In this article we will briefly discuss what it takes to develop a wireless HART product with intrinsic safety. Intrinsic safety is a protection technique for safe operation of electrical equipment in hazardous areas by limiting the energy, electrical and thermal, that can cause ignition. In signal and control circuits that can operate with low currents and voltages, the intrinsic safety approach simplifies circuits and reduces installation cost over other protection methods. Areas with dangerous concentrations of flammable gases or dust, that will benefit fom the intrinsic safety approach, are found in areas such as petrochemical refineries and mines.
What is wireless HART
WirelessHART is a global IEC-approved standard (IEC 62591) that specifies an interoperable self-organizing mesh technology in which field devices form wireless networks that dynamically mitigate obstacles in the process environment. The WirelessHART field networks (WFN) communicate data back to host systems securely and reliably and can be used for both control and monitoring applications. By removing the physical constraints of wiring and power as well as reducing weight and space, wireless networks increase flexibility in project execution, providing solutions which can mitigate risk and improve project schedules.
WirelessHART technology provides an opportunity to execute and build a more efficient plant compared to the traditional approach. The high cost of traditional field wiring limits the number of points that are able to be connected in a project for process monitoring, control and safety applications. Since WirelessHART does not require wires for communication or power, the lower costs enable inclusion of more process points during the project.
The design process.
For one of our assignments we are asked to develop an intrinsically safe monitoring device that can be installed in a wireless HART network. The following image shows at a high level the steps taken during the design process.
The target product is required to perform very accurate measurements and provide a time stamp with the measured data. To achieve this the signal conditioning is using linear circuits and an accurate time base. Special care is taken to only include low power and high-performance sensors.
Low Power Design
At every step of the design the power consumption is taken into account, both for meeting the intrinsic safety requirement and for maximizing battery life. To reach this goal it is vital to limit all leakage currents by minimizing currents required for pull up resistors, choosing low power components and making sure power is only applied to the circuits when they are being used. To provide a good insight of the remaining battery life parameters, such as temperature, battery voltage and power consumption are logged to predict the battery derating.
The WirelessHART protocol has been specifically designed to provide the highest level of data security to users of field devices in the process industries. While it is very easy for the end user to achieve a very reliable and secure network, it is a daunting and complex task for the product designer. There are hundreds of HART commands and test specifications that cover al the communication layers from physical layer, data link layer, network layer all the way up to the application layer. Designers are required to implement a response for all these commands and prove its interoperability with the test automation kit designed by the FieldComm Group, the standards organization maintains the HART standard. This then ensures interoperability of the product with other products that are typically part of the distributed control systems in a plant.
In this specific case the product will be installed in open environments and as such there is a need to design a dust- and waterproof enclosure which can also house the antenna. Besides keeping the product dry and clean the designer is also required to make sure the product looks elegant and aesthetically good. The ingress protection is achieved by employing different sealing techniques such as gaskets and o-rings.
Another requirement is that the product can be operated in a hazardous area. For this special care is taken in the design to be able to get approval as intrinsically safe device. Steps that are taken into account are, among others, limiting the maximum currents towards the devices used inside the product, limiting the maximum surface temperature and preventing all excess current flows in case of a fault condition This again requires careful hardware design. All this to make sure the product will pass the certification testing.
As mentioned earlier it is required to make sure the product complies to the HART standard. Once the product is completed it is required to register the product with the FieldComm group as a HART compliant device. This is done by running special tests, that are part of the test kit provided by the FieldComm Group, that also generate test reports. The test reports ultimately show that the product complies with the HART standard.
As you can see there are many aspects that should be taken into account while designing an intrinsically safe product that is compliant to the wirelessHART standard. You will need in your design team expertise in the fields of, safety standards, electronics design, mechanical design, product design embedded software design and application software design. At Flex-ES we are able to offer you the expertise in all this fields and can help you successfully design your products.
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