WO1997005456A1

WO1997005456A1 - Compact disc based high-precision optical shaft encoder

Info

Publication number: WO1997005456A1
Application number: PCT/US1996/012298
Authority: WO
Inventors: Markus P. Hehlen
Original assignee: The Regents Of The University Of California
Priority date: 1995-07-25
Filing date: 1996-07-25
Publication date: 1997-02-13
Also published as: AU6681196A

Abstract

A compact disc based optical shaft encoder compises a shaft (14) to be measured for rotation and a compact disc (13) attached to the shaft. The compact disc defines at least one track of concentrically located equally separated pits. Laser means (11, 12) are in optical communication with the compact disc for optically interrogating the pits with laser light. Detector means (11, 12) receive the laser light from the pits for sensing rotational movement of the shaft.

Description

COMPACT DISC BASED HIGH-PRECISION OPTICAL SHAFT ENCODER

FIELD OF THE INVENTION

The present invention generally relates to angular measurement, and, more specifically, to the use of compact discs to provide precise angular measurement. This invention was made with Government support under Contract No. -7405-ENG-36 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

The present invention relates to a Provisional Application filed July 25, 1995, which received serial number 60/001,423.

Precise angular measurement is important in a great many areas, including robotics, machine tools, surveying equipment, tracking systems for astronomical telescopes, gyroscopes for aviation, wind sensing, and sensors for ground-vehicle guidance systems. These devices normally employ what are called optical shaft encoders (OSE) to provide the required angular measurement. The OSE are also used as key elements that allow computer control of many devices which have moving parts.

In the field of industrial robotics, medium angular resolutions of 1,000 to 10,000 pulses per revolution (ppr) are typically used, and the respective OSE, for example, are applied for determining translation distances and rotation frequencies of moving elements in production lines. Significantly higher resolutions of greater than 20,000 ppr are required in a variety of high-precision applications such as machine tools, triangulation equipment for surveying, tracking systems for astronomical telescopes, gyroscopes for aviation, wind sensing, or sensors for ground-vehicle guidance systems.

Conventional OSE measure the transmission of light from a LED (light-emitting diode) through a moving, coded disk to determine the turning angle. This approach is limited by both the resolution of photographically prepared code disks and the spectral properties of the LED output. Conventional high- precision OSE typically use up to 5,400 lines as angular information on a 6.1 cm (2.4 in.) diameter disk. However, by applying additional advanced pulse-multiplier electronics, resolutions of up to 86,400 ppr can be realized. However, no standard OSE exist for the many applications in which extensive interpolation required by currently available OSE is not acceptable, in which highly miniaturized encoders are required, or in which even higher resolution than is available with current OSE is necessary.

As a result of the diffraction limit on the minimum diameter of the focal region of a collimated light source, highest spatial definition is achieved by focusing monochromatic light of short wavelength emitted from a laser. Several laser-based information storage concepts exist, the most common being the compact disc or CD.

Commercial CD data-storage technology applies semiconductor diode lasers operating in the red spectral region in combination with silicon detectors used to sense data bits written to and read from a reflecting surface. For read/write CD-technology applying red light the typical diameter of a single bit (pit) is approximately 1.25 mm. This small bit size is not only the basis for the huge data storage capacity but also for the very high angular resolution of a CD. Specifically, the outermost track of a standard CD can hold up to 138,000 single-spaced pits which divide the 360° circumference in angular steps of approximately 9.4 arc seconds (45.5 mrad) .

The present invention applies this inherently high angular resolution of compact discs (CD) to provide an OSE having extremely high angular resolution and/or being highly miniaturized. A CD-based OSE, according to the present invention, consists of a laser/detector optical system which senses a sequence of equally spaced, laser-written pits on the surface of a CD, and which subsequently generates a sequence of counter pulses the number of which is proportional to the CD turning angle. It is therefore an object of the present invention to provide an Optical Shaft Encoder having extremely high angular resolution.

It is another object of the present invention to provide an extremely accurate Optical Shaft Encoder which can be miniaturized for use in limited space environments.

Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the apparatus of this invention comprises a shaft to be measured for rotation with a compact disc attached to the shaft. The compact disc contains at least one track of concentrically located equally separated pits. Laser means are in optical communication with the compact disc for optically interrogating the pits with laser light. Detector means receive the laser light from the pits for sensing rotational movement of the shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:

FIGURE 1 is a plot comparing a) the intrinsic angular resolution (pulses per revolution and per cm diameter, pprd) of a data-storage CD, experimentally determined using a red helium- neon laser, (b) the estimated highest intrinsic angular resolution achievable using one red laser sensing a track of equally spaced diffraction-limited pits on a CD of the same diameter, and (c) the intrinsic angular resolution typically used in conventional OSE using a line disk with the same diameter.

FIGURE 2 is a cross-sectional side view of one embodiment of the present invention comprising a 60.96 mm (2.4 in.) diameter CD, two laser/detector systems operating in the red and sensing a concentrical track consisting of equally spaced diffraction- limited bits at the surface of the CD in a quadrature arrangement, and one detector/laser system sensing a zero-marker track for calibration purposes.

FIGURE 3 is a plot of the processed signals from two lasers operating in accordance with the principles of the present invention, illustrating the use of the quadrature arrangement to determine the turning angle of the compact disc, and enhance the intrinsic resolution.

FIGURE 4 is a top view of a portion of a CD-based OSE according to the present invention, showing the positioning of a third laser/detector system used to detect a zero marker on a separate track on the surface of a compact disc.

DETAILED DESCRIPTION

The present invention provides apparatus for the high- precision measurement of angles . It accomplishes this high precision measurement of angles through the application of a combination laser/detector system sensing a track consisting of equally spaced pits at the surface of a compact disc (CD) . The invention is best understood through reference to the drawings.

Figure 1 presents the angular resolution experimentally determined for a commercially available data-storage CD which is indicated by line (a) . Since commercial data-storage CD use bit compression techniques, the measured angular resolution of 10174 pprd is, as expected, a factor of 1.24 smaller than the estimated 12566 pprd resolution expected for a custom-made CD, having equally spaced 1.25 mm diffraction-limited bits for the CD-based OSE according to the present invention. This resolution is illustrated by line (b) .

As is made clear by Figure 1, a CD-based technique for OSE clearly outperforms the typical intrinsic angular resolution of approximately 886 pprd shown by line (c) for a standard OSE. The

CD-based techniques outperform the standard OSE resolution by more than a factor of 11, and a the preferred custom-made CD, operated by a red laser, by approximately a factor of 14, .

The use of this intrinsically high angular resolution of a CD for a CD-based OSE in principle requires only one laser/detector system, that is, a conventional laser combined with a conventional laser detection system, sensing one track of equally spaced pits on the surface of a CD. Such a simple device however does not provide any information on the turning sense of the shaft and lacks any ability for calibration of the system.

One embodiment of a high-precision CD-based OSE .10. according to the present invention which provides both turning sense and calibration information is schematically shown in a cross- sectional view in Figure 2. As illustrated laser/detector systems I∑, 12 are positioned below compact disc .13_. to sense a track of equally spaced pits (not shown) on surface 13a of compact disc .13_.. In other embodiments, laser/detector systems 11, 12 as well could be located above compact disc 13_, or in any arrangement that allow laser/detector systems 1JL, .12_. to be in optical communication with surface 13a. Compact disc 13. is attached to shaft 14. and also to plate and shaft JL5_.. Plate and shaft 15 terminate in bearing JL6. which is attached to enclosure 17.

Compact disc 13_ may be of any diameter that is appropriate for the intended application. In one embodiment, compact disk 13 may have a diameter of 60.96 mm (2.4 in.) .

Laser/detector systems lλ. 12 may operate in a quadrature arrangement. That is, the pulse trains generated by laser/detector systems lJL, 12 which interact with the pits on surface 13a, are shaped by electronics 18. and are phase shifted by 90° through precise relative positioning of laser/detector systems IJ-_., JL2_.. As shown, electronics 18_ also can be located within enclosure 37, and contain conventional pulse shaping and quadrature circuits. Interface 18a, mounted to enclosure 17, allows connection of power to electronics 18_ and to laser/detector systems H, 12_., and the output of signals from electronics 18_ indicative of the angular movement of shaft 14. The various waveforms generated by CD-based OSE 0 through electronics 18_. upon movement of shaft 14. are illustrated schematically in Figure 3. The quadrature arrangement provided by the present invention, and graphically illustrated in Figure 3, not only allows for the determination of the turning sense, but also allows for a four-fold increase in the intrinsic angular resolution of CD-based OSE 10_.. The intrinsic angular resolution of a general compact disk is given by:

(4π-d_CD)/( 2-d_pits)= pulses per revolution (ppr) (10) where d_CD = the diameter of a particular concentrical pits track on the compact disc, and d_pits = the diameter of the pits on the surface of the compact disc. Using red semiconductor diode lasers in laser/detector systems 11, 12_. and equation 10, with CD- based OSE 10_. having a concentrical pits track diameter, d_CD, of 60.96-10^"3 meters and a pit diameter, d_pic, of 1.25-10^-6 meters, CD-based OSE IQ_. is characterized by an intrinsic angular resolution of :

(4p-60.96-10^"3)/(2-1.25-10-⁶)= 306,418 ppr. (20)

This result corresponds to an intrinsic angular resolution of 4.23 arc seconds (20.51 mrad) . Without using any additional pulse multiplier logic in electronics JL8., CD-based OSE 10 outperforms high-end (interpolated) standard-technology OSE of the same disc size as an embodiment of compact disc 13 by more than a factor of three. With the incorporation of additional conventional electronic interpolation procedures within electronics 18, the improvement is more than an order of magnitude.

Figure 4 shows a top view of a portion of CD-based OSE 10, showing the positioning of a third laser/detector system 19, used to detect a zero marker on a separate track on surface 13a of compact disc 13.. This extra track, and laser/detector 9 provide a pulse that is used for calibration of CD-based OSE 10.

An example of one possible application of the devices shown in Figures 2 and 4 is with astronomical telescope tracking. The minimum angular resolution required for high-performance telescope tracking is 60 arc seconds, with a periodic tracking error of less than 5 arc seconds. CD-based OSE 1_ (Figures 2 and 4) provides an angular resolution of 4.23 arc seconds with an estimated angular error of less than ~0.4 arc seconds, far better than the high-performance resolution requirements. Since this resolution is achieved without pulse multiplier logic in electronics 18, CD-based OSE 10 can easily operate at the extremely low shaft rotational frequencies typical for astronomical tracking. Additionally, CD-based OSE 10_. has the potential of being compact, light-weight, durable, and of having a low power consumption.

In an appropriate application, the angular resolution of CD-based OSE 10_. can be increased by increasing the diameter of a concentrical track of bits on compact disc H, and/or by applying gears between shaft 14 and CD-based OSE 10. However, the intrinsic angular resolution is determined by the diffraction- limited diameter of a single pit on surface 13a of compact disc 13. This limit can be decreased, and accordingly the intrinsic angular resolution increased, by using laser light of shorter wavelength (1) . For example, the use of a blue (1=450 nm) instead of a red (1=900 nm) laser source in laser/detectors 11, 12 , and 19 for writing and reading on compact disc 13. increases the angular resolution by a factor 2, and thus improves the intrinsic angular resolution of CD-based OSE 10 to 2.12 arc seconds (10.26 mrad, i.e. 25132 pprd) , still without using additional electronic interpolation procedures.

In another variation of the present invention, the pits (not shown) on surface 13a (Figure 1) could be irregularly shaped, as perhaps in the shape of a teardrop. Such defined irregularity in the shape of the pits would provide a method for discerning the .direction of rotation of shaft 14. (Figure 1) using only a single laser/detection system, either laser/detection system H, or laser/detection system 12 (Figure 1) .

The foregoing description of the preferred embodiments of the invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

Claims

WHAT IS, CLAIMED IS:

1. A compact disc based optical shaft encoder comprising: a shaft to be measured for rotation; a compact disc attached to said shaft, said compact disc containing at least one concentrical track of equally separated pits; at least one laser means in optical communication with said compact disc for optically interrogating said pits with laser light; and at least one detector means receiving said laser light from said pits for sensing rotational movement of said shaft.

2. The compact disc based optical shaft encoder as described in Claim 1 wherein said at least one track of equally separated pits comprises two concentrical tracks of equally separated pits, said at least one laser means comprises two laser means, and said at least one detector means comprises two detector means for enabling use of a quadrature technique.

3. The compact disc based optical shaft encoder as described in Claim 1 wherein said at least one track of equally separated pits comprises one concentrical track of equally separated pits, said at least one laser means comprises two laser means, and said at least one detector means comprises two detector means for enabling use of a quadrature technique.

4. The compact disc based optical shaft encoder as described in Claim 1, further comprising a concentrical track of pits for providing a zero-marker track for calibration purposes.

5. The compact disc based optical shaft encoder as described in Claim 1 wherein said laser means operates at a short wavelength.

6. The compact disc based optical shaft encoder as described in Claim 1 wherein said equally separated pits are irregularly shaped to directly provide rotation direction detection.