Apparatus for determining and/or monitoring a process variable of a medium

Patent metadata

Field	Value
Publication number	US-9448097-B2
Application number	US-201113807396-A
Country	US
Kind code	B2
Filing date	Jun 20, 2011
Priority date	Jul 1, 2010
Publication date	Sep 20, 2016
Grant date	Sep 20, 2016

Abstract

An apparatus for determining and/or monitoring at least one process variable of a medium in a container which comprises a mechanically oscillatable structure, which has at least one oscillation characteristic dependent on the process variable, an electromechanical transducer having at least one piezoelectric element, which excites the structure, by means of an excitation signal supplied to the transducer, to execute mechanical oscillations, and which converts the resulting oscillations of the structure into a received signal, which corresponds to a superpositioning of the excitation signal and a wanted signal representing the oscillation. A reference element in parallel with the transducer and supplied with the excitation signal, via which a reference signal independent of the oscillation and corresponding to the excitation signal is tapped, and an electronics, which, based on the received signal and the reference signal, extracts the wanted signal, and, based on the wanted signal, determines and/or monitors the process variable, wherein, for automatic measuring of temperature of the transducer, the electronics includes a temperature measuring device, which supplies the transducer and the reference element, in temperature measurement operation, with an auxiliary signal, whose frequency lies outside a frequency range predetermined by a resonant frequency of the oscillatory structure, and, based on the wanted signal extracted in temperature measurement operation, determines a temperature of the transducer.

First claim

The invention claimed is:

1. An apparatus for determining and/or monitoring at least a density, a viscosity or a fill level of a medium in a container, comprising:
a mechanically oscillatable structure, which has at least one oscillation characteristic dependent on the density, viscosity or fill level;
an electromechanical transducer having at least one piezoelectric element, which excites said mechanically oscillatable structure, by means of an excitation signal supplied to said electromechanical transducer, to execute mechanical oscillations, and which converts the resulting oscillations of said mechanically oscillatable structure into a received signal, which corresponds to a superpositioning of the excitation signal and a wanted signal representing the oscillation;
a reference element in parallel with said electromechanical transducer and supplied with the excitation signal, via which a reference signal independent of the oscillation and corresponding to the excitation signal is tapped, said reference element comprising a series circuit of at least a compensation resistor having an electrically adjustable variable and a capacitor with a predetermined reference capacitance, wherein a control apparatus is provided for adjusting the variable of said compensation resistor and connected to the temperature measuring device, which controls the variable of said compensation resistor to a value, at which a magnitude of an amplitude of the wanted signal in temperature measurement operation is a minimum; and
an electronics, which, based on the received signal and the reference signal, extracts the wanted signal, and based on the wanted signal, determines and/or monitors the density, viscosity or fill level, wherein:
said electronics includes a temperature measuring device, which supplies said electromechanical transducer and said reference element, in temperature measurement operation, with an auxiliary signal, whose frequency lies outside a frequency range predetermined by a resonant frequency of said mechanically oscillatable structure, and, based on the wanted signal extracted in temperature measurement operation, determines a temperature of said electromechanical transducer.

2. The apparatus as claimed in claim 1 , further comprising:
a memory, in which a characteristic curve of the apparatus is stored, which shows the dependence of the value of the adjustable variable on the temperature of said electromechanical transducer, and wherein:
temperature is determined based on the controlled value and the characteristic curve.

3. The apparatus as claimed in claim 1 , further comprising:
a rectifier, which is fed in temperature measurement operation with the wanted signal, wherein:
the wanted signal rectified by means of said rectifier is fed to a logic unit, especially a microcontroller or a field programmable gate array; and
said logic unit determines an amplitude of the rectified wanted signal, and controls the variable of said adjustable component, in temperature measurement operation, to the value, at which the magnitude of the amplitude of the wanted signal is a minimum.

4. The apparatus as claimed in claim 3 , wherein:
said rectifier includes a comparator and a multiplier in parallel with said mechanically oscillatable structure and connected to said reference element;
the auxiliary signal lies, in temperature measurement operation, parallel on said mechanically oscillatable structure, on said reference element and on said comparator;
an output signal of said comparator and the wanted signal lie on said multiplier; and
said multiplier is connected to a lowpass filter, whose output signal is applied as rectified, smoothed, wanted signal to said logic unit.

5. The apparatus as claimed in claim 1 , further comprising:
a sampling apparatus for sampling said wanted signal, wherein:
said sampling apparatus determines the amplitude of said wanted signal in temperature measurement operation for sampling points in time, at which an amplitude of the auxiliary signal has extrema, and connected to said sampling apparatus, a logic unit is provided, which executes a comparison of the sign of amplitudes of said auxiliary signal and said wanted signal for sampling points in time; and
based on the comparison, ascertains a direction and, based on amplitudes of said wanted signal in temperature measurement operation, a magnitude, by which the value of the variable of said compensation resistor is to be corrected.

6. The apparatus as claimed in claim 1 , wherein:
there lies on said electromechanical transducer and said reference element a signal with a periodically continuously variable frequency, whose periods, in each case, comprise a time span, in which the frequency of the signal continuously rises or falls, and in which the signal forms said excitation signal, and
whose periods, in each case, comprise a time interval, in time, leading this time span, and/or, in each case, a time interval, in time, following this time span;
in which time interval the frequency of the signal is constant and lies outside of the frequency range predetermined by a resonant frequency of said mechanically oscillatable structure; and
in which time interval the signal forms said auxiliary signal.

7. The apparatus as claimed in claim 1 , wherein:
said excitation- and auxiliary signal, are: both sinusoidal signals, both trapezoidally shaped signals, both triangularly shaped signals, or both rectangular signals.

8. The apparatus as claimed in a claim 1 , wherein:
said excitation signal in temperature measurement operation is a rectangular signal;
said auxiliary signal is a part of said excitation signal, whose frequencies lie above the frequency range predetermined by the resonant frequency of said oscillatable structure;
said temperature measuring device has a highpass filter, which is fed with said wanted signal and which filters out from said wanted signal a measurement signal, whose frequencies lie above the frequency range predetermined by the resonant frequency of said mechanically oscillatable structure; and
said temperature measuring device determines temperature of said electromechanical transducer based on the measurement signal.

9. The apparatus as claimed in claim 8 , wherein:
said reference element has at least one component having an electrically adjustable variable; and
connected to the temperature measuring device, a control apparatus is provided for adjusting the variable of said at least one component, said control apparatus controls the variable of said at least one component to the value, at which the magnitude of the amplitude of said measurement signal in temperature measurement operation is a minimum.

10. The apparatus as claimed in claim 9 , further comprising:
a memory, in which a characteristic curve of the apparatus is stored, which shows the dependence of the value of the adjustable variable on temperature of said electromechanical transducer, wherein:
temperature is determined based on the controlled value and the characteristic curve.

11. The apparatus as claimed in claim 9 , wherein:
said reference element includes a series circuit of said at least one component having an electrically adjustable variable and a capacitor with a predetermined reference capacitance; and
said component is a compensation resistance.

12. The apparatus as claimed in claim 1 , wherein:
said electronics includes a compensation apparatus, which, based on the temperatures determined in temperature measurement operations, compensated at least one temperature dependence of an oscillation characteristic of said mechanically oscillatable structure dependent on the density, viscos

Assignees

• Lopatin Sergej
• Urban Martin
• Endress Hauser Gmbh Co Kg

Inventors

• Lopatin Sergej
• Urban Martin

Classifications

• G01D3/0365 Primary

  the undesired influence being measured using a separate sensor, which produces an influence related signal · CPC title

• G01F23/00 Primary

  Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm · CPC title

• G01F23/2967

  for discrete levels · CPC title

• G01N9/002

  using variation of the resonant frequency of an element vibrating in contact with the material submitted to analysis (G01N9/34 takes precedence) · CPC title

• G01N11/16

  by measuring damping effect upon oscillatory body · CPC title