stepper motors.pptx

7/30/2019 Stepper Motors.pptx

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A stepper motor is a pulse-driven motorthat changes the angular position of the rotorin steps It is a digital electromechanicaldevice

Define = the step angle (per input pulse)

Resolution = the number of steps/revolution

= total angle traveled by the rotor = X No of steps

n = the shaft speed = ( X fp) / 360 fp = No of pulses/second
http://www.sherline.com/images/67127pic.jpg


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1. 1. Input to SM is in the form of electric pulseswhereas input to CM is a constant voltagesource.

2. 2. A CM has a free running shaft , SM movesthrough angular steps.

3. 3. No feedback required when SM used.

4. 4. SM is a digital electromechanical device, CMis an analog electromechanical device.


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Stepper motors are operated open loop, while most DC

motors are operated closed loop. Stepper motors are easily controlled with microprocessors,

however logic and drive electronics are more complex. Stepper motors are brushless and brushes contribute

several problems, e.g., wear, sparks, electrical transients. DC motors have a continuous displacement and can be

accurately positioned, whereas stepper motor motion isincremental and its resolution is limited to the step size.

Stepper motors can slip if overloaded and the error can goundetected. (A few stepper motors use closed-loop control.) Feedback control with DC motors gives a much faster

response time compared to stepper motors.


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1. The rotation angle of the motor is proportional to the input pulse.2. The motor has full torque at standstill (if the windings are energized)3. Precise positioning and repeatability of movement since good steppermotors have an accuracy of 3 5% of a step and this error is noncumulative 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 of the motor is simply dependant on the life of thebearing.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 speed synchronous rotation with a loadthat is directly coupled to the shaft.8. A wide range of rotational speeds can be realized as the speed isproportional to the frequency of the input pulses.


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1. Resonances can occur if not properly controlled.2. Not easy to operate at extremely high speeds.3. Low torque capacity compared to dc motor.4. High level of vibration due to stepwise motion.

Open Loop OperationOne of the most significant advantages of a stepper motor is its

ability to be accurately controlled in an open loop system. Openloop control means no feedback information about position isneeded. This type of control eliminates the need for expensivesensing and feedback devices such as optical encoders. Yourposition is known simply by keeping track of the input step pulses.


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Aliasgar Kutiyanawala 6

1.Variable Reluctance

-Does not have permanent magnet

-Low torque

2. Permanent Magnet -Employ permanent magnet

-Low speed, relatively high torque

3. Hybrid -Multi-toothed stator poles and a permanent magnet

rotor

-High static and dynamic torque


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Permanent-magnet (PM) stepper motor This kind ofmotor creates rotation by using the forces between apermanent magnet and an electromagnet created by

electrical current. An interesting characteristic of thismotor is that even when it is not powered, the motorexhibits some magnetic resistance to turning.Variable-reluctance (VR) stepper motor Unlike thePM stepper motor, the VR stepper motor

does not have a permanent-magnet and creates rotationentirely with electromagnetic forces. This motor does notexhibit magnetic resistance to turning when the motor isnot powered.


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Step angle = 3600 / S = 300

S=mNr (3x4) =12

m= number of phases (3)

Nr=number of rotor teeth (4)
http://upload.wikimedia.org/wikipedia/commons/f/f8/Stepper_motor_1.png


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http://upload.wikimedia.org/wikipedia/commons/f/f8/Stepper_motor_1.pnghttp://upload.wikimedia.org/wikipedia/commons/1/10/Stepper_motor_2.png


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http://upload.wikimedia.org/wikipedia/commons/1/10/Stepper_motor_2.pnghttp://upload.wikimedia.org/wikipedia/commons/2/25/Stepper_motor_3.png


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http://upload.wikimedia.org/wikipedia/commons/2/25/Stepper_motor_3.pnghttp://upload.wikimedia.org/wikipedia/commons/2/2a/Stepper_motor_4.png


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http://upload.wikimedia.org/wikipedia/commons/2/2a/Stepper_motor_4.png


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Permanent Magnet / Variable Reluctance

Unipolar vs. Bipolar

Number of Stacks

Number of Phases Degrees Per Step

Microstepping

Pull-In/Pull-Out Torque Detent Torque


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Principle of Operation:

Reluctance of the magneticcircuit formed by the rotor

and stator teeth varies withthe angular position of therotor

Here, energize coils A andA (Phase A)

Rotor steps to align rotorteeth 1 and 4 with statorteeth 1 and 5

Toothed Rotor and Toothed Stator


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Energize coils B and B(Phase B)

Rotor steps forward

Rotor teeth 3 and 6 align with Statorteeth 1 and 5

Let Ns = No of teeth on the stator

Nr = No of teeth on the rotor

= Step Angle in spacedegrees

360s r

s r

N N

N N


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Energize Phase C

Rotor steps forward another 15


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Energize Phase D

Rotor steps forward another 15


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Repeat the sequence

Energize Phase A

Rotor steps forward again


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Switching Circuit for thestepper motor

Close switches in order 1, 2, 3, and4 to turn the rotor clockwise

Close switches in reverse order - 4,3, 2, and 1 to change rotation to theopposite (counter-clockwise)direction


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The following are the most common drive modes. Wave Drive (1 phase on) Full Step Drive (2 phases on)

Half Step Drive (1 & 2 phases on)Microstepping (Continuously varying motorcurrents)


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One phase on stepping sequence for two phase motor.

PM wave drive sequence (a) 1+ , (b) 2+ , (c) 1- , (d) 2-.


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Two phase on stepping sequence for two phase motor.


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One & Two phase on stepping sequence for two phase motor.


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H b id t


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A Hybrid stepper motor in which therotor has an axial permanent magnet atthe middle and ferromagnetic teeth at theouter section as shown in fig. A Hybridstepper motor combines thecharacteristics of both PMSM and VRSM.HM have a small step angle. 0.9, 1.8, 3.6degree.

Hybrid steppermotor


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Operation of Hybrid stepper motor

2-ph, 4/6-pole Hybridstepper motor


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1. Provides dtente torque with windings de-energized.

2. Less tendency to resonate.

3. Higher holding torque capability. 4. Better damping due to presence of rotor

magnet.

5. High stepping rate capability.


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1. What is Stepper Motor? Describe the operatingprinciple of Stepper Motor. 2. Describe the constructional features and

working of a single-stack 3-phase, 6/4-polevariable reluctance stepping motor.

3. Describe the 2-phase, 8/4-pole permanentmagnet stepping motor.

4.Describe the constructional features and workingof Hybrid stepper motor.

5. Explain the multistack variable reluctanceStepper Motor. 6. Comparison of VR, PM and Hybrid stepper

motor.

stepper motors.pptx

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