Rewriting the classics?

Figure 1 shows the classic bus-based motion board architecture that uses a controller card and separate amplifier modules. Cables connect the motion card to the amplifiers, the amplifiers to the motors, and the motors to the card. Because the popular standards for motion cards have evolved to support PCI, PC/104, compact-PCI and others, this architecture is still relevant today. But its weakness is the number of cables required to interconnect everything. The cost of this complexity is measured in dollar terms as well as reliability; since connectors are an important source of failures and signals on wires can degrade with distance.

The attendant costs of this approach represent a major dilemma in motion control, and solving this problem through the use of alternate architectures has been a major priority in the past ten years. Generally speaking, the goal is greater integration at the card level to eliminate connectors and cables.

Figure 2A shows an alternate control architecture which integrates the controller function and the amplifier function onto a single card. 2B shows a corollary version where the host software is also included on the control card, in the form of an oncard microprocessor. This second architecture is sometimes referred to as a machine controller, since it provides all control functions necessary to run the machine. Either of these approaches offer the advantage that the controller and the amplifier do not require cables to be interconnected, and the amplifier is much less expensive since it is integrated at the IC level rather than purchased as a standalone unit.

When is it appropriate to integrate the motion controller with the amplifier onto a single card? There are many aspects to this question, but the single most important factor is the power rating of the amplifier. Combining the motion controller with the amplifier tends to be most viable in lower power systems, such as those that drive NEMA 34-sized motors or smaller.

Another consideration is bus architecture. If other parts of the control system use a standard parallel bus, then it makes sense to locate the motion card on the bus, and separate the amplifier function. Although convenient in one respect, this represents a constraint to integrating the controller with the amplifier. By comparison, network/serial buses do not have this limitation, since there are no physical form factors to mate the control card to. This is one of the reasons that there has been so much excitement about distributed networks for motion control.

Bus-based motion card architecture

2A) Integrated controller/amplifier; (2B) Machine controller(intergrated motion controller with host microprocessor)