Parameter (technique)

Technical parameter - a physical quantity characterizing any property of a technical device, system, phenomenon or process. The number characterizing this parameter (value) is its value .

A parameter is a generic name for a specific physical, geometric, or other property of a device ( process ). This can be, for example, size, speed, voltage, etc. Metrology is engaged in the study of the types of parameters, measurements, methods and means of ensuring their unity and methods for achieving the required accuracy.

Content

Types of technical parameters

Parameters are divided into input, internal and output.

Input (external) parameters reflect the external requirements for the technical device (process), their values or the nature of the change is known with varying accuracy. Some of these parameters that significantly affect the state and characteristics of the device (process) are called control.

Part of the input parameters that characterize the function performed by the device (process) are referred to functional parameters . These parameters are known during the design process.

Internal parameters characterize the state and properties of the device (process) itself. Their values are determined or specified in the design process. They are necessary to justify the decisions made, characterize the properties of the device and other purposes.

Part of the input parameters and the calculated internal parameters of the device (process) can be used as source data for another, interconnected device (process) or its model . Such parameters are called output parameters for the considered device (process) and input parameters for the newly considered one.

For example, for the “ elevator ” device, the input parameters will be, for example, the mass of the load (functional parameter) and its lifting height, the service life (they are set, come from outside), and the internal ones, for example, the diameter and material of the cable, the dimensions of the elevator car (they are determined , characterize the device and initially unknown). For the “elevator shaft” device, the previously found dimensions of the elevator car will be the input parameters and, therefore, the output parameters for the “elevator” device.

Some parameters can act as generalized parameters that combine a number of properties. These parameters are used when excessive concretization in solving the problem is not required, or causes the need for additional special knowledge. However, with this parameter, there should be a link to the document that unambiguously reveals its contents.

For example, the grade (name) of the material: steel 45 GOST 1050-88 “Long products, calibrated, with special surface finish made of high-quality carbon structural steel. General specifications. " It contains data on the composition, manufacturing conditions and other properties of the material and is a generalized parameter, say, for the designer, but not for the material scientist or metallurgist.

Depending on what the parameters characterize - the real device (process) or its model, the parameters are divided into normalized and real.

Normalized parameter

The normalized parameter (or, more correctly, the normalized value of the parameter ) is a theoretical value, the value of which is established by regulatory and technical documents and characterizes the characteristics of the model of the corresponding technical device. It is expressed by the maximum permissible parameter values . The product, the parameters of which will be inside the interval formed by these maximum permissible values, is considered workable and can be used for its intended purpose.

For example, the length of the rod indicated in the drawing is 98 ... 104 mm. This is the normalized value of the parameter established by the drawing , and 98 and 104 are its maximum permissible values (the smallest and largest maximum permissible values of the parameter ).

If one of the limit values is zero or infinity, then it is not indicated, but implied. For example, the surface hardness of a part is not less than HB180, which means 180 ... ∞. Or, for example, the lifted load is 200 kg, which corresponds to 0 ... 200.

For a grade of material, for example, steel, the maximum permissible values are contained in the corresponding GOST .

The value of the interval limited by the limiting values of the parameters is called the tolerance of the parameter. It is indicated by the letter T (in the previous example, T = 104–98 = 6 mm). The region of admissible parameter values itself is called the tolerance field .

Valid parameter

The actual parameter (or the actual value of the parameter ) characterizes the characteristics of a particular real product . It is determined by testing ^[1] or a measuring experiment with an accuracy sufficient to control this parameter.

Usually, each measured actual value is unique, since its value depends on external conditions, manufacturing conditions, method and accuracy of measurement, and many other factors. In order to increase the reliability of knowledge of the parameter value, a series of measurements are carried out, the results of which will have a scatter within some interval. For this reason, the actual value of the parameter is set by the range. The coincidence of the actual values of the same parameters of products from their party is possible only within the limits of measurement accuracy.

For example, a rod length of 97 ... 98 mm was established by measurements. This is the actual value of the parameter, the true value of which lies within the range specified by the total measurement error. Improving the accuracy of measurements will narrow this range, for example, to 97.6 ... 98.1 mm.

The accuracy is estimated by the measurement error , which is the difference between the real and true values of the parameter. For the true value of the parameter, the ideal value is taken, to which the actual value of the parameter tends to increase the accuracy of the measurement. The true value cannot be determined experimentally, since all measuring instruments have some measurement error. Instead of the true value, to evaluate the measurement error, take the actual value of the parameter determined by another means of measurement, the error of which is an order of magnitude less than the allowable value for this purpose.

The measurement error includes components, the causes of which are the means of measurement, the measurement method and the operator (subject).

Rating parameter

For convenience, the parameters are recorded using the nominal ^[2] parameter (the nominal value of the parameter ), that is, its value that serves as the starting point for the actual and maximum permissible deviations. Subjectively appointed by a person or is the result of operations with the same nominal parameters.

For example, the rod length indicated on the drawing can be written as 101 ± 3 mm. Here 101 is the nominal value, ± 3 are deviations setting the limit values of the parameter (98 ... 104). In the given example, the nominal value is selected from the middle of the interval and, as a result, the deviations will be symmetrical. If we take a “round” value of 100 as the nominal value, then the recording form of this normalized parameter will take, for example, the following form $\varnothing 100_{-2}^{+4}$ ${\ displaystyle \ varnothing 100 _ {- 2} ^ {+ 4}}$ , where +4 is the value of the upper limit deviation (100 + 4), −2 - the lower (100 + (- 2)).

The nominal parameter can be considered the grade of the material given without reference to the corresponding GOST, for example, steel 45.

Often they operate only with nominal values of parameters, for example, indicate the length of the rod as 100 mm. Solving equations with parameters specified in this form is more convenient, although the feeling of accuracy not only of the source data, but also of the result of the calculations, is lost.

However, the product is considered suitable if the actual values of its parameters fall into the interval specified by the limiting values of the normalized parameter. If only the nominal value of the normalized parameter is indicated, then formally the value of the interval is equal to zero and it is practically impossible to fall into such an interval and, therefore, each product will be rejected for this parameter. Therefore, in the documentation (especially intended for other users - the customer, contractor, buyer, other specialists) it is customary to provide normalized parameter values, and not indicate only their nominal values.

To eliminate the excessive variety of nominal values of the parameters, they are recommended to be normalized, that is, brought into line (for example, to round the calculated values) with the preferred numbers .

Estimation of a technical parameter value

Parameter values can be evaluated as follows:

measurement and, if necessary, subsequent conversion into the necessary characteristics;
calculation based on mathematical models ;
statistical methods or direct piece -by -piece recounting;
organoleptic methods , that is, through vision, hearing, smell, taste, touch. For these purposes, experts are attracted, special devices are used, etc.
sociological surveys , for example, taking into account the opinions of a certain group of people;
expert judgment methods .

Notes

↑ GOST 15.309-98. Development and production systems for production. Testing and acceptance of products. Key Points
↑ The word “nominal” comes from the Latin word “nominalis” (translated as “nominal”) and has the meaning of “such by one name”, “only called, but not fulfilling its purpose, fictitious”.

Literature

A.I. Yakushev, L.N. Vorontsov, N.M. Fedotov . Interchangeability, standardization and technical measurements. 6th ed., Revised. and add. - M.: Mechanical Engineering, 1986. - 352 p.
Khoroshev A.N. Introduction to Design Management of Mechanical Systems: A Training Manual. - Belgorod, 1999 .-- 372 p. - ISBN 5-217-00016-3 . 2011 electronic version

[1] GOST 15.309-98. Development and production systems for production. Testing and acceptance of products. Key Points

[2] The word “nominal” comes from the Latin word “nominalis” (translated as “nominal”) and has the meaning of “such by one name”, “only called, but not fulfilling its purpose, fictitious”.