What is Profibus DP/FMS/PA?

ProfiBus (PROcess FIeld BUS) is a widely accepted international networking standard, generally found in process control and in larger assembly and material dealing with machines. It helps single-cable wiring of multi-input sensor blocks, pneumatic valves, complex intelligent devices, smaller sub-networks (corresponding AS-i), and operator interfaces. ProfiBus is nearly common in Europe and likewise popular in North America, South America, and parts of Africa and Asia. It's an open, vendor independent standard. It adheres to the OSI model and ensures that devices from a variety of different vendors can communicate together easily and effectively. It has been standardized under the German National Standard as DIN 19 245  part  1 and 2 and, in addition, has also been ratified under the European National Standard EN 50170 Volume 2.

The development of ProfiBus was initiated by the BMFT (German Federal Ministry of Research  and Technology) in cooperation with a several  of automation manufacturers in 1989. The bus interfacing hardware is carried out on ASIC (application specific integrated  circuit) chips produced by a number of vendors, and is based on the RS-485 standard  as well as the European EN50170 Electrical Specification. The standard is supported by the ProfiBus Trade Organization, whose website can be found at www.profibus.com. ProfiBus uses 9-Pin D-type connectors (impedance terminated) or 12mm quick disconnect connectors. The number of nodes is limited to 127. The distance supported is up to 24 Km (with repeaters and fiber optic transmission), with speeds varying from 9600 bps to 12 Mbps. The message size could be up to 244 bytes of data per node per message (12 bytes of overhead for a most message length of 256 bytes), while the medium access control mechanisms are polling and token passing.


ProfiBus supports two main types of devices, namely, masters and slaves:
• Master devices control the bus and once they have the right access the bus, they might transfer messages without remote request. These are referred to as active stations
• Slave devices are usually peripheral devices  i.e. transmitters/sensors and actuators. They might solely acknowledge received messages or, at the request of a master, transmit messages to that master. These are additionally referred to as passive stations.

There are several versions of the standard, particularly, ProfiBus DP (master/slave), ProfiBus FMS (multi-master/peer to peer), and ProfiBus PA (intrinsically safe).
• ProfiBus DP (distributed peripheral) permits the use of multiple master devices, by which case every slave device is assigned to one master. This implies that multiple masters can read inputs from the device but only one master can write outputs to that device. ProfiBus-DP is designed for high speed data transfer at the sensor/actuator stage (as opposed to  ProfiBus- FMS which tends to focus on the higher automation level) and relies around DIN 19 245 parts 1 and 2 since 1993. It is suitable as a replacement for the costly wiring of 24V and 4-20 mA measurement signals. The data exchange for ProfiBus-DP is usually cyclic in nature. The central controller, which acts as the master, reads the input data from the slave and sends the output information back to the slave. The bus cycle time is much  shorter than the program cycle time of the controller (lower than 10 mS).


ProfiBus supports two main types of devices, namely, masters and slaves:
• Master devices control the bus and once they have the right access the bus, they might transfer messages without remote request. These are referred to as active stations
• Slave devices are usually peripheral devices  i.e. transmitters/sensors and actuators. They might solely acknowledge received messages or, at the request of a master, transmit messages to that master. These are additionally referred to as passive stations

There are several versions of the standard, particularly, ProfiBus DP (master/slave),
ProfiBus FMS (multi-master/peer to peer), and ProfiBus PA (intrinsically safe).
• ProfiBus DP (distributed peripheral) permits the use of multiple master devices, by which case every slave device is assigned to one master. This implies that multiple masters can read inputs from the device but only one master can write outputs to that device. ProfiBus-DP is designed for high speed data transfer at the sensor/actuator stage (as opposed to  ProfiBus- FMS which tends to focus on the higher automation level) and relies around DIN 19 245 parts 1 and 2 since 1993. It is suitable as a replacement for the costly wiring of 24V and 4-20 mA measurement signals. The data exchange for ProfiBus-DP is usually cyclic in nature. The central controller, which acts as the master, reads the input data from the slave and sends the output information back to the slave. The bus cycle time is much  shorter than the program cycle time of the controller (lower than 10 mS)
• ProfiBus FMS (Fieldbus message specification) is a peer to peer messaging format, which permits masters to communicate  with one another. Simply as in ProfiBus DP, up to 126 nodes are available and all will be masters if desired. FMS messages consume more overhead than DP messages
• ‘COMBI mode’ is when FMS and DP are used simultaneously  in the same network, and some units (such as  Synergetic's DP/FMS masters) support this. That is mostly used in conditions where a PLC is being used in conjunction with a PC, and the primary master communicates with the secondary master  through FMS. DP messages are sent via the same network to I/O devices

The ProfiBus PA protocol is the same as the most recent ProfiBus DP with V1 diagnostic extensions, except that voltage and current ranges are lowered to meet the requirements of intrinsic security (class I division II) for the process industry. Many DP/FMS master  cards support ProfiBus PA, but barriers are required to convert between DP and PA. PA devices are usually powered by the network at intrinsically safe voltage and current level, utilizing the transmission technique specified in IEC 61158-2. (which Foundation Fieldbus H1 uses as well)

The Variable Area Flowmeter

The variable-area flowmeter (Figure 1) is among the oldest technologies out there and arguably probably the most well -known. It's constructed of a tapered tube (usually plastic or glass) and a metal or glass float. The

volumetric flowrate by means of the tapered tube is proportional to the displacement of the float. 

Fluid shifting by means of the tube type bottom to top causes a pressure drop throughout the float, which produces an upward force that causes the float to move up the tube. As this occurs, the cross-sectional space between the tube partitions and the float (the annulus) will increase (therefore the term variable-area). As a result of the variable-area flow meter relies on gravity, it have to be installed vertically (with the flow tube perpendicular to the ground). Some variable-area meters overcome this slight inconvenience by spring loading the float within the tube (Figure 2). Such a design can simplify installation and add operator flexibility, particularly when the meter have to be put  in a good physical  area and a vertical installation will not be possible. 

Two varieties of variable-area flow meters are usually available: direct-reading and correlated. The direct-reading meter allow  the user  to read the liquid or gas flowrate in engineering units (i.e., gal/min and L/min) printed directly on the tube, by aligning the top of the float with the tick mark on the flowtube. The benefit of a direct-reading flowmeter is that the flowrate is actually read straight off the flowtube. Correlated meters, however, have a unitless scale (sometimes tick marks from 0  to 65, or 0 to 150 ), and come with a separate data sheet that correlates the scale reading on the flowtube to the flowrate in a particular engineering unit. The correlation sheets often give 25 or so data factors alongside the dimensions of the flowtube, allowing the consumer to decide the precise flowrate in gal/min, L/min, or whatever engineering unit is needed.

The advantage of the correlated meter is that the identical flowmeter can be used for numerous gases and liquids (whose stream is represented by completely different units) by choosing the suitable correlation sheets, the place further direct-reading  meters would be required for different fluid applications. Equally, if strain or temperature parameters change for a given application, the person would simply use a special correlation sheet to reflect these new parameters. By comparability, for a direct-reading meter, a change in

operating parameters will compromise the meter's accuracy, forcing it to be returned to the factory for recalibration. Usually, the typical accuracy of a variable-area flowmeter is ±2-4% of fullscale flow.

Advantages: The main benefit of the variable-area flowmeter is its relative low cost and ease of installation. Due to its simplicity of design, the variable-area meter is virtually maintenance-free and, hence, tends to have a long working life.

Another advantage is its flexibility in dealing with a wide range of chemicals. As we speak, all-Teflon meters

can be found to resist corrosive injury by aggressive chemicals. The advantage of a Teflon flowmeter with a built-in valve is that you would be able to not only monitor the fluid flowrate, but you can control it, as well, by opening and shutting the valve. If the application requires an all-Teflon meter, chances are the fluid is fairly corrosive, and plenty of users would like the option of controlling the flowrate by merely turning a valve that is constructed into the flowmeter itself.

Disadvantages: One potential drawback of a variable-area flowmeter occurs when the fluid temperature and pressure deviate from the calibration temperature and pressure. As a result of temperature and strain variations will cause a gas  to increase and contract, thereby changing density and viscosity, the calibration of a specific variable-area flowmeter will now not be valid as these circumstances fluctuate. Manufacturers sometimes calibrate their gas flowmeters to a standard temperature and pressure (often 70°F with the flowmeter outlet open to the environment, i.e., with no back pressure). During operation, the flowmeter accuracy can rapidly degrade as soon as the temperatures and pressures start fluctuating from the standard calibration temperature and pressure. Meters used for water have a tendency to indicate much less variability, since water viscosity and density changes little  with normal temperature and pressure fluctuations. While there's a strategy to correlate the circulation from precise operating situations back to the calibration conditions, the conventional formulas used are very simplified, and don't bear in mind the impact of viscosity, which can cause large errors.

Primary Definitions and Terminology used in the field of Process Control

As we'll observe two of the most important indicators used in process control are referred to as Process Variable or PV and the Manipulated Variable or MV. 

In industrial process management, the Process Variable or PV is measured by an instrument in the area and acts as an input to a pc based mostly automatic controller which takes action primarily based on the value of it. Alternatively the PV will be an input to computer primarily based hardware system and its value displayed in some method in order that the operator can perform manual control and supervision.

The variable to be manipulated, so as to have management over the PV, is called the Manipulated variable or MV. If we control a particular movement for occasion, we manipulate a valve to regulate the flow. Here, the valve position  known as the Manipulated Variable  and the measured move becomes the Process Variable.

Within the case of a easy automated controller, the Controller Output Signal (OP) drives the Manipulated Variable. In additional complicated computerized control techniques, a controller output signal could not all the time drive a Manipulated Variable within the field. In apply, the time period Manipulated Variable isn't
used. Most individuals involved in course of control seek advice from the output of a controller and it is assumed that one knows the purpose of it. The perfect worth of the PV is usually known as Target value. and within the case of an automated management, the term Set Point Value is preferred.

Varieties or Modes of Operation of Available Control Methods
There are 5 basic forms of management accessible in Course of Control. These are:
• On-Off
• Modulating
• Open Loop
• Feed Forward
• Closed loop

Probably the most primary control idea is ON-OFF Management as found in a contemporary iron in our households. This is a very crude type of control, which however ought to be considered as an inexpensive and effective means of control if a reasonably large fluctuation of the PV (Process of Variable) is acceptable.

The damage and tear of the controlling component (solenoid valve and so on) wants particular consideration. Because the bandwidth of fluctuation of a PV is increased, the frequency of switching (and thus put on and tear) of the controlling element decreases.

Modulating Control

If the output of a controller can transfer via a variety of values, we've got modulating control. It is understood that modulating control takes place inside an outlined operating range (with an higher and lower limit) only.

Modulating control can be utilized in each open and closed loop control systems.

Open Loop Control

We have now open loop control, if the control motion (Controller Output Signal output) is just not a perform of the PV (Process  of Variable) or load changes. The open loop control does not self-correct, when these PV’s drift.

Feed Forward Control

Fairly often it is a form of control based on measured disturbances (Feed Forward Control). It's a type of Open Loop control as the PV is not utilized in the control action. Feed forward is a more direct form of control than discovering the correct value of the manipulated variable (MV) by trial and error as occurs in feedback control. In feed forward the foremost process variables are fed right into a model to calculate the Manipulated Variable (MV) required to manage at the Set Point (SP).

The proper industrial use of this sort of control is, for instance, to incorporate this with suggestions; or closed loop control. Then the imperfect feed forward control can correct as much as 90% of the upsets, leaving the feedback  system to appropriate the 10% bias left by the feed forward component.

Closed Loop or Feedback Control

We have now a Closed Loop Control System if the PV, the target of control, is

used to find out the control action.The idea of Closed Loop Control is to measure the PV (Process Variable); examine this with the SP (Setpoint), which is the desired or target value ; and decide a control motion which ends up in a change of the Output value of an automated controller. Most often, the Error (ERR) time period is used to calculate the OP value. ERR = PV - SP. If ERR = SP -PV has to be used, the controller needs to be set for REVERSE control action.