Communications: directive radio wave systems and devices (e.g. – Determining distance – Material level within container
Reexamination Certificate
1999-10-26
2001-06-19
Lobo, Ian J. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Determining distance
Material level within container
C367S908000
Reexamination Certificate
active
06249244
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an apparatus for the determination of the fill state of solid or liquid material in a container with a signal generating/transmitting unit which generates high-frequency signals with a predetermined transmission repetition frequency and transmits them in the direction of the surface of material. The high-frequency signals are reflected at the surface of the material and a receiving unit receives the reflected signals. A time-delay circuit transforms the high-frequency signals into low-frequency signals and an evaluating unit determines the fill state in the container using the transit time of the signals.
Processes for the determination of the fill state using the transit time of signals exploit behavior following the laws of physics according to which the transit interval is equal to the product of the transit time and propagation speed. In the case of fill state measurement the transit interval corresponds to twice the distance between the antenna and the surface of the material. The actual effective echo signal and its transit time are determined with the aid of the so-called echo function, the digital envelope curve or the low-frequency signals where the envelope curve or the low-frequency signals reproduce the amplitudes of the echo signals as a function of the distance from the antenna to the surface of the material. The fill state itself can then be determined from the difference between the known distance of the antenna from the base of the container and the distance of the surface of the material from the antenna as determined by the measurement.
In DE 31 07 444 A1 a high-resolution pulse radar process is described. A generator generates first microwave pulses and radiates them via an antenna with a predetermined transmission repetition frequency in the direction of the surface of the material. An additional generator generates reference microwave pulses which are equal to the first microwave pulses but differ slightly from them in their transmission repetition rates. The echo signal and the reference signal are mixed. At the output of the mixer an intermediate frequency signal is present. The intermediate frequency signal has the same curve as the echo signal but is extended with respect to it by a time-delay factor which is equal to a quotient of the transmission repetition factor and the frequency difference between the first microwave pulses and the reference microwave pulses. For a transmission repetition frequency of several megahertz, a frequency difference of a few hertz, and a microwave frequency of several gigahertz, the frequency of the intermediate frequency signal lies below 100 kHz. The advantage of using the intermediate frequency is that relatively slow and therefore cost-effective electronic components can be used for the monitoring of signals and/or signal evaluation.
Determination of the time extension or time delay by means of sequential sampling assumes that the time differential between two sampling points is always the same. Until now two processes were known which are suitable for the satisfying of this requirement: the mixer principle and the ramp principle. The ramp principle quantizes and works only approximately continuously.
In the case of the mixer principle two oscillators generate two oscillations with slightly different frequencies. By the slight “detuning” of the two oscillations a phase shift, which increases linearly with each period, arises which corresponds to a linearly increasing time delay.
A disadvantage of the mixer principle is the relatively high power consumption so that the power supply of a 4-20 mA current loop is only possible in the case of measurement rates of one measured value per second. Furthermore, disproportionately high demands must be made on the hardware and software in order to maintain the same time differential between the sampling points.
The mixer principle provides a comparatively small number of measured values per meter for sequential sampling. The result of this is low sensitivity of measurement for measurements with microwave pulses.
In the case of the ramp principle the same time differential from sampling point to sampling point is generated with the assistance of an RC circuit. The RC circuit is preset by a step voltage or linearly increasing ramp voltage, and therefore receives a certain offset, and is then charged and discharged in the rhythm of the transmission repetition frequency. The voltage offset in the RC circuits increases with increasing ramp voltage whereby reaching the operating point is delayed as a function of the level of the ramp voltage.
In the case of the ramp principle the time extension factor is critically dependent on the time constant of an RC circuit. The dependence of the RC circuit on the temperature has, depending on circuit technology, a great effect on the scaling of the time transformation. In order to eliminate this problem it is necessary to compensate for the temperature-dependent changes via a control circuit. However, even with this control circuit the scaling of the time transformation changes with the temperature in such a way that requirements for highly precise measurements are not fulfilled. A disadvantage of the ramp principle is furthermore the high expenditure which must be made for the generation of the ramp voltage.
BRIEF SUMMARY OF THE INVENTION
It is an objective of the present invention to provide a highly precise and cost-effective time-delay circuit for use in the determination of the fill state of solid or liquid materials.
The objective is realized according to the present invention by a time-delay circuit including a first circuit element having a time constant
T
1
which is driven by pulses synchronized with the transmission repetition frequency and which generates a non-linear output signal, a second circuit element having a time constant
T
2
(
T
1
<<
T
2
) which is driven by pulses whose clock frequency is smaller than the transmission repetition frequency and which generates a non-linear output signal, and a third circuit element which detects in each period of the transmission repetition frequency the intersection between the output signal of the first circuit element and the output signal of the second circuit element and supplies an output signal to an evaluation unit.
According to the present invention a circuit is provided which insures that the pulses which are present at the input of the first circuit element and the second circuit element have the same voltage level and the same ramp behavior.
A feature of the present invention is that the time-delay circuit is constructed of very few circuit elements. It is thus correspondingly cost-effective. Moreover it is substantially insensitive to changes of the external parameters such as, say, temperature. It is thus suitable for precise fill state measurements.
In order to reduce the discharge time of the second circuit element an electronic component, for example a resistor, is connected to the second circuit element. If the output signal of the second circuit element reaches a predetermined voltage value the second circuit element is connected in series with the electronic component via a switch. The second circuit element is discharged, where the discharge time can be optimized as a function of the resistance connected.
Another cost-effective feature of the apparatus according to the present invention is that the third circuit element is a comparator. Further, the evaluating unit contains only the sampling points for the evaluation which arise by the intersections between the output signals of the first circuit element and the second circuit element and which lie in the linear operational range of the comparator. It has been shown that comparators work approximately linearly until the second circuit element has reached approx. 63% of its maximum charging voltage. For precise measurements a range of approximately 40% maximum of the maximum charging voltage of the second circuit element is preferred.
According to another fea
Bose McKinney & Evans LLP
Endress + Hauser GmbH + Co.
Lobo Ian J.
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