Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2002-04-18
2004-02-10
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S719000, C324S691000
Reexamination Certificate
active
06690183
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a resistance measurement method and to a circuit arrangement suitable for performing the method.
BACKGROUND OF THE INVENTION
Such methods and circuits can e.g. be used for determining resistance values of strain gauges and therefore for the measurement of strains, e.g. for determining forces, pressures or torques, or also for producing temperature-dependent signals.
Particularly in connection with temperature measurements, it is known to feed a temperature-dependent resistor with a precisely known constant current and to digitize the resulting voltage drop at the resistor by means of a precise analog-digital converter. This requires high quality and therefore very expensive analog components, whose characteristic curve must not shift significantly over a wide temperature range.
It is also known in a first step to set a current of a constant current source with the aid of a reference resistor, to integrate a voltage up to a certain, specific value on a capacitor by means thereof and to measure with a counter the time by which said voltage value is reached and then to store the count. In a second step the constant current is set and inverted with a temperature-dependent measuring resistor and with said current the capacitor is discharged. Once again the time up to the complete discharge of the capacitor is measured and the corresponding count established. The ratio of the two counts is the ratio of the two resistors, which gives the measured temperature. Once again very precise and therefore expensive equipment components are needed, because otherwise the influences of errors would very rapidly become intolerable.
DE 36 42 862 C2 discloses a circuit arrangement for producing a temperature-dependent signal, a reference resistor and a temperature-dependent resistor being provided, by means of which a reference time and a measurement time can be produced and from the comparison of these times can be derived a digital signal associated with the measured temperature. For this purpose a charging capacitor is alternately charged across the reference resistor and the temperature-dependent resistor to the same, predetermined threshold values, the charging times are measured, so that in the case of a known reference resistance from the ratio it is possible to determine the value of the temperature-dependent measurement resistance and therefore a temperature value.
The advantage is this circuit and the corresponding method compared with the known types is that no analog components are required and no special demands are made on the quality of the digital components. However, it is disadvantageous that the measurement takes place during the charging process when a relatively high current must be flowed and in many possible uses this has to be supplied by a battery or a solar module. The internal resistance of the battery or module gives rise to an interfering influence with respect to the resistance determination of the reference and measuring resistor.
Another essential disadvantage is that by means of the determination of the charging time it is not the resistance value of the reference or measuring resistor alone, but instead the resistance value as such is measured, which is given by the indicated resistance and the internal resistance of the electronic switch/transistor connected in series therewith and which is unknown. For as long as the resistance value of the switch, which with CMOS transistors is 10 to 20 ohms with respect to the resistance value of the reference and measuring resistor and with NTC (Negative Temperature Coefficient) resistors is in the range 15 to 20 kohms, is negligible the known method and circuit operate in a satisfactory manner. However, for highly precise temperature measurements it is not possible to use such NTC resistors due to their non-linearity and lack of long-term stability. It is then necessary to use platinum resistors, which have much lower resistance values of approximately 100 to 500 ohms. Thus, as a result of th is with the aforementioned switch resistance values a significant error arises, particularly if it is borne in mind that a platinum resistor in the case of a temperature change of 1° C., changes its resistance value by only 0.4%. The indicated problem can also not be solved by the use of expensive, external power MOSFETs. The latter admittedly have the resistance value of approximately 10 to 15 mohms (milli-ohms), but with a platinum resistor PT100 a temperature change of 0.01° C. leads to a resistance change of 4 mohms and which is therefore of the order of magnitude of the resistance of said MOSFET switch. In addition, such power MOSFETs are not only expensive, but are also unsuitable for measurement electronics for other reasons.
An important disadvantage of strain gauges is the limited shift in the resistance change of such strain gauges. Typically the resistance of a strain gauge varies by approximately 0.2% from 0 to full scale deflection. This value is normally expressed as a change in parts per million (ppm). 0.2% corresponds to the value of 2,000 ppm. Thus, a strain gauge typically has a 2,000 ppm shift. As opposed to this a typical temperature-dependent, platinum resistor (e.g. PT500) changes its resistance per temperature difference degree by 3920 ppm or for a typical temperature shift of 100% by 392000 ppm or 39.2%. Thus, it is not readily possible to transfer to strain gauges methods known from temperature measurements, because the strain gauge shift is approximately 200 times lower. In addition, with such small measurement quantities, a decisive part is increasingly played by the unavoidable noise effects of the electronic components or circuits used, which leads to an additional deterioration of the measurement precision and resolution of a method or circuit arrangement for measuring such quantities.
With time or time-resolved measuring methods, which are used for avoiding the influences of different curve shapes of the measurement signals of a threshold switch, e.g. a Schmitt trigger, due to the time lag of the threshold switch a further problem arises, because such a lag cannot generally be ignored. It is particularly noticeable with strain gauges in the measured result with values of up to 10 ppm. As it is also highly dependent on the temperature and voltage, the threshold switch time lag is also noticeable as a temperature error.
Known electrical resistance measurement methods, such as are e.g. known from DE 44 20 998 C2, use signal processing means in the form of processors or rapid counters for determining time intervals.
In the processor sector at present using conventional processes maximum clock frequencies of approximately 20 MHz can be implemented. When using hardware-based, rapid counters this can be raised to approximately 200 MHz. Beyond this value significantly increased costs and high current consumption or power loss make such a device uncompetitive and can only therefore be used to a limited extent as a result of its restricted time resolution.
The problem of the invention is to provide a method and a circuit for the precise measurement of resistances, whilst avoiding the aforementioned disadvantages.
SUMMARY OF THE INVENTION
In the case of a method of the aforementioned type, the invention solves this problem in that a capacitor is repeatedly charged and discharged and the charging or discharging time is measured by means of at least one resistor and at least one first switch connected in series therewith, at least one second switch connected in series with the resistor and juxtaposed, parallel-connected switches in series with the resistor, whilst using a threshold switch. The invention also solves the set problem in the case of a circuit arrangement for resistance measurement by providing signal processing means, at least one capacitor and with respect thereto mutually parallel-connected, at least two resistors, a first switch being in each case connected in series with the resistors and wherein at least one second switch is connected in parallel with th
Acam-messelectronic GmbH
Le N.
McGlew and Tuttle , P.C.
Nguyen Vincent Q.
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