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Automatic retry can double the noise immunity of real-time industrial Ethernet-based motion networks.
Most modern motion control systems employ Ethernet-based networks to transmit data among various electrical and electronic components. The electrical noise immunity of these networks is critical to operation, as are the methods employed to deal with interruptions in data transmission due to electrical noise and other factors.
Designers of real-time motion control systems expect Ethernet-based motion networks to transport cyclic command and feedback data at specified intervals with perfect data integrity. The designer's selection of the motion control system's gains and trajectories is predicated on this fundamental assumption.
But in many industrial applications, Ethernet cabling must be located in the presence of electrical noise caused by power switchgear, large motors, or other electrically noisy equipment. If such noise interferes with the network and causes data loss, the designer's assumptions are invalid and the system will not behave as designed. Problems such as control loop instability and tracking errors can result, as can other operational issues.
To optimize system performance when real-time Ethernet networks must be operated in electrically noisy environments, potential data loss due to noise must be characterized and accounted for in the system design.
One strategy to reduce data loss is to use a network protocol that incorporates retry, which is a mechanism for automatic retransmission of corrupt or missing data within the same transmission cycle. If retry is built into the network hardware, no explicit action is required by master or slave to detect errors or trigger data retransmission.
This article quantifies the contribution of retry to improved noise immunity by testing the noise immunity performance of two real-time industrial Ethernet protocols and comparing the results. The two real-time industrial Ethernet protocols are Mechatrolink-III, which includes retry, and network X, which does not. Although the trade name of network X isn't specified in this article, its noise immunity performance is similar to other Ethernet-based motion control networks that don't incorporate retry.
Design factors
Factors that influence the noise immunity of a motion network include:
* The noise immunity of the physical layer. Relevant design factors include properties of the network cabling (shielding), the signaling scheme (single-ended vs. differential), and details of the transmit and receive circuitry (isolation, impedance, filtering,...