The measurement system of the universal testing machine is implemented The core module for precise acquisition of the three core physical quantities of "force value displacement strain" can be summarized as follows: using dedicated sensors to convert mechanical/geometric signals into electrical signals, after signal conditioning and analog-to-digital conversion, the control system processes them into quantifiable test data, and finally generates mechanical curves and parameter reports. The entire process needs to meet the requirements of "high precision, high response, and low noise". The following analysis will be conducted from three aspects: core components, workflow, and key technical details (combined with equipment operation principles):

1、 The core components and functions of the measurement system

The measurement system consists of The four core units of "sensor → signal conditioning → data acquisition → control system" are composed, and each component has clear division of labor and works together to ensure distortion free signal transmission

2、 Measurement principles of the three core physical quantities (workflow breakdown)

The core of a measurement system is precise capture The measurement logic of "force value, displacement, strain" is similar but designed for different physical quantities. The following is a detailed workflow:

1. Force measurement: The working logic of strain gauge force sensors (the most critical measurement step)

Force measurement is the foundation of all mechanical tests, and the core relies on strain gauge force sensors. The workflow is as follows:

① Load transmission: During testing, the force on the sample is transmitted to the elastic body of the force sensor (such as an alloy steel beam) through the fixture, and the elastic body undergoes slight deformation under the load (usually ≤ 0.1mm, invisible to the naked eye);

② Strain electrical signal conversion: Strain gauges (metal resistance wires or semiconductor materials) attached to the surface of an elastic body stretch/compress with the deformation of the elastic body, resulting in a change in the resistance value of the strain gauges (following the "strain resistance effect": the greater the deformation, the greater the resistance change);

③ Bridge circuit amplification: Strain gauges form a Wheatstone bridge, which is supplied with a stable voltage (usually 5V or 10V) by a bridge excitation power supply. Resistance changes are converted into weak voltage signals output by the bridge (usually in the μ V level, such as a 100 μ V signal corresponding to a 1000N load);

④ Signal conditioning: Weak voltage signals are amplified by a signal amplifier (1000~10000 times to mV level), and electromagnetic interference (such as high-frequency noise generated by motors and power supplies) is filtered through a low-pass filter;

⑤ Analog to digital conversion and data processing: The A/D converter converts the amplified analog signal into a digital signal, and the controller converts the digital signal into the actual force value (such as 100mV corresponding to 1000N) based on the calibration coefficient of the sensor (such as 1mV corresponding to 10N), displays and stores it in real time.

2. Displacement measurement: working logic of grating ruler/laser displacement meter

Displacement measurement is divided into The core equipment for "fixture displacement" (macroscopic displacement) and "sample strain" (microscopic deformation) are grating rulers (mainstream) and laser displacement meters (high-precision scenarios):

（1) Grating ruler displacement measurement (applicable to most devices)

① Structure composition: The grating ruler consists of a ruler grating (fixed on the frame) and an indicator grating (fixed on the moving fixture), with a small gap (about 0.1mm) between the two gratings;

② Displacement optical signal conversion: When the fixture moves, the indicator grating slides relative to the scale grating, and the light source (LED light) illuminates the grating to produce "Moir é fringes" (interference fringes). The speed of fringe movement is proportional to the displacement speed of the fixture;

③ Optical electrical signal conversion: photodetectors (such as photoresistors) capture the brightness changes of Moir é fringes and convert them into pulsed electrical signals (the larger the displacement, the more pulses there are);

④ Data processing: The controller calculates the actual displacement (displacement=number of pulses x grid spacing) based on the grid spacing of the grating ruler (e.g. 20 μ m/grid). For example, 1000 pulses correspond to a displacement of 20mm, achieving accurate measurement.

（2) Laser displacement measurement (suitable for high-precision equipment of 0.1 level and above)

Principle: By emitting a laser beam to irradiate the surface of the fixture or sample, receiving the reflected beam, and utilizing The "time-of-flight method" (measuring laser round-trip time) or "triangulation method" (measuring reflected beam offset angle) calculates distance changes and directly outputs displacement data;

Advantages: Non contact measurement, no friction error, and high resolution0.001mm， Suitable for small displacement and high-precision testing (such as micro spring deformation measurement).

3. Strain measurement: working logic of extensometer (optional, for micro deformation)

Strain is the amount of deformation per unit length of a material（ε=Δ L/L, where Δ L is the deformation and L is the gauge length, needs to be measured using an extensometer

① Installation and fixation: Fix the two clamping arms of the extensometer on the gauge length section of the sample (such as the 50mm area in the middle of the metal sheet), ensuring that the clamping is firm and does not damage the sample;

② Deformation transmission: When the sample is stretched/compressed, a small deformation occurs in the gauge length section, which drives the elastic element of the extensometer to deform and triggers a change in the resistance of the built-in strain gauge (based on the same principle as the force sensor);

③ Signal processing: After the electrical signal output by the extensometer is conditioned and converted, the controller calculates the strain based on the gauge length (such as gauge length of 50mm, deformation of 0.05mm, strain=0.05/50=0.001=1000 μ m/m);

Application scenario: High precision parameter testing of metal yield strain, composite material elastic modulus, etc., which requires a strain control mode to achieve closed-loop control.

3、 Key technical details of the measurement system (ensuring accuracy and stability)

1. Signal anti-interference technology

Electromagnetic interference shielding: The sensor cable adopts shielded wire, and the signal conditioning module is grounded (grounding resistance)≤ 4 Ω), to avoid electromagnetic noise generated by motors and frequency converters affecting the signal;

Temperature compensation: Force sensors and strain gauges are equipped with built-in temperature compensation resistors to offset zero drift caused by environmental temperature changes (such as every temperature change)10 ℃, drift ≤ 0.005% FS).

2. Calibration and error correction

Factory calibration: The manufacturer calibrates the sensor using standard force gauges and laser interferometers to establish Corresponding relationship between input physical quantity and output electrical signal, storing calibration coefficients;

Regular calibration: Users need to use it annuallyCalibration of CNAS certified standard instruments (such as standard measuring blocks and standard force sensors), updating calibration coefficients to ensure measurement accuracy;

System error correction: The software automatically corrects system errors such as mechanical clearance (such as screw travel) and frame deformation, for example, by eliminating screw clearance through preloading and correcting displacement errors caused by frame deformation through stiffness compensation.

3. Synchronous collection and data association

The collection of the three major physical quantities needs to be strictly synchronized (with consistent sampling rates, such as1000Hz）， Ensure that the force values, displacement, and strain data at each time point correspond one-to-one in order to draw accurate "force displacement" and "stress-strain" curves;

Data caching and completion: during high-frequency testing (such as5kHz sampling), temporarily storing data through a caching module to avoid data loss due to insufficient processing speed and ensure smooth curves without breakpoints.

4、 Performance indicators and selection references for measurement systems

summary

The essence of the measurement system of the universal testing machine is The conversion and processing system of "physical signal → electrical signal → digital signal" relies on three major technologies: high-precision sensors, low-noise signal conditioning, and high-speed synchronous acquisition. The workflow can be simplified as follows:

Sensors convert force, displacement, and strain into weak electrical signals;

Signal conditioning unit amplifies signals and filters noise;

The A/D converter converts analog signals into digital signals;

The controller calculates actual physical quantities based on calibration coefficients, synchronizes associated data, and generates curvesreport

When selecting, special attention should be paid to the accuracy of the sensor (force sensor)≥ 0.05 level, displacement sensor ≥ grating scale level), sampling rate (≥ 1000Hz), calibration compatibility (supporting CNAS calibration), regular calibration of sensors and maintenance of signal cables are required during use to ensure long-term stable and reliable data output of the measurement system.