Motion Control Networks

Chuck Lewin, President & CEO of Performance Motion Devices (download as PDF)

The trend toward distributed motion control and motion networks is driven by a desire to reduce wiring, lower cost, and increase reliability. When launching a development project to tap into these benefits, you will face choices regarding network buses, protocols, and a host of technical issues. In the end it is the architecture and physical configuration of your machine that will dictate which motion network you select and how you distribute the control problem. The cost savings and flexibility offered by motion networks can be substantial, but the key is knowing what kind of solution will work best in your control application.

Architecture

Architecture means the structure and organization of the control problem. Broadly speaking there are flat motion control applications, where a number of motors all must be controlled more or less equally by the central PC (we use PC here to mean the software program that controls the overall flow of the machine but this could be a microprocessor, or even a PLC), and there are hierarchical applications where the axes are clustered into 2, 3, or more functional axes. Figures 1 and 2 show this.

An example of a flat motion control problem is a printing press with multiple servo-controlled spools. In this application timing is critical, and the central controller, usually a PC or PLC, must drive all axes in synchrony. Typical commands in such a system are “move axis #1 to position X, move axis #2 to position Y,” etc.

An example of a hierarchical motion control application is a semiconductor wafer handling system that has a central robot (4 axes), a wafer aligner (3 axes), and a valve controller (1 or 2 axes). In this architecture the network typically connects the local robot or valve controllers to a central PC, but the actual motion control is local to the robot, aligner, or valve. Thus the overall machine controller doesn’t give commands such as “move robot axis #2 to position 12345,” it gives commands such as “extend robot arm” which the local robot controller interprets and executes.