Use of digital current ramping to reduce audible noise in...

Details Use of digital current ramping to reduce audible noise in... Use of digital current ramping to reduce audible noise in...

1999-12-03

2001-03-27

Nappi, Robert E. (Department: 2837)

Electricity: motive power systems

Positional servo systems

With particular motor control system responsive to the...

C318S696000, C417S045000

Reexamination Certificate

active

06208107

ABSTRACT:

FIELD OF THE INVENTION
This invention generally relates to a power source for a stepper motor, and more specifically, to a stepper motor having a power source that is digitally controlled to limit a slope of the power signal applied to the stepper motor.
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 variable 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.
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 directions 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.
Stepper motors are typically positioned by a sequence of command pulses that are received by a drive circuit portion of a stepper motor driver, which produces outputs signals to drive the stator windings (i.e., “coils” in the motor). This sequence of command pulses corresponds to one of the 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 are made with reference to
FIGS. 2A-2B
and
3
A-
3
B.
FIG. 3A
shows a typical six-wire unipolar drive circuit. In order to drive a unipolar stepper motor, it is necessary to energize the windings of the motor in a predetermined sequence. This procedure can be accomplished through the use of four switches
50
,
52
,
54
, and
56
(e.g., Darlington pair switches or field-effect transistors), each of which is connected to ground at one terminal, and connected to a respective winding at the other terminal. A positive supply voltage is provided at common or center taps
58
and
60
. Current can be caused to flow through windings corresponding to motor phases A, {overscore (A)}, B, and {overscore (B)} by respectively closing switches
50
,
52
,
54
, and
56
, each of which provides a path to ground through their corresponding winding. When current flows through the windings, a magnetic field is generated in accord with the right-hand rule, as discussed above, which causes the motor rotor to rotate so that it is aligned with the magnetic fields generated by stators A and B.
A somewhat more complex scheme is used for driving a bipolar motor. As shown in
FIG. 3B
, a typical bipolar drive circuit comprises a pair of H-bridge circuits, one for each winding. Each of the H-bridge circuits comprises four switches
62
,
64
,
66
, and
68
. The branches at the top of the bridges are connected to a positive supply voltage, while the branches at the bottom of the bridges are connected to ground. By selectively closing the H-bridge switches, current can be caused to flow through windings
70
and
72
in a desired direction, thereby producing motor phases A, {overscore (A)}, B, and {overscore (B)}. For example, to produce a current flow in winding
70
from right to left (i.e., motor phase A), switches
64
and
66
are closed, while switches
62
and
68
are kept open.
In a wave drive for a stepper motor, only one winding is energized at any given time. The windings on the stators are energized according to the sequence A→B→{overscore (A)}→{overscore (B)}, causing the rotor to step through positions
8
→
2
→
4
→
6
. For unipolar and bipolar wound motors with the same winding parameters, this excitation mode will result in the same mechanical position of the rotor. The disadvantage of this drive mode is that in a unipolar wound motor, only 25% of the total motor winding is energized at any given time, and in a bipolar motor, only 50% of the total motor winding is used. Thus, the maximum potential torque output of the motor is not realized.
In a full-step drive for a stepper motor, two phases are energized at any given time. The windings on the stators are energized according to the sequence AB→{overscore (A)}B→{overscore (A)} {overscore (B)}→A{overscore (B)}, causing the rotor to step through positions
1
→
3
→
5
→
7
. When using the full-step mode, the angular movement will be the same as was discussed above for a wave drive, but the mechanical position is offset by one-half step. The torque output of a unipolar wound motor when using full-stepping is lower than for a bipolar motor (i.e., for motors with the same winding parameters), since the unipolar motor uses only 50% of the available winding, while the bipolar motor uses the entire winding.
The half-step drive mode combines both wave and full-step (one and two phases on) drive modes. As shown in TABLE 1 (below), the number of phases that are energized alternates between one and two phases during every other step. The windings on the stators are energized according to the sequence AB→B→{overscore (A)}B→{overscore (A)}→{overscore (A)} {overscore (B)}→{overscore (B)}→A{overscore (B)}→A, causing the rotor to step through positions
1
→
2
→
3
→
4
→
5
→
6
→
7
→
8
. This procedure results in angular movements that are half of those discussed above for wave and full-step drive modes. Half-stepping can reduce a phenomena referred to as resonance, which sometimes occurs when using the wave or full-step drive modes at certain step rates.
TABLE 1
Normal Full-Step
Wave Drive
Drive
Half-step Drive
Phase
1
2
3
4
1
2
3
4
1
2
3
4
5
6
7
8
A
&Circlesolid;
&Circlesolid;
&Circlesolid;
&Circlesolid;
&Circlesolid;
&Circlesolid;
B
&Circlesolid;
&Circlesolid;
&Circlesolid;
&Circlesolid;
&Circlesolid;
&Circlesolid;
{overscore (A)}
&Circlesolid;
&Circlesolid;
&Circlesolid;
&Circlesolid;
&Circlesolid;
&Circlesolid;
{overscore (B)}
&Circlesolid;
&Circlesolid;
&Circlesolid;
&Circlesolid;
&Circlesolid;
&Circlesolid;
Resonance can be observed as a sudden loss or drop in torque at certain speeds, which can result in missed steps or loss of synchronism, and creates undesired no

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