And

1
Ultra Sonic Rangers tell the robot how far away
objects are.

• And

Light sensors measure light intensity.
Sensors for a robot
Heat Sensors which measure temperature.
1. Resistive 2. Infra-red 3. Light 4. Sonar 5.
Other
Part 1. Resistive Sensors
gyroscopes tell the robot which direction is up.
Based on book by Fred Martin
Touch sensors tell the robot when it bumps into
something.
2
The simplest possible use of sensors

• The diagram serves to illustrate the general case
  of sensing a specific phenomenon. In this case it
  is the presence or absence of light.
• The sensor in this case is a photo-resistor.
• When sufficient light strikes it, its internal
  resistance is reduced to several hundred Ohms.
• When no light strikes it its resistance is
  typically several million Ohms.

light
Remember Breitenbergs Vehicles?
3
What Is a Sensor?

• Anything that detects the state of the
  environment.
• For instance, we already used sensors in the
  Braitenberg vehicles.
• Are the following, sensors?
• Positioning devices
• Encoders
• Vision
• Mine detectors (detector vs. sensor)

The material presented in our textbook and here
relates to HandyBoard, but the same principles
are true for Robix, Lynxmotion, Lego, etc. Read
the manuals.
4
What can a robot do without sensing?

• Simple Sensors
• Can be used without much processing
• Still require electronics (and connectors)
• The basic electronics laws to know
• Ohm's law
• combining resistance
• dividing voltage

Review from ECE 201
5
What you (and the robot) can do without sensors?

• Close your eyes. Plug your ears. Hold your nose.
  Tie your hands behind your back.
• Shut your mouth. Tie your shoelaces together.
  Spin yourself around a few times.
• Now walk. How does it feel? That's exactly what
  your robot feels nothing - without sensors.
• You have been given many types of sensors that
  can be used in a variety of ways to give your
  robot information about the world around it.
• We will explain each of the sensors you can find
  in the lab, how it works, what it's good for, and
  how to build it.

6
Biological Analogs

• All of the sensors we describe in this and next
  parts exist in biological systems
• Touch/contact sensors with much more precision
  and complexity in all species (spiders?)
• Polarized light sensors in insects and birds
• Bend/resistance receptors in muscles
• and many more...

7
You have to understand sensors

• Before we can teach you what sensors do, we need
  to make one point very clear
• Sensors are not magical boxes.
• All information you get from sensors must be
  decoded by you, the human builder and programmer.
• Sensors convert information about the environment
  into a form that can be used by the computer.
• The sensors that are on the robot can be related
  to sensors found in humans.
• Touch sensors embedded in your skin, visual
  sensors in your retina, and hair cells in your
  ears convert information about the environment
  into neural code that your brain can understand.
• Your brain needs to understand the neural code
  before you can react.
• Since you will be programming the robot, you will
  need to understand the output of the sensors
  before you can program your robot to react to
  different stimuli.
• Learn about sensors in animals and think how to
  use this knowledge in your projects.

8
Some types of Sensors

• Ladar (laser distance and ranging)
• Time of flight
• Phase shift
• Sonar
• Radar
• Infra-red
• Light sensing
• Heat sensing
• Touch sensing

9
Sensors and their use

• Topics to be discussed
• What are sensors?
• Types of sensors (many examples)
• Sensor complexity
• Signals -gt symbols
• Levels of processing
• Poor and good design of perception
• Biological perception and lessons
• Sensor fusion

Im Mr. Sensitivity. . .
Not every quarter
10
Gas Sensor
Gyro
Accelerometer
Metal Detector
Pendulum Resistive Tilt Sensors
Piezo Bend Sensor
Gieger-Muller Radiation Sensor
Pyroelectric Detector
UV Detector
Resistive Bend Sensors
CDS Cell Resistive Light Sensor
Digital Infrared Ranging
Pressure Switch
Miniature Polaroid Sensor
Limit Switch
Touch Switch
Mechanical Tilt Sensors
IR Sensor w/lens
IR Pin Diode
Thyristor
Magnetic Sensor
Polaroid Sensor Board
Hall Effect Magnetic Field Sensors
Magnetic Reed Switch
IR Reflection Sensor
IR Amplifier Sensor
IRDA Transceiver
IR Modulator Receiver
Radio Shack Remote Receiver
Solar Cell
Lite-On IR Remote Receiver
Compass
Compass
Piezo Ultrasonic Transducers
11
What are the types of Sensors?

• Active
• send signal into environment and measure
  interaction of signal w/ environment
• e.g. radar, sonar
• Passive
• record signals already present in environment
• e.g. video cameras
• GPS

In our lab we used infrared, light (photodiodes,
phototransistors), compass, volt and
amperometers, ions, pH, magnetic, temperature,
voice, sound, camera, sonars and of course all
resistance based micro-switches and pads..

• We will discuss sensing using the following
• Touch
• Active Light
• Passive Light
• There are many more ways
• (sound, heat, magnetic field, smell...)

12
Passive versus Active Sensors

• All of the sensors that will be presented in this
  part are passive in that the stimulus, i.e., the
  physical property, they were measuring, comes
  from the environment.
• In contrast, active sensors provide their own
  signal/stimulus (and thus typically require extra
  energy), and use its interaction with the
  environment as the property to be measured.
• Active sensors include
• reflectance and break-beam infra-red (IR)
  sensors,
• ultrasound sensors,
• laser range finders,
• and others.
• They will be presented in next parts.

13
How to Choose a Sensor?

• There are four main factors to consider in
  choosing a sensor.
• Cost
• sensors can be expensive
• you can buy cheap sensors but often without good
  documentation
• knowing main principles and experimentation is
  useful when you purchase such sensors (usually
  military old sensors)
• Environment
• there are many sensors that work well and
  predictably inside, but that choke and die
  outdoors.
• Range
• Most sensors work best over a certain range of
  distances.
• If something comes too close, they bottom out,
  and if something is too far, they cannot detect
  it.
• Choose a sensor that will detect obstacles in the
  range you need.
• Field of View
• depending upon what you are doing, you may want
  sensors that have a wider cone of detection.
• A wider field of view will cause more objects
  to be detected per sensor
• But it also will gives less information about
  where exactly an object is when one is detected.

Tell our stories about sensors in lab as examples
14
Types of Sensors according to their purpose in a
robot

• Exteroceptive deal with external world
• where is something ?
• how does is look ? (camera, laser range-finder)
• Proprioceptive deal with self
• where are my hands ? (encoders, stretch
  receptors)
• am I balanced ? (gyroscopes)

No experience yet

• Interoceptive
• what is my thirst level ? (biochemical)
• what is my battery charge ? (voltmeter)

15
Try to understand sensors practically

• Take time to play with each of the sensors you
  find in the lab, especially in Lego, Lynxmotion
  and Robix kits.
• Figure out how they work.
• Look at the range of values they return.
• Check under what conditions they give those
  values.
• Look to code of previous students related to
  sensors.
• The time you spend here will greatly ease your
  integration of hardware and software later.
• The better you understand your sensors, the
  easier it will be for you to write intelligible
  control software that will make your robot appear
  intelligent.
• So as you read about the sensors, you should
  assemble a bunch of sensors as shown in Webpages
  of previous classes.

16
There can be no feedback without sensors!
Remember this!!

• Sensors provide feedback to your program about
  the environment.
• Feedback is important in any controlled
  situation.
• So far, we were discussing mostly open-loop, or
  timed programs that simply follow a pattern
• but have no real knowledge of the world.
• Sensors can provide the feedback necessary to let
  a robot make decisions about how to act in its
  environment.
• They will make these programs smarter.
• The feedback mechanism is very important in an
  environment that is continually changing.

17
There can be no feedback without sensors

• During the rounds of the contest, the objects on
  the playing field will be changing their location
  (i.e., the other robot moves, the drawbridge
  closes, or you bump into a block). Robot soccer,
  robot theatre
• We strongly encourage you to use closed-loop
  feedback design when planning and implementing
  your strategy.
• There will be a smaller chance of random errors
  completely messing up your game if you use
  sensors wisely.
• Read Chapter 6 of Martin about sensors.
• Read Chapter 8 of Martin for more information on
  the control problems you may encounter.

18
Electric Sensorsdigital
19
Example of Sensor Interfacing
Handy Boards Sensor Input Banks

• Each sensor ports provides three signals to the
  sensor
• 5v power - middle row
• Ground - lower row
• Sensor signal line - upper row
• Not all sensors require 5v power, e.g., switches
  and photocells may be wired between sensor signal
  and ground lines

• Handy Board has two banks for sensors
• Digital inputs, numbered 15 to 7 on the left
• Analog inputs, numbered 6 to 0 on the right

20
Location of Digital and Analog Ports on the
HandyBoard

• The digital ports on the main board are labeled
  from 0-7.
• There are also four analog ports on the main
  board, but when you use the expansion board, the
  analog ports get remapped to the connectors on
  the right side of the expansion board.
• The ports are all arranged in the same format.
• The innermost row of pins are the signals,
  followed by a space, then microprocessor power,
  and finally on the outer side is the ground.

21
Analog versus Digital Sensors

• In all our robotics kits the sensors are digital
  or analog.
• For instance, in HandyBoard, analog sensors can
  be plugged into the analog sensor ports, which
  return values between 0 and 255.
• Digital sensors can be plugged into either the
  digital ports or the analog ports, but will
  always return either 0 or 1.
• ANALOG 0 lt x lt 255
• DIGITAL 0 or 1
• Each type of sensor has its own unique uses.
• Think about new uses, not shown in these slides
  and tell me your ideas. May be we will use them.

22
Figure 5.1 Generic Digital Sensor Schematics.
23
Digital Sensors

• Digital inputs all have pull-up resistors
  connected to them as shown in Figure 5.1.
• Digital switches are wired such that the sensor
  is wired across the signal pin and ground.
• This means that when the digital sensors is
  closed, the signal is grounded or LOW.
• When the switch is open, the signal pin outputs
  5V, or HIGH.
• This value is INVERTED by software, so reading
  the digital port with the switch open returns 0,
  while reading the digital port with the switch
  closed returns 1.
• With nothing plugged in, the value of a digital
  port should be 0.
• Digital sensors can be used in the analog ports
  on the Controller board (such as 6.270 board) as
  well, relieving any restrictions the small number
  of digital inputs may cause.

24
Digital Sensors used in Analog Parts of the
HandyBoard

• In this board, for instance, the typical analog
  values for digital sensors are somewhat above 250
  for an open switch, and less than 20 for a closed
  switch.
• When using the IC command, digital(port) where
  port is an analog port number (i.e., greater than
  7)
• the sensor value is compared to a threshold
  value,
• and the command returns
• a 0 if the analog value is above the threshold
• or a 1 if the analog value is below it (remember
  the inversion of the actual signal that digital
  does?).
• This threshold's default value is 127, but it can
  be changed
• (See the section on IC commands for information
  on this).

25
Digital Sensors used in Analog Ports

• A good way to get digital information from an
  analog sensor is to plug the analog sensor into a
  analog port and call it with the digital(port)
  command.
• For example, a reflectance sensor would return a
  0 for black or a 1 for white if read with the
  digital command - provided the threshold is
  properly set.
• This can reduce some of the programming
  complexity by abstracting away the thresholding.
• You should however experiment with the sensors to
  determine the range of thresholds you get and
  under what conditions these thresholds are valid.

26
Analog sensors in digital ports?

• It is not recommended to plug analog sensors into
  digital ports, however, because the digital ports
  threshold to conventional logic levels which
  cannot be adjusted to suit each analog sensor.
• The valid analog readings may fall into the
  invalid range for digital logic.
• Read in book about some mountings and uses for
  some digital sensors in the 6.270 kit.

27
Switch Sensors

• Switches are perhaps the simplest sensors of all.
  
• They work without processing, at the electronics
  (circuit) level.
• Their general underlying principle is that of an
  open vs. closed circuit.
• If a switch is open, no current can flow if it
  is closed, current can flow and be detected.
• This simple principle can (and is) used in a wide
  variety of ways.

Think about all possible uses of switch sensors
in robot arms, mobile robots and robot-animals of
various kinds
28
Switch Sensors

• Switch sensors can be used in a variety of ways -
  recall which were already discussed and shown in
  lab.
• You have seen many kinds of switches already
• button switches,
• mouse switches,
• key board keys,
• phone keys, etc.
• Go to Shops (like Wacky Willy or Tek Country) and
  you will find plenty of cheap industrial switches
  useful for your robot project

Various Switches
One dollar switch
29
What are the ways that Switch Sensors can be
used?

• Contact (touch) Sensing
• detect when the sensor has made physical contact
  with another object
• triggers when a robot grabs an object
• contact of whiskers
• a robots body runs into a wall,
• a robots gripper closes around a cube
• Limit Sensing
• triggers when a gripper is as open as it can be
• a limit sensor detects when a mechanism has
  moved to the end of its range of travel,
  signaling that the motor should be turned off
• Shaft Encoding
• an axle may be fitted with a contact switch that
  clicks once per revolution.
• Software counts the clicks and determines the
  amount and speed of the axles rotation.
• e.g., triggers for each turn, allowing for
  counting rotations

Various Switches
1. Bumpers 2. Limit in robot arms 3. Shaft
encoders
30
Use of Dip Switches on Robots

• There are four dip switches on the Expansion
  Board 6.270.
• They can be used to select user program options
  during testing.
• One dip switch will be used in the starting code
  for the contest to determine the side your robot
  starts on and at which frequencies it transmits
  and receives the modulated IR.
• They can also be useful for outside control of
  program parameters, like enabling certain
  functions or selecting programs to run.
• While these switches are connected to the analog
  port, they are really digital switches.

31
Analog Sensors and Thresholding

• Analog sensors, such as photo-resistors, can tell
  you how far the sensor has bent, or how much
  light is hitting the sensor.
• They answer questions with more detail.
• Analog sensors, however can be converted to
  digital sensors using thresholding.
• Instead of asking the question How much is the
  sensor bent? you can ask the question Is the
  sensor bent more than half way?
• The threshold can be determined by playing around
  with the specific sensor.

32
How to interface a Digital Sensor to Handy Board?
Digital Inputs
Similar to Robix

• Nine digital sensor ports connect to circuitry
  on the HB that interprets each sensors Vsens
  voltage as a digital true/false
• Vsens gt 2.5 v, signal is logic one
• Vsens lt 2.5 v, signal is logic zero
• To connect switch to digital input circuit
• Wire between the sensor signal line and ground

Vsens
33
Sensor Interfacing to Digital Inputs

• normally open switch
• Switch is released it is open, so there is no
  connection between the Vsens sensor line and
  ground. The 47KW pull-up resistor on the HB then
  provides the default value of 5v or logic one to
  the sensor input circuitry.
• Switch is pressed it connects the Vsens sensor
  line to ground, the zero volt level. Then the
  sensor input circuitry detects a logic zero
  reading.
• Switch reading is inverted in software digital()

34
Mostly using micro-switches

• Touch sensors

35
Double Pull Micro-Switches

• The two micro switches are double pull, which
  means they can be wired so that they return a one
  or a zero when not depressed.
• The only major difference is how you think about
  the device in your code.
• Reading a sensor can be thought of as asking a
  question.
• Here, the question could be, Are you open?" or
  Are you closed?"
• If you wire the switch normally open, the answers
  are yes and no, respectively,
• where they would be no and yes for a switch wired
  normally closed,
• all for the same situation where the switch is
  not depressed.

36
Normally open and Normally closed switch

• Depending on how you wire a switch, it can be
  normally open or normally closed.
• This would of course depend on your robot's
  electronics, mechanics, and its task.
• The simplest yet extremely useful sensor for a
  robot is a "bump switch"
• it tells when it's bumped into something, so
  robot can back up and turn away.
• You'll find that even for such a simple idea,
  there are many different ways of implementation.

37
Figure 5.2 Microswitch Assemblies
Normally closed
Normally open
38
Switch Sensor Construction
Microswitch Normally Open Configuration
Pushbutton Switch Wiring Diagram
Microswitch Normally Closed Configuration
39
Possible arrangements for touch switches

• Touch switches should be wired in a normally open
  configuration, so that the signal line is brought
  to ground only when the switch is depressed.
• In some cases, a slight advantage may result from
  one of these arrangements, because there may be a
  difference between the position where the open
  side makes contact and the closed side breaks
  contact.
• When this is the case, the choice of normally
  open or normally closed will affect how sensitive
  the switch is to outside forces.
• This can allow you to make a very touchy sensing
  device or help block out noise.
• The small black switches with the white lever arm
  respond to a shorter arm movement when wired
  normally open and require a little more movement
  to cause a transition in the normally closed
  configuration.

40
Switch Sensors
Switch Sensor Applications
These are not standard touch sensors in Lego. You
can add them inexpensively buying in standard
hardware store rather than through Lego.
Left- and Right-Hand Switch Construction
Design for a Simple Touch Bumper
41
Micro-Switches as object detectors

• The standard kit includes three types of small
  switches, two micro switches and a small push
  button.
• These make great object detectors, so long as you
  are only interested in answering the question, Am
  I touching something right now? with a yes or no.
  
• This is often enough for responding to contact
  with a wall or the other robot or for actuator
  position sensing.
• Using a switch for actuator position sensing
  (called a limit" switch) can be a good way to
  protect drive mechanisms which self destruct when
  over driven.

42
Other uses of Micro-Switches in our robots

• Actuator position sensing.
• This could be handy for limiting the motion of
  hinged joints or linear actuators by requiring
  that a switch be open (or closed, depending upon
  the situation) before running the motor and
  monitoring it while things are moving.
• They could also be used for extended user
  interface for testing and development purposes.

43
Bouncing and Debouncing of microswitches

• Bouncing is a problem found in many switches. At
  the point where the switch goes from open to
  close or vice versa, the output from the switch
  is very glitchy.
• The switch may output several transitions.
• Bounciness occurs especially when the switch is
  used in a sensitive mode.
• One way to debounce the switch is to add a delay
  between samples of the digital input.
• If the sampling is sparse enough, the bouncing
  section of the data will not be collected.

Discuss debouncing using NAND latches and recall
asynchronous state machines from ECE 271
44
Touch Sensors other than microswitches

• Whiskers, bumpers etc.
• mechanical contact leads to
• closing/opening of a switch
• change in resistance of some element
• change in capacitance of some element
• change in spring tension
• ...

45
What are three types of sensing with touch?

• Normally done to avoid collisions
• Avoiding is a lot better than Detecting
• There are basically 3 forms
• Bumper Switch
• Whisker
• Pressure Pad

46
Bumper Switch

• Mounted on the chassis of the robot
• When plunger depressed collision is about to
  occur
• Characteristics
• small surface area
• low cost low sensitivity

47
Use of Touch Sensor as Bumper
48
Bumper Example 1
49
Another Bumper Design Examples
Example 2

• Design for Bi-Directional Touch Bumper
• can detect pressure from front or behind
• movement in either direction pushes levered arm
  away from contact sensor
• rubber bands pull arm back onto switch when
  pressure is released

• Bumper Design
• rotational and sliding pivot points allow the
  bumper to react to pressure from any forward
  direction

50
Touch Sensors bumper skirt

• When the robot runs into a wall the bumper skirt
  hits a micro switch
• which lets the robot controller know that the
  robot is up against a wall.

51
Whiskers

• Extends sensing like a cat extends its sensing
  through its whiskers
• Care should be taken in determining things like
• length
• weight
• shape

Cat whiskers measure space. If a whisker touches
the cat knows that it will not be able to go
through an opening as the whiskers define the
size of entrance it is capable of moving through.
Things like suspended ceiling wire, coffee sticks
or tooth picks can all act as whiskers. They
should no interfere with the actual sensing
element.
52
Analog Sensors
53
Ohm's Law

• Ohm's law explains the relationship between
  voltage (V), current (I), and resistance (R)
• V I R
• Simply put the voltage between two points in an
  electronic circuit is equal to the product of the
  amount of current flowing through them and the
  amount of resistance between them.
• Voltage is measured in Volts (V), current in
  Amperes (A), and resistance in Ohms (Omega).

54
Combining resistances

• It's not hard to figure out how much resistance
  one resistor gives (since they are labeled!).
• But what happens if you put one resistor R1 after
  another R2, i.e., connected them in series?
• The current I flowing through any number of
  resistors has to be equal, since it has only one
  route to flow on, as it goes from one resistor to
  the next.
• What happens to the voltage V?
• Recall Ohm's law V I R
• I (R1 R2)
• I R1 I R2
• Suppose we measure the voltage across R1, i.e.,
  the voltage between the input point V and the
  connection between R1 and R2, would would it be?
• It would be I R1 Volts. Similarly, if we measure
  the voltage across R2, i.e., the voltage between
  the connection between R1 and R2 and ground, what
  will it be? It will be I R2.
• The total voltage in an electronic circuit has to
  add up therefore, the input voltage V has to
  equal the output voltage, after the drop across
  the two resistors, R1 and R2.
• Therefore, since voltages in a series add, so do
  resistances in a series.

Practical use of your undergraduate electronics
55
Dividing voltage

• Suppose we take the voltage out at the point
  between R1 and R2, what will the amount of that
  voltage Vout be?
• Use Ohm's law again V I R gt I V / R
• V / (R1 R2)
• Then the voltage drop across R2, is the product
  of the above current I and R2
• Vout V R2 / (R1 R2)
• What if R1 R2?
• V R2 / 2 R2
• V / 2
• This is a voltage divider. To summarize voltage
  can be divided by using two equal-value resistors
  in series.
• You will learn in the lab how to bridge the gap
  between this type of laws of electronics to
  physical sensors all the way to robot behavior.

56
Analog Sensors

• The analog ports all have a pull up resistor
  which is a 47K resistor between 5 volts and the
  signal input.
• The analog readings are generated by measuring
  the amount of current flow through the pull up
  resistor.
• If no current flows through the resistor, the
  voltage at the signal input will be 5 volts and
  the analog value will be 255.
• The voltage at the signal pin can be simply
  calculated by
• V sig 5
• check if one sensor fell out write a piece of
  code that checks the values of the analog ports
  that you have sensors plugged into.
• If that value is above 250 or so, have it tell
  you to check the sensor.

57
Figure 5.4 Analog Sensors Schematics
58
Resistive Sensors

• The resistance of resistive analog sensors, like
  the bend sensors or potentiometers, change with
  changes in the environment, either an increase in
  light, or a physical deformation.
• The change in resistance causes a change in the
  voltage at the signal input by the voltage
  divider relation.

59
Transitive Analog Sensor

• Transitive analog sensors, like the photo
  transistors and reflectance sensors, work like a
  water faucet.
• Providing more of what the sensor is looking for
  opens the setting of the valve, allowing more
  current to flow.
• This makes the voltage the voltage at the signal
  decrease.
• A photo transistor reads around 10 in bright
  light and 240 in the dark.
• One problem that may occur with transitive
  sensors is that the voltage drop across the
  resistor may not be large enough when the
  transistor is open.
• Some transitive devices only allow a small amount
  of current to flow through the transistor.

60
Transitive Analog Sensor

• A larger range for the sensor can be accomplished
  by putting a larger pull-up resistor.
• By having a larger resistor, the voltage drop
  across the pull-up resistor will be proportional
  to the resistance.
• We will give example uses and mountings for each
  type of sensor.
• Keep in mind that these are only simple examples
  and are not the only possible uses for them. It's
  up to you to make creative use of the sensors you
  have.

61
Sensor Interfacing to Analog Inputs
photocell element

• Vsens voltage at the center tap of the two
  resistors is proportional to the ratio of the two
  resistances.
• Rphoto 47KW, Vsens 2.5 v (exactly)
• Rphoto ltlt 47KW, Vsens gnd
• Rphoto gtgt 47KW, Vsens 5 v

Two resistors form voltage divider circuit
Also possible to connect circuits that generate
a voltage
62
Sensor Interfacing to Analog Inputs
0 to 5 volts are converted into 8bit numbers 0
to 255 (decimal) (A/D conversion)

• When the photocell resistance is small
• (brightly illuminated), the Vsens 0v
• When the photocell resistance is large
• (dark), Vsens 5 v

63
Resistive Position Sensors
Potentiometers. Glowes. Pads. Bend Sensors.
Other.?
64
Pressure Pad
You can purchase such pad for Nintendo games
65
Pressure Pad

• LM339 is a quad comparator circuit
• Output will be 6V
• Another approach is to use ohm meter to detect
  the resistance change which would be proportional
  to amount of pressure applied.

66
Potentiometer the main ideas

• Potentiometers are very common for manual
  tuning you know them from some controls (such as
  volume and tone on stereos).
• Typically called pots, they allow the user to
  manually adjust the resistance.
• The general idea is that the device consists of a
  movable tap along two fixed ends.
• As the tap is moved, the resistance changes.
• As you can imagine, the resistance between the
  two ends is fixed, but the resistance between the
  movable part and either end varies as the part is
  moved.
• In robotics, pots are commonly used to sense and
  tune position for sliding and rotating mechanisms.

67
Potentiometers versus resistance sensors

• Fixed Rotation Sensors
• Easy to find, easy to mount
• Light Sensor
• Good for detecting direction/presence of light
• Non-linear resistance
• Slow response

Potentiometer
Look to catalogs
Cadmium Sulfide Cell
HANDYBOARD Gleason Research. http//www.gleasonre
search.com/ http//handyboard.com DISTRIBUTOR OF
AGE BEND SENSOR Images Company
http//www.imagesco.com PITSCO LEGO DACTA,
JAMECO, ETC - see the book and my webpage.
68
Potentiometers

• Manually-controlled variable resistor, commonly
  used as volume/tone controls of stereos
• Mechanical varieties
• Linear and rotational styles - make position
  sensors for both sliding mechanisms and rotating
  shafts
• Resistance between the end taps is fixed, but
  the resistance between either end tap and the
  center swipe varies based on the position of the
  swipe
• Electrical varieties
• Linear taper - linear relationship between
  position and resistance. Turn the pot 1/4 way,
  the resistance between the nearer end and the
  center is 1/4 of end-to-end resistance
• Audio taper - logarithmic relationship between
  position and resistance. At one end, 1/4 turn
  would swipe over a small bit of total resistance
  range, while at the other end, 1/4 turn would be
  most of the range

69
Figure 5.5 Potentiometer Assemblies

• Kits contain several sizes of potentiometers,
  also known as variable resistors.
• There are rotary and linear pots.
• Potentiometers should be wired with Vcc and
  ground on the two outside pins, and the signal
  wire on the center tap.
• This will, in effect, place the resistance of
  the potentiometer in parallel with the 47K
  pull-up on the expansion board and is more stable
  than just using one side and the center tab to
  make a plain variable resistor

70
Potentiometers as Resistive Position Sensors
works best when the potentiometer resistance is
small enough such that a 47K resistance in
parallel with the pots resistance has only a
small effect
3-terminal potentiometer
2terminal potentiometer works best when the
pots value is large
2-terminal potentiometer
71
Various uses of Potentiometers

• Potentiometers have a variety of uses.
• In the past, they have been used for menuing
  programs and angle measurement for various
  rotating limbs or scanning beacons.
• They can be used with a motor to mimic servos,
  but that's a difficult task.
• It is important to notice that the pots are not
  designed to turn more than about 270 degrees.
• Forcing them farther is likely to break them.

Tell about current project of animation inverse
kinematics robot with many pots and A/D board.
72
Various uses of Potentiometers

• A potentiometer can be attached to a LEGO beam
  such that it can be used in place of a bend
  sensor.
• The rotation of the beam will produce a rotation
  in the potentiometer.
• See if you can come up with an assembly that can
  be used in place of a bend sensor.
• The advantage to such a sensor is that it is much
  sturdier than the bend sensor.
• The disadvantage is that it is bulkier.

73
Linear Potentiometers and their use in HandyBoard

• A linear potentiometer can be used to measure
  precise linear motion, such as a gate closing, or
  a cocking mechanism for ring balls or blocks.
• Frob-knob
• The frob knob is the small white dial on the
  lower left corner of the Expansion Board.
• It returns values between 0 and 255 and provides
  a handy user input for adjusting parameters on
  the y or for menuing routines to select different
  programs.
• You may find it useful to glue a small LEGO piece
  to the frob knob to make turning it easier.

74
Homework Assignment

• Try to find in your storage any kind of sensors
  that you do not use and bring them to the
  robotics labs.
• The ECE 271 and the high school students will
  possibly use it for projects if you will not.
• Look around the lab and try to identify sensors
  and devices that we talked about.

75
Resistive (Analog) Position Sensors
76
Resistive Position Sensors bending

• We said earlier that a photocell is a resistive
  device, i.e., it senses resistance in response to
  the light.
• We can also sense resistance in response to other
  physical properties, such as bending.
• The resistance of the device increases with the
  amount it is bent.
• These bend sensors were originally developed for
  video game control (for example, Nintendo
  Powerglove), and are generally quite useful.
• Video game accessories are in general useful for
  robotics and virtual reality and very cheap.

77
Resistive Bend Sensors

• Resistance 10k to 35k
• Force to produce 90deg 5 grams
• www.jameco.com 10

78
Bend Sensors
You can remove it from Nintendo gloves

• Useful for contact sensing and wall-tracking
• The bend sensor is a simple resistance
• As the plastic strip is bent (with the silver
  rectangles facing outward), the resistance
  increases

79
Resistive Position Sensors

• Mechanically, the bend sensor is not terribly
  robust, and requires strong protection at its
  base, near the electrical contacts.
• Unless the sensor is well-protected from direct
  forces, it will fail over time.
• Notice that even in a good arrangement, repeated
  bending will wear out the sensor.
• Remember a bend sensor is much less robust than
  light sensors,
• although they use the same underlying resistive
  principle.

80
Applications of Resistive Analog Sensors
Sensor

• Measure bend of a joint
• Wall Following/Collision Detection
• Weight Sensor

Sensors
Sensor
81
Inputs for Resistive Sensors
V1
Voltage divider You have two resisters, one is
fixed and the other varies, as well as a constant
voltage V1 V2 (R2/R1R2) V
R1
V
Analog to Digital (pull down)
R2
V2
micro
measure
Known unknown
micro

Binary Threshold
Single Pin Resistance Measurement
-
Comparator if voltage at is greater than at -,
high value out
82
Sensor Assembly

• You should have read the section on the chapter
  of Martins book on the types of connectors used
  with the 6.270 board.
• This is an important concept to understand before
  building your sensors.
• When building your sensors, do not make your
  wires too long.
• Excess wiring has a tendency to get caught in
  gears and other mechanisms.

83
Sensor Assembly Homework

• Start out with sensor wires no longer than 1 foot
  long and when your finally decide on your robot
  configuration, you can modify to length.
• Just build a few of each type so you can play
  with them.
• Start out with building simple sensors like one
  or two switches.
• The more complicated ones will be the analog
  sensors that use IR.
• Go to lab and familiarize yourself with Lego kit
  sensors and how to use them.
• I purchased many good sensors from Wacky Willy,
  Tek Country Store and Radio Shack. In Goodwill
  you can buy old toys like Nintendo gloves or
  jumping pads that can be used. They are in the
  lab and you can use them. You have to notify me
  or lab assistant.

84
Sources

• A. Ferworn
• Saúl J. Vega
• Daisy A. Ortiz
• Raúl E. Torres
• Maja Mataric
• Ali Emre Turgut
• Dr. Linda Bushnell
• Web Site http//www.ee.washington.edu/class/462/a
  ut00/
• Robotic Explorations A Hands-on Introduction to
  Engineering, Fred Martin, Prentice Hall, 2001.