Page 749 - Mechatronics with Experiments

PROGRAMMABLE MOTION CONTROL SYSTEMS 735
calculated and commanded. In this case, the feedrate acts as the master (although it is not
an axis), and all of the motion axes are slaved to it. Another alternative is to select one
of the axis as master and define its motion, then derive the desired motion of other axes so
that the tool traces the desired path. CNC machine tools are an example of the coordinated
motion control application where contour coordination is most commonly implemented.
Consider a two-axis motion control application where the two axes are required to
trace a contour, that is an XY stage holding a tool, and the tool is required to trace a path
in x-y plane. Gearing coordination would generate a path that is a straight line. Contouring
is a more general form of coordination and requires the ability to trace any path in the
motion space.
The path can be defined as function of:
1. Time, each axis motion is separately defined as a function of time to generate the
path.
2. One axis is set as master and its motion for the path is defined, the other axis’ motion
is set as a function of the master axis position.
3. Path length or speed parameter – each axis motion is defined in terms of a path
parameter.

CNC Programming Computer numeric control (CNC) programming is a program-
ming language used to define the motion of machine tools. The most common CNC
language in use today is the G-code. The G-code standard is defined by the Electronics
Industry Association (EIA) standard 274-D. Although this standard exists, there are minor
variations in the implementation of G-code from one manufacturer to another. The meaning
of some codes also varies from one machine tool type to another. For example, G70 code
means programming in units of inches in machining centers, whereas the same G70 code
means edge finding in electric discharge machines (EDM).

10.5.4 Sensor Based Real-time Coordinated Motion
As the sophistication of computer controlled machines increases, the type of motion
demands placed on them becomes more and more complicated. Particularly in robotic
manipulator applications, the desired motion may not be known in advance. The machine
is required to sense its environment (i.e., using vision systems), and decide on a motion
strategy and generate the motion command profiles for each individual axis. The motion
synchronization used may be different for different phases of its motion. Furthermore, this
decision as to what kind of strategy to use is determined online by the control software.
Interpretation of sensory data, and generating intelligent motion planning strategies not
planned in advance, is one of the current challenges of programmable motion control in
robotics devices.

10.6 COORDINATED MOTION APPLICATIONS

10.6.1 Web Handling with Registration Mark

The following applications have very similar motion coordination requirements
(Figures 10.12, 10.13):

1. rotating cutting knife to cut a web (i.e., paper, plastic) to a fixed length,
2. rotating printing head to print over a web,
3. rotating sealing head to seal a web or bag.