Networking Technologies for Motion Control Applications

Overview

Intense global competition is pressuring device or machine builders to deliver equipment that increases throughput while reducing operating costs. Rising energy costs and environmental awareness are prompting engineers to design for lower energy consumption as well. Because of this, equipment builders have switched from designing single-purpose machines to creating flexible and effective multipurpose devices by adopting modern control systems and sophisticated algorithms and integrating high-end electronics and communication technologies into mechanical structures.

Motion control is critical to these mechatronic systems. To optimize the mechanical systems, machine builders often substitute mechanical parts with electronic solutions. One example is the elimination of rigid shafts to perform camming operations. These shafts are replaced by a combination of drives and motors that rely on control software to provide camming functionality. Such systems and devices are mechanically more flexible, easier to maintain, and smaller. However, caveat emptor: These machines also contain more electronic components and require complex control and deterministic and reliable communication.

Figure 1. Traditional Approach with Mechanical Components

 

 

Figure 2. Today's Approach with Multiple Synchronized Axes

 

In a typical system, the motion controller takes most of the burden of the increased complexity. By handling multiaxis synchronization, these controllers offer gearing and camming functionality as well as all the additional safety features such as limits switches, drive enable, and emergency stop. Besides that, the motion controller still needs to provide the traditional functions and run the control algorithms for maximum performance and efficiency.

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Motion Controller Technology Trends

Traditionally, motion control applications used a dedicated and separate motion controller to handle greater control complexity. The increasing performance of today’s automation controllers, such as programmable automation controllers or programmable logic controllers, is fueling a trend to integrate the functionality of the motion controller directly into the automation controller and run it as a high-priority task with other automation tasks.

Since the early 1980s, automation systems have been based on digital buses to perform tasks such as transferring process data and performing industrial communication. Compared to analog buses, they are more reliable and robust, especially for communication via long distances. In addition, digital networks simplified the wiring because they allow connecting multiple elements in a serial fashion instead of connecting each element individually. This results in significantly cheaper cabling and easier-to-maintain systems.

Because of the performance requirements, for many years the motion industry took a different approach and connected to the drives via an analog bus or specific motion buses. This additional communication scheme increased the complexity of the overall system. With the digital drive trend and high-performance digital buses, it is now possible to simplify the system architecture by using the same bus for motion control and process data and, at the same time, gain significant performance improvements through the advantages of digital buses such as EtherCAT, CANopen, PROFIBUS, Ethernet POWERLINK, or SERCOS.

Especially in applications that perform distributed multiaxis motion control, digital buses offer a lot of advantages. They provide higher flexibility and allow the development of systems with distributed processing power and decision making down to the level of the drives. With common standards, customers can easily combine systems from different vendors and choose the best solution for their individual tasks.

By using digital drives that communicate over digital buses, vendors and manufacturers are able to create drives that use the digital bus to not only exchange control-relevant data but also transfer status information or set up parameters. One of the big questions automation customers ask is which is the right protocol for industrial automation applications in general and motion applications in particular.

The most common Ethernet-based protocols for motion applications are the following:

  • EtherCAT
  • SERCOS II/III
  • CANopen
  • Modbus IDA
  • PROFINET
  • PROFIBUS
  • EtherNet/IP
  • Ethernet POWERLINK

Because of the large number of different bus standards and protocols, customers need to make sure that the components they want to use provide the right interface. This means manufacturers are often forced to develop several different versions of their components. This adds development cost and the requirement to engage in multiple standardization organizations. With their efforts to provide open systems, companies like National Instruments incorporate interfaces to various bus standards and protocols.

While offering support for all of the major industrial protocols and providing connectivity to all of the standard industrial networks, National Instruments selected EtherCAT as the premiere communication protocol for motion control applications.

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EtherCAT Technology for Motion Control Applications

EtherCAT (Ethernet Control Automation Technology) is a high-performance, industrial communication protocol for deterministic Ethernet. It extends the IEEE 802.3 Ethernet standard to transfer data with predictable timing and precise synchronization. Managed by the EtherCAT Technology Group, this open standard has been published as part of the IEC 61158 and is commonly used in applications such as machine design and motion control. 

EtherCAT implements a master/slave architecture over standard Ethernet cabling. EtherCAT masters from National Instruments consist of real-time controllers with dual Ethernet ports such as NI CompactRIO, PXI, and industrial controllers. Each NI slave also contains two ports that permit daisy chaining from the master controller.  

The EtherCAT protocol transports data directly within a standard Ethernet frame without changing its basic structure. When the master controller and slave devices are on the same subnet, the EtherCAT protocol merely replaces the Internet Protocol (IP) in the Ethernet frame. 

Figure 3. Ethernet Frame Structure with EtherCAT

Data is communicated between master and slaves in the form of process data objects (PDOs). Each PDO has an address to one particular slave or multiple slaves, and this “data and address” combination (plus the working counter for validation) makes up an EtherCAT telegram. One Ethernet frame can contain multiple telegrams, and multiple frames may be necessary to hold all the telegrams needed for one control cycle.

High-Speed Performance

EtherCAT is designed for reaching high performance and high-channel counts for single-point applications such as control. Because slave reads and writes can occur in the same frame, the EtherCAT telegram structure is optimized for decentralized I/O. Plus, the complete protocol processing takes place within hardware and is thus independent of the run time of protocol stacks, CPU performance, or software implementation.

Timing and Synchronization

Another factor in achieving deterministic networks is the master controller’s responsibility to synchronize all slave devices with the same time using distributed clocks. Out of all the slave devices, one of them must contain the master clock that synchronizes the other slave devices’ clocks. For NI implementation, the first slave device is designated with the master clock, and the master controller sends a special synchronization telegram to read the master clock in every scan cycle. This telegram then updates and realigns the clocks on all other slave devices to prevent drifting.

Accurate synchronization is particularly important in cases where widely distributed processes require simultaneous actions, such as coordinated motion between motion axes. NI uses timestamps to measure the time difference between the leaving and returning frame. In this way, propagation delay is calculated between nodes, and precise synchronization (less than 1 µs) can be achieved by the exact adjustment of distributed clocks.

Due to high-speed performance and its tight timing and synchronization features, EtherCAT is an ideal solution for networked or distributed motion control applications where a powerful real-time controller acts as the EtherCAT master and runs the motion control application communicating to external, distributed EtherCAT slave drives.

National Instruments EtherCAT Servo Motor Drives

NI EtherCAT servo drives combine performance with flexibility, scalability, and power range to meet the unique performance requirements of nearly any application – from basic torque and velocity applications to multiaxis motion control using graphical programming with NI LabVIEW and the LabVIEW NI SoftMotion Module.

Standard Ethernet cables significantly simplify the cabling and the capability to daisy chain up to 128 axes. A high-performance real-time controller allows the setup of a distributed motion control system within minutes. Using the LabVIEW project to configure and validate the motion control system simplifies the setup and graphical programming. With the high-level motion functions or the property invoke node API, customers can implement custom motion applications through the ease of use of graphical programming.

Figure 4. Graphical Axis Configuration through the LabVIEW Project

NI real-time controllers, AKD EtherCAT servo drives, NI LabVIEW, and the LabVIEW NI SoftMotion Module make EtherCAT technology easily accessible so customers can implement networked or distributed motion control applications.

Next Steps

 

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