Motion control: To network or not to network?
Machine controller cards
The major alternative to distributed drives is a machine controller card, also called a motion control card. The distinction is that a motion control card connects via a backplane bus to a separate motherboard or processer card, but here we will refer to stand-alone single-card controllers and backplane motion cards as machine controller cards.
In the machine controller approach a microprocessor holds the machine's application code, and a motion controller IC, also called a motion processor, generates profiles, does servo loop closure, and manages the time-critical elements of axis control. Note that it is possible, particularly for simple control applications, for the machine application microprocessor and the motion processor to be one and the same.
The advantages of the machine controller card approach are many-fold including easier serviceability since repair of the entire controller card is a simple swap-out. Another advantage is reduced wiring since the amplifiers are located on the card. Finally, the physical form factor of the card along with the connector interfaces can be tailored to suit the application.
There are two major variations of machine controller cards: off-the-shelf and custom built. Off-the-shelf cards, particularly bus-connected motion cards, have been around for a long time and are available from several different vendors.
Custom built cards, while more work on the design side, are also a strong choice. The most important trend here is integration of the amplifier, either IC or module-based, directly onto the card.
Another trend is use of off-the-shelf IC-based motion controllers. These units provide profile generation, servo loop closure, commutation, and a myriad of time-critical functions such as automatic safety responses, programmable breakpoints, and other types of automatic motion axis management.
How to choose a network
Here’s how to select a motion control network. Certain factors may make one architectural approach more suited than another.
When considering a distributed motion network, try to anticipate the kinds of signaling that will be required in your application. Does the behavior of the motion depend on the status of signals located on another part of the machine? Will you place sensors, and other non-motion controlled actuators, such as relays, on the network bus? How quickly does the motion have to shut down if an error occurs?
Another important consideration regarding how, and how much, you can use a network-based approach is the mechanical organization of the connected machine. This issue addresses questions such as, “How will the machine be serviced if electronics are physically distributed throughout the machine?” Although the traditional card rack that the technician services may be a mess of wires, there is something to be said for keeping everything under one roof. Serviceability and lifetime ownership cost strongly affect control system design choices.
Remember also that distributing the control by placing amplifiers near the motors may not always be feasible for weight, heat, or other environmental reasons. The traditional control rack cabinet can be air conditioned and insulated from the machine operating environment. This is often not possible if the controls are distributed.
When is one control approach used over another? There is no easy or simple answer, and sometimes two architectures can be used equally well for a given application.
In broad terms, the more cost sensitive the application, the more likely it is that the person designing the motion application will design a card and, depending on power level, integrate on-board amplifiers. When designing a card, it is possible to choose exactly the connectors needed and set the form factor of the card for that particular motion application.
Highly synchronized applications such as machine tools will gravitate toward multi-axis motion cards or a tightly coupled distributed drive approach. These drives allow a lot of flexibility in motor type and power range. Don't forget that a motion control card will be needed for overall path generation, or you will use a PC running dedicated G-code software.
A large middle ground of applications, such as medical automation, semiconductor automation, scientific instrumentation, and low-power general automation, can be served by several approaches including off-the-shelf machine controller cards, custom-built machine controller cards, or loosely coupled distributed drives.
- Chuck Lewin is founder and vice president of engineering, Performance Motion Devices. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, mhoske(at)cfemedia.com.
At bottom of this article, look for related articles on motion control.
- Understanding motion architectures can help with machine design and network selection and design.
- Two motion control devices include distributed drives and machine controller cards.