Sensors 4 FRC Robots
By Hugh Meyer
Red Alert Robotics
FRC Team 1741
Center Grove High School
Greenwood Indiana
October 5, 2013
Why Do We Need Sensors?
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Protect robot from self
destruction
Protect robot from driver
destruction
Provide closed loop
control
Sense position
Feedback for
autonomous mode
Limit range of motion
Confirmation system is
functioning properly
Gives electrical team
something to do
Mechanical Limit Switches
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Lever or roller actuated
Several sizes available
VR7 is popular, easy to work with,
inexpensive, supplied in kit of parts
Simple to wire – connect red and black
wires to NC and NO connections,
connect white signal wire to the COM
connection
Orient the lever so it wipes across the
cam – Avoid having the switch become
your mechanical limit
Mechanical Limit Switches
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Signal is on/off or true/false or
1/0
Connected to digital I/O on the
Digital Side Car
Software reads a one or zero
Switch connections will be labeled
Mechanical Limit Switches
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Common
Normally Closed
Normally Open
Common terminal
is the output
NO and NC
would be ground
and + 5 volts
Magnetic Reed Switches
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High reliability – Reed contacts are
sealed in a small glass cylinder
Easy to wire – Connect one terminal
to the black wire and the other to the
white wire
Used on Pneumatic cylinders, but the
cylinder must be ordered with the
magnetic ring option
Magnet is the element in motion to
be sensed by the reed switch
Fixed Pressure
Switch
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Used to control air compressor
Switch closes when pressure is
below a predefined low limit
value and opens when pressure is
above a high limit value
Simple to wire – Black and white
wire connect to the switch
Pressure Transmitter
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Provides an analog or digital signal
indicating the exact pressure
Analog wiring is simplest – Black
and red wires supply power to
sensor and the white connects to
the analog input
Digital device will connect to a
SPI, I2C, or serial port
Push Buttons
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Momentary contact
Suggest connecting all three wires
– Black and red to NC & NO
contacts and white wire to the
COM contact
Available in many different sizes,
shapes, color, and styles
Driver station ultimate control
Toggle Switches
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Generally not momentary action – switch
stays at position when released
Available in many different contact
arrangements
FRC usage can generally use the simple
SPDT (single pole double throw)
Center off is available if appropriate
Quadrature Encoders
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Measures speed, distance, and
position
Signal can return to the cRIO or
Jaguar, or both
Requires two digital inputs if you
want directional information and
highest resolution
Signals are 90 degrees out of
phase
Quadrature Encoders
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Different sizes available
Several styles available
Kit of parts encoders
must be installed
carefully
Encoder disk must be
correct distance from
optical element
Need oscilloscope to
verify correct operation
Quadrature Encoders
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Available in different resolutions
The line count specifies the resolution
Higher count gives the I and D
components of PID faster information
for better response
Lower count is more tolerant of
alignment errors
Several functions in WPILib are available
depending if you want speed or distance
Incremental device – If you want
absolute position then you need to home
the system, or always start from a known
position
Potentiometer Absolute Encoder
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Always know where you are
Standard pot supports up to 270 degrees of rotation
Multi-turn pots can support more than one rotation
Home switch and sequence not required
Analog voltage output is proportional to position
Black and red wires go to outer connections on pot
Signal comes from the wiper terminal
Absolute Magnetic Shaft Encoder
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Similar to a single turn pot, but allows full 360
degree rotation with no stops
Available with an analog or a pulse width
modulated digital output
Output signal is proportional to the absolute
shaft position
Different bearing configurations are offered
Can be used as operator control as well as
sensor on robot
Connected to analog bumper on analog I/O
module
Rotary Magnetic Encoder
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No mechanical contact
Accurate alignment is important
Data sheet is on KOP webpage
Several IO options
Linear Motion Magnetic Encoder
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No mechanical contact
Accurate alignment is
important
Data sheet is on KOP
webpage
Several IO options
Quad outputs
Linear Motion Optical Encoder
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High precision
Any length encoder strip
Quad output
US Digital
Resolution of sensor and
strip must match
Includes index mark for
home position
Gear Tooth Encoder
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Alignment not critical
Works with steel chain
sprockets or gears
Pulse width modulated
output
WPI Library has a
function to read these
type of sensors
Ultrasonic Distance Sensor
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Measures distance to a target
Two styles of sensors available –
Dual element and single element
Available with analog or digital
outputs
Digital output would be the
preferred method
Good for localization for
autonomous mode
Sensitively spec is used for scaling
voltage data to feet or inches
Get that information from the data
sheet
Gyro - Inertial Turn Rate Sensor
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Measures the rate of turn
Can be used for heading data for autonomous drive
mode
Can be used to create a field oriented style drive scheme
Can be used to create a target oriented style drive
scheme
If oriented properly can be a tilt sensor, although the
accelerometer can do that naturally with the Z axis data
Inertial Accelerometer Sensor
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Measures rate of change in motion and the
direction of gravity
Three axis sensor – X, Y, Z
Z axis measures direction of gravity and can be
used as a tilt sensor
X axis, if oriented properly, will measure
forward and backward motion
Y axis, if oriented properly, will measure
sideways motion
Inertial Sensor
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Should be mounted on a mass that is
suspended in foam
This acts as a low pass filter and will
significantly reduce high frequency noise
Accelerometer output is available in digital
or analog outputs
Analog outputs require one analog input
for each axis
Digital outputs require four digital I/O
connections on the digital side car
Use the SPI interface software functions
to get the data
Use the digital outputs – This prevents the
bias from becoming an issue
Inertial Sensor
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Rate gyro output is available as an
analog output
Connect this output to the analog
bumper input
Use shielded wire for a clean signal
System must measure the bias when
the sensor is initialized
System must be absolutely stationary
when the system is measuring the
bias
If your gyro is drifting it is likely
because the system was in motion
during the initialization period
Inertial Sensor
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If you have a onboard air compressor it should not run
during the gyro bias measurement time
Sensitivity must be passed to the gyro class when it is
initialized
Sensitively is found on the data sheet for the device
Accelerometer sensitivity will be on the order of 300
mV/g
Gyro sensitivity will be on the order of 7
mV/degrees/s
Read the data sheet provided by FIRST and the data
sheet from the manufacturer
The FPGA in the cRIO does the integration for you to
get an angle
Inertial Sensors More Information
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Kevin Watson has some good information
about these types of sensors
He has some well commented code if you really
want to get into the details
http://kevin.org
The FPGA in the cRIO really does the hard
work integrating the gyro rate data to get an
angle
Optical Transmitted Beam
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Consists of two elements – a transmitter and a
receiver
A signal is output when the light beam is blocked
Alignment is important
Simple packaging is readily available off the shelf
Power must be supplied to both the TX and RX
Optical Transmitted Beam
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Use a regulated power output.
Most require 24 volts which
you can get using the solenoid
bumper wired to the 24 volt
supply
Detected object must be large
enough for the sensor to see
Optical Retro Reflective
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Uses a retro reflective target
Only one device
Target can be a flag in motion
Target can be stationary and
have an opaque flag block the
light – The flag must not be
reflective, or it will be detected
Optical Vane Transmitted Beam
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One piece housing in a U shape – also
called slot sensor
Available in several different sizes
Will detect presence of any opaque
object that is inserted into the U
Very effective for home sensors when
used with incremental encoders
No mechanical contact for high
reliability
Simple to actuate
Does not require precise alignment or
mounting
LED & Photo Transistor
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Roll your own for a fraction of the
cost
Flexible designing package since
you make your own
LED (LTE-4208) and Photo
transistor (LTR-3208E) must be a
matched pair
IR light is best because that will
reduce the effect of ambient light
Works over a wide distance range
LED & Photo Transistor
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Requires resistors in the circuit to work
properly
LED needs a current limiting resistor in
series with it to drop the 5 volts down to
1.2~1.6 volts
Size this resistor to draw 15 ~ 30 milliamps
Check the spec sheet for the target current
A good range will be 180 ~ 221 ohms if
using a 5 volt power supply, I used 221
ohms for this device
You can calculate the value or measure it if
you have an ammeter
LED & Photo Transistor
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Photo transistor needs a pull up or down
resistor depending on the wiring scheme
you select
A good range will be 2k ~ 4k ohm, I
used 3.16k for this device
Resistor can be above the transistor or
below it
Above you get a high when the light
beam is blocked
Below you get a low when the light beam
is blocked
LED & Photo Transistor
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R = 221 ohms
RL = 3.16k ohms
Current Sensor
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Measures current flowing to a motor
Indicates if the motor is running within load specifications
Helps prevent “magic smoke” from exiting the motor
Hal effect sensors can measure DC current
IC based sensors can be inserted into the circuit
Jaguar speed controllers have current measuring ability built
right in – you just need to ask it
Test Rig
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Always measure
motor current when
building proto types
12” piece of 10
AWG wire makes a
good shunt resistor
1 milli-volt equals 1
amp current flow
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Two types provided over the years
from the kit of parts
Axis M1011
Axis 206
Any others are legal if they are IP
based
Library provided by FIRST is very
advance
Several teams got vision to work for
them
Can be connected directly to the
cRIO or radio
Vision processing can be done on
cRIO or driver station
Cameras
Tools Needed
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Oscilloscope is very helpful
Voltmeter
Ammeter
Soldering Equipment
Connectors
Shielded wire
Software You Need to Write
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Code that will read the sensor
Code that will log the data from the sensor
First thing you should do is hook up sensors you
plan to use and start reading data
Don’t wait
Connect sensors and start reading data as soon
as possible!!
Log The Data
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Log the data to a file on the cRIO
Send the data to the driver station
and log to a file on the DS
Add external device that logs to a
memory card
Logomatic by Spark Fun Electronics
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https://www.sparkfun.com/products/10216
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Sensor Guidelines
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It always needs a sensor
If a mechanism has limited travel then it needs an end
of travel (EOT) limit sensor on both ends!
Start reading data from sensors as soon as you can so
you will learn what is required and the type of data
Design your sensors into the system. Add on after
thought mounting schemes tend to be unreliable
Kit of Parts Sensors
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Use the resources provided by FIRST at the kit
of parts website
They have data sheets and instructions specific
to our application
http://www.usfirst.org/roboticsprograms/frc/2
012-kit-of-parts-sensors
Suppliers
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AndyMark - http://www.andymark.com/Default.asp
US Digital - http://usdigital.com/
Digikey - http://www.digikey.com/
Mouser Electronics - http://www.mouser.com/
Newark Electronics - http://www.newark.com/
Allied Electronics - http://www.alliedelec.com/
Rockwell Automation - http://www.ab.com/catalogs/
Automation Direct http://www.automationdirect.com/adc/Shopping/Catalog/Sensors_-z-_Encoders
Spark Fun Electronics - http://www.sparkfun.com/
Parallax Inc. - http://www.parallax.com/tabid/768/ProductID/92/Default.aspx
MaxBotix - http://www.maxbotix.com/
Suzo Happ - http://na.suzohapp.com/pushbuttons/pushbuttons.htm
Penny+Giles - http://www.pennyandgiles.com/Joystick-Controllers-pg-28,2,,.php
State Electronics - http://www.potentiometers.com/select_joystick.cfm
Questions
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Hugh Meyer
hmeyer@redalert1741.org
317 786-9214
Webpage:
http://rar.meyermat.net/workshops/sensors/index.html