Abstract:
The method and the device serve to monitor the alignment of a measuring instrument, specifically a balance. To perform this function, the monitoring device is equipped with an inclination sensor based on the principle of a spirit level, with a container filled with a fluid in which a bubble is formed. According to the invention, the position of the bubble is measured optically by means of a radiating element that is arranged on one side of the bubble and serves to emit a radiation, and a sensor element that is arranged on the opposite side of the bubble and serves to receive the radiation. The radiating element, preferably a light-emitting diode, and the sensor element, preferably a photodiode, together define the sensor axis (sx) on which the bubble is centered as long as the sensor axis runs parallel to the axis of the gravity force. Furthermore, the sensor element is flanked by at least two reference elements that are likewise receiving the radiation. The reference elements serve to test whether the intensity of the radiation is within a permissible range. A function test is performed automatically inside the monitoring device in order to verify that the monitoring device is functioning correctly.

Description:
BACKGROUND  
       [0001]     The present invention relates to a method and a device for monitoring the alignment of a measuring instrument, wherein the monitoring device is equipped with an inclination sensor. The invention further relates to a measuring instrument, specifically a balance, which is equipped with the monitoring device.  
         [0002]     Measuring instruments, in particular gravimetric measuring instruments such as for example thermo-gravimetric instruments, gravimetric moisture-determination instruments, or balances, have to meet special requirements in regard to how they are set up at the place where they are used. This applies in particular to balances equipped with a weighing cell and a load receiver which are used for the gravimetric measurement of weights. Ideally, a balance is set up in a position where the measurement axis of the balance—i.e., the axis that should coincide with the line of action of the weight force of a weighing object to be measured—runs in the direction of the gravity field. This ideal position can also be called the reference position of the balance. If the measurement axis of the balance, which is normally perpendicular to the plane of the weighing pan, is inclined at an angle relative to the gravity field, the weighing result will have a value that reflects the actual weight of the object multiplied by the cosine of the angle of inclination.  
         [0003]     This is the reason why balances that conform to the requirements for official certification are often equipped with an inclination sensor and with a leveling device that allows the balance to be set to the reference position which is indicated by the inclination sensor. The sensor signals of an electrical inclination sensor, which are for example delivered to a display unit, indicate by how much the sensor axis which is normally aligned with the measurement axis of the balance deviates from the direction of the gravity field. The leveling device, which includes for example two axially adjustable feet of the balance, allows a deviation of the sensor axis and thus of the measurement axis from the gravity axis to be corrected.  
         [0004]     A balance with an electrical inclination sensor is disclosed in DE 32 34 372 A1 [1], where the sensor signals are not used to correct the position of the balance, but to digitally correct the inclination-dependent error of the balance. According to [1], the inclination sensor can have either a pendulum mass or a partially filled liquid container with a gas bubble, where the position of the movable element is detected by optical or inductive means.  
         [0005]     An electrical inclination sensor consisting of a sprit level with a container holding a partial filling of an electrically conductive liquid with a gas bubble is disclosed in JP 61 108 927 A2 [2]. According to [2], this inclination sensor is used in a balance and triggers an acoustical alarm when the out-of-level condition reaches a limit value.  
         [0006]     The principal construction of a spirit level is described in detail in DE 38 00 155 A1 [3]. With the concept proposed in [3] the angle of inclination can be read directly from the spirit level.  
         [0007]     An arrangement is disclosed in JP 58033114 with a spirit level that has a light-emitting element on one side and a plurality of optical sensors on the other side. A light-transmitting colored liquid is enclosed in a hemispherical container in such a way that a bubble is formed. The container itself is enclosed in a cube-shaped transparent housing. The optical sensors are arranged on the outside of the cube-shaped housing. The light rays from the light-emitting element pass through the liquid and the bubble and fall on the optical sensors. If the spirit level is put in an inclined position, the bubble moves out of place and the signals of the optical sensors change. Both the angle and the direction of the inclination are detected with this device.  
         [0008]     An optoelectronic inclination measuring system with a deformable pendulum configured as a dual parallel spring linkage that cooperates with an emitter and a receiver unit is described in DE 43 16 046 C1. In addition to a sensor diode, there can be reference diodes arranged on the receiver side to detect and compensate the effects of undesirable extraneous factors such as changes in temperature and voltage.  
         [0009]     An inclinometer of very high sensitivity which works in two dimensions is described in DE 199 31 679 A1. It has a spirit level with a light source arranged at the underside. An optoelectronic sensor, preferably of a type that is based on CCD (charge-coupled device) technology, extends across the top of the spirit level.  
         [0010]     If the balance is not provided with an automatic inclination-monitoring arrangement, it is a requirement in weighing procedures that are relevant to product quality to inspect the spirit level and verify the leveled position of the balance before the weighing process is started. However, this rule is not always adhered to in practice. With automatic monitoring, on the other hand, there can be failures in the monitoring device. It is possible on the one hand that an alarm is triggered although the angle of inclination has not passed its prescribed limit, and on the other hand it can occur that the limit is exceeded even for a long period of operation without triggering an alarm. Both kinds of errors can have serious consequences. With the first kind of error, the false alarms which are also referred to as “false positives” can cause an unnecessary interruption of the measurement or production process. With the second kind of errors, also referred to as “false negatives”, the measurements and/or production processes continue in spite of the fact that the prescribed tolerance limits have been exceeded.  
       SUMMARY  
       [0011]     The present invention therefore has the objective to provide an improved method and an improved inclination-sensing device for monitoring the condition of a measuring instrument, specifically of a balance, and to provide a measuring instrument, specifically a balance, that is equipped with the monitoring device.  
         [0012]     In particular, the objective of the present invention calls for a method and a monitoring device that ensure a precise detection when at least one limit value for the inclination angle is exceeded and substantially avoid the problem of false messages or false alarms.  
         [0013]     In addition, the monitoring device should have a simple configuration and a cost-effective design and it should be simple to use in a balance or in other measuring devices. Furthermore, the monitoring performance should not be negatively affected by changes in the properties of components such as electrical and optical elements nor by extraneous factors such as stray light.  
         [0014]     The measuring instrument according to the invention should therefore receive an optimal monitoring surveillance in regard to an out-of-level position.  
         [0015]     A solution that satisfies the foregoing objectives is provided through a method, a monitoring device, and a measuring instrument with the features specified, respectively, in claims  1 ,  8  and  14 . Advantageous further developed embodiments of the invention are defined in additional claims.  
         [0016]     The method and the device serve to monitor the alignment of a measuring instrument, specifically a balance. To perform this function, the monitoring device has an inclination sensor based on the principle of a spirit level, i.e., with a container that is partially filled with a liquid so that a bubble is formed.  
         [0017]     The position of the bubble is measurable optically by means of a radiation-emitting element on one side of the bubble and a radiation-sensing element on the opposite side of the bubble. The emitter element, preferably a light-emitting diode, and the sensor element, preferably a photodiode, define a sensor axis that passes through the center of the bubble if the sensor axis if parallel to the direction of the gravity field.  
         [0018]     Furthermore, at least two radiation-sensing reference elements are arranged laterally flanking the sensor element to verify that the radiation intensity is within a permissible range. To check the monitoring device, a function test is performed automatically within the monitoring device.  
         [0019]     Thus, it is possible to detect and correct changes in the intensity of the radiation emitted by the emitter element and/or received by the reference elements, for example due to a change in the power supplied to the emitter element or due to a color change of the elements in the light path. If the light intensity from the emitter element is, e.g., too low, the foregoing arrangement prevents the problem that the bubble is erroneously assumed to be lying in the sensor axis and to be attenuating the light from the emitter element while the bubble is in fact in a position outside the sensor axis. In other words, a situation is prevented where the inclination is erroneously registered as being within the tolerance range. Conversely, if the light intensity from the emitter element is too high, the inventive arrangement also prevents the problem that the bubble is erroneously assumed to be lying outside the sensor axis where it would not attenuate the light from the emitter element while the bubble is in fact centered on the sensor axis. Thus, the inventive arrangement also prevents a situation where the inclination is erroneously registered as being outside of the tolerance range.  
         [0020]     The inclination sensor which is of an uncomplicated design as described above can thus register when the intensity of the radiation from the emitter is too high or too low, and this can be corrected by the simple measures which will now be described.  
         [0021]     The electronically monitored inclination sensor or spirit level can be arranged in an enclosed space inside the housing of the measuring instrument. With this arrangement, the inclination sensor is isolated from the outside light. However, the position of the bubble can no longer be verified by visual inspection.  
         [0022]     In a preferred embodiment, the inclination sensor or spirit level is arranged on the housing in such a way that the position of the bubble can be visually verified, so that the electronic monitoring is supplemented with the possibility of a visual verification by the user. Because the radiating element is of a small size, it does not stand in the way of the visual inspection of the bubble. A possible interference from outside light with the electronic monitoring of the bubble is preferably prevented by selecting radiating, sensing, and reference elements working in a range of wavelengths such as for example the infrared range which lies substantially outside of the range of the interfering outside light.  
         [0023]     Extraneous influences on the measurement can be further suppressed by emitting the radiation in the form of periodic or aperiodic pulses, preferably in intervals of 5 to 15 milliseconds and with a pulse width of 5 to 15 microseconds. For example, the pulses could follow each other at constant or slightly fluctuating period intervals of 10 milliseconds and have a pulse width of 5 microseconds. The desired radiation intensity is set by changing the height of the pulses. Interfering signals of a periodic nature can further be suppressed by using a fluctuating period length.  
         [0024]     The reference elements are arranged preferably along a straight line that also runs through the sensor element, so that the reference elements are not receiving rays that have passed through the bubble if the latter is centered on the sensor axis. The sensor element therefore delivers a sensor signal corresponding to a radiation intensity that is attenuated by the bubble, while the reference elements deliver sensor signals corresponding to a radiation intensity that is not attenuated by the bubble. The radiation intensity indicated by the reference elements can therefore be used as a reference for correcting the radiation intensity. With a more complex electronic circuit, the signal of the sensor element could also be normalized, i.e., measured as a ratio of a signal produced by the reference elements which represents the non-attenuated radiation level, so that the result would be a signal that is independent of the intensity of the radiation produced by the radiation-emitting element.  
         [0025]     Most advantageously, however, the signals emitted by the sensor element and the reference elements are evaluated by means of comparators. According to this concept, the output signal of the sensor element is compared to a first threshold value corresponding to the distance of the bubble from the sensor axis which delimits the permissible range of inclination. The output signals of the two reference elements are compared by means of a window comparator to a lower, second threshold value and an upper, third threshold value, where the latter two threshold values define the permissible range of the radiation intensity and thus the range of electrical power to be supplied to the radiation-emitting element.  
         [0026]     The output signals of all comparators are periodically interrogated and evaluated in a processor, wherein preferably 
        each time a first threshold value which indicates an out-of-level condition is not attained or is exceeded, an inclination counter is changed, respectively, towards a first or second limit value, and/or     each time a second threshold value which discriminates between a sufficient and an insufficient intensity of the radiation is not attained, an intensity counter is changed in the direction from a third towards a fourth limit value, and/or     each time the third threshold value which indicates an excessive intensity of the radiation is exceeded, the intensity counter is changed in the direction from the fourth towards the third limit value, and/or     after an error has been registered during a test sequence, a function counter is changed in the direction towards a fifth limit value, and/or     after an error has been registered during the inclination measurement, the function counter is changed in the direction towards the fifth limit value, or an error counter is changed in the direction towards a sixth limit value.        
 
         [0032]     With the use of the inclination counter, the intensity counter, the function counter and the error counter, it is possible to suppress momentary irregularities that may in some cases occur only once, so that unnecessary error messages are avoided.  
         [0033]     When the first or second limit value is reached, a signal is given that the inclination is within or outside of the tolerance range, and a measurement or production process may be stopped if necessary.  
         [0034]     When the third or fourth limit value is reached, the intensity of the radiation or, more specifically, the electric power supplied to the radiation-emitting element is changed as needed to bring the radiation intensity back into a permissible range.  
         [0035]     When the fifth or sixth limit value is reached, a signal is triggered to indicate the error condition, and if a measurement or production process is underway, it may be stopped if necessary.  
         [0036]     Of course, it is also possible to process the error messages without filtering them.  
         [0037]     All of the filter functions described above can be realized inexpensively by means of a software program. All of the limit values are preferably stored in an electronic memory and selectively changeable. The threshold values are preferably adjustable selectively by means of resistors that can be controlled by the processor, for example transistors. Likewise, the operating voltage that is applied to the radiation-emitting element in the form of pulses is preferably controllable by way of the processor. 
     
    
     BRIEF DESCRIPTION  
       [0038]     A more detailed description of the invention is presented below with reference to the drawings, wherein:  
         [0039]      FIG. 1  represents a balance  1000  in accordance with the invention, with an inclination sensor  1  that is integrated in the balance housing in such a way that it is visible to the user;  
         [0040]      FIG. 2  represents an inclination sensor  1 ′ whose sensor axis sx is aligned with the axis gx of the gravity force and which consists of a spirit level with a cylindrical container  10  that is closed off at both ends by transparent plates and filled with a liquid  11  leaving a bubble  12 , with a radiation-emitting element D 1  arranged above and a sensor element D 2  arranged below the inclination sensor;  
         [0041]      FIG. 3  represents the inclination sensor  1 ′ of  FIG. 2  in an inclined position tilted to the right;  
         [0042]      FIG. 4  represents an inclination sensor  1  in accordance with the invention consisting of a spirit level analogous to  FIG. 2 , with a radiation-emitting element D 1  arranged above and a sensor element D 2  as well as two reference elements D 3 , D 3 ′ arranged below the inclination sensor;  
         [0043]      FIG. 5  represents the inclination sensor  1  of  FIG. 4  in an inclined position tilted to the left;  
         [0044]      FIG. 6  represents a circuit arrangement connected to the inclination sensor  1  of  FIG. 4 ;  
         [0045]      FIG. 7  represents the time profile of the input signals u E11 , u E12  of a first comparator CMP 1  assigned to the sensor element D 2  in the circuit arrangement of  FIG. 6  when the inclination sensor  1  is in the leveled position shown in  FIG. 4 ;  
         [0046]      FIG. 8  represents the time profile of the input signals u E11 , u E12  of the first comparator CMP 1  when the inclination sensor  1  is in the position shown in  FIG. 5 ;  
         [0047]      FIG. 9  represents a typical time profile of the respective input signals u E21 , u E22  and u E31 , u E32  of a second comparator CMP 2  and a third comparator CMP 3  that are assigned, respectively, to reference elements D 3  and D 3 ′;  
         [0048]      FIG. 10  represents the monitoring device according to the invention with a processor  4 , which receives the output signals of the inclination sensor  1  by way of a comparator stage  3  that serves as an A/D converter, which is further connected by way of a D/A converter  2  to the radiation-emitting element D 1  of the inclination sensor  1 , and which is also connected to an input/output unit  5 ;  
         [0049]      FIG. 11  represents a first flowchart diagram which illustrates the operating steps that occur in the processor  4 ;  
         [0050]      FIG. 12  represents a second flowchart diagram which illustrates the operating steps in the sensor test that is shown in  FIG. 11 ; and  
         [0051]      FIG. 13  represents an inclination sensor  1  of a second preferred configuration. 
     
    
     DETAILED DESCRIPTION  
       [0052]      FIG. 1  illustrates a balance  1000  according to the invention with an inclination sensor  1  which is integrated in the balance housing  1001  in such a way that it is visible to the user. The inclination sensor  1 , which operates according to the principle of a spirit level, is part of a monitoring device according to the invention. Thus, while the output signals of the inclination sensor  1  are processed in the monitoring device by means of a processor, the inclination can also be monitored through visual inspection by the user. Error messages of the monitoring device can therefore be verified easily through one glance at the spirit level or the inclination sensing device  1  and as a result of this arrangement, the operating convenience of the balance  1000  is enhanced. An out-of-tolerance inclination of the balance can be corrected by means of height-adjustable set-up feet  1002 .  
         [0053]      FIG. 2  schematically illustrates an inclination sensor  1 ′ whose sensor axis sx is aligned with the axis gx of the gravity force and which consists of a spirit level with a cylindrical container  10  that is partially filled with a liquid  11  so that a bubble  12  is formed. A radiation-emitting element D 1  is arranged on top of the inclination sensor, and a sensor element D 2  is arranged at the underside. The same inclination sensor  1 ′ is shown in  FIG. 3  tilted to the right at an angle α, so that the sensor axis sx is inclined by the angle α in relation to the gravity axis gx. The container  10  is closed off at both ends by transparent plates. The inside wall of the upper plate where the bubble is floating is slightly curved. In  FIG. 2 , the bubble  12  is located on the sensor axis sx which is defined by the positions of the radiation-emitting element D 1  and the sensor element D 2 . In  FIG. 3 , the bubble  12  has moved to the left in response to the tilting of the container  10 . In the situation illustrated in  FIG. 2 , the radiation emitted by the radiating element D 1  therefore passes through the bubble and is attenuated by the effects of refraction and reflection. In contrast, in the situation shown in  FIG. 3 , the radiation is not attenuated by the bubble  12 , so that the sensor element D 2  produces a stronger output signal. Consequently, the output signal of the sensor element D 2  can be presented to a comparator, for example the comparator CMP 1  shown in  FIG. 6 , where the output signal is compared to a threshold value u E11  which is selected so that the output signal of the sensor element D 2  lies below the threshold value u E11  if the radiation received by the sensor has been attenuated by the bubble, and above the threshold value u E11  if the radiation received by the sensor has not been attenuated by the bubble. Thus, a logic level  1  of the output signal u OUT1  of the comparator CMP 1  indicates an angle position of the inclination sensor  1  corresponding to a leveled condition of the balance  1000  that is within the tolerance range, while a logic level  0  indicates an out-of-tolerance inclination.  
         [0054]     Comparators of this type are described in reference [4], U. Tietze, Ch. Schenk, Halbleiterschaltungstechnik (Semiconductor Circuit Design), 11 th  edition, 2 nd  printing, published by Springer Verlag, Berlin 1999, pages 610-612.  
         [0055]     However, if there is a change in the radiation intensity of the radiating element D 1 , for example due to a change in the energy supply, temperature-dependent properties of components, increased attenuation along the path of the radiation caused for example by color changes of the transparent plates, or due to changes of the characteristics of the sensor element D 2 , it is possible that the output signal of the sensor element D 2  changes to such an extent that the inclination of the balance  1000  can no longer be monitored correctly on the basis of the given threshold value u E11 . To correct the situation, one could consider readjusting the threshold value u E11 , but this solution would involve considerable complexity and expense.  
         [0056]     The invention therefore calls for the use of an inclination sensor  1  consisting of a spirit level in accordance with  FIG. 2 , which has a radiation-emitting element D 1  arranged on top and a sensor element D 2  as well as two reference elements D 3 , D 3 ′ arranged at the underside. As in the arrangement described in the preceding paragraph, the sensor axis sx is defined by the radiating element D 1  and the sensor element D 2 . As long as the sensor axis sx is in approximate alignment with the axis gx of the gravity force, the bubble  12  lies on the sensor axis. The reference elements D 3 , D 3 ′ are arranged on either side of the sensor element D 1 , preferably at locations where the radiation received from the radiating element is not attenuated by the bubble  12  when the latter is centered on the sensor axis.  
         [0057]     To show an example,  FIG. 4  further illustrates parts of the container  10  which has an upper glass plate  101  and a lower glass plate  102  that are held in a cylindrical tube section  103  and enclose the liquid  11  in which a bubble  12  is formed. The lower glass plate  102  is further overlaid with a light barrier  14  with openings that allow radiation emitted by the radiating element D 1  to fall on the sensor D 2  and reference elements D 3 , D 3 ′ but block the passage of extraneous light coming from other directions. As another known possibility of reducing the effects of extraneous light, the radiating element D 1  and/or the sensor element D 2  and reference elements D 3 , D 3 ′ can be equipped with an optical filter of a narrow bandwidth.  
         [0058]      FIG. 5  shows the inclination sensor  1  tilted to the left by the angle α, so that the bubble  12  moves to the right and the radiation from the radiating element D 1  arrives at the sensor element substantially without being attenuated. The bubble now lies in the light path from the radiating element D 1  to the second reference element D 3 ′. The first reference element D 3  still receives the non-attenuated level of radiation. The reverse conditions apply if the inclination sensor  1  is tilted to the right by the same angle α, in which case the second reference element D 3 ′ receives the radiation from the radiating element D 1  substantially without attenuation.  
         [0059]     Thus, the intensity of the radiation received can be monitored by means of the reference elements D 3 , D 3 ′. By comparing the output signals of the reference elements D 3 , D 3 ′ to the two threshold values u E21 , u E31  (shown as input voltages to the comparators CMP 2 , CMP 3  in  FIG. 6 ) it can be verified whether the intensity of the radiation is within a permissible range. If one of the output signals of the reference elements D 3 , D 3 ′ exceeds the higher threshold value u E31 , the radiation intensity is too high. If the output signals of both of the reference elements D 3 , D 3 ′ fall short of the lower threshold, then the radiation intensity is too low. Based on this evaluation of the radiation intensity, it is possible to make a correction if necessary.  
         [0060]     As a means for correcting the radiation intensity, the sender module  100  in the circuit arrangement of  FIG. 6  includes a controllable current source  111  which supplies the radiating element D 1  with an operating current i D1  in the form of pulses whose length and period interval depend on a control signal u PT  and whose pulse height depends on a reference voltage U REF . The reference voltage U REF  is provided by a voltage source  110  which has a switch S 1  that is controlled by means of a control signal u PWM  which charges and discharges a capacitor C 1  through a resistor R 1  in accordance with the duty cycle ratio of the control signal u PWM  which is delivered at the first output terminal of a processor  4 . By changing the duty cycle ratio or, in other words, by modulating the pulse width of the control signal u PWM , the capacitor C 1  is charged to the required control voltage U ST . At periodic or aperiodic intervals of preferably 5 to 15 milliseconds, the control signal u PT  coming from a second output terminal  412  of the processor  4  switches the current source  111  on and off to release a pulse with a pulse width in the range of 5 to 15 microseconds. Particularly preferred are a period interval of about 10 milliseconds and a pulse duration of 8 to 10 microseconds. With coordinated, slightly delayed timing, the processor  4  interrogates the outputs of the comparators CMP 1 , CMP 2  and CMP 3  (as well as the comparators CMP 2 ′ and CMP 3 ′ for the second reference element D 3 ′ which are not shown in the drawing), which are connected to the processor  4  through respective inputs  421 ,  422 ,  423 ,  424  and  425 . The comparator CMP 1  receives the output signal of the sensor element D 2 , while the comparators CMP 2 , CMP 3  receive the output signal of the reference element D 3  and the comparators CMP 2 ′, CMP 3 ′ receive the output signal of the reference element D 3 ′. The comparator modules  200 ,  300  and  300 ′ (module  300 ′ indicated only in a schematic manner) in  FIG. 6 , perform an analog/digital conversion of their respective input signals, while the sender module  100  performs a digital/analog conversion of the signal coming from output terminal  411  of the processor  4 .  
         [0061]      FIG. 6  also schematically illustrates the radiation-emitting element D 1  which is tied into the circuit of the sender module  100 , the sensor element D 2  which is tied into the circuit of the first comparator module  200 , and the reference element D 3  which is tied into the circuit of the second comparator module  300 .  
         [0062]     The output signal of the sensor element D 2  is transmitted through the R/C high-pass filter with the resistors R 2 , R 3  and the capacitor C 2  to the inverting input of the first comparator CMP 1 , whose non-inverting input is connected to a voltage divider formed of the resistors R 4  and R 5  which supplies the voltage u E  representing a first threshold value. The output of the first comparator CMP 1 , which shows a logic 0 if the first threshold value has been exceeded, is connected to the input  421  of the processor  4 .  
         [0063]     The output signal of the sensor element D 3  is transmitted through the R/C high-pass filter with the resistors R 6 , R 7  and the capacitor C 3  to the inverting inputs of the second comparator CMP 2  and third comparator CMP 3  which together form a window comparator (see [4], pages 611-612). The non-inverting inputs of the comparators CMP 2  and CMP 3  are connected to a variable voltage divider formed of the resistors R 8 , R 9 , R 11 , R 12 , R 14  and the variably controllable resistor R 10 . The variable voltage divider is configured in such a way that the non-inverting input of the second comparator CMP 2  receives the voltage u E21  representing a second, lower threshold value, while the non-inverting input of the third comparator CMP 3  receives the voltage u E31  representing a third, upper threshold value. The outputs of the second comparator CMP 2  and of the third comparator CMP 3 , which indicate whether the second or possibly also the third threshold value has been exceeded, are connected to the respective inputs  422  and  423  of the processor  4 .  
         [0064]     A condition where the third, upper threshold value has been exceeded implies that the lower, second threshold value has also been exceeded, so that the output signal u OUT2  of the second comparator should in this case likewise indicate a logic 0. If this is not the case, the evaluation of the input signals received by the processor  4  is preferably programmed to conclude the presence of an error (see Table “Evaluation of Comparator Output Signals” below).  
         [0065]      FIG. 7  illustrates the time profile of the current i D1  in the radiation-emitting element D 1  during the emission of a radiation pulse as well as the corresponding time profiles of the input signals u E11 , u E12  received by the first comparator CMP 1 , if the inclination sensor  1  is in the leveled condition shown in  FIG. 4 . After the steep rise of the current i D1  at the time T A , the voltage u E12  increases but does not rise above the voltage u E11  representing the first threshold value, so that the output signal u OUT1  of the comparator CMP 1  remains unchanged. Thus, at the time T B  the output of the first comparator CMP 1  is found to be at the logic level  1 .  
         [0066]      FIG. 8  illustrates the time profile of the current i D1  in the radiation-emitting element D 1  during the emission of a radiation pulse as well as the corresponding time profiles of the input signals u E11 , u E12  received by the first comparator CMP 1 , if the inclination sensor  1  is in the out-of-level condition shown in  FIG. 5 . In this case, the voltage u E12  at the inverting input of the first comparator CMP 1  at the time T C  rises above the voltage u E11  representing the first threshold value, so that at the time T B  the output of the first comparator CMP 1  is found to be at the logic level  0 .  
         [0067]      FIG. 9  represents a typical time profile of the respective input signals u E21 , u E22  and u E31 , u E32  at the second comparator CMP 2  and third comparator CMP 3 . A situation is illustrated where the voltage u E22  or u E32  (u E22 =u E32 ) at the inverting inputs of the comparators CMP 2  and CMP 3  at the time T D  rises above the voltage u E21  representing the second, lower threshold value, so that at the time T B  the output of the second comparator CMP 2  is found to be at the logic level  0 . However, the voltage level u E31  representing the third, upper threshold value is not exceeded, so that at the time T B  the output of the third comparator CMP 3  is found to be at the logic level  1  which indicates that the radiation intensity is within the prescribed range.  
         [0068]      FIG. 10  represents a block diagram of the monitoring device  150  according to the invention with the inclination sensor  1 , a comparator group  3  containing the comparator modules  200 ;  300 ,  300 ′, . . . , a D/A converter  2  serving for the control of the radiation-emitting element D 1 , and an input/output unit  5  connected to the processor  4  (man/machine interface MMI). The input/output unit  5  includes an arrangement of annunciator elements  51 ,  52 ,  53 , for example light-emitting diodes, an indicator unit  54 , for example a liquid crystal display, and an input unit  55 , for example a keyboard or a touch-sensitive display unit. The processor  4 , which is equipped with an operating program  43  stored in a memory unit  41 , can be constituted for example by the main processor of the balance  1000  or by a separate processor in which an applications program  42  is implemented that serves to evaluate the comparator signals u OUT1 , u OUT2  and u OUT3 , and which further serves to control the radiation-emitting element D 1 , to signal the condition of the balance  1000 , and if applicable to control measuring and production processes that are dependent on the condition of the balance.  
         [0069]      FIG. 11  shows a first flowchart diagram with the operating steps that need to be executed under the applications program  42 . Following a first wait cycle, a pulse is sent out at the time T A , whereupon at the time T B  the digital values of the comparator signals u OUT1 , u OUT2  and u OUT3  are taken in (as well as the comparator signals u OUT2  and u OUT3  of the second comparator module  300 ′ which is connected to the second reference element D 3 ′, analogous to the first comparator module  300  and therefore not detailed in the drawing). Subsequently, a status value which corresponds to the combination of detected comparator signals u OUT1 , u OUT2 , u OUT3 , u OUT2 , and u OUT3  and describes the status of the balance  1000  and the monitoring device  150  is looked up from a table.  
         [0070]     Following is an excerpt of this table with some typical combinations of the comparator signals u OUT1 , u OUT2 , u OUT3 , u OUT2  and u OUT3 . A logic value of 0 means in each case that the threshold monitored by the respective comparator CMP 1 , . . . , has been exceeded.  
                                                                   TABLE                           Evaluation of Comparator Output Signals                Comparator   Comparator   Comparator               Module 200   Module 300   Module 300′                Case   CMP1   CMP2   CMP3   CMP2′   CMP3′           #   u OUT1     u OUT2     u OUT3     u OUT2′     u OUT3′     Status                1   0   0   0   0   0   Intensity                               too high        2   0   0   1   0   0   Intensity                               too high        3   1   1   1   1   1   Intensity                               too low        4   0   1   0   0   1   Error        5   0   0   1   1   0   Error        6   0   0   1   1   1   Out-of-level                               condition        7   1   0   1   0   1   Level within                               tolerance       . . .   . . .   . . .   . . .   . . .   . . .   . . .       32                  
 
         [0071]     In the cases  1  and  2  of the table, the upper threshold value in one of the window comparators, i.e., the reference value of one or both of the comparators CMP 3  and CMP 3 ′ is exceeded, and as a result the radiation intensity is registered as being too high.  
         [0072]     In case  3 , none of the threshold values is exceeded and accordingly the radiation intensity is registered as being too low.  
         [0073]     In the cases  4  and  5 , one of the third, upper threshold values is found to be exceeded without a simultaneous finding that the traversing of the lower, second threshold value has triggered the respective comparator CMP 2  or CMP 2 ′. This indicates a malfunction in the comparator modules  200 ,  300 ,  300 ′.  
         [0074]     In case  6 , the first comparator CMP 1  has switched to 0 while one of the window comparators, in this case the comparator combination CMP 2 /CMP 3 , indicates that the radiation intensity lies within the prescribed range. This indicates an out-of-level condition.  
         [0075]     In case  7 , the first comparator CMP 1  has not switched to 0 because the bubble  12  has remained in a centered position relative to the sensor axis sx. This indicates a correctly leveled condition.  
         [0076]     To prevent false alarms after a single incidence of detecting a functional error, an out-of-level error or an out-of-tolerance intensity, the evaluation results are tallied by incrementing and decrementing individual counters, i.e., an inclination counter, an error counter, and an intensity counter. If the intensity is found to be too high or too low, the intensity counter is incremented or decremented and the error counter is decremented. Signals are turned on only when a limit value is reached in one of the counts, for example by switching the light-emitting diodes  51 ,  52 ,  53  which indicate, respectively, the conditions of INCLINATION IN TOLERANCE—INCLINATION OUT OF TOLERANCE—SYSTEM ERROR. If a limit value has been reached which indicates that the radiation intensity is outside the prescribed range, this condition is corrected by adjusting the supply of electrical power to the radiating element D 1 , i.e., the diode current i D1 .  
         [0077]     If the first wait cycle has not yet ended, the program loops through a test to determine whether a second wait cycle has ended. If this is the case, the function test is performed which is shown in a separate diagram in  FIG. 12  and which serves to verify whether the modules and components are functioning correctly.  
         [0078]     In performing the function test, a first step consists of setting the radiation intensity to zero (i D1 =0) or raising it only into a range where the comparators CMP 1 , . . . , are not yet allowed to switch their logic outputs. Consequently, if a comparator CMP 1 , . . . , switches its output in this test, this is noted as an error which causes a function counter to be decremented. In a second step, the radiation intensity is raised into a range in which at least the first comparator CMP 1  (inclination comparator) and the comparators CMP 2 , CMP 3  or CMP 2 ′, CMP 3 ′ of one of the window comparators should switch from logic 1 to logic 0. If the comparators CMP 2 , CMP 3  or CMP 2 ′, CMP 3 ′ fail to switch, an error is registered and the function counter is decremented. If no switching failure is found, the function counter is incremented. Subsequently, the content of the function counter is evaluated, and a function error is signaled if the respective limit value has been reached.  
         [0079]     The method according to the invention, the monitoring device  150  and the balance  1000  have been described in preferred embodiments. However, based on the concepts taught by the invention it is possible to realize further embodiments. In particular the inventive inclination sensor  1  can also be used with differently configured comparator circuits and evaluation programs. The evaluation of the comparator signals with the applications software  42  as described herein is particularly advantageous. However, by using the inventive concepts as a basis, individuals of ordinary skill in the art will be able to adapt this applications program to given requirements.  
         [0080]     To satisfy other design requirements, the inclination sensor or more specifically the spirit level can be realized in further configurations.  FIG. 13  illustrates a top view of an inclination sensor according to the invention, but slightly inclined towards the lower right. The bubble  12  still lies in the light path between the radiation-emitting element D 1  and the sensor element D 2  but has moved towards a control circle  13  which allows a visual determination whether the inclination is still within the permissible range, and which provides a visual reference for the magnitude of the inclination inside or outside the permissible range and for the direction of the inclination gradient. As mentioned above, there are special advantages in using a visual surveillance by the user to supplement the electronic monitoring, but the magnitude and gradient direction of the inclination can also be determined electronically by using at least one or two further reference elements D 32 , D 33 . As a preferred arrangement, the reference elements D 3  and D 3 ′ are arranged on a first measurement axis mx and the two further reference elements D 32 , D 33  are arranged on a second measurement axis my which runs perpendicular to the first measurement axis mx. As long as the bubble  12  moves along the second measurement axis my, the monitoring device registers a condition according to case # 7  in the foregoing evaluation table of comparator output signals, but a point may be reached where the first comparator CMP 1  switches from logic 1 to logic 0. The direction in which the bubble  12  has migrated along the second measurement axis my can now be determined from the further reference elements D 32 , D 33  and their associated comparators. For example, if the comparator that is connected to the reference element D 32  switches its logic level, the bubble  12  has migrated to a position between the radiating element D 1  and the reference element D 32 .  
         [0081]     The inclination sensor  1  and the monitoring device  150  can be used to particular advantage in a balance  1000 . However, as is self-evident, the inclination sensor  1  and the monitoring device  150  can also be used in any other kind of measuring apparatus.  
       List of Literature References  
       [0000]    
       
          [1] Published patent application DE 32 34 372 A1  
          [2] Published patent application JP 61 108927 A2  
          [3] Published patent application DE 38 00 155 A1  
          [4] U. Tietze, Ch. Schenk, Halbleiterschaltungstechnik, 11 th  Edition, 2 nd  Printing, Springer Verlag, Berlin 1999  
       
     
       List of Reference Symbols  
       [0000]    
       
           1  inclination sensor  
           10  container  
           11  liquid  
           12  bubble  
           13  control circle  
           14  light barrier  
           100  sender module  
           101  upper glass plate  
           102  lower glass plate  
           103  cylindrical tube section  
           110  voltage source  
           111  current source  
           150  monitoring device  
           1000  balance  
           1001  balance housing  
           1002  adjustable feet  
           1003  entry unit  
           2  digital/analog converter  
           200  first comparator module  
           3  analog/digital converter-comparator group  
           300 ,  300 ′ second comparator module  
           4  processor  
           41  memory unit  
           411 ,  412  output terminals  
           42  software application program  
           421 - 425  input terminals  
           430  bus/system bus  
           43  operating system  
           5  input/output unit  
           51 ,  52 ,  53  light-emitting diodes  
           54  indicating unit, liquid crystal display  
           55  keyboard, touch-sensitive display screen  
          C 1 , C 2  capacitors  
          CMP 1 , comparators, operational amplifiers  
          D 1  radiating element (light-emitting diode)  
          D 2  sensor element (photodiode)  
          D 3 , D 3 ′ reference elements (photodiodes)  
          R 1 , R 13  resistors  
          S 1  switch  
          T 1  switching transistor