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Jake Ten Haken, P.E. and Jeff White, CAP
Presentation at the 2003 AOM Expo

As process control continues to evolve and change it is becoming clear that days of 4-20 mA control may not always be around. Terms like ASI bus, DeviceNet, Profibus, and Foundation Fieldbus are starting to become more common. In general, these terms all represent similar styles of systems that retrieve process data and pass it along to the control system via some type of networking generically referred to as a fieldbus.

Fieldbus is a digital, two-way, multi-drop communication link among intelligent control devices that will more than likely change how we look at discrete and analog signals in a control system.

Fieldbus is a technology that is becoming commonplace in industries such as: Automotive, Food and Beverage, Petro-Chemical, but has not yet made its way into the Flour Milling industry. Figure A indicates how the “standard” for analog control has changed, however the speed by which fieldbus will capture the market depends largely on how fast the users are trained.

One does not feel confident to buy something if one does not understand how it works. The purpose of this paper is to explain: What Fieldbus is, why it is used, and how it may impact the Flour Milling Industry.

What is Field Bus?
Fieldbus is a digital, two-way, multi-drop communication link among intelligent control
devices that will more than likely change how we look at discrete and analog signals in a
control system. Fieldbus is characterized by the following criteria:

• Digital Signal Transmission
  • Two-Way Capable
  • Multi-Variable Capable
  • Multi-Drop Capable
• Interoperable Multi-Vendor

Digital Signal Transmission
First of all, fieldbus is digital. This is a change from most instruments (e.g. valves, pressure transducers, flow switches, etc.), which traditionally have used analog or discrete signals to communicate. For example, an analog value of 4 mA might correspond to a flow of 0Tons/hr while a value of 20 mA might correspond to a 1000 Tons/hr flow value and then a value of 12 mA would correspond to 500 Tons/hr.

With discrete devices, such as flow switches or pressure switches, the presence of a 110-volt or 24 Volt signal might represent a “closed" or "alarm" condition while the absence of a signal might represent "open" or “normal". In the traditional analog and discrete scenario, the information flow is only one way, either from the control system to the instrument or from the instrument to the control system.

A digital signal differs from the analog signal in that it is not a continuous signal; it is a series of “ons” and “offs”, much like the Morse code. Technological advances now allow for this data to be transmitted at very rapid rates of speed. In addition, the devices are addressed and also have the intelligence to interpret the digital signals. This digital signal as well the circuitry embedded in the devices allow the information to be transmitted both to and from the controllers and the devices, this is commonly referred to as two-way communication.

Two-Way
This two-way communication means that the instrument is not only sending information back to the control system, but it is now possible for the control system to write to the instrument. For example, the calibration constants associated with a particular sensor can now be stored directly in the device itself and changed as needed directly from the control system.

If needed, the instrumentation technician can read the current calibrations constants up from the instrument, make some necessary changes and write them back to the instrument all from the control system.

Multi-Variable
Multi-variable functionality allows the instrument to send more information than just the process variable information back to the control system. For instance, some pressure transmitters are capable of sending both a pressure measurement back as well as a temperature measurement.

The multi-variable capability of Fieldbus allows a wealth of information that is currently stored in the instrument to be sent back to the control system.(See figure B for further clarification.) This is similar to today’s digital cell phones, not only are they capable of receiving the signal telling the phone to ring, it will now also have the display indicate who is trying to call you.

Multi-Drop
With traditional analog and discrete devices, a separate cable needs to be run between the end device and the control system because only a single analog or discrete signal can be represented on the circuit. Modern distributed systems partially solve this problem by locating remote multiplexing devices out in the field. The ultimate solution however is to be able to connect a reasonable number of sensors all located in the same area to the same cable. This is exactly what fieldbus allows.  

Interoperable Multi-Vendor
The ability of 4-20 mA devices to replace any other device of the same type is called ‘interoperability'—roughly meaning compatible. Fieldbus offers the same capability. Fieldbus began as a standardized work by ISA (Instrumentation, Systems and Automation society), just like the 4-20 mA standard that started in the 70’s. Several fieldbus platforms are getting worldwide recognition.

Interoperability allows a brand 'Y' transmitter to be replaced by a brand 'X' transmitter of the same type any time, without loss of functionality, and can interface to another brand 'Z' device. Fieldbus forces interoperability between the devices, which are complying with this standard, and it is also available to all manufacturers and users without licensing agreements. It is 'open', it is fully disclosed, and there are no 'secrets'.

The user or a third party can make their own configurations and software. They may mix and match the best of each type, just as they could with the 4-20 mA. They do not have to choose a device manufactured by a certain company just to match other devices of the same brand already installed. Another benefit of interoperability is that system software does not have to be upgraded when new products are introduced.

Why it’s Used
The “why” it is used will vary drastically between the end users. The most common reasons are:

• Lower Installation Costs
• Data Accuracy and Integrity
• Future Flexibility

Lower Installation Costs
Reduction in wiring and simplification is achieved through connection of several devices on a single pair of wires – multi-dropping. Connection is a simple task, since everything is in parallel and terminal number matching is at a minimum. The multi-dropping will allow for fewer wires between the control system enclosure and the field devices. This reduction in the quantity of wires will lead to smaller conduits as well as a reduction in control system I/O cards and therefore smaller control system enclosures and a lower cost installation.

In addition, the multi-variable feature of an instrument has the potential of allowing multiple variables be brought back from one instrument, that could lead to the requirement of fewer instruments. For instance, a pressure transmitter that is also capable of measuring the product temperature may eliminate the need for a temperature transmitter to be purchased and installed.

Data Accuracy and Integrity
With the two-way communications and the multi-variable features of fieldbus instrumentation, instruments now have the capability of returning process information that is more accurate. The higher accuracy can be attributed to digital communication, since the microprocessors in a transmitter and a controller may talk directly, rather than going through Digital to Analog (D/A) and Analog to Digital (A/D) conversions.

The instrument generates the data in a digital format and the control system needs the information in a digital format. When you utilize analog signals, both ends need to perform data manipulation. Greater data integrity can be achieved since the instrument can also pass a rating back of how accurate the data is. All data is checked and guaranteed free from distortion due to noise or an impedance mismatch that may affect analog undetected signals. It is therefore possible to determine if the information is reliable or not.

Future Flexibility
All aspects of the definition of fieldbus support future flexibility. First of all multi-variable will allow the use of the vast amounts of information that may not be used initially at a later date with the addition of little or no hardware. This may allow for quick and easy assessment of process related problems.

Secondly, multi-drop will allow the easy installation of added instruments to existing multi-drop installations. Small process changes or process related experiments could be done with very little electrical investment.

Thirdly, the multi-vendor requirement will not limit the end user to any one instrumentation supplier. The open design will allow users to select a device based on price, performance, quality, and delivery time.

Fieldbus Concerns
Fieldbus has the potential to provide huge benefits. However, with all new technological advances, there are some concerns that must be understood before one can confidently move forward.

First of all, the increased functionality that will now need to be in the new instruments will add costs. In fact, the additional instrumentation cost could eat up a majority of the installation cost savings.

Secondly, the new technology will require a different type of troubleshooting and support base, which may mean retraining existing plant personnel or the addition of more technically skilled staff.

Thirdly, although this paper has concentrated on the generic definition of fieldbus, there are multiple open field buses in existence. For example, Foundation Fieldbus, DeviceNet, Profibus, ASI, etc just to name a few. Each digital-bus format has particular strengths and weaknesses, and some are exceptional for discrete signals and some for analog signals. Fortunately, most control systems can support several buses, running simultaneously.

How will it impact the milling industry?
Why or when should Fieldbus applications be considered in the milling industry?

In order to evaluate this question one should consider the following:

• Are there similar technologies currently being used in the milling industry today?
• What Fieldbus applications being used in other industries could be applied in today’s milling industry?
• What benefits can be realized in the milling industry by using Fieldbus?

There are technologies currently used in the milling industry that are very similar to or even considered fieldbus applications. Some common uses of similar technology currently applied in the milling industry include various industrial busses that adhere to RS-485 or other serial communication applications used on scales and flow metering equipment.

These industrial busses can be applied with a multi-drop system to monitor and control numerous parameters on each scale on the system. In addition, instruments such as continuous level monitoring devices may incorporate a remote I/O communication standard as a method of communicating data to a control system. Although these applications may not be considered a fieldbus technology, they provide some insight to the potential benefits of fieldbus.

Consideration
When we take into account the fieldbus applications utilized in other industries it initially seems there are few obvious applications that could be applied in the milling industry. However, there are several applications that may warrant consideration. Following are several specific applications in the milling industry that are worth consideration:

• Flow sensors (switches) and level switches located on lift lines, bin levels, etc. In flour mills many of these sensors are densely located in certain areas of the plant.
• Roller mill controllers. Many of the most modern roller mills have PLC controllers located locally at the roller mill. These controllers can typically be networked to a centrally located control system. The information obtained from the controllers could be gathered via a fieldbus network. Even without these controllers, roller mills contain enough control devices for consideration of installation on a fieldbus system.
• Continuous bin level monitoring systems – especially in areas with many bins located in one area.
• Motor control centers. Many of the manufacturers offer ”smart” motor control center options.
• Power monitoring equipment. We are seeing more installations of power monitoring  equipment in MCCs, Switchboard, Switchgear, and other power distribution equipment that could be placed on a fieldbus network to transmit the data to a central system.
• Scales and flow controllers typically provide multiple levels of data for monitoring by a central yield management system.
• Grain tempering equipment. In most cases grain tempering equipment will include multiple control devices that a plant may want to monitor.
• Ingredient feed systems.
• Bucket elevator/leg alignment and bearing temperature sensing systems.
• Valves, gates, diverters, etc. Many of these devices are found throughout areas of the mill. They typically contain a single or multiple solenoids and multiple limit switches.

The use of fieldbus I/O interface modules creates an opportunity to utilize additional types of devices that may not be available as “smart” fieldbus devices. This will allow a plant to utilize the same devices types installed in other areas of the plant and therefore maintain a smaller spares parts inventory.

Benefits
When considering these applications listed above specifically for the milling industry, we must consider some of the benefits that may be realized with a fieldbus application. Following are some of the key benefits that may be realized with these applications in the milling industry:

• Expansion of a flour mill could require less construction effort and be less expensive if a fieldbus installation already exists. For example, the installation of a couple lift line sensors onto an existing system could involve connecting these devices to the nearest available existing device on a fieldbus segment, without the need to wire the new devices back to a centrally located control room.
• The advantage of reduced control system enclosure sizes may be a huge advantage to mills where real estate is at a premium and possibly not available for the control equipment. The reduction in I/O cards necessary for a fieldbus application will allow the user better space utilization for the control panels.
  
• Several of the applications listed above are specifically intended to reduce wiring costs by taking advantage of the multi-drop and multivariable capabilities of fieldbus. For example, a diverter valve with two solenoids and two limits may traditionally require six wires brought back to a control panel for power and I/O requirements on a conventional system. The fieldbus network would address all of these needs on the single pair of bus wires.
• Additional data would become available for monitoring critical machines in a mill
  without the need for additional wiring. This may involve data that is currently
  available from roller mills, scales, flow controllers, etc but not utilized because it would require extra wiring and costs. Fieldbus provides a way to readily obtain this data without the additional wiring requirements.
• Power monitoring of electrical systems has recently become a higher priority at many facilities. Power monitors can transmit data to a central control system without the use of fieldbus, however the use of fieldbus may allow the plant a means for obtaining a greater amount of this data without additional wiring. This additional data may allow the control system to perform functions such as initiating load shedding or monitoring critical load for unusual operating characteristics as part of a preventive maintenance program.

We hope this provides you with a better understanding of the basics of a fieldbus and why it is used. Once people in the milling industry have a better understanding of fieldbus, we feel the impact it may have on the flour milling industry may finally be realized such as it has in other industries.

More Information
If you have any questions or comments, please call Jake Ten Haken at (712) 722-1663, extension 157, or Jeff White at (712) 722-1663, extension 182.

For visual examples associated with fieldbus technology, please view the .pdf version of this article.

 

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