Download Mechanical Actuation System

Electrical Actuation System
Lecture 9
(Chapter 9)
Introduction
• The electrical systems used as
actuators are:
1. Switching devices – mechanical
switches and solid-state switch
2. Solenoid type device
3. Drive system – motors (AC, DC,
Stepper)
SME 3252: Mechatronics
Lecture 7
7.5: DC Motor
• Has six basic parts -- axle, rotor (a.k.a., armature),
stator, commutator, field magnet(s), and brushes
• The stator is the stationary part of the motor -- this
includes the motor casing, as well as two or more
permanent magnet pole pieces
• The rotor (together with the axle and attached
commutator) rotates with respect to the stator. The
rotor consists of windings (generally on a core),
the windings being electrically connected to the
commutator
SME 3252: Mechatronics
Lecture 7
SME 3252: Mechatronics
Lecture 7
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/motdc.html
7.5.1: DC Motor Operation
SME 3252: Mechatronics
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/motdc.html Lecture 7
a) Current in DC Motor
SME 3252: Mechatronics
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/motdc.html Lecture 7
b) Magnetic Field in DC Motor
SME 3252: Mechatronics
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/motdc.html Lecture 7
c) Force in DC Motor
SME 3252: Mechatronics
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/motdc.html Lecture 7
d) Torque in DC Motor
SME 3252: Mechatronics
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/motdc.html Lecture 7
Voltage
• DC motors are non-polarized - can reverse
voltage without any bad things happening
• Typical DC motors are rated from about 6V-12V
• Motors operate at different voltages because
voltage is directly related to motor torque
• More voltage, higher the torque
• A DC motor is rated at the voltage it is most
efficient at running
• Apply too few volts, it just won’t work. If apply
too much, it will overheat and the coils will melt
• General rule - try to apply as close to the rated
voltage of the motor
• Recommendation - do not surpass 12V
SME 3252: Mechatronics
Lecture 7
Torque
• two torque value ratings given: operating
torque - the torque the motor was designed
to give and stall torque - the torque required
to stop the motor from rotating
• Torque ratings can change depending on the
voltage applied
SME 3252: Mechatronics
Lecture 7
3 ways to brake DC motors
• Controls Method
Requires an encoder placed onto a rotating part of DC motor. Write
an algorithm that determines the current velocity of motor, and
sends a reverse command to H-bridge until the final velocity equals
zero
• Mechanical Method
The mechanical method is what is used on cars. Something with
very high friction and wear resistance is needed, e.g. a servo
actuated brake l.
• Electronic Method
The least reliable, but the easiest to implement. The basic concept
of this is that if the power is shorted and ground leads motor, the
inductance created by motor in one direction will power motor in the
opposite direction. Although motor will still rotate, it will greatly resist
the rotation.
SME 3252: Mechatronics
Lecture 7
• Connect a MOSFET
(transistor) and a relay as
shown
• The MOSFET turns on the
relay, which creates a short
between the motor leads
• Turn the MOSFET on (set C
high) with microcontroller to
brake the motor
• Basically the motor will still
have an H-bridge for normal
control
• When braking, the H-bridge
is turn off and use the
braking circuit
SME 3252: Mechatronics
Lecture 7
7.5.4: Control of d.c. motor
• Control d.c. motor speed from microprocessor –
pulse width modulation or PWM
• Taking a constant dc supply voltage and
chopping it so that average voltage is varied
• PWM can be obtained using basic transistor
circuit
• Transistor is switch on and off based on signal
applied to its base (for one direction movement)
• H circuit (four transistors) – enable motor to be
in forward and reverse direction
SME 3252: Mechatronics
Lecture 7
SME 3252: Mechatronics
Lecture 7
SME 3252: Mechatronics
Lecture 7
Closed-loop control
•
Closed-loop control – use feedback to
control motor speed if conditions change
a) Feedback signal by tachogenerator which gives analog signal
b) Feedback signal by encoder – digital
signal
c) System is digital with PWM is used to
control average voltage applied to
armature
SME 3252: Mechatronics
Lecture 7
SME 3252: Mechatronics
Lecture 7
SME 3252: Mechatronics
Lecture 7
FINAL PROTOTYPE
PIC Microcontroller
Motor Driver
Analog Distance Sensor
DC Geared Motor
IR Sensor
SME 3252: Mechatronics
Lecture 7
Advantage and disadvantage of
d.c. motor
SME 3252: Mechatronics
Lecture 7
7.6: AC Motor
• AC motors can be classified into 2 – single
phase and three phase AC motors
• To have 3 phase power on a robot, use
either big and expensive DC to AC
converter, or tether it to a wall socket
• AC motors is used if the robot is
stationary, such as a robot arm
SME 3252: Mechatronics
Lecture 7
Voltage
• Polarized (current cannot be reversed)
• Typically from 120-240V AC, usually to
match mains power
• Higher voltages generally mean more
torque, but also require more power
• Rarely used on mobile robots due to
power requirements
SME 3252: Mechatronics
Lecture 7
Current
•
1.
2.
•
•
•
•
•
2 types of current: stall and operating current (max and
minimum)
Stall Current - The current a motor requires when
powered but held so that it does not rotate
Operating Current - The current draw when a motor
experiences zero resistance torque
It is best to determine current curves relating voltage,
current, and required torque for optimization
When a motor experiences a change in torque (such as
motor reversal) expect short lived current spikes
Current spikes can be up to 2x the stall current, and
can fry control circuitry if unprotected
Use diodes to prevent reverse current to your circuitry
Check power ratings of your circuitry and use heat
sinks if needed
SME 3252: Mechatronics
Lecture 7
Torque
• When buying a motor, consider stall and
operating torque (max and minimum)
• Stall Torque - The torque a motor
requires when powered but held so that it
does not rotate
• Operating Torque - The torque a motor
can apply when experiencing zero
resistance torque
SME 3252: Mechatronics
Lecture 7
Control method
• Modifying the AC frequency can alter speed and
torque
• Encoder - device which counts rotations of wheel
or motorshaft to determine velocity for a control
feedback loop
• Tachometer - device which measures current
draw of motor to control output torque
• To maintain constant torque at diff speed when
freq. varied, a.c. in converted to d.c. by a
converter and converted back to a.c. by inverter
SME 3252: Mechatronics
Lecture 7
SME 3252: Mechatronics
Lecture 7
Flexible
Coupling
Magnetic
Brake
inverter is to vary the input voltage
(stator voltage) into the induction
motor
Inductive
Proximity Sensor
Timing Plate
Extension Shaft
PSM student’s project – Wong Kien Fatt
SME 3252: Mechatronics
Lecture 7
Advantage and disadvantage of AC
motor
• Advantage – more rugged, reliable and
maintenance free
• Disadvantage – more complicated
compared to d.c. motor,
SME 3252: Mechatronics
Lecture 7
7.7: Stepper Motor
• Stepper motors come in two main types –
Variable Reluctance and Permanent
Magnet.
Permanent Magnet (PM or tin-can)
SME 3252: Mechatronics
Lecture 7
How it works (VR)
1. The upper
electromagnet is
activated and the teeth of
the central cog line up
accordingly.
2. The upper
electromagnet is
deactivated and the right
one turned on. The closest
cog teeth then jump to line
up with this. This causes a
step (e.g. 1.8° turn).
3. The right electromagnet is
deactivated and the lower
one is turned on. The cog
teeth then jump to line up
with the bottom
electromagnet. This causes
another step.
4. The bottom electromagnet is
deactivated and the left-most one
turned on. The cog teeth then jump to
line up with this. This causes another
step. On a motor which has a step
angle of 1.8°, 200 steps are required
for a full rotation.
SME 3252: Mechatronics
Lecture 7
http://www.societyofrobots.com/member_tutorials/node/28
Advantages and disadvantages
of stepper motor
SME 3252: Mechatronics
Lecture 7
Servo Motor
• DC motors with built in gearing and feedback control
loop circuitry, require no driver
• Extremely popular with robot
• Most servo motors can rotate about 90 to 180 degrees or
a full 360 degrees or more
• unable to continually rotate – e.g. driving wheel
• precision positioning makes them ideal for robot arms and
legs, rack and pinion steering
• To use a servo, simply connect the black wire to ground,
the red to a 4.8-6V source, and the yellow/white wire to a
signal generator (e.g. from microcontroller)
• Vary the square wave pulse width from 1-2ms and your
servo is now position/velocity controlled.
SME 3252: Mechatronics
Lecture 7
SME 3252: Mechatronics
Lecture 7
Servo wiring
• All servos have three wires - Black or Brown is for
ground, Red is for power (~4.8-6V), Yellow, Orange, or
White is the signal wire (3-5V).
• Servo Voltage (Red and Black/Brown wires)
• Typical operation is from 4.8V to 6V
• Recommended is 6V, higher torque
• Signal Wire (Yellow/Orange/White wire)
• Signal wire is what you use to command the servo
• Simply send an ordinary logic square wave to servo at a
specific wave length, and the servo goes to a particular
angle (or velocity if your servo is modified)
• The wavelength directly maps to servo angle.
SME 3252: Mechatronics
Lecture 7
Voltage Regulator
Diode
(IN4001)
IN
OUT
7805
Battery Source
9V
330 uF
10 uF
GND
5V
Connect to
Microcontroller
0.1 uF
From Voltage
Regulator
(7805)
Microcontroller power source
circuit schematic
5V
10kΩ
VDD
MCLR
Switch
RA0
Start
Button
30pF
RB7
Servo 1
RB6
Servo 2
RB5
Servo 3
RB4
Servo 4
RB3
Servo 5
RB2
Servo 6
RB1
Servo 7
RB0
Servo 8
OSC1
30pF
OSC2
Crystal
20MHz
18F452
Led 9
RC3
Led 10
RC2
RD0
RD1
RD7
Led 1
RD6
Led 2
RD5
Led 3
RD4
Led 4
RX
Connect to Boatloader, 9
TX
Connect to Boatloader, 10
RC5
Led 5
RC4
Led 6
RD3
Right Sensor
RD2
Left Sensor
Limit
SME 3252:Switch
Mechatronics
Lecture 7
Vss
Led 7
Led 8
Square wave signal
• The standard time vs. angle is
represented in this chart:
SME 3252: Mechatronics
Lecture 7
Advantages and disadvantages
of servo motor
SME 3252: Mechatronics
Lecture 7
6V / 4.5AH
battery for all
outputs
Sensor system
Servo motor
9V battery for
microcontroller
SME 3252: Mechatronics
Lecture 7
End of Lecture 9