The present invention relates to a method of forming a magnetic substrate for an encoder scale.

Magnetic encoders having passive magnetic scales are known. Such encoders include a scale that comprises a series of markings that have a different magnetic permeabi lity than the surrounding material. The magnetic permeability variations of the scale can be sensed using an associated readhead comprising a magnet and a plurality of magnetic field sensors (e.g. Hall sensors).

It has been described previously in JP63098501 how encoder scale marking can be formed in magnetic material by using a laser beam to heat small regions of material. These heated regions are converted from a magnetic material to a non- magnetic material.

An object of the present invention is to provide an improved method of producing a magnetic substrate for an encoder scale. Such a magnetic substrate produced by the present invention may, for example, subsequently have scale markings applied thereto using a laser marking process.

According to a first aspect of the present invention, there is provided a method for producing a magnetic substrate for an encoder scale, the method comprising the step of mechanical ly working the substrate, wherein the substrate is cooled prior to the mechanical working step. Advantageously, the substrate is cooled after the mechanical working step. The substrate may also be cooled during the mechan ical working step.

The combination of mechanical working (also termed cold forming) and cooling of the substrate has been found to increase the amount of magnetic material formed during substrate processing. This improves the magnetic permeability d ifference between subsequently formed scale markings and the substrate thereby providing improved magnetic encoder performance.

Conveniently, the substrate comprises a metal. The substrate may comprise steel. Preferably, the substrate comprises stainless steel. Advantageously, the stainless steel is an austenite grade of stainless steel; e.g. A1S 1 304L grade stainless steel may be used. The substrate preferably comprises a non-amorphous (e.g.

crystall ine) material. The substrate is thus preferably formed from a material that can adopt a non-magnetic phase and a magnetic phase. This may, for example, comprise a material that can adopt an austenite (non-magnetic) and martensite (magnetic) phase.

Advantageously, the mechanical working step is performed on a substrate that has been cooled to a temperature below room temperature. Preferably, the mechanical working step is performed on a substrate that has been cooled below 0° Celcius. In a preferred embodiment, the mechanical working step is performed on a cryogen ically cooled substrate. Such cryogenic cooling may be provided by immersion of the substrate in a bath of liquid nitrogen.

The substrate may be formed in any suitable shape. Preferably, the substrate is elongate. Advantageously, the substrate comprises a rod. Conveniently, the substrate may take the form of a tape.

Any suitable mechanical working step may be performed on the substrate. For example, pressing, hammering, beating etc. Advantageously, the mechanical working step comprises drawing the substrate. Preferably, the mechanical working step comprises the step of rolling the substrate. Conveniently, the mechanical working step comprises the step of perform ing a plurality of working operation on the substrate. The substrate may comprise an elongate substrate, such as a rod or tape, and a thickness of the elongate substrate may be reduced by each working operation. For example, the rod diameter or the tape thickness may be reduced by each working (e.g. each drawing or rol ling) operation. Preferably, the desired final thickness of the elongate substrate is obtained in a plurality of steps. Conven iently, a thickness of the elongate substrate is reduced by no more than 20% during each working operation. Preferably, a thickness of the elongate substrate is reduced by approximately 5 to 10% during each working operation.

The method may comprise a step of heating the substrate after completion of the mechanical working step. The heating step may comprise heating the substrate to an elevated temperature for a prolonged period. For example, the substrate may be heated above 100°C, above 200°C or above 300°C . Advantageously, the substrate may be heated to around 450°C. The elevated temperature may be maintained for at least an hour or at least two hours. Preferably, the elevated temperature is below any phase transition temperature above which the material reverts to a nonmagnetic (e.g. austenitic) state.

The method may comprise the additional step of applying a surface hardening step after the step of mechanically working the substrate. For example, the surface hardening step may conveniently comprise pulsed plasma nitriding.

A step may also be performed of using a laser to locally heat the substrate to form non-magnetic markings therein that define an encoder scale. This may be done before the surface hardening step.

The present invention also extends to a magnetic substrate for an encoder scale produced using the method described above. Preferably, the encode scale is a passive magnetic scale. It should be noted that a "passive" magnetic scale is not magnetised in any way (i.e. it does not generate a magnetic field) but has local magnetic permeabil ity variations that affect the magnetic field generated by the magnet of an associated magnetic scale reader unit. This should be contrasted to "active" magnetic scales in which north and south magnetic poles are formed.

The invention wi l l now be described, by way of example only, with reference to the accompanying drawing (Figure 1 ) that shows the relative content of magnetic phase material as a function of the area reduction provided by the drawing process.

In a preferred embodiment of the method that will now be described in detail, the present invention allows a high content of martensite phase to be formed in an austenite grade of stainless steel. The resulting stainless steel substrate (which may be provided in the form of a tape or rod) can then be made into an encoder scale by creating small non-magnetic regions using a suitable marking process (e.g. laser marking). Martensite is formed in austenitic stainless steel during cooling below room temperature (thermally) or by mechanical working (also called "cold forming" because the material is not heated during working). The present inventors have found that the amount of martensite formed in a rod or tape can be increased by using both cooling and mechanical working.

The temperature at which martensite starts form ing when cooled depends on the carbon (C) and nitrogen (N) content of the steel. The lower the C and N content, the higher the temperature associated with martensite forming. The stainless steel material A1SI 304L was found to be a suitable material.

The magnetic content of the steel was detected using Feritscope MP-30. It should be noted that the results (percentage of ferrite grade) of the magnetic content measurements presented herein are not actual (absolute) values of magnetic content. Such measurements are thus provided purely for comparative purposes.

A first experiment was performed by cooling 2mm thick (flat) tapes of AISI 304L stainless steel down to between -30 and -70 °C before reducing their thickness. This cooling was done by immersing the tapes in a bath of liquid nitrogen prior to drawing the tapes through drawing dies.

The best magnetic contents achieved were around 25% if cooling was done only prior to deformation. It was, however, found that if the tapes were cooled before deformation and also cooled again just after the deformation, the magnetic contents increased to around 40 %.

Further increases in the magnetic content of the tapes were obtained by performing the deformation step in multiple stages. Immersing the tape in liquid nitrogen before and after deformation provided magnetic contents in range of 55% to 60 % if the deformation occurred in several stages. It is thought that if the deformation is high (e.g. the reduction of thickness is 30 - 40% in one pass) the temperature of the tape increases during the deformation thereby slowing the formation of martensite. The best results were achieved when the tape's thickness was reduced by 5 to 1 0% at each pass through the cyl inders.

The process was repeated with stainless steel (A1S1 304L grade) rods by drawing the rods step by step through the drawing dies. The rods were cooled to - 196 °C before drawing and immersed again in liquid nitrogen just after passing the dies. The magnetic contents achieved were again in range from 55 to 60%.

The accompanying figure 1 shows the relative content of magnetic phase material of the rod as a function of the percentage area reduction. It seems that approximately 55% of magnetic phase is obtained if the rod's area is reduced by 25 to 35 %. An additional increase of magnetic content of approximately 10% was observed if the rod was heated up to 450°C in vacuum for several hours after the cooling/forming process. Following rod (or tape) formation, a surface hardening process may be performed. This can improve mechanical robustness of the scale and may be done after any required scale markings are formed in the rod. This surface hardening process may comprise, for example, a plasma nitriding process of the type described below.

A rod made using the method outlined above was taken that has a relative content of magnetic phase of 62%. This rod was m ^' trided at 400 °C for 1 0 hours in a 25% N2 and 75% H ₂ atmosphere. The rod had locally heat treated regions of approximately semicircular shape (i.e. scale markings); these were about 0.13 mm deep and 0.28 mm wide (on the surface). These regions are non-magnetic (austenite) markings formed in the surface of the rod using a laser treatment process.

Following the nitriding process, the hardness of the rod's core increased from 460 HVio to 580 HV,o (620 HV ₀₀₁ ). The surface hardness of the rod itself is 13 1 8 HV001 after nitriding, while surface hardness of the rod on the top of the heat treated regions is 470 H V0.01 - Hardness within the heat treated area (i.e. within austenite region) is 295 HV ₀ .oi - The depth of the nitrided layer was found to be approximately 8μm in martensite and 3.5 - 4 μηι in austenite regions.

A small increase (in percents. not in tens of percents) of signal amplitude was observed in the nitrided sample. This effect is thought to occur due to a small amount of ε martensite (i.e. martensite that is paramagnetic) being transformed into ferromagnetic (α') martensite because of the treatment at temperatures of approximately 400 to 450 °C for several hours. It is important to note that the above is merely one example of the present invention. The method may be applied to different materials and the substrates may be used for purposes other than making encoder scale. The various temperature and processing parameters outlined above are also merely il lustrative and the skilled person would readily appreciate how the process could be adapted to other materials.