Reach Maximum Productivity
Discover how redundant and non-redundant methods can help you achieve high availability of control systems.
By Art Pietrzyk, TUV FSExp and Brian Root, Redundancy Marketing Manager, Process Initiative, Rockwell Automation; and Paul Gruhn, P.E., CFSE, Training Manager, ICS Triplex
Many engineers think of redundancy as the only method for achieving higher availability. However, redundancy increases the number of components, which increases the number of potential component failures. Therefore, redundancy, if improperly applied, can decrease system availability. So, should engineers consider alternate methods?
Availability is the probability that a system is successfully operating. Many people use the term high availability to encompass productivity, including reliability and maintainability. Reliability is the likelihood that a device will perform its intended function during a specific time, often called the mission time.
But even the most robust and reliable system may not be the most available. To be available, a system must be easy to troubleshoot, modify and repair during the mission time – in other words, maintainable.
How maintainability impacts availability
Qualified maintenance personnel are important to maintainability, and so are physical characteristics. You should be able to remove, replace or add modules or components to the system without interrupting the mission and with no rewiring or reprogramming. Control technology features that have improved maintainability include:
• Adding or removing modules under power
• Adding I/O online
• Online edits and partial downloads
• Soft switching of processor’s producer/consumer communication
• Internal diagnostics to detect failures
• Diagnostics of field circuit problems
• Configurable fault response: hold last state or turn off
• HART and other fieldbus technology with sensor and actuator diagnostics
• Self-learning or inherent machine diagnostics
• Adding sensors, I/O and tags online without interruption
HART and other fieldbus devices communicate with more intelligent sensors, instruments and actuators, and provide their own level of device diagnostics. This diagnostic functionality, along with additional process data that these devices provide, are married to software that benefits users with up-front alarms, calibration and model information for easier replacement and inventory management. Other innovations, such as state-based control and self-learning diagnostic routines, have raised the controller’s ability to detect, annunciate and describe problems within machinery.
Redundancy, fault tolerance
Redundancy or fault tolerance might be needed for critical applications. Redundancy means duplication or triplication of equipment that’s needed to operate without disruption if primary equipment fails during the mission. Fault tolerance is the system’s ability to tolerate faults and continue roperly operating. Redundant components needed for high availability include:
• Uninterrupted Power Supply (UPS). It’s important to provide continuous power. Bad power can cause unexpected behaviour to running microprocessor-based equipment. Therefore, the control system only is as reliable as the power provided to it. The key is to attach the output power of the UPS to the primary controller, which filters surges and minimises system recovery when power is re-established
• Redundant Power Supplies
• Redundant Components. Chassis, processors, I/O modules, sensors and actuators, PCs/human-machine interface (HMI), networks, media, servers, databases.
Redundant Field Devices to determine if redundant I/O modules are needed for achieving higher availability, consider sensors and end devices. The reliability and diagnostics from sensors and actuators is a magnitude less than that of the logic solver. Engineers often implement redundancy on the input side by monitoring the same process variable with two sensors wired to two I/O modules
The importance of design
Many times, you can achieve an acceptable level of availability through design, which includes the controller, HMI and information system. The designer must be willing to accept the fact that anything can fail, and design the facility around this notion. The equipment or plant can be designed to continue running if a machine were to fail. This often has been referred to as modular distributed design, and involves the following areas:
• Distributed control architectures
• Distributed control design with independent line, zones, etc.
• Distributed HMI
• Distributed databases
In continuous and batch processing operations, following the S88 model helps achieve availability by allowing recipes and procedures to be ported to various equipment, lines and plants. Human interface into a process or operation is crucial. If data is critical, then provisions should secure it from data loss, such as when a single server goes down. One of the most economical methods is to keep some of the more recent data stored in the controllers. Use redundant or fault-tolerant communications if a network is determined to be a weak link. This could include Ethernet rings, with or without redundant media. Networks can be configured with redundant paths using switches or routers.
More than redundant components
Although redundancy is the traditional method for achieving high availability, the way to achieve high availability requires more than just thinking about redundant components. A system with no redundant components can still be very available.
For more information, please e-mail us at: info_at@ra.rockwell.com with ref: Redundancy
| Predictive Maintenance Can Help The goal of a Condition-based Maintenance (CbM) programme is to increase reliability and availability of your machinery, while minimising downtime, labour and repair costs. You can perform CbM by scheduling downtime, labour and materials based on machinery health. The results can be dramatic, and the documented cost savings can be significant. A CbM programme has many benefits, including overall equipment effectiveness (OEE); reduced mean time to repair (MTTR); increase Return on Net Assets (RONA); reduced unplanned downtime and planned downtime duration; and lower inventory costs. Three types of condition monitoring services are:
Rockwell Automation Condition Monitoring Services provide tailored solutions to help you implement your CbM programme. A vibration analyst can come to your facility and provide both data collection and analysis of your equipment, or Rockwell Automation can teach you how to collect data and perform analysis. For more information, visit www.rockwellautomation.com/go/tjcm |
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