One of the biggest advances in automation has been the development and spread of smart sensors. But what exactly is a "smart" sensor? Experts from six sensor manufacturers define this term.
A good working "smart sensor" definition comes from Tom Griffiths, product manager, Honeywell Industrial Measurement and Control. Smart sensors, he says, are "sensors and instrument packages that are microprocessor driven and include features such as communication capability and on-board diagnostics that provide information to a monitoring system and/or operator to increase operational efficiency and reduce maintenance costs."
No failure to communicate
"The benefit of the smart sensor," says Bill Black, controllers product manager at GE Fanuc Automation, "is the wealth of information that can be gathered from the process to reduce downtime and improve quality." David Edeal, Temposonics product manager, MTS Sensors, expands on that: "The basic premise of distributed intelligence," he says, is that "complete knowledge of a system, subsystem, or component's state at the right place and time enables the ability to make 'optimal' process control decisions."
Adds John Keating, product marketing manager for the Checker machine vision unit at Cognex, "For a [machine vision] sensor to really be 'smart,' it should not require the user to understand machine vision."
A smart sensor must communicate. "At the most basic level, an 'intelligent' sensor has the ability to communicate information beyond the basic feedback signals that are derived from its application." says Edeal. This can be a HART signal superimposed on a standard 4-20 mA process output, a bus system, or wireless arrangement. A growing factor in this area is IEEE 1451, a family of smart transducer interface standards intended to give plug-and-play functionality to sensors from different makers.
Smart sensors can self-monitor for any aspect of their operation, including "photo eye dirty, out of tolerance, or failed switch," says GE Fanuc's Black. Add to this, says Helge Hornis, intelligent systems manager, Pepperl+Fuchs, "coil monitoring functions, target out of range, or target too close." It may also compensate for changes in operating conditions. "A 'smart' sensor," says Dan Armentrout, strategic creative director, Omron Electronics LLC, "must monitor itself and its surroundings and then make a decision to compensate for the changes automatically or alert someone for needed attention."
Many smart sensors can be re-ranged in the field, offering "settable parameters that allow users to substitute several 'standard' sensors," says Hornis. "For example, typically sensors are ordered to be normally open (NO) or normally closed (NC). An intelligent sensor can be configured to be either one of these kinds."
Intelligent sensors have numerous advantages. As the cost of embedded computing power continues to decrease, "smart" devices will be used in more applications. Internal diagnostics alone can recover the investment quickly by helping avoid costly downtime.
Monday, August 18, 2008
Tutorial: Thermal mass flowmeters (insertion type) for gas applications
In our last issue, we began considering the problems of gas flow measurement, given the problems associated with its compressibility. Mass flow measurement approaches for gas are immune to this problem, and there are two main options: thermal and Coriolis. Thermal mass flowmeters are based on the understanding that a given mass of fluid will remove a known amount of heat from a given body. While some designs handle liquids as well as gasses, liquid applications are far less common for this sensor type.
Within the thermal mass flowmeter technology, there are two approaches: the in-line type and insertion type. We will consider the in-line type in a future issue. Insertion type sensors use a probe that is inserted through the pipe wall into the process gas line. The probe includes a heating element and multiple temperature sensors, typically RTDs. One sensor measures the temperature of the ambient gas stream. Another measures the temperature of the heating element.
A sensor uses one of two approaches. One feeds a specific amount of current into the heating element and calculates flow by measuring how much lower the actual temperature is than it should be for the amount of current. The other heats the element to a specific temperature, and calculates flow by measuring how much current it takes to maintain that temperature. The current levels and temperature differences provide the data to calculate mass flow. Most designs are relatively precise and offer accuracy in the ±2% or less range, along with wide turndown ratios. Since the sensor calculates a mass reading, the gas density must be known to convert to a volumetric reading.
This technology has some practical application considerations. (These are generalities, so discuss specifics with your supplier.)
--Since the design uses a heating element, it operates continuously (or at least with long on periods) at high power levels, so battery powering isn’t a practical option.
--Given the probe size, insertion designs are best with relatively large pipes and ducts.
--The nature of the measurement is for a small section of the gas stream. The normal practice is to place the sensing point in the center of the pipe and the final flow calculation is based on normal flow profiles for that size pipe. This means that the probe length has to be adjustable or fixed for a specific pipe size. It also means that turbulence has to be minimal, which calls for flow stabilizers or long sections of straight pipe up and downstream.
--Some probe designs depend on careful axial, or yaw, positioning. If the probe is twisted relative to the gas stream, the reading can be affected.
--For large ducts or where turbulence is unavoidable, some suppliers offer probes with multiple sensors. These take readings at a range of points across the flow profile and correct for poor gas distribution.
--Dirty and corrosive gas streams can leave deposits on critical surfaces and interfere with measurements or damage fragile sensing points. Some designs are more tolerant of these than others, so discuss these potential problems with your suppliers
Within the thermal mass flowmeter technology, there are two approaches: the in-line type and insertion type. We will consider the in-line type in a future issue. Insertion type sensors use a probe that is inserted through the pipe wall into the process gas line. The probe includes a heating element and multiple temperature sensors, typically RTDs. One sensor measures the temperature of the ambient gas stream. Another measures the temperature of the heating element.
A sensor uses one of two approaches. One feeds a specific amount of current into the heating element and calculates flow by measuring how much lower the actual temperature is than it should be for the amount of current. The other heats the element to a specific temperature, and calculates flow by measuring how much current it takes to maintain that temperature. The current levels and temperature differences provide the data to calculate mass flow. Most designs are relatively precise and offer accuracy in the ±2% or less range, along with wide turndown ratios. Since the sensor calculates a mass reading, the gas density must be known to convert to a volumetric reading.
This technology has some practical application considerations. (These are generalities, so discuss specifics with your supplier.)
--Since the design uses a heating element, it operates continuously (or at least with long on periods) at high power levels, so battery powering isn’t a practical option.
--Given the probe size, insertion designs are best with relatively large pipes and ducts.
--The nature of the measurement is for a small section of the gas stream. The normal practice is to place the sensing point in the center of the pipe and the final flow calculation is based on normal flow profiles for that size pipe. This means that the probe length has to be adjustable or fixed for a specific pipe size. It also means that turbulence has to be minimal, which calls for flow stabilizers or long sections of straight pipe up and downstream.
--Some probe designs depend on careful axial, or yaw, positioning. If the probe is twisted relative to the gas stream, the reading can be affected.
--For large ducts or where turbulence is unavoidable, some suppliers offer probes with multiple sensors. These take readings at a range of points across the flow profile and correct for poor gas distribution.
--Dirty and corrosive gas streams can leave deposits on critical surfaces and interfere with measurements or damage fragile sensing points. Some designs are more tolerant of these than others, so discuss these potential problems with your suppliers
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