WO2016171630A1

WO2016171630A1 - Incremental magnetic motion or rotation encoder with a reference pulse

Info

Publication number: WO2016171630A1
Application number: PCT/SI2016/000012
Authority: WO
Inventors: Janez Novak
Original assignee: Rls Merilna Tehnika D.O.O.
Priority date: 2015-04-22
Filing date: 2016-04-13
Publication date: 2016-10-27
Also published as: SI24975A

Abstract

An incremental magnetic motion or rotation encoder with a reference pulse of the invention is characterized in that a non-magnetic scale is applied over at least part of the basic magnetic scale as a thin layer and there are at least two types of sensors within a reading head, wherein one or several sensors of the first type detect magnetic properties of the scale, while one or several sensors of the second type detect properties of the non-magnetic scale and the reference pulse is generated by the non-magnetic scale.

Description

INCREMENTAL MAGNETIC MOTION OR ROTATION ENCODER WITH A REFERENCE PULSE

The object of the invention is an incremental magnetic motion or rotation encoder with a reference pulse.

Prior Art

A position encoder is a device that detects a physical change that appears at motion or rotation and converts it to an analogue or digital electrical signal. It generally consists of two parts that move relative to one another: a scale and a reading head containing sensors that detect physical changes while a reading head passes along a scale. Many different types of encoders are available nowadays. Depending on the principle applied for sensing physical changes there are magnetic, optical, capacitive, inductive and other encoders. Encoders also differ as to their type of output signals. Incremental and absolute encoders are known. Incremental encoders report a relative change and a direction of a change in motion or rotation of a reading head with respect to a scale. The reading head of an incremental encoder generates periodic sinusoidal or pulse shaped signals, the number of which is proportional to a change in a reading head position.

Absolute encoders measure and generate the proper position of a reading head with respect to a scale immediately at power-up.

Incremental encoders require calibration after each power-up since the periodic nature of generated signals makes it impossible to determine, on which part of the scale the reading head is present. Calibration requires a reference mark; this is a special spot on a scale that represents a starting point or home. The reading head generates an additional signal in the reference mark, the so-called reference pulse that allows a unit counting periodic signals to report the position of the reading head with respect to the reference mark. The reference mark on linear scales is usually in the centre of the scale or at its ends. A scale may also have a plurality of reference marks, among which a user can select which one is to be used, e. g. with an external switch.

There are several ways of applying a reference mark to a magnetic encoder scale. A few ways are disclosed in patent document US8405387. The scale may be divided in two strips. One strip is made of a periodic sequence of magnetized regions, which is read by one magnetic sensor within the reading head. Another strip is made up of a periodic sequence of magnetized regions, yet this sequence is changed at one or several spots along the scale. Another magnetic sensor in the reading head, which is shifted transversally to the first one, detects a spot of change in the sequence on the second strip and reproducibly allows generation of a reference pulse. One of advantages of this solution is that there is no need for additional external switches that would increase the dimensions of the magnetic encoder. On the other hand, one of drawbacks of the described solution is that the minimum width of the scale is limited since a strip with periodically magnetized regions and a strip with a change must be arranged on the scale in parallel. The minimal width of one strip is of an order of one period of the magnetized region, a total of two in fact. If the strip with periodic magnetization and the strip, on which the periodic magnetization is changed on one spot, were narrower than the period of magnetization, the first sensor of a magnetic field would be so close to the second strip that it would not only detect the periodic sequence but also a change in the sequence and this would be reflected in an error of periodic signals and consequently in poorer accuracy of such an encoder when the reading head were in the vicinity of the reference mark.

The mentioned drawbacks are solved by the incremental magnetic encoder with a reference pulse of the invention, wherein a non-magnetic scale is applied over at least part of the basic scale as a thin layer, preferably a cover film with a non-magnetic type of information, and there are two types of sensors within the reading head, wherein one or several sensors of the first type detect magnetic properties of the scale, while one or several sensors of the second type detect properties of the nonmagnetic scale. The reference pulse is generated by the non-magnetic scale which is applied over the magnetic scale. Detection in change of non-magnetic scale properties is used for generation of a reference pulse and for determination of a reference mark.

The invention will now be explained in more detail by way of embodiments with reference to the accompanying drawings.

Figure 1 illustrates a first embodiment of a magnetic encoder of the invention.

Figure 2 shows a typical electrical signal from an optical sensor when a reading head passes a region with reduced reflectivity on a cover layer, and its conversion to a reference pulse.

Figure 3 illustrates a second embodiment of a magnetic encoder of the invention. Figure 4 illustrates a third embodiment of a magnetic encoder of the invention, where an inductive sensor is integrated in the reading head.

Figure 5 shows a typical electrical signal from an inductive sensor, when the reading head passes over a short conductive layer glued or applied onto a magnetic strip.

Description of Embodiments

In the first embodiment illustrated in Figure 1, a scale in the form of a magnetic strip 3 that comprises only periodic magnetized regions is provided with a thin cover film 4 glued thereon, whose upper surface is highly reflective. The cover film 4 has a region 5 where the reflectivity of the film changes. The cover film 4 is made of a non-magnetic material that does not interfere with the magnetic field between the magnetic strip 3 and a reading head 6. Such materials are for instance austenitic stainless steel and aluminium. Typically, both materials can be adequately treated, e. g. by grinding and polishing, to reach high reflectivity of the surface. Within the reading head 6, an optical sensor 2 is added to a magnetic sensor 1 for detecting changes in the magnetic field along the magnetic strip 3. The optical sensor 2 in the embodiment has a source and a detector of reflected light arranged within the same housing. Since the electrical signal from the optical sensor 2 is proportional to the amount of the reflected light, it is suitable to detect presence of the cover film underneath the reading head. In fact, the value of an electrical signal from the sensor 2 is high if a cover film is present underneath the optical sensor and low in the absence of a cover film or if its light reflectivity is considerably lower. When the optical sensor in the reading head passes over the region 5 where the reflectivity of the cover film 4 gets changed, the value of the electrical signal from the optical sensor 2 gets changed. Through comparison with an adequate comparative level and by negation, such change in the value can be converted into a shape suitable for a reference pulse. This is shown in Figure 2. A distinctive change in the reflectivity of the surface of the cover film can be realized in several ways: by laser marking, by a mechanical increase in surface roughness, by applying a dark paint, etc. One of the possibilities for a distinctive change in reflectivity is to have a cover film 4 with a hole, through which the magnetic strip 3 is visible, and since the latter contains dark brown ferrite particles, it can be very well differentiated from the cover film 4. Since the optical sensor 2 is added to the magnetic sensor 1 in the axis of the strip 3 and not transversally to the strip 3 as in the case of the above-mentioned known patent document, the minimal width of the strip 3 is equal to one period of magnetization. And since the cover film 4 is non-magnetic, it does not change the magnetic field detected by the magnetic sensor 1. A reference mark thus has no influence on the accuracy of the encoder. A dimension of the region 5 in direction of movement of the reading head does typically not exceed the length of one period of the magnetized regions, while the width of the reference pulse is set by an adequate comparative level upon conversion of the electrical signal from the optical sensor 2.

In a second embodiment illustrated in Figure 3, a short strip 7 made of the same material as the cover film 4 described in the first embodiment is glued onto the magnetic strip 3. The strip 7 in this embodiment functions as the region 5 in the first embodiment. The reflectivity of the surface of the magnetic strip 3 is poor due to the colour of the ferrite particles it contains. Thus, the electrical signal of the optical sensor 2 is low when the optical sensor 2 is located above the magnetic strip 3, yet increases when the optical sensor 2 is above the glued strip 7. Again, this can be used for generation of a reference pulse through comparison with the comparative level. The typical length of the strip 7 in this case does not exceed one period of the magnetized regions.

The thickness of the cover film 4 is limited by a maximal allowed distance between the strip and the reading head, at which the encoder still properly operates. A tolerance region of the distance is typically from 0.1 mm to ½ of the period of magnetization, which represents from 0.1 to 1 mm in case of a 2 mm period. It is desirable that the film 4 should be as thin as possible, for instance 0.1 mm, since the tolerance region of the distance between the reading head and the magnetic strip does not get considerably reduced by the cover film.

The regions with a distinctive change in reflectivity of the surface of the magnetic strip 3 are also realized in a way that a layer of paint is applied instead of the cover film. In this case, the thickness of such a layer is typically of an order of magnitude of one-hundredth of one millimetre.

While the linear scale has been described above, the invention is equally applicable to axially symmetrical scales that are read both radially and axially.

In a third embodiment illustrated in Figure 4, a nonconductive film with a copper line 8 is glued to the strip 3. In this embodiment, the line 8 functions as the region 5 from the first embodiment and as the strip 7 from the second embodiment. The reading head contains a sensor 9 that detects changes in inductivity while passing over the copper line. It consists of a primary excitation coil 10 that encloses two secondary or reading coils 11 and 12, schematically shown in Figure 5. The coils of the sensor 9 lie in a plane parallel to the upper side of the strip 3. Alternating current with a frequency of an order of magnitude of 1 MHz runs through the primary coil. The alternating magnetic field excited by the primary coil induces voltage in secondary coils. A signal from the sensor 9 is proportional to a difference in the two voltages generated in the reading coils. When the sensor 9 is located far away from the copper line 8, the two voltages which are induced in the reading coils 11 and 12 are equal and their difference is zero. When the sensor 9 is in a position where the copper line 8 is underneath the reading coil 12, eddy currents are induced in the copper line and this is why the induced voltage in the coil 12 is smaller than the induced voltage in the reading coil 11. A difference in these two voltages is negative and so is the signal from the sensor 9. Similar conditions are observed when the copper line 8 is underneath the coil 11, only that a lower induced voltage is present in the reading coil 11 and the signal is positive. A typical course of difference in amplitudes of the voltages induced in the reading coils while the sensor 9 passes over the copper line is illustrated in Figure 6. Like in the first two embodiments, the reference pulse is generated through a comparison of the signal from the sensor 9 with adequate comparative levels. Although the voltage is induced in the reading coils 11 and 12 also when the reading head travels over the incrementally magnetized strip 3, the frequency of this contribution to the total induced voltage is smaller by several orders of magnitude than the frequency of excitation of an adequate coil and can therefore be easily filtered. Instead of copper a different material can be used to produce the line 8, a well conductive and nonmagnetic material, e. g. aluminium, silver or gold.

The embodiments have illustrated solutions with an adequate film glued over the scale, i. e. magnetic strip 3. It is obvious to a person skilled in the art that an adequate coating may be used instead of the film, for instance a layer of a paint having adequate light reflectivity or a coating such as a layer of an electrically conductive material which represent regions of changes in nonmagnetic properties. The invention comprises application of a layer of a material having adequate light reflective and electrical properties. It goes without saying that the light also means the light outside of the visible spectrum. In the incremental magnetic motion or rotation encoder with a reference pulse of the invention, a non-magnetic scale is applied over at least part of the basic magnetic scale as a thin layer and there are at least two types of sensors within the reading head, wherein one or several sensors of the first type detect magnetic properties of the scale, while one or several sensors of the second type detect properties of the non-magnetic scale and the reference pulse is generated by the non-magnetic scale. To the non-magnetic scale a layer of a region having high and/or a region having low light reflectivity is added and there are two types of sensors in the reading head, and a second type of sensors is added to the first type of sensors in the axis of the strip of the magnetic scale, wherein one or several sensors of the first type detect the magnetic properties of the scale, while one or several sensors of the second type detect the changes in the light reflective properties of the added layer and the changes in the light reflectivity are used for the generation of a reference pulse and herewith for the application of the reference mark. In the second embodiment, to the non-magnetic scale a layer of a region having high and/or a region having low electric conductivity is added and there are two types of sensors in the reading head, and a second type of sensors is added to the first type of sensors in the axis of the strip of the magnetic scale, wherein one or several sensors of the first type detect the magnetic properties of the scale, while one or several sensors of the second type detect the changes in the electrical properties of the added layer and the changes in the electrical properties are used for the generation of a reference pulse and herewith for the application of the reference mark. The thickness of the added layer ranges preferably from 0.01 mm to ½ of the period of magnetization. The dimension of the region with changed reflectivity or electric conductivity for the generation of a reference pulse in direction of movement of the reading head is preferably up to one period of the magnetized regions, while the width of the reference pulse is set by a comparative level upon conversion of the electrical signal from the sensors of the second type.

Claims

1. An incremental magnetic motion or rotation encoder with a reference pulse characterized in that a non-magnetic scale is applied over at least part of the basic magnetic scale as a thin layer and there are at least two types of sensors within a reading head, wherein one or several sensors of the first type detect magnetic properties of the scale, while one or several sensors of the second type detect properties of the non-magnetic scale and the reference pulse is generated by the non-magnetic scale.

2. The encoder according to claim 1 characterized in that to the non-magnetic scale a layer of a region having high and/or a region having low light reflectivity is added and there a re two types of sensors in the reading head, and a second type of sensors is added to the first type of sensors in the axis of the strip of the magnetic scale, wherein one or several sensors of the first type detect the magnetic properties of the scale, while one or several sensors of the second type detect the changes in the light reflective properties of the added layer and the changes in the light reflectivity are used for the generation of a reference pulse and herewith for the determination of the reference mark.

3. The encoder according to claim 1 characterized in that to the non-magnetic scale a layer of a region having high and/or a region having low electric conductivity is added and there are two types of sensors in the reading head, and a second type of sensors is added to the first type of sensors in the axis of the strip of the magnetic scale, wherein one or several sensors of the first type detect the magnetic properties of the scale, while one or several sensors of the second type detect the changes in the electrical properties of the added layer and the cha nges in the electrical properties are used for the generation of a reference pulse and herewith for the determination of the reference mark.

4. The encoder according to claims 2 and 3 characterized in that the thickness of the added layer ranges preferably from 0.01 mm to Vi of the period of magnetization.

5. The encoder according to claims 2 and 3 characterized in that the dimension of the region with changed reflectivity or electric conductivity for the generation of a reference pulse in direction of movement of the reading head is preferably up to one period of the magnetized regions, while the width of the reference pulse is set by a comparative level upon conversion of the electrical signal from the sensors of the second type.