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Engineering

E n g i n e e r i n g

Introduction toM o tio n C o ntro l Te c hno lo g y

I nt r o d u c t i o n t o M o t o r Te c h n o lo g i e s

DC Motor SystemsDC motors are used with IDC s DC motor controls tocontrol velocity and position. With these simplecontrols, limit switches attached to the actuator or thecustomer s load provide position feedback. DC motorsare also used with analog position controls and a linearpotentiometer for position feedback. The result is anabsolute positioning system. They can also be used inclosed loop servo systems with an incremental encoderfor position feedback.

DC motors have windings on the rotor of the motor. Tosupply current to the rotor, a set of brushes ride on asegmented commutator, which supplies current to theappropriate internal windings depending on the positionof the rotor. Brushes may need to be replacedperiodically, depending on the load, speed and dutycycle. The commutation method limits the top speedthat can be reached before arcing over the commutatorsegments occurs. The heat generated in the windingsmust travel across the air gap and the through stator tobe dissipated. This long thermal path results in a motorthat is less thermally efficient and is therefore larger thana brushless motor with an equivalent power rating. Thewindings also add inertia to the rotor, which results inlower peak acceleration rates than a similar brushlessmotor.

Many different components are used in a variety of combinations to create a complete motion control orpositioning system. IDC offers the broadest range of products spanning the complete spectrum frommechanical actuators to microstepping and brushlessservo drives to programmable motion controllers. A

successful application depends on choosing the rightcombination of actuator, motor, drive, and controltechnology. More than one technology may meet therequirements of your application. In this case, factorssuch as performance, cost, flexibility, and simplicity maydetermine your selection.

C o m m o n A p p lic a t io n s Air cylinder replacement Clamping, pressing Transfer mechanismsA d v a n t a g e s Lower cost than brushless Simple controlsD i s a d v a n t a g e s High maintenance (brushes). Larger motor Less responsive

RotorWindings

Stator Magnets

Brushes

Commutator

PCor

PLCMotionControl Drive Motor

FeedbackDevice

MechanicalSystem

Host Computer

ProgrammableController

ProgrammableMotion

Controller

Limit Switch Control

Microstepping Drive

Brushless Servo Drive

DC Motor

Brushless Servo

Step Motor

Linear Servo Motor

Encoder

Linear Potentiometer

Linear Actuator

Linear Positioning Table

Direct Drive Linear Motor

Rotary Table

GearheadSmart Drive

Programmable Drive Control

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Industrial Devices Corporation

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Engineering

E n g i n e e r i n g

Introduction toM o tio n C o ntro l Te c hno lo g y

B r u s h le s s S e r v o S y s t e m s

IDC brushless servo systems use a three-phase brushlessservo motor with an incremental encoder for positionfeedback.

Brushless servo motors are constructed with the

windings in the stator or outer portion of the motor andhave permanent magnets attached to the rotor. Thelocation of the windings next to the finned surface of the motor provides a shorter path for heat generated inthe windings to escape. This improved thermalefficiency when compared to a dc motor results inhigher power ratings for a similar sized motor. Lessrotor inertia and no mechanical commutator limits resultin higher speed and acceleration capabilities.

The servo controller and drive use the encoder feedbacksignal to continuously adjust the motor torque so thatthe desired position is maintained. This is referred to asa closed loop servo system. The electronics required tooperate a brushless motor and close the loop aretherefore more complex and expensive thanmicrostepping or dc motor controls.

Unlike DC motors, which use a mechanical commutatorto deliver current to the appropriate torque producingwindings, the brushless motor requires a feedbackdevice to sense the position of the permanent magnetrotor in relation to the motor s windings. The servodrive must maintain the correct relationship betweenthe magnetic vector created by the windings and therotor position to produce torque.

IDC brushless servo drives employ proprietary vectortorque control and advanced servo algorithms to provideunmatched closed loop servo performance (see pageK-32 for a more thorough discussion).

C o m m o n A p p lic a t io n s Higher speeds and accelerations Changing loads Clamping, pressingA d v a n t a g e s Closed loop control Highest torque at high speeds No maintenance (brushless) Efficient operationD i s a d v a n t a g e s Higher cost

L in e a r B r u s h le s s S e r v o M o t o r s

A special type of brushless servo motor, IDC direct drivelinear motors use an innovative design that houses thepermanent magnets in a cylindrical thrust rod. Themotor s windings reside in the thrust block whichsurrounds the rod. A linear incremental encoder is usedfor closed loop position feedback along with hall sensorsfor commutation.All of the electromagnetic force is utilized to producethrust directly, eliminating the need for rotary to linearconversion mechanisms. This eliminates backlash, leaderror and other mechanical system innacuracies.

C o m m o n A p p lic a t io n s Highest linear speeds and accelerations Repeatable linear positioningA d v a n t a g e s Direct production of thrust High reliability Efficient operationD i s a d v a n t a g e s Higher cost

Magnets

Windings

N

S

N

S

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In t r o d uc t io n t o F e e d b a c k D e v ic e s

Incremental encoderOptical encoders generate an output signal as theencoder disk rotates. The disk passes between a lightsource and photo detectors. The number of lines on thedisk determines the number of on-off cycles of eachoutput during a revolution. The detectors are arrangedsuch that the two output signals are in quadrature (shifted in phase 90 ). A once per revolution indexchannel is usually provided for establishing a home orzero position upon power up.

Electronic circuitry in the motion controller counts thepulses from the encoder to determine the position of themotor. The quadrature signal allows the controller tocount up when rotating in one direction and down inthe other, thus always remembering the absoluteposition of the system as long as power is applied. Thereare four unique transitions per cycle and controllers willgenerally count each transition. The number of countsper revolution is often referred to as the postquadrature resolution. For example, an encoder with1000 lines will generate 4000 counts per revolution post quadrature .

Incremental encoder with commutation channels The process of steering current through the appropriatemotor windings in order to produce output torque iscalled commutation. In brush motors, commutation isperformed electro-mechanically via brushes and thecommutator. In brushless motors, commutation isperformed by electronically steering the current to theappropriate winding. To do this, the rotor position mustbe determined. A circuit board containing the Halldevices is aligned with a magnet on the rotor, so that therelationship shown between the Hall outputs and themotor back EMF can be established.

When a brushless motor is used in a servo applicationrequiring position feedback, room must be made forboth the commutation circuit board as well as theencoder. This generally adds both cost and complexityto the motor package. Encoders are now available withintegrated commutation outputs equivalent to thoseproduced by Hall devices. The result is a more compactsystem, reduced alignment time and superior switchingaccuracy, due to the much lower hysteresis of theencoder when compared to a Hall device. Addingadditional data tracks to the encoder disk provides thecommutation outputs.

Channel A

Channel B

Index

Vab Vbc Vca

MotorBack EMF

HallDevices

0 60 120 180 240 300 360 60

Degrees

Hab

Hbc

Hca

M o t o rTechnology

Introduction toM o tio n C o ntro l Te c hno lo g y