Sensor with strain gauges

Abstract

The present invention relates to a strain gauge, in particular a weighing sensor (1), comprising a proof body (2) and at least one strain gauge (4) mounted on the proof body (4), wherein the sensor (1) further comprises a first temperature sensor (7) arranged on the proof body (2) as close as possible to the strain gauge (4). Preferably, the sensor further comprises an acquisition circuit board (5) to which the one or more strain gauges (4) and the first temperature sensor (7) are connected and which is intended to transform the analogue signal from the gauges (4) and from the first temperature sensor (7) into a digital signal. A second temperature sensor (8) is placed on or in the immediate vicinity of the acquisition circuit board (5) and connected thereto in order to allow digital compensation for thermal drift of the sensor caused by the sensor's environment and the acquisition board.

Description

[0001] Strain gauge sensor

[0002] The present invention relates to a strain gauge sensor intended to measure a force and in particular a weighing sensor intended to measure weight.

[0003] Strain gauge sensors, and in particular weighing sensors, are used in a wide range of areas: supermarket scales, automotive industry, food industry (filling, etc.), transport industry, etc.

[0004] Such sensors generally consist of a test body on which the strain gauge(s) are applied (incorrectly called a strain gauge, but it is the strain that is measured to determine the stress). The test body is usually made of steel or aluminum and undergoes deformations during the measurement: the test body exhibits minimal elastic behavior so that it deforms slightly when subjected to a load and returns to its initial position when this load is removed. Strain gauges are measuring grids made of a conductive material photo-etched on a specific support, chosen in particular for its deformation properties. When the support is stretched, the measuring grids lengthen and the resistance increases; when the support is compressed, the grids retract and the resistance decreases. This variation in resistance makes it possible to determine the stress applied to the support.The gauge support is glued to the test body and therefore undergoes the same deformations as the latter.

[0005] The gauges are generally placed on the test body in the area where there is the most deformation. They can be arranged and wired in several ways:

[0006] • full Wheatstone bridge: four gauges of equivalent resistance, two of which are subjected to tension and two to compression; • half bridge, where only two gauges are sensitive elements (one in tension, one in compression), the other two are fixed resistances; and

[0007] • quarter bridge: a single gauge is a sensitive element for three fixed gauges.

[0008] In a load cell, the weight is determined by suspending or applying a load to the test body. Unlike other load cells, the load applied to a load cell must always be aligned in the direction of gravity. Several types of load cells are known: central fulcrum load cell; flexural load cell; compression load cell; and tensile load cell.

[0009] Depending on the application, the load cell and the test body in particular may have a particular design or special material characteristics. Analog sensors must be connected to a measuring amplifier, while digital sensors have integrated electronics that can process the measurement results received from the strain gauges and make these results available in a specific format.

[0010] The environment in which the sensor is placed plays an important role. In particular, temperature has a significant influence on the measurement. The test body and strain gauges elongate and contract in response to heat and cold. The electrical resistance of conductive materials also varies with temperature. Thermal drift in the absence of stress is unpredictable depending on the sensor and intrinsic to the strain gauge bridge used. It is determined by practical tests at various temperatures. To compensate for the thermal drift in the absence of stress, a resistor (generating a drift opposite to that measured during testing) is added to one branch of the strain gauge bridge (branch chosen according to the direction of the drift). This resistor can, for example, be a copper winding. Sensitivity differences due to ambient temperature during measurements are due to the materials used for the test body and the strain gauge support.These deviations can also be quantified during measurement tests. Mounting the strain gauges in a full Wheatstone bridge reduces the deviations. It is also possible to add a resistor, for example made of nickel, to each power supply branch of the strain gauge bridge. These resistors will modify the supply voltage at the terminals of the strain gauge bridge in order to compensate for the sensitivity differences due to temperature.

[0011] To determine the resistors to be added to the strain gauge bridge to compensate for the effects of temperature on the sensor, it is necessary to test the sensor under variable temperature conditions (for example between -10° and + 40°). The thermal drifts at no-load and during measurement are thus determined, then the resistors necessary to compensate for these drifts. To install the resistors intended to compensate for these thermal drifts, it is necessary to reopen the sensor. This work is entirely manual and therefore requires time and attention to detail, particularly because the resistors to be added are small in an assembly that requires great precision, especially in the case of a weighing sensor for which the precision requirement is very high.

[0012] In addition, thermal drift effects are generally determined by stabilized measurements, i.e. measurements taken when the sensor has reached a uniform temperature (which can take a few hours). However, some applications, such as rotary dosing machines for filling, require high-temperature cleaning cycles, cycles which subject the force sensor to significant temperature variations over a very short time. During these transient phases, when the sensor has not reached a uniform temperature but is subjected to rapid temperature variations, analog compensation by adding resistive elements will be less accurate because its elements do not have the same response time to temperature variations as the test body and the strain gauges.In transient temperature phases, with analog compensation by adding resistive elements as described above, a temporary drift in the measurement can be observed.

[0013] The aim of the present invention is therefore to provide a strain gauge force sensor, in particular a weighing sensor, in which the parasitic effects due to temperature are compensated and corrected, when empty and during measurement, in a simple and precise manner, without it being necessary to reopen the sensor to install additional resistors intended to compensate for thermal drifts. Another aim of the present invention is to provide a strain gauge force sensor suitable for uses in which the sensors are subjected to significant temperature variations over short periods of time.

[0014] The present invention relates to a force sensor according to claim 1.

[0015] Figure 1 is a schematic sectional view of a force sensor according to one embodiment of the present invention.

[0016] In the embodiment illustrated in Figure 1, the force sensor according to the invention is a weighing sensor 1 with a central support point. The weighing sensor 1 comprises in particular a test body 2, which, in the illustrated embodiment, is essentially parallelepipedal in shape.

[0017] The test body 2 comprises a 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 in further detail.

[0018] In the illustrated embodiment, the weighing sensor 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 and intended to process the information received from the strain gauges 4 and to make results available in an appropriate format, or to convert the analog signal into a digital signal. For this, the strain gauges 4 are connected to the electronic card 5 and the latter comprises an output and power supply cable 6 for its power supply and the provision of the measurement results.

[0019] According to the prior art, the method for compensating and correcting temperature-related drifts consists of:

[0020] • determine these drifts by a series of measurement tests at different temperatures, empty and under stress;

[0021] • open the sensor and add to one or more of the branches wiring the strain gauges 4 a resistance chosen to compensate for the measured drifts.

[0022] As we have seen, this method is time consuming and requires reopening the sensor and installing very small resistors in a very careful construction.

[0023] According to the invention, the weighing sensor 1 therefore comprises a first temperature sensor 7 placed in the gauge cavity 3 as close as possible to the strain gauges 4 and connected to the electronic acquisition card 5. The first temperature sensor 7 is therefore arranged to capture the temperature in the gauge cavity 3 as close as possible to the strain gauges 4 and transmit this information to the electronic acquisition card 5. Preferably, this first temperature sensor 7 is a thermistor of the NTC (negative temperature coefficient) type whose resistance decreases, relatively uniformly, when the temperature increases, and vice versa. This NTC thermistor is preferably soldered to the circuit for connecting the gauges in the gauge cavity 3.The advantage of such a sensor is that it is not necessary to add an additional wire for the connection between the first temperature sensor 7 and the acquisition card 5, a wire from the gauge bridge 4 can be used (for example, the Ov cable). The value for the temperature is obtained by measuring the voltage across the NTC resistor mounted as a divider bridge with a resistor mounted on the electronic acquisition card 5. If necessary, a linearization of the measurement will be carried out, for example, by a polynomial correction of order 4 or 5. Alternatively, any type of sensor making it possible to determine a temperature can be used as the first temperature sensor 7.

[0024] In the illustrated embodiment, a second temperature sensor 8 is placed on or in the immediate vicinity of the electronic acquisition card 5 and is arranged to sense the temperature of this card. The second sensor 8 is also connected to the electronic card 5 to transmit the temperature data to said card. This sensor can be of any suitable type.

[0025] In the illustrated embodiment, the electronic acquisition card 5 receives measurements from the strain gauges 4, and from the first and second temperature sensors 7, 8. It is arranged to process these measurements and make a result available via its power cable and output 6.

[0026] To compensate for the thermal drifts of the weighing sensor 1 according to the embodiment illustrated, we start with a series of tests at different temperatures (-10° to +40° for example) to determine the drifts due to temperature. On the basis of these tests, we then program in the acquisition card 5, the appropriate equation to correct the measurement and compensate for these drifts according to the information provided by the first and second temperature sensors and the strain gauges.

[0027] Thus, it is not necessary to reopen the weighing sensor 1 after its final assembly, the compensation of thermal drifts being done by an algorithm in the acquisition card 5. In addition, the precision of the compensation is increased since it is no longer physical (choosing, cutting and installing a resistor with the right properties in a branch of the gauge bridge, operations which depend on the operator's experience) but entirely digital: exact calculation based on the measurements of the temperature sensors and the strain gauges.

[0028] The weighing sensor 1 therefore makes it possible to compensate for thermal drifts very precisely without complicating the manufacturing process of the sensor, even lightening the final calibration steps. In fact, we thus produce a digital weighing sensor which makes it possible to limit the effects of temperature in a simple, efficient and precise manner. There are three distinct phenomena which produce a measurement drift:

[0029] • Absolute temperature has an effect on the signal;

[0030] • Variations in environmental temperature affect the signal. These variations can be sudden and significant or more gradual;

[0031] • Powering up and therefore heating the electronic card 5 has an effect on the temperature. This latter phenomenon becomes significantly more pronounced with communication technologies such as Ethernet, which consume more and therefore dissipate more heat.

[0032] The presence of a temperature sensor as close as possible to the strain gauges makes it possible to determine the amplitude of the first two phenomena above, while the last phenomenon is characterized by the temperature sensor near the acquisition card. The various filters and algorithms programmed to process the measurement signal then make it possible to limit and compensate for the effects of temperature by numerical calculation.

[0033] In particular, for a weighing sensor used on a packaging or filling line in which regular high-temperature cleaning is necessary, the present invention makes it possible to determine even transient temperature changes while the test body has not reached a uniform temperature. Indeed, whether with the first sensor alone or with the second sensor placed close to the acquisition card, it is possible to identify rapid temperature changes and adapt the compensation accordingly.

[0034] In the illustrated embodiment, the force sensor comprises two temperature sensors. Alternatively, one or more further temperature sensors could be arranged on the sensor test body to further refine the compensation calculation. One sensor could thus be dedicated to measuring the ambient temperature.

[0035] In the illustrated embodiment, the force sensor according to the invention is a weighing sensor. Alternatively, any other strain gauge sensor could be considered.

[0036] In the illustrated embodiment, the load cell is a central fulcrum sensor with a substantially parallelepipedal test body. In addition, the strain gauges are housed in a cavity included in the test body. Alternatively, other types of load cell are conceivable with other shapes of test body (for example, compression load cell with cylindrical test body). Similarly, the strain gauges are generally mounted on the test body by any suitable means and are not necessarily housed in a cavity of the test body.

[0037] In general, the present invention relates to a strain gauge sensor, in particular a weighing sensor, 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 arranged on the test body as close as possible to the strain gauge.

[0038] Preferably, the force sensor comprises four strain gauges forming a Wheatstone bridge.

[0039] Preferably, the strain gauges are housed in a cavity of the test body.

[0040] Preferably, the sensor further comprises an electronic acquisition card to which the strain gauge(s) and the first temperature sensor are connected and intended to transform the analog signal from the gauges and the first temperature sensor into a digital signal.

[0041] Preferably, the electronic acquisition card is mounted on or in the test body. Preferably, the force sensor further comprises a second temperature sensor placed on or in close proximity to the electronic acquisition card and connected thereto.

[0042] Preferably, the electronic acquisition card is intended to process the measurements provided by the strain gauges and the first and second temperature sensors to compensate for the effects of temperature on the sensor and provide a weighted and accurate measurement.

Claims

Claims: A strain gauge sensor, in particular a load cell (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) as close as possible to the strain gauge (4). A sensor according to claim 1, characterized in that it 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. A sensor according to claim 1 or 2, characterized in that the test body (2) comprises a cavity (3) in which the strain gauge(s) (4) and the first temperature sensor (7) are housed.Sensor according to one of the preceding claims, characterized in that it further comprises an electronic acquisition card (5) to which are connected the strain gauge(s) (4) and the first temperature sensor (7) and intended to transform the analog signal of the gauges (4) and of the first temperature sensor (7) into a digital signal.

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

6. Sensor according to claim 4 or 5, characterized in that it further comprises a second temperature sensor (8) placed on or in the immediate vicinity of the electronic acquisition card (5) and connected to the latter.

7. Sensor according to any one of claims 3 to 5, characterized in that the electronic acquisition card (5) is designed to process the measurements provided by the strain gauges and the first and second temperature sensors to compensate for the effects of temperature on the sensor and provide a weighted and accurate measurement.

8. Sensor according to any one of the preceding claims, characterized in that it is a weighing sensor, preferably a central support point weighing sensor.

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