Sensors - 1 Intro to Sensors

Sensors - 2 Physical Principles of Sensing Generation of electrical signals in response to nonelectrical influences Electric Charges, Fields and Potentials Magnetism Induction Resistance Piezoelectric Effect Pyroelectric Effect Hall Effect Seebeck and Peltier Effects Mechanical Measurements Sound Waves Optical Properties of Material

Sensors - 3 Physical Principles of Sensing Generation of electrical signals in response to nonelectrical influences Electric Charges, Fields and Potentials –Coulomb’s Law Capacitance Magnetism –Faraday Law Induction –Varying magnetic field produces a current Resistance –Electrical resistance Piezoelectric Effect –Generate an electric charge due to stress Pyroelectric Effect –Generate an electric charge due to heat Hall Effect –Interaction between moving electric carriers and an external magnetic field Seebeck and Peltier Effects –The basis for a thermocouple Mechanical Measurements –Kinematics, Dynamics, Time Sound Waves Optical Properties of Material

Sensors - 4 Resistive Sensors Potentiometers –Symbol: Strain Gauges (Piezoresistive Effect) Resistive Temperature Detectors –Metals (Linear, positive temperature coefficient) –Symbol: Thermistors –Semiconductors (non-linear, negative temp coef) –Symbol: +tº-tº

Sensors - 5 Resistive Sensors Light-Dependent Resistors –Symbol: Cadmium Sulfide Photoresistor How would you: 1.increase reaction time? 2.increase sensitivity?

Sensors - 6 Packaging

Sensors - 7 Signal Conditioning for Resistive Sensors Voltage Divider Differential Amplifiers Instrumentation Amplifiers +v–+v– R1R1 R2R2 v2v2

Sensors - 8 Capacitive Sensors SENSORS AND SIGNAL CONDITIONING, 2 nd Edition, by Ramon Pallas-Areny and John Webster.

Sensors - 9 Electromagnetic Sensors Sensors based on Faraday’s Law Hall Effect Sensors Electronic Compass Magnetic Field Sensor Board Uses a 3A Hall-effect sensor

Sensors - 10 Piezo Gyro Piezoelectric Sensors Appearance of electric polarization in a material that strains under stress, and vice versa. No DC component (a constant stress initially generates a charge that slowly dissipates) Accelerometer

Sensors - 11 Pyroelectric Sensors Change in temperature causes a change in electric charge Fast (thin with high sensitivity) Incident radiation must be modulated due to parasitic charges that may neutralize the surface charge induced

Sensors - 12 Distance Sensors IR Distance Sensor Sonar IR Distance Sensor Hagisonic StarGazer Robot Localization System

Sensors - 13 Actual Differential Photocell Sensor Schematic Differential Sensor +5V R photo1 R photo2 V sensor 47K

Sensors - 14 Statistical Considerations How many times do you need to sample a sensor? –Mean (  ) What is the error? –Variance (  2 ) –Standard Deviation (  ) Problem: definition of  requires us to know the distribution’s mean, , not the approximation

Sensors - 15 Statistical Considerations Thought: –Expect Consider Degrees of Freedom and redefine:

Sensors - 16 Cd Sensor Homework  90  Angle Analog Readout Handyboard

Sensors  90  Light to the Right Light to the Left Ambiguous Region Ambiguous Region OK! How to Follow a Light Source 1.Determine the Unique Cd Sensor Parameters (Homework) 2.Rotate until light is located (How do you know the light has been found?)

Sensors - 18 What would you select for: Upper limit Lower limit

Sensors - 19 What would you select for: Upper limit Lower limit

Sensors - 20 CdS Structure

Sensors - 21 The Search Behavior Exploring the use of PID Controllers

Sensors  90  Light to the Right Light to the Left Ambiguous Region Ambiguous Region OK! CdS Homework / Lab Homework :Determine the thresholds for each region Lab: Program the robot to follow the light!

Sensors  90  Light to the Right Light to the Left Search Analog Readout Limits Upper Limit Lower Limit

Sensors  90  Light to the Right Light to the Left Search Reduced Limits → Reduced Oscillations? Upper Limit Lower Limit

Sensors  90  Light to the Right Light to the Left Search The Limit Setpoint Limit

Sensors - 26 A Short Introduction to Modern Control Theory ControlPlant u(t)u(t)e(t)e(t)d(t)d(t)y(t)y(t)

Sensors Light to the Right Light to the Left Search Lab Using a PD Controller Setpoint Limit Make the speed of the turns proportional to the distance from the setpoint power = reading - setpoint 0 When is power negative? When is it positive? How would you implement this in your Search code?

Sensors - 28 Implementing Proportional Feedback Control “Follow” Motor Command power = reading – setpoint If power > 0, Turn ? If power < 0, Turn ? {motor( left_motor, ); motor( right_motor,); }

Sensors - 29 Proportional Feedback How to increase the speed of the correction? power = pgain*(reading – setpoint)

Sensors - 30 Proportional Feedback How to increase the speed of the correction? power = pgain*(reading – setpoint) Problem: Instability – can begin oscillating with ever increasing amplitude

Sensors - 31 Modification: Add Differential Controls PD Controllers use both proportional and differential controls. Think of differential control as a velocity term. If you need to change direction and your velocity is large, you do not need as large of a correction compared to when the speed was low.

Sensors - 32 Modification: Add Differential Controls power = pgain*(reading – setpoint) - dgain*velocity where velocity = (reading – old_reading) PD Controllers use both proportional and differential controls. Think of differential control as a velocity term. If you need to change direction and your velocity is large, you do not need as large of a correction compared to when the speed was low.

Sensors - 33 Summary of PID Controller Characteristics ControllerBenefitsDrawbacks PSimple Cheap Steady state error Large overshoot & settling time Poor transient response Prone to instability (large gains) IEliminates steady-state errorPoor damping Large overshoot & settling time Poor transient response Reduces stability DIncreases damping Reduces overshoot Reduces settling time Improved transients Improves stability Steady-state error Cannot be used alone

Sensors - 34 Lab Report Be sure to contrast and compare how the robot performs using: –Thresholds (e.g., no controller) –Proportional Controller –Proportional-Differential Controller PS Bring a Bright light with a Wide beam (12V flashlight if available!)