Increasingly elaborate Rube Goldberg schemes for controlling step-motors have developed over the years that try to pace stepping to avoid missteps. This direction of development is fundamentally misguided. This article tells why.
The industry-standard interface between step-motor drivers and controllers consists of essentially two lines: direction and step. The controller determines when the steps are taken by pulsing the step line. As a consequence, all the system dynamics depends on the controller.
A step-motor nowadays is a permanent-magnet synchronous (PMS) motor with some residual variable-reluctance torque production. It can basically be regarded as a PMS motor which is sometimes referred to as a “servo motor”. Permanent magnets are attached to the rotor.
The stator has two phase-windings, which is the minimum needed to rotate a field-referred current vector in a plane, the plane normal to the spin of the rotor. Three phase-windings reduce the amount of drive electronics over two phases, but two were chosen historically because electronic power drivers and control were implemented with electron tubes and had to be kept simple. By center-tapping the two windings, four leads result. Combinations of them can be switched to ground to allow winding current to flow when the center-tap is connected to the supply voltage, Vg . The four half-windings can be labeled A+, A–, B+, and B–.
Stator electromagnets rotate a magnetic field, represented by the field-referred current vector of the stator, that attracts the rotor magnets, like a donkey to a carrot held out in front. The stator keeps ahead of the rotor, to pull its magnets along rotationally. Maximum torque is produced when the stator field vector leads the rotor field vector by 90o in the direction of rotation. In the diagram below, the stator field is not quite advanced enough CCW in phase for maximum torque, as both vectors rotate in synchronism together CCW.
When Vg is connected across a half-winding of either the A or B windings, a current flows which produces a vector pointing in one of the four cardinal directions (0o , 90o , 180o , 270o ) which can be labeled by the winding names. The result is a rather crude sequencing of the stator current vector phase with four “full steps” per electrical cycle. In more refined drives, the stepping has more phase resolution and the jerkiness of discrete steps is reduced. The mainstream development for step-motors has attempted such a refinement, called microstepping , by increasing the number of intermediate steps.
The basic idea of torque generation is to produce an attracting current vector that is rotated just enough ahead in phase of the rotor magnet to pull it along with maximum torque. The phase is the electrical phase and this is related to the mechanics by the number of pole-pairs. For each mechanical cycle (revolution) of the rotor there are a pole-pair number of electrical cycles. Each electrical cycle advances the rotor by a pole-pair.
Maximum torque is produced when the stator current (or flux) vector is maintained at 90 degrees electrical ahead of the rotor magnet flux vector. It is basically that simple and the result is called field-oriented control because the stator field is kept oriented 90o ahead of the rotor field. When this is maintained, none of the adverse dynamic behaviors of step-motors occur and the motor behaves essentially like a brush motor. With field-orientation, the motor model is shown below, where Vg is the peak vs .
In a brush motor, the phase of the stator current vector is controlled electromechanically by the switch sequencing (or commutation ) of the windings with commutator bars on the rotor. These bars rub against stationary (relative to the stator) graphite brushes to complete the moving electrical connection to the spinning rotor winding. PMS motors eliminate the brushes by eliminating the rotor winding and replacing it with magnets.
Open-Loop Versus Phase Control
Field-oriented control is simply phase control of the stator current vector. The magnitude of the torque generated is proportional to winding current and a current controller circuit thus controls the current magnitude as a different subsystem of the motor drive. In early step-motor drives, neither magnitude nor phase was controlled. In mainstream step-motor drives, the phase is open-loop controlled by schemes that try to model what the motion system will do when driven in various ways. In other words, an elaborate compensator is inserted in the forward path of the control system and there is no phase feedback from the motor. As a result, it is logical to then have a command interface that consists of direction and step pulses. As pulses are received, they are “buffered” by the motion controller and the motor drive is fed these step pulses at a rate and with timing that will minimize spurious mechanical motor behavior.
This overly-complicated and only partially successful direction of development can be replaced by using the motor phase as feedback and letting the advancement or stepping of the stator-current drive vector be controlled to keep it 90o ahead of the actual motor electrical phase. Then nothing bad happens. It is just that simple, yet step-motor drive development has overlooked basic (and generally known!) facts about motors and for many years has failed to apply basic motor theory to drive design.
It is no longer optimal to maintain a step-direction interface from motion controller to drive. The command interface instead becomes direction and step enable , where step enable is no longer a sequence of pulses but a logic level that enables stepping. In addition, the actual motor steps and their direction are fed back to the motion controller so that it can count them to determine position and control the step-enable line. In this scheme, the motor steps at its own pace, driven by the system dynamics and constrained in time to keep field orientation. The motion dynamics for the motor itself have been moved from motion controller to motor drive.
A better, though less backward-compatible, interface between motion controller and motor drive has torque magnitude and direction inputs to the drive. This allows for indirect speed control and the motor drive achieves phase control of the motor without involving the motion controller at all. This is how step-motor systems should have been designed from the mid-1960s and thereafter, but were not. The requisite motor theory to implement field-oriented control existed ever since it was refined and simplified by Paul Krause in 1965. (His book, from Purdue U., on Analysis of Electric Machinery is the standard on explaining motor theory, though a subsequent book co-authored by Oleg Wasynczuk titled Electromechanical Motion Devices , McGraw-Hill, 1989, is a better place to start for an introductory treatment.)
It was not until years later, when solid-state power electronics developed, that field-oriented control was applied to large three-phase induction and synchronous motors but not to step motors. The reason is probably that step-motor technology continued to be viewed as minimalist-electronics technology and was treated as such far after it would have been simpler to apply motor theory to step-motors. And to this day, this lesson has not been completely learned.
It is not that field-oriented control lacks design challenges. Sensing the motor phase is one, and has two categories of schemes. One is to sense electrical phase by sensing mechanical phase with a shaft encoder on the rotor shaft. Step-motors have a large number of pole-pairs – usually 50 – and hence electrical phase resolution from an electromechanical encoder, while possible, results in higher cost and the limitations in reliability of an electromechanical component. The encoder wheel must be adjusted just-so for electrical alignment. More practically, after the encoder is attached to the shaft, calibration using a dynamometer acquires phase offset compensation for the encoder.
The more elegant category of schemes uses the induced voltage across the windings of the motor to sense phase, for the induced-voltage waveforms are the true determinant of motor electrical phase and can be used directly. One problem in sensing them is that the motor windings are also being driven at the same time, but this can be overcome – and is left for the next article in this series.