Stepper Motors: Types, Uses, and Working Principles - Black keyhole

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Stepper Motors: Types, Uses, and Working Principles

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Stepper motors, or steppers, are digitally controlled brushless motors that rotate a specific number of degrees (a step) every time a clock pulse is applied to a special translator circuit that is used to control the stepper. The number of degrees per step(resolution) for a given stepper motor can be as small as 0.72° per step or as large as 90° per step. Common general-purpose stepper resolutions are 15 and 30° per step.

Unlike RC servos, steppers can rotate a full 360° and can be made to rotate in a continuous manner like a dc motor (but with a lower maximum speed) with the help of proper digital control circuitry. Unlike dc motors, steppers provide a large amount of torque at low speeds, making them suitable in applications where low-speed and high-precision position control is needed. For example, they are used in printers to control paper feed and are used to help a telescope track stars. Steppers are also found in plotter-and sensor-positioning applications.


Here is a simple model depicting a 15ç per step variable-reluctance
stepper.The stationary section of the motor, called the stator, has eight poles
that are spaced 45ç apart. The moving section of the motor, called the
rotor, is made from a ferromagnetic material (a material that is attracted
to magnetic fields) that has six teeth spaced 60ç apart. To make the rotor
turn one step, current is applied, at the same time, through two opposing
pole pairs, or coil pairs. The applied current causes the opposing pair
of poles to become magnetized. This in turn causes the rotor’s teeth to
align with the poles, as shown in the figure. To make the rotor rotate 15°
clockwise from this position, the current through coil pair 1 is removed
and sent through coil pair 2. To make the rotor rotate another 15° clockwise
from this position, the current is removed from coil pair 2 and sent
through coil pair 3. The process continues in this way. To make the rotorspin counterclockwise, the coil-pair firing sequence is reversed.


Kinds of Stepper Motors 


   The model used in the last example was based on a variable-reluctance
stepper. As it turns out, this model is incomplete—it does not show how a real variable-reluctance stepper is wired internally. Also, the model does not apply to a class of steppers
referred to as permanent-magnet steppers. To make things more realistic, let’s take a
look at some real-life steppers.


 


 




There are three types of stepper motor. They are,

1.variable reluctance motor.

2.Hybrid stepper motor.

3.permanent magnet stepper motor.

In these three types mostly hybrid stepper motor is used. The stepper motor used in robotic arm at industry application



Variable-Reluctance Steppers


 
picture shows a physical model and schematic diagram of a 30° per step variable-reluctance stepper. This stepper consists of a six-pole (or three-coil pair) stator and a four-toothed ferromagnetic rotor. Variable-reluctance steppers with higher angular resolutions are constructed with more coil pairs and/or more rotor teeth. Notice that in both the physical model and the schematic, the ends of all the coil pairs are joined together at a common point. (This joining of the coil ends occurs internally within the motor’s case.) The common and the coil pair free ends are brought out as wires from the motor’s case. These wires are referred to as the phase wires. The common wire is connected to the supply voltage, whereas the phase wires are grounded in sequence


Permanent-Magnet Steppers (Unipolar, Bipolar, Universal)

UNIPOLAR STEPPERS

These steppers have a similar stator arrangement as the variable-reluctance steppers,
but they use a permanent-magnet rotor and different internal wiring arrangements.
above picture shows a 30° per step unipolar stepper. It consists of a four-pole (or two-
coil pair) stator with center taps between coil pairs and a six-toothed permanent-magnetic rotor. The center taps may be wired internally and brought out as one wire or may be brought out separately as two wires. The center taps typically are wired to the positive supply voltage, whereas the two free ends of a coil pair are alternately grounded to reverse the direction of the field provided by that winding. As shown in the figure, when current flows from the center tap of winding 1 out terminal 1a,the top stator pole “goes north,” while the bottom stator pole “goes south.” This causes the rotor to snap into position. If the current through winding 1 is removed,sent through winding 2, and out terminal 2a, the horizontal poles will become energized, causing the rotor to turn 30°, or one step.
 In above picture, three firing sequences are shown. The first sequence provides full stepping action (what we just discussed).
The second sequence, referred to as the power stepping sequence, provides full stepping action with 1.4 times the torque but twice the power consumption. The third sequence provides half stepping (e.g., 15° instead of the rated 30°). Half stepping is made possible by energizing adjacent poles at the same time. This pulls the rotor in-between the poles, thus resulting in one-half the stepping angle. As a final note, unipolar steppers with higher angular resolutions are constructed with more rotor teeth. Also, unipolars come in either five-or six-wire
types. The five-wire type has the center taps joined internally, while the six-wire
type does not.


BIPOLAR STEPPERS

These steppers resemble unipolar steppers, but their coil pairs do not have center taps. This means that instead of simply supplying a fixed supply voltage to a lead, as was the case in unipolar steppers (supply voltage was fixed to center taps), the supply voltage must be alternately applied to different coil ends. At the same time, the opposite end of a coil pair must be set to the opposite polarity (ground). For example, in above picture, a 30° per step bipolar stepper is made to rotate by applying the polarities shown in the firing sequence table to the leads of the stepper. Notice that the firing sequence uses the same basic drive pattern as the unipolar stepper, but the “0” and “1” signals are replaced with “+” and “-” symbols to show that the polarity matters. As
you will see in the next section, the circuitry used to drive a bipolar stepper requires an H-bridge network for every coil pair. Bipolar steppers are more difficult to control than both unipolar steppers and variable-reluctance steppers, but their unique polarity-shifting feature gives them a better size-to-torque ratio. As a final note,bipolar steppers with higher angular resolutions are constructed with more rotor teeth.


UNIVERSAL STEPPERS
 
These steppers represent a type of unipolar-bipolar
hybrid. A universal stepper comes with four independent windings and eight leads. By connecting the coil windings in parallel, as shown in above picture, the universal stepper can be converted into a unipolar stepper. If the coil windings are connected in series, the stepper can be converted into a bipolar stepper.









                  
 

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