Permanent Magnet Motor: Micromotors

1. What is a micromotor?

A micromachine is defined as a very small electromechanical apparatus (1µm to 1cm) for conversion of electrical energy into mechanical energy or/and vice versa. Most micromotors, microgenerators or microactuators have external dimensions in the millimeter or submillimeter range; however, the developed torque or thrust should be high enough to overcome the losses.

Practical rotary micromotors, microgenerators and microactuators operate as electromagnetic or electrostatic devices, which use forces arising from the field energy changes.

Electrostatic micromachines are micromachined out of silicon. Silicon is unquestionably the most suitable material. It has a modulus of elasticity of 110.3 GPa, about 52 to 60% of that of carbon steel, higher yield-strength than stainless steel, low specific mass density of 2330 kg/m^3, higher strength-to- mass ratio than aluminum, high thermal conductivity of 148 W/(m K) and a low thermal expansion coefficient about 2.8 × 10^-6 1/K. Although silicon is difficult to machine using normal cutting tools, it can be chemically etched into various shapes.

The manufacturing and development infrastructures for silicon are already well established. The raw materials are well known. Their availability and processing expertise have the experience of more than 40 years of development in the IC industry. All this infrastructure and know-how have been implemented to advanced micro electromechanical systems (MEMS), while enjoying the advantages of batch processing, i.e., reduced cost and increased throughput.

Most of the applications have focused on electrostatic micro-drives having typical rotor diameters of 100µm. Electrostatic micromotors have been fabricated entirely by planar IC processes within the confines of a silicon wafer.

This is done by selectively removing wafer material and has been used for many years for most silicon pressure sensors. Over the last two decades, surface micromachining, silicon fusion bonding, and a process called LIGA-based on a combination of deep-etch X-ray lithography, electroforming and molding processes - have also evolved into major micromachining techniques. These methods can be complemented by standard IC processing techniques such as ion implantation, photolithography, diffusion, epitaxy, and thin-film deposition.

[1. The term LIGA is an acronym for the German terms meaning lithography (Litographie), electroforming (Galvanoformung) and molding (Abformung).]

2. Permanent magnet brushless micromotors

The magnetic micromotor is an attractive option in applications with dimensions above 1 mm and where high voltages, needed in electrostatic micromotors, are unacceptable or unattainable. PM brushless micromotors dominate for rotor dimensions above 1 mm as micromachines of both cylindrical or disk construction. High energy rare earth PMs are used for rotors. The magnets move synchronously with the rotating magnetic fields produced by very small copper conductor coils or current gold paths on silicon substrates.

FIG. 1. Cross sections of cylindrical brushless PM micromotors with stator outer diameter greater than 2 mm: (a) with slotless stator; (b) with slotted stator.

2.1 Cylindrical micromotors

Cylindrical micromotors are usually designed as three-phase or two-phase PM brushless machines with a small number of rotor poles. The stator can be slotless (FIG. 1a) or slotted (FIG. 1b). Typically, the resistance of the stator winding is much higher than the synchronous reactances, i.e., R1 >> Xsd and R1 >> Xsq, especially in slotless machines, in which the winding inductance is very small. This is why in overexcited micromotors (negative d-axis current) the phase voltage V1 >Ef (FIG. 2).

FIG. 2. Phasor diagram of an overexcited micromotor in which R1 >> Xsd, R1 >> Xsq, V1 >Ef and ? = f + d.

FIG. 3. Torque constant kT as a function of the load angle d for a 0.1 to 0.2 W, 80,000 rpm, 1 V PM brushless micromotor.

FIG. 4. Construction of cylindrical brushless PM micromotors: (a) synchronous micromotor designed at Eindhoven University of Technology, Netherlands, (b) 4 mW, 2-V micromotor designed by Toshiba. 1 - stator coil, 2 - stator yoke, 3 - rotor PM, 4 - stainless steel shaft, 5 - bearing.

FIG. 5. Expanded view of the smallest in the world electromechanical drive system with PM brushless micromotor and micro-planetary gearhead. 1 - housing (enclosure) of micromotor,2-endcap,3-bearing support,4-bearing of micromotor, 5-PM, 6-shaft, 7-armature winding,8-washer, 9-end cover, 10 - satellite carrier, 11 - satellite gear, 12 - sun gear, 13 - planetary stage, 14 - output shaft, 15 - housing of micro-planetary gearhead, 16 - bearing cover, 17 - retaining ring. The outer diameter of housing is 1.9 mm. Source: Faulhaber Micro Drive Systems and Technologies - Technical Library, Croglio, Switzerland.

Table 1. PM brushless micromotors manufactured by Minimotor SA, Faulhaber Group, Croglio, Switzerland.

Xsd = Ef - V1 cos d + IaR1 cos(f + d) Ia sin(f + d) (12.3) Xsq = V1 sin d - IaR1 sin(f + d) Ia cos(f + d) (eqn.4)

The angle between the armature current Ia and q-axis is ? = f + d. The torque constant for a given type of PM brushless micromotor is very sensitive to the load angle d ( FIG. 3).

Cross sections of magnetic circuits of cylindrical micromotors with stator outer diameter in the range of a few millimeters are shown in FIG. 1. The micromotor shown in FIG. 1a has a slotless stator winding with distributed parameters. Such a winding in the shape of a thin-wall cylinder is fabricated using round copper magnet wires encapsulated in a resin. Then, two halves of the stator ferromagnetic ring are placed around the stator winding. The stator core can be made of very thin laminations (Permalloy, amorphous alloy or cobalt alloy). The rotor PM is of cylindrical shape with round hole for the shaft. To obtain adequate rotor stiffness, the shaft behind the rotor magnet is of the same diameter as the magnet.

In the micromotor shown in FIG. 1b the stator coils are fabricated together with the stator ferromagnetic poles and then placed inside the stator ring-shaped core (yoke). The outer diameter of the stator is bigger than the diameter of the slotless micromotor shown in 1a.

Examples of prototypes of low speed cylindrical PM brushless micro-motors are shown in FIG. 4. The stator (armature) coils consist of a few turns of flat copper wire (typical thickness 35 µm).

Specifications of very small PM brushless micromotors for surgical de vices, motorized catheters and other clinical engineering devices are listed in Table 1. The smallest in the world electromechanical drive system with PM brushless micromotor and micro-planetary gearhead is shown in FIG. 5.

The rotor has a 2-pole NdFeB PMs on a continuous spindle. The maximum output power is 0.13 W, no-load speed 100 000 rpm, maximum current 0.2 A (thermal limit) and maximum torque 0.012 mNm (Table 1).

2.2 Fabrication of magnetic micromotors with planar coils

Planar coils can be fabricated, e.g., by local electroplating2 of gold. A large cross section of the gold lines is necessary to keep the power consumption and thermal load at acceptable levels. For NdFeB magnets with a typical dimension of 1 mm, forces of 150 µN, torques of 100 nNm and maximum speeds of 2000 rpm can be achieved. The PMs are guided in channels or openings in the silicon itself or in additional glass layers.

FIG. 6. Fabrication sequence for a magnetic micromotor: (a)polyimide deposition, dry etching; (b) bottom core electroplating; (c) patterning of conductor; (d) magnetic via hole and rotor pin electroplating; (e) photosensitive polyimide deposition and developing; (f) top core and stator pole electroplating; (g) rotor and stator micro-assembly.

The fabrication process of a magnetic micromotor ( FIG. 6) would start with a silicon wafer as substrate, onto which silicon nitride is deposited. Cr- Cu-Cr layers are deposited onto this substrate using electron-beam evaporation, to form an electroplating seed layer. Polyimide is then spun on the wafer to build electroplating molds for the bottom magnetic core. A 40 µm thick polyimide layer is built-up. After curing, the holes which contain bottom magnetic cores are etched until the copper seed layer is exposed. The electroplating forms are then filled with NiFe permalloy.

The stator winding can also be made of interleaved, electroplated copper coils that are dielectrically isolated from a 1-mm thick NiFeMo substrate by a5 µm polyimide (high temperature engineering polymer) layer ( FIG. 7).

2. Electroplating is the process of using electrical current to reduce metal cations in a solution and coat a conductive object with a thin layer of metal.

FIG. 7. Stator winding patterns for 8-pole micromachine: (a) 2 turns per pole; (b) 3-turns pole [16]. Photo courtesy of Georgia Institute of Technology, Atlanta, GA, U.S.A.

FIG. 8. Micromotor with rotating PM.

FIG. 9. An example of four layer winding for a PM micromotor. Courtesy of TU Berlin, Germany.

2.3 Disk-type micromotors

A disk-shaped rare earth PM, magnetized radially, which rotates on the silicon chip surface driven by four surrounding planar coils is shown in FIG. 8. The planar coils on silicon substrate generate a rotating field which is predominantly horizontal, while the magnet is held in position by the guide hole in the glass sheet.

FIG. 10. Etched winding disk type PM motor: (a) schematic, (b) layout of etched windings. 1 - windings (2+2 phases), 2 - back iron, 3 - bearing, 4 - shaft, 5 - PM. Courtesy of TU Berlin, Germany.

FIG. 11. Construction of penny-motor: 1 - shaft, 2 - soft steel yoke cover, 3 - PM ring, 4 - ball bearing, 5 - stator winding, 6 - flange,7-bottom steel yoke.

According to a study, a very small PM with its height of 1.0 mm and diameter of 1.4 mm can be used in a planar coil micromotor. The diameter of the guide hole is 1.4 mm. Each planar coil extends over an angle of 80 degree and has a resistance of 1.4 Ohm. The synchronous speed of 2000 rpm was obtained at the current of 0.5 A. This current in two opposite coils generates a mean lateral magnetic field of 90 A/m. For a perpendicular orientation of field and magnetization, a maximum torque of 116 nNm is produced by the motor shown in FIG. 8.

Owing to their simple construction and advances in IC manufacture, it is possible that the size of these PM micromotors can be reduced further. PMs with a diameter of 0.3 mm have already been manufactured.

Etched windings are also used in some slotless axial flux micromachines. The advantage of the slotless winding design is the elimination of the cogging torque, the tooth saturation and tooth losses. The disadvantage is that the coils are stressed by the electromagnetic forces and by the mechanical vibration. Thus, these micromotors are not robust enough for all applications.

A four-layer etched winding is shown in FIG. 9. The conducting material for the prototypes is gold with some addition of palladium. Extremely high accuracy is necessary in the etching process of multilayer windings using the thick film technology. For substrate, different ceramic materials and glass have been used. The cross section of the conductors is from 3750 to 7500 µm^2 and the distance between conducting paths varies from 150 µm to 200 µm.

The current density in a multilayer etched winding is very high: from 1000 to 10, 000 A/mm^2. To reduce the nonferromagnetic air gap, a thin ferromagnetic liquid layer with µr ˜ 10 can be added.

FIG. 10 shows a disk type 8-pole prototype micromotor with etched winding. The dimensions of the NdFeB PM are: thickness 3 mm, outer diameter 32 mm and inner diameter 9 mm. Although the stator laminated core reduces losses, it is difficult to manufacture. In a four-layer etched stator winding as in FIG. 10b the current conducting path has a width of 0.4 mm and a thickness of 0.1 mm. The electric time constant of the stator winding per phase is L1/R1 =0.7 µs. The four-phase stator winding is fed by four transistors. Magnetoresistive position sensors have been used. The sensor PM has the dimensions 3×3×1 mm. The torque of 0.32 mNm at a speed of 1000 rpm and input voltage 3.4 V has been developed.

Ultra-flat PM micromotor, the so-called penny-motor, is shown in FIG. 11. The thickness is 1.4 to 3.0 mm, outer diameter about 12 mm, torque constant up to 0.4 µNm/mA and speed up to 60,000 rpm. A 400 µm eight-pole PM and three-strand 110 µm disk shaped lithographically produced stator winding have been used. Plastic bound NdFeB magnets are a cost effective solution. However, the maximum torque is achieved with sintered NdFeB magnets. A miniature ball bearing has a diameter of 3 mm. Penny motors find applications in miniaturized hard disk drives, cellular phones as vibration motors, mobile scanners and consumer electronics.

3. Applications

Micromotors and micro-actuators are used in high precision manufacturing, glass-fiber and laser mirror adjustments, military and aerospace industry, medical engineering, bioengineering, and microsurgery. By inserting a micro motor intra-vascularly, surgery can be done without large openings of vessels.

3.1 Motorized catheters

A brushless motor with planetary gearhead and outer diameter below 2 mm has many potential applications such as motorized catheters, minimally invasive surgical devices, implantable drug-delivery systems and artificial organs. An ultrasound catheter shown in FIG. 12a consists of a catheter head with an ultrasound transducer on the motor/gearhead unit and a catheter tube for the power supply and data wires. The site to be examined can be reached via cavities like arteries or the urethra4. The supply of power and data to and from the transmit/receive head is provided via slip rings.

[3. Catheter is a tube that can be inserted into a body cavity, duct or vessel.

4. Urethra is a tube which connects the urinary bladder to the outside of the body.]

FIG. 12. Ultrasound motorized catheter: (a) general view; (b) 1.9-mm diameter PM brushless motor (Table 1). Source: Faulhaber Micro Drive Systems and Technologies - Technical Library, Croglio, Switzerland.

The stator of the brushless motor is a coreless type with skewed winding.

The outer diameter of the motor is 1.9 mm, length of motor alone is 5.5 mm and together with gearhead is 9.6 mm. The high-precision rotary speed setting allows analysis of the received ultrasound echoes to create a complex ultrasound image.

3.2 Capsule endoscopy

Capsule endoscopy helps doctors to evaluate the condition of the small intestine. This part of the bowel cannot be reached by traditional upper endoscopy or by colonoscopy.

The most common reason for doing capsule endoscopy is to search for a cause of bleeding from the small intestine. It may also be useful for detecting polyps, inflammatory bowel disease (Crohn's disease), ulcers, and tumors of the small intestine.

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5. Endoscopy is the examination and inspection of the interior of body organs, joints or cavities through an endoscope. An endoscope is a device that uses fiber optics and powerful lens systems to provide lighting and visualization of the interior of a joint.

6. Colonoscopy is a procedure that enables a gastroenterologist to evaluate the appearance of the inside of the colon (large bowel) by inserting a flexible tube with a camera into the rectum and through the colon.

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FIG. 13. Intracorporeal video probe IVP2. 1 - Q-PEM motor, 2 - transmission shaft, 3-LEDs, 4-transparent cover, 5-optics,6-CMOS sensor, 7-fixation point, 8 - camera chip, 9 - localization chip, 10 - electrical wires, 11 - batteries, 12 - data transmission chip. Courtesy of Scuola Superiore Sant' Anna, Pisa, Italy.

FIG. 14. Camera tilting mechanism with wobble motor.

FIG. 15. Wobble motor: (a) cross section; (b) cam mechanism. 1 - stator core, 2 - stator coil, 3 - PM rotor wheel.

Approximately the size of a large vitamin, the capsule endoscope includes a miniature color video camera, a light, a battery and transmitter. Images captured by the video camera are transmitted to a number of sensors attached to the patient's torso and recorded digitally on a recording device similar to a walkman or beeper that is worn around the patient's waist.

In next generation capsule endoscopy, e.g., Sayaka capsule endoscope, a tiny stepper motor rotates the camera as the capsule passes through the digestive tract, allowing it to capture images from every angle. Sayaka capsule is characterized by a double structure made up of an outer and an inner capsule. Whereas the outer capsule traverses through the gastrointestinal tract, the inner capsule alone spins. The spinning is produced by a small PM stepping motor with a stepping angle of 7.50. This stepping rotation is necessary to prevent fluctuation or blurring in the images. An 8-hour, 8-m passage from entrance to exit will yield 870,000 photos, which are then combined by software to produce a high-resolution image.

The autonomous intra-corporeal video probe (IVP) system contains a CMOS image sensor with camera, optics and illumination, transceiver, system control with image data compression unit and a power supply. The optical part is located on a tiltable plate, which is driven by a wobble motor. The basic concept is to use a frontal view system with a vision angle up to 1200 and a tilting mechanism able to steer the vision system (optics, illumination and image sensor) between about ±300 in one plane ( FIG. 14).

By exploiting this technique, the device will perform an optimal view between ±90 degrees in the xy plane. The tilting mechanism consists of a wobble mo tor ( FIG. 15a) and simple mechanical parts, such as one cam and one shaft fixed to the vision system. The cam system transforms the rotational action of the motor in a linear action to the shaft ( FIG. 15b). The so-called Q-PEM stepping motor can be controlled with a precision of 340 steps per revolution. The motor with outer diameter of 4 mm and thickness of 3 mm draws about 100 mW of power.

Numerical examples

Numerical example 1

A three-phase, 0.15 W, 80,000 rpm, two-pole slotless PM brushless micromotor has a single layer stator winding consisting of two full pitch stator coils per phase. There are N1 = 8 turns per phase wound with da =0.1024 mm diameter wire. The number of parallel wires is aw = 2, the d.c. bus voltage is Vdc =1.0 V, the amplitude modulation index ma =0.96, the armature current Ia =0.448 A and input power Pin =0.455 W at the load angle d =30. The peak value of the magnetic flux density in the air gap at no load is Bmg =0.65 T, stator core losses ?PFe =0.1 W and the stator winding phase resistance R1 =0.28 Ohm at 750 C. The dimensions of the motor (Fig. 1a) are as follows: rotor outer diameter D2out = 1 mm, shaft diameter in side the PM dsh =0.4 mm, air gap (mechanical clearance) g =0.2 mm, radial thickness of the winding hw =0.2 mm, stator outer diameter D1out =2.4mm and the length of the stator stack equal to the axial length of PM is Li =4 mm. Find the synchronous reactances in the d and q-axis, output power, efficiency, power factor, shaft torque, EMF constant and torque constant. The armature reaction, windage losses and losses in PMs (slotless stator) can be neglected.

Solution

The total number of coils is Nc =2 × 3 = 6. For a single layer winding the number of "slots" is 2Nc =2 × 6 = 12. The number of "slots" per pole per phase is q1 =2Nc/( 2pm1)=12/(2 × 3) = 2. The winding factor according to eqns (A.1), (A.6) and (A.3) is kw1 =0.966 × 1=0.966. The stator core inner diameter D1in = D2out +2(g + hw)=1.0 + 2(0.2+0.2)=1.8mm and the pole pitch t = p × 1.8/2=2.83 mm. The input frequency is f = 1×80 000/60 = 1333.3 Hz. Thus, the fundamental harmonic of the magnetic flux without armature reaction is Ff1 = 2 p 0.65 × 0.00283 × 0.004 = 4.68-6 Wb and the stator EMF per phase excited by the rotor magnetic flux is [...]