Procedure for controlling a crane

Abstract

 The invention relates to a procedure for controlling the traversing motion of a crane when the length (L) of its hoisting cable changes. In the procedure, the traversing motion of the crane is controlled by using motor acceleration periods (u₁) determined on the basis of control commands and the length (L) of the hoisting cable is measured. According to the invention, when the length (L) of the hoisting cable changes, at least one motor acceleration period (u_k) proportional at least to the hoisting speed (L') of the load and to the angular velocity (Θ') of the load oscillation is generated.

Description

[0001] The invention relates to a procedure according to the preamble of claim 1 for controlling the traversing motion of a crane. In this context, traversing motion means both the traversing motion of the trolley and the motion of a carriage supporting the trolley or other essentially horizontal motion used to move an element which has a suspension point for a hoisting cable or on which a hoisting drum or equivalent for winding up the hoisting cable is installed.

[0002] A problem in the control of the traversing motion of a load suspended from the hoisting cable of a crane is how to damp the load oscillation to a minimum. In relevant industry, several solutions are known which aim at stopping the load without oscillation at its point of destination. For example, patent application FI 920751 presents a procedure in which the control commands for the travesing motor of the crane are so modified that, when the commands are being issued, acceleration periods are formed which both implement the control commands and compensate the oscillation of the load at the end of the traversing motion.

[0003] This previously known solution makes use of a differential equation representing the oscillation of the load is utilized. However, for easier practical implementation, it employs approximations, which again result in inaccuracies in the control methods if the cable length changes during the hoisting movement. For this reason, this procedure cannot completely compensate the oscillation generated during traversal.

[0004] The object of the present invention is to achieve a procedure for controlling a crane whereby the oscillation of the load can be compensated even when the oscillation length of the hoisting cable changes during traversal. This is implemented via the actions defined in the characterization part of claim 1.

[0005] In a manner defined by the invention, the changes in the hoisting cable length, which has an effect on the oscillation, are taken into account in the control procedure. Therefore, the control substantially follows the actual oscillation of the load and changes the motor acceleration periods accordingly. Even in the case of high hoisting speeds, the oscillations can be compensated. With the procedure of the invention, the load can be moved without oscillation when the hoisting height changes even without speed feedback by using acceleration periods determined in the control procedure. Deceleration periods are to be understood as corresponding to acceleration periods, only the direction of the change of velocity is reversed.

[0006] In the following, the invention is described in detail by the aid of one of its embodiments by referring to the drawings, in which

• Fig. 1 presents the principle of a crane,
• Fig. 2 presents the acceleration periods of a crane during the control of the traversing motion when the length of the hoisting cable remains constant,
• Fig. 3 presents acceleration periods when the length of the hoisting cable changes, and
• Fig. 4 illustrates the crane control according to the invention.

[0007] As illustrated by Fig. 1, the trolley 1 of the crane is moved by its traversing motor along the supporting rails 2 in direction x at a speed v determined by the control. A hanging load 4 is supported by a hoisting cable 3 fixed to the trolley 1. During traversal, oscillations of the hoisting cable and the load supported by it are generated as a result of control actions, these oscillations being determined by the oscillation equation of the system. The length of the hoisting cable is calculated here as the distance between the cable suspension point in the trolley 1 to the centre of gravity of the load 4. As the load 4 oscillates, the hoisting cable 3 forms an angle Θ with respect to the vertical direction y. The following oscillation equation now applies:





where

L

= length of hoisting cable

L'

= rate of change of hoisting cable length, i.e. hoisting speed

Θ

= angle of deflection

Θ'

= 1st time derivative of the angle of deflection, i.e. oscillation velocity

Θ''

= 2nd time derivative of the angle of deflection, i.e. angular acceleration

u

= acceleration of trolley, i.e. 2nd time derivative of trolley position

g

= acceleration of free fall = 9,81 m/s².

[0008] Correspondingly, the time T of the oscillation cycle of the oscillating motion is given by the equation:





If the length L of the hoisting cable remains constant during traversal or control, the second term in the right-hand part of equation (1) will be omitted because the time derivative L' of the cable length L is zero. In this case, the oscillation time T of the oscillating motion remains constant. The oscillation equation (1) is reduced to the form:





The solution of equation (3) is a periodic function whose cycle duration, i.e. the oscillation cycle, is T. If the motor of the trolley is controlled using equal acceleration periods with a phase difference of T/2 between them, the oscillation resulting from the change of acceleration will be compensated at the end of the acceleration periods, as is implemented e.g. in the solution of patent application FI 920751. A situation according to equation (3) is illustrated by Fig. 2a), where an acceleration period 21 of magnitude u₁ is started at instant

and ended at instant

. At instant

begins another acceleration period 22 of a magnitude and duration equal to those of period 21, ending at instant

. In a corresponding manner, when the motor is to be decelerated, deceleration periods 23 and 24 are started at instants t₄ and t₆ and ended at instants t₅ and t₇, respectively. The broken line represents the variation of velocity v during the traversal.

[0009] Fig. 2b) illustrates the variation of the oscillation angle Θ with respect to time resulting from the acceleration periods in Fig. 2a). The broken lines represent the oscillations caused by individual changes in acceleration while the solid lines represent the oscillation angle of the total load oscillation.

[0010] When the length of the hoisting cable changes during motor control as in Fig. 2a) while the load is being hoisted by the hoisting machinery, the oscillation compensating effect of the control will not be completely realized because the hoisting-speed-dependent term of equation (1) has not been taken into account and because the duration of the oscillation cycle changes simultaneously according to equation (2).

[0011] In the procedure of the present invention, the motor acceleration period is modified by including in it a correction term which takes the effect of the hoisting speed into account. For this purpose, the change in the length of the hoisting cable as well as the hoisting speed of the load are determined. When the acceleration period 21 presented in Fig. 2 is corrected by the quantity

, the resulting control period will be

. When the acceleration quantity u in equation (1) is replaced by the value of U, equation (1) will be reduced to the form of equation (3), where u has the value of u₁. Since the correction term u_k is a function of time depending on the time derivative of the oscillation angle and the hoisting cable length, the value of u_k changes as a function of time.

[0012] The diagrams in Fig. 3 illustrate the generation of the control period according to the invention. Fig. 3a) shows an additional control period 31 according to the correction term at the beginning of the traversal and a corresponding additional control period 31' at the end of it. In the example depicted, the hoisting speed is assumed to be constant during the control period. To clarify the presentation, the corrections made in connection with acceleration and deceleration are depicted as having equal magnitudes and beginning at an instant when the oscillation angle and the oscillation velocity are zero.

[0013] Additional control period 31 affects the velocity of the trolley, increasing it to a value larger than the desired value (=control command). Therefore, the additional control has to be altered so that the target speed is reached. Since the control period occurring in the opposite direction after half the period generates an oscillation of a corresponding magnitude, this can be achieved by dividing the control period 31 into two parts 32 and 33 as shown in Fig. 3b). The magnitude of these parts 32,33 is half that of additional control 31 and the latter part begins at an instant when the former part of the additional control is in opposite phase. The latter additional control also occurs in the opposite direction, i.e. if the first additional control is in the accelerating direction at a given instant, then the second part is in the decelerating direction after T/2, T being the duration of the prevailing oscillation cycle. In this way, the additional control periods 32 and 33 compensate the velocity change generated by each other, and at the same time they compensate the oscillation resulting from the change in hoisting height. The effect of the change in hoisting height on the control period is taken into account by scaling in a manner determined by equation (2). Fig. 3c) depicts the total control periods 34 and 35 in solid lines and the acceleration periods 21-24 generated on the basis of the control in broken lines.

[0014] Fig. 4 presents a diagram of the control principle for implementing the procedure of the invention. Based on the crane operator's control actions, the control periods are generated in unit 41, whose output is taken into a correction unit 42, where the control is modified by a correction term u_k. The input quantities of unit 41 are the cable length L, the oscillation angle Θ. The input quantities of unit 42 are, in addition to the correction term u_k, the output quantity u₁ of unit 41 and the duration T of the oscillation cycle. The correction term u_k is generated by unit 43, whose input quantities are the angular velocity Θ' of the oscillation and the rate of change L' of the cable length. The output quantity obtained from unit 42 is the motor control period U as a function of time.

[0015] The invention has been described above by the aid of one of its embodiments. However, the presentation is not to be regarded as limiting the invention, but instead the embodiments of the invention may vary within the limits defined by the following claims.

Claims

1. Procedure for controlling the traversing motion of a crane when the length (L) of its hoisting cable (3) changes simultaneously, in which procedure the traversing motion of the crane is controlled by using motor acceleration periods (21-24) determined on the basis of control commands and in which procedure the length (L) of the hoisting cable is determined, characterized in that, when the length (L) of the hoisting cable changes, at least one additional motor acceleration period (31-33,31'-33') proportional at least to the hoisting speed (L') of the load and to the angular velocity (Θ') of the load oscillation is generated.

2. Procedure according to claim 1, characterized in that the additional acceleration periods (31-33) are proportional to the product of the hoisting speed (L') of the load (4) and the angular velocity (Θ') of load oscillation.

3. Procedure according to claim 1 or 2, characterized in that the additional acceleration period is divided into two parts (32,33) which are effected with an interval of half an oscillation cycle (T/2) between them.

Drawing