外文翻译---混合式步进电机及其驱动.docx

附录AHybrid Stepping Motors and DrivesHybrid stepping motors derive their name from the fact that their construction is a hybrid between permanent magnet and reluctance motor topologies. Their inherent positional accuracy makes them suitable for a wide range of motion control and industrial positioning applications. This article explains the construction and operation of the hybrid stepping motor. Power converter topologies are presented which are commonly used in hybrid stepping motor drives. Methods are discussed of enhancing performance beyond that achievable by traditional methods. It is shown that the fall in cost of power electronic devices is enabling further enhancements of stepping motor drive technology, broadening the range of applications for this class of motor.by J. D. Wale and C. PollockHybrid stepping motor drive applicationsHybrid stepping motors derive their name from the fact that their construction is a hybrid between permanent magnet and reluctance motor topologies. Hybrid stepping motors range from miniature motors having outputs measured in uNm and uW to motor producing up to 60Nm of torque and 2kW of shaft power. Stepping motors have traditionally been used in applications at speeds up to2000/3000 r/min, but recent enhancements in drive technology and improvements in motor design are enabling the operating speed of stepping motors to be increased. Hybrid stepping motors are inherently suited to providing motion in discrete, small steps. As a result, they are ideal in applications where a piece of equipment must be moved and positioned accurately, usually according to instructions from a digital controller. Stepping motors are suited to driving low-speed high-friction loads and are ideal for point-to-point positioning systems and applications needing short rapid moves. They have the advantage of being able to operate without feedback devices and this, coupled with the absence of brushes, makes stepping motors virtually maintenance free. They give a non-cumulative error over any distance and are inherently digital in operation as their operation is based on converting electrical pulses into mechanical movement in fixed increments. Stepping motors find applications in many industrial areas such as packing machines, conveyor systems, robot arm movement, lift and stack machine tools.Despite the extent of its use in specialist positioning applications the construction and operation of the hybrid stepping motor is not as well known as other brushless drives, such as induction, brushless DC and switched reluctance motors. This article highlights how its unusual construction gives rise to a unique ability to deliver high-precision positioning without requiring particularly complex control circuits.Hybrid stepping motor driveA block diagram of a hybrid stepping motor and drive is shown in Fig. 1 and consists of a hybrid stepping motor, power converter and controller.The hybrid stepping motor is a synchronous excited by AC current supplied by the power converter. It is usual for the power converter to incorporate a phase current sensor so that the excitation level can be controlled to suit the motor rating and load conditions. The controller is responsible for generating the phase excitation sequence such that the drive acts according to the requirements given by the user input. Whilst a stepping motor drive does not require a shaft position sensor simply to rotate, greater control of performance can be achieved where position information is available.Hybrid stepping motors usually have two, three or five electrical phase windings. Two-phase motors are the most common, as this keeps the power electronic circuits relatively simple. Five-phase motors3 can, however, offer greater positional accuracy without requiring a shaft position sensor. Since the principles of operation are common for the whole motor family, this article will concentrate on the construction and operation of the two-phase hybrid stepping motor and drive.Two-phase hybrid stepping motor：construction, operation and analysisThe two-phase hybrid stepping motor is illustrated in cross-section in Fig. 2 and consists principally of a stator and rotor, both of which are typically constructed from stacked laminations of electrical grade steel. The stator has eight poles, four for each phase, which widen at the tips to a group of teeth. Each stator pole has a winding in the form of a short pitched phase coil. These are connected, usually in series, to form two electrically independent motor phases, designated A and B in Fig. 2.The rotor has a large number of teeth (typically 50) and incorporates a permanent magnet which produces static flux along the axis of the machine. The teeth at one end of the rotor are offset by half a tooth pitch with respect to those at t.he other end of the rotor, as shown in Fig. 3. The rotor tooth pitch is the same as that of the stator. Fig. 4 shows a stator with the eight coils and the rotor clearly showing the permanent magnet in the centre of the rotor.With the number of rotor teeth chosen to be a non-integer multiple of four and with regular angular displacement of the stator poles, an arrangement is set up such that two poles of one phase can be set with their teeth in alignment with one end of the rotor and the other pair of poles of the same phase are in alignment at the other end of the rotor. At this position, all teeth of the other phase poles are equally unaligned with the rotor teeth at both ends of the motor. This is the position shown in Fig. 2 (rotor teeth at rear end not shown).Fig. 5 shows more clearly the half-tooth pitch difference between each end of the rotor and how this relates to the stator teeth. Fig. 6 is a magnetic plot obtained by finite-element analysis, which illustrates the way in which magnetic flux links the front and back ends of the rotor. The .magnetic paths within the motor are complex, involving axial, radial and tangential components.The direction of winding of the coils is important, as they are arranged in such a way as to set up a flux path which passes axially along the rotor and hence through the permanent magnet. With the rotor as shown in Fig. 2, suppose that the magnetisation of the rotor produces a magnetic north pole at the front and a south pale at the rear. Stator poles 1, 3, 5 and 7 all belong to phase A, and it can be seen that poles 1 and 5 have teeth in alignment with the rotor at the front, and poles 3 and 7 have alignment with the rotor at the rear. Suppose now that phase A is energized with current such that poles 1 and 5 become magnetic south poles and poles 3 and 7 become north poles. Magnetic flux will travel from poles 3 and 7 to the teeth at the rear of the rotor, through the rotor and then out from the front of the rotor to poles 1 and 5, where the flux path will be closed by the flux flowing back through the stator to the rear of the motor. The flux path hence follows a twisted three-dimensional loop, as seen in Fig. 6.At the position shown in Fig. 2, this energisation will tend to hold the rotor in the position shown. To cause the rotor to rotate, it needs to be moved on by one step. This can be achieved by now de-energising phase A and energising phase B. In the position shown, all teeth of the phase B poles 2,4, 6 and 8 are at a position midway between two rotor teeth both front and rear. From this position, the rotor could turn in either direction. The actual direction in which it turns is governed by the direction of current in the phase B windings in relation to the permanent magnet excitation of the rotor.If the energisation is such that poles 2 and 6 become south poles and poles 4 and 8 become north, the rotor will tend to rotate clockwise (as viewed from the front) as opposite poles of rotor and stator attract and like poles repel each other. Had the current in phase B been in the opposite direction, the magnetic polarity of the stator poles would have been reversed and rotation would have been in the anticlockwise direction. Once the rotor has turned to meet the tooth aligned positions, it can be moved on another step by now de-energising phase B and reenergizing phase A. The direction in which phase A needs to be energised, however, is in the opposite direction to that two steps previously Continuous stepwise rotation can be achieved by alternately energising phases A and B with alternative polarities, for example A+, B+, A-, B-, A+, B+ etc. A consequence of this excitation pattern is the need for bipolar phase excitation. It can be seen that the purpose of the permanent magnet is to provide flux which interacts with the flux from the phase windings to produce torque. The permanent magnet flux, being present even with no phase excitation, gives rise to a small alignment or cogging torque which tends to make the rotor take up the nearest position of alignment with a phase and can be useful in maintaining shaft position in some equipment when the power is switched off.The pole geometry of the hybrid stepping motor is such that there is minimal change in either self or mutual phase inductance as the rotor is moved.4 There is, however, a large position-dependent influence in the flux linkage of the phase winding coils due to the rotor permanent magnet. This can be observed when the rotor of the hybrid stepping motor is rotated by another machine, and the phase voltage signals observed with the oscilloscope. Fig. 7 shows the open-circuit rotational EMF produced by one phase of a stepping motor manufactured by Stebon Ltd. in the UK.It is apparent from Fig. 7 that the rotational EMF from this motor is approximately sinusoidal, although there is a small third harmonic component. The sinusoidal approximation to the rotational EMF in Fig. 7 has the same RMS voltage value as the real signal and is clearly very similar in shape. The time integral of the rotational EMF represents the phase winding flux linkage due to the rotor permanent magnet. This is also almost sinusoidal, showing how the action of the rotor and stator poles passing each other produces a smooth variation in flux linkage between the magnet and each phase coil.The flux linkage waveform of the other motor phase (not shown in Fig. 7), has identical shape and magnitude, but is shifted in phase by YO. It is therefore conceptually convenient to describe the rotor magnet flux as a phasor of fixed magnitude, rotating at a speed related to the shaft rotation, inducing voltage in the two orthogonal phase windings. The motor phases can be considered as static viewpoints from which the effect of the rotor flux can be observed as it passes by. This concept is illustrated in Fig. 8, where is the rotor flux, w is the electrical speed and tis time. The components of the rotor flux linking phases A and B can be expressed as cos(wt) and sin(wt), respectively.The electrical speed is related to the mechanical speed by the number of rotor teeth.For a typical 50-tooth rotor, the electrical speed is 50 times the mechanical speed. In other words, 50 electrical cycles are performed for each complete rotation of the motor shaft. In a hybrid stepping motor torque is generated, as with any other electrical machine, by the interaction of magnetic fluxes. Stator flux is generated by the circulation of current in the phase windings, which interacts with the rotor flux to produce torque.Since the two phase windings are magnetically orthogonal, the resultant stator flux can be made to act at any desired angle by appropriate excitation of the two phases. Hence the position of the rotor can be changed by selection of a flux vector at the appropriate angle. Fig. 9 shows how eight different basic flux vector angles cm be generated by various combinations of phase excitations.From the table in Fig. 9, a number of excitation sequences can be developed for continuous rotation. The simple excitation scheme given in the earlier description of motor operation uses just vectors 1,3, 5 and 7 in sequence. Clearly, the direction of rotation can be reversed by making the energisation sequence 7, 5, 3, 1 etc. A superior excitation scheme makes use of positions 2, 4, 6 and 8, where both phases are always excited. This provides improved torque stiffness at each holding position as the stiffness (given by the gradient of the torque/position characteristic) is least in the direction of the reference axes and greater elsewhere. Additionally, since both windings are now involved in torque production, the total available torque for any given winding current is increased by a factor of .Both of the previously described schemes generate four discrete positions per electrical cycle, known as full step mode. Given that, with 50 rotor teeth, there are 50 cycles per revolution, either of these simple schemes can position the machine rotor in any one of 200 unique positions. The discrete positioning characteristic of the hybrid stepping motor makes it ideal for use in digitally controlled motion applications. Higher positional resolution can be achieved by using all eight positions, known as the half step mode of operation. An extension of this, known as microstepping creates many more unique flux directions by using a greater range of different phase current combinations.Power converters for hybrid stepping drivesSince hybrid stepping motors generate torque using a magnetically polarised rotor, the polarity flux in the machine is important, which results in the requirement for bipolar stator flux excitation.One method of providing bidirectional flux is to have bifilar wound phase coils: a technique frequently applied to small stepping motors where drive simplicity is required. Such motors are frequently known as unipolar, since the current in any particular winding only needs to flow in one direction. Fig. 10 shows the typical power circuitry used in a small drive employing a bifilar wound motor.The circuit of Fig. 10 achieves full motor control with just four power switches, all of which have a common connection at OV Several semiconductor manufacturers produce integrated circuits containing four switches and the necessary sequencing logic to directly drive a small unipolar stepping motor under the control of digital inputs for direction and step increment.The circuit of Fig. 10 has a major drawback in that it relies on the resistance of the phase windings to dissipate energy and hence to allow the current in a winding to fall to zero when it is switched off. The circuit of Fig. 10 is therefore limited to motors with high resistance windings. A simple improvement to the circuit is to add a power Zener diode, as shown in Fig. 11.The addition of the Zener diode allows a negative voltage to appear across the phase coil at turn off, which rapidly reduces the current to zero. Energy is dissipated in the Zener diode as well as the phase resistance while this happens. The choice of Zener voltage is a compromise, as a large voltage is required to maximise the rate of change in current, but the peak voltage appearing at the switches is the supply voltage plus the Zener voltage. The Zener voltage must therefore be chosen so as not to require excessive voltage ratings of the switching devices.The primary disadvantage of the bifilarwound unipolar excitation scheme is that, in each phase, only half the total winding area can be used to carry current at any one time. This results in higher resistive losses in the machine which is particularly serious in larger machines with high power outputs. In the larger drives, an increase in the complexity of the power electronics can be justified by improved performance of the drive. In these cases, each phase has just one winding which then requires bipolar excitation. The circuit of Fig. 12 is commonly used to drive such motors.The circuit of Fig. 12, known as an H-bridge due to the appearance of the schematic layout, requires eight power switching devices to control the bipolar energisation of two motor phases. With this arrangement, care must be taken in the power switch control to avoid shoot-through where two switches in series across the DC power supply conduct simultaneously with destructive results.It was shown in the preceding analysis of motor operation that torque production in a stepping motor is due to a flow of current in the stator windings. The action of the power converters presented here is to impose a particular voltage .across the phase winding rather than a particular current through it.In a small stepping motor, the relatively high resistance of the phase windings naturally limits the current when voltage is applied. Sometimes each phase winding has an added series resistor, known as a forcing resistor, which makes the constant voltage output of the power converter more like a constant current source.Whilst resistance is a practical and costeffective way to limit current in a small drive, the inefficiencies of this are intolerable in a large drive. In these cases, a current sensor can provide feedback to a control system which regulates current to a preset level. This is achieved by using the power converter repetitively to switch. the voltage supply to the motor phase on and off. The ratio of on time to off time is varied according to the feedback level such that the average current level in the winding is equal to the control systems preset level. This method of current control, known as switching regulation or pulse width modulation, can achieve very high efficiencies and is ubiquitous in high-power stepping drives.Further developmentsThe production of rotation by incremental steps is well established in commercially available stepping drives. This approach in either full or half step forms can be achieved with simple control circuitry, but has some undesirable features. Operating the motor in discrete steps involves accelerating