Absolute positioning and incremental positioning

ABSOLUTE POSITIONING AND INCREMENTAL POSITIONING

The machine tools require the position of cutting tool and workpiece. They move relative to each other. Absolute and incremental positions are concerned with whether the tool positions are defined relative to the origin of the coordinate system or relative to the previous location of the tool. There are two ways of positioning the too in relative to the workpiece as follows:

1. Absolute positioning

2. Incremental positioning.

Absolute positioning means that the tool locations are always defined in relation to zero point.

Incremental positioning means that the next tool location must be defined with reference to the previous tool location.

In Figure 5.6, the tool head is presently at point (20, 20) and it is to be moved to point (40, 50). In absolute positioning, the movement is specified from the origin as X = 40, Y = 50; but in incremental positioning, the movement is specified from the present tool position as X = 20, Y = 30.

The absolute system has the following two significant advantages over the incremental system.

1. Interruptions caused by, for example, tool breakage (or tool change, or checking the parts), would not affect the position at the interruption. If a tool is to be replaced at some stage, the operator manually moves the table, exchanges the tool, and it has to return the table to the beginning of the segment in which the interruption has occurred. In the absolute mode, the tool is automatically returned to the position. In the incremental mode, it is almost impossible to bring it precisely to that location unless the part program is repeated.

2. Easy change of dimensional data

The incremental mode has the following two advantages over the absolute system.

1. Inspection of the program is easier because the sum of position commands for each axis must be zero. A non-zero sum indicated as an error. Such an inspection is impossible with the absolute system.

2. Mirror image programming (for example, symmetrical geometry of the parts) is simple by changing the signs of the position commands.

Both systems are incorporated in most CNC systems. For an inexperienced operator, it is wise to use incremental mode.

Connection between the choice of coordinate system and the position sensor used in the machine tool:

Among many position sensors used, the most preferred one is the linear optical scale or Inductosyn which directly measures the movement of the table slide based on the fixed parts of the machine. Some other kinds are the rotary sensors such as shaft angle encoders and resolvers. The translational motion of the slide is assumed from ball screw pitch constant. Among them, the linear optical scale and the rotary encoder can either give absolute or incremental displacement based on the encoding on the grating. Though, in fact incremental output is more common, better resolution can be achieved. So, this output should be integrated to generate absolute positions by electronic means in an absolute coordinate system. On the other hand, the resolver or Inductosyn naturally produce absolute positions with respect to a fixed origin. The reason is that these signals have to be differenced from the current origin of the incremental system to generate incremental coordinate positions.