Lecture_8_Stepper_motors.pdf

Lecture 8. Stepper Motors

STEPPER MOTOR – an electromagnetic actuator. It is an incremental drive

(digital) actuator and is driven in fixed angular steps.

This mean that a digital signal is used to drive the motor and every time it

receives a digital pulse it rotates a specific number of degrees in rotation.

•Each step of rotation is the response of the motor to an input pulse (or

digital command).

•Step-wise rotation of the rotor can be synchronized with pulses in a

command-pulse train, assuming that no steps are missed, thereby

making the motor respond faithfully to the pulse signal in an open-loop

manner.

•Stepper motors have emerged as cost-effective alternatives for DC

servomotors in high-speed, motion-control applications (except the high

torque-speed range) with the improvements in permanent magnets and

the incorporation of solid-state circuitry and logic devices in their drive

systems.

•Today stepper motors can be found in computer peripherals, machine

tools, medical equipment, automotive devices, and small business

machines, to name a few applications.

Stepper motors are usually operated in open loop mode.

TYPES OF MOTORS AVAILABALE

DC MOTORS VS. STEPPER MOTORS

•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 is incremental and its

resolution is limited to the step size.

•Stepper motors can slip if overloaded and the error can go undetected. (A

few stepper motors use closed-loop control.)

•Feedback control with DC motors gives a much faster response time

compared to stepper motors.

ADVANTAGES OF STEPPER MOTORS

•Position error is noncumulative. A high accuracy of motion is possible,

even under open-loop control.

•Large savings in sensor (measurement system) and 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.

•Torque capacity and power requirements can be optimized and the

response can be controlled by electronic switching.

•Brushless construction has obvious advantages.

DISADVANTAGES OF STEPPER MOTORS

•They have low torque capacity (typically less than 2,000 oz-in) 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.

STEPPER MOTOR BASICS

STEPPER MOTOR STATES FOR MOTION

The above figure is the cross-section view of a single-stack variable-reluctance

motor. The stator core is the outer structure and has six poles or teeth. The

inner device is called the rotor and has four poles. Both the stator and rotor are

made of soft steel. The stator has three sets of windings as shown in the figure.

Each set has two coils connected in series. A set of windings is called a “phase”.

The motor above, using this designation, is a three-phase motor. Current is

supplied from the DC power source to the windings via the switches I, II, and, III .

Starting with state (1) in the upper left diagram, note that in state (1), the winding

of Phase I is supplied with current through switch I. This is called in technical

terms, “phase I is excited”. Arrows on the coil windings indicate the magnetic

flux, which occurs in the air-gap due to the excitation. In state I , the two stator

poles on phase I being excited are in alignment with two of the four rotor teeth.

This is an equilibrium state.

Next, switch II is closed to excite phase II in addition to phase I . Magnetic flux is

built up at the stator poles of phase II in the manner shown in state (2), the upper

right diagram. A counter-clockwise torque is created due to the “tension” in the

inclined magnetic flux lines. The rotor will begin to move and achieve state (3),

the lower left diagram. In state (3) the rotor has moved 15 ° .

When switch I is opened to de-energize phase I , the rotor will travel another 15 °

and reach state (4). The angular position of the rotor can thus be controlled in

units of the step angle by a switching process. If the switching is carried out in

sequence, the rotor will rotate with a stepped motion; the switching process can

also control the average speed.

STEP ANGLE

The step angle, the number of degrees a rotor will turn per step, is calculated as

follows:

Step Angle(

Θ=

)

360

SmN

m number of phases

N number of rotor teeth

For this motor:

SmN

= = =

Θ= = °

360

per step

34 12

BASIC WIRING DIAGRAM

Stepper Motor

Driver

Chip

TWO PHASE STEPPER-MOTOR WIRING DIAGRAM

The above motor is a two-phase motor. This is sometimes called UNIPOLAR.

The two-phase coils are center-tapped and in this case they the center-taps are

connected to ground. The coils are wound so that current is reversed when the

drive signal is applied to either coil at a time. The north and south poles of the

stator phases reverse depending upon whether the drive signal is applied to coil

1 as opposed to coil 2.

STEP SEQUENCING

There are three modes of operation when using a stepper motor. The mode of

operation is determined by the step sequence applied. The three step

sequences are:

Full

H = HIGH = +V

Half Stepping

L = LOW = 0V

Wave

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