Stepper Motor Report

STEPPER MOTOR DEFINITION: A stepper motor is a brushless, synchronous electric motor that converts digital pulses into mechanical shaft rotation. 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.

PHYSICAL STRUCTURE:

MECHANISM/PRINCIPLE OF OPERATION:

There are 4 coils with 90o angle between each other fixed on the stator. The way that the coils are interconnected, will finally characterize the type of stepper motor connection. In the above drawing, the coils are not connected together. The above motor has 90 o rotation step. The coils are activated in a cyclic order, one by one. The rotation direction of the shaft is determined by the order that the coils are activated. The following animation demonstrates this motor in operation. The coils are energized in series, with about 1sec interval. The shaft rotates 90o each time the next coil is activated:

APPLICATIONS:

Stepper motors have become an essential component to applications in many different industries. The following is a list of industries making use of stepper motors:

• Aircraft – In the aircraft industry, stepper motors are used in aircraft instrumentations, antenna and sensing applications, and equipment scanning • Automotive – The automotive industry implements stepper motors for applications concerning cruise control, sensing devices, and cameras. The military also utilizes stepper motors in their application of positioning antennas • Chemical – The chemical industry makes use of stepper motors for mixing and sampling of materials. They also utilize stepper motor controllers with single and multi-axis stepper motors for equipment testing • Consumer Electronics and Office Equipment – In the consumer electronics industry, stepper motors are widely used in digital cameras for focus and zoom functionality features. In office equipment, stepper motors are implemented in PC-based scanning equipment, data storage drives, optical disk drive driving mechanisms, printers, and scanners • Gaming – In the gaming industry, stepper motors are widely used in applications like slot and lottery machines, wheel spinners, and even card shufflers • Industrial – In the industrial industry, stepper motors are used in automotive gauges, machine tooling with single and multi-axis stepper motor controllers, and retrofit kits which make use of stepper motor controllers as well. Stepper motors can also be found in CNC machine control • Medical – In the medical industry, stepper motors are utilized in medical scanners, microscopic or nanoscopic motion control of automated devices, dispensing pumps, and chromatograph auto-injectors. Stepper motors are also found inside digital dental photography (X-RAY), fluid pumps, respirators, and blood analysis machinery, centrifuge • Scientific Instruments –Scientific equipment implement stepper motors in the positioning of an observatory telescope, spectrographs, and centrifuge • Surveillance Systems – Stepper motors are used in camera surveillance

TYPES: 1.

Permanent Magnet Stepper :

The rotor and stator poles of a permanent magnet stepper are not teethed. Instead the rotor have alternative north and south poles parallel to the axis of the rotor shaft.

When a stator is energized, it develops electromagnetic poles. The magnetic rotor aligns along the magnetic field of the stator. The other stator is then energized in the sequence so that the rotor moves and aligns itself to the new magnetic field. This way energizing the stators in a fixed sequence rotates the stepper motor by fixed angles.

2.

Variable reluctance stepper :

The variable reluctance stepper has a toothed non-magnetic soft iron rotor. When the stator coil is energized the rotor moves to have a minimum gap between the stator and its teeth.

The teeth of the rotor are designed so that when they are aligned with one stator they get misaligned with the next stator. Now when the next stator is energized, the rotor moves to align its teeth with the next stator. This way energizing stators in a fixed sequence completes the rotation of the step motor.

3.

Hybrid stepper :

A hybrid stepper is a combination of both permanent magnet and the variable reluctance. It has a magnetic teethed rotor which better guides magnetic flux to preferred location in the air gap.

The magnetic rotor has two cups. One for north poles and second for the south poles. The rotor cups are designed so that that the north and south poles arrange in alternative manner.

ADVANTAGES 1. The rotation angle of the motor isproportional to the input pulse. 2. The motor has full torque at stand still (if the winding are energized) 3. Precise positioning and repeatability of movement since good stepper motor s have an accuracy of 3 –5% of a step and this error is non cumulative from one step to the next. 4. Excellent response to starting stopping reversing. 5. Very reliable since there are no contact brushes in the motor. Therefore the life to the motor is simply dependanton the life of the bearing. 6. The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control. 7. It is possible to achieve very low speedsynchronous rotation with a load that is directly coupled to the shaft. 8. A wide range of rotational speed is proportional to the frequency of the input pulses.

DISADVANTAGES 1. Resonance can occur if not properly controlled. 2. Not easy to operate at extremely high speeds.

DRIVING MODES

Stepping Modes There are three stepping modes of a stepper motor. The stepping mode refers to the pattern of sequence in which stator coils are energized. 1.

Wave drive (One phase ON at a time)

2.

Full drive (Two phase ON at a time)

3.

Half drive (One and two phase ON at a time)

1.

Wave drive :

In wave drive stepping mode only one phase is energized at a time.

2.

Full Drive :

In full drive, two phases are energized at a time.

3.

Half Drive :

In half drive, alternately one and two phases are energized. This increases the resolution of the motor.

Wave drive or Single-Coil Excitation

The first way is the one described previously. This is called Single-Coil Excitation, and means that only one coil is energized each time. This method is rarely used, generally when power saving is necessary. It provides less than half of the nominal torque of the motor, therefore the motor load cannot be high.

This motor will have 4 steps per full cycle, that is the nominal number of steps per cycle.

Full step drive

The second and most often used method, is the Full step drive. According to this method, the coils are energized in pairs. According to the connection of the coils (series or parallel) the motor will require double the voltage or double the current to operate that needs when driving with Single-Coil Excitation. Yet, it produces 100% the nominal torque of the motor.

This motor will have 4 steps per full cycle, that is the nominal number of steps per cycle. Half stepping

This is a very interesting way to achieve double the accuracy of a positioning system, without changing anything from the hardware! According to this method, all coil pairs can be energized simultaneously, causing the rotor to rotate half the way as a normal step. This method can be single-coil or two-coil excitation as well. The following animations make this clear:

Single-Coil excitation

Two-Coil excitation

With this method, the same motor will have double the steps per revolutions, thus double the accuracy in positioning systems. For example, this motor will have 8 steps per cycle! Microstepping

Microstepping is the most common method to control stepper motors nowadays. The idea of microstepping, is to power the coils of the motor NOT with pulses, but with a waveform similar to a sin waveform. This way, the positioning from one step to the other is smoother, making the stepper motor suitable to be used for high accuracy applications such as CNC positioning systems. Also, the stress of the parts connected on the motor, as well as the stress on the motor itself is significantly decreased. With microstepping, a stepper motor can rotate almost continuous, like simple DC motors. The waveform that the coils are powered with, is similar to an AC waveform. Digital waveforms can also be used. here are some examples:

Powering with sine wave

Powering with digital signal

Powering with high resolution digital signal

The microstepping method is actually a power supply method, rather than coil driving method. Therefore, the microstepping can be applied with single-coil excitation and full step drive. The following animation demonstrated this method:

Although it seems that the microstepping increases the steps even further, usually this does not happen. In high accuracy applications, trapezoidal gears are used to increase the accuracy. This method is used to ensure smooth motion.

►Full Step Sequence In the full step sequence, two coils are energized at the same time and motor shaft rotates. The order in which coils has to be energized is given in the table below.

Full Step Sequence

►Half Step Sequence In Half mode step sequence, motor step angle reduces to half the angle in full mode. So the angualar resolution is also increased i.e. it becomes double the angular resolution in full mode. Also in half mode sequence the number of steps gets doubled as that of full mode. Half mode is usually preffered over full mode. Table below shows the pattern of energizing the coils.

Half Step Sequence

►Full Step Sequence In the full step sequence, two coils are energized at the same time and motor shaft rotates. The order in which coils has to be energized is given in the table below. Full Mode Sequence Step

A

B

A\

B\

0

1

1

0

0

1

0

1

1

0

2

0

0

1

1

3

1

0

0

1

The working of the full mode sequence is given in the animated figure below.

►Half Step Sequence In Half mode step sequence, motor step angle reduces to half the angle in full mode. So the angualar resolution is also increased i.e. it becomes double the angular resolution in full mode. Also in half mode sequence the number of steps gets doubled as that of full mode. Half mode is usually preffered over full mode. Table below shows the pattern of energizing the coils.

Half Mode Sequence Step

A

B

A\

B\

0

1

1

0

0

1

0

1

0

0

2

0

1

1

0

3

0

0

1

0

4

0

0

1

1

5

0

0

0

1

6

1

0

0

1

7

1

0

0

0

The working of the half mode sequence is given in the animated figure below.

- See more at: http://www.8051projects.net/stepper-motor-interfacing/step-sequence.php#sthash.FzS8MVI1.dpuf

There are three commonly used excitation modes for step motors; these are full step, half step and microstepping.

In full step operation, the motor moves through its basic step angle, i.e., a 1.8° step motor takes 200 steps per motor revolution. There are two types of full step excitation modes. In single phase mode, also known as "one-phase on, full step" excitation, the motor is operated with only one phase (group of windings) energized at a time. This mode requires the least amount of power from the driver of any of the excitation modes. See Fig. 5a.

In dual phase mode, also known as "two-phase on, full step" excitation, the motor is operated with both phases energized at the same time. This mode provides improved torque and speed performance. Dual phase excitation provides about 30% to 40% more torque than single phase excitation, but does require twice as much power from the driver. See Fig. 5b.

Half step excitation is alternating single and dual phase operation resulting in steps that are half the basic step angle. Due to the smaller step angle, this mode provides twice the resolution and smoother operation. Half stepping produces roughly 15% less torque than dual phase full stepping. Modified half stepping eliminates this torque decrease by increasing the current applied to the motor when a single phase is energized. See Fig. 6. Microstepping is a technique that increases motor resolution by controlling both the direction and amplitude of current flow in each winding. Current is proportioned in the windings according to sine and cosine functions.

Microstepping can divide a motor's basic step up to 256 times. Microstepping improves low speed smoothness and minimizes low speed resonance effects.

Wave Drive In this mode only one electromagnet is energized at a time. Generated torque will be less when compared to full drive in which two electromagnets are energized at a time but power consumption is reduced. It has same number of steps as in the full drive. This drive is preferred when power consumption is more important than torque. It is rarely used. Wave Drive Stepping Sequence Step

A

B

C

D

1

1

0

0

0

2

0

1

0

0

3

0

0

1

0

4

0

0

0

1

Full Drive In this mode two electromagnets are energized at a time, so the torque generated will be larger when compared to Wave Drive. This drive is commonly used than others. Power consumption will be higher than other modes.

Full Drive Stepping Sequence Step

A

B

C

D

1

1

1

0

0

2

0

1

1

0

3

0

0

1

1

4

1

0

0

1

Half Drive In this mode alternatively one and two electromagnets are energized, so it is a combination of Wave and Full drives. This mode is commonly used to increase the angular resolution of the motor but the torque will be less, about 70% at its half step position. We can see that the angular resolution doubles when using Half Drive. Half Drive Stepping Sequence Step

A

B

C

D

1

1

0

0

0

2

1

1

0

0

3

0

1

0

0

4

0

1

1

0

5

0

0

1

0

6

0

0

1

1

7

0

0

0

1

8

1

0

0

1

Now we will see how to implement these drives.