Electromechanical balance with string vibrator.

0001The invention relates to an electromechanical balance with a frequency-giving force sensor and a microprocessor for reshaping the measurement variable, a first quartz oscillator being provided as a clock generator for the microprocessor and as a timer for measuring the vibration frequency of the force sensor, and also with a frequency-giving temperature sensor for measuring the balance temperature. A string oscillator can, for example, be provided as the frequency-generating force sensor.

0002To improve the accuracy of such scales, it is known to compensate for the temperature dependence of the force sensor or the oscillation frequency of the same in such a way that a sensitivity of the scale that is independent of the temperature is achieved. Other temperature-dependent weighing parameters also need to be corrected, for example to keep zero point shifts and linearity deviations as small as possible.

0003It is also known to either directly compensate for the temperature influence on the weighing parameters by means of measures which are based on direct counteracting effects, or to calculate a correction quantity to be related to the measured variable on the basis of the continuously measured weighing temperature. In both cases, difficulties can arise if the weighing parameters as well as any compensation means or temperature sensors are subject to strong scatter and have non-linearities, so that an adjustment of each balance, possibly with several temperature values, is necessary in order to achieve an exact compensation effect. From this point of view, therefore, components with little scatter and as precisely and linearly as possible should be aimed for, which, on the other hand, can lead to excessively expensive compensation solutions.

0004From US Pat. No. 4,464,725 a scale of the type mentioned is known, in which a separate frequency-giving temperature sensor is provided for a temperature compensation carried out on the basis of arithmetically determined correction quantities and as a timer for measuring the temperature-dependent oscillation frequency of the temperature sensor of the clock generator the microprocessor.

0005This solution does not use any direct compensation measures. The temperature sensor measures the temperature of the scale, whereby the measured temperature values ​​are evaluated for the arithmetical correction of the measured variable. Such comprehensive temperature compensation of the weighing parameters is, however, only possible with a relatively high outlay in terms of hardware and software, if extensive adjustment work is to be avoided.

0006The invention now shows a way to solve the problem mentioned in a simple manner and with less effort. Starting from a balance of the type mentioned at the outset, the invention consists in that the first quartz oscillator has a temperature-dependent oscillation frequency and serves as a temperature sensor for measuring the balance temperature, that the temperature dependence of the oscillation frequency of this first quartz oscillator is adapted to that of the force sensor, and that a second Quartz oscillator with an oscillation frequency that is at least approximately independent of the temperature is provided, which serves as a timer for measuring the temperature-dependent oscillation frequency of the first quartz oscillator.

0007The first quartz oscillator, which serves as a clock for the microprocessor and as a timer for measuring the oscillation frequency of the frequency-generating force sensor, is a relatively expensive component, primarily due to the tight quality tolerances required with regard to the aforementioned tasks. In the solution according to US Pat. No. 4,464,725, an equally high-quality component is, for example, for the temperature sensor in the form of a quartz oscillator, necessary to meet the specified accuracy requirements. In the solution according to the invention, the functions of the two quartz oscillators with respect to the temperature measurement are to a certain extent interchanged by the clock generator of the microprocessor also performing the function of a temperature sensor, the second quartz oscillator only serving as a timer, for which cheaper designs are available. The first quartz oscillator is also used for direct temperature compensation of the force sensor. The latter measure not only has the advantage that the existing high-quality quartz oscillator is better utilized; rather, temperature compensation, which is based on direct counteraction, is more accurate under the given conditions than the arithmetical correction method. Since the characteristic of the force sensor also represents the most important weighing parameter, the proposed measure is of particular importance. The decisive factor here is the fact that frequency-giving force sensors, for example string transducers, can be produced with only very little variation in the characteristic curve, so that a comparison in this regard is unnecessary.

0008The exact temperature measurement is used for arithmetic correction of the weighing result based on the other temperature-dependent weighing parameters. For this purpose, a second quartz oscillator is only required as a timer in the solution according to the invention. A commercially available quartz oscillator designed for watches can be used for this. So-called watch crystals are very inexpensive, but still have the accuracy required for the aforementioned purpose and are largely independent of temperature. The use of a temperature-independent quartz oscillator as a timer for the frequency-giving temperature sensor has the further advantage that, as is usually the case, two temperature-dependent quartz oscillators with different temperature dependencies of the frequency, but only a single quartz oscillator, are decisive for temperature detection. In this way the interference potential is reduced.

0009In the case of the balance according to US Pat. No. 4,464,725, however, a corresponding solution with clock quartz is not possible because a clock quartz with an oscillation frequency of usually 32.768 kHz can be used as a timer for measuring the oscillation frequency of the force sensor and as a clock generator of the microprocessor not suitable in high-resolution scales (clock frequency approx. 16 MHz).

0010In order for the microprocessor to work properly, its clock must be in its immediate vicinity. On the other hand, the clock, which now also works as a temperature sensor, should be arranged as close as possible to those parts of a load cell where the temperature-dependent effects to be compensated occur. Therefore, the print carrying the microprocessor is preferably mounted on the load cell containing the force sensor and arranged together with it in a common housing in order to achieve the most accurate possible detection of the temperature profile in the load cell.

0011The temperature as a measured variable can be derived, for example, from the oscillation frequencies of the two quartz oscillators using the temperature functions shown as power series, for example for an initial temperature of 25 ° C. For crystals with the temperature coefficients ∝₁, β₁ ... or. ∝₂, β₂ ... can be found for the temperature T = 25 ° + ΔT:<maths id="math0001" num=""><math display="inline"><mrow><mtext>Quartz 1: f₁ = f₁ 25 ° C (1 + α₁ΔT + β₁ΔT² + ...) (1)</mtext></mrow></math><img file="EP0504487A2_D0001.tif" /></maths><maths id="math0002" num=""><math display="inline"><mrow><mtext>Quartz 2: f₂ = f₂ 25 ° C (1 + α₂ΔT + β₂ΔT² + ...) (2)</mtext></mrow></math><img file="EP0504487A2_D0002.tif" /></maths><maths id="math0003" num=""><img file="EP0504487A2_D0003.tif" /></maths> As can be seen, the quotient of the two frequency values ​​is only a function of the temperature. The temperature information results from equation (3).

0012According to the invention, a type is provided for the second quartz oscillator, the frequency of which is independent of the temperature at least to a first approximation (temperature coefficient )₂ = 0). As a result, only the temperature of the first quartz oscillator is decisive for the temperature detection.

0013A further advantage results if types are selected for the quartz crystals of the two quartz oscillators, in which the coefficients (β) of the quadratic element of the temperature function shown as a power series are equal to one another. In this case there is a linear dependence of the temperature according to equation (3). An adjustment at a certain temperature value (eg 20 °) is sufficient to obtain the absolute temperature as the measured variable. In many cases, however, the adjustment mentioned can even be dispensed with, since the relative accuracy of the temperature measurement is usually sufficient for compensation purposes.

0014An exemplary embodiment of the invention is explained in more detail below with reference to the accompanying drawing. In the drawing, a block diagram is shown, which shows the essential parts of the electronic equipment of a string scale for understanding the invention.

0015In the present example, a string vibrator 1 with a string 4 clamped between two brackets 2 and 3 serves as the frequency-generating force sensor and is caused to oscillate by an oscillator 5. At the output of the oscillator 5 there is a signal whose frequency changes with the tension force of the vibrating string 4 acting on the holders 2 and 3, ie with the load on the scale. This signal is fed via a pulse shaper 6 to a counter 7 which is controlled by a microprocessor 8. A first quartz oscillator 9, to which a pulse shaper 10 is assigned, serves as the clock generator for the microprocessor 8 and the counter 7. By counting the counting pulses, the frequency value f 1 corresponding to a certain scale load is determined in the microprocessor 8 in a manner known per se and, taking any corrections into account, the weight value GW to be displayed by the scale is calculated.

0016The first quartz oscillator 9 is also designed as a frequency-giving temperature sensor, ie it is provided with a quartz Q1, whose temperature coefficient ∝ 1 has a non-zero value. The counting pulses taken from the pulse shaper 10 of the quartz oscillator 9 are fed to a further counter 13 to determine the temperature value. A second quartz oscillator 11 serves as the timer, the quartz of which has a temperature coefficient ∝₂ = 0 and which is also assigned a pulse shaper 12. The microprocessor 8 calculates from the counted pulses of the counter 13 in a similar manner to the counter 7, the frequency value f₂ and finally the quotient of the two frequency values ​​f₁ and f₂. The result is a digital temperature value TW, which is available for compensation purposes.

0017In order to achieve a sensitivity of the balance that is independent of the temperature, the temperature characteristic curve of the first quartz oscillator 9 is further adapted to that of the string oscillator 1. In addition, the quartzes Q1 and Q2 used for the two quartz oscillators 9 and 11 are advantageously chosen such that the temperature coefficients β_x have the same value. This results in a linear relationship between the determined temperature value TW and the temperature prevailing at the temperature sensor 9.