Fieldbus: Commissioning with Advanced Diagnostic Tools

In process plants with fieldbus plant operators benefit from the use of fieldbus for measurement and control tasks. Intelligent field devices provide higher levels of accuracy. Only one translation from an analog to a binary value is performed – and the transmission of this binary value is ensured using checksums and other mechanisms common to any digital transmission. This leads to control loops acting with higher accuracy thus creating better quality output with higher consistency.

Modern day field devices can transmit additional status information such as the break-away and run time of valves or other status referring to device usage. Predictive maintenance is performed based on the actual status of the field device versus time-based at regular intervals. DCS manufacturers respond with comprehensive asset management systems. They facilitate need-based maintenance activities sometimes doubling time intervals compared to standard procedures.

Additionally, remote configuration via the fieldbus speeds up change over activities i.e. in batch processes or when a field device is exchanged during plant operation. With all these and more benefits gained, fieldbus has proven its value even in the conservative segment of plant process automation.

Many novice users view the fieldbus network itself as a black hole or at least a black channel. Data is mysteriously sent by the DCS into that channel and received by a field device and vice versa. There is a simple reason: When multiple devices are connected in parallel to the same cable, communication signals have a higher degree of complexity. This is obvious when compared to 4…20 mA technology. One device is connected per cable and the load current shows it all: measurement value, cable breakage as well as short circuit conditions.

Operations and maintenance personnel describe obscure plant behavior in ambiguous words such as: “The device had been operating fine for the whole day – and then it disappeared for a few hours only to re-appear again.” Or: “The LCD’s of field devices were energized but the devices were in the live list just for a short time.” In most cases, this is because handheld tools cannot decipher data signals and inform the user about details necessary to see clearly, what is going on in the fieldbus cable.

Modern day fieldbus diagnostic tools – such as the Advanced Diagnostic Module for the FieldConnex® Power Hub – bring transparency to the fieldbus physical layer and communication. The module performs measurements such as supply voltage, load current, signal level, line noise or jitter. The module listens to the communication and distinguishes between segment and device information. Clear and concise displays within the Diagnostic Manager show readouts to the user – this brings transparency to a plant condition that seems ambiguous.

Many everyday scenarios exist undetected that have a detrimental effect on the fieldbus physical layer and communication:

• An existing cable was used with improper capacitive and inductive values
• An improperly closed housing may allow water ingress. Corrosion will cause the signal transmitted by the field device to slowly decay
• Connections may degrade due to vibrations
• Cable or field device components deteriorate over time
• Wire strands from the shield may cause bad isolation between the two poles or against ground
• Improperly chosen grounding and shielding techniques may cause loops that enable induction of noise

The compound of multiple cases often causes situations leading to the above user descriptions. Each individual condition may push some values of the physical layer out of specification; but it would not be enough to actually disrupt communication. Fieldbus is just that robust. However, several factors may already be pushing the limit so that one additional disturbance – such as that one frequency converter starting an induction motor close by the fieldbus trunk may be just enough to push this segment beyond its level of resistance. As a result, communication is at least temporarily disturbed.

The fieldbus physical layer is no mystery; it requires clear indication provided by the right tools such as the Advanced Diagnostic Module. All of the above conditions can easily be detected and taken care of at a convenient point in time. During commissioning, the quality of the installation will be better thus granting communications a higher level of fault tolerance. And that is not enough: Users want forewarning about disturbances before they can become critical to the process.

The Advanced Diagnostic Module monitors fieldbus communication online and in real-time. Operations and maintenance personnel view all measurements in the control room. There is no need for personnel to be exposed to the hazardous area. There is no need to access control cabinets or field junction boxes and upset the wiring for connection of hand-held devices. Information is available immediately.

During commissioning, all relevant measurements receive warning levels with dead bands. Any relevant change to the fieldbus cable causes summary warnings and alarms at the operator station. Information that operators can now access via the Advanced Diagnostics Module and the Diagnostic Manager to enable smart decision making: Increased levels of noise and minor earth leakage, indicative of a worsening condition, are within view. The plant operator can decide whether to immediately set repair work in motion, schedule additional review for the next shift, or even wait for the next planned outage – all while the plant is operating at its best quality and production levels.

Everyone is rewarded with reduced plant upsets and fewer unplanned outages. Less than optimal condition of the fieldbus will cause a certain percentage of faulty transmissions to go undetected while at the same time increasing the bus cycle time. Reduced system response time may also be a contributing factor to product quality.

A finely tuned and proactively maintained fieldbus system and field devices will not only increase the overall system availability and product quality. It also puts plant managers, operators and maintenance crews at ease – with the knowledge that the fieldbus installation formerly perceived as the black hole is now brightly lit and monitored by the FieldConnex® Advanced Diagnostic Module from Pepperl+Fuchs.