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).
Types and characteristics of the drive elements –
Examples of Pneumatics
• Cylindrical Muscles.
Types and characteristics of the drive elements –
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.
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.
DC Servo Motor – Working principle
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
DC Servo Motor – Working principle
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
DC Servo Motor – Working principle
• 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。
DC Servo Motor – Working principle
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.
3 DC servo motor-Working principle
• 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.
3 DC servo motor-Working principle
• 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)
3 DC servo motor-Variable voltage control
•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
3 DC servo motor-Variable voltage control
•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-Variable voltage control
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.。
DC Servo Motor – Working principle
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.
3 DC servo motor-Working principle
• 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.
3 DC servo motor-Working principle
• 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)
3 DC servo motor-Variable voltage control
•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
3 DC servo motor-Variable voltage control
•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-Variable voltage control
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.
Thank you