Tech Tips March 2007
March 20, 2007
TECH TIP OF THE WEEK:
Factors affecting network bandwidth.
Bandwidth is defined as the carrying ca-pacity of a circuit. It's usually measured in bits per second for digital circuits, or in hertz for analog circuits. Though a hard-wired network is also limited by the speed of its devices and microprocessors, a typical 4-20 mA signal over typical 18-gauge wire usually has about a 10-ms response time. Fieldbus response times can range from 2-50 ms or more.
'Network bandwidth comes down to two measures: 1) transactions per second-how many communication cycles is the network accomplishing per second, and 2) latency- how long did it take to get the data from the time you wanted it?' says Wally Pratt, chief engineer at the HART Communication Foundation. 'When you're talking about throughput, bytes per second, latency means how many times can you measure your pri-mary variable? When you're talking about a multi-drop network, you have to take trans-actions per second and divide by the number of devices on the network, which will show the number of times you can talk to each one. Then, you have to ask is that time enough for the application?'
Perry Marshall, author and consultant, adds that users need to determine their speed and data requirements. 'Does your I/O device need to be scanned every 2 ms? Or is 500 ms okay? Do your devices send a few bits of data, or hundreds of bytes? Are you mixing simple and complex devices on the same network?' asks Marshall. 'To answer these questions, determine the 'hard limits' and worst-case requirements of your design. Which networks are available on specific devices you've chosen? This issue may force your hand, or at least limit your choices, if you are required to use a specific brand of PLC or other component. 'There's only a loose relationship between bandwidth and speed. The real tradeoff is among bandwidth, response time, determinism, and noise immunity.'
To read the article and its 'Fieldbus Comparison Chart,' click here .
March 13, 2007
TECH TIP OF THE WEEK:
Get the least bang for the buck!.
In general, engineers want to get the most bang for the buck when specifying machine and motion control components. The 'bang' refers to whatever action the component is supposed to produce. For example, if the component is a motor, you want to get the highest ratio of torque, horsepower, or whatever for the purchase price. Actually, we're trying to maximize a ratio by specifying the numerator (the bang) and minimizing the denominator (the bucks).
When the bang is actually a real bang—that is, mechanical shock created by applying full power to a de-energized motor, or suddenly cutting power to a motor running at full tilt—it's the numerator we want to minimize. In this case, we want to minimize the bang for the buck.
The technology we want to use is called 'soft start.' A soft starter stands in the place of a variable speed drive in simple motor applications. The simplest configuration is to have a contactor, which is nothing more than a big relay, apply unmodified power from the electric mains to the motor. The user (who could be a digital I/O line from a complex supervisory system) energizes the contactor, which closes to apply full mains power to the motor. You push the button, Max, and the contactor goes 'bang' and the motor tries to ramp from dead stop to full speed instantaneously. To stop, the contactor simply opens to cut motor power. At best, there might be capacitors across the contacts to absorb energy stored in the motor's inductive load.
The soft start configuration puts a more complex 'box'—the soft starter—in place of the relay. The 'start' signal now really becomes a digital bit going high to tell the soft starter to begin its ramp up. The soft starter, for its part, has been sending zero power to the motor, but now begins to ramp the motor speed from zero to maximum along a predefined trajectory, achieving full speed at some predefined later time. To stop, the soft starter applies a separately programmed ramp-down trajectory to bring the motor to a controlled stop.
The variable speed drive (VSD) is, of course, the usual variable speed drive that allows full motor-speed control under direction of a PLC or other controller. The difference is that, while the soft starter ramps the motor speed along a predefined trajectory, the VSD controls the motor speed along a trajectory that is infinitely flexible and modifiable on the fly.
There are two styles of soft starter: analog and digital. Analog soft start uses analog power circuits to more-or-less slowly apply power. There are two analog controls—basically potentiometers with knobs—to set the ramp up time and ramp down time individually. The trajectory is invariably a linear ramp. Digital soft starters, on the other hand, employ an embedded computer providing real-time control of the motor's speed. Generally, they still control the speed by controlling the power transmitted to the motor, but embedded software can (theoretically) allow the user to program in fairly complex trajectories.
Soft starters prevent large motors from creating large mechanical and electrical transients during start up and shut down operations. Suppressing such transients reduces wear and tear on the mechanical systems, improves electric power quality throughout the facility, and extends the life of electrical switching gear.
C.G. Masi , Control Engineering senior editor
For more information about soft starting equipment, visit the Control Engineering Website at www.controleng.com , and type 'soft starter' into the search box
March 6, 2007
TECH TIP OF THE WEEK:
Learn a second language.
There are 3 good reasons why the idea of learning a second language (and I don't mean another programming language) is anathema to most U.S. engineers:
English is the international language of engineering and science, anyway, so the most important technology ends up published in English;
Learning a foreign language is hard, especially for engineers and engineering students who have plenty of hard stuff to learn already;
Engineering schools as well as employers seldom require foreign language competence from engineers.
While Johnny Wang, product marketing manager with Via Technologies in Fremont, CA, doesn't dispute that these reasons are valid, he feels strongly that U.S. engineers should be encouraged to learn foreign languages, anyway. In fact, he characterizes the current lack of foreign-language competence among American engineers as 'dangerous.'
Johnny is a transplant from Taiwan, and he has held engineering, sales and management positions in a number of multi-national semiconductor and robotics companies, so he's in a position to know. 'The most important developments are always published in the local language first,' he told me at lunch last week. 'When those developments occur in non-English-speaking countries, they therefore appear in Japanese, Korean, or some other language first, and the developers have little or no motivation to translate them into English.'
That means, unless it was invented here (here being an English-speaking country), U.S. engineers are the last to know about it!
Of course, that makes perfect sense. It doesn't take some sinister global conspiracy to hide technical advances from Americans. People publish their findings to communicate them to their peers. If Timbuktu is the world center for frammis manufacturing, and the engineers in Timbuktu all speak Timbuktuese as their native language, then publishing frammis-technology advances in Timbuktuese makes perfect sense. Why would frammis developers translate their papers into English first?
Without knowing Timbuktuese, U.S. engineers are closed off from the latest frammis technology.
The same holds true for any technology whose main activities take place outside English-speaking countries. Take semiconductor manufacturing, for example, Japan, Korea, and Malaysia as well as a handful of other countries sport large, advanced semiconductor manufacturing industries large enough to support local-language semiconductor manufacturing publications. Many of them also have engineers capable of developing significant technological advances. Having a dearth of engineers capable of reading those languages puts American semiconductor companies at a disadvantage.
Part of Wang's complaint is his difficulty in finding people to translate even technical manuals into English. 'I can find plenty of people who speak, read, and write Japanese,' he reports, 'but they seem to all be humanities majors with no understanding of the technology being described. What I can't find are engineers—the people who understand technology—who speak, read and write Japanese.'
Wang's message to American engineers and American engineering companies is simple: learn a second language.
People in developing and developed countries all start learning English in grade school. It's required. By the time they reach college and start studying engineering in earnest, they already have a good command of English. Then, it's easy for them to pick up technical English.
Looking back at the three reasons not to learn a foreign language, Wang shows why U.S. engineers should dismiss them:
While technical advances generally end up published in English, many of them appear in English only after significant delays, which give the advantage to overseas competitors;
While learning a foreign language is hard, it's no harder to learn than many skills engineers are expected to master;
While engineering schools and employers don't require engineers to learn a foreign language, knowing a foreign language makes them more desirable.
Johnny Wang, after all, is still looking for semiconductor engineers with a good command of technical Japanese.
C.G. Masi , Control Engineering senior editor