RU175968U1 - Optical design of an ultra-precision holographic linear displacement sensor with a controlled mirror unit - Google Patents

Abstract

The optical scheme of the ultra-precision holographic linear displacement sensor includes sources of coherent optical radiation, a collimating system that transmits the diffraction grating, a block of receivers detecting diffracted optical beams. The device also contains a block of mirrors, which divides the collimated radiation into several beams by the number of mirrors in the block. The number of mirrors is equal to the number of radiation receivers, each mirror has the ability to independently change the angular position, due to which a phase shift is introduced. The technical result is to increase the accuracy of the sensor. 6 ill.

Description

Technical field

The utility model relates to the field of precision linear displacement sensors using optical measuring instruments by monitoring the parameters of light beams that are diffracted by various combinations of diffraction gratings operating according to the interference principle (pairwise interference of the diffracted beams with each other) and finally detected by photocells (photodetectors).

State of the art

Known sensor company Heidenhain (Germany), operating on the interference principle. The principle of operation and the optical design of this sensor (see Fig. 1) are described in US Pat. No. 4,776,701 DISPLACEMENT MEASURING APPARATUS AND METHOD (IPC G01B 11/00; G01D 5/38; (IPC1-7): G01B 9/02, publ. 1988-10-11) and can be taken as one of the analogues of the proposed utility model. This patent also had another patent analogue - USSR patent SU 1560068 (IPC G01B 11/00; published on April 23, 1990) A DEVICE FOR MEASURING MOVEMENTS. The objective of the invention of this device was to increase the resolution due to the use of diffraction gratings with a pitch of 40 microns. When illuminating gratings, one of which is transparent (transmissive), is attached to the object and made with a toothed profile, and the other is reflective and attached to another object, an interference pattern is formed, recorded using detectors located respectively in the zone of formation of zero, positive and negative first orders of diffraction image. The electrical signals from the detectors judge the direction and magnitude of the movement. In other words, this sensor contains a moving reflective grating and a non-moving optical head assembly consisting of a radiation source, an optical system, transmission gratings and four radiation receivers (detectors). Thus, this system is multi-channel - with 2 axial and 2 inclined channels.

The disadvantage of this optical sensor circuit is the increased dimensions of the optical head assembly along the length (along the long side of the reflective array). This leads to the fact that when the reflective grating is displaced in such a way that displacement is measured at the edge of the reflective grating, half of the optical head assembly extends beyond the reflective grating of length L (see Fig. 2, notes on the notation of Fig. 2: * For measurement of identical movements in the analog sensor, the length of the moving reflecting array 4 is required to be ΔL longer than in the proposed optical sensor circuit; ** The optical head in the analog sensor has a size ΔL1 larger than in the proposed optical sensor circuit). This can complicate the use of the sensor on some types of objects (for example, in machines), since it requires the presence of free space (in length) around the moving reflective grating. Another disadvantage is that for measuring displacements of magnitude L, it is necessary to make the reflective grating longer, since its symmetric illumination relative to the axis of the optical head is required (Fig. 1, 2).

The same drawback of the increased dimensions is also inherent in a number of other optical schemes of linear displacement sensors, patented in several other US patents.

US 5,430,546 OPTICAL DEVICE FOR MEASURING RELATIVE POSITION OF OR ANGLE BETWEEN TWO OBJECTS (IPC G01B 11/00; G01B 11/26; G01D 5/38; (IPC1-7): G01D 9/02, publ. 1995-07-04 ) - this system is multi-channel, but, like most devices of this type, it uses several diffraction directions, which leads to significant dimensions of the device.

US patent 5,120,132 POSITION MEASURING APPARATUS UTILIZING TWO-BEAM INTERFERENCES TO CREATE PHASE DISPLACED SIGNALS (IPC: G01D 5/38; (IPC1-7): G01B 9/02, publ. 1992-06-09) - multi-channel system. Its main drawback is the use of additional massive optical components — prisms — to produce beams, which leads to significant design dimensions.

периода по нарастающей, то есть

Т,

Т и

Т. Таким образом данная система является четырехканальной. Данные каналы являются осевыми (то есть направления излучения, падающего на приемники излучения (G3a, G3b, G3c, G3d) параллельны направлению излучения, сформированного после коллимирующей системы (объектива) (2). Принцип работы данной оптической схемы датчика заключается в том, что определение координаты происходит за счет получения и логической обработки оптических сигналов, продифрагировавших несколько раз на решетках и попарно проинтерферировавших друг с другом, на четырех приемниках излучения (PD1…PD4). На каждом из приемников сигнал формируется за счет интерференции двух пучков (интерференционный способ оценки перемещения) несколько раз продифрагировавших сигналов. Синусоидальное изменение интенсивности сигналов (при равномерном перемещении) на каждом из приемников происходит при перемещении отражающей решетки (G2) относительно пропускающей дифракционной решетки (G1). В данной схеме минимизированы габариты неперемещаемого узла оптической головки (она состоит из источника излучения (1), коллимирующей системы (объектив) (2), пропускающей дифракционной решетки (G1), четырех вторичных дифракционных решеток (G3a, G3b, G3c, G3d) и четырех приемников излучения (PD1, PD2, PD3, PD4).The closest analogue (prototype) of the proposed optical scheme of an ultra-precision topographic linear displacement sensor can be recognized as a sensor scheme according to US patent US 005569913A (B2) OPTICAL DISPLACEMENT SENSOR (IPC: H01J 3/14, publ. 1996-10-29) (see Fig. 3). The optical scheme of this high-precision linear displacement transducer contains an unmovable optical head assembly consisting of a coherent optical radiation source (laser) (1), a collimating system (lens) (2), a transmission diffraction grating (G1), and a composite component (as a multi-section phase element in the form of a single optical element with several (four) zones, made end-to-end with each other with the possibility of these phases introducing phase shifts into diffracting optical beams and transmitting radiation reflection reflected from a movable reflecting grating) in the form of four secondary diffraction gratings (G3a, G3b, G3c, G3d) and four radiation detectors (photodetectors) (PD1, PD2, PD3, PD4), as well as a moving reflective grating (G2). Moreover, the period of all gratings is constant and unified, but four secondary diffraction gratings (G3a, G3b, G3c, G3d) are shifted relative to each other by

growing period, i.e.

T

T and

T. Thus, this system is four-channel. These channels are axial (that is, the directions of the radiation incident on the radiation receivers (G3a, G3b, G3c, G3d) are parallel to the direction of the radiation formed after the collimating system (lens) (2). The principle of operation of this optical sensor circuit is that the definition the coordinates are due to the receipt and logical processing of optical signals that diffracted several times on the gratings and pairwise interfered with each other, on four radiation detectors (PD1 ... PD4). They are due to the interference of two beams (an interference method for estimating the displacement) of several times diffracted signals.The sinusoidal change in the signal intensity (with uniform displacement) at each of the receivers occurs when the reflective grating (G2) is moved relative to the transmission diffraction grating (G1). dimensions of the non-moving optical head assembly (it consists of a radiation source (1), a collimating system (lens) (2), a transmission diffraction grating (G1), four second egg diffraction gratings (G3a, G3b, G3c, G3d) and four radiation detectors (PD1, PD2, PD3, PD4).

However, a significant drawback of this implementation is that a composite element in the form of four secondary diffraction gratings (G3a, G3b, G3c, G3d) is difficult to manufacture, since the necessary accuracy of the displacement of the secondary diffraction gratings relative to each other (in their manufacture) to determine the displacement with accuracy of the order of 1 nm should be of the order of fractions of a nanometer. This can only be achieved by lithographic production (but with limitations) or the use of complex optical schemes.

Also a common drawback of all the above sensor implementation schemes is that the phase relationships introduced into the interfering beams (by measuring the intensity of which the movement is determined) are implemented using optical components with parameters that are constant over time. This does not allow the use of interchangeable diffraction gratings in the sensors, and also does not allow to dynamically (in time) change the phase relationships introduced into the interfering beams during the operation of the sensor.

Utility Model Disclosure

The use of an optical component that changes the phase relations in the beams during the operation of the sensor allows one to increase the accuracy of position determination (with the same degree of signal sampling), since it allows one to obtain in real time the maximum (or correspondingly large) signal increment (changes in the beam intensity at the receiver) with (or, accordingly, small) change in the controlled parameter (movement of the reflecting grating).

The technical result achieved by the utility model consists in increasing the accuracy of the sensor by using a prototype in the optical circuit as a replacement for a single optical element with several zones made end-to-end with each other with the possibility of these phases introducing phase shifts into diffracting optical beams and transmitting radiation reflected from the transferred reflecting grating, an additional block of mirrors (or micromirrors) installed after the collimating system and including a set of mirrors, which allows you to get a set of optical signals that are phase shifted relative to each other (due to the independent controlled rotation of each of the mirrors) and propagating in the same direction. Due to these controlled turns, a phase shift is introduced into the optical beams, which allows one to further change the phase relations of the received optical beams at the receivers of the indicated block of receivers; with the possibility of obtaining, at one of the receivers of the indicated block of receivers, a large increment in the intensity of the detected beam with a small displacement of the movable reflecting diffraction grating.

The result is achieved by the fact that the optical scheme of the ultra-precision holographic linear displacement sensor contains an unremovable optical head assembly consisting of a source of coherent optical radiation (laser), a collimating system (lens) that transmits a diffraction grating, which transmits radiation in the direction of a moving reflective diffraction grating, which is not included the composition of the optical head, and the receiver unit of the recorded diffracted optical beams. The period of all diffraction gratings is constant and uniform. In this case, the circuit further comprises a mirror unit installed after the collimating system and allowing to divide collimated radiation into several beams by the number of mirrors in the unit, which propagate further parallel to each other and have the same intensity. The number of mirrors is equal to the number of radiation receivers or zones in a multi-section receiver, each mirror has the ability to independently controlled change of its angular position, due to which a phase shift is introduced into these beams, which allows further changing the phase relations of the received optical signals at these receivers. The movement of the indicated movable reflective diffraction grating is associated with the control of the angular positions of the mirrors and the associated phase shifts of the beams to obtain at least one of these receivers a large increment in the intensity signal of the detected beam with a small displacement of the indicated movable reflective grating.

In the proposed optical sensor circuit (Fig. 4), one final direction is used in which several channels are combined. That is, all the receivers 6 (or the multi-section receiver) in the proposed scheme are located in the same diffraction order of the grating 4. The rotation of the mirrors in the mirror block introduces phase delays α (1) ... α (n) into the light beams diffracted by the transmission diffraction grating and moved reflecting diffraction grating and pairwise interfering to obtain the final signals at the radiation receivers.

The main advantage of the proposed optical sensor circuit compared to analogues and prototype is to increase the determination of the accuracy of the position of the moving reflective diffraction grating due to the use of a mirror unit in the sensor, in which each of the mirrors has independent control of its position - rotation. When each of the mirrors is rotated, the phase relations in the corresponding diffracting and then pairwise interfering beams change. These phase relationships change due to the fact that the optical paths for these beams change. An increase in the accuracy of position determination (with an identical degree of signal discretization) is achieved due to the fact that a dynamic change in the phase relations in the interfering beams (which are achieved by independent rotation of each of the mirrors) allows one to obtain (at least at one of the receivers of the receiver block or zone of a multi-section receiver) in real time the maximum signal increment (intensity change at the receiver) with a minimum change in the controlled parameter (moving about reflects the lattice).

The use of a block of mirrors in the initial beam (the beam emerging from the laser and collimated by the lens) allows you to save the minimum dimensions of the scheme along the displacement of the moving reflective diffraction grating, proposed in the prototype.

Diffraction gratings 4 and 5 (Fig. 4) can best be performed by holographic technology due to registration of the interference field created in the zone of intersection of two radiation beams with plane wave fronts. That is, these lattices are topographic according to the method of their preparation.

List of figures

FIG. 1 is an optical diagram of an analog of a holographic linear displacement transducer from Heidenhain (Germany);

FIG. 2 is an optical diagram of a prototype sensor explaining the principle of operation;

FIG. 3-optical scheme of the prototype linear displacement sensor;

FIG. 4 is a proposed optical design of an ultra-precision holographic linear displacement sensor;

FIG. 5 is an illustration of a galvanometric scanner explaining a device that allows you to change the position of an individual mirror in the mirror unit due to its rotation;

FIG. 6 is an exemplary diagram of signal changes at the receivers during the operation of the sensor (when moving a movable reflective diffraction grating).

Utility Model Implementation

In FIG. 4 of the proposed optical scheme, the following general elements are indicated: 1 - a source of coherent radiation (for example, a semiconductor laser); 2 - collimating optical system (lens); a block of mirrors 3, a transmission diffraction grating 4, a movable reflecting diffraction grating 5, 6 — a set of receivers located in joint to each other, or a single composite signal receiver. The block of mirrors 3 allows you to split the radiation coming out of the laser 1 and collimated by the lens 2, into several beams that propagate parallel to each other and have the same intensity. Further, these radiation beams are diffracted on a transmission diffraction grating 4 and on a movable reflecting diffraction grating 5 and interfere in pairs. The interfering beams fall on the block of radiation receivers 6 (or a multi-section receiver). All receivers in the proposed scheme are located in the same diffraction order of the transmission grating 4 - in a single direction (channel). With this implementation, the sensor is multi-channel, but the total radiation of all channels propagates when it falls on the block of receivers 5 in one direction (as in one channel). Separate mirrors of the mirror block during rotation introduce phase delays in the corresponding interfering beams α (1) ... α (n). This is due to the fact that the optical path in the directions of diffracted (and then pairwise interfering) beams is different when the direction of incidence of radiation on the transmission diffraction grating changes. When the sensor is initially set up when implementing a block of mirrors in the form of three mirrors, the starting phase delays can be set as 0, π / 2, π (taking into account the parameters of diffraction gratings 4 and 5). Also, the starting phase delays introduced by rotation of the corresponding mirrors can be adjusted based on maximizing the difference between the signals received at the receivers of the receiver unit 6. When the sensor starts to work, depending on the rate of change of the signals, the phase delays are changed due to the rotation of the corresponding mirrors in order to obtain at least at one receiver of the receiver block 6 of the maximum signal increment with minimal movement of the reflecting array 4.

When adjusting the sensor due to rotation of the mirrors (3), the required initial phase difference in the interfering beams is created. In this case, a significant phase difference (of the order of several tens of degrees) can be introduced into the signals due to the rotation of the mirrors both around the y axis and around the x axis at extremely small angles (of the order of fractions of angular minutes). During the operation of the sensor (when moving the movable reflecting diffraction grating), depending on the speed of change of signals at the receivers (which depend on the speed of moving the moving reflecting grating 5), the phase delays at the receivers are changed due to rotation of the corresponding mirrors (in the block of mirrors 6) the goal is to obtain at least one receiver the maximum signal increment with a minimum movement of the reflecting array 5. In this case, an increase in the determination of exactly the position of the movable reflecting diffraction grating 5 with identical geometric parameters of the sensor (e.g., its dimensions) and identical parameters of its components (e.g., periods of diffraction gratings). With all the turns of the mirrors in the block of mirrors 3, the radiation beams must not go beyond the respective radiation receivers of the block of radiation receivers 6 (or the corresponding sections of the multi-section receiver).

The block of mirrors can be made in the form of a set of mirrors mounted on the shafts of individual galvanometric scanners (Fig. 5). In this case, each of the mirrors of the mirror unit is controlled independently.

In more detail, the scheme of changing the phase difference for changing the signals at the respective receivers to obtain at least one of them the maximum signal increment (change in intensity at the receiver) with a minimum change in the controlled parameter (movement of the reflecting grating) will be explained using an example of a block of mirrors with two mirrors (3 ₁ and 3 ₂ ) and two receivers (6 ₁ and 6 ₂ ). The initial phase relations in the beams are set so that the signals at the receivers 6 ₁ and 6 ₂ have the form shown in FIG. 6 (step 1).

In this case, during the first measurement of the movement of the reflecting array (obtaining the first measurement reference) at the first receiver 6 _{1, the} maximum signal increment (changes in the intensity at the receiver) will be measured with a minimum change in the controlled parameter (movement of the reflecting array). At the second receiver 6 _{2, the} signal must differ from the signal at the receiver 6 ₁ , since the direction of movement of the reflecting grating is determined by their mutual change.

In the second measurement of the movement of the reflecting array (obtaining a second measurement reference), mirror 3 _2, due to its rotation, changes the ratio of the optical paths in the interfering beams and introduces a phase delay so that the signal at receiver 6 ₂ takes the form shown in FIG. 6 (step 2). At the same time, for the signal at receiver 6 _{2, the} condition for the maximum increment of the signal (change in intensity at the receiver) will be satisfied with a minimum change in the controlled parameter (movement of the reflecting grating).

In the third measurement of the movement of the reflecting array (obtaining the third measurement reference), the mirror 3 ₁ introduces a phase delay so that the signal at the receiver 6 ₁ takes the form shown in FIG. 6 (step 3). At the same time, for the signal at receiver 6 _{1, the} condition for the maximum increment of the signal (change in intensity at the receiver) will be satisfied with a minimum change in the controlled parameter (movement of the reflecting grating).

Next, there is a sequential change in phase delays due to the rotation of mirrors 3 ₁ , and 3 ₂ to receive signals at the respective receivers 6 ₁ and 6 ₂ , for which the condition of maximum signal increment would be fulfilled alternately when measuring the corresponding step (obtaining the next measurement reference) with a minimum change controlled parameter (movement of the reflecting grating).

Since the speed of movement of the reflecting grating can be significant, the frequency of changes in phase relations introduced by the rotating mirrors 3 will also be high. Currently, in order to reduce the requirements for the rotation speed, it makes sense to use a block of mirrors with the number of mirrors greater than 2. Then the frequency of rotation of the mirrors will decrease by a multiple of the number of zones used in the phase modulator. In this determination of the final movement of the reflecting grating, the summation of the samples takes place. These samples are obtained after digitizing the analog signal. To simplify the scheme of digitizing signals and summing the samples, it is necessary to use additional mirrors in the block of mirrors. In this case, one of the mirrors due to its rotation should provide a phase delay in the signal at the corresponding receiver with the addition of the π / 2 phase to the phase at the receiver, providing a change with the maximum increment of the signal with a minimum change in the controlled parameter. With such an implementation, displacement is calculated using the standard sin - cos model.

Implementation example

This utility model was developed as part of the theme “Research and development of an experimental sample of an ultra-precision holographic linear displacement sensor” under agreement of October 14, 2015 No. 14.577.21.0197 of MSTU named after N.E. Bauman with the Ministry of Education and Science of the Russian Federation in the framework of the federal target program “Research and Development in Priority Directions for the Development of the Scientific and Technological Complex of Russia for 2014-2020”. A prototype of the proposed optical scheme of a high-precision linear sensor was designed. The period of all gratings is T = 1 μm. The scheme used the option with 3 mirrors in the mirror unit and 3 radiation detectors. Each of the mirrors is mounted on the shaft of an independent small-sized galvanometric scanner. During the operation of the sensor, the measurement of the position of the moving reflective grating was worked out to speeds of 200 mm / min.

The approximate dimensions of the optical head assembly with a laser 1, a lens 2, a mirror unit 3, a grill 4 and a receiver block 6 can be, for example, 30 × 70 × 100 mm (excluding the size of galvanometric scanners). At the same time, the accuracy of the sensor when measuring linear displacements is higher than that of similar sensors with identical parameters and using static components for changing phase delays (phase masks; composite diffraction gratings offset from each other), namely, the error in the measurement of displacement is 0.5 nm (this accuracy corresponds to the concept of "ultra-precision" of the sensor) when moving the reflecting grating by 1 μm (when shifting by one period of the grating equal to 1 μm, we have two periods of change of the measuring signal).

Claims (1)

The optical scheme of an ultra-precision holographic linear displacement sensor in the form of an optical head, consisting of a coherent optical radiation source, a collimating system, transmitting a diffraction grating, transmitting radiation in the direction of a moving reflective diffraction grating, and a block of receivers of detected diffracted optical beams, while the period of diffraction gratings is constant and one, characterized in that it further comprises a mirror unit installed after the collie of the developing system and allowing to divide collimated radiation into several beams according to the number of mirrors in the block, which propagate further parallel to each other and have the same intensity, while the number of mirrors is equal to the number of radiation receivers, each mirror has the ability to independently controlled change of its angular position, due to which a phase shift into the indicated beams, which allows one to further change the phase relations of the received optical signals at these receivers.

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