Wireless Sensor Network – Performance Evaluation in the field

K. B. Batra, Saswati Srivastava

Keywords: battery life, gateway, wireless HART, interoperability, ISA100.11, WSN, performance evaluation, wireless technology, update rate, wireless sensor network

Abstract- With continuing developments in communication technology, use of wireless devices and networks is increasing at a rapid pace. However end users are skeptical of the reliability and satisfactory performance of wireless technology especially for process control applications. The confidence to use wireless technology can be built by first using and evaluating the technology for monitoring non-critical applications. A case study of implementation of wireless network through sensors in a Generator testbed has been presented with an expectation to imbibe confidence amongst users for use of wireless in industrial applications. This is followed by performance evaluation of the wireless sensor network w.r.t checking of the reliability of instruments, ruggedness of network in presence of obstacles, electromagnetic interference, battery life deterioration etc. It is then concluded that wireless sensor network can be safely used for monitoring applications. However for time critical, fast response and high reliability applications it is still not ready.

I. INTRODUCTION We are witnessing an explosive growth in wireless communications and mobile devices. However its spin off effect on use of wireless sensor networks (WSN) for industrial process control has been limited. In this paper, we will try to find out the reasons for this by using a case study on implementation of a WSN in an industrial environment and evaluation of its performance.

The actual case study, the basic technology itself, its advantages over wired networks, challenges faced and potential areas of application are discussed. Later on, the configuration of wireless sensor network and its implementation are described. Performance of the network has been evaluated w.r.t reliability, update times, battery life etc. Finally the results of the study have been presented. The Wireless sensor network includes three main elements: sensors, gateway and propagation protocols. These are briefly described:-

Financial support:-This work is supported by Bharat Heavy Electricals limited (BHEL Haridwar (HEEP).

Sh K. B. Batra is with BHEL Haridwar, District Uttarakhand , India, Pin 249403 (email: kbbatra@bhelhwr.co.in) Ms Saswati Srivastava is with BHEL Haridwar, District Uttarakhand , India, Pin 249403 (email: saswati@bhelhwr.co.in)

A. Sensors with Battery power

The sensors at the field are at the foundation of any process that sense and detect the parameter related to the process such as temperature, pressure, level, flow, acceleration, vibration, position, humidity etc. The technological evolution has resulted in these wireless sensors getting smarter, lighter and cheaper. As these devices are battery powered, battery life becomes a major factor in selection of wireless instruments for a process.

B. Gateway

The Gateway enables communication between the field devices and host application connected to high speed backbone.

1) It Powers the wireless sensor network for deliveringwireless connectivity with field devices.

2) It acts as an Interface Module for handlingcommunication between field devices and hostapplication.

3) It acts as a Network manager for creation of policiesw.r.t field device configuration, communication ofperformance and operational status.

4) It acts as a Security manager for Authentication andValidation of data packages.

C. Propagation Protocol

The wireless communication comprises of the transmission protocol and method of propagation of data.

1) Transmission protocolStarting from various proprietary protocols, the worldhas narrowed down to two standard protocols namelyWireless HART and ISA 100.11.

a) Wireless HART has been developed by HARTCommunication Foundation. This is a wireless versionof 4-20mA industrial monitoring standard. It includesIEEE 802.15.4 standard radios using TDMAtechnology. It offers time synchronized self organizing,self healing and mesh architecture and supportsoperation in 2.4 GHz ISM band. Each device can serveas a router for messages from other devices. It achievesreliability through devices by providing alternate routesfor transmission of data. Also security and effectivepower management are its key capabilities. Referfigure 1 for a typical HART network.

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Fig. 1. Wireless HART Network

b) ISA 100.11 is developed by ISA and is intended to be asingle “universal” network for the wireless transport ofinformation from all types of industrial protocols such asHART, Profibus, Fieldbus and Modbus. It includes 2.4GHz IEEE 802.15.4 and 802.11 standard radios usingTDMA technology. It offers both mesh and startopology of field devices. There are some field devicesin the network which route messages and some of theserouting devices are configured to be connected torouters. Refer figure 2 for a typical ISA network.

Fig. 2. ISA 100.11 network

The routing devices located at the edge of network do not have routing capabilities. This network is attached to a host application through a gateway. The gateway provides transition from ISA100.11 into host application. Additionally, ISA100.11a uses Internet-conforming IP addressing to make data from field devices addressable remotely. It achieves reliability of data through multiple gateways rather than multiple devices.

2) Propagation method

The industrial wireless sensor network uses electromagnetic air waves (radio and infrared) for data propagation from one point to another without any physical connection. The radio devices in the antenna transmit and receive signals in the form of packets in the licence free 2.4 GHz ISM band. The networks mostly use Spread Spectrum technology i.e. either Frequency Hopping Spread Spectrum (FHSS) or Direct Sequence Spread Spectrum (DSSS). Refer figure 3

FHSS transmits data in a narrow frequency that hops around a wide frequency band. The frequency changes in a pattern known to both transmitter and receiver. It avoids interference by hopping over different frequencies.

DSSS transmits data by spreading over a wide frequency range. It avoids interference due to concentration of desired signal and dilution of any interfering signal. This enables recovery of original data even if parts of it are damaged during transmission

Using spread spectrum significantly increases the reliability of data and thus end users confidence on wireless.

Fig. 3. FHSS and DSSS

II. ADVANTAGES OF WIRELESS SENSOR NETWORK OVER TRADITIONAL WIRED NETWORK

Wireless sensor networks are offering many advantages and outperforming wired networks in the following respects:

1) Reduction of installation and operational costs due to less infrastructure and elimination of cabling.

2) Elimination of constraints associated with re-wiring.

3) Lowering of labour costs by elimination of cumbersome manual monitoring techniques.

4) Improvement in process safety by automating activities previously unmanageable with a wired solution.

5) The wireless devices can communicate the diagnostic data such as health of device or process with a “warning” from a failing device. Thus due to possibility of monitoring and predicting device health, the devices can be used efficiently.

III. CHALLENGES

Despite having several advantages, the implementation of wireless sensor network is not easy and is filled with challenges for vendor as well as end user.

A. Users are having apprehension regarding effects of wireless sensor network on existing wired network or on existing wired instrumentation.

B. They are facing challenges from using equipments of multi vendors and from multi protocol in a given wireless sensor network. Thus interoperability due to different market participants has become a major challenge and is further aggravated by the association of heterogeneous network protocols. At present vendor products conforming to ISA100.11a cannot communicate with vendor products conforming to wireless HART.

C. Some critical control loops in the industries have

response time requirements as low as 10msec. Wireless sensor networks are definitely not suited for such fast applications. In fact they may not be able to go below 1 sec response time at the best.

D. Due to electromagnetic noise interference with wireless

communications, RF signal can corrupt or even eliminate the data signal. Vendors have to meet the challenge for selection of the right techniques to improve the reliability and accuracy of data.

E. During wireless implementation some users want to

make use of existing infrastructure or product. Thus they would like to select product with features of plug and play options for their process applications thus posing compatibility issue for vendors.

IV. POTENTIAL AREAS FOR WIRELESS APPLICATION Challenges should be looked upon as opportunity and potential areas of industrial applications can be explored where wired networks have not been able to provide solutions for possibility of implementation of wireless. Not only will we be able to overcome the hindrance posed by reduced manpower and harsh environments, but also we can begin to have confidence in wireless. Following are examples of such areas:-

A. The Industrial Environment such as plants, factories, refineries are full of remote sensing applications such as coal yard, ash handling areas, cooling ponds, feed water inlets, water treatment areas, sanitary sewer flow etc. The environment here is either too hot or electrically noisy causing safety threats to workers. When the distance is beyond the capabilities of typical cabling, simple wireless instrumentation can serve the purpose.

B. The unreachable and immeasurable areas such as rotating kilns, portable skids, boiling water flow and other moving equipment have process fluid temperatures considerably higher or lower than ambient conditions. The measurement can become viable through surface mounted temperature sensors mounted on them and feed input to respective wireless instruments.

C. The SCADA applications such as Electrical power

transmission and distribution, oil and gas pipeline monitoring etc can use wireless as back-up communications link in case of normal network failure.

D. Certain competitive companies monitor material levels

to track the rate of material usage. These can use wireless for effective monitoring to enable timely replenishment.

Table. 1. Wireless HART Network

Fig. 4. Layout of wireless items at testbed site

E. Certain applications having gas leakages in pipelines, steam leakages due to stuck up steam traps etc, go unattended and result in huge loss. The wireless acoustic technology can come to the forefront and solve the leak detection problem.

V. CASE STUDY DETAILS

Despite several ways for evaluation of the wireless technology, BHEL HWR decided to gain hands on experience by conceiving a small wireless sensor network at Generator test bed at BHEL works. The application chosen for case study was related to power plant environment, primarily monitoring of Turbo generator auxiliaries for safe operation of Generator. The test bed facility is used for testing of large size Turbo Generators of upto 800 MW rating. Wireless HART technology with mesh network and propagation through FHSS was chosen for configuring the network. Table-1 refers to details of instruments, w.r.t the type, location and update rate assigned to each instrument. Two nos. each of wireless pressure transmitters (PT1 and PT2) and wireless temperature transmitters (TT1 and TT2) were chosen. All the four measurements were used for monitoring/display of parameters and not for any real time control. A tentative layout of these items in testbed site has been shown in figure 4 followed by a brief explanation and details of the application along with our objectives for this case study.

Sl no

WIRELESS INSTRUMENTS -TAG

DESCRIPTION MOUNTING LVL

BATTERY UPDATE RATE

1 PT1- MKG10CP002 H2 gas pressure in manifold 0 M 8 sec

2 PT2- MKW31CP003 Seal oil pressure in header (-6 M) 16 sec 3 TT1- MKW11CT001B Seal oil temperature before coolers (-6 M) 32 sec 4 TT2- ACW02CT003B Auxiliary cooling water temp before H2 coolers (-4.5 M) 64 sec 5 hg1420 Gateway with detachable antenna 0 M

VI. APPLICATION DETAILS

The Application involved monitoring of three of generator auxiliaries namely Gas Control System, Generator Shaft Seal System and Stator Cooling Water System. MKG10CP003 refers to “Hydrogen (H2) gas pressure in manifold” after release from hydrogen cylinders. Hydrogen acts as a coolant for the stator windings during running operation of TG. ACW02CT003B refers to “Auxiliary cooling water temperature before H2 coolers”. The heated hydrogen gas is continuously cooled by water coolers. The temperature of the auxiliary water to the coolers should not exceed beyond limit for efficient cooling of hydrogen. MKW31CP003 refers to “Seal Oil pressure in header”. The right amount of seal oil is pressure is meant for preventing H2 gas escape through the gap.

MKW11CT001B refers to “Seal oil temperature before coolers”. Some seal oil during injection at shaft seals enters the hydrogen casing and thus gets heated. This is collected and cooled by coolers prior to re-circulation.

VII. OBJECTIVES OF IMPLEMENTATION OF WIRELESS SENSOR NETWORK

1) Successful establishment of wireless communication

A typical plant is crowded with metallic obstructions, walkways above and below resulting in inaccessible locations to reach. Table. 2. Battery life dependence on update rate

VIII. INSTALLATION AND CONFIGURATION

With the objectives clear in our minds, following steps were carried out for configuring the network. 1) Installation of wireless Instruments

The wireless instruments were installed in parallel to the existing wired instruments at the same location. Existing tapping was used for the pressure instrument using a Tee connection. For thermocouples & RTD, second spare element was connected to temperature transmitter.

Also there might be hazardous leaks, emissions, noise and interference due to machines such as motors etc. Thus establishment of wireless sensor network in an industrial site would imply that the system is rugged and can withstand harsh site and interference conditions.

2) Verification of reliability of data captured over wireless sensor network

Another important goal of any measuring network is reliable data delivery. It is difficult to verify wireless data reliability as the data available is not time stamped. Hence the task requires certain tools for comparison. Thus data integration from third party systems onto DCS is necessary along with creation of trends and reports to enable comparison of the wireless values with those of existing instrumentation in DCS for interpreting reliability and integrity of the network.

3) Evaluation of battery life Vendors claim long battery life of wireless instruments of upto 10 years with largest update rate. Higher battery consumption can disrupt the process. Hence the claims need to be verified so that appropriate planning for replacement of batteries can be put in place without disruption of the process. A typical battery life consumption for pressure and temperature transmitters has been indicated below. Refer table 2 However it is necessary to determine a similar pattern for implemented devices by plotting battery voltage with time.

2) Configuration Configuration of wireless Instruments to Gateway can be done by means of Hand held communicator or by PC through dedicated software. In this case Hand held communicator was used.

3) Establishment of ethernet Communication

Establishing communication between wireless Gateway to PC (host application) through ethernet ports on Gateway via existing Ethernet switch (already connected to DCS)

Update 4 sec 8 sec 16 sec 32 sec 60 sec 120 sec 300 sec

Pressure Transmitter (no. of years)

2.0 3.1 5.2 8.0 10 10 10

Temperature Transmitter (no. of years)

0.7 1.3 2.3 4.1 6.4 9.3 10

4) Establishment of serial Communication Communication between wireless Gateway to PC can also be established through serial ports on Gateway via cross cables with suitable connectors. A serial converter is also required for communication between RS485 serial port on gateway and RS232 port on PC.

5) IP Addressing This comprised of aligning the IP address of Gateway (hg1420-gateway) in line with IP address of BHEL DCS and accessing the webpage of Gateway through the new IP address.

6) Selection of update rate Selection of the “update rate” of instruments is done to optimize between desired output and longer battery life. Different update rates were selected for various instruments to see the affect on battery life as detailed in Table-1

7) Noise Interference RSSI (radio signal strength interference) values corresponding to (-20dBm) to (-90dBm) signify noise interference within limits during wireless propagation. The values should be kept within limits to avoid unnecessary loading on battery .

8) Antenna placement The antenna should be mounted at a location where there is no movement of any physical obstacle. Care should be taken to properly place the lightning arrestor and fuse connected to antenna.

IX. ACHIEVEMENTS AND INTERPRETATION

1) Successful establishment of wireless communication

9) Antenna Cable It was observed during installation that the cable joint between Gateway and antenna cable is very fragile and pulling can cause a crack. This needs to be handled carefully.

10) Path stability After implementation of all conditions satisfactorily, the measured values of the transmitters on web page of Gateway and path stability were checked. 100% path stability indicated the successful establishment of the wireless sensor network

11) Communication with DCS DCS gateway was used for establishing third party communication with wireless gateway. On the workstation, a page was designed and necessary linking was done to obtain both wired data from conventional hard wired instruments as well as wireless data from wireless instruments mounted in parallel to enable comparison between them.

12) Trending and Report generation

The tabular data for enabling comparison between wired and wireless values is not sufficient to give information regarding delay in transmission, missed updates etc. Hence trending of data and creation of reports was carried out. The reports are detailed in the following section.

Report 1. Main screen on Web page of Wireless gateway

Interpretation-This is a web page in gateway which gives information regarding the successful establishment of mesh network with 100% reliability. The node state with green indication indicates the health of devices. The active neighbours indicate the devices acting as repeater along with being transmitter. The path of device communicating with gateway only will have less stability. The no. of joins indicates the several connects and disconnects such as system shutdown, initialization, maintenance etc.

2) Verification of reliability of data captured over wireless sensor network through tabular comparison

Verification of reliability of data captured over wireless sensor network through trend generation

3) Evaluation of battery life

Report 2. DCS-Wireless integration on main screen

Report 3. Trend on main screen

Report 4. Battery trending in web page of gateway

The above tabular comparison and trend generation give information for comparison of wireless and DCS values. It is seen that values from both sources are similar with tolerable variation. There is automatic updation of values due to linking with the source data.

The above plot shows the trend of battery voltage with time and is helpful in determining the battery life. The variables impacting battery life are mainly Device Power Consumption for standard operations such as sensor power, data acquisition and conversion, digital output actuation etc, communication and transmission retries due to poor RF connectivity, data transmission Update rate frequency, radio Diagnostics requirement, Local display, temperature and humidity effects, battery construction quality type etc. The transmitter will not be able to transmit when battery voltage falls below a minimum value.

X. CONCLUSIONS

It can be concluded that despite physical obstacles and noise interferences existing in the plant, the data could be acquired accurately without loss. It was seen that alternate paths for transmission of data were available and were working satisfactorily. Also it was possible to interpret wireless data accuracy and data reliability through successful data integration in the plant DCS.

However, maturity and reliability of wireless technology for better control applications is still to be explored. Update times would need to be substantially improved to enable the use of wireless for control applications. Several critical issues such as interoperability, compatibility etc are needed to be resolved through better design for wider acceptance. As the network was installed only few months back, battery life is still under evaluation w.r.t update times

At the time when the network was installed, redundant gateway configuration was not available with the vendor. This reduces the reliability of the network. At present redundant gateways are under development. Continuing future research and development promises to resolve several key issues such as interoperability.

Wireless sensor network may be commercialized as an add-on component for meeting customer needs in select areas where it offers definite advantages.

This will enable gradual change in mindset from traditional wired system to another promising system with significant advantages. Potential users can gain experience of energy saving, power management, smart routing etc. They can be prepared for meeting customer future requirement and stay on top. When wireless technology and sensor network become fully matured, they promise to introduce a paradigm shift in the process control industry.

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[4] Gang Zhao, ”Wireless Sensor Networks for Industrial Process

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