Impulse wrench rotation detection

Abstract

A method for detecting deviations between an actual angular displacement in a screw joint being tightened by an impulse type power wrench and an angular displacement measured between a power wrench housing and an impulse unit of the power wrench, includes determining a first angular interval (.sub.1) between an end point (A.sub.E) of a first delivered impulse (A) and an end point (B.sub.E) of a succeeding second delivered impulse (B), determining a second angular interval (.sub.2) between a start point (B.sub.S) of the second delivered impulse (B) and a start point (C.sub.S) of a succeeding third delivered impulse (C), comparing (.sub.1) with (.sub.2) to determine a difference between the intervals, and determining that a deviation exists between the actual angular displacement in the screw joint and the angular displacement measured between the power wrench housing and the impulse unit based on the determined difference between the intervals (.sub.1 and .sub.2).

Claims

1. A method for detecting deviations between an actual angular displacement in a screw joint being tightened by an impulse type power wrench and an angular displacement measured between a power wrench housing and an impulse unit of the power wrench, the method comprising: determining a first angular interval (.sub.1) between an end point (A.sub.E) of a first delivered impulse (A) and an end point (B.sub.E) of a succeeding second delivered impulse (B); determining a second angular interval (.sub.2) between a start point (B.sub.S) of the second delivered impulse (B) and a start point (C.sub.S) of a succeeding third delivered impulse (C); comparing the first angular interval (.sub.1) with the second angular interval (.sub.2) to determine a difference between the first and the second angular intervals (.sub.1 and .sub.2); and determining whether a deviation exists between the actual angular displacement in the screw joint being tightened by the impulse type power wrench and the angular displacement measured between the power wrench housing and the impulse unit of the power wrench based on the determined difference between the first and said second angular intervals (.sub.1 and .sub.2).

2. The method according to claim 1, wherein the end and start points (A.sub.E, B.sub.E and B.sub.S, C.sub.S) of the first, second, and third delivered torque impulses (A, B, C) are chosen as certain rotation speed change levels of the impulse unit of the power wrench.

3. The method according to claim 2, wherein the determination whether a deviation exists is based on a comparison between the determined difference between the first and second angular intervals (.sub.1 and .sub.2) and a pre-set threshold value.

4. The method according to claim 3, further comprising outputting an alert signal when a deviation is determined to exist.

5. The method according to claim 2, further comprising outputting an alert signal when a deviation is determined to exist.

6. The method according to claim 1, wherein the determination whether a deviation exists is based on a comparison between the determined difference between the first and second angular intervals (.sub.1 and .sub.2) and a pre-set threshold value.

7. The method according to claim 6, further comprising outputting an alert signal when a deviation is determined to exist.

8. The method according to claim 1, further comprising outputting an alert signal when a deviation is determined to exist.

9. A non-transitory computer readable medium storing a program executable by a computer to detect deviations between an actual angular displacement in a screw joint being tightened by an impulse type power wrench and an angular displacement measured between a power wrench housing and an impulse unit of the power wrench, the program being executable by the computer to cause the computer to perform operations comprising: determining a first angular interval (.sub.1) between an end point (A.sub.E) of a first delivered impulse (A) and an end point (B.sub.E) of a succeeding second delivered impulse (B); determining a second angular interval (2) between a start point (B.sub.S) of the second delivered impulse (B) and a start point (C.sub.S) of a succeeding third delivered impulse (C); comparing the first angular interval (.sub.1) with the second angular interval (.sub.2) to determine a difference between the first and the second angular intervals (.sub.1 and .sub.2); and determine whether a deviation exists between the first and the second angular intervals, .sub.1 and .sub.2, based on the determined difference between the first and second angular intervals (.sub.1 and .sub.2).

10. A device for detecting deviations between an actual angular displacement in a screw joint being tightened by an impulse type power wrench and an angular displacement measured between a power wrench housing and an impulse unit of the power wrench, the device comprising: a determination circuit adapted to determine a first angular interval (.sub.1) between an end point (A.sub.E) of a first delivered impulse (A) and an end point (B.sub.E) of a succeeding second delivered impulse (B), and adapted to determine a second angular interval (.sub.2) between a start point (B.sub.S) of the second delivered impulse (B) and a start point (C.sub.S) of a succeeding third delivered impulse (C); and a comparator adapted to compare the first angular interval (.sub.1) with the second angular interval (.sub.2) to determine a difference between the first and the second angular intervals (.sub.1 and .sub.2); wherein the determination circuit is further adapted to determine whether a deviation exists between the first and the second angular intervals, .sub.1 and .sub.2, based on the determined difference between the first and the second angular intervals (.sub.1 and .sub.2).

11. A power wrench comprising the device according to claim 10.

12. A power wrench controller unit comprising the device according to claim 10.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The invention will now be described in further detail in the following specification and claims with reference to the accompanying drawings, in which.

(2) FIG. 1 shows schematically a side view, partly in section, of an impulse type power wrench illustrating the impulse generating unit with an angle encoder,

(3) FIG. 2 shows a diagram illustrating the angular speed over time of the inertia drive member of the power wrench impulse generating unit in relation to the power wrench housing, and

(4) FIG. 3 is a flow chart illustrating some procedural steps performed when detecting an angular deviation.

DETAILED DESCRIPTION

(5) The method according to the invention is intended to be applied on an impulse type power wrench having a manually supported housing 10 with a rotation motor and an impulse generating unit 12. The latter comprises an inertia drive member 13 coupled to a motor and arranged to transfer kinetic energy intermittently to an anvil member connected to the output shaft 14 of the, typically hand held, power wrench. The energy transfer is accomplished via a hydraulic medium confined in the impulse generating unit. Since the impulse generating unit is of a common well known type and design a further detailed description thereof is left out of this specification.

(6) The impulse generating unit also comprises an angle sensor such as an angle encoder 16 for indicating and measuring the rotational movements of the inertia drive member 13 of the impulse unit 12 in relation to the housing 10 during each generated torque impulse to thereby enable measurement of the rotational displacements of the output shaft 14 and, hence, the stepwise angular displacements of a screw joint being tightened. The sum of these angular displacements corresponds to the clamping force obtained in the screw joint. This is easily calculated in view of the thread pitch of the screw joint. The angle encoder 16 in accordance with the embodiment shown in FIG. 1 is of a prior art type and comprises a circumferential band 17 mounted on the inertia drive member 13 and having a large number of magnetized transverse stripes. Other angle sensors can be used in other embodiments. A sensor 18 is secured in the housing 10 and being activated by the magnetized stripes of the band 17 at rotation of the inertial drive member 13.

(7) The rotation speed/time diagram of FIG. 2 illustrates parts of three succeeding torque impulses wherein start points as well as end points of these impulses are indicated. To the left in the diagram an end point A.sub.E of a first impulse A is chosen as a certain degree of rotation speed increase of the inertia drive member in relation to the housing. The following impulse B is preceded by an acceleration phase X of the inertia drive member, and the start point B.sub.S of the impulse B is chosen as a decrease of the inertia drive member rotation speed. The impulse B is completed at an end point B.sub.E and is succeeded by a third impulse C which is preceded by an acceleration phase Y. The Impulse C begins at a start point C.sub.S.

(8) Each one of the points A.sub.S, A.sub.E, B.sub.S, B.sub.E and C.sub.S, C.sub.E represents an angular position of the inertia drive member 13 relation to the housing 10, which means that it is possible to determine an angular interval .sub.1 between the end point A.sub.E of impulse A and the end point B.sub.E of impulse B as well as an angular interval .sub.2 between the start point B.sub.S of impulse B and the start point C.sub.S of impulse C.

(9) If the power wrench housing during impulse delivery has been kept completely immobile during generation of the torque impulses A, B and C the angular intervals .sub.1 and .sub.2 should be identical. Accordingly, if an angular displacement of the power wrench housing has occurred during the impulse delivery a difference between the angular intervals .sub.1 and .sub.2 can be observed. Such a difference in angular displacement indicates that a movement of the power wench housing has occurred and that the measured and calculated sum of the angular displacements of the inertia drive member does not truly correspond to the angular displacement actually imposed on the screw joint being tightened. This means that the accomplished clamping force or tightening level of the screw joint does not truly correspond to the indicated and calculated sum of angular displacements of the inertia drive member of the impulse unit and that the tightening level actually obtained should be checked for ascertaining the quality and safety of the screw joint. In accordance with some embodiments an indicator such as a light or sound generating device is provided on the power wrench or in the vicinity of the operator operating the power wrench to indicate that screw joint should be checked.

(10) In FIG. 3 a flow chart illustration some procedural steps that can be performed by a computer program when performing the method as set out herein. First in a step 301 a first angular interval 1 between an end point AE of a first delivered impulse A and an end point BE of a succeeding second delivered impulse B is determined.

(11) Next, in a step 303 a second angular interval 2 between a start point BS of said second delivered impulse B and a start point CS of a succeeding third delivered impulse C is determined. Thereupon, in a step 305 the first angular interval 1 is compared with the second angular interval 2) to determine the difference between said first and said second angular intervals (.sub.1 and .sub.2). An occurring difference between said first and said second angular intervals (1 and 2) indicates a deviation between the angular displacement measured between the power wrench housing (10) and the impulse unit (12) and the angular displacement actually imposed on the screw joint being tightened. If a difference is determined to exist between the first and the second angular intervals 1 and 2, respectively in a step 305, a deviation between the angular displacement measured between the power wrench housing 10 and the impulse unit 12 and the angular displacement actually imposed on the screw joint being tightened is determined to exist in a step 307. A signal alerting the operator or a control system can then be output to make the operator of the power wrench aware of a potentially inferior screw joint in a step 309.

(12) In accordance with one exemplary embodiment a difference is determined to exist in step 307 if the difference exceeds a pre-set threshold value. The pre-set threshold value can be set differently for different screw joint to compensate for different tolerances acceptable for different screw joints.

(13) In accordance with some embodiments, data relating to a determined potentially inferior screw joint can also be logged for follow-up purposes and to form a basis for statistics regarding a particular power wrench or a particular operator to determine if a problem exists with a particular power wrench or a particular operator. This can for example be determined by comparing a frequency at which a potentially inferior screw joint is determine with some predetermined value(s). The data that is logged can for example comprise one or more of: time, identity of power wrench, identity of operator, and magnitude of the determined difference.

(14) By the methods and devices according to the invention it is possible to detect uncertainties of the delivered tightening movements imposed on a screw joint being tightened and, hence the finally obtained tightening level of the screw joint, without adding any extra equipment in the form of for instance gyros to the power wrench. Occurring angular displacements of the power wrench housing during impulse delivery may be detected by an angle sensor such an angle encoder in the impulse unit which is standard equipment in most impulse wrenches today.

(15) The method can be implemented using suitable software executed on a computer. The software can be stored on a non-volatile device. In accordance with some embodiment the power wrench it-self houses the hardware such as a central processor unit and an associated memory that comprises the software enabling the execution of the method as described herein. In accordance with some other embodiments the method is executed in a power tool controller located remote from the power wrench.