PITCH ERROR CORRECTION SYSTEM

Abstract: A system for correcting a pitch error of a ball screw in a numerical control machine tool. Pitch error calculation means (4) reads a current value from a current value register (3), reads pitch error correction data (εn,εpn) of a corresponding zone and outputs equidistantly the pitch error correction quantities in the zone as a pitch error correction pulse (CP). This pitch error correction pulse (CP) is added to an interpolation pulse (CP) by an adder (5). Accordingly, since pitch error correction quantities are not outputted all at once, the movement of the machine becomes smooth and a cut surface can be improved.

Description: The present invention relates to a system for compensating for a pitch error of a ball screw in a numerically controlled machine tool, and more particularly to a pitch error compensating system suitable for precision machining. Background Art Ball screws for controlling movable components of numerically controlled machine tools do not have accurately constant pitches. Pitch errors of such ball screws are compensated for by computerized numerical control apparatus to increase positioning accuracy. According to a general pitch error compensating system, the strokes of respective axes are divided into suitable periods, and pitch error corrective quantities are measured in the respective periods. The measured corrective quantities are then stored in a nonvolatile memory. Each time an axis moves, the pitch error corrective quantity that corresponds to the present value of the axis is read from the memory, and added to or subtracted from interpolated pulses, thereby compensating the pitch error. FIG. 6 of the accompanying drawings shows, by way of example, pitch error corrective pulses in a conventional pitch error compensating system. In FIG. 6, a first period P₀ - P₁ has a pitch error corrective quantity ε1, a second period P₁ - P₂ a pitch error corrective quantity ε2, and a third period P₂ - P₃ has a pitch error corrective quantity ε3, each period being indicated as a pitch error correcting period ℓ. The pitch error corrective quantity in each period is outputted as a pitch error corrective pulse at an intermediate point of the period, and added to interpolated pulses. A pitch error corrective quantity is outputted at once as a pitch error corrective pulse. If the pitch error corrective quantity is large, then the corresponding error corrective pulse is also large. The large error corrective pulse that is outputted at once is not preferable from the stand point of positioning accuracy. Such pitch error corrective pulse may lower the quality of the machined surface in precision machining. Disclosure of the Invention In view of the aforesaid drawbacks of the conventional system, it is an object of the present invention to provide a pitch error compensating system which is capable of smoothing the movement of a machine. To achieve the above object, there is provided a system for compensating for a pitch error of a ball screw of a numerically controlled machine tool, comprising a memory for storing pitch error corrective data, a present value register for integrating output pulses from interpolating means and storing a present value, pitch error calculating means for reading said present value and outputting pitch error corrective quantities at equal intervals in a period having said value, and an adder for adding interpolated pulses from said interpolating means and said pitch error corrective quantities, and outputting sum pulses to an axis control circuit. The pitch error calculating means reads a present value, reads pitch error corrective data, and outputs pitch error corrective quantities as pitch error corrective pulses at equal intervals in the period. The pitch error corrective pulses are added to interpolated pulses. Since the pitch error corrective quantities are not outputted all at once, the machine tool moves smoothly, and the quality of the machine surface of the workpiece is improved. Brief Description of the Drawings FIG. 1 is a block diagram of a pitch error compensating system according to the present invention; FIG. 2 is a block diagram of the hardware arrangement of a computerized numerical control (CNC) apparatus for implementing the pitch error compensating system according to the present invention; FIG. 3 is a diagram showing the manner in which pitch error corrective pulses are outputted; FIG. 4 is a diagram illustrative of the manner in which a pitch error calculating means calculates pitch error corrective pulses; FIG. 5 is flowchart of a processing sequence of the pitch error calculating means; and FIG. 6 is a diagram illustrative of pitch error corrective pulses in a conventional pitch error compensating system. Best Mode for Carrying Out the Invention An embodiment of the present invention will hereinafter be described with reference to the drawings. FIG. 1 shows in block form a pitch error compensating system according to the present invention. For the sake of brevity, the pitch error compensating system is shown as compensating for a pitch error of a single axis. In the pitch error compensating system shown in FIG. 1, a machining program 14a is read by a preprocessing means 1, and decoded thereby into pulse-interpolatable data that is sent to an interpolating means 2. The interpolating means 2 interpolates the data and outputs interpolated pulses IP to an adder 5. The interpolated pulses IP are also supplied to a present value register 3 which intergrates the interpolated pulses IP and holds a present value. Pitch error corrective data 14b represent data in the stroke of the axis as it is divided into equal intervals, and has a pitch error corrective quantity εn and an accumulated pitch error corrective quantity εpn up to a first point of the present value in each of the periods. The pitch error compensating system also has a pitch error calculating means 4 that reads the present value of the axis from the present value register 3, and also reads a pitch error corrective quantity εn corresponding to the period that has the present value and an accumulated pitch error corrective quantity εpn up to a first point of the present value, in constant periodic cycles. The pitch error calculating means 4 then produces pitch error corrective pulses CP from the read data, and outputs them at equal intervals to the adder 5. The adder 5 adds the interpolated pulses IP and the pitch error corrective pulses CP, and output the added pulses as output pulses to an axis control circuit 18. In reponse to the output pulses, the axis control circuit 18 controls a servomotor (described later on). The pitch error calculating means 4 outputs the pitch error corrective quantity at equal intervals in the period, as described in the detail later on. FIG. 2 shows in block form the hardware arrangement of a computerized numerical control (CNC) apparatus which implements the pitch error compensating system according to the present invention. The computerized numerical control apparatus includes a processor 11 for controlling the apparatus in its entirety according to a system program stored in a ROM 12. The ROM 12 comprises an EPROM or an EEPROM. A RAM 13, which comprises an SRAM, stores various data. A nonvolatile memory 14 stores a machining program 14a, pitch error corrective data 14b, parameters, etc. The nonvolatile memory 14 comprises a CMOS memory or the like that is backed up by a battery, so that it holds the stored program and data even after the power supply of the computerized numerical control apparatus is turned off. The computerized numerical control apparatus also has a graphic control circuit 16 for converting the data stored in the RAM 13, such as the present position and displacement of each axis, into display signals, and transmitting the display signals to a display unit 15, which displays the display signals. The display unit 15 may comprise a CRT, liquid crystal display, or the like. Various data can be entered into the apparatus through a keyboard 17. An axis control circuit 18 receives position commands from the processor 11, i.e., the output pulses produced by adding the interpolated pulses IP and the pitch error corrective pulses CP described with reference to FIG. 1, and outputs a speed command signal to a servoamplifier 19 to control a servomotor 20. The servoamplifier 19 amplifies the speed command signal, and energizes the servomotor 20 with the amplified speed command signal. To the servometer 20, there is coupled a pulse coder 21 for feeding position feedback pulses back to the axis control circuit 18. A position detector such as a linear scale or the like may be used in place of the pulse coder 21. A ball screw 31 is also coupled to the servomotor 20 and is threaded through a nut 32 fixed to a table 33 for controlling movement of the table 33. A pitch error of the ball screw 31 is compensated for by the pitch error corrective pulses CP for improving the accuracy with which the table 33 is positioned. Since the pitch error corrective pulses CP are outputted at equal intervals according to the present invention, the table 33 moves smoothly, improving the surface of a workpiece which is being machined. There are actually as many axis control mechanisms as the number of axes required, but only the axis control mechanism for one axis is illustrated here as the axis control mechanisms are identical to each other. A PMC (programmable machine controller), an input/output circuit connected thereto, a spindle control circuit for controlling a spindle, a spindle amplifier, a spindle motor, and other elements are omitted from illustration. While only one processor is used in the illustrated embodiment, a multi-processor system composed of the of a plurality of processors may be employed depending on the entire system. FIG. 3 shows the manner in which pitch error corrective pulses are outputted. Pitch error corrective quantities εn in respective periods are given below. Usually, one pulse has a duration of 1 µm. However, one pulse may be 10 µm or may be expressed in an angular unit. Indicated by ℓ is the length of a pitch error correcting period. Between P₀ and P₁ : 4 pulses Between P₁ and P₂ : 2 pulses Between P₂ and P₃ : 3 pulses Therefore, the pitch error calculating means 4 shown in FIG. 1 reads the pitch error corrective quantities (4 pulses, 2 pulses, and 3 pulses in the illustrated embodiment) corresponding to the periods that have a present value, and outputs the pitch error corrective quantities at equal intervals as shown in FIG. 3. FIG. 4 shows the manner in which the pitch error calculating means 4 calculates pitch error corrective pulses. It is assumed that a machine origin is indicated by P₀, a present value by ABSM, a first point of the period having the present value ABSM by P n , a coordinate of the point P n by ABSP n , an ending Point of the period by P n+1 , a pitch error corrective quantity in the period by ε, and the length of a pitch error correcting period by ℓ. FIG. 5 is flowchart of a processing sequence of the pitch error calculating means. The numerals following the letter S indicate step numbers in FIG. 5. [S1] The Present value ABSM is determined by referring to the present value register 3. [S2] Accumulated pitch error corrective quantities εpn from the machine origin P₀ up to the point P n , a pitch error corrective quantity εn in the period P n - P n+1 , and previous accumulated pitch error corrective quantities PA m-1 are read from the pitch error corrective data 14b. [S3] Accumulated pitch error corrective quantities PA m at the present value ABSM are calculated by adding, to the accumulated pitch error corrective quantities εpn to the point P n , pitch error corrective pulses to be outputted at equal intervals up to the present value in the period, as determined according to the following equation : εn·(ABSM - ABSP n )/ℓ = en · (ℓa/ℓ). This equation represents a value produced by proportionally distributing the pitch error corrective quantities εn to a distance ℓa in the pitch error corrective period ℓ, where the distance ℓa extends from the point Pn to the present value ABSM. [S4] pitch error corrective pulses C p are determined as the difference between the accumulated pitch error corrective quantities PA m at the present value ABSM and the previous accumulated pitch error corrective quantities PA m-1 . Also, the previous accumulated pitch error corrective quantities PA m-1 are replaced with the new accumulated pitch error corrective quantities PA m . The accumulated pitch error corrective quantities PA m are then stored among the pitch error corrective data 14b. [S5] When the value of pitch error corrective pulses CP becomes 1 or more, one pulse is outputted. According to the above process, the pitch error calculating means 4 can output pitch error corrective pulses at equal intervals. While more than one pitch error corrective pulse may be outputted when the speed of movement is very high, one pitch error corrective pulse is normally outputted at a time. The accumulated pitch error corrective quantities are employed in the above embodiment because the calculating period of the pitch error corrective means 4 may not necessarily be in conformity with the pitch error correcting period. With the present invention, as described above, since pitch error corrective pulses are outputted at equal intervals in each pitch error correcting period, the machine moves smoothly, thereby improving the quality of the machined surface of the workpiece.

Claims: A system for compensating for a pitch error of a ball screw of a numerically controlled machine tool, comprising: a memory for storing pitch error corrective data; a present register for integrating output pulses from interpolating means and storing a present value; pitch error calculating means for reading said present value and outputting pitch error corrective quantities at equal intervals in a period having said present value ; and an adder for adding interpolated pulses from said interpolating means and said pitch error corrective quantities, and outputting sum pulses to an axis control circuit. A system according to claim 1, wherein said pitch error calculating means comprises means for adding accumulated pitch error quantities from a machine origin up to a first point of the period having said present value and pitch error corrective pulses to be outputted from said point up to the present value, thereby producing accumulated pitch error quantities at the present value, and for determining pitch error corrective pulses as the difference between said accumulated pitch error corrective quantities and accumulated pitch error corrective quantities in a previous calculating cycle.

Inventors: EGUCHI RYOUJI FANUC MANSION HA, FUJIBAYASHI KENTARO, OTSUKI TOSHIAKI, SASAKI TAKAO, EGUCHI, RYOUJI, FANUC MANSION HARIMOMI 7-109, FUJIBAYASHI, KENTARO, OTSUKI, TOSHIAKI, SASAKI, TAKAO
Applicants: FANUC LTD, FANUC LTD.
Application year: 1991
Main classification: B23Q15
IPC codes: G05B19