case studies based on ni hardware

8/12/2019 Case Studies Based on NI Hardware

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Case studies based on NI

Hardware


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To drive stepper motor with different driving

mode

To control the speed of DC Motor

Temperature control based application


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Stepper Motor


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Overview

A stepper,orstepping motorconverts electronicpulses into proportionate mechanical movement. Eachrevolution of the stepper motor's shaft is made up of aseries of discrete individual steps. A step is defined asthe angular rotation produced by the output shaft each

time the motor receives a step pulse.


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Types of stepper motor

Permanent Magnet Stepper Motor

Variable Reluctance Stepper Motor

Hybrid Stepper Motor


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Permanent Magnet Stepper Motor

The first and most basic type of stepper

motors is the Permanent Magnet (PM).

The rotor of the PM motor carries a

permanent magnet with 2 or more poles,in a shape of disk. The stator coils will

attract or repulse the permanent magnet

on the rotor and will generate the torque.

Here is a sketch of a PM motor:


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Variable Reluctance Stepper Motor

The VR motor does not have a

permanent magnet on the rotor. Instead,

the rotor is made of soft iron, and

performs a teethed disk like a gear. Thestator has more than 4 coils. The coils

are energized in opposite pairs, and will

attract the rotor. It has a great

advantage. These motors have nodetent torque.


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Hybrid Stepper Motor

The hybrid stepper motors are named so, because

they combine the characteristics from both VR and

PM stepper motors. They have excellent hold and

dynamic torque, and very small step angles, from 0.9o

to 5o, giving them A+ in accuracy. Their mechanical

parts can rotate at high speeds relatively to the other

stepper motor types. This is the type of motor used for

high end CNC and robots. The major disadvantage isthe cost.


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A permanent magnet with 50 North and 50 South poles whichs is notpossible to be manufactured...Therefore two disks are placed on topand bottom of a cylindrical permanent magnet


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Driving Mode


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Single-Coil Excitation

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 steppermotor connection. This motor has

90o rotation step. The coils are

activated in a cyclic order, one by

one. The rotation direction of theshaft is determined by the order

that the coils are activated.

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.


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The second and most often usedmethod, 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.

Full Step Driving Mode


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This is a very interesting way to achieve double the accuracyof a positioning system, without changing anything from thehardware! According to this method, all coil pairs can beenergized simultaneously, causing the rotor to rotate half theway as a normal step. This method can be single-coil or two-coil excitation as well.

Single-Coil excitation Two-Coil excitation

Half Step Driving Mode


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Microstepping is the most commonmethod to control stepper motorsnowadays. The idea of microstepping, isto power the coils of the motor NOT with

pulses, but with a waveform similar to a

sin waveform. This way, the positioningfrom one step to the other is smoother,making the stepper motor suitable to beused for high accuracy application

Powering with sine wave Powering with digital signal Powering with high resolution

digital signal

Microstep Driving Mode


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DC Motor


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Overview

The DC Motor or Direct Current Motor isthe most commonly used actuator for

producing continuous movement and whose

speed of rotation can easily be controlled,making them ideal for use in applications

where speed control, servo type control, and/or

positioning is required. A DC motor consists oftwo parts, a "Stator" which is the stationary

part and a "Rotor" which is the rotating part.


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Types of DC Motor

Brushed Motor

Brushless Motor

Servo Motor


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Brushed Motor

This type of motor produces a magnetic field in a

wound rotor (the part that rotates) by passing an

electrical current through a commutator and carbon

brush assembly, hence the term "Brushed". The stators(the stationary part) magnetic field is produced by using

either a wound stator field winding or by permanent

magnets. Generally brushed DC motors are cheap, small

and easily controlled


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Brushless Motor

This type of motor produce a magnetic field in therotor by using permanent magnets attached to it andcommutation is achieved electronically. They are

generally smaller but more expensive thanconventional brushed type DC motors because theyuse "Hall effect" switches in the stator to producethe required stator field rotational sequence but they

have better torque/speed characteristics, are moreefficient and have a longer operating life thanequivalent brushed types.


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Servo Motor

This type of motor is basically a brushed DC

motor with some form of positional feedback

control connected to the rotor shaft. They are

connected to and controlled by a PWM typecontroller and are mainly used in positional

control systems and radio controlled models.


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DC Motor Switching and Control

Small DC motors can be switched "On" or "Off" by means of switches, relays, transistors or

mosfet circuits with the simplest form of motor control being "Linear" control. This type of circuit

uses a bipolarTransistor as a Switch

(A Darlington transistor may also be used were a highercurrent rating is required) to control the motor from a single power supply. By varying the amount

of base current flowing into the transistor the speed of the motor can be controlled for example, if

the transistor is turned on "half way", then only half of the supply voltage goes to the motor. If the

transistor is turned "fully ON" (saturated), then all of the supply voltage goes to the motor and it

rotates faster. Then for this linear type of control, power is delivered constantly to the motor as

shown below.

Unipolar Transistor Switch

The simple switching circuit shows the connections for a Uni-directional (one direction only)

motor control circuit. A continuous logic "1" or logic "0" is applied to the input of the circuit to

turn the motor "ON" (saturation) or "OFF" (cut-off) respectively, with the flywheel diode

connected across the motor terminals to protect the switching transistor or MOSFET from any

back emf generated by the motor when the transistor turns the supply "OFF".

As well as the basic "ON/OFF" control the same circuit can also be used to control the motors

rotational speed. By repeatedly switching the motor current "ON" and "OFF" at a high enough

frequency, the speed of the motor can be varied between stand still (0 rpm) and full speed (100%).

This is achieved by varying the proportion of "ON" time (tON) to the "OFF" time (tOFF) and this

can be achieved using a process known asPulse Width Modulation.
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Pulse Width Speed Control

The rotational speed of a DC motor is directlyproportional to the mean (average) value of its supplyvoltage and the higher this value, up to maximumallowed motor volts, the faster the motor will rotate. Inother words more voltage more speed. By varying theratio between the "ON" (tON) time and the "OFF" (tOFF)time durations, called the "Duty Ratio", "Mark/Space

Ratio" or "Duty Cycle", the average value of the motorvoltage and hence its rotational speed can be varied.


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For simple unipolar drives the duty ratio isgiven as:

Mean DC output voltage fed to the motor is given as: Vmean = x Vsupply. Then by

varying the width of pulse a, the motor voltage and hence the power applied to the

motor can be controlled and this type of control is called Pulse Width Modulationor

PWM.


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