Patent Publication Number: US-10767344-B2

Title: Hydraulic drive control

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is based on and claims the benefit of U.S. provisional patent application Ser. No. 62/670,360, filed May 11, 2018, the contents of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     This disclosure is directed toward power machines. More particularly, this disclosure is directed toward power machines having hydraulic systems, and to methods of controlling the hydraulic systems in cold temperature conditions. 
     Power machines, for the purposes of this disclosure, include any type of machine that generates power for the purpose of accomplishing a particular task or a variety of tasks. One type of power machine is a work vehicle. Work vehicles are generally self-propelled vehicles that have a work device, such as a lift arm (although some work vehicles can have other work devices) that can be manipulated to perform a work function. Work vehicles include loaders, excavators, utility vehicles, tractors, and trenchers, to name a few examples. 
     Power machines having hydraulic systems sometimes operate in cold temperature conditions. Operation of the hydraulic system with the temperature of the hydraulic oil in the hydraulic system being too low can result in performance degradation, damage to components, or other undesirable results. 
     The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. 
     SUMMARY 
     Disclosed embodiments include power machines having hydraulic systems and controllers configured to control, and sometimes limit or prohibit, operation of the hydraulic systems based upon measured temperature of the hydraulic oil in the system. Disclosed embodiments also include methods of controlling the hydraulic systems. In the methods implemented by the controllers, determinations are made as to whether the temperature of the hydraulic oil is below a first set point temperature, between the first set point temperature and a second set point temperature, or above the second set point temperature. The controllers then either prohibit hydraulic system operation, allow limited hydraulic system operation, or allow full hydraulic system operation, based upon the measured temperature in comparison to the two set point temperatures. In other embodiments, only a single set point temperature is utilized, and operation of at least part of the hydraulic system can be prevented at oil temperatures below the single set point temperature and allowed above the set point or alternatively operation is partially allowed or can be allowed with a ramping up of hydraulic system operation as a function of time. 
     In an exemplary embodiment, a disclosed power machine ( 100 ;  200 ;  350 ) comprises a hydraulic system ( 300 ) including a drive system having at least one drive pump ( 303 ) and at least one drive motor ( 226 A;  226 B;  306 ) which selectively receives hydraulic oil from the at least one drive pump to move the power machine in forward and reverse directions of travel. The drive pump has a variable hydraulic displacement controllable by controlling a percentage of a full pump stroke allowed. A temperature sensor ( 318 ) of the power machine is configured to measure a temperature of hydraulic oil in the hydraulic system and to provide a temperature signal indicative of the measured temperature. A user input device ( 322 ) of the power machine is configured to provide a user input signal responsive to actuation of the user input device by a user. A controller ( 320 ) of the power machine is coupled to the user input device, to the temperature sensor and to the at least one drive pump of the drive system. The controller is configured to receive the user input signal from the user input device and the temperature signal from the temperature sensor and to responsively generate drive control signals to control the at least one drive pump. The controller is configured to generate the drive control signals to: control the at least one drive pump to not move the power machine if the temperature of the hydraulic oil is below a first set point temperature; control the at least one drive pump responsive to the user input signal to allow a first percentage of the full pump stroke if the temperature of the hydraulic oil is at the first set point temperature and to allow the pump stroke to increase with temperature increases toward a second percentage of the full pump stroke, higher than the first percentage of the full pump stroke, at a second set point temperature higher than the first set point temperature; and control the at least one drive pump responsive to the user input signal to allow the full pump stroke if the temperature of the hydraulic oil is above the second set point temperature. 
     In some embodiments, the first percentage of the full pump stroke causes the power machine to move at a first speed, the second percentage of the full pump stroke causes the power machine to move at a second speed higher than the first speed, and the full pump stroke causes the power machine to move at a full speed higher than the second speed. In some embodiments, the second percentage of the full pump stroke is less than full pump stroke. 
     In some embodiments, the controller is configured such that when the temperature signal from the temperature sensor indicates that the temperature of the hydraulic oil has risen from below the second set point temperature to above the second set point temperature, the controller controls the at least one drive pump responsive to the user input signal to allow the full pump stroke only after the user input device has first been returned to a neutral position. 
     In some embodiments, the at least one drive motor is a two-speed drive motor configured to operate in a low range displacement mode and a high range displacement mode, and the controller is configured to control the at least one drive motor to prevent operation in the high range displacement mode when the temperature of the hydraulic oil is below the second set point temperature, but allow operation in the low range displacement mode when the temperature of the hydraulic oil is between the first set point temperature and the second set point temperature. 
     In some embodiments, the at least one drive pump comprises at least one hydrostatic drive pump. 
     In another exemplary embodiment, a power machine ( 100 ;  200 ;  350 ) includes a drive system having at least one drive pump ( 303 ) and at least one drive motor ( 226 A;  226 B;  306 ) which selectively receives hydraulic oil from the at least one drive pump to move the power machine in forward and reverse directions of travel, with the drive pump having a variable hydraulic displacement up to a maximum displacement corresponding to a full pump stroke. A temperature sensor ( 318 ) is configured to measure a temperature of hydraulic oil and to provide a temperature signal indicative of the measured temperature. A user input device ( 322 ) is configured to provide a user input signal, to command movement of the power machine using the drive system, responsive to actuation of the user input device by a user. A controller ( 320 ) is coupled to the user input device to receive the user input signal, to the temperature sensor to receive the temperature signal, and to the at least one drive pump of the drive system. The controller is configured to generate drive control signals to control the at least one drive pump responsive to the user input signal such that the controller prevents the drive system from moving the power machine if the temperature of the hydraulic oil is below a first set point temperature, to allow the drive system to move the power machine using a drive pump displacement varying between a first percentage of the full pump stroke when the temperature of the hydraulic oil is at the first set point temperature and a second percentage of the full pump stroke when the temperature of the hydraulic oil is at a second set point temperature higher than the first set point temperature, and to allow the drive system to move the power machine using the maximum displacement corresponding to the full pump stroke if the temperature of the hydraulic oil is above the second set point temperature. 
     In some embodiments, the first percentage of the full pump stroke causes the power machine to move at a first speed, the second percentage of the full pump stroke causes the power machine to move at a second speed higher than the first speed, and the full pump stroke causes the power machine to move at a speed higher than the second speed. In some embodiments, the second percentage of the full pump stroke is less than the full pump stroke. 
     In some embodiments, the controller is configured to generate the drive control signals to control the at least one drive pump responsive to the user input signal such that when the temperature signal from the temperature sensor indicates that the temperature of the hydraulic oil has risen from below the second set point temperature to above the second set point temperature, the controller allows the drive system to move the power machine using the maximum displacement corresponding to the full pump stroke only after the user input device has first been returned to a neutral position after the temperature of the hydraulic oil has risen to above the second set point temperature. 
     In some embodiments, the at least one drive motor is a two-speed drive motor configured to operate in a low range displacement mode and a high range displacement mode, and the controller is configured to control the at least one drive motor to prevent operation in the high range displacement mode when the temperature of the hydraulic oil is below the second set point temperature, but allow operation in the low range displacement mode when the temperature of the hydraulic oil is between the first set point temperature and the second set point temperature. 
     In another exemplary embodiment, a method ( 400 ) of controlling a drive system of a power machine ( 100 ;  200 ;  350 ) is provided. The drive system has at least one drive pump ( 303 ) and at least one drive motor ( 226 A;  226 B;  306 ), and the at least one drive pump has a variable hydraulic oil displacement up to a maximum displacement corresponding to a full pump stroke. The method comprises: measuring ( 402 ) a temperature of hydraulic oil in a hydraulic system ( 300 ) including the drive system; receiving a user input signal from a user input device ( 322 ); determining ( 406 ) whether the temperature of the hydraulic oil is below a first set point temperature; controlling ( 408 ) the at least one drive pump of the drive system to prevent movement of the power machine by the drive system if the temperature of the hydraulic oil is below the first set point temperature; determining ( 410 ), if the temperature of the hydraulic oil is above the first set point temperature, whether the temperature of the hydraulic oil is above or below a second set point temperature; controlling ( 414 ) the at least one drive pump of the drive system to move the power machine, responsive to the user input signal, using a drive pump displacement varying between a first percentage of the full pump stroke when the temperature of the hydraulic oil is at the first set point temperature and a second percentage of the full pump stroke when the temperature of the hydraulic oil is at the second set point temperature; and controlling ( 412 ) the at least one drive pump of the drive system to move the power machine, responsive to the user input signal, using the maximum displacement corresponding to the full pump stroke, if the temperature of the hydraulic oil is above the second set point temperature. 
     In some embodiments, the first percentage of the full pump stroke causes the power machine to move at a first speed, the second percentage of the full pump stroke causes the power machine to move at a second speed higher than the first speed, and the full pump stroke causes the power machine to move at a full speed higher than the second speed. 
     In some embodiments, the second percentage of the full pump stroke is less than the full pump stroke. For example, the first percentage of the full pump stroke can be approximately 10 percent and the second percentage of the full pump stroke can be approximately 60 percent. 
     In some embodiments, controlling ( 414 ) the at least one drive pump of the drive system to move the power machine, responsive to the user input signal, using the drive pump displacement varying between the first percentage of the full pump stroke and the second percentage of the full pump stroke further comprises varying the drive pump displacement as a function of temperature between the first percentage of the full pump stroke and the second percentage of the full pump stroke. 
     In some embodiments, when the temperature of the hydraulic oil has risen from below the second set point temperature to above the second set point temperature, then controlling ( 412 ) the at least one drive pump of the drive system further comprises controlling the at least one drive pump to move the power machine using the maximum displacement corresponding to the full pump stroke only after the user input device has first been returned to a neutral position. 
     In some embodiments, the at least one drive motor is a two-speed drive motor configured to operate in a low range displacement mode and a high range displacement mode, and the method further comprises controlling the at least one drive motor to prevent operation in the high range displacement mode when the temperature of the hydraulic oil is below the second set point temperature. 
     This Summary and the Abstract are provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       DRAWINGS 
         FIG. 1  is a block diagram illustrating functional systems of a representative power machine on which embodiments of the present disclosure can be advantageously practiced. 
         FIGS. 2-3  illustrate perspective views of a representative power machine in the form of a skid-steer loader of the type on which the disclosed embodiments can be practiced. 
         FIG. 4  is a block diagram illustrating components of a hydraulic system of a power machine such as the loader illustrated in  FIGS. 2-3 . 
         FIG. 5  is a flow diagram illustrating a method of controlling the hydraulic system of a power machine in cold temperature conditions using multiple set point temperatures. 
         FIG. 6  is a flow diagram illustrating another method of controlling the hydraulic system of a power machine in cold temperature conditions using only a single set point temperature. 
     
    
    
     DETAILED DESCRIPTION 
     The concepts disclosed in this discussion are described and illustrated with reference to exemplary embodiments. These concepts, however, are not limited in their application to the details of construction and the arrangement of components in the illustrative embodiments and are capable of being practiced or being carried out in various other ways. The terminology in this document is used for the purpose of description and should not be regarded as limiting. Words such as “including,” “comprising,” and “having” and variations thereof as used herein are meant to encompass the items listed thereafter, equivalents thereof, as well as additional items. 
     Disclosed embodiments establish hydraulic temperature zones of operation and control operation of hydraulic systems of power machines based upon what temperature zone hydraulic fluid in the power machine is in, the hydraulic temperature zone of operation being determined based upon a measured temperature of the hydraulic oil in the system. More particularly, the disclosed embodiments provide hydraulic temperature zones of operation for controlling operation of hydraulic systems when hydraulic oil is colder and sometimes significantly colder than the temperature of hydraulic oil during normal operation. In one embodiment, three zones of operation are defined. In some embodiments, three hydraulic temperature zones of operation are defined. The first zone of operation is defined as when the measured temperature of hydraulic oil in the system is below a first set point temperature. The second zone of operation is defined as when the hydraulic oil is above the first set point temperature and below a second set point temperature. The third zone of operation is defined as when the hydraulic oil is above the second set point temperature, above a second (and higher) set point temperature, or between the first and second set point temperatures. The hydraulic systems are then either prohibit from operation responsive to a user input, allowed limited operation responsive to the user input, or allowed full operation responsive to the user input, based upon the hydraulic temperature zone of operation. In other embodiments, only two hydraulic zones of operation are defined, with a first zone of operation being a single set point temperature is utilized, and hydraulic system operation can be prevented at oil temperatures below the single set point temperature, with increasing hydraulic system operation allowed as a function of time. In other embodiments, additional zones of operation are defined, which can allow for additional control schemes to be incorporated beyond those described above. 
     These concepts can be practiced on various power machines, as will be described below. A representative power machine on which the embodiments can be practiced is illustrated in diagram form in  FIG. 1  and one example of such a power machine is illustrated in  FIGS. 2-3  and described below before any embodiments are disclosed. For the sake of brevity, only one power machine is illustrated and discussed as being a representative power machine. However, as mentioned above, the embodiments below can be practiced on any of a number of power machines, including power machines of different types from the representative power machine shown in  FIGS. 2-3 . Power machines, for the purposes of this discussion, include a frame, at least one work element, and a power source that is capable of providing power to the work element to accomplish a work task. One type of power machine is a self-propelled work vehicle. Self-propelled work vehicles are a class of power machines that include a frame, work element, and a power source that is capable of providing power to the work element. At least one of the work elements is a motive system for moving the power machine under power. 
       FIG. 1  is a block diagram that illustrates the basic systems of a power machine  100 , which can be any of a number of different types of power machines, upon which the embodiments discussed below can be advantageously incorporated. The block diagram of  FIG. 1  identifies various systems on power machine  100  and the relationship between various components and systems. As mentioned above, at the most basic level, power machines for the purposes of this discussion include a frame, a power source, and a work element. The power machine  100  has a frame  110 , a power source  120 , and a work element  130 . Because power machine  100  shown in  FIG. 1  is a self-propelled work vehicle, it also has tractive elements  140 , which are themselves work elements provided to move the power machine over a support surface and an operator station  150  that provides an operating position for controlling the work elements of the power machine. A control system  160  is provided to interact with the other systems to perform various work tasks at least in part in response to control signals provided by an operator. 
     Certain work vehicles have work elements that are capable of performing a dedicated task. For example, some work vehicles have a lift arm to which an implement such as a bucket is attached such as by a pinning arrangement. The work element, i.e., the lift arm can be manipulated to position the implement for the purpose of performing the task. The implement, in some instances can be positioned relative to the work element, such as by rotating a bucket relative to a lift arm, to further position the implement. Under normal operation of such a work vehicle, the bucket is intended to be attached and under use. Such work vehicles may be able to accept other implements by disassembling the implement/work element combination and reassembling another implement in place of the original bucket. Other work vehicles, however, are intended to be used with a wide variety of implements and have an implement interface such as implement interface  170  shown in  FIG. 1 . At its most basic, implement interface  170  is a connection mechanism between the frame  110  or a work element  130  and an implement, which can be as simple as a connection point for attaching an implement directly to the frame  110  or a work element  130  or more complex, as discussed below. 
     On some power machines, implement interface  170  can include an implement carrier, which is a physical structure movably attached to a work element. The implement carrier has engagement features and locking features to accept and secure any of a number of implements to the work element. One characteristic of such an implement carrier is that once an implement is attached to it, it is fixed to the implement (i.e. not movable with respect to the implement) and when the implement carrier is moved with respect to the work element, the implement moves with the implement carrier. The term implement carrier as used herein is not merely a pivotal connection point, but rather a dedicated device specifically intended to accept and be secured to various different implements. The implement carrier itself is mountable to a work element  130  such as a lift arm or the frame  110 . Implement interface  170  can also include one or more power sources for providing power to one or more work elements on an implement. Some power machines can have a plurality of work element with implement interfaces, each of which may, but need not, have an implement carrier for receiving implements. Some other power machines can have a work element with a plurality of implement interfaces so that a single work element can accept a plurality of implements simultaneously. Each of these implement interfaces can, but need not, have an implement carrier. 
     Frame  110  includes a physical structure that can support various other components that are attached thereto or positioned thereon. The frame  110  can include any number of individual components. Some power machines have frames that are rigid. That is, no part of the frame is movable with respect to another part of the frame. Other power machines have at least one portion that is capable of moving with respect to another portion of the frame. For example, excavators can have an upper frame portion that rotates with respect to a lower frame portion. Other work vehicles have articulated frames such that one portion of the frame pivots with respect to another portion for accomplishing steering functions. 
     Frame  110  supports the power source  120 , which is configured to provide power to one or more work elements  130  including the one or more tractive elements  140 , as well as, in some instances, providing power for use by an attached implement via implement interface  170 . Power from the power source  120  can be provided directly to any of the work elements  130 , tractive elements  140 , and implement interfaces  170 . Alternatively, power from the power source  120  can be provided to a control system  160 , which in turn selectively provides power to the elements that capable of using it to perform a work function. Power sources for power machines typically include an engine such as an internal combustion engine and a power conversion system such as a mechanical transmission or a hydraulic system that is configured to convert the output from an engine into a form of power that is usable by a work element. Other types of power sources can be incorporated into power machines, including electrical sources or a combination of power sources, known generally as hybrid power sources. 
       FIG. 1  shows a single work element designated as work element  130 , but various power machines can have any number of work elements. Work elements are typically attached to the frame of the power machine and movable with respect to the frame when performing a work task. In addition, tractive elements  140  are a special case of work element in that their work function is generally to move the power machine  100  over a support surface. Tractive elements  140  are shown separate from the work element  130  because many power machines have additional work elements besides tractive elements, although that is not always the case. Power machines can have any number of tractive elements, some or all of which can receive power from the power source  120  to propel the power machine  100 . Tractive elements can be, for example, track assemblies, wheels attached to an axle, and the like. Tractive elements can be mounted to the frame such that movement of the tractive element is limited to rotation about an axle (so that steering is accomplished by a skidding action) or, alternatively, pivotally mounted to the frame to accomplish steering by pivoting the tractive element with respect to the frame. 
     Power machine  100  includes an operator station  150  that includes an operating position from which an operator can control operation of the power machine. In some power machines, the operator station  150  is defined by an enclosed or partially enclosed cab. Some power machines on which the disclosed embodiments may be practiced may not have a cab or an operator compartment of the type described above. For example, a walk behind loader may not have a cab or an operator compartment, but rather an operating position that serves as an operator station from which the power machine is properly operated. More broadly, power machines other than work vehicles may have operator stations that are not necessarily similar to the operating positions and operator compartments referenced above. Further, some power machines such as power machine  100  and others, whether or not they have operator compartments or operator positions, may be capable of being operated remotely (i.e. from a remotely located operator station) instead of or in addition to an operator station adjacent or on the power machine. This can include applications where at least some of the operator controlled functions of the power machine can be operated from an operating position associated with an implement that is coupled to the power machine. Alternatively, with some power machines, a remote control device can be provided (i.e. remote from both of the power machine and any implement to which is it coupled) that is capable of controlling at least some of the operator controlled functions on the power machine. 
       FIGS. 2-3  illustrate a loader  200 , which is one particular example of a power machine of the type illustrated in  FIG. 1  where the embodiments discussed below can be advantageously employed. Loader  200  is a skid-steer loader, which is a loader that has tractive elements (in this case, four wheels) that are mounted to the frame of the loader via rigid axles. Here the phrase “rigid axles” refers to the fact that the skid-steer loader  200  does not have any tractive elements that can be rotated or steered to help the loader accomplish a turn. Instead, a skid-steer loader has a drive system that independently powers one or more tractive elements on each side of the loader so that by providing differing tractive signals to each side, the machine will tend to skid over a support surface. These varying signals can even include powering tractive element(s) on one side of the loader to move the loader in a forward direction and powering tractive element(s) on another side of the loader to mode the loader in a reverse direction so that the loader will turn about a radius centered within the footprint of the loader itself. The term “skid-steer” has traditionally referred to loaders that have skid steering as described above with wheels as tractive elements. However, it should be noted that many track loaders also accomplish turns via skidding and are technically skid-steer loaders, even though they do not have wheels. For the purposes of this discussion, unless noted otherwise, the term skid-steer should not be seen as limiting the scope of the discussion to those loaders with wheels as tractive elements. 
     Loader  200  is one particular example of the power machine  100  illustrated broadly in  FIG. 1  and discussed above. To that end, features of loader  200  described below include reference numbers that are generally similar to those used in  FIG. 1 . For example, loader  200  is described as having a frame  210 , just as power machine  100  has a frame  110 . Skid-steer loader  200  is described herein to provide a reference for understanding one environment on which the embodiments described below related to track assemblies and mounting elements for mounting the track assemblies to a power machine may be practiced. The loader  200  should not be considered limiting especially as to the description of features that loader  200  may have described herein that are not essential to the disclosed embodiments and thus may or may not be included in power machines other than loader  200  upon which the embodiments disclosed below may be advantageously practiced. Unless specifically noted otherwise, embodiments disclosed below can be practiced on a variety of power machines, with the loader  200  being only one of those power machines. For example, some or all of the concepts discussed below can be practiced on many other types of work vehicles such as various other loaders, excavators, trenchers, and dozers, to name but a few examples. 
     Loader  200  includes frame  210  that supports a power system  220 , the power system being capable of generating or otherwise providing power for operating various functions on the power machine. Power system  220  is shown in block diagram form, but is located within the frame  210 . Frame  210  also supports a work element in the form of a lift arm assembly  230  that is powered by the power system  220  and is capable of performing various work tasks. As loader  200  is a work vehicle, frame  210  also supports a traction system  240 , which is also powered by power system  220  and is capable of propelling the power machine over a support surface. The lift arm assembly  230  in turn supports an implement interface  270 , which includes an implement carrier  272  that is capable of receiving and securing various implements to the loader  200  for performing various work tasks and power couplers  274 , to which an implement can be coupled for selectively providing power to an implement that might be connected to the loader. Power couplers  274  can provide sources of hydraulic or electric power or both. The loader  200  includes a cab  250  that defines an operator station  255  from which an operator can manipulate various control devices  260  to cause the power machine to perform various work functions. Cab  250  can be pivoted back about an axis that extends through mounts  254  to provide access to power system components as needed for maintenance and repair. 
     The operator station  255  includes an operator seat  258  and a plurality of operation input devices, including control levers  260  that an operator can manipulate to control various machine functions. Operator input devices can include buttons, switches, levers, sliders, pedals and the like that can be stand-alone devices such as hand operated levers or foot pedals or incorporated into hand grips or display panels, including programmable input devices. Actuation of operator input devices can generate signals in the form of electrical signals, hydraulic signals, and/or mechanical signals. Signals generated in response to operator input devices are provided to various components on the power machine for controlling various functions on the power machine. Among the functions that are controlled via operator input devices on power machine  100  include control of the tractive elements  219 , the lift arm assembly  230 , the implement carrier  272 , and providing signals to any implement that may be operably coupled to the implement. 
     Loaders can include human-machine interfaces including display devices that are provided in the cab  250  to give indications of information relatable to the operation of the power machines in a form that can be sensed by an operator, such as, for example audible and/or visual indications. Audible indications can be made in the form of buzzers, bells, and the like or via verbal communication. Visual indications can be made in the form of graphs, lights, icons, gauges, alphanumeric characters, and the like. Displays can be dedicated to provide dedicated indications, such as warning lights or gauges, or dynamic to provide programmable information, including programmable display devices such as monitors of various sizes and capabilities. Display devices can provide diagnostic information, troubleshooting information, instructional information, and various other types of information that assists an operator with operation of the power machine or an implement coupled to the power machine. Other information that may be useful for an operator can also be provided. Other power machines, such walk behind loaders may not have a cab nor an operator compartment, nor a seat. The operator position on such loaders is generally defined relative to a position where an operator is best suited to manipulate operator input devices. 
     Various power machines that can include and/or interacting with the embodiments discussed below can have various different frame components that support various work elements. The elements of frame  210  discussed herein are provided for illustrative purposes and frame  210  is not the only type of frame that a power machine on which the embodiments can be practiced can employ. Frame  210  of loader  200  includes an undercarriage or lower portion  211  of the frame and a mainframe or upper portion  212  of the frame that is supported by the undercarriage. The mainframe  212  of loader  200 , in some embodiments is attached to the undercarriage  211  such as with fasteners or by welding the undercarriage to the mainframe. Alternatively, the mainframe and undercarriage can be integrally formed. Mainframe  212  includes a pair of upright portions  214 A and  214 B located on either side and toward the rear of the mainframe that support lift arm assembly  230  and to which the lift arm assembly  230  is pivotally attached. The lift arm assembly  230  is illustratively pinned to each of the upright portions  214 A and  214 B. The combination of mounting features on the upright portions  214 A and  214 B and the lift arm assembly  230  and mounting hardware (including pins used to pin the lift arm assembly to the mainframe  212 ) are collectively referred to as joints  216 A and  216 B (one is located on each of the upright portions  214 ) for the purposes of this discussion. Joints  216 A and  216 B are aligned along an axis  218  so that the lift arm assembly is capable of pivoting, as discussed below, with respect to the frame  210  about axis  218 . Other power machines may not include upright portions on either side of the frame, or may not have a lift arm assembly that is mountable to upright portions on either side and toward the rear of the frame. For example, some power machines may have a single arm, mounted to a single side of the power machine or to a front or rear end of the power machine. Other machines can have a plurality of work elements, including a plurality of lift arms, each of which is mounted to the machine in its own configuration. Frame  210  also supports a pair of tractive elements in the form of wheels  219 A-D on either side of the loader  200 . 
     The lift arm assembly  230  shown in  FIGS. 2-3  is one example of many different types of lift arm assemblies that can be attached to a power machine such as loader  200  or other power machines on which embodiments of the present discussion can be practiced. The lift arm assembly  230  is what is known as a vertical lift arm, meaning that the lift arm assembly  230  is moveable (i.e. the lift arm assembly can be raised and lowered) under control of the loader  200  with respect to the frame  210  along a lift path  237  that forms a generally vertical path. Other lift arm assemblies can have different geometries and can be coupled to the frame of a loader in various ways to provide lift paths that differ from the radial path of lift arm assembly  230 . For example, some lift paths on other loaders provide a radial lift path. Other lift arm assemblies can have an extendable or telescoping portion. Other power machines can have a plurality of lift arm assemblies attached to their frames, with each lift arm assembly being independent of the other(s). Unless specifically stated otherwise, none of the inventive concepts set forth in this discussion are limited by the type or number of lift arm assemblies that are coupled to a particular power machine. 
     The lift arm assembly  230  has a pair of lift arms  234  that are disposed on opposing sides of the frame  210 . A first end of each of the lift arms  234  is pivotally coupled to the power machine at joints  216  and a second end  232 B of each of the lift arms is positioned forward of the frame  210  when in a lowered position as shown in  FIG. 2 . Joints  216  are located toward a rear of the loader  200  so that the lift arms extend along the sides of the frame  210 . The lift path  237  is defined by the path of travel of the second end  232 B of the lift arms  234  as the lift arm assembly  230  is moved between a minimum and maximum height. 
     Each of the lift arms  234  has a first portion  234 A of each lift arm  234  is pivotally coupled to the frame  210  at one of the joints  216  and the second portion  234 B extends from its connection to the first portion  234 A to the second end  232 B of the lift arm assembly  230 . The lift arms  234  are each coupled to a cross member  236  that is attached to the first portions  234 A. Cross member  236  provides increased structural stability to the lift arm assembly  230 . A pair of actuators  238 , which on loader  200  are hydraulic cylinders configured to receive pressurized fluid from power system  220 , are pivotally coupled to both the frame  210  and the lift arms  234  at pivotable joints  238 A and  238 B, respectively, on either side of the loader  200 . The actuators  238  are sometimes referred to individually and collectively as lift cylinders. Actuation (i.e., extension and retraction) of the actuators  238  cause the lift arm assembly  230  to pivot about joints  216  and thereby be raised and lowered along a fixed path illustrated by arrow  237 . Each of a pair of control links  217  are pivotally mounted to the frame  210  and one of the lift arms  232  on either side of the frame  210 . The control links  217  help to define the fixed lift path of the lift arm assembly  230 . 
     Some lift arms, most notably lift arms on excavators but also possible on loaders, may have portions that are controllable to pivot with respect to another segment instead of moving in concert (i.e. along a pre-determined path) as is the case in the lift arm assembly  230  shown in  FIG. 2 . Some power machines have lift arm assemblies with a single lift arm, such as is known in excavators or even some loaders and other power machines. Other power machines can have a plurality of lift arm assemblies, each being independent of the other(s). 
     An implement interface  270  is provided proximal to a second end  232 B of the lift arm assembly  234 . The implement interface  270  includes an implement carrier  272  that is capable of accepting and securing a variety of different implements to the lift arm  230 . Such implements have a complementary machine interface that is configured to be engaged with the implement carrier  272 . The implement carrier  272  is pivotally mounted at the second end  232 B of the arm  234 . Implement carrier actuators  235  are operably coupled the lift arm assembly  230  and the implement carrier  272  and are operable to rotate the implement carrier with respect to the lift arm assembly. Implement carrier actuators  235  are illustratively hydraulic cylinders and often known as tilt cylinders. 
     By having an implement carrier capable of being attached to a plurality of different implements, changing from one implement to another can be accomplished with relative ease. For example, machines with implement carriers can provide an actuator between the implement carrier and the lift arm assembly, so that removing or attaching an implement does not involve removing or attaching an actuator from the implement or removing or attaching the implement from the lift arm assembly. The implement carrier  272  provides a mounting structure for easily attaching an implement to the lift arm (or other portion of a power machine) that a lift arm assembly without an implement carrier does not have. 
     Some power machines can have implements or implement like devices attached to it such as by being pinned to a lift arm with a tilt actuator also coupled directly to the implement or implement type structure. A common example of such an implement that is rotatably pinned to a lift arm is a bucket, with one or more tilt cylinders being attached to a bracket that is fixed directly onto the bucket such as by welding or with fasteners. Such a power machine does not have an implement carrier, but rather has a direct connection between a lift arm and an implement. 
     The implement interface  270  also includes an implement power source  274  available for connection to an implement on the lift arm assembly  230 . The implement power source  274  includes pressurized hydraulic fluid port to which an implement can be removably coupled. The pressurized hydraulic fluid port selectively provides pressurized hydraulic fluid for powering one or more functions or actuators on an implement. The implement power source can also include an electrical power source for powering electrical actuators and/or an electronic controller on an implement. The implement power source  274  also exemplarily includes electrical conduits that are in communication with a data bus on the excavator  200  to allow communication between a controller on an implement and electronic devices on the loader  200 . 
     Frame  210  supports and generally encloses the power system  220  so that the various components of the power system  220  are not visible in  FIGS. 2-3 .  FIG. 4  includes, among other things, a diagram of various components of the power system  220 . Power system  220  includes one or more power sources  222  that are capable of generating and/or storing power for use on various machine functions. On power machine  200 , the power system  220  includes an internal combustion engine. Other power machines can include electric generators, rechargeable batteries, various other power sources or any combination of power sources that are capable of providing power for given power machine components. The power system  220  also includes a power conversion system  224 , which is operably coupled to the power source  222 . Power conversion system  224  is, in turn, coupled to one or more actuators  226 , which are capable of performing a function on the power machine. Power conversion systems in various power machines can include various components, including mechanical transmissions, hydraulic systems, and the like. The power conversion system  224  of power machine  200  includes a pair of hydrostatic drive pumps  224 A and  224 B, which are selectively controllable to provide a power signal to drive motors  226 A and  226 B. The drive motors  226 A and  226 B in turn are each operably coupled to axles, with drive motor  226 A being coupled to axles  228 A and  228 B and drive motor  226 B being coupled to axles  228 C and  228 D. The axles  228 A-D are in turn coupled to tractive elements  219 A-D, respectively. The drive pumps  224 A and  224 B can be mechanically, hydraulic, and/or electrically coupled to operator input devices to receive actuation signals for controlling the drive pumps. 
     The arrangement of drive pumps, motors, and axles in power machine  200  is but one example of an arrangement of these components. As discussed above, power machine  200  is a skid-steer loader and thus tractive elements on each side of the power machine are controlled together via the output of a single hydraulic pump, either through a single drive motor as in power machine  200  or with individual drive motors. Various other configurations and combinations of hydraulic drive pumps and motors can be employed as may be advantageous. 
     The power conversion system  224  of power machine  200  also includes a hydraulic implement pump  224 C, which is also operably coupled to the power source  222 . The hydraulic implement pump  224 C is operably coupled to work actuator circuit  238 C. Work actuator circuit  238  includes lift cylinders  238  and tilt cylinders  235  as well as control logic (such as one or more valves) to control actuation thereof. The control logic selectively allows, in response to operator inputs, for actuation of the lift cylinders and/or tilt cylinders. In some machines, the work actuator circuit also includes control logic to selectively provide a pressurized hydraulic fluid to an attached implement. 
     The description of power machine  100  and loader  200  above is provided for illustrative purposes, to provide illustrative environments on which the embodiments discussed below can be practiced. While the embodiments discussed can be practiced on a power machine such as is generally described by the power machine  100  shown in the block diagram of  FIG. 1  and more particularly on a loader such as track loader  200 , unless otherwise noted or recited, the concepts discussed below are not intended to be limited in their application to the environments specifically described above. 
     Referring now to  FIG. 4 , shown are components of a power machine  350 , such as those discussed above with reference to  FIGS. 1-3 , which provides a hydraulic system  300  and a controller  320  configured to control the hydraulic system over a range of different hydraulic oil temperatures. Hydraulic system  300  includes at least one hydraulic pump  302  configured to provide pressurized hydraulic fluid or oil to power one or more actuators. In an exemplary embodiment, hydraulic system  300  includes one or more control valves  304  to selectively control the application of hydraulic oil flow to the actuators. Control valve  304 , in some embodiments is a multiple spool, open center valve assembly, the spools being independently actuable to control hydraulic actuators. Examples of hydraulic actuators lift actuator(s)  308  configured to raise and lower a lift arm, tilt actuator(s)  310  configured to rotate an implement carrier and any attached implement relative to the lift arm or relative to the frame of the power machine, and auxiliary function actuators  312  such as those found on hydraulically powered implements. These example actuators are not required in all embodiments, and hydraulic system  300  can include, in some embodiments, other types of actuators, instead of or in addition to those discussed above, represented in  FIG. 4  as actuators  314 . Hydraulic pump  302  is a constant displacement gear pump, but in other embodiments can be a variable displacement pump. Hydraulic system  300  can also include one or more drive pumps  303 , which are coupled to one or more drive motors  306  to selectively move the power machine in forward and reverse directions of travel. Drive pumps  303  are hydrostatic pumps. In other embodiments, hydraulic drive systems that include control valves to direct flow from hydraulic pumps can be employed for drive systems, but these embodiments are not shown for the sake of brevity. The one or more drive pumps  303  are shown as being mechanically linked to pump  302 . In various embodiments, all the hydraulic and hydrostatic pumps are driven by a single prime mover, such as an engine or electric motor, although that need not be the case in every embodiment. In some embodiments, drive motors  306  are so-called two-speed motors, with two different displacements, including a low range displacement and a high range displacement. In those embodiments, controller  320  can communicate with the drive motors  306  to control their displacement. Other embodiments can include drive motors with multiple displacements or infinitely variable displacements. Controller  320  can be configured to communicate with these types of drive motors as well to control their displacement. 
     Hydraulic system  300  also includes at least one temperature sensor  318  configured to measure the temperature of hydraulic oil in the hydraulic system and to provide a temperature signal indicative of the measured temperature to controller  320 . Temperature sensor  318  can be positioned at any suitable location within the hydraulic system to measure the temperature of the hydraulic oil. Further, in some embodiments, multiple temperature sensors and/or switches such as temperature sensor  318  are positioned at various different locations within the hydraulic system. For example, temperature sensors can be positioned at one or more of the input or output of the hydraulic pump, at the input or output of the control valve  304 , at the input or output of any of the illustrated actuators, or hydraulic cooling elements (which are not shown in  FIG. 4 ), and so forth. 
       FIG. 4  illustrates a user input device  322  that is in communication with controller  320 . User input device  322  is manipulable by an operator to allow an operator to communicate an intention to operator the power machine to controller  320 . In various embodiments, any number of user input devices, such as joystick controllers, levers, foot pedals, and the like are configured to be actuated by an operator of the power machine and to provide user input signals indicative of intentions to control various actuators to controller  320 . In addition, in some embodiments, a hydraulic system enable input  324  is also included and is required to be actuated by the user prior to operation of some or all of the hydraulic functions of the power machine hydraulic system  300 . For example, an enabling input such as a button, a seat bar sensor, or other types of inputs or any combination thereof can be included and required to be actuated prior to operation of the hydraulic system. Such hydraulic system enabling inputs  324  are not required in all embodiments, and for purposes of the present disclosure, are not further discussed with regard to implementation of the disclosed control methods for controlling hydraulic system  300 . Nevertheless, those skilled in the art will understand that, when hydraulic system enabling inputs are included or required, such inputs can prevent operation of the hydraulic system  300  when not properly actuated even if an operator has actuated one or more user input devices  322 . Some embodiments such as the one shown in  FIG. 4  include display device  326  that is in communication with controller  320  and is configured to display information to the operator, including information discussed below with reference to operational limitations allowed in different oil temperature zones of operation. 
     In exemplary embodiments, controller  320  is configured to control operation of hydraulic system  300 , responsive to user input signals from the one or more user input devices  322  over a range of temperatures including cold temperature operation. This requires the temperature of the hydraulic oil to be increased to a sufficient temperature prior to allowing full operation of the hydraulic system. In various embodiments, one or more set point temperatures are used to define hydraulic temperature zones of operation during which the hydraulic system is disabled, limited, or fully operational. 
     Referring now to  FIG. 5 , shown is a flow diagram illustrating a method  400  of controlling hydraulic system  300  according to one illustrative embodiment. Controller  320  can be configured to implement method  400 , or similar methods, in implementing oil temperature based hydraulic system control. As shown at block  402 , the oil temperature in hydraulic system  300  is measured using one or more temperature sensors and/or switches. The user input signal will typically correspond to a user actuating the user input device in an attempt to cause the power machine to perform a work operation such as travel, raising or lowering the lift arm, rolling the implement carrier and any attached implement forward or backward using at tilt actuator, etc. However, in some embodiments, the user input can simply be a signal from the user input device indicating that the user input device remains in a neutral position. It yet other embodiments, no user input signal is required. For instance, as will be discussed below, the control methods can include displaying information identifying that hydraulic system functionality is prohibited or limited whether a user input is received or not. 
     Next, as shown at decision block  406 , controller  320  determines based on inputs from the temperature sensors and/or switches, whether the hydraulic oil temperature is below a first set point temperature. If the hydraulic oil temperature is below the first set point temperature, the controller  320  operates under the rules of a first hydraulic temperature zone of operation. The first set point temperature represents a temperature below which the hydraulic system functions are disabled to prevent the operator from performing work functions while the hydraulic oil warms. Thus, in the first zone of operation, the controller  320  is configured to prevent operation of the hydraulic system, even responsive to user input signals. The rules of the first hydraulic temperature zone of operation are shown at block  408 . When the power machine is operating in the first hydraulic temperature zone of operation, in some embodiments, controller  320  also controls display  326  to show the operator a “COLD” warning and/or to indicate that no machine travel or work functions are available. In other embodiments, in the first hydraulic temperature zone of operation, the controller  320  may prevent operation of only some hydraulic functions. For example, the controller  320  in some embodiments may prevent operation of the drive system while allowing for operation of other functions such as auxiliary functions and lift and tilt functions. In some embodiments where a variable displacement implement pump  302  is employed, the controller  320  can actuate the implement pump  302  even when functions that obtain pressurized hydraulic fluid from the implement pump are not actuated. This can allow pressurized hydraulic fluid to move through the control valve  304  and cause the hydraulic fluid to warm more quickly. 
     If the controller determines at decision block  406  that the measured oil temperature is above the first set point temperature, the controller determines, at decision block  10 , whether hydraulic fluid temperature is above a second set point temperature. For example, the second set point temperature can be 20° F., though other second set point temperatures can be used. The second set point temperature represents a temperature above which normal or full hydraulic system operation is allowed. If it is determined at decision block  410  that the oil temperature is above the second set point temperature, then the controller  320  determines that it should operate in the hydraulic temperature third zone of operation at block  412  and allow full and unrestrained (at least because of hydraulic fluid temperature) hydraulic system operation responsive to the user input. If the operator is manipulating one or more user input devices as the controller  320  transitions from a second hydraulic temperature zone of operation (discussed below) to the third hydraulic temperature zone of operation, in some embodiments the controller  320  is configured to require that any or all user input devices that are being manipulated be returned to a neutral or non-manipulated position before the controller allows unrestrained operation hydraulic functions that were being restrained in the second hydraulic temperature zone of operation. This prevents a sudden and rapid increase in the travel speed or other hydraulic function once the hydraulic fluid temperature reaches the second set point temperature. Also, in some embodiments, once the hydraulic fluid temperature reaches the second set point temperature, controller  320  controls the display  326  to provide an indication of normal hydraulic system functionality. 
     If the controller  320  determines, at decision block  410 , that the hydraulic fluid temperature is below the second set point temperature, controller  320  operates in the second hydraulic temperature zone of operation and controls the hydraulic system to allow limited operation of hydraulic system  300  as shown at block  414 . This limited functionality can be controlled in various manners to transition between no operation of the hydraulic system and partial or full operation of the hydraulic system. The transition can include further set points between the first and second set point temperatures to create additional oil temperature zones or regions of hydraulic system control, though additional set points are not required in all embodiments. 
     In one exemplary embodiment, controller  320  is further configured to utilize a third set point temperature, between the first and second set point temperatures, and to control the hydraulic system differently depending upon whether the measured temperature is between the first and third set point temperatures or between the third and second set point temperatures. For example, in one embodiment, a third set point temperature of 10° F. is used for the third set point temperature. If the controller determines that the measured oil temperature is between the first set point temperature (e.g., 0° F.) and the third set point temperature (e.g., 10° F.), then the controller allows the drive pumps  303  to function at a limited percentage of the full pump stroke. For example, in some embodiments, between the first and third set point temperatures, pump  302  can be allowed only to operate at only 10% of the full pump stroke. Other maximum pump stroke percentages can also be used. Further, the controller  320  can control the drive motors  306  (when they are two-speed motors or infinitely variable displacement motors) to prevent operation in the high range displacement or, in the case of infinitely variable displacement motors prevent operation of the motors above a minimum displacement, while allowing low range operation. Display device  326  can similarly be controlled to notify the operator of the “COLD” condition and limited available operation of the hydraulic system. 
     If the controller determines that the measured oil temperature is between the third set point temperature (e.g., 10° F.) and the second set point temperature (e.g., 20° F.), then the controller allows the drive pumps  303  to ramp up slowly from the limited maximum percentage of pump stroke of the previous temperature range, to a higher maximum pump stroke percentage. For example, in one embodiment, controller  320  allows pumps  303  to slowly ramp from 10% of maximum pump stroke to 60% of maximum pump stroke as the temperature increases from the third set point temperature to the second set point temperature. The transition between the limited maximum percentage of pump stroke of the previous temperature range (e.g., 10%) to the higher maximum pump stroke percentage (e.g., 60%) can be a linear function of temperature, a piecewise linear function of temperature, a step function of temperature, an exponential function of temperature, or controlled by other types of functions correlating temperature to maximum percentage of pump stroke allowed. In some embodiments, between the third and second set point temperatures, only low range operation is allowed, while high range operation is still prohibited. Display device  326  can again be controlled to notify the operator of the “COLD” condition and limited available operation of the hydraulic system. 
     Although some disclosed embodiments utilize multiple set point temperatures and three (or more) zones of operation, this need not be the case in all embodiments, and instead a single set point temperature can be utilized. For instance, in some embodiments, a single set point temperature is utilized (e.g., the first set point temperature 0° F.), and if the controller determines that the measured oil temperature is below the first set point temperature, hydraulic system operation can initially be prevented, but increasingly allowed based upon time elapsed. One such example of such an embodiment is method  500 , which is illustrated in the flow diagram shown in  FIG. 6 . 
     As shown at block  502  of  FIG. 6 , the oil temperature in hydraulic system  300  is measured using one or more temperature sensors. Next, as shown at decision block  506 , a determination is made as to whether the oil temperature is below a first set point temperature. For example, the first set point temperature can be 0° F., though other first set point temperatures can be used. If it is determined that the measured oil temperature is above the first set point temperature, then at block  508  the controller  320  allows at least partial operation of the hydraulic system responsive to user input signals. This can include allowing full operation of the hydraulic system in some embodiments. 
     If it is determined that the measured oil temperature is not above the first set point temperature, then at block  510  the controller initially prevents operation of at least part of the hydraulic system responsive to user inputs but sets a timer at block  512 . Then, as shown at block  514 , the controller allows increasing operation of the hydraulic system based upon the elapsed time indicated by the timer. For instance, at predetermined amounts of elapsed time, the controller can increase drive pump stroke percentage to predetermined and increasing values. Other types of time-based increased hydraulic system operation are also possible, for example allowing control of different actuators after different amounts of elapsed time (e.g., travel before lift arm operation), allowing high range motor operation, and the like. Alternatively, using the single temperature set point, operation of at least part of the hydraulic system is not allowed below the temperature set point and fully allowed above the temperature set point. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the scope of the discussion.