EtherCAT is a real-time Ethernet protocol conceived explicitly for industrial applications. It is characterized by high communication efficiency, which permits control loops to be closed with short cycle times, and is provided with a suitable mechanism, known as distributed clock (DC), that enables synchronized operations to take place across the controlled system. These features can be profitably adopted, for instance, to support motion control applications. In this paper, the performance of the DC mechanism is evaluated by means of a thorough campaign of experimental measurements carried out on a real network setup. A number of factors have been taken into account that can affect accuracy and precision, and their effects studied in depth.

Evaluation of EtherCAT Distributed Clock Performance

The Controller Area Network (CAN) provides the basis for many cost-effective distributed embedded systems. In this paper, we present a novel approach to networked control systems (NCS) analysis and design that provides increased control performance for a set of control loops that share a single CAN network. This is achieved by enabling the following functionality for each control loop: first, standard periodic messaging is guaranteed to ensure stability, and second, non-periodic additional messaging is added whenever bandwidth is available in such a way that the aggregated control performance for all control loops is improved. To validate the presented approach, a proof-of-concept implementation is presented, and the extensive experimental results show the type of benefits that can be achieved as well as the resulting behavior depending on several key parameters. Moreover, it demonstrates that the approach can be implemented in practice.

https://esaii.upc.edu/people/pmarti/10TII.pdf

Runtime Allocation of Optional Control Jobs to a Set of CAN-Based Networked Control Systems

Despite the significant advantages of the Controller Area Network (CAN) there is an extended belief that CAN is not suitable for critical applications, mainly because of several dependability limitations. During the CANbids project each one of these limitations has been addressed and a complete architecture for CAN-based fault-tolerant systems has been devised. This architecture allows building highly-reliable systems. This paper describes the design of such an architecture and the prototyping of its fundamental parts.

The design of the CANbids architecture

The controller area network (CAN) bit stuffing mechanism, albeit essential to ensure proper receiver clock synchronization, introduces a significant, payload-dependent jitter on message response times, which may worsen the timing accuracy of a networked control system. Accordingly, several approaches to overcome this issue have been discussed in literature. This paper presents a novel software payload encoding scheme, which is able to guarantee that no stuff bits will ever be added to the data field by the CAN controller during transmission and, hence, lessens jitters considerably. Particular care has been put in its practical implementation and its subsequent evaluation to show how the simplicity and inherent high performance of the scheme make it suitable even for low-cost, embedded architectures.

Fixed-Length Payload Encoding for Low-Jitter Controller Area Network Communication

In islanded microgrids, parallel operation of voltage source inverters is commonly enabled by primary droop control to achieve power sharing and secondary control that removes the frequency deviation by mimicking at some extend the operation of an integral controller. Often the main task of the distinct secondary control approaches is to compute a corrective term considering the frequency error that is then used by the droop mechanism for frequency compensation. This paper considers the secondary control approach where local integral controllers that compute the frequency corrective term are implemented at each inverter. And considering that integrals are performed in different nodes, the paper shows that global time between nodes achieved through clock synchronization is mandatory for accurate power sharing and frequency restoration.

Synchronization of local integral controllers for frequency restoration in islanded microgrids

The controller area network (CAN) is facing a great opportunity. The maturity of this technology makes many researchers believe that CAN may be adopted in more critical systems. However, the suitability of CAN for these challenging applications strongly depends on our capacity to integrate all the solutions already available into a single, comprehensive architecture. We claim that clock synchronization plays a fundamental role in such architecture. Therefore, the means to achieve a solution fulfilling the expected requirements on reliability, cost, and precision must be deeply investigated. This paper discusses the relevance of clock synchronization in the future of CAN systems and describes a novel solution to supply this service. This solution exhibits several advantages: it provides very high precision, causes very low communication and computation overhead, and includes mechanisms to provide fault tolerance. Moreover, and in contrast to previous proposals, it is designed to be orthogonal to the rest of the system. Thus, it can be directly incorporated to any CAN system, without having to replace any of the components, which reduces the cost increment caused by the new service.

S. Roca

Papers

Orthogonal, Fault-Tolerant, and High-Precision Clock Synchronization for the Controller Area Network