The P-NET Toolset for Building Data Acquisition Systems

By Jörg Böttcher, International P-NET User Organization, 1998.

Contents:

• What's Inside this Article?
• Data Acquisition in Industrial Processes
• P-NET as an Easy to Use Toolset for Data Acquisition
• Three Application Examples
• Four Steps for Building a Data Acquisition System with P-NET
• Available Modules and Tools
• Contacts

What's Inside this Article ?

This article describes the simple steps required to use P-NET modules and tools for building small, medium and large data acquisition systems. P-NET is an easy to use fieldbus system, which has been designed to connect distributed process components, such as PCs, I/O-modules, sensors, actuators, field and central controllers, PLCs etc. Furthermore, three typical application examples and a four step method for building a data acquisition system with P-NET, are provided in the article.

P-NET is one of the three bus systems which are standardized within the European fieldbus standard EN 50170. Among these, P-NET is the system which needs a minimum of configuration effort. Therefore, P-NET is well suited for data acquisition applications, such as those described in this article.

Data Acquisition in Industrial Processes

Industrial Processes typically deal with a number of physical and/or chemical values. Among these are analogue values, such as temperature, weight, pressure, flow, humidity, speed etc., or standardized current or voltage signals (e.g. 0/4...20 mA, 0...100 mV etc.). In addition, there are often numerous digital signals inside the plants, such as valve states, limit switch signals, motor signals etc.

Data Acquisition in these industrial processes typically mean:

• Long term registration and data logging of some of the process signals, in order to allow for future analysis of a process (e.g. for the purpose of audits etc.)
• Short and medium term logging of process signals, in order to study the process and optimize it
• Making available all the data at a host station (e.g. PC) needed for visualisation (HMI)

Industrial processes with data acquisition systems can be divided up into two different groups:

• Processes without control loops: This means processes where the collection of data is the sole purpose. Nothing needs to be controlled. Examples are:  transport of food and other goods (e.g. fuel), test systems for machines and components, measurement systems for meteorology, environmental control, tank management systems etc.
• Processes with control loops: Here, PLCs and industrial PCs etc. are typically used for controlling a production process. In addition to the production equipment, a separate data acquisition system has to often be installed for quality assurance purposes. Whereas production equipment and data acquisition system are sometimes installed at the same time, a new data acquisition system has to often be integrated into an existing production system. Examples of relevant production areas are: food production, textile industry, energy production, chemical plant, asphalt and concrete production, building automation, animal feeding systems etc.

P-NET as an Easy to Use Toolset for Data Acquisition

Since data acquisition means the collection of analogue and digital signals from all over the process, from the technical and the economical point of view, a system based on fieldbus I/O-modules is usually the best one. Among the fieldbuses available, P-NET has some exciting features, which often makes it first choice when searching for the appropriate bus system:

• P-NET is very easy to use
• P-NET needs only a few configuration steps to achieve operation
• there are a wide variety of different process module types available, all being 100 % compatible
• the process modules can be configured in a way to allow them to perform some intelligent process functions by themselves, like limit switch control, PID regulation, automatic weighing functions, advanced mathemical calculations etc.
• step-by-step expansion of an installation from a small scale, with only two or three modules, to systems with many thousands of process signals
• P-NET offers a few special modules which are optimized for typical data acquisition tasks, such as small programmable field controllers with LCDisplay and some keys, or adapters for field devices with RS-232 interface
• P-NET supports transparent and standardised access to process data by PC-based applications, running under Windows®, written in Visual Basic, Visual C, Delphi, LabVIEW, InTouch, FactoryLink, Excel, Access, PRO-view etc.
• coupling between P-NET and LAN/WAN/GSM systems etc., or other fieldbus protocols such as PROFIBUS or CAN, is very easy, and supported by software and hardware products.

Indeed, many of the many thousands of P-NET installations known, have been designed for data acquisition purposes. Among these, there are many installations where conventional PLC systems are used for process control, and a parallel P-NET system is responsible for data acquisition.

Three Application Examples

The following three application examples show how P-NET is used for data acquisition. There are similarities with a wide variety of other P-NET data acquisition installations. The P-NET modules which are utilised within these application examples, can be seen in more detail in the table in chapter "Available Modules and Tools".

Example 1: Collecting Tank Weight Data with P-NET

In this system the liquid level of four tanks is measured via load cells at the bottom of the tanks. The load cells are directly connected to P-NET weight modules PD 3230.
 

A Local Display - being a programmable PD 4000 controller - is used for showing the actual liquid levels in the field. A PC, which can be installed outside the field, collects all weight data around the clock, to allow precise registration of any filling or out flow. There are many existing applications which are similar to the one mentioned here, and many of them have an approval e.g. by German Lloyd, Bureau Veritas, Norske Veritas or Lloyds Register of Shipping.

Example 2: Quality Assurance with P-NET, in the Food Industry

During the process of producing and transporting food many analogue values have to be monitored. Among those are temperature (which is usually the most important!), flow, pressure, pH-value, weight etc. The drawing shows an installation where P-NET collects temperatures and CO₂ concentrations, within a plant for roasting nuts.

Whereas the controlling part is operated by conventional PLCs, the complete data acquisition part is handled by P-NET. Modules such as the PD 3240 with 16 analogue inputs, or the PD 3221 (mixed analogue/digital inputs/outputs) can be used. Within the PC, there is a PD 3920 P-NET card installed, which is operated by the VIGO fieldbus driver. On the top of VIGO, two Windows® NT-applications - Excel and LabVIEW - are running. In this application VIGO, together with a VIGO add on, handles the coupling to a host computer via a TCP/IP based Ethernet LAN. Therefore, the collected quality data can be monitored and visualized on the local PC, and in parallel, transferred to a central quality data base.

Example 3: P-NET for Power Monitoring within Building Automation

The last example deals with a P-NET installation which is used for monitoring power and additional digital signals, such as door detectors and window switches, within a larger building automation system. One module type used here is the PD 3260 - a module which has power measurement channels for three power lines and eight additional digital inputs/outputs, as well as two switching outputs. For monitoring the high number of digital signals, PD 3120 (with 16 digital inputs/outputs) and PD 3221 (with 6 digital and 3 analog inputs/outputs) are used.
 

An Access data base is used for storing the overall building data. Some of the Building Systems designed with P-NET, have many thousands of process signals handled by the P-NET modules. Very often, the systems are not only used for data acquisition, but also for implementing the complete control functions for such useful things like automatic lighting, power management, air conditioning etc.

Four Steps for Building a Data Acquisition System with P-NET

When a data acquisition system is planned to be implemented with P-NET, the following four steps should be carried out:

First Step: Selecting the Appropriate P-NET Modules and Installing Network

By analyzing the location, the kind and the number of digital/analogue inputs and outputs which have to be handled by the P-NET data acquisition system, one has to select the appropriate number and type of P-NET modules. The table in chapter "Available Modules and Tools" will help an engineer in his selection. Of course, before any module or program can be started, the whole P-NET network, including all modules, must be installed, and the inputs and outputs must be connected to sensors, transmitters and actuators.

Second Step: Installing Software

In many data acquisition applications - but not in all ! - one or more PCs are used. Typically, the PCs will be running under Windows®-95/NT. Therefore, the PC software must be installed. For P-NET, the driver software VIGO must be installed from disk or CD. Doing this, some very useful additional P-NET tools, which should be used in the following steps, are automatically installed. Of course, the application software written by the customer himself or by an engineering office/company must be installed too.

When there are programmable devices among the P-NET modules - like the field controller PD 4000 or the so called "calculator channels" inside many standard P-NET modules - the programs can be downloaded from a PC to any P-NET device which is connected to the P-NET cable ("remote download"). This is very useful, since nobody has to go to any of the modules in the field.

Third Step: Configuring

When a PC is used, the VIGO fieldbus driver must be configured (see drawing below). This means that with a special tool from the VIGO software package - called the MIB-Editor (Manager Information Base), a file containing all project data about installed P-NET modules, their node addresses and individual input/output configurations etc. can be created. Normally, the developer of the application generates this file. Thus, the end user only has to switch the system on, without having to do any configuration work.

With the VIGO tools, remote configuration of the P-NET modules can also be performed. Because the module configuration data is stored within the VIGO system, this can be achieved by making a few mouse clicks from the PC. 
   

Fourth Step: Putting into Operation

Now the system can be switched from "instalation" to "operational mode". Usually, the only thing to be done here, is to start the application program(s). On the PC side, VIGO supports the so called OLE2 Automation Interface, which is a standardised software interface within all 32 Bit Windows® systems. Therefore, every application generated by means of a standard software package like Visual Basic, Visual C, LabVIEW, InTouch, FactoryLink, Excel, Access, PRO-view etc. can be coupled to VIGO in a simple, yet powerful way.

If anything seems to be wrong, an easy to use monitor program, also being part of the VIGO package (see last drawing), can be started, in order to test the P-NET communication between the modules.
 

Available Modules and Tools

The following tables show some of the general purpose P-NET modules and tools from the PD-family of P-NET products. It can be used as a selection guide when designing a P-NET data acquisition system. Within one P-NET segment (up to 1,200 m cable), there can be up to 125 modules. Among those can be up to 32 modules with P-NET Master functionality. Using one of the PD 50XX Controllers, one can build up a P-NET multi-net system, where this Controller type works as bridge between two P-NET segments .

Input/Output-Modules (P-NET-Slave-Functionality)

Special Modules

PC Coupling (P-NET-Master-Functionality)

Controllers (P-NET-Master-Functionality)

Controllers of type PD 4000 and PD 50XX can be programmed in PROCESS-PASCAL which is standard ISO-Pascal plus some extensions for simple P-NET communication and for multitasking. The PROCESS-PASCAL development tools come together with the Controllers. PD 4500 applications can be programmed in C, and compiled on an IAR compiler; a library of C functions can be provided, in order to make development of applications easier.

Additionally, special modules and tools for remote data access via the GSM mobile telephone system, and for coupling to other bus systems and to PLC systems (e.g. Siemens S7 systems and controllers with Modbus

interface) are also available.

Contacts

Should you have any question about P-NET equipment and P-NET applications, please don't hesitate to contact one of the following addresses:

International P-NET User Organization (IPUO)
 
IPUO Headquarters, Denmark:
phone +45 87 200 396, fax +45 87 200 397
e-mail p-net@post4.tele.dk

IPUO Germany:
phone +49 991 270302-0, fax +49 991 270302-99

IPUO UK:
phone +44 1491 828 200, fax +44 1491 828 201
e-mail pnet@easynet.co.uk

IPUO Portugal:
phone +351 56 412 789, fax +351 56 412 792
e-mail ajp@tecnocon.pt

IPUO Canada:
phone +1 905 840 6800, fax +1 905 840 6799
e-mail conflow@orbonline.net

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