Hydraulic actuator piston measurement apparatus and method

A method and device for use with a hydraulic system is adapted to measure a position, velocity and/or acceleration of a piston of a hydraulic actuator based upon differential pressure measurement. The device of the present invention utilizes a differential pressure flow sensor to establish a flow rate of a hydraulic fluid flow traveling into and out of a cavity of the hydraulic actuator, from which the position, velocity and acceleration of the piston can be determined.

BACKGROUND OF THE INVENTION

The present invention relates to hydraulic systems. More particularly, the present invention relates to position, velocity, and acceleration measurement of a hydraulic actuator piston of a hydraulic system based upon a differential pressure measurement.

Hydraulic systems are used in a wide variety of industries ranging from road construction to processing plants. These systems are generally formed of hydraulic valves and hydraulic actuators. Typical hydraulic actuators include a hydraulic cylinder containing a piston and a rod that is attached to the piston at one end and to an object at the other end. The hydraulic valves direct hydraulic fluid flows into and out of the hydraulic actuators to cause a change in the position of the piston within the hydraulic cylinder and produce a desired actuation of the object. For many applications, it would be useful to know the position, velocity, and/or acceleration of the piston. By these variables, a control system could control the location or orientation, velocity and acceleration of the objects being actuated by the hydraulic actuators. For example, a blade of a road grading machine could be repeatedly positioned as desired resulting in more precise grading.

One technique of determining the piston position is described in U.S. Pat. No. 4,588,953 which correlates resonances of electromagnetic waves in a cavity, formed between a closed end of the hydraulic cylinder and the piston, with the position of the piston within the hydraulic cylinder. Other techniques use sensors positioned within the hydraulic cylinder to sense the position of the piston. Still other techniques involve attaching a cord carried on a spool to the piston where the rotation of the spool relates to piston position.

There is an on-going need for methods and devices which are capable of achieving accurate, repeatable, and reliable hydraulic actuator piston position measurement. Furthermore, it would be desirable for these methods and devices to measure the velocity and acceleration of the hydraulic actuator piston.

SUMMARY

A method for measuring position, velocity, and/or acceleration of a piston, which is slidably contained within a hydraulic cylinder of a hydraulic actuator is provided. In addition, a device that is adapted to implement the method of the present invention within a hydraulic system is provided. The method involves measuring a differential pressure across a discontinuity positioned in a hydraulic fluid flow which is related to the position, velocity, and acceleration of the piston. The position, velocity, and/or acceleration is then calculated as a function of the differential pressure measurement.

The device includes a differential pressure flow sensor and a calculating module. The differential pressure flow sensor is adapted to measure the differential pressure and produce a first signal that is indicative of a flow rate of the hydraulic fluid flow. The calculation module is adapted to receive the first signal and responsively provide a second signal, which is of the position, velocity, and/or acceleration of the piston.

Elements of the figures which are identified by the same or similar labels are intended to represent the same or similar elements.

DETAILED DESCRIPTION

The present invention provides a method and device for use with a hydraulic system to measure the position, velocity and/or acceleration of a piston of a hydraulic actuator-based upon differential pressure measurement. In general, the present invention utilizes a differential pressure flow sensor to establish a flow rate of a hydraulic fluid flow traveling into and out of a cavity of the hydraulic actuator, from which the position, velocity and acceleration of the piston can be determined. The position of the piston is directly related to a volume of hydraulic fluid that is contained in a cavity of the hydraulic actuator. The velocity of the piston is directly related to the flow rate of the hydraulic fluid flow. Finally, the acceleration of the piston is directly related to the rate of change of the flow rate of the hydraulic fluid flow.

FIG. 1shows a simplified block diagram of an example of a prior art hydraulic system10, to which embodiments of the present invention can be applied. Hydraulic system10generally includes at least one hydraulic actuator12, hydraulic control valve13, and a sources of high and low pressure hydraulic fluid (not shown) delivered through, for example, hydraulic lines14. Hydraulic control valve13is generally adapted to control a flow of hydraulic fluid into and out of cavities of hydraulic actuator12, which are fluidically coupled to a ports16through fluid flow conduit17. Alternatively, hydraulic control valve13could be configured to control hydraulic fluid flows into and out of multiple hydraulic actuators12. Hydraulic control valve13could be, for example, a spool valve, or any other type of valve that is suitable for use in a hydraulic system.

The depicted hydraulic actuator12is intended to be an example of a suitable hydraulic actuator to which embodiments of the present invention may be applied. Hydraulic actuator12generally includes hydraulic cylinder18, piston20, and rod22. Piston20is attached to rod22and is slidably contained within hydraulic cylinder18. Rod22is further attached to an object (not shown) at end24for actuation by hydraulic actuator12. Piston stops25can be used to limit the range of motion of piston20within hydraulic cylinder18. Examples suitable hydraulic actuators12will be discussed in greater detail with reference toFIGS. 2A and 2B.

Hydraulic actuator12A, shown inFIG. 2A, includes first and second ports26and28, respectively, which are adapted to direct a hydraulic fluid flow into and out of first and second cavities30and32, respectively, through fluid flow conduit17. First cavity30is defined by interior wall36of hydraulic cylinder18and surface38of piston20. Second cavity32is defined by interior wall36of hydraulic cylinder18and surface40of piston20. First and second cavities30and32of hydraulic actuator12A are completely filled with hydraulic fluid and the position of piston20is directly related to the volume of either first cavity30or second cavity32and thus, the volume of hydraulic fluid contained in first cavity30or second cavity32. As pressurized hydraulic fluid is forced into first cavity30, piston20is forced to slide to the right thereby decreasing the volume of second cavity32and causing hydraulic fluid to flow out of second cavity32through second port28. Similarly, as pressurized hydraulic fluid is pumped into second cavity32, piston20is forced to slide to the left thereby decreasing the volume of first cavity30and causing hydraulic fluid to flow out of first cavity30through first port26.

Hydraulic actuator12B, shown inFIG. 2B, includes only first port26through which hydraulic fluid flows into and out of first cavity30. A spring42is adapted to exert a force on rod22to bias piston20toward first port26. As hydraulic fluid is pumped into first cavity30, piston20is forced to slide to the right thereby decreasing the volume of second cavity32and compressing spring42. As hydraulic fluid is pumped out of first cavity30, spring42expands and piston20slides to the left. Here, the position of piston20is directly related to the volume of hydraulic fluid contained within first cavity30.

The present invention provides piston position, velocity, and/or acceleration measurement based upon a differential pressure measurement taken within the hydraulic fluid flow traveling into and out of first cavity30of hydraulic cylinder12. Those skilled in the art understand that the following method and equations could be equally applied to hydraulic fluid flows traveling into and out of second cavity32of hydraulic actuator12A. As mentioned above, a position x of piston20is directly related to the volume V1of hydraulic fluid contained within first cavity30. This relationship is shown in the following equation:x=V1-V0A1Eq.⁢1
where A1is the cross-sectional area of first cavity30and V0is the volume of first cavity30that is never occupied by piston20due to the stops25positioned to the left of piston20.

As the hydraulic fluid is pumped into or out of first cavity30, the position x of piston will change. For a given reference or initial position x0of piston20, a new position x can be determined by calculating the change in volume ΔV1of first cavity30over a period of time t0to t1in accordance with the following equations:Δ⁢⁢V1=∫t0t1⁢Qv1⁢Eq.⁢2x=x0+Δ⁢⁢V1A1=x0+1A1⁢∫t0t1⁢Qv1Eq.⁢3
where QV1is the volumetric flow rate of the hydraulic fluid flow into or out of first cavity30. Although, the reference position x0for the above example as shown inFIGS. 2A and 2Bas being set at the left most stops25, other reference positions are possible as well. A similar method can be used to determine the position of piston20of hydraulic actuator12A based upon a the volume of hydraulic fluid contained in second cavity32.

The velocity at which the position x of piston20changes is directly related to the volumetric flow rate QV1of the hydraulic fluid flow into or out of first cavity30. The velocity υ of piston20can be calculated by taking the derivative of Eq. 3, which is shown in the following equation:v=ⅆxⅆt=Qv1A1Eq.⁢4
Finally, the acceleration of piston20is directly related to the rate of change of the flow rate QV1, as shown in Eq. 5 below. Accordingly, by measuring the flow rate QV1flowing into and out of first cavity30, the position, velocity, and acceleration of piston20can be calculated.a=ⅆvⅆt=ⅆⅆt⁢(ⅆxⅆt)=1A1⁢(ⅆQv1ⅆt)Eq.⁢5

The general method of the present invention for measuring the position, velocity, and/or acceleration of piston20of hydraulic actuator12is illustrated in the flowchart shown in FIG.3. At step44, the differential pressure across a discontinuity positioned in a hydraulic fluid flow travelling into or out of first cavity30of hydraulic cylinder18is measured. Next, at step46, a flow rate QVof the hydraulic fluid flow is calculated as a function of the differential pressure measurement using methods which are known in the art. Finally, the position, velocity, and/or acceleration of piston20is calculated as a function of the flow rate QV, at step48, in accordance with the above equations. The position, velocity, and acceleration information can be provided to a control system, which can use the information to control the objects being actuated by hydraulic actuator12.

Implementation of the above method can be accomplished using measuring device50, an embodiment of which is shown in FIG.4. Measuring device50generally includes a differential pressure flow sensor52and a calculation module54. Differential pressure flow sensor52is coupled to conduit17and is adapted to measure a pressure drop across a discontinuity placed in the hydraulic fluid flow. The differential pressure sensor produces a first signal, based upon the pressure drop, which is indicative of the flow rate QV1of the hydraulic fluid flow flowing into and out of first cavity30. Calculation module54is adapted to receive the first signal from differential pressure flow sensor52over a suitable physical connection, such as wires56, or a wireless connection, in accordance with a communication protocol. The first signal can be a differential pressure signal relating to the pressure drop across the discontinuity, a flow rate signal relating to the flow rate QV1, a compensated pressure drop signal, or a compensated flow rate signal. The compensated pressure drop and flow rate signals are generated in response to, for example, the temperature of the hydraulic fluid, a static pressure measurement, or other parameter that affects the pressure drop measurement or the relationship between the pressure drop and the flow rate QV1.

Calculation module54is generally adapted to produce a second signal, based upon the first signal, that is indicative of the position, velocity, and/or acceleration of piston20. The second signal is preferably provided to control system11over a physical connection, such as wire55, or a wireless connection, in accordance with a communication protocol. Calculation module can be an integrated into differential pressure flow sensor52, separated from differential pressure flow sensor52, or located within control system11. If necessary, calculation module can calculate the flow rate QV1of the hydraulic fluid flow, when the first signal is a differential pressure signal, based upon various parameters of the hydraulic fluid flow, the geometry of the object forming the discontinuity, and other parameters in accordance with known methods. Calculation module54samples the varying flow rate QV1at a sufficiently high rate to maintain an account of the current volume V1of first cavity30or position x0. This information can then be used to establish the position x of piston20using Eqs. 1-3 above. The flow rate QV1can also be used to calculate the velocity and acceleration of piston20in accordance with Eqs. 4 and 5 above, respectively.

In this manner, control system11can obtain piston position, velocity, and acceleration information, which can be used in the control of hydraulic actuator12. Furthermore, hydraulic system10can incorporate multiple measuring devices50to monitor the position, velocity, and acceleration of pistons20of multiple hydraulic actuators12. Thus, control system11can use the information to coordinate the actuation of multiple hydraulic actuators12.

Measuring device50can be configured to filter or compensate the first or second signal for anomalies that develop in the system. For example, the starting and stopping of piston20can cause anomalies to occur in the hydraulic fluid flow which are detected in the form of transients in the pressure drop. These errors can be filtered by differential pressure flow sensor52or calculation module54. Alternatively, control system11can be configured to provide the necessary compensation.

FIG. 5shows a simplified block diagram of a hydraulic control valve13which includes various additional embodiments of the invention.

Hydraulic control valve13generally includes at least one port60that is fluidically coupled to a source of hydraulic fluid, valve body62, flow control member64, and at least one port16that is inline with a cavity of a hydraulic actuator, such as first cavity30(FIGS.2A and2B). Ports16and60are placed inline with flow control member64through fluid flow passageways66. Flow control member64is contained within valve body62and is adapted to control hydraulic fluid flows through ports16and60using methods that are known to those skilled in the art. Here, at least one flow sensor52of measuring device50is placed proximate a port16or60to measure the flow rate of the hydraulic fluid passing therethrough. Calculation module54can be a formed within valve body62, attached to valve body62, or separated from valve body62. Here, calculation module54is adapted to receive first signals from one or more flow sensors52through a suitable physical connection, such as wires68, and produce the second signal that can be provided to control system11over a physical (e.g., wire14) or a wireless connection as described above. Furthermore, calculation module54can be adapted to control flow control member64in response to control signals from control system11.

In one embodiment, flow sensor52of measuring device50is positioned proximate at least one port16of hydraulic control valve13to monitor the flow rate of the hydraulic fluid flow into first cavity30(or second cavity32) of hydraulic actuator12. Flow sensors52can also be placed at each port16to monitor hydraulic fluid flows to different hydraulic actuators12. Alternatively, a pair of flow sensors12can monitor a single direction of the fluid flow to a hydraulic actuator12or be used as a redundant pair whose measurements can be verified by comparison. Here, the comparison can be used for diagnostic purposes (e.g., leak detection). In another embodiment (not depicted), flow sensor52could be positioned proximate port60, which couples hydraulic control valve13to a high or low pressure source of hydraulic fluid, to establish the flow rate of hydraulic fluid into and out of hydraulic control valve13, which in turn can be used to measure the position, velocity, and acceleration of a piston20.

One embodiment of differential pressure flow sensor52is shown in the simplified block diagram of FIG.6. In this example, differential pressure flow sensor52is shown installed inline with conduit17. However, this embodiment of flow sensor52could also be installed proximate a port16or60of hydraulic control valve13, as shown in FIG.5. Flow sensor52is adapted to produce a discontinuity within the hydraulic fluid flow traveling to and from a cavity, such as first cavity30(FIGS.2A and2B), and measure a pressure drop across the discontinuity. The pressure drop measurement is indicative of the direction and flow rate QVof the hydraulic fluid flow. Furthermore, flow sensor52is adapted to produce a first signal that is indicative of the flow rate QV, as discussed above.

Flow sensor52generally includes flow restriction member72and differential pressure sensor74. Flow sensor52can be installed in conduit17or proximate hydraulic control valve13using nuts and bolts76. O-rings78can be used to seal the installation. Flow restriction member72, shown as an orifice plate having an orifice80, forms the desired discontinuity in the hydraulic fluid flow by forming a flow restriction. Preferably, flow restriction member72is configured to operate in bi-directional fluid flows due to the symmetry of flow restriction member72. Those skilled in the art will appreciate that other configurations of flow restriction member72that can produce the desired pressure drop could be substituted for the depicted flow restriction member72. These include, for example, orifice plates having concentric and eccentric orifices, plates without orifices, wedge elements consisting of two non-parallel faces which form an apex, or other commonly used bi-directional flow restriction members.

Differential pressure sensor74is adapted to produce a differential pressure signal that is indicative of the pressure drop. Differential pressure sensor74can comprise two separate absolute or gauge pressure sensors arranged to measure the pressure at first and second sides81A and81B of member72such that a differential pressure signal is generated by differential pressure sensor74that relates to a difference between the outputs from the two sensors. Differential pressure sensor74can be a piezoresistive pressure sensor that couples to the pressure drop across flow restriction member72by way of openings82. One of the advantages of this type of differential pressure sensor is that it does not require the use of isolation diaphragms and fill fluid to isolate sensor74from the hydraulic fluid. If needed, a coating84can be adapted to isolate and protect differential pressure sensor74without affecting the sensitivity of differential pressure sensor74to the pressure drop. Differential pressure sensor74could also be a capacitance-based differential pressure sensor or other suitable differential pressure sensor known in the art.

Another embodiment of flow sensor52includes processing electronics86that receives a differential pressure signal from differential pressure sensor74and produces the first signal that is indicative the flow rate QVof the hydraulic fluid flow based upon the differential pressure signal. The first signal can be transferred to calculation module54(FIGS. 4 and 5) of measuring device50through terminals88in accordance with a communication protocol. Flow sensor52can include additional sensors, such as temperature and static pressure sensors to provide additional parameters relating to the hydraulic fluid and flow sensor52. The temperature and static pressure signals can be provided to processing electronics86or calculation module54, which can use the signals to compensate the first or second signal for the environmental conditions. Alternatively, processing electronics86can perform the function of calculation module54by producing the second signal in response to the differential pressure signal received form differential pressure sensor74.

FIG. 7shows another embodiment of flow sensor52coupled to a port16of valve body62and fluid flow conduit17. Alternatively, this embodiment of flow sensor52, as well as the other embodiments discussed herein, could be mounted elsewhere within hydraulic system10(FIG. 1) such that it is inline with the hydraulic fluid flow that is to be measured. As with the previous embodiment shown inFIG. 6, this embodiment of flow sensor52includes flow restriction member72and differential pressure sensor74. Flow restriction member72is preferably a bi-directional flow restriction member that forms a discontinuity within the hydraulic fluid flow traveling between hydraulic control valve13and a cavity of a hydraulic actuator12thereby producing a pressure drop across first and second sides81A and81B, respectively. This embodiment of flow sensor52also includes first and second pressure ports90A and90B corresponding to first and second sides81A and81B, respectively. First and second ports90A and90B respectively couple the pressure at first and second sides81A and81B to differential pressure sensor74. Differential pressure sensor74is preferably a piezo-resistive pressure sensor, however, other types of pressure sensors may be used as well as mentioned above. Flow restriction member72can be formed of first and second flow restriction portions92A and92B, each of which have varying flow areas which constrict the fluid flow and form the desired discontinuity. Although second flow restriction portion92B is shown as having a threaded portion94that mates with port16of valve body62, second flow restriction portion92B could also be formed integral with valve body62. Bleed screws or drain/vent valves (not shown) can be fluidically coupled to first and second pressure ports90A and90B to release unwanted gas and fluid contained therein. Seals96can provide leakage protection and retain the static pressure in conduit17and hydraulic control valve13. First and second flow restriction portions92A and92B can be joined using a suitable fastener such as the depicted nuts and bolts76.

Flow sensor52is preferably adapted to generate a first signal that is indicative of a flow rate QVof the hydraulic fluid flow as well as a direction that the flow is traveling. This is preferably accomplished using a flow restriction member72that is symmetric about a horizontal plane98running parallel to the hydraulic fluid flow and a vertical plane (not shown) running perpendicular to plane90and dividing flow restriction member72into equal halves. However, those skilled in the art understand that non-symmetric flow restriction members72could also provide the desired bi-directional function. The flow rate QVrelates to the magnitude of the pressure drop and can be calculated in accordance with known methods. The direction of the hydraulic fluid flow depends on whether the pressure drop is characterized as a positive pressure drop or a negative pressure drop. For example, a positive pressure drop can be said to occur when the pressure at first side81A is greater than the pressure at second side81B. This could relate to a positive fluid flow or a fluid flow moving from left to right in the sensors52shown inFIGS. 6 and 7, which could indicate a flow moving out of first cavity30of hydraulic actuator12. Accordingly, a negative pressure would occur when the pressure at first side81A is less than the pressure at second side81B. The negative pressure drop would then relate to a right-to-left hydraulic fluid flow or one traveling into first cavity30. Consequently, the pressure drop can be indicative of both the direction of the fluid flow and its flow rate QV.

FIG. 8shows a simplified block diagram of calculation module54of measuring device50in accordance with the various embodiments discussed above. Calculation module54generally includes one or more analog to digital (A/D) converters100, microprocessor102, input/output (I/O) port104, and memory106. The optional temperature sensor108and static pressure sensor110can be provided to module54to correct for flow variations due to the temperature and the static pressure of the hydraulic fluid, as mentioned above. Piston position module54receives the first signal112from a first differential pressure flow sensor52A, in accordance with an analog communication protocol, at A/D converter100which digitizes the first signal. The first signal can be a standard 4-20 mA analog signal that is delivered over, for example, wires56(FIG. 4) or wires68(FIG.5). Alternatively, A/D converter100can be eliminated from calculation module54and microprocessor102can receive the first signal directly from flow sensor52A when the first signal is in a digital form that is provided in accordance with a digital communication protocol. Suitable digital communication protocols, which can be used with the present invention include, for example, Highway Addressable Remote Transducer (HART®), FOUNDATION™ Fieldbus, Profibus PA, Profibus DP, Device Net, Controller Area Network (CAN), Asi, and other suitable digital communication protocols.

Microprocessor102uses the digitized first signal, which is received from either A/D converter100or flow sensor52, to determine the position, velocity, and/or acceleration of piston20within hydraulic cylinder18(FIGS.2A and2B). Memory106can be used to store various information, such as the current position x0of piston20, an account of the volume V1of hydraulic fluid contained in first cavity30, applicable cross-sectional areas of hydraulic cylinder18, such as area A1, and any other information that could be useful to calculation module54. Microprocessor102produces the second signal114which is indicative of the position, velocity, and/or acceleration of piston20within hydraulic cylinder18. The second signal can be provided to control system11through I/O port104.

As mentioned above, calculation module54can also receive differential pressure, static pressure and temperature signals from flow sensor52, or from separate temperature (108) and static pressure (110) sensors as shown in FIG.8. These signals can be used by microprocessor102to compensate for spikes or anomalies in the flow rate signal which can occur when the piston starts or stops as well as the environmental conditions in which flow sensor52is operating. Temperature sensor108can be adapted to measure the temperature of the hydraulic fluid, the operating temperature of differential pressure sensor74, and/or the temperature of flow sensor52. Temperature sensor108produces the temperature signal116that is indicative of the sensed temperature, which can be used by calculation module54in the calculation of the flow rate QV. Temperature sensor108can be integral with or embedded in flow restriction member72(FIGS.6and7). The static pressure signal118from static pressure sensor110can be used by calculation module54to correct for compressibility effects in the hydraulic fluid.

In another embodiment of the invention, additional flow sensors52, such as second flow sensor52B, can be included so that the hydraulic fluid flows coupled to first and second cavities30and32(FIG.4), respectively, or at different ports16(FIG. 5) of a hydraulic control valve13can be measured. The first signals received from the multiple flow sensors52can be used for error checking or diagnostic purposes.

In summary, the present invention provides a method and device for measuring the position, velocity, and/or acceleration of a hydraulic piston operating within a hydraulic system. These measurements are taken based upon a differential pressure measurement taken across a discontinuity that is placed in a hydraulic fluid flow which is used to actuate the piston. The differential pressure measurement is then used to establish a flow rate of the hydraulic fluid flow, which can be used to determine the position, velocity, and/or acceleration of a piston contained within a hydraulic cylinder of a hydraulic actuator.

The measuring device includes a differential pressure flow sensor and a calculation module. The differential pressure flow sensor is positioned inline with a cavity of the hydraulic actuator that receives the hydraulic fluid flow. The flow sensor can be positioned proximate a port of a hydraulic control valve or a port of the hydraulic actuator corresponding to the cavity, or inline with fluid flow conduit through which the hydraulic fluid flow travels. The flow sensor produces a first signal which is indicative of the flow rate of the hydraulic fluid flow and is based upon a differential pressure measurement. The calculation module is adapted to receive the first signal and produce a second signal based thereon, which is indicative of the position, velocity, and/or the acceleration of the piston.