Types of Stepper Motors There are few types of stepper motors: Unipolar motor Bipolar motor Variable Reluctance motor Permanent Magnet motor Hybrid motor

Unipolar Motor Two windings per phase, one for each direction of current. A magnetic unipolar stepper motor requires only one power source. It has logically pole that can be reversed without switching the direction of current. The commutation circuit can be made very simple for each winding using a single transistor. There are three leads per phase and six leads for a typical two phase motor.

Unipolar Motor (Cont..) Fig-1: Unipolar Motor

Bipolar Motor Bipolar motors require two power sources. They have logically a single winding per phase. The current in a winding needs to be reversed in order to reverse a magnetic pole, so the driving circuit must be more complicated. There are two leads per phase. Because windings are better utilized, they are more powerful than a unipolar motor of the same weight.

Bipolar Motor (Cont..) Fig-2: Bipolar Motor

Variable Reluctance Motor Variable Reluctance (VR) motors have a soft iron multiple rotor and a wound stator. They generally operate with step angles from 5 degrees to 15 degrees at relatively high step rates, and have no detent torque.

Variable Reluctance Motor (Cont..) When phase A is energized, four rotor teeth line up with the four stator teeth of phase A by magnetic attraction. In the next step A is turned off and phase B is energized, rotating the rotor clockwise 15 degrees. Continuing the sequence, C is turned on next and then A again. Counter clockwise rotation is achieved when the phase order is reversed.

Permanent Magnet Motor Permanent magnet motors has permanent magnet rotors with no teeth, and are magnetized perpendicular to the axis. In energizing the four phases in sequence, the rotor rotates as it is attracted to the magnetic poles.

Permanent Magnet Motor (Cont..) The motor take 90 degree steps as the windings are energized in sequence ABCD. PM's generally have step angles of 45 or 90 degrees and step at relatively low rates, but they exhibit high torque and good damping characteristics.

Hybrid Motor Combining the qualities of the VR and the PM, the hybrid motor has some of the desirable features of each. They have high detent torque and excellent holding and dynamic torque, and they can operate at high stepping speeds. Normally, they exhibit step angle of 0.9 to 5 degrees.

Hybrid Motor (Cont..) Bi-filar windings are generally supplied, so that a single-source power supply can be used . If the phases are energized one at a time, in the order indicated, the rotor would rotate in increments of 1.8 degrees. This motor can also be driven two phases at a time to yield more torque, or alternately one then two then one phase, to produce half steps or 0.9 degree increments.

Motor Stepping There are three commonly used excitation stepping modes: Full-step Half-step Micro-step

Full-Step This is the usual method for driving the motor. Both phases are always on. Each stepping covers 90 degree of rotation. In single phase full-step excitation the motor is operated with only one energized phase at-a-time. This mode should only be used where torque and speed performance are not important.

Full-Step (Cont..)

Full-Step (Cont..) In dual phase full-step excitation the motor is operated with two energized phase’s at-a-time. This mode provides good torque and speed performance with a minimum of resonance problems.

Full-Step (Cont..)

Half-Step Half-step excitation is alternate single and dual phase operation resulting in steps one half the normal step size. Each step completes 45 degree of rotation. Motors can be operated over a wide range of speeds.

Half-Step (Cont..)

Half-Step (Cont..)

Micro-Step In the micro-step mode, a motor's natural step angle can be divided into much smaller angles. Typically, micro-step modes range from divide-by-10 to divide-by-256. This mode is only used where smoother motion or more resolution is required.

Step Angle Calculation The number of degrees a rotor will turn per step is called step angle. It is denoted by s. It is calculated by Step angle, s =360°/S where S=m*Nr m=number of phases Nr=number of rotor teeth

Step Angle Calculation (Cont..) For this motor, m=3 Nr=4 so S=3*4=12 Step angle, s =360°/12 =30° per step Fig-6: Cross Section of Stepper Motor

Working Procedure of Unipolar Motor Fig-7:Single Phase

Working Procedure of Unipolar Motor (Cont..) Four voltage sources are connected using wire to the stators of motors through transistors. Current goes to the stators through coil. As a result a magnetic field is created at the at the charging pole. For magnetism the charging pole attract the permanent magnet of the motor.

Working Procedure of Unipolar Motor (Cont..) Though there are four voltages sources with stepper motor there can be 8 input combination. In the previous figure input combination highlighted is 0001 i.e. inputs comes through first wire. Corresponding pole becomes charged and attracts the magnetic pole to it. The right sided picture shows the position of the magnet.

Working Procedure of Unipolar Motor (Cont..) Fig-8: Dual Phase

Working Procedure of Unipolar Motor (Cont..) In the previous figure input combination highlighted is 1010 i.e. inputs comes through second & fourth wires. Corresponding poles become charged and attract the magnetic pole to them. Though both charging poles attract the magnet, the magnet positions in the middle of them which is shown in the right sided figure.

Working Procedure of Unipolar Motor (Cont..) Fig-9: Working procedure of Unipolar Motor.

Interfacing with Computer Fig-10:Interfacing a unipolar stepper motor with computer using parallel port

Interfacing with Computer (Cont..) ULN2003: The ULN2003 is a monolithic high voltage and high current Darlington transistor arrays. It consists of seven NPN Darlington pairs that features high-voltage outputs with common-cathode clamp diode for switching inductive loads. The collector-current rating of a single Darlington pair is 500mA. High-voltage outputs: 50V

Interfacing with Computer (Cont..) ULN2003: The ULN2003 has a 2.7kW series base resistor for each Darlington pair for operation directly with TTL or 5V CMOS devices.

Interfacing with Computer (Cont..) 25 –pin parallel port: Fig: Pin Configuration of 25-pin parallel port

Variants of Stepper Motor Small Stepper Motor - PM20S Small Compact Size. Good Torque Characteristics. Step Angle: 18°. Ideal for small projects and robots. Dimensions: 20mm Wide, 14mm High .

Variants of Stepper Motor (Cont..) Small Stepper Motor - PM25S Good output characteristics . Step Angle: 7.5°. Ideal for small projects and robots. Dimensions: 25mm Wide, 12.5mm High.

Variants of Stepper Motor (Cont..) Small Stepper Motor - MP24GA High Torque . Superior running quietness and stability . Step angle: 7.5°, 4 Phase, 12Vdc, Unipolar . Dimensions: 24mm Wide, 20mm High . Used in car mirror controls and automated door windows .

Variants of Stepper Motor (Cont..) Medium Stepper Motor - PM35S High Output Torque . Step angle: 7.5°, 4 Phase, 24Vdc, Unipolar . Dimensions: 35mm Wide, 15.5mm High . Easy to drive with accurate step control . Used in printers, scanners and small toys .

Variants of Stepper Motor (Cont..) Large Stepper Motor - M55SP-1 High Output Torque. Heavy duty for large applications. Dimensions: 55mm diameter, 23mm high. Step Angle 7.5°, 48 Steps, 24Vdc, Unipolar type. Ideal for use in cutting machines and large robots.

Advantages of Stepper Motor A high accuracy of motion is possible, even under open-loop control. Large savings in controller costs are possible when the open-loop mode is used. Because of the incremental nature of command and motion, stepper motors are easily adaptable to digital control applications. No serious stability problems exist, even under open-loop control. Low power requirement.

Disadvantages of Stepper Motor They have low torque capacity compared to DC motors. They have limited speed (limited by torque capacity and by pulse-missing problems due to faulty switching systems and drive circuits). They have high vibration levels due to stepwise motion. Large errors and oscillations can result when a pulse is missed under open-loop control.

Applications of Stepper Motor Factory Automation: X-Y plotters, laser processors, electric discharge processors, NC machines, sewing machines, etc. Semiconductor fabrication equipment: Wafer processing devices, wafer conveyors, IC bonders, dicing machines, IC inspection devices, etc. Automation and labor-saving devices: ATMs, ticket machines, postal sorters, laboratory systems, bill counters, vending machines, etc.

Applications of Stepper Motor (Cont..) Medical equipment: Analytical instruments, blood pumps, centrifuges, spectrographs, etc. Office automation: Copiers, faxes, word processors, printers, optical and magnetic disk devices, etc.