Determining the rigidity of a drivetrain of a machine, in particular a machine tool or production machine

Abstract

A drivetrain for linear movement of a machine component along a linear guide of a machine includes a motor with a motor-measuring system. A length-measuring system is assigned to the linear guide for determining a position of the machine component. To determine rigidity of the drivetrain, a constant acceleration for the machine component is predetermined by a numerical controller for performing closed-loop control of the movement of the machine component. The numerical controller determines a difference between a position of the machine component derived from the motor-measuring system and a position of the machine component measured at the same time by the length-measuring system during the acceleration phase, and the difference is assigned to the acceleration or to a force required for the acceleration and storage in the numerical controller of the pair of values established in this way and/or of a rigidity value emanating from the pair of values.

Claims

1. A method for determining a rigidity of a drivetrain having a screw drive for moving a machine component along a linear guide of a machine, said method comprising the steps of: under control of a numerical controller, imparting during an acceleration phase on a rotary motor having a first position sensor and being operatively connected to the linear screw drive a constant acceleration for the machine component; measuring during the acceleration phase with the first position sensor a first position of the machine component; measuring simultaneously during the acceleration phase with a length-measuring system operably connected to the linear guide a second position of the machine component along the linear guide; determining a difference between the first position and the second position; associating the difference with the constant acceleration or with a force producing the constant acceleration; and storing in the numerical controller a value pair composed of the determined difference and the constant acceleration or a rigidity value derived from the value pair.

2. The method of claim 1, wherein the machine is a machine tool or a production machine.

3. The method of claim 1, further comprising repeating the method steps for different machine components and/or for different constant accelerations until an abort criterion is reached.

4. The method of claim 3, further comprising determining a characteristic curve from the value pairs established for different constant accelerations, and storing the characteristic curve in the numerical controller.

5. The method of claim 1, further comprising determining a friction force to be overcome for moving the machine component, and compensating the friction force.

6. The method of claim 5, wherein the friction force is determined from a plurality of measurements of a power consumption of the motor, during which the machine component is moved at different constant speeds.

7. The method of claim 5, further comprising establishing a Stribeck curve using measurement technology for determining the friction force as a function of a speed with which the machine component is moved.

8. The method of claim 7, further comprising determining a plurality of Stribeck curves as a function of the position of the machine component in relation to the linear guide.

9. The method of claim 1, further comprising specifying to the numerical controller a mass of the machine component.

10. The method of claim 9, wherein the mass of the machine component is derived by the numerical controller from the force producing the constant acceleration.

11. The method of claim 1, further comprising determining the rigidity of the drivetrain at different points in time, and inferring from a comparison between the rigidity determined at different points in time characteristic properties of the drivetrain.

12. The method of claim 1, further comprising determining the rigidity of a number of drivetrains of identical design installed at a number of machines, and inferring from a comparison of the rigidities established for the drivetrains of the identical design characteristic properties of the respective drivetrains.

13. The method of claim 11, wherein the numerical controller carries out predetermined functions depending on the characteristic properties of the drivetrain.

14. The method of claim 12, wherein the numerical controller carries out predetermined functions depending on the characteristic properties of the respective drivetrains.

15. A numerical controller for closed-loop control of a movement of a machine component of a machine moving along a linear guide, said numerical controller being configured to impart during an acceleration phase on a rotary motor having a first position sensor and being operatively connected to a linear screw drive which moves the machine component a constant acceleration on the machine component; measure during the acceleration phase with the first position sensor on the motor a first position of the machine component; measure simultaneously during the acceleration phase with a length-measuring system operably connected to the linear guide a second position of the machine component along the linear guide; determine a difference between the first position and the second, postion; form a value pair composed of the determined difference and the constant acceleration or a force producing the constant acceleration; and store the value pair and/or a rigidity value derived from the value pair.

16. A machine, comprising: a machine bed; a machine table; a linear guide secured to the machine bed and guiding the machine table along the machine table; a length-measuring system operably connected to the linear guide for measuring a first position of the machine component relative to the machine bed; a drivetrain comprising a screw drive which converts a rotational movement of a rotating motor into a translational movement of the machine table; a motor-measuring system comprising a motor sensor which determines from a known transmission ratio of transmission elements of the drive train a second position of the machine table in relation to the machine bed; and a numerical controller configured to impart during an acceleration phase a constant acceleration on the machine component along the linear, measure during the acceleration phase simultaneously the first position and the second position, and determine a difference between the measured first position and second position, associated the difference with the constant, acceleration or with a force producing the constant acceleration, and store a value pair composed of the determined difference and the constant acceleration or and/or a rigidity value derived from the value pair.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The invention will be explained in greater detail below on the basis of exemplary embodiments. In the figures:

(2) FIG. 1 shows a drivetrain of a machine tool,

(3) FIG. 2 shows a rigidity curve of the drivetrain,

(4) FIG. 3 shows a Stribeck curve for establishing and compensating for the friction, and

(5) FIG. 4 shows the method steps when carrying out an inventive method.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(6) FIG. 1 shows a schematic diagram of an example of a drivetrain of a machine tool. This comprises a rotating servo motor 1 which drives a ball screw drive 5 fastened to a machine bed 4 via a clutch 2 and a toothed belt 3. The ball screw drive 5 converts the rotational movement of the motor via a spindle and a nut into a translational movement of a machine table 7. For precise guidance of the movement of the machine table 7 there are linear guides 6 present. The position of the machine table 7 in relation to the machine bed 4 can be determined via the motor sensor 8 in conjunction with the known transmission ratio of the transmission elements. Moreover this position can also be determined directly via the length-measuring system 9.

(7) For positioning the machine table 7 a numerical controller 10 is further provided, which activates the motor 1 via power converters (not shown) and to which the sensor signals of the motor sensor 8 and of the length-measuring system 9 are supplied for controlling the movement of the motor 1 or of the machine table 7.

(8) The layout shown in FIG. 1 involves a typical drivetrain for exact positioning of a workpiece or of a tool within a machine tool. This layout can be used without further aidsexcept for suitable softwareto determine the rigidity of the drivetrain. For the subsequent calculations the mass of the machine table 7 must be known in the controller 10. In the simplest case the mass M of the machine table 7 is known in advance and only has to be stored in the controller 10 by corresponding user entries. If the mass M of the machine table 7 is not known, it can be established on the basis of measurements of the motor torque during a movement of the machine table 7 with a predetermined acceleration.

(9) In a first method step for determining the rigidity of the drivetrain, a constant acceleration a for the movement of the machine table 7 is now predetermined by the numerical controller 10. At at least one specific point in time during this acceleration phase the position of the machine table 7 is determined, both on the basis of the sensor signal of the motor sensor 8 and also on the basis of the measured value concerned of the length-measuring system 9. Because of the finite rigidity of the drivetrain, a difference x of the two measured position values is produced. This difference x is stored in the controller 10 for the predetermined acceleration or for the force required to reach the acceleration. Subsequently the process is repeated for a number of different accelerations. It should be noted that the measurement processes can also be carried out for negative accelerations, i.e. braking processes. A plurality of measurements enable a characteristic curve of the force F over the position difference x to be specified in this way. An example of this type of characteristic curve K is shown in FIG. 2, which describes the force F or the force F reduced by the friction plotted against the change in length x. The striking aspect here is thatunlike for an ideal springa non-linear relationship is involved. In particular the force F or F increases disproportionately as x increases.

(10) The accuracy of the characteristic curve established in this way naturally increases with the number of measurements carried out. However a corresponding characteristic curve can already be determined with few measurement points by means of mathematical methods known per se. Since the characteristic curve is of interest, especially at its zero crossing, for information about the state of the machine, it is recommended to place the measurement points close to one another, particularly in this area.

(11) As well as the predetermination of different accelerations, the relationship sought can also be obtained from machine components of different mass. However this requires a change to the machine between the individual measurements and is therefore associated with greater effort. For example the machine component can involve a tool holder in conjunction with a tool clamped therein. Then, by using different tools, measurements for machine components can be carried out, which differ in respect of their mass. This procedure too is largely able to be automated through the option of automatic tool changing available with many machine tools.

(12) More precise results in relation to the established rigidity are produced when the friction also present in the drivetrain is taken into consideration. In this case the friction is advantageously determined by moving the axis at different constant speeds by evaluation of the power consumption needed for the respective constant travel. The description of the curve of the friction established in this way as a function of the speed of movement is also known as the so-called Stribeck curve. An example of such a Stribeck curve S for describing the friction p plotted against the speed v is shown in FIG. 3. The striking aspect here is the jump at the zero crossing point, which results from the adhesion friction.

(13) If the friction in accordance with the Stribeck curve is known for the relevant speed range, then the friction can also be taken into consideration for the previously described accelerated movement of the machine table 7. The share of the friction F.sub.R can thus be calculated out using the relationship F=m*af.sub.R during the determination of the rigidity.

(14) The calculating out of the friction has the advantage that through this there is not a mixture of two causes for the machine characteristics changed over the runtime of the machine, which would make it more difficult to determine the cause of a specific changed machine property. Unlike with a changed rigidity of the drivetrain, the causes of increased friction namely lie for example in the area of the bearings and guides or the lubrication of said drivetrain.

(15) Both the established rigidity and also the friction can depend on the position of the machine table 7 in relation to the machine bed 4. Thus it can be sensible to determine the rigidity or the friction for different positions of the machine table 7 along the linear guide, as explained above.

(16) The rigidity is a significant property of a drivetrain. In particular from a comparison of the rigidities between machines of the same design or from a comparison of the rigidities established at different points in time during the lifetime of a specific machine, conclusions can be drawn about specific characteristics or the state of the machine at a specific point in time, in particular about the current state of the machine. A corresponding evaluation of the results produced by a number of rigidity measurements can be carried out directly by means of the controller 10. Naturally a corresponding evaluation can also be undertaken however in a computing device connected to the numerical controller 10. In particular a flattening off of the characteristic rigidity curve at the zero crossing points to a loss of the pre-tensioning of a ball screw. The controller or the external computing device can react to this with different measures. Thus an appropriate function of the controller can consist of alerting the user that maintenance is required. With a serious deviation there can even be an automatic decommissioning of the machine involved. As well as the reactions given by way of example, the controller 10 can however also adapt its closed-loop control behavior automatically to the changed circumstances. Thus it is of advantage if, with a loss of pre-tension of the ball screw, the amplification of the position control circuit is reduced automatically. Further the loss of pre-tension is also accompanied by a deterioration of the dynamic accuracy, so that advantageously the values predetermined by the numerical controller 10 in relation to the maximum jerk and the maximum acceleration are reduced.

(17) FIG. 4 successively describes the significant method steps of a preferred inventive method for determining the rigidity of a drivetrain for linear movement of a machine component along a linear guide of a machine, in particular a machine tool or production machine, wherein the drivetrain comprise a motor with a motor-measuring system, wherein the linear guide is assigned a length-measuring system to determine the position of the machine component and wherein the machine comprises a numerical controller for closed-loop control of the movement of the machine components:

(18) In a first method step S1 the friction force is advantageously first established from a plurality of measurements of the power consumption of the motor, in which the machine component is moved at different speeds, but is moved at a constant speed during the respective measurement. The relationship between speed and friction is preferably established in the form of a Stribeck curve and stored in the numerical controller.

(19) In a subsequent second method step S2 a constant acceleration for the machine component is predetermined by the numerical controller and the machine component is accelerated accordingly.

(20) Subsequently, in a third method step S3, a difference x between a position of the machine component derived from the measurement system and a position of the machine component measured at the same time by the length-measuring system during the acceleration phase is determined by the numerical controller.

(21) Thereafter, in a fourth method step S4, the difference x is assigned the acceleration a defined at the start or a force F required for the acceleration and the pair of values thus established and/or a rigidity value emanating from the pair of values is stored in the numerical controller. In the assignment of x to F the influence of the friction established in method step S1 is advantageously calculated out in accordance with the formula F=m*aF.sub.R.

(22) The method steps S1 to S4 are subsequently repeated for a plurality of measurements with different accelerations and/or different masses, until such time as the desired precision in relation to the relationship between x and F or F is achieved.

(23) Preferably the measurement points are connected to form the characteristic curve in a method step S5 by mathematical methods known per se and are accordingly stored in the numerical controller.