The PLC and PC no longer suit today’s control needs.
Applications demand integrated rather than add-on functions.
A new control, the PAC, promises to address the new requirements.
James Davis
Senior Systems Engineer
Opto 22
Modern industrial applications present a daunting mix of control requirements. A typical control system must interface with signals from simple sensors and actuators, yet it must also offer advanced control functions, network connection, device interoperability, and enterprise data integration.
These modern requirements extend beyond the traditional PLC role of discrete-logic-based control of input/output (I/O) signals. Most PLCs are programmed using ladder logic, which has its origins in the wiring diagrams that describe the layout and connections of discrete physical relays and timers in a control system. Applications that diverge from this model are hard to program in ladder logic. For example, proportional integral-derivative (PID) loops used for temperature control involve floating-point arithmetic. To perform these calculations, PLCs often need separate, and separately programmed, hardware cards.
The tasks for modern applications are usually more suited to the capabilities of a computer (PC). A PLC would need additional processors, network gateways or converters, “middleware” software running on a separate PC, and special software for enterprise systems.
On the other hand, to make an industrial PC perform PLC-like tasks, such as machine or process control, also requires expansion. For example, additional I/O expansion cards or special extensions may need to be integrated into the PC’s operating system to provide deterministic or near-deterministic operation.
To meet the increased requirements of modern industrial applications, manufacturers are blending the advantages of PLC-style deterministic machine and process control with the flexible configuration and enterprise integration strengths of PC-based systems. Such a device has been termed a programmable automation controller, or PAC.
Blending PLC and PC-based technologies has been attempted before, but usually through the add-on approach where additional middleware, processors, or both are used in conjunction with one or more PLCs. A PAC, however, has these broader capabilities built in. A PAC includes the processing hardware to perform counting, latching, PID loop control, and data acquisition and delivery.
A PAC also uses open architectures for interconnection with other devices and business systems. Not only do these devices offer efficient processing and I/O scanning, they also offer ways to integrate with enterprise business systems.
SNAP PACs from Opto 22 are multi-domain, Ethernet-based industrial
automation controllers for applications in automation and control,
remote monitoring, and data acquisition. They are well-suited to
Original Equipment Manufacturers, system integrators, and end-users in
process control, discrete manufacturing, or hybrid industries and
applications. They include comprehensive automation software suites for
programming, debugging, human-machine interface (HMI) development, OPC
and database connectivity, and utilities. Two versions are available:
the SNAP PAC S-series and the SNAP PAC-R series.
Characteristics of a PAC
Credit for inventing the name PAC goes to the industrial analyst ARC Advisory Group. ARC coined the term for two reasons: to help automation hardware users better define their application needs, and to give automation hardware vendors a term that more clearly communicates their products’ capabilities.
A programmable automation controller must fulfill the following requirements:
• Operate using a single platform in multiple domains, including logic, motion, drives, and process control.
• Employ a single development platform using common tagging and a single database for development tasks across a range of disciplines.
• Tightly integrate controller hardware and software.
• Be programmable using software tools that can write control programs to support a process that “flows” across several machines or units.
• Operate on open, modular architectures that mirror industry applications, from machine layouts in factories to unit operation in process plants.
• Employ de-facto standards for network interfaces, languages, and protocols for data exchange as part of networked multi-vendor systems.
• Provide efficient processing and I/O scanning.
The integrated hardware and software of a PAC is an advantage when programming because it includes a single tag-name database shared by all development tools. PACs use one software package for existing and future automation needs, instead of multiple software packages and utilities from various vendors.
In PACs, modular processor hardware can be replaced without ripping out existing sensor and actuator wiring. PACs are also compact to conserve valuable cabinet space.
With their network and communication capabilities, data are collected accurately and quickly, making production information available in or near real time.
The overall cost of the PAC control system is low because hardware is less expensive and needs less development and integration time. Other benefits include a greater return on assets, reduced lifecycle costs, and lower total cost of ownership because the control can be applied to a wider range of applications — also known as its domain expertise.
Cash flow also improves-the ability to add I/O as separate modules means that just the minimum number of modules can be used during initial configuration, and the remaining modules added at the end of the project.
PACs offer easy programming through
Structured Query Languages and other programs that support processes
that flow across several machines or systems.
Single platform operating in multiple domains
A single PAC operates in multiple domains to monitor and manage a production line, a chemical process, a test bench, and shipping activities. The PAC simultaneously manages analog values such as temperatures and pressures; digital on/off states for valves, switches, and indicators; and serial data from inventory tracking and test equipment. At the same time, it exchanges data with an Object Linking and Embedding for Process Control (OPC) server, an operator interface, and a Structured Query Language (SQL) database. The hallmark of a PAC is its ability to simultaneously handle these tasks without need of additional processors, gateways, or middleware.
In a factory example, a PAC, operator and office workstations, testing equipment, production line and process sensors and actuators, and barcode reader connect to the factory 10/100 Mbps Ethernet network. In some instances, devices without built-in Ethernet connectivity, such as temperature sensors, connect to I/O modules on an intermediate Ethernet-enabled I/O unit, which in turn communicates with the PAC.
Through this Ethernet network, the PAC communicates with remote racks of I/O modules to read/write analog, digital, and serial signals. The network also links the PAC with an OPC server, an operator interface, and a SQL database. A wireless segment is part of the network, so the PAC can also communicate with mobile assets like the forklift and temporary operator workstation.
The PAC can control, monitor, and exchange data with this variety of devices and systems because it uses the same standard network technologies and protocols they use. This example includes wired and wireless Ethernet networks, Internet Protocol (IP) network transport, OPC, and SQL. In another control situation, common application-level protocols such as Modbus®, Simple Network Management Protocol (SNMP), and point-to-point protocol (PPP) over a modem could be required. The PAC has the ability to meet these diverse communication requirements.
In another factory example, the PAC exchanges manufacturing, production, and inventory data with an enterprise SQL database. This database in turn shares data with several key business systems, including an enterprise resource planning (ERP) system, operational equipment effectiveness (OEE) system, and supply chain management (SCM) system. Because the PAC constantly and automatically updates data from the factory floor, business systems continually have timely and valuable information.
Through PACs’ human-machine interface, the
control exchanges manufacturing, production, and inventory data with
enterprise SQL databases, which in turn share with key business
systems, such as enterprise resource planning (ERP), operational
equipment effectiveness (OEE), and supply chain management (SCM).
The PAC as alternative to other controllers
The PAC can take the place of remote terminal units (RTU). RTUs are usually deployed in distant geographic areas to monitor, acquire, and control remote assets such as pipelines, wellheads, lift stations, or telecommunications facilities. Traditional PLCs don’t natively possess the communication capabilities needed for these applications. Also, PLCs typically don’t offer the flexible I/O configuration required in most RTU applications.
However, legacy RTU communication capabilities are usually outdated, as they were developed in a time of private radio or leased-line networks. Today’s open, IP-based wired and wireless local- and wide-area networks are more flexible and often less expensive. Because of this, retrofitting an existing RTU or implementing new applications using outdated RTU technology doesn’t make good business or technical sense.
PACs, in contrast, provide the communication capabilities, versatile I/O options, broad temperature and shock specifications, and use today’s leading communication standards. These features make PACs a suitable alternative to legacy RTUs in the field, especially when older, proprietary networks are retired due to maintenance costs or obsolescence.
PACs can be an alternative to data acquisition (DAQ) systems. The majority of DAQ systems are PC-based, limiting their use to laboratory locations. PACs offer versatile and flexible signal sensing, conditioning, and multiplexing. A PAC’s processors and global memory can aggregate, collate, and convert raw data into engineering units before sending them to a database or other application. Data may also be archived locally.
Buyer beware
Not all PACs are really programmable automation controllers. Several industrial automation vendors offer PAC products that are more PLC or industrial PC. As with any product, some vendors have offered these devices longer than others. Look for a company with a successful track record of providing PAC functionality several years before the term itself entered the mainstream.
A PAC provides a single compact control with advanced control features, network connectivity, device interoperability, and enterprise data integration capabilities. With these features, the PAC will be an integral part of meeting the new and diverse requirements demanded in a modern industrial automation application.
:: Design World ::
Source: :: Design World ::
