Tech Tips May 2005
MAY 31, 2005
TECH TIP OF THE WEEK:
Learning level-sensing technologies.
There are many different ways to measure the level of products in industrial storage and process vessels. One of the most commonly used devices is the differential pressure (dp) transmitter. A dp device actually measures the height of material in the vessel and its density. These two variables multiplied, result in the amount of pressure exerted on the diaphragm, which then can be translated into an indication of level. Dps are relatively economical and easy to install. This 'comfortable' technology is fairly accurate and dependable when used to measure the level of clean liquids. However, density compensation is required for accurate measurements. New installations require additional piping and isolation valves that add to initial installation cost.
One of the simplest devices for measuring level is the float . Floats are classified by the type of position sensor (reed switch, cable-and-pot, magnetostrictive, and sonic or radar) . Advantages to using floats are unlimited tank height, excellent accuracy (depending on the float type), and comparatively low cost. However, they are intrusive sensors. Additionally, these mechanical devices are subject to wear, corrosion, mechanical failure, and 'getting stuck.' Floats are subject to material buildup, which can affect their weight and therefore, accuracy.
Application can narrow choice
Related to the float principle of level measurement is the displacer . Displacer technology is based on Archimedes' principal. Although they have fewer moving parts than typical float devices, actual mechanical motion is limited. Displacers are frequently placed in external 'cages,' which can affect accuracy if the vessel/cage level is misaligned. Long-span displacement devices may be very expensive.
Sonic instruments determine level by measuring the length of time it takes for a sound pulse to return to a piezoelectric transducer after bouncing off the process material. For maximum accuracy, the transmitter must be mounted at the top of the vessel and positioned so the internal structure of the vessel will not interfere with the signal path. Sonic devices are non-contact and minimally intrusive. Dust, solvent vapors, foam, surface turbulence, and ambient noise effect accuracy. Elevated process temperatures can limit application.
Radar-based devices beam microwaves at the process material's surface. A portion of that energy is reflected back and detected by the sensor. Time for the signal's return determines the level. Technologies in use include:
Frequency modulated continuous wave (FMCW) is very accurate, ignores vapors, and is immune to changes in physical characteristics (except dielectric constant) of process materials. Applications include 'still,' but not turbulent, fluids. Cost is quite high in comparison to other technologies ($5,000-$10,000 per point).
Pulsed time of flight (PTOF) is lower powered and lower priced. Due to its lower power, its performance can be limited by the presence of vessel obstructions, agitation, foam, elevated pressure, and low dielectric materials (Dielectric constant less than 2).
Time domain reflectometry (TDR)—Unlike FMCW and PTOF, TDR is an intrusive measurement that uses a rod or flexible cable to 'channel' the microwave pulse. It can measure normal (low K on top) interface levels in immiscible fluids. It is low cost, can measure long spans, and provides good performance in lower dielectric materials.
Some methods intrude
Radio frequency (RF), based on capacitance or admittance, can handle a wide range of process conditions. Process temperature and pressure are limited only by the material system of the sensing element. Level transmitters of this type sense the change of electrical impedance that occurs with the change of level on the sensor. RF devices ignore material buildup on sensor and work with all types of process material. It is an intrusive technology.
Teresa Parris, marketing communications manager, and John Roede, application engineer, Drexelbrook Engineering Co., a supplier of level sensing devices.
Source: Teresa Parris, John Roede, 'Level Sensing Technologies,' Back to Basics, Control Engineering, June '99, p. 104.
MAY 24, 2005
TECH TIP OF THE WEEK:
Power-switching device acronyms.
The heart of any medium-voltage ac drive is the type of power electronics device that handles the rapid and controlled switching of large voltages and currents. Translating the 'Tower of Babel' of related power device acronyms:
GTO (gate turn-off) thyristor is a power semiconductor similar to an SCR (see below), but handles lower currents and can also be turned off by a negative gate terminal signal. Its switching frequency is higher than an SCR. Low power factor, low efficiency, and need for output filtering add to its application costs.
SCR (silicon-controlled rectifier) is a one-directional, solid-state switch offering high current handling capability that retains its usage in very large MV drives. Current going to the gate terminal controls breakover voltage, the point at which conduction starts. Turn-off occurs as current is reduced below a holding value. Drawbacks include relatively slow switching speed and large size of the resulting drive. GTOs and SCRs are mature power devices.
IGBT (insulated-gate bipolar transistor) combines best features of a MOSFET (metal-oxide semiconductor field-effect transistor) input and a bipolar transistor output in a newer power-switching device. Very rapid switching results, since no junction effect exists at the input. Power consumption is small due to the insulated gate. Standard IGBTs have voltage-switching limits and need to be connected in series for MV drive usage. A still newer high-voltage device, HV-IGBT , extends operating voltages to those required by MV drives. This eases the need to gang standard devices. See Online Extra at www.controleng.com for more details.
IGCT (integrated gate-commutated thyristor) combines the high-switching frequency and low switching losses of IGBTs with the high voltage handling capability and low on-state (conduction) losses of GTOs. The integrated diode and gate unit lowers the parts count, resulting in increased reliability.
SGCT (symmetrical gate-commutated thyristor) is a modified version of a GTO device and similar to the IGCT. SGCTs block voltage in both directions while allowing only one-directional current to flow. However, SGCTs do not need a series diode or anti-parallel diode, as IGCTs do. This is said to result in the lowest possible component count. Presently, IGCTs and SGCTs have fewer suppliers than IGBTs.
Source: Frank Bartos, 'Medium-Voltage AC Drives Shed Custom Image,' Control Engineering, Feb '00, p. 86.
MAY 17, 2005
TECH TIP OF THE WEEK:
Picking a robot controller.
The following 10 questions can help when selecting a robot controller, according to Joe Campbell, Adept Technology's VP:
Is the application path intensive or pick and place? Software optimized for path functions will not deliver the performance required for high-speed applications.
How fast is the required I/O response? While most I/O devices respond happily in milliseconds, some functions like stop-on-force or motion latching require microsecond response.
Where is the sensor? If the sensor is external to the controller, assure enough processing and communication bandwidth.
Are you safe? Assure compliance with safety standards including ANSI 15.06.
Going international? If so, make sure controller (or systems) has the CE mark.
Is your 'open' architecture closed? If you plan to add third-party boards or software, make sure the vendor allows it.
Do you know your networks? Make sure you know exactly what hardware connections and software protocols are available.
Need dual robots? If using two robots in a single cell, determine if you can live with one controller or if two will be necessary.
Do you have enough software power? Match software power to the application.
It's just another controller, isn't it? Apply traditional control engineering guidelines in determining I/O capacity; selecting and designing the graphical user interface:, and providing power isolation and backup, enclosures, and other interconnects.
Source: Gary Mintchell, 'One (or More) Controller for Every Application,' Control Engineering, Feb '02, p. 61.
MAY 10, 2005
TECH TIP OF THE WEEK:
Barcode scanning basics
Many, if not most, manufacturing facilities incorporate bar coding on some level. Applications range from parts tracking and quality control to finished goods identification. The question is, are manufacturers using barcode scanners best suited to the application? Just as there are many barcode symbologies, different types of barcode scanners are available.
A barcode scanner is composed of a light source and a light detector. No matter what the form factor, all types of barcode scanners work by scanning and detecting the contrast between the dark bars and the spaces on a bar code label. The first requirement for a good read is a nicely contrasting print with a minimum of smudging.
The data signal is converted from analog to digital so it can be processed by a decoder, which is either built into the scanner or is a separate plug-in device. If the barcode is legible and the symbology is recognized as valid, the symbol is decoded. The decoded data is then transmitted to application-based software and can be displayed on a CRT, batch terminal, or other screen, or used in other enterprise database operations.
All scanners are available in one of two form factors—handheld or fixed position. Handheld scanners are pointed at stationary items. Fixed position scanners read barcodes from items as the items themselves pass by the scanner. In industrial applications, items with barcode labels generally pass by the scanner on a conveyor belt.
Handheld or fixed position
Handheld and fixed position scanners for industrial applications operate using one of two technologies: charged-coupled device (CCD) and laser. CCD scanners operate by flooding the barcode with light, which reflects the barcode symbol back to an array of sensors. Laser scanners use a beam created by a laser diode that is scanned across the bar code via a rapidly moving mirror.
Handheld CCD scanners are available in single-line array and two-dimensional (2-D), while fixed-position CCD scanners are available in single-line array and 2-D array (vision systems). Handheld and fixed position laser scanners are available in single line (or linear); raster (used when the bar code is not precisely positioned and when readability must be increased); vibrating vane (sweeps a laser across a bar code for better readability and to read some 2-D barcodes); and omnidirectional (can read a bar code regardless of orientation).
While the most common barcode is a single line array, 2-D codes see increasing use. The barcode on a retail product may only contain two numbers—a vendor identification number and a unique product part number. 2-D codes are capable of packing much larger amounts of data into a small space. This is useful, for example, in tracking work in process in a manufacturing plant.
Source: Omron Electronics, 'Scanners find the code,' Back to Basics, Control Engineering, April '99, p. 124.
Advantages and shortcomings of scanner types
Source: Control Engineering with input from Omron Electronics
Bar code application matrix
Linear barcodes, Stationary product
Laser Fixed position
Linear barcodes, Moving product
Single line array CCD Handheld
Linear barcodes, Stationary product, Short read ranges
Single line array CCD Fixed position
Linear barcodes, 2D barcodes, Moving product
2D Array CCD Handheld
2D barcodes, Linear barcodes
2D Array CCD Fixed position
2D barcodes, Linear barcodes, Moving product
Source: Control Engineering with input from Omron Electronics
MAY 3, 2005
TECH TIP OF THE WEEK:
Protecting process instruments.
Too frequently, instrumentation protection is not considered until the snow is flying and then a problem is discovered. The summer months are the best time to examine instrumentation winterization problems and take measures to ensure the instrumentation performs accurately and repeatedly when the mercury falls.
When evaluating protection options, passive protection methods are preferred over active methods. In order of preference, six popular ways of ensuring instruments perform well in harsh winter elements are:
Locate instruments indoors;
Use instruments immune to cold weather;
Install nonfreezing liquid in impulse lines and meters to form a liquid seal;
Apply insulation and/or heat trace to impulse lines and meters to keep contents above freezing;
Purge impulse lines with a dry gas to keep liquids or vapors out; or
Purge impulse lines with a nonfreezing liquid to keep process liquids out.
If outside, enclose
Every effort should be made to locate instruments indoors or in instrument housings. Even if the enclosure is not heated, instrument reliability and life will be extended if the enclosure takes the beating from snow, rain, hail, and falling ice.
Manufacturers constantly improve product performance and broaden environmental limits that affect instrument performance. Paying a premium for an instrument that does not require winter protection may actually be a bargain when all factors are considered.
Liquid seals come in two forms, open and closed. Open liquid seals most often use seal pots to form an interface between the process media and the nonfreezing liquid in the impulse lines and meter bodies.
Closed, or integral, seals use a diaphragm and flexible liquid-filled capillary system. Process pressure changes cause slight deflections on the diaphragm. The capillary system hydraulically transmits the change to the instrument.
When liquid seals won't work, the use of insulation and/or heat trace is required.
If the process media is heated, mounting the instrument very close to the process piping or vessel and insulating everything may provide adequate protection.
Heat trace may be electric, steam, salt solution, glycol, or other media. Any of these choices increase operating cost, and require periodic maintenance. Unless a temperature controller is used, it's important to remember to turn continuous heating heat tracing off when the weather warms and on again when it gets cold.
A variety of reasons make purges the least desirable protection. Adding hardware, piping, and proximity of the purge media creates a miniature process. Also, unless purge flow rates are tightly maintained, accuracy and repeatability suffer. Purges have been successful in measuring liquid level changes in open vessels, but opportunities for this application are limited.
Avoiding instrument failures
For all the ways to protect instrumentation, there are an equal number of ways to improperly apply and/or install protection. Keep reading to learn three of the most common mistakes made while designing and/or installing instrument protection.
First, many flow meters have two impulse lines. When heat tracing is used, both lines must be equally protected. Avoid splitting a single steam trace line into two lines, one for each impulse line, and then rejoining the lines ahead of the steam trap. Steam follows the path of least resistance. The line offering the most resistance will stop flowing, the steam will condense, freeze, and possibly rupture the tracer line. It's okay to split the lines, but provide each line its own steam trap.
Secondly, when using an instrument housing, especially 'home built housings,' avoid temptations to mount the instrument on the same metal pedestal as is used to mount the instrument housing. Thermodynamics applied to the pedestal will cause heat, supplied to protect the instrument, to migrate toward the cold end of the pedestal. The instrument housing should be mounted on top of a pedestal and a separate instrument-mounting stand should be located inside the housing.
Finally, when using finned heaters to warm instrument housings, small buildings, etc., the fins should be in a vertical position. Finned heaters rely on convection to move heat away from the heater. When fins are horizontal, air cannot flow between the fins, and little or no heat is transferred.
Dave Harrold, contributing editor
Source: Dave Harrold, 'Protect process instruments to ensure performance,' Back to Basics, Control Engineering, July '99, p. 108.
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