Zero-point return method

Details Zero-point return method Zero-point return method

1986-03-07

1987-07-14

Dobeck, Benjamin

Electricity: motive power systems

Positional servo systems

Multiple mode systems

318592, 318468, G05B 1118

Patent

active

046805206

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

1. Technical Field
This invention relates to a method of returning a movable element of a machine to a zero point and, more particularly, to a zero-point return method applied to a movable element of a machine such as machine tool controlled by a numerical control unit.
2. Background Art
A method shown in FIGS. 1(a), (b) is generally employed in the prior art to return, to a zero point, a movable machine element such as the table of a machine tool controlled by a numerical control unit. Specifically, in the conventional method, a dog DG for zero-point return is provided on a movable machine element (table) TB and a deceleration limit switch DLS is provided in the vicinity of a zero point of a fixed machine part MC. When the numerical control unit controlling the machine is set to a zero-point return mode, the movable element (table) TB is moved toward the zero point at a rapid-traverse velocity V.sub.R. The movable machine element (table) TB arrives in the vicinity of the zero point and the dog DG actuates the deceleration limit switch DLS at a time t.sub.1 [FIG. 1(a)], whereupon a deceleration signal XDL goes to "1" as shown in FIG. 1 (a). In response to the leading edge of the deceleration signal XDL, the numerical control unit lowers the velocity at which the return to the zero point is carried out so that the velocity reaches zero as shown by the dashed line after a predetermined time (at time t.sub.2) or reaches a velocity V.sub.L at which the motor can be stopped instantaneously, as shown by the solid line. Thereafter, the movable machine element TB is moved toward the zero point at the velocity V.sub.L. At time t.sub.3, the dog DG separates from the decelerating limit switch DLS so that the limit switch is restored and the deceleration signal reverts to "0". Since the numerical control unit considers the first grid position (a position, described below, at which a grid signal GRS is generated) encountered since restoration of the limit switch to be the zero point, the movable machine element is stopped at time t.sub.4 (grid position G.sub.0) to complete the zero-point return operation. Let us now explain the grid positions. FIG. 1(b) is a view for describing grid positions. A servomotor is provided with a pulse coder which generally produces one pulse of a position signal per revolution of the servomotor. Specifically, let one revolution of the pulse coder be equivalent to movement L (mm) of the movable element, and plot the position of the movable element along the horizontal axis, as shown in FIG. 1(b). In such case, the movable element will generate one position signal GRS per amount of movement L (mm). The points G.sub.0, G.sub.1, G.sub.2, G.sub.3 . . . at which the position signal GRS is generated are referred to as grid points.
With a system that uses the leading and trailing edges of a single dog as a deceleration dog and zero-point position dog, as in the case of the zero-point return dog DG shown in FIG. 1(a), it is difficult to judge the direction in which the table will initially operate when zero-point return is started. In other words, it is difficult to judge on which side the table is located with respect to the dog. Though a mechanical zero-point return method has been practiced using two dogs, namely a deceleration dog and a zero-point position dog, the problem with this method is the same as that encountered with the method shown in FIG. 1(a).
Use of a flat dog has been proposed in order to eliminate the difficulty encountered in judging the operating direction of a table at the time of a zero-point return operation. The term "flat dog" used herein refers to a dog capable of indicating only an on signal or off signal state. The dog can possess only one point at which a signal makes an on/off changeover in a stroke. Judging the operating direction of a table at the time of a zero-point return operation is facilitated by using such a dog that possesses only one point at which a signal makes an on/off changeover in a stroke.
However, since a flat dog has only one point at

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patent: 3995206 (1976-11-01), Aronstein
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patent: 4312033 (1982-01-01), Sweeney et al.
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