Transcript ME 486 - Automation Building Blocks

Automation Building
Blocks
Sensors
Analyzers
Actuators
Drives
Vision systems
ME 486 - Automation
By
Ed Red
Objectives
• To review basic building blocks for implementing automation
• To consider application conditions
• To introduce assessment criteria
• To test understanding of the material presented
ME 486 - Automation
Building Blocks
• Sensors
Inductive proximity
sensors
• Analyzers
• Actuators
• Drives
• Vision system (integrated sensor/analyzer)
ME 486 - Automation
Building Blocks – sensor features
• Accuracy and repeatability
• Precision
• Range
• Response time
• Calibration methods
• Minimum drift
• Costs and reliability
• Sensitivity
ME 486 - Automation
Building Blocks – sensors & moving objects
We can characterize a sensor’s capability
by it’s operating frequency or by its
response time. Both determine how well the
sensor might measure the desired property
(proximity, length…) of a moving object.
Using a sensor’s specification, how might
we determine how fast a moving object
might move past the sensor and the sensor
still read the object parameter correctly?
ME 486 - Automation
Building Blocks – sensor devices
See text for in depth description!
• Photoelectric sensors
Inductive proximity
Linear
sensors
• Proximity switches (inductive
and capacitive)
encoder
Absolute rotary
encoder
• Range sensors (ultrasonic/acoustic, laser reflectors…)
• Transducers (encoders)
ME 486 - Automation
Building Blocks – analyzers
Encoder example – An absolute optical encoder has 8 rings, 8 LED
sensors, and 8 bit resolution. If the output pattern is 10010110, what is the
shaft’s angular position?
Ring
1
2
3
4
5
6
7
8
Angle (deg)
180
90
45
22.5
11.25
5.625
2.8125
1.40625
Pattern Value (deg)
1
180
0
0
1
22.5
0
1
5.625
1
2.8125
0
Total 210.94
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Building Blocks – drives
• Stepper Motors (index by open-loop control)
• AC/DC servomotors (PID feedback control, holds torque
when at rest)
• Kinematic devices (intermittent
operation, e.g., geneva mechanism)
• Digital drives
ME 486 - Automation
Building Blocks – present drives
Controller
Servocard
Application
ME 486 - Automation
Motion
Planning
&
Control
Servo-loops
Building Blocks – digital drives
• Microprocessors and Digital Signal Processors (DSP’s) are replacing
analog components with digital components (i.e., digital drives).
•EIA RS-431, the outdated ±10V standard, no longer need constrain control
resolution.
• Revolutions in computer operating systems, applications, and
networking.
• Networking standards, such as IEC 61491 and IEEE 1394, are changing
motion control architectures and hardware configurations.
• Need for A/D and D/A interfaces is rapidly declining, being replaced by a highspeed network between the master host (a PC) and the distributed digital
slave devices.
ME 486 - Automation
Building Blocks – digital drives
DMAC
Ormec’s servowire implementation of IEEE 1394
ME 486 - Automation
PWM and digital drives
(binary control!)
PWM – Pulse Width Modulation - a constant frequency, two-valued
signal (e.g., voltage) in which the proportion of the period for which the
signal is on and the period for which it is off can be varied.
• Percentage of time on is called the duty cycle.
• Voltage value will depend on the application
If direction is to be changed,
requires another PWM signal.
• PWM frequency must be high enough so that motor cannot respond
to a single PWM signal
On
25% duty cycle
Off
On
50% duty cycle
Off
T
2T 3T 4T
ME 486 - Automation
T
2T 3T 4T
A/D Signal Conversion
Resolution of A/D is represented by number of conversion bits n:
Nq = number of quantitization levels = 2n
R = conversion resolution = Voltage range/(Nq – 1)
±R
Variable
(or Voltage)
Time
ME 486 - Automation
(± 10 V)
A/D Signal Conversion
Successive approximation method is similar to the method we used
to extract the encoder value from the binary output but
backwards. Here is simple example:
Range (± 10 V)
6.8 V
1.8
1.8
0.55
0.55
0.2375
0.08125 = error
ME 486 - Automation
Quantitizations Bit (on or off)
5
1
2.5
0
1.25
1
0.625
0
0.3125
1
0.15625
1
Value
5
1.25
0.3125
0.15625
6.719 V
D/A Signal Conversion
The decoding equation is:
Eo = Eref [0.5 B1 + 0.25 B2 + 0.125 B3+…+(2n)-1Bn]
where
Eo = output analog signal value
Eref = ref voltage
For example: 10010 means B1 = 1, B2 = 0, B3 = 0, B4 = 1, B5 = 0
ME 486 - Automation
Electromagnetism
B
F=IlxB
I
F
l
I
B
Current flow produces magnetic field and associated flux.
Changing field (flux) through a coil induces a reactive electromotive force (emf) e:
e = -N dF/dt
(Faraday’s Law)
N = # turns in coil; F is flux in webers
This in turn generates an induced current in opposite direction and a resulting opposing flux
as described by:
e = -L di/dt
ME 486 - Automation
L = inductance in henrys
AC motors
Stator structure is composed of steel laminations
shaped to form poles around which are wound
copper wire coils. These primary windings connect
to, and are energized by, the voltage source to
produce a rotating magnetic field. Three-phase
windings spaced 120 electrical degrees apart are
popular in industry.
Rotor (or rotating secondary) is another assembly
of laminations over a steel shaft core. Radial slots
around the laminations’ periphery house rotor
bars—cast-aluminum or copper conductors
shorted at one end and positioned parallel to the
shaft (see photo).
The motor’s name comes from the alternating current (ac) “induced” into the rotor by the
rotating magnetic flux produced in the stator. Motor torque is developed from interaction of
currents flowing in the rotor bars and the stator’s rotating magnetic field.
ME 486 - Automation
(new tech)
Linear motors
Two basic classes: 1) permanent magnet (PM) brushless,
and 2) asynchronous linear induction motors (LIMs).
PM brushless motors abound in various subclasses, such
as the moving coil and moving magnet types. Ironless
refers to a core containing only copper coils (and epoxy
encapsulation). Smooth "cog-free" motion is produced
since no attractive force exists between coil and magnet-but at the cost of lower force output.
Tubular linear motor
Slot-less refers to a special design of steel laminations where the windings go through holes in the
stator rather than slots. The result is a smoother surface facing the magnet. This design
also reduces cogging by eliminating variation in attractive force.
Tubular linear motors roll up the unit about an axis parallel to its length. In one style, an outer
thrust block carrying the motor coils envelops and moves along a stationary thrust rod that
houses magnets. Another style has a central rod with magnets that moves relative to an outer
stator member. Travel is limited since the thrust rod must be supported at both ends (or at one
end for the moving-rod version).
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(new tech)
Switched reluctance motor
Reluctance - opposition of a material to magnetic lines of force
Both stator and rotor of the switched reluctance motor have
projecting poles. In the image, poles 1 and 1' are energized.
These are wired in series. The rotor has no permanent magnets
or windings. Thus when one of the four phases of the stator is
energized, the closest set of poles of the rotor (made up of
reluctance magnets) are pulled into alignment. By turning off
phase 1 and energizing phase 2, you can visualize how the rotor
will rotate 15' CCW to align the rotor poles closest to phase 2.
A four phase converter capable of accepting feedback is used to energize the coils in order to
control the switched reluctance motor. The feedback is necessary to run the motor in selfsynchronous mode, which enables a continuous smooth speed operation. By energizing the phases
in reverse sequence, the motor can also run CW. The switched reluctance motor along with the
four phase converter are meant to be used as a precise speed control device, and they are
approximately 2% more efficient than the other AC speed control systems.
ME 486 - Automation
Control Servo
DCI
Set Points
Motion Planning
& Control
CPU 2
RTOS
Controls Application
CPU 1
IEEE 1394,
USB2,
Fiber Optic,
etc.
Windows Application
PC
DMAC
ME 486 - Automation
Digital Drive
Network
Building Blocks Assessment
1. Who are major vendors of proximity switches,
servomotors?
2. What are the limits to sensor proximity distances?
3. What types of proximity accuracies might you expect
from proximity sensors?
4. Which sensors work on which materials?
5. Are sensors affected by speed by which materials move
past them?
6. What are weight to torque ratios for common
servomotors?
ME 486 - Automation
Building Blocks Assessment
7. What does torque speed curve look like for the
motors typically used to control robots?
8. What is difference between absolute encoder and
relative encoder? How do encoders measure
directional changes?
9. What is difference between a resolver and digital
encoder?
10. Costs of sensors, motors, etc.?
11. How do the new linear drives work, and what are
their response characteristics?
ME 486 - Automation
Building Blocks – machine vision
Definition – “Machine vision is the capturing of an image (a
snapshot in time), the conversion of the image to digital
information, and the application of processing algorithms to
extract useful information about the image for the
purposes of
Algorithm
pattern recognition, part inspection, or part positioning
and
PC
orientation”….Ed Red
ME 486 - Automation
Building Blocks – machine vision
Equipment:
• Computer
• Frame grabber
Types:
• Camera (CCD array)
Front
• Lenses
Back
• Lighting
Side
• Calibration templates
Structured
• Algorithms
Strobe
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Machine Vision – structured lighting
Structured Lighting is used in a front
lighting mode for applications requiring
surface feature extraction. Structured
lighting is defined as the projection of a
crisp line of light onto an object. The
patterned light is then used to determine
the 3-D characteristics of an object from
the resulting deflections observed.
Note the non-typical
approach of
projecting a grid
array of light on an
object to detect
features
ME 486 - Automation
Machine Vision – image processing
Segmentation – Define and separate regions of interest
Thresholding – Convert each pixel into binary (B or W) value
by comparing bit intensities
Edge detection – Locate boundaries between objects
Feature extraction – Determine features based on area and
boundary characteristics of image
Pattern recognition – Identify objects in midst of other objects
by comparing to predefined models or standard values (of
area, etc.)
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Machine Vision – applications
Dimensional measurement
Object verification
Proper position/orientation
Flaws and defects
Counting
Guidance and control (offsets, tracking)
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Machine Vision – example
• 8-bit image of metallic iron as it appears in iron
ore (lighter objects in the image represent the metallic iron)
• Histogram displays pixel intensity distribution
…background appears at gray level 40, ore shows
up at gray level 70, and high-intensity iron turns
up at gray levels above 150. Image clearly
differentiates components.
• Blob analysis - set threshold to gray level 148…all
the pixels with gray levels of 148 or lower get set to
zero. Pixels with gray levels of 149 or higher get set
to one
• Morphology functions slightly change or eliminate
the shapes of objects so imaging software can
easily count them.
ME 486 - Automation
Machine Vision – example
Suppose we wish to calculate the area
and centroid of the selected binary
region in the last figure, how would
you do it? Assume that you have a
camera such that the pixels are
square and you have a matrix of
pixel values as depicted in the figure
shown.
Y
What equations would you apply?
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X
Machine Vision Assessment
1.
Who are major vendors of vision systems and the various
components?
2.
What are typical camera resolutions?
3.
What are typical camera calibration techniques?
4.
What is camera distortion?
5.
Is color vision imaging used? In what applications?
6.
How long does it take to process images? As a function of image
processing function?
7.
What are typical costs for imaging systems? For frame grabbers,
cameras, lenses, lighting?
ME 486 - Automation
Building Blocks
What have we learned?
ME 486 - Automation