Motion System Design

Logic and I/O


James • Opto 22: There might have been a time when companies were limited in the type of I/O available to them. So, if you picked a Brand X control system for whatever reason (e.g., you liked the software), you'd be stuck with whatever I/O choices they had. Back then, instances would sometimes arise when you might need an I/O module that handled a particular type of signal and the platform you had didn't support that kind of module.

I think this is less of an issue today because the majority of PC, PLC, and PAC control systems can handle most if not all common industrial control signals. Motion controllers are a little different in that they may not offer the breadth or variety of I/O. Here, the choices may be more specialized to complement the type of motion for which the controller is designed.

Jason • Phoenix: Control systems that don't support major bus networks automatically limit I/O flexibility. Many motion networks support I/O, but limit the amount of data, because they don't want to affect their main motion control. It's important for the control system to support a secondary network if your application requires large amounts of I/O data.

Mark • WAGO: The choice of a control system can substantially impact the availability and flexibility of I/O options. Regardless of the device chosen, each system offers a variety of fieldbus interfaces; however, some are fixed (the fieldbus interface is built in) while others are modular (the fieldbus interface is selectable). Bottom line: The flexibility of the fieldbus interface will “make or break” the number of I/O options available to users. Additionally, the nature of fieldbus interfaces, open vs. proprietary, dictates the number of suppliers and I/O components that a user can select, which can limit system versatility.

Bob • Rockwell: PLCs typically offer a broad range of discrete, analog, and specialty I/O, available as local I/O or distributed networked I/O. Traditional PLCs often provide limited motion control capability via specialized local modules and are connected to standalone motion controllers via a network interface for more advanced motion control. A PAC may be based on a modular platform, which allows for extensive local I/O support as well as networked, distributed I/O or it may be based on an “embedded PC platform” that relies on networked distributed I/O. PACs offer similar I/O functions as PLCs, but often include advanced features like integrated motion support.

Standalone digital motion controllers typically offer limited I/O options and I/O functions. Hard PC-based controllers or soft controls usually rely on networked distributed I/O. Soft controls typically provide a broad range of I/O functions and distributed I/O support, whereas hard PC-based controllers usually have limited I/O functions and distributed I/O support.


James • Opto 22: This is so broad. Challenges could include things relating to choosing local versus remote I/O, wiring the I/O, density and expandability of the I/O (Will I have to buy more racks? Are there high density I/O options?), and ease of migration to the next generation of I/O products. It's hard to make sweeping generalizations as far as the challenges of integrating the I/O. I think it depends on the application.

Jason • Phoenix: When integrating I/O into control systems, the first question should be, “What I/O network does the control system support?” I/O networks all serve the same function of replacing parallel wiring to the control cabinet. The difference is in the way they perform this task. Variations can be found in the approach to network topology, transmission speeds, I/O capacity, and diagnostics.

Network topology (ring, star, tree, and trunk) is important because it determines how I/O devices are connected together, as well as the network's cabling limitations. Transmission speed is important because it can affect I/O update rate. But the biggest problem with higher speed is its susceptibility to industrial noise, meaning that the fastest network is not always the best network for the application.

I/O capacity is also important because each network has limitations in the number of devices it can support along with a maximum digital and analog I/O count. Diagnostics is one of the most important and most overlooked options when choosing an I/O network. If the networks don't contain accurate diagnostic routines, time can be wasted troubleshooting network problems.

Mark • WAGO: There appears to be a perception among end users that learning curves are the main challenge to working with various control systems and I/O integration. As an industry, it's important to combat this perception by pointing out that the general interoperability between suppliers and systems is good. Whether users realize it or not, they have created this by demanding that suppliers minimize the variances between systems and components — most designers, understandably, do no want to be forever “tied” to one supplier and system.

International organizations such as ODVA and PTO have been established to ensure consistency across the industry in the development and production of technology. These organizations guide worldwide suppliers in adhering to open communication standards and protocols, ensuring that products will operate in a multiple-supplier environment.


James • Opto 22: As motion controllers adopt more computerlike development platforms, they can become more tightly integrated with hardware technologies and other systems they might interact with that employ those same platforms. For example, motion controllers with programming platforms that offer computerlike features — like easily understood GUI (such as flowcharting) and plain English commands — allow users to create motion control processes that flow, share, and integrate more easily with machines and systems like second party HMIs or a backend database.

Ladder logic, in comparison, is by its very nature much more esoteric and less “open,” relying on an interface of diagrammed rungs, rails, and coils, plus register numbers and tag mapping. The result is that ladder logic controllers do not integrate or communicate as easily with second party HMIs, backend databases, and those other more computerlike systems.

Mark • WAGO: From an I/O standpoint, the new generation of motion controllers is creating a greater need for distributed I/O. These evolved motion controllers appear to be replacing PLCs, mitigating the need for local, rackmounted I/O.

Bob • Rockwell: The general trend for motion programming has been to move to high level, standardized, and open programming languages and allow multiple languages to be used in the same project. This enables more efficient development of complex algorithms, better use of control resources, and higher performance applications.

The IEC 61131-3 standard that supports ladder diagram (LD), structured text (ST), instruction list (IL), function block diagram (FBD), and sequential function chart (SFC) is being adopted by many vendors. The best implementation lets users mix different programming languages in the same project and choose the language that best fits the specific task. For example, LD may be used for basic machine interlocking and ST may be used for advanced algorithms and math intensive code development.


James • Opto 22: Standardization for the purposes of integration is a must. But it's also necessary so we can all keep our sanity. For those looking to accomplish motion control, there should be solutions out there that offer a cohesive development platform for applications that require motion control along with analog, digital, and serial control. Let's face it, most applications today are hybrid apps — not confined to just one of the above.

To combine and handle stepper motor and other motion control along with process control, sequential logic, string and data handling, math, etc., we will start to see multidomain systems becoming more prevalent. Some of the characteristics of these systems are modular architectures, higher density I/O, and de-facto standards for network interfaces, programming languages, and communication protocols.

Bob • Rockwell: Distributed I/O will continue to grow and EtherNet adoption for distributed I/O control will increase at a rapid rate. EtherNet will drive a single network solution for a broad range of I/O, closed and open loop drives, safety I/O, and specialty devices. Time synchronization services on EtherNet will provide the ability for high performance control across distributed networks of controls and devices.

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Meet the experts

James Davis
Opto 22

Jason Haldeman
Phoenix Contact

Mark DeCramer
WAGO Corp.

Bob Hirschinger
Rockwell Automation

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