Frequency Modulated Continuous Wave (FMCW) Radar Level Measurement Systems

Radar (radio detection and ranging) level measurement systems are very successfully utilized for assessing the filling level of liquids in tanks and of bulk solids in silos. They continuously demonstrate their advantages against other techniques in a very wide field of industrial applications. The major benefit is the contactless measurement principle, which is based on using microwave based time-of-flight distance measurements in the free space above the measured medium.

Reliable measurements are possible under various practical conditions even with aggressive media like acids and bases and even in low reflecting products like solids. Given their high frequency basis, the microwaves easily propagate through dust and mist in the air. Furthermore, in most industrial applications, their propagation speed is constant and independent of the composition of the air and of parameters like pressure, temperature, and others. Accordingly, radar systems also allow for accurate and very repeatable level measurements even in ‘dirty’ applications, where the radar antenna may be covered by bulk solid build-up, condensation, or anything else.

In the field of radar level measurement systems, two concepts are commonly used. The first is known as FMCW which stands for Frequency-Modulated Continuous-Wave, the other is simply known as pulse. FMCW radars have many advantages, the most important of which is the substantial measurement sensitivity (dynamic range). This is why since the very first industrial radar was introduced by KROHNE in 1989, we have opted to exclusively use FMCW as the basis for all its OPTIWAVE radar level measurement systems.

RADAR SYSTEMS

RADAR SYSTEMS DESCRIPTION

Radar level measurement is based on discerning the round-trip time-of-flight based on emitting microwaves from an antenna and receiving those reflected from the measured product’s surface along with any which are backscattered from anything else in the tank.

Pulse :

In pulse radar systems, this is achieved by emitting a short pulse signal waiting for a while and then directly acquiring the resulting echo signal. A sequential of sampling technique is typically employed. Instead of a single pulse, a sequence repeated pulses is issued and the echo signal is sampled using a second sequence of pulses with a slightly different repetition time period. The energy of each transmitted pulse is relatively small because the peak amplitude is very limited. Therefore with the sequential sampling, this generally results in a relatively small dynamic range and limited signal-to-noise ratio (SNR) performance which is acceptable for most simple installations but could be problematic in some use cases.

FMCW :

The concept of FMCW radar systems is completely different in order to achieve a much better SNR and for the broadest applicability range. In FMCW radars, acontinuous-wave signal is generated and emitted. This signal has very large time duration and more energy than the emitted signal in a pulse radar system (even with equivalent peak amplitude). Depending on the device, the frequency of the continuous-wave signal is linearly modulated over time, in a saw tooth pattern, from low to high according to its frequency sweep capability. For example some devices have 1, 2 or 4 GHz sweeps starting at different nominal frequencies. With this approach the ‘sweep-duration’ can be chosen independently from the bandwidth and the signal can simply be generated by means of a voltage-controlled oscillator (VCO) and the resulting spectral purity of the sig-nal is very high.

The approach for processing the continuous-wave signal echo is to mix the received signal with the transmit signal. After low pass filtering, the so-called intermediate-frequency signal is directly obtained.
As another advantage of the FMCW concept, this signal can directly be digitized using a low-cost, low sampling frequency, analogue-to-digital converter (ADC) and no sequential sampling has to be performed.

In the past, the challenge was to accurately generate a highly linear and stable frequency sweep in a loop-powered instrument. Early on KROHNE solved this requirement in the OPTIWAVE radar by employing phase-locked loop (PLL) technology.

RADAR SYSTEM PARAMETERS

Each radar level measurement system and an antenna is characterized by a set of technical parameters as follows:

• Utilized frequency band (nominally this is the center frequency and bandwidth)
• Antenna size (diameter and length)
• Type of antenna (horn antenna, dielectric ‘drop’ antenna, dielectric lens antenna)
• Antenna gain and efficiency

The angular beam width of the antenna radiation field is inversely proportional to the aperture diameter of the antenna and to the center frequency. The choice of antenna is mostly dependent on the given application conditions.

On the one hand, the angular beam width should be small in the case of tall narrow tanks or silos in order to avoid the unwanted ‘illumination’ of the tank or silo wall or to avoid any disturbing echo signal from tank internals (e.g. agitators or reinforcement structures).

On the other hand, the beam width should not be too small, for example, if movements (‘waves’) are given on the liquid surface or if a perpendicular incident direction towards the planar surface of the liquid cannot be guaranteed. The latter might be the case, for example, if the flange on a given tank is not perfectly horizontal. In either of these scenarios, the reflected microwaves would not arrive back at the radar if the angular beam width is too small and consequently no echo would be detected by the radar.

The transmission loss is the ratio between the transmitted power and the received power, and this parameter is largely dependent on the properties of the antenna (gain and efficiency), the utilized frequency, and also of course, on the reflection or backscattering properties of the liquid or bulk solid respectively. While the reflection coefficient of the planar liquid surface does not change with frequency, the backscattering at fine granulated bulk solids largely increases with increasing frequency. Accordingly, the penetration of microwaves into the bulk solid heap decreases.

As a general rule of thumb, the received power generally increases with increasing frequency and with increasing diameter of the antenna. For this reason, the echo signal level can generally be increased by using a high frequency and an antenna with a large diameter. At first glance it looks advantageous to always configure a radar system for minimum transmission loss and minimum beam width, but there are exceptions and each specific application should be reviewed with a technical specialist.

Range resolution is another interesting parameter of the radar system. It describes the ability to separate different radar targets from each other over distance. This parameter is inversely proportional to the bandwidth. For this reason, a large bandwidth is required to allow a good separation between echoes from the filling medium and from other ‘disturbing’ objects such as the antenna outlet reflectionwhich is often the root cause for the so-called ‘upper dead zone’. Another example is the weld seams in the tank or silo wall. Typically, the bandwidth of a radar system is proportionally increasing with its center frequency.

Radar antennas :

A variety of radar antennas based on different designs concepts are available to address many applications.

Dielectric lens antenna :

This type of antenna is available for the 80 GHz frequency range radar systems. Here, a dielectric lens is given at the interface between the radar and the tank or silo and can directly fulfill the function of a ‘barrier element’. Advantageously with this design, the overall length of the antenna is very small, while offering a good electrical matching.

Horn antenna :

This is the standard type of antenna, especially for the 10 GHz and 24 GHz frequency ranges, and horn antennas usually have very good applicability in most applications. The overall length of horn antennas is relatively large, but they provide a very good electrical matching to the free space. Additionally, antenna wave-guide feeds including a so-called ‘metaglass’ or plastic component are available as a ‘barrier element’ between the tank or silo and the environment. This is relevant for means of explosion protection if an inflammable or explosive atmosphere is given inside the tank or silo.

Dielectric ‘drop’ antenna :

This type of antenna has been specially designed and optimized for offering a smaller overall length with the same (and even better) performance as a horn antenna of the same diameter and for directly functioning as a ‘barrier element’.

Furthermore, the ‘drop’ antenna is the perfect choice for bulk solid applications and ‘dirty’ environments, because adherences of bulk solid material, condensed water, etc. are largely avoided by thesmooth surface of the antenna. Also, adherences are generally less critical with the volumetric plastic body of the antenna. The ‘drop’ antenna shows a good electrical matching and is available for the 24 GHz frequency range.

The overall length of the antenna is especially interesting with regard to the decay of disturbing echoes (‘antenna ringing’) over distance, which result from multiple reflections inside the antenna caused by the (always given) mismatching at the front face of the antenna and at its feeding point. This decay is faster the smaller the overall length of the antenna is. Accordingly, a short antenna is very favorable if level measurements should also be done at very small distances, i. e. close to the antenna.

SUMMARY AND CONCLUSIONS

The answer to every challenging level measurement application cannot be given by only one radar system. The solution is always based on the proper selection of the radar system and the antenna combination selected for the specific application.

Now a complete range of FMCW radar systems and a large variety of corresponding antennas is the real enabler to finding the right solution for contactless level measurement in nearly every application.

It should be noted that given the intrinsic technical advantages of FMCW, and with recent advances and availability of monolithic microwave integrated circuits (MMIC) other radar vendors are now migrating from pulse radar systems to the FMCW radar technology.

While a large variety of pulse radars is still available in the market, KROHNE will persist with FMCW as its core technology.

FMCW Level Transmitter Range