Schneider instrumentation accuracy

It refers to the degree to which the average value measured repeatedly under certain test conditions is consistent with the true value, which is indicated by error. It is used to indicate the magnitude of the error in the system. In practice, the control test is generally carried out with standardized substances or standardized methods, and when there is no standardized substance or standardized method, the pure substance with the measured components is often added to carry out the recovery test to estimate and determine the accuracy. When the error is small, the average value of the truth μ can be used as an estimate from the average value of the repeated parallel measurements. Good precision is a prerequisite for achieving outstanding accuracy, and generally speaking, if the precision is not good, it will not be possible to have outstanding accuracy. Regarding an ideal analysis method and analysis results, it is required to have both good precision and good accuracy.

Schneider instrumentation accuracy

Measurement accuracy refers to the degree of consistency between the measured results and the measured true value; The accuracy of the measuring instrument refers to the ability of the measuring instrument to give an echo close to the true value. Precision is only a qualitative concept, not a quantitative expression. The ** value of the measurement error is large, and its accuracy is low. But accuracy doesn't equal error. Accuracy is only available as: high, low; Large, small; Expressions of pass and fail. Regarding the accuracy of the measuring instrument, there are also grades or other expressions. It is wrong to give the precision of the dosage value, e.g. the precision is 0.5 mg, where it is not clear what 0.5 mg is.

Accuracy: It is used to indicate the degree of systematic error and random error in the measurement result.

The degree to which the average value of the measured value is close to the true value.

error

Physical experiments are inseparable from the measurement of physical quantities, which are both direct and indirect. Because of the constraints of instruments, test conditions, environment and other factors, the measurement can not be infinite, and there will always be a definite difference between the measured value of the physical quantity and the objective real value, which is the measurement error.

Let the measured true value (true size) be a, the measured value be x, and the error be ε, then

　　x——a=ε

Mistakes are different from faults, mistakes should and can be avoided, while mistakes cannot be avoided. From the principle of the test, the instrument used in the test and the adjustment of the instrument, to each measurement of the physical quantity, there are unavoidable errors, and throughout the whole test.

The difference between the measured value and the true value is called an error.

When measuring, because various elements will constitute a small number of errors, it is necessary to understand these elements and effectively solve them, so that the errors in the entire measurement process can be reduced to the smallest. When measuring, the main errors that constitute errors are system errors and random errors, and system errors have the following conditions: misreading, miscalculation, parallax, scale error, wear error, touch force error, deflection error, cosine error, Abbe error, thermal deformation error, etc. The size of the system error is unchanged in the measurement process, which can be obtained by accounting or experimentation, that is, it can be guessed, and can be corrected or adjusted to reduce it. These elements are grouped into five broad categories, which are described below:

1. The human element

Because of the errors caused by human elements, including misreading, miscalculation and parallax. Misreading often occurs in measuring tools such as vernier rulers and centimeter cards. The vernier scale is likely to misread a small reading, such as 10.02 mm or 9.98 mm at 10.00 mm. The centimeter card scale is easy to misread the size of a pitch, such as 10.70 mm or 9.70 mm at 10.20 mm. Miscalculations often occur when accounting errors or when entering faulty data. Parallax often occurs when the direction of reading the measurement value is different or the scale plane is not in the same plane, and the difference between the two scale planes is about 0.3-0.4 mm, and if the reading scale is not perpendicular to the scale plane, the error amount will occur. In order to eliminate this error, the manufacturer of the measuring tool plans the carving of the vernier to be the same height or close to the same height as the engraving of the ruler, (the vernier is engraved with an arc-shaped composition and the ruler is almost the same height, the vernier is concave V-shaped and the ruler is convex V-shaped, thus constituting two carved equals.)

2. Gage elements

Because the errors caused by the elements of the measuring tool include scale errors, wear errors and unproofed before use. Whether the scale division is accurate or not, it is necessary to calibrate and trace it through a more delicate instrument. After a period of use, the measuring tool will be worn to a considerable extent, so it is necessary to calibrate or send it for repair before it can be used again.

3. Strength Factor

Because of the use of touch force or the deflection error caused by touch. According to Hooke's law, when measuring the scale, assuming that the axle and the machine part are touched by a certain measuring force, the axle and the machine will be partially or fully elastically deformed, in order to avoid this elastic deformation, the axle and the machine should be made of the same data. Secondly, according to Hertz's law, if both the axle and the machine are made of steel, the amount of error caused by its elastic deformation

When the workpiece is measured with a gauge, the gauge is fixed on the support, and the bracket will constitute elastic deformation because of the measured force, as shown in Figure 2-4-3, the cross-sectional secondary moment of the length is , the long pillar is , and the longitudinal elastic coefficients are respectively , so when the measured force is P, the deflection is . To avoid such errors, the strut can be enlarged and the length of the measuring axis can be minimized. In addition, larger gages such as centimeter cards, vernier rulers, straight gauges and long gauge blocks are bent due to their own weight and load. In general, when the fulcrum bearing of the two end faces parallel to the vertical line is 0.577 full length, the two end faces can remain parallel, and this fulcrum is called the Airey Points. The fulcrum of the line scale gauge is 0.5594 in the direction of its full length, and its full-length bending error is *small, which is called Bessel Points here

4. Measure the elements

When measuring, errors caused by poor instrument planning or placement include cosine error, Abbe error, etc. The cosine error occurs when the measurement axis and the appearance to be measured are skewed at the viewpoint, as shown in Figure 2-4-5, and the error amount is , which is the practical measurement length. In general, cosine errors occur in two measurement directions, and it is necessary to be especially careful. For example, when measuring the inner hole, the radial measurement scale needs to be *large scale, and the axial measurement needs to take *small scale. In the same way, when measuring the outside, it is also necessary to pay attention to its correct orientation. It is necessary to choose the anvil and the appearance of the workpiece to be measured carefully, such as the spherical anvil when the surface of the workpiece to be measured is flat, and the planar anvil should be selected when the workpiece is cylindrical or spherical. Abbe' Law states that the axis of the measuring instrument and the axis of the workpiece to be measured should be on the same line. Otherwise, an error occurs, and this error is called Abbe's error. In general, if the axis of the measuring instrument and the axis of the workpiece to be measured cannot be together, the interval should be shortened as much as possible to reduce the error value. If the vernier ruler measures the workpiece as an example, as shown in Figure 2-4-6, the error is, so if you want to reduce the measurement error of the vernier ruler, you need to reduce the angle formed by the gap between the ruler and the vernier ruler and should be as close to the scale mark as possible when measuring. If the scale measures the workpiece as an example, as shown in Figure 2-4-7, the probe of the gauge is spherical, the workpiece is cylindrical, and the two axes have an offset amount, and the amount of error in its touch is . If the probe and workpiece of the gauge are both flat, if the two planes are skewed, the amount of error in their touch is as shown in Figure 2-4-8, and this error is called sinusoidal error. Figure 2-4-9 shows the error analysis diagram of the cam in the organization planning, in order to reduce wear, the end of the follower is often planned into a ball or cylinder with a radius of , and the pressure angle between the two is , thus causing the error to .

5. Environmental elements

When measuring, depending on the environment or site, the errors that may be constituted include thermal deformation errors and random errors. Thermal deformation errors generally occur due to room temperature, human touch and the temperature of the workpiece after processing, so it is necessary to touch the workpiece and measuring tools by hand under the control of temperature and humidity, and the workpiece is measured after cooling after processing. However, in order to shorten the processing time, it is necessary to measure in real time during processing, so it is necessary to consider the thermal expansion coefficient of various data as compensation, so as to cope with the error caused by the different thermal expansion coefficients of temperature data. The thermal expansion coefficients of commonly used materials are shown in Table 2-4-2. In general, it is necessary to use the following formula: CMR: The length of the workpiece at 20°C.

Schneider instrumentation accuracy

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