Strain gauge sensor

Abstract

The invention relates to a strain gauge, in particular a weighing sensor (1), comprising a test body (2) and at least one strain gauge (4) mounted on the test body (4), the sensor (1) further comprising a first temperature sensor (7) arranged on the test body (2) as close as possible to the strain gauge (4). Preferably, the sensor further comprises an electronic acquisition card (5) to which the one or more strain gauges (4) and the first temperature sensor (7) are connected, and which is used to convert analog signals from the strain gauges (4) and from the first temperature sensor (7) into digital signals. The second temperature sensor (8) is placed on or adjacent to the electronic acquisition card (5) and is connected with the electronic acquisition card so as to digitally compensate the environment of the sensor and the sensor thermal drift generated by the acquisition board.

Description

Technical Field

[0001] The present invention relates to a strain gauge sensor for measuring force, and in particular to a load cell for measuring weight. Background Art

[0002] Strain gauge (starin gauge) sensors - in particular load cells - are used in many areas: supermarket scales, automotive industry, food industry (filling, etc.), transport industry, etc.

[0003] Such a sensor usually consists of a test body to which one or more strain gauges are applied. The test body is usually made of steel or aluminum and deforms during the measurement: the test body has minimal elastic behavior and therefore deforms slightly when subjected to a load and returns to its initial position if the load is removed. The strain gauge is a measuring grid made of a conductive material photoetched onto a specific support, specifically chosen for its deformation characteristics. When the support is stretched, the measuring grid stretches and the resistance increases; when the support is compressed, the grid retracts and the resistance decreases. This change in resistance can be used to determine the stress applied to the support. The instrument support is bonded to the test body and is therefore subject to the same deformation as the test body.

[0004] The gauges are usually placed in the area of ​​the test body where the deformation is greatest. They can be arranged and wired in a variety of ways:

[0005] Complete Wheatstone bridge: four gauges of equivalent strength, two in tension and two in compression;

[0006] Half-bridge, where only two of the meters are sensing elements (one tension, one compression) and the other two are fixed resistors; and

[0007] Quarter Bridge: A single meter is the sensitive element of three fixed meters.

[0008] In a load cell, the weight is determined by suspending or applying a load to a test body. Unlike other sensors, the load applied to the sensor must always be aligned with the direction of gravity. Several types of load cells are known: sensors with a central support point; flexion sensors; compression sensors and traction sensors.

[0009] Depending on the application, the sensor and the test body may have a specific design or special material properties. Analog sensors need to be connected to a measuring amplifier, while digital sensors have integrated electronics that process the measurements received from the strain gauge and make them available in a specific format.

[0010] The environment in which the sensor is placed plays an important role. In particular, the temperature has a significant influence on the measured values. The test body and the strain gauge expand and contract in response to heat and cold. The resistance of the conductive material also changes with temperature. The thermal drift under no-stress is unpredictable, depends on the sensor and is intrinsic to the strain gauge bridge used. It is determined by actual tests at different temperatures. In order to compensate for the thermal drift under no-load conditions (no stress), a resistor (producing a drift opposite to the drift measured during the test) is added in one branch of the strain gauge bridge (the branch selected according to the drift direction). This resistor can be, for example, a copper coil.

[0011] Differences in sensitivity to ambient temperature during measurement are due to the materials used for the test body and the instrument support. These differences can also be quantified during the measurement test. The full Wheatstone bridge mounting of the strain gauges attenuates these deviations. It is also possible to add resistors, for example made of nickel, to each supply branch of the strain gauge bridge. These resistors modify the supply voltage across the strain gauge bridge to compensate for temperature-related sensitivity changes.

[0012] In order to determine the resistors to be added to the strain gauge bridge to compensate for the temperature effects on the sensor, the sensor must be tested under variable temperature conditions (e.g. between -10° and +40°). The thermal drifts without load and during measurement are then determined and subsequently the resistors required to compensate for these drifts. The sensor must be reopened in order to install the resistors required to compensate for these thermal drifts. This work is entirely manual and therefore requires time and attention to detail, especially since the resistors to be added are of small size in an assembly that requires high precision, especially in the case of sensors where very high precision is required.

[0013] Additionally, thermal drift effects are usually determined by stabilization measurements, i.e. measurements taken when the sensor has reached a uniform temperature, which can take several hours. However, certain applications, such as rotary metering machines for filling, require high-temperature cleaning cycles, which subject the force sensor to significant temperature changes in a very short time. During these transient phases, when the sensor has not yet reached a uniform temperature but is subject to rapid temperature changes, simulated compensation by adding resistive elements will be less accurate, as these elements do not respond to temperature changes in the same time as the test body and strain gauges. During the transient temperature phase, temporary drifts in the measurements can be observed by simulated compensation by adding resistive elements as described above. Summary of the invention

[0014] It is therefore an object of the present invention to provide a strain gauge sensor, in particular a load cell, in which parasitic effects due to temperature are compensated and corrected in a simple and accurate manner in idle state and during measurement, without the need to reopen the sensor to install additional resistors to compensate for thermal drift. Another object of the present invention is to provide a strain gauge sensor suitable for applications where the sensor is subjected to significant temperature changes within a short period of time.

[0015] The invention relates to a strain gauge sensor according to claim 1 . BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a schematic cross-sectional view of a stress sensor according to an embodiment of the present invention. DETAILED DESCRIPTION

[0017] exist Figure 1 In the embodiment shown, the sensor according to the invention is a load cell 1 with a central support point. In particular, the load cell 1 comprises a test body 2 which, in the embodiment shown, is substantially parallelepiped in shape.

[0018] The test body 2 comprises a strain gauge cavity 3 in which there is at least one but preferably four strain gauges 4 arranged to form a Wheatstone bridge. The strain gauges 4 may be of any suitable type and will not be described further.

[0019] In the embodiment shown, the load cell 1 is a digital sensor and the test body 2 further comprises an electronic acquisition card 5 placed in or on the test body 2 for processing the information received from the strain gauge 4 and making the results available in an appropriate format or converting analog signals into digital signals. To this end, the strain gauge 4 is connected to the electronic card 5, which comprises an output and a power supply line 6 for powering the electronic card 5 and providing the measurement results.

[0020] According to the prior art, methods for compensating and correcting drift caused by temperature include:

[0021] These drifts are determined through a series of measurement tests at different temperatures, in vacuum and under stress;

[0022] Open the sensor and add resistors to one or more of the branches connected to the strain gauges 4 to compensate for the drift of the measurement.

[0023] As we can see, this method is time consuming and requires reopening the sensor and installing very small resistors in a very delicate configuration.

[0024] Therefore, according to the present invention, the weighing sensor 1 includes a first temperature sensor 7, which is placed in the strain gauge cavity 3 as close as possible to the strain gauge 4 and connected to the electronic acquisition card 5. Therefore, the first temperature sensor 7 is arranged as close as possible to the strain gauge 4 to sense the temperature in the strain gauge cavity 3 and transmit this information to the electronic acquisition card 5. Preferably, the first temperature sensor 7 is a NTC (negative temperature coefficient) type thermistor, whose resistance decreases relatively uniformly when the temperature increases, and vice versa. This NTC thermistor is preferably soldered to the instrument connection circuit in the instrument cavity 3. The advantage of such a sensor is that there is no need to add additional wires to the link between the first temperature sensor 7 and the acquisition card 5, because the wires from the strain gauge bridge 4 (for example, 0v cables) can be used. The temperature value is obtained by measuring the voltage at the terminals of the NTC resistor installed in the bridge voltage divider, where the resistor is installed on the electronic acquisition card 5. If necessary, the measured value is linearized, for example by a 4th or 5th order polynomial correction. Optionally, any type of sensor for determining temperature can be used as the first temperature sensor 7.

[0025] In the embodiment shown, the second temperature sensor 8 is placed on or in close proximity to the electronic acquisition card 5 and is arranged to sense the temperature of the card. The second sensor 8 is also connected to the electronic card 5 to transmit temperature data to the electronic card. The sensor can be of any suitable type.

[0026] In the embodiment shown, the electronic acquisition card 5 receives the measurements from the strain gauges 4 and the first temperature sensor 7 and the second temperature sensor 8. It is arranged to process these measurements and make the results available via its power supply and output cables 6.

[0027] In order to compensate for the thermal drift of the load cell 1 shown in the figure, a series of tests are first carried out at different temperatures (for example, -10° to +40°) to determine the drift due to temperature. Then, based on these tests, appropriate equations are programmed in the acquisition card 5 to correct the measured values ​​and compensate for these deviations based on the information supplied by the first and second temperature sensors and the strain gauge.

[0028] In this way, after the final assembly of the load cell 1, it is not necessary to reopen the load cell 1, since the compensation for the thermal drift is performed by an algorithm in the acquisition card 5. In addition, the accuracy of the compensation is improved, since the compensation is no longer physical (selection, cutting and installation of resistors with the right characteristics in the branches of the strain gauge bridge, operations that depend on the operator's experience), but completely digital: a precise calculation is performed based on the measured values ​​of the temperature sensor and the strain gauge.

[0029] As a result, load cells 1 can very accurately compensate for thermal drift without making the sensor manufacturing process more cumbersome or even reducing the final calibration step. The result is a digital load cell that simply, effectively and accurately limits the effects of temperature. There are three different phenomena that produce measurement drift:

[0030] Absolute temperature has an effect on the signal;

[0031] Temperature changes in the environment have an effect on the signal. These changes may be sudden and dramatic, or more gradual;

[0032] The temperature is affected by the powering up and thus heating up of the electronic card 5. This phenomenon becomes more pronounced in the case of communication technologies such as Ethernet, since Ethernet consumes more power and therefore dissipates more heat.

[0033] The presence of a temperature sensor as close as possible to the strain gauge makes it possible to determine the amplitude of the first two phenomena mentioned above, while the latter phenomenon is characterized by a temperature sensor close to the acquisition card. Various filters and algorithms programmed to process the measured signal then make it possible to limit and compensate for the effects of temperature by digital calculations.

[0034] In particular, for load cells on packaging or filling lines that require regular high-temperature cleaning, the invention makes it possible to determine instantaneous temperature changes even when the test body has not reached a uniform temperature. In fact, whether by a separate first sensor or a second sensor placed close to the acquisition card, rapid temperature changes can be identified and the compensation adjusted accordingly.

[0035] In the embodiment shown, the force sensor includes two temperature sensors. Optionally, one or more further temperature sensors may be arranged on the sensor test body to further refine the compensation calculation. For example, one sensor may be dedicated to measuring the ambient temperature.

[0036] In the embodiment shown, the force sensor according to the invention is a load cell. Alternatively, any other strain gauge sensor may be considered.

[0037] In the illustrated embodiment, the load cell is a central support point sensor of a test body having a substantially parallelepiped shape. In addition, the strain gauge is housed in the cavity of the test body. Alternatively, other types of sensors with test bodies of other shapes can be envisaged (e.g., a compression sensor with a cylindrical test body). Similarly, the strain gauge is generally mounted on the test body by any suitable means and is not necessarily housed in the cavity of the test body.

[0038] In general, the invention is directed to a strain gauge sensor, in particular a load cell, comprising a test body and at least one strain gauge mounted on the test body. According to the invention, the sensor further comprises a first temperature sensor, which is arranged on the test body as close as possible to the strain gauge.

[0039] Preferably, the force sensor comprises four strain gauges forming a Wheatstone bridge.

[0040] Preferably, the strain gauge is housed in a cavity of the test body.

[0041] Preferably, the sensor further comprises an electronic acquisition card, to which the strain gauge or the plurality of strain gauges and the first temperature sensor are connected, and the electronic acquisition card is designed to convert analog signals from the plurality of strain gauges and the first temperature sensor into digital signals.

[0042] Preferably, the electronic acquisition card is installed on or in the test body.

[0043] Preferably, the stress sensor further comprises a second temperature sensor, which is placed on the electronic acquisition card or is close to the electronic acquisition card and connected to the electronic acquisition card.

[0044] Preferably, the electronic acquisition card is designed to process the measurements provided by the strain gauge and the first and second temperature sensors to compensate for the effects of temperature on the sensors and to provide weighted and accurate measurements.

Claims

1. A strain gauge sensor, in particular a sensor (1), comprising a test body (2) and at least one strain gauge (4) mounted on the test body (4), wherein the sensor (1) further comprises a first temperature sensor (7) arranged on the test body (2) closest to the strain gauge (4).

2. The sensor according to claim 1, characterized in that The invention comprises four strain gauges (4) arranged in or on the test body (2) to form a full Wheatstone bridge, a half bridge or a quarter bridge.

3. The sensor according to claim 1 or 2, characterized in that: The test body (2) comprises a cavity (3), in which the strain gauge or strain gauges (4) and the first temperature sensor (7) are accommodated.

4. The sensor according to claim 1, characterized in that It also includes an electronic acquisition card (5), to which the strain meter or multiple strain meters (4) and the first temperature sensor (7) are connected, and the electronic acquisition card (5) is used to convert analog signals from the multiple strain meters (4) and the first temperature sensor (7) into digital signals.

5. The sensor according to the preceding claim, characterized in that The electronic acquisition card (5) is installed in the test body (2) or on the test body (2).

6. The sensor according to claim 4 or 5, characterized in that It also includes a second temperature sensor (8), which is placed on the electronic acquisition card (5) or in close proximity to the electronic acquisition card (5) and is connected to the electronic acquisition card (5).

7. The sensor according to any one of claims 3 to 5, characterized in that The electronic acquisition card (5) is used to process the measurement values ​​supplied by the plurality of strain gauges and the first temperature sensor and the second temperature sensor to compensate for the influence of temperature on the sensors and to supply weighted and accurate measurement values.

8. The sensor according to claim 1, characterized in that The sensor is a load cell, preferably a load cell having a central support point.

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