Analysis of the Causes of Accuracy Differences in Weighing Sensors for Raw Materials in the Same Batch

Analysis of the Causes of Accuracy Differences in Weighing Sensors for Raw Materials in the Same Batch

1. Deviation in Cutting Precision
    
   Although CNC machining equipment has high precision, tool wear after long-term operation (such as blunting of milling cutter edges) and fixture positioning errors (such as elastic body clamping offset of 0.005mm due to fixture wear) will cause dimensional differences in the "strain area" (a key area for pasting strain gauges) of elastic bodies from the same batch. For example, a strain area designed to be 5mm thick may fluctuate between 4.995mm and 5.005mm after actual processing. For every 0.001mm deviation in the thickness of the strain area, the deformation sensitivity will change by approximately 0.2%, directly affecting the linearity of the sensor's output signal.
2. Uneven Surface Roughness
    
   The pasting of strain gauges has extremely high requirements for the surface roughness of the elastic body (requiring Ra0.8-0.4μm). If the grinding wheel speed is unstable during the polishing process (such as fluctuating from 3000rpm to 3200rpm) or the polishing pressure is inconsistent, some elastic body surfaces will have tiny scratches or unevenness, leading to differences in the bonding degree between the strain gauges and the elastic body. The parts with insufficient bonding will produce "signal lag", resulting in increased repeatability errors of the sensor (for example, some products have a repeatability error of 0.02% FS, and some reach 0.04% FS).
3. Fluctuations in Heat Treatment Process
    
   Although elastic bodies from the same batch are annealed in the same furnace, uneven temperature distribution in the furnace (such as a core temperature of 850℃ and an edge temperature of 830℃) and differences in cooling speed (such as elastic bodies near the furnace door cooling faster) will lead to inconsistent internal grain structures of the metal, thereby causing fluctuations in the elastic modulus (for example, the standard elastic modulus is 200GPa, and the actual fluctuation range is 198GPa-202GPa). Differences in elastic modulus will directly affect the proportional relationship between deformation and weight, ultimately manifesting as a range deviation.

II. Component Assembly Link: Superposition of Discreteness and Operational DeviationsIn addition to the elastic body, the inherent discreteness of core components such as strain gauges and compensation resistors, as well as manual operation deviations during the assembly process, are another important source of accuracy differences.(A) Characteristic Discreteness of Core Components
Performance Differences of Strain Gauges
Although strain gauges from the same batch are marked with "gauge factor 2.0±0.1", the actual gauge factor may fluctuate between 1.95-2.05 in testing. At the same time, the temperature coefficient (a performance parameter affected by temperature) of strain gauges also has discreteness (for example, the temperature coefficient of some products is 5ppm/℃, and that of some reaches 8ppm/℃). These differences will lead to: even if the deformation of the elastic body is the same, the electrical signals output by different strain gauges are different, which ultimately manifests as differences in sensor zero drift and range error.
Precision Deviation of Compensation Resistors
Temperature compensation resistors need to match strain gauges to offset temperature effects. Although compensation resistors from the same batch are marked with "precision ±0.1%", there may be slight differences in actual resistance values (for example, designed as 1kΩ, actual 999.8Ω-1000.2Ω). Resistance deviations will lead to inconsistent compensation effects—some sensors have zero drift ≤0.002% FS/℃ at high and low temperatures, while others reach 0.005% FS/℃, thereby affecting accuracy stability.
(B) Human Deviations in Assembly Operations
Differences in Position and Pressure of Strain Gauge Pasting
Strain gauges need to be accurately pasted at the center of the strain area of the elastic body (deviation ≤0.1mm). However, during manual pasting, if the positioning marks are blurred or the pressure of the pressing block is unstable (for example, some products apply 0.1MPa pressure, and some apply 0.15MPa), the strain gauges will be offset or have different degrees of tight bonding. Offset strain gauges will "mis-capture" the deformation of non-target areas, increasing the deviation between the output signal and the actual weight. Insufficient bonding is prone to "signal virtual connection", leading to an increase in repeatability errors.
Fluctuations in Lead Welding Quality
Differences in soldering iron temperature (e.g., set at 320℃, actual fluctuation of 20℃) and soldering time (e.g., standard 1 second, actual 0.8-1.2 seconds) during welding will lead to different solder joint resistances (e.g., some solder joint resistances are 0.1Ω, some are 0.3Ω). Solder joint resistance deviations will introduce additional signal loss, reducing the output signal amplitude of some sensors, and thus resulting in insufficient range (e.g., standard output is 2mV/V, some products are only 1.95mV/V).

1. Impact of Temperature on Adhesive Curing
    
   The epoxy resin adhesive used for pasting strain gauges needs to be cured in a constant temperature oven at 60-80℃. If the temperature distribution in the constant temperature oven is uneven (such as a temperature difference of 5℃ between upper and lower parts) or there is a deviation in curing time control (such as a standard of 3 hours, actual 2.5-3.5 hours), the curing degree of the adhesive will be different. Insufficiently cured adhesive will shrink slowly in subsequent use, causing slight displacement between the strain gauge and the elastic body, leading to sensor zero drift. Excessive curing will make the adhesive brittle, affecting the strain transmission efficiency and leading to linearity deviation.
2. Impact of Humidity on Insulation Performance
    
   The circuit assembly link needs to ensure that the insulation resistance is ≥500MΩ. If the workshop humidity fluctuates (such as standard RH40%-60%, actual RH30%-70%), when the humidity is high, the surface of the elastic body is prone to absorb moisture, leading to a decrease in the insulation resistance between the circuit and the elastic body. Some sensors will have signal leakage due to insufficient insulation resistance (such as only 300MΩ), reducing the stability of the output signal and thus affecting accuracy.
    
   (B) Random Impact of Electromagnetic Interference
    
   Frequency converters and welding equipment in the workshop generate electromagnetic radiation during operation. If the sensor assembly station is close to the interference source (such as some stations are 3 meters away from the frequency converter, and some are 5 meters away), or the shielding measures are not in place (such as some cables are not sheathed with metal corrugated pipes), electromagnetic interference will couple into the circuit. Sensors with strong interference will have clutter mixed in their output signals, leading to "false signals" being misjudged as valid signals during the calibration process, and ultimately increasing the accuracy deviation after calibration (for example, some products have a linear error of 0.03% FS, and some reach 0.06% FS).

IV. Calibration Link: Subtle Deviations in Operation and EquipmentCalibration is a key link to "endow" sensors with accuracy. If the calibration equipment has insufficient accuracy or the operation process is not standardized, even if the previous links are consistent, it will lead to differences in the final accuracy.(A) Accuracy Fluctuation of Calibration Equipment
Precision Deviation of Standard Weights
Calibration requires the use of standard weights with accuracy three grades higher than that of the sensor (for example, if the sensor is grade 0.1, the weight needs to be grade 0.01). However, the same set of weights will wear out after long-term use (for example, a 10kg weight actually weighs 9.998kg-10.002kg). If the weights are not calibrated regularly, the applied "standard weight" will have differences. For example, when a "10kg" weight is applied to the same batch of sensors, the actual weights are 9.998kg and 10.002kg respectively, and the sensor will have a range deviation of ±0.02% FS after calibration.
Errors of Calibration Bench and Instruments
The calibration bench needs to ensure levelness (error ≤0.1mm/m). If the bench surface deforms after long-term use (such as a local depression of 0.05mm), it will cause uneven force on the elastic body. If the signal acquisition instrument used for calibration (such as a multimeter) has accuracy drift (such as the error increases from 0.01% to 0.02%), it will lead to signal reading deviation. These equipment errors will be directly transmitted to the sensor calibration results, resulting in accuracy differences.
(B) Process Differences in Calibration Operation
Deviation in Preheating Time and Loading Sequence
Sensors need to be preheated for 30 minutes before calibration. If some products are only preheated for 20 minutes, the circuit does not reach a stable working state, which will lead to zero drift. When loading weights, if some products are loaded in the order of "20%-40%-60%-80%-100%" and some are loaded in the order of "100%-80%-60%-40%-20%", and the loading speed is not strictly controlled (such as some fast loading causing impact deformation), the output signals under the same weight will differ, thereby affecting the linearity calibration result.
Human Judgment Deviation in Parameter Adjustment
During calibration, the zero point and range compensation resistors need to be adjusted manually, and the adjustment depends on the operator's judgment of the instrument reading (for example, the standard output is 2.000mV/V, some operators stop when adjusting to 1.998mV/V, and some adjust to 2.002mV/V). This subtle judgment deviation will lead to inconsistent output signal benchmarks of the same batch of sensors, ultimately resulting in accuracy differences.

Summary：The accuracy difference of load cells from the same batch of raw materials is essentially the result of the "cumulative effect of subtle deviations": from the micron-level dimensional fluctuations in elastic body processing, to the characteristic discreteness of strain gauges, and then to the subtle deviations in environmental variables and calibration operations, the tiny differences in each link will be transmitted and amplified, eventually leading to inconsistent accuracy of finished products. To reduce this difference, efforts should be made from three aspects: first, introduce automated equipment (such as automatic strain gauge pasting machines and intelligent calibration systems) to reduce human deviations; second, optimize the production environment (such as constant temperature and humidity workshops, electromagnetic shielding stations) to control environmental variables; third, establish a full-process quality traceability system (such as recording the parameters and equipment status of each process) to locate the source of deviations in a timely manner. Only through "refined management + automation upgrade" can the accuracy difference of products in the same batch be minimized, and the consistency and reliability of sensors be improved.