Abstract:
A method and apparatus for operating a load lifting device to move a load and hold the load steady is disclosed. In the conventional arts, a lifted or lowered suspended load had a tendency to move from its desired position, as hydraulic fluid in the lifting system cooled, e.g. a stick slip condition. The present invention monitors a fluid pressure in the lifting system and compensates the fluid pressure to accommodate any pressure drop due to fluid cooling. Thereby, the load can be effectively held steady at a desired position, and the stick slip condition can be avoided.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a control system for a hydraulic cylinder. More particularly, the present invention relates to a control system which can stabilize a hydraulic cylinder under load. 
     2. Description of the Relevant Art 
     A hydraulic cylinder is often employed in a load lifting device, such as a crane. Fluid is supplied to, or removed from, the hydraulic cylinder to cause a piston to move within the hydraulic cylinder. Movement of the piston enables a boom of the load lifting device to lift or lower a load. 
     When the load is lifted, or lowered, to a desired height, an operator of the load lifting device deactivates a control to stop the fluid flow relative to the hydraulic cylinder. At this point, movement of the boom stops. Then, workers in the vicinity of the load remove, modify or otherwise interact with the load. 
     A natural phenomenon is known to occur once lifting or lowering of the load is stopped at the desired height. Specifically, the load will sometimes slightly lower, despite the fact that the operator has set the control to stop movement of the load. This phenomenon has been called a “stick slip condition” in the art. 
     The “stick slip condition” can be very concerning, particularly when workers are in the vicinity of the load. For example, a worker could be injured under the load, pinned between a shift in the load, or could lose their balance when the load moves. 
     The “stick slip condition” occurs because of a cooling of the hydraulic fluid. When fluid is repeatedly pumped into, and evacuated from, hydraulic components in the system, the temperature of the fluid in the cylinder will be raised significantly. Further, the temperature of the mechanical system handling the fluid will rise. Once the operator controls the load lifting device to stop movement of the load, fluid is no longer pumped into or evacuated from the hydraulic cylinder. As the fluid and mechanical system sit idle, they cool. This causes the pressure in the hydraulic cylinder to decrease. The pressure decreases because of a change in the energy of the fluid as it cools (i.e. a thermal fluid contraction), and a change in the static friction of the mechanical system as it cools. 
     Eventually, the pressure in the hydraulic cylinder will decrease to a point where the force on the piston in the cylinder, due to the load attached thereto, is greater than the system&#39;s static pressure in the hydraulic cylinder supporting the piston, plus the mechanical friction. When this occurs, the piston will move, and hence the load will slightly lower, until a new equilibrium inside the hydraulic cylinder is established. If the mechanical system static friction is large enough to support the load to a significant degree, then the piston motion that results from that static friction finally being overcome can be substantial and very sudden (i.e. the “stick slip condition”). This cycle may repeat itself numerous times as the fluid continues to cool. 
     Once the fluid cools to the environmental temperature, the lowering cycles of the load will stop and the “stick slip condition” will cease. However, in the typical operation of a crane, it would be undesirable to allow a load to remain at a desired height for the amount of time needed for the fluid to completely cool, and the possibility of a “stick slip condition” to pass. Such a practice would greatly increase the time and money required in typical construction projects. Therefore, there is a need in the art for a system which can effectively reduce or eliminate the occurrence of a “stick slip condition” immediately upon raising or lowering a load to a desired height. 
     A first solution in the background art has been to provide a pinning system. In the first solution, once the load is elevated or lowered to the desired height a physical pin is inserted through aligned holes in moveable sections of the boom to physically link the boom sections together. The weight of the load is essentially held by the pins. Hence, if the pressure in the hydraulic cylinder drops, the load will not lower. 
     The first solution has drawbacks. The cost and maintenance associated with a pinning system must be added to the boom. The pinning system itself adds weight to the boom. Further, the drilling of holes through the boom sections makes it necessary to enlarge the size of the boom sections in order to maintain a suitable strength for the boom sections 
     Another drawback of the first solution is that only a finite number of holes are provided in the boom sections. Therefore, the load has to reside at one of only a few possible heights in order for the pins to pass through the aligned holes in the moveable boom sections. Often, the closest “lockable” height for the load is not the optimum, or even a desirable, height in a particular circumstance. 
     As an alternative to the pinning solution, a second solution has been proposed in the background art. In the second solution, hydraulic pressure is maintained on the piston or rod side of the hydraulic cylinder. This pressure acts to buffer or dampen any movement of the piston as the hydraulic fluid cools. 
     The second solution does not prevent the “stick slip condition,” but rather smoothes the decent of the load as its lowers, by preventing a violent downward lurch in the load. The second solution reduces the likelihood that the load will shift, and may provide some additional time for a worker in the vicinity of the load to react by getting out of the way, or maintaining their balance when working around the load. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to address one or more of the drawbacks associated with the background art. 
     Further, it is an object of the present invention to provide a method and system to prevent a “stick slip condition.” 
     Further, it is an object of the present invention to provide a control system and method for operating a load lifting device which improves the safety and accuracy of its operation. 
     Further, it is an object of the present invention to provide a control system and method for operating a load lifting device which maintains a constant pressure in a hydraulic cylinder sufficient to hold a load at any desired height. 
     Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitative of the present invention, and wherein: 
     FIG. 1 is a block diagram illustrating the component parts of a control system, in accordance with the present invention; 
     FIG. 2 is a flow chart illustrating a method of operation for the control system of FIG. 1; 
     FIG. 3 is a flow chart illustrating a method of operation for the control system when multiple hydraulic cylinders are involved; and 
     FIG. 4 is a block diagram illustrating the component parts of a control system, in accordance with an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates one embodiment of a control system, in accordance with the present invention. The control system controls movement of a piston  10  within a hydraulic cylinder  12 , which in turn controls movement of a load  14  connected to a rod  16  of the piston  10 . 
     Primary movement of the piston  10  in one direction may be achieved via any known conventional manner. For example, the piston  10  is moved by activating a main control valve  20  to supply pressurized hydraulic fluid to a piston head-side  24  of the hydraulic cylinder  12 . The hydraulic fluid causes the piston  10  to move. Meanwhile, hydraulic fluid leaves the hydraulic cylinder  12 , via the main control valve  20 , in fluid communication with a piston rod-side  18  of the hydraulic cylinder  12 . 
     Primary movement of the piston  10  in the opposite direction may again be achieved via any known conventional manner. For example, the main control valve  20  directs fluid to enter the piston rod-side  18  of the hydraulic cylinder  12 , while a counter balance valve  22  allows hydraulic fluid to leave the piston head-side  24  of the hydraulic cylinder  12 . 
     Now, with reference to FIG. 1, the components associated with the compensation control system to hold the piston&#39;s position steady to avoid a “stick slip condition” will be explained. A microprocessor  26  is provided to oversee the control system. Of course, the microprocessor  26  would have associated RAM and ROM memory, either internal or external, to facilitate its operation. A rod dump valve  23  is controlled by an output  46  to relieve the rod-side pressure  18  in the cylinder. This element of the system eliminates the need to compensate for the effects of the rod-side fluid, which would also be cooling. 
     A compensator pump/motor  28  is controlled by a first output  29  of the microprocessor  26 . The compensator pump/motor  28  is operable to draw hydraulic fluid from a common reservoir  30  and deliver the hydraulic fluid, via a first conduit  32 , to a valve, such as a compensator control valve  34 . A relief valve  36  is also in fluid communication with the first conduit  32  in order to limit the pressure of hydraulic fluid in the first conduit  32 . 
     The compensator control valve  34  is normally in an “off” condition, such that pressurized hydraulic fluid may not pass therethrough. However, the compensator control valve  34  is electrically controlled by a second output  35  of the microprocessor  26 . The second output  35  may take the form of an output pulse having a “high” state and a “low” state. The “high” state of the second output  35  causes a solenoid of the compensator control valve  34  to activate an internal valve, so as to place the compensator control valve  34  into an “on” condition. In the “on” condition, the compensator control valve  34  allows hydraulic fluid to flow therethrough. 
     Hydraulic fluid passing through the compensator control valve  34  travels, via a third conduit  49  into the piston head-side  24  of the hydraulic cylinder  12 . Hence, the pressure on the piston head-side  24  of the hydraulic cylinder  12  can be subsidized, when the compensator pump/motor  28  and the compensator control valve  34  are activated by the microprocessor  26 . 
     A transducer  38  is connected to the piston head-side  24  of the hydraulic cylinder  12 . The transducer  38  measures a pressure of the hydraulic fluid on the piston head-side  24  of the hydraulic cylinder  12 . The measured pressure is transmitted to the microprocessor  26 , via a control line  40 . The measured pressure is an analog signal, which is converted into a digital signal via an analog to digital (A/D) converter of the microprocessor  26 . 
     Of course, the microprocessor  26 , which controls the inventive compensation control system for the piston  10 , could also be used to control the conventional primary movement system for the piston  10 . For example, the microprocessor  26  could receive first and second inputs  42 ,  44  to signal that the load is to be extended or retracted, respectively. Further, in addition to output  46 , provided to control the rod dump valve  23 , similar outputs (not illustrated) could control the other necessary components for primary movement, such as the main control valve  20 . As illustrated in FIG. 1, it is also contemplated that the microprocessor  26  would have additional inputs, such as a reset input  48  to receive a reset signal from a user activated control panel. 
     Now, with reference to FIG. 2, a flowchart explaining an operational procedure for the present invention will be explained. It is contemplated that a series of self-diagnostic tests or error checks would be performed prior to initiation of the operational procedure to ensure that all of the system&#39;s components were fully operational. To this end, lights, gauges, or other indicators would be provided in the operator&#39;s area to indicate faults, an activated state, and/or pressures measured during the operational procedure 
     In step S 100 , it is determined whether or not the compensation control system has been activated. If not, the procedure waits until the compensation control system has been activated. If so, processing proceeds to step S 102 . Activation of the compensation control system could be accomplished via a switch located in the user&#39;s area of the load lifting device. 
     In step S 102 , it is ascertained if the lifting device is in operation. For example, it is ascertained if a hoist is being operated, a boom is being extended, retracted, tilted or swiveled, etc. If operation of the lifting device is ongoing, the procedure continues to monitor the operation until the operation stops. Once operation of the lifting device stops, the process continues to step S 104 . 
     “Stopping” of the load lifting device may be defined as a lack of operation of the load lifting device for a predetermined time, such as five seconds, or activation of a “stop” switch by a user of the load lifting device. Once the load lifting device is stopped, the main control valve  20  is closed and the rod dump valve  23  is opened. 
     In step S 104 , the hydraulic pressure in the hydraulic cylinder  12  is measured, via the transducer  38 . Next, in step S 105 , the measured pressure is stored in a memory as P(start) by the microprocessor  26 . In step S 106 , the microprocessor obtains two values X and Y. The values X and Y relate to changes in the pressure of the hydraulic fluid in the hydraulic cylinder  12 , which can occur without causing a stick slip condition to occur, or the load to be lifted, respectively. The value X would be a tolerable drop in psi in the hydraulic cylinder  12 , which could occur without the occurrence of a stick slip condition. The value Y would be a tolerable increase in psi in the hydraulic cylinder  12 , which could occur without resulting in any lifting of the load. 
     In step S 108 , the current pressure P in the hydraulic cylinder  12  is measured using the transducer  38 . Next, in step S 110 , the current pressure P is compared to P(start)−X. If the current pressure P is not less than P(start)−X, the procedure returns to step S 108 . By this arrangement, the procedure continually monitors the current pressure P in the hydraulic cylinder until the current pressure drops below P(start)−X. 
     Once the current pressure P drops below P(start)−X, the procedure moves to step S 112 . In step S 112 , the pressure in the hydraulic cylinder  12  is increased to P(start)+Y, such as by activation of the compensator pump/motor  28  and the compensator control valve  34 . 
     The pressure P(start)+Y is insufficient to cause movement of the piston  10 , and hence lifting of the load  14 . The reason why the pressure is increased to a value above P(start) is because the pressure in the hydraulic cylinder  12  will have to be adjusted less frequently, which results in less wear and tear on the associated valves, pumps, and motors. Of course, it would be possible to obtain the benefits of the present invention by simply raising the pressure in the hydraulic cylinder  12  up to only P(start), if so desired. In such an event, Y would equal zero and there would be no need to include or process the variable Y. Also, Y could equal a negative number, which would result in raising the pressure in the hydraulic cylinder  12  up to a value less than P(start). Compensating the pressure to a point below the initial pressure P(start) would also be adequate to prevent motion of the load. Further, compensating the pressure to a point below the initial pressure P(start) could prove to be more certain in preventing unwanted movement of the load. 
     Once the pressure is raised to P(start)+Y, the procedure returns to step S 108 . The procedure continues to monitor the pressure in the hydraulic cylinder  12  and to supplement that pressure should it drop below P(start)−X. 
     In a preferred manner of operation, the step S 102 , wherein it is ascertained if the lifting device is being operated, would function as an interrupt signal to the microprocessor  26 . In other words, if the lifting device is being operated by a user, such as by having its hoist or boom operated, the procedure would stop executing and return to step S 100 . The interrupt procedure prevents the compensatory hydraulic fluid system from operating at the same time as the primary hydraulic system. 
     The values X and Y may be fixed numbers based upon the type of components employed in the lifting device. However, more preferably, the values X and Y are variables stored in a look-up table, which is indexed by a P(start) value. Alternatively, the values X and Y could be determined by an equation, having P(start) as a variable. In either event, the values of X and Y will be dependant upon the value of P(start). By this arrangement, the values of X and Y when the load lifting device is holding a heavy load steady will be different from the values of X and Y when the load lifting device is holding a relatively lighter load steady. Further, the values of X and Y could be influenced by the ambient temperature. A typical range of the pressures involved might be: P=500 to 3,000 psi; X=10 to 100 psi; and Y5 to 20 psi. 
     Of course, if more than one hydraulic cylinder  12  is employed to lift a load  14 , one would apply the teachings of the present invention to each hydraulic cylinder supporting the load  14 . However, it is envisioned that the microprocessor  26  would be able to control the entire system if multiple hydraulic cylinders  12  were used. Further, it would be possible to use a common compensator pump/motor  28  and a separate compensator control valve  34  for each cylinder to provide compensatory hydraulic fluid to multiple hydraulic cylinders. 
     FIG. 3 is a flow chart illustrating an operational procedure when plural hydraulic cylinders are employed in the load lifting device. Steps S 100  and S 102  are identical to the steps described in association with FIG.  2 . 
     In step S 104 ′, the pressures P(k) of each hydraulic cylinder used to move the load are measured, where k equals 1, 2, . . . up to the number of the cylinders used. In step S 105 ′, the measured starting pressures are stored as P(start)(k) for each of the cylinders, e.g. P(start)( 1 ), P(start)( 2 ), etc. 
     In step S 106 ′, the values X and Y are obtained by the microprocessor  26 . The values of X and Y may be the same for each cylinder, or there may be specific X(k) and Y(k) values for each cylinder, particularly if the cylinders are of different types or sizes. 
     Next, in step S 200 , a variable k is set equal to 1 (meaning that the first hydraulic cylinder will be analyzed first). In step S 108 ′, the current pressure in hydraulic cylinder (k) is measured. 
     Next, in step S 110 ′, the current pressure P(k) is compared to P(start)(k)−X. If P(k) is less then P(start)(k)−X, then the pressure in hydraulic cylinder k is increased to P(start)(k)+Y in step S 112 ′. If P(k) is not less than P(start)(k)−X, the procedures moves to step S 202 . 
     In step S 202 , it is evaluated if k equals the total number of hydraulic cylinders used by the load lifting device to lift the load  14 . In other words, has the last cylinder been analyzed? If not, the variable (k) is incremented in step S 203 , and the next hydraulic cylinder is analyzed by proceeding to step S 108 ′. If the last cylinder has been analyzed, the procedure moves to step S 200 , where the variable k is reset to equal 1, and the procedure goes back to measure the current pressure in the first hydraulic cylinder. 
     By the method of FIG. 3, multiple hydraulic cylinders may be monitored and compensatory hydraulic fluid may be applied thereto to prevent a stick slip condition. 
     FIG. 4 is a view illustrating an alternative embodiment for a control system, in accordance with the present invention. Like component parts have been assigned same reference numerals. Basically, the alternative control system employs an electro-proportional pressure reducing valve  50  instead of the compensator control valve  34 , described in conjunction with FIG.  1 . 
     The electro-proportional pressure reducing valve  50  receives a control signal  52  from the microprocessor  26 . The control signal  52  includes a pulse width modulated signal which controls the electro-proportional pressure reducing valve  50 , such that the hydraulic fluid flow therethrough can be controlled. The electro-proportional pressure reducing valve  50  is able to more accurately control the compensatory hydraulic fluid applied to the hydraulic cylinder  12 , as compared to the solenoid-driven, compensator control valve  34  of FIG.  1 . 
     The invention being thus described, it will be obvious that the same may be varied in many ways. For example, the outputs and inputs of the microprocessor  26  have been illustrated as hardwired, however wireless signals may be transmitted by and received at the microprocessor  26 . Although the illustrated transducer  38  has an analog output, the transducer  38  could be replace by a transducer having a digital output. This would obviate the analog to digital conversion which takes place in the microprocessor  26 . 
     Although the present invention compensates for a pressure change in a hydraulic cylinder  12  by applying compensatory hydraulic fluid, it is envisioned that the pressure could be held constant by controllably changing the volume of the hydraulic cylinder  12 . For example, a compensatory piston could be provided on the piston head-side  24  inside the hydraulic cylinder  12 , or a bladder, so as to form the circular wall of the piston head-side  24  of the hydraulic cylinder  12 . By moving the compensatory piston of the bladder toward the primary piston  10 , one could increase the hydraulic pressure on the piston head-side  24 , so as to compensate for a loss of hydraulic pressure due to a drop in temperature. 
     Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.