Control Engineering's Weekly News Tech Tips of the Week

07/10/2007


Touchscreen mechanics

A touchscreen is a computer input device that enables users to make a selection by touching the screen, rather than typing on a keyboard or pointing with a mouse. Computers with touchscreens have a smaller footprint, can be mounted in smaller spaces, have fewer movable parts, and can be sealed. Touching a screen is more intuitive than using a keyboard or mouse, which translates into lower training costs.

Three common components

All touchscreen systems have three components. To process a user's selection, a sensor unit and a controller sense the touch and its location, and a software device driver transmits the touch coordinates to the computer's operating system. Touchscreen sensors use one of five technologies: resistive, capacitive, infrared, acoustic wave, or near field imaging.

Resistive touchscreens typically include a flexible top sheet and a glass base separated by insulating dots. Each layer is coated with a transparent metal oxide on its inside surface. Voltage applied to the layers produces a gradient across each. Pressing the top sheet creates electric contact between resistive layers, essentially closing a switch in the circuit.

Capacitive touchscreens are also coated with a transparent metal oxide, but the coating is bonded to the surface of a single sheet of glass. Unlike resistive touchscreens, where any object can create a touch, capacitive touchscreens require contact with a bare finger or conductive stylus. The finger's capacitance, or ability to store an electric charge, draws some current from each corner of the touchscreen, where voltage has been applied.

Infrared touchscreens are based on light-beam interruption technology. Instead of placing a layer on the display surface, a frame surrounds it. The frame has light sources, or light-emitting diodes (LEDs), on one side, and light detectors, or photosensors, on the opposite side, creating an optical grid across the screen. When any object touches the screen, the invisible light beam is interrupted, causing a drop in the signal received by the photosensors.

Acoustic wave touchscreens use transducers mounted at the edge of a glass screen to emit ultrasonic sound waves along two sides. The ultrasonic waves are reflected across the screen and received by sensors. When a finger or other soft-tipped stylus touches the screen, the sound energy is absorbed, causing the wave signal to weaken. In surface acoustic wave (SAW) technology, waves travel across surface of the glass, while in guided acoustic wave (GAW) technology, waves also travel through the glass.

Near field imaging (NFI) touchscreens consist of two laminated glass sheets with a patterned coating of transparent metal oxide in between. An ac signal is applied to the patterned conductive coating, creating an electrostatic field on the surface of the screen. When a finger--gloved or ungloved--or other conductive stylus comes into contact with the sensor, the electrostatic field is disturbed.

 

Source: Morse, Elizabeth, “How touchscreens work,” Back to Basics, Control Engineering, Sept. ’98, p. 238.



Turbine flowmeter installation rules of thumb.

Similar to most flowmeters, the accuracy of turbine flowmeters is highly dependant on the ability of the installation to ensure non-swirling conditions.

Even at constant flow rates, swirl can change the angle of attack between the fluid and the rotor blades, causing varying rotor speeds and thus varying flow rate indications.

Effects of swirl can be reduced or eliminated by ensuring sufficient lengths of straight pipe—a combination of straight pipe and straightening vanes, or specialized devices, such as Vortab's flow conditioner—are installed upstream and downstream of the turbine flowmeter.

Turbine flowmeters for liquid applications perform equally well in horizontal and vertical orientations, while gas applications require horizontal flowmeter orientation to achieve accurate performance.

When installing turbine flowmeters in intermittent liquid applications, it's recommended that the flowmeter be mounted at a low point in the piping.

Turbine flowmeters are designed for use in clean fluid applications. Where solids may be present, installation of a strainer/filter is recommended. Also, because the strainer/filter can introduce swirl, it needs to be located beyond the recommended upstream straight pipe lengths.



10 reasons to use adjustable-speed ac drives

Modern ac adjustable-speed drives (ASDs) have come a long way in their own right and as an alternative solution to dc drives. There are many reasons for using ASDs. Here’s a summary of their benefits from “Drives at Work—Do You Know These 10 Benefits?” by Mark Kenyon, ABB Inc. product manager, presented at ABB Automation World Conference & Exhibition, in Houston, TX (April 2005).

Energy savings
Since more than 65% of industrial electric energy is consumed by electric motors, it makes sense to adjust motor operating speed to demands of the load. Varying load applications like centrifugal pumps and fans in particular benefit from ASDs. For example, when pump speed can be cut in half, resulting power consumption is reduced by a factor of eight! However, you need to know the load’s duty cycle to get most out of energy savings.

Controlled starting current
High starting currents of ac motors (6-10 times full-load amps) stress windings, generate heat, and shorten motor life. ASDs start at zero frequency and voltage, extending motor life.

Reduced power line disturbances
Adjustable-speed ac drives virtually eliminate voltage sags caused by the staring of large or numerous motors. This minimizes tripping of voltage-sensitive equipment, reducing down time.

Lower power demand at start
Reducing “demand charges”—or highest average demand recorded during any one time period within a billing period—also can reduce energy costs. Less power used during motor start means lower demand charges.

Controlled acceleration
This ASD feature reduces stress on the motor, as well as on upstream power system components (transformers, switchgear, cabling, etc.). Customer equipment and sensitive products also are protected.

Infinitely adjustable operating speed
Adjustable-speed ac drives provide the right speed for the “job,” allowing a production process to be optimized. Ability to easily make process changes widens applications.

Adjustable torque limit
An ASD can limit torque (current) supplied to the ac motor to protect against machinery damage or jamming. It also protects the product being manufactured, which can be fragile.

Reverse operation
By changing the firing order of its output devices, the ac drive can electronically swap two output phases to reverse an ac motor’s rotation. Eliminating separate contactors decreases panel space needed and lowers maintenance costs.

Elimination of components
ASDs can eliminate external components—mechanical (belts, transmissions, gear motors), electrical (PLCs, contactors, motor starters), and other process controllers.

Controlled stopping
Control of motor deceleration is as important as acceleration. Stopping time must suit the application, which ASDs provide either internal or external to the drive, without the need for a mechanical brake. Benefits are improved productivity and less scrap produced.


 

The two most common ways of measuring industrial temperatures are with resistance temperature detectors (RTDs) and thermocouples. But when should control engineers use a thermocouple and when should they use an RTD? The answer is usually determined by four factors: temperature, time, size, and overall accuracy requirements.

  • What are the temperature requirements? If process temperatures fall from -328 to 932°F (-200 to 500°C), then an industrial RTD is an option. But for extremely high temperatures, a thermocouple may be the only choice.

  • What are the time-response requirements? If the process requires a very fast response to temperature changes--fractions of a second as opposed to seconds (i.e. 2.5 to 10 sec)--then a thermocouple is the best choice. Keep in mind that time response is measured by immersing the sensor in water moving at 3 ft/sec with a 63.2% step change.

  • What are the size requirements? A standard RTD sheath is 0.125 to 0.25 in. dia., while sheath diameters for thermocouples can be less than 0.062 in.

  • What are the overall requirements for accuracy? If the process only requires a tolerance of 2°C or greater, then a thermocouple is appropriate. If the process needs less than 2°C tolerance, then an RTD is the only choice. Keep in mind, unlike RTDs that can maintain stability for many years, thermocouples can drift within the first few hours of use.

Although not a technical point, price may be another consideration. An average thermocouple costs approximately $35, while an average RTD costs $55. Cost of extension wire must also be considered. Thermocouples require the same type of extension wire material as the thermocouple, which can cost up to $1 per ft. Standard nickel-plated, teflon-coated RTD wire averages pennies per ft.

RTD basics

Once parameters are defined, the type of RTD or thermocouple is chosen. RTDs provide a resistance vs. temperature output and are passive devices, needing no more than 1.0 mA to run. The most common RTD is a 100 ohm, platinum sensor, with an alpha coefficient of 0.00385 ohms/ohm/C. It can be ordered as DIN A or DIN B which specifies the initial accuracy at 0°C (ice point) and the interchangeability over the operating range. IEC 751 states that DIN A is 0.15°C

RTDs can also be constructed from nickel, copper, or nickel/iron. Each metal has a different alpha coefficient and operating range. An RTDs alpha coefficient must be matched to its instrumentation or an error of several degrees can occur.

About thermocouples

Thermocouples can be made with any combination of two dissimilar materials. ISA recognizes twelve thermocouples. Eight of the 12 have letter designations including Type J, Type K, Type T and Type E.

The most common determining factor for chosing thermocouple type is the temperature range of its intended application. Type J is suitable for a temperature range of 32 to 1,400°F (0 to 759.99°C). Type K is appropriate for a temperature range of 32 to 2,300°F (0 to 1,259.99°C). Type T handles a temperature range of -300 to 700°F (-184.44 to 371°C). Type E fits a temperature range of 32 to 1,600°F (0 to 871.11°C).

Standard limits of error and special limits of error must also be considered. These values relate to the purity of the wire used to manufacture the thermocouple. For very little additional cost, thermocouple specifiers can often improve accuracies greatly (100% or greater).

Specifying the correct thermocouple or RTD for an unconventional application may be a difficult task. Many manufacturers of RTDs and thermocouples offer applications engineering support to help customers select the right combination of temperature measurement equipment.

Source: Sulciner, Jim, “Choosing RTDs and thermocouples,” Back to Basics, Control Engineering, Feb. ’99, p. 152.


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.


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.

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- 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





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