EP0305754A1 - Method for controlling a screwdriver - Google Patents

Abstract

The method for controlling a screwdriver for the automatic tightening of screws and/or nuts is characterised in that a screw descent analysis is carried out, in which the course of the torque required for tightening a screw or a nut is determined during a yield point-controlled screwing process. The desired tightening torque, inter alia, for the screw is fed in by the user and the appropriate screwing method is determined and set automatically therefrom. <IMAGE>

Description

State of the art

The invention is based on a method for controlling a screwdriver for automatically tightening screws and / or nuts, and on a control for a screwdriver of the type mentioned according to the preamble of claim 8. Such a method and a control of this type are known. In the so-called two-stage screwing process, a screw or nut is screwed in or screwed on at a high speed. The actual tightening then takes place at a lower speed. The switch from high to a lower speed takes place when the application torque is reached, which is achieved when the screw head or the nut touches the surface. The screwing process ends when the switch-off criterion is reached, i.e. when the desired one Tightening torque or a certain tightening angle is reached. The user must set the switchover time for each screwdriving application, the switchover torque being small for a hard screwdriving application with a steep torque increase and larger for a soft screwdriving application with a flat torque increase. Errors in the setting lead to deviations from the desired tightening torque and also to the screws being torn off.

Advantages of the invention

The method according to the invention with the features characterized in the main claim has the advantage that the switching torque for switching the high speed to a lower speed is automatically determined in the two-stage screwing method. In this respect, errors due to the user entering incorrect data are excluded. The data required to control the screwdriver are obtained using a screwdriving case analysis, i.e. at a certain, low speed, at which the screw is prevented from being torn off, a screw or nut is achieved by means of a yield point-controlled screwing process until the yield or yield point is reached tightened and determined the necessary torque. In this case, the torque applied when the screw or the nut comes into contact with the base is preferably determined and stored as a first torque value, the torque prevailing when the yield point is reached as a second torque value and the associated times. The control according to the invention has the advantage that the screw case analysis can be evaluated immediately, in that the quantities obtained can be stored in a memory of the control and are available for later display or a comparison with a desired tightening torque. A control is particularly preferred which, after the desired tightening values have been entered, is automatically adapted to the present screwing case.

The measures listed in the dependent claims are advantageous developments and improvements of the method and the control specified in the main claim. Claim 8 possible. It is particularly advantageous that the method is also suitable for specifying the desired angles of rotation and is therefore very versatile.

drawings

• Figur 1 ein Diagramm des Drehmomentenverlaufs über der Zeit während des für die Schraubfallanalyse durchge­führten Schraubvorgangs,
• Figur 2 eine Tabelle für die Schraubfallanalyse nach Figur 1,
• Figur 3 ein Diagramm eines Drehmoments über der Zeit für eine beispielshaft ausgeführte Schraubfallanalyse und
• Figur 4 eine aus der Schraubfallanalyse gem. Figur 3 gewonnene Tabelle.

An embodiment of the invention is explained in more detail with reference to the drawing and the following description. Show it:

• FIG. 1 shows a diagram of the torque curve over time during the tightening process carried out for the tightening case analysis,
• FIG. 2 shows a table for the screwdriving case analysis according to FIG. 1,
• FIG. 3 shows a diagram of a torque over time for an example screw connection analysis and
• Figure 4 a from the screwdriving case analysis acc. Figure 3 obtained table.

Description of the embodiment

FIG. 1 shows a diagram of the torque applied to a screw or nut during a test run for a screwdriving case analysis. The test run is carried out at a low speed, at which the screw is certainly prevented from being torn off.

The torque required to screw in the screw increases only slightly during the screwing in. It is determined by the friction of the threads.

If the screw head or the nut touches the base at the time T _min , the torque reaches the application torque with the value M _min , which is clearly above the torque values during the screwing-in phase; the screwing process now goes into the tightening process. A constant, predetermined value or a value calculated from the torque curve in FIG. 1 can be selected as the application torque M _min .

If the torque does not increase even when the screw is tightened further, the yield point or yield point of the screw has been reached. The maximum achievable torque M _max is then present.

The course of the torque over time is detected by suitable sensors; whose signals are sent to a memory of the control of the screwdriver and stored there. Likewise, the angle of rotation and / or the screw depth over time can be recorded and stored using appropriately designed sensors.

The screw is tightened even further after the screwdriver has been switched off, because it will brake during braking phase continues due to its inertia. The braking time T _{B of} the screwdriver depends on the system. It is assumed that T _{B is} proportional to the speed. It therefore applies

T _B = kf (1),

where f is the speed, measured here in Hz, and k is a constant, the value of which is available or is determined using an idle test before the test run.

At a given speed f *, the deceleration time results from the following equation:

T _{Bf *} = k * f * (2).

T _{Bf *} is the time it takes for the screwdriver to come to a standstill at a speed f * after the drive has been switched off.

In a two-stage screwing process, there is a high speed f 1 during the screwing-in phase and a low speed f 2 during the tightening phase.

The time passes until braking from the high speed f 1 to a standstill

T _Bf1 = k · f₁ (3).

This time is transformed as a reference point for the scraping process to the speed used in the diagram:

From the time T _min at which the application torque M _{min was} reached and the deceleration time constant T _f1 according to Equation (4) uses the diagram shown in FIG. 1 to determine a torque limit value M _Gr , which is given at the time t = T _min + T _f1 .

In addition, a value M _{p is} calculated from the maximum torque M _max , for which with

0 <p <1 (5)

the following equation applies:

M _p = PMM _max (6).

For example, p = 0.8 is selected. These values are also stored in the controller's memory.

From the value of p, the moment M _p can be calculated, from which the yield point method must be selected as the screwing method for reasons of screw connection. If a screw is to be tightened with a torque M whose value is greater than M _p , the yield point method must be selected as the screw method. If the desired tightening torque M of a screw is below the value M _p , the torque or angle of rotation method can also be used instead. In the rotation angle method, for example, the angle counting begins when the application torque M _{min is reached} at the time T _min .

The values obtained from the screwdriving case analysis are shown in the table in accordance with Figure 2 used.

It can be seen from FIG. 2 that the screwing method is determined by the value M _p : if the desired tightening torque M of the screw is in the range M _p ≦ M ≦ M _max , the yield point method can be used as the screwing method; for all values M _min ≦ M ≦ M _p , the torque or angle of rotation method must be selected.

The switchover from the higher speed f₁ to the lower speed f₂ is the determined from the diagram according to Figure 1 torque limit M _Gr determined: Applies for the desired torque M a screw the relationship M ≧ M _Gr, then the speed switching point is determined by the during the Torque applied to the screw determined. If a screw is to be tightened with a torque M that is less than the limit torque M _Gr , the screwing process must be controlled by the screwing depth L, because according to the inertia of the screwdriver or the braking time T _f1 . Equation (4) after reaching the torque M _min clearly above the screwing torque even when switching the screwdriver at time T _min to the lower speed f 2 a torque lying above the desired tightening torque, namely M _{Gr is} reached.

The table also shows that the type of screwing method can also be determined from the maximum angle of rotation ω _{max by the} value ω _p which can be calculated from the equation (6).

The table shown in Figure 2 was drawn up without taking tolerances into account. In practice, the calculated braking time T _f1 should therefore be replaced by a somewhat larger value.

After the screwdriving case analysis for the two-step screwdriving process, the user can enter or change the following parameters in the screwdriver control unit:
high speed f₁
low speed f₂

maximum torque M _max
minimum torque M _min
Start of angle counting at M₁ = M _min
(in Hz or in rpm)

Alternative values are entered as the specified torque or angle of rotation values:
Torque M _should
Angle of rotation ωsoll

In connection with the table, these values result in Figure 2 shows the screwing process used.

The speed changeover time is determined by the following values

Torque M _o
Depth L _o

The values of M _o and L _o are calculated for the screwdriver in such a way that the screwing process is carried out in a time-optimized manner. Ie the speed is reduced as late as possible from the high speed f₁ to the lower speed f₂. In the calculation of the Drehzahlumschaltzeitpunkts Abbremszeitkonstante of the screwdriver is taken into account that, after braking from the higher speed immediately the desired tightening torque of the screw _to M is reached.

In addition, the following monitoring values can be entered to monitor the tightening process:
Torque
M _OG *
M _UG *
Angle of rotation
ω _floor *
ω _UG *
depth
L _OG *
L _UG *.

The user can change the values marked with *. The remaining values are obtained from the screwdriving case analysis.

Two cases can be defined for the speed changeover time:

T _set - T ≧ T _{min f1} (7)

the following torque value results for a changeover by torque

M _o = M (T _target - T _f1 ) (8)

and for

T _target - T _f1 <T _min (9)

the following screw depth value results for a switchover by screw depth:

L _o = L (T _should ¡- T _f1 ) (10).

If the higher speed f₁ is changed, the braking time T _f1 will automatically acc. Equation (4) changed accordingly and the result of the tightening case analysis updated.

The angle count does not necessarily have to start at the application torque M _min , instead a higher value M 1 ≧ M _{min can be selected} as the start of the rotation angle count, the other angle values being adapted accordingly.

In the present case, the screwdriver is provided with a display that shows the values determined during the screwdriving case analysis and possibly also the values specified by the user.

If it emerges from the values determined or calculated in the screwdriving case analysis that the speed changeover is determined by the screwing depth for a desired tightening torque of the screw or nut, the screwdriver must be equipped with a screwing depth sensor to carry out the desired screwing. If this is not the case, the display on the control system indicates that the desired screw connection cannot be carried out in this way.

In addition, it is calculated whether the desired screwing case can be realized by reducing the higher speed f 1 and the speed switching instead of using the screw depth by means of the torque.

= T_soll - T_min      (11)
A reduced speed f₁ is calculated as follows:

T _f

= T _target - T _min (11)

und aus Gleichung (1) die Abbremszeit

T_f

= k· f

.      (13)
The relationship results from equation (4)

and the deceleration time from equation (1)

T _f

= k · f

. (13)

:

This finally results in the following equation for f

:

If this calculation results in a speed value that makes sense for a screw connection, then this calculated speed is indicated on the display panel.

FIG. 3 shows a diagram of a torque curve obtained over time in a screwdriving case analysis. Besides the well known of Figure 1 values of the feed torque M _min, the torque limit M _Gr, the maximum torque M _max and the value calculated from equation (6) the value M _p and the switching time T _o is located, as the target torque M _soll.

M_Gr = M(T_min + T_f1) = 40 Nm
M_p = p· M_max = 0,8· 60 Nm = 48 Nm.Some numerical examples are given below:
M _max = 60 Nm
M _min = 2 Nm
T _max = 1200 ms
T _min = 900 ms
f₁ = 200 Hz
f₂ = 50 Hz

M _Gr = M (T _min + T _f1 ) = 40 Nm
M _p = PMM _max = 0.860 Nm = 48 Nm.

The table in FIG. 4 shows the screwing method and the speed changeover point that results for each screwing case.

The yield point screw method results for torque values in the range 48 ≦ M ≦ 60 Nm or for angles of rotation of 40 ≦ ω ≦ 50 °. The speed changeover point is determined by the torque.

If the screw is to be tightened with a torque M in the range 40 ≦ M ≦ 48 Nm or with a rotation angle ω of 33 ≦ ω ≦ 40 °, the torque or Angle screw method application. The speed changeover point is in turn determined by the torque.

With a desired tightening torque M of the screw in the range 2 ≦ M ≦ 40 Nm or an angle of rotation ω of 0 ≦ ω ≦ 33 °, the torque or angle of rotation screw method is also used. Here, however, the speed changeover time is determined by the screw depth L.

The data determined during the screwdriving case analysis are fed to a memory of the screwdriver and stored there. This data is displayed by means of the display device connected to the memory, for example on a liquid crystal display panel.

After the screwdriving case analysis, the user enters the following values:

high speed f₁ = 200 Hz
lower speed f₂ = 50 Hz

maximum torque M _max = 60 Nm
minimum torque M _min = 2 Nm

Start of angle counting at M₁ = 2 Nm.

The screw should _be tightened with the following torque M:
M _nominal = 30 Nm.

It can be seen from the table or from the display of the control unit that a torque-controlled screwing process is initiated automatically, in which the speed changeover point is determined by the screwing depth.

The following monitoring values are also entered
Torque
M _OG = 31.5 Nm (± 5% tolerance)
M _UG = 28.5 Nm
Angle of rotation
_ωOG = 28 ° (± 10% tolerance)
_ωUG = 22 °.

It can thus be seen that the screwing method can only be carried out if the screwdriver is equipped with a sensor for detecting the screwing depth. If this is not the case, the speed would have to be reduced to f'₁ = 167 Hz. In this case there is a braking time of the screwdriver which is so short that the desired torque M Soll _is not exceeded even with a torque-controlled screwing process.

To evaluate the data obtained during the analysis and the data entered by the user, comparison stages can be provided in the control of the screwdriver, which compare the stored with the specified torque or Compare the angle of rotation values and use them to automatically switch the control of the screwdriver. Switching commands can also be given to the user on the display, who then changes the control of the screwdriver by entering the displayed data.

Claims (11)

1. A method for controlling a screwdriver for the automatic tightening of screws and / or nuts, characterized by a screwdriving case analysis, in which the course of the torque required for tightening a screw or a nut is determined during a yield point-controlled screwdriving process. 2. The method according to claim 1, characterized in that a first torque value (M _min ) when the screw or nut is applied to the base and a second torque value (M _max ) when the yield point of the screw is reached are determined and stored. 3. The method according to claim 1 or 2, characterized in that the time (T _min ) when reaching the first torque value (M _min ) and the time (T _max ) when the second torque value (M _max ) is reached and determined. 4. The method according to any one of claims 1 to 3, characterized in that by means of a known or determined from a test run deceleration time constant (T _f1 ) a torque limit value (M _Gr ) is determined and stored, which is given at the time (T _min + T _f1 ) is. 5. The method of claims 1 to 4, characterized in that a third torque value (M _p) is calculated and stored from the second torque value (M _max) which is smaller than the second torque value (M _max) is for one. 6. The method according to claim 1, characterized in that a first given at the yield point rotation angle value (ω _max ) is stored. 7. The method according to claim 6, characterized in that a second angle of rotation value (ω _p ), which is smaller than the first angle of rotation value (ω _max ), is calculated and stored. 8. Control of a screwdriver for the automatic tightening of screws and / or nuts, characterized by a memory in which the screwdriver analysis according to. one of claims 1 to 7 determined values can be stored. 9. Control according to claim 8, characterized in that the desired torque for tightening the screw or nut can be entered and then the screwing method suitable for this torque is automatically set. 10. Control according to claim 8, characterized in that the desired angle of rotation value for tightening the screw can be entered and then the screwing method suitable for this angle of rotation is automatically set. 11. Control according to one of claims 8 to 10, characterized by a display device for displaying the values obtained during the screwdriving case analysis and / or the entered values.

Images