1.4 A review of magnetic encoders related technologies
1.4.2 Rotary types of multi-pole magnetic components
Except linear types, the rotary types of multi-pole magnetic components are also widely used in precise control systems, as shown in Fig. 1-9 [20]. The components are magnetized in the axial and radial directions for various applications. Figure 1-10 demonstrates a rotary type of magnetizing fixture (or head) is employed to magnetize the magnetic component into an eight-pole structure in the axial direction.
Axial direction
(a) (b) Radial direction
Fig. 1-9. Rotary types of multi-pole magnetic components (a) in the axial and (b) in the radial directions for various applications.
+ Current input
Magnetic component Groove
- Current output
Fixture base Fixture base
Piece
(a)
Current I
Different directions of magnetic fields Bending angle of grooves
Base
Magnetizing fixture Current input
Current output
(c) (b)
Fig. 1-10. (a) Schematic view of the magnetizing fixture in the axial direction. (b) Configuration of the winding pattern in the magnetizing fixture. (c) Photos of the fixture base and magnetizing fixture.
Both of fixture bases (12 and 12’) are also made of a permalloy, which has a very high permeability that can concentrate the magnetic flux to enhance the magnetic field for magnetization [23-24]. The surfaces of fixtures are divided into eight equal pieces (16~30 and 16’~30’) through the line-cutting process. The magnetizing coils (15’ and 34) are wound into the grooves. An alternate multi-pole magnetic field distribution is formed from using an appropriate layout of magnetizing coils, as illustrated in Fig. 1-10 (b). Both terminals of current input (36) and output (38) are connected to a magnetization machine. A large magnetic field is induced instantaneously after the magnetization machine releases a large magnetizing current.
The magnetic component (40) is then magnetized into an eight-pole structure.
Unfortunately, the same problem also occurs in this rotary type of magnetizing fixture, as discussed in the linear type. The insulating layer of magnetizing coils can not withstand the stress and finally results in break. Consequently, a short circuit
happens between the bases of magnetizing fixtures. Since both bases are made of a permalloy, the magnetizing coils and fixtures are often exploded during the magnetization process. The risk of explosion can not be avoided because the large magnetic field is required for reversing the magnetic moments inside the magnetic material. The sum of groove width and piece size is defined as the magnetic pole pitch, which is also limited by machining techniques. Accordingly, it is not easy to have a fine magnetic pole pitch of less than 1mm through this approach.
Leakage gap Air gap
(a)
Unique magnetizing head
Unique magnetizing head
Spindle motor Magnetic component
(b) Base
Fig. 1-11. (a) Configuration and (b) photo of a unique magnetizing head.
In order to overcome the limitation of 1mm in fabricating the fine magnetic pole pitch, a single-pulse magnetization method, which resembles the magnetic recording technology, was introduced [25]. Figure 1-11 (a) shows the magnetizing coils are wound and fixed on a unique magnetizing head. A leakage gap exists in the head to leak out the magnetic flux to record magnetic pole pairs (i.e. N and S pole) onto the surface of the plastic ferrite permanent magnet wheel (magnetic component). The magnetic pole pitch of less than 1mm can be achieved by using this method. Before magnetization, the magnetic component is mounted on a base that is usually supported and rotated through a high-precision spindle motor, as shown in Fig. 1-11
(b). The precise position control of the spindle motor is highly required; otherwise, an asymmetric magnetic field distribution will appear on the multi-pole magnetic component after magnetization. The asymmetric magnetic field distribution is not useful to the subsequent processing of signals.
After magnetization, a multi-pole magnetic drum (magnetic component) with a fine magnetizing pitch λ is obtained, as shown in Fig. 1-12. A precise MR element is employed to determine the field distribution generated from the multi-pole magnetic drum.
(a) (b)
Multi-pole magnetic drum
Base
Fig. 1-12. (a) Schematic view and (b) photo of a multi-pole magnetic drum (magnetic component) with a fine magnetizing pitch λ mounted on the base.
A precise multi-pole magnetization system comprises a magnetizing head and a magnetization machine, as shown in Fig. 1-13 (a). The red mark in Fig. 1-13 (b) represents the unit of the magnetizing head, as discussed in Figs. 1-11(a) and 1-12(a).
The blue area indicates the unit of the magnetization machine, as illustrated in Fig. 1-13 (b). The field distributions of multi-pole magnetic components are detected by using a Gauss meter with a precise Hall-effect probe. The precise position of the spindle motor is controlled through the µP controller. The magnetic pole pairs are recorded intermittently onto the surface of the magnetic component through the pulse
width modulation (PWM) generation circuit and drive circuit. All tasks are carefully handled by the personal computer.
Precise multi-pole magnetization system
Magnetizing head
Magnetization machine
(a) (b)
PWM : P ulse Width Modulation
Fig. 1-13. (a) Photo and (b) schematic view of a precise multi-pole magnetization system including a magnetizing head and a magnetization machine.
During the magnetization process, the magnetic component will often collide with the head if its radial run-out is too large and thus results in damages on both of them.
Consequently, the dimension of the magnetic component must be fabricated uniformly with a small radial run-out. Moreover, the leakage gap in the head and the air gap between the head and magnetic component must be properly adjusted to obtain the desired magnetic pole pitch. As the magnetic pole pitch gets smaller, both of leakage and air gaps need to be narrowed as well. These are key factors to affect the size of the magnetic pole pitch in magnetization.
Furthermore, the waveform of the magnetizing current from the magnetization system is required to modify to fit the magnetic material property of the magnetic
component. In addition to the tiny radial run-out and material homogeneity, the magnetic component must be mounted on a spindle motor under a precise position control. All requirements for magnetization can be achieved only through the precise multi-pole magnetization system.
Despite the fact that the single-pulse magnetization method can narrow the magnetic pole pitch to be less than 1mm, the fabrication process is very difficult and complicated. Additionally, the precise machining technique, magnetizing head and magnetization machine are also necessary. Consequently, the single-pulse magnetization method is costly in fabricating a multi-pole magnetic component with a fine magnetic pole pitch.
1.4.3 Summary
Several methods have been discussed to fabricate a multi-pole magnetic component with a fine magnetic pole pitch. For practical applications, some critical issues are required for narrowing the magnetic pole pitch as mentioned as the following :
n Precise machining
To fabricate the magnetic component with a small radial run-out for magnetization.
n Magnetizing head (fixture)
To magnetize the magnetic component into a multi-pole structure with a fine magnetic pole pitch.
n Magnetization machine
To supply and modify the magnetizing current to fit the material characteristic of the magnetic component.
1.5 Motivation and objective of this dissertation
A wide magnetic pole pitch is insufficient for high-resolution control applications.
Traditionally, a magnetizing head, which is used to magnetize the magnetic component into a multi-pole structure, is fabricated through the line-cutting process.
Since the process is highly depended upon the precision of machining tools, it is difficult to have a fine magnetic pole pitch of less than 1mm. The single-pulse magnetization method, which resembles the magnetic recording technology, is also mentioned to fabricate a multi-pole magnetic component. Finally, the limitation of 1mm in the magnetic pole pitch is overcome by using this costly and complicated method.
In view of foregoing problems in narrowing the magnetic pole pitch, the precise magnetizing head and magnetization machine are required. Additionally, the precise machining techniques are essential to produce the magnetic component with a small radial-runout for magnetization. In order to overcome the different technical barriers, creating an innovative method with a simple process to fabricate a multi-pole magnetic component with a fine magnetic multi-pole pitch of less than 1mm is the motivation of this dissertation.
PCB manufacturing technology has been applied to many products and gained a great success at various industrial applications. More and more electric devices can be placed on a substrate and the corresponding wire circuit density is thus higher and higher than before. According to electromagnetism, applying a current to a long and
straight wire will induce an annular magnetic field around the wire. The magnetic flux density is proportional to the current input, but inversely proportional to the distance [26]. Consequently, an alternate and regular magnetic field distribution can be obtained from designing a special wire circuit pattern with an appropriate layout.
Thus, a multi-pole magnetic component can be accomplished by forming the special wire circuit pattern on the PCB. The objective of this dissertation is to fabricate a multi-pole magnetic component with a fine magnetic pole pitch by using the PCB manufacturing technology.
At present, the minimum wire width and the gap between any two adjacent wires on the circuit can be achieved are about 3mils (~75µm, 1mil = 25.4µm) through the PCB manufacturing technology. Correspondingly, it is highly possible to have a fine magnetic pole pitch of less than 1mm. The first goal of this thesis is to design and fabricate a special wire circuit pattern having a multi-pole structure with a fine magnetic pole pitch of less than 1mm on the PCB.
Since the induced magnetic fields among the wire circuits are very small, the field distributions in the fine magnetic pole pitch are not easy to detect. Accordingly, the second goal of this thesis is to develop a high-precision magnetic field measuring system to determine the field distributions in the fine magnetic pole pitch. It combines a precise Gauss meter, a Hall-effect probe, a probe holder, a PCB sample holder, an X-Y micro-stage, and an X-Y-Z micro-stage. All of them are mounted on an optical table to prevent vibrations.
1.6 Organization of this dissertation
This dissertation is organized as the following : An introduction is reported in Chapter 1. Design and fabrication of a multi-pole magnetic component with a fine
magnetic pole pitch by using the PCB manufacturing technology are discussed in Chapter 2. Both of linear and rotary types of wire circuit patterns are presented to form different multi-pole magnetic components on the PCB. In Chapter 3, the field formulae for computing the magnetic flux density distribution in the fine magnetic pole pitch are derived through the theoretical analysis. In Chapter 4, a precise magnetic field measuring system is designed and set up to determine the field distributions induced from the wire circuits. In Chapter 5, the field enhancement using a dual-layered wire circuit structure is studied to improve the field strength for measurements. The field optimization in the fine magnetic pole pitch is also investigated. Moreover, the field variations along different measuring routes are analyzed in Chapter 6. Finally, the conclusions with a summary of main results and areas for future works in this dissertation are presented in Chapter 7. All programs for computing the field distribution in the fine magnetic pole pitch are given in Appendix.
Chapter 2
Design and fabrication
An innovative method by using the printed circuit board (PCB) manufacturing technology is employed to fabricate a multi-pole magnetic component with a fine magnetic pole pitch of less than 1mm. Neither the precise machining technique or the magnetizing head and magnetization machine is required. This innovative method is not only a simple but also a cost-effective method to enable mass production easily. Additionally, different pole numbers and pitch sizes can be also easily achieved by modifying an appropriate wire circuit pattern on the PCB.
2.1 Introduction
Several technologies have been described to fabricate a multi-pole magnetic component with a fine magnetic pole pitch in Chapter 1. However, the costly and complicated machining technique, magnetizing head and magnetization machine are required. Unfortunately, these techniques are difficult to be held simultaneously.
PCB manufacturing technology has been widely used and gained a great success in many products for various industrial applications. More and more electric devices can be placed on a substrate. Consequently, the wire circuit density is thus higher and higher than before. Since the wire circuit fabricated on the PCB is made of copper, it can be treated as composed of many straight wires that are located at different positions on the substrate.
According to electromagnetism [27], an annular magnetic field is induced around
a long and straight wire after applying a steady current, as shown in Fig. 2-1. The magnetic flux density Br
inside the wire is given by
∫
Br⋅dlr =µ0I è B⋅2πr=µ0Iππdr22 è Br =µ0I 2πrd2 φˆ, (2-1)where r is the distance, d is the radius of the wire, I is the current input, φˆ is the unit vector in cylindrical coordinates and µ is the permeability of free space. Outside0 the wire, the magnetic flux density Br
is proportional to the current input I but inversely proportional to the distance r, and it is
∫
Br⋅dlr =µ0I è B⋅2πr =µ0I è Br = µ2π0rI φˆ. (2-2)Distance r d
Magnetic flux density B
d I Bmax
π µ 2
= 0
Radius d Current I
Distance r
Fig. 2-1. Magnetic flux density distribution in a long and straight wire.
A linear wire circuit pattern with a periodic structure is designed as in Fig. 2-2 (a).
This periodic structure provides a loop allowing the current to flow in the opposite directions for inducing different magnetic fields among the wire circuit.
Correspondingly, an alternate and regular magnetic flux density distribution is generated when a steady current is applied to the wire circuit, as shown in Fig. 2-2 (b).
Thus, a linear type of multi-pole magnetic component with a uniform pole profile is accomplished. Various plus and minus marks denote the induced magnetic field
along different z directions.
Current I
Magnetic flux density distribution
Magnetic pole pitch Down
Up Magnetic field directions
(a) (b)
Fig. 2-2. (a) Linear wire circuit pattern with a periodic structure. (b) The magnetic flux density distribution induced from the linear wire circuit pattern.
Annular wire circuit pattern is proposed and illustrated in Fig. 2-3 (a) to produce different magnetic fields in the radial direction. An annular multi-pole magnetic field distribution is obtained after applying a steady current to the annular wire circuit, as shown in Fig. 2-3 (b). Consequently, a rotary type of multi-pole magnetic component with a uniform pole profile is achieved. It is seen that the different multi-pole magnetic components with a uniform field distribution can be acquired without magnetization. Both of linear and annular wire circuit patterns can be easily fabricated on the PCB for different applications.
Down
Up Magnetic flux density distribution
Current I
(b) (a)
Fig. 2-3. (a) Annular wire circuit pattern with a multi-pole configuration in the radial direction. (b) The magnetic flux density distribution induced from the annular wire circuit pattern.
Currently, the minimum wire width and the gap between any two adjacent wires on the circuit can be achieved about 3mils (~75µm, 1mil = 25.4µm) through the PCB manufacturing technology. Accordingly, it is highly possible and feasible to fabricate a multi-pole magnetic component with a fine magnetic pole pitch of less than 1mm. A comparison among different methods in fabricating a multi-pole magnetic component with a fine magnetic pole pitch is summarized on Table 2-1.
Table 2-1 Comparison among different methods Basic requirements
Methods
Precise machining
Magnetizing head
Magnetization machine
Minimum pole pitch
Cost Conventional
magnetization
Yes Yes Yes ∼1mm High
Single-pulse magnetization
Yes Yes Yes ∼100µm Very
high PCB manufacturing
technology
No No No ~150µm Low
Obviously, all precise machining technique, magnetizing head and magnetization machine are necessary in conventional and single-pulse magnetization methods. The manufacturing cost is thus very high and the minimum pitch sizes are around 1mm and 100µm, respectively. However, the innovative method of PCB manufacturing technology provides a simple process to fabricate a multi-pole magnetic component with a fine magnetic pole pitch, neither the precise machining technique nor the magnetizing head and magnetization machine is required. The magnetic pole pitch can be minimized to around 150µm. Although this pitch size is slightly larger than 100µm produced by using the single-pulse magnetization method, it has been significantly reduced to be smaller than 1mm.
From above discussions, PCB manufacturing technology is not only a simple but also a feasible and convenient method to fabricate a multi-pole magnetic component with a fine magnetic pole pitch. Consequently, PCB manufacturing technology is
hired to narrow the magnetic pole pitch to be less than 1mm for this dissertation work.
The detailed design and fabrication process will be discussed in the following sections.
2.2 Design
A special wire circuit pattern was designed as in Fig. 2-4. All dimensions of wire segments on the circuit should be taken into account to obtain a desired pitch size. The spacing between any two adjacent wires is defined as the gap G. The sum of wire width T1 and gap G is defined as the magnetic pole pitch. A straight wire d on both sides is designed to connect to the current source. Various wire segments of a and b are related by L=a+b. The segment a is equal to b at the condition along the bisection line.
Point B Area C T1 Point A
Enlarging area C
d d
b
Wire width = T1 and Gap = G Magnetic pole pitch = T1 + G Bisection line
G L
a
L=a+b
Fig. 2-4. Special wire circuit pattern designed for fabricating a multi-pole magnetic component with a fine magnetic pole pitch.
The schematic view of the cross section on the special wire circuit pattern along the bisection line is illustrated in Fig. 2-5 (a), indicating the wire thickness is T2.
According to Ampere’s law, an alternate and regular magnetic field distribution is induced after applying a steady current to the wire circuit, as shown in Fig. 2-3 (b).
Thus, a multi-pole magnetic component with a fine magnetic pole pitch is obtained.
Additionally, different pole numbers and pitch sizes can be also easily achieved by designing an appropriate wire circuit pattern.
Gap G Wire thickness T2
Wire width T1 Copper wire
Magnetic pole pitch (a)
(b) Magnetic flux density distribution
Fig. 2-5. (a) Schematic view of the cross section on the special wire circuit pattern along the bisection line. (b) The magnetic flux density distribution generated from the special wire circuit pattern.
Wire thickness T2 on Layer 1
Gap G Wire width T1
Insulaying layer t
Layer 2 Layer 1 Copper wire
Wire thickness T2 on Layer 2
Fig. 2-6. Schematic view of the cross section on a dual-layered wire circuit structure along the bisection line.
A dual-layered wire circuit structure is considered to enhance the field strength in the fine magnetic pole pitch. Figure 2-6 shows the schematic view of the cross section on a dual-layered wire circuit structure along the bisection line. The geometrical structures of single-layered and dual-layered wire circuits are depicted in Fig. 2-7. An insulating layer t is inserted between two layers in the dual-layered wire circuit structure to prevent a short circuit. Both of Layer 1 and Layer 2 induce
the magnetic fields simultaneously after applying a steady current to the wire circuit.
The total magnetic field is the sum of Layer 1 and Layer 2. Consequently, the field strength can be effectively enhanced using the dual-layered wire circuit structure.
The large field strength in the wire circuit is useful to the signal detection. The details can be found in Chapter 5. Both of single-layered and dual-layered wire circcuit structures can be easily realized through the PCB manufacturing technology.
T1
T2
Single-layered structure Dual-layered structure
Insulating layer t Layer 1
Layer 2
(a) (b)
Wire width T1 Wire thickness T2
Fig. 2-7. Geometrical structures of (a) single-layered and (b) dual-layered wire circuits.
2.3 Fabrication
2.3.1 Drawing
First, the special wire circuit pattern with an appropriate layout must be designed and drawn by using drawing software such as AutoCAD, SolidWorks or Pro/E, etc.
[28-30]. Here, AutoCAD is employed to do this job. In this step, the wire width, gap and corresponding position must be drawn and arranged accurately to acquire the desired pitch size and pole number.
[28-30]. Here, AutoCAD is employed to do this job. In this step, the wire width, gap and corresponding position must be drawn and arranged accurately to acquire the desired pitch size and pole number.