A new method has been developed for decreasing the fixed error in a conventional FM-CW radar, with a view to developing a burden level meter in a blast furnace.
In this method, a sinusoidal wave of angular frequency ω1 is mixed to the beat signal, whose frequency is proportional to the measured distance, and the upper side band is detected. Next, another sinusoidal wave of angular frequency ω2 is mixed to the upper side band, and filtered to produce the lower side band.
In this manner, the term of (ω1-ω2)t is added to the phase of the beat signal, that is, the phase of the beat signal is modulated to a degree of 2π in a period of 2π/(ω1-ω2). During one cycle of this phase modulation, the numbers of zerocrossings of the beat signal are averaged and so the fixed error is reduced.
When the maximum frequency deviation was 80MHz, the standard deviation of the residual error was found to be less than 20mm, while the fixed error in the conventional FM radar is 450mm. And even when coke and ore was used as targets, the distance was measured with an accuracy of 50mm. Thus this method was found to be sufficiently accurate for level measurement in a blast furnace.
Since this method requires merely a narrow frequency band, the configuration of the radar is simple and compact.
This burden level meter is now in successful operation in a blast furnace at Kakogawa Works of Kobe Steel, Ltd..
