SCALE WITH OVERLOAD DIAGNOSTIC

Description

[0001] The present invention relates to a scale operating according to the principle of electromagnetic force compensation. In such known scales, a weight force is often introduced into a force-transmitting lever mechanism. A support coil is arranged on one of the levers. This coil is immersed in the magnetic field of a preferably stationary magnet and thereby assumes a predetermined target position relative to the magnet. "Directly supporting" systems, i.e., those without a lever mechanism, also operate according to the same principle.

[0002] To keep the lever or coil in the desired position at all times, even under varying weight forces, a correspondingly controlled counterforce is generated by the coil. A current flows through the coil, generating a Lorentz force acting in the magnet's magnetic field. The current is provided by an electronic unit, referred to below as the "output stage," which can be configured to supply the coil with either a unipolar or bipolar current. The strength of the controlled coil current is a measure of the counterforce generated by the coil, which in turn is a measure of the weight force or load applied to the lever mechanism.

[0003] To evaluate the coil current, a measuring resistor is connected in series with the coil, and the voltage drop across the measuring resistor, possibly preprocessed by amplifiers and / or filters (V) or other electronic components, is fed to an A / D converter. This converts the incoming voltage into digital signals, which are then fed to a control unit for evaluation, for example, to display the weight value or to trigger weight-dependent control processes.

[0004] An A / D converter used in this way operates within a predefined measuring range. If the applied load or the resulting voltage drop across the measuring resistor is outside (below or above) this measuring range, an underload and / or overload condition exists (for the purposes of this application, the term "overload" represents both underload (tension) and overload (compression). In the prior art, an overload condition is only determined if the voltage detected by the A / D converter is outside its intended measuring range. No statement can be made about the magnitude of the overload. However, the duration, magnitude, number, type, and timing of the overload affect weighing operation and weighing accuracy, as the scale may require a certain recovery time after an overload condition, or even require a zero point setting or readjustment.Document US4930588 A discloses a scale operating on the principle of electromagnetic force compensation according to the prior art.

[0005] The object of the invention was therefore to offer a scale with overload detection or overload diagnosis, which enables the evaluation of overload cases and a dependent control of the scale, as well as a check of the functionality of the overload protection and the evaluation of the trigger threshold.

[0006] The object is achieved by a scale according to claim 1 and a method according to claim 9.

[0007] The invention is based on the realization that an overload situation can be analyzed more precisely with the help of an auxiliary measuring voltage UH , which is recorded in addition to the voltage UR dropping across the measuring resistor R. According to the invention, the auxiliary measuring voltage is the voltage which either a) between the output of the power amplifier and the coil or b) between the coil and the measuring resistor.

[0008] In variant a), the auxiliary measuring voltage can preferably be tapped directly at the output of the power stage. Alternatively, a protective resistor could be provided between the output of the power stage and the coil, for example, for explosion protection purposes. The auxiliary measuring voltage can then also be tapped between the protective resistor and the coil. Variant b) could be somewhat simpler in terms of design or circuitry, although here the auxiliary measuring voltage could be close to the measuring resistor voltage.

[0009] In the event of an overload, the auxiliary measuring voltage (UH ) lies outside the range or the voltage (UR ) of the measuring resistor that could be correctly detected within the measuring range of the associated A / D converter set for the measuring resistor voltage (UR ). However, by separately evaluating the auxiliary measuring voltage according to the invention, the overload can be detected quantitatively and / or qualitatively. The measuring range for detecting the auxiliary measuring voltage preferably corresponds essentially to the maximum output voltage of the power stage.

[0010] The auxiliary measuring voltage is also fed to an A / D converter. By switching the input voltage, this can be the same A / D converter (AD 1 ) that is also used to measure the measuring resistor voltage (UR ). To be able to measure the then higher (or lower) voltage, a different, suitable measuring range can be set on the A / D converter (two measuring ranges arranged "one above the other" for the measuring resistor voltage (UR ) and the auxiliary measuring voltage (UH ) are also considered a "different" measuring range here). Alternatively, the auxiliary measuring voltage, which, like the measuring resistor voltage, is preferably amplified with an amplifier V before being introduced into the A / D converter, can be amplified less than the measuring resistor voltage, so that the A / D converter operates unchanged, but the auxiliary measuring voltage is supplied "reduced" and can therefore be evaluated.

[0011] In one embodiment of the invention, the auxiliary measuring voltage is therefore fed to the same A / D converter that detects the voltage (UR ) of the measuring resistor during normal operation. For this purpose, the input of the A / D converter is switched from the measuring resistor voltage (UR ) to the auxiliary measuring voltage (UH ) as soon as the measuring resistor voltage (UR ) lies outside the intended measuring range or approaches the upper limit, i.e. exceeds a predetermined threshold. The switching can be initiated and / or carried out by the A / D converter itself. The switch can be implemented externally or be an integral part of the A / D converter. In the latter case (integrated), the A / D converter can have two input connections. In the first case (external), the A / D converter can have a control output for controlling the switch.

[0012] By switching the signal source applied to the input of the A / D converter, either the measuring range of the converter is also switched (for example, the sensitivity of the converter can be halved and thus the measuring range doubled), or the gain of the auxiliary measuring voltage is appropriately reduced without having to change the measuring range of the A / D converter.

[0013] In an alternative embodiment of the scale, the measuring resistance voltage (UR ) is fed to a first A / D converter AD 1 , while a second A / D converter AD 2 , designed separately from the first A / D converter AD 1 , is provided for the auxiliary measuring voltage (UH ). Here, the measuring range of the second A / D converter can be selected from the outset to be sufficiently large to be able to record and evaluate the incoming auxiliary measuring voltage. Alternatively, the gain of the auxiliary measuring voltage (UH ) could be selected so that the two A / D converters can be operated with the same measuring range.

[0014] In electromagnetic force compensation, the "carrying load" is defined by the maximum compensation current that the output stage can deliver to keep the coil in the target position. For loads exceeding this, the output stage can no longer provide sufficient coil current, causing the coil and any lever mechanism supporting it to deflect and move out of the target position.

[0015] To prevent overloading a scale, mechanical overload protection devices are known to protect the lever mechanism and mechanically prevent (possibly irreversible) deformation of the lever mechanism or other scale components. However, an overload from a weighing perspective is defined as occurring when the measuring resistance voltage (UR ) lies outside the measuring range of the associated A / D converter. A mechanical overload that endangers the mechanics of the scale does not necessarily have to be present. However, because the measuring range of the A / D converter has been exceeded, the force generated at the coil can no longer be quantitatively determined from the measuring resistance voltage (UR ).

[0016] Mechanical overload protection devices are designed to intervene preferably before (but in some cases only after) permanent damage occurs. In the preferred case, the load or the maximum output voltage of the power stage has not yet been reached, and the coil or its lever has not yet left the target position because the power stage can supply sufficient compensation current for the coil. In this case, the resistance voltage (UR ) lies outside the measuring range provided by the associated A / D converter, but the tapped auxiliary measuring voltage (UH ) can be quantitatively evaluated in an A / D converter with a larger measuring range or by reducing the amplification in order to be able to analyze the overload as follows: During normal operation of the scale, the compressive or tensile load introduced into the scale causes a resistance measuring voltage (UR ) within the measuring range of the associated A / D converter in the manner described above.If the weight force continues to increase, the resistance measurement voltage increases accordingly. If it exceeds the specified measuring range of the A / D converter, the coil current can no longer be determined. However, as the force acting on the scale platform (load application) increases / decreases, the auxiliary measurement voltage also increases.

[0017] This voltage continues to rise, for example, with increasing load force until the maximum output voltage of the power stage is reached and / or a mechanical overload protection device intervenes. If the overload protection device intervenes before the maximum output voltage of the power stage is reached, the coil current has reached a maximum and the auxiliary measuring voltage no longer increases, but remains essentially constant. Its curve and maximum value then allow conclusions to be drawn about the tripping force and the functionality of the overload protection device.

[0018] The curve and / or magnitude of the auxiliary measuring voltage can provide information about the overload protection. The mere exceedance of predefined, even specifically adjustable, limit values can already qualitatively indicate that the overload protection has been triggered. The quantitative evaluation of the auxiliary measuring voltage also allows the assignment of a specific load value (e.g., force in Newtons, weight in g, kg, etc.) that caused the overload protection to be triggered, provided the maximum output voltage of the power stage has not yet been reached.

[0019] Preferably, a control unit is provided for the scale for evaluating the measuring resistance voltage (UR ) and the auxiliary measuring voltage (UH ) or their digital values provided by the associated A / D converter(s). The control unit can be designed, among other things, to detect overloads, to define or save limit values for overloads in a fixed or variable manner (tracking), to issue warning messages, for example visually or acoustically, to interrupt (block) weighing functions temporarily or permanently, and to set, request, or initiate zero points or adjustments. It can also be designed to process, save, or output the measuring resistance voltage (UR ) and / or the auxiliary measuring voltage (UH ) or values or signals derived from them, or to detect and evaluate the exceeding of predefined limit values according to type, number, level, duration, time, etc. (e.g.to estimate the remaining service life of individual components of the scale, such as the overload protection, or for the entire scale) and, depending on this, to output control or alarm signals to the scale control system, a higher-level system control system or an operator.

[0020] Parameters for operating the scale, in particular for defining limit values to determine overload cases, can be transferred manually or in digital format to the control unit or an associated storage unit via suitable interfaces.

[0021] To verify the function and position of the trigger threshold or limit value of a mechanical overload protection device, forces or test loads (known, possibly traceable standard weights) can be applied to the scale platform (load introduction) during a test run in order to assign certain values of the auxiliary measuring voltage to the triggering of the overload protection device. However, a purely qualitative indication of an overload arises from the fact that the measuring resistance voltage (UR) is outside the measuring range of its A / D converter.

[0022] The inventive method for detecting an overload on a scale according to the invention comprises detecting and evaluating the auxiliary measuring voltage. This makes it possible to analyze the behavior of the scale within the intended load range of the output stage (maximum output voltage, equivalently the maximum output current) and above the measuring range provided for the measuring resistance voltage (UR).

[0023] Preferably, the method also includes the output of an overload signal if the auxiliary measuring voltage exceeds a predeterminable limit value or exhibits a temporal characteristic corresponding to an overload. The latter would be the case, for example, if the auxiliary measuring voltage initially increases monotonically up to a maximum value that remains constant within a predeterminable period of time. This would indicate that a mechanical overload protection device has been triggered, which relieves the output stage of the need to provide a continuously increasing coil current.

[0024] Conveniently, the method also includes the output and / or display and / or storage of the auxiliary measuring voltage or a value corresponding thereto (the terms "value" or "signal" in this application are to be equated with the measured or processed voltages insofar as values or signals can be derived from the voltages or assigned to them. A digital or analog signal, for example, can correspond to a concrete (numerical) value that characterizes, for example, a voltage or a load).

[0025] The output can be provided by suitable display devices (display, screen, binary signal, etc.), whereby the reaching of a threshold value could also be indicated, for example, visually or acoustically. The auxiliary measuring voltage or the corresponding values can be transmitted as digital data to a suitable higher-level controller for evaluation. To create a history, the voltages recorded in the scale or the corresponding values can be saved, preferably with a time stamp and other additional information (e.g., temperature, humidity, air pressure, and various data derived from them), and output as needed.

[0026] Depending on the overload situation, it may be necessary to temporarily or permanently shut down the scale, or at least prevent weight output and, if necessary, activate additional protective measures. Alternatively, it may be necessary to set a new zero point for the scale or readjust the scale. These requirements can be displayed, requested, and / or automatically initiated or performed within the scope of the method according to the invention in order to ensure optimal operation of the scale, taking overload situations into account.

[0027] The scale according to the invention and the associated method can be advantageously used for quality assurance in production and / or for self-diagnosis in the field, for example to be able to make a comparison with a delivery state or historical data.

[0028] The circuit of a scale according to the invention will be described in more detail below with reference to three figures. Fig. 1 shows a first embodiment of the scale according to the invention with two separate A / D converters, Fig. 2 shows an alternative embodiment with only one A / D converter, and Fig. 3 shows a further embodiment of the scale according to the invention with two separate A / D converters and modified auxiliary measuring voltage.

[0029] Fig. 1 shows a schematic representation of the simplified circuit of a scale according to the invention in a first embodiment. An output stage E is designed to output a coil current IL, which is derived through a support coil L and a measuring resistor R connected in series with it. The coil L interacts with a magnet M and is arranged at the end of a lever K, which absorbs a force F introduced into the lever. By regulating the coil current IL, a counterforce is generated at the coil L, which holds the lever almost motionless in its desired position. With a larger force F, a higher coil current IL is necessary to generate the counterforce, while a smaller force F requires a correspondingly lower coil current IL (the coil current can be unipolar or bipolar).

[0030] To determine the force F, the voltage UR dropped by the coil current IL across the measuring resistor R is tapped, amplified and / or conditioned in an amplifier / filter V 1 and fed to a first A / D converter AD 1 with a measuring range M 1. The A / D converter AD 1 outputs a digital value corresponding to the measuring resistor voltage UR to a control unit C as long as the measuring resistor voltage UR remains within the measuring range M 1. Since the measuring resistor voltage UR is a measure of the force F, this force can be determined by evaluating the data transmitted to the control unit C.

[0031] At the output A of the output stage E, an output stage voltage UE is present, which in this embodiment corresponds to the auxiliary voltage UH detected according to the invention. The auxiliary voltage UH is in turn amplified and / or conditioned by an amplifier / filter V 2 and fed to a second A / D converter AD 2 , the measuring range M 2 of which is either larger than the measuring range M 1 of the first A / D converter AD 1 or is connected to it in such a way that a voltage lying outside the measuring range M 1 of the first A / D converter AD 1 lies within the measuring range M 2 of the second A / D converter AD 2 . The signals output by the second A / D converter AD 2 are also fed to the control unit C for evaluation.

[0032] As long as the measuring resistor voltage UR remains within the measuring range M 1 , a weight value corresponding to the force F can be determined from the signals provided by the A / D converter AD 1 in the control unit C. However, if the force F increases to such an extent that the measuring resistor voltage UR lies outside the measuring range M 1 , the force F can no longer be quantified using the A / D converter AD 1 . The auxiliary voltage UH , which is recorded in addition to the measuring resistor voltage UR and which also increases with increasing force F, can, however, be recorded and quantified via the second A / D converter AD 2 as long as the auxiliary voltage UH lies within the measuring range M 2 of the second A / D converter AD 2 . As the force F increases, the auxiliary measuring voltage UH will then continue to rise until a) the maximum output voltage (modulation) of the power stage E is reached, so that it can no longer provide sufficient coil current IL to generate sufficient counterforce in the coil L and to maintain the lever K in the desired position, or b) a mechanical overload protection T is triggered, so that the coil current IL is set to a constant value.

[0033] In both cases, the overload of the scale can be determined by the fact that the measuring resistor voltage UR leaves the measuring range of the A / D converter AD 1 or exceeds a threshold that is shortly before that, or the A / D converter outputs a corresponding signal. In the aforementioned case a), the force triggering the overload protection can only be determined qualitatively because the power stage E has reached its load and the auxiliary voltage UH no longer has a defined relationship with the force F or exceeds the measuring range M 2. In case b), however, the fact that the overload protection T has been triggered can be determined from the temporal characteristic of the auxiliary measuring voltage UH and the resulting maximum value of this voltage, and the triggering force can be determined quantitatively because the auxiliary measuring voltage UH is still within the measuring range M 2.

[0034] Fig. 2 shows a opposite Fig. 1 A modified solution according to the invention, in which the first A / D converter AD 1 is used selectively to detect the measuring resistor voltage UR or the auxiliary voltage UH. If the measuring resistor voltage UR supplied to the A / D converter AD 1 exceeds the measuring range M 1 or a definable threshold shortly before it, the A / D converter AD 1 is switched (or switches itself) to detect the auxiliary voltage UH instead of the measuring resistor voltage UR.

[0035] By switching the signal source applied to the input of the A / D converter AD 1, the measuring range of the converter can also be switched (preferably internally by the converter itself). For example, the sensitivity of the converter can be halved, thus doubling the measuring range. Alternatively, the gain V 2 of the auxiliary measuring voltage could be reduced sufficiently so that even the higher auxiliary measuring voltage lies within the measuring range M 1.

[0036] The switch W which causes the switching between the two voltages can be arranged outside or inside an A / D converter board, so that it can be controlled directly via the control of the A / D converter or, for example, by the control unit C.

[0037] In Fig. 3 is one opposite Fig. 1 modified embodiment of the invention. Unlike in Fig. 1 The auxiliary measuring voltage is not tapped between the output stage E and the coil L, but between the coil L and the measuring resistor R (depending on the point of application and the line resistance, the auxiliary measuring voltage UH can also differ from the measuring resistor voltage UR in this case). The A / D converter AD 2 can, as already mentioned, Fig.1 As described, the auxiliary measuring voltage is again recorded in a suitably amplified size and / or in a suitable measuring range M 2 and passed on to the control unit for evaluation.

Claims

1. Scale which operates according to the principle of electromagnetic force compensation and has a carrying coil (L), wherein the carrying coil (L) is movable relative to a magnet (M), and a coil current (IL) can be supplied to the carrying coil from an output stage (E), wherein a) the carrying coil (L) is connected in series with a measuring resistor (R), and b) the measuring resistor voltage (UR) dropping across the measuring resistor (R) is used to determine a weight measured value, characterized in that c) the scale is designed to record and separately evaluate an auxiliary measurement voltage (UH) with respect to the electrical ground, in addition to recording and evaluating the measuring resistor voltage (UR), in order to detect an underload and / or overload, wherein the auxiliary measurement voltage (UH) is tapped d1) between the output (A) of the output stage (E) and the coil (L), or d2) between the coil (L) and the measuring resistor (R).

2. Scale according to claim 1, characterized in that the measuring resistor voltage (UR) can be supplied to a first A / D converter (AD1), and the auxiliary measurement voltage (UH) a) can either be supplied to the first A / D converter (AD1) by switching the input of the first A / D converter (AD1) between the measuring resistor voltage (UR) and the auxiliary measurement voltage (UH), or b) can be supplied to a second A / D converter (AD2) provided in addition to the first A / D converter (AD1).

3. Scale according to claim 2, variant a), wherein i) the auxiliary measurement voltage (UH) supplied to the first A / D converter (AD1) is subject to a lower amplification (V2) than an amplification (V1) of the measuring resistor voltage (UR), and / or ii) the measuring range (M1) of the first A / D converter (AD1) for the measurement of the auxiliary measurement voltage (UH) is selected to be different, preferably larger, than for the measurement of the measuring resistor voltage (UR) before switching.

4. Scale according to claim 2, variant b), wherein i) the auxiliary measurement voltage (UH) supplied to the second A / D converter (AD2) is subject to a lower amplification (V2) than an amplification (V1) of the measuring resistor voltage (UR), and / or ii) the measuring range (M2) of the second A / D converter (AD2) for the measurement of the auxiliary measurement voltage (UH) is selected to be different, preferably larger, than the measuring range (M1) of the first A / D converter (AD1) for the measurement of the measuring resistor voltage (UR).

5. Scale according to any of the preceding claims, characterized in that a control unit (C) is provided for outputting an overload signal if the auxiliary measurement voltage (UH) exceeds a specifiable limit value.

6. Scale according to any of the preceding claims, characterized in that a mechanical overload protection means (T) is provided, wherein the scale is designed a) to generate a signal corresponding to a triggering force of the overload protection means or a value derived therefrom by evaluating the auxiliary measurement voltage (UH), and / or b) to detect the triggering of the overload protection means by comparing the auxiliary measurement voltage (UH) to one or more specifiable threshold values and / or by analyzing the temporal profile of the auxiliary measurement voltage (UH).

7. Scale according to any of the preceding claims, characterized in that a) recorded voltages (UH, UE, UR) or signals generated therefrom can be stored in and retrieved from a memory, and / or b) in that the point in time and / or the number, type, level and duration of instances of exceeding specifiable voltage limit values, or the number of generated signals or recorded measured values can be recorded and / or evaluated and / or output by a control unit (C).

8. Method for detecting an underload and / or overload on a scale according to any of the preceding claims, comprising the following method step: a) recording and evaluating the auxiliary measurement voltage (UH) and / or the point in time, number, type, level and / or duration of the instances of exceeding specifiable voltage limit values.

9. Method according to the preceding claim, further comprising b) outputting an overload signal if the auxiliary measurement voltage (UH) exceeds one or more specifiable limit values.

10. Method according to either of the two preceding claims, further comprising controlling the scale c) to output and / or display and / or store the auxiliary measurement voltage and / or a value corresponding to the auxiliary measurement voltage (UH) and / or the point in time, number, type, level and / or duration of the instances of exceeding the at least one limit value, and / or d) to interrupt the regular weighing operation and / or to prevent the output of a weight value for a duration which is specifiable in each case, and / or e) for zeroing and / or adjusting the scale.

11. Method for checking an overload protection means in a scale according to any of claims 1 to 8, comprising the following steps a) recording and quantitatively evaluating the auxiliary measurement voltage (UH) in order to determine a value corresponding to the triggering force of the overload protection means, to evaluate the value, and to adjust the overload protection means as required, b) recording and qualitatively evaluating the auxiliary measurement voltage (UH) by comparison with one or more specifiable limit values in order to determine the occurrence of the overload protection means being triggered.