Non-contacting radar transmitters use either pulse or frequency modulated continuous wave modulation techniques to perform level measurements. In this guest feature, Emerson’s Ingemar Serneby explains the differences and argues that FMCW technology provides superior accuracy and reliability.
Accurate and reliable level measurements are fundamental to the safe and efficient operation of process plants. A broad range of level measurement technologies is available to end users, but there are certain challenging applications for which non-contacting radar is the most suitable option. This technology measures liquids, sludges, slurries and some solids, and provides the key advantage of being unaffected by process conditions such as density, viscosity, conductivity, coating and vapours.
Non-contacting radar level transmitters provide top-down, direct measurement of the distance to the process media surface, which is highly accurate and reliable. It is a suitable technology for tanks containing moving objects or corrosive products, and high temperature and pressure applications, as these factors do not affect its measurement accuracy. With no moving parts, built-in diagnostics and straightforward installation and commissioning, non-contacting radar transmitters offer both ease-of-use and low maintenance requirements.
Non-contacting radar level transmitters use one of two main modulation techniques – either pulse or frequency modulated continuous wave (FMCW) – to perform continuous level measurements.
Devices based on pulse technology emit tens of thousands of short radar pulses per second from an antenna positioned at the tank top directly towards the process material below. The pulses reflect off the surface of the material being measured and return to the transmitter. The transmitter measures the time delay between the transmitted signal and the received echo signal. An on-board microprocessor then calculates the distance to the process media surface using the following formula: distance = (speed of light x time delay) ÷ 2.
Once the transmitter has been programmed with a reference gauge height – usually the bottom of the tank or chamber – the level measurement can be calculated.
Devices using FMCW technology also transmit a radar signal from an antenna at the tank top, but the transmitted radar frequency increases over time to create a signal sweep.
After the signal is reflected from the process media surface, the echo is picked up by the antenna. Because the transmitted signal is constantly varying in frequency, the received echo always has a slightly different frequency compared to the signal being transmitted at that specific moment.
The difference between these frequencies is directly proportional to the echo delay – in other words, the distance from the transmitter to the process media surface – which enables the level to be accurately measured.
Critically, because the process variable information is then in the frequency domain, rather than the amplitude modulated (AM) or time difference domain, this enables more accurate signal conversion. Most tank noise sources are in the amplitude domain, so FM signal processing can ignore them, ensuring that measurement accuracy is not affected.
A major benefit of transmitters based on FMCW technology is that their sensitivity can be more than 30 times higher than pulse transmitters. This means signal strength is much greater, which enables superior measurement accuracy and reliability to be provided.
Consequently, transmitters based on FMCW technology are regarded as the most stable level measurement devices available and are the preferred solution for many challenging applications within the manufacturing and process industries.
For example, if an application involves a turbulent liquid surface, this can potentially lead to a pulse being lost. As a result, the radar will misregister and lock onto the next available pulse.
Without intelligent software to identify this misregistering, the gauge will display an erroneous measured value, typically 25 millimetres out, and the user has no way of knowing that this is a misregistered value. For large tanks or vessels storing thousands of gallons, this type of error can be significant. Because FMCW does not use a time-of-flight technique, an error such as this cannot occur, helping to ensure the accuracy and reliability of measurements.
Although non-contacting radar devices are unaffected by high process temperatures and pressures, their accuracy can be significantly affected by variations in ambient temperature. Devices based on FMCW technology are better able to compensate for these variations than those based on pulse technology.
With pulse radar transmitters, there is often no temperature compensation because there is no reference available for continuous performance and calibration checks, leading to the possibility of unobserved non-linearity and inaccuracy. Some devices may use an analogue reference (a wound cable of known length), but these are susceptible to inaccuracy through thermal expansion or contraction of the reference cable.
To achieve the highest precision, the radar sweep produced by FMCW transmitters must be absolutely linear. A crystal oscillator is therefore used for on-line adjustment of the transmitted frequency. This gives consistent accuracy at dynamic ambient temperature conditions. For example, in a typical process tank with an ambient range of 120°C (minus 40°C to plus 80°C), an FMCW radar would offer measurement stability over the entire ambient temperature range, with a maximum error of 12 millimetres.
For a pulse radar, however, a typical manufacturer’s specification states the influence of ambient temperature at between 0.05% and 0.1% per 10°C, which would translate to a maximum error of 100-200 millimetres for the same tank.
Two-wire FMCW technology
Although FMCW technology provides advantages over the pulse radar technique in terms of accuracy and sensitivity, a drawback has been its need for more processing power. Older devices are seen as being ‘power hungry’ and as a result, FMCW has typically only been deployed within four-wire devices.
The installation of such devices often necessitates putting additional cable infrastructure in place. This has led to some users sacrificing the additional accuracy and reliability of FMCW devices and installing two-wire gauges based on pulse technology. The problem of high processing power requirements for FMCW technology has been overcome by Emerson’s latest non-contacting radar, the Rosemount 5408 level transmitter.
Unique radar-on-chip technology, which replaces the traditional circuit board, enables it to be less power-hungry and more energy efficient. These devices only require two wires for power and communication, enabling end users to benefit from the superior accuracy and sensitivity of FMCW technology without needing to install additional infrastructure. The radar-on-chip technology removes sources of electromagnetic compatibility noise which cause signal disturbance. This enables further improvements in measurement accuracy and reliability.
- Ingemar Serneby is marketing manager at Emerson