Download ert 457 – design of automation systems

ERT 457 – DESIGN OF
AUTOMATION SYSTEMS
LECTURE 3.3
Electrical Actuation
Systems
MUNIRA MOHAMED NAZARI
PPK BIOPROSES, UnIMAP
Course Outcome
CO 2
Ability to design (C5) automation system for
agricultural and biological production system.
Introduction

Elements of electrical systems used in control systems as
an actuator.
 Switching
 Control


devices
signal switches on or off some electrical device – motor
Mechanical switches – relays
Solid-state switches – diodes, thyristors and transistors.
 Solenoid
 Current
type devices
through solenoid is used to actuate a hydraulic /pneumatic
flow.
 Drive
systems
 Current
motor.
through a motor is used to produce rotation – d.c and a.c
Electrical Actuators, Drive Systems and
Motion Control

Electrical motors
DC
 DC servo
 AC
 Stepper motor


Drive system
Open-Loop positioning system
 Close-Loop positioning system


Motion control
Motor driver
 Numerical control (NC)

Electrical Motor


Electric motor converts electrical power into mechanical
power.
Consists of two basic components - stator and rotor.
Stator – ring shaped stationery component
Rotor – cylindrical part that rotate inside
the stator.
- assembled around shaft, supported
by bearing.
Shaft can be coupled to machinery
components such as
Gears
Pulley
Lead screw
Spindle
DC Motor


Powered by constant current and voltage.
Two types

Brushed DC Motor
 Used commutator as rotary switching device.



Brushless DC Motor
 Used solid state circuit as switching device.


Commutator rotate with the rotor and pick up current from set or carbon
brushed.
Disadvantage – result in arcing, worn brushes and maintenance problem.
Advantage – reducing inertia of rotor assembly and higher speed operat
Two reason to used DC motor,


Convenience of using DC power – eg: car battery supply.
Torque speed relationships are attractive in many apllication compare to
AC motor.
DC Motor

Brushed DC Motor
D.C. motor: (a) basics, (b) with two sets of poles
DC Motor

Brushless DC Motor
(a) Brushless permanent magnet motor, (b) transistor switching
DC Servomotor



Used feedback loop to achieve speed control.
The torque produced by motor and torque by the load
must be balanced.
Operating point – amount of torque in steady state
operation.
DC Servomotor

Advantage of DC servo
Ability to deliver a very high torque at starting velocity of zero.
 Variable speed motor and bi-directional.


Calculation for DC servo operation
 Torque
, T = Kt i
 Kt
= torque constant for motor
 i = current
 Back
 Kv
e.m.f, vb = Kvω
= back e.m.f constant for motor.
 ω = angular velocity
DC Servomotor

DC motor with equivalent circuit
 Starting
current, i = V/R
 Starting torque, T = Kt V/R
 Current, i = V – Kvω
R
 Kv
= back e.m.f constant for motor.
 ω = angular velocity
 R = resistance
 V = voltage
 Torque,
T = Kt (V – Kvω)
R
Calculation for DC servo operation

A DC servomotor has a torque constant = 0.088 N-m/A
and a voltage constant 0.12 V/ (rad/sec). The armature
resistance is 2.3 ohms. A terminal voltage of 30 V is used
to operate the motor. Determine:
a)
b)
c)
The starting torque generated by the motor just as the
voltage is applied. T = 1.148 Nm
The maximum speed at a torque of zero.
Power delivered by the motor.
AC Motor



Can be classified into two groups, single phase and
polyphase, with each group being further subdivided into
induction and synchronous motors.
Single-phase motors tend to be use for low-power
requirements while polyphase motors are used for higher
powers.
Induction motors tend to be cheaper than synchronous
motors and are thus very widely used.
AC Motor

Single-phase squirrel-cage induction motor
 Consist
of a squirrel cage rotor – copper or aluminum bars
that fit into slots in end rings to form complete electric circuits.
AC Motor

Three-phase induction motor
Similar to the single-phase induction motor but has a stator with
three windings located 120 degree apart, each winding being
connected to one of the three lines of the supply.
 The rotation of the magnetic field is much smoother than with the
single-phase motor.
 Has a great advantage over the single-phase motor of being selfstarting.

AC Motor

Synchronous motors
 Have
stators similar to induction motors but a rotor which is a
permanent magnet.
 The magnetic field produced by the stator rotates and so the
magnet rotates with it.
 With one pair of poles per phase of supply, the magnetic
field rotates through 360° in one cycle of the supply and so
the frequency of rotation with this arrangement is the same as
the frequency supply.
 Are used when a precise speed is required. They are not selfstarting and some system has to be employed to start them.
Three-phase synchronous motor
AC Motor
AC Motor
AC Motor
AC Motor
Stepper Motor



Stepper motors use a magnetic field to move a rotor.
Stepping can be done in full step, half step or other
fractional step increments.
Voltage is applied to poles around the rotor. The voltage
changes the polarity of each pole, and the resulting
magnetic interaction between the poles and the rotor
causes the rotor to move.
Stepper motors provide precise positioning and ease of
use, especially in low acceleration or static load
application.
Stepper Motor

Important performance specifications to consider when
searching for stepper motors include:
 Shaft
speed
 The
no-load rotational speed of output shaft at rated terminal
voltage. The terminal voltage is the design DC motor voltage.
 The
current per phase
 The
 The
maximum rated current or winding for a stepper motor.
continuous output power
 The
 Static
 The
mechanical power provided by the motor output.
or holding torque
maximum torque a motor can develop to hold its rotor in a
stationary position.
Stepper Motor

Motor types for stepper motors can be permanent
magnet, variable reluctance, or hybrid.
 Permanent
 Use
magnet (PM) motors
a permanent magnet on the rotor. Step angles range from 1.8
to 90 degree.
 The most common and versatile stepper motor.
Permanent magnet two-phase stepper motor with 90° steps.
(a), (b), (c) and (d) show the positions of the magnet rotor as
the coils are energized in different directions
Stepper Motor
 Variable
 Have
reluctance (VR) motors
a free-moving rotor, no residual torque is produced due to
lack of a permanent magnet.
 The rotor is instead composed of a soft iron metal and also
composed of its own very prominent poles, tending to stick out
more than a rotor found on the PM version.
 Step angles : 7.5 to 15 degree.
Variable reluctance stepper motor
Stepper Motor
 Hybrid
motors
 Consist
of a heavily toothed PM rotor and toothed stators, plus
prominent rotor poles like a VR rotor.
 They are capable of very fine step angles: 0.9 to 1.8 degree and
have a high-speed capability.
 There is higher available torque than PM or VR stepper motors.
 Most effective but most expensive stepper motor type.
Total number of steps/revolution = nm
n = motor phase on the stator
m = number of teeth on the rotor
Calculation for Stepper Motor Operation

A stepper motor has a step angle = 7.5°.
 a)
How many pulses are required for the motor to
rotate through five complete revolutions?
 b) What pulse frequency is required for the motor
to rotate at a speed of 200 rev/min?
a) 7.5° = 1 pulse
n pulses = 360° / 7.5°
= 48 pulses/rev
so, n pulses for 5 revolution,
= 48 pulses/rev x 5 rev
= 240 pulses.
b) fp = np Nm
= 48 pulse/rev x 200 rev/min
60 sec/min
= 160 pulses/sec
= 160 Hz
Stepper Motor

Stepper motor specifications
 Terms
commonly used in specifying stepper motors:
 Phase


The number of independent windings on the stator (eg: four-phase
motor). The current required per phase and its resistance and inductance
will be specified so that the controller switching output is specified.
Two-phase motor – light duty application, three-phase motor – variable
reluctance stepper, four-phase and above motor – higher power
application.
 Step

angle
The angle through which the rotor rotates for one switching change for
the stator coils.
 Holding

torque
Maximum torque that can be applied to a powered motor without
moving it from its rest position and causing spindle rotation.
Stepper Motor
 Pull-in

torque
Maximum torque against which a motor will start, for a given pulse rate,
and reach synchronism without losing a step.
 Pull-out

Maximum torque that can be applied to a motor, running at a given
stepping rate, without losing synchronism.
 Pull-in

rate
Maximum switching rate at which a loaded motor can start without losing
a step.
 Pull-out

rate
Switching rate at which a loaded motor will remain in synchronism as the
switching rate is reduced.
 Slew

torque
range
The range of switching rate between pull-in and pull-out within which the
motor runs in synchronism but cannot start up or reverse.
Stepper motor characteristics
Motor Selection

When selecting a motor for a particular application,
factors that need to be consider are:
 Inertia
matching
 Torque requirements
 Power requirements
Motor Selection
 Inertia
 For
matching
maximum power transfer, the moment inertia of the load should
be similar to that of the motor.
 When IM = IL, torque to obtain a given angular acceleration will
be minimized.
Motor Selection
 Power
requirements
 Total
power (P) required is the sum of the power required to
overcome friction and that needed to accelerate the load.
 As power is the product of torque and angular speed, then the
power required to overcome the frictional torque Tf is Tfω and that
required to accelerate the load with angular acceleration α is
(ILα)ω, where IL is the moment of inertia of the load.
P = Tfω + ILαω
Drive System

Open Loop Control System
 Normally
used stepper motor.
 Operates without verifying that the actual position achieved
in the move is the desired position.
Drive System

Closed Loop Control System
 Normally
used servomotor (DC, AC & stepper motor).
 Used feedback measurements to confirm that the final
position of the worktable is the location specified in the
program.
Motion Control


Motion control can refer to simple on-off control or
sequencing of events, controlling the speed of a motor
or other actuator, moving objects from one point to
another, or precisely constraining the speed,
acceleration, and position of a system throughout a
move.
Motion controllers are components that range from
ON/OFF devices with simple linear controllers to
complex, user programmable modules that act as
controllers within complex integrated multi-axis motion
systems.
Motion Control


Motion control is an important part of robotics, CNC and
machine tools.
Important performance specifications to consider when
searching for motion controllers include:
 Number
of axes.
 Update time.
 D/A resolution.
 Type of motion supported.
Motion Control
 The
number of axes
 Usually
 Update
 The
 D/A
correlates to number of motor outputs.
time
time between position, speed or other feedback updates.
resolution
 Represent
the “fineness” of the analog drive signal as converted
from the digital command signal.
 The
type of motion supported
 The
ability for coordinated/interpolated motion of multiple axes.
They include simple, linear and/or circular, complex ad user
defined.
Numerical Control (NC)



Form of programmable automation in which the
mechanical actions of a machine tool or other equipment
are controlled by a program containing coded
alphanumeric data.
The alphanumeric data represent relative positions
between a workhead (cutting tool) and a workpart.
When the current job is completed, a new program can
be entered for the next job.
Numerical Control (NC)

Applications of NC
 Machine
tool applications
 Milling,
drilling, grinding
 Punch presses, thermal cutting machine
 Other
applications
 Component
insertion machines in electronics
 Coordinate measuring machines
 Drafting machine
Numerical Control (NC)

Basic components of an NC system
 Program
 Part
instructions
program in machining
 Machine
control unit
 Controls
the process
 Processing
 Performs
equipment
the process
Numerical Control (NC)

Motion control systems in NC
 Point
to point systems
 System
moves to a location and performs an operation at that
location (eg: drilling).
 Continuous
 System
path systems
performs an operation during movement (eg: milling ).
Numerical Control (NC)

NC positioning system
 Typical
motor and leadscrew arrangement in an NC
positioning system for one linear axis.
 For
x-y capability, the apparatus would be
piggybacked on top of a second perpendicular axis.
Numerical Control (NC)

NC positioning system
 Two
types of NC positioning systems,
 Open-loop

No feedback to verify that the actual position achieved is the desired
position.
 Closed-loop

Uses feedback measurements to confirm that the final position is the
specified position.
Numerical Control (NC)

Analysis of Open Loop Positioning Systems
 One
axis of an NC positioning system is driven by a
stepping motor. The motor is connected to a lead screw
whose pitch is 4.0 mm, and the lead screw drives the table.
Control resolution for the table is specified as 0.015 mm.
determine
a) the number of step angles required to achieve the
specified control resolution
b) size of each step angle in the motor, and
c) linear travel rate of the motor at a pulse frequency of
200 pulses per second.
Numerical Control (NC)

Solution
Numerical Control (NC)

Analysis of Open Loop Positioning Systems
A
DC servomotor is used to drive one of the table axes of
an NC milling machine. The motor is coupled directly to the
lead screw for the axis, and the lead screw pitch = 5mm.
The optical encoder attached to the lead screw emits 500
pulses per revolution of the lead screw. The motor rotates at
a normal speed of 300 rev.min. Determine
a) control resolution of the system, expressed in linear travel
distance of the table axis.
b) frequency of the pulse train emitted by the optical
encoder when the servomotor operates at full speed.
c) travel rate of the table at normal rpm of the motor.
Numerical Control (NC)

Solution