Mechatronics Engineeringsoftrobotics.buaa.edu.cn/PPT/Chapter 4 Actuator.pdf · Types and...

Wang TianMiao Wen Li

Robotics Institute, Beihang University

Mechatronics Engineering

Problems?

• Types of drives? Advantages and Disadvantages?

• DC motor; working and control?

Driving element

1. Types and characteristics of the drive elements.

2.DC Servo motor.

Types and characteristics of the drive elements.

• Driving element (Actuators): For all types of robots, CNC

machine tools and other mechanical and electrical

integration system of energy conversion devices; under its

control in microelectronic devices, various forms of input

energy is converted to mechanical energy.

• Power of a traditional machine components, electro-

mechanical drive components in the control system, not only

to provide the movement, also have a controllable character.

Types and characteristics of the drive elements -

Hydraulics

• Includes: Hydraulic cylinders, hydraulic

motors, etc.

• Advantages: Output power, smooth action,

can be directly driven operating

mechanism, strong overload capacity, able

to servo control.

• Disadvantages: Complex structure,

requires a corresponding hydraulic power

source, large area, easy oil spills causing

pollution of the environment, not as good

as servo motor control in terms of

performance.

• Application: Load, slow motion occasion.

Types and characteristics of the drive elements –

Examples of Hydraulic systems

• Boston Dynamics

PETMAN (robot)

• Boston Dynamics

Cheetah (robot)

Types and characteristics of the drive elements -

Pneumatics• Actuators: Gas motor

• Driving element: Air cylinder

• Advantages: Gas-source convenience, low cost, fast

• Disadvantages: The output power is small (compared to hydraulic and electric), large volume, large operating noise, it is difficult to servo control.

• Applications: Due to the compressibility of the air, air pressure drive cannot be used in precision positioning, only used in simple point-to-point fixed movements (clamping device).


Examples of Pneumatics

• Cylindrical Muscles.


Electric Motors

Includes: Stepper motor, DC, AC servo motor;

are small electro-mechanical drive elements most

commonly used in the systems which are small sized, needs

less speed, to simplify mechanical transmission system.

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Types and characteristics of the drive elements

– ComparisonType Features Advantages Disadvantages

Electric M

oto

rs

commercial power can

be used, same direction

signal transmission to

power, both AC and

DC power source can

be used, should pay

attention to the voltage.

Simple, easy programming,

easy to achieve positioning

servo, fast response, easy

computer connectivity, small

volume, large power, no

pollution

Instantaneous output power;

overload difference; especially

when stuck for some reason, it

will cause burn accident,

vulnerable to external noise

Pneu

matic

Air pressure source

pressure (5 ~ 7) ×105Pa, requires skilled

operators

Gas source, low cost, no

spills, speed operation is

relatively simple

Power is small, bulky, not

smooth operation, easy

miniaturization, difficulties in

long-distance transmission,

working noise, difficult to

achieve servo effect

Hydrau

lic

Requires skilled

operators; hydraulic

pressure source (20 ~

80) × 105Pa

Output power, speed, smooth

movement, positioning servo

can be realized easily

connected to the computer,

fast response

Miniaturization of the device

is difficult; requirements or

hydraulic oil pressure source

(impurities, oil quality) strictly

leakage and contamination

Types and characteristics of the drive elements

– The basic requirements of the drive element

• Small inertia, big momentum

• Small size, light weight

• Ease of maintenance, installation

• Appropriate for computer control

DC Servo Motor

•Features:

–First electric motor

–The earliest realization of speed-regulating motor

•Advantages:

Fast response, high precision and efficiency,

broader speed range, large load capacity, excellent

control, and dominated in the past.

• Disadvantages:

Require regular maintenance, the speed must not

be too high, power must not be too large.

DC Servo Motor – Basic structure

• Stator: Generating stator magnetic field.

• Rotor: Embedded surface coils, when direct current passes; electromagnetic

torque with a load in the stator rotating magnetic field.

• Brushes and commutator: In order to maintain a constant electromagnetic

torque generated in the direction to ensure continuous rotation of the rotor, the

external DC power supply is connected with the brush, commutator and

armature coil connection.

• According to the structure and shape of the armature:

Smooth armature type, hollow slotted armature type.

• According to the way stator magnetic field generated:

Permanent magnet excitation.

• According to the size of the rotor inertia: Large inertia, small

inertia servo motor.

DC Servo Motor – Classification

• Permanent magnet DC motor working principle:

– DC voltage between a and b Electric brush causes electric current to flow, outflow from the b, AB and CD under the action of electromagnetic torque rotate in the counterclockwise direction.

– As the armature turns 90o, armature coil is in neutral surface of the magnetic poles; brush and commutator pieces open, so there is no torque at this moment.

– Under inertia, armature continues to rotate at an angle, when brush is contacted again to film, conductor ab and cd exchange position, ab and cd of the current is changed, thus ensuring thearmature subjected to electromagnetic torque direction change, therefore, the armature can be rotated continuously.

DC Servo Motor – Working principle

• Separately excited DC motor principle:

– Magnetic excitation and compensating windings are on the stator while

rotor winding through a powered brush.

– The rotor and the stator magnetic field is always orthogonal to produce

torque, rotor rotation.


From the principle, when a current in the armature windings, the

magnetic field generated in the fixed magnetic moment M is given by:

MM k I

- Moment coefficient, depends on the structural parameters of the

motor itself;

I -The armature winding current.

In stable mode the electromagnetic torque of the

balanced motor shaft and the processing should be

equal to load torque.

Mc is the overall load on the motor shaft including

friction，Eddy current resistance of the motor 。

cMM


Mk

• DC servo motor torque balance equation:

IkM Mc

Physical explanation the over load in torque equilibrium must be coupled with the stability of

power, and the electromagnetic torque in addition to resistance 。

The load torque increases with the increase in electromagnetic currents in the motor, therefore the

armature current is determined by the load summary


• Further analysis in a simple theory from the front as you can see,

when the motor rotation, the rotor windings in the stator magnetic

field will be cut magnetic lines, then produce EMF in the rotor

winding, its direction as against the current direction, and the

applied voltage of opposite polarity, they were called back EMF, is

proportional to its size and the rotational speed of the rotor ，即：

ekE

Types：E-----reverse electrical momentum

Ke-----back motor EMF coefficient，structural parameters of

the motor。ω ------speed motor。


ke and km are the structural parameters of the

motor itself, the relationship between the two is

as follows

MM60

π2025.1 k

gkke

3 DC servo motor-Working principle

Effects of inductance and inertia to characteristics of

the motor•When the control voltage is applied to the DC servo motor, because

of the winding inductance, the current is not established immediately.

•More importantly, when the electromagnetic torque overcome and

exceed the load torque, the inertia of the motor shaft due (including

the inertia of the load), the speed of the motor is not built up

instantly.

•The establishment of current and speed requires a certain time, the

former is called electromagnetic time constant of the motor (Ta), and

the main inductance of the windings; the latter is called an

electromechanical motor time constants (TM), which mainly

depends on the inertia of motor axis ; TM is generally far greater

than Ta.


• After considering the winding inductance and motor inertia,

the motor balance equation is:

tJMM

ikM

t

iLRiEU

d

d

d

d

c

M

a

La－－Loop inductance;

J －－Moment of inertia of the motor shaft;

i－－ Instantaneous current of armature circuit;

ω－－The instantaneous speed of the motor.


• Variable voltage control:

• Variable magnetic control；

• Mixing the first two .

RIkRIEU e

3 DC servo motor- Speed control method

Variable voltage control

If the moment of inertia of the motor Jm and armature

resistance Ra are very small and negligible , there is ：

U=ke ω

The motor speed ω is proportional to the armature voltage U.

3 DC servo motor-Variable voltage control

VT represents switch. When it is turned on, the motor having

a voltage Us at both ends; after closing the output goes low.

Average voltage winding ：

Duty cycle α represents the ratio of a switch conduction cycle

and cycles; adjusting α means terminal voltage can be changed,

so as to achieve the purpose of speed control.

PWM

（Pulse Width

Modulation）


•Take motor forward for example, the M + side generates PWM wave, M-

connects low. When M+ is high, the motor is transferred; when low, the motor

stops.

•Changing the duty cycle of the PWM wave can change the motor speed, the

greater the duty cycle is, the faster motor can reach.

•Pulse width is the high-level time.

ondutt

Sycy cycle

le T

Rotation time

ON OFF

M+

Motor forward

PWMH

L


•Single polarity PWM amplifier

VT1 ~ VT4 is doing the role of high-power switching transistors, VD1 ~ VD4

as a freewheeling diode. The base control voltage of VT1 and VT2 are oppsite,

which make VT1, VT2 work in an alternate switching state. The other side of

the armature, VT4 is in a saturated conduction state, VT3 in the off state. When

you need to change the output voltage to change the direction of motor rotation

direction, you can close VT1, VT2, and make VT3 and VT4 work alternately.


3 DC servo motor-Control Loop

3 DC servo motor-Closed-loop control

• Servo motor should be able to provide enough

power to the load, according to the need regular

exercise.

• Therefore, servo motor output torque, speed and

power should be able to meet the requirements of

drag load movement, control characteristic shall

ensure the required speed range and torque range.

3 DC servo motor-How to choose

Design of DC motor servo drive system based on

L298 IC

L298 DRIVER IC

DC motor,

encoder interface

Motor

driving

module

Motor Module

Enable switch

L298

DC motor with the screw

According to the input-output relationship of L298. Enable

control terminal ENA connects to a manual switch. When port

ENA is high, the PWM signal input terminal In1 and In2 can

control the motor.

•When In1 is PWM signal, Output In2 is low, motor move forward;

•When In2 is PWM signal, Output In1 is low, motor move reversely ；

•When ENA is low, The four transistors in the drive axle are all off , which

make motor stop.

Motor speed is determined by the duty cycle of the PWM

signal, which is adjusted by microcontroller.

L298 control DC motor

Take Atmega64 develop suit as example.

Introduce the method that L298 control motor.

Hardware Design

Microcontroller, driver circuit, direction-distinguishing circuit,

limit switch

Hardware Design

•Before the motor experiment, ensure that all interfaces are

connected, then turn on the power.

•When using the motor module, first turn on the power switch, and

then open the L298 enable switch; to close, turn off the L298 enable

switch first, then turn off the power switch.

L298

enable switch

Power

Switch

Operating instructions

PWM DC motor speed control

AVR microcontroller output pulse width modulation (PWM)

signal need to be subjected to a power amplifier to drive the

motor. L298 has two kinds of working mode of unipolar and

bipolar.

Unipolar operation mode refers to a PWM cycle, the motor

armature only withstand unipolar voltage; bipolar mode of

operation is the voltage of the motor armature in a PWM cycle

were positive and negative alternantly.

Speed control system uses unipolar work mode. PB4 and PB5

respectively connect with In1, In2 pin of L298, use the output

compare unit of Timer0 or 1, you can control the the motor

speed, direction of rotation.

RIkRIEU e

Voltage balancing of DC servo loop

equation

•式中：U applied voltage；R total loop resistance of armature；I current in armature

•物理意义：applied voltage on the armature circuit

resistance consumed，second part is used to generate the

motor speed。More current in the armature circuit, lowers

the speed.。


ke and km are the structural parameters of the

motor itself, the relationship between the two is

as follows

MM60

π2025.1 k

gkke


Effects of inductance and inertia to characteristics of

the motor•When the control voltage is applied to the DC servo motor, because

of the winding inductance, the current is not established immediately.

•More importantly, when the electromagnetic torque overcome and

exceed the load torque, the inertia of the motor shaft due (including

the inertia of the load), the speed of the motor is not built up

instantly.

•The establishment of current and speed requires a certain time, the

former is called electromagnetic time constant of the motor (Ta), and

the main inductance of the windings; the latter is called an

electromechanical motor time constants (TM), which mainly

depends on the inertia of motor axis ; TM is generally far greater

than Ta.


• After considering the winding inductance and motor inertia,

the motor balance equation is:

tJMM

ikM

t

iLRiEU

d

d

d

d

c

M

a

La－－Loop inductance;

J －－Moment of inertia of the motor shaft;

i－－ Instantaneous current of armature circuit;

ω－－The instantaneous speed of the motor.


• Variable voltage control:

• Variable magnetic control；

• Mixing the first two .

RIkRIEU e

3 DC servo motor- Speed control method

Variable voltage control

If the moment of inertia of the motor Jm and armature

resistance Ra are very small and negligible , there is ：

U=ke ω

The motor speed ω is proportional to the armature voltage U.
