DE3710512A1 - SCREW METHOD AND SCREWDRIVER FOR THE AUTOMATIC TIGHTENING OF SCREWS AND / OR NUTS - Google Patents

Description

State of the art

The invention is based on a screwing method the genus of the main claim and a screwdriver the genus of claim 9. Known screwdriver this Kind have a screw spindle with a - possibly integrated - sensor unit, a drive unit with a motor and with a power unit as well as a Control unit on. The one related to the sensor unit of the torque, the speed and / or the screw deep information is delivered in the control unit  processed which for the given Tightening process required target signal, such as the target turning number, supplies. It is also from the so-called two stage fen-screw method known the speed of the screw spindle on reaching the application torque at which the Screw head or the nut touches the base, to reduce from a high value to a low value adorn to the screwdriver when the target torque is reached to be able to brake quickly.

The accuracy of the achieved when screwing Tightening torques are determined by the braking time of the screwdriver determined after reaching the target torque. Therefore the speed in time after reaching the landing moments can be reduced. However, the speed may not be withdrawn too early so the clock times are not extended unnecessarily. Disadvantage of this Screwdriver is that the switch point from case to case determined and set by an operator must be, with a hard screwdriving with steep increase in momentum for that as a switchover point serving application torque a small value is selected.

Advantages of the invention

The screwing method according to the invention with the main has features marked against it the advantage that the adaptation to the respective Screwdriving takes place automatically. This happens there by that the screwdriver has a control loop that regulates the screw speed continuously. Disturbances such as the speed that occurs during the screwing process dependent friction are thereby in the regulation of the Screwing process included. Particularly advantageous  it is that both measured quantities as control parameters like torque, angle of rotation or screw depth as well derived sizes such as B. the gradient of one Screw can be selected.

By the measures listed in the subclaims are advantageous further developments and improvements the screwing method specified in the main claim possible. It is particularly advantageous that, as a rule circle parameters, such as B. the braking time of the screwdriver can be entered by which the rule characteristic can be changed.

Screwdrivers are used to carry out the screwdriving process proposed whose control unit the torque that Angle of rotation and / or the screw depth or derivatives of these sizes according to time or angle as a rule Recycle parameters.

drawing

Three embodiments of the invention are in the Drawing shown and in the description below exercise explained in more detail. Show it:

Figure 1 is a circuit diagram of a first game Ausführungsbei the control circuit of a screwdriver.

Figure 2 shows the course of the torque over time when tightening a screw or nut.

3 shows the course of the rotation speed of the nutrunner during the braking phase.

Fig. 4 curves a family of curves of different torque;

Fig. 5 is a circuit diagram of a second game Ausführungsbei a control loop and

Fig. 6 is a circuit diagram of a third exemplary embodiment of a control loop.

Description of the embodiments

Fig. 1 shows a control circuit as the first embodiment, in which as the control parameter M _to the target torque has been selected, to be tightened with a screw or a nut. The moment given by the drive 1 of the screwdriver, not shown here, is denoted by M.

The difference between the target torque M is applied to a first comparison or addition point 3 _{is intended} and, therefore formed the actual torque to the currently delivered torque M, the torque M is applied to the junction 3 via the connection. 4

In a differentiator5 becomes the actual moment M the derivation after the time dM/ dt=  won. The reciprocal of this derivative is in a he most multiplier7 with the differenceM _should -M  multiplied. A weighting can also be pre-selected here be taken. The result of this multiplication is in a second multiplier9 with the factor k multiplied and thus the target speedω _should  won nen. On the factork will be discussed below.

Instead of the derivative according to time, the derivative of the moment M according to the angle can alternatively be obtained as d M / d α in a differentiation element 6 and the reciprocal of this derivative can be fed to the multiplication element 7 . This is shown in dashed lines in Fig. 1 Darge.

Both the value of the target speed ω _should and the speed currently output by the drive 1 of the screwdriver ω are fed to a second comparison or addition point 11 , the value of l being inverted. Thus, it is the difference between ω _to ω and formed and this value to a controller 13 supplied to the actuator. 1 The reference junction 11 , the controller 13 and the drive 1 form the drive unit 14 of the screwdriver which is shown in broken lines in FIG. 1.

With 15 the controlled system is designated, which forms the behavior of the mechanical components of the screwdriver. A screw is tightened with the angle of rotation α . The angle-dependent load torque, which is dependent on the respective screw, is multiplied in the control process by the factor k _s , which will be discussed below. This is represented by the multiplier 17 .

A torque-controlled screwing method is to be explained on the basis of the control loop in FIG. 1.

The speed ω of the screwdriver is calculated from the increase in torque with respect to time, i.e. from d M / d t , or from the increase in torque with respect to the angle of rotation, i.e. from d M / d α , and from the difference between the predetermined target torque M _soll derived and the actual torque M. Additional parameters also affect the speed ω , so that the following general formula for ω can be given:

Here, the factor k is used as a parameter, which is referred to as the braking time constant and is explained on the basis of the diagram in FIG. 3: The diagram shows that the speed ω of the screwdriver cannot drop to zero immediately after switching off. Rather, this value is only reached after a braking time T _B due to the sluggishness of the screwdriver. It can be assumed that the decrease in speed with respect to time is approximately constant for each individual screwdriver and can be determined using a switch-off or braking attempt. The following relationship applies to the braking time

With

It is readily apparent that the braking time T _{B increases} with increasing speed.

So while the constant or factor k depends on the size and structure of the screwdriver, the factor k _{s is determined} by the yield point of the screw to be fastened. It is assumed here that the following equation applies:

So while tightening a screw before the yield limit, the torque required to tighten the screw per angular unit increases, when the yield limit is reached, it can be seen that the torque no longer increases even if the screw is turned further. In the present embodiment, it is assumed in equation (3) that the torque change per angular unit is constant and has not yet assumed the value zero. I.e. So, the specified tightening torque M _{Soll of} the screws is not so large that the yield point is reached.

In general, the torque increase with

designated. From this equation yields the time T _M that passes to be paid starting in currently constant rotation from the actual torque M _is the value of the target torque M _to achieve. The following equation results for linearized evaluation:

Since, due to the inertia of the screwdriver, a standstill after the drive 1 has been switched off can only be reached after the braking time T _{B has} elapsed, the following conditions must be met:

T _B ≦ T _M (6)

To ensure that the specified tightening torque M _should not exceed the screw or nut, the screwdriver must be switched off in time so that it does not tighten the screw too tightly during the braking time after the drive 1 has been switched off.

The condition set out in equation (6) then becomes fulfilled if follow for the speed of the screwdriver The equation applies:

In order to avoid a division by zero in equation (7), M must not fall below a predetermined value. The following therefore applies:

It is assumed in the following that the speed control that can be achieved with the control circuit according to FIG. 1 can be understood as a proportional element with respect to the dynamics of the screwdriver.

Under the condition

ω = ω _should (9)

and provided that Equation (3) for the closed circuit in FIG. 1 gives the following equation for the torque M acting on the screw or nut to be tightened by the screwdriver or drive 1 :

The condition set out in equation (8) also applies here supply.

To avoid numerical problems is in the final stages the screw connection instead of a variable value For  a fixed value _min  given.

Otherwise applies to  the following equation:

where for c equation (12) applies:

c = k · k _s (12)

 follows from this

By separating the variables, substitution and an closing integration one gets the following equation:

M = M _target - k _o ² + 2 k _o t - ct ² (14)

In the following, k _{o is determined} under the following conditions: t = 0; M = M _o , where M _{o is} the tightening torque at which the controlled screwing process starts. Equation (14) results in

wherein for c in turn, equation (12). It follows from this that with the aid of the braking time constant k , which was derived from FIG. 3, the control characteristic of the control circuit in FIG. 1 can be changed. A change in the factor k results in the family of curves shown in FIG. 4 of the various torque curves.

After all, it can be seen that by changing the braking time constant or the factor k in the second multiplier 9 of the control circuit in FIG. 1, the user can choose a steeper or flatter momentum curve after the application torque M _{o has been reached} . In any case, however, it is ensured that the tuning of the screwing on the serving as the switching point de application torque M _{o is} automatically guaranteed.

The exemplary embodiment shown in FIG. 1 was described on the basis of the constant k determined by the braking behavior of the screwdriver and on the basis of the constant k _s determined by the yield point of the screws to be tightened. However, additional parameters such as minimum or maximum speeds can also be entered into the control loop without any problems, in order to ensure timely braking, for example in the case of hard screwdriving, or to apply the necessary torque. Limit values can also be specified for the measured values, when they are switched off. If, for example, the torque increases too quickly, it is concluded that the screw is seizing and the screwdriver is switched off.

If in the torque curve z. B. due to mechanical Fault in the gearbox or the swan drive motor occur, there are two different regulations ways to differentiate:

In the instantaneous value control, an attempt _{is made to} maintain the instantaneous torque, the actual torque M , or to set it again.

In the peak value control, the torque output is adjusted to the peak value M _{sp of} the instantaneous torque which occurs in the event of fluctuations, so that the following relationship results:

M _sp = max (M _ist ) (16)

At the inFig. 1 described embodiment the derivative of the torque becomes the control parameter after the time  used. However, it can also second and higher derivatives over time as a rule parameters are used. In addition to the first derivative of the moment after the angle also higher Derivatives are used. The one in the derivation obtained values, especially the values of higher derivatives are preferably smoothed.  

Fig. 5 shows a second embodiment of a screwdriver, whose control unit as control parameter the rotation angle α of the screw or nut or the derivative of the rotation angle α with respect to time used. In FIGS. 1 and 5, corresponding parts are provided with identical reference numerals. Their description is omitted here.

The angle counting starts at a pre-selectable starting point, e.g. B. when reaching the application torque M _o or when reaching a certain screw depth L _o . In the embodiment shown in FIG. 5, the starting point is determined by the application torque M _o . A sensor 19 is used to detect the torque or application torque M _o , which emits a start signal to a counter 21 when the application torque M _{o is reached} . After the occurrence of a start signal, this counts the angle of rotation α with which a screw or nut is turned. The angle of rotation determined by the counter is passed on the one hand to the comparison point 3 , at which the difference between a predetermined angle of rotation a _should and the determined angle of rotation is formed, and on the other hand to a differentiation member 5 , which is the derivative of the angle over time d α / d t = α forms. This derivative is fed to the multiplication element 7 , which multiplies the difference obtained at the comparison point 3 by the reciprocal value of the derivative of the angle over time, that is to say by 1 / α . The same result can be achieved with a division member that divides this difference by deriving the angle.

Both in multiplication and in a divi a weighting factor can be taken into account.

As in the first exemplary embodiment, the result of the first multiplication is multiplied in a further multiplication element 9 by the factor k, and the setpoint speed ω setpoint _is thus obtained. Drive unit 14 , the controlled system 15 and the multiplication element 17 correspond to those of the exemplary embodiment according to FIG. 1, so that their description can be dispensed with here.

In this embodiment, therefore, is of from a specific starting point, which is here by the application moment M _o determines the rotation angle α of the screw or nut, starting from the value α = 0 counted and the set value A _is compared. Ω _to the target rotational speed is continuously regulated and finally reduced to the value 0 when the desired angle of rotation a _{is intended} is reached. To avoid numerical problems, it is assumed that when multiplying by the factor 1 / α, the value of α does not fall below a minimum value.

Fig. 6 shows an embodiment for a screw depth-controlled screwing process. In Figs. 1, 5 and 6, corresponding elements are designated by identical reference numerals, their description is omitted here.

In FIG. 6, an addition or junction 3 is shown, at which the difference between a determined by a depth sensor 23 screw depth L and a predetermined screw depth L _{is to} be formed. From the screw depth L , a differentiation member 5 forms the derivative according to the time d L / d t ge. Instead of this, the derivation of the screw depth according to the angle of rotation d L / d α can also be formed in a differentiation element 6 , which is shown in dashed lines in FIG. 6.

The difference obtained at the comparison point 3 is multiplied in the multiplier 7 by the reciprocal of the derivation of the screwing depth over time. By a further multiplication in the multiplication member 9 by the factor k , the deceleration time constant, the target speed ω setpoint _is obtained, which is input to the drive unit 14 . For the Antriebsein unit 14 , the controlled system 15 and the multiplier 17 , the above applies to Fig. 1.

From Fig. 6 that the target speed is continu ously controlled and lowered to the value 0 when the screw amount measured _to the value L reaches arises. Here too, in order to avoid numerical problems, it is assumed that the derivation of the screw depth does not fall below a minimum value over time.

It is thus shown that various screwing methods can be carried out by using control loops with disturbance variables. In addition to the drehmomentgeregel th bolting, in which the desired target rotational moment M is given _to, including rotational angle and depth-controlled tightening processes can be realized in which the desired angle of rotation α _to or a certain screw amount L is set _to. In any case, the speed ω of the drive unit is continuously adapted to the currently measured values of the torque, the angle of rotation or the screw depth.

Claims (11)

1. Screwing method for automatically tightening screws and / or nuts, characterized in that a difference signal is formed from a predetermined or predeterminable and a detected value and is used as a measure of the speed. 2. Screwing method according to claim 1, characterized characterized in that the difference signal is derived by Sizes of the measured value is influenced. 3. Screwing method according to claim 1, characterized characterized in that the derived quantities are the change in the Measured variable used after the time or after the angle of rotation becomes. 4. Screwing method according to claim 1 or 3, characterized characterized in that the derived quantities are the second  Derivation and / or higher degree derivatives used will. 5. Screwing method according to one of claims 2 to 4, characterized in that derived sizes with be communicated. 6. Screwing method according to one of claims 1 to 5, characterized in that the upper speed in Dependency on the tightening case is specified. 7. Screwing method according to one of claims 1 to 6, characterized in that the control unit Para meters to design the screwing process will. 8. Screwing method according to one of claims 1 to 7, characterized in that limit values for the measured values values are specified for which the screwdriving process is interrupted. 9. screwdriver for the automatic tightening of screws and / or nuts with a drive unit, a control unit for determining the speed for the drive unit and with means for detecting the torque and / or the angle of rotation and / or the screwing depth for carrying out the screwing method according to one of the Claims 1 to 8, characterized in that the control unit has a comparison point ( 3 ) for forming the difference between the desired tightening torque (M _soll ) and the measured torque (M) , a difference element ( 5 ) for deriving the torque over time ( d M / d t) , a first multiplication element ( 7 ) for multiplying the difference (M _soll - M) , with the reciprocal of the derivative of the torque and a second multiplication element ( 9 ) for multiplying the product by a parameter to form the Has target speed for the drive unit ( 1 , 13 ). 10. Screwdriver for automatically tightening screws and / or nuts with a drive unit, a control unit for determining the speed for the drive unit and with means for detecting the torque and / or the angle of rotation and / or the screw depth, characterized in that the Control unit has the following elements: a comparison point ( 3 ) for forming the difference between the desired angle of rotation and the determined angle of rotation, a differentiator ( 5 ) for deriving the angle of rotation over time, a first multiplier ( 7 ) for multiplying the difference by the reciprocal deriving the angle of rotation over time and a second multiplier ( 9 ) for multiplying the product by a parameter for forming the target speed for the drive unit ( 1,13 ). 11. Screwdriver for the automatic tightening of screws and / or nuts with a drive unit, a control unit for determining the speed for the drive unit and with means for detecting the torque and / or the angle of rotation and / or the screw depth, for carrying out the screwing process one of claims 1 to 8, characterized in that the control unit has a comparison point ( 3 ) for forming the difference between the desired screwing depth and the screwing depth measured by means of a depth sensor ( 23 ), a differentiating element ( 5 ) for deriving the screwing depth over time , a first multiplication element ( 7 ) for multiplying the difference by deriving the measured screw depth over time and a second multiplication element ( 9 ) for multiplying the product by a parameter for forming the target speed for the drive unit ( 1,13 ).