Tech Tips November-December 2005

By Control Engineering Staff March 22, 2007

December 27, 2005


Make use of schematic design software—Part 2

As mentioned, advantages of using electrical schematic or panel design software include faster and more efficient wiring layouts, lower design and development costs, and less training time for users. Some of the software integrates with Autodesk AutoCAD; others are stand alone.

The following is a continuation of reasons for using this type of software. (The first five guidelines appeared in last week’s ‘Tip of the Week’):

Add features or libraries to scale/expand layout as needed. Some software fits electrical, process control, hydraulic, and pneumatic applications.

Support multiple languages. For example, Microsoft Visio 2003 is available in 17 languages, including enhanced support for Asian and bidirectional text.

Integrate tightly with other software, enabling easier import and export of information, or to create a bill of materials. For instance, Eplan 5 software is certified for ABB’s ‘Industrial IT’ product lifecycle management framework. Microsoft Visio integrates with other Microsoft Office-based tools allowing integration of its Outlook calendars or generation of reports to Excel.

Automate many functions, such as cross-referencing (also aids in navigation), searching, wire numbering, list generation, dimensioning, and error checking/alarms. SCADA Systems Ltd. EDS (Elecdes Design Suite) takes information from 2-D control drawings and produces 3-D layouts for electrical panels and cable trays.

Gain features through regular upgrades. In general, vendors support old software only for a certain time. As hardware platforms and software operating systems advance, a point is reached when a next-generation leap must be made to the newer framework. It pays to keep tabs on vendors’ announcements offering an upgrade because incentives often are provided.

Vendors offer demo or sample software for ‘test drives’ and many provide tutoring, user groups, and Web-based training.

Source: Control Engineering, February 2004 Technology Update, ‘ 10 reasons to use panel or schematic design software .’

December 20, 2005


Make use of schematic design software—Part 1

The following reasons offer some practical guidance for applying electrical schematic or panel design software. Advantages include faster and more efficient wiring layouts, lower design and development costs, and less training time for users. Some of the software integrates with Autodesk AutoCAD, while others are stand-alone.

Here are the first five (of 10) reasons you should consider this type of generic CAD software:

Eliminate repetitive drafting functions by using libraries or database of specific symbols and functions, including manufacturer-specific devices. Enable greater reuse of information and codes. Menus help organize information to speed selection and use. Associations and groupings permit creation of larger or new objects.

Preview/undo: Apply changes to multiple and related documents and see the outcome before committing to a change.

Maintain compatibility with existing DWG files, DXF format, and IEC/DIN and ANSI/JIC standard page and symbol formats—among others.

Collaborate with secure, Web-based features to allow simultaneous modifications with notations of who modified what and when. This allows information sharing with others in a project design team.

Use project management features to help coordinate various parameters and phases of the project.

Vendors offer demo or sample software for ‘test drives’ and many provide tutoring, user groups, and Web-based training.

Part 2, with another five reasons and guidance follows in next week’s ‘Tip of the Week.’

Source: Control Engineering, February 2004 Technology Update, ‘ 10 reasons to use panel or schematic design software .’

December 13, 2005


How to differentiate among reliability parameters MTTF, MTBF, and MTTR

Reliability is a metric first developed to determine probability of successful operation of a system for a specific ‘mission time.’ Measurement of reliability requires that the system work successfully for some period of time.

One of the most widely used reliability parameters, mean time to failure (MTTF) has been formally defined as the ‘expected value’ of the random variable, time to fail, T . However, MTTF has been misinterpreted as ‘guaranteed minimum life.’ Formulas for MTTF are derived and often used for products during their useful life period, but this method excludes wearout failures.

Mean time to restore (MTTR) —changed from ‘mean time to repair’ in IEC 61508 to clear up confusion over what ‘times’ are included in the term—is the expected value of the random variable restore time . It includes diagnostic time required to detect/identify a failure occurrence and actual time needed to make the repair. For estimating purposes, MTTR also must include time to obtain spare parts, time for repair team response, time for documenting all tasks, and time to put the equipment back into service.

Mean time between failures (MTBF) is defined as the average time period for a failure/repair cycle, including time to failure and any time to detect/repair the equipment. For a simple repairable component, MTBF = MTTF + MTTR. Because MTTR is typically much smaller than MTTF, MTBF is approximately equal to MTTF, and is often substituted for MTTF. MTBF applies to repairable as well as nonrepairable systems.

Source: ‘A Guide to the Automation Body of Knowledge,’ Vernon L. Trevathan, editor, ISA-The Instrumentation, Systems, and Automation Society (2006), Chapter 18: Reliability, ISBN 1-55617-961-8.

December 06, 2005


Resolving wireless security problems

With the burgeoning of industrial wireless solutions the concerns and issues of system security are likewise on the rise. One expert suggests that most wireless security problems can be resolved with some combination of two elements: encryption and data authentication.

Encryption typically involves enabling the 128- or 194-bit capabilities already in most wireless equipment, and establishing a process for managing the keys that encrypt or decrypt data running over a network. These keys feed into algorithms that allow data encryption and—using newly available ‘dynamic change’ keys—prevent unauthorized data sniffing.

Data authentication , similar in role to passwords that verify user identities, ensures that a wireless access point is truly the authorized point it claims to be. This is accomplished by passing back and forth keys and other pre-programmed information known only by the client device and its host. Methods for doing this include Wi-Fi Protected Access (WPA) and Cisco’s Lightweight Extensible Authentication Protocol (LEAP).

Source : Control Engineering, May 2005, ‘Security for wireless” sidebar in article, ” Wireless: Simple, Safe, Secure, Successful. ”

November 29, 2005


Guidelines for creating practical manufacturing databases—Part 2

The following simple guidelines represent a practical approach to develop databases where a manufacturing execution system (MES) cannot be justified. (The first five guidelines appeared in last week’s ‘Tip of the Week’):

Use transactional control. Most SQL databases provide some form of transaction control, allowing multiple database changes that either all succeed, or if any one fails, then all roll back. It further reduces the application’s error-handling code.

Use stored procedures, which are SQL statements executed on database events, such changing a data field or deleting a record. This can significantly reduce the application code.

Provide an option so the application can create the database. This makes testing and rollout much easier, including the stored procedures. It is easier to track one application source file than multiple files and versions.

Don’t hard-code database paths, instead use the operating system registry database to find environment information. Hard-coded paths, such as ‘C:My Database,’ provide an execution constraint that is hard to maintain. Relocating the database without recompiling the application is more supportable.

Use Visual Basic (or equivalent) to reduce code and provide a good user interface. Many free, or low-cost, high-level development environments exist that can reduce coding effort and make a database application cost-effective.

These simple guidelines—including those in Part 1, Tip of the Week for 11/22/05—have helped the author develop long-lasting database applications when an MES is not available or cost justified.

Source : Control Engineering, October 2005 Insight, ‘ How to create practical manufacturing databases ,’ Dennis Brandl, BR&L Consulting.

November 22, 2005


Guidelines for creating practical manufacturing databases—Part 1

The following simple guidelines represent a practical approach to develop databases where a manufacturing execution system (MES) cannot be justified. Typical database applications involve recording an event, a person, a place, a material, and a time—for example, tracking production artifacts, such as test labels and samples.

Here are the first five (of 10) guidelines for manufacturing database applications:

Keep the functionality small and stick to transactional problems rather than real-time or procedural ones. Transactional problems rarely need more functionality than create, report, update, and delete.

Use an SQL-based database. This standard-based method provides scalability and long-term maintainability of the application needed in many manufacturing applications.

Use a database server. While small applications often can be run on one computer, locating the database on a server provides a supportable solution. It also offers backup support and redundancy.

Use integrity constraints. Most SQL databases support some form of foreign key, not null, unique, and range checking functions. This ensures that only valid data are added to the database, providing checks that need not be included in code.

Create ‘abnormal’ tables (similar in structure to normal tables), but without integrity constraints to hold abnormal situations. If an error occurs in normal operation, an operator often will not have enough information to solve the problem. Then, information stored in abnormal tables can be used for analysis and correction of problems.

Part 2, with another five guidelines follows in next week’s ‘Tip of the Week.’

Source: Control Engineering, October 2005 Insight, ‘ How to create practical manufacturing databases ,’ Dennis Brandl, BR&L Consulting.

November 15, 2005


4 ways to integrate engineering and maintenance

In pursuing ever-increasing efficiencies, manufacturers may be overlooking existing opportunities within their organizations to effectively strengthen processes, performance, and overall productivity. Here are four recommendations.

1. Create efficiencies in maintenance and engineering functions by more closely aligning these activities within your production operation. Most manufacturers encourage collaboration between engineering and maintenance functions, but have traditionally managed each separately—possibly missing out on opportunities to improve asset performance due to poor coordination and lack of communication.

2. Involve engineering and maintenance-support staff during the install and operate phases of any project. Active participation from both groups is essential to uncover the most effective methods of designing and optimally maintaining production equipment.

3. Maintenance personnel also must become better informed about production specifications and operating parameters of plant-floor equipment. When a problem occurs, maintenance teams often repair equipment to its most recent production levels, rather than the machine’s optimal design capacity. Therefore, maintenance teams should compare their equipment’s actual productivity with the project engineer’s original design specifications to maximize operating potential.

4. To help maintenance and engineering staffs collaborate more effectively, many manufacturers use third-party suppliers to provide insight into opportunities for process improvements and synergies. Understanding your organization’s unique challenges, engineering service providers help integrate engineering and maintenance to maximize uptime and optimize ROI.

Many manufacturers have reported substantial returns after 18 to 36 months of functional integration. The author has witnessed many advantages of more closely integrating engineering and maintenance functions, among them: lower costs; improved process efficiency; and better machine operation/output. Although integration requires an up-front investment, bridging the gap between engineering and maintenance can reap substantial, long-term rewards for all types of manufacturers.

Opinions presented in the Soapbox column are those of the author and not necessarily those of Control Engineering or Reed Business Information.

Source: Control Engineering, Feb 2005 Soapbox, ‘ 4 paths to engineering, maintenance integration ,’ opinion by Mike Laszkiewicz, vice president, customer support and maintenance, Rockwell Automation.

November 8, 2005


Troubleshooting RS-485 networks

RS-485 technology remains the mainstay of myriad communication networks. Here’s an eight-step recipe for checking common failures and making troublesome RS-485 networks work.

1 . RS-485 uses an unbalanced differential wire pair. To minimize data line noise, every network device must be connected to ground through a signal return. In a noisy environment, data conductors should consist of a twisted pair of wires plus a shield.

2 . Termination causes many problems. To check which nodes are terminated, power down each one and disconnect it from the network. Measure between A and B or + and – lines of the receiver with an ohmmeter. Terminated nodes will usually read less than 200 ohm; unterminated nodes will read greater than 4,000 ohm.

3 . Differentiating between A and B lines can be difficult because different manufacturers have adopted different labeling conventions; however, B line should always be more positive in the idle condition. Thus, A line equates to (-) and B line equates to (+). Check the idle network with a voltmeter. If B line isn’t more positive than A line, there’s a connection problem.

4 . A tristate condition occurs in an RS-485 network when no devices are transmitting. With all devices listening, this can cause all drivers to go into a high impedance state, leaving floating wires feeding into all RS-485 receivers. A node designer typically cures this precarious state by installing pull-down and pull-up resistors on the receiver’s A and B lines to simulate an idle condition.

5 . In a two-wire-plus-ground RS-485 network no two transmitters can successfully talk at once. There’s a slice of time immediately after the last bit has been sent when the network appears to be idle, but in fact the node has not yet put its driver into the tristate condition. A collision with unpredictable results will occur if another device tries to talk during this interval. To check for collisions, use a digital oscilloscope to capture a few bytes of ones and zeros. Identify the time it takes for a node to reach the tristate condition at the end of a transmission. Make sure the RS-485 software is not trying to respond to a request faster than one byte time (a little more than 1 ms at 76.8 kbit/s).

6 . RS-485 has no built-in isolation. It’s up to the system designer to ensure the network does not include any ground loops. Isolating every node will increase the network’s reliability by orders of magnitude.

7 . Isolation is the first line of defense against power surges, but adding a multistage surge suppressor takes the edge off larger surges, keeping them in the range that the network’s isolation can tolerate. It’s best to install a surge suppressor at one network location with a high-quality ground. Tie it to earth ground at the same point as the rest of the network equipment or the facility’s electrical system.

8 . Once the RS-485 network is operating, write down every detail of its configuration. Include information on termination, biasing, wire types, and spare parts. Buy some spares now, if they’re affordable.

More information is available at the source below.

Source : Control Engineering, March 2005 (Back to Basics),’ Troubleshoot RS-485 networks .’

November 1, 2005


Select induction motor types to suit different application loads

Popularity of the ‘workhorse’ ac induction motor has led to certain standardized motor designs that result in manufacturing efficiencies and attractive pricing. The National Electrical Manufacturers Association, (NEMA) has developed specifications for so-called NEMA design A, B, C, and D motor types that standardize typical motor characteristics, such as starting current, slip, and torque points to suit various application loads. Here’s a brief rundown on NEMA motor types:

Design A has normal starting torque (typically 150-170% of rated) and relatively high starting current. Breakdown torque is the highest of all NEMA types. It can handle heavy overloads for a short duration. Slip is less than or equal to 5%. A typical application might be powering of injection-molding machines.

Design B is the most numerous type of ac induction motor sold. It has starting torque similar to, but somewhat lower than, Design A, and offers lower starting current. However, its locked rotor torque still allows starting many loads encountered in industrial applications. Slip is less than or equal to 5%. Motor efficiency and full load power factor are comparatively high, contributing to the popularity of the design. Typical applications include pumps, fans, and machine tools.

Design C provides high starting torque (greater than Design A or B and typically more than 200% of rated). It’s useful for driving heavy breakaway loads. These motors are intended for operation near full speed without great overloads. Starting current is low. Slip is less than or equal to 5%.

Design D offers the highest starting torque of all the NEMA motor types. Starting current and full-load speed are low. High slip values (5-13%) make this motor suitable for applications with changing loads and attendant sharp changes in motor speed, such as in machinery with flywheel energy storage. Several design subclasses cover the rather wide slip range. This motor type is usually considered a ‘special order’ item.

These motors exhibit different speed-torque characteristics. Locked-rotor torque (starting torque) refers to minimum torque generated with the rotor at rest, and rated voltage and frequency applied. Breakdown torque is maximum torque generated before an abrupt drop in motor speed occurs as rated speed is approached (at rated voltage and frequency). Pull-up torque is the minimum torque generated over the motor’s speed range from rest to the speed point where breakdown torque is developed

Motors manufactured for European and international markets conform to a different set of designs and specifications defined by the International Electrotechnical Commission (IEC). However, one IEC induction motor type, called Design N , has operating characteristics comparable to NEMA Design A and B.

Source: Control Engineering, December 1999 (Online Extra), ‘ AC Induction Motor Designs, Types .’