Motion Control in Factories: Choosing the Right Solution
The need for Ethernet-based communication within the factory has grown over the past 10 years. Detailed information about the tasks and performance of machines at every stage is critical to productivity. As capabilities have expanded from performance reporting to predictive maintenance warnings and beyond, the need to connect multiple controllers within the same machine and elsewhere in the factory has also grown.
Machine Control Formats
There are two formats when incorporating high-performance servo motion control in a machine. In Figure 1, the motion and machine controllers are separate. The motion controller focuses on motion and the machine control — a resident programmable logic controller (PLC) or personal computer (PC) — handles the rest of the machine control. The advantage of having a separate motion controller is that it will often have more functionality and processing power dedicated to motion performance.
More motion types such as camming and more operational modes such as torque, speed, gearing, and more time-critical I/O (Input/Output) are benefits of standalone motion controllers. In addition, with motion control as the central focus, higher motion performance and precision is often possible when motion and machine controllers are separate.
The second type of machine control combines both the machine and motion control into one controller, as demonstrated in Figure 2. Thanks to increasingly powerful processors, there are now motion controllers that include PLC functionality. This can eliminate the need for the PLC or a PC-based machine controller, which increases the potential for lower total machine control cost.
Motion controllers are used in a variety of industries including medical, laboratory automation, robotics, printing, labeling, material forming, pharmaceutical, packaging, food and beverage, tire and rubber, and postal sorting. The primary data out of the motion controller is performance-related. The data can include how efficiently the machine is making widgets, how many widgets have been made, warnings about machine errors or limitations, and unexpected variations such as a heater in the machine operating at the incorrect temperature.
While knowing that a machine is operating properly and at peak efficiency provides peace of mind, knowing when it is not running properly can save money, time, and resources. Suppose you have a motor drawing more current than it should, indicating a mechanical problem, or a cut-to-length machine making improper cuts. Having a system programmed to provide machine performance data is critical to identifying and fixing these issues quickly.
Information moving from the machine controller to the motion controller is command-focused. These commands could be instructions or recipes for machine setup; for instance, part numbers or the parameters for making a specific widget type. They could also include motion specs such as distance, speed, camming motion points, and other operational details like periodic reminders for machine operators to perform routine maintenance.
Three types of information transfer mediums are:
Ethernet-PLC Oriented Networks — These are the common Industrial Ethernet Fieldbuses incorporated into traditional PLCs such as Ethernet/IP, Profinet, EtherCAT, SERCOSIII, vendor-specific, and others.
Ethernet-PC Oriented Networks — These are traditional PC-based networks such as UDP and HTTP.
Other Methods — Web servers, SD cards, remote memory, FTP, and VPN/eWON.
Ethernet Communications Basics
Transportation medium is copper wire, CAT5 or CAT6 cable with RJ45 termination. Cables can be up to 100 meters in length between each node.
Connection topology can be a line, a star, or a ring, although many network implementations only support a subset of the three.
Built-in electrical isolation is needed in a lot of motion or machine applications to help eliminate part variances and maintain precision motion control.
Deterministic or non-deterministic transfer of information, depending on the network and how it is configured.
Update times can be 500 milliseconds or higher, and down to 250 microseconds, depending on the applications and the network.
Automatic network configuration and integrity checks are available to ensure the quality of the network transmission.
Information passed between the motion controller and the machine controller or external controller is often called parameters, variables, or tags in the form of single objects or in the form of a data array or structure.
The Ethernet-based Modbus TCP/IP is built on the Modbus RTU protocol. It is an industrial standard used with and supported by a variety of devices and control products. Modbus TCP/IP has a standard address block scheme for transferring binary and non-binary data in 32-bit or 16-bit formats. It is non-deterministic, and the data update time can vary. In general, the performance range for update times between two devices is 20 to 500 milliseconds.
Motion Design Magazine
This article first appeared in the October, 2018 issue of Motion Design Magazine.