Wireless Network: The Next Generation Communication Methods of Field Instruments
Sugata Bandyopadhyay
Consultant (Instrumentation & Control) at
TATA Consulting Engineers Ltd.

C. Sailaja
General Manager (Instrumentation & Control)
TATA Consulting Engineers Ltd.

The Industry 4.0 and IIOT had defined a digital domain to the advantage of the Industry & New area of business prospects are picking up. The prime considerations that are ushered in are digital codification of equipment and devices and communication through wireless for information exchanges. Thus wireless communication had become a necessity now and not a preference. This paper will address the requirement of digital wireless communication, its technical aspects and its challenges with reference to Wireless Networking of smart process sensing devices.

The digital wireless communication had a much wider purview covering human & material identification, their movement, acquiring condition monitoring information of plant and equipment and other plant information including its logistics data. We will however address here the wireless communication as pertinent in the process industry. Wireless technology is more often considered for monitoring applications than for Control applications, especially in remote / inaccessible places in process areas where it is difficult to lay the cables. They are also used hey are also used in trial applications in Pilot plants as also models in R & D which are temporary in nature.

The Objective of this paper is to discuss the Wireless technology in Process Plants, the considerations for implementing a Wireless Sensor Network (WSN), including its Architecture, advantages & limitations and the Data security and interoperability between different protocols and devices.

Wireless Technology in Process Plants:

Wireless is a term used to describe telecommunications in which electromagnetic waves (rather than wired connection) carry the signals over part or whole of the communication network. Wireless communication involves codification or modulation of the signal on a carrier frequency, transmitting the packaged information over a longer distance without the use of wires or cables. The distance over which communication may be established can vary from a few meters to thousands of kilometers. Depending on the distance of communication, the type of wireless communication can be selected. In Shortrange wireless communication, signals travel from a few centimeters to several meters. In contrast, signals in Medium-range wireless communication can travel up to 100 meters or so, while signals in Wide-area wireless communication can travel from several kilometers to several thousand kilometers.

Frequency Bands

The frequency of communication is one of the most important factors which need to be considered when implementing a wireless sensor network. The 2.4 gigahertz (GHz) communication frequency is the most widely used band for industrial applications for the following reasons:
  • World wide availability for unlicensed use.
  • High data rate (250 Kbps).
  • Lower power requirement.
Wireless Solutions for the Process Industry

The Industrial Wireless technologies can be classified into three main categories based on their range / area of coverage. They are Global canopy, a Site backbone and a Field mesh. The Global canopy is also known as long range wireless communications which is used for long distance data communication. It comprises private networks joining two sites / locations which are hundreds of kilometers apart or Public network like Internet or satellite communication for long distance data transmission.

A site backbone is a communication method in which data is transmitted over a few miles from node to node. Although the distances covered are shorter than with a global canopy, a site backbone network can still be used to transmit data over relatively long distances.

The field mesh or WSN is used for transmitting a few kilobytes (kB) of data over a short range up to a few hundred meters. This field wireless network comprises sensors and actuators, field mobile devices and these types of networks will be discussed in this paper.

The modern wireless technology utilizes IEEE 802.15.4 compatible DSSS (Direct Sequence Spread Spectrum) radios and operates in the 2.4GHz ISM radio band. The IEEE 802.15.4 also supports multiple bands. Two standards using the IEEE 802.15.4 radio technology are IEC62591 (Wireless HART) and ANSI/ISA100.11a-2011 (ISA100.11a/ IEC62734), which are the most important standards accepted by the industry. In wireless networks, typical network topologies are Star, Mesh or Hybrid network (a combination of star and mesh). All field devices in Wireless HART support full Mesh topology, as they have routing capability. On the other hand, the I/O devices in the ISA100.11a network can be defined as nodes with or without routing capability. It thus supports both star, mesh and hybrid topology.

Factors to be Considered When Implementing A Wireless Sensor Network (WSN):

(a)Understanding Measurement Requirements
While designing the system, the basic considerations are the parameters that require monitoring, measuring or controlling with the WSN system, the Sensors used to acquire this data and how the sensors communicate with the Data Acquisition System. Power is an important factor for WSN systems. . The sensors in WSN application are powered by batteries, considering they could be located in remote / inaccessible areas. Hence external power availability is not always feasible. The internal battery provides upto 50mA at 12V which powers the sensors. Lower sampling rates (100 Hz or below) are considered to conserve the battery power and extend its life.

Hence the parameters considered for monitoring and its response time are important factors. Considering the latency of the Process parameters, like pressure, temperature, level, flow etc, the sample rate adequately meets the requirement.

(b) Selecting a Wireless Protocol

The Wireless standards can be selected based on the power requirements, throughput and range. Cellular and WAN standards such as GPRS, WiMAX and EDGE provide significant throughput and range. However, they consume more amount of power. The Bluetooth protocol requires very little power, but does not provide adequate range. For WSN systems communication distance requirements are of the order of 100 to 200 mts. However, Standards such as IEEE 802.11 and IEEE 802.15.4 consume a reasonable amount of power & can provide adequate throughput and range for wireless measurement systems.

Table1 brings out the distinct differences between IEEE 802.11 & IEEE 802.15.4 devices. The IEEE 802.11 devices have the advantage of high bandwidth which is associated with increased power consumption. On the contrary, IEEE 802.15.4 devices such as ZigBee, 6LoWPAN, Wireless HART and ISA 100.11a, have the advantage of range & power.

( c) Software and Data Access

The other important factor is the deployment of the appropriate software responsible for acquisition, data handling and analysis, transmission to local close & far away nodes & presentation in appropriate Format.

Functional decentralization of intelligence and controlling capabilities for the remote nodes enhances system performance & optimization.

( d ) Wireless Network Topologies

The various Topologies for WSN are Star, Tree, Mesh and Cluster. The star topology is preferred when concern for battery power preservation is the prime consideration while the other topologies are considered, when the limitation of power supply does not exist.












The simplest network topology is the star having one node at the center which both sends and receives data directly from all of other nodes in the network. Thus, the center node handles both sending and receiving operations. The other nodes have the only distinct function of transmitting information to the node at the center. This result in energy saving as the end nodes beyond data transmitting to the center node does not have any other function. Further any end node failure does not stop functioning of the other part of the network as long as the center node is operating. In the mesh topology each of the nodes can both send and receive data & hence can each exchange data with the other nodes network. Thus the mesh topology ensures more reliability as in case of any node failure the other nodes establishes new route of communication ensuring that the balance part of the network is functional. With a combination of star and mesh topology all other network topologies may be configured.

Architecture of WSN



WSN's are networks for real time applications comprising a number of nodes. The function of these nodes is to sense the process parameters such as Pressure, level, temperature, humidity etc. After sensing, these nodes, the data is stored in their memory and passed to the Control system for further processing. Any device to connect itself to another device needs a protocol stack. The protocol stack of WSN comprises of 5 layers.
  • Physical Layer
    In the Physical layer, transmission,reception and modulation functions are implemented.
  • Data Link Layer
    In the Data link layer, noise elimination, avoiding collisions and a well defined interface with the higher layers are implemented.
  • Network Layer
    In the Network layer functions like addressing, connecting and forwarding the data sent by the above layers are addressed.
  • Transportation layer
    Transport layer ensures and organizes the data flow in the network.
  • Application layer
    Application layer assists in building various applications based on the sensing data.
In addition, a central system manager is associated with routing, communication & scheduling of the network in totality.

The above 5 layers are aided by three modules as mentioned below:
  • Power management module:
    As the nodes are battery power dependant, this module organises optimal utilization of the power.
  • Mobility management module:
    This module manages the movement of the nodes along with their neighbors
  • Task management module:
    This module balances and schedules the sensing tasks in a specific region of the network
Advantages of WSN
  • Implementation cost is cheaper than wired network
  • In some situations where wired connections are exposed to extreme conditions, a wireless replacement can actually be more reliable. In other situations, a redundant wireless connection can serve as a backup for wiring.
  • It eliminates cabling.
  • Introduction of new devices as node can be accommodated at any time
  • Easy conversion of an existing plant having Wired Smart Transmitters with 4-20mA over HART to Wireless network using Adapters. (Note: External power is needed for Adapters while the communication is Wireless)
Limitations
  • Unless adequate protection is taken, the system is susceptible to hacking. Performance may be affected by presence of blocking elements like building walls, interference from devices like Microwave oven etc.
(e) Data Security and Interoperability between different Protocol Devices

In the wireless frequency band 2.4 GHz, there are many users with technologies like Wi-Fi, Wireless HART, ZigBee, UWB and others and hence if proper security measures are not adopted, there are chances of interference and data corruption.

The methodology to be adopted to ensure no interference & data corruption occur:
  • Encryption: Proper encryption of messages. In ISA100.11a data communicator follows a 128-bit Advanced Encryption Standard (AES) .
  • Authentication: - Authentication of members participating in communication whether it be transmitting or receiving. Only members authenticated by the system or security controller are allowed to participate in sending & receiving data.
  • Integrity: To retain integrity of communication, each network node uses a Media Access Control address (MAC address) which is a distinct property assigned to network interfaces for communications to confirm data integrity and transport security. MAC addresses are used as a network addres for most IEEE 802 network technologies, including Ethernet and WiFi.
  • Key Management: All the network nodes must have a join key that may be assigned to be the password recognised by devices to grant entry into the network.This is adopted in the ISA100.11a protocol. By adopting this methodology, the security of the network may be increased & and unauthorized participants may be restricted from joining the network.
(f) Requirement to establish secured communication in a platform of multiple simultaneous wireless users:

To ensure security, reliability and data integrity, measures for co-existent operation of other users in RF environment must be ensured. In IEEE 802.11, there is provision for 11 channels in the 2.4GHz frequency range. The channel width is 22 MHz and each channel is separated by 5 MHz. IEEE 802.15.4 also supports 14 channels in 2.4GHz frequency range, with 2 MHz separation, thus having reduced bandwidth than the Wi-Fi channels at the same frequency.

Conclusion

To sum up, it may be concluded that IEEE 802.15.4 had become the choice of all the users and manufacturers of Wireless instruments and networks. IEEE 802.15.4 supports IEC62591 (Wireless HART) and ANSI/ISA100.11a - 2011 (ISA100.11a / IEC62734). The primary choice had been decided on the battery life which is very important for a wireless device which has no other source of power, while the speed, range and bandwidth meet the process requirement adequately. Further the features and protections adopted to maintain the signal integrity has been time proven and so user shall have no hesitation to switch to wireless network. With advent of new technology of solar cell with light sensitive nanoparticles, soon we will have facilities of powering the smart wireless instruments by integral body mounted solar cells by which we can extend the life of the batteries. Hence in the near future the process networking through wireless will become the only choice.

REFERENCES:

1. National Instruments - White Paper
2. Five factors to consider when implementing a Wireless Sensor Technology - NI.com
3. White Paper - ISA100 - Application, Technology & Systems