The industry’s successes in environmental control tend to induce in us a smug confidence in our industrial control systems. Often without us being too aware, the environment throws us a curve ball. While some effects are only a nuisance, there are times when these can be adding needlessly to the running costs – and there are times when its effects can be downright disastrous. This article presents a low-cost method of protection against the influence of ambient factors on the results of measurements.
An objective of this research and development project was to create a technology, which will introduce a correction, or prevent the use of measurement results, if they are distorted by ambient factors. Such ambient factors can be: magnetic and electro-magnetic fields of different frequencies and field strengths; temperature fluctuations; different radiations; wind force; mechanical shocks; geomagnetically induced currents, and others. Because of these factors, a technology was developed which can detect the influence of all these factors, measure them directly in desired units (such as kg or km/h), and in addition ensure that measured parameters are specifically targeted, and not some other foreign object targeted.
This technology contributes to the improved reliability of operations such as: legal metrology; military and space measurements and control; air, sea and space navigation; the measurement and control of chemical, petrochemical, petrol refineries and nuclear processes; mining operations, and others.
It is a known fact that an electromagnetic field generated by a radio transmitter - such as a cellphone - may influence the operation of TV, telephone, radio or any other electronic equipment.
There are different standards that prescribe at what field strength, and range of frequencies of electromagnetic radiation, specific equipment must be tested and may operate. There is usually no information available to the operator about the value and/or effect of the distortion created by this electromagnetic field at the time of the physical parameter measurement.
For example, how many kilograms will be added or subtracted from the mass measurement result if there was a 100 MHz electromagnetic field influencing the equipment, with an EMF in the vicinity of a loadcell of say 5 V/m - and in the vicinity of the indicator, an EMF of 3 V/m?
The technical objectives of the Ambient Influence Detector (AID) are:
* To detect all external factors, which are presently influencing measurement results.
* To determine a value of the influence in units of the main measurement.
* To indicate the total value of all influences, and to assess if this is within permissible limits and tolerances, or not.
* To prevent the misidentification of the object of testing or measurement.
Typical ambient factors which may influence the operation of devices or systems.
* Temperature.
* Wind.
* Mechanical shocks.
* Electromagnetic field generated by: Radio communication devices, special devices such as radar, geomagnetically induced currents (GIC).
Taking a look at the effects that these influences have:
Temperature
In the typical case of a loadcell, the method measures temperature influence on the loadcell output. The following factors are influenced by temperature: material elasticity, expansion of steel, electrical resistance of all electrical components of the loadcell, the influence of the direction of force application to the loadcell. The influence is measured in mV/V, or in kg, or in any desired units.
Electromagnetic field generated by radio communication device
In processing plants, especially chemical and petrochemical plants, two-way radios are used to communicate between the plant and the control room. Such radio communication, GIC or other interferences may induce currents into control circuit via the current to pressure converter (I/P), transmitter (PT) or wires and create undesirable effects such as the closing or opening of control valve (PCV-101). This may precipitate production losses or even explosions. The AID can detect this influence and prevent any undesirable changes in the process control equipment.
Electromagnetic field generated by special devices
Strong magnetic field, like that from a powerful radar, can influence the electronic circuits on a modern aircraft and cause undesirable action such as the release of a bomb, or mid-air crashes. AID can detect and prevent tragic actions of this type.
Geomagnetically induced currents (GIC) and electromagnetic fields
Solar flares (eruption on the sun) may create a false radio beacon and influence navigational equipment. AID can detect this false radio beacon and take action to prevent navigational error and a possible catastrophe.
Wind
When a truck is weighed on a weighbridge - in addition to the affects caused by loadcell temperature fluctuations, magnetic fields created by radio transmissions, magnetic field created by other sources - the sailing surface of a truck may be as large as 2,6 x 22 m = 57,2 m2. These influences can all contribute to the final mass measurement. If the influence of these environmental conditions is within acceptable limits, then it may be ignored. If it is above an acceptable level, then preventative action will be taken by AID to avoid the mistake.
Mechanical shocks
As a result of strong mechanical shock an electromechanical relay may close a normally open (NO) contact and trigger undesirable action. The AID method can prevent this action.
Geomagnetically induced currents
The underestimation of GIC can be a reason for serious disasters. This phenomenon was discovered over 150 years ago. The interaction of solar wind with the magnetic field of the Earth creates terrestrial electric currents of millions of amps. Ambient influence Detection provides protection against some of the devastating effects of GIC.
GIC so damaged a transformer in the Canadian province of Quebec on 13 March 1989, that the province was without electricity for nine hours. At the same time, GIC also caused damage to the transformer at the Salem nuclear generating station in New Jersey, USA, which was burnt out - the effects of this GIC were felt as far south as Mexico.
The financial consequences of a transformer failure go far beyond the replacement cost of the transformer. The transformer at the Salem nuclear generating station, cost several million dollars to replace. Thus the effect of GIC on low voltage instruments is obvious.
Misidentification of the object of the test or measurement
A typical example of misidentification of the object of a test could be vehicle speed measurement by means of a laser speed trap device. The diameter of the laser beam at the distance of 300 m is more than 1 m. If a traffic officer is aiming at a headlight, the beam could conceivably be reflecting off a nearby vehicle - and not from the vehicle belonging to that headlight, thus giving a false speed indication for the targeted vehicle.
The ambient influence detector method
The basic principle makes use of components that are identical or similar to those making the measurements in the existing operational circuit. These additional components are not connected directly to the same measured process, but they are exposed to the same ambient conditions. And so the ambient conditions sensing circuit (ACSC) performs detection or measurement of the ambient conditions influence on the operation of a device or a system.
How this system would be implemented in the examples mentioned:
The solution for the mechanical shock scenario
An example in simplified form may be: an identical relay with normally closed (NC) contacts which will operate (open contacts) when the NO contacts of the operational relay will close as the result of a mechanical shock.
Solution for the measurement of influences on the weighbridge
The ACSC components may be scaled up or down. For the purpose of detecting and quantifying ambient condition influence on the operation of the weighbridge, ACSC components were scaled down. A smaller loadcell from the same manufacturer, made from the same materials was placed in close proximity to the operational loadcell. ACSC wires were placed next to the weighbridge wires. The ACSC indicator was from the same manufacturer as the operational indicator and placed in close proximity to the operational indicator. Such installation ensures that ACSC is subjected to the same influence of electromagnetic field and temperature as the operational circuit. To establish the influence of temperature on the elasticity of the loadcell, a weight, with a known mass, was attached to the ACSC loadcell. To establish the influence of wind, a plate, scaled similarly to the ACSC load cell, was attached, at the height corresponding to the height of the truck, to the ACSC loadcell. The total influence of all ambient factors is measured directly in kg.
Solution for the speed measurement by laser
The ACSC with identical laser diode and electronic circuit will be attached to an operational gun and will measure speed in close proximity to the target, where there should be no moving object. If ACSC indicates any speed, it will mean that reflection is received from foreign objects and the readings of the operational unit will be rejected.
Reduction of measurement uncertainty
Ambient factors such as the environment - temperature, air pressure, humidity, electromagnetic fields and many other conditions - can affect the measuring instrument or the item being measured. During calibration of the instrument, significant uncertainty can be introduced by ambient factors. This uncertainty will be 'imported' by the calibrated instrument and will affect all measurements up to the next calibration. During a measurement process, significant distortions of measurement results can occur as a result of ambient interferences, especially when electronic instruments are used and the operator does not have adequate training in measurement techniques.
Conclusion
Sensors and associated circuitry can only deliver an accurate indication when all influences other than the quantity being measured are known and accounted for. AID technology makes it possible to know when a measurement is not to be trusted. This technology provides a cost-effective means of reducing measurement uncertainty, preventing catastrophes, sabotage - and the associated legal and financial implications that go along with damaged equipment and harm to persons.
For more information: D. Lechtman, Amalgamated Consulting Engineers of South Africa, 082 451 3031, [email protected]
© Technews Publishing (Pty) Ltd | All Rights Reserved
printer friendly version