Application Of A Stepper Motor

ABSTRACT

This work is on a stepper motor is defined as an electric motor whose main feature is that its shaft rotates by performing steps, that is, by moving by a fixed amount of degrees. This feature also makes it fit for a wide range of applications.

In this seminar, we will cover the basics of stepper motors. You will learn about the working principles, construction, control methods, uses, and types of stepper motors, as well as its advantages and disadvantages and finally the general application of stepper motor.

 TABLE OF CONTENTS

COVER PAGE

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

CHAPTER ONE

1.0      INTRODUCTION

• BACKGROUND OF THE PROJECT
• PROBLEM STATEMENT
• OBJECTIVE OF THE PROJECT
• SCOPE OF THE PROJECT
• ADVANTAGES OF STEPPER MOTORS
• DISADVANTAGES OF STEPPER MOTORS
• TYPES OF STEPPER MOTORS
• MODES OF OPERATION IN STEPPER MOTOR
• CALCULATING THE STEPS PER REVOLUTION FOR STEPPER MOTOR
• WORKING PRINCIPLE
• OPERATION OF STEPPER MOTOR
• STEPPER MOTOR CONSTRUCTION
• APPLICATIONS OF STEPPER MOTOR
• CONCLUSION

REFERENCES

 CHAPTER ONE

1.0                                                        INTRODUCTION

1.1                                         BACKGROUND OF THE PROJECT

A stepper motor, also known as step motor or stepping motor, is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor’s position can then be commanded to move and hold at one of these steps without any position sensor for feedback, as long as the motor is carefully sized to the application in respect to torque and speed.

Switched reluctance motors are very large stepping motors with a reduced pole count, and generally are closed-loop commutated.

Brushed DC motors rotate continuously when DC voltage is applied to their terminals. The stepper motor is known by its property of converting a train of input pulses (typically square wave pulses) into a precisely defined increment in the shaft position. Each pulse moves the shaft through a fixed angle.

Stepper motors effectively have multiple “toothed” electromagnets arranged around a central gear-shaped piece of iron. The electromagnets are energized by an external driver circuit or a micro controller. To make the motor shaft turn, first, one electromagnet is given power, which magnetically attracts the gear’s teeth. When the gear’s teeth are aligned to the first electromagnet, they are slightly offset from the next electromagnet. This means that when the next electromagnet is turned on and the first is turned off, the gear rotates slightly to align with the next one.

1.2                                                  PROBLEM STATEMENT

Stepper motors was invented to overcome the low life time found in other types of motors. Stepper motor is considered to have high life time than normal DC or servo motor

1.3                                  AIM AND OBJECTIVES OF THE PROJECT

The main aim of this work is to discuss the application of a stepper motor. At the end of this study student involved shall be able to achieve the following objectives:

1. Describe the operation of a stepper motor
2. Describe various application of the motor

• Describe various types of motor.

1.4                                                 SCOPE OF THE PROJECT

In this work a stepper motor which is a brushless, synchronous electric motor that converts digital pulses into mechanical shaft rotation shall be discussed. Its normal shaft motion consists of discrete angular movements of essentially uniform magnitude when driven from sequentially switched DC power supply.

The stepper motor is a digital input-output device. It is particularly well suited to the type of application where control signals appear as digital pulses rather than analog voltages. One digital pulse to a stepper motor drive or translator causes the motor to increment one precise angle of motion. As the digital pulses increase in frequency, the step movement changes into continuous rotation.  Some industrial and scientific applications of stepper motors shall be discussed.

1.5                                     ADVANTAGES OF STEPPER MOTORS

One major advantage of stepper motor is that it has excellent position control and hence can be used for precise control application. Also it has very good holding torque which makes it an ideal choice for robotic applications. Stepper motors are also considered to have high life time than normal DC or servo motor.

1.6                                  DISADVANTAGES OF STEPPER MOTORS

Stepper Motors rotates by taking small steps it cannot achieve high speeds. Also it consumes power for holding torque even when it is ideal thus increasing the power consumption.

1.7                                          TYPES OF STEPPER MOTORS

There are mainly three types of stepper motors based on construction, which are:

• Variable reluctance stepper motor:  They have iron core rotor which is attracted towards the stator poles and provide movement by minimum reluctance between stator and rotor.
• Permanent magnet stepper motor:  They have permanent magnet rotor and they are repelled or attracted towards the stator according to pulses applied.
• Hybrid synchronous stepper motor:  They are combination of Variable reluctance and permanent magnet stepper motor.

Apart from this, stepper motors can be classified as Unipolar and Bipolar based on the type of stator winding.

• Bipolar Stepper Motor: The stator coils on this type of motor will not have a common wire. The driving of this type of stepper motor is different and complex and also the driving circuit cannot be easily designed without a microcontroller.
• Unipolar Stepper Motor: In this type of stepper motor we can take the center tapping of both the phase windings for a common ground or for a common power as shown below. This makes it easy to drive the motors, there are many types in Unipolar stepper motor as well

1.7                           MODES OF OPERATION IN STEPPER MOTOR

Since the stator of the stepper mode is built of different pairs of coils, each coil pair can be excited in many different methods, this enabling the modes to be driven in many different modes. The following are the broad classifications

Full Step Mode

In full step excitation mode we can achieve a full 360° rotation with minimum number of turns (steps). But this leads to less inertia and also the rotation will not be smooth. There are further two classifications in Full Step Excitation, they are one Phase-on wave stepping and two phase-on mode.

1. One phase-on stepping or Wave Stepping: In this mode only one terminal (phase) of the motor will be energised at any given time. This has less number of steps and hence can achieve a full 360° rotation. Since the number of steps is less the current consumed by this method is also very low. The following table shows the wave stepping sequence for a 4 phase stepper motor

2. Two Phase-on stepping: As the name states in this method two phases will be one. It has the same number of steps as Wave stepping, but since two coils are energised at a time it can provide better torque and speed compared to the previous method. Although one down side is that this method also consumes more power.

Half Step Mode

The Half Step mode is the combination of one phase-on and two-phase on modes. This combination will help us to get over the above mentioned disadvantage of the both the modes.

As you might have guessed it since we are combining both the methods we will have to perform 8-steps in this method to get a complete rotation. The switching sequence for a 4-phase stepper motor shown below

Micro Step Mode

Micro stepping mode is the complex of all, but it offers very good precision along with good torque and smooth operation. In this method the coil will be excited with two sine waves which are 90° apart. This way we can control both the direction and amplitude of the current flowing through the coil which helps us to increase the number of steps the motor has to make for one complete rotation. Micro stepping can take as high as 256 steps to make one complete rotation, this makes the motor to rotate faster and smoother.

1.8    CALCULATING THE STEPS PER REVOLUTION FOR STEPPER MOTOR

It is important to know how to calculate the steps per Revolution for your stepper motor because only then you can program/drive it effectively.

Lets assume we will be operating the motor in 4-step sequence so the stride angle will be 11.25° since it is 5.625°(given in datasheet) for 8 step  sequence, it will be 11.25° (5.625*2=11.25).

Steps per revolution = 360/step angleHere, 360/11.25 = 32 steps per revolution.

1.9                                               WORKING  PRINCIPLE

Every revolution of the stepper motor is divided into a discrete number of steps, in many cases 200 steps, and the motor must be sent a separate pulse for each step. The stepper motor can only take one step at a time and each step is the same size.

Since each pulse causes the motor to rotate a precise angle, typically 1.8°, the motor’s position can be controlled without any feedback mechanism. As the digital pulses increase in frequency, the step movement changes into continuous rotation, with the speed of rotation directly proportional to the frequency of the pulses.
Stepper motors are used every day in both industrial and commercial applications because of their low cost, high reliability, high torque at low speeds and a simple, rugged construction that operates in almost any environment.

• The rotation angle of the motor is proportional to the input pulse.
• The motor has full torque at standstill (if the windings are energized).
• Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3 to 5% of a step and this error is non-cumulative from one step to the next.
• Excellent response to starting/stopping/reversing.
• Very reliable since there are no contact brushes in the motor. Therefore the life of the stepper motor is simply dependent on the life of the bearing.
• The stepper motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control.
• It is possible to achieve very low speed synchronous rotation with a load that is directly coupled to the shaft.
• A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses.

1.10                        OPERATION OF STEPPER MOTOR

Brushed DC motors rotate continuously when DC voltage is applied to their terminals. The stepper motor is known by its property of converting a train of input pulses into a precisely defined increment in the shaft position. Each pulse moves the shaft through a fixed angle.

Stepper motors effectively have multiple “toothed” electromagnets arranged around a central gear-shaped piece of iron. The electromagnets are energized by an external driver circuit or a micro controller. To make the motor shaft turn, first, one electromagnet is given power, which magnetically attracts the gear’s teeth. When the gear’s teeth are aligned to the first electromagnet, they are slightly offset from the next electromagnet. This means that when the next electromagnet is turned on and the first is turned off, the gear rotates slightly to align with the next one. From there the process is repeated. Each of those rotations is called a “step”, with an integer number of steps making a full rotation. In that way, the motor can be turned by a precise angle.

The circular arrangement of electromagnets is divided into groups, each group called a phase, and there is an equal number of electromagnets per group. The number of groups is chosen by the designer of the stepper motor. The electromagnets of each group are interleaved with the electromagnets of other groups to form a uniform pattern of arrangement.

Electromagnets within the same group are all energized together. Because of this, stepper motors with more phases typically have more wires (or leads) to control the motor.

1.11                       STEPPER MOTOR CONSTRUCTION

The performance of a stepper motor — both in terms of resolution (or step size), speed, and torque — is influenced by construction details, which at the same time may also affect how the motor can be controlled. As a matter of fact, not all stepper motors have the same internal structure (or construction), as there are different rotor and stator configurations.

Rotor

For a stepper motor, there are basically three types of rotors:

• Permanent magnet rotor: The rotor is a permanent magnet that aligns with the magnetic field generated by the stator circuit. This solution guarantees a good torque and also a detent torque. This means the motor will resist, even if not very strongly, to a change of position regardless of whether a coil is energized. The drawbacks of this solution is that it has a lower speed and a lower resolution compared to the other types. Figure 1 shows a representation of a section of a permanent magnet stepper motor.

Figure 1: Permanent Magnet Stepper Motor

• Variable reluctance rotor:The rotor is made of an iron core, and has a specific shape that allows it to align with the magnetic field. With this solution it is easier to reach a higher speed and resolution, but the torque it develops is often lower and it has no detent torque.
• Hybrid rotor:This kind of rotor has a specific construction, and is a hybrid between permanent magnet and variable reluctance versions. The rotor has two caps with alternating teeth, and is magnetized axially. This configuration allows the motor to have the advantages of both the permanent magnet and variable reluctance versions, specifically high resolution, speed, and torque. This higher performance requires a more complex construction, and therefore a higher cost. Figure 1 shows a simplified example of the structure of this motor. When coil A is energized, a tooth of the N-magnetized cap aligns with the S-magnetized tooth of the stator. At the same time, due to the rotor structure, the S-magnetized tooth aligns with the N-magnetized tooth of the stator. Real motors have a more complex structure, with a higher number of teeth than the one shown in the picture, though the working principle of the stepper motor is the same. The high number of teeth allows the motor to achieve a small step size, down to 0.9°.

Figure 4: Hybrid Stepper Motor

Stator

The stator is the part of the motor responsible for creating the magnetic field with which the rotor is going to align. The main characteristics of the stator circuit include its number of phases and pole pairs, as well as the wire configuration. The number of phases is the number of independent coils, while the number of pole pairs indicates how main pairs of teeth are occupied by each phase. Two-phase stepper motors are the most commonly used, while three-phase and five-phase motors are less common (Figure 2 and Figure 3).

Figure 2: Two-Phase Stator Winding (Left), Three-Phase Stator Winding (Right)

Figure 3: Two-Phase, Single-Pole Pair Stator (Left) and Two-Phase, Dipole Pair Stator (Right). The Letters Show the Magnetic Field Generated when Positive Voltage is Applied between A+ and A-.

1.12                             APPLICATIONS OF STEPPER MOTOR

The Stepper motor is manufactured in various sizes ranging from milliwatts to hundreds of watts. Its maximum torque value ranges up to 15 Newton Meter and the step angle ranges from 1.8 to 90 degrees. The Stepper Motor Applications have a wide range. Some of the applications are given below.

1. As the stepper motor are digitally controlled using an input pulse, they are suitable for use with computer controlled systems.
2. They are used in numeric control of machine tools.
3. Used in tape drives, floppy disc drives, printers and electric watches.
4. The stepper motor also use in X-Y plotter and robotics.
5. It has wide application in textile industries and integrated circuit fabrications.
6. The other applications of the Stepper Motor are in spacecrafts launched for scientific explorations of the planets etc.
7. These motors also find a variety of commercial, medical and military applications and also used in the production of science fiction movies.
8. Stepper motors of microwatts are used in the wrist watches.
9. In the machine tool, the stepper motors with ratings of several tens of kilowatts is used

1.13                                                   CONCLUSION

At the end of this work, a stepper motor which is an electromagnetic device that converts digital pulses into mechanical shaft rotation was discussed. Advantages of step motors are low cost, high reliability, high torque at low speeds and a simple, rugged construction that operates in almost any environment. The main disadvantages in using a stepper motor is the resonance effect often exhibited at low speeds and decreasing torque with increasing speed.

References

1. Liptak, Bela G. (2005). Instrument Engineers’ Handbook: Process Control and Optimization. CRC Press. p. 2464. ISBN978-0-8493-1081-2.
2. Tarun, Agarwal. “Stepper Motor – Types, Advantages & Applications”.
3. “Friction and the Dead Zone” by Douglas W Jones https://homepage.divms.uiowa.edu/~jones/step/physics.html#friction
4. “electricmotors.machinedesign.com”.
5. com, microstepping
6. “Microstepping: Myths and Realities – MICROMO”. micromo.com.
7. step motor: http://www.applied-motion.com/videos/intro-amps-ip65-rated-motors-motordrives
8. “Advanced Micro Systems – stepper 101”. http://www.stepcontrol.com.