Tech Tips May 2006


May 30, 2006


Applying thermocouples

Thermocouples are the most commonly used devices for temperature measurement. Constructed of two dissimilar metals joined at one end and open at the other (output end), they produce a voltage for a given temperature. A thermocouple's voltage signal increases as temperature rises.

Thermocouples have many advantages over other types of sensors, such as resistance temperature detectors (RTDs) and thermistors. Thermocouples (T/Cs) are self-powered, requiring no external source. Other T/C benefits include extreme ruggedness, ability to withstand harsh environments, low cost—compared to RTDs and thermistors—and availability in a wide variety of types offering wide temperature range,

On the downside, nonlinearity of T/Cs requires cold-junction compensation (CJC) to linearize the device. Also, voltage signals are low—typically, in the tens to hundreds of microvolts. This requires careful techniques to eliminate noise and drift in a low voltage measurement. A further drawback is thermocouples' lower accuracy compared to RTDs or thermistors. Depending on thermocouple type, a one degree temperature change could mean a few microvolts change in signal.

Set-up advice
Avoiding some common mistakes when setting up and using thermocouples will result in better temperature measurements. A frequent problem is that CJC is not configured or compensated properly, or not done at all. This leads to inaccurate or nonlinear temperature measurements. Ensure that CJC is accounted for in the setup.

Another mistake is to use copper wire from the thermocouple connection to the measurement device. This in effect 'introduces' another thermocouple into the measurement, because any junction of dissimilar metals forms a thermocouple. A compensation method is provided in the measurement system to offset this effect.

On the measurement device side, the voltmeter being used may not be sensitive enough for thermocouple measurements. Make sure the voltmeter is sensitive and accurate enough for the low voltage (micro- to millivolt) measurements associated with thermocouples.

Source: Control Engineering, Feb. 2006, ' How to apply thermocouples .'

May 23, 2006


Using safety integrity levels—Part 2

Applying the safety integrity level (SIL) concept has been ongoing in process industries but is relatively new in production automaton. SIL ratings (1, 2, 3, and 4) use the probability of a dangerous failure over time as a way to measure range of risk reduction afforded by a piece of equipment.

Part 1, in the previous Tip of the Week, discussed a general approach to the SIL selection process and described the four SIL probability ratings. Here's further guidance and description of the top-level risk assessment steps to use at any facility embarking on a risk reduction program.

Determine an acceptable level of risk . Select a SIL- (or category-level) rating that the assessment should meet.

Start by choosing who will look for hazards . Cross-functional teams can offer perspectives beyond one person's view. A control engineer, operator, maintenance technician, and custodian all may have insights into risks and risk avoidance in varied circumstances.

Look for areas to be assessed . This includes a device, a machine design, general area, or full manufacturing line or system.

List the hazards . Examine each situation from multiple perspectives and circumstances. Ensure clear delineations among commissioning, operations, and maintenance modes; look at each and transitions among them. Ensure all designed and installed safety equipment become a documented part of training for anyone with access to the area.

Analyze risks according to severity and probability. Evaluation and probabilistic analysis are based on failure rate and failure mode data. For any potential source of harm, figure out how bad risks are, how often they could occur, and combine the two.

Typically for a protective function, a machine often will have more than one means of protection, perhaps a primary-perimeter guard or electronic guard, such as light curtains. How bad would it be if they fail; how often could they fail?

Mitigate risks , if needed: A shrink-wrap machine could present pinching or mechanical hazards. A guard or isolation with other protective device might stop injury. Over-temperature protection or a flame detector guard might be used if a flame or heater was involved.

For some cases, design changes may be needed; there's not a piece of safety equipment that can mitigate every hazard. Fortunately, most things in life require no safety function at the end of the discussion.

Source: Control Engineering, Jan. 2006, ' How to Assess Risk .'

May 16, 2006


Using safety integrity levels—Part 1

Applying the SIL (safety integrity level) concept has been ongoing in process industries but is relatively new in production automaton. It offers different, perhaps more flexible, means of assessing risk and augmenting safety at discrete manufacturing sites. SIL uses statistics to represent the reliability of a safety-instrumented system for a process that occurs through the requirement of the system.

SIL ratings (1, 2, 3, and 4) offer probability of a dangerous failure over time and can be used to measure range of risk reduction afforded by a piece of equipment. SIL selection determines how much risk reduction is needed. Risk reduction with SIL safety measures is:

  • Level 1—between 10 and 100 times;

  • Level 2—between 100 and 1,000 times;

  • Level 3—achieves a risk reduction of 1,000 to 10,000 times; and

  • Level 4—achieves a risk reduction of >10,000 times.

SIL 3 is considered the highest risk reduction level achievable using one programmable electronic system. Need for SIL 4-rated applications is rare-for example, in some areas of a nuclear-power generating plant. Standards caution that one programmable safety system shouldn't be used to meet SIL 4 requirements.

Here is a suggested general approach to the SIL selection process, leading to required level of risk reduction with cost-effectiveness:

  • Estimate the consequence —analyze all outcomes of an unwanted event;

  • Estimate the likelihood —combine initiating events and failure of devices designed to prevent accidents;

  • Integrate the risk —determine impacts on people, property, profits, and the environment; and

  • Select the required risk reduction —this is the difference between baseline risk and what's considered tolerable.

Additional guidance and insight into SIL selection process will be provided in next week's Tip of the Week.

Source: Control Engineering, Jan. 2006, ' How to Assess Risk .'

May 9, 2006


Voltage step-down configuration choices for FPGAs

Field-programmable gate arrays (FPGAs) operate with different voltages to power their various circuits. Operating current for these voltages varies widely (typically in the 100 mA to 20 A range)—depending on application-related factors, such as FPGA speed and the device's capacity loading. Input voltage must be stepped down and regulated because it is usually higher than the several operating voltages required onboard the FPGA.

Three of the most commonly used voltage step-down configurations for FPGAs are known as synchronous buck , non-synchronous buck , and linear regulator . The choice of regulator depends on a combination of input voltage, output voltage, and output current. Here's a rule of thumb to help in the selection:

  • Use a linear regulator if power dissipation in less than 1 W.

  • Use a non-synchronous buck regulator if voltage input/output ratio is &2:1 and output current is &3 A.

  • Use a synchronous buck regulator if the voltage input/output ratio is >2:1 and output current is >5 A.

Source: National Semiconductor Corp., Power designer, No. 102 (2004)

May 2, 2006


Using RSS in manufacturing

RSS, short for 'Really Simple Syndication,' is an XML-based method for any Web site or FTP site to become an information publisher. RSS represents a way to select and collect Web accessible information—with applications extendable to manufacturing tasks.

Lack of a common standard for RSS readers has slowed adoption of the RSS method, however most RSS readers will accept all available standards. Thus, manufacturing systems are able to use RSS.

RSS is not a Web-publishing system, but a format for an XML file. The publisher puts information on an accessible Web (or FTP) site and updates one XML RSS file, called an RSS stream. The top-level element in an RSS stream file is a channel , which defines an area of interest. Channels can be identified with production areas, such as separate RSS stream files for packaging, main production, dispensing, filling, and assembly. Channels also can be defined for specific manufacturing report categories, such as variance, utilization, and throughput. If a report fits into multiple channels, then different RSS streams can point to the same report.

Each channel contains items , which usually include at least a title, description, publication date, and link. The link is a URL that points to the complete Web page, document, or file. An RSS publisher places these files in an accessible area and then updates the RSS stream with a new item pointing to the file.

Most manufacturing report packages can generate XML files and formatted reports. Automatically generated reports can be saved in an accessible location, such as an FTP directory. The same report package can generate a summary XML document, containing an 'item' description of the report—which can contain pertinent summary or exception data. A simple application would then read the generated items and update the RSS stream file.

RSS allows each production manager to 'subscribe' to streams important to him or her and setup key word filters to select only the data needed. RSS provides a way to change our patterns for finding important and pertinent information contained in manufacturing system reports.

Source: Control Engineering, Jan. 2006, ' RSS finds pertinent manufacturing data .'

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