The past is prologue

Up until about ten years ago, there were just two basic types of motion controllers. The first is shown in Figure 1, and is one form or another of a multi-axis motion card. In this architecture, the motion card connects to external amplifiers, which generally accept +/- 10V analog signal input, and control torque or sometimes velocity of the motor. Early on, the path planning and servo feedback required a dedicated numerical computer and was often rack mounted. Today, variations on this approach center around card bus type. Popular formats today include PCI, PC/104, compact PCI, and Ethernet.

This architecture has a number of important advantages, primary among them flexibility. The motion controller is independent of the motor power level, and often even the motor type. For example if the motion controller outputs a single phase +/- 10V signal, this can be connected to a DC Servo motor amplifier, or a Brushless DC motor amplifier which in turn performs commutation. If the user wants to increase the power of the motor, or change the motor type, the motion card doesn't need to be changed, only the amplifier.

Another important advantage of multi-axis motion cards is that synchronization among axes is straightforward, and essentially automatic. This is because most such systems generally use a single CPU (central processing unit) or DSP (digital signal processor) to “time-slice” its computation for each axis. Taking advantage of this capability, some motion cards provide builtin multi-dimensional profiling commands such as “draw an arc with radius x” or even support complete machine tool instruction sets such as G-code.

Multi-axis motion card

The primary disadvantage of this architecture is wiring complexity, and cost. For a typical servo motion axis, there are 15-25 wires that connect to and from each motor axis, depending on whether differential signals are used, and whether the controller or the amplifier performs commutation. Imagine building a controller for a ten-axis system using this approach. You would need bundles carrying hundreds of wires through the machine. This is a complex, costly, and potentially failure-prone design.