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The article provides comparative analysis of autocollimators models available on the domestic market, with special attention paid to digital autocollimators AC-D manufactured by scientific and production corporation Diagnostica
Manufacturers and consumers
Autocollimators (Fig. 1) are used to measure a range of micro- and macro-optical details, optical details gluing, reflective surface movement angles, as well as to control surface plates and sliding guides. Currently, on the Russian market, most part of laboratories and optical enterprises use visual autocollimators manufactured by "PA "Novosibirsk Instrument-Making Plant" (e.g. models АKU-0.2, AKU-0.5 and AKU-1 (Fig. 2)) [1]. Such popularity of these devices is primarily associated with their relative low cost and traditional reputation.
These models possess weak points typical of visual devices, i.e. low efficiency (due to labor-intensive adjustment and use of visual reading method); high probability of subjective operator’s errors (fatigue); visual reading method adversely effects the operator’s vision; low accuracy in measurement.
As a foreign alternative, expensive import digital autocollimators produced by companies Trioptics (e.g. TriAngle models) [2] or Tailor Hobson (Elcomat series models (Fig.3)) [3] are used.
Given that all drawbacks of work with visual devices are closely associated with the dependence thereof on operator’s physiological characteristics, and the obvious fact that the modern equipment and technique supposes wide use of a computer for measuring and data processing, the Research-and-production company NPK DIAGNOSTIKA has developed a range of Digital Autocollimators. Thus, several years ago, the market of digital autocollimators was challenged by relatively inexpensive domestic autocollimators AC-03D, AC-05D and AC-1D [4].
All autocollimators of AC-D series (Fig. 4) are included in the State Register of Measurement Equipment, have a Measurement Equipment Quality Medal. Autocollimators manufactured by NPK DIAGNOSTIKA are successfully used at the relevant optical enterprises such as State Optical Institute OJSC, Research Institute for Optical and Electronic Instrument OJSC, Research and Production Company Precision Manufacturing System OJSC, Kuybyshev Oil Refinery Plant OJSC and at a number of commercial organizations. Autocollimators allow to make automatic angle measurements both for static and dynamic objects. Measurement objects reflecting surfaces may differ in size and reflection index.
We can now return to the keystone – the theory of autocollimators, then compare technical characteristics and criteria for evaluation of modern autocollimators.
Theory of calculation and production
Designers and manufacturers always do their best to achieve maximum possible instrument precision. However, for a variety of reasons, actual equipment usually is less precise. The errors depend on many factors. One of the influencing factors is a theoretical error of an instrument, i.e. the deflection of an actual scheme or method of calculation from the strictly theoretical one. In this specific case, autocollimator is clear of a theoretical error. Another factor is an error resulted from inaccurate manufacture of individual parts and final accuracy of adjustment operations. As it is well known, all details, both optical, and mechanical, are produced not according to the nominal size, but allowing for some tolerance, depending on an accuracy class. In the calculation and design of a device, nominal dimensions should be taken into consideration. Some errors are likely during the assembly. According to the degree of impact on instrumental accuracy, the errors can be divided into two groups: those not subject to correction during assembly (e.g. non-uniform ruing), and those, which impact may be significantly reduced when adjusting the equipment (e.g. unbalanced scale). Final accuracy of reduction of effect of errors shall be set allowing for a tolerance for adjustment. Thus, in an assembled and adjusted device, there is a number of factors (initial errors) influencing instrumental accuracy, i.e. the accuracy of measurement of some or other value. Therefore, in the calculation, to reveal all initial errors, to define the degree of influence thereof upon outcome via position function of the mechanism, and compute random instrumental error is of prime importance. After continuous operation, for a variety of reasons (negligent storage, negligent operation, reasonable tear and wear of parts in operation, temperature difference, and humidity), measuring accuracy of the equipment may change. That is why errors of indication shall be checked on a regular basis.
Measurement theory
Autocollimators (AC) are one of common and well-known high-accuracy angle measurement devices used in a number of machine-building industries, instrument engineering, geodesy and measuring equipment.
AC represents an optical device based on autocollimation phenomenon. AC is used for accurate measurements of a mirror angular position on an object under control. AC may be used for control of straightness and flatness of sliding guides (e.g. of a machine). Extensive use of AC in control equipment field is determined by high sensitivity of autocollimation method to insignificant angular movement oh the mirror.
Recall that autocollimation is an automatic return of a light beam leaving the lens focus back to the focus (i.e. integration of a collimator and a telescope in the same device), if a flat mirror is located behind the lens strictly perpendicularly to the optical axis. During autocollimation, an AC optical system light beam sent is self-directional to its axis. The general principal of operation of autocollimator consists in the following. In the focal plane M of lens 2, a light mark A is placed (Fig.5).
Beams leaving this point, having passed the lens 2, travel in a parallel beam to get onto the flat mirror 1. If the mirror is located perpendicularly to the beam axis, beams reflected from the mirror shall return in the same way, and upon passing through the lens shall form an image coinciding completely with a fluorescent mark. If the mirror bends to the axis of the beam falling onto it under angle α, the reflected beam will travel back under the angle 2α to the initial direction, and image of the point A will form in the focal plane F in the point А’ at a distance C from it. This distance may be calculated according to the formula: C = F tg 2 α.
With a measuring scale placed in the focal plane of the lens M and knowing the lens focus distance F, the value of the scale intervals for any angle of the mirror position can be calculated. The value C for one scale division will be the division interval 2α – a division value for autocollimation beam path.
Classification of autocollimators
According to the number of coordinate directions where measurements are taken, AC are subdivided into the following subclasses: single-axis for measuring angular swivel of an object in one dimension, two-axis for measuring angular swivel in two inter-perpendicular dimensions, three-axis for additional measuring of a twist angle, and multifunctional ones. The more coordinate directions are used, the more complex the circuit and design of a device [5].
According to the stage of involvement of a supervisor in measurements, all autocollimators are divided into two classes: visual and photoelectric autocollimators (PEAC) [5].
Main characteristics of visual AC are given in [6]. In PEAC, the main parameter readings are taken with photoelectric receivers without a human eye, and the degree of automation is determined by their purpose, it varying.
Mostly, the AC considered represents the analogue devices characterized by corresponding weak points. Analogue PEAC normally have optical compensators, which serve for image-motion compensation. Introduction of compensators lead to further errors.
With digital PEAC, a control element swivel angle is determined, e.g., by metering standard pulses filling the time interval between a reference pulse and an operating pulse arising when radiation flow reflected by the control element gets onto the analyzer. In recent years, with availability of multielement photodetectors (CCD arrays), digital AC of a new generation have been developed.
The principle of operation, design and application area of import digital PEAC are described in detail in [7, 8], while of up-to-date domestic ones – in [9].
PEAC with CCD arrays does not require use of compensators and allows for increase in the measurement range by several degrees. Owing to more accurate and convenient connection with other units of a measuring or monitoring electrooptical equipment PEAC with CCD camera, currently replace analogue AC [10]. Besides, digital PEAC are easier to adjust to the PC as compared to the analogue ones.
Compare the autocollimators models AC-D (NPK DIAGNOSTIKA) reviewed at the beginning of the article according to three main parameters: accuracy, performance and cost effectiveness, and compare different models of autocollimators according to their performance specifications (Table 1).
Accuracy
Digital Autocollimator AC-03D is the most precise one of all series of autocollimators AC-D. It complies with the first-class measurement equipment. Unique data processing algorithms and high-precision CCD camera ensure high accuracy of measurements. Due to quality manufacture of optical and mechanical elements, total accuracy of this Digital Autocollimator is ±0.3arc-sec. Autocollimator ensures efficient handling of the objects having small reflecting area surface, as well as having surfaces with low reflection index – about 4%.
Automation of statistical processing of repeated measurements is ensured by software Autocollimator (Fig. 6). In this autocollimator the RMS error does not exceed 0.1 arc-sec. At the end of measurement series, a report with measurement result datasheet is displayed on the PC.
To minimize the errors (aberrations) created by an autocollimator optical system, the illuminator is providently equipped with a red monochromatic source of light. The source of light reduces to minimum external thermal effect, it being designed based on a superbright LED with the wavelength of 650 nm.
The next most precise autocollimators are: a Digital Autocollimator AC-05D (total accuracy ±0.5arc-sec) and AC-1D (total accuracy ±1arc-sec).
Performance
Digital autocollimators AC-D make it possible to control angles and angular displacements in a wide range and to the highest precision, as well as assist in solving the problems of assembly control and optical detail gluing. Small size and weight of the AC allow for lower costs of an operator’s working place arrangement.
Due to direct visualization of the measurement process (displaying the autocollimator field of view onto the computer monitor), and to automation of the computations, the autocollimator features high speed of measurements and minimized operator’s errors.
Measurement range of autocollimators AC-D (field of view) is quite wide, which adds convenience during initial adjustment of details under control, and further operation efficiency. Intuitive and graphic English software (Fig. 7) and automated principle of measurements as included in the operation algorithm will not take long time for the operator to prepare and study the device in operation.
Cost effectiveness
Autocollimators AC-D are completely made by domestic manufacturers. High-skilled experts of the enterprise perform quality mechanical assembly of units of the equipment and accurate adjustment of optical elements. Proven technologies of manufacture and unification of assembly procedures allow for relatively low market value of the device. The design does not require frequent maintenance, therefore, current expenses for servicing are almost unnecessary. Easy preparation for work with the equipment, computer processing and taking readings need minimum training.
Typically, products with optical details are the output of high optical technologies. These technologies are the key ones for industries to meet challenges. Technological novelties are, certainly, associated with control and measuring equipment. Autocollimation devices (Digital Autocollimators of AC-D series (Fig. 8)) manufactured by NPK DIAGNOSTIKA will be of help to optical enterprises to increase the output of proper products, raise their quality, and create truly innovative and competitive products.
We invite all industry experts and glad to show you our products on the upcoming exhibition "PHOTONICS. WORLD OF LASERS AND OPTICS-2014" in the Central Exhibition Complex "EXPOCENTER" Moscow 25-27.03.2014, Pavilion 7, Hall 1. Our Booth is 71B13.
Autocollimators (Fig. 1) are used to measure a range of micro- and macro-optical details, optical details gluing, reflective surface movement angles, as well as to control surface plates and sliding guides. Currently, on the Russian market, most part of laboratories and optical enterprises use visual autocollimators manufactured by "PA "Novosibirsk Instrument-Making Plant" (e.g. models АKU-0.2, AKU-0.5 and AKU-1 (Fig. 2)) [1]. Such popularity of these devices is primarily associated with their relative low cost and traditional reputation.
These models possess weak points typical of visual devices, i.e. low efficiency (due to labor-intensive adjustment and use of visual reading method); high probability of subjective operator’s errors (fatigue); visual reading method adversely effects the operator’s vision; low accuracy in measurement.
As a foreign alternative, expensive import digital autocollimators produced by companies Trioptics (e.g. TriAngle models) [2] or Tailor Hobson (Elcomat series models (Fig.3)) [3] are used.
Given that all drawbacks of work with visual devices are closely associated with the dependence thereof on operator’s physiological characteristics, and the obvious fact that the modern equipment and technique supposes wide use of a computer for measuring and data processing, the Research-and-production company NPK DIAGNOSTIKA has developed a range of Digital Autocollimators. Thus, several years ago, the market of digital autocollimators was challenged by relatively inexpensive domestic autocollimators AC-03D, AC-05D and AC-1D [4].
All autocollimators of AC-D series (Fig. 4) are included in the State Register of Measurement Equipment, have a Measurement Equipment Quality Medal. Autocollimators manufactured by NPK DIAGNOSTIKA are successfully used at the relevant optical enterprises such as State Optical Institute OJSC, Research Institute for Optical and Electronic Instrument OJSC, Research and Production Company Precision Manufacturing System OJSC, Kuybyshev Oil Refinery Plant OJSC and at a number of commercial organizations. Autocollimators allow to make automatic angle measurements both for static and dynamic objects. Measurement objects reflecting surfaces may differ in size and reflection index.
We can now return to the keystone – the theory of autocollimators, then compare technical characteristics and criteria for evaluation of modern autocollimators.
Theory of calculation and production
Designers and manufacturers always do their best to achieve maximum possible instrument precision. However, for a variety of reasons, actual equipment usually is less precise. The errors depend on many factors. One of the influencing factors is a theoretical error of an instrument, i.e. the deflection of an actual scheme or method of calculation from the strictly theoretical one. In this specific case, autocollimator is clear of a theoretical error. Another factor is an error resulted from inaccurate manufacture of individual parts and final accuracy of adjustment operations. As it is well known, all details, both optical, and mechanical, are produced not according to the nominal size, but allowing for some tolerance, depending on an accuracy class. In the calculation and design of a device, nominal dimensions should be taken into consideration. Some errors are likely during the assembly. According to the degree of impact on instrumental accuracy, the errors can be divided into two groups: those not subject to correction during assembly (e.g. non-uniform ruing), and those, which impact may be significantly reduced when adjusting the equipment (e.g. unbalanced scale). Final accuracy of reduction of effect of errors shall be set allowing for a tolerance for adjustment. Thus, in an assembled and adjusted device, there is a number of factors (initial errors) influencing instrumental accuracy, i.e. the accuracy of measurement of some or other value. Therefore, in the calculation, to reveal all initial errors, to define the degree of influence thereof upon outcome via position function of the mechanism, and compute random instrumental error is of prime importance. After continuous operation, for a variety of reasons (negligent storage, negligent operation, reasonable tear and wear of parts in operation, temperature difference, and humidity), measuring accuracy of the equipment may change. That is why errors of indication shall be checked on a regular basis.
Measurement theory
Autocollimators (AC) are one of common and well-known high-accuracy angle measurement devices used in a number of machine-building industries, instrument engineering, geodesy and measuring equipment.
AC represents an optical device based on autocollimation phenomenon. AC is used for accurate measurements of a mirror angular position on an object under control. AC may be used for control of straightness and flatness of sliding guides (e.g. of a machine). Extensive use of AC in control equipment field is determined by high sensitivity of autocollimation method to insignificant angular movement oh the mirror.
Recall that autocollimation is an automatic return of a light beam leaving the lens focus back to the focus (i.e. integration of a collimator and a telescope in the same device), if a flat mirror is located behind the lens strictly perpendicularly to the optical axis. During autocollimation, an AC optical system light beam sent is self-directional to its axis. The general principal of operation of autocollimator consists in the following. In the focal plane M of lens 2, a light mark A is placed (Fig.5).
Beams leaving this point, having passed the lens 2, travel in a parallel beam to get onto the flat mirror 1. If the mirror is located perpendicularly to the beam axis, beams reflected from the mirror shall return in the same way, and upon passing through the lens shall form an image coinciding completely with a fluorescent mark. If the mirror bends to the axis of the beam falling onto it under angle α, the reflected beam will travel back under the angle 2α to the initial direction, and image of the point A will form in the focal plane F in the point А’ at a distance C from it. This distance may be calculated according to the formula: C = F tg 2 α.
With a measuring scale placed in the focal plane of the lens M and knowing the lens focus distance F, the value of the scale intervals for any angle of the mirror position can be calculated. The value C for one scale division will be the division interval 2α – a division value for autocollimation beam path.
Classification of autocollimators
According to the number of coordinate directions where measurements are taken, AC are subdivided into the following subclasses: single-axis for measuring angular swivel of an object in one dimension, two-axis for measuring angular swivel in two inter-perpendicular dimensions, three-axis for additional measuring of a twist angle, and multifunctional ones. The more coordinate directions are used, the more complex the circuit and design of a device [5].
According to the stage of involvement of a supervisor in measurements, all autocollimators are divided into two classes: visual and photoelectric autocollimators (PEAC) [5].
Main characteristics of visual AC are given in [6]. In PEAC, the main parameter readings are taken with photoelectric receivers without a human eye, and the degree of automation is determined by their purpose, it varying.
Mostly, the AC considered represents the analogue devices characterized by corresponding weak points. Analogue PEAC normally have optical compensators, which serve for image-motion compensation. Introduction of compensators lead to further errors.
With digital PEAC, a control element swivel angle is determined, e.g., by metering standard pulses filling the time interval between a reference pulse and an operating pulse arising when radiation flow reflected by the control element gets onto the analyzer. In recent years, with availability of multielement photodetectors (CCD arrays), digital AC of a new generation have been developed.
The principle of operation, design and application area of import digital PEAC are described in detail in [7, 8], while of up-to-date domestic ones – in [9].
PEAC with CCD arrays does not require use of compensators and allows for increase in the measurement range by several degrees. Owing to more accurate and convenient connection with other units of a measuring or monitoring electrooptical equipment PEAC with CCD camera, currently replace analogue AC [10]. Besides, digital PEAC are easier to adjust to the PC as compared to the analogue ones.
Compare the autocollimators models AC-D (NPK DIAGNOSTIKA) reviewed at the beginning of the article according to three main parameters: accuracy, performance and cost effectiveness, and compare different models of autocollimators according to their performance specifications (Table 1).
Accuracy
Digital Autocollimator AC-03D is the most precise one of all series of autocollimators AC-D. It complies with the first-class measurement equipment. Unique data processing algorithms and high-precision CCD camera ensure high accuracy of measurements. Due to quality manufacture of optical and mechanical elements, total accuracy of this Digital Autocollimator is ±0.3arc-sec. Autocollimator ensures efficient handling of the objects having small reflecting area surface, as well as having surfaces with low reflection index – about 4%.
Automation of statistical processing of repeated measurements is ensured by software Autocollimator (Fig. 6). In this autocollimator the RMS error does not exceed 0.1 arc-sec. At the end of measurement series, a report with measurement result datasheet is displayed on the PC.
To minimize the errors (aberrations) created by an autocollimator optical system, the illuminator is providently equipped with a red monochromatic source of light. The source of light reduces to minimum external thermal effect, it being designed based on a superbright LED with the wavelength of 650 nm.
The next most precise autocollimators are: a Digital Autocollimator AC-05D (total accuracy ±0.5arc-sec) and AC-1D (total accuracy ±1arc-sec).
Performance
Digital autocollimators AC-D make it possible to control angles and angular displacements in a wide range and to the highest precision, as well as assist in solving the problems of assembly control and optical detail gluing. Small size and weight of the AC allow for lower costs of an operator’s working place arrangement.
Due to direct visualization of the measurement process (displaying the autocollimator field of view onto the computer monitor), and to automation of the computations, the autocollimator features high speed of measurements and minimized operator’s errors.
Measurement range of autocollimators AC-D (field of view) is quite wide, which adds convenience during initial adjustment of details under control, and further operation efficiency. Intuitive and graphic English software (Fig. 7) and automated principle of measurements as included in the operation algorithm will not take long time for the operator to prepare and study the device in operation.
Cost effectiveness
Autocollimators AC-D are completely made by domestic manufacturers. High-skilled experts of the enterprise perform quality mechanical assembly of units of the equipment and accurate adjustment of optical elements. Proven technologies of manufacture and unification of assembly procedures allow for relatively low market value of the device. The design does not require frequent maintenance, therefore, current expenses for servicing are almost unnecessary. Easy preparation for work with the equipment, computer processing and taking readings need minimum training.
Typically, products with optical details are the output of high optical technologies. These technologies are the key ones for industries to meet challenges. Technological novelties are, certainly, associated with control and measuring equipment. Autocollimation devices (Digital Autocollimators of AC-D series (Fig. 8)) manufactured by NPK DIAGNOSTIKA will be of help to optical enterprises to increase the output of proper products, raise their quality, and create truly innovative and competitive products.
We invite all industry experts and glad to show you our products on the upcoming exhibition "PHOTONICS. WORLD OF LASERS AND OPTICS-2014" in the Central Exhibition Complex "EXPOCENTER" Moscow 25-27.03.2014, Pavilion 7, Hall 1. Our Booth is 71B13.
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