Reference point return method

Details Reference point return method Reference point return method

1987-10-14

1988-11-01

Shoop, Jr., William M.

Electricity: motive power systems

Positional servo systems

Program- or pattern-controlled systems

318571, 318572, 318573, 36416701, 36447434, G05B 1918

Patent

active

047822759

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION



Reference Point Return Method



BACKGROUND 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 performing a reference point return operation in a numerically controlled machine tool employing a digital servo-circuit for digitally generating a velocity command to transport a movable element.
In a numerically controlled machine tool, it is required that a movable element be returned to a predetermined reference point, thereby establishing coincidence between the machine position and a present position in a numerical controller before numerical control starts.
FIG. 1 is a block diagram of a section for practicing a conventional reference point return method, and FIG. 2 is the associated timechart.
If a move command is provided as NC data by an NC tape 10 when a reference point return command ZRN is "0" (i.e. when a reference point return mode is not in effect), a numerical controller 11 calculates, at every predetermined time, a minute traveling distance in the direction of each axis along which a movable element is to be transported during the predetermined time, and inputs the minute traveling distance to a pulse distributor for each axis for each predetermined time until the destination is reached. On the basis of the minute traveling distance inputted thereto, the pulse distributor 12 performs a pulse distribution calculation to generate command pulses P.sub.c. An error counter 13 comprising a reversible counter counts the command pulses P.sub.c. A DA converter, not shown, inputs a voltage proportional to the count to a velocity controller 14 as a velocity command, thereby rotating a servometer 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 servometer. 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 RTS whenever it makes one full revolution. Accordingly, the amount of rotation of the servometer 16 is detected by the rotary encoder 15 and is inputted to the error counter 13 as the feedback pulses P.sub.F, thereby diminishing the contents of the counter in the zero direction. When a steady state prevails, the data (error) in the error counter 13 becomes substantially constant and the servometer 15 rotates at a substantially constant velocity. When the command pulses Pc stop arriving and the number of feedback pulses P.sub.F generated is equal to the number of command pulses, the data in the error counter 13 becomes zero and the servometer 15 stops rotating.
When the referenc point return command ZRN is "1" (i.e. when the reference point return mode is in effect), on the other hand, the numerical controller 11 calculates a minute traveling distance every predetermined time along each axis for transporting the movable element at a rapid-traverse velocity, and inputs these traveling 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. 2.
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 inputted to a reference counter 18, just as to the error counter 13. The reference counter 18 is comprises a reversible counter and has a capcity N (equivalent to the number of feedback pulses P.sub.F generated during one revolution of the rotary encoder). The contents REF of the reference counter 18 agree with the contents (error) E of the error counter 13.
When the actual velocity V.sub.a exceeds a predetermined first reference velocity VR, the controller 11 generates a control signal D01. After the control signal D01 is generated, a gate control circuit 19 closes the gate 17 in response to generation of the first one-revolution signal RTS. Thereaft

REFERENCES:
patent: 4597040 (1986-06-01), Buizer

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