Classifications

• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
  • H02P6/00—Arrangements for controlling synchronous motors or other dynamo-electric motors using electronic commutation dependent on the rotor position; Electronic commutators therefor
  • H02P6/10—Arrangements for controlling torque ripple, e.g. providing reduced torque ripple
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B62—LAND VEHICLES FOR TRAVELLING OTHERWISE THAN ON RAILS
  • B62D—MOTOR VEHICLES; TRAILERS
  • B62D5/00—Power-assisted or power-driven steering
  • B62D5/04—Power-assisted or power-driven steering electrical, e.g. using an electric servo-motor connected to, or forming part of, the steering gear
  • B62D5/0457—Power-assisted or power-driven steering electrical, e.g. using an electric servo-motor connected to, or forming part of, the steering gear characterised by control features of the drive means as such
  • B62D5/046—Controlling the motor
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02K—DYNAMO-ELECTRIC MACHINES
  • H02K29/00—Motors or generators having non-mechanical commutating devices, e.g. discharge tubes or semiconductor devices
  • H02K29/03—Motors or generators having non-mechanical commutating devices, e.g. discharge tubes or semiconductor devices with a magnetic circuit specially adapted for avoiding torque ripples or self-starting problems
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02K—DYNAMO-ELECTRIC MACHINES
  • H02K29/00—Motors or generators having non-mechanical commutating devices, e.g. discharge tubes or semiconductor devices
  • H02K29/06—Motors or generators having non-mechanical commutating devices, e.g. discharge tubes or semiconductor devices with position sensing devices
  • H02K29/08—Motors or generators having non-mechanical commutating devices, e.g. discharge tubes or semiconductor devices with position sensing devices using magnetic effect devices, e.g. Hall-plates, magneto-resistors
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
  • H02P2209/00—Indexing scheme relating to controlling arrangements characterised by the waveform of the supplied voltage or current
  • H02P2209/07—Trapezoidal waveform

Definitions

• the invention is directed to electric power steering systems, particularly towards a torque ripple free system for electric power steering.
• EPS Electric power steering
• BACKGROUND OF THE INVENTION Electric power steering (EPS) has been the subject of development by auto manufacturers and suppliers for over a decade because of its fuel economy and ease-of-control advantages compared with traditional hydraulic power steering .
• commercialization of EPS systems has been slow and is presently limited to small and midget-class cars due to cost and performance challenges.
• TRF Torque-Ripple Free
• a sinusoidal inverter/controller to switch currents into the three-phases to be in synchronism with the rotor at all times.
• the currents are controlled to have sinusoidal waveforms
• FIG. 3 illustrates a schematic of the TRF motor cross-section for illustration.
• a composite iron stator yoke to replace laminated yoke and eliminate its associated whining noise. It simultaneously functions also as a housing for the stator, thus reducing the stator manufacturing cost and defraying some of the added cost of the more expensive magnet and position sensor required for this concept.
• a composite reinforced plastic rotor core and shaft which is another cost saver because it can be molded to shape instead of machined, h addition it will help with lowering the rotor inertia due to its lower density compared with steel, thus compensating for the increased inertia due to a needed larger magnet. While using a plastic rotor core tends to reduce its torsional stiffness, it is found to yield acceptable angular strain.
• Figure 1 illustrates a schematic diagram of an electric power steering system.
• Figures 2 A-C illustrate a series of current, voltage, and instantaneous torque waveforms in a PM brushless motor.
• Figure 3 illustrates a schematic of an embodiment of a TRF motor cross section.
• Figure 4 illustrates the measured magnetic flux density at the surface of a sinusoidally magnetized magnet.
• Figure 5 illustrates a magnetoresistive position encoder configuration
• Figure 6 illustrates a magnetic flux density distribution above a magnet surface, with and without a ferromagnetic layer.
• Figure 7 illustrates a test set-up.
• Figure 8 illustrates a simplified schematic of a TRF-EPS power stage.
• Figure 9 illustrates a digital signal processor based TRF-EPS controller architecture.
• Figures 10 A and B illustrates a position sensor signal and the corresponding position initialization logic.
• Figure 11 illustrates a TRF motor cross-section and physical dimensions.
• Figure 12 illustrates no-load induced voltages at 2000 rpm.
• Figures 13 A and B illustrate MR signals in a high resolution array.
• Figures 14A and B illustrate the current waveform and frequency at 1.35 Nm and 1560 rpm.
• Figure 15 illustrates the torque ripple of a baseline motor at 10 rpm.
• Figure 16 illustrates the torque ripple of a TRF motor at 10 rpm.
• Figures 17A-17C illustrate an audible noise spectrum in a baseline motor, a motor with double insulation and a TRF motor with engine idling and steering at 30 rpm.
• Figure 1 shows a column-mounted EPS system
• the steering assist torque is provided by an electronically controlled motor 12 in the amount demanded by the driver's use of the steering wheel 14.
• the driver's need is sensed and communicated to the motor controller by an in-line torque sensor 16.
• the assist torque is transmitted to the wheels via the conventional rack-pinion assembly 18, usually by applying the torque to the steering column 15, though it is also known to apply torque to the rack 17 directly.
• reduction gears 20 are typically placed at the motor shaft. A mechanical link is maintained between the steering wheel and the wheels for safe manual operation in case of failure of the EPS system.
• An Electronic Controller 22 and Power Module 24 complete the EPS system.
• the choice of motor type is a crucial one, because it determines the characteristics of the drive and the requirements on the power switching devices, controls, and consequently cost.
• Leading contenders are the Permanent Magnet (PM) brushless motor, the Permanent Magnet (PM) commutator-type, the Induction Motor (IM) and the Switched Reluctance (SR) motor, each of the four options has its own inherent advantages and limitations.
• the PM brushless motor is preferred, chosen on the strength of years of experimenting with commutator- type motors.
• the large motor size and rotor inertia of commutator-type motors limit their applicability to very small cars with reduced steering assist requirements.
• SR drives offer an attractive, robust and low cost option, but suffer from inherent excessive torque pulsation and audible noise, unless special measures are taken to reduce such effects.
• the motor is located within the passenger compartment and therefore must meet stringent packaging and audible noise requirements that the present SR motor technology may not satisfy. Therefore, the PM brushless motor with its superior characteristics of low inertia, high efficiency and torque density, compared to commutator motors, appears to have the potential for not only meeting the present requirements but also of future high performance EPS systems of medium and large vehicles.
• Pulsating torque produced by motors would be felt at the steering wheel, if not reduced to very low levels.
• the EPS audible noise is mainly emanating from the motor and gearbox.
• the gear noise is mechanical and is the result of lash caused by manufacturing tolerances.
• the motor-caused noise is mainly a result of structural vibration excited by torque pulsation and radial magnetic forces in brushless motors and, additionally, by the commutator/brush assembly in commutator motors.
• Torque Ripple Causes and Remedies Torque ripple is the primary cause of imperfect steering feel.
• the cogging torque (also known as detent torque) is caused by the magnetic interaction between the permanent magnets and the slotted structure of the armature. It exists in both brushless and brush-type machines at all speeds and loads, including no-load.
• the instantaneous value of the cogging torque varies with rotor position and alternates at a frequency that is proportional to the motor speed and the number of slots.
• the amplitude of the cogging torque is affected by some design parameters, such as slot opening/slot pitch ratio; magnet strength; and air gap length, while its profile could be altered by varying the pole arc/pole pitch ratio. Careful selection of these parameters can lead to reducing cogging torque, but this approach is limited by practical and performance constraints.
• a more common and effective approach is by skewing either the stator teeth or the rotor magnet longitudinally, which provides for a gradual transition as the magnet moves under a stator tooth.
• a skew amount of one slot pitch should eliminate cogging.
• due to practical factors such as magnetic leakage end effects, skew variation attributable to tolerances, and eccentricity, some undesirable cogging remains.
• the commutation torque ripple results from the interaction of the harmonic contents of the stator currents and rotor field, as the instantaneous torque is proportional to the product of the stator current and induced voltage.
• Typical current and voltage waveforms in a conventional PM brushless motor are shown in Figure 2 A.
• the trapezoidal voltage waveform 32 results from a magnetic field distribution in the air gap, which is nearly constant under the magnet poles and changing polarity between poles as in most PM machines.
• the double-hub shape of the current waveform 34 is caused by current switching (commutation) between the motor three phases in a typical six step inverter.
• the noise generation mechanism in a conventional PM brushless motor is multifaceted:
• the main contributor to the motor noise is the structural vibration excited by torque pulsation.
• causes and remedies of torque pulsation have been discussed above.
• the TRF concept disclosed below substantially eliminates the torque ripple and thus the origin for this noise contributor.
• Noise is also generated by the structural vibration excited by the radial magnetic forces. These are forces exerted by the magnets on the individual stator teeth in a conventional motor, causing the stator structure to cyclically flex and vibrate as the magnets rotate within the stator bore. A toothless configuration as embodied in the TRF concept will not experience such forces and the associated noise.
• the high frequency components of the stator non-sinusoidal currents and those introduced by the PWM current or voltage control of EPS motors produce magnetic fields and forces that cause the laminated structure of conventional brushless motors to vibrate at these frequencies producing an audible noise known as magnetic whine.
• the absence of laminations in the TRF concept will eradicate this noise source. Another source of audible noise is windage.
• Substantial air movement through the motor air gap can cause an audible whistling.
• This air flow is the result of nonuniform gaps between stator teeth and or magnet arcuates with interpolar air passages.
• a smooth cylindrical structure as proposed in the TRF concept will reduce such windage noise.
• the motor bearings can also contribute to audible noise. This contribution is insignificant for a motor that produces smooth torque and minimal torque pulsation.
• sinusoidal drive is meant the combination of a sinusoidal inverter with sinusoidally magnetized permanent magnets, or, more to the point, with magnetic fields created by permanent magnets and distributed sinusoidally in the airgap, as will be described in greater detail below.
• a sinusoidal drive eliminates commutation torque, though cogging torque remains unaffected. Nevertheless, a sinusoidal drive may eliminate as much as 60% of the total torque ripple.
• An important element of the present invention is the use of sinusoidal currents to eliminate commutation torque ripple in power steering systems.
• PM permanent magnet
• Such machines are called “synchronous permanent magnet machines” to distinguish them from “brushless dc machines” which typically use trapezoidal or square current waveforms.
• the present application teaches cost-effective ways to use a PM machine driven by sinusoidal currents in order to eliminate torque ripple in a power steering system.
• PM synchronous machines have been generally considered to be high performance, but high cost, drives, thus inapplicable to automotive applications such as power steering systems.
• the present invention retains the desired high performance, particularly low torque ripple, but at a moderate price.
• an important cost item is the position encoder.
• NdFeB magnets like samarium-cobalt magnets, do not show any significant demagnetization characteristics.
• the coercivity of the NdFeB magnet falls off rapidly beyond a "knee” and, hence, demagnetization can occur. Because the demagnetization force applied to the magnet is proportional to armature current, a conventional design using NdFeB magnets will have limited peak current and, therefore, low peak torque despite the higher energy product magnets.
• the stator winding is a multi-phase winding contained wholly within the magnetic air gap so that there are no saturation constraints in the magnetic circuit and flux densities above 7 kilogauss in the air gap can be used.
• the ratio of the magnet length to the gap length is in the range of 0.5 to 2.0.
• the ratio of the interpolar distance to the radial gap length is greater than 1.3.
• a comparison of samarium-cobalt (Sm CO 17 ) magnet servo motors with motors of comparable size and weight made according to the Schultz patent invention indicates about a 70% increase in the dynamic continuous torque speed output performance and about an 80% increase in the intermittent performance.
• the winding In order to achieve the improved results it is important to properly secure the winding within the surrounding back iron cylindrical shell that provides the flux return path. Because the stator teeth are eliminated the winding must be secured to the stator structure with sufficient adhesion to withstand the maximum motor torque force throughout a range of operating temperatures. The winding must be rigid because movement of the conductors adversely affects the ability to generate torque.
• the winding is encapsulated and bonded to the cylindrical stator shell by a ceramic filled epoxy selected to provide (1) a good mechanical strength (i.e., compressive strength, tensile strength, tensile shear), (2) good thermal conductivity, and (3) a coefficient of thermal expansion equal to or greater than that of other material in the stator structure.
• a ceramic filled epoxy selected to provide (1) a good mechanical strength (i.e., compressive strength, tensile strength, tensile shear), (2) good thermal conductivity, and (3) a coefficient of thermal expansion equal to or greater than that of other material in the stator structure.
• a suitable material is Nordbak 7451- 0148/7450-0027 epoxy made by Rexnord Chemical Products, Inc.
• Stycast 2762 made by Emerson and Cummings, a division of W.R. Grace & Co.
• the Schultz patent further includes a method for assembling a motor with the winding in the air gap.
• the winding is formed using a cylindrical support with a reduced diameter section at one end.
• a fiberglass sleeve is placed aro ⁇ nd the cylindrical support in the uniform diameter portion and thereafter preformed coils are placed in position. It is understood that the fiberglass sleeve is not necessary to support the coils and that other embodiments do not use a sleeve.
• the Schultz invention is a disc drive data storage system having a motor with an "ironless" stator winding.
• the spindle motor includes a housing, a stationary member and a rotatable member.
• a bearing interconnects the rotatable member with the stationary member such that the rotatable member is rotatable about a central axis.
• a magnet is attached to the rotatable member and forms a portion of a rotor for the spindle motor.
• the "ironless" stator winding is coaxial with the rotor magnet and provides a rotating magnetic field that drives the rotor magnet.
• the spindle motor further includes a hydrodynamic bearing and a magnetic field focusing member or back-iron that is attached to the stator winding.
• the back-iron concentrates the magnetic flux that is generated by the stator winding.
• the stator winding is "ironless” in that the back-iron is external to the winding. There is no stator core or other magnetic material within the stator winding.
• the spindle motor includes first and second rotor magnets disposed about the stator winding.
• the hydrodynamic bearing is integrated with the motor such that the bearing has a first bearing surface formed by the first rotor magnet and a second bearing surface formed by the stator winding. The combination of the hydrodynamic bearing and the "ironless" stator winding reduces the forcing functions that give rise to pure tone vibrations in data storage disc drives. Hydrodynamic spindle motors are much quieter than spindle motors having mechanical bearings.
• the disclosures of the Rutz patent are incorporated by reference herein in their entirety.
• the method comprises treating the iron particles with phosphoric acid to form a layer of hydrated iron phosphate at the surfaces of the iron particles, heating the iron particles in an inert atmosphere at a temperature and for a time sufficient to convert the hydrated layer to an iron phosphate layer, and coating the particles with a thermoplastic material to provide a substantially uniform, circumferential coating of such material surrounding the iron phosphate layer.
• the mixtures comprise iron core particles having a weight average particle size of approximately 20-200 microns, wherein the particles have a layer of iron phosphate at their surfaces and a substantially uniform circumferential coating of a thermoplastic material surrounding the iron phosphate layer.
• the thermoplastic material constitutes about 0.2%> to about 15.0% by weight of the coated particles.
• a reservoir microcapsule that has a thin, yet strong, protective inner layer of parylene as a primer coating well suited to join to itself a layer of thermoplastic material that molds well with the corresponding layer of thermoplastic material of like microcapsules.
• the Versic patent teaches a microcapsule that has multiple hydrophobic layers of coatings around a solid core particle, an inner one of which coatings is conformal to the surface of the core particle, is pinhole-free, is built up by molecular deposition, and can be built up to any desired thickness.
• the Versic patent also teaches a microcapsule that has a solid core of magnetic material coated by a layer formed by molecular deposition of free radicals of a monomer that polymerize to form a conformal, pinhole-free layer on the surface of the solid core particle, the radicals being non-reactive with the surface.
• the Versic patent also teaches a microcapsule having multiple, hydrophobic coating layers encapsulating a solid inner particle, the inner layer being a pinhole-free, electrically insulating layer conformal to the surface of the particle and having ,a higher melting temperature than a layer external to it.
• solid particles which may or may not have ferromagnetic properties and which have a diameter in the range from about a micron to as much as about 500 microns, are encapsulated by a pinhole-free layer of a poly-para-xylylene, known by the generic name of parylene.
• This layer is formed over the entire surface of each solid particle by molecular deposition of free radicals of parylene monomer that polymerize in place on the surfaces as the particles are tumbled in the presence of a cloud of the monomer.
• the parylene layer can be deposited using the techniques and apparatuses shown and described in United States Patent Nos. 4,508,760 and 4,758,288, the disclosures of which are incorporated by reference herein in their entirety.
• the initial molecular layer of the polymerized parylene thus produced conforms exactly to the configuration of the surface of each particle as a strong and unbroken electrically insulating barrier to the passage of oxidizing materials or to the electrical current that would be necessary for oxidation, or corrosion, to take place.
• the pinhole-free parylene is not only an unbroken insulator over the whole core particle but a good primer coat to tie the next encapsulating layer to the parylene-coated core particle.
• the next layer can be any thermoplastic or heat-curable resin capable of serving as a binder when the microcapsules are later formed into a shaped structure, e.g., ABS, epoxy, nylon, polyethylene, polypropylene, polysulfone, polyethersulfone, polyetheretherketone, and phenolic resin and others. Polysulfone and polyethersulfone have been found to be particularly satisfactory.
• the binder material should also serve as a lubricant in the process of forming a shaped structure. It is important that the temperature at which the binder material can be shaped into a desired structure be lower than the melting temperature of the parylene used so that the effectiveness of the latter as a barrier layer will not be adversely affected by any heat required in the shaping process.
• TRF Torque-Ripple-Free Motor
• a novel stator with (a) a slotless air gap winding 68 and (b) a composite iron yoke 70, preferably made from powdered iron, that also acts as a housing.
• a novel rotor is disclosed having a (c) high energy magnet 66 that is sinusoidally magnetized, (d) a molded composite plastic shaft 62, and (e) a new higher resolution position sensor with magnetoresistive sensing elements and steel wheels.
• the motor will preferably be driven with a sinusoidal inverter.
• the combination of sinusoidal inverter, sinusoidally magnetized magnets, and position sensor is what is referred to in this specification as the "sinusoidal drive.”
• the coils of the slotless winding are typically prewound separately on mandrels using high speed winders, arranged in the desired 3 -phase configuration, then placed, pressed and glued onto the inside surface of a cylindrical stator yoke or flux carrier.
• a suggested winding for the TRF motor is built by ELINCO, Inc., which holds Hendricks, H.F., US 4,556,811, for COIL UNIT AND COIL FORM FOR ELECTRICAL MACHINES, issued December 3, 1985, on the process for making such windings, the disclosures of which are incorporated by reference herein in their entirety.
• the winding in the exemplary embodiment is housed directly into a one-piece composite iron cylinder, 70.
• a microencapsulated powder iron material features a low electrical conductivity and reasonable magnetic and mechanical properties. The low conductivity eliminates the need for laminations.
• the magnetic properties are adequate to carry the magnetic flux circulating between the poles, and the mechanical strength is improved by postbaking to withstand the high- pressure forces exerted during assembly of the winding into the housing.
• this simple stator construction presents a cost advantage that defrays some of the added cost in the magnet and sensor.
• the rotor will most preferably use high energy product (e.g., Nd- Fe-B) ring magnets.
• the ring magnets are preferably assembled onto the rotor in an unmagnetized state for ease of assembly and then magnetized in place.
• Using ring magnets instead of several magnet arcuates that need to be properly spaced and retained, may lower cost for brushless motors in high volume applications.
• One of the challenges in using high energy ring magnets is the difference in the thermal expansion coefficient of the magnet and the steel core usually attached to it, which result in excessive hoop stresses in magnets leading to their failure under temperature cycling conditions. To overcome this problem, a proprietary process for attaching the magnet to its core has been developed and successfully applied to the TRF motor and many other prototype motors.
• the magnets are magnetized sinusoidally, as shown in Figure 4 for the field measured over the magnet surface.
• the deviation from an exact sinusoidal is not crucial in a machine with an air gap winding because the harmonic content is filtered out as the field travels through the large magnetic gap (winding) region.
• Magnetizing a magnet sinusoidally does not constitute any additional cost compared to conventional trapezoidal magnetization because it is only a matter of using the appropriate magnetizer geometry.
• the penalty is that the amount of useful flux provided by the magnet is estimated to be about 18% less than in an equivalent trapezoidally magnetized magnet, hi other words, to realize a given amount of flux with a sinusoidally magnetized magnet, one would need 18% longer magnet (more cost) than with the trapezoidal case.
• Sinusoidally magnetized ring magnets mounted on the surface of the rotor constitute the preferred embodiment.
• the key to this invention is a permanent magnet assembly on the rotor that produces an essentially sinusoidal magnetic field waveform in the airgap.
• Other permanent magnet assemblies, other than ring magnets, are thus possible. They include, but are not limited to, surface mounted, sinusoidally magnetized magnet arcuates; so-called “Halbach” magnet arrays; and, various configurations of buried permanent magnet structures.
• the ring magnets and their flux-carrying core are placed over a shaft preferably made of composite plastic.
• composite thermoplastic materials such as "ULTRAMID” made by BASF and the “ULTEM 2300” by GE, can be cost effective and easily molded to shape while integrating other structural components such as the worm and flux carrier during the molding process.
• the shaft will also carry the toothed wheels of the position sensor, whose sensing elements are housed on the stationary end bells. The sensor is described in more detail below.
• the inverter switching devices In order to generate motor currents with a sinusoidal shape, the inverter switching devices (MOSFETs, for instance) must be turned on and off at specific rotor angular positions. Therefore, the position of the rotor must be known at all times and an encoder is needed. This requirement is one of the factors adding to the cost of sinusoidal drives, hence traditionally limiting their application to high-performance applications.
• EPS is indeed a high-performance drive, yet it must meet stringent cost limits if it is to be practical for general automotive use. Therefore, a new type of encoder is used that combines high resolution and low cost.
• Optical encoders are temperature limited and susceptible to dirt.
• Semiconductor-based magnetic sensors such as Hall effect sensors or magnetoresistors (MRs), on the other hand, can work at higher temperature, and are already used in automotive applications.
• a preferred angular position sensor 80 having a set of magnetoresistors 82 (MR) mounted on a stationary permanent magnet 84.
• the magnet faces a steel wheel 86 with several tracks, 88 and 90, each of which has teeth and slots on its periphery.
• the teeth and slots modulate the magnet's field and these variations in magnetic field are sensed by the MR's 82.
• the various tracks on the wheel allow the sensor to perform several functions at the same time.
• a high resolution track 88 provides an incremental signal to enable the generation of sinusoidal currents in the motor.
• the high resolution track 88 is so named because it has many more teeth about its periphery than the low resolution tracks 90, and it's corresponding MR is designated the incremental position sensor 82i.
• the three other tracks 90 provide absolute signals every 60 electrical degrees and their associated MR's are designated the absolute position sensors 82a. These absolute signals are used for motor commutation, that is, to direct the current to the appropriate phases, which is important at start-up.
• An encoder for a TRF device needs to have the highest resolution possible while keeping the sensor simple enough for low cost.
• the sensor resolution is therefore proportional to the sensor wheel diameter.
• a single MR would provide approximately 4 mechanical degrees resolution, which is not sufficient. Therefore, several MRs are used to generate additional signals and increase resolution to a satisfactory level.
• the difficulty in designing a sensor with multiple MRs resides in the fact that the MR signals must all be of similar magnitudes.
• An MR signal is, typically, an oscillating signal with a dc bias. In order to obtain the final square- wave signal output, the dc bias must be eliminated. The resulting signal zero crossings are then used to trigger a flip-flop and generate a square wave.
• the dc bias is difficult to predict as it varies with air gap, wheel concentricity, temperature, doping of the MR material, etc.
• the MR signals are, therefore, best compared with one another as this provides automatic, internal compensation for many of these variations.
• the sensor must be designed to ensure maximum uniformity among the various MRs.
• a configuration with an array of several MRs facing the same track is chosen.
• An exemplary number is four. With this approach the MR chips are located close to one another, therefore ensuring air gap and magnetic field uniformity.
• the various MRs may be designed to come from essentially the same location on the semiconductor wafer from which they originate, thus minimizing MR material variations. The comparison between the various MRs and the elimination of the dc bias is achieved as follows.
• the MRs located at both ends of the MR array are spaced exactly one half of a tooth pitch apart, so that their signal output is half a period out of phase. Averaging their output, therefore, yields the dc bias. This provides a resolution of 1.25 mechanical degrees (2.5 electrical degrees with 2 pole-pairs). The testing of the overall drive established that this resolution is sufficient for this application.
• a thin ferromagnetic layer (on the order of 0.13 -mm thick) is placed on the magnet surface below the MR array.
• the effect of a ferromagnetic layer is shown in Figure 6, where there is plotted the results of a test conducted on a stationary magnet with and without a ferromagnetic layer. The magnetic flux density was measured across the magnet surface in the presence of a steel target emulating the target wheel.
• Figure 7 is illustrative of the experimental set up. Without a ferromagnetic layer, the flux density pattern is dome shaped.
• magnetic-based sensors are used, rather than optical sensors, mostly for temperature reasons as well as because optical sensors are susceptible to dirt.
• Various magnetic sensors are also possible, in a variety of configurations. For instance, Hall sensors can sense a pattern of North and South poles imprinted on a moving permanent magnet assembly.
• Electrical coils can also be designed to use the variable reluctance principle, or to sense varying self or mutual inductances.
• Magnetoresistors were chosen in the preferred embodiment because of their higher temperature capability, as well as higher intrinsic signals compared to that of Hall sensors. The particular configuration herein described was chosen because it groups all the active sensing elements close to one another on a single surface, that of a stationary permanent magnet, and this an important cost lowering factor.
• a Hall sensor-based commutation sensor readily available commercially
• an MR-based, Hall effect- based, or sealed optical encoder mounted separately on the unit.
• various known methods such as "phase-lock loop" may be used to enhance the resolution of the sensor signal.
• the essential elements of the encoder described above, as they pertain to the present invention are: First, the use of an incremental encoder complemented with an index pulse to obtain at low cost absolute position during operation, other than during the motor power-up phase.
• the power-up phase comprises the phase of initial rotation of the steering motor.
• a power-up phase may be deliberately performed, as a way to initialize the system, when the vehicle in which the system is installed is first started.
• the approximate absolute encoder comprises a set of three "commutation” sensors, where "commutation sensor” should be understood as they are used in conventional brushless dc motors.
• a gear reducer is much preferred in the EPS system to magnify the torque produced by the electric motor to the level required to assist the vehicle steering effort. The higher the gear ratio, the smaller the motor size and cost. This is particularly desirable with the slotless configuration of the TRF concept, which inherently penalizes the permanent magnet weight and cost.
• Gear drives particularly well suited for this application include, but are not limited to, worm gear drives, harmonic gear drives, hypoid gear drives, and face gear drives.
• the torque produced by the TRF-EPS motor can be controlled by either controlling the amplitude and phase of the sinusoidal winding currents (current-mode control) or by controlling the sinusoidal applied voltage (voltage- mode control). Both control approaches have been implemented and evaluated for this invention. hi current-mode control, where the currents are controlled to be in phase with the back-emf voltages, the torque produced is directly proportional to the amplitude of the current reference.
• Current-mode control requires phase current feedback and proportional-integral (PI) control in order to achieve high accuracy.
• Current-mode control provides the advantages of maximum torque per ampere, lower torque ripple, fast and precise torque control, inherent overcurrent protection and insensitivity to motor speed and parameter variations.
• the main drawbacks of current-mode control are the requirements of at least two isolated current sensors and a more complex controller for implementing a fully digital current-mode controller.
• the applied voltage In order to achieve constant torque, the applied voltage must be controlled as a function of speed and machine parameters R and Ke.
• the voltage-mode control does not require current feedback and hence is simpler to implement. However, clean speed feedback signal (free of noise) and knowledge of machine parameters are essential in order to achieve accurate torque control.
• the EPS Controller Power Stage is similar to that of conventional trapezoidal-motors with the exception of the phase current sensors.
• the three-phase full-bridge inverter, 100 uses power MOSFET switches 102 in order to minimize the switching losses at high switching frequencies.
• Power is connected through a relay 104 in series with the positive side of the battery to the input of the inverter.
• the relay can be opened in case of a fault, thereby isolating the battery from the inverter.
• Boot-strap gate drive circuits 106 are used to drive the ⁇ -channel MOSFETS 102. This method eliminates the need for costly, isolated power supplies and level shifters for driving the upper - channel MOSFETS.
• the ON/OFF control signals to the gate drive circuits are generated by the digital controller as explained later.
• the PWM switching frequency of the MOSFETS is set to approximately 19 kHz, so that audible noise due to inverter switching is minimized. High switching frequency is also necessary in the case of TRF-EPS motor to minimize the current ripple because the motor inductance is very low due to air-gap winding.
• the TRF-EPS 10 system requires two phase current sensors 108 with matched gain and zero offset, in order to control the motor torque precisely. Because the three phase currents add to zero in a Y- connected motor, the third phase current can be obtained from the other two phase currents. While a precision resistor can be used for current measurement, it is not favored in this case because the signal to noise ratio in a PWM inverter is very poor. Instead, the power stage uses two, low cost, open-loop Hall-Effect current sensors at its output, which provide isolated current feedback signals. Each current sensor 108 comprises a linear Hall sensor placed in the gap of a ferrite toroid, with a single turn of the current carrying conductor passing through the center of the toroid.
• An op-amp is used at the output of the sensor to adjust the gain and offset.
• Each sensor is calibrated to provide 2.50V at zero current (to be compatible with the input of the A/D converter) and an incremental signal of 20 mV/A.
• the linear range of the current sensors is -100A to +100A, for which the sensor output ranges from 0.5V to 4.5V. Both sensors can be calibrated for gain and zero offset simultaneously in a production environment with standard laser trimming techniques.
• Conventional analog implementation of sinusoidal PWM is realized by comparing a triangular carrier wave at the switching frequency, with a sine wave reference signal, where the crossover points determine the instants of switching the inverter power devices.
• High carrier frequency assures low harmonic distortion even when the carrier signal is not synchronized with the reference waveform.
• the amplitude of the fundamental output voltage is varied by varying the amplitude of the reference signal relative to that of the carrier.
• three sinusoidal reference signals are needed that are synchronized to the motor back-emf using the rotor absolute position information.
• analog methods suffer from gain and offset drift problems.
• the sinusoidal current control of the TRF-EPS motor requires a lot of non-linear elements such as multipliers, trigonometric function generators etc. Implementing them in analog hardware is expensive and makes the controller tuning more difficult.
• the PWM signals are generated by digital comparators fed by digitally synthesized carrier and reference signals.
• the all-digital controller hardware 110 generally comprises a TMS32OC14 DSP chip operating at 25 MHz, 112 a 4- channel, 3 Ns, 10-bit analog to digital converter (ML2375), 114 with 2-channel simultaneous sampling capability, a Programmable Logical Device (PLD) for absolute position-detection and address decoding (explained later), 116 two current sense amplifiers and a serial DAC interface to the system controller.
• the DSP chip contains high speed ALU that can multiply two 16-bit numbers in 160 ns, 4K of EPROM for program storage, 118, 256 bytes of data RAM, 6 channels of digital PWM comparators 122 and two timer counters for speed measurement 124.
• Other peripherals included in the controller are an RS-232 serial interface to a host computer 126 and a 256-byte EEPROM 120 for storing system parameters such as the index position offset and the current loop integral and proportional gains.
• the operating mode can be selected to be either voltage-mode or current-mode, using an external switch 128.
• the torque command is received as an analog signal via the serial DAC 118 and is digitized by the A/D converter 114 along with the current feedback signals.
• the DSP 112 reads the rotor absolute position from the PLD, the torque command and feedback currents from the ADC and using the proper algorithm, generates the 3-phase PWM signals using internal digital comparators. These PWM signals are further processed by the PWM output logic to insert a dead- time between the top and bottom MOSFET control signals before applying them to the gate drive circuits on the power stage.
• the entire control algorithm execution time is less than 100 ⁇ sec and has proven to be fast enough to achieve pure sinusoidal currents and low torque ripple.
• the rotor position sensor used in the TRF-EPS system provides an incremental high resolution signal (obtained by combining quadrature signals EA and EB) with 2.5 ° resolution (electrical), a direction signal (derived from quadrature signals EA and EB, from ways known in the art, and not shown) and three commutation signals (HI, H2, H3) that provide absolute position information with 60° resolution, as illustrated in Figure 10.
• degrees must be understood as “electrical degrees”. It is therefore necessary to derive the absolute position using these signals.
• This is achieved by deriving an index pulse from the three commutation signals (HI, H2, H3), and combining it with the incremental signal. That is, each time the system generates an index pulse, position is reset, then it is adjusted according to the incremental signal. More specifically, the direction of rotation signal is used to add, or subtract, one step each time that an incremental signal is generated.
• the index pulse may be derived as follows. First, a zero reference angle must be chosen. For instance, one may choose the positive zero crossing of the back-emf of phase A ,when the motor is rotating in the forward direction, as the zero reference angle. Then, the controller is designed in such a way that one of the transitions in signals (HI, H2, H3) corresponds to that zero reference angle. This can be achieved by first placing the sensor during assembly so that signal HI (for instance) corresponds to the back-emf of phase A, and also by placing it in such a way that the transitions in signal HI from level 0 to level 1 (and vice-versa) correspond to the zero crossings of that back-emf.
• the method adopted for determining the absolute position then uses an 8-bit up/down position counter that counts the high resolution pulses using the direction signal to determine whether to count up or down.
• the maximum count of this counter is set to 143, corresponding to 357.5°.
• the counter is initialized with an angle value that is equal to the midpoint of the 60° sector determined by the three commutation signals.
• the initial position can have a maximum error of ⁇ 30°.
• the controller can then switch over to sinusoidal current control as soon as a transition in either one of HI, H2 or H3 is sensed, or as soon as a first index pulse is generated.
• any coarse absolute position sensor could be used that would define large angular intervals such as the 60° intervals mentioned earlier.
• Commutation sensors providing signals HI, H2 and H3 in Figure 10A are proposed here as the preferred embodiment because they are a technology commonly used with brushless motors, hi a sense, this approach enables to start the system as a brushless motor using currents with square waveforms, and switch over to the more desirable sinusoidal control as soon as a transition in either HI, H2 or H3 is sensed, or as soon as a first index pulse is generated.
• the position counter initialization logic is shown in Figure 10, where the rising edge of commutation signal HI is assumed to be aligned with the positive zero crossing of the back-emf of phase A for increasing angles.
• transition of any commutation signal can be used for setting the rotor position counter to the absolute value, from which the counter can keep track of the position at 2.5° increments.
• only one commutation signal (HI) may be used for correcting the absolute position. This simplifies the logic and minimizes the effects of non- uniform spacing of the commutation signals.
• the position counter is reset to 0° on the rising edge of HI signal for increasing angles and preset to 357.5° on the falling edge of HI for decreasing angles.
• This scheme enables tracking of the absolute position within one electrical cycle from the time of starting the system. All the logic for detecting the absolute position is implemented in an Altera EPM5128 Programmable Logic Device (PLD). The position counter initialization and reading of the absolute position is carried out by the DSP. Any offset of the reference edge of HI from the zero reference point of phase A back- emf is corrected in the controller software.
• PLD Altera EPM5128 Programmable Logic Device
• the maximum line to line RMS fundamental voltage obtainable with this modulation method is 0.6124 N ⁇ j c , where Na c is the inverter bus voltage.
• the RMS fundamental voltage can be increased while keeping the currents sinusoidal because the Y-connected motor does not allow triplen currents to flow.
• the TRF-EPS system utilizes space vector modulation where the modulating function sin( ⁇ ) is replaced by:
• This modulation method provides an RMS fundamental line to line voltage of 0.7071 Ndc, which is 15.5% higher than that of a simple sine modulation.
• the space vector modulation function as well as the 3-phase digital PWM functions are implemented by the DSP chip.
• the sine function is obtained from a look-up table and is multiplied by the reference voltage in order to generate the PWM duty cycle values.
• the resolution of the digital PWM signals is 40 ns and the PWM frequency is set to 19 kHz.
• the bench testing comprises a) verification of system performance as measured by the quality of the sinusoidal voltage and current waveform, sensor signal and resolution; and b) evaluating the torque- ripple content in comparison with the base-line motor.
• the bench testing comprises a) verification of system performance as measured by the quality of the sinusoidal voltage and current waveform, sensor signal and resolution; and b) evaluating the torque- ripple content in comparison with the base-line motor.
• the term "sinusoidal" with respect to the currents is to be interpreted to mean “nearly” or “essentially” sinusoidal.
• the torque ripple in the motors of this invention will be within the range of 0 to 3%, preferably 0 to 2%, more preferably 0 to 1%, and most preferably less than 1%.
• Figures 14A and 14B show the phase current waveform and its frequency spectrum, respectively, with the machine running at 1560 rpm and delivering a torque of 1.35 N-m to a dynamometer load.
• the current waveform shows a very good sine wave quality.
• the frequency spectrum shows a component at the fundamental frequency of 52 Hz and a very small component at the PWM frequency of 19 kHz. This good sine wave quality was observed over the speed and load range.
• the torque ripple content was evaluated at a test stand equipped with automated data acquisition system. The results were compared with those of a trapezoidal motor used by the assignee to serve as a base line.
• the torque ripple tests were conducted for a voltage-mode operation (mainstream approach at the time of testing) and at a low motor speed (10 rpm), because at high speed the system inertia (of both tested and dynamometer motors) tends to mask any present torque ripple. This is a known measurement difficulty that has not been overcome.
• the data were collected over two mechanical revolutions as displayed in Figures 15 and 16 for the base line and the TRF motors, respectively. It is clear that the TRF system exhibits a much less torque ripple content, (peak- peak)/average, than the base line system (2.5% versus 18%).
• the noise level at 600 Hz was reduced dramatically from 47 dB A to 31 dBA with the engine off and from 47 dBA to 36 dBA with the engine idling while steering at 30 rpm.
• a conventional motor achieved levels of 34 and 24 dBA under the same conditions, respectively.
• the TRF motor noise was down to 28 from 42 dBA with engine off versus 37 dBA with isolation, and down to 31 from 40 dBA with the engine idling versus 36 with isolation.
• the TRF concept demonstrated a much quieter operation than conventional motors, to a level perhaps achievable only with rubber isolation of the shaft and housing.
• the housing isolation adds to the motor size and affects packageability.
• Sinusoidal inverter and conventional motor A possible embodiment combines a sinusoidal inverter (sinusoidal current) with a conventional machine (trapezoidal voltage).
• the magnitude of the commutation torque ripple and noise could be less than in the basic system, depending on the width of the flat top of the trapezoidal voltage waveform.
• the cogging torque is reduced in a traditional fashion by skewing either the magnet or the stator teeth. In this option, a conventional motor with a less expensive high energy magnet can be used.
• Sinusoidal inverter and a motor having sinusoidally magnetized magnet and a conventional stator Another embodiment is the same as the previous embodiment, but with sinusoidally magnetized magnets.
• the machine active length must be increased by about 18% to compensate for the reduction in magnetic flux associated with a sinusoidal waveform as opposed to trapezoidal.
• this option has the potential of approaching the low torque ripple level of the TRF system. It does not benefit from the simplicity of the slotless winding, but also does not necessarily need the expensive high energy magnets as for a slotless construction.
• the decision between less expensive vs. expensive magnets, in this case, should be based on packaging and economic constraints.
• Another possible alternative embodiment comprises a conventional rotor producing a trapezoidal magnetic field waveform in the airgap, along with a slotless stator.
• a conventional rotor producing a trapezoidal magnetic field waveform in the airgap, along with a slotless stator.
• Such an arrangement would completely eliminate cogging torque, and significantly reduce the radial forces that are a source of vibration and audible noise. As long as trapezoidal currents and magnetic field waveform are used, commutation torque ripple would still be present.
• Rotary machines constitute the most common technology for electric power steering. Rotary machines will be of the radial-field type, as shown in the figures included in this application, or could be of the axial-field type as well. However, inasmuch as linear motors are an option (US 5,924,518), the various aspects of the present invention could be used in a linear motor configuration to reduce "force" ripple.

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• 2000
  • 2000-09-06 WO PCT/US2000/024416 patent/WO2002020293A2/en active Application Filing
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