EP1412135B1 - Method for controlling an intermittently operating screw tool - Google Patents

Description

The invention relates to a method according to the preamble of independent claim 1 for controlling a screwing tool that performs intermittently rotating strokes and has a torque sensor and a rotation angle sensor.

Such a method is from the document EP 0 340 999 A known.

It is known to use for tightening screws hydraulic power wrench having a piston-cylinder unit which drives a ratchet lever back and forth. The ratchet lever drives a ring element, which is coupled via a key nut with the screw to be rotated. By the rotating strokes of the ratchet lever in one direction, the screw is tightened while the return stroke of the ratchet lever is empty.

For screws that are tightened with power wrenches, often a predetermined preload must be strictly adhered to, so that the screw on the one hand the associated bolt in a defined Way spans, on the other hand, but not spanned the bolt. In order to achieve a defined tension, it is not sufficient to measure the hydraulic pressure in a hydraulic power screwdriver and to end the screwing process when the hydraulic pressure reaches a limit value. When tightening screws, unforeseen obstacles may appear, such as hooking or jamming caused by missing threads or rust. The screw resistance is a suitable measure for achieving defined tightening conditions.

The invention is based on the object of specifying a method for controlling a screwing tool with which a high degree of accuracy and reproducibility of the screwing operation is achieved, so that the screwing operations performed with this method provide the security of the proper tightening of the screw.

• Durchführung drehender Hübe unter Messung des Drehmoments in einem Drehmomentmodus,
• bei Erreichen eines vorgegebenen Fügemoments: Durchführung eines Drehwinkelmodus durch Hochzählen des Drehwinkels bis zum Ende des laufenden Hubes und Speicherung der am Hubende erreichten Werte von Drehwinkel und Drehmoment,
• bei jedem nachfolgenden Hub: Fortsetzung des Hochzählens des Drehwinkels dann, wenn das Drehmoment einen Wert erreicht, der dem Drehmoment am Ende des vorhergehenden Hubs entspricht, und Speicherung der am Hubende erreichten Werte von Drehwinkel und Drehmoment,
• Beenden des Schraubvorganges, wenn der hochgezählte Drehwinkel einen Zielwinkel erreicht.

The solution of this object is achieved according to the invention with the features specified in claim 1. Thereafter, the screwing is carried out with the following steps:

• Performing rotating strokes while measuring the torque in a torque mode,
• upon reaching a predetermined joining torque: carrying out a rotation angle mode by counting the angle of rotation up to the end of the current stroke and storing the values of angle of rotation and torque reached at the stroke end,
• at each successive stroke: continue incrementing the angle of rotation when the torque reaches a value equal to the torque at the end of the previous stroke corresponds to, and storage of the values of angle of rotation and torque reached at the stroke end,
• Ending the screwing process when the increased rotation angle reaches a target angle.

In the method according to the invention, a torque mode is first carried out, in which the screw is tightened to a joining moment. The previously defined joining torque is dimensioned such that the parts to be joined have a certain hold, so that the screw connection is already fundamentally secured upon reaching the joining moment. Upon reaching the joining moment, the turning angle mode is entered, in which a certain predetermined rotation angle, which is referred to as the target angle, is swept over. The sweeping of the rotation angle is performed by counting increments of the rotation angle supplied from a rotation angle sensor.

To achieve the target angle several strokes of the screwing tool are required. With each stroke, torque and angle of rotation are increased and in the subsequent return stroke, the torque returns to zero. In the following stroke, the torque increases very quickly. According to the invention, the further counting of the angle of rotation takes place only when the same torque has been reached in a subsequent stroke, in which the previous stroke has ended. This torque at the end of a stroke is stored in a memory, as well as the accumulated until this time angle of rotation.

The method according to the invention enables secure control of the screwing operation. It is assumed that the joining torque in torque mode reproducible and with high accuracy can be determined. From reaching the joining moment is transferred to the rotation angle mode, in which an angle measurement is performed until reaching the target angle. The termination of the screwing thus takes place only as a function of the angle of rotation, which was covered after reaching the joining moment.

According to a preferred embodiment of the invention, it is provided that the rotation angle mode is only started when the joining moment is reached out of the movement. If the joining torque is achieved, for example, at the end of a stroke while the turning process has come to a complete or near standstill, there are no defined friction conditions at the screw connection. It may also be the case that by temporarily hooking or blocking the torque increases above the value of the joining torque, so that a random condition would be assumed for the beginning of the rotation angle mode. To avoid this, the achievement of the joining moment is assumed only if the screwing operation takes place in a linear region, and at a certain distance from the stroke end.

In a preferred variant of the method according to the invention, in the event that, after reaching the joining moment during a stroke, the count-up remains below a predetermined limit value, the achievement of the joining moment is not utilized and the utilization is postponed to the next stroke. This condition corresponds to the case that the joining moment is reached at the end of a stroke. In this case, the torque mode is maintained and it is performed after the next return stroke, a new stroke in the torque mode, in which then the joining torque is reached again. This second achievement of the joining moment is evaluated to form the zero point of the angle count.

The method according to the invention also makes it possible to determine and utilize the differential quotient of the relationship between torque and angle of rotation.

In a specific embodiment of the method, this differential quotient is determined and stored even before the joining moment has been reached. Based on the respectively measured torque and the stored differential quotient, it is predetermined whether the joining moment is reached at the stroke end. The torque indicates the actual state and the differential quotient allows an extrapolation, so that it can be predetermined whether the joining torque is reached at the stroke end. If this is the case, then the stroke is stopped before reaching the stroke end, so that the achievement of the joining moment is shifted to the next following stroke.

The differential quotient of the dependence between torque and angle of rotation can also be used for the control of the rotation angle mode, wherein the screwing operation is discarded if, during the counting up of the rotation angle, an out-of-tolerance deviation from the stored value is detected. In this way, anomalies can be detected, such as the blocking of a screw or too high a screw resistance. Such a condition occurs when the screwing tool is applied to a screw which has already been tightened. Even screws that are too smooth after reaching the joining moment, can be detected and discarded.

In an angular range before reaching the target angle, a narrower special tolerance range is expediently defined. This will ensure that the target angle is approached only with a differential quotient, which is close to the stored predetermined differential quotient. It is prevented that the target angle is reached with a jerk. If the differential quotient is outside the special tolerance range, the screwing process is discarded.

Within the scope of the invention, it is also possible to measure the duration of the individual strokes and to reject the screwing operation if the duration is too long. Thus, such screw are excluded in which irregularities are present.

In the following an embodiment of the invention will be explained in more detail with reference to the drawings.

Fig. 1

eine schematische Darstellung eines hydraulischen Kraftschraubers mit Drehmomentsensor und Drehwinkelsensor,

Fig. 2

einen schematischen Schnitt entlang der Linie II/II von Fig. 1,

Fig. 3

ein Diagramm des Drehmoments über dem Drehwinkel bei einem Schraubvorgang und

Fig. 4

eine Darstellung der Ermittlung des Differentialquotienten des linearen Astes eines Hubes.

Show it:

Fig. 1

a schematic representation of a hydraulic power wrench with torque sensor and rotation angle sensor,

Fig. 2

a schematic section along the line II / II of Fig. 1 .

Fig. 3

a diagram of the torque over the angle of rotation in a screwing and

Fig. 4

a representation of the determination of the differential quotient of the linear branch of a stroke.

In the Fig. 1 and 2 is ein.hydraulische power screwdriver shown. This has a drive part 10 and a functional part 11. The drive member includes a hydraulic cylinder in which a piston 12 is slidably guided. Of the Drive of the piston 12 takes place in the feed direction (acc. Fig. 1 to the left) and in the retraction direction (to the right) each hydraulically. A pivotable connection device 13 has a pressure port and a return port.

The functional part 11 has a housing 14 in which a ratchet lever 15 moves. The ratchet lever 15 is connected to the piston 12 via a piston rod 16. In a transverse bore of the housing 14, a shaft 17 is rotatably mounted. The shaft 17 has in the interior of the housing 14 has a circumferential toothing 18, in which engages a (not shown) toothing of the ratchet lever 15. At each stroke of the piston 12, the shaft 17 is rotated by a certain angle about its axis. Thereafter, the return stroke of the ratchet lever 15, in which the shaft 17 is not taken.

The shaft 17 has at one end a driving device in the form of a Einsteckausnehmung 21 of hexagonal cross-section. In a cavity 22 of the shaft 17 is a torsion sensor 23 in the form of the measuring strips, which are glued to the peripheral wall. The region of the shaft 17 carrying the torsion sensor 23 forms the measuring section 25.

At the rear end of the shaft 17, a data transmission element 28 is provided. From the torsion sensor 23, a cable channel 29 extends to the data transmission element 28. The data transmission element 28 is, for example, a slip ring assembly which connects an external cable 30 to the torsion sensor 23 which is rotatable with the shaft 17. Alternatively, the transmission can also be wireless. The cable 30 leads to a cable connection 31 (FIG. Fig. 1 ), which is provided on the housing 14 and to which a control unit can be connected.

The hydraulic power wrench is further equipped with a rotation angle measuring device 33. This has a code disc 34 which is fixed to the shaft 17, and an angle sensor 35 which responds to the bars of the code disc 34 and thereby generates rotational angle pulses. The angle sensor 35 consists of a fork-type photoelectric sensor into which projects the radially projecting from the shaft 17 code disk. From the angle sensor 25, a cable 38 leads to the cable terminal 31 so that both the torsion sensor 23 and the angle sensor 35 are electrically accessible at the cable terminal 31.

The signals of the torque sensor 23 and the rotation angle sensor 33 are fed to a (not shown) control unit, which in turn controls a valve which can interrupt the pressure supply in the hose connections 13. Moreover, the operation of the power wrench is controlled in such a way that the two hydraulic connections of the power wrench are alternately connected to a pressure line and a return line, wherein the reversing either mechanically effected by actuation of a reversing valve when the piston 12 has hit against the relevant stop and no further movement takes place, or by automatic change of direction.

Fig. 3 shows for a certain screw the dependence between the torque M _D and the rotation angle α. At the first stroke of the power wrench, first a non-linear increase 50 of the torque with respect to the rotation angle α and then, when the screw engages, a linear increase 51, in which the bolt is stretched. During the return stroke of the power wrench, the torque M _D in region 52 drops to zero, after which the second stroke then follows.

Until reaching a predetermined mating torque M _F , the screws in the torque mode D _M , so under measurement of the torque. When the torque has reached the value of the joining torque M _F , the mode enters the rotation angle mode DWM. At the time of reaching the joining moment, the angle of rotation α = 0 is defined, so that the subsequent counting up of the angle of rotation is in each case related to the angle of rotation at which the joining moment M _{F has been} reached.

The joining moment M _F is traversed in the movement, ie the change of the mode from DMM to DWM takes place without the stroke being interrupted. At the end of the relevant stroke, the torque reaches the value M _HE1 , which refers to the stroke end 1 after reaching the joining moment. In the next return stroke, the torque goes back to 0 and the third stroke is initially a non-linear increase 53 until reaching the torque M _HE1 and then joins a linear region 54, in which the screw is tightened further. At the end of each stroke, the value of the torque at the stroke end M _HE1 , M _HE2 and M _{HE3 is} stored, as well as the associated rotation angle α _HE1 , α _HE2 , α _HE3 . If at the next following stroke the torque has reached the same value as the torque end of the previous stroke, the further counting up of the rotation angle α starts. The angle α _HE1 , which was stored at the end of the second stroke, at the same time forms the initial angle α _HA2 , during which the count continues in the linear region 54 during the third stroke. At the end of the 3rd stroke the final value α _{HE2 is} stored and at the 4th stroke the incrementing of the angle starts with the value α _HA3 , which is equal to α _HE2 .

The screwing is terminated when a target angle α _{Z is} reached, for example, is set to 90 ° (after reaching the joining torque M _F ). Then the power screwdriver is switched off. The screw is now tightened in a defined manner, the desired voltage of the bolt is reached.

For the rotation angle detection, the condition exists that an incrementing of the rotation angle takes place only if the simultaneously measured moment has at least the height of the joining moment M _F. This ensures that the angle of rotation is basically only detected from the moment of joining.

Another condition is that the counting up of the angle of rotation takes place only when, in the previous stroke, an incrementing of the angle of rotation has taken place and the associated torque has been detected. A summation to the already stored rotation angle takes place only if a torque which was stored at the end of the last stroke minus a tolerance range, e.g. 5%, is achieved. The counting continues, however, only when the final torque of the last stroke is reached. In this way it is ensured that the summation of the angle of rotation takes place only when the nut rotates and not out of the state.

The same applies to the achievement of the mating moment M _F. The achievement of the joining moment should only be determined when the linear part of the tension line is traversed, in the middle region between the end points. If the joining moment is reached in the upper end of the linear range, a new determination of the achievement of the joining moment takes place. The transition from the torque mode DMM to the rotational angle mode DWM must be made from the rotary motion, ie not at the end a stroke. This is necessary to determine the zero point α = 0 defined with sufficient reproducibility. If only a small angular range, which is below a limit value of, for example, 2 °, is reached after reaching the joining moment, the detection of the joining moment is discarded and shifted to the next following stroke. Such operation is possible both with manual control of the power wrench and with automatic control.

In the case of automatic control, the following criterion is additionally or alternatively possible:

Even before reaching the joining moment, the differential quotient of the dependence between torque and angle of rotation is determined, ie the slope of the straight line. Based on the respectively measured torque and the differential quotient, it is predetermined whether the joining moment is reached at the stroke end. If it is detected that the joining moment is reached at the stroke end, the stroke is terminated prematurely by the automatic and a new stroke is initiated, at which the joining torque in the linear range is then reached.

Fig. 4 shows the determination of the differential quotient Q in the linear region of the curve M _D over α. The differential quotient, ie the slope is calculated to $$\mathrm{Q}\mathrm{=}\begin{array}{l}{\mathrm{M}}_{\mathrm{D}\mathrm{2}}\mathrm{-}{\mathrm{M}}_{\mathrm{D}\mathrm{1}}\\ {\mathrm{\alpha }}_{\mathrm{1}}\mathrm{-}{\mathrm{<figure-callout\; id="1"\; label="\alpha "\; filenames="imgf0002.png"\; state="\{\{state\}\}">\alpha </figure-callout>}}_{\mathrm{1}}\end{array}$$

Here, M _D1 means the torque measured at a certain rotation angle α ₁ after reaching the joining torque, and the torque M _D2 is the torque measured at a higher rotation angle α ₂ .

The differential quotient Q can also be used for other tests, for example, to check whether a screw is already tightened. In this case, the power screwdriver works at very high torque without any further rotation. Consequently, the differential quotient is out of tolerance. The screwing process is then canceled.

The differential quotient can also be evaluated immediately before reaching the target value. Here, a special tolerance range for the differential quotient is defined and the target value is considered to be reached only if the differential quotient was previously determined in the particular tolerance range. In this way it is avoided that the target angle is achieved by a sudden jerk.

Another possibility is to measure the duration of the individual strokes, wherein the screwing is discarded at too long a period of time. For example, it is possible to measure a number of durations of the individual strokes in a plurality of screwing operations for a particular screwing and then define an average stroke duration, which is stored. Likewise, for the differential quotient Q, a typical value may be averaged or otherwise determined from numerous previously measured values.

Claims (7)

1. A method of controlling a screw tool which carries out intermittent rotating strokes and includes a torque sensor (23) and a rotation angle sensor (33), comprising the steps of:

   - carrying-out strokes while measuring the torque (M_D) in a torque mode (DMM), and

   - upon attaining a predetermined pre-torque (M_F): transitioning to a rotation angle mode (DWM) and thereby counting-up the rotation angle (α) up to the end of the current stroke, and storing the rotation angle (α_HE1) and torque (M_HE1) values attained at the end of the stroke,

   characterized in that

   - during each subsequent stroke: continuing the counting-up of the rotation angle (α) when the torque (M_D) has reached a value corresponding to the torque (M_HE) at the end of the previous stroke, and storing the rotation angle (α_HE) and torque (M_HE) values attained at the end of the stroke, and

   - terminating the screw process when the counted-up rotation angle has reached a target angle (α_Z).
2. The method according to claim 1, characterized in that the rotation angle mode (DWM) is started only when the pre-torque (M_F) is obtained during the movement.
3. The method according to claim 1 or 2, characterized in that if after attainment of the pre-torque (M_F) during a stroke the counted-up value remains below a predetermined limit value, the attained pre-torque (M_F) is not utilized and the utilization of the pre-torque (M_F) is shifted to the next stroke.
4. The method according to any one of claims 1-3, characterized in that before attaining the pre-torque (M_F) the differential quotient (Q) of the interdependence of torque (M_D) and rotation angle (α) is determined and stored, and on the basis of the respective measured torque and the differential quotient (Q) a determination is made whether the pre-torque (M_F) will be reached at the end of the stroke.
5. The method according to any one of claims 1-4, characterized in that the differential quotient (Q) of the interdependence of torque (M_D) and rotation angle (α) is determined and stored, and the screw process is discarded when during counting-up of the rotation angle (α) an out-of-tolerance deviation of the differential quotient (Q) from the stored value is detected.
6. The method according to claim 5, characterized in that a narrower special tolerance range is defined in an angle range before attaining the target angle (α_z) with the screw process being discarded when the special tolerance range is exceeded.
7. The method according to any one of claims 1-6, characterized in that the duration of the individual strokes is measured, and the screw process is discarded when the duration is too long.

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