There are serveral types of stepper motor currently availbale, with permanent magnet or soft iron rotors, and there are hybrids. Permanent magnet stepper motors have found the widest application because they have good dynmaic and static characteristics and a relatively high efficiency. Also, they have a static characteristics and a relatively high effciency. Also, they have a static holding torque when not energised, which the soft-iron rotor motor does not have. A further advantage is that they have good damping. Therefore the following discussion is limited to stepper motors with permanent magents and to hybrid motors.
The characteristic property of the stepper motor is the step-by-step turning of the motor shaft. One complete turn of the shaft is made up from an exactly specified number of steps, which is determined by the motor design. This property meets the requirement for operation directly digital signals. The stepper motor can thus be the bridge between digital information and incremental mechanical displacement. If a stepper motor drive is to be secure and interference-free, certain fundamental points must be taken into account.
Drive systems with stepper motors bring together the following properties:
1 precise step-by-step positioning – without feedback – following a predetermined number of control pulses
2 high torque at low speeds, and with single steps
3 in a powered standstill condition a large holding torque with self-locking.
Problems occur in stepper motors drives during starting, accelerating, decelerating and stopping, and these are associated with the dynamic structure of the drive.
In stepper motor drive systems attention must therefore always be paid to the characteristics of the stepper motor itself, the mechanical system to be driven and the necessary electronic control ciruits, because all three together determine the dynamic structure of the system.
Poisition setting drives can be effected without feedback,i.e.with open loop control. Then there are none of the problems of closed loop control, particularly that of instability. But stepper motors are also found in closed loops and, in certain applications, produce very good results.
A 2 phase stepper motor can cause oscillation problems when operated at and around its natural resonant frequency, in which case damping must be incorporated somehow. Further, there are transients during stopping which result from the dynamic properties of the motor and its load and from the control circuits. If at the design stage of a system with stepper motors certain fundamental points are taken into account, the dynamic problem can be largely overcome and a neat dynamically excellent drive system can be built up that fully justifies the cost of the electronic controls.