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F.I.R.S.T Robotics
Sensors 101
January 5th 2008
Montgomery High School
Skillman, NJ
Michael Reffler
Mentor FRC Team 1089
Hightstown, NJ
[email protected]
Agenda
Sensors 101
General Points
CMUCam-2
Yaw Rate/Gyro
Accelerometer
Gear Tooth
Encoders
Potentiometers
Sonic Range Finder
Other Sensors
Sensor Navigation
Information Resources
Vendors and Suppliers
Questions ?
General Points
Sensors 101
Balance your drivetrain mechanically
Programming Techniques
– State Machines
– Function Driven
– Multiple fallback implementations for when sensors
or appendages break
Pay close attention to LIMITS when designing
for sensors
Use RF shielded cabling/wiring
Physical protection/project boxes
CMUCam2
Sensors 101
What can you do with the camera?
– Track colors, e.g., illuminated target
– Locate the scoring goal
– Orient the robot or a turret to the goal
Heading & angle or distance
– Drive to the goal
CMUCam2
Sensors 101
CMUCam2 Camera
– Labview / Focusing and Calibrating
– CMUCam2 GUI / Focusing and Calibrating
– RC default camera code-Kevin Watson
CMUCam2
Labview Interface
Camera Focus
Load Sample Configuration
Track Colors / View Results
CMUCam2
Kevin Watson Camera Code
Just does the camera tracking
Load initial calibration data
Tracking.h Settings
– PAN/TILT servo pwm assignment
– Reversing servo direction
Camera/tracking menus through Hyperterminal
Camera settings stored permanently on the RC
Use Get_Tracking_State()
Use Pan/Tilt angles for direction and distance
CMUCam2
Finding Range to the Light
range = Height_of_Light – Height_of_Camera / tan(tilt_angle_radians)
Yaw Rate Gyro
Sensors 101
Yaw Rate Gyro (ADXRS150)
– Measures angular rate (150o/sec) along the Z
axis.
– Supply voltage 5V.
– Output – analog.
Alternative Yaw Rate Gyro (ADXRS300)
– Measures angular rate (300o/sec) along the Z
axis
Yaw Rate Gyro
Sensors 101
What can you do with the gyro sensor ?
– Application: Stability control, guidance.
– Rotate robot left or right while the gyro reading
is less than Xo.
– If robot rotates too fast (the rate of change of
the gyro sensor) then reduce motor speed
(good in any mode).
Dual Axis Accelerometer
Sensors 101
Dual Axis Accelerometer (ADXL 311)
– Measures dynamic and static acceleration
on both X and Y axis.
– Supply voltage 5 V.
– Output – analog. Bandwidth 3KHZ.
– Sampling frequency > 6 KHZ.
Dual Axis Accelerometer
Sensors 101
What can you do with the accelerometer?
– Application: tilt or motion sensor.
– Am I standing straight?
Measure pitch and roll in degrees.
Pitch = asin(Ax/1g); Roll asin(Ay/1g).
– Orient an arm.
– Collision detection.
Gear Tooth
Sensors 101
Gear Tooth Sensors (2)
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–
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–
–
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AT642LSH Peak Detecting Gear Tooth Sensor.
Generates a pulse when a gear tooth is detected.
Speed of the gear: pulse rate.
Cannot easily determine direction of the gear
Digital Output.
Supply voltage: 12 V.
Gear Tooth
Sensors 101
What can you do with the gear tooth
sensors?
– Control the speed of the wheel.
– Adjust for relative speed difference
between wheels.
– AUTONOMOUS MODE:
Move d feet distance.
Circumference of the wheel.
2 * PI * R / Gear Teeth = Inches per Pulse.
Quadrature Encoders
Sensors 101
Encoders are commonly used as feedback
devices for motor controllers.
– An encoder is a sensor that uses light to sense
the speed and direction of a rotary shaft.
– Encoders produce quadrature outputs which
indicate the speed and direction of the shaft.
– Inherently free from contact wear and the digital
outputs are bounce less.
– Provide greater accuracy and precision.
– Digital Output (2) (A & B).
– Supply Voltage 5 V.
Quadrature Encoders
Sensors 101
What can you do with encoders?
– Everything you can do with a gear tooth sensor.
– PLUS :
Direction indication (you can now tell which direction
your going even if you are being pushed in an opposite
direction).
Greater Precision/Accuracy
Higher Speeds
– AUTONOMOUS MODE:
Move d feet distance.
Circumference of the wheel.
2 * PI * R / CountsPer = Inches per Pulse.
Potentiometers
Sensors 101
Measures the rotation angle in terms of
electrical degrees.
– 0o to 300o-340o depending on the pot.
– Use Linear Taper NOT Audio Taper.
– Wire Wound, Conductive Plastic.
– Limited number of revolutions.
–
–
–
–
Unlimited versions are available, but rare.
Sliders are available instead of rotational.
100K ohm for the OI.
10K, 5K, 1K ohm for the RC.
Analog Output.
Supply Voltage 5V.
Potentiometers
Sensors 101
What can you do with potentiometers?
– Measure the angle of an elbow on an arm
Move the arm up or down until Xo is reached
– Measure the turn of a turret
Turn left or right until Xo is reached
– Raise or lower a telescoping arm
Move up or down until Xo is reached
Sonar/Sonic Rangers
Sensors 101
Emit a sound and measure the time of flight
distance.
Ranges: 6 inches – 20 feet, with a field of view
of approximately 30o.
Other versions available with smaller/narrower
beam widths.
Analog Output ~9.8mv inch
Supply Voltage 3.3-5.5V
Sonar/Sonic Rangers
Sensors 101
What can I do with a sonic range finder?
– Measure the distance to an object.
– Listen for approaching objects.
Another robot is approaching so move away.
Touch/Bumper Sensors
Sensors 101
If pressure is applied to it, an electrical signal is
generated.
What can you do with it?
– I already bumped into something. I better get back,
stop, or move around it.
Infrared Sensors
Sensors 101
Emit modulated infrared (IR) energy and measure
amount of (IR) returned.
Range: inches to several feet.
Led IR sensors have ranges of 3-5 inches.
Problems: Most venues use halogen/tungsten lighting.
90% of the energy emitted is infrared radiation.
Light Sensors
Sensors 101
Red LED emits light and a phototransistor measures the
incoming light.
What can you do with a light sensor?
– Recognize objects of certain colors.
– Follow a line.
Problems: Ambient light and battery level affect sensor
readings.
Autonomous Navigation
Sensors 101
Find the light.
– CMUCam2 Get_Tracking_State().
– Get direction left or right degrees (pan angle).
– Get distance to the light (tilt angle and a lookup table).
Turn towards the light.
– Use the gyro to turn to the matching pan angle.
Drive towards the light.
– Encoders / Gear tooth (Counts per inch).
– Gyro keep driving straight (+/- a few degrees).
Fine tune.
– Sonic Ranger creep in until inches away.
Raise the arm.
– Potentiometer measure the upward angle.
Information Resources
Sensors 101
ChiefDelphi – White Papers
– Working the Angles
http://www.chiefdelphi.com/media/papers/1755
Sample Gyro code
– Quadrature Encoders
http://www.chiefdelphi.com/media/papers/1490
Sample Encoder code
Kevin Watson FAQ’s and Sample Code
– http://www.kevin.org/frc/camera/
– http://www.kevin.org/frc/gyro/
– http://www.kevin.org/frc/encoder/
Vendors and Suppliers
Sensors 101
Digi-Key Electronics
– http://www.digikey.com/
Newark in One
– http://www.newark.com/
Mouser Electronics
– http://www.mouser.com/
Jameco Electronics
– http://www.jameco.com/
Sparkfun
– http://www.sparkfun.com/
USDigital
– http://www.usdigital.com/
Questions ? Answers?
Sensors 101
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