Design and Implementation of an Ethernet-Based Linear Motor Drive for Industrial Transport Systems

Ethernet-based motor drives are hard real-time control systems used to operate servomotors through the industrial Ethernet. Recently, Ethernet-based drives have drawn attention as a solution for industrial transport systems where numerous linear motor drives move magnetic shuttles individually or collectively to accurately position production parts. This paper presents the design and implementation of an Ethernet-based motor drive that enables delay analysis for synchronized motor actuation and sensing to build a scalable and precise industrial transport system. Our software design constructs the drive function using periodic tasks run by a rate-monotonic real-time scheduler and performs worst-case response analysis to determine the end-to-end delay required for the control host to actuate or sense the motor in the drive. Based on the calculated drive delays, clock-based I/O using Ethernet-provided global time realizes synchronized motor operation across multiple drives. In the Ethernet-networked control system, different phases of the host cycle with respect to the drive cycle can result in different actuation and sensing delays. To reduce the delay, we propose a phase-shifted loop method and present a heuristic to find the best phase that minimizes the normalized drive delays. Experimental results obtained using a prototype EtherCAT drive show that the phase-shifted loop significantly reduces the difference between the commanded and feedback currents while properly managing tracking errors. Performance evaluations are performed to investigate the impact of different Ethernet technologies on delays. Elaborated delay models are developed for EtherCAT and Ethernet Powerlink, and a comparative study of delay performance is conducted for various parameters, such as the number of drives, the message size, the network topology, and the bandwidth.