Abstract:
A prime mover transfers power to a separate hydraulic circuits via a power take off (PTO). In some situations, a peak torque demand in one of the hydraulic circuits may cause the total torque of all hydraulic circuits to exceed a rated torque at the PTO. An apparatus measures hydraulic fluid pressure in a first hydraulic circuit and adjusts the hydraulic fluid pressure in a second hydraulic circuit. Because pressure is directly correlated to torque at a pump, by adjusting the pressure in the second circuit based on the current pressure in the first hydraulic circuit, the total torque may be capped below the maximum torque rating at the PTO. The first hydraulic circuit may have a higher priority function, such as steering, compared to that of the second hydraulic circuit which may, for example, power a tool.

Description:
TECHNICAL FIELD 
     The present disclosure relates to hydraulic implements and more particularly to limiting torque presented by a hydraulic implement. 
     BACKGROUND 
     A power takeoff (PTO) unit may be used to transfer power between a prime mover, such as a diesel engine, and one or more hydraulic pumps. In applications where two or more hydraulic pump are in use, the total torque of the combined hydraulic pumps may, especially during transient conditions, exceed the torque rating of the PTO and/or the prime mover. Catastrophic failure of the prime mover may result. 
     An example is a road compactor that uses one hydraulic pump to drive a steering circuit and another hydraulic pump to drive a compactor unit. The startup of the compactor unit can cause very high transient pressures in the hydraulic circuit of the compactor hydraulic pump. If this occurs when a steering maneuver is in process, such as turning over a bump, the resulting total pressure demands in the respective hydraulic circuits can, in turn, create very high torques at the hydraulic pumps. The combined torque reflected back through the PTO to the prime mover can cause mechanical failures in critical components of a gear train of the PTO or the prime mover. 
     U.S. Published Patent Application 20070079533 (the &#39;533 application) discloses a hydraulic system that uses a sensor on a hydraulic circuit with a compactor in a machine used for snow surface preparation. A sensor in the hydraulic circuit opens a shunt valve to bypass the compactor when the pressure in the hydraulic circuit exceed a pre-programmed pressure value. The &#39;533 application fails to teach measuring the total pressure in multiple hydraulic circuits to determine when pressure should be reduced in only one of the multiple hydraulic circuits. 
     SUMMARY 
     According to one aspect of the disclosure, a power management system includes a power generation apparatus configured to deliver mechanical power to a plurality of hydraulic pumps. The apparatus may also include a first hydraulic circuit with a first hydraulic pump powered by the power generation apparatus and a second hydraulic circuit with a second hydraulic pump that is also powered by the power generation apparatus. The power management system may further include a valve coupled to the first hydraulic circuit and the second hydraulic circuit. The valve may be configured to limit a pressure in the second hydraulic circuit as a function of a torque load placed on the power generation apparatus by the first hydraulic pump. 
     According to another aspect of the disclosure, an apparatus for use in managing power in a mechanical power system may include a first hydraulic circuit having a first pressure and a second hydraulic circuit having a second pressure. The apparatus may also include a pressure sensor that measures the first pressure in the first hydraulic circuit and a pressure relief valve coupled to the second hydraulic circuit that reduces the second pressure based on a combination of the first pressure and the second pressure. 
     According to yet another aspect of the disclosure, a method of managing power in a mechanical power system may include developing a first pressure in a first hydraulic circuit using a first hydraulic pump driven by a power generation apparatus and developing a second pressure in a second hydraulic circuit using a second hydraulic pump driven by the power generation apparatus. The method may include calculating a torque of the first hydraulic pump based on the first pressure and calculating an allowable pressure in the second hydraulic circuit. The allowable pressure in the second hydraulic circuit may be based on a torque capacity of the power generation apparatus and a current torque of the first hydraulic pump. The method may also include venting the second hydraulic circuit when a pressure in the second hydraulic circuit reaches the allowable pressure. 
     These and other benefits will become apparent from the specification, the drawings and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified and representative view of a first embodiment of a torque limit control apparatus; 
         FIG. 2  is a simplified and representative view of another embodiment of the torque limit control apparatus of  FIG. 1 ; 
         FIG. 3  is a simplified and representative view of another embodiment of a torque limit control apparatus; 
         FIG. 4  is a block diagram of a controller suitable for use in the torque limit control apparatus of  FIG. 3 ; and 
         FIG. 5  is a flowchart of a method of using a torque limit control apparatus. 
     
    
    
     DESCRIPTION 
       FIG. 1  is a simplified and representative view of a first embodiment of the torque limit control apparatus  100 . A prime mover  102 , such as a diesel engine, may mechanically drive a power take off (PTO) unit  104 . In turn, the PTO  104  may drive a first hydraulic pump  106  and a second hydraulic pump  108 . The first hydraulic pump  106  may drive a first hydraulic motor  110  via a first hydraulic circuit  112 . The first hydraulic circuit  112  may be completed at tank  124 , which may be coupled in a known manner to the hydraulic pump  106 . In an embodiment, the first motor  110  may drive a steering unit in a vehicle. 
     The second hydraulic pump  108  may drive a second hydraulic motor  114  via a second hydraulic circuit  116  and tank  122 . In many exemplary embodiments, the tank  122  and the tank  124  may be common with the tank associated with the pumps  106  and  108 . The second hydraulic motor  114 , in an exemplary embodiment, may be drive a compactor used to compress asphalt for pavement. Other applications using two or more hydraulic pumps and corresponding motors are equally applicable. 
     A hydro-mechanical pressure relief valve  118  may be connected to vent hydraulic fluid from the second hydraulic circuit  116  to a tank  120  or another low pressure element, such as a cooling fan (not depicted). As discussed above, in many embodiments, there is only one common tank for the entire machine. The pressure relief valve  118  can be set to open when a pressure in the second hydraulic circuit  116  reaches a set-point pressure. The set-point pressure may be a function of the pressure in the first hydraulic circuit  112 , such that a higher pressure in the first hydraulic circuit  112  lowers the set-point pressure at which the pressure relief valve  118  opens. 
     A proportional relationship may be configured at the pressure relief valve  118  so that hydraulic pressures representing the torque at the hydraulic pumps  106  and  108  do not exceed the rated torque at the PTO  104 . That is, as the pressure in the first hydraulic circuit  112  increases, the pressure in the second hydraulic circuit  116  required to open the pressure relief valve  118  may be lowered. Therefore, the sum of the torques of the first and second pumps  106 ,  108  will not exceed the torque maximum of the PTO  104 . In doing so, both the PTO  104  and the prime mover  102  are protected from damage to their respective gear trains or other internal components. 
     The load of the one of the motors  110 ,  114  may not have as high a priority as the other. In the exemplary embodiment of the road compactor, the first motor  110  may drive the vehicle steering assembly (not depicted) and the second motor  114  may drive a vibratory assembly (not depicted) that does the compacting. Because reducing or eliminating power to the steering assembly could cause a safety issue, it is desirable that only power to the second motor  114  be adjusted to avoid over-torquing the PTO  104 . Therefore, the pressure in the first hydraulic circuit  112  is allowed to range up and down and the pressure in the second hydraulic circuit  116  is controlled to ensure that the output of the two pumps  106  and  108  does not exceed the torque rating at the PTO  104 . 
       FIG. 2  illustrates a variation on the embodiment of  FIG. 1 . An intermediate valve  119  is illustrated inserted between the first hydraulic circuit  112  and the pressure relief valve  118 . In this embodiment, pressure in the first hydraulic circuit  112  must reach an initial threshold value to open the intermediate valve  119  before the pressure in the first hydraulic circuit  112  will influence the set point of the pressure relief valve  118 . When the pressure in circuit  112  reaches an initial threshold value, intermediate valve  119  opens. Flow from intermediate valve  119  through orifice  126  creates a pressure at line  128  that acts on pressure relief valve  118  to reduce or lower the pressure at which  118  opens and begins to relieve or unload circuit  116 . 
       FIG. 3  is a simplified and representative view of another embodiment of a torque limit control apparatus  150 . As above, a prime mover  152 , such as a gas or diesel engine may mechanically drive a power takeoff (PTO) unit  154 . The PTO  154  may drive a first hydraulic pump  156 . The first hydraulic pump  156  may drive a first motor  158  via a first hydraulic circuit  160 . In an embodiment, the first motor  158  may drive a steering assembly for a vehicle, but other applications using a hydraulic motor would be equally supported. As in many hydraulic circuits, the first hydraulic circuit  160  and first hydraulic motor  158  may discharge hydraulic fluid into a tank  161  that returns the fluid to the first hydraulic pump  156 . 
     A second hydraulic pump  170  may also be driven by the prime mover  152  and PTO  154 . The second hydraulic pump  170  may provide hydraulic power to a second motor  172  via a second hydraulic circuit  174 . In an embodiment, the second motor  172  may drive a vibrator assembly (not depicted) for use compacting asphalt paving. Other applications for the second hydraulic motor  172  would be equally supported. For example, in an excavator, the first hydraulic motor  158  may drive the stick and bucket, and the second hydraulic motor  172  may swing the cab and/or move its tracks. As discussed above, the second hydraulic circuit  174  and second motor  172  may discharge fluid to a tank  176  that returns fluid to the second hydraulic pump  170 . In an embodiment, the tanks  161  and  176  may be the same tank or reservoir or may be separate. 
     In contrast to the hydro-mechanical valve  118  of  FIGS. 1 and 2 , the embodiment of  FIG. 3  uses a pressure sensor  162  that may measure the pressure in the hydraulic circuit  160  and send a signal via electrical connection  164  to a controller  166 . The controller  166  may be a standalone processing unit, discussed below with respect to  FIG. 3 , or may be part of an existing controller, such as an engine controller or machine controller. 
     An electro-hydraulic pressure relief valve (EH valve)  178  may allow discharging the second hydraulic circuit  174  to a tank  180  or other low pressure device, such as a cooling fan (not depicted). The EH valve  178  may be any of several commercially available products that use an electrical signal to set a pressure at an input  179  required to open the valve  178 . In the exemplary embodiment, the input pressure is the pressure in the second hydraulic circuit  174 . 
     The controller  166  may evaluate the pressure in the first hydraulic circuit  160 . Using known characteristics of the first hydraulic pump  156 , the pressure in the first hydraulic circuit  160  may be converted to a torque of the first hydraulic pump  156 . This process is discussed in more detail below. By subtracting the torque at the first hydraulic pump  156  from a rated torque or a torque capacity of the prime mover  152  and/or PTO  154 , an allowable torque of the second hydraulic pump  170  may be calculated. Using a process similar to that of calculating the torque at the first hydraulic pump  156  from the pressure in the first hydraulic circuit  160 , an allowable pressure may be calculated for the second hydraulic circuit  174  using the allowable torque at the second hydraulic pump  170 . 
     The controller  166  may then generate a signal via electrical connection  168  that determines a set point of the EH valve  178  to open at the allowable pressure. In some embodiments, the signal is a voltage that controls the set point, in other embodiments the signal may be a current. In other embodiments, the signal may be a digital value. A transfer curve or other formula allows the controller  166  to select an appropriate signal value that sets the opening pressure of the EH valve  178  to the calculated allowable pressure of the second hydraulic circuit  174 . 
       FIG. 4  is a block diagram of an exemplary controller  166  that may be used in an embodiment of a torque limit control apparatus  100 . The controller  166  may include a processor  200  and a physical memory  202  including RAM, ROM, EEPROM, flash memory, bubble memory, magnetic media, or other hardware embodiments, but excludes carrier waves and propagated media. The memory  202  may be coupled to the processor  200  by a data bus  204  that may also couple to an input  206  from a pressure sensor  162  via an electrical connection  164 . The processor  200  may generate a signal to a valve control output  208 . The valve control output  208  may drive the EH valve  178 , with the appropriate signal, be it voltage, current, digital, etc. 
     The memory  202  may include an operating system  210  and may also include conversion routines for performing the math required to calculate torque from pressure and allowable pressure from allowable torque. The memory  202  may also include configuration data including, but not limited to, hydraulic pump displacement values and efficiencies used in the torque/pressure calculations. The configuration data may further include a transfer curve, formula or table that allows the correct output signal for the EH valve  178  based on the allowable pressure of the second hydraulic circuit  174 . 
     INDUSTRIAL APPLICABILITY 
       FIG. 5  is a flowchart  300  of a method of using a torque limit control apparatus  150 . At a block  302 , a first hydraulic pump  156  may pressurize a hydraulic circuit  160  using power supplied by a power generation apparatus, such as a prime mover  152  and power takeoff unit (PTO)  154 . At a block  304 , a second hydraulic pump  170  may pressurize a second hydraulic circuit  174  using power supplied by the power generation apparatus. In an embodiment, the first hydraulic circuit  160  may drive a motor  158  used to steer a vehicle and the second hydraulic circuit  174  may drive one or more motors  172  coupled to vibratory units used to compact asphalt paving, although other embodiments supporting different applications may be supported. In the exemplary embodiment, the motor  158  may have a higher priority during operation because, for example, one motor&#39;s function is associated with safety, while the other motor&#39;s function is not time critical. 
     At a block  306 , torque at the first pump  156  may be determined. In general, torque in a hydraulic pump may be calculated as 
                   τ   =       d   ⁢           ⁢   Δ   ⁢           ⁢   P       η   TOR               Eq   .           ⁢   1               
where τ=torque; d=pump displacement; ΔP=outlet pressure−inlet pressure; and η TOR =mechanical efficiency of the pump. In the exemplary embodiment of  FIG. 3 , the inlet pressure is atmosphere so ΔP=outlet pressure. For a fixed displacement pump, displacement d is a constant and the efficiency η TOR  may be assumed to be a constant so that the term
 
             d     η   TOR           
is a constant and the calculation of torque reduces to a fairly simple arithmetic problem.
 
     Therefore, the torque at the first pump  156  may be determined using the pressure in the first hydraulic circuit  160  as measured at a pressure sensor  162  and multiplying by the constant. The pressure in the first hydraulic circuit  160  is a function of activity at the motor  158 . In an embodiment where the motor  158  is part of a steering assembly, the motor  158  may demand very little torque from the first pump  156  during straight-line, flat surface operation of a vehicle. However, during turns, maneuvering over uneven terrain, or combinations of those may result in fairly significant demands on the first pump  156  so that it&#39;s torque output may increase significantly over its quiescent state. 
     At a block  308 , an allowable torque in the second hydraulic pump  170  may be determined. The allowable torque in the second hydraulic pump  170  may simply be the maximum torque available at the power takeoff  154 , or other associated powertrain, minus the torque at the first hydraulic pump  156 , or
 
τ pump 2 max =τ PTO max −τ pump 1   Eq. 2
 
     Calculating the allowable pressure in the second hydraulic circuit  174  is performed using a different form of Eq. 1, so that at a block  310 , a maximum allowable pressure in the second hydraulic circuit  174  may be calculated. 
     
       
         
           
             
               
                 
                   
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     In an embodiment, an EH valve  178  may be set at a block  312  to the calculated maximum pressure for the second hydraulic circuit by a controller  166  using known transfer curves for the EH valve  178 . At a block  314 , when the pressure in the second hydraulic circuit  174  reaches the allowable pressure, the EH valve  178  may open. Then, at a block  316 , the EH valve  178  may discharge the second hydraulic circuit  174 . In an embodiment, the discharge may halt operation of the second motor  172 . In other embodiments, particularly when the over pressure situation is an impulse function, the EH valve  178  may close as soon as the pressure drops and the function of the second motor  172  may only be temporarily interrupted so that an operator may not notice the effect. 
     In an embodiment, the controller  166  may be coupled to a body or implement controller (not depicted) so that events that cause an impulse in the second motor  172  may be monitored. For example, in an embodiment, the most likely time for a torque impulse is when the second motor  172  is turned on and the vibratory assembly starts from a full stop. Because the controller  166  may have information about an operator&#39;s activation of the vibratory assembly, the controller  166  may be able to proactively adjust down the set point of the EH valve  178  and thereby limit the maximum pressure in the second hydraulic circuit  174  during the startup period. Because the impulse in the second hydraulic circuit  174  may thus be avoided, a possible torque overload at the PTO  154  and prime mover  152  may also be avoided. 
     Similarly, the controller  166  may monitor the operator controls (not depicted) to predict a change or spike in pressure in the first hydraulic circuit  160  so that the set point of the EH valve  178  may be adjusted down proactively to more readily vent the second hydraulic circuit  174  and thereby avoid a torque overload due to the impending change of pressure in the first hydraulic circuit  160  and corresponding increase in torque at the first hydraulic pump  156 . 
     Because the embodiment of  FIG. 3  uses a controller  166  to monitor and analyze conditions in the hydraulic circuits of a machine, the torque limiting function can be applied to more than the two hydraulic circuits illustrated. By setting a priority of operation and by monitoring pressures and pending actions, additional EH valves in those additional circuits can be dynamically adjusted to provide their associated functions in a priority combination while preserving the total torque demand on the PTO  154  and prime mover  152 . 
     While the above method is described in view of the embodiment of  FIG. 3 , an equivalent process can be applied to the hydro-mechanical valve embodiment of  FIGS. 1 and 2 . The torque conversions are inherent in the calibration of the relief pressure settings of the pressure relief valve  118 . 
     The various apparatus and method discussed above benefit both manufacturers and operators of large hydraulically-driven equipment. Manufacturers increase equipment reliability and reduce maintenance costs when the equipment operates within the limits of the various components, even under impulse loads. Operators simply benefit when their equipment performs reliably and does not need to be taken out of service for repairs or other maintenance. 
     In accordance with the provisions of the patent statutes and jurisprudence, exemplary configurations described above are considered to represent a preferred embodiment of the invention. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.