Sensors and Actuators! Robert Stengel! Robotics and Intelligent Systems, MAE 345, Princeton University, 2013"Copyright 2013 by Robert Stengel. All rights reserved. For educational use only.!http://www.princeton.edu/~stengel/MAE345.html!• Biological Antecedents"• Critical Elements for System Observation and Control"• Control Effecters"• Output Sensors"• Navigation"Biologically Inspired Control"• Declarative Planning"• Procedural Formatting"• Reflexive Control"• Sensory input"• Motor output"Feedback Control Requires Sensors and Actuators"• Sensors and actuators have their own dynamic characteristics"• Desirable properties"– High bandwidth ( faster than system to be controlled)"– Accuracy"– Precision"– Large dynamic range"– Sufficient power for control"– Reliability"– Low cost"Peripheral Sensory and Motor Neurons"Synapse: chemical or electrical axon-dendrite connection!Sensory and Motor Signal Paths to the Brain"Reflexive response is processed in the spinal roots"Declarative and procedural response is processed in the brain"Skeletal Muscle"• Attached to the skeleton to produce motion of limbs, torso, neck, and head"• Agonist-antagonist muscle pairs produce opposing motion"• End-effecter strength depends on lever arm and varies with joint angle"• Voluntary (declarative) commands from somatic central nervous system"Sensory Neuron Receptors"Neuron Receptors (corpuscles, disks, cells, muscle spindles) generate action potentials that are sensed by the neuron soma"Cutaneous and Sub-Cutaneous Receptors"The Eye"Retinal Cross Section"Rod and Cone !Cells!Retinal Ganglion !Cells!Amacrine and !Horizontal Cells!Biological Inertial Measurement: The Inner Ear"Measures linear and angular acceleration"Integration with eye motion"ActuatorsRubbertuator"Pneumatic analog of muscle"Contraction under pressure"Agonist-antagonist action produces rotation"• Robot arm"Princetons SLIM Robot, 1993!Hydraulic Actuator"Electric Actuator !Brushed DC Motor"• Current flowing through armature generates a magnetic field"• Permanent magnets torque the armature"• When armature is aligned with magnets, commutator reverses current and magnetic field"• Multiple poles added to allow motor to smooth output torque and to start from any position "Two-pole DC Motor!Electric Actuator !Brushless DC Motor"• Armature is fixed, and permanent magnets rotate"• Electronic controller commutates the electromagnetic force, providing a rotating field"• Advantages"– Efficiency"– Noise"– Lifetime"– Reduced EMI"– Cooling"Electric Actuator !Stepper Motor"• Brushless, synchronous motor that moves in discrete steps"• Precise, quantized control without feedback"• Armature teeth offset to induce rotary motion"Actuation Linkages"Belt Linkage!http://www.youtube.com/watch?v=FV_P7GBAAgo"• Gearing, leverage"• Gears"• Belts, Chains, Cables"• Bellcranks"Reaction Wheel, Control-Moment Gyro "• Flywheel on a motor shaft"• Reaction wheel rpm is varied to trade angular momentum with a spacecraft for control"– Three orthogonal wheels vary all components of angular momentum"– Fourth wheel at oblique angle provides redundancy""• Control moment gyro rpm is fixed, axis is rotated to impart torque"from Joe Munder, Lockheed!SensorsMagnetometer "• Flux gate “compass”"– Alternating current passed through one coil"– Permalloy core alternately magnitized by electromagnetic field"– Corresponding magnetic field sensed by second coil"– Distortion of oscillating field is a measure of one component of the Earths magnetic field"• Three magnetometers required to determine Earths magnetic field vector"Sun Sensor "• Distance from centerline measured by sensed pattern, which determines angle, α"• With index of refraction, n, angle to sun, α, is determined"• Photodetectors may provide digital (coarse) or analog (fine) outputs"tanα= d / hsinα' = nsinα(Snell 's law)n = index of refractionPotentiometer, Synchro, and Tachometer"Synchro!Potentiometer!Tachometer!Angular Encoder"Linear Variable Differential Transformer"Tactile Sensors"Photoelectric Key!Capacitive Touchpad!Pressure-Sensitive Touchpad!Strain Gauge"ε=ΔRRo⎛⎝⎜⎞⎠⎟Gauge FactorWheatstone Bridge!Force Sensors"Force = Stiffness x Displacement(Strain)Pressure and Temperature Sensors "Deflection of Diaphragm Between Chambers at Different Pressure"Deflection of Bi-Metallic Element"Thermistors"! Variation in "! Capacitance or"! Resistance"! Mercury switch - on/off"! Variation in Resistance"Air Data Sensors "Radar and Sonar"Tracking (Pulse) Radar"Adaptive Cruise Control Radar"Doppler Effect (wave source moving to the left)"(Doppler) Radar Gun"Handheld Sonar"Active Electronically Steered Array Tracking Radar"http://www.youtube.com/watch?v=LOgRBtbEuig"Ultrasonic Rangefinder"Transmit/Receive Unit!Antenna Pattern!Chirp Spectrum!SensComp (Polaroid ) Devices!Transmitted!Received!• 5 chirps/s"– 8 pulses @ 60kHz"– 8 pulses @ 57kHz"– 16 pulses @ 53kHz"– 24 pulses @ 50kHz"Triangulation Rangefinders"Video and Computer Vision"CCD Sensor"Optic Flow"CMOS Device"Spring Deflection Accelerometer"Proof Mass"• Deflection is proportional to acceleration"• Damping required to reduce oscillation" Δx = −ksΔx m Δx =mksΔxForce Rebalance Accelerometer" Δx = fxm =torque moment armm⇒ Δx  0• Torquer voltage required to re-center the proof mass becomes the measure of acceleration"• Example of closed-loop control "MicroElectroMechanical System (MEMS) Accelerometer"3-DOF MEMS Accelerometer"Mechanical Gyroscope "• Body-axis moment equation " MB=hB+ωBhB= IBωB+ωBhBConstant nominal spin rate, n, about z axis"Ixx = Iyy << Izz "Small perturbations in ωx and ωy" ˙ ω B= IB−1MB−˜ ω BIBωB( )Angular momentumhB= IBωBTypes of Mechanical Gyroscope "! Two-degree-of-freedom gyro"– Free gyro mounted on a gimbaled platform"– Gyro stores reference direction in space"– Angle” pickoffs” (encoders) on gimbal axes measure pitch and yaw angles"! Single-degree-of-freedom gyro"– Gyro axis constrained to rotate in its case with respect to the output axis, y, only"– Synchro measures axis rotation, and