Patent Application
20120130600
This patent disclosure relates generally to control of grading operations and, more particularly to a method and system for controlling wheel slip during a grade to level operation.
Motor graders are often used for construction, road-building, rural road resurfacing, shallow ditching, field preparation, and other industrial activities requiring the preparation of a flat earthen or particulate surface. A motor grader typically includes a ground engaging element such as a plurality of wheels that convey the machine over the ground, and a large blade extending from the underside of the machine and disposed generally transverse to both the underlying surface and the direction of travel. The blade can generally be manipulated in a plurality of directions and dimensions by the machine operator.
As the machine traverses over the ground, the blade removes and displaces surface material that comes into contacting engagement with the blade. The vertical amount of material removed in a pass is referred to as the cut depth. Typically, the cut depth is operator controlled depending upon a subjective analysis of the operating conditions as well as the final desired grade level. It will be appreciated that, for larger cut depths, the impedance to the blade, and hence to the movement of the machine itself, can become rather large. In extreme cases, the machine wheels may begin to slip, causing the motor grader to mar or otherwise cause unevenness in the surface being treated. This can result is a substantial loss of productivity as the surface must now be repaired in many cases. Alternatively, to avoid the likelihood of wheel slip, the operator may take multiple passes of overly shallow cut depths. This also results in a substantial loss of productivity.
Current electronic blade control systems such as GPS-guided grading systems provide very precise elevation determination by precisely tracking the location of the blade edge. However, such systems do not prevent the machine from spinning out, i.e., spinning one or more wheels against the underlying surface, if the operator attempts too deep of a cut in a single pass. Although an operator may prevent spinouts by taking many light passes, removing only a small amount of material each time, this technique is also damaging to productivity. In particular, this technique requires substantially more time and fuel than would be needed if the machine were operated closer to its traction limit.
It will be appreciated that this background description has been created by the inventors to aid the reader, and represents concepts known to the inventors. It is not a discussion of, nor reference to, prior art, nor is this section intended to imply that any of the indicated problems were themselves appreciated in the art. While the principles described herein can, in some regards and embodiments, avoid the problems described, it will be appreciated that the scope of the protected innovation is defined by the attached claims, and not by the ability of the claimed invention to solve any specific problem noted herein.
The described system and method are implemented with a motor grader or other machine for grading of surfaces. The machine includes a ground engaging element, as well as one or more blades for removing surface material. In this context, the described system and method prevent slippage of the ground engaging element against the underlying surface. A torque limit is applied to the ground engaging element based upon available traction. The torque limit corresponds to a torque that is less than that required for slippage, thus avoiding the problems caused by both overly aggressive and overly conservative cut depth strategies.
In one embodiment, the torque limit is a settable limit that is determined by a user based upon an observation of current operating conditions of the ground engaging element. In another embodiment, the torque limit is determined by detection of available traction through any, or a combination of, a traction control system, wheel speed sensors, radar, or other suitable mechanism.
In general, this disclosure relates to motor grader machines used for grading of surfaces, i.e., smoothing an earthen or other particulate surface for construction, road building, and other application that requires the creation of a relatively smooth or flat surface. As noted above, a motor grader typically includes a plurality of wheels that convey the machine over the ground, as well as one or more large blades beneath the machine for removing surface material. In this context, the described system prevents a loss of productivity due to spinouts. In particular, the system allows the operator to set the wheel torque limit below a slip point that may be determined based upon current conditions, thus preventing any wheel spin that would cause the finished surface to be repaired. In one realization of the described system, a CVT (continuously variable transmission) of the motor grader is used to limit the torque delivered to the wheels.
With the wheel torque set at a level that provides a maximum level of torque while still avoiding wheel slip, the system minimizes the number of passes that an operator executes to reach the desired grade. The system thus assists the operator by avoiding the problems caused by overly aggressive and overly conservative cut depth strategies.
With this overview in mind, specific details of the described principles and system will now be discussed.
The engine frame 102 supports an engine (not visible), which is protected from the elements by an engine cover 116. The engine provides the power necessary to propel the motor grader 101 as well as to operate the various actuators and systems of the motor grader 101. As can be appreciated, other machines may have different configurations and/or various other implements associated therewith. In a hydrostatically operated machine, the engine in the engine frame 102 may be associated with a hydrostatic pump (not shown), which may be part of a propulsion system of the motor grader 101. In the embodiment shown, the motor grader 101 is driven by two sets of drive wheels 118 (only one set visible), with each set including two drive wheels 118 that are arranged in a tandem configuration along a beam 120. Two such beams 120 are pivotally connected on the ends of a shaft or axle 122 at a respective pivot joint or bearing 123, with one beam 120 disposed on either side of the motor grader 101.
The axle 122 is connected to the engine frame 102 of the motor grader 101 via mounting plates and stabilizer bars such that the drive wheels 118 can effectively propel the motor grader 101. In an alternative embodiment, the axle 122 may be omitted and the beams 120 may instead be pivotally connected directly to the engine frame 102. At least one or both of the two drive wheels 118 on the beam 120 may be actively rotated.
Although a motor grader such as motor grader 101 shown in
The motor 202 comprises a similar arrangement including a number of pistons 206 in respective chambers. The pistons 206 of the motor 202 are slidably engaged upon a fixed swash plate 207. The chambers of the pistons 205 of the pump 201 are in fluid communication with the chambers of the pistons 206 of the motor 202 via hydraulic fluid that fills the chambers and intervening conduits (not shown). The chambers for the pistons 206 are formed in a motor carrier 210 that rotates the motor output shaft 211. As the angle of the swash plate 203 is varied, the amount of fluid displaced by the pistons 205 of the pump 201 (and thus the fluid volume received or taken from the chambers of the pistons 206) varies.
Because of these interrelationships, the torque and/or output speed of the motor 202 varies with the angle of swash plate 203. In overview, the swash plate actuator 204, which in this example operates on differential hydraulic pressure, is driven via solenoid valves (not shown in
The pistons 300, 301 are joined by a bar 306 which has a central pivot pin 307 mounted thereon. The central pivot pin 307 interferes within a slot 308 in a swash plate arm 309, such that the lateral position of the bar 306 establishes the position of the swash plate arm 309 and hence the angle of the swash plate itself (not shown). The bar 306 is biased to a central position by opposing springs 312. As the bar 306 is displaced from this central position, there is a restoring force exerted by springs 312 that is proportional to the displacement.
The lateral position of the bar 306 is determined by the positions of the pistons 300, 301 within the cylinders 302, 303. The positions of the pistons 300, 301 are determined by the difference in hydraulic pressure between the pressure chambers 304, 305. Respective pressure valves 310, 311 independently control the pressure within chambers 304, 305. In an example, the pressure valves 310, 311 are solenoid valves that supply hydraulic fluid at a pressure that is set by an applied current within limits set by a supply pressure. Thus, in the illustrated example, each valve 310, 311 has at least a current input (illustrated as inputs A and C) and a fluid input (illustrated as inputs B and D). Typically, solenoid valves can supply fluid at any pressure between zero and the fluid pressure at the fluid input B, D. The pressure response of a solenoid valve such as solenoid valves 310 and 311 to a current input is a function of various components and their tolerances.
Because the distance between the pistons 300, 301 is fixed by the length of the bar 306, it is the pressure differential between chambers 304, 305 rather than the absolute pressure within each chamber 304, 305 that establishes the position of the bar 306. In particular, when the bar 306 is in such a position that the net displacement force differential between the pistons 300, 301 is equal to the net restoring force exerted by springs 312, the system is in equilibrium.
Considering
The present disclosure is not limited to the particular electro hydraulic solenoid controlled pump and motor arrangement described in conjunction with an illustrated embodiment. To the contrary, other pump and/or motor controlled technologies that are mechanical or hydraulic may be used to supply torque to the wheels. For example, the disclosure may utilize other pump and motor constructions without axial pistons and swash plate arrangements. Such systems may not use hydraulic pumps or motors at all so long as an output torque limit may be set in accordance with the principles described herein.
To better understand the torque control provided by the presently describe principles, the flow chart of
In an embodiment, the torque limiting user interface element is a knob, slider, or other multi-setting or continuously variable setting device. In an aspect of this embodiment, the user preferably sets the torque limiting user interface element to a position corresponding to a value less than a highest value that does not permit wheel slippage on the underlying surface. Thus, for example, on a hard wet surface or a hard surface covered by loose debris such as sand, the maximum torque attainable without slippage may be much lower than the maximum torque usable on another surface type. To identify the appropriate torque limit in any given environment, the user may perform a trial cut, or brake the non-driven wheels, or provide other resistance, while increasing the applied torque until slippage occurs. Such a trial cut may be performed as the operator first uses the equipment at the beginning of a shift, at the beginning of the day, or at any other convenient time.
Once the selected torque limit Tmax, is provided to the controller at stage 401, the controller establishes an appropriate maximum set point with respect to a variable that varies as a function of output torque. In the described example, the controller establishes an actuator pressure limit Pmax at stage 403 to limit the available circuit pressure within the variator. As will be appreciated, Pmax is selected such that at an actuator pressure of Pmax, the output torque of the variator, multiplied or reduced as required by any driveline ratio, corresponds to the maximum output torque Tmax.
At stage 405, the controller receives a torque command Tcom from the user interface, e.g., via an accelerator lever or pedal. Optionally, the torque command Tcom may be received from an automated source, pursuant to a user speed control command. The received torque command Tcom is converted to a required motor displacement D, which may correspond to an actuator movement command Acom that is necessary to achieve the required motor displacement at stage 407. That is, the actuator movement command Acom corresponds to an actuator position that establishes a desired swash plate angle, which in turn achieves a motor displacement D. This conversion may be executed via a mapping, a table, a calculation or other suitable means of deriving an actuator pressure to provide a desired variator output torque.
At stage 409, the process 400 determines whether the motor displacement D results in an excess circuit pressure. In the illustrated embodiment, the process determines whether the motor displacement results in a circuit pressure Pcir that exceeds the maximum pressure Pmax. If it is determined that the circuit pressure Pcir does exceed the maximum pressure Pmax, then the process flows to stage 411. At this stage, the controller commands an actuator pressure of Pmax, to set the swash plate angle, and thereby the displacement of the motor 202. If it is instead determined that Pcir does not exceed Pmax, then the process 400 flows to stage 413, wherein the controller commands an actuator pressure of Pcir, corresponding to the current motor displacement
The process 400 repeats at a rate determined by the controller cycle, receiving new torque commands, and providing an actuator pressure command that is either Pcom or Pmax, depending upon their relative magnitudes. In this way, the system prevents wheel slippage without the operator being required to grade the surface by employing multiple overly shallow passes.
The operational impact of the described torque limiting control system can be seen in the simplified grade schematics of
The first cut 505 removes the top portion of the obstruction 503. A second cut 507 removes a portion of a shallow second layer, and a third cut removes the remainder of the shallow second layer. The transition 511 between the second cut 507 and the third cut 509 will generally be a gradual transmission caused by the operator raising the blade at the end of the second cut 507 while continuing to move forward.
The schematic 520 shows another series of cuts used to partially reduce a similar obstruction 523 of the same height h, with the goal of eventually reaching a desired grade 533. In contrast to schematic 501, the cuts shown in schematic 520 assume that the described system of torque control is being employed, and that the operator is using the maximum torque that will not result in wheel slippage.
The first cut 515 removes a large portion of top of the obstruction 523. The second cut 517 removes a portion of a deep second layer, and the third cut removes the remainder of the deep second layer. The transition 521 between the second cut 517 and the third cut 519 is now abrupt rather than gradual, allowing the operator to easily identify the end of the second cut 517, i.e., the location at which to begin the third cut 519. The abrupt nature of the transition 521 is due to the fact that the operator simply drives into the second cut 517 until the torque limit is reached, at which point the machine stops. At this point, the operator raises the blade and/or backs away, and then approaches for the third cut 519.
At stage 601 of the process 600, the machine controller obtains a torque limit Tmax value from any of a number of sources, such as a value stored in memory or via a torque limiting user interface as described above. During operation of the machine after this point, torque commands of greater than Tmax will be reduced to Tmax, while torque commands of less than Tmax will be applied without modification. The controller may also set a timer, e.g., for one hour, at stage 601. At stage 603, the controller detects whether a slip event has occurred. By way of example, the controller may obtain a signal from a traction control system that determines an overspin condition exists, such as when the wheel speed of one wheel relative to another wheel exceeds a threshold. If the controller determines at stage 603 that a slip event has occurred, then the process proceeds to a stage 605 and sets a new torque limit Tmax, which may correspond to a maximum torque that is equal to, or somewhat less than, the torque at which the slip event occurred. From stage 605, the process returns to stage 601.
If the controller determines at stage 603 that a slip event has not occurred, then the process proceeds to stage 607, wherein the controller determines whether the timer has elapsed. The timer may allow the controller to periodically alter the maximum torque limit if warranted by current conditions even in the absence of wheel slip. If at stage 607 the controller determines that the timer has elapsed, the process proceeds to stage 609 and resets the torque limit setting Tmax to a maximum available torque or other value higher than the present Tmax. The new torque limit Tmax being greater than the current torque limit setting Tmax enables the machine to operate at greater torque, such as when conditions permit such operation. This permits system testing and updates to occur on a regular basis. On the other hand, if at stage 607 the controller determines that the timer has not elapsed, the process returns to stage 603. As noted above, the torque control actions executed by the machine are executed by a controller in one implementation. The controller may be of any suitable construction; however, in one example it comprises a digital processor system including a microprocessor circuit having data inputs and control outputs, operating in accordance with computer-readable instructions stored on a computer-readable medium. Typically, the processor will have associated therewith long-term (non-volatile) memory for storing the program instructions, as well as short-term (volatile) memory for storing operands and results during (or resulting from) processing.
The described principles are applicable to machines that are used for grading applications and which include a ground-engaging mechanism, e.g., wheels, tracks, etc. A primary example of such a machine is a motor grader. Within such applications, the described principles provide a user-settable torque limiting function to avoid wheel spin and attendant surface marring while grading a surface. In this way, the operator of the motor grader or other grading machine is able to make a fewer number of more aggressive cuts to complete a surfacing operation with greater efficiency without sacrificing surface quality. It will be appreciated that the described principles also apply to machines used for operations other than grading where wheel slippage is undesirable. Moreover, other ground engaging mechanisms such as tracks are also usable within the described principles.
Various other modifications may also be employed. For example, the maximum torque threshold may determined in any number of ways, such as through user setting or machine determination based upon historical data concerning the available traction of the ground engaging elements. The amount of available traction may be determined either manually or automatically, such as through any of, or a combination of, radar, GPS, wheel speed sensors and direct observation. Also, the torque limiting functionality of this disclosure may be used in conjunction with a suitable warning system. By way of example, the system may provide a warning signal or indication to the user when the maximum torque threshold has been reached. In this way, the user may take appropriate action if desired. Alternatively or in addition, the torque limiting functionality of this disclosure may be used in conjunction with a GPS-guided system to more precisely determine depth of cut or other sequences of operation. For example, such an integrated system may be used to determine a real-time cutting sequence or the like for a particular application.
Thus, although it will be appreciated that the foregoing description provides useful examples of the disclosed system and technique, it should be appreciated that other implementations of the disclosed principles will differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for the features of interest, but not to exclude such from the scope of the disclosure entirely unless otherwise specifically indicated.
Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein.
