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Isochronous Wireless Network for Real-time Communication in Industrial Automation

Authors:
  • rt-solutions.de

Abstract

In industrial automation systems the deployment of wireless technologies is more and more common. This is mainly due to applications which consist either of moving components, for example rotating machine parts, or require a high degree of flexibility. However, due to their high real-time requirements the implementation of applications, such as wireless networked control systems (NCS), is rather limited or even impossible with existing wireless technologies. The objective of this dissertation is to design an isochronous wireless network for industrial control applications with guaranteed latencies and jitter. The main challenges are a non-deterministic medium access of existing systems, the uncontrolled, shared wireless medium and its limited capacity, as well as the asynchronous behaviour of wireless and wired communication systems degrading the temporal behaviour of such hybrid systems. Based on the requirements analysis of real industrial applications and the characterisation of the wireless channel for the application scenario, a solution approach is presented consisting of a deterministic, TDMA-based medium access control, a dynamic resource allocation and the provision of a global time base for the wired and the wireless network. The global time base allows a seamless and synchronous integration into existing wired Real-time Ethernet systems. An implementation prototype of the proposed wireless system and a simulation case study are used for the evaluation of the solution approach in two case studies. The prototype is used for the evaluation in a real factory environment and for the validation of the simulation model. Due to given scalability constraints of the prototype, a second case study based on a realistic simulation model is conducted. A realistic channel model for the simulation, implemented based on the channel characterisation, allows more realistic simulation results. The obtained evaluation results show that latencies ≤ 10 ms and a maximum jitter ≤ 100 μs can be achieved with the presented solution approach as long as all components are active. Thus the solution approach allows a deployment within NCSs with the given requirements. As soon as components are deactivated, the behaviour of the network is significantly degraded and the requirements cannot be satisfied any more.

Chapters (8)

This chapter provides a general state of the art and relevant background for this work. Both wired and wireless communication systems for industrial automation have been considered with their characteristics, even though the main focus is put on wireless systems. Existing technologies and solutions are analysed with respect to the relevant application requirements as identified in Sect. 1.1. The related work for the three main components of the solution approach, i. e., medium access control, resource allocation, and the provision of a global time base is discussed at the beginning of their corresponding chapter.
The industrial environment significantly differs from well-known office and home environment. Besides posing several challenges on technologies and systems due to harsh environmental conditions, such as increased temperatures or vibrations, it is remarkably different in terms of deploying wireless technologies. Mainly with respect to two aspects, the industrial traffic characteristics and the industrial wireless channel. Hence, both aspects are further investigated in this chapter by means of extensive experiments in real environments. In Sect. 3.1, two factory automation applications representing the RMS application category are analysed with respect to their temporal requirements.
In this chapter the solution approach of an isochronous wireless network for realtime communication in industrial control applications is presented. The system is based on IEEE 802.11 [70] to allow a seamless integration of the wireless system into existing RTEs and to ensure a sufficient capacity. The approach has been published in [164, 168].
In this chapter an approach for a deterministic medium access control for industrial real-time traffic is proposed. The determinism is achieved by a central coordination of the medium access using a TDMA-based approach. According to [19] this is only solution to avoid random delays when accessing the shared wireless medium. The presented approach is using appropriate mechanisms on layer 2 of the communication system to achieve this goal.
In this chapter an approach for the resource allocation component for the IWN is proposed. The resource allocation is responsible for allocating the required resources to nodes in the network. It considers currently admitted traffic flows of the AP and the available capacity of the channel. It also guarantees that the application requirements can be met by all admitted flows.
In order to allow a synchronization of all IWN system components, including the wired communication network, a global time base must be established by means of clock synchronization. Establishing a system wide synchronization has two major advantages. First, different communication systems can be operated synchronously, i. e., the deterioration of the system capabilities, especially in terms of temporal characteristics, caused by asynchronously running communication systems can be reduced or even minimized.
In this chapter the key components of the presented solution approach of an Isochronous Wireless Network (IWN) are evaluated to prove its validity based on our main application scenario. The experiments are conducted based on a prototypical implementation and a simulation case study.
The realisation of a highly flexible, complex, labour divided, and geographically distributed production is one of the main topics of the German initiative “Industry 4.0". In order to achieve this goal, new ways of interconnecting industrial facilities are necessary, for instance by using innovative wireless technologies. In addition to this, the efficiency and flexibility of control systems can be increased significantly. This is mainly due to applications which consist either of moving components, such as rotating machine parts, or require a high degree of mobility.
... More recently, an extension of IsoMAC [17] has been proposed in which new functionalities are added. Among others, the incorporation of a variable duration for frame retransmission just after the uplink slots This latter one can be seen in Fig. 1. ...
... The AP will be considered the centralized controller of the wireless network and will coordinate the access to the wireless medium through TDMA-based MAC respecting the control cycle of the system. The control cycle of the system is considered as the time interval from the reading of the sensor until the PLC output is applied by the actuator [17]. To control the wired network, another cyclic TDMA MAC will be implemented. ...
... To control the shared medium of the wireless extension a deterministic way, a cyclic TDMA that is based on [17]. Because NCS is periodic real-time traffic, time slot assigned. ...
... Thus, the transmission of unnecessary copies of the packets is avoided. In [12] IsoMAC is proposed, which consider a TDMA-based superframe. This superframe implements a scheduled phase for the transmission of both uplink and downlink real-time traffic and a contention phase for the Best-Effort (BE) traffic and synchronization messages. ...
... Similarly, the PLC output must be provided to all actuators in the network at a defined time instant. The time interval from the reading of the sensors until the PLC output is applied by the actuator, or what is the same, the time interval between two readings is called the control cycle of the system [12]. In the middle of this control cycle, all the data processing that includes the transmission times occurs. ...
... This scheduler can be fixed, i.e., it will always follow the same structure superframe after superframe, or it can be calculated dynamically according to the WIOD requests. This mechanism of sending the scheduler at the beginning of each superframe by means of beacon use is used in IsoMAC [12], a MAC proposal previously described in Section II. ...
... A numerical analysis of network dimensioning, cost analysis, and network performance and capability using an NS3 simulator is presented in Sect. 6. Finally, conclusions are drawn and future research directions are identified in Sect. ...
... The survey doesn't discuss the coexistence of these networks. An IEEE 802.11 based isochronous wireless network is presented in [6] to provided seamless connectivity and sufficient capacity for real-time communication in industrial control applications. Similarly, an LTE Advanced-Pro and 5G based wireless network in terms of modified picocell and modified distributed antenna systems are presented in [7] and [8], respectively and a comparative study is reported on IEEE 802.11an/ac protocols. ...
Article
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To attain automation across different applications, industries are beginning to leverage advancements in wireless communication technologies. A ”one-size-fits-all” solution cannot be applied since wireless technologies are selected according to application needs, quality of service requirements, and economic restrictions. To balance the trade-off between technical and economic requirements, a multi-band heterogeneous wireless network architecture is presented and discussed in this paper. Wireless local area network (WLAN) and distributed antenna system (DAS) with Long Term Evolution (LTE) are considered as the backbone for the multi-band heterogeneous network into which other wireless technologies can be integrated. The technical and economic feasibility of the network are evaluated through a techno-economic analysis (TEA). The economic feasibility of the proposed network is measured in terms of net present value while the technical feasibility is measured in terms of network throughput and latency. Finally, network performance for DAS with LTE and WLAN are verified using an NS3 simulator for machine-to-machine, real-time video, and high-definition video data transmissions. The TEA analysis showed that the number of DAS units required to achieve technical feasibility is less than WLAN units, but the overall cost of DAS units are higher compared to WLAN units, even without taking into consideration industrial, scientific, and medical band technologies.
... An example of such a TDMAbased communication protocol for IEEE 802.11 above MAC layer is presented in [18]. In [19], [20], the authors have proposed TDMA-based access with modifications directly in the MAC layer. The major pre-requisite for these TDMAbased approaches is accurate CS in the range of ≤ 1 µs. ...
... For such networks as well, there is a need to provide temporal consistency to enable a synchronous communication between the wired and the wireless entities of the system. Hence, a global timebase is required, which ensures integration of wireless subsystems with the base wired network to enable a seamless real-time performance [19]. This ultimately requires availability of synchronized schemes for not only wired, but also wireless communication. ...
Article
Just like Ethernet before, IEEE 802.11 is now transcending the borders of its usage from the office environment towards real-time communication on the factory floor. However, similar to Ethernet, the availability of synchronized clocks to coordinate and control communication and distributed real-time services is not a built-in feature in WLAN. Over the years, this has led to the design and use of a wide variety of customized protocols with varying complexity and precision, both for wired and wireless networks, in accordance with the increasingly demanding requirements from real-time applications. This survey looks into the details of synchronization over IEEE 802.11 with a particular focus on the infrastructure mode which is most relevant for industrial use cases. It highlights the different parameters which affect the performance of clock synchronization over WLAN and compares the performance of existing synchronization methods to analyze their shortcomings. Finally, it identifies new trends and directions for future research as well as features for wireless clock synchronization which will be required by the applications in the near future.
... The synchronized clocks are not only required for control and monitoring of devices but are also critical for time-based contention-free channel access schemes such as time division multiple access (TDMA). For example, authors of [4] and [5] have shown that high accuracy clock synchronization over WLAN in the range of few microseconds is a pre-requisite for TDMA-based communication for various industrial applications. ...
Conference Paper
IEEE802.11 Wireless Local Area Networks are an appealing complement, if not an alternative, to Ethernet-based industrial solutions because it not only can match Ethernet’s high throughput but also leads to reduced costs and more system- design flexibility. However, like in Ethernet, clock synchronization service for applications has not been inherently present in IEEE802.11. This paper analyzes the clock synchronization mechanisms of IEEE802.11, which has become a major com- munication technology to establish the Internet of Things in industries, and how they can be used to provide high precision clock synchronization. In doing so, this work discusses the different parameters which can affect the performance of clock synchronization over the wireless channel, such as timestamping quality, clock adjustment, and synchronization overhead. An outlook to the future includes the new trends for synchronization for wired-wireless hybrid and fully wireless mesh networks where the IEEE802.1AS audio video bridging networks and IEEE802.11s mesh networks are leading the line, respectively.
... For instance, in [18] it was shown, how a cross-domain manager (CDM) is able to translate in real-time the state information (e.g., latency in the network) from the communications domain in order to optimize the performance of the control domain (e.g., through adaptive controller redesign). In [22] it was shown how very accurate timestamps can be generated even over standard WiFi systems, enabling a global clock synchronization over both domains, enabling sophisticated control algorithms as timestamped messages and an accurate clock in the receiver enable to calculate precise delay-adjusted estimation values. These examples show that there are many interconnections between the two domains that can be jointly optimized. ...
Chapter
Dependable wireless communications is considered to be one of the key enablers for digital, flexible, and smart production systems in industry. However, what are the design requirements of industrial radio systems (IRSs) if highly dependable industrial applications are to be implemented with them? Does an IRS need to guarantee maximum packet loss rates on a link level of, e.g., 10⁻⁹? Are new and more precise performance metrics and design approaches needed to characterize, evaluate, and improve radio access technologies to become reliable for ultra-dependable and time-critical industrial applications? This chapter highlights selected aspects and approaches to assist in finding answers to these questions.
... Both approaches, red and green, require a joint understanding of the current time, which translates to tight requirements of synchronizing clocks over wireless channels. For instance, in [22], a wireless clock synchronization scheme is presented that is capable of keeping the clock offset between a master clock (here: controller) and local clock (here: plant) lower than 1.5 µs (95%-ile) even with commercial-off-the-shelf WLAN hardware. It is assumed that with specialized hardware and more sophisticated clock synchronization strategies, this accuracy will be even outperformed. ...
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Control over wireless channels promises to be a great enabler for an interconnected world. Historically, the “control engineering” and “wireless communications” domains were seen as separate, but with upcoming 5G networks, joint design of wireless control systems promises large gains in both the domains for a wide range of applications. By means of a typical industrial use case of the automated guided vehicles (AGVs), we present a methodology to analyze the latency requirements along with the wireless links from a controller to a plant (downlink) and from a plant to its controller (uplink). From the perspective of a Wireless Communications Engineer, we present a framework to analyze the basic properties of the resulting control cycle in order to derive feasible latency values that differ from the commonly found values in the communications literature. Also, we highlight an approach to derive the proportional-derivative (PD) controller parameters that yield the best control performance according to the integral of absolute error (IAE) criterion. At last, we present the idea of a cross-domain manager (CDM) that is able to translate (in real-time) the current network performance metrics to optimal controller gains.
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