Electricity: motive power systems – Positional servo systems – With particular motor control system responsive to the...
Reexamination Certificate
1999-10-29
2001-09-04
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Positional servo systems
With particular motor control system responsive to the...
Reexamination Certificate
active
06285155
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to stepper motors, and more specifically, to a method and apparatus for driving a stepper motor that provides improved efficiency and eliminates overshoot and ringing.
BACKGROUND OF THE INVENTION
Stepper motors are well known in the art and are used in a wide variety of devices, including printers, disk drives, and other devices requiring precise positioning of an element. Stepper motors provide many advantages over other types of motors, most notably the ability to rotate through controlled angles of rotation, called steps, based on command pulses from a driver circuit. The accuracy of the stepped motion produced by a stepper motor is generally very good, since there is not a cumulative error from one step to another. The ability to incrementally rotate a shaft through a defined number of fixed steps enables stepper motors to be used with open-loop control schemes (i.e., applications in which a position feedback device such as an optical encoder or resolver is unnecessary), thereby simplifying the motion control system and reducing costs.
The speed of stepping motors can be readily controlled based on the pulse frequency employed, enabling stepping motors to achieve very low speed synchronous movement of a load that is directly coupled to the drive shaft of the motor. Furthermore, stepper motors are reliable, since they do not include contact brushes that can wear out. Typically, the only parts in a stepper motor susceptible to wear are the motor bearings.
There are three basic types of stepper motor, including a variable-reluctance (VR), a permanent magnet (PM), and a hybrid (HB). A VR stepper motor comprises a soft iron multi-toothed rotor and a wound stator. When the stator windings (also commonly referred to as the motor “coils”) are energized with a DC current, a magnetic flux is produced at the poles of the stator. Rotation occurs when the rotor teeth are magnetically attracted to the energized stator poles. PM stepper motors have permanent magnets added to the motor structure. The rotor no longer has teeth, as in the VR motor. Instead, the rotor includes permanent magnets with the alternating north and south poles disposed in a straight line, parallel to the rotor shaft. These magnetized rotor poles provide an increased magnetic flux intensity, resulting in improved torque characteristics when compared with VR stepper motors.
An HB stepper motor is more expensive than a PM stepper motor, but provides better performance with respect to step resolution, torque and speed. Typical step angles for the HB stepper motor range from 3.6° to 0.9° (100-400 steps per revolution). The HB stepper motor combines the best features of both the PM and VR type stepper motors; its rotor is multi-toothed, like the VR motor, and includes an axially magnetized concentric magnet around its shaft. The teeth on the rotor provide an even better flux path, which helps guide the magnetic flux to preferred locations in the air gap between the rotor and the stator teeth. This configuration further increases the detent, holding, and dynamic torque characteristics of the HB stepper motor, when compared with both the VR and PM stepper motors.
Stepper motors generally have two phases, but three, four and five-phase motors also exist.
FIG. 1
shows a typical two-phase motor, comprising a stator A and a stator B, each of which produce a magnetic flux with opposite poles at end faces
300
when a respective phase A winding
302
and phase B winding
304
are energized with an electric current. The direction of the magnetic flux is determinable by applying the “right-hand rule.” In
FIG. 1
, a current I
B
flows through the phase B windings, creating a magnetic flux in stator B, as indicated by the direction of the arrows. This flux produces a torque applied to the rotor, causing the rotor to turn so that the magnetic field produced by the poles in the rotor are aligned with the magnetic field produced by stators A and B. In this case, the rotor will rotate clockwise so that its south pole aligns with the north pole of stator B at a position
2
, and its north pole aligns with the south pole of stator B at a position
6
. To continually rotate the rotor, current is applied to the phase A and phase B windings in a predetermined sequence, producing a rotating magnetic flux field.
The output torque of the motor drive shaft is proportional to the intensity of the magnetic flux generated when the winding is energized. The basic relationship determining the intensity of the magnetic flux is defined by:
H
=(
N×i
)÷
l
(1)
where N is the number of winding turns, i is the current, H is the magnetic field intensity, and l is the magnetic flux path length. This relationship shows that the magnetic flux intensity, and consequently the torque, is proportional to the number of turns in the winding and the current, and is inversely proportional to the length of the magnetic flux path. In addition, stepper motors that include permanent magnets produce a built-in “detent” torque. This detent torque results from the magnetic flux generated by the permanent magnets, and is what produces the “cogging” effect that is felt when turning a PM or HB stepper motor that is not energized.
As shown in
FIGS. 2A and 3A
, a unipolar motor has one winding with a center tap per phase (two phase motors), or four windings with one winding per phase, typically sharing a common tap. (Some unipolar stepper motors are genuine four-phase motors, while other unipolar stepper motors are erroneously referred to as four-phase motors, even though they have only two phases.) Unipolar motors typically have either five or six leads. In comparison, as shown in
FIGS. 2B and 3B
, a bipolar motor generally comprises two phases, wherein each phase has a corresponding winding. Bipolar motors typically have four leads. Motors that have two separate windings per phase also exist and can be driven in either bipolar or unipolar mode.
A pole can be defined as a region on a magnetized body where the magnetic flux density is concentrated. Both the rotor and the stator of a stepper motor have poles.
FIGS. 1
,
2
A, and
2
B show simplified motors for illustrative purposes, while in reality, several more poles are normally included in both the rotor and stator structure in order to increase the number of steps per revolution of the motor (i.e., decrease the step angle). A PM stepper motor contains an equal number of rotor and stator pole pairs. Typically, the PM stepper motor has 12 pole pairs, and the stator has 12 pole pairs per phase. An HB stepper motor has a rotor with teeth that is split into two parts, separated by a permanent magnet, making half of the teeth south poles and half north poles. The number of pole pairs is equal to the number of teeth on one of the rotor halves. The stator of an HB motor also has teeth that increase the number of equivalent poles (i.e., smaller pole pitch, since the number of equivalent poles equals 360/teeth pitch) compared to the main poles, on which the winding coils are wound. Usually four main poles are used for 3.6° hybrid stepper motors and eight main poles are used for 1.8° and 0.9° stepper motors.
It is the relationship between the number of rotor poles and the equivalent stator poles, and the number of phases that determine the full-step angle of a stepper motor:
Step angle=360÷(
N
Ph
×Ph
)=360/
N
(2)
where N
Ph
is the number of equivalent poles per phase or the number of rotor poles, Ph is the number of phases, and N is the total number of poles for all phases.
There are four drive modes that are typically used to move and position stepper motors, including the wave drive (one phase on), full-step drive (two phases on), half-step drive (one and two phases on), and microstepping (continuously varying phase currents). The following discussion of these various drive modes references
FIGS. 2A and 2B
, and
FIGS. 3A and 3B
.
FIG. 3A
shows a typical six-wire unipolar drive circuit. In or
Bunyard Marc R.
Holst Peter A.
Maske Rudolph J.
Abbott Laboratories
Ro Bentsu
Vrioni Beth A.
Woodworth Brian R.
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