Radar level transmitter developments
22 March 2016
Doug Anderson explains how radar level transmitter technology has developed over the years to become an easy to use and predictably accurate process level measurement solution.
Given the choice, most engineers would prefer a ‘non contact’ process measurement solution. Today’s contactless radar devices are almost unrecognisable from earlier devices in both commercial and technical aspects. After some varied user experiences with first generation process radar sensors, today’s devices offer better focussing and processing technology. Their overall performance and usability has also improved markedly.
Costs of early systems were high, reflecting the cutting edge nature of the technology. Set up was a complex task and the mixed performance outcomes were due to both the nature of the hardware and software used at the time, as well as the tough applications that the transmitters were being specified for. Today, sensor costs have reduced significantly and the designs have been refined, enabling them to be used in many more ‘everyday’ applications, across many industry sectors. SIL conformance, overfill certification, and nuclear industry approvals further demonstrate how reliable this technology has now become.
Even after 20 years, there is still some confusion about whether radar transmitters are the same as ultrasonic sensors. While the principle of ‘contactless time of flight’ measurement of radar is similar to ultrasound – for example they both measure distance directly to compute level and volume, while being immune to density change – there is a distinct difference.
An ultrasound wave travelling in air can have its accuracy compromised or signal blocked by conditions such as temperature, vapours, gases, noise, dust, vacuum, pressure and even weather conditions if used outside. Radar utilises a more advanced technique, utilising ‘microwave’ signals travelling in electromagnetic spectrum and the measurement is virtually immune to these same conditions. Its growing popularity in other sectors – such as in the automotive sector, where it is being used in driver assistance, parking and lane control applications – clearly demonstrate this. Indeed, some process radar devices were purchased and used as car sensors in the very early stages of autonomous vehicle development.
‘Contactless’ or ‘through air’ process radar devices use either Pulse technology or Frequency Modulated Continuous Wave (FMCW) techniques, which direct and receive signals generally via a cone, rod, ‘flush’, parabolic, or planar antenna. For the Pulse technique, a microwave generator directs a continuous beam of very short, microwave pulses (millions per second) that are transmitted to, and reflected back from, a product surface. The elapsed time period between transmission and reception of the signal (at the speed of light) is measured. Using a special sampling technique it is calculated and converted to a distance, vessel level or volume. FMCW systems employ a modulating frequency transmission signal and then a shift detection technique to make the same measurement. Both offer accurate, continuous and real-time readings.
The measured product Dielectric Constant (DK) or Relative Permittivity has an effect on reflectivity to the radar signals. The higher the figure (higher conductivity) the more reflective it is to radar signals. Military aircraft and ships demonstrate this as they are built with ‘composite materials’ to make them less reflective to radar. This ‘DK value’ was an important application consideration. However, with modern devices it is now much less of an issue. The newer higher sensitivity radar devices are able to work with much lower ‘reflectivity’ products. This phenomenon also has advantages, for example enabling radar sensors to look at liquid levels from completely outside of some processes, through ‘low reflectivity’ plastic vessel walls and glass windows. Accuracy is also improved and some process radars used in storage vessels, can achieve <2mm accuracy and mm resolution.
Many years ago, all radar devices required a separate power supply for the electronics to enable them to handle the large amounts of signal processing needed. In recent years, in common with many other technologies, state of the art electronics and processing systems have enabled devices to employ more powerful processors, to be loop powered and use low voltages – from 9-12 VDC. Set up is also simple. Microwave emission levels are also very low, typically 100 times below those of the average mobile phone. Radar devices are suitable for contactless microwave transmission in closed vessels and modern devices will also meet the new European LPR EN 302729 standard required for ‘open air’ applications, such as rivers, open storage vessels and stockpile sites. Power consumption is now so low on some devices that they can even be solar and battery powered at remote telemetry sites.
Is it all in the software?
Regardless of the radar ‘technique’ and the frequency, it will be fully electronic and will measure at the speed of light, which means that it is a virtually drift free technology. Good software is important to enable effective analysis of data and to reliably follow and deliver the correct accurate level measurement. This should be constantly refined through a manufacturers application experience. Just as important – if not more so – it is vital to be able to set up a device easily, to ensure programing errors are minimised and to ensure that it delivers the best performance.
The latest generation of contactless radar sensors should also have the ability to adapt to process changes, using algorithms based on application experience. This is what enables the devices to offer the minimum complexity during set up, yet still work reliably if process conditions change somewhat from those anticipated.
Is there anywhere they can’t go?
There are some areas where a ‘guided wave’ radar (with a rod, co-axial or cable probe that guides the measurement path) works more effectively. For example, in external side chambers or bridles on vessels, the guided radar signal format sees less interference from the side connections into the vessel. On interface measurement (such as oil and water), a guided wave radar will also be the better solution. Although with a rod in the process, build up can be a significant factor. A good full-range radar supplier will be able to advise on this and provide the right technology for a particular application.
Where to now for radar?
The latest generation of high sensitivity contactless process radar level transmitters can offer highly effective measurement solutions, even in what have, traditionally, been considered to be challenging liquid and solid applications. Applications with foaming liquid surfaces, for example, or very low reflectivity plastic powders and liquid hydrocarbons or those requiring fast response for wave height measurement and machine/conveyor positioning over long distances. Installations with all kinds of agitators, mountings with narrow apertures and measurement of small target areas and environments with high combustion and process temperatures or full vacuums are just a few of the areas where radars are working reliably.
Future radar developments will no doubt feature even wider application capability in extremes of process and environmental conditions, more process connection options, both longer and shorter measuring range capability, as well as offering easier set up and communication options for a more refined user experience.
Doug Anderson is marketing manager at VEGA Controls Ltd
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