Motors and Drives
4 Stepper motors are very often the best solution for positioning applications, being low cost and straightforward. But they are not a universal panacea as Gerard Bush explains.
4 Bien souvent, les moteurs pas-à-pas sont souvent la meilleure solution pour positionner des applications, en raison de leur faible coût et de leur simplicité. Cependant, ils ne constituent pas une panacée universelle, comme Gerard Bush nous l’explique.
4 Schrittmotoren sind sehr oft die beste Lösung für Positionieranwendungen, da sie kostengünstig und unkompliziert sind. Aber sie sind kein universelles Patentrezept, wie Gerard Bush erklärt.
The fundamental principles of stepper motors
S
tepper motors are self- positioning, and therefore simple to use. They do not require an encoder to maintain their position nor do they need a
servo control loop. Their main drawbacks are vibration and noise, plus a limited speed range. Also they require external ‘commutation’ using a multi-phase drive or amplifier. Stepper motors are multi-phase devices,
meaning they have two or more motor coils which must be electronically driven to create motion. (Most have two coils -also called phases - although 3-phase and 5-phase designs also exist.) The amplifier converts unphased command
information such as pulse and direction signals into a set of correctly sequenced voltage commands for each coil of the motor. There are several possible phasing techniques used by the amplifiers, including full step, half step and microstep control.
Power levels
These different techniques refer to the number of power levels that are applied to each motor coil during an electrical cycle. A full step drive uses an ‘all-positive’ or
‘all-negative’ technique; a half step drive can separate the current into three distinct levels (all positive, zero, all negative). Both of these produce a somewhat crude output (of waveform) from the motor. A microstep drive, in contrast, can generate
a more or less sinusoidal signal. The more the waveform resembles a sinusoid, the smoother and more precise the degree of control. In all three drive methods, the motor moves
forward or backward by altering the electrical phasing. A ‘full’ step means one 90 electrical degree
movement. Stepper motors usually come constructed with 1.8 mechanical degrees per full electrical step (90 electrical degrees). So this means a 1.8 degree stepper has
200 full steps per mechanical motor rotation. If a microstepping scheme is used, you can
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calculate your positioning resolution (not accuracy - see next paragraph) by multiplying the microsteps per full steps by the degrees per full step rating. Thus, if you use a microstep drive with
64 microsteps per full step and you use a 1.8 degree step motor with 200 full steps per motor rotation, you have 64 x 200 = 12,800 different commandable positions per motor rotation. This figure is the ‘resolution’ with which
you can control the motor position. Note that with stepper motors, accuracy and resolution are not exactly the same. This is because a stepper motor’s mechanical response to the amplifier’s output signals is not perfectly linear, nor is the magnetic holding torque perfectly stiff.
These differences are generally small, but
may be important in some applications. Motor principles Like most motors, a stepper can be likened to a spinning bar magnet that interacts with a magnetic field. The rotor contains permanent magnets
which interact with the stator’s magnetic field (the stator is the outer portion of the motor that does not move) when its coils are energized (magnetised) with current. When the stator coils are excited, a roughly
sinusoidal force ‘valley’ is created which drives the step motor to settle at a specific position. The force valley is simply the torque on the rotor as it moves through a 360 degree electrical cycle. Whenever the curve is horizontal there
is no torque, and wherever the curve is at its steepest the torque is the largest. To create motion, the electronic controller moves this valley forward or backward (depending on the commanded direction of motion) by adjusting the coil phasing. The motor then ‘falls’ forward or backward
in response. Think of a ball settling to the bottom of a trough or a surfer riding a wave. With these analogies in mind, several aspects of stepper motor operation become clear. Thinking of the torque valley as a wave
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