WO2015105448A1 - Capacitive position measuring of electro-mechanical linear actuator - Google Patents

Capacitive position measuring of electro-mechanical linear actuator Download PDF

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Publication number
WO2015105448A1
WO2015105448A1 PCT/SE2014/051512 SE2014051512W WO2015105448A1 WO 2015105448 A1 WO2015105448 A1 WO 2015105448A1 SE 2014051512 W SE2014051512 W SE 2014051512W WO 2015105448 A1 WO2015105448 A1 WO 2015105448A1
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WO
WIPO (PCT)
Prior art keywords
moveable member
stationary member
linear actuator
electro
moveable
Prior art date
Application number
PCT/SE2014/051512
Other languages
French (fr)
Inventor
Pär HÖGBERG
Original Assignee
Aktiebolaget Skf
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Filing date
Publication date
Application filed by Aktiebolaget Skf filed Critical Aktiebolaget Skf
Publication of WO2015105448A1 publication Critical patent/WO2015105448A1/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/12—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means
    • G01D5/14—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage
    • G01D5/24—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying capacitance
    • G01D5/241—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying capacitance by relative movement of capacitor electrodes
    • G01D5/2412—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying capacitance by relative movement of capacitor electrodes by varying overlap
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16H—GEARING
    • F16H25/00—Gearings comprising primarily only cams, cam-followers and screw-and-nut mechanisms
    • F16H25/18—Gearings comprising primarily only cams, cam-followers and screw-and-nut mechanisms for conveying or interconverting oscillating or reciprocating motions
    • F16H25/20—Screw mechanisms
    • F16H25/2015—Means specially adapted for stopping actuators in the end position; Position sensing means
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D2205/00—Indexing scheme relating to details of means for transferring or converting the output of a sensing member
    • G01D2205/20—Detecting rotary movement
    • G01D2205/22—Detecting rotary movement by converting the rotary movement into a linear movement
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02K—DYNAMO-ELECTRIC MACHINES
    • H02K7/00—Arrangements for handling mechanical energy structurally associated with dynamo-electric machines, e.g. structural association with mechanical driving motors or auxiliary dynamo-electric machines
    • H02K7/06—Means for converting reciprocating motion into rotary motion or vice versa

Definitions

  • the present invention relates to position measuring of electromechanical linear actuators. More specifically, the present invention relates to a method for determining the position of an electro-mechanical linear actuator, the position being the relative position between a moveable member and a stationary member of the actuator, which electro-mechanical linear actuator converts rotary motion into linear motion by a screw and nut unit.
  • the present invention also relates to an electro-mechanical linear actuator arrangement comprising a screw and nut unit.
  • Electro-mechanical (EM) linear actuators are arranged to convert rotary motion from an electric motor into linear motion of a moveable member by using a screw and nut unit, and are utilized for providing linear motion in a wide range of applications, such as in machines, industrial machinery, valves, dampers, etc.
  • a position sensor such as a potentiometer and a position control unit or a rotary position sensor and a counter.
  • the function of a potentiometer may for example be based on electrical resistance, and be provided with a wiper contact linked to a mechanical shaft that can be either angular or linear, i.e. slider type, in its movement, and which causes the resistance value between the wiper/slider and the two end connections to change giving an electrical signal output that has a
  • a position sensor unit may also log the number of revolutions of the motor or screw.
  • installing a position sensor, such as a potentiometer or other sensors, in an EM linear actuator requires space. In turn, this leads to increased size and complexity of the actuator, as wells as increased manufacturing cost in terms of cost for parts and a more time consuming manufacturing process.
  • a general object of the present invention is to provide an improved method for determining the relative position between a moveable member and a stationary member of an electro-mechanical linear actuator, and to an improved electro-mechanical linear actuator.
  • Another object is to provide a more cost efficient and more durable determination of position of an electro-mechanical linear actuator.
  • the present invention relates to a method for determining the relative position between a moveable member and a stationary member of an electro-mechanical linear actuator, which linear actuator is arranged to transform rotary motion from a motor into translation of the moveable member in a linear direction by a screw and nut unit.
  • the method comprises arranging the moveable member in a relative position in relation to the stationary member, in which relative position a first conducting portion of the moveable member is overlapping a second conducting portion of the stationary member in the linear direction.
  • the method further comprises measuring a parameter C indicative of the capacitance of the first and second conductive portions, and determining the relative position between the moveable member and the stationary member based on the measured parameter C.
  • the invention is based on the realization by the inventors that by configuration a linear actuator as a position dependent capacitance system, the position of the moveable member in relation to the stationary member of the actuator may be determined by measuring the capacitance of the capacitance system formed by the first and second conductive portions.
  • the overlapping first and second conductive portions are arranged to form a capacitor having a variable capacitance that is dependent on the position of the linear actuator in the linear direction.
  • the capacitance of the capacitance system formed of the first and second conductive portions is dependent on the overlapping area of the first and second portions and the dielectric constant of the separation space between the first and second portions, wherein the extension of the overlapping area in the linear direction is determined by the linear movement and position of the moveable member in relation to the stationary member.
  • the position of the linear actuator may be determined in an
  • the first and second portions form essential members forming part in the electromechanical actuator, no additional or extra equipment is required and the conventional structural members forming a conventional electromechanical actuator may advantageously be used.
  • the first and second conductive portions form part of the basic actuator design, for example as part of the force bearing structure of the moveable and stationary members forming the main constitutional members of the linear actuator.
  • capacitance is the ability of a body, such as the tubular bodies of the movable and stationary members, to store electric charge.
  • the position determination of the linear actuator is provided according to the present by measuring the ability of structural members of the linear actuator to store electric energy and to determined the position of the linear actuator as a function of the ability to store electric energy.
  • the parameter C will be influenced by various properties, such as the material and electrical properties of the moveable and stationary members and other components, such as a center lead screw unit being rotated by the motor.
  • properties such as the material and electrical properties of the moveable and stationary members and other components, such as a center lead screw unit being rotated by the motor.
  • the parameter C gives the characteristics of the first and second conductive portions ability to store charge and their electric coupling to each other depending on the position of the linear actuator.
  • the first conductive portion may be formed of only a portion of the moveable member or the main part of the complete moveable member.
  • the second conductive portion may be formed of only a portion of the stationary member or the main part of the complete stationary member.
  • the first or second conductive portions may further comprise or be electrically connected to portions of the lead screw located radially inside a tube-formed moveable member.
  • the electro-mechanical (EM) linear actuator has a moveable member formed of a metallic moving member, also known as push tube or moving pillar part, and a stationary member formed of a metallic stationary member, also known as protection tube or stationary pillar part.
  • a metallic moving member also known as push tube or moving pillar part
  • a stationary member formed of a metallic stationary member, also known as protection tube or stationary pillar part.
  • the first conducting portion is electrically insulated from the second conducting portion.
  • the complete moveable member is electrically insulated from the stationary member.
  • the method further comprises separating the moveable member from the stationary member with one or more non-conducting guiding members.
  • the guide members may be arranged between the moveable member and stationary member.
  • the guide member is made of low friction material allowing low friction relative gliding movement between guide member and moveable member and/or between guide member and the stationary member.
  • a guide member may be fixed to the moveable member undergoing a linear movement, or be fixed to the stationary member.
  • the guide member comprises or is formed of a thermoplastic, or similar, material. The guide member provides a non- conductive position of a conductive moveable member in relation to a conductive stationary member.
  • the method further comprises providing a dielectric material between the first and second conductive portions.
  • the dielectric material comprises or is constituted of air or vacuum, or other materials, such as polymers etc., each having a known or approximate dielectric constant, also called the permittivity of a material.
  • a dielectric material can be described as an insulator, meaning that no current will flow through the material when voltage is applied, during normal working conditions. However, the dielectric material becomes polarized when voltage is applied. A good dielectric becomes polarized easy, wherein the dielectric constant represents the ability of the material of to concentrate electrostatic lines leading to storage of energy.
  • the moveable member comprises a force transmitting first tubular body and the stationary member comprises second tubular body, wherein the first conductive portion forms a part of the first tubular body and the second conductive portion forms part of the second tubular body.
  • the first and second conductive portions may both fully encircle the center axis of the linear actuator, which axis coincides with the linear direction.
  • the cross-section of the tubular bodies forming the moveable and stationary members may by circular, oval, triangular, rectangular, or have other suitable geometries, such l-shape, U- shaped, H-shaped, or alternative simple polygon shapes, such as pentagon, hexagon, heptagon, etc.
  • the tubular body of the moveable member is co-axial with the tubular body of the stationary member.
  • the moveable member is arranged travel at least partially into the stationary member.
  • the electro-mechanical linear actuator may further have a telescopic structure, wherein one or more moveable members are arranged inside the stationary member.
  • the method further comprises measuring the parameter C with a measuring unit.
  • the measuring unit is integrally formed in structure or housing of the linear actuator.
  • the measuring unit may also be arranged separated from the conventional housing of the linear actuator, or a combination thereof.
  • the measuring unit may be place in close proximity to the body of the linear actuator comprising the moveable and stationary member.
  • the step of measuring the parameter C further comprises probing the first conductive portion via a first electrical conductor, and probing the second conductive portion via a second electrical conductor.
  • the electrical conductor may be formed of a wire, flexing wire, or a stationary conductor connected to connection means for enabling signal transfer to the moveable member.
  • the connection means e.g. may be form of a contact member, such as brush, incorporated into a guiding member, such as a guiding member located in the front of the stationary member.
  • the electrical conductors may be connected directly to the first and second conductive portions, respectively.
  • the electrical conductors may also be connected to the moveable and stationary members, respectively, wherein the moveable and stationary members are electrically coupled to the first and second conductive portions, respectively.
  • Various probing techniques may be employed. For example, electric measuring of the capacitance using the measuring unit by signal transfer and signal processing using electric circuits forming a capacitance meter.
  • the operation for determining the parameter C indicative of the capacitance of the capacitance system may be based on or comprise various approaches. Non- limiting examples thereof are charging the first and second portions, discharging the first and second portions, feeding a current to the first and second portions, or by driving the capacitance system formed of the first and second conductive portions with a varying voltage.
  • the capacitance-measuring operation may comprise charging and discharging the capacitance system with a known current and measuring the rate of rise of the resulting voltage.
  • the capacitance-measuring operation may also or alternatively comprise passing a known alternating current with suitable frequency through the device under test, and measuring the resulting voltage across it.
  • the capacitance system may also be measured using a bridge circuit configuration, by arranging the capacitance system into a bridge circuit and varying the values of the legs in the bridge.
  • the capacitance measuring may also output parameters indicative of the other properties of the capacitance system formed of the first and second conductive portion, which properties may be used for determining the position of the linear actuator.
  • Other properties may for example be leakage and/or inductance of the capacitance system.
  • the method further comprises varying the capacitance of the first and second conductive portions by performing a linear movement of the moveable member in relation to the stationary member, which linear movement changes the overlap between the first and second conductive portions.
  • the method further comprises determining the relative position between the moveable member and the stationary member of the electro-mechanical linear actuator by using a position sensor, and comparing the relative position determined based on the measured parameter C and the relative position determined based on the position sensor.
  • a redundant system for determining the position of the linear actuator is advantageously provided, which allow for improved durability and operation.
  • a redundant system involves the duplication of the positioning determining system with the intention of increasing reliability of the position determining system, usually in the form of a backup or fail-safe.
  • the variation of capacitance varies substantially linear with the actual movement of the moveable member in the linear direction of the actuator.
  • the actual position of the moving member can be decided by measuring the capacitance.
  • the variation of the capacitance parameter C as a function of the movement of the moveable member may also be mapped and logged as a function and/or stored as data in a memory device, which function and/or memory device is operatively accessible by the measuring unit in order to determine the actual position.
  • the method further comprises calibrating the measuring of the parameter C, and/or the mapping of the parameter C into an actual position of the actuator.
  • the calibration may involve actuating the electromechanical linear actuator between a known calibration start position and a known calibration end position, and to calibrate the measuring unit based on the difference of parameter C between these two calibration positions.
  • the present invention relates to an electro-mechanical linear actuator comprising a stationary member having a tubular shape, a screw unit connected to an electric motor for rotating the screw unit, and a moveable member having a tubular shape, which moveable member is attached to a nut unit being screwed to the screw unit, wherein the moveable member and nut unit is arranged to move linearly in a linear direction by rotation of the screw unit.
  • the electro-mechanical linear actuator comprises a measuring unit electrically coupled to a first conductive portion of the moveable member via a first electrical conductor, and electrically coupled to a second conductive portion of the stationary member via a second electrical conductor, wherein measuring unit is arranged to measure a parameter C indicative of the capacitance of the first and second conductive portions, and to determine the relative position of the moveable member in relation to the stationary member is based on the parameter C.
  • the moveable member comprises a load-bearing tubular body, wherein the first conductive portion forms part of the load-bearing tubular body.
  • the first conductive portion forms an integral part of the moveable member.
  • the second conductive portion may be form an integral part of the stationary member.
  • the actuator further comprises non-conducting guiding members separating the moveable member from the stationary member, wherein the moveable member is electrically insulated from the stationary member.
  • the actuator may further, according to an embodiment, comprises a separating space 8 between the first conductive portion of the moveable member and the second conductive portion of the stationary member, which separating space comprises a dielectric material.
  • the actuator is further provided with a secondary position sensor for determining the relative position D of the moveable member in relation to the stationary member.
  • the relative position D may advantageously be determined by two separate and independent processes, allowing for improved and redundant positioning of the actuator.
  • the actuator further comprises a comparison unit arranged to compare the relative position D determined by the measuring unit based on the parameter C, and the relative position D determined by the secondary position sensor.
  • the secondary position sensor may for example be a potentiometer sensor or a rotary position sensor and a counter.
  • Fig. 1 is a schematic flow-chart of an embodiment of the method according to the present invention.
  • Fig. 2 is a schematic cross-sectional view of an exemplifying embodiment of the electro-mechanical linear actuator according to the present invention.
  • Fig. 3 is a schematic view of an exemplifying embodiment of a low- performance electro-mechanical linear actuator according to the present invention.
  • Fig. 4 is a schematic view of an exemplifying embodiment of a high- performance electro-mechanical linear actuator according to the present invention.
  • Fig. 5 is a schematic view of an exemplifying embodiment of a high- performance electro-mechanical linear actuator according to the present invention.
  • Fig. 6 is a schematic view of an exemplifying embodiment of a pillar type actuator arrangement according to the present invention.
  • FIG. 1 an exemplifying flow-chart of an embodiment of the method
  • the method 1 involves determining the position of an electro-mechanical linear actuator, such as the actuator 1 depicted in Fig. 2.
  • determining the position of the actuator is meant determining a value, signal, or other indication that is indicative of the relative position of a moveable member and a stationary member, i.e. the actuated position of the moveable member in relation to a reference point fixed to the stationary member.
  • the method 1 comprises a step 101 involving arranging overlap of the moveable and stationary members.
  • this may be achieved by arranging the moveable member in a relative position in relation to the stationary member, in which relative position a first conducting portion of the moveable member is overlapping a second conducting portion of the stationary member in the linear direction.
  • the overlap is represented by the overlapping area of the first conducting portion in relation to the second conducting portion.
  • the step 101 may involve actuating the moveable member in relation to the stationary member, but this is not required.
  • the 101 may also involve selecting a linear actuator for which the position is to be determined, i.e. without first actuating the moveable member in relation to the stationary member.
  • the linear actuator may be in its indented position, which position is to be measured.
  • the step of selecting a linear actuator may involve selecting a linear actuator in a system of linear actuators.
  • the step of selecting a linear actuator may also involve the activation of a position- measuring process of a particular linear actuator.
  • the method 1 further comprises a step 102 involving measuring a parameter C indicative of capacitance of the overlap, i.e. the capacitance of the capacitance system formed of the first and second conductive portions defining the overlap. Furthermore, the method 1 comprises a step 103 involving determining the position, i.e. the relative position between the moveable member and the stationary member, based on the measured parameter C. For example, the position is determined by comparing the parameter C with known values for different actuator positions. The position may also be determined by calculating a position value defined by e.g.
  • FIG. 2 a schematic cross-sectional view of an exemplifying embodiment of an electro-mechanical (EM) linear actuator 1 according to the present invention, is shown.
  • the electro-mechanical linear actuator 1 hereafter also referred to as only the actuator 1 , comprises a stationary member 3 having a tubular shape, a screw unit 4b connected to an electric motor 50 for rotating the screw unit 4b, and a moveable member 2 having a tubular shape, which moveable member is attached to a nut unit 4a being screwed to the lead screw unit 4b.
  • the lead screw unit 4b and nut unit 4a form a combined screw and nut unit 4 arranged to cooperate such that the moveable member 2 and nut unit 4a move linearly in a linear direction L by rotation of the screw unit 4b.
  • the movement is in the positive or negative linear direction L depending on the rotational direction of the screw unit 4b, wherein a movement in the positive linear direction L corresponds to an extension of the actuator 1 and a movement in the negative linear direction L corresponds to a retraction of the actuator 1 .
  • the actuator 1 further comprises a measuring unit 20 electrically coupled to a first conductive portion 2a of the moveable member 2 via a first electrical conductor 21 , and electrically coupled to a second conductive portion 3a of the stationary member 3 via a second electrical conductor 22.
  • the measuring unit 20 is arranged to measure a parameter C indicative of the capacitance of the capacitance system formed by the first and second conductive portions 2a and 3a, and to determine the relative position D of the moveable member 2 in relation to the stationary member 3 based on the parameter C.
  • the position of the actuator 1 is determined based on the overlap 10 between the moveable member 2 and the stationary member 3, which overlap 10 defines the active areas of the first and second conductive portions 2a and 3a forming the capacitance system for which the parameter C is measured.
  • the overlap 10 will increase or decrease thereby changing the capacitance of the system, allowing for a determination of the position of the actuator based on capacitance measurement.
  • the extension of the first conductive portion 2a of the moveable member 2 in the linear direction is indicated by 10a.
  • the extension of the second conductive portion 3a of the stationary member 3 in the linear direction is indicated by 10b.
  • the full length of the tubular body of the moveable and stationary members 2 and 3 may form part of the first and second conductive portions 2a and 3a, respectively.
  • the moving member 2 is arranged partially inside the stationary member 3 and protrudes in the linear direction L out from an axial end opening 3b of the tubular body of the stationary member 3.
  • the moveable member 2 is electrically insulated from the stationary member 3.
  • the moveable member 2 is separated from the stationary member 3 by non-conducting guiding members 5 located at the axial end opening 3b of the stationary member.
  • the moveable member 2 is further separated from the stationary member 3 by non-conducting guiding member 6 located closer to, or at, an axially inner end 2b of the moveable member 2.
  • the guiding members 5 and 6 electrically insulate the moveable member 2 from the stationary member 3, facilitate accurate linear movement of the movable member in relation to the stationary member during actuation, and provides for a separating space 8 between the first and second conductive portions 2a and 3a.
  • the space 8 forms a dielectric separation between the first and second conductive portions, thereby making part in the capacitance system together with the first and second conductive portions 2a and 3a.
  • a dielectric material may be arranged in the space 8.
  • the lead screw unit 4b is electrically connected to the motor 50 and to the stationary member 3, and the non-conducting guiding member 6 electrically insulates the moveable member 2 from the screw unit 4b and the nut unit 4a.
  • the nut unit 4a and the guide member 6 may also be integrally formed into one non-conducting guide and nut unit.
  • the lead screw 4b is further supported by guide member 7 arranged between an axially outer end, in the linear direction L, of the lead screw 4b and a radially inner contact surface of the moveable member 2.
  • Guide member 7 is formed of non-conducting material and provides for correct alignment of the lead screw 4b in relation to the moveable member 2, mitigate undesired movements of the lead screw 4b, and/or electrical insulation between lead screw 4b and moveable member.
  • Fig. 3-5 alternative embodiments of the linear actuator 1 are illustrated. These actuators 1 are arranged according to the actuator 1 described in relation to Fig. 2, unless stated or illustrated differently.
  • a drive axis and worm gear 13 being coupled to an electric motor (not shown), is arranged to rotate the lead screw 4b via gear 12 connected at an axially inner end of the lead screw 4b.
  • the lead screw is rotatably supported and axially locked in relation to the stationary member 3 via rolling element bearing 1 1 .
  • Fig. 3 illustrates a low performance EM linear actuator 1 .
  • the lead screw 4b is electrically connected to the stationary member 3 via bearing 1 1 .
  • the nut unit 4a and guide member 6 are integrally formed of a thermo plastic material and arranged to convert a rotational movement of the lead screw 4b into a linear movement of the moveable member 2, and arranged to electrically insulate and guide the moveable member 2 in relation to the stationary member 3.
  • variable capacitance formed by the structural design of the actuator in Fig. 3 may be described as a set of capacitances.
  • a first main variable capacitance is formed between the metallic first portion 2a of the moveable member 2 and the metallic second portion 3a of the stationary member 3.
  • a secondary variable capacitance is formed between the first portion 2a and the metallic lead screw 4b, which are also separated by nonconducting guide members 6 and 7.
  • the secondary capacitance is less than, or approximately equal to, a 1/5-th, 1 /10-th, of a 1 /100-th of the first main capacitance, due to larger separation space between the lead screw 4b and the first portion 2a, smaller circumference of the lead screw 4b in relation to the circumference of the moveable member 2, and due to obstruction by threads of the surface of the lead screw 4b.
  • the secondary capacitance may be treated as a disturbance or used for position measuring.
  • the secondary capacitance may be added to the first main capacitance and measured, neglected and treated as a measuring fault, or omitted and isolated from the capacitance circuit of the first main
  • the measuring unit 20 is
  • First and second electrical conductors 21 and 22 are schematically represented by 21 and 22, and are connected to the first and second conducting portions 2a and 3a.
  • a high performance EM linear actuator 1 is shown.
  • the nut unit 4a is arranged for higher load applications and/or increased performance, and may for example comprise a metal body and rolling elements 4c, such as metal ball members.
  • the nut unit 4a may be part of a ball screw or a roller screw arrangement in cooperation with the lead screw 4b, wherein low friction rolling elements engage the threads of the lead screw 4b.
  • the moveable member 2 is electrically separated from the nut unit 4a via guiding member 6, which also serves to guide the nut unit 4a and the moveable member 2 in relation to the tubular body of the stationary member 3, via slide-action between radially outer contacting-surfaces of guiding member and radially inner contacting surface of the stationary member.
  • the electric connection between the nut unit 4a and lead screw 4b may vary and/or have irregular characteristics due to lubrication film between contacting surfaces of nut unit 4a, lead screw unit 4b and the rolling elements 4c.
  • the measuring unit is separately arranged from the actuator 1 , and may for example be located in a central control unit for monitoring one or more actuators.
  • First electrical conductor 21 is connected to the first conductive portion 2a via a connections means 21 a, such as a brush.
  • the connection means 21 a may for example be arranged next to the guide member 5, or be incorporated into guide member 5 at the axial outer end opening of the stationary member 3.
  • the second electrical conductor 22 is electrically coupled directly to the second conductive portion 3a.
  • a high performance EM actuator 1 which differs from the actuator 1 described with reference to Fig. 4 in that the moveable member is not electrically insulated from the nut unit 4a and the lead screw unit 4b. Instead, the lead screw 4b is electrically insulated from the stationary member by a non-conducting bearing separator 14 arranged between the rolling bearing 1 1 and the stationary member 3. The lead screw 4b is further electrically insulated from the motor and stationary member 3, for example by a non-conducting gear 12 and/or non-conducting axis and/or worm gear 13. The actuator is further provided with a conductor channel 23 accommodating connection between the measuring unit 20 and the first and second conductive portions 2a and 3a.
  • the members creating the capacitance being measured for determination of the position of the actuator are electrically insulated from each other, such as the first and second conducting portions. This is often the case for conventional EM linear actuators.
  • electrical insulation between the members forming the capacitor may also require electrical insulation from conditions imposed on the actuator in its working milieu.
  • the internal condition in the actuator is often relatively controlled, and is typically occupied by air and/or grease having weak electrical conducting properties.
  • the exterior surface of the actuator may be more affected by interference from the working milieu. For example, in moist milieus, isolation between the moveable member and the stationary member may be improved by providing a protective coating or paint covering the actuator members. Also, the distance between the moveable member and the stationary member provided by the guiding members reduce the risk of electrical interference between the moveable and stationary members.
  • the electric connection of the measuring unit to the conductive portions may be achieved by various methods.
  • electrical wire may be connected to the moveable and stationary members and to the measuring unit.
  • the wire may be a flexing wire and may be arranged internally or externally of the actuator members.
  • the measuring unit may also be connected to the moveable member via a connection means, for example comprising a brush incorporated into the guiding members or into the stationary member such that the brush is contacting the moveable member.
  • the measuring unit may further comprise two separate measuring circuits using separate conductors, sensors and/or measuring techniques for determining the position of the actuator by two separate and independent systems, thereby allowing for increased redundancy.
  • an actuator being arranged as described with reference to Fig. 2 may have an outer diameter of 35mm of the moveable member, an inner diameter of 40mm of the stationary member, and space 8 filled with air having dielectric constant 1 .
  • Guiding members having e.g.
  • dielectric constant 3 may be omitted in the example.
  • a typical movement range of the moveable member in the linear direction L is 200mm, giving an overlap of the first and second conductive portions of the moveable and stationary members between 50 and 250 mm.
  • the capacitance which may be approximated by a parallel plate capacitor that is cut and unfolded, can be calculated by area times permittivity, i.e. a constant times the dielectric constant for the space divided by distance between the first and second conductive portions.
  • the example may e.g. gives a resulting capacitance of 20pF for 50mm overlap of the first and second conductive portions, and 100pF for 250mm overlap of the first and second conductive portions. This example implies a change in capacitance of 0.4pF per millimeter movement between the moveable and stationary members of the actuator, in the linear direction.
  • a schematic view of an exemplifying embodiment of a pillar type actuator arrangement 60 comprising an electro-mechanical linear actuator is shown.
  • the linear actuator 1 is internally arranged in the pillar type actuator construction, inside a base member 61 and linearly extendible shrouding formed of outer shroud 62, middle shroud 63, and inner shroud 64 comprising top surface 65.
  • the shrouds 62, 63 and 64 may for example be made of metal, such aluminum tubes, and provided with non-metallic upper guiding members 62a and 63a, and non-metallic lower guiding members 63b and 64b.
  • the shrouds 62, 63, and 64 are overlapping each other, which overlap will vary during linear actuation of the pillar type actuator 60.
  • the shrouding is used for determining the position by measuring a parameter C indicative of the capacitance, wherein the moveable and stationary members and associated first and second conductive portions forming the capacitance system are formed of any combination of the shrouds 62, 63 and 64.
  • the varying capacitance between the overlapping portions of the shrouds is used for determining the position of the pillar type actuator 60.
  • shroud 62 forms the stationary member
  • shroud 63 forms the moveable member.
  • the stationary member may also be represented by shroud 63, wherein the moveable member is represented by shroud 64.
  • the capacitance between the shroud members 62, 63 and/or 64 may be measured with measuring unit 20, for example by being connected to shroud 62 via conductor 25, and/or to shroud 63 via conductor 26, and/or to shroud 64 via conductor 27, as illustrated.
  • the parameter C is determined by measuring the capacitances between all the shrouds in the layered shroud arrangement by the measuring unit, and by combining the separate capacitances into a resulting capacitance.

Abstract

The present invention relates to a method for determining the relative position between a moveable member and a stationary member of a electromechanical linear actuator, which linear actuator is arranged to transform rotary motion from a motor into translation of the moveable member in a linear direction by a screw and nut unit, the method comprising measuring a parameter C indicative of the capacitance of a first conductive portion of the moveable member and a second conductive portion of the stationary member, and determining the relative position between the moveable member and the stationary member based on the measured parameter C. The present invention also relates an electro-mechanical linear actuator.

Description

CAPACITIVE POSITION MEASURING OF ELECTRO-MECHANICAL
LINEAR ACTUATOR
Field of the Invention
The present invention relates to position measuring of electromechanical linear actuators. More specifically, the present invention relates to a method for determining the position of an electro-mechanical linear actuator, the position being the relative position between a moveable member and a stationary member of the actuator, which electro-mechanical linear actuator converts rotary motion into linear motion by a screw and nut unit. The present invention also relates to an electro-mechanical linear actuator arrangement comprising a screw and nut unit.
Background Art
Electro-mechanical (EM) linear actuators are arranged to convert rotary motion from an electric motor into linear motion of a moveable member by using a screw and nut unit, and are utilized for providing linear motion in a wide range of applications, such as in machines, industrial machinery, valves, dampers, etc. In order to determine the position of the sliding moveable member in relation to the stationary member and housing of the actuator, it is known to provide the actuator with a position sensor, such as a potentiometer and a position control unit or a rotary position sensor and a counter. The function of a potentiometer may for example be based on electrical resistance, and be provided with a wiper contact linked to a mechanical shaft that can be either angular or linear, i.e. slider type, in its movement, and which causes the resistance value between the wiper/slider and the two end connections to change giving an electrical signal output that has a
proportional relationship between the actual wiper position on the resistive track and its resistance value. A position sensor unit may also log the number of revolutions of the motor or screw. However, installing a position sensor, such as a potentiometer or other sensors, in an EM linear actuator requires space. In turn, this leads to increased size and complexity of the actuator, as wells as increased manufacturing cost in terms of cost for parts and a more time consuming manufacturing process.
Summary of the Invention
In view of the above-mentioned and other drawbacks of the prior art, a general object of the present invention is to provide an improved method for determining the relative position between a moveable member and a stationary member of an electro-mechanical linear actuator, and to an improved electro-mechanical linear actuator.
Another object is to provide a more cost efficient and more durable determination of position of an electro-mechanical linear actuator.
These and other objects are met by the subject matters provided in the independent claims. Preferred embodiments of the invention are presented in the dependent claims.
According to a first aspect thereof, the present invention relates to a method for determining the relative position between a moveable member and a stationary member of an electro-mechanical linear actuator, which linear actuator is arranged to transform rotary motion from a motor into translation of the moveable member in a linear direction by a screw and nut unit. The method comprises arranging the moveable member in a relative position in relation to the stationary member, in which relative position a first conducting portion of the moveable member is overlapping a second conducting portion of the stationary member in the linear direction. The method further comprises measuring a parameter C indicative of the capacitance of the first and second conductive portions, and determining the relative position between the moveable member and the stationary member based on the measured parameter C. The invention is based on the realization by the inventors that by configuration a linear actuator as a position dependent capacitance system, the position of the moveable member in relation to the stationary member of the actuator may be determined by measuring the capacitance of the capacitance system formed by the first and second conductive portions. In other words, the overlapping first and second conductive portions are arranged to form a capacitor having a variable capacitance that is dependent on the position of the linear actuator in the linear direction.
In more detail, the capacitance of the capacitance system formed of the first and second conductive portions is dependent on the overlapping area of the first and second portions and the dielectric constant of the separation space between the first and second portions, wherein the extension of the overlapping area in the linear direction is determined by the linear movement and position of the moveable member in relation to the stationary member. Hence, the position of the linear actuator may be determined in an
advantageous manner by measuring a position dependent capacitance value being characteristic of the position. Furthermore, by allowing the first and second portions to constitute essential members forming part in the electromechanical actuator, no additional or extra equipment is required and the conventional structural members forming a conventional electromechanical actuator may advantageously be used. In other words, the first and second conductive portions form part of the basic actuator design, for example as part of the force bearing structure of the moveable and stationary members forming the main constitutional members of the linear actuator.
By providing capacitive position detection of the linear actuator, there is a reduced need for operative parts and members arranged in the linear actuator, which reduce the risk for failing components, such as position sensor members, etc., during operation and handing. Also, many
conventional electro-mechanical linear actuator designs may be used in combination with the method without or with few modifications.
Generally, capacitance is the ability of a body, such as the tubular bodies of the movable and stationary members, to store electric charge. In other terms, the position determination of the linear actuator is provided according to the present by measuring the ability of structural members of the linear actuator to store electric energy and to determined the position of the linear actuator as a function of the ability to store electric energy.
Depending on the architectural design and the structural members of the linear actuator, the parameter C will be influenced by various properties, such as the material and electrical properties of the moveable and stationary members and other components, such as a center lead screw unit being rotated by the motor. By being indicative of the capacitance of the
capacitance system formed of the first and second conductive portions having a variable overlap depending on the position of the linear actuator, the parameter C gives the characteristics of the first and second conductive portions ability to store charge and their electric coupling to each other depending on the position of the linear actuator.
For example, according to various exemplifying embodiment, the first conductive portion may be formed of only a portion of the moveable member or the main part of the complete moveable member. In analogy, the second conductive portion may be formed of only a portion of the stationary member or the main part of the complete stationary member. Also the first or second conductive portions may further comprise or be electrically connected to portions of the lead screw located radially inside a tube-formed moveable member.
For example, the electro-mechanical (EM) linear actuator has a moveable member formed of a metallic moving member, also known as push tube or moving pillar part, and a stationary member formed of a metallic stationary member, also known as protection tube or stationary pillar part. Various designs and applications for the electro-mechanical linear actuator are conceivable.
According to an exemplifying embodiment, the first conducting portion is electrically insulated from the second conducting portion.
According to an exemplifying embodiment, the complete moveable member is electrically insulated from the stationary member.
According to an exemplifying embodiment, the method further comprises separating the moveable member from the stationary member with one or more non-conducting guiding members. The guide members may be arranged between the moveable member and stationary member.
For example, the guide member is made of low friction material allowing low friction relative gliding movement between guide member and moveable member and/or between guide member and the stationary member. A guide member may be fixed to the moveable member undergoing a linear movement, or be fixed to the stationary member. According to an exemplifying embodiment the guide member comprises or is formed of a thermoplastic, or similar, material. The guide member provides a non- conductive position of a conductive moveable member in relation to a conductive stationary member.
According to an exemplifying embodiment, the method further comprises providing a dielectric material between the first and second conductive portions.
For example, the dielectric material comprises or is constituted of air or vacuum, or other materials, such as polymers etc., each having a known or approximate dielectric constant, also called the permittivity of a material. A dielectric material can be described as an insulator, meaning that no current will flow through the material when voltage is applied, during normal working conditions. However, the dielectric material becomes polarized when voltage is applied. A good dielectric becomes polarized easy, wherein the dielectric constant represents the ability of the material of to concentrate electrostatic lines leading to storage of energy.
According to an exemplifying embodiment of the method, the moveable member comprises a force transmitting first tubular body and the stationary member comprises second tubular body, wherein the first conductive portion forms a part of the first tubular body and the second conductive portion forms part of the second tubular body. For example, the first and second conductive portions may both fully encircle the center axis of the linear actuator, which axis coincides with the linear direction. The cross-section of the tubular bodies forming the moveable and stationary members may by circular, oval, triangular, rectangular, or have other suitable geometries, such l-shape, U- shaped, H-shaped, or alternative simple polygon shapes, such as pentagon, hexagon, heptagon, etc. Typically, the tubular body of the moveable member is co-axial with the tubular body of the stationary member. For example, the moveable member is arranged travel at least partially into the stationary member. The electro-mechanical linear actuator may further have a telescopic structure, wherein one or more moveable members are arranged inside the stationary member.
According to an exemplifying embodiment, the method further comprises measuring the parameter C with a measuring unit.
According to an exemplifying embodiment, the measuring unit is integrally formed in structure or housing of the linear actuator. The measuring unit may also be arranged separated from the conventional housing of the linear actuator, or a combination thereof. In order to avoid stray capacitance and/or disturbance the measuring unit may be place in close proximity to the body of the linear actuator comprising the moveable and stationary member.
According to an exemplifying embodiment of the method, the step of measuring the parameter C further comprises probing the first conductive portion via a first electrical conductor, and probing the second conductive portion via a second electrical conductor.
For example, the electrical conductor may be formed of a wire, flexing wire, or a stationary conductor connected to connection means for enabling signal transfer to the moveable member. The connection means e.g. may be form of a contact member, such as brush, incorporated into a guiding member, such as a guiding member located in the front of the stationary member. The electrical conductors may be connected directly to the first and second conductive portions, respectively. The electrical conductors may also be connected to the moveable and stationary members, respectively, wherein the moveable and stationary members are electrically coupled to the first and second conductive portions, respectively.
Various probing techniques may be employed. For example, electric measuring of the capacitance using the measuring unit by signal transfer and signal processing using electric circuits forming a capacitance meter. The operation for determining the parameter C indicative of the capacitance of the capacitance system may be based on or comprise various approaches. Non- limiting examples thereof are charging the first and second portions, discharging the first and second portions, feeding a current to the first and second portions, or by driving the capacitance system formed of the first and second conductive portions with a varying voltage. For example, the capacitance-measuring operation may comprise charging and discharging the capacitance system with a known current and measuring the rate of rise of the resulting voltage. The capacitance-measuring operation may also or alternatively comprise passing a known alternating current with suitable frequency through the device under test, and measuring the resulting voltage across it. The capacitance system may also be measured using a bridge circuit configuration, by arranging the capacitance system into a bridge circuit and varying the values of the legs in the bridge.
According to various exemplifying embodiments, the capacitance measuring may also output parameters indicative of the other properties of the capacitance system formed of the first and second conductive portion, which properties may be used for determining the position of the linear actuator. Other properties may for example be leakage and/or inductance of the capacitance system.
According to an exemplifying embodiment, the method further comprises varying the capacitance of the first and second conductive portions by performing a linear movement of the moveable member in relation to the stationary member, which linear movement changes the overlap between the first and second conductive portions.
According to an exemplifying embodiment, the method further comprises determining the relative position between the moveable member and the stationary member of the electro-mechanical linear actuator by using a position sensor, and comparing the relative position determined based on the measured parameter C and the relative position determined based on the position sensor. Hence, a redundant system for determining the position of the linear actuator is advantageously provided, which allow for improved durability and operation. For example, a redundant system involves the duplication of the positioning determining system with the intention of increasing reliability of the position determining system, usually in the form of a backup or fail-safe.
According to an exemplifying embodiment, the variation of capacitance varies substantially linear with the actual movement of the moveable member in the linear direction of the actuator. Hence the actual position of the moving member can be decided by measuring the capacitance. The variation of the capacitance parameter C as a function of the movement of the moveable member may also be mapped and logged as a function and/or stored as data in a memory device, which function and/or memory device is operatively accessible by the measuring unit in order to determine the actual position.
According to an exemplifying embodiment, the method further comprises calibrating the measuring of the parameter C, and/or the mapping of the parameter C into an actual position of the actuator.
For example, the calibration may involve actuating the electromechanical linear actuator between a known calibration start position and a known calibration end position, and to calibrate the measuring unit based on the difference of parameter C between these two calibration positions.
According to a further aspect thereof, the present invention relates to an electro-mechanical linear actuator comprising a stationary member having a tubular shape, a screw unit connected to an electric motor for rotating the screw unit, and a moveable member having a tubular shape, which moveable member is attached to a nut unit being screwed to the screw unit, wherein the moveable member and nut unit is arranged to move linearly in a linear direction by rotation of the screw unit. Furthermore, the electro-mechanical linear actuator comprises a measuring unit electrically coupled to a first conductive portion of the moveable member via a first electrical conductor, and electrically coupled to a second conductive portion of the stationary member via a second electrical conductor, wherein measuring unit is arranged to measure a parameter C indicative of the capacitance of the first and second conductive portions, and to determine the relative position of the moveable member in relation to the stationary member is based on the parameter C. Thereby an improved electro-mechanical linear actuator is provided, which is advantageous in similar manners as described in relation to the first aspect of the present invention. In particular, the EM linear actuator according to the present invention allows for a compact design and efficient
determination of position.
According to an exemplifying embodiment, the moveable member comprises a load-bearing tubular body, wherein the first conductive portion forms part of the load-bearing tubular body. In other words, the first conductive portion forms an integral part of the moveable member. In a similar manner, the second conductive portion may be form an integral part of the stationary member.
According to an exemplifying embodiment, the actuator further comprises non-conducting guiding members separating the moveable member from the stationary member, wherein the moveable member is electrically insulated from the stationary member.
The actuator may further, according to an embodiment, comprises a separating space 8 between the first conductive portion of the moveable member and the second conductive portion of the stationary member, which separating space comprises a dielectric material.
According to an exemplifying embodiment, the actuator is further provided with a secondary position sensor for determining the relative position D of the moveable member in relation to the stationary member. Hence, the relative position D may advantageously be determined by two separate and independent processes, allowing for improved and redundant positioning of the actuator.
According to an exemplifying embodiment, the actuator further comprises a comparison unit arranged to compare the relative position D determined by the measuring unit based on the parameter C, and the relative position D determined by the secondary position sensor. The secondary position sensor may for example be a potentiometer sensor or a rotary position sensor and a counter. Generally, other objectives, features, and advantages of the present invention will appear from the following detailed disclosure, from the attached dependent claims as well as from the drawings are equally possible within the scope of the invention.
Brief Description of Drawings
Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, wherein:
Fig. 1 is a schematic flow-chart of an embodiment of the method according to the present invention.
Fig. 2 is a schematic cross-sectional view of an exemplifying embodiment of the electro-mechanical linear actuator according to the present invention.
Fig. 3 is a schematic view of an exemplifying embodiment of a low- performance electro-mechanical linear actuator according to the present invention.
Fig. 4 is a schematic view of an exemplifying embodiment of a high- performance electro-mechanical linear actuator according to the present invention.
Fig. 5 is a schematic view of an exemplifying embodiment of a high- performance electro-mechanical linear actuator according to the present invention.
Fig. 6 is a schematic view of an exemplifying embodiment of a pillar type actuator arrangement according to the present invention.
It should be understood that the drawings are not true to scale and, as is readily appreciated by a person skilled in the art, dimensions other than those illustrated in the drawings are equally possible within the scope of the invention. Detailed Description of Embodiments of the Invention
In the drawings, similar, or equal elements are referred to by equal reference numerals.
In Fig. 1 , an exemplifying flow-chart of an embodiment of the method
100 according to the present invention is shown. The method 1 involves determining the position of an electro-mechanical linear actuator, such as the actuator 1 depicted in Fig. 2. By determining the position of the actuator is meant determining a value, signal, or other indication that is indicative of the relative position of a moveable member and a stationary member, i.e. the actuated position of the moveable member in relation to a reference point fixed to the stationary member.
As illustrated, the method 1 comprises a step 101 involving arranging overlap of the moveable and stationary members. In more detail, this may be achieved by arranging the moveable member in a relative position in relation to the stationary member, in which relative position a first conducting portion of the moveable member is overlapping a second conducting portion of the stationary member in the linear direction. The overlap is represented by the overlapping area of the first conducting portion in relation to the second conducting portion. The step 101 may involve actuating the moveable member in relation to the stationary member, but this is not required. The step
101 may also involve selecting a linear actuator for which the position is to be determined, i.e. without first actuating the moveable member in relation to the stationary member. The linear actuator may be in its indented position, which position is to be measured. The step of selecting a linear actuator may involve selecting a linear actuator in a system of linear actuators. The step of selecting a linear actuator may also involve the activation of a position- measuring process of a particular linear actuator.
The method 1 further comprises a step 102 involving measuring a parameter C indicative of capacitance of the overlap, i.e. the capacitance of the capacitance system formed of the first and second conductive portions defining the overlap. Furthermore, the method 1 comprises a step 103 involving determining the position, i.e. the relative position between the moveable member and the stationary member, based on the measured parameter C. For example, the position is determined by comparing the parameter C with known values for different actuator positions. The position may also be determined by calculating a position value defined by e.g.
geometrical and material properties of the capacitance system, such as area of overlap, capacitance properties of neighboring actuator elements giving rise to stray capacitances influencing the capacitance properties of the capacitance system, and dielectric properties of the gap material between the first and second conducting portions and other structural actuator elements neighboring the first and second conductive portions.
In Fig. 2 a schematic cross-sectional view of an exemplifying embodiment of an electro-mechanical (EM) linear actuator 1 according to the present invention, is shown. The electro-mechanical linear actuator 1 , hereafter also referred to as only the actuator 1 , comprises a stationary member 3 having a tubular shape, a screw unit 4b connected to an electric motor 50 for rotating the screw unit 4b, and a moveable member 2 having a tubular shape, which moveable member is attached to a nut unit 4a being screwed to the lead screw unit 4b. The lead screw unit 4b and nut unit 4a form a combined screw and nut unit 4 arranged to cooperate such that the moveable member 2 and nut unit 4a move linearly in a linear direction L by rotation of the screw unit 4b. The movement is in the positive or negative linear direction L depending on the rotational direction of the screw unit 4b, wherein a movement in the positive linear direction L corresponds to an extension of the actuator 1 and a movement in the negative linear direction L corresponds to a retraction of the actuator 1 .
The actuator 1 further comprises a measuring unit 20 electrically coupled to a first conductive portion 2a of the moveable member 2 via a first electrical conductor 21 , and electrically coupled to a second conductive portion 3a of the stationary member 3 via a second electrical conductor 22. The measuring unit 20 is arranged to measure a parameter C indicative of the capacitance of the capacitance system formed by the first and second conductive portions 2a and 3a, and to determine the relative position D of the moveable member 2 in relation to the stationary member 3 based on the parameter C. In other words, the position of the actuator 1 is determined based on the overlap 10 between the moveable member 2 and the stationary member 3, which overlap 10 defines the active areas of the first and second conductive portions 2a and 3a forming the capacitance system for which the parameter C is measured. By actuating actuator 1 , the overlap 10 will increase or decrease thereby changing the capacitance of the system, allowing for a determination of the position of the actuator based on capacitance measurement. The extension of the first conductive portion 2a of the moveable member 2 in the linear direction is indicated by 10a. The extension of the second conductive portion 3a of the stationary member 3 in the linear direction is indicated by 10b. However, the full length of the tubular body of the moveable and stationary members 2 and 3 may form part of the first and second conductive portions 2a and 3a, respectively.
As further illustrated, the moving member 2 is arranged partially inside the stationary member 3 and protrudes in the linear direction L out from an axial end opening 3b of the tubular body of the stationary member 3. In order to form a suitable capacitor of the first portion 2a of the movable member 2 and the second portion 3a of the stationary member, the moveable member 2 is electrically insulated from the stationary member 3. The moveable member 2 is separated from the stationary member 3 by non-conducting guiding members 5 located at the axial end opening 3b of the stationary member. The moveable member 2 is further separated from the stationary member 3 by non-conducting guiding member 6 located closer to, or at, an axially inner end 2b of the moveable member 2. The guiding members 5 and 6 electrically insulate the moveable member 2 from the stationary member 3, facilitate accurate linear movement of the movable member in relation to the stationary member during actuation, and provides for a separating space 8 between the first and second conductive portions 2a and 3a. The space 8 forms a dielectric separation between the first and second conductive portions, thereby making part in the capacitance system together with the first and second conductive portions 2a and 3a. A dielectric material may be arranged in the space 8.
With reference to Fig.2, the lead screw unit 4b is electrically connected to the motor 50 and to the stationary member 3, and the non-conducting guiding member 6 electrically insulates the moveable member 2 from the screw unit 4b and the nut unit 4a. The nut unit 4a and the guide member 6 may also be integrally formed into one non-conducting guide and nut unit.
The lead screw 4b is further supported by guide member 7 arranged between an axially outer end, in the linear direction L, of the lead screw 4b and a radially inner contact surface of the moveable member 2. Guide member 7 is formed of non-conducting material and provides for correct alignment of the lead screw 4b in relation to the moveable member 2, mitigate undesired movements of the lead screw 4b, and/or electrical insulation between lead screw 4b and moveable member.
In Fig. 3-5, alternative embodiments of the linear actuator 1 are illustrated. These actuators 1 are arranged according to the actuator 1 described in relation to Fig. 2, unless stated or illustrated differently. A drive axis and worm gear 13, being coupled to an electric motor (not shown), is arranged to rotate the lead screw 4b via gear 12 connected at an axially inner end of the lead screw 4b. The lead screw is rotatably supported and axially locked in relation to the stationary member 3 via rolling element bearing 1 1 .
In more detail, Fig. 3 illustrates a low performance EM linear actuator 1 . The lead screw 4b is electrically connected to the stationary member 3 via bearing 1 1 . The nut unit 4a and guide member 6 are integrally formed of a thermo plastic material and arranged to convert a rotational movement of the lead screw 4b into a linear movement of the moveable member 2, and arranged to electrically insulate and guide the moveable member 2 in relation to the stationary member 3.
The variable capacitance formed by the structural design of the actuator in Fig. 3 may be described as a set of capacitances. A first main variable capacitance is formed between the metallic first portion 2a of the moveable member 2 and the metallic second portion 3a of the stationary member 3. A secondary variable capacitance is formed between the first portion 2a and the metallic lead screw 4b, which are also separated by nonconducting guide members 6 and 7. For example, depending on the structural design and material properties, the secondary capacitance is less than, or approximately equal to, a 1/5-th, 1 /10-th, of a 1 /100-th of the first main capacitance, due to larger separation space between the lead screw 4b and the first portion 2a, smaller circumference of the lead screw 4b in relation to the circumference of the moveable member 2, and due to obstruction by threads of the surface of the lead screw 4b. The secondary capacitance may be treated as a disturbance or used for position measuring. According to various embodiments, the secondary capacitance may be added to the first main capacitance and measured, neglected and treated as a measuring fault, or omitted and isolated from the capacitance circuit of the first main
capacitance in suitable manner. As shown, the measuring unit 20 is
connected to, and forms part of, the actuator body. First and second electrical conductors 21 and 22 are schematically represented by 21 and 22, and are connected to the first and second conducting portions 2a and 3a.
In Fig. 4, a high performance EM linear actuator 1 is shown. The nut unit 4a is arranged for higher load applications and/or increased performance, and may for example comprise a metal body and rolling elements 4c, such as metal ball members. In other words, the nut unit 4a may be part of a ball screw or a roller screw arrangement in cooperation with the lead screw 4b, wherein low friction rolling elements engage the threads of the lead screw 4b. Due to metallic connection between the nut unit 4a and lead screw 4b, via the rolling elements 4c, the moveable member 2 is electrically separated from the nut unit 4a via guiding member 6, which also serves to guide the nut unit 4a and the moveable member 2 in relation to the tubular body of the stationary member 3, via slide-action between radially outer contacting-surfaces of guiding member and radially inner contacting surface of the stationary member. The electric connection between the nut unit 4a and lead screw 4b may vary and/or have irregular characteristics due to lubrication film between contacting surfaces of nut unit 4a, lead screw unit 4b and the rolling elements 4c. As further shown, the measuring unit is separately arranged from the actuator 1 , and may for example be located in a central control unit for monitoring one or more actuators. First electrical conductor 21 is connected to the first conductive portion 2a via a connections means 21 a, such as a brush. The connection means 21 a may for example be arranged next to the guide member 5, or be incorporated into guide member 5 at the axial outer end opening of the stationary member 3. The second electrical conductor 22 is electrically coupled directly to the second conductive portion 3a.
In Fig. 5, an alternative embodiment of a high performance EM actuator 1 is shown, which differs from the actuator 1 described with reference to Fig. 4 in that the moveable member is not electrically insulated from the nut unit 4a and the lead screw unit 4b. Instead, the lead screw 4b is electrically insulated from the stationary member by a non-conducting bearing separator 14 arranged between the rolling bearing 1 1 and the stationary member 3. The lead screw 4b is further electrically insulated from the motor and stationary member 3, for example by a non-conducting gear 12 and/or non-conducting axis and/or worm gear 13. The actuator is further provided with a conductor channel 23 accommodating connection between the measuring unit 20 and the first and second conductive portions 2a and 3a. In general, the members creating the capacitance being measured for determination of the position of the actuator are electrically insulated from each other, such as the first and second conducting portions. This is often the case for conventional EM linear actuators. However, electrical insulation between the members forming the capacitor may also require electrical insulation from conditions imposed on the actuator in its working milieu. The internal condition in the actuator is often relatively controlled, and is typically occupied by air and/or grease having weak electrical conducting properties. The exterior surface of the actuator may be more affected by interference from the working milieu. For example, in moist milieus, isolation between the moveable member and the stationary member may be improved by providing a protective coating or paint covering the actuator members. Also, the distance between the moveable member and the stationary member provided by the guiding members reduce the risk of electrical interference between the moveable and stationary members.
In order to achieve a suitable measurement of the parameter C indicative of the capacitance of the capacitance system formed by the first and second conductive portions, the electric connection of the measuring unit to the conductive portions may be achieved by various methods. For example, electrical wire may be connected to the moveable and stationary members and to the measuring unit. The wire may be a flexing wire and may be arranged internally or externally of the actuator members. The measuring unit may also be connected to the moveable member via a connection means, for example comprising a brush incorporated into the guiding members or into the stationary member such that the brush is contacting the moveable member. The measuring unit may further comprise two separate measuring circuits using separate conductors, sensors and/or measuring techniques for determining the position of the actuator by two separate and independent systems, thereby allowing for increased redundancy.
According to an example, an actuator being arranged as described with reference to Fig. 2 may have an outer diameter of 35mm of the moveable member, an inner diameter of 40mm of the stationary member, and space 8 filled with air having dielectric constant 1 . Guiding members, having e.g.
dielectric constant 3, may be omitted in the example. Furthermore, a typical movement range of the moveable member in the linear direction L is 200mm, giving an overlap of the first and second conductive portions of the moveable and stationary members between 50 and 250 mm. The capacitance, which may be approximated by a parallel plate capacitor that is cut and unfolded, can be calculated by area times permittivity, i.e. a constant times the dielectric constant for the space divided by distance between the first and second conductive portions. Based on the above assumptions, the example may e.g. gives a resulting capacitance of 20pF for 50mm overlap of the first and second conductive portions, and 100pF for 250mm overlap of the first and second conductive portions. This example implies a change in capacitance of 0.4pF per millimeter movement between the moveable and stationary members of the actuator, in the linear direction.
In Fig. 6, a schematic view of an exemplifying embodiment of a pillar type actuator arrangement 60 comprising an electro-mechanical linear actuator, is shown. The linear actuator 1 is internally arranged in the pillar type actuator construction, inside a base member 61 and linearly extendible shrouding formed of outer shroud 62, middle shroud 63, and inner shroud 64 comprising top surface 65. The shrouds 62, 63 and 64 may for example be made of metal, such aluminum tubes, and provided with non-metallic upper guiding members 62a and 63a, and non-metallic lower guiding members 63b and 64b. As illustrated, the shrouds 62, 63, and 64 are overlapping each other, which overlap will vary during linear actuation of the pillar type actuator 60. According to an embodiment, the shrouding is used for determining the position by measuring a parameter C indicative of the capacitance, wherein the moveable and stationary members and associated first and second conductive portions forming the capacitance system are formed of any combination of the shrouds 62, 63 and 64. In other words, the varying capacitance between the overlapping portions of the shrouds is used for determining the position of the pillar type actuator 60. For example, shroud 62 forms the stationary member and shroud 63 forms the moveable member. The stationary member may also be represented by shroud 63, wherein the moveable member is represented by shroud 64.
The capacitance between the shroud members 62, 63 and/or 64 may be measured with measuring unit 20, for example by being connected to shroud 62 via conductor 25, and/or to shroud 63 via conductor 26, and/or to shroud 64 via conductor 27, as illustrated. According to an embodiment, the parameter C is determined by measuring the capacitances between all the shrouds in the layered shroud arrangement by the measuring unit, and by combining the separate capacitances into a resulting capacitance.
It should be noted that the invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.
Also, features and advantages described in relation to a certain embodiment or method step may be attributed to other described
embodiments or method steps in a corresponding manner.
In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single member or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain features or method steps are recited in mutually different dependent claims does not indicate that a combination of these features or steps cannot be used to advantage.

Claims

1 . Method (100) for determining the relative position between a moveable member (2) and a stationary member (3) of an electro-mechanical linear actuator (1 ), which linear actuator is arranged to transform rotary motion from a motor (4) into translation of the moveable member in a linear direction (L) by a screw and nut unit (4), the method comprising:
- arranging (101 ) the moveable member in a relative position (D) in relation to the stationary member, in which relative position a first conducting portion (2a) of the moveable member is overlapping a second conducting portion (3a) of the stationary member in the linear direction,
characterized by
- measuring (102) a parameter C indicative of the capacitance of the first and second conductive portions (2a, 3a), and
- determining (103) the relative position (D) between the moveable member and the stationary member based on the measured parameter C.
2. The method (100) according to any one of the preceding claims, wherein the first conducting portion (2a) is electrically insulated from the second conducting portion (3a).
3. The method (100) according to any one of the preceding claims, further comprising,
- separating the moveable member (2) from the stationary member (3) with one or more non-conducting guiding members (5, 6).
4. The method (100) according to any one of the preceding claims, further comprising
- providing a dielectric material in the space (8) between the first and second conductive portions (2a, 3a).
5. The method (100) according to any one of the preceding claims, wherein the moveable member (2) comprises a force transmitting first tubular body and the stationary member (3) comprises second tubular body, wherein the first conductive portion (2a) forms a part of the first tubular body and the second conductive portion (3a) forms part of the second tubular body.
6. The method (100) according to any one of the preceding claims, further comprising:
- measuring the parameter C with a measuring unit (20).
7. The method (100) according to any one of the preceding claims, wherein the step of measuring the parameter C comprises:
- probing the first conductive portion (2a) via a first electrical conductor (21 ), and
- probing the second conductive portion (3a) via a second electrical conductor (22).
8. The method (100) according to any one of the preceding claims, further comprising
- varying the capacitance of the first and second conductive portions
(2a, 3a) by performing a linear movement of the moveable member in relation to the stationary member in the linear direction (L), which linear movement changes the overlap (10) between the first and second conductive portions.
9. The method (100) according to any one of the preceding claims, further comprising
- determining the relative position between the moveable member and the stationary member of the electro-mechanical linear actuator by using a position sensor,
- comparing the relative position determined based on the measured parameter C and the relative position determined based on the position sensor.
10. An electro-mechanical linear actuator (1 ) comprising:
a stationary member (3) having a tubular shape,
a screw unit (4b) connected to an electric motor (50) for rotating the screw unit, and
a moveable member (2) having a tubular shape, which moveable member is attached to a nut unit (4a) being screwed to the screw unit, wherein the moveable member and nut unit is arranged to move linearly in a linear direction (L) by rotation of the screw unit,
characterized in that the electro-mechanical linear actuator (1 ) comprises a measuring unit (20) electrically coupled to a first conductive portion (2a) of the moveable member via a first electrical conductor (21 ), and electrically coupled to a second conductive portion (3a) of the stationary member via a second electrical conductor (22),
wherein the measuring unit (20) is arranged to measure a parameter C indicative of the capacitance of the first and second conductive portions (2a, 3a), and to determine the relative position (D) of the moveable member in relation to the stationary member is based on the parameter C.
1 1 . The electro-mechanical linear actuator (1 ) according to claim 10, wherein the moveable member (2) comprises a load-bearing tubular body, wherein the first conductive portion (2a) forms part of the load-bearing tubular body.
12. The electro-mechanical linear actuator (1 ) according to any one of claims 10 to 1 1 , further comprising one or more non-conducting guiding members separating the moveable member (2) from the stationary member (3), wherein the moveable member (2) is electrically insulated from the stationary member (3) by the guiding members.
13. The electro-mechanical linear actuator (1 ) according to any one of claims 10 to 12, further comprising a separating space (8) between the first conductive portion (2a) of the moveable member (2) and the second conductive portion (3a) of the stationary member (3), which separating space (8) comprises an dielectric material.
14. The electro-mechanical linear actuator (1 ) according to any one of claims 10-13, further comprising a secondary position sensor for determining the relative position (D) of the moveable member (2) in relation to the stationary member (3).
15. The electro-mechanical linear actuator (1 ) according to claim 14, further comprising a comparison unit arranged to compare the relative position (D) determined by the measuring unit based on the parameter C, and the relative position (D) determined by the secondary position sensor.
PCT/SE2014/051512 2014-01-07 2014-12-16 Capacitive position measuring of electro-mechanical linear actuator WO2015105448A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
SE1400003-8 2014-01-07
SE1400003 2014-01-07

Publications (1)

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20070008230A1 (en) * 2005-07-06 2007-01-11 Tadashi Osaka Linear actuator
WO2009039853A2 (en) * 2007-09-27 2009-04-02 Linak A/S Linear actuator
US20090091287A1 (en) * 2006-05-13 2009-04-09 Linak A/S Linear Actuator

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20070008230A1 (en) * 2005-07-06 2007-01-11 Tadashi Osaka Linear actuator
US20090091287A1 (en) * 2006-05-13 2009-04-09 Linak A/S Linear Actuator
WO2009039853A2 (en) * 2007-09-27 2009-04-02 Linak A/S Linear actuator

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