MXPA96002705A - Method to tighten a screw with a timeopt - Google Patents

Abstract

The present invention relates to a method for tightening a screw at an optimum time, comprising the steps of: setting at least an initial speed of a motor, a target torque and a tolerable error of a final torque relative to the torque Objective: to produce a plurality of torsion velocity curves based on the initial velocity, target torque and tolerable error, select an arbitrary curve from a predetermined plurality of torsion velocity curves, operate a motor at speeds obtained from the selected curve, monitor the torques produced while the motor is being operated, determine a final curve over which the target torque is reached in a minimum of time, and perform a screw tightening operation using the curve over which the target torsion moment was reached

Description

METHOD TO TIGHTEN A SCREW WITH AN OPTIMUM TIME D E S C R I P C L I N.

FIELD OF THE INVENTION: The present invention relates to a method for tightening a screw or nut with a predetermined maximum torque (torque) by the use of a mechanical nutrunner.

Prior art: When tightening a screw by the use of a mechanical nutrunner, an over-tightening results if the speed of the nutrunner is set to zero when the torque has reached a predetermined value. This is due to the inertia of the motor and the delay time before the speed control to the motor arrives. In order to prevent excessive torsion from being applied to the screw, the motor speed should be as close as possible to zero precisely before the torque is applied to the screw to achieve a desired value. However, decreased speeds result in an increased time required to tighten the screw and therefore a decreasing speed is not practical. Setting the speed close to zero can cause the motor to stop before the torque reaches the desired value, if the friction is relatively large. Conventionally, taking into account the data and experiences of the past, the programmer writes a control program that is controllably triggers the stepper step by step. Writing a program requires considerable experience and skill. Most users can not write that program and an experienced programmer should spend considerable time writing the control program.

SUMMARY OF THE INVENTION.

The present invention is made in view of the aforementioned problems. An object of the invention is to provide a method for tightening a screw at an optimum time, which method does not require an additional program to control the speed once the optimal internal parameters have been determined through several learning cycles.

In the present invention, first a desired twist is pre-set and the mechanical nutrunner is subjected to several recognition action cycles so that the nutrunner automatically determines those internal parameters in order to stop the engine immediately before the moment arrives. torsion, requiring a minimum of time to tighten the screw.

BRIEF DESCRIPTION OF THE DRAWINGS.

Other objects and features of the present invention will become more apparent from the description of the preferred physical embodiments, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of an apparatus of the invention for tightening screws with a constant torque (moment).

Figure 2 is a flow chart showing the operation of the apparatus of Figure 1.

Figure 3 shows simple curves of torsion against rotational speed S.

Figure 4A shows the curve P8 of figure 3 and figure 4B shows the cave P2 of figure 3.

Figure 5 shows the torque / speed table produced according to the curve P (s) shown in figure 3; and Figure 6 illustrates the changes in torsion, showing less twisting after a desired twist is achieved.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.

The principle of an apparatus for tightening a screw with an optimum time in accordance with the present invention will be described in detail below. The kinetic energy of the body of rotation, in general, is given by: F = I dx / dt + dX ... (1) where F is an input (desired speed), I is a moment of inertia, D is a decay constant, x is an angular velocity and t is time.

Equation (1) implies that after the desired velocity F is changed in steps, the operation condition of the nutrunner is determined by the variables I, D and x, with x being the angular velocity immediately after the desired velocity F changes . The torque is proportional to the integration of x with respect to the time after the nut driver has reached the work (load) to tighten the screw, ie, after the torque has been generated.

If the nutrunner is operated so that the nutrunner runs slowly until the screw head reaches the work and the input speed F is set infinitely close to Dx after the work is reached, then the applied torsion to work is approximated slowly to the target torsion. If the input speed F is set to zero when the target torque is reached, no over-tension will result but there will be a very long time before the tightening is completed. In contrast, if the input speed F is set to a large value so that the target torque is achieved at the input speed F, it is set to zero as soon as the target torque is reached, and then the nut driver operates according to the equation 2: 0 = 1 dx / dt + DX ... (2) with an over-voltage given by equation 3, J I dt (3 the overvoltage still exists at x = 0.

Figure 3 shows sample curves of the torsion against the rotational speed S, the rotational speed S is plotted on the vertical axis and the torsion on the coordinate. It is assumed that the rotational speed S is a maximum value when the torsion is zero and the input speed F is set to zero when the torsion reaches the objective torsion. The operation of the motor according to curve 8 allows tightening of the nut in a minimum time. If the motor fails to follow curve P8, then there will be an overvoltage.

An embodiment of the present invention will now be described with reference to Figures 1 and 2. Figure 1 is a block diagram of an apparatus for tightening constant torque screws and Figure 2 is a flow chart showing the operation of the apparatus. The reference number 1 indicates a CPU. The selector switch 2 is used to select one of the curves in Figure 3 along which the screw tightening apparatus undergoes recognition cycles. A memory 3 stores the table shown in Figure 5, which shows the normalized values of the rotational speed and torsion in Figure 3. A torsion sensor 5 is incorporated in a nut driver 4. The reference number 6 signals an alarm.

In the SI stage, the switch 2 is operated to set an initial value of the rotation speed S based on the characteristics of the nut driver 4, target torque and tolerant error E. In step S2, a plurality of curves is produced based on the rotation speed S, target torque and tolerable error E. In step S2A, the curve P (s) is arbitrarily selected, P (s) is the ratio of P in • £? P (s) < ^ Pmax and s is l, 2, 3 ... 8. The curve P (s) can be a curve used in the past. CPU1 reads the values of T (0) and S (0) for j = 0 of the curve P (s) and the value of the torsion T (n) target for j = n.

Figure 3 shows the curves representing the relationships between speed and torsion for different parameters, which represent a speed required for a given torsion.

Figure 4A shows the curve P8 of figure 3 and figure 4B shows the curve P2, the curves P2 and P8 are shown on a semilogarithmic scale. Each curve represents a value S (j) of speed S of rotation corresponding to a given value T (j) of torsion T, where j = 1, 2, ..., n. A larger value of n allows more precise control.

In step S3, a torque / speed table as shown in Fig. 5 is produced according to the curve P (s) shown in Fig. 3 and proceeds to step S4 where the tightening operation begins.

At the start of the tightening operation, the CPU1 monitors the torsion values t by means of a torsion sensor 5 incorporated in the nutrunner 4. In step S6, the CPU1 determines whether the torsion t is equal to or greater than T (j ). If the answer in step S6 is YES, then the program proceeds to step S7 where a check is made to determine whether the torsion T is equal to or greater than the target torque T.

If answered in step S7, then the value of j is incremented by 1 in step S8 and the program returns to step S5. Steps S5-S8 are repeated until the torsion reaches or exceeds the target value T (n). When the response in step S7 is YES, then the program proceeds to step S9 where the motor is stopped.

In step S10, a logical test is made to determine whether a difference between the final detected torque t and the target torque T (n) is equal to or less than the tolerable E error. If the difference is larger than the tolerable E error, the program proceeds to step S15 where a logical test is made to determine if the difference is less than the error that resulted from the previously selected curve. If the answer in step S10 is YES, then the curve P (s + 1) is selected in the stage Sil. A check is made to determine if the curve P (s + 1) is really Pmax or a higher curve. If the answer is NO in step S12, then the control proceeds to step S3 to repeat steps S3-S12 until the curve P (s + l) is actually the Pmax curve or a higher curve.

If the answer in step S12 is YES, then the curve Pmax is stored in step S14 and the recognition cycle is completed.

If the answer in step S15 is YES, the control jumps to S16 where the previously tested curve is finally employed and stored in step S14.

If the answer in step S15 is NO, then the curve P (sl) is selected in step S17 and the control proceeds to step S18 where a check is made to determine whether the curve is actually the Pmin curve or a curve more low. If the answer in step S18 is NO, then the control jumps to S3 and the steps S3-S10 and S15-S18 are performed until the curve is actually Pmin or a lower curve. If the response in step S18 is YES, then the tightening operation fails and the control triggers the alarm operation. If the answer is YES, in step S15, the program jumps to step S16.

Tightening the screw according to the optimum curve determined through the aforementioned recognition cycle may result in insufficient tightening as shown in Figure 6 due to a loss in torsion occurring a short time after the target torque is achieved. . This is due to the fact that the tension is tightened and brought into intimate contact with the screw or nut. Therefore, in the present invention, the output torque with the nutrunner still subject to tension is used to provide additional rotations to compensate for the loss in torsion.

That is, the motor current is allowed to continue to flow for a short period of time after the objective torque is reached. This prevents the twisting of the screw from decreasing, which ensures that the screw is tightened with the target torsion.

Claims (2)

R E I V I N D I C A C I O N E S.

1. A method for tightening a screw at an optimum time, comprising the steps of: setting at least an initial speed of a motor, a torque of objective torque and a tolerable error of a final torque relative to the target torque; producing a plurality of torsion velocity curves based on the initial velocity, target torque and tolerable error; selecting an arbitrary curve from a predetermined plurality of torsion velocity curves; operate a motor at the speeds obtained from the selected curve; monitor the torques produced while the engine is being operated; determining a final curve over which the target torque is reached in a minimum of time; and perform a screw tightening operation using the curve on which + or the target torque was reached.

2. - Method according to clause 1, which further includes the step of: allowing a current passing through the motor to continue to flow a predetermined time subsequently after the target is reached, that current is of a value at which the current is reached torsional moment, which compensates for a loss in torsion. SUMMARY . A method to tighten a screw in optimum time that does not require an additional program to control the speed once the optimal internal parameters has been determined through several learning cycles. A target torque is set first and the nutrunner is subjected to several learning cycles in such a way that the nut driver automatically determines those internal parameters in order to stop the engine immediately after the target torque is achieved, requiring a minimum time to tighten the screw.