Features and Applications of the New Programmable Automation Controller (PAC)

Preface

Today, when designing and building control systems, engineers always hope to use fewer devices to achieve more functions.

Especially today, they need control systems that can not only handle digital I/O and motion, but also integrate visual functions and modular instruments for automated monitoring and testing, and must also be able to process control algorithms and analysis tasks in real time and transmit data back to the enterprise. This means that products are required to combine advanced functions and reliability. For this complex application, it is difficult to rely solely on incomplete solutions of PLC or PC. PC-based industrial control has the following weaknesses:

Stability - General-purpose operating systems are often unstable and production lines are subject to system crashes and unpredictable reboots.

Reliability - PCs are prone to failure due to the spinning magnetic hard disk and components like power supplies that are not as rugged as industry standards.

Unfamiliar programming environment - When a system stops, plant operators need to recover the system. With ladder logic, operators know how to manually activate a coil or add code to quickly recover a system. But with a PC system, operators need to learn new tools.

It is obviously out of reach to add comprehensive automation technologies such as vision, motion, instrumentation and analysis functions to PLC.

If you want to have the best solution of both PC functions and PLC (programmable controller) reliability, then PAC (Programmable Automation Controller) is such a platform, which can best combine the advantages of PC and PLC (see Figure 1), and it provides open industrial standards, scalable field functions, a general development platform and some advanced performance. This is the need for the development of today's design and establishment of control systems, and is a relatively complete emerging controller in the field of industrial automation.

PAC defines a new type of controller that combines the advantages of PC processor, RAM and software with the inherent reliability, ruggedness and distribution characteristics of PLC. PAC uses existing commercial off-the-shelf technology (COTS) and is very suitable for industrial environments. It has the characteristics of scalability, easy maintenance and low failure time. [page]

PAC Platform

The rapidly growing PAC platform is based on PXI. Because PXI combines the circuit characteristics of the PCI bus and the rugged Eurocard mechanical structure of Compact PCI, this structure has been successfully used in industrial environments for many years. Today, suppliers such as NI, Chroma, LeCroy and JTAG now offer more than 1,000 unique I/O modules, including analog I/O, digital I/O, vision, motion and high-precision data acquisition. The following four PAC hardware platforms are typically available.

PXl improves on the industrial PC with a real-time OS, standard cooling, an optional non-rotating solid-state drive, and built-in inter-module synchronization. The PXl standard requires all chassis to provide 25W of air flow cooling for each module slot, so that even when using high-power relays, high-speed PXl or CompactPCI cards, the operating system will not overheat or shorten its life. PXl also provides the ability to tightly synchronize the modules, so engineers can design motion, vision and I/O systems for high-speed control applications, including product packaging and semiconductor device processing.

Compact FieldPoint uses industrial-grade components to resist strong shock and vibration, and its operating temperature range is -40°C to 70°C. It also uses conduction cooling instead of rotating fans, which improves reliability by not using moving parts. With a floating-point processor running a real-time OS, the Compact FieldPoint system has PC functions, a CompactFlash drive for recording data, and an Ethernet port for communication.

The Compact Vision system is a rugged controller designed specifically for machine vision. It uses the IEEE standard 1394 FireWire interface to communicate with 16 cameras in vision applications. The Compact Vision system also uses inactive components and conduction cooling, so the system can be fixed near the machine. It provides 29 built-in digital I/O channels that can be directly controlled by LabVlEWRT or an embedded FPGA using LabVlEWFPGA.

CompactRIO is a new type of reusable embedded system based on LabVlEWFPGA and LabVlEW real-time technology. The CompactRIO system uses a 3 million gate FPGA chip to control modular digital and analog I/O. These FPGA chips can run the code embedded in the chip, with a digital cycle rate of up to MHz and an analog cycle rate of 150kHz. The FPGA can transmit information back to the floating-point processor running LabVlEWRT for advanced calculations and data logging and communication. Due to the metal housing and conduction cooling, the controller is very suitable for harsh environments.

PAC characteristic performance

The characteristic performance of PAC (Figure 2) is as follows:

Provide a universal development platform and a single database to meet the needs of multi-domain automation system design and integration;

A lightweight control engine that can realize functions in multiple fields: logic control, process control, motion control and human-machine interface, etc., as a unified platform;

Allow users to run multiple applications with different functions on the same platform according to the requirements of system implementation, and allocate system resources among various programs according to the design requirements of the control system;

Adopt open modular hardware architecture to realize free combination and matching of different functions, reducing the cost of system upgrade;

Supports IEC 61158 fieldbus specification, which can realize highly decentralized factory automation environment based on fieldbus;

Supports de facto industrial Ethernet standards and can be easily integrated with the factory's EMS and ERP systems;

For network protocols, languages, etc., use established de facto standards to ensure data exchange in multi-vendor networks.

PAC can perform many advanced tasks:

Real-time vibration analysis, image processing, motion control and CAN;

Perform automatic adjustment of PID control, or PID control with adjustable gain, fuzzy logic;

Communicate using the built-in Web server, FTP server, and e-mail capabilities. [page]

PAC Application Examples

Use PAC combined with NIPXI and graphics software LabVIEW RT (real-time) to achieve high-precision monitoring and high-reliability closed-loop control system for steel bars - automated processing design solution.

Project Design

Since the metal industry system requires cold rolling mills to produce uniform thickness steel bars (in the form of steel coils), these rolling mills need a highly reliable closed-loop control system that monitors the specifications of the steel bars produced. This is the design task of this PAC application.

System Technical Requirements

The system can perform high-precision monitoring and control with faster response time (than traditional PLC); it can ensure continuous high-reliability operation 24 hours a day, seven days a week, and minimize manual operation.

Since NI's PXI-based and real-time PAC platform is very suitable for high-precision monitoring, NI's graphics software LabVIEW RT (real-time) and PXI hardware can be used to develop a specification monitoring and control system based on NI programmable automation controller (PAC). The PAC control system monitors the thickness of the steel sheet in real time and controls the meter with a fast and deterministic response time.

The PAC control system has two rollers, one fixed and the other movable, which are connected to hydraulic cylinders. The movable roller presses the steel sheet according to the input signal to ensure that the steel sheet has a suitable thickness.

After comparing various platforms that can be used for this complex application, we chose the NI PAC platform and graphics software LabVIEW RT (real-time) and PX1 hardware. Using NI's PAC products not only shortens the development time, but also reduces costs for our customers.

The control system uses a thickness gauge to measure the thickness of the steel sheet. It measures the deviation from the positioning point with micron-level accuracy and forms an analog signal to input into the PXI hardware. Then an analog voltage signal is formed according to the pressure of the hydraulic cylinder.

The displacement sensor determines the position of the hydraulic cylinder and provides a digital input signal to the controller that is associated with the metal gauge (or the gap between the two rolls). The rotary encoder (TRD-K) is fixed to the rotating lifting roller, which shows the speed at which the steel bar is rolled out, and its schematic diagram is shown in Figure 3. By receiving data from various sensors, LabVlEW RT running on the PXI controller responds to the input and generates an output signal that changes the hydraulic pressure, thereby controlling the specifications of the steel bar. Both the thickness gauge and the displacement sensor can be implemented using fiber optic sensors.

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Figure 4 is a block diagram of the design scheme of the high-reliability closed-loop control system-automatic processing for monitoring the specifications of steel bars (or sheet materials) using PAC. It can be seen from the figure that the design scheme selects signal sensing and detection parts such as air pressure, displacement, thickness sensors and encoders, graphic software LabVIEW RT (real-time), PX1/CompactPCI multi-function high-precision acquisition card, and programmable automatic controller (PAC).

The control system is implemented using PXI control running LabVIEW RT (real-time). The PXI system uses three different modules (see Figure 4). We use the NI PXI-6070 multi-function high-precision acquisition card to complete the analog input or output thickness and pressure data. In addition, the NI PXI-6608 counter/timer card is used to measure the encoder signal and determine the roller speed (the schematic diagram is the same as Figure 3); the NI PXI-6711 analog output module is used to control the servo valve to change the position of the hydraulic cylinder, thereby controlling the specification or thickness of the steel bar. The NI PXI-6527 industrial digital I/O module (including 24 isolated optical isolation input lines and 24 isolated solid-state relay output lines) is used to implement a standing button control panel to solve the problem of unexpected accidents such as disconnection between the real-time system and the host. This is the development of a reliable control system using PXI and LabVIEW.

Compared with traditional instruments and PLCs, we use the NI PAC platform to increase flexibility, reduce response time and improve product quality. It is well known that for a typical PLC-based system, its control loop rate is 100 to 500ms. Using a PXI-based control system, we reduced the system's cycle time to 10ms, thereby improving the quality of the output. We also use the PXI backplane to synchronize measurements. Finally, by utilizing a single development environment and flexible hardware, we reduced the system's response time by one-sixth, thereby reducing the overall cost of ownership of the project.

It should be noted that the high-precision monitoring and high-reliability closed-loop control system of steel bars applied by the PAC can also be applied to the plate industry (or construction industry).

Conclusion

Since PAC can add the required PC functions for advanced control, real-time analysis or connecting to enterprise databases, while maintaining the reliability of PLC. If you need more than just integrated digital I/O and motion control, or if you need faster computer processing power, PAC may be a very good choice. For this reason, in addition to PLC control, today's engineers have PAC as the best choice. It is occupying the field of automation, and the PAC concept will play an important role in the automation of factories today and in the future.