Pressure Transducer Selection

While selecting a right kind of pressure transducer for an application one must consider following important points:
Corrosion protection: Decide whether the sensor is required to be isolated from the medium being measured. If the medium is a clean and a non corrosive gas or liquid then a non isolated transducer is adequate whereas for corrosive, high-temperature, or viscous media, isolation is usually required. Often, a metal or ceramic diaphragm with or without a fill fluid is integrated.
Accuracy: Another key selection criterion is the performance level i.e. accuracy required for the application. In general, high accuracy devices have improved performance both with temperature changes and over time. However this better stability comes at a premium price.
Pressure range: Available pressure ranges exist from vacuum to 60,000 psi-in steps-with vacuum, gauge, absolute or differential pressure references. While selecting a transducer’s range, it is desirable for the application’s normal operating pressure to be 50-90% of the range chosen.
Temperature effects: This is another crucial selection criterion. Temperature variations can have considerable effect on a pressure transducer’s environmental performance. Nearly all manufacturers provide temperature compensation specifications which define thermal effects over a specified range. Performance shown as a coefficient or error band is guaranteed over that temperature range. Beyond that range, larger errors should be estimated.
Vibration/shock effects: Since these are highly application-specific environmental issues, they should be reviewed carefully for fit with manufacturer’s specifications.
Electrical effects: In today’s operating environment, radio frequency interference (RFI), electromagnetic interference (EMI), and electrostatic discharge (ESD) protection have become an integrated feature of pressure transducers. “CE”- marked products generally have RFI, EMI and ESD protection built into the transducer’s electronics.
Hazardous area applications: If transducers are to be used in a hazardous environment then they must be approved explosion-proof or intrinsically safe models.
Hydraulic Applications: If transducers are to be employed in hydraulic systems, it may possibly be required to consider use of “Snubbers”. Snubbers are dampening devices used to dampen hydraulic spikes. They can avoid sensor failure because of over range readings from phenomena called “water hammer.”
Outputs: Transducer output is another key decision area. Outputs are available in following forms:

  • industry-standard
  • millivolt
  • Voltage or current signals.
  • Digital outputs with communication capability

A few of the regular outputs are 0-30 mV, 0-100 mV, 4-20 mA, 0-5 V dc and 0-10 V dc. The 4-20 mA output is the simplest as it is usually a two-wire configuration. Other nonstandard outputs are by and large the result of specific requirements of a large-volume OEM.
Electrical connections: Another important decision in selecting a pressure transducer is based upon type of electrical connection needed. Following Electrical terminations are possible:

  • conduit
  • cable
  • circular
  • DIN style

DIN-style connectors, both full size and miniature, are the popular options across the application spectrum since the advantage of screw terminals and moderate cost is associated with them.

There are two conflicting considerations which appear in the selection procedure of pressure transducers. They are:

  1. Accuracy
  2. Overpressure Protection

From an accuracy point of view, the range of a transmitter should be low so that error, generally a percentage of full scale, is reduced. Whereas on the other side, one must always consider the consequences of overpressure damage due to operating errors, faulty design or failure to isolate the instrument during pressure-testing and start-up. Consequently, it is imperative to specify not only the requisite range, but also the amount of overpressure protection needed. Almost all pressure instruments are granted with overpressure protection of 50% to 200% of range.

Automation Protocol Converters

Strain Gauge

Any external force applied to a stationary object produces stress and strain. The object’s internal resisting forces are referred to as stress while the displacement and deformation that occur is termed as strain. Strain can be either compressive or tensile and is usually measured by strain gauges. In general, strain gauges are devices used to measure displacement, force, load, pressure, torque or weight.

Main Features

Following are the key features of a strain gauge:

  • Strain-gauge sensor is one of the most commonly used means of load, weight, and force detection.
  • It is a device which is used for measuring the changes in distances between points in solid bodies that happen when the body is deformed.
  • Resistance strain gauge is a helpful tool in the field of experimental stress analysis. It operates on the principle that the electrical resistance of a copper or iron wire changes when the wire is either stretched or compressed.
  • Usual strain gauge resistances range from 30 Ohms to 3 kOhms (unstressed).
  • Size of strain gauges is normally smaller than a postage stamp.
  • An ideal strain gage is small in size and mass, low in cost, easily attached, and highly sensitive to strain but insensitive to ambient or process temperature variations.
  • The ideal strain gauge would undergo change in resistance only because of the deformations of the surface to which the sensor is coupled. However, in real applications, there are many factors which influence detected resistance such as temperature, material properties, the adhesive that bonds the gage to the surface, and the stability of the metal.

Gauge Factor

Essentially, all strain gauges are designed to convert mechanical motion into an electronic signal. The strain experienced by the sensor is directly proportional to the change in capacitance, inductance, or resistance of the gauge used. For instance, if a wire is held under tension, it gets slightly longer and its cross-sectional area is reduced. This causes a change in its resistance proportional to the strain sensitivity of the wire’s resistance. When a strain is introduced, the strain sensitivity, which is also known as the gage factor (GF) of the sensor, is given by:

GF =  ΔR/RG
            ε

Where, RG is the resistance of the undeformed gauge,
ΔR is the change in resistance caused by strain, and
ε is strain.

Applications

Strain gauges are frequently used following areas

  1. In mechanical engineering research and development to measure the stresses generated by machinery
  2. Aircraft component testing: Tiny strain-gauge strips glued to structural members, linkages, and any other critical component of an airframe measure stress

Strain Gauge Characteristics

Every strain gage wire material has its own characteristic

  • Gauge factor
  • Resistance
  • Temperature coefficient of gage factor
  • Thermal coefficient of resistivity
  • Stability

Strain Gauge Materials

Typical materials include:

  1. Constantan (copper-nickel alloy)
  2. Nichrome V (nickel-chrome alloy)
  3. Platinum alloys (usually tungsten)
  4. Isoelastic (nickel-iron alloy)
  5. Karma-type alloy wires (nickel-chrome alloy)
  6. Foils
  7. Semiconductor materials

The most popular alloys employed for strain gages are copper-nickel alloys and nickel-chromium alloys.

Temperature Effects

  • High temperatures can affect the internal structure of strain-sensing materials like copper.
  • Temperature can influence not only the properties of a strain gage element, but also can amend the properties of the base material to which the strain gage is attached.
  • Variation in expansion coefficients between the gage and base materials may cause dimensional changes in the sensor element.
  • Expansion or contraction of the strain-gage element or the base material can result in errors which are extremely intricate to correct. For example, a change in the resistivity or temperature coefficient of resistance of the strain gage element can modify the zero reference used to calibrate the unit.

Measuring Circuits

For measurement of strain via a bonded resistance strain gage, it must be connected to an electrical measuring circuit which can measure even the minute changes in resistance corresponding to strain. Modern strain-gage transducers usually employ a grid of four strain elements electrically connected to form a Wheatstone bridge measuring circuit. A Wheatstone bridge is a divided bridge circuit employed for the measurement of static or dynamic electrical resistance. The output voltage of the Wheatstone bridge is expressed in millivolts output per volt input. Besides, this bridge circuit is appropriate for temperature compensation. A quarter bridge strain gauge circuit is shown in the figure below:

Quarter Bridge Strain Gauge Circuit
 

Installation Diagnostics

Following steps should be followed while checking strain gauge installations:

  • First of all measure the base resistance of the unstrained strain gage after its proper mounting but before complete wiring.
  • Check for surface contamination by measuring the isolation resistance between the gauge grid and the stressed force detector specimen by means of an ohmmeter, if the specimen is conductive. This should be done before connecting the lead wires to the instrumentation.
  • Also, check for irrelevant induced voltages in the circuit by reading the voltage when the power supply to the bridge is disconnected. Ensure that Bridge output voltage readings for each strain-gage channel are practically zero.
  • Now, connect the excitation power supply to the bridge and verify both the correct voltage level and its stability.
  • Test out the strain gage bond by applying pressure to the gage. The reading should not be affected.

 CAS BACnet Explorer

Ethernet/IP Installation challenges

Implementing Ethernet/IP includes following problems

  1. Lack of trained staff: Installation of Ethernet/IP requires trained personnel who can understand both the IT fundamentals and the automation network. Both the Automation Team and the IT staff must work together to install and implement an Ethernet/IP system.
  2. Proper network configuration: Accurate planning of Ethernet factory automation infrastructure is critical. Careful documentation of the pathways, spaces, cabling, devices, and device connections is critical to meet the networks Intended operation, as well as, having to choose the correct routers and switches. Ethernet/IP once installed correctly requires little maintenance.
  3. Various competitors to Ethernet/IP: Competitors include Modbus/TCP, ProfiNet, HSE Fieldbus and many other proprietary protocols. In Opposition to Ethernet, it is always argued that Ethernet lacks the level of strength needed in automation applications. However, this argument doesn’t hold true in today’s scenario since many intelligent switches have mostly eliminated this argument. Switches create separate collision domains that offer the reliability required of just about all automation applications.

Ethernet/IP Main Features

FactoryTalk Batch Benefits

FactoryTalk Batch helps us to create processes that help our operators to do their jobs more efficiently and make quality products over and over again. It offers following major benefits:

  1. Share resources, maximize use of expensive equipment or swiftly switches equipment in the case of a failure.
  2. Visualizes and develops complex recipe structures in a graphical sequential function chart format by means of the recipe editor.
  3. Reduces the number of recipes required by using class-based recipes.
  4. Automatically records all actions that FactoryTalk Batch monitors and controls, allowing full recovery and system redundancy in the event of a control system failure.
  5. Supports 21 CFR Part 11 compliance with comprehensive electronic signatures.
  6. Streamlines electronic signature setup and maintenance through signature templates.

FactoryTalk ViewPoint

FactoryTalk ViewPoint enables us view real-time plant floor operations data just by logging onto an Internet browser. It is basically a Web-enabled HMI application that extends the access to FactoryTalk View Site Edition (SE) displays and dashboards to users everywhere resulting in improved real-time decision making. FactoryTalk ViewPoint is an add-on to FactoryTalk View SE and PanelView Plus, hence delivers a rich and interactive experience. Via this, Simplicity of standard browser navigation and new degrees of access to critical FactoryTalk View information both gets easily available in a web browser. The thin-client configuration means no client software to install and maintain, lowering total cost of ownership.

RSLogix Emulate Main Features

  • RSLogix 5000 Enterprise Series provides support for the S88 equipment phase state model for batch and machine control applications using the PhaseManager optional feature.
  • RSLogix 5000 version 12.0 or higher is used to program the emulator and observe its operation whereas RSLinx Classic version 2.50 or higher is required for communicating with the emulator.
  • For development of application programs, RSLogix Emulate 5000 supports relay ladder, structured text, function block diagram, and sequential function chart editors.
  • RSLogix Emulate can now be used for Troubleshooting, Ladder logic scanning options and Serial device Emulation.

FactoryTalk VantagePoint Main Features

FactoryTalk Vantage Point

  • Provides unified access to virtually all manufacturing/plant data sources.
  • Produces web-based reports like dashboards, trends, X-Y plots, Microsoft Excel reports and much more which can be used by manufacturing operators, engineers, supervisors, management and executives all over the plant (for managing cost, quality, production, assets and resources more successfully).
  • Connect to Logix and FactoryTalk Historian to enable users to generate meaningful and pre-configured reports just about instantly.
  • Measures time-to-value in hours only and not in days or weeks.