GB2177420A - Magnetic medium used for magnetic scale - Google Patents

Abstract

The magnetic scale material is prepared from a Cr-Co-Fe base alloy consisting of 15 to 40% by weight of Cr, 5 to 35% by weight of Co, 0.1 to 5% by weight of any one of Ti, V, Zr, Nb, Mo, W, Mn, Ni, Si, Cu, Zn, Ge, and Ta, or a combination thereof, with Fe as the remainder. The magnetic scale material has a predetermined coercive force, residual magnetic flux density, hardness, thermal expansion coefficient, Young's modulus of elasticity, the number of settled particles is defined to range between 5.4 x 10<14>/mm<3> and 5.1 x 10<9>/mm<3>, and the number of foreign particles whose size range from 0.1 microns to 10 microns is limited to 589/mm<2> or fewer.

Description

SPECIFICATION Magnetic scale This invention relates to an instrumentation system involving a magnetic scale, and more particularly to the improvement of a magnetic medium used with said magnetic scale.

As is well known, an instrumentation system for measuring a length or angle by means of a magnetic scale and which can be adapted to digital display means is characterized by a high precision of measurementand its applicability invarious industrial fields. Recently, the magnetic scale type instrumentation system has also been widely accepted in the field of automatic control. An instrumentation system provided with the above-mentioned type of magnetic scale comprises a magnetic scale in which a plurality of AC magnetized patterns are arranged at a prescribed pitch, and a movable magnetic head, made so that it can face any required one of said plural AC magnetized patterns. With the above-mentioned instrumentation system, eitherthe magnetic scale or magnetic head is fixed, and the other is coupled to a movable body.A signal issued from the magnetic head,when said movable body is carried, is electrically processed thereby to measure the distance covered by said movable body.

The magnetic medium used with the aforementioned magnetic scale is required to have a magnetic property, such as a coercive force (Hc) higherthan 300 oersteds, to be spared from the effect of an external magnetic field, and further, a residual magnetic flux density (Br) higherthan 1,000 gauss, in orderto issuea more powerful signal. To date, a magnetic medium which satisfies the above-mentioned magneticre- quirements and has excellent workability has been prepared from an alloy based on copper (Cu), nickel (Ni) and iron (Fe). This alloy material has been manufactured with a composition of, for example, 65 to 75% by weight of Cu, 17to 30% by weight of Ni, and morethan 5% or more byweightofFe.

When subjected to aging at a temperature of around 600 C, the above-mentioned alloy is increased in coercive force and this facilitates cold working after the aging treatment. Therefore, said alloy offers the advantage of being easily fabricated into a plate or round rod to be used as a magnetic scale. Nevertheless, it has been found that said alloy still possesses a number of drawbacks.

The present invention has experimentally discovered the following facts: In the first place, a Cu-Ni-Fe alloy has a structure rich in copper nickel and iron nickel, with nickel as a common component. Conse quentlythe copper portion of said Cu-Ni structure is subject to segregation. Said segregation leads to a changeinthe number of particles which have settled out in the region of such segregation,thefailureto ensure uniform magnetic properties, and a decline in the precision of a magentic scale. As a result, said Cu-Ni-Fe alloy, as a whole, fails to have a uniform magnetic prnperty.Thisfact has been experimentally discovered by the present inventors. Therefore, the elimination ofthe above-mentioned segregation is of great importanceforthe magnetic medium ofthe above-mentioned type.

In the second place, Cu-Ni-Fe alloy has a thermal expansion coefficient of 13.5 x 10-6to 14.0x0-6/ C.

On the other hand, a support for holding a magnetic scale prepared from the aforementioned Cu-Ni-Fe alloy is generallyformed of iron. Iron has a lower thermal expansion coefficient(10.Ox 10-6to 11 Ox 10-6/ C) than said alloy. When, therefore, the magnetic scale composed ofthe aforementioned alloy is held by said ion support at both ends,the magnetic scale is subject to sagging, depending on temperaturn changes, or experiences tension, thereby leading to instrumentation errors.

The present inventors' experiments prove that when a magnetic scale 1 meter in length and 2 mm in diameter has been prepared from Cu-Ni-Fe alloy having a thermal expansion coefficient of 13.5 x 1 0s/0C, a length difference of 35 microns appears between said magnetic scale and an iron support having a thermal expansion coefficient of 10.0 x 1 0-6/0C, when a temperature change of 10"C appears.

Inthethird place, Cu-Ni-Fe alloy has a small Young's modulus, such as 13,000 to 14,000 kg/mm2. When, therefore, the above-mentioned magnetic scale, 2 mm in diamater and 1 meter in length, is prepared from the aforesaid Cu-Ni-Fe alloyand is held bytheiron support at both ends, sagging occurring at the central portion ofthe magnetic scale becomes too noticeable to be overlooked. In other words, the magnetic scale is prominently inclined in the direction in which the magnetic head travels. Th is drawback, too, brings about instrumentation errors.

This invention has been accomplished in view of the above-mentioned circumstances, and is intended to provide a magnetic medium fora magnetic scale as is unlikelyto bring about the aforesaid segregation, which contributes to the improvement of the instrumentation precision of the magnetic scale, which minimizes instrumentation errors arising from the thermal expansion ofthe components, and has a high wear resistance.

To attain the above-mentioned object, this invention provides a magnetic medium for a magnetic scale, said magnetic medium being prepared from a Cr-Co-Fe base alloy composed of 15to 40% by weight of Cr, 5 to 35% by weight of Co, 0.1 to 5% by weight of any one ofTi, V, Zr, Nb, Mo, W, Mn, Ni, Si, Cu, Zn, Ge, and Ta, ora combinationthereof,with Fe asthe remainder.

Said magnetic medium has a coercive force Hc of 300-1000 oersteds, a residual magneticflux density Br of 7000 to 15000 gauss, hardness Hv 350 to 600, thermal expansion coefficientof9.5-1 x 106/'C, and Young's modulus of 20,000-23,000 kg/mm2.

The number ofparticleswhich have settled outfalis within the range between 5,4 x 1014/mm3 and 5,1 x 1 09/mm3, and the number of intervening particles is limited two 589/mm2 or fewer.

The above-mentioned alloy of this invention, constituting a magnetic medium for a magnetic scale, has excellent magnetic properties such as a coercive force Hc of 300 to 1,000 oersteds and a residual magnetic flux density Brof 7,000 to 15,000 gauss. However, the following facts should be noted with respect to the subject alloy. If the content of Co of the main components (Cr, Co and Fe) falls below 5% by weight, then the residual magnetic flux density Brfalls. If the Co content rises above 35% by weight, then the coercive force decreases too much. If the Cr content exceeds 40% by weight, the residual magnetic flux density Br of said alloy drops, and if the Cr content falls below 15% by weight, the coercive force Hc of said alloy reduces too much.

The application ofthe above-listed additives Ti, V, Zr, Nb, Mo, W, Mn, Ni, Si, Cu, Zn, Ge, and Ta is intended to improve the workability ofthe alloy.

However, the addition of Mo in uncombined form deteriorates the workability of the resultant alloy, making it necessaryto apply Mo combined with any of the aforementioned other additives. Therefore the alloy prepared in the above-mentioned process has magnetic properties (for example, coercive force and density of magnetic fluxes) adapted forthe fabrication of a magnetic scale.

The Cr-Co-Fe base alloy ofthis invention, prepared as described above, has a coercive force Hc of 300-1,000 oersteds and a residual magneticflux density Br of 7,000-15,000 gausses, and constitutes an excellent magnetic medium to be used with a magnetic scale. The subject alloy is readily plastic worked and cut, and in this aspect, too, it is desirable as a magnetic medium for a magnetic scale.

The subject alloy possesses the undermentioned properties to be applied as a magnetic medium for a magnetic scale.

(1) Thermal expansion coefficient: If, as previously mentioned, a difference occurs between the thermal expansion coefficient of the material of a magnetic scale and that of its support member, holding the magnetic scale at both ends, the temperature change is likely to give rise to sagging of the magnetic scale, or may apply an unnecessary tension to said scale, thereby leading to instrumenta tion errors. Conventional Cu-Ni-Fe a alloy has a a thermal expansion coefficient of 13.5 xl - - 14.0 10-6/ C, which widely differs from the thermal expansion coefficient (1 O.0x 1 0-e - 11.0 x 10-6/ C) of standard industrial iron used as a support member, thus resulting in noticeable instrumentation errors.

In contrast, the above-mentioned Cr-Co-Fealloyof this invention has a thermal expansion coefficient ranging between 9.0 x 10-6 and 13.5 x 10-6/ C, thus making it possible to select a Cr-Co-Fe alloy having any thermal expansion coefficient falling within said range. In otherwords, it is possible to provide an alloy whose thermal expansion coefficient matches that of the iron material used as the support of the magnetic scale, thus preventing instrumentation errors from arising, due to the differentthermal expansion coeffi cients ofthe magnetic scale and its iron support.

In this connection, it will be noted that the thermal expansion coefficient ofthe subject alloy can be varied with the treatment conditions of said alloy after fusion, namely,the initial orthermaltemperatures, cooling rate or aging effect. For instance, when water cooled for 30 minutesfrom atemperature of 1,000 C,the subject alloy shows a thermal expansion coefficient of 13.2x10-6/ C.When further cooled down to 500 C and later subjectedto final aging, the subject alloy indicates a thermal expansion coefficientof9.1 x 10-6/ C. When cooled from 635 Cto 500 Cata decrement of 13"C per hour,the subjectalloy has a thermal expansion coefficient of 10.2 x 1 10-6C.

Therefore, itis possible to manufacture an alloy having the desired thermal expansion coefficient, by properly selecting the aforementioned treating conditions.

(2) The number of settled particles: The present inventors have experimentally clarified thatthe number of particles which settle out as the result of aging treatment is very closely related to the occurrence of errors in measurement by a magnetic scale. It has been disclosed that the recording property ofthe recording section of a magnetic scale is harmfully affected, no matter whether a magnetic scale contains too many or too few settled particles.

This fact clearly shows that the flux value appearing on the surface of a magnetic scale varies with the number of particles which have settled out, giving rise to measurement errors. Now, let it be assumed that it is intended to limit measurement errors to less than + 1.0 micron per 1 meter of a magnetic scale. In such case, it is required thatthe number of particles which may settle out be limited to a range between 5.4x 1014 and 5.1 x 109/mm3. However, it is preferred that the number of particles which may settle out be limited to a range between 1.4 x 1014 and 6.2 x 1010/mm .The aforementioned Cr-Co-Fe alloy offers the advantage that since the nu mber of particles which may settle out is defined by the aging treatment carried out after dissolution, it is possible therefore, to control said number by properly selecting the conditions of said aging treatment.

(3) The number of nonmetallic foreign particles: The present inventors have discovered that the number offoreign particles, such as nonmetallic compounds (mainly oxides) coexistent with settled particles,also acts to vary the flux value and deteriorate the recording property of the recording section of the magnetic scale, and consequently, it is necessary to reduce the number of said foreign particles as much as possible. If it is intended to limit the measurement errors of a magnetic scale to less than + 1.0 micron per 1 m of said scale, it is necessary, therefore, to reduce the numberofforeign particles to less than 589 per mm2, or preferably to less than 152; namely, to decrease the number of said foreign particles to the lowest possible extent.Further, if it is intended to reduce measurement errors to less than + 1.0 micron per 1 m of the magnetic scale, it is desirable, therefore, to reduce the size offoreign particles acting to change thefluxvalue,to a range between 0.1 micron to 10 micron.

When, as previously mentioned, a material constituting the magnetic scale segregates, a change takes place in the magnetic property of said material, leading to the occurrence in instrumentation errors in the segregated portions of said magnetic scale component. Conventional Cu-Ni-Fe alloy is rich in copper nickel and iron nickel. The copper portion of the copper nickel structure often tends to segregate, thereby failing to provide an alloy having a uniform magnetic property.

In contrast, it has been disclosed tha the Cr-Co-Fe alloy ofthis invention can betreated with sufficient care to prevent the segregation of any structure. If, on a magnetic scale is produced with a length of,for example, about 1 meter, and if it it required to let instrumentation errors resulting from the segregation fall within an allowable range, it is desirable, to reduce the segregation extentto less than 10 microns.

(4) Young's modulus: When a long magnetic scale is supported at both ends, the central portion ofthe scale is subjectto sagging as previously described, thereby resulting in instrumentation errors. The maximum extentS of said sagging may be expressed as 5=kl/E where: k=a constant I=the length ofthe magnetic scale E=Young's modulus It can be seen from the above equation that the maximum extentS of sgging can be progressively reduced, as Young's modulus is increased. On the other hand, Young's modulus has to be extended accordingly as the length of the magnetic scale is increased.

The above-mentioned Cr-Co-Fe alloy ofthis invention has a Young's modulus of 20,000 to 23,000 kg/mm2,thusfully conforming to the above-mentioned requirements. In contrast, conventional Cu-Ni Fe alloy has a Young's modulus as small as 13,000 to 14,000 kg/mm2, thus failing to fully meet the aforesaid requirements.

It was experimentally discovered that when a magnetic scale composed of the alloyofthis invention eomprising 11.5% byweightofCo,33% by weight of Cr, 0.5% by weight of Ti and Fe as the remainder, was formed with a length of 750 mm and a diameterof2 mm# and both ends of said magnetic scale were supported, the maximum extents ö of saging at the central portion of said scale indicated 0.55 mm. In contrast, the conventional magnetic scale prepared from an alloycomprising 22% byweightof Ni,8% by weight of Fe and Cu as the remainder, was prepared with the same measurements as mentioned above.

Thismagneticscale indicated a maximum sagging of 1.0 mm at the central portion when both ends of said conventional magnetic scale were supported.

As described above, the magnetic medium of this invention has been proved to be suitable for use with a magnetic scale in respect of coercive force Hc, residual magnetic flux density Br, workability, thermal expansion coefficient, segregation and young's modulus. In addition, the subject magnetic medium offers the advantages that the Cr-Co-Fe base alloy has a hardness Hvof350-600, a level higherthanthe hardness (240-260) of conventional Cu-Ni-Fe base alloy; and the subject magnetic medium has noticeably improved abrasion lesistance, thereby preventing the space between the magnetic scale and magnetic head from being changed as a result of abrasion.

Conventional Cu-Ni-Fe alloy has a Curie point of 480 C, and decreases in coercive force Hc and residual magneticflux density Br with rise in temper ature. In contrast, the Cr-Co-Fe base alloy of thins invention is characterised in that it has a Curie point higher than 650 C, and no change appears in the magnetic properties, such as the coercive force He and residual magneticflux density Br, uptoa temperature of 450 C. Consequently, a magnetic scale composed of the Cr-Co-Fe base alloy ofthis invention ensures accurate measurement at an elevated temperature.

The Cr-Co-Fe base alloy ofthe present invention offers the fu rther advantage that the number of particles which may settle out as the result of aging treatment and the number offoreign particles are limited to the predetermined range, thereby minimizing changes in thefluxvalue on the surface of a magnetic scale material, and reducing the measurement errors to such an extent as presents no practical difficulties.

The attached Table 1 shows six examples ofthe Cro-Fe base alloys of this invention, in which the main components are varied in their content, together with one control.

Table 1

Proportions (# by weight) Samples Cr Co Ti or Cu Fe 1 20 25 Ti - 1 the remainder 2 25 12 Ti - l Rxam- 3 33 11 Cu - 2 pies 4 35 5 Ti - 1 5 34 16 Ti - l 6 10 35 Ti - 1 Mi Fe Cu Control (% by (% by as the wt.) wt.) remainder 20 20 Referring to Table 1 above, Samples 1-4 represent the alloys ofthis invention. Sample 5 has a composition falling within the range ofthe invention, but has a thermal expansion coefficient failing outside of said range. Sample 6 has a composition slightly falling outside of said range. Sample 7 or Control represents the conventional Cu-Ni-Fe base alloy. The properties ofthe seven samples shown in Table 1 above are indicated in Property Tables 1 and 2 below.

Property Table 1

Sample Coercive force BC Residual magnetic flux No. (oersted) density Br (kilogausses) 1 600 - 740 8.8 - 10.1 2 500 - 630 14 - 15 3 750 - 850 11-12 4 430 - 480 9.5 - 11.0 5 700 - 800 9.5 - 10.5 6 380 - 450 9.8 - 11.2 7 600 - 800 6.0 - 6.5 Property Table 2

Sample Hardiness Thermal expansion Young's mosulun So. coefficient (/ C) 1 400 - 460 11.5 x 10-6 22,000 - 23,000 2 400 - 420 10.5 x 10-6 22,000 - 23,500 3 480 - 500 10.6 x 10-6 22,000 - 23,500 4 320 - 350 11.3 x 10-6 22,000 - 23,000 5 360 - 400 13.3 x 10-6 21,000 - 22,000 6 400 - 460 12.2 x 10-6 22,000 - 23,000 7 230 - 255 13.5 - 14.5 x 10 11,000 - 14,500 Note: Even when an additive, for example, Ti was changed to another additive, no noticeable difference appeared in the properties of the resultant alloy, thus providing substantially the same results as the data given in PropertyTabies land 2.

The relationship between the number of settled particles,which has been determined bya large number of experiments carried out bathe present invention,andthe measurement precision is setforth in PropertyTable 3. The relationship between the numberofforeign particle and measurement precision is indicated in PropertyTable4.

Property Table 3

Sample No. Number of Settled3Particle Measurement Pre, (particles/mm cision ( m) micron per meter 1 2.8 x 1015 1.48 2 5.4 x 1014 +1.00 3 1.4 x 1014 +0.64 4 2.0 1012 4 +0.41 5 6.2 x 1010 +0.62 6 5.1 x 109 1 +1.00 7 5.4 x 108 ! +1.62 Property Table 4

Sample No, Number of Poreign2Particles Measurement Pre (particles/mm ) cision meter micron per meter 1 152 +0.64 2 332 +0.80 3 589 +1.00 4 765 +1.50 5 790 +2.00

Claims (5)

1. A magntic medium for a magnetic scale comprising: a Cr-Co-Fe base alloy prepared from 15to 40% by weight of Cr, 5 to 35% by weight of Co, 0.1 to 5% by weightofanyone ofTi,V,Zr, Nb, Mo,W, Mn, Ni,Si, Cu, Zn, Ge, and Ta, ora combination thereof, with Fe as the remainder, said magnetic medium having a coercive force He of 300-1000 oersteds, a residual magneticflux density Br of 7000 to 15000 gauss, hardness Hv 350 to 600, thermal expansion coeffi ciency of 9.5-11.5 x 10-%C, and Young's modulus of 20,000-23,000 kg/mm2 and the number of settled particles ranging between 5.4x 014/mm a-nd 5.1 109/mm3; the number of foreign particles having sizes ranging from 0.1 micron to 10 microns indicates 589/mm2 or fewer.

2. The magnetic medium according to claim 1, wherein the number of settled particles ranges between 1,4x1014/mm3 and 6.2x1013.

3. The magnetic medium according to claim 1, wherein the number offoreign particles is 2/mm2 or fewer.

4. The magnetic medium according to claim 2, wherein the number of foreign particles is 1 52/mm2 orfewer.

5. A magnetic scale, substantially as hereinbefore described with reference to Examples.