Patent Description:
Construction machines, such as excavators, have implements for modifying a surface. A typical excavator implement includes a hydraulically driven boom, stick, and bucket members each with a respective hydraulic cylinder and can be moved by applying hydraulic fluid pressure to the cylinder. Various valves are used to apply the hydraulic fluid pressure to the cylinders based on input from a user.

One problem associated with these valves is that they can cause a delay between user input and movement of an implement. This delay is caused, at least in part, by static friction, which prevents immediate movement of a valve component in response to hydraulic fluid pressure urging the component to move. Static friction is the friction occurring between two surfaces that resists movement of the surfaces relative to each other. As hydraulic fluid pressure urging the component to move increases, static friction is overcome and only kinetic friction remains, which requires less force than static friction to overcome. For example, in a pilot style system in which pilot valves actuate in response to user input, a pilot valve applies increasing hydraulic fluid pressure urging a hydraulic component to actuate, static friction is overcome and only kinetic friction remains. These static friction delays can make control of movement of the members of an implement by a user more complex and confusing. Document <CIT> shows a valve for a crane, controlled with a dither signal for reducing friction. Document <CIT> shows two valves controlled with two dither signals having a phase shift of <NUM>° for reducing vibrations. <CIT> discloses a method of controlling a construction machine including a work machine including a boom, an arm, and a bucket, the method comprising: determining a speed limit according to a distance between the bucket and a target excavation landform based on the target excavation landform and bucket position data and limiting a speed of the boom so that a speed at which the work machine approaches the target excavation landform is equal to or smaller than the speed limit; operating an operating device in order to drive a movable member including at least one of the arm and the bucket; detecting an amount of operation of the operating device; setting a limited amount of operation for limiting a speed of the movable member based on a detection result of the detection device; and outputting a control signal so that the movable member is driven with the limited amount of operation. <CIT> discloses a control for an excavator of the type having a plurality of hydraulic cylinders for moving excavator components such that digging is accomplished at a worksite with an excavator bucket or other excavator implement, includes a plurality of hydraulic control valves, each of which is associated with a respective one of the hydraulic cylinders for controlling the application of hydraulic fluid pressure to the respective one of the hydraulic cylinders, and a plurality of manually actuated joystick valves for supplying hydraulic fluid pressure to the respective hydraulic control valves to control the movement of the hydraulic cylinders. The control includes a sensor arrangement for sensing the position of one or more excavator components.

The present invention relates generally to hydraulic valves, and more particularly to techniques for mitigating delays between user input and movement of a hydraulic cylinder caused by static friction.

A method for mitigating static friction ("stiction") according to the invention is provided in claim <NUM>. A partial aspect of this method includes the steps of dithering a first hydraulic valve (i.e., continuous back and forth motion of the valve) to produce a first periodically varying hydraulic fluid pressure applied to a first input of a second hydraulic valve and dithering a third hydraulic valve to produce a second periodically varying hydraulic fluid pressure applied to a second input of the second hydraulic valve second hydraulic valve. Outputs of each of the first hydraulic valve and the second hydraulic valve are connected to inputs of a main hydraulic valve. The main hydraulic valve dithers in response to hydraulic fluid pressure applied to its inputs that occur due to dithering of the first hydraulic valve and the second hydraulic valve. The first periodically varying hydraulic fluid pressure and the second periodically varying fluid pressure applied to the first input and the second input of the second hydraulic valve cause the second hydraulic valve to dither. The dithering of the second hydraulic valve causes hydraulic fluid pressure to be applied to a first input of a hydraulic cylinder and a second input of the hydraulic cylinder, wherein the hydraulic fluid pressure applied is a value lower than a value required to actuate the hydraulic cylinder. User input is received to actuate a hydraulic cylinder associated with the main valve. A controller transmits a signal to the first hydraulic valve which causes hydraulic fluid pressure to be applied to one of the inputs of the main valve in response to the user input. The hydraulic cylinder associated with the main valve is actuated by the application of hydraulic fluid pressure from one of the outputs of the main valve in response to the hydraulic fluid pressure applied to a corresponding input of the main valve.

An apparatus according to claim <NUM> and an excavator according to claim <NUM> in which hydraulic valves are dithered to mitigate static friction are also disclosed.

The methods and apparatus described herein mitigate static friction, referred to herein as "stiction. " Stiction is the general inability of a hydraulic valve or cylinder to respond immediately and fully to a command (e.g., electrical signal or hydraulic fluid pressure) transmitted to it when it is not currently in motion. For example, an electro-mechanical hydraulic valve that is not receiving a command remains at rest in a particular position. The valve while at rest experiences static friction which is higher than kinetic friction. Since the static friction is much higher than the kinetic friction, more force is required to begin actuation of the hydraulic valve when it is at rest than when the valve is moving. Stiction causes a delay from a time when an input is received to when a respective hydraulic cylinder actuated by hydraulic valves moves. Such delays can result in difficulty in controlling movement of a component driven by a hydraulic cylinder as used in various machines, such as construction machines.

<FIG> depicts hydraulic valve <NUM> having two inputs <NUM>, <NUM> for receiving hydraulic fluid pressure and one output <NUM> for applying hydraulic fluid pressure. Hydraulic valve <NUM> has a slider <NUM> located within valve body <NUM>. Slider <NUM> is a cylindrical object sized to fit within the associated cylindrical cavity of valve body <NUM> as shown in <FIG>.

Hydraulic valve <NUM> operates as follows. Hydraulic fluid pressure applied to input <NUM> urges slider away from input <NUM> toward input <NUM>, compressing spring <NUM>. Hydraulic fluid pressure applied to input <NUM> urges slider <NUM> away from input <NUM> toward input <NUM>, compressing spring <NUM>. If the hydraulic fluid pressures applied to input <NUM> and input <NUM> are substantially the same, slider <NUM> remains stationary. If hydraulic fluid pressure applied to one input is higher than hydraulic fluid pressure applied to the other input, slider <NUM> will be urged to move away from the input having the higher hydraulic fluid pressure. Sufficient movement of slider <NUM> uncovers output <NUM> which allows hydraulic fluid pressure to be applied from either input <NUM> or input <NUM>, depending on which input has a higher hydraulic fluid pressure applied.

Slider <NUM> does not move in response to increased hydraulic fluid pressure because of static friction between slider <NUM> and the inner surface of valve body <NUM>. When hydraulic fluid pressure applied to input <NUM> is sufficiently higher to overcome static friction, slider <NUM> begins to move and kinetic friction, which is lower than the static friction, occurs between slider <NUM> and inner surface of valve body <NUM>. The static friction can cause a delay between when actuation of hydraulic valve <NUM> is requested and when hydraulic valve <NUM> is actuated. In one embodiment, slider <NUM> is sized to fit within inner surface of valve body <NUM> to prevent the flow of hydraulic fluid between slider <NUM> and valve body <NUM>. In another embodiment, O-rings are used but stiction still occurs between slider <NUM> and valve body <NUM>, and in many cases the resulting stiction is higher than without O-rings.

<FIG> shows a construction machine, specifically excavator <NUM>. Excavator <NUM> has a boom <NUM>, a stick <NUM>, and a bucket <NUM> each of which can be controlled by a user located in cab <NUM> of excavator <NUM>. Boom <NUM>, stick, <NUM>, and bucket <NUM> together are referred to as an implement (e.g., a surface modifying implement) of excavator <NUM>. Cab <NUM> is part of what is referred to as the body of excavator <NUM> which can include treads or other means of conveyance. In one embodiment, the user actuates a control device (e.g., a joystick) located in cab <NUM> to move boom <NUM>, ultimately via hydraulic fluid pressure applied to hydraulic cylinder <NUM>. The user actuates another control device to move stick <NUM> via hydraulic fluid pressure applied to hydraulic cylinder <NUM>. The user actuates an additional control device to move bucket <NUM> via hydraulic fluid pressure applied to hydraulic cylinder <NUM>.

<FIG> depicts a schematic of components of excavator <NUM> related to control of boom <NUM> according to an embodiment. Controller <NUM> can be an electric control device such as a programmable logic controller, application specific integrated circuit (ASIC), field programmable gate array (FPGA), etc. In one embodiment, controller <NUM> is implemented using a computer. Controller <NUM> contains a processor <NUM> which controls the overall operation of the controller <NUM> by executing computer program instructions which define such operation. The computer program instructions may be stored in a storage device <NUM>, or other computer readable medium (e.g., magnetic disk, CD ROM, etc.), and loaded into memory <NUM> when execution of the computer program instructions is desired. Thus, the method steps of <FIG> (described below) can be defined by the computer program instructions stored in the memory <NUM> and/or storage <NUM> and controlled by the processor <NUM> executing the computer program instructions. For example, the computer program instructions can be implemented as computer executable code programmed by one skilled in the art to perform an algorithm defined by the method steps of <FIG>. Accordingly, by executing the computer program instructions, the processor <NUM> executes an algorithm defined by the method steps of <FIG>. One skilled in the art will recognize that an implementation of a controller could contain other components as well, and that controller <NUM> is a high level representation of some of the components of such a controller for illustrative purposes.

Sensors <NUM>, represents one or more sensors for detecting a state of excavator <NUM>, such as an orientation of the implement and operating parameters such as fluid pressures and temperatures. In one embodiment, the orientation of the implement is determined using linear or rotary sensors and/or inertial measurement units for determining the position boom <NUM>, stick <NUM>, and bucket <NUM> of the implement.

Inputs <NUM>, <NUM> and <NUM> represent various input devices for operating excavator <NUM>. In one embodiment, input <NUM> can include one or more control devices (e.g. joysticks) for moving boom <NUM>, stick <NUM>, and bucket <NUM>. For example, a boom joystick can be actuated by the user to command boom <NUM> to raise or lower. Similarly, a stick joystick (i.e., a joystick for controlling movement of stick <NUM>) can be actuated by the user to command stick <NUM> toward body of excavator <NUM> or away from body of excavator <NUM>. A bucket joystick can be actuated by the user to command bucket <NUM> to move toward body of excavator <NUM> or away from body of excavator <NUM>. In one embodiment, inputs associated with joysticks are signals from sensors associated with each respective joystick. Input <NUM> can also include inputs from a user via input devices such as touch screens, buttons, and other types of inputs.

Display <NUM>, in one embodiment, is located in the cab of excavator <NUM> and displays information to a user. Display <NUM> can be any type of display such as a touch screen, a light emitting diode display, a liquid crystal display, etc. Display <NUM> presents various information to a user concerning a related machine, a current site plan, a desired site plan, etc..

Controller <NUM> is connected to multiple electro-mechanical control valves (e.g. <NUM>, <NUM>, and others not shown) each associated with movement of boom <NUM> of excavator <NUM>. An electro-mechanical control valve <NUM> receives electric signals from controller <NUM> and, in response, applies hydraulic fluid pressure to its output. Controller boom-up valve <NUM>, in one embodiment, is used to control upward movement of boom <NUM> of excavator <NUM> by directing hydraulic fluid pressure to a first input of hydraulic main valve <NUM> that controls cylinder <NUM> associated with boom <NUM>. Controller boom-down valve <NUM> is an electro-mechanical control valve that is used to control downward movement of boom <NUM> of excavator <NUM> by directing hydraulic fluid pressure to a second input of hydraulic main valve <NUM> connected to hydraulic cylinder <NUM> associated with boom <NUM>. Controller <NUM> would typically also be connected to electric joystick control valves, via input <NUM> (not shown) for controlling stick <NUM> and bucket <NUM> or other machinery associated with excavator <NUM>. The electro-mechanical control valves for controlling stick <NUM> and bucket <NUM> operate in a manner similar to the electro-mechanical control valves for controlling boom and are therefore not shown.

In one embodiment, controller <NUM> receives data from input <NUM> and sensors <NUM>. Controller <NUM> analyzes the received data and determines excavator operation information for display to a user via display <NUM> and determines if outputs should be sent to controller boom-up valve <NUM> and/or controller boom-down valve <NUM> to control boom <NUM>. In one embodiment, controller <NUM> outputs signals to controller boom-up valve <NUM>, and/or controller boom-down valve <NUM>, in the absence of control inputs from a user to mitigate stiction as described below.

<FIG> shows a schematic of a portion of a hydraulic system <NUM> of excavator <NUM> for controlling movement of boom (<NUM> of <FIG>). Hydraulic systems of excavator <NUM> for controlling movement of stick (<NUM> of <FIG>) and bucket (<NUM> of <FIG>) are similar and therefore not shown. Hydraulic cylinder <NUM> is connected to boom <NUM> which it moves in response to hydraulic fluid pressure applied from main valve <NUM>. Main valve <NUM> is a hydraulic valve that applies hydraulic fluid pressure to hydraulic cylinder <NUM> via output <NUM> or output <NUM> in response to hydraulic fluid pressure applied to input <NUM> or input <NUM> of main valve <NUM>. For example, when hydraulic fluid pressure is applied to input <NUM> and no hydraulic fluid pressure is applied to input <NUM>, main valve <NUM> outputs hydraulic fluid pressure to output <NUM> which is applied to hydraulic cylinder <NUM> causing it to actuate and move boom (<NUM> of <FIG>) upward. When hydraulic fluid pressure is applied to input <NUM> and no hydraulic fluid pressure is applied to input <NUM>, main valve <NUM> outputs hydraulic fluid pressure to output <NUM> which is applied to hydraulic cylinder <NUM> causing it to actuate and move boom (<NUM> of <FIG>) downward.

Input <NUM> receives hydraulic fluid pressure from controller boom-up valve <NUM> which receives signals from controller <NUM>, in response to user boom-up input <NUM> or from internally generated signals.

Input <NUM> receives hydraulic fluid pressure from controller boom-down valve <NUM> which receives signals from controller <NUM>, which receives signals from controller <NUM> based on user input received via user boom-down input <NUM>, or from internally generated signals.

Main valve <NUM> experiences stiction which can cause a delay from the time a valve is actuated by controller <NUM> to the time when hydraulic cylinder <NUM> begins to move. In one embodiment, the stiction of main valve <NUM> is mitigated by dithering main valve <NUM> via its inputs <NUM> and <NUM>.

<FIG> depict various examples of valves being dithered, with various amplitudes. <FIG> depict graphs in which controller boom-up valve <NUM> and controller boom-down valve <NUM> are both dithered, but the dithering of those valves is insufficient to cause dithering in their outputs. <FIG> depict graphs in which controller boom-up valve <NUM>, and controller boom-down valve <NUM> are both dithered, with a signal level greater than in <FIG>, but their output pressure variations are present but insufficient to cause dithering in main valve <NUM>. <FIG> depict graphs in which controller boom-up valve <NUM> and controller boom-down valve <NUM> are dithered, with sufficient amplitude to produce dithered pressure control signals at main valve inputs <NUM> and <NUM>.

<FIG> depict graphs of dithering electrical signals applied to controller boom-up valve <NUM>, controller boom-down valve <NUM> by controller <NUM> and resulting hydraulic fluid pressures <NUM> applied to main valve <NUM> via <NUM> and <NUM>. The graphs shown in <FIG> have the same time scale and signal events are shown with respect to times T<NUM>, T<NUM>, T<NUM>, and T<NUM>, etc. <FIG> shows that insufficient dither amplitude produces no dither in the outputs of either <NUM> or <NUM>.

<FIG> depicts graph <NUM> showing voltage over time of dithering electrical signal <NUM>. In this embodiment, dithering electrical signal <NUM> is a square wave that is added to controller boom-up valve <NUM> by controller <NUM>. Dithering electrical signal <NUM> applied to controller boom-up valve <NUM> causes hydraulic fluid pressure to be output from controller boom-up valve <NUM> which is applied to main valve <NUM>. <FIG> depicts graph <NUM> showing voltage over time of signal <NUM>. Signal <NUM> is applied to controller boom-down <NUM> by controller <NUM>. Signals <NUM> and <NUM> are pulse width modulated signals having duty cycles selected to modulate hydraulic fluid pressure on the outputs of <NUM> and <NUM>. In one embodiment, signals <NUM> and <NUM> also have an additional signal that is changed depending on a desired hydraulic fluid pressure to be output from valves <NUM> and <NUM>.

As shown in <FIG>, dithering electrical signals <NUM> and <NUM> are <NUM> degrees out of phase. As shown in <FIG>, at time T<NUM>, signal <NUM> is high and signal <NUM> is low. At time T<NUM>, signal <NUM> is low and signal <NUM> is high. The combination of the amplitude of signals <NUM> and <NUM> and being out of phase causes periodically varying hydraulic fluid pressure to be applied to inputs <NUM> and <NUM> of boom main valve <NUM>. Since signals <NUM> and <NUM> are out of phase, the hydraulic fluid pressures applied to inputs <NUM> and <NUM> will also be out of phase. Main valve <NUM> applies hydraulic fluid pressure to hydraulic cylinder <NUM> in response to hydraulic fluid pressure at input <NUM> of main valve <NUM> from boom-up valve <NUM>.

<FIG> depicts graph <NUM> of hydraulic fluid pressure over time at input <NUM> of main valve <NUM>. Output pressure <NUM> is shown in <FIG> having a constant value that, in one embodiment, can range from zero up to a value prior to hydraulic fluid pressure that will cause main valve <NUM> to actuate. The operation of controller boom-up valve <NUM> as shown in <FIG>, with minimal variation of hydraulic fluid pressure applied to input <NUM> of main valve <NUM> as shown by output pressure <NUM> in <FIG>, results in no movement in main valve <NUM> and no reduction in its stiction.

Boom-down valve <NUM> can be operated in a manner similar to the operation of boom-up valve <NUM> as described above.

<FIG> depict graphs of hydraulic fluid pressures applied to inputs <NUM> and <NUM> of main valve <NUM> when boom-up valve <NUM> and boom-down valve <NUM> are dithered as shown, for example, in <FIG> and no user inputs are being received. The graphs shown in <FIG> have the same time scale and events are shown with respect to times T<NUM>, T<NUM>, T<NUM>, and T<NUM>, etc..

<FIG> depicts graph <NUM> showing hydraulic fluid pressure values at input <NUM> of main valve <NUM> over time. Hydraulic fluid pressure <NUM> is shown having values over time forming a sinusoidal shape that is the response of the valve to the dithering signal.

<FIG> depicts graph <NUM> showing hydraulic fluid pressure values at input <NUM> of main valve <NUM> over time. Hydraulic fluid pressure <NUM> is shown having values over time forming a sinusoidal shape that is in response to dithering boom-down valve <NUM>.

<FIG> show that sinusoidal waveforms <NUM> and <NUM> are out of phase by <NUM> degrees. As shown in <FIG>, at time T<NUM> hydraulic fluid pressure shown by waveform <NUM> is climbing higher while hydraulic fluid pressure shown by waveform <NUM> is descending lower. At time T<NUM>, waveform <NUM> is shown descending lower while waveform <NUM> is climbing higher. In one embodiment, this alternating high and low of waveforms <NUM> and <NUM> continues as long as user input commanding boom <NUM> to move is not received. The amplitudes of waveforms <NUM> and <NUM> shown in <FIG> are insufficient to cause main valve <NUM> to dither.

<FIG> depicts graph <NUM> of hydraulic fluid pressure over time at output <NUM> and output <NUM> of main valve <NUM> in response to hydraulic fluid pressures applied to inputs <NUM> and <NUM> of main valve <NUM> as depicted in <FIG>, respectively. Hydraulic fluid pressure <NUM> at output <NUM> is shown in <FIG> having a constant value that, in one embodiment, can range from zero up to a value prior to hydraulic fluid pressure that would cause hydraulic cylinder <NUM> to move. Hydraulic fluid pressure <NUM> at output <NUM> is shown in <FIG> having a constant value that, in one embodiment, can range from zero up to a value prior to hydraulic fluid pressure that would cause hydraulic cylinder <NUM> to move.

<FIG> depict graphs of hydraulic fluid pressures applied to inputs <NUM> and <NUM> of main valve <NUM> when no user inputs are being received. The graphs show increased dither amplitude, and also show that sinusoidal waveforms <NUM> and <NUM> are still out of phase by <NUM> degrees The graphs shown in <FIG> have the same time scale and events are shown with respect to times T<NUM>, T<NUM>, T<NUM>, and T<NUM>, etc..

<FIG> depicts graph <NUM> showing hydraulic fluid pressure values at input <NUM> of main valve <NUM> over time. Hydraulic fluid pressure <NUM> is shown having values over time forming a sinusoidal shape that is in response to dithering of boom-up valve <NUM>.

It should be noted that waveforms <NUM> and <NUM> are similar to waveforms <NUM> and <NUM>. Each of waveforms <NUM>, <NUM>, <NUM>, and <NUM> depicts periodically varying hydraulic fluid pressure at a particular point. The amplitudes of waveforms <NUM> and <NUM> are higher than the amplitudes of waveforms <NUM> and <NUM>. The higher amplitudes of waveforms <NUM> and <NUM> cause main valve <NUM> to dither which mitigates stiction of main valve <NUM>.

<FIG> depicts graph <NUM> showing hydraulic fluid pressure applied to input <NUM> and hydraulic fluid pressure applied to input <NUM> over time. As shown in <FIG>, waveform <NUM> is out of phase with waveform <NUM> by <NUM> degrees. The alternating pressures applied via inputs <NUM> and <NUM> are in response to dithering valve <NUM> and <NUM> with an amount of dither that exceeds the amount necessary just to reduce their stiction. It should be noted that the dithering of main valve <NUM> overcomes the stiction of main valve <NUM>. However, the hydraulic fluid pressure applied to inputs <NUM> and <NUM> do not contain enough sinusoidal variations cause variations in the outputs <NUM> and <NUM>, and therefore hydraulic cylinder <NUM> does not move in response to the dither. Thus, the stiction of main valve <NUM> is mitigated without causing movement of hydraulic cylinder <NUM>.

The graph of signal <NUM> in <FIG> can be modified with the addition of a control signal, such that the shape remains the same but the average pressure level is higher, causing main valve <NUM> to shift and create pressure at <NUM>, extending cylinder <NUM> and raising boom <NUM>.

The graph of signal <NUM> in <FIG> can be modified with the addition of a control signal, such that the shape remains the same but the average pressure level is higher, causing main valve <NUM> to shift and create pressure at <NUM>, retracting cylinder <NUM> and lowering boom <NUM>.

The net amount of dither to main valve <NUM> can be adjusted by varying the amplitudes dither signals <NUM> and <NUM>. This net amount can also vary based on the value of the control signal added in graph <NUM> or <NUM>, such that the net difference to main value <NUM> remains the same but the inactive opposite side reaches zero and it corresponding dither disappears, replaced by dither only on the active side. This remaining active dither + control signal would be equal to the amount needed to both control output <NUM> or <NUM>, and reduce stiction in main valve and the corresponding active controller valve.

<FIG> depicts a flowchart of a method <NUM> for mitigating stiction of valves (i.e., two pilot control valves and a main valve) of a hydraulic system according to an embodiment. At step <NUM>, a first hydraulic valve is dithered, with a signal beyond what is needed to remove its inherent dither. In one embodiment, boom-up control valve <NUM> shown in <FIG> is dithered. At step <NUM>, a second hydraulic valve <NUM> is dithered, also with a signal beyond what is needed to remove its inherent dither. The dithering of boom-up valve <NUM> and boom-down valve <NUM> causes hydraulic fluid pressure to be applied to inputs <NUM> and <NUM> of main valve <NUM> as shown in <FIG>. At step <NUM>, main valve <NUM> is dithered by the hydraulic fluid pressure applied to inputs <NUM> and <NUM>. Dithering of main valve <NUM> reduces or eliminates stiction in spool <NUM> of the main valve <NUM>. In one embodiment, the variations in pressures at <NUM> and <NUM> are sufficient to mitigate stiction of spool <NUM> in main valve <NUM>, but are insufficient to cause hydraulic fluid pressure variations in outputs <NUM> and <NUM> and to cause hydraulic cylinder <NUM> to move in response.

At step <NUM>, an input to actuate hydraulic cylinder <NUM> is received by controller <NUM> shown in <FIG>. In one embodiment, input is received from a joystick of input <NUM> shown in <FIG>. At step <NUM>, controller <NUM> outputs a signal to one of controller boom-up <NUM> or controller boom-down <NUM> shown in <FIG> in response to the joystick input. The signal causes hydraulic fluid pressure to be added to the dither signal and applied by valve <NUM> to input <NUM> or valve <NUM> to input <NUM> of main valve <NUM>. Valve <NUM> responds to the net difference in pressure at inputs <NUM> and <NUM>, and, at step <NUM>, the hydraulic cylinder <NUM> is actuated by the hydraulic fluid pressure applied via outputs <NUM> or <NUM> of main valve <NUM>.

It should be noted that stiction of other types of hydraulic valves for various applications can be dithered in a similar manner to mitigate stiction. Accordingly, the stiction associated with hydraulic valves for moving stick <NUM> and bucket <NUM> of excavator <NUM> can be mitigated using methods similar to those described above in connection with boom <NUM>.

Claim 1:
A method comprising:
dithering a first hydraulic valve (<NUM>) to produce a first periodically varying hydraulic fluid pressure applied to a first input (<NUM>) of a second hydraulic valve (<NUM>); and
characterized by:
dithering a third hydraulic valve (<NUM>) to produce a second periodically varying hydraulic fluid pressure <NUM> degrees out of phase with the first periodically varying hydraulic fluid pressure and applied to a second input (<NUM>) of the second hydraulic valve,
wherein the first periodically varying hydraulic fluid pressure and the second periodically varying fluid pressure applied to the first input and the second input of the second hydraulic valve cause the second hydraulic valve to dither, and the dithering of the second hydraulic valve causes hydraulic fluid pressure to be applied to a first input (<NUM>) of a hydraulic cylinder (<NUM>) and a second input (<NUM>) of the hydraulic cylinder, wherein the hydraulic fluid pressure applied is a value lower than a value required to actuate the hydraulic cylinder.