WO2018069897A1

WO2018069897A1 - Measurement system and method for measuring displacements of a structure elements

Info

Publication number: WO2018069897A1
Application number: PCT/IB2017/056386
Authority: WO
Inventors: Artiom KOMARDIN; Przemyslaw GALAZKA
Original assignee: Sense Monitoring Spolka Z Ograniczona Odpowiedzialnoscia
Priority date: 2016-10-15
Filing date: 2017-10-14
Publication date: 2018-04-19
Also published as: PL240201B1; PL429602A1; PL419127A1

Abstract

According to the invention, the measurement system for measuring the displacements of a structure provided with a measurement device (la) with a central processing unit, a memory for storing measurement results, a communication unit for transferring the measurement results, a display, and a rangefinder with a measurement axis (01) directed toward a measurement target (3), in which the measurement device (la) and the measurement target (3) are adapted to be attached opposite each other, one of them to the element subjected to displacement (B) and the other one to the reference element (A) of the structure, is distinguished in that the rangefinder is a first laser rangefinder with the substantially horizontal measurement axis (01), the measurement device (la) is provided with a self-leveling system (2) adapted to be attached to elements of the structure, and the measurement target (3) is provided with a conical surface (6) with the substantially vertical axis of revolution (02) and an angle of opening ranging from 80o to lOOo. The measurement target (3) is provided with a self-righting system (5). According to the invention, the method for measuring the displacements of a structure, in which difference of the horizontal distance between the reference element and the movable element of the structure (4) is measured, is distinguished in that it is carried out using the measurement system according to the invention. The measurement device (la, lb) and the measurement target are attached to the movable element (B) of the structure (4) and the reference element (A) of the structure, respectively, and an alarm is triggered when the indication of the distance sensor (la) meets a predefined criterion.

Description

MEASUREMENT SYSTEM AND METHOD FOR MEASURING DISPLACEMENTS OF A STRUCTURE ELEMENTS

[ 0001 ] The invention relates to a measurement system for carrying out the method for measuring the vertical displacements of a structure and the measurement method, which are suitable for monitoring roof structures of large-area facilities, load tests of bridges, or for other types of such monitoring.

[ 0002 ] The vertical displacement of a structure, and especially the deflection of the structure, is one of the structure's safety criteria. Static calculations allow one to determine deflection limits (so-called service limit state) . Exceeding the deflection limits is unacceptable and can result in a construction disaster, a transport disaster, or a mine collapse.

[ 0003 ] The deflection of a structure, apart from the stresses (cross-sectional force densities), is an important measure of the structure's effort resulting from an external load. It is advantageous to reliably measure the deflection of the structure during its use to determine its current state of safety.

[ 0004 ] A periodic measurements of deflection values can be associated with a monitoring and/or early warning system, for example, to evacuate the facility or to perform snow removal.

[ 0005 ] One of the many sources of loads acting on a structure is snow. Incremental deflections from this kind of load are usually of several or over a dozen millimeters, which requires appropriate measurement precision.

[ 0006 ] Simple monitoring systems for measuring the deflections of structural elements at the center of their span, sometimes supplemented by measurements of the displacements of the support points of the controlled element, are known. Such displacements are usually measured by geodetic devices, such as total station, rangefinders , or hydrostatic level sensors.

[ 0007 ] Polish patent application No. 393402 discloses a method for monitoring the vertical components of the displacements of selected points and the vertical components of the deflection changes at those points of structural elements of a facility, particularly structural elements of a roof or their coverings or parts, consisting in that the distance, preferably vertical, from each monitored point of the structural element to a fixed element or a fixed base is measured, next, for each monitored point the vertical component of the displacement that has occurred since the monitoring initiation is calculated, and then for each monitored structural element the vertical component of the deflection change in the monitored point that has occurred since monitoring initiation is determined. The invention also relates to a system for implementing this method.

[0008 ] Also known is Polish patent application No. 381670. This invention relates to a method for monitoring structures, primarily roofs of industrial, storage, or service halls, and for early warning against overrunning limits of usage and load capacity. The method is characterized by sensors attached to the structural elements for recording states of these structural elements, the sensors being connect to a system capable of processing, transmission and visualization data from these sensors so that an alarm is triggered before the limits are overrun.

[ 0009 ] Polish patent application No. 381578 discloses a method consisting in directing the light ray below the beams of a roof structure. Deflection of at least one beam provided with a dedicated screen higher than a certain admissible value results in breaking the light ray and triggering a sound and visual alarm. The device consists of light rays transmitter and receiver installed under the roof at the opposite ends of the monitored facility. The beams are provided with the screens arranged on a straight line above the light ray. The receiver is connected to an alarm device.

[ 0010 ] Japan patent No. 8093230 discloses a method for monitoring the deformations of a roof structure with use of a number of laser rangefinders . The system is coupled with an early warning system. The rangefinders measure the mutual distances of structural elements of the roof.

[ 0011 ] Also known is the international patent application No. WO/2001/061301, in which the collected measurement data is transferred to a data processing unit, where the data is analyzed to obtain deformation information for various measurement points of the structure. The measurement data is collected and processed in real-time.

[ 0012 ] Solutions known in the art are capable of carrying out measurements in a discontinuous mode for detecting certain deflection of the structure, as well as in a continuous mode for continuous monitoring changes of a certain characteristic. These solutions use dedicated computer software that processes and archives the collected data.

[ 0013 ] In the known solutions, a certain deflection of a structure or change of a certain characteristic triggers an alarm signal in visual or audible form. This signal indicates the threat to the safety of the facility and consequently allows appropriate remedial actions, for example removing snow from the roof

[ 0014 ] A use of state-of-the-art solutions is difficult and requires the equipment to be set up by highly skilled personnel. Reliable reading of deformation magnitude depends on configuration of the measurement system and sometimes requires additional calculations. It may be cumbersome if the measurement system is arranged in a difficult-to-reach place and requires manual inspection. It may be especially cumbersome to use radio transmission for transferring measurement results from tight spaces with a high number of obstacles, e.g. from attics, spaces under bridges, or mines. In such places radio transmission can be easily disturbed and the reading of the measurement signal must be carried out by a human.

[ 0015 ] Setting up the measurement system in hard-to-reach places can also be very cumbersome if it requires determining and writing down the parameters and the complex positioning of the system components .

[ 0016 ] The object of the invention is to provide a measurement system and a measurement method characterized in that the system elements can be readily and quickly set up in the structure to be measured, and operation control and/or emergency reading by a human can by readily carried out without any processing of displayed data. The object of the invention is furthermore to limit the required frequency of emergency readings.

[ 0017 ] According to the invention, the measurement system for measuring the displacements of a structure provided with a measurement device with a central processing unit, a memory for storing measurement results, a communication unit for transferring the measurement results, a display, and a rangefinder with a measurement axis directed toward a measurement target, in which the measurement device and the measurement target are adapted to be attached opposite each other, one of them to the element subjected to displacement and the other one to the reference element of the structure, is distinguished in that the rangefinder is a first laser rangefinder with the substantially horizontal measurement axis, the measurement device is provided with a self-leveling system adapted to be attached to elements of the structure, and the measurement target is provided with a conical surface with the substantially vertical axis of revolution and an angle of opening ranging from 80° to 100°. The measurement target is provided with a self-righting system. The use of target with conical surface allows the target to be illuminated from many directions and reduces the problem of correctly setting the angle of the target in relation to the measurement axis to the problem of setting the vertical position. The self-leveling and self-righting systems solve the problem of setting the cone axis in relation to the measurement axis. With an angle of opening of the cone of 80° to 100°, the difference in measured horizontal distance corresponds to the vertical deformation of the structure under test with a construction tolerance of less than 20%.

[ 0018 ] Advantageously, the measurement target is further provided with a level arranged so that it indicates the horizontal position when the axis of revolution of the conical surface is positioned vertically. Providing the levels reduces the risk of improper mounting of the target by an unskilled worker, and allows for easy checking of whether the operating measurement system is not damaged or deformed, e.g. by hit or collision with another object.

[ 0019 ] Advantageously, the measurement target is provided with the first level and the second level, the levels being mutually perpendicular and perpendicular to the axis of revolution of the conical surface. Using two levels makes it easy to hang the target and to control the operation of the self-leveling system.

[ 0020 ] Advantageously, the measurement device is provided with a level arranged so that it indicates the horizontal position when the measurement axis of the laser rangefinder of the measurement device is positioned horizontally.

[ 0021 ] Even more advantageously, the measurement device is provided with a digital inclinometer and means for generating an alarm when the inclinometer indicates deviation from the horizontal position greater than 2°. With this solution, if the measurement device is hit and/or the self-leveling system fails, an alarm can be generated that will bring the worker to the repair. This solution decreases the frequency of necessary inspections. The alarm may be generated at the measurement device or alternatively transmitted via the communication module.

[ 0022 ] Advantageously, the communication unit of the measurement device is a radio transceiver utilizing Orthogonal Frequency- Division Multiplexing, OFDM. This modulation technique is resistant to multipath phenomena typical for closed spaces comprising obstacles that reflect and diffract radio waves.

[0023 ] Advantageously, the communication unit of the measurement device is a wired communication unit. Making wire connections is tedious and cumbersome, but wires are usually the most reliable medium, provided that they are not led through busy places where they may be subjected to mechanical damage.

[ 0024 ] Advantageously, the measurement system is provided with the second measurement device with the measurement axis set at an angle of 60° to 90° to the measurement axis of the first measurement device. The use of two measurement devices arranged in this way allows the detection of deformations at all angles even if the deformations are not vertical. With one measurement device, there is a non-vertical direction of deformation, which may be undetected.

[ 0025 ] Advantageously, the conical surface of the measurement target is provided with a scale. The scale allows one to note the position of a laser spot when the structure is not deformed and its element is not displaced, and then to perform an emergency reading on the basis of a change of the position of the laser spot on the conical surface.

[0026 ] According to the invention, the method for measuring the displacements of a structure, in which a measurement of the difference of the horizontal distance between the reference element and the movable element of the structure is performed, is characterized in that it is carried out using the measurement system according to the invention. The measurement device and the measurement target are attached to the movable element of the structure and the reference element of the structure, respectively, and an alarm is triggered when the indication of the distance sensor meets a predefined criterion. The predefined criterion is usually exceeding the maximal value, buy it is also possible to apply more complex criteria, taking into account time changes of the deformation . [ 0027 ] Advantageously, in the method the measurement system according to the claim 8 is used, and in the predefined criterion the difference of the indication of the first distance sensor in relation to the first reference value and the difference of the indication of the second distance sensor in relation to the second reference value are taken into consideration.

[ 0028 ] The use of the solution according to the invention allows one to measure the displacements of structural elements of a facility, road infrastructure, or even a mine, by measuring the horizontal distance between the fixed point of the structure, for example a column or a wall, with the vertical component of the deflection equal to zero, considered as a measurement base, and the observed point located on the deflected structural element, for example on the roof girder.

[ 0029] The solution according to the invention allows one to measure the displacements in a structure, in particular the deflections of roof girders of large-area facilities, which in turn allows one to determine the safety status of the facility in case of external loads, especially snow.

[ 0030 ] The measurement results can be processed, visualized, archived, transmitted via a monitoring system to inform the user of the facility about the current state of the structure's effort. The information obtained can be used to manage the facility, for example to indicate the need to remove snow from the roof, and to determine the safety of persons and property, for example to identify the need to evacuate the facility.

[ 0031 ] Embodiments of the present invention are described below with reference to attached drawing in which:

Fig. 1 schematically illustrates an embodiment of the measurement system according to the invention;

Fig. 2 schematically illustrates an alternative embodiment of the measurement system according to the invention;

Fig. 3, 4, and 5 illustrate detection of the deflection caused by snow in a structure provided with the embodiment of the system according to the invention shown in Fig. 1;

Fig. 6 schematically illustrates geometrical relationships of the measurement in the vertical cross-section comprising the measurement axis, in the embodiment of the system according to the invention shown in Fig. 1; Fig. 7 schematically illustrates geometrical relationships of the measurement in the vertical cross-section comprising the measurement axis, in the embodiment of the system according to the invention shown in Fig. 2;

Fig. 8 schematically illustrates an embodiment of the measurement system according to the invention with two measurement devices in a top view;

Fig. 9 shows a block diagram of the measurement device used in an embodiment of the measurement system according to the invention; Fig. 10 shows the target of the measurement system according to the invention .

[ 0032 ] Fig. 1 schematically illustrates a measurement system for carrying out the method for measuring vertical displacements of a structure composed of a measurement device la equipped with a laser rangefinder performing measurements along an axis 01 and a measurement target 3 with a conical surface 6 having a vertical axis of revolution 02. The measurement device la, the block diagram of which is shown in Fig. 9, is provided with a central processing unit 10 and the following elements connected to said central processing unit: a memory 11, a laser rangefinder 13, a self- leveling system 2, a display 14, an inclinometer 15, and a communication unit 12 capable of transferring measurement results to an external server. The measurement axis 01 of the rangefinder 13 of the measurement device passes through the conical surface 6 of the measurement target. Fig. 1 illustrates the measurement performed in the vertical plane defined by the horizontal measurement axis 01 and the vertical axis of revolution 02 of the conical surface 6. The measurement device la is attached via the self-leveling system 2 to the fixed reference element A. This element may be a part of the structure to be measured for deflection, or may belong, as in Fig. 1, to a fixed object independent of the structure. The measurement target 3 is attached to a movable element B of the structure 4 to be measured for deflection by means of a self-righting system 5. The angle of opening of the conical surface 6 is in the range of 80° to 100°, and in consequence an indication of the rangefinder differs by no more than 20% from the actual deflection measured with the measurement system.

[ 0033 ] Fig. 2 shows the measurement system in which the target 3 is attached to the reference element A and the measurement device la to the movable element B. The block diagram of the measurement device la used in the embodiments shown in Fig 1 and 2 is shown in Fig. 9. It is equipped with the central processing unit 10 adapted to perform arithmetic operations and read/write operations from peripheral devices. The communication unit 12, the memory 11, the display 14, the laser rangefinder 13, and the inclinometer 15 are connected to the central processing unit 10.

[0034 ] Fig. 3, 4 and 5 illustrate the method according to the invention employed in the measurement system configured as discussed above with reference to Fig. 1, in the situation where the roof structure is deflected under the weight of snow. Fig. 3 shows the measurement system in the situation of no snow, i.e. when the structure in not deflected. The measurement distance lo between the reference element A and the movable element B is measured with the measurement device la and stored in memory 11 for such a condition. Optionally, the measurement result may also be sent to an external server by means of the communication unit, either wired or wireless. The value lo of the distance measured in the absence of deflection is the reference measurement.

[ 0035 ] Fig. 4 shows the roof structure deflected under the weight of snow accumulated therein. As the measurement target 3 moves downward along the vertical axis 02, the measurement axis 01 moves over the conical surface 6 and pierces it at a different height than before. Indication of the laser rangefinder 13 changes from the value lo stored in the memory 11 to the measured value l_m. The difference between 1₀ and l_m is proportional to the amount of displacement, where the coefficient of proportionality is the tangent of the half of the angle of opening a of the conical surface 6 of the measurement target 3.

[ 0036 ] Fig. 5 shows a greater deflection of the roof structure due to the increase in the snow cover. The deflection exceeds the alarm threshold. The measurement result and/or the warning signal are sent to an external unit not shown in the figure via the transceiver 12 of the measurement device la. The warning signal is also displayed on the display 14 of the measurement device la. The criterion for triggering the alarm may be simply exceeding a certain value. This criterion may also include the rate of change - then the alarm will also trigger at lower deflections when the deflection grows faster than 10% of the maximum value per day. [ 0037 ] Fig. 6 presents in the form of a mathematical diagram the method for measuring vertical displacements of a structure employed in the measurement system illustrated in Fig. 1. In fig. 6, the measurement device la and the surface 6 are schematically shown in a cross-section by the vertical plane defined by the measurement axis 01 and the axis of revolution 02 of the conical surface 6. In this example, the measurement device la is attached to the reference element A. First, the reference horizontal distance lo from the element A to the point C of the intersection of the conical surface 6 with the measurement axis 01 is measured. After a vertical displacement of the structure along the displacement direction k indicated by an arrow occurs, the horizontal distance from the fixed point A to the point C' is measured, where the point C' is defined as the point in which the measurement axis 01 intersects with the conical surface 6 after the displacement. In this way another value of the distance l_m is obtained. Then the difference X between the reference value lo and the currently measured value l_m is calculated. This difference corresponds to the displacement of the structure with an accuracy of 20% due to the fact that the angle of opening of the surface 6 is in the range of 40° to 80 °.

[0038 ] Fig. 7 presents, also in the form of a mathematical diagram, the method for measuring the vertical displacements of a structure in the system illustrated in Fig. 2. In this example, the measurement target is attached to the measurement base. In fig. 7, the measurement device la and the surface 6 of the measurement target 3 are schematically shown in a cross-section by the vertical plane defined by the measurement axis 01 and the axis of revolution 02 of the conical surface 6. First, the horizontal distance from the first movable point B of the structure 4 to the point C of the intersection of the measurement axis 01 with the conical plane 6 of the measurement target 3 is measured, and thereby the reference value of the distance lo is determined. After a vertical displacement of the structure along the displacement direction k indicated by an arrow occurs, the horizontal distance from the movable point B' to the point C of the intersection of the measurement axis 02 with the surface 6 is measured, and thereby the current value of the distance l_m is determined. Then the difference X between the determined final value l_m and the reference value lo is calculated. This difference corresponds to the displacement of the structure with an accuracy of 20% due to the fact that the angle of opening of the surface 6 is in the range of 40° to 80°. As a result, the worker controlling the system can read the value l_m directly from the display 14 of the measurement device la.

[0039 ] Because the surface 6 of the target 3 is cone-shaped, the above method works properly independently of the direction of measurement - provided that the measurement axis 01 is horizontal and the axis of revolution 02 of the conical surface 6 of the target 3 is vertical.

[0040 ] The measurement device la is attached to structural elements by means of the self-leveling system 2, whereas the measurement target 3 is attached to structural elements by means of the self- righting system 5. Numerous examples of self-leveling and self- righting systems are known in state of the art and a person skilled in the art can easily utilize them. Self-leveling devices are even integrated in some of the laser rangefinders available on the market; so are logic, memory, and wired or wireless communication units .

[0041 ] In an alternative embodiment one can use a measurement device la with a simple rangefinder with no self-leveling system and an external self-leveling system, as it is disclosed in U.S. patent publication No. 20120128406. The self-leveling system disclosed in this document may also be used as a self-righting system by attaching it to the base of the conical surface 6.

[ 0042 ] Because the correct operation of the self-leveling and self- leveling systems has an impact on safety-critical values, it is advantageous to provide the measurement target 3 with a circular level or, even more advantageous, a first linear level 6a and a second linear level 6b perpendicular to the first one, both levels being perpendicular to the axis of revolution 02 of the conical surface 6. This allows periodic inspection of the orientation of the measurement target by the personnel. The levels also make the installation process easier, so it can be performed by less skilled workers .

[ 0043 ] The linear level may also be used in the measurement device la. It should be positioned along the measurement axis 01 of the laser rangefinder 13. However, it is preferable to provide the measurement device la with a digital inclinometer 15 connected to the central processing unit 10 and to provide means for generating an alarm signal via the transceiver 12 and/or the display 14 when the inclinometer indicates a non-horizontal position.

[ 0044 ] The wired transceiver 12 is usually the most reliable solution. However, in some structures wiring is not allowed, and in some structures wiring is not a transmission medium that is most resistant to interference and environmental exposure. In this case the solution is radio communication. Due to obstacles in the radio propagation environment typical for buildings, civil engineering structures or mines, it is justified to use multipath-resistant modulation technique, such as Orthogonal Frequency-Division Multiplexing (OFDM), e.g. as in the IEEE 802. llg standard.

[ 0045 ] Reliable detection of displacement, especially in the case of complex displacements directed not necessarily in the vertical direction, can be achieved using two measurement devices: the first la and the second lb. They should be arranged so that their measurement axes 01 and 01' form an acute angle β, ranging from 60° to 90°. A measurement system of this kind is easy to set up because the surface 6 is symmetrical with respect to the axis of symmetry 02. Such configuration of measurement system is shown in Fig. 8.

[ 0046 ] During periods of significant and variable loads, such as during a storm or during the winter, the measurements should be carried out at intervals adequate to load increase and response time needed for facility evacuation or removing snow in order to keep the displacement values comparable to the - maximum - allowable displacement value in the monitoring IT system, which automatically sends to the user or the administrator of the facility the message indicating the current safety level of the facility.

[ 0047 ] It is possible to send the measurement results to a server that analyzes the results obtained for multiple points of the same structure. Communications between the server and the measurement devices can be provided via a wired or wireless connection. In the latter case, transceiver 12 utilizing OFDM technique is employed.

[ 0048 ] Providing a vertical scale on the surface 6 of the measurement target 3 allows yet another type of emergency reading. The spot of the laser of the rangefinder is visible on the surface 6. Owing to the scale provided, it is enough to note the position of the spot during periodic inspections. This allows the deformation of the structure to be detected even if the electronic system fails. An example of a measurement target 3 with a scale 6c applied on the conical surface 6 is shown in Fig. 10. The measurement target 3 shown in Fig. 10 is further provided with two levels 6a and 6b, mutually perpendicular and perpendicular to the vertical axis of revolution 02 of the conical surface 6. The target also has a simple self-righting system 5 adapted to be attached to elements of the structure by a screw connection.

[ 0049 ] The solution according to the invention allows monitoring high traffic areas such as shopping malls, exhibition halls, sports facilities such as swimming pools where vertical measurement relative to water surface is impossible, indoor stadiums even during sport events, or facilities where the ceiling is covered with an insulating layer.

[ 0050 ] The invention can be applied in buildings where the traffic is chaotic, e.g. in shops with frequent change of exposures, logistic and reloading halls, production facilities with variable equipment arrangement, as well as in mines or civil engineering structures .

Claims

1. A measurement system for measuring displacements of a structure provided with a measurement device (la) having a central processing unit (10), a memory (11) connected to the central processing unit for storing measurement results, a communication unit (12) for transferring measurement results, a display (14), and a rangefinder (13) with a measurement axis (01) directed towards a measurement target (3), wherein the measurement device (la) and the measurement target (3) being adapted to be attached opposite each other, one of them to an element subjected to displacement (B) and the other one to a reference element of the structure (A) , characterized in that

the rangefinder is a first laser rangefinder with the substantially horizontal measurement axis (01),

the measurement device (la) is provided with a self-leveling system

(2) adapted to be attached to elements of the structure, and the measurement target (3) is provided with a conical surface

(6) with the substantially vertical axis of revolution (02) and an angle of opening ranging from 80° to 100°, and has a self-righting system (5) .

2. The measurement system according to claim 1, characterized in that the measurement target (3) is further provided with a level (6a, 6b) arranged so that it indicates the horizontal position when the axis of revolution (02) of the conical surface (6) is positioned vertically .

3. The measurement system according to the claim 2, characterized in that the measurement target is provided with a first level (6a) and a second level (6b), perpendicularly to each other and perpendicular to the axis of revolution (02) of the conical surface (6) .

4. The measurement system according to the claim 1 or 2 or 3, characterized in that the measurement device (la) is provided with a level arranged so that it indicates the horizontal position when the measurement axis of the laser rangefinder of the measurement device (la) is positioned horizontally.

5. The measurement system according to the claim 1 or 2 or 3, characterized in that the measurement device (la) is provided with a digital inclinometer (15) and means for generating an alarm when the inclinometer indicates deviation from the horizontal position greater than 2°.

6. The measurement system according to any claim form 1 to 5 characterized in that the communication unit (12) of the measurement device (la) is a radio transceiver utilizing Orthogonal Frequency- Division Multiplexing, OFDM.

7. The measurement system according to any claim form 1 to 5 characterized in that the communication unit (12) of the measurement device (la) is a wired communication unit.

8. The measurement system according to any claim form 1 to 7 characterized in that it is provided with the second measurement device (lb) with the measurement axis (01') set at an angle of 60° to 90° to the measurement axis (01) of the first measurement device (la) .

9. The measurement system according to any claim form 1 to 8 characterized in that the conical surface (6) of the measurement target (3) is provided with a scale.

10. A method for measuring the displacements of a structure characterized in that the method is carried out by performing a measurement of the difference of the horizontal distance between the reference element and the movable element of the structure (4) using the measurement system according to any claim from 1 to 7, the measurement device (la, lb) and the measurement target are attached to the movable element (B) of the structure (4) and the reference element of the structure (A) , respectively, and an alarm is triggered when the indication of the distance sensor (la) meets a predefined criterion.

11. A method for measuring the displacements of a structure according to the claim 10 characterized in that the measurement system according to the claim 8 is used, and in a predefined criterion a difference of the indication of the first distance sensor (la) in relation to a first reference value and a difference of the indication of the second distance sensor (lb) in relation to a second reference value are taken into consideration.