boom 130 can be used.

Positioning of the work platform 120 is accomplished by pivoting the extendable boom 130 horizontally and vertically as required and then extending the

lift assembly to a desired elevation. In this manner, the personnel lift 100 can be used to elevate a load L of personnel and materials in the work platform 120 to a desired location. The horizontal distance at the desired location between the work platform 120 and the rotation point 132 is defined as distance D in Figure 1. For any given position of the work platform 120, the load L multiplied by the horizontal distance D results in a load-moment LM about the rotation point 132. The load- moment LM resulting from the load L places a compression load on the lift cylinder 150. Conversely, a reverse load-moment RLM resulting from a reverse load RL, such as would be experienced if the work platform 120 were landed onto another structure during operation, accordingly places a tension load on the lift cylinder 150.

Any tension or compression load on the lift cylinder 150 can be resolved into two components: a component that is colinear with the line 190 and a component that is perpendicular to the line 190 along an axis 192. Because of the alignment of the first pivot point 212, the second pivot point 216, and the rotation point 132, the colinear component is not related to the load-moment LM or the reverse load-moment RLM, and as such is not measured by the load-moment sensing apparatus 110. The perpendicular component along the axis 192, however, is linearly proportional to the load-moment LM, and as such can be directly measured by the load-moment sensing apparatus 110.

If at any time during operation the load-moment LM or reverse load- moment RLM exceeds the counter-balancing capability of the personnel lift 100, the personnel lift may become unstable and tip about either the wheel-track 186 or the wheel-base 184, depending on the particular orientation of the extendable boom 130 at the time. Since the component of the load in the lift cylinder 150 along the axis 192 will at all times be linearly proportional to the load-moment about the rotation point 132, monitoring this component and preventing it from exceeding a selected magnitude can provide a means for preventing the load-moment from exceeding a selected magnitude. And if this selected load-moment magnitude corresponds to the stability limits for the personnel lift 100, then monitoring this component of the load in the lift cylinder 150 and preventing it from exceeding a selected magnitude can also provide a means for preventing the personnel lift 100 from becoming unstable.

Figure 2 is an enlarged partial cut-away isometric view of the load- moment sensing apparatus 110 of Figure 1 useable for monitoring the load on the lift

cylinder 150 and preventing it from exceeding a preselected magMjen. accordance with an embodiment of the invention. The load-moment sensing apparatus 110 can be provided as original equipment on a lift assembly, or it can be provided as a retrofitable assembly for retrofitting to an existing lift assembly. The load-moment sensing apparatus 110 includes the pivot member 210 disposed between the left and right side members 141 and 143 of the boom chassis 140. The pivot member 210 includes a left side plate 211 spaced apart from a right side plate 213 defining a space therebetween. A lower bearing portion 217 is fixedly attached to the bottom edge of the left and right side plates 211 and 213 and spans the space therebetween toward a free end 220 of the pivot member 210. An upper bearing portion 218 is similarly fixedly attached to the upper edges of the left and right side plates 211 and 213 opposite the lower bearing 217.

The pivot member 210 is pivotally coupled to the chassis'left and right side members 141 and 143 by a pin 214, thereby defining the first pivot point 212.

The pin 214 also extends through the pivot member's left and right side plates 211 and 213 and interleaves between the left and right side members 141 and 143. The end of the lift cylinder 150 extends between the left and right side plates 211 and 213 and is pivotally coupled to the pivot member 210 at the second pivot point 216 offset from the first pivot point 212. A pin 219 extends through the lift cylinder 150 and is fixedly supported on one side by the left side plate 211 and on the other side by the right side plate 213.

A lower mechanical stop 242 is fixedly attached to the left and right side members 141 and 143 of the boom chassis 140. The lower mechanical stop 242 is positioned adjacent to, and spaced apart from, the lower bearing portion 217 defining a space therebetween. In the illustrated embodiment, an upper mechanical stop 244 is also attached in a similar fashion to the left and right side members 141 and 143 of the boom chassis 140. The upper mechanical stop 244, however, is positioned adjacent to, and spaced apart from, the upper bearing portion 218 defining a space therebetween.

In the illustrated embodiment, a lower deformable member 232 is securely mounted to the lower mechanical stop 242 in the space between the lower bearing portion 217 and the lower mechanical stop. An upper deformable member 234 is similarly mounted to the upper mechanical stop 244 in the space between the

upper bearing portion 218 and the upper mechanical stop. AccordinU wh-én7wa sufficient force is exerted on the lift cylinder 150 to pivot the pivot member 210 about the first pivot point 212, the pivot member presses the lower or upper bearing portion 217 or 218 against the respective lower or upper deformable member 232 or 234.

The respective upper or lower deformable member in turn press against the respective rigid lower or upper mechanical stop 242 or 244. Thus, the mechanical stops 242 and 244 and the deformable members 232 and 234 resist the pivoting motion of the pivot member 210 about the first pivot point 212 by blocking movement of the respective bearing portions 217 and 218.

In one embodiment of the invention, the deformable members 232 and 234 are resilient elements capable of compressing toward their respective mechanical stops 242 and 244 in proportion to a load applied by the adjacent bearing portions 217 and 218, respectively, and then elastically returning to their original shape when the load is removed, In one aspect of this embodiment, the deformable members 232 and 234 can be compressible polyurethane rubber members. In another embodiment, the lower and upper deformable members 232 and 234 can be mechanical compression spring elements. And in other embodiments, other compressible and resilient elements can be used for the lower and upper deformable members 232 and 234, respectively.

In an alternate embodiment, the lower and upper deformable members 232 and 234 can be attached directly to the respective lower and upper bearing portions 217 and 218 and spaced apart from the respective lower and upper mechanical stops 242 and 244. In yet another embodiment, the lower and upper deformable members 232 and 234 can be compressible non-elastic members that can compress toward their respective mechanical stops 242 or 244 in proportion to an applied load, but not elastically return to their original shape when the load is removed. In another alternate embodiment, the lower and upper deformable members 232 and 234 can be mechanical tension spring elements. In the tension spring embodiment, each deformable member would be attached to its adjacent mechanical stop and to its adjacent bearing portion. Pivoting motion of the pivot member 210 would be resisted by extension, rather than compression, of the deformable members.

Figure 3 is a cross-sectional view taken substantiattyatongf tth'e-SL Figure 2 in accordance with an embodiment of the invention with the load-moment sensing apparatus shown in an unloaded position. The load-moment sensing apparatus 110 includes a lower limit switch 320 mounted to the lower mechanical stop 242 positioned below the lower bearing portion 217. The lower limit switch 320 has a depressible contact 321 adapted and positioned to be engaged by the lower bearing portion 217 so as to close or activate the contact. The lower limit switch 320 is also operably connected to the lift control system 160 of Figure 1 (not shown) so as to provide a signal to the lift control system when the contact is activated. An upper limit switch 322 is similarly mounted to the upper mechanical stop 244 adjacent to the upper bearing portion 218. The upper limit switch 322 also has a depressible contact 323 positioned to be engaged by the upper bearing portion so as to close or activate the contact. The upper contact is also operably connected to the lift control system 160 of Figure 1 so as to provide a signal to the lift control system when the contact is activated.

The lower limit switch 320 is positionable between an operate mode and an activated, interrupt mode. The switch 320 will remain in the operate mode as long as the contact 321 is not fully depressed. In the operate mode, the switch 320 transmits no signal to the lift control system 160, and the extendable boom 130 (Figure 1) will be fully operable and capable of extending or rotating within its full range of movement. Conversely, full depression of the contact 321 places the switch 320 in the interrupt mode. In the interrupt mode, the switch 320 sends an interrupt signal to the lift control system 160 causing the lift control system to prevent any further extension or rotation of the extendable boom 130 that would increase the load-moment LM (Figure 1) and potentially make the personnel lift 100 unstable. The upper limit switch 322 mounted to the upper mechanical stop 244 is also positionable between the same operate and interrupt modes and functions in a substantially similar manner as the lower limit switch 320 described above.

In the illustrated embodiment, the limit switches 320 and 322 are electrical limit switches configured to send an electrical interrupt signal to the lift control system 160 of Figure 1 (not shown) when the respective contacts 321 or 323 are fully depressed. The use of electrical switches to control hydraulic systems is well known to those of ordinary skill in the relevant art. In other embodiments, other

switching devices can be used to interrupt destabilizing boom motion when fully depressed. As will be apparent to those of skill in the relevant art, the limit switches 320 and 322 of the illustrated embodiment act as movement detectors configured to detect pivotal motion of the pivoting member 210 and send a signal to the lift control system 160 when the detected motion reaches a preselected amount. It will also be apparent to those of skill in the relevant art that numerous other known devices can be configured in accordance with the present invention to detect motion-even minute motion-of the pivot member 210 past a selected position. For example, a piezoelectric sensor device can be embedded into compressible polyurethane rubber deformable members in an alternate embodiment of the invention.

As best seen in Figure 1, when the load L is placed on the work platform 120, the load-moment LM introduces a compression force into the lift cylinder 150.

This compression force is shown in Figures 1 and 3 as Fo. The compression force Fs has a component that acts along the axis 192 perpendicular to the line 190. This component has a tendency to rotate the pivot member 210 about the pivot point 212 in a counterclockwise direction CCW. This counterclockwise rotation causes the lower bearing portion 217 to bear on the lower deformable member 232, applying a compression force to the lower deformable member which is linearly proportional to the load-moment LM.

The lower deformable member 232 can be configured to prevent the lower bearing portion 217 from fully depressing the contact 321 until the load-moment LM reaches a preselected magnitude. This preselected magnitude can be selected to correspond to a load-moment LM approaching the limits of stability for the personnel lift 100 (Figure 1). For ease of reference, such a load-moment will be referred to here as a"critical"load-moment. Thus, as the load-moment LM approaches the critical load-moment, the component of the compression force Fo along the axis 192 will cause the lower bearing portion 217 to compress the lower deformable member 232 until the lower bearing portion fully depresses the contact 321. The fully depressed contact 321 places the lower limit switch 321 in the interrupt mode causing the lift control system 160 (Figure 1) to interrupt any further motion of the extendable boom 130 that will increase the load-moment LM. Examples of motions that would be interrupted because they increase the load-moment LM

would be extensions or rotations of the extendable boom 130 that increase lbef, horizontal distance D shown in Figure 1.

Referring again to Figure 1, when a reverse load RL is applied to the work platform 120, the reverse load-moment RLM introduces a tension force into the lift cylinder 150. This tension force is represented by Ft in Figures 1 and 3. The tension force Ft has a component along the axis 192 that has a tendency to rotate the pivot member 210 about the pivot point 212 in a clockwise direction CW This clockwise rotation causes the upper bearing portion 218 to bear on the upper deformable member 234, applying a force to the upper deformable member which is linearly proportional to the reverse load-moment RLM.

As explained above with reference to the lower deformable member 232, the upper deformable member 234 can be configured to prevent the upper bearing portion 218 from fully depressing the contact 323 until the reverse load- moment RLM reaches a preselected magnitude. This preselected magnitude can be selected to correspond to a reverse load-moment RLM approaching the limits of stability for the personnel lift 100 (Figure 1). For ease of reference, such a reverse load-moment will be referred to here as a"critical"reverse load-moment. Thus, as the reverse load-moment RLM approaches the critical reverse load-moment, the component of the tension force Ft along the axis 192 will cause the upper bearing portion 218 to compress the upper deformable member 234 until the upper bearing portion fully depresses the contact 323. The fully depressed contact 323 also places the upper limit switch 322 in the interrupt mode causing the lift control system 160 (Figure 1) to interrupt any further motion of the extendable boom 130 that will increase the reverse load-moment RLM. An example of motion that would be interrupted because it increases the reverse load-moment RLM would be further rotation of the extendable boom 130 downward against another structure that the work platform was landed against during operation.

Figure 4A is a side elevational view of the load-moment sensing apparatus 110 for the purpose of illustrating its use in accordance with the foregoing discussion. In this embodiment, assume that the load-moment LM (Figure 1) on the personnel lift 100 has reached the critical value such that the personnel lift is on the verge of instability. This critical load-moment could be the result of overloading the work platform 120, overextending the extendable boom 130, or a combination of

these two factors. The critical load-moment LM results in a maximum force 492 along the axis 192 as shown in Figure 4A.

The force 492 tends to rotate the pivot member 210 about the first pivot point 212 in the counterclockwise direction CCW. This counterclockwise rotation is resisted by the lower deformable member 232 bearing on the lower bearing portion 217. The force 492 accordingly compresses the lower deformable member 232 enough to permit the lower bearing portion 217 to fully depress the contact 321 on the lower limit switch 320, as is shown in Figure 4A. As a result, the lower limit switch 320 transmits an interrupt signal to the lift control system 160. The interrupt signal will command the lift control system 160 to limit the motion of the extendable boom 130 (Figure 1) to only motions, such as retractions or upward rotations, that will reduce the horizontal distance D between the work platform 120 and the rotation point 132 (Figure 1). Reducing the horizontal distance D will accordingly reduce the load-moment LM.

As the load-moment LM is reduced, the force 492 will decrease proportionately. As the force 492 decreases, the lower bearing portion 217 is rotated off of the contact 321. This places the lower limit switch 320 back in the operate mode, thus terminating the interrupt signal sent to the lift control system 160 (Figure 1). As a result, the personnel lift 100 becomes fully operational and the operator is again free to position the lift assembly as desired. Thus, by preventing the extendable boom 130 from being positioned in a way that would exceed the critical load-moment, the load-moment sensing apparatus 110 can automatically prevent the personnel lift 100 from being operated in an unstable manner.

Figure 4B is a side elevational view of the load-moment sensing apparatus 110 for the purpose of illustrating its use in accordance with another embodiment of the invention. In this embodiment, assume that the reverse load- moment RLM (Figure 1) on the personnel lift 100 has reached the critical value such that the personnel lift is on the verge of instability. This critical reverse load-moment RLM could be the result of landing the work platform 120 onto another structure and, in an attempt to lower the extendable boom 130 even further, retracting the lift cylinder 150 to the point of lifting the aft wheel-set 182 (Figure 1) off of the ground.

The critical reverse load-moment RLM results in a maximum force 493 along the axis 192 in the opposite direction as the force 492 of Figure 4A, as shown in Figure 4B.

The force 493 tends to rotate the pivot member 210 about the pivot point 212 in the clockwise direction CW. This clockwise rotation is resisted by the upper deformable member 234 bearing on the upper bearing portion 218. The force 493 accordingly compresses the upper deformable member 234 enough to permit the upper bearing portion 218 to fully depress the contact 323 on the upper limit switch 322, as shown in Figure 4B. As a result, the upper limit switch 322 transmits an interrupt signal to the lift control system 160 (Figure 1). The interrupt signal can command the lift control system 160 to limit the motion of the extendable boom 130 (Figure 1) to only movements, such as retractions or upward rotations, that will accordingly reduce the reverse load-moment RLM.

As the reverse load-moment RLM is reduced, the force 493 will decrease proportionately. As the force 493 decreases, the upper bearing portion 218 rotates off of the contact 323. This places the upper limit switch 322 in the operate mode, thus terminating the interrupt signal sent to the lift control system 160 (Figure 1). As a result, the personnel lift 100 becomes fully operational and the operator is again free to position the lift assembly as desired. Thus, by preventing the extendable boom 130 from being positioned in a way that would exceed the critical reverse load-moment, the load-moment sensing apparatus 110 can automatically prevent the personnel lift 100 from being operated in a manner that results in an unstable condition.

The load-moment sensing apparatus 110 of the present invention is configured to sense the maximum allowable boom extension of an elevated work platform or similar machine at the maximum rated lifting capacity. The allowable boom extension at the maximum rated lift capacity will vary according to the angle of the extendable boom. By allowing the extendable boom 130 to approach--but not exceed--the critical load-moment, the load sensing apparatus 110 allows the extendable boom to achieve the maximum extension at the maximum rated lifting capacity for any given boom angle. Further, by holding the load-moment constant, the load-moment sensing apparatus 110 can be configured to automatically adjust boom extension as the angle of the boom is changed, thereby providing maximum boom extension for any boom angle.

For the purposes of illustration, the limit switches 320 and 322 of the foregoing embodiments are shown as being positioned adjacent to the upper and

lower bearing portions 217 and 218 and attached to the respective mechanical stops 242 and 244. It will be apparent to those of ordinary skill in the relevant art that the limit switches can be used in other configurations, and with other pivoting members, to sense load-moments in accordance with the present invention. For example, the limit switch 320 could be attached to the bearing portion 217 of the pivot member 210 adjacent to the mechanical stop 242, and the function of the load-sensing apparatus would remain essentially the same as described above. Thus, other positions of the limit switches 320 and 322, where pivoting motion of the pivot member 210 in response to an applied load-moment will actuate the switches, will be within the scope of the present invention. Those of skill in the ordinary art will also recognize that the use of two deformable members is not imperative to the invention. For example, in the event that reverse loading is not a concern, the upper compressible member and attendant limit switch could be omitted entirely.

In an alternate embodiment of the invention, the limit switches 320 and 322 can be operably connected to other warning systems, instead of, or in addition to, the lift control system 160 (Figure 1). For example, in one such alternate embodiment the limit switches 320 and 322 can be connected to a visible alarm system, such as a light system, that illuminates if the load-moment approaches a critical magnitude, thus alerting the lift operator to the situation. In another embodiment, the limit switches can be connected to an audible alarm system that sounds a suitable audible alarm when the load-moment approaches the critical magnitude. Those of skill in the art will also recognize that the signals sent by the limit switches can be employed in numerous other embodiments to alert the lift operator when the load-moment approaches a critical magnitude.

One advantage of the present invention is the relative ease with which it can be adjusted. For example, if the load-moment the personnel lift 100 can accommodate changes, the load-moment sensing apparatus 110 can be adjusted by simply changing the compressibility of the relevant deformable member as required.

That is, if the allowable load-moment increases, then a less compressible material can be used for the relevant deformable member. The less compressible material will require a larger load-moment to pivot the pivot member until the applicable limit switch contact is depressed or activated. Conversely, if the allowable load-moment decreases, then a more compressible material can be used. The more compressible

material will require a smaller load-moment to pivot the pivot member until the applicable limit switch contact is depressed or activated. Alternatively, another way to adjust the load-moment sensing apparatus of the present invention is to simply reposition the applicable limit switch either closer to, or further from, the adjacent bearing portion of the pivot member 210. For example, if the allowable load-moment increases, then positioning the limit switch further from the bearing portion will require a larger load-moment to depress the limit switch. Conversely, if the allowable load- moment decreases, then positioning the limit switch closer to the bearing portion will require a smaller load-moment to depress the limit switch.

Figure5 is an enlarged partial cut-away isometric view of a load- moment sensing apparatus 501 in accordance with an alternate embodiment of the invention. In one aspect of this alternate embodiment, the load-moment sensing apparatus 501 can be mounted to the boom chassis 140 and incorporated into the personnel lift 100 of Figure 1 in a substantially similar manner as the load-moment sensing apparatus 110. The load-moment sensing apparatus 501 includes a pivot member 510 having a left side plate 511 space apart from a right side plate 513 defining a space therebetween. The pivot member 510 is rotatably coupled to the boom chassis 140 by a pin 514 at a first pivot point 512. The pin 514 passes through the pivot member 510 and is supported on one side by the left side member 141 and on the other side by the right side member 143. The lift cylinder 150 is operatively coupled to the pivot member 510 by a pin 519 at a second pivot point 516 offset from the first pivot point 512. The pin 519 passes through the lift cylinder 150 and is supported by the left and right side plates 511 and 513. In one aspect of this embodiment, the first pivot point 512, the second pivot point 516, and the rotation point 132 (Figure 1) are aligned on a common line. In other embodiments, these points do not need to be aligned to function in accordance with the present invention.

A deformable member 530 passes through the left and right side plates 511 and 513 at a distal end 520 of the pivot member 510. In the illustrate embodiment, the deformable member 530 is a cylindrical bearing pin having a left end 532 that protrudes beyond the left side plate 511, and a right end 534 that protrudes beyond the right side plate 513. The left end 532 mates with a left mechanical stop 541 comprising an adjacent support hole in the left side member 141 of the boom chassis 140, and the right end 534 similarly mates with a right mechanical stop 543

comprising an adjacent support hole in the right side member 143. A movement detector such as a suitable strain gauge 532 or similar analog measuring device is attached to the deformable member 530 between the left and right side plates 511 and 513, respectively. The strain gauge 532 is operably connected in this embodiment to the lift control system 160 shown in Figure 1.

As best seen in Figure 1, when a load-moment LM is applied to the personnel lift 100 through the extendable boom 130, a tension or compression force will be introduced into the lift cylinder 150. As explained above with regard to the pivot member 210, this force will tend to rotate the pivot member 510 in either a counterclockwise direction CCW or a clockwise direction CW. Either way, this rotation will be resisted by the deformable member 530 mated to the mechanical stops 541 and 543 as shown in Figure 5. The resulting deflection of the deformable member 530 will be linearly proportional to the load-moment LM applied to the personnel lift 100 if the first pivot point 512, the second pivot point 516, and the rotation point 132 (Figure 1) are aligned. The strain gauge 532 can accordingly be calibrated so that when the load-moment LM, or the reverse load-moment RLM (Figure 1), reaches the critical magnitude capable of destabilizing the personnel lift 100, an interrupt signal will be transmitted to the lift control system 160 that will limit further motion of the extendable boom 130 to only those movements which will accordingly reduce the load-moment or reverse load-moment. By not permitting the extendable boom 130 to exceed the critical load-moment, the load-moment sensing apparatus 510 automatically precludes the personnel lift 100 from being operated in a manner that would result in an unstable condition.

In an alternate embodiment of the invention, the pivot member 510 can be omitted entirely and the lift cylinder 150 can be connected directly to the deformable member 530. In this embodiment, the deformable member can be at least substantially similar to a bolt attaching the lift cylinder 150 to the boom chassis. A load-moment LM on the personnel lift 100 will accordingly cause a load on the lift cylinder 150, which in turn will be reacted by the deformable member 530. The resulting deflection of the deformable member 530 can be used to measure the load- moment LM applied to the personnel lift 100. The strain gauge 532 can accordingly be calibrated so that when the load-moment LM, or the reverse load-moment RLM (Figure 1), reaches the critical magnitude capable of destabilizing the personnel lift

100, an interrupt signal will be transmitted to the lift control system 160 that will limit further motion of the extendable boom 130 to only those movements which will accordingly reduce the load-moment or reverse load-moment. Thus, the load- moment sensing apparatus of this alternate embodiment only requires a deformable member 530 and a suitably calibrated movement detector 530 to limit load-moments on the personnel lift 100.

Although specific embodiments of, and examples for, the present invention are described for illustrative purposes, various equivalent modifications can be made without departing from the spirit or scope of the present invention, as will be appreciated by those of skill in the relevant art. For example, the teachings provided for a load-moment sensing apparatus can be applied not only to the exemplary mobile elevating work platform described above, but to other extendible systems wherein instability resulting from over extension or overloading is a concern.

These and other changes can be made to the invention in light of the above-detailed description. Therefore, the terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed, but in general should be construed to include all load-moment sensing apparatuses that operate in accordance with the claims. Accordingly, the invention is not limited by this disclosure, but instead its scope is to be determined entirely by the following claims.