Abstract
Guaranteeing isochronous transfer of control commands is an essential function for networked motion control systems. The adoption of real-time Ethernet (RTE) technologies may be profitable in guaranteeing deterministic transfer of control messages. However, unpredictable behavior of software in the motion controller often results in unexpectedly large deviation in control message transmission intervals, and thus leads to imprecise motion. This paper presents a simple and efficient heuristic to guarantee the end-to-end isochronous control with very small jitter. The key idea of our approach is to adjust the phase offset of control message transmission time in the motion controller by investigating the behavior of motion control task. In realizing the idea, we performed a pre-runtime analysis to determine a safe and reliable phase offset and applied the phase offset to the runtime code of motion controller by customizing an open-source based integrated development environment (IDE). We also constructed an EtherCAT-based motion control system testbed and performed extensive experiments on the testbed to verify the effectiveness of our approach. The experimental results show that our heuristic is highly effective even for low-end embedded controller implemented in open-source software components under various configurations of control period and the number of motor drives.
Highlights
Cyber Physical Systems (CPS) are physical and engineered systems whose operations are monitored, coordinated, controlled and integrated by a computing and communication core [1]
On an EtherCAT-based motion control system test-bed, we evaluated the jitters of the intervals between successive control frames observed at the motor drives for varying number of motor drives and control cycle
In a modern Motion control systems (MCS) considered in this paper, a motion controller and a number of motor drives, which are interconnected through industrial communication links, cooperate with each other in a synchronized manner
Summary
Cyber Physical Systems (CPS) are physical and engineered systems whose operations are monitored, coordinated, controlled and integrated by a computing and communication core [1]. A typical MCS consists of computational units such as a motion controller and motor drives, and physical components such as actuators and sensors, that are tightly coupled and collaborate with each other in synchronized manner. The end-to-end actuation delay refers to the time interval from the dispatch of control commands at the controller to the corresponding actuation at a motor drive. In a modern MCS considered in this paper, a motion controller and a number of motor drives, which are interconnected through industrial communication links, cooperate with each other in a synchronized manner. The motion controller periodically generates control messages containing the commands of target position or velocity and transmits them to the motor drives. From the industrial robot example shown, we can derive real-time constraints generally imposed by the modern MCS
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