Abstract:
Vehicles with open loop hydraulic steering systems may suffer from jerky steering due to the necessity to have safety valves in the open loop system to prevent uncontrolled vehicle movement. Traditional open loop steering arrangements allow steering by controlling fluid quantities flowing from the pump to the drive motors. The disclosed vehicle has an open loop hydraulic drive system including first and second variable displacement motors for driving ground engaging mechanisms at first and second sides of the vehicle, respectively. The system includes a control mechanism configured for steering the vehicle by changing the displacement of one of the first and second motors. This allows the operator to change the drive motor speed range during travel.

Description:
PRIORITY STATEMENT 
     This application is the National Stage, filed under 35 U.S.C. §371, of International Application PCT/EP2006/012395 having an International Filing Date of Dec. 21, 2006, and published Jul. 5, 2007, as International Publication No. WO/2007/073925. Applicant claims benefit of priority under 35 U.S.C. §119(a) and §365(b) of European Patent Application No. 05113048.2 filed Dec. 28, 2005. 
     TECHNICAL FIELD 
     This disclosure relates to a steering arrangement and method for a work machine. In particular, but not exclusively, it relates to a steering arrangement and method for skid steer or tracked vehicles with open loop hydraulic drive arrangements. 
     BACKGROUND 
     Skid steer or tracked vehicles such as mini hydraulic excavators (MHE) commonly have an open loop hydraulic transmission for propelling and steering the vehicle. One simplified example of an open loop system may have a fluid reservoir for a fluid such as hydraulic oil, of which a pump can draw a volume of fluid. The pump pressurizes the fluid and sends this to at least one of two motors (one for each track) which is coupled to a track such that the rotating motor will turn the track thereby moving the vehicle. Once the fluid has passed through the motor the fluid is returned to the reservoir. For comparison, in a closed loop system the return flow from the motor would return to the pump instead of the reservoir. Closed loop systems tend to provide superior vehicle control, but generally require more sophisticated pumps, motors and control software resulting in higher levels of complexity and expense. 
     In a closed loop arrangement the return flow can flow no faster than what the pump can take in. For example, on an inclined terrain the vehicle will tend to roll downwards whereby the track is inclined to drive the motor instead of the motor driving the track. However, the motor cannot displace more fluid than the pump can take in, therefore the motor is prevented from speeding up and the operator remains in control of vehicle speed. In an open loop system without any additional controls in place, the motor would be able to freely dispose of the fluid into the tank leading to potentially uncontrolled vehicle behavior such as a run away condition of the machine. To prevent this situation, over center lock valves may be fitted in the motor return line. As the motor is bi-directional, an over center lock valve is fitted in both supply/return lines to each motor. 
     Steering of vehicles with open loop steering arrangements is achieved by providing the two motors with different fluid quantities or opposite flows, thereby causing the tracks to have different speeds and/or directions. One problem associated with open loop systems is the repeated switching of the over center lock valves in response to varying flows and pressures created during the steering process. Operator control may not always be smooth and the system is affected by changing torque and power requirements during a steering maneuver. This causes the over center valve to open and close repeatedly making the steering maneuver jerky and uncontrolled. This may lead to problems with modern machinery where high travel speeds are demanded to reduce travel times when moving between sites. To avoid those problems, an operator may have to slow down the vehicle or come to a complete standstill before engaging a turn. To increase smoothness during a turn the machine may be equipped with cross line relief valves between the motor inlet and outlet ports as this enables a continued rotation of the braked motor as fluid is moved across the relief valve during turning hence softening the turn to a degree. As the range relief valve settings are limited due to system relief valve settings, cross line relief is only a partial alleviation of some of the problems. 
     The following disclosure is directed to one or more improvements in the existing technology. 
     SUMMARY 
     In one aspect, a vehicle includes an open loop hydraulic drive system. The open loop hydraulic drive system includes a first and a second variable displacement hydraulic motor for driving ground engaging mechanisms at opposite sides of the vehicle and a control mechanism for steering the vehicle by changing the displacement of one of the first and second motors. 
     In another aspect, a method of turning a vehicle includes an open loop hydraulic drive system, wherein the open loop hydraulic drive system includes a first and a second variable displacement hydraulic motor for driving ground engaging mechanisms at opposite sides of the vehicle. The method includes selecting a desired vehicle direction and changing the displacement of at least one of the first and second motors in response to the selection of a desired vehicle direction to thereby turn the vehicle. 
     Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 . is a schematic of a vehicle with an open loop steering system in accordance with an embodiment of the current disclosure. 
         FIG. 2  is a flowchart of an exemplary method for controlling the vehicle of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic for a vehicle  10  with a steering arrangement such that steering is achieved by inducing different speeds between opposite ground engaging driving mechanisms such as typically found on skid steer type machines or tracked vehicles. Examples of such machine are tracked hydraulic excavators or skid steer loaders, either tracked or equipped with non-steerable wheels. For exemplary purposes only, an embodiment of the current disclosure, such as vehicle  10  as shown in  FIG. 1 , may be a mini hydraulic excavator (MHE) with tracks, but by no means is the application of this disclosure limited to a MHE with tracks. 
     Vehicle  10  has a left hand side track  14  and a right hand side track  114  for driving opposite sides of the vehicle  10 . Both tracks are connected with a hydraulic system  30  via shaft and gearbox arrangements  20  and  120  for driving and steering the vehicle  10 . 
     The hydraulic system  30  may vary in design but may be characterized as an open loop system as will be elaborated upon below. The hydraulic system  30  can generally be described as having a first circuit  31  for driving track  14 , a second circuit  33  for driving track  114  and one or more further circuits for other hydraulic systems present on the vehicle  10 . The tracks  14  and  114  are driven by independent but substantially similar circuits and only one will be explained in more detail on the understanding that the second circuit operates in a substantially similar fashion. Components involved in driving tracks  14  and  114  with like numerals have like functions. Although the circuits for driving the tracks are separate to a degree, they may share components where convenient such as the fluid reservoir  32  and the fluid supply  34 . 
     The vehicle  10  has one or more reservoirs  32  that may be interconnected, for holding an actuating fluid such as hydraulic oil. A fluid supply such as pump  34  draws fluid from the reservoir  32 . The fluid supply may be an engine driven single pump with either variable or fixed displacement, for supplying fluid to all systems on the vehicle  10 , such as, for example, driveline, steering, work arms, and implements, or it may be a combination of pumps with more dedicated supply arrangements. For simplicity sake the fluid supply will be described as a fixed displacement pump  34  of which the fluid output depends on its rotational speed and associated losses. The pump  34  supplies fluid to a control valve  36  which may be of any suitable type. The control valve  36  shown is a valve which is normally in a position such that no fluid flows from pump  34  towards the motor  52 , and such that no fluid flows from the motor  52  to the reservoir  32 . The control valve  36  may be a proportional valve having an infinite number of positions to control the direction and volume of the fluid flowing through the control valve  36 . The control valve  36  may be operated by an operator via a suitable input device  38 . The input device  38  may be a mechanical arrangement such as a lever or an electrical/electronic arrangement such as a proportional roller switch. From control valve  36  the pressurized fluid can flow into either line  40  or  42 , depending on the desired vehicle direction as selected by the operator. In this example the pressurized fluid from the pump  34  travels through line  40  towards the check valve  44 . The check valve  44  may be a 2-way, 3-position over center valve being spring biased to a neutral position as shown and may be controlled by pilot lines  46  and  48 . When the check valve  44  allows so, the pressurized fluid can travel via a line  50  towards a motor  52 . The motor  52  is a variable displacement axial piston drive motor which may in principle have an infinite number of available swash angles. In one embodiment only a discrete number of those positions are selectable. For example, in one embodiment the motor may be classified as a two speed motor as only a first and second swash angle are selectable. From motor  52 , the fluid returns via the line  56 , the check valve  44 , the line  40 , the control valve  36 , and the line  58  to the reservoir  32 . 
     The swash angle position of the motor  52  is in this example controlled by an arrangement  54  which includes a hydraulic valve  60  and a shuttle valve  62 . Another suitable arrangement may be selected, if preferred, or additional restrictors or orifices may be fitted to enhance flow characteristics, for example to soften the impact of the change while shifting the swash angle position of the motor  52 . The valve  60  is of the electro-hydraulic solenoid operated type and is controlled by a logic element  65  such as an ECU. The logic element  65  receives inputs from an operator via several input devices located in the operator&#39;s environment. An input device  64 , which may for example be a switch, controls both the valves  60  and  160  and requests the logic element  65  to trigger a similar effect in both the motors  52  and  152 . Working on the theoretical principle that both systems are set up identically and no differences in set up and tolerances are present, the operation of the input device  64  will place both motors  52  and  152  in identical positions, i.e., either they are both in their first swash angle positions or they are both in their second swash angle positions. For motor  52  this is achieved by shifting the valve  60  in a position such that the line with the highest pressure, i.e., one of the lines  50  and  56 , will provide pressurized oil via shuttle valve  62  to a swash angle controller  66  on the motor  52 . The motor may also be spring biased such that the motor is in a default position unless it receives a signal from the logic element  65 . The functions of input devices  68  and  168 , which in this example may be switches, are very similar to that of the input device  64  except for that each of the input devices  68  and  168  do not control both the motors  52  and  152 . The input device  64  controls both the motors  52  and  152 , the input device  68  controls motor  52  but not the motor  152 , and the input device  168  controls the motor  152  but not the motor  52 . In one embodiment the input devices  68  and  168  are biased switches and return to their default position when the operator no longer engages them, while the input device  64  remains in the position as selected by the operator. 
     One embodiment of the method for steering the vehicle  10  is shown in  FIG. 2 . It is to be noted that the flowchart is not exhaustive and that more steps and routines may be added or that certain steps may be in a different order. The box  70  represents the process of receiving a vehicle direction change request as selected by the operator or another arrangement such as an at least partially automated management system. In this example the direction change request corresponds to a request for a turn towards the left. The change request may be received by the logic element  65  which determines the current speed ranges of both motors  52  and  152 . In this embodiment the speed range of the left hand side motor  52  is abbreviated to SRL and the speed range of the motor of the right hand side motor is abbreviated to SRR. HIGH means the motor has a swash angle position corresponding to the high vehicle speed range, while LOW therefore corresponds to the low vehicle speed range. The logic element  65  may determine SRL and SRR in any suitable way such as, for example, measuring or sensing electrical or electronic signals, sensing the actual physical position of a component of the motors  52  and  152 . To fulfill the left turn request, the left hand side track  14  must run slower than the right hand side track  114 , hence the motor  52  must run slower than the motor  152 . If both SRL and SRR are in the high speed position as shown in the box  72 , the process moves on to the box  74  and SRL is changed to LOW. This results in the motor  52  and corresponding track  14  slowing down and the vehicle  10  turns to the left. If both SRL and SRR are in HIGH as shown in the box  76 , the process of the box  78  is followed and SRR is changed to high. This results in the motor  152  and corresponding track  114  speeding up and the vehicle  10  turns to the left. 
     In one embodiment, SRL and SRR are not equal as long as one of the input devices  68  and  168  is activated. SRL and SRR can be equalized by either deactivating the one input device that is activated or by activating both the input devices  68  and  168 . By equalizing SRL and SRR the vehicle will again commence traveling in a straight line as long as no other factors that may influence vehicle direction are present. 
     INDUSTRIAL APPLICABILITY 
     When the vehicle is not moving, any undesired movement of the vehicle  10  is prevented by check valves  44  and  144  blocking the return lines from the motors  52  and  152  to the tank, therefore holding the motors  52  and  152  and the associated track  14  and  114  in a fixed position as the motors cannot dispose of any fluid. 
     During operation the operator actuates both the control valves  36  and  136  to the same extent to start moving the vehicle  10  in a straight line. For the exemplary embodiment as shown in  FIG. 1  the controls for the control valves  36  and  136  may be two manual and adjacently mounted levers that can be pulled or pushed. As again the function of both the circuits for the tracks  14  and  114  is similar only one will be explained in more detail. 
     To overcome the friction associated with moving the vehicle, the pressure in the motor  52  has to increase. The increase in pressure is sensed by the check valve  44  via the pilot line  46 . Once the pressure has reached a certain level the valve  44  shifts to the right hence engaging the left portion of the valve  44  into the circuit. This opens the connection between the return lines  50  and  42  from the motor to the tank after which the vehicle will start to move. 
     The check valve  44  has the function of preventing an uncontrolled run-away condition of the vehicle  10  while traveling down a slope. While on the slope the vehicle  10  is inclined to travel at a higher speed than selected by the operator. The track  14  tries to increase the speed of the motor  52  which leads to a drop in pressure  56  and hence in pilot line  46 . This allows the check valve  44  to shift back to its neutral position and therefore blocks the connection between the return lines  50  and  42 . This in turn will prevent the motor  52  from rotating and therefore brakes the vehicle. Pressure will rise subsequently in the lines  40  and  56  thereby shifting the check valve  44  once again to the right resulting in the vehicle  10  commencing controlled straight line movement once more. Normally this cycle will be short such that the machine is likely not to come to a complete standstill, although the machine behavior may be jerky. 
     At any time during the operation of the vehicle  10  the operator may engage the input device  64 . The input device  64  will place both the motors  52  and  152  simultaneously in similar swash angle positions as described above. When the minimum swash angle is selected the vehicle  10  will be in a high speed, low torque mode suitable for operations such as long distance travel. When the maximum swash angle is selected the machine is in the low speed, high torque mode which may be more suited to work operations or delicate maneuvering. 
     A combination of the controls mentioned above therefore enables the operator to select a speed range with input device  64  and a speed selection within the selected speed range by operating the control valves  36  and  136 . This type of system is usually operated with a fixed engine speed, but if a variable engine speed regime is adopted this will of course influence the fluid output of the pump  34  and therefore the vehicle speed. 
     A change in direction of the vehicle  10  can be achieved by selecting a different position for the control valve  36  than for the control valve  136  and vice versa. By selecting different control valve positions each of the motors  52  and  152  is provided with a different flow rate and one of the tracks  14  and  114  will run faster than the other track hence inducing a turn of the vehicle  10 . However, varying the flow rate through one of the control valves  36  and  136  to induce such a turn leads to downstream pressure fluctuations which may impact on the behavior of the check valves  44  and  144 . The check valves  44  and  144  are sensitive to pressures in the lines to which they are connected as described above and may therefore open and close repeatedly making the turning movement jerky. Especially at high speeds this may lead to dangerous situations where the jerking movement may be exaggerated as a sudden jerk may lunge the operator forward or backward which in turn then may induce an uncontrolled movement by the operator of the control valves  36  and  136 . An operator may therefore decide to significantly reduce the speed of the vehicle or to bring it to a standstill before instigating a turn with the valves  36  and  136 . 
     Instead of controlling the control valves  36  and  136  the operator may decide to operate one of the input devices  68  and  168 . In one embodiment the input devices may be mounted on the levers that control the control valves  36  and  136  so the operator does not have to move his hands from the levers. If for example the operator wants to make a left hand turn, the left hand track  14  must be running at a lower speed than the right hand track  114 . If the motors  52  and  152  are both in the high speed mode, i.e., in the minimum swash angle position, engaging the input device  68  will trigger the logic element  65  to place the motor  52  in the maximum swash angle position thereby reducing the speed of the track  14 . If the motors  52  and  152  are both in the low speed mode, i.e., in the maximum swash angle position, engaging the trigger  68  will trigger the logic element  65  to place the motor  152  in the minimum swash angle position thereby increasing the speed of the track  114 . The logic element  65  may therefore be programmed such that attempts are made to have a consistent response of the vehicle  10  in response to activating one of the input devices  68  and  168 . For example, activating the input device  68  may, where possible, always result in a left hand turn, while activating the input device  168  may, where possible, always result in a right turn. It will be clear from the above that this may require a different action by the logic element  65  in that it may need to operate the motor  52  in certain conditions and the motor  152  in other conditions. The conditions may have to take into account the state of the motors  52  and  152  at the moment of receiving the steering request, but it may also depend on the type of vehicle. For example, hydraulic excavators are commonly equipped with an operator platform which is rotatably mounted on a tracked undercarriage. The platform may therefore be rotated such that the operator faces either towards the front or the rear of the machine as defined by the undercarriage. This will of course have implications for what the operator perceives to be left and right. To make the controls more operator friendly the logic element  65  may be programmed and equipped such that it can determine the orientation of the operator or the cab and adapt the response to the steering request accordingly. 
     Steering the vehicle  10  by engaging either of input devices  68  and  168  results in minimal disturbance of the position of the check valves  44  and  144 . Of course the check valves  44  and  144  may momentarily be influenced but to a lesser degree than occurs when steering by operating the control valves  36  and  136 . Of course the check valves  44  and  144  may still influence the vehicle behavior if any of the maneuvers take place on a slope as described above. 
     Steering the vehicle  10  by changing the swash angle position of one of the motors  52  and  152  to a preset position will lead to a turning circle having a fixed radius as the turn is not proportionally controllable. This may be preferred by an unskilled operator to, for example, make minor corrections to straight line travel as the length of turn is dictated by the duration of actuation of one of the input devices  68  and  168  rather than the duration and degree of displacement of the control levers of the proportional valves  38  and  138 . 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed embodiments without departing from the scope of the disclosure. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the embodiments disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims.