Stepper Motor-1©1998-2003 by M. Zarrugh ISAT 303 Module II Electromechanical Actuators The objectives of this module are to –understand the concept of

Stepper Motor-1©1998-2003 by M. Zarrugh

ISAT 303 Module IIElectromechanical ActuatorsThe objectives of this module are to

– understand the concept of actuation.– learn about types of actuators from the simple

on/off switch to servo motor drives.– understand the physics and applications of stepper

motors.– learn how to control output speed and position in

a closed loop to create a desired motion profile.– learn how to select a stepper motor for a

particular load level


Actuators:What are They? Actuators are devices which accept a small control

signal to produce a large effect in the system output. Actuator Types: electrical, mechanical (hydraulic or

pneumatic) Electrical: On/off switches and motors

ActuatorPlant orSystem

ActualOutput

DesiredOutput

Sensor

Controller


Mechanical Actuators:Hydraulic and Pneumatic Cylinders

Cylinder

4-Way control valve

Work being actuated(pushed)

Piston

Hydraulic cylinder: working fluid is liquidPneumatic cylinder: working fluid is air

Working fluid from and to pump

Fluid flowing from the cylinder

Fluid flowing to the cylinder


Actuators:On/Off Switches Electromechanical switches

(relays): a low voltage control signal energizes a electromagnetic coil which closes a switch to close or open a high voltage (or current) circuit.

Programmable limit switches: to produce a regular sequence of on/off conditions.

Alarms/Annunciators: for monitoring process conditions and sounds an alarm if a pre-set condition is reached.

Return spring

Electro-magnet

When electro-magnet is energized, contacts close and deliver power to the load

+


Drive Pros Cons Typical Use

Motor Drives: Types, Pros and Cons

StepperMotor

DC Motor

AC(brushless)

• Lowest cost

• Compact size

• Digital control

• Limited power

• Small payload

• Low efficiency

• Easy to design

• Easy to maintain

• Quiet operation

• high speed

• Low maintenance

• Largest payload

• Complex implementation

• Motor adds inertia load

• Require bulky & costly inverters

• Cost the most

• Moderate conditions

• Low speed

• High power

• Inertial loads (high torque)

• Very high speed

• Hazardous use

• No maintenance


Motor Drives: Steppers Fundamentals Convert a series of input pulses

(steps) into a proportional angular movement.

The motor shaft position is determined by the pulse count and its speed by the pulse rate.

The permanent magnet rotor is propelled by successively energizing peripheral electromagnets (stator poles or windings).

Motor must start and stop in the start & stop zone, but can operate in the slew zone.

Start &Stop zone

Torque

Stepping Rate(speed)

Starting and stopping limit “L-curve”

Max. speedlimit “U-curve”

Slew zone

N

S


Motor Drives: Steppers FundamentalsExample: A constant load torque of 70 oz.in is supplied by a steppermotor with following approximate torque-speed relations:

L-curve: T (oz.in) = 200 - 0.050 V(steps/s)

U-curve: T (oz.in) = 200 - 0.025 V(steps/s)

T (oz.in)

V (103 steps/s)8

U200

4

LFind: (a) Maximum allowable starting speed and running speed

(b) Starting time if the motor runs 13 pulses at the max starting speed

Solution:

(a) The maximum starting speed VL is determined by equating the load torque to the supply torque given in the L-equation:

70 = 200 – 0.05 VL or VL = 2600 steps/s

The maximum running speed VU is similarly determined. VU =5200 steps/s

(b) During the starting period, the speed (or frequency) is 2600 steps/s; thus

t = (no. of steps) / (steps/s) = 13 steps / 2600 steps/s = 5 ms.

Load70

VLVU


Stepper Motor:Current Variations The pole windings are wired into two separate sections

called phases. The phases are sometimes divided further into two parts. Phases allow for many variations in current patterns: – Full-step current: both phases are always energized– Half-step current: the two phases are not always energized (most

common)

Standard 2-phase 200-step stepper motor– 50 teeth on each of the two rotor pole sections– stator has 8 poles each with 5 teeth– current in stator pole windings is sequenced to allow 1/4 tooth

“effective” rotor rotation per step which results in the 200 steps.


Stepper Motors: Drives The drive is a separate section in the motor system. The

drive responds to speed and position control commands by delivering appropriate electrical current to the motor to achieve the desired motion.

The translator section of the drive translates the step and direction signal into a set of pole switches to sequence the rotor movement.

TranslatorSwitch

Set

Stepper Drive Subsystem

Direction

Phase 1

Phase 2Motor

Step


Stepper Motors: Performance Curves Torque/speed curves are called

motor performance curves. These curves are the fundamental

indicators of dynamic (motion and power) behavior. They specify combinations of torque and speed for safe operation.

At low speed, output torque depends on drive current setting.

At high speeds, output current depends on drive supply voltage.

Torque

Speed

“U-curve”

Inputpulse

Motor Current


Stepper Motors: Performance Curves The two windings halves in an 8-lead 200-step stepper

motor can be connected either in series or in parallel. The series connection doubles the number of winding turns

and increases the output torque at low speeds, but reduces it at high speeds due to 4-fold increase in inductance.

Series Parallel

Series

ParallelTor

que

Speed


Motor Selection:Design Considerations Motor torque/speed supply

characteristics Load torque/speed

requirements Load and motor inertia Friction Torque operating margin

(>50%) Duty cycle

Tor

que

Speed

Motor

ConstantLoad

Load varieswith speed

Spe

ed

Time

Duty Cycle

Motor and Load Characteristics


Motor Selection:Dynamic Considerations Motor torque must overcome friction and

inertia: Total torque = Friction torque + Acceleration torque

Tt = Tf + Ta

The friction torque, Tf is a constant to be determined experimentally.

Motor

The acceleration torque, Ta depends on the moment of inertia J (oz-in2) and angular acceleration rad/s2:

Ta = J

Loadm, J

Ta

Tf

Tt


Motor Selection:Kinematic (Motion Profile) Considerations In most positioning applications, a

typical cycle of movement can be represented with a trapezoidal motion profile.

The cycle begins with a constant acceleration stroke, continues with a constant velocity stroke and ends with a constant deceleration segment.

Angular acceleration, speed and displacement are related by similar equations to linear motion.

Spe

ed,

Time, tt1 t3t2

Accelerate = max/t1

Const. speed, max

Decelerate = max/t3

Linear Angular

v = v0 + at = 0 + t

d =vot + at2/2 = ot + t2/2

v2 = vo2 + 2ad 2 = o

2 + 2


Motor Selection:Continuous Torque Requirement At a given maximum

operating angular speed, the motor torque must be at least 1.5 times the required continuous load torque.

The total required load torque Tt is computed from the known friction torque and the time t required to accelerate to max.

Motor

Tor

que

Speed, max

Continuousload torqueTt

>0.5 Tt

Tm

Given: Tf = 12 oz-in max = 10 rps = 10 rev/s t = 0.2 s Jtot = 50 oz-in2

Find: Total load torque Minimum motor torque at max

MaximumOperatingtorque


Stepper Motor:PROBLEM SOLUTION

= max/t = (10 rev/sec)/0.2 sec) = 50 rev/sec2

= 50 rev/sec2(2 rad/rev) = 100 rad/sec2

TT = TF + T and TF was given as 12 oz-in. T = J* = 50 oz-in2(100 rad/sec2) T = 50 oz-in2(100 rad/sec2)(1/386 in/sec2)# T = 40.7 oz-in

TT = TF + T = 12 + 40.7 = 52.7 oz-in = total load torqueMinimun Motor Torque at max =1.5 TT = 1.5(52.7)

=79.1 oz-in.#conversion factor for converting gravitational

units(in/s2) to units of mass(oz-in2) The pole windings are wired into two separate sections called phases. The phases are sometimes divided further into two parts. Phases allow for many variations in current patterns:

–Full-step current: both phases are always energized

–Half-step current: the two phases are not always energized (most common)

Standard 2-phase 200-step stepper motor

–50 teeth on each of the two rotor pole sections

–stator has 8 poles each with 5 teeth

–current in stator pole windings is sequenced to allow 1/4 tooth “effective” rotor rotation per step which results


Stepper Motor:APPLICATIONS

COMPUTER PERIPHERALS– FLOPPY DISC-POSITION MAGNETIC PICKUP– PRINTER-CARRIAGE DRIVE

BUSINESS MACHINES– CARD READER-POSITION CARDS– COPY MACHINE-PAPER FEED

PROCESS CONTROL– IN-PROCESS GAGING-PART POSITIONING– I.C. BONDING-CHIP POSITIONING

MACHINE TOOL– MULTI-AXIS MACHINES-X-Y-Z POSITIONING– GRINDING MACHINES-AUTO WHEEL DRESSING

The pole windings are wired into two separate sections called phases. The phases are sometimes divided further into two parts. Phases allow for many variations in current patterns:

–Full-step current: both phases are always energized

–Half-step current: the two phases are not always energized (most common)

Standard 2-phase 200-step stepper motor

–50 teeth on each of the two rotor pole sections

–stator has 8 poles each with 5 teeth

–current in stator pole windings is sequenced to allow 1/4 tooth “effective” rotor rotation per step which results

Documents

Stepper Motor-1©1998-2003 by M. Zarrugh ISAT 303 Module II Electromechanical Actuators The objectives of this module are to –understand the concept of