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Profibus: small installation benefits

02 September 2014

The ability of Profibus to connect to an instrument remotely is generally considered to be a feature employed only in larger plants. However, this situation is now changing, according to James Powell, business developer, PI Communications at Siemens Industry Automation.

Working with a customer in the oil and gas industry, Siemens was able to demonstrate the advantages of Profibus in small-scale applications. Furthermore, it has been able to deploy this solution in a cost-effective manner.

The customer had a large number of sites – ranging in size from five to 30 tanks – that were using SITRANS LR250, Siemens’ liquid radar transmitter. Communication issues had been experienced and the company needed assistance. Operators were losing communication with the transmitters at various rates. For most of the sites, the problem only occurred sporadically. But at about 9% of the sites, the issue was more problematic. 

The customer was using a HART to Modbus RTU converter connected to a micro PLC. We were able to determine that the equipment was fine. However, the company was having problems sizing the HART resistor and were running into some limitations with the HART physical layer.

For the HART message to be transmitted and received, at least 250 ohms of overall loop resistance is required. Determining the value required for the resistor becomes more difficult in the case of multi-drop and long runs. In addition, each HART device has a minimum startup voltage. Depending on the various voltage drops and the available power supply, this was found to cause problems. 

Another issue was that both the power supply and loop resistance change slightly at low temperatures. The sites are located in the northern United States, and problems occurred whenever the temperature dropped to -40°C.

HART uses frequency shift keying (FSK) to transmit the ones and zeroes over the 4-20 mA signal. One of the known limits to HART is in high-noise environments, particularly when variable frequency drives (VFDs) are involved. Our experience has shown that HART works very well in point-to-point and short distances. However, the longer the distance – and particularly in the case of multi-dropping – the greater the chance that problems will occur. 

We were able to fix many of the issues by simply better sizing the HART resistor and increasing the voltage of the power supply. However, this did not solve all of the problems. Furthermore, costs were incurred whenever problems arose. In this application the HART physical layer, when used in this multi-drop way, was not robust enough to always withstand the noise generated by the VFDs. 

This led the company to look at Profibus PA. We already knew from experience on many other sites that the PA physical layer is robust and can withstand the noise generated by VFDs. What we did not know when we first started to consider using this solution was that it was also economical.

Changing market
When Profibus was first released, it was only available on high-end DCSs and PLCs. The solution was cost-effective, but only for plants that had a large number of I/O. It did not make good economic sense to purchase a processor that could serve 500 I/O. 

Over the last eight years, however, Profibus has moved down into the smaller processors and is now even found in micro-PLCs. The Siemens SIMATIC S7-1200, for example, has a Profibus interface. Cost-effective Ethernet to Profibus gateways are also now available.

The PA physical layer has good noise immunity. This was demonstrated at a site where a PA segment that was terminated in water was found to still work. The same site also had a number of variable speed drives, yet everything still worked.

Profibus PA is also easy to design and assemble and it eliminates the need for a correctly sized resistor. The design rules are also very simple, especially for a small site. PA also offers wiring versatility. A trunk line can be used and simple ‘T’ junctions created to reach each instrument. For these junctions, simple terminal strips can be used.

Two other features, provided by Profibus PA in this application, included the ability to wire according to the Intrinsically Safe (IS) method, which limits the power flowing to the field, eliminating the chance that a ‘spark’ can occur. 

The second, was that it provided transparency – giving the ability to communicate to the instrument over the network – down to the instrument level. In this application, we offered two levels of transparency. The first level enabled workers to connect in at the panel in the safe area and to program and troubleshoot the radar transmitters from this location. 

The second level of transparency involved adding a GSM/GPRS modem and giving the customer remote access to the site via the Internet, using a secure Virtual Private Network (VPN). This gives the ability to resolve problems remotely.

Because the customer did not want to switch PLCs, we needed an Ethernet to Profibus gateway. We selected the HMS Anybus gateway which provided all the features required. 

Since the initial investigations, we have found a way to further lower the costs – in fact, they are now even lower than the original HART solution. This was achieved by changing the PLC to a Siemens SIMATIC S7-1200 and using an isNet PNgate from IFAK for the Ethernet to Profibus gateway.The SIMATIC S7-1200 has a Profinet interface, which makes the integration of the network very easy. 

The costs of implementing a Profibus PA system have dropped considerably. Furthermore, this type of system can be cost-effectively deployed, even at plants with a smaller number of I/O. 

This solution is easy to put together and offers features such as good noise immunity, IS, and the transparency that the process market needs. 

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