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Extending Stepper Motor Algorithm Immunity in Open-Loop Applications

IP.com Disclosure Number: IPCOM000044348D
Original Publication Date: 1984-Dec-01
Included in the Prior Art Database: 2005-Feb-05
Document File: 2 page(s) / 14K

Publishing Venue

IBM

Related People

Hallman, BL: AUTHOR

Abstract

The performance of a stepper motor algorithm is affected by variations in the excitation voltage and winding current. The method described yields an algorithm whose performance is very insensitive to these parametric variations, and whose form is very simple. The key to this method is to filter the abruptness of the motor steps so that the load experiences a smoothed excitation, to synchronize winding energizations (commutations) with occurrences of zero rotor velocity, and to make use of windage losses to dissipate small amounts of rotor energy. To implement this method, first determine the rise and fall times of the winding current. Next, verify that the system is compliantly coupled.

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Extending Stepper Motor Algorithm Immunity in Open-Loop Applications

The performance of a stepper motor algorithm is affected by variations in the excitation voltage and winding current. The method described yields an algorithm whose performance is very insensitive to these parametric variations, and whose form is very simple. The key to this method is to filter the abruptness of the motor steps so that the load experiences a smoothed excitation, to synchronize winding energizations (commutations) with occurrences of zero rotor velocity, and to make use of windage losses to dissipate small amounts of rotor energy. To implement this method, first determine the rise and fall times of the winding current. Next, verify that the system is compliantly coupled. This can be done by executing a single-step move and comparing the periods of the rotor oscillations of an unloaded motor to that of a loaded motor. Systems in which the load is compliantly coupled to the motor will have periods which are shorter than that of the unloaded motor. The opposite holds for rigidly coupled loads. The elastic coupling is used to store energy, and as such, is the mechanism which smooths out the motor's motion and delivers to the load a more uniform excitation force. The current rise and fall times and the single step period of the loaded motor are used to determine the commutation timing. By way of an example, assume that this method is to be used on a 3-phase motor, which is to be driven in a half-stepping mode. Two phases of the motor are initially energized, and the motor is allowed sufficient time to stabilize. The first half-step move is executed by turning off one of the phases. The circuit driver must remove all of the phase current by the time the rotor reaches the commanded position. The rotor will continue to move past the commanded position, slowing down as it does. Time the current turn-on in the next phase of the half-stepping sequence so that the phase is fully energized by the time the rotor arrives at the second commanded position. Repeat this two half-step sequence to form any desired move-length. The algorithm is insensitive to variations in voltage and current because the commutations occur only when the rotor velocity is at or near zero. If the energy of the rotor changes due to a variation in one of these parameters, the rotor will achieve zero velocity at slightly different times. The effects of these small time variations are much easier to correct when the rotor velocity is near zero than when t...