The S-type force sensor is named after its structural shape which is similar to the English letter "S". The currently available S-type force sensor products on the market cover a range of force measurement from a few Newtons to several hundred kilonewtons, capable of withstanding both tension and compression forces. They have high measurement accuracy, reaching levels of 0.3 grade, 0.1 grade, or even higher, and are widely used in various scenarios where force measurement is required, from industrial automated production lines to various equipment for precise measurement of item weights, as well as professional measurement research and testing instruments.
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Assembly situation of the pressure direction fixture for S-type force sensors
In the field of metrology, S-type force sensors and related indicating instruments are combined to form a standard force measuring instrument with traceability of measurement values. Usually, according to their measurement requirements, they come with accompanying pressure direction ball joints, bases, tension transfer components and other fixture components, which are connected to the force sensor body through threads. During daily work, it was found that when S-type force sensors are used to measure pressure values, the installation of the accompanying pressure direction ball joint and base has two main situations: one is that the accompanying pressure direction ball joint and base are tightened by threads, and the working plane of the fixture contacts the force sensor body, in this connection method, the force of the force sensor body is transmitted through the ball joint, base and threads. The other is that the accompanying pressure direction ball joint and base are tightened by threads and then reversed by half a turn, the working plane of the fixture does not contact the force sensor body, and the force of the force sensor body is transmitted only through the threads. These two assembly situations will lead to different force states of the force sensor, becoming an important factor affecting the measurement repeatability of this force sensor. 1
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S-type force sensor compression load test
Select an S40AC3-1T S-type force sensor from HBM.
Carry out the compression load test according to the different installation modes of its ball joint and base.
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Simulation test: We conducted a simulation test on the S40AC3-1T force sensor (Figure 1) using relevant software. The test results showed the force state of the sensor under the following conditions: Figure 2 shows the force state of the force sensor when a 1kN load is applied after the ball joint, base, and force sensor body are in contact and fitted. Figure 3 shows the force state of the force sensor when a 1kN load is applied after the ball joint, base, and force sensor body are not in contact and not fitted. Figure 4 shows the force state of the force sensor when a 10kN load is applied after the ball joint, base, and force sensor body are in contact and fitted. Figure 5 shows the force state of the force sensor when a 10kN load is applied after the ball joint, base, and force sensor body are not in contact and not fitted. Based on the simulation test results, it can be concluded that under the same load conditions, whether the ball joint, base, and force sensor body are in contact and fitted or not, the pressure value at the corresponding position of the force sensor after being subjected to force is different. According to the strain force output principle of the force sensor, the output signal (mV/V, i.e. voltage ratio) of the force sensor is different. In the two installation modes, load simulation tests were conducted at two load points of 10% and 100% of the range of the force sensor. From the simulation test results, it can be clearly seen that the force state of the force sensor within its effective range is affected by the assembly method of the ball joint, base, and force sensor body in compression.
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Force standard machine test: Select an S40AC3-1T S-type force sensor from HBM, and an HBM DMP-41 precision digital measuring instrument as the indicating instrument, and an 0.01-class DWM-10 10kN static load force standard machine as the reference standard for this test (Figure 6).
According to JJG 144-2007 "Standard Force Meter Verification Regulations", the laboratory environment temperature is controlled at (22 ± 0.5) °C, and the relative humidity is 48%. During the test, the laboratory environment temperature fluctuation does not exceed 1°C.
The assembly scheme of the force sensor's press-fit ball joint and base: Scheme one: The ball joint and base are tightened by threads, and the workpiece plane is in contact with the force sensor body; Scheme two: The ball joint and base are tightened by threads and then reversed by half a turn, and the workpiece plane is not in contact with the force sensor body. According to JJG 144-2007 "Standard Force Meter Verification Regulations", using the 0.01-class DWM-10 10kN static load force standard machine as the reference standard, load tests were conducted on the assembly schemes of the ball joint and base of the two force sensors. The output voltage ratio (mV/V, i.e. voltage ratio) of the force sensor under the corresponding load was read by the DMP-41 precision digital measuring instrument. Summary of force sensor compression test: Through the simulation test and physical test of the S-type force sensor, it can be verified that the force state of the S-type force sensor is directly affected by the assembly method of the ball joint, base, and force sensor body. The force transmission methods of the ball joint, base, and force sensor body in the two different assembly schemes are also different. Scheme one: When the ball joint, base, and force sensor body are in contact, some force is transmitted through the contact surface to the force sensor, and some is transmitted through the threads to the force sensor. This force value transmission method affects the force received by the force sensor. It has brought certain impacts, and at some load points, the straightness changes. Scheme Two: When the ball pair, base and the force sensor body do not fit properly, all the force is transmitted to the force sensor body through the threads of the ball pair and the base. By comparing the two sets of measured data of schemes one and two, the output of scheme one is always slightly higher than that of scheme two. The difference range is from -0.00003 mV/V (1kN) to -0.00058 mV/V (10kN), and the difference gradually increases slightly as the load increases. The straightness of scheme two is significantly better overall than that of scheme one.
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S-shaped force sensor pull test
Two assembly schemes for the pull test fixtures were designed. Scheme One: The pull test fixture is manually tightened to the limit through threads, without applying force for a pre-tightening action. The pull head of the pull test fixture has no gap with the force sensor body. Fang
Case 2: The pull-down fixture is manually tightened by threads until the threads of the pull-down fixture fully contact the threaded holes of the force sensor. The tightening stops once this condition is met. The pull head of the pull-down fixture and the body of the force sensor leave a certain gap. The pull-down fixture for the S-type force sensor and the installation of the force sensor are completed. After installation, the force direction of the load is opposite to the body of the force sensor, and there is no situation where the pull-down fixture and the sensor body are compressed. Whether the pull-down fixture is in contact with the force sensor body after being tightened or not, the force value transmission is completed by threads. From the data of this test, it can be seen that the voltage ratios (mV/V) output by the two test schemes at each load point are basically the same, and the straightness error is similar. Therefore, it can be verified that whether the pull-down fixture is in contact with the body of the force sensor basically does not affect the pull-down test performance of the S-type force sensor. In addition, foreign countries have solved the problem of the pull-down load influence of the S-type force sensor by using pull heads with joint bearings, and have achieved good application results. When S-type force sensors are used in the metrology professional field to play their value transmission traceability role, the assembly mode of the pressure ball pair and the base directly affects the accuracy of the value transmission traceability. Through simulation test and force standard machine test data analysis, it can be obtained that under the same load conditions, the data measured by the assembly scheme two, which is the reverse rotation of half a turn after tightening the ball pair, the base, and the force sensor body through threads, is significantly better than the data measured by the assembly scheme one, which is tightening the pull-down fixture and the force sensor body through threads and the plane of the fixture is in contact with the force sensor body. However, when it bears the pull-down load, whether the threads of the pull-down fixture are tightened to the point of contact with the force sensor body basically does not affect the force state of the force sensor. Therefore, when S-type force sensors are used for pressure load measurement, it is recommended that during the assembly of the ball pair and the base, the threads be tightened by reversing half a turn to make the plane of the ball pair and the base not contact the force sensor, and this assembly scheme can better improve the performance of this S-type force sensor. While it is used for pull-down load measurement, one can choose whether to tighten the threads of the pull-down fixture to the point of contact with the force sensor body at will.