Sensors
Brad Miller
Associate Director, WPI Robotics
Resource Center
Why Use Sensors?
• Robot Internal State
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Banner sensors
Limit switches
Potentiometers on arms
Accelerometers for tilt angle
Gyros for direction
Encoders for speed
• Robot position (usually) field relative
– Rangefinders (ultrasonic or infrared)
– Encoders for distance
This Stuff is Easy!
• People seem to think autonomous
operations is only for advanced teams
– Hard to program
– Hard to connect sensors
– Hard to understand
• It’s NOT HARD – the FLL 7th graders can
do it, their robots are all autonomous
• You just need the right tools!
Wiring Sensors
• Connects to robot via 3 pin PWM
connector
– PWM shells, connectors and crimp tool are
available from Jameco and DigiKey
– Same for digital and analog I/O
– All FRC connectors should be female
– Vex connectors are male
What makes a machine a robot?
Sensing
information
about the
environment
where
is the
truck?
Planning
Acting
action
on the
environment
What is sensing?
Sensing is converting a quantity that you
want to measure into a useable signal
(usually electronic).
Perception is the interpretation or understanding of these signals.
Example:
Sensing: Sound waves -> vibrating eardrums -> signals to brain
Perception: Understanding that I am talking to you about sensors.
One Use of Sensors
• Control your motors so they go at the
speed and distance you ask them to
– How do we know how fast the motor is
really going?
– How do we know how far the robot really
went?
– How do we know when the angle of the arm
is what we asked it to be?
Controlling the Robot
• Open loop control has no feedback
– Set the speed and “hope” that the motors
go the requested speed and/or distance.
• Closed loop control (with feedback)
– Use sensors to measure what’s going on
then do something about it!
– Apply correction to the input
• Desired speed is higher – Increase motor power
• Desired speed is lower – Decrease motor power
Simple feedback – On-Off
• On-Off (“Bang-Bang”) controller
• Thermostats have a set (desired)
temperature
– House too cold – turn on furnace
– House too warm – turn off furnace
• Very of “jerky” response
• Hysteresis helps (dead band)
Proportional Control
• The speed of the robot (in this case) is
proportional to the error signal
– R(t) = Kp * (Ddes(t) – Dact(t))
– The difference between the desired value and
the actual value is called the error function
• The constant Kp sets the response time
– Higher Kp (controller gain) means faster
response, but too high and oscillations and
overshoot
– Lower Kp and the system is slow and mushy
Proportional Control
Potentiometers
• Directly provides an angle of a piece of
the robot
• You should always use potentiometers
on anything that rotates or slides
– Limit switches can provide stops, but not
position
• Examples
– Robot arm joint position
– Lift (elevator) position
Potentiometers
• Variable resistor used to
indicate position of
something
• Single turn (about 270
degrees) or multi-turn for
elevators
• Always use linear pots – not
logarithmic volume controls
• Connect to the power,
ground, and signal
Potentiometer Examples
Encoders
• A sensor of mechanical motion that
translates motion such as speed,
direction or shaft angle into an
electrical signal
• Types of encoder outputs
– Standard
– Quadrature
– Other outputs: Grey Code, Binary
Encoders
• Usually optical and
measure changes in
rotating disk
• Standard encoders
have one output and
can’t sense direction
• Quadrature encoders
have multiple outputs
for direction
Encoder Examples
Characteristics
• Resolution – Cycles/revolution
– Typically 32, 64, 128, or greater
– Trade resolution for processing
requirements
• Types of outputs
– Absolute: binary and gray code
– Incremental: single channel (tachometer),
quadrature
– Optionally provide index output
• Maximum rotational speed
Measuring Distance
Methods of Measurement
• Mechanical touch sensors
– Simple to build but you need to be in
contact
– The bumper sensor is a good example
• Light sensors
– Infrared sensors measure reflected light
amplitude, angle, time of flight (TOF), or
contrast
• Sonar
– Bounce sound off target and time
Mechanical Sensors
• Just switches:
– Commercial switches like limit switches and
bump sensors
– Can be as simple as whiskers
• Generally only tell you a single distance
• Connect to a digital input on robot
controller – we’re either in contact (1) or
not in contact (0)
Limit Switches
• Can detect a fixed position
• Useful for knowing which position a
mechanism is in
– Pneumatic pistons with the reed switches
• Measuring the position
– Polling – can easily miss the transition
– Interrupts – better, but maybe more complex
– Interrupt watchers (easyC and WPILib)
• Banner sensors are optical version of limit
switch
Sonar Sensor
• Using the sensor
– Start the sensor to begin, and stop it when finished
– Read the range whenever it’s needed
How WPILib Computes Distance
• Speed of sound (in air): about 343 m/s or 1125 ft/s
– Distance to object is 1/2 the round trip time
– EasyC does the work for us – but does not give distance
directly – that’s our job
– Two connections, interrupt port and digital output
Output
port
Interrupt
port
Sharp IR Rangefinders
• Very inexpensive
• Easy to use
• Very cool!
Triangulation
• IR emitting diode is located at the focal point (F0),
producing parallel rays of output
Triangulation 2
Triangulation 3
Triangulation 4
Triangulation 5
• PSD is located at the back focal plane (f)
Sharp: Limitations
• Back side of
curve can give
confusing
readings – can be
disastrous – easy
to deal with
• Need to deal with
nonlinear output
Cross Firing Detectors
• Get around the minimum distance
Beam Pattern
• Wider beam pattern
using two crossing
detectors
• Can be used with a
servo to sweep an
area
• Widest portion of
beam is about 16cm
Choices of Sharp IR Sensors
Detector
Output Type
Range
Enable Method
On Current
Off Current
~25 mA
~2 uA
GP2D02
Byte value read serially
from device
10cm - 80cm
Each reading
triggered by
an external
clock
GP2D05
Boolean value (1 or 0)
based on distance
threshold
10cm - 80cm adjustable threshold
with small integrated
potentiometer
Each reading
triggered by
an external
clock
~25 mA
~2 uA
GP2D12
Analog value (0V to ~3V)
based on distance
measured
10cm - 80cm
Continuous
readings
~38ms per
reading
~25 mA
*
factory preset to 24cm
Continuous
readings
~38ms per
reading
~25 mA
*
GP2D15
Digital (0 or 1) output
* detector continuously reads (always on)
Gyros
• Measure rate of rotation
– Integrate to get the angle
– Initialize before using sensor
– Connects to analog port
• Easy to get robots driving straight or on
predetermined headings using proportional
control
• Be careful of specs – especially the range
(maximum degrees/sec)
• Parts usually come from Analog Devices
Gyro Programming
• With easyC or WPILib
– InitializeGyro – runs calibration
– StartGyro – starts background readings
– GetGyroAngle – returns angle in 0.1 degrees
– StopGyro – stops background readings
• Works with various models of gyros
including kit parts – 80, 150, 300
degrees/sec
Sample Gyro Program
#include "BuiltIns.h"
void main(void)
{
InitGyro(1);
StartGyro(1);
TwoWheelDrive(1, 2);
while (1)
{
int error;
int heading = GetGyroAngle(1);
error = heading - 0;
Drive(60, - error / 2);
}
}
• Program always
drives straight (0
degrees)
• Proportional
control: Kp = 0.5
• Replace 0 with
900 for 90 degree
turn, then drive
FIRST Infrared Remote Sensor
• Detects which of 4 IR Remote buttons is
pressed
– You program the sensor for a set of remote
codes
• Put in program mode and teach it each code
• It then detects those codes by setting one of 4
wires to 5v for 100ms. Maybe a good candidate
for interrupt watchers
• Be careful if polling
– 7-15v power supply
Programming IR Sensor
Sample IR Receiver Program
#include "BuiltIns.h"
#define LEFT 5
#define RIGHT 6
#define FORWARD 8
#define BACKWARD 7
void main(void)
{
TwoWheelDrive(1, 2);
while (1)
{
if (GetDigitalInput(FORWARD))
Drive(127, 0);
else if (GetDigitalInput(BACKWARD))
Drive (-127, 0);
else if (GetDigitalInput(LEFT))
Drive(0, 127);
else if (GetDigitalInput(RIGHT))
Drive(0, -127);
}
}
Programming Sensors
• Programming tools
– MPLab – Microchip tools that ship in the kit
– Eclipse – Very popular open source
programming environment
– easyC – supplied in the kit from Intelitek
• Represents the softest entry into the world of
programming
Libraries and Frameworks
• Raw IFI code base
– Very basic – a set of starter files
– You’re responsible for all sensor code,
interrupts, timers, etc.
• Kevin Watson’s code
– Complete source code base that you augment
to add your code
• WPILib
– Modular high level library that has “drivers” for
all sensors, timing, interrupts
– easyC is based on WPILib
Program Structure
• Basically you write three functions:
– void Initialize(void) // init sensors here
– void Autonomous(void) // autonomous code
– void OperatorControl(void) // operator code
• The library does all the rest of the work
for you
Questions?
Why do robots need sensors?
What is the angle of my arm?

internal information
Why do robots need sensors?
Where am I?
?
localization
Why do robots need sensors?
Will I hit anything?
obstacle detection
Sensing for specific tasks
Where is the cropline?
Autonomous
harvesting
Sensing for specific tasks
Where are the forkholes?
Autonomous material handling
Sensing for specific tasks
Where is the face?
Face detection & tracking
Control Systems
• Open loop: the sequence of commands in the
program is carried out irrespective of
consequences
– Example: microwave oven set to defrost for 2
minutes. Shuts off in two minutes – ready or not.
– Driving a fixed distance by timing
• Closed loop: feedback is used to modify the
commands in the program based on current
conditions
– Example: home heating system – the current
temperature is fed back to the control system
– Driving a fixed distance using wheel encoders
People do Closed Loop Feedback
• We stay in our lane.
How do we do this:
– If we stray out of
the lane we apply a
correction
– Amount of
correction is
usually related to
the amount of error
(wild turns on
waking up!)
What are some examples
• Keeping your house at a constant
temperature
• Following a wall at a fixed distance
• Keeping the arm on our FIRST robot at a
desired angle regardless of load
• Automotive cruise controls
• Airplane autopilots – near and dear to
my heart
Derivative Control
• Going fast makes sense when far away.
– The problem is momentum carries actuators
past the setpoint
• Rate of change of error over time – is the
error getting smaller over time?
Solving the Steady State Error
Problem
• The problem is: the system has internal friction
– At low values (near 127) the motors just don’t turn and
the robot doesn’t make that final correction.
• But there is still an error – this is the steady state error.
• This is where the integral term helps: sum the error and
reapply it. The longer the robot sits parked at 13” the
larger the error becomes until it is enough to move the
robot.
• The same thing happens when the robot is too close to
the wall. This time, a negative correction accumulates.
Integral Control
• The Integral of the error (error
accumulated over time)
– Acts on the “history” of the error
– Eliminates steady state error
– While the error is positive, the correction is
increasing. While the error is negative, the
correction is decreasing.
Example: Driving Straight
• Gyro returns the rate of rotation
– Integrate rate to get the heading
– Different gyros have different sensitivities
Problem: Don’t Know the
Distance Until Hitting the Object
Other Echolocation Users
Enter: Echolocation (Sonar)
• Sound Navigation and
Ranging (Sonar)
– Use speaker to send out
sound and microphone to
listen for echo
– More correctly called
echolocation
– Works in air or water
Demonstration: Looking at the
Rangefinder Signals
• We can measure the
voltage from the echo
return on the
rangefinder.
– That lets us see the
rangefinder operating
in real-time using a
software oscilloscope.