Till 1980’s industry used pneumatic instruments and process data communication was normally 3 to 15 psi pressure. The evolution of semiconductor technology brought the popular current communication such as 4 – 20 mA where instrument can use up to 4 mA for its own power consumption and passive measurements. The basic aspect of these standards for measurement is safety. These signals does not create hazard to plant if used safety instruments like zener barriers. In India, still 4-20 mA is most popularly used considering its flexibility and ease of use.
User Need Finding:
Since 1990, digital communication development has been started considering following basic needs of the industry:
Solutions to user needs:
Intrinsic safety technique can be employed to the instrument in case instrument and its digital communication is designed for low power and low temperature rise of the electronic components of the circuit. HART communication became very popular as HART modem is using very low power and works on low frequency.
Now wireless instruments are gaining popularity because of availability of system on chip (SoC) solution works on low power excellent receiver sensitivity of the order of -92dBm and output power up to 3 dBm. These wireless instruments works on duty cycle basis for example it wakes up for 50 milliseconds senses and sends data after every 2 seconds and instrument goes to sleep mode for next approx. 2 seconds.
Ease of Installation:
There are many requirements such as two wire transmission i.e. giving power and signal on the same cable and better battery life for wireless instruments, easy to connect and form the network. HART and fieldbus physical layers are designed such that they will use the same cable for power and digital communication. The software is designed such that it produces diagnostics features and configure it easily using remote work stations. Batteries with high energy density chemistry such as Lithium Thionyl Chloride are used. The wireless sensor software is designed in such a way that it forms ad hoc network and finds alternate path in case of breaking a path. Device description language is developed for hassle free commissioning of instruments.
To achieve high reliability required for process control, communication protocols used task priority based on severity of data communication, fault reporting. In wireless networks, protocols are designed for time deterministic communication by using techniques such as beacon enabled communication where a fixed time slot is allocated to every instrument in the network, so that there is no collision in the air.
To enable end user to form a network of instruments from different vendors different foundations are formed who gives compliance certificates by vigorously testing the instruments for different use cases. HART communication foundation, Fieldbus Foundation, ISA100 Wireless Compliance Institute are making efforts in this direction.