ERT 457 – DESIGN OF AUTOMATION SYSTEMS

LECTURE 3.3LECTURE 3.3

Electrical Electrical Actuation Actuation SystemsSystems

MUNIRA MOHAMED NAZARIPPK BIOPROSES, UnIMAP

Course OutcomeCourse 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 devices

Control signal switches on or off some electrical device – motor

Mechanical switches – relays Solid-state switches – diodes, thyristors and transistors.

Solenoid type devices Current through solenoid is used to actuate a

hydraulic /pneumatic flow. Drive systems

Current through a motor is used to produce rotation – d.c and a.c motor.

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 asGearsPulleyLead screwSpindle

DC Motor Powered by constant current and voltage. Two types

Brushed DC Motor Used commutator as rotary 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.

Brushless DC Motor Used solid state circuit as switching device.

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 e.m.f, vb = Kvω Kv = 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) The starting torque generated by the motor

just as the voltage is applied.b) The maximum speed at a torque of zero.c) Power delivered by the motor.

T = 1.148 NmT = 1.148 Nm

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 self-starting.

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 self-starting 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 maximum rated current or winding for a stepper motor.

The continuous output power The mechanical power provided by the motor output.

Static or holding torque The 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 magnet (PM) motors

Use 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 reluctance (VR) motors

Have 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

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/revso, n pulses for 5 revolution,

= 48 pulses/rev x 5 rev = 240 pulses.

a) 7.5° = 1 pulse n pulses = 360° / 7.5°

= 48 pulses/revso, 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

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 torque 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 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 matching For 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 + ILαωP = 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 correlates to number of motor outputs.

Update time The time between position, speed or other feedback

updates. D/A 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 instructions

Part program in machining Machine control unit

Controls the process Processing equipment

Performs 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 path systems

System 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