EP0112903A4

EP0112903A4 - An improved magnetic rotational velocity sensor.

Info

Publication number: EP0112903A4
Application number: EP19830902390
Authority: EP
Inventors: Kenneth S Kordik
Original assignee: Kenneth S Kordik
Current assignee: Applied Motion Products Inc
Priority date: 1982-07-06
Filing date: 1983-06-23
Publication date: 1984-11-07
Also published as: EP0112903A1; WO1984000422A1

Abstract

Improved magnetic rotational velocity sensor (20) adapted for use with stepper motors. The sensor (20) includes a rotor portion (22) and a stator portion (26) arrayed circumferentially about the rotor (22). The rotor portion (22) is in the form of an alternately radially polarized magnetic disk (24). The stator portion (26) includes an insulated conducting ring (28), a plurality of conducting windings (30) and electrical leads (36) for connecting the sensor (20) to external analysis and control apparatus. The rotor portion (22) is concentrically rigidly mounted upon the rotational shaft (16) of a stepper motor (10) such that rotation of the shaft (16) generates inducted electrical signals in the conducting windings (30). These signals provide analogs for the rotational velocity and the position of the shaft (16). The sensor (20) is protected from external signal interference by a conducting cap (40). The predominant current usage of the invention is in combination with electrical stepper motors utilized in disk drives for data storage and retrieval.

Description

AN IMPROVED MAGNETIC ROTATIONS'VELOCITY SENSOR

TECHNICAL FIELD

The present invention relates generally to velocity sensing devices and more particularly to magnetic rotational velocity sensors. The predominant current usage of the improved magnetic rotational velocity sensor of the present invention is in connection with electrical stepper motors such as those used with computer disk drives and electronic printers.

BACKGROUND ART

Precise positioning of various components at high speed has become very important to the modern data and information processing industries. The technology of data retrieval from memory requires an ability of the equipment to find specific locations in memory extremely quickly. In the common data storage and retrieval systems utilizing magnetic disk media, the precise positioning is accomplished by the use of electrical stepper motors. Stepper motors are also extensively utilized in high speed printers and electric typewriters.

An electrical stepper motor is a device wherein the stator and windings and the rotor are constructed in such a manner that the rotational positioning of the shaft is energetically urged to several specific distinct positions in the rotation. These energetically preferred positions are known as steps. Thus the shaft will only come to rest at certain locations in the rotation_. and is not a true freely rotating shaft. Commonly used stepper motors frequently include as many as 400 distinct steps in a rotation.

Electrical stepper motors of the type with which the present invention is intended to be utilized are disclosed in U.S. Patent No. 3,845,335 issued to H. . Oguey and U.S. Patent No. 3,946,298, issued to H. Van de Loo.

Since it is of extreme importance in data retrieval systems and printers to obtain the positioning as rapidly as possible, it is desirable to derive methods for causing the stepper motor to reach the precise center of the step in the most rapid manner possible. Since stepper motors have a tendency to overshoot and oscillate about the center of the step for a period of time, forms of damping are ordinarilly necessary to maximize efficiency.

The methods commonly utilized to minimize the overshoot and oscillation and thus facilitate the rapid settling of the position of the shaft at a given step have previously utilized open loop schemes. The open loop methods of damping are such that the performance may be affected by minor variances in the magnitudes of the voltage, current and/or load factors.

Any alteration of these variables may result in increased damping time and lower efficiency. The damping efficiency of any type of scheme may be enhanced by adding feedback information into the control method. Feedback-added schemes typically have better damping time than those schemes which do not utilize the feedback addition. Various methods of damping and feedback-added damping have been utilized in the prior art. One such class of methods utilizes optical position measurement. Optical encoding, as a method of calculating the positioning of the shaft in the rotation, and thus properly adding the feedback, has been disclosed as an applicable stepper motor adjustment method in U.S. Patent No. 4,228,387, issued to W.S♦ Brown and in U.S. Patent No. 4,258,622, issued to S. Estrabaud, et al. The prior art optical methods, utilizing exterior sensing and control components, have added to the complexity and cost of stepper motor equipment while resulting in only moderate improvements in efficiency. Furthermore, the improvement in efficiency of measurement is proportional to the number of measuring pulses utilized per revolution. As the number of pulses per revolution is increased, the cost rises proportionally. Other electronic damping and controlling methods for use with stepper motors have been disclosed by U.S. Patent No. 4,286,202, issued to D.E. Clancy, et al , U.S. Patent No. 4,074,179, issued to B.C. Kuo, et al, and U.S. patent No. 4,215,302, issued to D. Chiang. Complex electronic sensing and damping methods such as those disclosed in Clancy, et al, Kuo, et al , and Chiang, have resulted in significant improvements in the rapid positioning of stepper motors. However, for many common stepper motor usages, such as electronic printers, the control system has proved to be more expensive than the motor itself. Thus, the high cost of the complex electronic damping systems has severely limited their usefulness. Another common method of sensing the position of a rotational motor, and thus providing a feedback adding capability, is the use of rotational velocity sensing by way of a magnetic tachometer. The velocity measurement, combined with a zero position sensor, provides an analog method of determining position. Furthermore, the velocity measurement itself is very useful in aiding damping.

Magnetic type tachometers for sensing rotational velocity have long been known. One prior art magnetic tachometric device is disclosed in U.S. Patent No. 3,504,208, issued to J.D. Rivers. Magnetic tachometers such as those disclosed by Rivers and other prior art tachometers tend to be complex, use a large number of either windings or magnetic poles, and require co mutating brushes which tend to increase system friction and require periodic replacement. Thus the prior art magnetic tachometers are complex and have limited usable lifetimes. They are therefore not efficacious in high velocity constant use applications such as stepper motors for disk drives or electronic printers.

The use of a rotational magnet with external coils in an axial air gap arrangement directly adapted for application to stepper motors has been attempted in the prior art by Sinano Electric Company, Ltd. , as applied to certain disk drives manufactured by Shugart Associates. It has also been observed that a magnetic velocity sensor using a bar magnet and a radial air gap has been attempted in some prototype devices. None of these prior art attempts result in the efficiency and low cost of production present in the present invention.

The prior art attempts to quickly and efficiently damp electronic stepper motors and failed to concurrently solve all the problems of rapid damping, long lasting mechanisms, low cost, and simplicity of construction. DISCLOSURE OF INVENTION Accordingly, it is an object of the present invention to provide an improved magnetic rotational velocity sensor, utilizing a radial air gap, for sensing the rotational velocity of an electric stepper motor. It is another object of the present invention to provide a simply constructed velocity sensor adaptable for use with conventional stepper motors.

It is a further object of the present invention to provide a velocity sensor which may be easily installed on presently existing stepper motors.

This invention relates to methods of sensing the rotational velocity, and therefore by analog logic application, the position of the rotational shaft of an electrical stepper motor. It is particularly adapted for sensing rotational velocity such that the rotational velocity information can be utilized with the appropriate attached components and logic, to provide adaptive back phasing and damping to the stepper motor. This utilization provides quicker and more accurate stopping of the stepper motor at a desired step. The primary utilizations of the improved magnetic rotational velocity sensor are on disk drives for data retrieval systems and on electronic printing equipment.

Briefly, a preferred embodiment of the present invention is an improved magnetic rotational velocity sensor for a stepper motor, especially adapted for attachment to commonly used stepper motors. The sensor includes a rotor in the form of an alternately radially polarized permanent magnet in the form of a disk. The disk is attached to the shaft of the stepper motor such that the permanent disk magnet rotates in conjunction.with the shaft. The stator portion of the magnetic tachometric device includes a ferromagnetically conducting ring concentric with the rotor and surrounding it with a narrow air gap formed therebetween. In the preferred embodiment the conducting ring is wrapped by three separate windings of electrically conducting wire.

The windings are positioned so that the wires are perpendicular to the magnetic field of the rotor such that when the radially polarized rotor rotates past the windings an electrical current is induced therein. The current is then carried by electrical leads from the windings to the logic and control systems for providing the adaptive back phasing and damping to the stepper motor. The rotor and stator portions of the velocity sensor are enclosed in a conducting cap which forms, with the stepper motor housing, an isolated volume which prevents flux leakage from the magnetic components and also prevents interference from exterior electrical and magnetic fields. In the preferred embodiment the permanent disk magnet is polarized in two opposite directions with approximately 150° polarized in one direction and 210° polarized in the opposite direction. The permanent disk magnet includes milled segments formed to minimize the effects on magnetic flux of edge flux build-up in the permanent magnet.

An advantage of the present invention is that it provides a simple and economically manufactured velocity sensor which is easily adapted to conventional stepper motors.

Another advantage of the present invention is that the improved velocity sensor utilizes significantly fewer windings and/or permanently magnetized components than conventional magnetic sensors. A further advantage of the .invention is that the required additional components for analyzing the velocity signals are minimized and simplified by the use of the radial air gap magnetic sensor.

Another advantage of the present invention is that no additional friction no additional wear is introduced into the stepper motor system by the inclusion of the improved sensor device.

These and other objects and advantages of the present invention will become clear to those skilled in the art in light of the description of the best presently known method of carrying out the invention and the industrial applicability of the preferred embodiment as described herein and as

OMPI illustrated in the drawing.

BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is an exploded perspective view of an electric stepper motor showing the improved magnetic rotational velocity sensor of the present invention installed thereon; Fig. 2 is a front elevational view of a single piece permanent disk magnet utilized in the rotor;

Fig. 3 is a schematic illustration illustrating the polarization of the various portions of the disk magnet of Fig. 2;

Fig. 4 is an exploded perspective view of an alternate disk magnet; and

Fig. 5 is a block diagram illustrating typical associated components and controls systems for utilizing the output of the velocity sensor to improve the performance of a stepper motor.

BEST MODE FOR CARRYING OUT INVENTION The best presently known mode of practicing the present invention is an improved magnetic rotational velocity sensor for use with an electrical stepper motor, as illustrated in the drawing. The magnetic velocity sensor device is especially adapted for being installed on conventional electrical stepper motors and for providing velocity, and through the application of logic, positional information regarding the stepper to control, systems such that the speed and accuracy of positioning of the stepper motor is improved.

The presently preferred improved magnetic rotational elocity sensor, as installed upon a conventional stepper lotor, is shown in an exploded perspective view in Fig. 1. The electrical stepper motor is designated by the general reference character 10. The stepper motor includes a housing 12, a plurality of armature windings 14, a central drive shaft 16, and a plurality of power and control input lines 18. The rotational velocity sensor is shown as an attachment to the conventional stepper motor 10 and the various components of the rotational velocity sensor are commonly designated by the general reference character 20. The rotational velocity sensor 20 includes a rotor portion 22. In the preferred embodiment the rotor portion 22 is in the form of a solid permanent disk magnet 24 which attaches concentrically to the shaft 16 of the stepper motor 10 such that the disk magnet 24 rotates concurrently with the shaft 16. The disk magnet 24 is radially polarized in such a manner that a portion of the disk magnaet 24 is polarized in one direction while the remainder of the disk magnet 24 is polarized in the opposite direction. The rotational velocity sensor 20 also includes a stator portion 26. In the preferred embodiment the stator portion 26 includes a magnetically and electrically conducting ring 28 with an insulating covering. The conducting ring 28 is attached to the housing 12 of the stepper motor 10 in such a manner as to radially surround the disk magnet

24. The conducting ring 28 is arranged so as to be concentric with the disk magnet 24. The ring 28 has an interior diameter which is slightly larger than the exterior diameter of the disk magnet 24 such that a narrow air gap is formed between the disk magnet 24 and the conducting ring 28 when the components are mounted on the stepper motor 10. The conducting ring 28 is provided with electrical windings at various positions about the ring. The electrical windings comprise a first winding 30, a second winding 32 and a third winding 34. First, second and third windings 30,32 and 34 are preferably evenly spaced about the conducting ring 28 such that the centers of the windings are situated at the 0°, 120° and 240° positions. The first winding 30, the second winding 32 and the third winding 34, are connected by seperate electrical leads 36 to the logic and control system which utilize the signals generated in the windings. A reference electrical lead 38 carries the reference signal, or relative ground of the windings, to the logic. The reference lead 38 is connected to all three windings. The rotor portion 22, the stator portion 26 and the end of motor shaft 16 are all enclosed by an electrically and magnetically conducting cap 40 which forms, with the electrically and magnetically conducting housing 12 of the stepper motor, an isolation volume. The isolation volume insures that the external and internal magnetic and electrical fields are isolated from each other. A small aperture 42 exists in the conducting cap 40 to allow the signal leads 36 and the reference lead 38 to exit and proceed to the control components. The isolation volume created by conducting cap 40 also prevents the leakage of magnetic flux from the permanent magnet 24 and further prevents stray or external magnetic or electrical fields from affecting the signals generated in the first second and third windings 30, 32, and 34.

The permanent disk magnet 24 is illustrated in a front elevational view in Fig. 2. In this view it may be seen that the magnet 24 includes an exterior circumferencial ridge 44. The ridge 44 is shaved or planed to form milled segments 46 in the vicinity of the interface between the opposing magnetic poles. The milled segments equalize the disks effective magnetic field by adjusting the air gap to minimize the edge effects construction on the flux. The permanent disk magnet 24 is preferably constructed by embedding ferromagnetic materials such as barium ferrite [Ba(Feθ2)2J ⁱⁿ a rigid plastic material such that the polarity is maintained in a constant direction. The magnetic polarity and flux of the permanent disk magnet 24 is illustrated schematically in Fig. 3. In this Fig. it can be seen that for approximately 210° of the arc of disk magnet 24 the magnetic polarity extends outward radially, whereas in the remaining 150° the magnetic polarity is directed inward radially. Fig. 3 also illustrates that the milled segments 46 cause the magnetic flux to be equalized in the vicinity of the stator. Since the deformation at the milled segments 46 causes an increased air gap between the rotor and the stator the increased flux due to edge effects is equalized by the increased air gap distance and the resultant induced current in the first second and third windings 30, 32 and 34 is maintained at the same level as if the full radius portions of the disk magnet 24 were situated opposite the windings.

Fig. 4 illustrates an alternate rotor construction. The alternate rotor 48 is constructed utilizing an interior rigid disk 50 which attaches to the drive shaft 16 of the stepper motor 10 in the same manner as does the permanent disk magnet 24 of the preferred embodiment. The rigid disk 50 is not magnetic in character and is merely structural. Attached around the circumference of rigid disk 50 are a pair of flexible magnetic strips 52. The flexible magnetic strips 52 are polarized such that the polarity is perpendicular to the long axis of the strip when it is extended and thus the polarity is radial to the rigid disk 50 when the magnetic strip 52 is wrapped about the circumference of the disk 50. The flexible magnetic strips 52 are selected and cut such that one strip is polarized in one direction and wrapped about a greater circumference of the rigid disk 50 and a shorter segment of the flexible magnetic strip 52 is polarized in the opposite direction such that the resultant magnetic polarity of the combination of the rigid disk 50 and the flexible magnetic strips 52 is similar to that illustrated in Fig.3. Although the strips 52 and disk 50 complete a disk volume similar to the permanent disk magnet 24 it is not necessary to provide indentations 46 since the construction of the magnetic strips 52 eliminates the edge effects.

A typical control loop system for utilizing the signals generated by the magnetic velocity sensor of the present invention is illustrated in Fig. 5 in a block diagram fashion. Fig. 5 shows the manner in which the velocity signals generated by the first winding 30, the second winding 32, and the third winding 34 and the reference signal generated in the conducting ring 28 are utilized. The typical control system utilized in conjuction with a velocity sensor of the present invention operates as follows. The rotation of the motor drive shaft 16, and consequently the rotation of the oppositely polarized magnetic disk 24 generates an induced electrical voltage in the first winding 30, the second winding 32, and the third winding 34. The amount of voltage induced is dependent upon the speed of rotation of the magnetic field and therefore proportional to the speed of rotation of the shaft 16 of the stepper motor 10. The voltage signals from each winding are delivered to a commutating switch. The reference signal, the reference ground of all three windings, is also delivered to the commutating switch via reference lead 38 to provide a reference value against which the individual voltage signals from the first, second and third wingings 30, 32 and 34 are compared. The commutating switch is directed by the logic as to which winding signal to deliver for processing. The commutating switch serves the same purpose as do the brushes in an ordinary magnetic tachometer, but performs this function by calculated electronic switching rather than by physical, direct electrical or mechanical switching.

The selected signal from the appropriate winding is then delivered to the adaptive back phase damping pulse source and logic board. The back phase damping pulse source and logic board combines the input from the com¬ mutating switch, which indicates the rotational velocity and direction of the stepper motor, with inputs from a pulse source or computer and from a zero position sensor. The zero position sensor provides information sufficient for the logic board to derive the postion of the motor from data providing the speed of rotation. The combined inputs are then integrated into outputs directing the power to be delivered to the motor and the direction in which the power is to be applied. The adaptive back phase damping pulse source and logic also provides the switching signals to the commutating switch which tell the commutating switch which of the winding signals to to select. This switching may also be dependent upon the direction of rotation of the stepper motor so a seperate directional input is required.

The output signals from the adaptive back phase da ping pulse source and logic are in the nature of clockwise and counter-clockwise power signals to the stepper motor. These signals direct the speed of desired rotation and the direction .of rotation in order to achieve most efficient stopping and reduction of oscillation. The clockwise and counter-clockwise signals are delivered through a ring counter device which serves the purpose of differentiating which phase of the four phase stepper motor to which to send the signal. The signals f om the ring counter are then delivered through a power amplifier which further receives an input from the motor power supply. The signals from the power amplifier are then delivered to the four phase stepper motor wherein they are converted to actual rotational motion or braking in the desired direction and magnitude.

The magnitude and direction of damping is directly controlled by knowing the position and rotational velocity of the stepper motor shaft. The applicable logic can then apply the proper damping force to minimize both the stopping time and the oscillation once the desired position is reached. The present invention is adapted for use with conventional stepper motors. It is desirable that the drive shaft 16 for the stepper motor continue through the housing 12 in the direction opposite the drive object. This provides a position for direct connection of the rotor portion 22 of the magnetic velocity sensor 20 to the drive shaft 16. It is also desirable to select a stepper motor 10 in which the motor housing 12 adjacent to the mounting position of the steppper motor is of an electrically and magnetically conductive material such that the application of the conducting cap 40 completes an isolation volume.

In the preferred embodiment the conducting ring 28 is constructed of steel insulated and wrapped by a teflon tape material to avoid direct conduction between the ring material and the windings and/or the housing 12 or the windings 30, 32 and 34. A preferred winding material is No. 40 copper magnet wire. The electrical leads 36 and the reference lead 38 are constructed of standard insulated copper wire .

The preferred rotor 22 is a barium ferrite [Ba(FeO_) ] magnetic matrix embedded in a nylon binder to form the permanent disk magnet 24. The presently preferred degree of magnetization is approximately 210° of arc mag¬ netized one direction and 150° of arc magnetized the opposite direction. The preferred indentation at the milled segments 46 extends approximately 30° of arc centered about each magnetic interface and includes a maximum indention of approximately 4% of the diameter of the disk 24.

The alternate rotor 48 is constructed utilizing a steel disk center portion 50 and the 3 M Company rubber Plastiform _TM IH magnetic strip material 52. When the magnetic strip material is used the edge effects are eliminated and no milled segments are required.

A preferred improved magnetic rotational velocity sensor 20 includes a rotor 22 having an outside diameter of 2.51 cm (1.0 inch) and a thickness of 0.50 cm (0.197 inch), The conducting ring 28 of the stator is selected to have an outside diameter of 2.858 cm (1.125 inch), an outside diameter of 3.411 cm (1.343 inch) and a thickness of 0.394 cm (1.55 inch). Thus, an air gap of 0.159 cm (0.0625 inch) is created between the rotor 22 and the conducting ring 28. This gap is greater when the bare ring 28 is opposite the indented segments 46 on the rotor and less at the positions where the rotor 22 is opposite the first, second and third windings 30,32, and 34. When No. 40 copper magnet wire is utilized the number of turns in each of the first, second and third windings 30, 32, and 34 is approximately 500 turns.

The magnetic materials of the permanent magnetic disk 24 and the flexible magnetic strip 52 of the alternate rotor 48 are selected such that the resultant magnetic field created by the rotor at the distance of separation of the radial air gap is approximately 1 kilogauss.

Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure is not intended as limiting. The appended claims are therefore to be interpreted as encompassing the entire spirit and scope of the invention.

INDUSTRIAL APPLICABILITY The improved magnetic rotational velocity sensors of the present invention are particularly adapted for use with electrical stepper motors of the type utilized in magnetic disk drives in data retrival systems and also in electronic printers.

The most common usage of the velocity sensors of the present invention is in the nature of analog position measurement by way of velocity ^"measurement in conjunction with control systems such as those illustrated in Fig. 5. In this manner the velocity measurement can be used to provide data such that the positioning of the motor drive shaft can be accomplished in the shortest possible period of time with the minimum oscillation possible.

The present invention is relatively simple in construction when compared with other velocity and position sensors utilized with electric stepper motors. Furthermore, it is adapted to be readily installed on presently existing conventional stepper motors. The velocity sensors of the present invention provide no direct friction and thus do not significantly decrease the lifetime of the stepper motor as do certain prior art position and, velocity sensing methods. Furthermore, the construction of the improved velocity sensor is such that magnetic saturation in the windings is avoided, as is not always possible in axial air gap systems.

It is therefore believed that the present invention will be widely applicable in electric stepper motor systems wherein it is desirable to measure the velocity and/or the position of the stepper drive shaft.

Claims

_l4-

IN THE CLAIMS

1. A velocity sensing device for measuring the rotational velocity of a rotating shaft comprising: a permanently magnetized rotor rigidly attached concentrically about the shaft, the rotor being radially symmetrical physically, with a continuous portion of the rotor being polarized radially outward and the remaining portion of the rotor being polarized radially inward: a stator arranged circumferentially about the rotor such that a radial air gap is formed therebetween, . the stator including a plurality of electrically conducting windings arranged such that a current is induced in said windings by the motion of the rotor; and means for delivering and processing signals derived from said current induced in said windings. 2. In a magnetic rotational velocity sensing device including a rotor and a stator having at least one conducting winding thereabout, the improvement comprising: the rotor is formed to be approximately radially symmetrical physically about a rotating member and the rotor is permanently magnetized such that one portion of the arc is polarized radially outward while the remainder is polarized radially inward; and the stator circumferentially surrounds the rotor to form a radial air gap therebetween. 3. The sensing device of claim 1 or 2 wherein: the rotor is in the form of a permanently magnetized disk. 4. The sensing device of claim 2 wherein: said permanently magnetized disk includes a ferromagnetic material embedded in a rigid binder. 5. The sensing device of claim 4 wherein: said permanently magnetized disk is formed to include indentations about the polarity interface such that the edge formation effects inherent in binding said ferro- magnetic material are minimized.

^.\AT_ 6. The sensing device of claim 1 or 2 wherein: approximately 210° of arc of the rotor is polarized in one direction; and the stator includes three similar windings equally spaced about an insulated conducting ring. 7. The sensing device of claim 1 or 2 wherein: the rotor is in the form of a rigid nonmagnetized central disk with two strips of opposingly polarized permanent magnetic material bonded about the circumference of said disk. 8. The sensing device of claim 1 or 2 wherein: the thickness of the rotor is greater than the thickness of the stator. 9. The sensing device of claim 1 or 2 wherein: the rotor and stator portions of the device are enclosed in a magnetically and electrically conducting enclosure formed so as to provide an isolated volume. 10. The sensing device of claim 1 wherein the means for delivering and processing include: commutator means; separate signal leads connecting each said winding to said commutator means ; a reference lead for delivering a reference ground signal from said windings to said commutator means; and logic means for processing the signals from said commutator means and controlling the rotation of the shaft. 11. The sensing device of claim 10 wherein: the means for delivering and processing are connected to an electrical stepper motor including a zero position sensor so as to provide feedback rotational velocity modification signals to the stepper motor to minimize oscillation.

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