JPH0321882A - Rotating speed measuring method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for measuring rotational speed using light, and is used to measure the rotational speed and unevenness of copying machine transfer drums, optical disks, and polygon mirror drive motors. This invention relates to a method for measuring the rotational speed of a crane, which is applied to the measurement of wow and flutter.

(Prior Art) Laser Doppler method is known as a method for determining the rotational speed 4I using light. Laser Doppler method is scattered by moving particles? This method takes advantage of the fact that the laser beam undergoes Doppler shift.
Among these, the differential type (incident light method) is the most well known.

In addition, as a method for measuring rotating bodies, the angular velocity measurement method (Japanese Patent Application Laid-open No. 63
-Refer to Publication No. 222267).

Conventional Example 1 Figure 5 shows an example of a differential type optical Doppler velocimeter. In the figure, light emitted from a laser light source 1 is divided into two optical paths by a half mirror HM and travels. One reaches the moving object 2 as the measurement target via mirrors Ml and M2, and the other also reaches the moving object 2 via the mirror M3. It is assumed that the moving object 2 is moving at a speed V in the direction of the arrow. The reflected light of each of the above-mentioned lights reaching the moving object 2 is detected by a photoelectric detector 3, and the detection signal is processed through an amplifier 4 to determine the speed V of the moving object 2.

Conventional Example 2 In the angular velocity measurement method, as disclosed in the above-mentioned publication, a circularly polarized light source with a frequency ω■ is attached to a rotating body, a circularly polarized light source with a frequency ω2 is provided externally, and both light beams are superimposed. Here, if the angular velocity of the rotating body is ω', the frequency ω is shifted and becomes ω,+ω'. Therefore, ω' can be found by detecting the optical beat signal ω1-ω2±ω' with a photodetector.

(Problem M to be solved by the invention) In the conventional example 1, the moving object to be measured is required to be at the intersection of two light beams, and therefore, if there is vibration or displacement in an out-of-plane direction, the measurement cannot be performed. It is difficult to measure the velocity of an object with a rough surface, and there are problems in that it is easily affected by speckle noise and there are restrictions on the arrangement of the measuring means.

Regarding Conventional Example 2, since the laser light source must be attached to the rotating object to be measured, it can only be applied to large objects for which space can be secured, and is limited in this respect. Furthermore, in order to generate two types of lasers and a beat, it is necessary to use a very stable laser, and such lasers are generally expensive and difficult to miniaturize.

An object of the present invention is to provide a rotational speed measuring method that allows easy arrangement of measuring means, is resistant to external disturbances, and is capable of measuring rotational speeds of relatively small objects. (Means for Solving the Problems) In order to achieve the above object, in the rotational speed measuring method of the present invention, counterclockwise and clockwise circular or elliptically polarized light is attached to the rotating surface of a rotating body as a measuring object. The beat signal of the light generated at this time was detected, and the rotation speed of the rotating body was determined from this beat frequency.

(Example) An example embodying the measuring method according to the present invention is shown in FIG.

In the figure, a polarizer 11 is attached via a mirror M to the end face of a rotating body 10 as a measurement target.

A laser light source or the like (not shown) is placed in front of the polarizer 11, and a 174-wavelength Fil2 and a half mirror HMI are sequentially placed in the middle, and a photoelectric detector 13 is placed on the optical path branched by the half mirror HMI. . First, linearly polarized light beams orthogonal to each other are made incident on a 174-wave plate. This 1/4 wavelength plate 12 has a fast axis of crystal that is 4 times different from each linearly polarized light.
174 wavelength plate 12
The light flux that has passed becomes right-handed circularly polarized light E8 and left-handed circularly polarized light E2. This light flux is incident on a polarizer 11 attached to a rotating body 10 rotating within the polarization plane of circularly polarized light. A polarizer converts left and right circularly polarized light into linearly polarized light LR, L[. and is reflected by a mirror M provided on the back surface of the polarizer 11.

The reflected light flux is guided to the photoelectric detector 13 via the half mirror HM1 placed in the optical path without undergoing any conversion by the polarizer 1l. Here, for example, when the polarizer 11 is stationary, left and right circularly polarized light passes through the polarizer 11 and becomes coherent and exhibits a certain intensity. Further, for example, when the polarizer 11 has an angular velocity ω. When the beam rotates at 200 degrees, a frequency shift occurs in the left and right circularly polarized light, and an optical beat signal is detected. This can be explained using Jones Matrix as follows.

Right-handed circularly polarized light ER and left-handed circularly polarized light E. Expressed mathematically, it becomes,
A polarizer rotating with an angular velocity ω can be expressed as When left and right circularly polarized light is incident on a rotating polarizer, equation (1). (2) is obtained by the action of equation (3). However, Eo is the amplitude.

Here, the light intensity when each of the circularly polarized lights in equations (4) and (5) interfere can be found by calculating I=lE1 +E,l'', and I=I.( 1 +sin(2 ωtt)i However, ■ is E1, which is the direct current of the light intensity.Therefore, by measuring the time change of the light intensity I, the rotational speed ω,, is determined regardless of the absolute values of I and Io. Seek.

The above equation indicates that the interference light is intensity-modulated at a frequency twice the angular velocity of rotation, and an optical beat signal is generated. Due to the rotation of the polarizer, the left and right circularly polarized light is ±ω. This means that a frequency shift of 2ω occurs, and a beat signal of 2ω corresponding to the frequency difference between the two light beams is generated due to interference.

In other words, since only the mutual frequency difference generates an optical beat signal, even if the oscillation wavelength of the laser beam changes, its influence is removed. Therefore, by detecting the generated beat frequency, the rotational speed of the object can be determined.

Furthermore, since both light beams pass through the same optical path, they are not affected by phase changes in the optical system along the way, and a stable signal can be obtained.

A light source having linearly polarized light as shown in FIG. 1 is already commercially available as a transverse Zeeman laser. but,
Since such lasers are generally expensive, the same effect can be obtained by using an inexpensive laser that emits only a single linearly polarized light. An example of the configuration in that case is shown in FIG.

In the figure, symbol l4 is a polarizing beam splitter, symbol 1
5 indicates a l/2 wavelength plate. Here, the 1/2 wavelength plate l5 is for rotating the vibration direction of the linearly polarized light coming from the light source so that the ratio of light amounts separated by the polarizing beam splitter 14 becomes equal. By making the light quantity ratio equal, the generated beat signal S/N can be increased.

Moreover, mutually orthogonal linearly polarized lights can be obtained by using a birefringent polarizing element such as a Savard plate or a Wollaston prism in place of the polarizing beam splitter l4.

Next, as shown in FIG. 3, the polarizer 11 attached to the rotating body 10 is made larger than the rotating body 10,
By arranging the photoelectric detector 13 at a position opposite to the mirror M via the polarizer l1, the light flux used in the examples of FIGS. Optical beat signals can be detected without using a half mirror HMI. According to this example, the number of parts is reduced and adjustment of the optical system is also very easy.

In the example shown in Fig. 4, as in the example shown in Fig. 2, orthogonal linearly polarized light is obtained by the polarizing beam splitter l6, and after separating into orthogonal polarized light, the 174 wavelengths are not superimposed. The plate 12 converts the light into left and right circularly polarized light, and the light is incident on a rotating polarizer t':. Lens 17 for reflected light
It is designed to condense light and cause interference. In this example, the two
Since the light beams are spatially close to each other and pass through the same optical element, it is possible to obtain a stable signal.

Note that an external resonance type LD is available as a light source that can obtain left-right circularly polarized light, and by using such an element, it is also possible to downsize the entire structure.

(Effect of the invention) The polarizer attached to the rotating body only needs to be large enough to reflect or transmit the light beam, and if a fine light beam is used, it can be made sufficiently small. It can also be measured without affecting rotation.

Furthermore, since the signal is detected directly by specularly reflected light or transmitted light, the signal strength can be high and speckle noise does not occur, so measurements with a high S/N ratio can be performed.

Further, since the optical system can be a common path optical system, stable measurement can be performed without being affected by external disturbances. It is not affected by out-of-plane displacement or vibration of rotating objects.

Furthermore, since we are measuring the frequency difference between left and right circularly polarized light,
Measurement can be performed regardless of wavelength fluctuations of the light source.

[Brief explanation of the drawing]

FIGS. 1 to 4 are diagrams explaining embodiments of the present invention, and FIG.
The figure is an explanatory diagram of the prior art. 10...Rotating body, l1...Polarizer. -548-

Claims (1)

[Claims] Counterclockwise and clockwise circular or elliptically polarized light is incident on a polarizer attached to the rotating surface of a rotating body to be measured, the beat signal of the light generated at this time is detected, and the rotational speed of the rotating body is determined from this beat frequency. A rotational speed measurement method characterized by determining .