Making sense of sensors
June 01, 1999
by Teresa Acklin
Sensors and instrumentation perform many functions in automated flour mills.
By David Sugden
Many types of sensors and instrumentation are used in flour mills to detect a disturbance, sense change, create change, command the speed of change (timers), create process stability and consistency, monitor and control. Information is processed locally or centrally by PLCs and computers to control machines and processes, track production, job queue, diagnose faults and economically provide management information.
Other instruments are used to optimize the manipulation of data. In the future, one can envisage the use of “knowledge-based expert systems” in milling, which will use the knowledge of an expert miller not just quantitatively but also qualitatively, according to judgment instead of the “yes/no” systems used today.
Current automated systems are good, but not by any means all embracing. Moreover, there is a huge temptation to overautomate, leading to undue cost (the law of diminishing returns) and plant instability.
So far as field instruments are concerned, millers would like to see a reliable, single “black box” on-line laboratory that is both non-invasive and non-destructive to products or processes, that can be used to verify and not just identify. It also is possible to envision greater use of techniques for measuring acoustically by means of sound waves, light, magnetic impedence, odor and imaging. Although some of these instruments are currently available, they are a long way from either full exploitation or acceptance.
Tomography, the technique similar to human body scanners used in hospitals, is a non-invasive, non-destructive method of measuring what is happening in real time inside an enclosed machine or conveyor. By employing external electrodes placed around the outside of equipment, flour or other product within can be detected by the emission of mild electrical current. This, in turn, is transferred onto a screen and recorded.
Among the benefits of a tomogram is the possibility to minimize power consumption during production. Much original research in this area has been done, and is being carried out, by Tom Dyakowski at the University of Manchester Institute of Science and Technology in Manchester, England.
The purpose of field sensors and instruments positioned at remote points throughout a flour mill is to relay quantitative or qualitative information for operational use. The sensors help to minimize human error and enable greater detailed and accurate supervision, reduce production costs and by varying degrees increase health and safety, protect machinery, monitor cost information and allow manipulation of data captured.
Distance instruments typically cover level controls of wheat or flour in bins or silos. These instruments come in a variety of styles, including physical (paddle wheel resistance), capacitance (electromagnetic field) and ultrasonic (reflected sound waves).
Limit switches detect the position of a valve or slide, the routing of a conveyance system or the status of an explosion relief panel.
Pressure gauges indicate the scaled pressure of air in compressors and blow lines. Steam pressure in boilers is displayed in kilograms per square centimeter or in pounds per square inch. A different type of pressure gauge is occasionally used on roller mills to determine the pressure at each end of a pair of rolls.
Manometer U-tubes on dust collectors measure the difference in air pressure across dust filter clothing. A high resistance indicates the need for sleeve replacement. Diaphragm switches are used in bin hoppers or as a plug or choke sensor.
Weight measurement instruments, such as electronic load cells, come in two categories: compression and tension. Compression-type measuring instruments can be found on weighbridges for weighing trucks or rail cars, and are used under bins to calculate the contents, to operate filling and discharging equipment and to log throughput.
Tension measuring instruments are frequently used on bagging or production weighers. Load cells work on the principle of mechanical deflection, which causes a change in electrical resistance between two circuits. Mechanical beam scales were the precursor of the load cell, and are still very much in use.
All of these instruments are capable of top-class accuracy when properly maintained, and allow the miller to buy and sell goods that have been measured and certified by government inspectors.
Flow rate also is determined by weighers. Where there is less need for near absolute accuracy, gravimetric feeders of the continuous belt weigher variety are installed. Electrical or electropneumatic-type impact feeders fall under this heading. Their precision is seldom claimed at better than 1%, although in many cases they are.
The function of these feeders includes blending of ingredients, flour, wheat and similar materials. A more sophisticated version the loss in weight feeder is particularly suitable for the addition of expensive or sensitive additives, such as gluten, self-rising powders and fungal enzymes. These work well in a feed back control loop configuration when working in tandem with on-line quality control devices.
Speed and its acceleration is measured by revolution counters, tachometers and underspeed detectors. The first two can typically be found on rollermill feed rolls, while the latter is found on the bottom drums or pulleys of bucket elevators as a malfunction or anti-explosion instrument.
Heat, measured by thermometers, is usefully detected in elevator silos to determine hot spots in stored grain. Electric motors are protected by semiconductor (thermistor) temperature sensors based on such material as metal oxides or silicon crystals. There are many other applications, including overload protection of circuits.
Optical sensors include infrared instruments that detect sifter discharge problems. Infrared light aligned under a line of sifter sleeves will show a malfunction situation, typically a choke. There also is a move toward the greater application of fiber-optics for ultra high-speed data transmission, particularly in troublesome locations, because they are quicker than electrical or pneumatic signals.
Electrical instruments encompass ammeters to detect the quantity of amperes (amps) or electrical current consumed by any electrical motor or an entire plant and voltmeters to monitor the strength of current. KVA meters (kilovolt ammeters) measure the maximum demand of electricity used by a mill at any particular time. In many countries, KVA meters are installed in flour mills by electricity utilities as an extra charge on top of normal meterage of units consumed.
These are some of the most common instruments. Near infrared (NIR) on-line measurement of protein and gluten addition can be found in several plants (see World Grain, July/August 1996, “On-Line Flour Testing”). Another example is the moisture addition feedback system (see World Grain, September 1997, “Wheat Damping”).