[0001] The present invention relates to position sensors, and more specifically to resistive position sensors with discrete resistors that may be utilized in different position

measurement applications, such as linear and rotational position measurements. In one embodiment, the invention relates to a hydraulic actuator incorporating a position sensor for measuring linear displacement of a piston in the actuator.

BACKGROUND

[0002] In the industry there is an increasing demand for sensors and control devices. Especially emerging robot technology requires a large number of sensing devices in order to perform complex tasks.

[0003] One example of a sector where robot technology is advancing, is within well intervention. Well intervention is a broad topic covering operations in oil or gas wells that are still operational, i.e. able to deliver gas or oil. Intervention operations will typically be carried out to improve the quality or extend the production life of the wellbore.

[0004] Intervention operations can be performed both in open-hole and cased wellbores, where examples of such operations can be to replace equipment in the well such as pumps, valves etc., adjusting or repairing such equipment, fishing operations, monitoring well parameters, well reconfiguration etc.

[0005] When performing well interventions operations, different types of intervention tools are used for different intervention types, and modular tools have been proposed to allow reconfiguration of the tools for different operations. Considering that intervention tools may be operating thousands of meters in vertical and horizontal wells below ground or sea level, it will be understood that automated operation and introduction of

autonomous or semi-autonomous operation becomes a requirement.

[0006] In an intervention operation, two major tool movements are commonly used; longitudinal movement, i.e. in the direction of the wellbore, and rotational movement. The movements may be distinct in time or performed simultaneously.

[0007] One way of performing the longitudinal movement is to use a linear actuator. A linear actuator has been described in international patent publication W02016010436. The application further discloses a petroleum well downhole mechanical services platform tool on a conveyor line (10) is disclosed where the conveyor line (10) comprises a power and signal line (12, 13, 11) for communicating with a surface system (160), and the power and signal line (11, 12, 13) is connected via a head assembly (20) to a communications module (30) and further connected to a master electronics module (70) for controlling a motor drive electronics module (40) connected to the power line (12) for driving a hydraulic power unit (50, 51, 52, 53, 54), the hydraulic power unit (50) providing hydraulic power to one or more of an anchor unit (60) for anchoring to a wall in the petroleum well and a linear actuator module (80) arranged for linearly actuating along the axis of the petroleum well a conveyed device (90, 901, 902, 100) such as a tool (90, 901, 902) and / or an instrument (100).

[0008] The operation of the linear actuator module requires detailed feedback from a position sensor to be able to perform complex intervention operations. In this case a linear position sensor is needed that can withstand the harsh environment in a wellbore, and be easily integrated with a linear actuator.

[0009] International patent publication W00077472 A1 describes an optical position detector for sensing the position of a movable member which moves along an axis relative to a stationary member. A nonrepeating N bit chain code embodied in a scale on the movable member runs along the axis. A detector fixed to the stationary member is positioned to sense a portion of the chain code. The detector has K elements (K>>N) generating a plurality of signals. A controller determines the position of the movable member relative to the stationary member as a function of the signals.

[0010] Some of the sensors above may sense direction of movement. However, this requires usually two sensors and quadrature encoding.

[0011] Potentiometers with variable resistance have been employed in various

configurations. However, it is difficult to obtain the required accuracy of standard potentiometers, especially in harsh environments.

[0012] French patent application FR2807159 A1 describes an actuator comprising a body and an actuator rod (14) that slides relative to the body. Movement is via a motor and screw mechanism or similar. The actuator includes a potentiometer whose resistance varies with the position of the rod relative to the body. The potentiometer comprises a resistive path (30) on the surface of the rod (14). A cursor (32) integral with the body slides up and down the rod making an electrical connection, thus changing the resistance.

SUMMARY OF THE INVENTION

[0013] A goal with the present invention is to disclose a novel position sensor and method for accurately measuring a position that can be used for relative and absolute position measurements at a wide range of temperatures. It is also a goal that the position sensor has inherent support for the direction of travel.

[0014] The invention is a resistive position sensor and a method for measuring a position according to the independent claims.

[0015] One effect of the invention is that a relatively long measurement range can be obtained.

[0016] The travel direction of the position sensor may be determined with only one signal channel. In many prior art incremental systems, quadrature encodings with two signal channels are needed.

[0017] The disclosed position sensor and method also offers the possibility to calibrate the position sensor while in operation. This may be of specific importance where operating or environmental conditions are changing.

[0018] In one embodiment, a double-acting linear actuator comprises:

- a control and processing unit (CPU) and a hydraulic power unit (HPU) connected to an actuator housing; the CPU and HPU configured and interconnected to at least operate the piston and register data provided by the resistive position sensor;

- a first chamber and a second chamber inside said housing, fluidly connected to a motor- and-pump module in the HPU via respective conduits; and wherein the volumes of said chambers are variably defined by the movement of a piston head; and wherein;

- the piston head is movable between a first end stop in the first chamber and a second end stop in the second chamber.

[0019] In one embodiment, the motor-and-pump module comprises a motor which is drivingly connected to a hydraulics pump via a shaft; and an RPM sensor is arranged to measure the drive shaft RPM; and the pump is a fixed displacement type pump.

[0020] It is also provided a method of determining end positions for a double-acting linear hydraulic actuator; characterized by

- moving the piston head towards a first stop or towards a second stop until a hydraulic functionality-related variable is detected;

- recording first (upper) or second (lower) end position, respectively, for the piston head based on information provided by the resistive position sensor at the time of recording the hydraulic functionality-related variable.

[0021] In one embodiment, said first and second end points and their corresponding resistive values are set as reference points for further measurements. The hydraulic functionality-related variable comprises a pressure spike in the chamber or a sudden increase in power consumption in a hydraulic power unit connected to the actuator, correlated with the pressure spike.

[0022] In one embodiment, the method comprises:

- recording the RPM of an output shaft from a motor in said HPU;

- calculating the volumetric displacement of a fixed displacement hydraulic pump drivingly connected to said the motor by said shaft, based at least on the recorded RPM and the pump's volumetric output per shaft revolution.

In one embodiment, the method further comprises:

- comparing the volumetric displacement of said pump with the anticipated volumetric displacement caused by a translation of said piston head, and

- generating an alert signal if the calculated volumetric displacement of said pump differs from the anticipated volumetric displacement caused by said piston head translation by a predetermined value.

[0023] The invented method may be used to automatically calibrate a position sensor in a double-acting linear hydraulic actuator and/or detect obstructions in a wellbore or other tubular element.

[0024] Although wellbore applications have been mentioned here, the resistive position sensor has a wide range of applications, in the same way as e.g. magnetic, optical, and inductive position sensors are used. The position sensor according to the invention may also be used as a control device to control e.g. an electric motor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure la illustrates the principle of a linear position sensor according to an embodiment of the invention.

[0026] Figure lb illustrates the same linear position sensor as in figure la, but where a subset of the resistors are signature resistors with signature resistive values different from the other resistors in the chain. More specifically, in the figure, resistance R5 has a signature resistive value, different from the other resistors.

[0027] A physical implementation where this principle is used, is further illustrated in figures 2a, 2b and 2c. A serial circuit comprising discrete resistors (Rl, R2.... RN) and contact points (Cl, C2,....CN) can be seen laid out on a printed circuit board (PCB) arranged inside a conducting tube (21). Further, a processing system (30) connected to the serial circuit and the conducting tube is shown. The processing system (30) is configured to calculate the position of the contact member (10) relative the printed circuit board (PCB) based on resistance measurements.

[0028] Figures 2a, 2b and 2c illustrate different views of an embodiment of a linear position sensor according to an embodiment of the invention.

[0029] Figure 2a illustrates to the left a first part of the position sensor and to the right a second part of the position sensor. The second part comprising the contact member (10) and the contact member shaft (11) is configured to move longitudinally with respect to the first part as illustrated by the horizontal arrow.

[0030] Figure 2b illustrates in a cross sectional view the same position sensor as in figure 2a. The first part comprises a tube (21) inside of which the contact member (10) and the contact member shaft (11) can move. Further, inside the tube (21) there is a printed circuit board (PCB) as illustrated in more detail in figure 2c. The circuit board comprises the serial circuit illustrated in figures la and lb comprising the discrete resistors (not shown) and the contact points (Cl, C2,....CN).

[0031] Figure 3 illustrates the principle of a rotational position sensor according to an embodiment of the invention.

[0032] Figure 4a is a side view of an embodiment of a linear double-acting actuator according to the invention. Figures 4b and 4c are views of the actuator illustrated in figure 4a, seen along the center longitudinal axis A-A, of the first (bottom) end and the second (top) end, respectively.

[0033] Figure 5a is a sectional side view along the axis A-A of figure 4a. Figure 5b is a sectional side view along the section B-B of figure 4b. Figure 5c is a sectional side view along the section C-C of figure 4b.

[0034] Figure 6a is a perspective view and partly cut-away drawing of the actuator illustrated in figures 4a-c. Figure 6b is an enlarged view of the area marked as "K" in figure 6a. Figure 6c is an enlarged view of the area marked as "L" in figure 6a.

[0035] Figure 7a is a side view and partly cut-away drawing of the actuator illustrated in figures 4a-c. Figure 7b is an enlarged view of the area marked as "M" in figure 7a. Figure 7c is an enlarged view of the area marked as "N" in figure 7a.

[0036] Figure 8 is an enlarged view of the area marked as "O" in figure 5a. [0037] Figure 9 is a schematic drawing of an embodiment of the invented actuator, connected to a control and processing unit (CPU) and a hydraulic power unit (HPU) and installed in a wellbore.

[0038] Figure 10 is an enlarged view of the area marked as "U" in figure 9. EMBODIMENTS OF THE INVENTION

[0039] The following description may use terms such as "horizontal", "vertical", "lateral", "back and forth", "up and down", "upper", "lower", "inner", "outer", "forward", "rear", etc. These terms generally refer to the views and orientations as shown in the drawings and that are associated with a normal use of the invention. The terms are used for the reader's convenience only and shall not be limiting. In the following description, various examples and embodiments of the invention are set forth in order to provide the skilled person with a more thorough understanding of the invention. The specific details described in the context of the various embodiments and with reference to the attached drawings are not intended to be construed as limitations. Rather, the scope of the invention is defined in the appended claims.

[0040] The embodiments described below are numbered. In addition, dependent embodiments defined in relation to the numbered embodiments are described. Unless otherwise specified, any embodiment that can be combined with one or more numbered embodiments may also be combined directly with any of the related or dependent embodiments of the numbered embodiment(s) referred to.

[0041] In a first embodiment the invention is a resistive position sensor (1) comprising a first and a second part movable relative each other as illustrated in figure la.

[0042] The first part comprises a sequence of serially interconnected discrete resistors (Rl, R2.... RN) that are interconnected in individually spaced apart contact points (Cl, C2,....CN), and the second part comprises a conducting contact member (10) configured for sliding over and contacting the contact points (Cl, C2,....CN) as the first and a second part moves relative each other, wherein a displacement of the second part relative to the first part indicates a position (PI, P2,...PN) to be sensed by the resistive position sensor (1, 100). The movement of the contact member is indicated by the double headed arrow in figure la, indicating movement in either direction.

[0043] In a first dependent embodiment, the contact member (10) is configured for sliding over and contacting at least one of the contact points (Cl, C2,....CN) at any given moment, when the first and a second part moves relative each other. [0044] In a second dependent embodiment, the contact member (10) is configured for sliding over and contacting the contact points (Cl, C2,....CN) separately, one at the time, when the first and a second part moves relative each other.

[0045] The position indicated is a discrete position indication related to the contact point (Cl, C2,....CN) that the contact member (10) contacts. This is illustrated in figure la, where the contact member (10) is shown in two different physical positions, both representing the same discrete position (P2). The resolution (Res) of the position sensor then becomes the distance from a specific location of one contact point to a corresponding location of the next contact point, e.g. from start to start or end to end of the contact points. Thus, resolution (Res) can be improved by reducing the length and increasing the number of the contact points for a given position indicator.

[0046] As described above, the contact points are spaced apart. In an embodiment, the distance between the contact points is small compared to their length. The relationship between the spaces and contact member surface may be designed to allow the contact member to contact the next contact point before leaving the previous, i.e. the contact member is longer than the space between the contact points.

[0047] The contact member may be e.g. a wiper or a brush.

[0048] In a first related embodiment the discrete resistors (Rl, R2.... RN) and the contact points (Cl, C2.... CN) are alternately arranged as illustrated in figure la. The contact points may here be seen as extensions as of the interconnections between the discrete resistors, and there is one contact point for every interconnection. This arrangement relates to the electric circuit configuration. The physical location may be different, e.g. the resistors may be placed directly above, below or at the same level as a contact point, depending on the arrangement of the contact member.

[0049] In a second related embodiment that may be combined with the first related embodiment above, the discrete resistors (Rl, R2.... RN) and the contact points (Cl, C2.... CN) are arranged in a serial circuit with a first and a second end (la, lb) as illustrated in figure la. The term serial is in this case related to the electric properties of the circuit and not the physical layout, which does not limit the physical configuration to be longitudinal. Other physical layouts where the circuit is serial may also be possible, as will be illustrated later.

[0050] In a third related embodiment that may also be combined with the first or second related embodiment above, the resistive position sensor (1) comprises an electric conductor (20) wherein the contact member (10) is arranged to provide an electric connection between an adjacent contact point (Cl, C2,....CN) and the electric conductor

(20).

[0051] In a second embodiment that may be combined with the first embodiment and any of its related embodiments, the discrete resistors (Rl, R2.... RN) and the contact points (Cl, C2.... CN) are arranged on a printed circuit board (PCB). This is illustrated in figure 2.

[0052] The discrete resistors (Rl, R2.... RN) may be surface mounted on the printed circuit board (PCB). Surface mounted design has several advantages over through hole implementation for the position sensor, especially where there is a requirements for high resolution resulting in a large number of components, e.g. in the order of thousand.

[0053] In a third embodiment that may be combined with any of the embodiments above, a subset of the discrete resistors (Rl, R2.... RN) are signature resistors with a different nominal resistance value than the other discrete resistors (Rl, R2.... RN). Thus, the discrete resistors (Rl, R2.... RN) in this case have at least two different nominal resistance values. As an example the signature resistors could have nominal values of 10 ohms, while the rest could have nominal values of 5 ohms.

[0054] The signature resistors may be arranged interleaved between the other discrete resistors (Rl, R2.... RN). In an example every fifth discrete resistor is a signature resistor.

If the same nominal values as exemplified above are used, every fifth resistor has a value of 10 ohms, while the remaining resistors have a value of 5 ohms. Additional subsets of resistors with other nominal values could also be used, such that e.g. every twentieth resistor has a nominal value of 20 ohms. The first and last resistors could also have specific resistance values different from the resistor values not in the subsets.

[0055] Other pre-defined patterns of interleaved resistors with specific nominal resistance values may also be used. One way would be to arrange the signature resistors at specific position locations, e.g. every 20 cm if it is a longitudinal measurement or every 30 degree if it is a rotational measurement.

[0056] The signature resistors may be used to define starting points for pre-defined measurement regions, as will be explained later.

[0057] Discrete resistors (Rl, R2.... RN) have nominal resistance values with a tolerance. The tolerance should in this case allow detection of a decrement or increment in resistor value from one contact point to the next. However, as will be explained later, tolerance may not be a critical issue in specific embodiments of the invention as long as a stepwise resistance increment or decrement can be detected. This depends on the algorithm used to calculate the position from the resistance measurements. [0058] The embodiments above may be applied in different physical position sensors, e.g. linear or rotational.

[0059] In a fourth embodiment that may be combined with any of the embodiments above, the contact points (Cl, C2,..., CN) are arranged longitudinally in line as illustrated in figure la, 2b and 2c. This configuration is typically used for measuring longitudinal position.

[0060] In a first related embodiment, the resistive position sensor (1) comprises an electric conductor (20) arranged equidistant from the contact points (Cl, C2,....CN). The contact member (10) is here arranged to provide an electric connection between the contact points (Cl, C2,....CN) and the electric conductor (20).

[0061] In a second related embodiment that may also be combined with the first related embodiment above, the electric conductor (20) may be fixed relative the first part, implying that it is also fixed relative the contact points (Cl, C2,....CN). The contact member will then establish an electric connection between the electric conductor (20) and the current or instant adjacent contact point.

[0062] In a third related embodiment that may be also combined with the first and second related embodiment above, the resistive position sensor (1) comprises a tubular element (21) wherein the contact points (Cl, C2,....CN), and the contact member (10) are arranged in the tubular element (21). This has been illustrated in Figures la, 2a and 2b. In figures 2a and 2b, the contact member 10 is shown as retracted from the tubular element (21) for illustration purposes, but in normal operation, the contact member will reside longitudinally movable inside the tubular element as illustrated by the arrow in figure 2a.

[0063] The tubular element (21) may have a conductive inner surface constituting the electric conductor (20). Thus, the contact member 10 will set up a connection between the inner surface of the tubular element, or the tubular element itself (if it is entirely in a conductive material) and the adjacent contact point.

[0064] In a fourth related embodiment that may also be combined with any of the embodiments above, the second part comprises a contact member shaft (11) fixed to the contact member (10). In the case above, where the tubular element is used, the contact member shaft is longitudinal and configured to, at least partly, enter into the tubular element.

[0065] In a fifth embodiment that may be combined with the fourth embodiment above, the printed circuit board (PCB) is longitudinal, and the first and second ends (la, lb) of the serial circuit are arranged in first and second ends (5a, 5b) of the longitudinal printed circuit board (PCB), respectively as can be seen from the illustration in figure la.

[0066] In a related embodiment, the longitudinal printed circuit board (PCB) comprises a connection (6) from the first end (la) of the serial circuit to the second end (5b) of the longitudinal printed circuit board (PCB), thereby providing connectivity to both the first and the second ends (la, lb) of the serial circuit from the second end (5b) of the printed circuit board (PCB). Since the tubular element is also fixed relative the printed circuit board and the second part, all electrical connections to the resistive sensor can be fixedly connected to the same side of the position sensor. This advantage becomes especially apparent when long printed circuit boards are required for position measurements of large longitudinal displacements, e.g. in the order of meters. No additional connection is required to the moving contact member.

[0067] The resistive position sensor (1) described above may be seen as encoding discrete positions related to the specific contact points and the position of the contact member. In the following, embodiments comprising a processing system (30) transforming these encoded values to position or values are explained.

[0068] In a sixth embodiment that may be combined with any of the embodiments above, and illustrated in figures la and 3 for a longitudinal and a rotational position senor, respectively, the resistive position sensor (1) comprises a processing system (30) having first, second and third input terminals (Tl, T2, T3) electrically connected to the first and second ends (la, lb) of the serial circuit, and to the electric conductor (20), respectively, wherein said processing system (30) is configured to calculate the position based on resistivity measurements between any of the first, second and third input terminals (Tl,

T2, T3). It should be understood that the term "position" is related to the actual position of the first part relative the second part, i.e. the position of the contact member relative to a specific contact point. As such, the position value calculated by the processing system (30) may be absolute or relative, depending on the configuration and input parameters of the processing system (30).

[0069] In a seventh embodiment that may be combined with the sixth embodiment above, the processing system (30) is configured for measuring an instant resistance (Ri) between the first, and third input terminals (Tl, T3) and/or the second and third input terminals (T2, T3). The resistance between the first and third terminals (Tl, T3) and the second and third terminals (T2, T3) will both vary according to the position of the contact member. In figure la the resistance between the first and the third terminal (Tl, T3) will increment when the contact member is moved to the right and decrement when moved left. For the resistance between the second and the third terminal (T2, T3) it will be opposite.

[0070] The term instant resistance is meant to define the resistance for the current position of the contact member and may be a single sample.

[0071] In a related embodiment the processing system (30) is in addition configured for;

- registering a number of discrete resistors (Rl, R2.... RN) in the resistive position sensor (1), i.e. the total number of resistors;

- measuring a total resistance (Rt) between the first input terminal (Tl) and the second input terminal (T2); and

- calculating an average resistance (Ra) by dividing the total resistance (Rt) by the number of discrete resistors (Rl, R2.... RN);

[0072] Since the resistor values may vary with temperature and other operating conditions, this method can be used to calibrate the position sensor before and during measurements.

[0073] The average value can then be used in a related embodiment by the processing system to calculate the position by;

- registering a resolution (Res), as defined previously;

- calculating a number of instant resistors by dividing the instant resistance (Ri) by the average resistance (Ra);

- calculating the position by multiplying the number of instant resistors with the resolution (Res).

[0074] The methods above based on calculated average resistance requires a certain resistance value tolerance to work properly. In one example the tolerance is better than 20 %.

[0075] The average-based method will in many applications be sufficient. However, it might be required to improve the error margin by implementing a different position calculation method in the processing system. It may also be desirable to reduce the need for resistance tolerance to simplify production of the printed circuit board.

[0076] In an eight embodiment that may be combined with the sixth embodiment above, the processing system (30) is configured for;

- registering a value representing a resistor value range for the discrete resistors (Rl,

R2.... RN). Just for exemplification the range could be 1 to 10 ohms. Another way of representing the range could be to register a nominal value and a tolerance such as e.g. 5 ohms +- 80%. - registering a value representing a resolution (Res), as defined previously;

- continuously measuring an instant resistance (Ri) between the first, and third input terminals (Tl, T3).

[0077] Further, the processing system (30) is configured to stepwise increment or decrement the position with the resolution (Res) every time the instant resistance (Ri) increases or decreases above or below the resistor value range.

[0078] Continuous measurement will in most practical terms be implemented as a samples with a sampling frequency sufficiently high to detect every increment in resistance as the contact member passes from one contact point to the next. E.g. if the resolution is 1 mm and the contact member can move with a maximum speed of 10 mm/s, the sampling frequency should preferably be 20Hz or higher. Thus, as long as the sampling frequency is detect every increment or decrement within the resistor value range, i.e. every step the contact member takes from one contact point to the next, the measurement can be said to be continuous for the purpose of the invention.

[0079] In a related embodiment the resistor value range may be obtained by performing an initial calculation of the average value of the discrete resistors instead of entering the resistor value range.

[0080] The processing system (30) is then further configured for;

- registering a number of discrete resistors (RI, R2.... RN) in the resistive position sensor

(1);

- measuring a total resistance (Rt) between the first input terminal (Tl) and the second input terminal (T2);

- calculating an average resistance (Ra) by dividing the total resistance (Rt) by the number of discrete resistors (RI, R2.... RN),

- registering a tolerance for the discrete resistors, wherein the resistor value range is calculated from the average resistance (Ra) and the tolerance;

[0081] E.g. if the average value is 5.5 ohms and tolerance 70%, the range is 1.65 to 9.35 ohms.

[0082] In an ninth embodiment that may be combined with the eighth embodiment, the processing system (30) is configured for;

- registering a value representing a resistance and a position for an adjacent contact point (Cl, C2,....CN) for signature resistors arranged interleaved between the other discrete resistors (RI, R2.... RN), wherein the signature resistors have a different nominal resistance value than the other discrete resistors (RI, R2.... RN). [0083] In figure lb, illustrating this embodiment, the resistor (R5) is a signature resistor and has a different nominal value than the other resistors. The adjacent contact point (C5) is located at a position (X5) relative to the position (XI). The resistor (R5) and the contact point (C5) are hatched for illustration.

[0084] In a related embodiment, the processing system (30) is configured for resetting the position value to the registered position when the increase or decrease in instant resistance (Ri) indicates that a corresponding signature resistor has been reached.

[0085] If e.g. the contact member is moving from left to right in figure lb and for some reason does not contact contact point (C3) properly so that it appears that the contact member (10) is still at contact point (C2) when it is actually at (C3), and at the next increment at contact point (C4) it appears as if the contact member is still at contact point (C3), the processing system (30) will reset the system to the correct position when the contact member reaches contact point (C5) because the increment in resistance is higher when moving to contact point to (C5) from the previous contact point, than between the other contact points. Since the position of the contact point (C5) is known to the

processing system (30), the next samples, in either direction, can start from this position. This means that the value may be reset to the expected value of contact point (C5) before continuing.

[0086] In a tenth embodiment that may be combined with any of the first to sixth embodiments above, and illustrated in figures la and 3 for a longitudinal and a rotational position senor, comprising the serial circuit with first and second ends (la, lb) and the electric conductor (20), the invention is a method for measuring a position with a resistive position sensor (1) comprising;

- calculating the position based on resistivity measurements between any of the first and second ends (la, lb) of the serial circuit and the electric conductor (20).

[0087] It should also here be understood that the term position is related to the actual position of the first part relative the second part, i.e. the position of the contact member relative to a specific contact point. As such, the position value calculated by the method may be absolute or relative, depending on the configuration and other input parameters of the method, as will be understood by a person skilled in the art.

[0088] In an eleventh embodiment that may be combined with the tenth embodiment above, the method comprises;

- measuring an instant resistance (Ri) between the first end (la) and the electric conductor (20) or the second end (lb) and the electric conductor (20). [0089] The resistance between the first end (la) and electric conductor (20) and the second end (lb) and electric conductor (lb) will both vary according to the position of the contact member. In figure la, the resistance between the first end (la) and electric conductor (20) will increment when the contact member is moved to the right and decrement when moved left. For the resistance between the second end (lb) and the electric conductor (20) it will be opposite.

[0090] The term instant resistance is meant to define the resistance for the current position of the contact member and may be a single sample.

[0091] In a related embodiment the processing system (30) is in addition configured for;

- registering a number of discrete resistors (Rl, R2.... RN) in the resistive position sensor

(1);

- measuring a total resistance (Rt) between the first and second ends (la, lb);

- calculating an average resistance (Ra) by dividing the total resistance (Rt) by the number of discrete resistors (Rl, R2.... RN);

[0092] In a related embodiment the resistor value range may be obtained by performing an initial calculation of the average value of the discrete resistors instead of entering the resistor value range.

[0093] The method then comprises;

- registering a resolution (Res);

- calculating a number of instant resistors by dividing the instant resistance (Ri) by the average resistance (Ra);

- calculating the position by multiplying the number of instant resistors with the resolution (Res).

[0094] The average-based method will in many applications be sufficient. However, it might be required to improve the error margin by implementing a different position calculation method in the processing system. It may also be desirable to reduce the need for resistance tolerance to simplify production of the printed circuit board.

[0095] In a twelfth embodiment that may be combined with the tenth embodiment above, the method comprises;

- registering a value representing a resistor value range for the discrete resistors (Rl,

R2.... RN);

- registering a value representing a resolution (Res);

- continuously measuring an instant resistance (Ri) between the first end (la) and the electric conductor (20), [0096] Further the processing system (30) is configured to stepwise increment or decrement the position with the resolution (Res) every time the instant resistance (Ri) increases or decreases above or below the resistor value range.

[0097] Continuous measurement will in most practical terms be implemented as a samples with a sampling frequency sufficiently high to detect every increment in resistance as the contact member passes from one contact point to the next. E.g. if the resolution is 1 mm and the contact member can move with a maximum speed of 10 mm/s, the sampling frequency should preferably be 20Hz or higher. Thus, as long as the sampling frequency is detect every increment or decrement within the resistor value range, i.e. every step the contact member takes from one contact point to the next, the measurement can be said to be continuous for the purpose of the invention.

[0098] In a related embodiment the resistor value range may be obtained by performing an initial calculation of the average value of the discrete resistors instead of entering the resistor value range.

[0099] The method then comprises;

- registering a number of discrete resistors (Rl, R2.... RN) in the resistive position sensor

(1);

- measuring a total resistance (Rt) between the first and second ends (la, lb);

- calculating an average resistance (Ra) by dividing the total resistance (Rt) by the number of discrete resistors (Rl, R2.... RN),

- registering a tolerance for the discrete resistors, wherein the resistor value range is calculated from the average resistance (Ra) and the tolerance;

[0100] In a thirteenth embodiment that may be combined with the twelfth embodiment, the method comprises;

- registering a value representing a resistance and a position for signature resistors arranged interleaved between the other discrete resistors (Rl, R2.... RN), wherein the signature resistors have a different nominal resistance value than the other discrete resistors (Rl, R2.... RN).

[0101] In figure lb, illustrating this embodiment, the resistor (R5) is a signature resistor, and has a different nominal value than the other resistors (Rl, R2, R3, R4, R6, R7 and RN) that are illustrated in the same figure. The adjacent contact point (C5) is located at a position (X5) relative to the position (XI). The resistor (R5) and the contact point (C5) are hatched for illustration. [0102] In a related embodiment the method comprises;

- resetting the position value to the registered position when the increase in instant resistance (Ri) indicates that a corresponding signature resistor has been reached.

[0103] Here, the increase in instant resistance (Ri) corresponds to a recognizable resistance value of one or more of the signature resistors. The recognizable resistance value may be seen as a signature for the registered position.

[0104] When the first and second parts are moved relative each other in one direction, the instant resistance (Ri) may increase stepwise before the increase caused by the signature resistor is reached. Correspondingly, in the other direction, the instant resistance (Ri) may decrease stepwise before the increase caused by the signature resistor has been reached.

[0105] E.g. when instant resistance (Ri ) is measured between the first and third terminals (Tl, T3) and the contact member (10) moves right in figure lb, resistance will increase stepwise for each contact point (Cl, C2, C3, C4) with the resistance values of resistors (RI, R2, R4, R4). From there, it will increase with the signature value of signature resistance (R5) when the contact member (10) reaches the contact point (C5).

[0106] If the contact member (10) for some reason does not contact contact point (C3) properly so that it appears that the contact member is still at contact point (C2) when it is actually at (C3) and at the next increment at contact point (C4) it appears as if the contact member is still at contact point (C3), the processing system (30) will reset the system to the correct position when the contact member reaches contact point (C5) because the increment, or spike, in resistance is higher when moving from contact point (C4) to (C5) than between the other contact points. Since the position (X5) of the contact point (C5) is known to the processing system (30), the next samples, in either direction, can start from this position. This allows the resistors to have a certain tolerance.

[0107] The signature resistors, may be arranged equally spaced. E.g. in figure lb, every fifth resistor (R5, R10, R15. ) could have a signature value.

[0108] In a related embodiment, the subset of the discrete resistors (RI, R2.... RN) have a higher nominal resistance than the other discrete resistors (RI, R2.... RN).

[0109] The nominal resistance of the signature resistors may in related embodiments be at least 2, 3, 5, 10 or 20 times higher than the nominal resistance of the other discrete resistors (RI, R2.... RN). [0110] The resetting may be seen as an override function of the standard measurement sequence to make the position sensor more fault tolerant.

[0111] In a fourteenth embodiment, the invention is an actuator, comprising a resistive position sensor (1, 100) according to any of the first to ninth embodiments above, wherein the resistive position sensor (1, 100) is configured to measure the actuation position of the actuator.

[0112] Any of the methods in embodiments nine to thirteen may be employed to measure the actuation position.

[0113] In addition, the method may in an embodiment comprise calibration of the resistive sensor with the first end position of the actuator, where the method comprises;

- actuating the actuator to a first end position,

- register a first end resistive value between the first end (la) and the electric conductor

(20).

[0114] In a corresponding related embodiment, the resistive sensor may be calibrated with the second end of the actuator, comprising;

- actuating the actuator to second end position,

- register a second end resistive value between the first end (la) and the electric conductor (20).

[0115] In a further related embodiment, the actuator may be a hydraulic actuator, more specifically a double-acting linear hydraulic actuator. This embodiment, as well as a method of calibration, is described in more detail below, with reference to figures 4a to 10.

[0116] The above embodiments may be used e.g. to calibrate position sensors of wellbore linear actuators, such as actuators used for intervention operations, in-situ.

[0117] In most of the examples above, a longitudinal position sensor have been illustrated. However, the same principle described in embodiments 1 to 3 and 6 to 14 may be used with different physical layouts, such as e.g. rotational position sensor.

[0118] In a fourteenth embodiment the invention is a resistive position sensor (100) comprising a first and a second part movable relative each other, where the first and the second parts may be connected to individually moving members, such as e.g. the case (150) and the piston (151) of an actuator to measure the rotational displacement of the piston relative the case. This is illustrated in figure 3.

[0119] Similar to the linear embodiments, the resistive position sensor (100) here comprises a first and a second part movable relative each other. The first part comprises a sequence of N serially interconnected discrete resistors (Rl, R2.... RN) that are interconnected in individually spaced apart contact points (Cl, C2,....CN).

[0120] This embodiment differs from the linear embodiments in that the individually spaced apart contact points are arranged in a circular formation. However, as can be seen, the ends of the serial circuit are not interconnected. Thus the electric circuit in this embodiment may be similar to the circuit in the first embodiment, but the location of the first and second end points (la, lb) are different.

[0121] Similar to the linear embodiments, the second part also here comprises a conducting contact member (10) configured for sliding over and contacting the contact points (Cl, C2,....CN) separately one by one as the first and a second part moves relative each other. In this embodiment the second part illustrated as a rotating core rotates relative the outer first part.

[0122] Any of the method related embodiments above may be implemented as computer programmed instructions stored on a computer-readable medium.

[0123] The processing system (30) may be any type of suitable electronic circuit for calculating a position. Typically the processing system (30) is computer based and may comprise a microprocessor.

[0124] The first second and third terminals (Tl, T2, T3) and the output terminal (31) may be separate physical connections or terminations in a common protocol. The circuits illustrated should be seen as connections that may be realized using different protocols and physical wiring.

[0125] The term registering is in this document meant to include any type of temporary or long term storage of values of the parameters mentioned. The parameter values may be stored in a memory accessible by the processing system (30). Sometimes the parameter values are determined by the physical configuration of the position sensor and therefore not altered during measurement, while others may be changed depending on the operation, e.g., as part of a calibration process or as a result of a calculation.

[0126] Another embodiment of the invention will now be described with reference to figures 4a to 10, in which the position sensor is incorporated in a hydraulic, linear double acting actuator. Unless otherwise noted, the features and methods described above with reference to figures 1 to 3, shall apply also to the embodiment illustrated in figures 4a to 10. [0127] Referring initially to figures 4a and 5a, the linear actuator 50 comprises in the illustrated embodiment a housing 55, a first (upper) end 50b with an interface structure 50c for e.g. a hydraulic power unit (HPU) 91 (not shown in figure 5a, see figures 9 and 10), and a second (lower) end 50a with a bottom nose assembly. A piston having a piston head 53 and a piston shaft 54 is arranged to reciprocate inside the actuator housing (illustrated in an intermediate position). The piston head defines a head chamber 51 (in the following also referred to as a first actuator chamber) and a return chamber 52 (in the following also referred to as a second actuator chamber) on opposite sides of the piston head. The piston head may move between a first (upper) stroke stop 66a and a second (lower) stroke stop 66b. The piston shaft 54 extends out from the actuator housing and is at its free end provided with a tool interface portion 50d for connection to (e.g.) a bottom adapter (not shown). As the skilled person will know, a typical bottom adapter may have a diameter corresponding to the diameter of the actuator housing.

[0128] Figure 5b identifies a hydraulics line (conduit) 51a, connecting the head chamber 51 to a hydraulic fluid reservoir in the HPU 91 (not shown in figure 5b). Figure 5c identifies a hydraulics line (conduit) 52a, connecting the return chamber 52 to a hydraulic fluid reservoir in the HPU 91 (not shown in figure 5c). Referring to figure 10, the hydraulic lines are interconnected via a manifold 94.

[0129] Referring now to figures 6a-c, 7a-c, and 8, the actuator comprises a first part having a printed circuit board PCB with a sequence of a plurality of serially interconnected discrete resistors Rx that are interconnected in individually spaced apart contact points Cx. It should be understood that "x" may be an integer greater than 1, and that the resistors Rx and contact points Cx correspond to the resistors Rl, R2.... RN and contact points Cl, C2,....CN described above with reference to figures la to 2c. The actuator also comprises a second part having a wiper 65 configured for sliding over and contacting the contact points Cx as the first and a second part move relative each other and a displacement of the second part (i.e. the piston) relative to the first part indicates a position to be sensed by the resistive position sensor, as described above. It will be understood that the wiper 65 in effect corresponds to the conducting contact member 10 described above with reference to figures la to 2c.

[0130] The PCB is arranged in a carrier tube 67 which is arranged in a sensor support tube 58 connected to a non-conductive sensor tube carrier 59. The carrier tube 67 is arranged inside a center tube 56. The wiper 65 is connected to a wiper carrier 60 which is connected to the piston shaft 54 via a sensor pin 57 (see e.g. figure 5b). Leads (wires) 61 and 62 connect the PCB to terminals T1 and T2, respectively, and leads (wire) 63 is connected to terminal T3 (ground). These connections are similar to the connections described above with reference to figures la to 3.

[0131] In figures 9 and 10, the actuator 50 is illustrated as part of a downhole tool assembly, arranged in a wellbore 92. The actuator is connected to a hydraulic power unit (HPU) 91 which in turn is connected to a control and processing unit (CPU) 90. Hydraulic lines (conduits) 93 extend from a reservoir and hydraulic motor-and-pump module 95 and a manifold 94 into the first (head) and second (return) actuator chambers 51, 52, via respective lines 51a and 52a, as described above. The hydraulic motor-and-pump module 95 is as such known in the art, and comprises a motor 95a which is drivingly connected to a hydraulics pump 95d via a driveshaft 95c. An RPM sensor 95b, for example a hall-effect sensor, is arranged to measure the drive shaft RPM. The pump is preferably a fixed displacement type pump, that outputs a fixed volume of hydraulic oil for each rotation (revolution) of the shaft 95c.

[0132] Reference number 96 denotes power and communication lines through the actuator, reference number 97 denotes power and communication lines to the CPU, reference number 98 denotes power and communication lines to and from the position sensor (such as the wiper and PCB), reference number 99 denotes power and

communication lines to and from the motor-and-pump module 95, and reference number 80 denotes power and communication lines to the manifold 94 (i.e. to control manifold solenoids).

[0133] Although the illustrated embodiment shows an actuator-HPU-CPU configuration, the invention is equally applicable to a configuration in which the CPU for example is arranged between the actuator and the HPU, or any other arrangement.

[0134] In operation, the invented actuator - incorporating the resistive position sensor described above, may be used to measure the actuation position of the piston shaft 54 and hence that of the tool interface portion 50c and its associated bottom assembly. In addition, the double-acting actuator 50 is capable of calibrating the actuator, automatically and in real-time, by utilizing its inherent hydraulic functionality to zero out the sensor at either end of the actuator (piston head) stroke. The hydraulic functionality-related variables which may be used to indicate the position of either end of the stroke (cf.

reference to first and second stroke stops 66a, b, above) may be the pressure spikes at both ends or the sudden increase in power consumption in the HPU correlated with the pressure spikes.

[0135] The real-time calibration is based on relating the hydraulic functionality-related variable (e.g. pressure spike) to a preset pressure p _r . The value for _r will be chosen to be higher than the following two different and distinguished pressures that can be measured before and during the calibration process:

i) The system pressure p _s . This is a static pressure for the actuator and its value is determined by the design of the HPU and can thus be calculated or measured.

ii) The free-running pressure P _f . This is dynamic pressure and defines the amount of pressure necessary to move the actuator piston freely inside the housing, between the two stroke ends, in the absence of any external load. The free-running pressure is a function of factors such as (for example) machined parts' tolerances, seal hardness and materials, and elastomers between the moving parts, as well as hydraulic oil viscosity and ambient temperature. As a non-limiting example, the free-running pressure P _f may be twice or four times the system pressure p _s . In general, the preset pressure p _r will be chosen to be significantly higher than p _s and Pf, i.e. p _r > > p _s and p _r > > Pf.

[0136] In one embodiment, the preset pressure p _r may be on the order of ten times the system pressure p _s in order to make it unique and easily detectable. In general, however, any pressure spikes up to or above the preset pressure p _r will indicate either one of the first and second stroke stops. Such pressure spike may also indicate an obstruction in the wellbore, which is discussed below.

[0137] A calibration method may in one embodiment comprise calibration of the resistive sensor with a first end position 66a of the actuator (i.e. piston head 53), where the method comprises: (i) actuating the piston head to a first end position 66a, and (ii) register a first end resistive value as described above with reference to figures la to 3.

[0138] Correspondingly, the resistive sensor may be calibrated with the second end position 66b of the of the actuator (i.e. piston head 53), comprising : (i) actuating the piston head to a second end position 66b, and register a second end resistive value as described above with reference to figures la to 3.

[0139] More specifically, in one embodiment the method comprises moving the piston head 53 towards the first stroke stop 66a or towards the second stroke stop 66b until a hydraulic functionality-related variable (e.g. pressure spike or sudden increase in power consumption in the HPU correlated with the pressure spike) is detected. When thus moving the piston head 53 towards the first (top) stroke stop 66a, the hydraulic functionality- related variable is detected in the second (return) chamber 52. When thus moving the piston head 53 towards the second (bottom) stroke stop 66b, the hydraulic functionality- related variable is detected in the first (head) chamber 51. [0140] The initial position of the piston head 53, before moving is initiated, may be at (and abutting) the first stroke stop, or at (and abutting) the second stroke stop, or at any intermediate position.

[0141] First (upper) and second (lower) end positions for the piston head may thus be recorded. These positions may be based on information provided by the resistive position sensor at the time of recording the hydraulic functionality-related variable. The first (upper) and second (lower) end positions may thus be set based on said hydraulic functionality-related variable, .

[0142] The first and second end points, and their corresponding resistive values, may thus be set as reference points for further measurements.

[0143] As mentioned above, the hydraulic functionality-related variable may be a pressure spike in the chamber or a sudden increase in power consumption in the HPU, correlated with the pressure spike.

[0144] These calibration methods may be implemented in the processing system 30, as discussed above.

[0145] The above embodiments may be used e.g. to calibrate position sensors of wellbore linear actuators, such as actuators used for intervention operations, in-situ.

[0146] While the methods described above are suitable for auto-calibration of a double acting hydraulic cylinder (actuator), the methods may in certain instances yield incorrect results, particularly when the actuator is part of a tool located in a subterranean wellbore. While the real-time calibration relies on the spike in pressure or power consumption in the HPU which correlates with the actuator piston reaching either ends of its stroke, the piston may be caused to stop in an intermediate position between the first and second stops 66a, b. Reasons for this may for example be (i) a restriction in the well ahead of the actuator piston shaft 54 which the shaft bottoms with or rams into upon extension, or (ii) a profile in the well with protruding edges which the above mentioned bottom adapter might catch upon retraction. Both of these events and any other obstacle will cause the pressure/ power consumption to spike momentarily and will make it look as if the piston has reached one of its end points, and thus provide a false input to the control system.

[0147] Such erroneous reading may be mitigated by using the volumetric displacement of the fixed-displacement pump 95d in the motor-and-pump module 95 in the HPU 91 as a secondary displacement sensor (i.e. a position estimator) which can be considered as a redundant (hence "secondary") position sensor in the actuator system during normal operation. The secondary position sensor is an internal function in the system and is controlled only by hydraulic control, which makes it independent and predictable.

[0148] The secondary position sensor serves as a redundant position sensor when the primary sensor (i.e. the resistivity-based sensor comprising the contact member (e.g. wiper)-PCB combination; e.g. reference numbers 10 and 65 as described above) is in action, and as an indication of full-stroke translation confirming that the actuator's piston head has reached one of its end stops. When the actuator is in operation, the position measurement obtained from the secondary position sensor runs in the background at all times and corresponds to a full stroke (within acceptable tolerances). However, when the deviation between the measurements obtained by the primary sensor and the secondary sensor is greater than a predetermined value, this deviation may be indicative of an obstruction in the wellbore, for example as described above (As a non-limiting example, the predetermined deviation may be in the order of 0.1%). Based on this information, an operator may decide that the present tool location in the wellbore is unsuitable for calibration, and for example pull the tool slightly upwards to another position for calibrating the actuator. The said deviation may be useful in providing information about the wellbore when the actuator encounters anomalies in the well (and thus provide crucial information on the tool and the well condition) where no anomalies are expected.

[0149] In the following, the primary sensor may also be referred to as a "resistivity position sensor" and the secondary sensor may be referred to as a "volumetric

displacement sensor". While multiple embodiments of the resistivity position sensor have been described above, the following is a non-limiting description of an embodiment of the volumetric displacement sensor.

[0150] As discussed above, the hydraulic motor 95a is furnished with an RPM sensor 95b (for example a hall-effect sensor), the output of which is an accurate measurement of the motor's RPM at its drive (output) shaft 95c. The motor output shaft is mechanically coupled with the hydraulic pump 95d. The pump 95d is a fixed displacement type pump that outputs a fixed volume of hydraulic oil for each rotation (revolution) from its input shaft, i.e. the shaft 95c. This arrangement makes it possible to accurately calculate the pump's volumetric displacement over a certain amount of time, based on recorded and logged RPM values. The result of such calculations is the accurate measurement of volumetric changes in the actuator's double acting cylinder, which corresponds directly to the piston position in reference to a start point.

[0151] Although the method of determining end positions for a double-acting hydraulic actuator has been described here with reference to the resistive position sensor as described above with reference to figures la to 3, it should be understood that concept of determining an actuator position based on one or more hydraulic functionality-related variables may be used in conjunction with other position sensor means or methods.

[0152] In the exemplary embodiments, various features and details are shown in combination. The fact that several features are described with respect to a particular example should not be construed as implying that those features by necessity have to be included together in all embodiments of the invention. Conversely, features that are described with reference to different embodiments should not be construed as mutually exclusive. As those with skill in the art will readily understand, embodiments that incorporate any subset of features described herein and that are not expressly

interdependent have been contemplated by the inventor and are part of the intended disclosure. However, explicit description of all such embodiments would not contribute to the understanding of the principles of the invention, and consequently some permutations of features have been omitted for the sake of simplicity or brevity. The scope of the invention, which includes all permissible embodiments, is defined by the claims.