Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01G—WEIGHING
  • G01G3/00—Weighing apparatus characterised by the use of elastically-deformable members, e.g. spring balances
  • G01G3/18—Temperature-compensating arrangements
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01G—WEIGHING
  • G01G3/00—Weighing apparatus characterised by the use of elastically-deformable members, e.g. spring balances
  • G01G3/12—Weighing apparatus characterised by the use of elastically-deformable members, e.g. spring balances wherein the weighing element is in the form of a solid body stressed by pressure or tension during weighing
  • G01G3/14—Weighing apparatus characterised by the use of elastically-deformable members, e.g. spring balances wherein the weighing element is in the form of a solid body stressed by pressure or tension during weighing measuring variations of electrical resistance
  • G01G3/1414—Arrangements for correcting or for compensating for unwanted effects
  • G01G3/1418—Arrangements for correcting or for compensating for unwanted effects for temperature variations

Definitions

• Strain gauge sensors and in particular weighing sensors are present in a large number of areas: supermarket scales, automobile industry, food industry (filling, etc.), transport industry, etc.
• Such sensors generally consist of a test body to which the deformation gauge(s) are applied (mistakenly called a strain gauge but it is the deformation which is measured to know the stress).
• the test body is usually made of steel or aluminum and undergoes deformation during the measurement: the test body presents a minimum elastic behavior so that it deforms slightly when subjected to a load and recovers 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 media is stretched, the measurement grids extend and the resistance increases, when the media compresses, 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.
• 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 arrangements:
• 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 measurement 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 specified format.
• the environment in which the sensor is placed plays an important role.
• the temperature has an important influence on the measurement.
• the test body and strain gauges expand and retract in response to heat and cold.
• the electrical resistance of conductive materials also varies with temperature. Thermal drift in the absence of constraints is unpredictable depending on the sensor and intrinsic to the gauge bridge used. It is determined by practical tests at various temperatures.
• a resistance (generating a drift opposite to that measured during the tests) is added in a branch of the gauge bridge (branch chosen according to the direction of the drift). This resistance can for example be a copper winding.
• Differences in sensitivity due to ambient temperature during measurements are due to the materials used for the test body and the gauge support. These deviations can also be quantified during measurement tests.
• a Wheatstone full-bridge strain gauge arrangement mitigates deviations. It is also possible to add a resistor, for example nickel, to each power branch of the gauge bridge. These resistors will modify the supply voltage across the gauge bridge so as to compensate for differences in sensitivity due to temperature.
• thermal drift effects are generally determined by stabilized measurements, that is to say measurements taken when the sensor has reached a uniform temperature (which can take a few hours).
• certain applications such as rotary dosing machines for filling, require cleaning cycles at high temperatures, cycles which subject the force sensor to significant temperature variations over a very short time.
• analog compensation by adding resistive elements will be less precise because its elements do not have the same response time to temperature variations as the test body and the strain gauges.
• transient temperature phases with analog compensation by adding resistive elements as described above, a temporary drift in the measurement can be observed.
• the subject of the present invention is a force sensor according to claim 1.
• Figure 1 is a schematic sectional view of a force sensor according to one embodiment of the present invention.
• the force sensor according to the invention is a weighing sensor 1 with a central support point.
• the weighing sensor 1 notably comprises a test body 2, which, in the embodiment illustrated, is of essentially parallelepiped shape.
• the test body 2 comprises a gauge cavity 3 in which there is at least one but preferably four strain gauges 4, arranged so as to form a Wheatstone bridge. Strain gauges 4 may be of any suitable type and will not be described in further detail.
• 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 strain gauges 4 and to make results available in an appropriate format, or to convert the analog signal into a digital signal.
• the strain gauges 4 are connected to the card electronic 5 and this includes an output and power cable 6 for its power supply and the provision of the measurement results.
• the method for compensating and correcting drifts due to temperature consists of:
• 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 sense 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.
• this first temperature sensor 7 is a thermistor of the CTN type ( negative temperature coefficient) whose resistance decreases, relatively uniformly, as the temperature increases, and vice versa.
• This NTC thermistor is preferably soldered to the gauge connection circuit 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 sensor temperature 7 and the acquisition card 5, a wire from the gauge bridge 4 can be used (for example, the cable Ov).
• the value for the temperature is obtained by measuring the voltage across the CTN 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.
• any type of sensor making it possible to determine a temperature can be used as the first temperature sensor 7.
• 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 temperature data to said card.
• This sensor can be of any suitable type.
• 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.
• the weighing sensor 1 therefore makes it possible to compensate for thermal drifts very precisely without burdening the sensor manufacturing process, even reducing the final calibration steps.
• 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 different filters and algorithms programmed to process the measurement signal then make it possible to limit and compensate for the effects of the temperature using digital calculation.
• the present invention makes it possible to determine even transient temperature changes while the test body is not. has not reached a uniform temperature. Indeed, whether with the first sensor alone or with the second sensor placed near the acquisition card, it is possible to identify rapid temperature changes and adapt the compensation accordingly.
• the force sensor comprises two temperature sensors.
• one or more other temperature sensors could be placed on the sensor test body to refine still the compensation calculation.
• a sensor could thus be dedicated to measuring the ambient temperature.
• the force sensor according to the invention is a weighing sensor.
• any other strain gauge sensor could be considered.
• the weighing sensor is a central support point sensor with an essentially parallelepiped test body.
• the strain gauges are housed in a cavity that includes the test body.
• other types of load cells can be envisaged with other forms of test body (for example, compression load cell with cylindrical test body).
• the strain gauges are generally mounted on the test body by any appropriate means and are not necessarily housed in a cavity of the test body.
• 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.
• the sensor further comprises a first temperature sensor arranged on the test body as close as possible to the strain gauge.
• the force sensor comprises four strain gauges forming a Wheatstone bridge.
• the strain gauges are housed in a cavity of the test body.
• the senor further comprises an electronic acquisition card to which the deformation 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.
• the electronic acquisition card is mounted on or in the test body.
• the force sensor further comprises a second temperature sensor placed on or in the immediate vicinity of the electronic acquisition card and connected to the latter.
• 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 precise measurement.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Measurement Of Force In General (AREA)

Applications Claiming Priority (1)

Publications (1)

Family

ID=82693839

Family Applications (1)

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• 2022
  • 2022-06-23 EP EP22747071.3A patent/EP4544271A1/de active Pending
  • 2022-06-23 CN CN202280097336.7A patent/CN119343585A/zh active Pending
  • 2022-06-23 WO PCT/FR2022/000060 patent/WO2023247837A1/fr not_active Ceased

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