Automation Building Blocks

1
Automation Building Blocks By Ed Red
Sensors Analyzers Actuators Drives Vision systems
2

• Objectives
• To review basic building blocks for implementing
  automation
• To consider application conditions
• To introduce assessment criteria
• To test understanding of the material presented

3

• Building Blocks
• Sensors
• Analyzers
• Actuators
• Drives
• Vision system (integrated sensor/analyzer)

4

• Building Blocks sensor features
• Accuracy and repeatability
• Precision
• Range
• Response time
• Calibration methods
• Minimum drift
• Costs and reliability
• Sensitivity

5
Building Blocks sensors moving objects
We can characterize a sensors capability by its
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 sensors specification, how might
we determine how fast a moving object might move
past the sensor and the sensor still read the
object parameter correctly?
6

• Building Blocks sensor devices
• See text for in depth description!
• Photoelectric sensors
• Proximity switches (inductive and capacitive)
• Range sensors (ultrasonic/acoustic, laser
  reflectors)
• Transducers (encoders)

7
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 shafts angular
position? Ring Angle (deg) Pattern Value
(deg)1 180 1 1802 90 03 45 04 22.5 1
22.55 11.25 06 5.625 1
5.6257 2.8125 1 2.81258 1.40625 0
Total 210.94
8

• 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

9
Building Blocks present drives
Controller
Servocard
Motion Planning Control
Servo-loops
Application
Amplifiers
Set Points
10
Building Blocks digital drives

• Microprocessors and Digital Signal Processors
  (DSPs) 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• EIA RS-431, the outdated 10V standard, no longer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 high- speed
  network between the master host (a PC) and the
  distributed digital slave devices.

11
Building Blocks digital drives
12
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
• PWM frequency must be high enough so that motor
  cannot respond to a single PWM signal

If direction is to be changed, requires another
PWM signal.
13
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) ( 10 V)
R
Variable(or Voltage)
Time
14
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) Quantitizations Bit (on
or off) Value 6.8 V 5 1 5 1.8 2.5 0 1.8
1.25 1 1.25 0.55 0.625 0 0.55 0.3125
1 0.3125 0.2375 0.15625 1 0.15625 0.08125
error 6.719 V
15
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
16
Electromagnetism
B
F I l x B
I
I
F
l
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 (Faradays 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 L inductance in
henrys
17
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
barscast-aluminum or copper conductors shorted
at one end and positioned parallel to the shaft
(see photo).
The motors 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
stators rotating magnetic field.
18
(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).
19
(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
self-synchronous 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.
20
DMAC
21
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?

22
Building Blocks Assessment

1. What does torque speed curve look like for the
   motors typically used to control robots?
2. What is difference between absolute encoder and
   relative encoder? How do encoders measure
   directional changes?
3. What is difference between a resolver and digital
   encoder?
4. Costs of sensors, motors, etc.?
5. How do the new linear drives work, and what are
   their response characteristics?

23
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 pattern recognition, part inspection,
or part positioning and orientation.Ed Red
24
Building Blocks machine vision
Equipment

• Computer
• Frame grabber
• Camera (CCD array)
• Lenses
• Lighting
• Calibration templates
• Algorithms

Types
Front Back Side Structured Strobe
25
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
26
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.)
27
Machine Vision applications
Dimensional measurement Object verification
Proper position/orientation Flaws and
defects Counting Guidance and control (offsets,
tracking)
28
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
  148all 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.

29
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. What equations would you apply?
Y
X
30
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?

31
Building Blocks
What have we learned?