Reference-point return method

Details Reference-point return method Reference-point return method

1989-10-18

1991-07-23

Ip, Paul

Electricity: motive power systems

Positional servo systems

Program- or pattern-controlled systems

318571, 318572, 318573, 36447434, G05B 1918

Patent

active

050346725

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to a reference-point return method and, more particularly, to a reference-point return method well suited for use in effecting return to a reference point in a numerically controlled machine tool employing a digital servo-circuit, which digitally generates a velocity command to move a movable element, and an absolute position detector.
2. Description of the Related Art
In a numerically controlled machine tool, it is necessary to return a movable element to a predetermined reference point before the start of numerical control, whereby coincidence can be achieved between machine position and the present position internally of a numerical control unit.
FIG. 4 is a block diagram of part of an apparatus for practicing a reference-point return method according to the prior art, and FIG. 5 is the associated time chart.
If a move command is given as NC data in a case where a reference-point return command ZRN is "0", a numerical control unit 11 computes a minute traveling distance, at a predetermined time interval, to be traversed along each axis during the time interval, and applies this minute traveling distance to a pulse distributor 12 for each axis at the abovementioned time interval until a destination is reached. Based on the minute traveling distance inputted thereto, the pulse distributor 12 performs a pulse distribution calculation to generate command pulses Pc. An error counter 13 constituted by a reversible counter counts the command pulses P.sub.c, and a DA converter (not shown) applies a voltage proportional to the count of the error counter 13 to a velocity controller 14 as a velocity command, thereby rotating a servomotor 15 to transport a movable element such as a tool or table, not shown.
A rotary encoder 16 is rotated by the rotation of the servomotor. The rotary encoder 16 is adapted to generate a single feedback pulse P.sub.F whenever it rotates by a predetermined amount and to generate a one-revolution signal whenever it makes one full revolution. Accordingly, the amount of rotation of the servomotor is detected by the rotary encoder 16 and is input to the error counter 13 as the feedback pulses P.sub.F, thereby diminishing the count of the error counter 13 in the zero direction. When a steady state prevails, the data (error) in the error counter 13 becomes substantially constant and the servomotor 15 rotates at a substantially constant velocity. When the command pulses stop arriving and the number of feedback pulses P.sub.F generated becomes equal to the number of command pulses, the data in the error counter 13 becomes zero and the servomotor 15 stops rotating.
When the reference-point return command ZRN is "1", on the other hand, the numerical control unit 11 calculates a minute traveling distance every predetermined time period along each axis for transporting the movable element at a rapid-traverse velocity, and inputs these distances to the pulse distributor 12, whereby the movable element is transported just as described above. It should be noted that the actual velocity V.sub.a at this time gradually rises to the rapid-traverse velocity V.sub.R, as shown in FIG. 5.
A gate 17 is open at the start of reference-point return. Therefore, the command pulses P.sub.C and the feedback pulses P.sub.F are input to a reference counter 18, just as to the error counter 13. The reference counter 18 is constituted by a reversible counter and has a capacity N (equivalent to the number of feedback pulses PF generated during one revolution of the rotary encoder). The contents REF of the reference counter 18 agrees with the contents (error) E of the error counter 13.
When the actual velocity V.sub.a exceeds a predetermined first reference velocity VR.sub.1, the numerical control unit 11 generates a control signal D0.sub.1. After the control signal D0.sub.1 is generated, a gate control circuit 19 closes the gate 17 in response to the generation of the first one-revolution signal RTS. Thereafter, the reference counter 18 counts u

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