Patent Publication Number: US-2013229272-A1

Title: Manual control device and method

Description:
TECHNICAL FIELD 
     This patent disclosure generally relates to manual control devices and, more particularly, to manual control devices providing haptic information to a user. 
     BACKGROUND 
     Machines having implements are typically controlled by a combination of control devices. For example, an operator may use one device to move the machine into a desired direction, for example, a steering wheel or yolk, a different device to accelerate and decelerate the machine, for example pedals or levers, and yet a different device, for example, a joystick, to operate an implement of the machine, such as a bucket or shovel. 
     When machines such as excavators are operating, they are often operating in confined areas and can be surrounded by either immovable objects, such as building structures, or hazardous conditions, such as power lines. In those conditions, it is desired to maintain accurate and precise control of the motion of the work implement to ensure safe machine operation. Currently, machines such as excavators, cranes and the like, use joystick-type control devices to control motion of their implements. These joysticks can have two, three or more degrees of freedom of motion, each of which corresponds to a particular direction or type of motion of the work implement. When an operator is manipulating the control, the operator can simply move the control in various fashions to achieve the desired placement and trajectory of the work implement. 
     When operating such machinery, it is advisable to carefully control the position and motion of the work implement such that overshoot in the position of the work implement is avoided. Until now, the careful positioning of the work implement is determined by the experience and perception of the operator. However, inexperienced or inattentive operators may, at times, overshoot the position of the implement or overcompensate the force required to move the implement when an obstruction is present, and as a result place the implement in an undesired location. These situations cannot be avoided at present. 
     SUMMARY 
     A machine includes an actuator operating to displace an implement based on a command provided by an operator. The command is provided in the form of a displacement of a handle of a manual control device by the operator. The displacement of the handle occurs in an activation direction of the handle. The machine includes a variable damper associated and displaceable with the handle. The variable damper is configured to selectively alter a stiffness thereof in response to a control signal. A displacement sensor is associated with the variable damper and configured to provide a displacement signal indicative of the displacement of the handle. A controller is associated with the variable damper, the manual control device, the displacement sensor and the actuator. The controller is disposed to determine a then present operating state of the actuator, determine a command provided to the actuator based on the displacement signal, and provide the control signal to stiffen the variable damper such that the displacement of the handle is limited to an additional displacement of the handle that corresponds to a difference between the then present operating state of the actuator and a maximum allowable operating state of the actuator. 
     In another aspect, the disclosure describes a method for providing haptic information to an operator of a manual control device for a system. The manual control device may include a handle adapted for use by the operator to issue commands, which are provided in the form of a displacement of the handle in an activation direction where the extent of displacement is indicative of a magnitude of each command. The method includes selectively altering a stiffness of a variable damper associated with the handle, determining a then present command based on the displacement of the handle, determining a maximum possible command that is allowable based on a capability of the system, and limiting the displacement of the handle to an additional displacement of the handle by stiffening the variable damper when the then present command approaches the maximum possible command. In one embodiment, the additional displacement of the handle corresponds to a difference between the then present command and the maximum possible command. 
     In yet another aspect, the disclosure describes a positive-force generating device mounted via at least one variable damper to a machine. The variable damper is configured to selectively alter a stiffness thereof in response to a control signal. The device is moveable in a direction of application of an impulse force by compression or extension of the variable damper. The positive-force generating device includes a displacement sensor associated with the variable damper and configured to provide a displacement signal indicative of a displacement of the device. A controller is associated with the variable damper, the device, and the displacement sensor. The controller selectively provides the control signal to alter the stiffness of the variable damper. A motor is responsive to a command signal from the controller has a mass connected to an output shaft of the motor. The mass has a center of gravity that is offset relative to an axis of rotation of the output shaft of the motor. An encoder is configured to provide a rotational signal to the controller that is indicative of a rotational position of the mass relative to the device. The controller is configured to provide the command signal to the motor and the control signal to the variable damper based on the rotational signal and the displacement signal such that the impulse force is selectively provided along a predetermined direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-3  are various views of a machine in accordance with the disclosure. 
         FIG. 4  is a block diagram of an implement control system for a machine in accordance with the disclosure. 
         FIG. 5  is a block diagram of a control in accordance with the disclosure. 
         FIG. 6  is a time chart of various signals to illustrate control principles for the directional application of force in accordance with the disclosure. 
         FIG. 7  is a block diagram for a force-feedback control in accordance with the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure describes an exemplary embodiment relative to a machine having a work implement. Operation of the work implement can be carried out by the selective control of actuators, which are responsive to control signals from a machine controller. In one embodiment, a manual control device is configured to control the actuators in response to user input through appropriate displacement of a control handle. The control handle is configured to provide haptic feedback to the user or operator that is indicative of the loading condition or operating condition of the implement actuators. The haptic feedback may be embodied in the form of a selectively variable resistance to handle displacement such that the issuance of commands that exceed the then present power capabilities are avoided. The haptic feedback can also include a positive force feedback tending to push the handle when the handle is indicating a command to the actuators that exceeds the then present capabilities of the system. Although the described embodiments relate to control of a machine implement, the structures and methods relating to the manual control device have universal applicability to applications involving human-machine interfaces and controls. 
     As used herein, the term “machine” may refer to any machine that performs some type of operation associated with an industry such as mining, construction, farming, transportation, marine or any other industry known in the art. For example, although an excavator is shown in certain figures, the machine may generally be an earth-moving machine, such as a wheel loader, excavator, dump truck, backhoe, motor grader, or may alternatively be any other type of machine, such as a material handler, a locomotive, paving machine or the like. Similarly, although an exemplary bucket is illustrated as the attached implement of the illustrated excavator, any implements may be utilized and employed for a variety of tasks, including, for example, loading, compacting, lifting, brushing, and include, for example, buckets, compactors, forked lifting devices, brushes, grapples, cutters, shears, blades, breakers/hammers, augers, and others. 
     With the foregoing in mind, an excavator  100  is shown for purpose of illustration in  FIG. 1 . The excavator  100  includes an undercarriage  102  and an upper structure  104 . The undercarriage  102 , which is also shown in  FIG. 3 , includes a generally H-shaped frame  106  that supports two crawler tracks  108  along its edges and includes a post  110  supporting a ring gear  112  close to its center. The crawler tracks  108  are moved by sprockets  107  that are rotated by hydraulic drive motors or electric drive motors  109  connected to the frame  106 . The ring gear  112  includes a plurality of teeth  114  arranged along its inner periphery, which mesh with a drive sprocket  116  powered by a swing motor  118 . In reference to  FIG. 2 , the swing motor  118  is connected to the upper structure  104  such that rotation of the drive sprocket  116  causes the relative rotation of the upper structure  104  relative to the undercarriage  102 . 
     In reference now to  FIGS. 1 and 2 , the upper structure  104  includes a boom  120  that is pivotally connected to an upper structure frame  121  and pivoted by use of two boom actuators  122 . An arm  124 , which is also commonly referred to as a stick, is pivotally connected at an end of the boom  120  and pivoted by an arm actuator  126 . A bucket  128  is connected at an end of the arm  124  and pivoted by a bucket actuator  130 . The boom actuators  122 , the arm actuator  126  and the bucket actuator  130  are embodied in the illustrations as linear hydraulic cylinders, which are configured to be extended and retracted by selective porting of pressurized fluid on one side of a hydraulic piston. The various functions of the machine  100  may be controlled in part by the appropriate handling of various control devices by an operator occupying a cab  132 . The swing motor  118  may be powered by hydraulic or electrical power. 
     A block diagram for an implement control system  400  for the machine  100  is shown in  FIG. 4 . The word “implement” is used herein to generally refer to any device on the machine  100  that is moved by an actuator. In the illustrated embodiment, the implement is considered to be the bucket  128  and the various actuators providing four degrees of freedom of motion of the bucket  128 , that is, the swing motor  118  that operates to rotate the boom  120  and arm  124  together with the bucket  128  relative to the undercarriage  102 , the boom actuators  122 , which operate to lift and lower the boom  120 , the arm actuator  126 , which pivots the arm relative to the boom, and the bucket actuator  130 , which tilts the bucket  128  relative to the arm  124 . As can be appreciated, operation of each of the swing motor  118 , boom actuators  122 , arm actuator  126 , and bucket actuator  130 , will independently cause a rotational or arcuate motion of the bucket  128  in one of four different trajectories in a three-dimensional space. 
     In reference to the implement control system  400 , a command provided to each of the afore-mentioned actuators causing displacement of the implement originates at a controller  402 . The actuators  404  controlling position and motion of a machine implement are generically represented in  FIG. 4 . The controller  402  may be an electronic controller configured to provide appropriate signals to components or systems of the machine  100  that operate to effect activation of each actuator  404 . The command signals provided by the controller  402  are based on command signals provided by a manual control device  406 . Information may be provided to the actuators through dedicated actuator communication lines  403 , which communicate with other actuator activation devices, such as electromechanical hydraulic-fluid valves and the like. The manual control device  406  may be one of a plurality of operator-controlled devices used to control operation of the machine  100 . Although one device  406  is illustrated, other control devices may also be connected to the controller  402  but are not shown in the illustration of  FIG. 4  for simplicity. 
     The controller  402  is further in communication with other machine systems  408  via a communication line  407 . The other machine sensors and systems  408  are generically shown collectively as a single block in  FIG. 4  and may include the engine or other prime movers of the machine, fluid pumps, transmissions, and others. Such devices or systems of the machine may provide feedback information to the controller  402  that is indicative of the operating state of each system or components and, in certain embodiments, may include information about the extent of power-output saturation of these systems. Power output saturation in this context is meant to describe the portion of available power output of each device relative to the total power output capability of that device. 
     Accordingly, the feedback information may include signals indicative of the duty cycles of those systems, the degree of power output of those systems as a percentage of the power input to those systems, and any other information that provides the controller  402  an indication of the rate and magnitude of power output is in a condition to provide in the event a maximum command is provided by the manual control device  406 . For example, when lifting a heavy load in the bucket  128  that approximates the lifting capacity of the boom actuators  122 , the corresponding sub-system that monitors and controls operation of those actuators may provide an indication to the controller  402  that one or more of the boom actuators  122  is/are near their output force capacity and have limited responsiveness to additional lifting force commands. Similar indications may be provided for each of the other actuators  404  of the machine that participate in moving the work implement of the machine or in conducting other machine functions. This and other information from the actuators  404  and machine systems  408  is provided to the controller  402  via the actuator communication lines  403  and/or the communication line  407 . 
     In the illustrated embodiment, the manual control device is a joystick-type control device having a handle  409  connected to three haptic control and feedback assemblies  410 . Each assembly  410  includes a damper device  412  having a stiffness and/or range of motion that is adjustable in response to a control signal provided by the controller  402  via a dedicated control line  414 . In the illustrated embodiment, each damper device  412  is a magnetorheological (MR) fluid-based force feedback damper. Dampers employing MR fluid-based properties may typically include MR fluids that are controlled by a magnetic field, which is typically induced by an electromagnet  416 . In this way, the damping characteristics of an MR damper can be finely controlled by appropriately controlling the intensity and other characteristics of the magnetic field. For example, the viscosity of the MR fluid in the damper can be controlled by controlling the current provided to the electromagnet. In alternative embodiments, each damper device may be a hydraulic piston arrangement in which a single piston or two opposing pistons is/are displaced when fluid passes into and out from piston volumes. The flow of fluid into and out from the piston volume may be controlled by an electromechanical valve operating to selectively modulate fluid flowing therethrough in response to the control signal provided by the controller. In this way, the stiffness of each damper device may be infinitely controlled through the control of the electromechanical valve. In the present disclosure, dampers having a variable stiffness capability may be generally referred to as variable dampers, which is a term contemplated to encompass any type of damper arrangement that has a variable stiffness capability, including MR fluid-based or hydraulic dampers having valves to modulate fluid flow therethrough as described. 
     Returning now to the illustrated embodiment, various types of MR fluid-based dampers are suitable for use with the damper devices  412 . One example of an MR fluid-based damper suitable for use in vehicle suspension systems can be found in U.S. Pat. No. 7,234,575. Another example of a MR fluid-based damper can be found in U.S. Pat. No. 7,775,333. Both these exemplary descriptions are incorporated herein in their entirety by reference. 
     In one embodiment of the present disclosure, a damper may include two chambers connected through a flow passage having a predetermined flow orifice therebetween. The area of the flow orifice may be within the effective range of an electromagnet  416 . Plungers configured to change the volume of the chambers when moved may be used to push fluid through the orifice when the damper undergoes compressive or tensile axial forces. The viscosity of the MR fluid passing through the orifice, which depends on the intensity of the field created when current passes through the electromagnet, will determine the force required to displace the damper. In alternative embodiments, other MR fluid-based damper arrangements may be used. For example, the MR fluid may be subject to a shearing stress when placed between concentric cylinders, or may be captured within a sponge that is disposed between two moveable walls. In either case, the force required to move the shearing bodies or walls will depend on the intensity of a magnetic field acting on a portion of the fluid. In yet another embodiment, a piston containing a magnet may be disposed inline with an electromagnet within a cylinder such that the force required to move the piston depends on the magnetic field and polarity of the electromagnet. 
     In the embodiment illustrated in  FIG. 4 , each of the damper devices  412  is configured to be axially compressed or extended by motion of the handle  409 . Magnets  416  associated with each device  412  are responsive to the signals from the controller  402  provided through lines  414  to change the force required to displace each damper  412 , and can even operate to selectively seize motion of each device  412  when desired. Each device  412  further includes a position sensor or encoder  417 , which is configured to provide a feedback signal indicative of the displacement state and displacement speed of each device  412  to the electronic controller  402 , for example, via the communication lines  414 . The extent and displacement of the devices  412  is indicative of the extent and speed of displacement of the handle  409 , which is taken as an indication of the extent and speed of implement actuation by the operator. 
     In the illustrated embodiments, the manual control device  406  further includes an optional buzzer or rotating mass assembly  418 . The assembly  418  includes a motor  420  having an eccentric weight  422  connected to an output shaft thereof such that a vibration is induced when the motor  420  is operating. The frequency of the vibration depends on the speed of the motor  420 , and the amplitude depends on the mass of the weight  422  and/or adjustability of the rotational moment of inertia of the weight  422 . A shaft encoder  424  may provide information indicative of the rotational position of the eccentric weight  422  relative to a reference orientation. Control of operation and speed of the motor  420 , as well as information from the shaft encoder  424 , may be exchanged between those devices and the electronic controller  402  through a buzzer communication and command line  426 . The buzzer  418 , however, is optional and may be omitted. For example, certain machine applications may inherently possess a predetermined or random vibration profile that is perceptible in the operator cab and, specifically, in the handle  409  of the manual control device. Such inherent vibrations may be the result of engine vibration of the machine, travel of the machine over uneven terrain, vibration of a work implement that is transferred to the cab, and other vibration sources. Examples of work implements that can induce a vibration include vibrators used on vibratory soil or asphalt compactor machines, pneumatic hammers, augers, and the like. 
     Alternatively, the buzzer may be embodied as a different structure that is configured to induce a vibration along one or more directions. As an illustrative example, the buzzer may include a generally elongate hollow shell having a ferrous or permanent magnet slug slidably disposed therewithin. Electromagnets disposed at each end of the shell such that alternating magnetic fields produced by the magnets can produce a reciprocal motion of the slug within the shell. In this example, a vibration induced by the buzzer would be generally axial along the reciprocal path of the slug. In one embodiment, such an axial vibration could be coupled in a collinear or other fashion, for example, in series with the variable damper, instead of being applied directly to the handle. As can be appreciated, when a vibratory device is coupled to a specific damper, multiple such vibratory devices may be used, each corresponding to a particular variable damper for applications having more than one variable damper. 
     A block diagram for a manual control  500  having haptic feedback capability is shown in the block diagram of  FIG. 5 . The control  500  may be a control algorithm embodied electronically or mechanically within the controller  402  ( FIG. 4 ) or a mechanical control arrangement. In the illustrated embodiment, the control  500  is embodied as a set of computer executable instructions stored in a tangible, non-volatile electronic storage medium of the controller  402 . A processor (not shown) of the controller  402  is configured to access the instructions and provide appropriate commands to other components and subsystems of the controller  402  that are arranged to transmute digital computer commands and signals to and from analog or other commands sent and received from machine systems and actuators. 
     As shown in  FIG. 5 , the control  500  is disposed to receive inputs indicative of the operating state of the machine. More specifically, a work signal  502  may be indicative of the operating state of an implement actuator. For example, the work signal  502  may be indicative of the loading of a particular actuator participating in the operation of an implement of the machine  100 . However, more than one actuator may participate in the motion of an implement. For example, the lifting and simultaneous scooping and tilting of the bucket  128  ( FIG. 1 ) will require simultaneous participation by the boom, arm and bucket actuators  122 ,  126  and  130  respectively. The work signal  502  may be indicative of the loading of either of these actuators, or may alternatively be indicative of the loading of a fluid pump (not shown) that provides hydraulic fluid to these actuators collectively. It should be appreciated that in the case of electrical, pneumatic or other types of actuators, the signal  502  may be indicative of the loading of those systems or of the device providing power to those systems irrespective of the type of energy used. In the case of an electric system, for example, the work signal  502  may be a voltage and/or current value present in a bus bar, alternator, storage array and/or the like, while in the case of pneumatic power the work signal  502  may be a pressure and/or flow rate of air provided by a compressor. 
     The control  500  further receives a limit signal  504 . The limit signal  504  is optional and is determined elsewhere in the controller  402  ( FIG. 4 ) (not shown) to be indicative of the power output saturation state of one or more actuators of the machine  100 . For example, in the case of a hydraulic piston actuator, the rate at which the hydraulic piston can extend may be limited by the rate at which the corresponding hydraulic pump can provide fluid to the actuator. Thus, even if the actuator has not reached its full motion, the rate at which it can extend may be limited. Alternatively, output saturation may be indicative of the force of the actuator. Using the hydraulic actuator again as an example, the force applied by the actuator may be limited by the maximum output pressure of the hydraulic pump. In the case of electrical actuators, power output saturation may similarly depend on the maximum output current and/or voltage of an electrical power source. These types of limitation may be monitored in the controller  402  to provide a limit signal  504 , which may be expressed as a percentage of the total possible actuator force or actuation rate at which the particular actuator is operating at any one time. 
     The work and limit signals  502  and  504  are provided to a monitor  506 , which outputs an inhibition signal  507 . The inhibition signal  507 , which may be expressed as a ratio between zero and one, is representative of the real-time operating state of an actuator and indicative of the capability of an actuator to respond to any command given by the machine operator, where zero indicates that the actuator is already at its saturation point and one indicates that the actuator is ready to receive and respond to a maximum command. The determination of the inhibition signal  507  may depend on various parameters in addition to the work and limit signals  502  and  504  such as the time-constant for a step response in the actuator, ambient temperature, machine age and various other parameters that may directly or indirectly affect the ability of an actuator to respond to commands. 
     Moreover, when more than one actuator are monitored at one time, the monitor  506  may be configured to receive numerous work and limit signals  502  and  504 , each corresponding to a particular actuator belonging to a group. In this case, the monitor  506  may output numerous inhibition signals  507  corresponding to each actuator or, alternatively, may select the lowest signal to be the inhibition signal  507  provided. Selection of the lowest signal may advantageously be implemented in machines where groups of actuators are operating in a predetermined and coordinated fashion to perform a single operation. 
     The control  500  may further include displacement signals  508  provided by each of the encoders associated with a manual control, for example, the encoders  417  ( FIG. 4 ). The displacement signals  508  may be collectively processed in a command processor  510  to provide a command signal  511 . The command signal  511  is indicative of the type and direction of motion of one or more actuators that is commanded by the machine operator by displacement of a control device in three or more dimensions, for example, by moving and/or twisting the handle  409  ( FIG. 4 ). In an alternative embodiment, the displacement signals  508  may be further processed to determine the nature, frequency and amplitude of a natural or induced machine vibration that is transferred to the handle  409 . In such embodiments, for example, a function such as a fast Fourier transform (FFT) may be used to calculate or otherwise determine the frequency of the natural vibration, and limit switches may be implemented to determine vibration amplitude in real time. This information can be used to control and limit the vibration of the handle if desired, and may further be exploited to induce a positive force feedback to the handle as will be described below. 
     Returning now to  FIG. 5 , the inhibition and command signals  507  and  511  are provided to a determinator function  512 . The determinator function is configured to compare on multiple dimensions the inhibition and command signals  507  and  511  to determine, in real time, whether the actuator(s) participating in a function are in a condition to respond to the operator command or whether, because of certain functional limitations, the operator command exceeds the capabilities of the machine. For example, when swinging the upper structure  104  in one direction at high speed and a change in swinging direction also at a high speed is desired, the machine operator may be tempted to swiftly swing the handle  409  from an extreme position to one side of a control to another side of the control. Physically, the machine may expend energy to slow the rotating structure before initiating motion in the opposite direction. Unless the operator is able to manage the force applied by the machine to accomplish this change in motion orientation, the operator may achieve the swing slower or faster than the machine is capable of achieving the change, in this way undershooting or overshooting the desired motion in the opposite direction. Whether undershooting or overshooting occurs will depend on the experience of the operator and, as a result, there may be loss in machine operating effectiveness and/or efficiency. 
     In the illustrated embodiment, however, such undershooting or overshooting of the machine, as well as potentially overloading of machine systems, may be avoided by the comparison between the inhibition and command signals  507  and  511  in the determinator function  512 . Specifically, the determinator function  512  may determine the readiness of each actuator to receive a different command based on that actuator&#39;s inhibition signal  507 , examine the command actually provided by the operator based on the command signal  511 , and determine whether the commanded motion by the operator is within the then present operational capability of the actuator(s). 
     When the determinator function  512  concludes, based on this comparison, that the operator command is within the capability of the system, the command signal is permitted to pass through to the actuators and no action is taken in this regard. However, when the determinator function  512  concludes that the command signal, if permitted to pass through to the actuators, would exceed the capabilities of the system, the determinator function  512  outputs a dampening signal  513 . The dampening signal  513  is tailored for the particular direction of motion of the handle  409  ( FIG. 4 ) that would yield a command to the actuator requiring a delimiting of the command provided to it. The dampening signal  513  may increase in value the closer an actuator is to a power output saturation point. 
     In one embodiment, the dampening signal  513  is proportional to the command sent to an electromagnet that is part of a MR fluid-based damper, for example, one of the devices  412  ( FIG. 4 ). In general, the dampening signal  513  is appropriate to appropriately adjust the stiffness of a variable damper such that motion of the handle  409 , as representative of the command provided to an actuator, is maintained within acceptable actuator operation limits. In such an embodiment, an increase of the dampening signal  513  would be perceived by the operator as a stiffening of the motion of the manual control device in the direction of increasing commands to the actuator. This stiffening would be interpreted by the operator as a haptic feedback indicative of a saturation in the power output condition of an actuator the operator is attempting to command such that the operator would be aware that operation of the machine is approaching its limits. Moreover, as a practical matter, stiffening of the control in that direction would also avoid or at least minimize the issuance of operator commands that would overload the system. 
     Nevertheless, it is possible that through the action of multiple actuators at the same time, a command that would overload the system may be present. For such conditions, the present embodiment provides a positive force-feedback function to the manual control device that would effectively not only stiffen motion of the control device towards an overloading command direction, but would also provide a force tending to move the control device away from the overloading command direction. In the illustrated embodiments, the ability to provide a force counter-acting the force of the operator applied to a manual control device in a direction tending to overload the system is provided by appropriate manipulation of a vibration present in the handle  409 , which can be provided naturally during machine operation, as previously described, and/or be induced artificially through a vibration device associated with the handle  409 , for example, the rotating mass assembly  418  as shown in  FIG. 4 . 
     More particularly, the determinator function  512  is configured to provide a force-feedback signal  526  when it is determined that the manual control device has already reached a position that would result in overloading of an actuator. The force feedback signal  526  is provided to a force feedback function  514 , which is also configured to further receive an eccentric mass orientation signal  524 , for example, provided by the encoder  424 . The eccentric mass orientation signal  524  is optional and may be replaced by a calculated natural vibration signal, as previously described. The force feedback function  514  is configured to coordinate the control of the one or more damper devices  412  with the natural vibration or, when present, with the rotating mass assembly  418  such that a net force is applied to the handle  409  ( FIG. 4 ) that tends to push the handle in a particular direction away from a direction in which an overloading command to an actuator is represented. Accordingly, the force feedback function  514  outputs signals  516  to each of the dampers in the system, for example, the damper devices  412 . When applicable, the function  514  also outputs an eccentric mass control signal  518 , which includes a motor signal  520  configured to command a particular rate of rotation of the eccentric mass that is coupled with an optional control signal  522  configured to set an appropriate moment of inertia to the rotating mass. The control signal  522  is optional and can be used in embodiments where the capability of setting amplitude of vibration is provided, for example, by setting the rotational radius of the rotating mass by a screw drive or other device. 
     The output and eccentric mass control signals  516  and  518  may be used to selectively control the direction and magnitude of the positive force-feedback applied to a control device, for example, the handle  409  ( FIG. 4 ). A time graph  600  illustrating the concept of creating a positive-force feedback using a rotating eccentric mass by the coordinated control of a MR fluid-based damper is shown in  FIG. 6 . The example using a rotating mass is illustrative for the sake of discussion but is should be appreciated that the control concept described relative thereto is applicable to any condition where a vibration is present in the handle  409 , whether the vibration is natural or artificially created, and is not limited to use of a buzzer. The graph  600  illustrates time-aligned signals for the sake of discussion. A first curve  602  represents a position, P, of the projection of the position of the rotating mass, M, onto a diameter, D, of its circular trajectory, T, relative to reference or zero position, R. Accordingly, the projection of the mass onto the diameter D will appear as a sinusoidal wave as it rotates around an axis. The curve will cross zero each time the mass it at diametrically opposite positions and lies onto a reference diameter, D′, which is predetermined and lies at 90 degrees relative to the reference diameter, D, and occupy positions P 1  and P 2  when it occupies diametrically opposite positions disposed on a the diameter D. As shown, P 1  can be positive and P 2  can be negative, even though those designations are solely for illustration. The first curve  602  may be created if the positional information from the encoder  424  is plotted over time. As can be appreciated, when the mass M is rotating, the vibration it creates will have a vector, V, tending to pull the mass into a continuously variable direction. Thus, when providing a force in a particular direction is desired, certain segments of the trajectory of the mass M may be selected for amplification, while the remaining portions be dampened. 
     In the graph  600 , a second curve  604  illustrates a control signal provided to a variable damper, for example, a MR fluid-based damper that lies in a particular orientation, over time. One example of such damper is the device  412  ( FIG. 4 ). In general, the stiffness of the variable damper is proportional to the intensity of the signal, S. Here, the damper is shown to receive a maximum signal, S, for the majority of the time except for certain force-feedback periods,  606 , during which the rotating mass M is a particular position. When the control signal is maximum, the corresponding damper is stiff to avoid displacement of the control handle  409 . During the periods  606 , the signal S is reduced such that the damper is allowed to move and thus the handle is displaced in the desired direction. Although a square wave is shown for the signal S, other shapes may be used. For example, the transition between maximum and minimum or any other intensities for the signal S can have any desired shape including a linear relationship. The coordinated activation of the damper with respect to the position and orientation of the force vector V of the rotating mass M in this fashion will create a directional and pulsed positive-feedback force in a selected direction, while force applied in other directions will be muted. 
       FIG. 7  is one embodiment for a block diagram of a force-feedback control  528  operating under this principle. The control  528  is disposed to receive information relative to the position and speed of a rotating mass associated with a manual control device, for example, the rotating mass assembly  418 . Specifically, the control  528  may receive a rotating speed signal or a rotational position signal  530 . In alternative embodiments, the control  528  may receive information relative to the natural vibration experienced at the operator cab or at the handle of a manual control device. The control  528  may also receive a desired direction of force application signal  532  relative to the manual control, as well as a position signal  534  indicative of the then present position of the manual control with respect to the desired direction of force application. This information is provided to a feedback force processor  536 , which calculates the appropriate time intervals, for example, the periods  606  ( FIG. 6 ), during which the stiffness of one or more variable dampers, for example, the power of magnets in MR fluid-based dampers or the valve setting in a variable hydraulic damper, either of which may be associated with the manual control, are adjusted to provide a positive force-feedback to the manual control, as previously described. In certain embodiments, for example, where a linear or one-dimensional vibration device is used as previously described, the position of a reciprocating slug need not be measured and can be determined based on the operation of the vibration device. Thus, control signals  538 A,  538 B and  538 C may be provided to three variable damper devices acting along three dimensions to control the force feedback in any direction. Although three such signals are shown here, fewer or more than three may be used depending on the type of manual control and the degrees of freedom it is designed to provide. In this way, a control that has exceeded the possible force response of an actuator may be pushed into a position that will not cause an overshoot of the actuator when the capability of the system to respond to a command is restored, as previously discussed 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is applicable to a wide array of applications in which a directional pulse of force is desirable during operation. In the embodiments discussed, the variable stiffness and positive-force feedback is provided to a manual control device, such as a joystick handle, which is configured to control operation of work implements in a machine. The variable stiffness ensures that the capability of the system is not exceeded, while the positive force-feedback is used to bring the handle back into an acceptable position that corresponds to the force output capability of the system and avoids overshoots in the event system capability is restored. 
     It should be appreciated that the control of the application of the force from a natural or from an induced vibration, for example, one provided by a rotating mass, into a selected direction by coordinated control of a variable damper such as a MR fluid-based damper has wide applicability in other fields that a haptic force-feedback can be provided to a manual control. For example, although a control operating machine implements is disclosed, any other type of manual control used in any other type of land, air or sea machine may be used. Moreover, other devices such as game or remote-device controllers where it is desired to make physical or machine limitations directly known to the operator may make use of the systems and methods disclosed herein without departing from the spirit of the disclosure. Further, the directional application of pulsing force may have application on a much larger scale, such as hydraulic hammers, subterranean drilling apparatus, and the like. 
     It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may 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 those features, but not to exclude such from the scope of the disclosure entirely unless otherwise 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. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context.