Stepper Motor
Introduction :
A stepper’s purpose is to rotate through a precise angle and halt. The speed and torque of the rotation are secondary concerns. As long as the stepper rotates through the exact angle and stops, its mission is accomplished. Each turn is called a step.
Due to their simplicity and precision, steppers are popular in electrical devices. Analog clocks, manufacturing robots, and printers rely on steppers for motion control. An important advantage is that the controller doesn’t have to read the stepper’s position to determine its orientation. If the stepper is rated for 2.5°, each control signal will turn the rotor through an angle of 2.5°.
For many applications, we want the step angle to be as small as possible. The smaller the motor’s step angle, the greater its angular resolution . Another important factor for Stepper Motor is torque, specially holding torque . A stepper is expected to hold its position when it comes to a halt, and holding torque identifies the maximum torque it can exert to maintain its position.
There mainly two types of Stepper Motors
(1) Permanent Motor (PM)
(2) Variable Reactance type Motor (VR)
(1) Permanent Motor :
Introduction :
To understand how a PM stepper operates, it’s crucial to see how its step angle is determined by the number of windings and rotor magnets. This discussion focuses on the motor depicted in Figure 1.1. Its stator has 12 windings and its rotor has six magnets mounted on its perimeter.
Construction :
PM steppers are generally two-phase motors. In the figure, the different phases are denoted A and B. The windings labeled A’ and B’ receive the same current as those labeled A and B, but in the opposite direction. That is, if A behaves as a north pole, A’ behaves as a south pole.
Each winding has one of three states: positive current, negative current, and zero current. For this discussion positive current implies a north pole and negative current implies a south pole.
Operation :
Now let’s see how these motors operate. Figure 1.1 illustrates a single turn of a PM stepper. In the windings, a small “N” implies that the winding behaves like a north pole due to positive current. A small “S” implies that the winding behaves like a south pole due to negative current. If a winding doesn’t have an N or S, it isn’t receiving current.
In Figure 1.1 (a) , A is positive (north pole), A’ is negative (south pole), and Phase B isn’t energized.The rotor aligns itself so that its south poles are attracted to the A windings and its north poles are attracted to the A’ windings.
In Figure 1.1 (b) , B is positive (north pole), B’ is negative (south pole), and Phase A isn’t energized.The rotor rotates so that its poles align with the B and B’ windings. The rotation angle equals the angle between the A and B windings, which means the rotor turns exactly 30° in the clockwise direction. This arrangement of eight windings and six poles is common for PM stepper motors, though others turn at angles of 15° and 7.5°. In case this isn’t clear, let’s look at a second movement. Figure 1.2 presents another 30° rotation of a PM stepper motor.
In Figure 1.2 (a), B is negative (south pole), B’ is positive (north pole), and A isn’t energized. The rotor is positioned so that its poles align with the B windings.
In Figure 1.2 (b) , A is positive (north pole), A’ is negative (south pole), and B isn’t energized. The rotor turns exactly 30° in the clockwise direction to align itself between the A windings.
The controller’s job is to deliver current to the windings so the rotor continues rotates in 30° increments.
(2) Variable Reluctance type Stepper Motor :
Introduction :
Just as resistance determines the flow of electric current, reluctance determines the flow of magnetic flux. In a variable reluctance (VR) stepper, the rotor turns at a specific angle to minimize the reluctance between opposite windings in the stator.
The primary advantage of VR steppers is that they have excellent angular resolution. The primary disadvantage is low torque.
This section presents VR steppers in detail. I’ll explain their internal structure first and then show how they rotate as their windings are energized.
Construction :
Structurally speaking, variable reluctance (VR) steppers have a lot in common with PM steppers. Both have windings on their stator and opposite windings are connected to the same current source. However, there are two primary differences between VR steppers and PM steppers:
(1) Rotor : Unlike a PM stepper, the rotor in a VR stepper doesn’t have magnets. Instead, the rotor is an iron disk with small protrusions called teeth .
(2) Phases : In a PM stepper, the controller energizes windings in two phases. For a VR stepper, the controller energizes every pair of opposite windings independently. In other words, if the stator has N windings, it receives N/2 signals from the controller.
Figure 1.3 illustrates the rotor and stator of a VR stepper. In this motor, the stator has eight windings and the rotor has six teeth.
The rotor doesn’t have magnets, but because it’s made of iron, its teeth are attracted to energized windings. In the figure, the A and A’ windings are labeled N and S, which shows how they’re energized by the controller. The teeth in the rotor align with these windings to provide a path for magnetic flux between A and A’.
Operation :
As illustrated in Figure 1.3 , only one pair of teeth is aligned with the windings at any time. When the controller energizes a second pair of windings, the rotor turns so that a different pair of teeth will be aligned. Because the teeth aren’t magnetized, it doesn’t matter whether a winding behaves as a north pole or as a south pole.This can be confusing, so Figure 4.6illustrates the rotation of a VR stepper. In this example, thestepper rotates 15° in a counterclockwise orientation.
In Figure 1.4 (a) , the controller has delivered current to the B and B’ windings, and the rotor has aligned itself accordingly. In Figure 1.4 (b) , the C and C’ windings are energized. The C and C’ windings attract the nearest pair of teeth, which moves the rotor 15° in the clockwise direction.
If you know the number of windings in the stator (N w) and the number of teeth on the rotor (N t), the step angle of a VR stepper can be computed with the following equation :
In Figure 1.3 , N w equals 8 and N t equals 6. Therefore, the step angle can be computed as 360(2/48) =15°.
The angular resolution can be improved by increasing the number of windings and teeth. With the right structure, the step angle can be made much less than that of a PM stepper.
However, there’s a problem. The torque of a VR stepper is so low that it can’t turn a significant load. For this reason, VR steppers are not commonly found in practical systems. In fact, I’ve only ever seen a handful of VR motors for sale.
Advantages :
Simple in construction
High holding torque
Pricise angle control
Good for robotics application
Open loop control system
Excellent starting , stoping and control response
Less costly most of variant
Less maintanence is required
Disadvantages :
Takes more current compared to DC motor
Less efficient
At high speed , control is not possible
At high speed value of torque reduce
Post a Comment