Light Sensors

1

• Light Sensors

2
Light Sensors
3
Light Sensors or Dark Sensors?

• Light sensors measure the amount of light
  impacting a photocell,
• photocell is basically a resistive sensor the
  light effects the amount of resistance.
• The resistance of a photocell is low when it is
  brightly illuminated,
• i.e., when it is very light it is high when it
  is dark.
• In that sense, a light sensor is really a "dark"
  sensor.
• In setting up a photocell sensor, you will end up
  using the equations we learned above
• you will need to deal with the relationship of
  the photocell resistance Rphoto, and the
  resistance and voltage in your electronics sensor
  circuit.

4
What can be measured by Light Sensors?

• Of course since you will be building the
  electronics and writing the program to measure
  and use the output of the light sensor, you can
  always manipulate it to make it simpler and more
  intuitive
• for example, most people invert the values, so
  low means dark and high means light.
• You can also be clever about what you put around
  a light sensor, to affect its properties.
• You can shield it and position it in various
  ways.
• If you use multiple sensors, you can arrange them
  in useful configurations and isolate them from
  each other with shields.
• Just like switches, light sensors can be used in
  many different ways
• Light sensors can measure
• light intensity (how light/dark it is)
• differential intensity (difference between
  photocells)
• break-beam (change/drop in intensity)
• Light sensors can be shielded and focused in
  different ways
• Their position and directionality on a robot can
  make a great deal of difference and impact

5
Figure 5.6 Photocell Light Sensor
The photocell is a special type of resistor
which responds to light. The more light hitting
the photocell, the lower the resistance it has.
The output signal of the photo-cell is an analog
voltage corresponding to the amount of light
hitting the cell. Higher values correspond to
less light. A photoresistor changes its
resistive value based on the amount of light that
strikes it. As the light hitting it increases,
the resistive value decreases. They are somewhat
sensitive to heat, but stand up to abuse well.
Try not to overheat when soldering wires to them.

• Photocell versus photoresistor

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Photocells versus photoresistors

• As with all the light-sensing devices, shielding
  is very important. A properly shielded sensor can
  make the difference between valid and invalid
  values reported by that sensor.
• The idea is simple restrict the amount of light
  striking the sensor to the direction you expect
  the light to be coming from.
• You do not want light from external sources
  (i.e., camera ashes or spot lights) to interfere
  with your robot.
• Black heat shrink tubing often works well to
  shield the photoresistor from external light
  sources.

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light-sensitive robot

• One good way to get a feel for how these sensors
  work, and how your robot and software interact,
  is to make a light-sensitive robot.
• With two or more photo-resistors, try to create a
  simple robot that can
• move around a room,
• either avoiding light
• or avoiding shadows in a controlled manner.
• Ambient light conditions play a major role in how
  to interpret the data from any light sensors.
• A combination of photocells, one pointed up and
  one pointed down, may be used to adjust for
  ambient light levels, which may be useful in some
  applications.

These robots are easy to build and many
high-school and undergraduate projects for
wheeled robots were build
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Photoresistors are the simplest light sensors

• Photoresistors are probably the only sensor
  required to be on your robot.
• A starting light will be used to start each
  contest round, and the robot must be able to
  sense that light. This is for all wheeled robot
  contests.
• You must place one photoresistor on the underside
  of your robot, probably near the center.
• Be sure to shield it as much as possible from the
  overhead ambient light.
• Write the starting code that reads the value of
  that sensor to start the match.

9
Photoresistors are the simplest light sensors

• Mounting the photoresistors doesn't tend to be
  difficult.
• You can use a small amount of hot glue to attach
  the photocell to a LEGO brick, or double-sticky
  tape will also work.
• Be inventive.

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Photo Transistors and Infra Red Light Emiting
Diodes
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Photo Transistors

• Phototransistors are usually tuned to a specific
  wavelength of light.
• The wavelength is usually near visible red, or in
  the infrared spectrum.
• They have similar properties to the
  photoresistors.
• The main difference is that the phototransistors
  are usually tuned to a specific wavelength.

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Photo Transistors

• The other important difference is that the time
  delay for a change in light conditions is much
  smaller for a phototransistor.
• This can be useful in doing fast control looking
  for polarized light.
• The time constant for a phototransistor is much
  smaller than a photoresistor, so it may be used
  in situations where timing is critical.

13
Infra Red LEDs

• An IR LED is a type of diode which emits
  radiation in the infrared range.
• This part could be used as a component in a break
  beam sensor or a reflectance sensor.
• We used two kinds of phototransistors, each of
  which are packaged in cylindrical brass-colored
  cans with a glass lens.
• The first kind is packaged individually, with no
  wires attached, and with three leads.
• The second is surplus parts, with wires already
  attached, and with each phototransistor paired
  with an LED.
• (Note surplus parts are usually overstocked or
  obsolete parts that didn't sell through retail
  channels. See the book's appendix on ordering
  electronics parts.)
• The individual Phototransistors cost 6.270 about
  1 each, about the same as an entire surplus
  assembly bundle of wires and phototransistors and
  LEDs.

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How to tell apart the phototransistors from the
LEDs

• Be careful to differentiate the phototransistors
  from the LEDs
• the phototransistors have relatively flat lenses,
  
• while the LEDS lenses are more convex.
• Fig 5.7 shows one of the LEDs.

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How to tell apart the phototransistors from the
LEDs

• Also, the two different kinds of phototransistors
  (surplus vs virgin manufactured) have very
  different characteristics, and cannot be used in
  sensors interchangeably.
• The surplus phototransistors respond almost
  exclusively to infrared light
• They have a resistance" of approximately 100 k
  when activated and 1 M when not activated.
• The individual, un-wired phototransistors, on the
  other hand, respond to visible light as well as
  infrared, and have resistances" about one
  hundred times smaller.

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Interfacing to the Board

• These phototransistors require pull-up resistors,
  a resistor connected between Vcc and the signal
  line, to work properly.
• In past years, all of these sensors required 47k
  pull-up resistors, but that is no longer the
  case.
• Each individually packaged phototransistor now
  can be used with a 220k pull-up, while the
  bundle of wires" phototransistors work well with
  100k pull-ups.
• This may present a slight problem if you have
  already installed RP6, one of the 47k pull-up
  resistor packs on your expansion boards.
• Fear not! By installing the pull-up resistors on
  the connector as shown in Fig 5.8.
• For the individually packaged phototransistors,
  the 2.2k resistor on the connector will be in
  parallel with the 47k pull-up on the board.

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Interfacing to the Board

• Since resistors in parallel add reciprocally, the
  combination of the two will electrically look
  like a2.2k resistor (approximately).
• However, if you have the bundle of wires"
  phototransistors, you will have to cut a trace on
  the bottom side of the expansion board to disable
  the 47k pull-up resistor, since it would
  otherwise dominate.
• Warning! Once you cut a trace, that analog port
  should be used only for the 220k
  phototransistors.
• This means that you will have to be sure to plug
  these sensors into the correct analog ports each
  time you use them.
• Ask me or a TA before you cut this trace!

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Visible Light sensor

• The phototransistors respond very well to visible
  (far-red, we hypothesize) light as well as
  infrared.
• They should be wired with a 2k to 4k resistor for
  best results (we recommend 2.2k).
• Because they respond to visible light, they are
  extremely susceptible to interference from
  ambient light.
• You may be able to use them as floor-color
  sensors using just ambient light.
• But if you want to use them for break-beam
  sensing, they will have to be very well-shielded.

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Light Sensors

• Light sensors are used to detect the presence and
  Intensity of light.
• These can be used to make a light seeking robot
  and are often used to simulate insect
  intelligence in robots.

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Light Sensors
CdS photocell (or other resistive sensor)
Analog sensor -- Change resistance in response to
light stimuli
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Shielding Photocell

• Read photocell values
• while (1) printf("d\n",
• analog(0))
• msleep(100L)
• Mounting photocell through Lego beam makes it
  easier to attach to robot
• Build optical shield to limit the amount of
  ambient light that is able to fall on the sensor

Photocell Sensors with Light Shields
Photocell Sensors Mounted on LEGO Technic Beam
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Single Photocell Light Sensor Circuit
Building light shields

• After building the photocell and test that it
  works (port 0)
• while (1) printf("d\n",
• analog(0))
• msleep(100L)
• Mount the photocells leads through holes of a
  LEGO Technic beam, making a sensor device that
  can easily be positioned anywhere on the robot
  and subsequently reused
• If your photocell easily floods from ambient
  room light, then the next order of business is to
  build an optical shield to limit the amount of
  ambient light that is able to fall on the sensor

Photocell Sensors with Light Shields
Photocell Sensors Mounted on LEGO Technic Beam
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Single Photocell Light Sensor Circuit

• Photocell element is connected to the circuit
  ground and the HBs sensor input line via a
  voltage divider circuit
• Vsens , resulting sensor voltage, varies as to
  the ratio between 47KW and Rphoto
• When the photocell resistance is small (brightly
  illuminated), the Vsens 0v
• When the photocell resistance is large (dark),
  Vsens 5 v
• Continuously varying range between extremes
• Sensor will report small values when brightly
  illuminated and large values in the dark
• May invert the sense of the readings from the
  HBs analog ports
• int light(int port) return 255 - analog(port)

dark sensor
Photocell Voltage Divider Circuit
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Differential Photocell Light Sensor Circuit

• Instead of comparing the single photocell to a
  fixed resistor value, the values of two
  photocells are compared to each other
• Differential sensor provides a signal that can
  be directly interpreted to indicate which side of
  the sensor is receiving more light, and by how
  much
• Rphoto2 Rphoto1, Vout 2.5 v
• Rphoto2 ltlt Rphoto1, Vout 5 v (R2 more light)
• Rphoto2 gtgt Rphoto1, Vout gnd

Ideal Photocell Sensor Differential photocell
sensor is constructed by wiring two like
photocells in the voltage divider configuration
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Differential Photocell Light Sensor Circuit
Actual Differential Photocell Sensor Schematic

• Considerations
• Use photocells with small dark resistance
  values, e.g., 10KW, Otherwise 47KW pull-up
  resistor on HB will bias sensor reading in the
  dark
• Mount a nose between two sensor elements to
  cast shadow on one element if there is a distinct
  source of light off to the side

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Differential Photocell Light Sensor Circuit
Program tests the value of the differential light
sensor to decide which way to turn If the value
is less than 128, the program causes HandyBug to
take a step to the left Otherwise, HandyBug
takes a step to the right
/ stepdiff.c - Light-Seeking Program for
HandyBug / int LEFT_MOTOR 0 int RIGHT_MOTOR
3 int DIFF_EYE 0 void main() while (1)
if (analog(DIFF_EYE) lt 128) / turn
to left / motor(RIGHT_MOTOR, 100)
sleep(0.1) off(RIGHT_MOTOR) else
/ turn to right / motor(LEFT_MOTOR,
100) sleep(0.1) off(LEFT_MOTOR)

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Polarized light

• "Normal" light emanating from a source is
  non-polarized, which means it (i.e., the light
  waves) travels at all orientations with respect
  to the horizon.
• However, if we put a polarizing filter in front
  of a light source, only the light waves of a
  given orientation (i.e., the characteristic
  plane) of the filter will pass through.
• This is useful because now we can manipulate this
  remaining light with other filters
• if we put it through another filter with the
  same characteristic plane, almost all of it will
  get through.
• But if we use a perpendicular filter (one with a
  90-degree relative characteristic angle), we will
  block all of the light.
• You can use polarized light to make specialized
  sensors out of simple photocells
• if you put a filter in front of a light source
  (i.e., an emitter) and the same or a different
  filter in front of a photocell, you can cleverly
  manipulate what and how much light you detect.
• Note that polarized light, like all other sensor
  types we have discussed today, has its equivalent
  in nature many insects and birds use polarized
  light.

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Polarizing Film

• Polarized film has printed or etched straight
  lines.
• The polarizing film allows the light to travel
  in parallel perpendicular planes rather than in
  all directions.
• Assume for this section that the lines are
  running up and down, and therefore the light
  waves will be traveling up and down.
• If a second film is placed such that the lines
  are horizontal, the light traveling past the
  first filter will not pass through the second
  filter.

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Polarizing Film

• Two pieces of film which are perpendicular to
  each other will block out most of the light.
• Parallel pieces will allow maximum light to go
  through.
• The Polarizing lm can be used to enhance the
  photo transistors and photo resistors.
• The beacons at each end of the playing field are
  emitting polarized light.
• One side is polarized at positive 45 degrees from
  the vertical and the other side is at negative 45
  degrees.
• You can detect the difference between one side
  and the other by placing a piece of polarizing lm
  in front of a phototransistor or photoresistor.

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Polarized Light Seeking for Light Sensor Circuits

• Two opposing goals consists of a light box with
  light filter one has the polarization filter
  aligned vertically, while the other has it
  aligned horizontally
• If polarized light is passed through a filter at
  a right angle to the plane of polarization, it is
  completely blocked out
• At angles in between the 0 - 90 deg, light
  passes through proportional to the ratio of the
  polarization angle
• Using differential sensor with polarized shields
  makes it easy to tell if robot is pointed at a
  light beacon or not
• Sensor readings above the midpoint indicate
  readings from one beacon and readings below the
  midpoint indicate readings from the other

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Polarized Light Seeking for Light Sensor Circuits

• Robot employs a pair of photocells, one with a
  45 deg rotation (right photocell) and one with a
  -45 deg rotation (left photocell). Depending on
  the polarization of the light source, either
• light will pass equally through both photocells
  filters (no polarization)
• be blocked in the left and transmitted in the
  right (45 deg polarization)
• be blocked in the right and transmitted in the
  left (-45deg polarization)

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Use of Light Sensors
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Two Thresholds for Hysteresis
Line Following performance run Setpoint 20

• Problem with single threshold variances in
  sensor readings
• Bump on floor may spike the readings
• Shiny spots on line may reflect as well as the
  floor, dropping the sensor readings up into the
  range of the floor

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Figure 5.8 Phototransistor body and connector
35
Two Thresholds for Hysteresis
Line Following performance run Setpoint 20

• Solution two setpoints can be used
• Imposes hysteresis on the interpretation of
  sensor values, i.e., prior state of system
  (on/off line) affects systems movement into a
  new state

int LINE_SETPOINT 35 int FLOOR_SETPOINT
10 void waituntil_on_the_line() while
(line_sensor() lt LINE_SETPOINT) void
waituntil_off_the_line() while
(line_sensor() gt FLOOR_SETPOINT)
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Light Sensor States

• Using two sensors to keep track of one of four
  states
• Light-on-Left
• Light-on-Right
• Light-in-Center
• No light
• Options
• Use hysteresis on each sensor individually and
  combine values into single state
• Combine sensor readings first and then apply
  hysteresis on resulting value
• Try to maintain light-in-center state make
  closed-loop motor changes when in other states

37
Simple Feedback Control Wall Following
You can use either bend sensor or reflective IR
sensor

• Robot turns towards wall if distance sensor
  indicates too far away turns away from wall if
  too close
• Single threshold for too far and too close
  goal variable

38
Simple Feedback Control Wall Following
void main() calibrate() ix 0
while (1) int wall analog(LEFT_WALL)
printf("goal is d wall is d\n", goal,
wall) if (wall lt goal) left() / too far
from wall -- turn in / else right() /
turn away from wall / dataix wall
/ take data sample / msleep(100L) / 10
iterations per second /
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Hard Turns Control
void left() motor(RIGHT_MOTOR,
100) motor(LEFT_MOTOR, 0) void right()
motor(LEFT_MOTOR, 100) motor(RIGHT_MOTOR, 0)
Hard turns

• Results with bend sensor
• HandyBug oscillates around setpoint goal value
• Never goes straight

40
Soft Turns Control
void left() motor(RIGHT_MOTOR,
100) motor(LEFT_MOTOR, 50) void right()
motor(LEFT_MOTOR, 100) motor(RIGHT_MOTOR,
50)

• Gentle Turning Algorithm
• Swings less abrupt
• HandyBug completes run in 16 sec (vs. 19 sec in
  hard turn version) for same length course
• In light following we want to include a
  go-straight function and a random-movement-to-find
  -light function as well

41
Separate Sensor State Processing from Control

• Functions might each make use of other sensors
  and functions need to decide how to implement
  each

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5.5.7 Reflectance Sensors

• A reflectance sensor is made up of a combination
  of an infrared or red LED and a phototransistor
  that is sensitive to the wavelength of light
  being emitted by the LED.
• Over dark surfaces, the light is absorbed,
  whereas over light surfaces, the light is
  reflected back to the phototransistor.
• A reflectance sensor (Figure 5.9) can be made
  using discrete components.
• The reflectance sensors are useful for detecting
  what color the floor is.
• They could also be used as object detectors, but
  they are very near sighted and quite responsive
  to outside lighting.

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5.5.7 Reflectance Sensors

• In any application, good shielding is an absolute
  requirement if any reliability is desired.
• The sensors are very sensitive to distance from
  the reflecting surface.
• Distances greater than an inch will give very
  poor reading, and distances that are too small
  will not allow the right to be reflected.
• The angle at which the light is reflected to the
  surface is important and can produce better or
  worse results at different distances.

44
5.5.8 Motor-Force Sensors

• There are four motor force sensors built into the
  6.270 Controller board, attached to motors 0
  through 3.
• These sensors are included to detect when the
  motors might be stalled.
• When the motors stall, they draw a large amount
  of current, which appears as a large voltage on
  the analog inputs to the 6811.
• When a motor force value increases sharply,
  that's a good sign, but not guaranteed, that the
  motor may be stalled.
• The value that it reaches will depend on the load
  attached to the motor.
• Experiment by stalling the motor yourself while
  printing the values on the LCD to determine a
  threshold that's right for your robot.
• Motor force values are not very accurate when you
  are driving the motors at anything less than
  100.
• Driving the motors at lower speeds will cause the
  motor force value to oscillate wildly, so it is
  recommended that you only use this information
  when you are driving a motor at full speed.

45
Modulation / Demodulation of Lightand Infra-Red
Sensors
46
Modulation and Demodulation of Light

• We mentioned that ambient light is a problem
  because it interferes with the emitted light from
  a light sensor.
• One way to get around this problem is to emit
  modulated light, i.e., to rapidly turn the
  emitter on and off.
• Such a signal is much easier and more reliably
  detected by a demodulator, which is tuned to the
  particular frequency of the modulated light.

47
Modulation and Demodulation of Light

• Not surprisingly, a detector needs to sense
  several on-flashes in a row in order to detect a
  signal, i.e., to detect its frequency.
• This is a small point, but it is important in
  writing demodulator code.
• The idea of modulated IR light is commonly used
  for example in household remote controls.
• Modulated light sensors are generally more
  reliable than basic light sensors.
• They can be used for the same purposes
• detecting the presence of an object
• measuring the distance to a nearby object (clever
  electronics required, see your course notes)

48
Infra Red (IR) Sensors

• Infra red sensors are a type of light sensors,
  which function in the infra red part of the
  frequency spectrum.
• IR sensors are active sensors they consist of an
  emitter and a receiver.
• IR sensors are used in the same ways that visible
  light sensors are as break-beams and as
  reflectance sensors.

49
Infra Red (IR) Sensors

• IR is preferable to visible light in robotics
  (and other) applications.
• This is because it suffers a bit less from
  ambient interference,
• because it can be easily modulated,
• because it is not visible.

50
IR Communication

• Modulated infra red can be used as a serial line
  for transmitting messages.
• This is is fact how IR modems work.
• Two basic methods exist
• bit frames (sampled in the middle of each bit
  assumes all bits take the same amount of time to
  transmit)
• bit intervals (more common in commercial use
  sampled at the falling edge, duration of interval
  between sampling determines whether it's a 0 or
  1)
• Notes
• you are strongly encouraged to pay careful
  attention to the exercises and problems given in
  your assigned readings.
• Projects, exams, homeworks and reports will use
  some of those, so it is in your interest to think
  about the answers to their questions, and work
  some of them out as practice.
• Also the additional recitations (Fridays)
  problems may appear on the exams.

51
Elimination of the effect of the stray IR light

• There is a lot of infrared light that is ambient
  in the air. Some components of this light are at
  40kHz, and straight output from the sensor would
  look very glitchy.
• The sun produces a lot of IR light, and in the
  sun, the sensor output bounces all over the
  place.
• To eliminate the effect of the stray IR light,
  the IR emitters are modulated at 100 or 125 Hz
  and the output of the IR Detectors is demodulated
  to look for these frequencies.
• (see section A.7 for more information on the IR
  transmission)
• The 40kHz frequency is known as the carrier
  frequency, and the other frequency is the
  modulated frequency.

52
Noise readings in infrared sensors and their
effect in the Khepera Miniature Robots
performance
53
Background

• Robots
• Management of hazardous waste
• Moving of heavy equipment
• Ocean and space exploration
• Fire extinguishing
• Artificial Intelligence
• Knowledge-based
• Behavior-based

54
Background (cont.)

• Behavior-based Artificial Intelligence
• Subsumption Architecture (SA)
• Build behaviors from smaller sub-behaviors
• SA rely heavily on sensory input
• Noise cause disturbance in robot operation

55
Problem Statement

• Avoid negative effect of fluorescent lamps on
  infrared sensory readings

56
Objectives

• Determine the effect of noisy readings on robot
  performance
• Determine the effect of filtered sensory on robot
  performance

57
Methodology

• Review of literature
• Simulation study
• Hardware implementation
• Real Khepera used in testing
• Filters design
• Testing-platform development
• Braitenberg vehicle algorithm
• Comparison of results

58
Shaft Encoder Exercises
1. Build a shaft encoder using a break-beam
optosensor and a perforated disk or LEGO pulley
wheel. Verify the raw sensor performancewhat
values represent the light beam being broken vs.
not broken? 2. Choose a suitable midpoint value
for determining encoder transitions. Write a
program in IC to implement the simple encoder
counting algorithm presented in the flowchart.
Use IC multi-tasking capability to display the
encoder counter variable while the counting
routine is running, and experiment with the
encoder. Can you determine the performance limit
of the algorithm in your implementation, in terms
of counts per second? What is a fundamental
problem with this implementation method? 3. Load
a library shaft encoder routine and experiment
with its performance. Capture raw data from the
encoder. Based on the graph of raw encoder
performance, choose suitable high and low
threshold values. Explain your choices. 4. One
limitation of current encoder routines, both the
IC and library versions, is that they cannot
determine which direction the shaft is rotating.
Can you think of a different approach for
determining the direction of rotation? 5.
Implement the trailer wheel idea discussed in the
text on your HandyBug. Write a program to make
HandyBug drive around and stop, back up, and turn
when the trailer wheels velocity is 0. Can you
think of other applications for knowing the
robots velocity, other than as a non-zero/zero
(i.e., moving/not moving) quantity? 6.
Instrument one of HandyBugs drive wheels with an
encoder, and write a program at attempts to
maintain constant velocity on the drive wheel by
varying the power level delivery to the motor.
Experiment with the system by holding HandyBug in
the air and applying pressure to the drive wheel.
Is the system able to maintain the velocity? What
happens if you suddenly remove the pressure?
59
Sources

• A. Ferworn
• Saúl J. Vega
• Daisy A. Ortiz
• Advisor Raúl E. Torres, Ph.D., P.E.
• Maja Mataric
• Ali Emre Turgut
• Dr. Linda Bushnell, EE1 M234, bushnell_at_ee.washingt
  on.edu
• Web Site http//www.ee.washington.edu/class/462/a
  ut00/
• Robotic Explorations A Hands-on Introduction to
  Engineering, Fred Martin, Prentice Hall, 2001.