Electricity: motive power systems – Positional servo systems – With particular motor control system responsive to the...
Patent
1995-03-29
1997-06-17
Martin, David S.
Electricity: motive power systems
Positional servo systems
With particular motor control system responsive to the...
318696, G05B 1940
Patent
active
056400756
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for controlling a stepping motor whose field windings are connected to a switching device which sets the magnitude and direction of the phase currents in the field windings as a function of the vector angle of a required vector which is switched onwards by one step angle per interval of a system clock, which step angle is dependent on stored values which are read out in the direction of increasing values for acceleration and in the reverse direction for braking.
2. Description of the Related Art
Such a method is known from U.S. Pat. No. 4,280,084. There, the current step angle is formed as the sum of a constant coarse step and a fine step whose value rises or falls from step to step and is read from a memory. In this way, the lag angle (angular difference between the field vector and the rotor position) and thus the torque are held at their initial maximum value despite the changing rotor speed.
The invention is based on the knowledge that, during acceleration/deceleration with the maximum moment at the start and end of a displacement, the shortest possible displacement time is never achieved and that the suddenly acting high moment excites the stepping motor to oscillate, which frequently leads to step losses. Finally, this also results in an unfavorable dimensioning of the stepping motor since its maximum moment is utilized only briefly.
SUMMERY OF THE INVENTION
In contrast, the invention has the an object of shortening the time for a rotor displacement process and of reducing the tendency of the stepping motor to oscillate, with the stepping motor being more favorably dimensioned.
The invention provides that the step angles themselves are stored as an angle table and their magnitudes increase from an initial position having an initial value of zero via microsteps to a final position having a final value in the order of magnitude of a coarse step (for example a quarter step to a full step). As a consequence, smooth and continuous acceleration is possible with a low torque which maintains a very wide safety margin from the stalling torque, so that the risk of stepping losses is virtually precluded.
The possibility for an automatic and smooth transition between microstepping operation for the purpose of high resolution positioning and full stepping operation at a high displacement speed is particularly advantageous; according to the invention, no changeover between coarse stepping operation and microstepping operation is required for this purpose.
The step angles in the angle table preferably extend over at least one control cycle of 360.degree. of the vector angle, which is governed by the phase currents. In the case of a two-phase stepping motor having 50 pole pairs, this corresponds to four full steps with a rotor angle of 1.8.degree. each; one revolution of the electrical vector angle thus corresponds to a rotor angle of 7.2.degree.. Larger adjusting movements are in this case carried our using the full stepping operation, the number of vector revolutions being stored in an overflow counter.
However, it is particularly favorable to dimension the step angles in the angle table for a given stepping motor that includes an actuating device such that the acceleration phase and the braking phase which directly follows the acceleration phase extend from the point of maximum speed as far as both limits of the maximum adjustment region: a positive or negative acceleration with a preferably constant torque thus takes place throughout the entire displacement. As a consequence, the shortest possible displacement time can be achieved, with a very low tendency to oscillate and with favorable dimensioning of the stepping motor.
If the stepping motor has to operate against a spring prestressing force in one direction, this means that the spring assists the braking process in this direction, and assists acceleration in the opposite direction. For this purpose, an optimum design requires different moments for acceleration and bra
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Brasseur Georg
Noichl Andreas
Trefalt Manfred
Turba Roman
Martin David S.
Siemens Sktiengesellschaft
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