Sensors and Actuators - Intranet DEIBhome.deib.polimi.it/gini/robot/docs/sensor-actuators_14_15.pdf · Robot actuators: – Motors of various ... The beam follows different optical

A.A. 2014/2015

Roboticsfor Computer Engineering students

Marcello RestelliDipartimento di Elettronica e InformazionePolitecnico di Milanoemail: [email protected]: 02-2399-4015

Sensors and Actuators

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Effectors and Actuators

Effector– Any device robot that has an impact on the

environment– Effectors must match a robot’s task– Controllers command the effectors to achieve the

desired task

Actuator– A robot mechanism that enables the effector to

execute an action

Robot effectors are very different than biological ones– Robots: wheels, tracks, legs, grippers

Robot actuators:– Motors of various types– Passive actuation

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Types of Actuators

Electric motorsHydraulicsPneumaticsPhoto-reactive materialsChemically reactive materialsThermally reactive materialsPiezoelectric materials

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Most Popular Actuators

First robots used pneumatic and hydraulic actuators– hydraulic actuators are expansive, weighing, and their

maintenance is hard● used only in big robots

– pneumatic actuators are used for application requiring stop-to-stop trajectories, such as pick-and-place

Nowadays the most common actuators are electrical motors both DC and AC

– since these motors reach high speeds they are typically reduced by gearing that make the dynamics more complex

– typically each joint has its own motor, but it may happen that the same motor may actuate several joints through transmissions

– for stepper motors internal sensors are not required, but when an error occurs their position is unknown

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DC Motors

DC (direct current) motors– Convert electrical energy into

mechanical energy– Small, cheap, reasonably efficient,

easy to use

How do they work?– Electrical current through loops of

wires mounted on a rotating shaft – When current is flowing, loops of wire

generate a magnetic field, which reacts against the magnetic fields of permanent magnets positioned around the wire loops

– These magnetic fields push against one another and the armature turns

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DC Motors

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DC Motors

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DC Motors

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DC Motors

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DC Motors

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DC Motors: Brushed and Brushless Motors

Brushes are used to change the magnetic polarity of the electromagnet

Brushed motors are cheap but have many drawbacks– Brushes eventually wear out– Brushes make noise– Limit the maximum speed– Hard to cool– Limit the number of poles

Brushless DC motors overcome these problems but they are more expensive

– Brushes are replaced by computer– Permanent magnets on the rotor– Electromagnets on the stator

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Operating/Stall Current

When provided with constant voltage, a DC motor draws current proportional to how much work it is doing

– Work = Force * Distance

When there is no resistance to its motion, the motor draws the least amount of currentWhen the robot pushes against an obstacle motors drain more currentIf the resistance becomes very high the motor stalls and draws the maximum amount of current (stall current) at its specified voltage

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Torque

Torque: rotational force that a motor can deliver at a certain distance from the shaftStrength of magnetic field generated in loops of wire is directly proportional to amount of current flowing through them and thus the torque produced on motor's shaftThe more current through a motor, the more torque at the motor's shaftStall torque: the amount of rotational force produced when the motor is stalled at its recommended operating voltage, drawing the maximal stall current at this voltageTorque units: ounces*inches or N*m

– 9.8 N*m torque means motor can pull a weight of 1kg through a pulley 1m away from shaft

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Power of a Motor

Power: product of the output shaft's rotational velocity and torqueIf there is no load on the shaft then P=0

– rotational velocity is maximum, but the torque is 0

– the motor is spinning freely

If the motor stalled then P=0– it is producing its maximal

torque– rotational velocity is zero

A motor produces the most power in the middle of its performance range

τm=τs(1−ωmωmax )

ωm=ωmax (1−τmτs )

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Efficiency of a Motor

Motor efficiency is power out divided by power in

Power out is mechanical energy

Power in is electrical energyPoutput=τ⋅ω

Pinput=V⋅I I=V s−V e

RV e=k e⋅ω

η=PoutputPinput

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Motor Efficiency and Operating Voltage

DC motors are not perfectly efficient– Due to friction some energy is wasted as heat– Industrial-grade motors (good quality): 90%– Toy motors (cheap): 50%– Micro-motors for miniature robots < 50%

To make the motor run, electrical power must be provided in the right voltage range

– if the voltage is lower than the motor runs fine even if it is less powerful

– if the voltage is higher the life of the motor becomes shorter

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How Fast do Motor Turn?

Free spinning speeds (most motors)– 3000-9000 RPM (50-150 Hz)

High speed, low torque– drive light things that rotate very fast

What happens with heavy robots or manipulators?– it is required more torque and less speed

The solution consists of using gearing– Trade-off high speed for more torque

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Gearing

Torque: T = F x r– rotational force generated at the

center of a gear is equal to the gear's radius times the force applied tangential at the circumference

Meshing gears: – by combining gears with different

ratios we can control the amount of force and torque generated

Example: r2 = 3r

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– Gear 1 turns 3 times while gear 2 turns only once

– T1*360 = T

2*1080

– T2 = 3*T

1 = T

1*r

2/r

1

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Gearing Effect on Speed

Combining gears has a corresponding effect on speed

– A gear with a small radius has to run faster to keep up with a larger gear

● Increasing the gear radius reduces the speed

● Decreasing the gear radius increases the speed

Torque – Speed tradeoff– when a small gear drives a large one,

torque is increased and speed is decreased

– analogously, when a large gear drives a small one, torque is decreased and speed is increased

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Designing Gear Teeth

Reduced backlash– the looseness between mashing gear teeth

Tight meshing between gears– increases friction

Proportionally sized gears– a 24-tooth gear must have a radius three times the

size of an 8-tooth gear

Example– Input (driving) gear: 8 teeth– Output (driven) gear: 24 teeth– Effect at the 24 teeth gear

● 1/3 reduction in speed ● 3 times increase in torque

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Gear Reduction in Series

By putting two 3:1 gear reductions in series (“ganging”) a 9:1 gear reduction is created

– the effect of each pair of reductions is multiplied

– key to achieve useful power from a DC motor

With such reductions, high speeds and low torques are transformed into usable speeds and powerful torques

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Motor Control: PWM

Motors can be controlled by modulating the input voltage (or current)

Use of linear amplifierPower inefficient and impractical

Alternative: Pulse Width Modulation (PWM)switch voltage ON/OFFfrequency from 2 to 20 kHz (against a 100Hz bandwith)higher frequencies are preferred (non audible), but...

over-heatvoltage spikesinterference become prominent

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Motor Control: PWM

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Motor Control: PWM

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Servo Motors

Specialized motors that can move their shaft to a specific positionFor DC motors is only possible to specify one direction“Servo”

– capability to self-regulate its behavior, i.e. to measure its own position and compensate for external loads when corresponding to a control signal

– often used in hobby radio control applicationsServo motors are built from DC motors by adding

– Gear reduction– Position sensor for the motor shaft– Electronics that tell the motor how much to turn and in

what directionMovement Limitations

– shaft travel is restricted to 180 degrees– sufficient for most applications

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Sensors

Sensors allow a robot to accomplish more complex tasks autonomouslyTwo main categories

– Internal sensors– External sensors

● sensors with contact● sensors without contact

Other classification– Passive sensors (measure a physical property)– Active sensors (emitter + detector)

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Encoders

An encoder is a sensor for converting rotary motion or position to a series of electronic pulsesLinear architecture

Consist of a long linear read track, together with a compact read head

Rotary architectureServe as measuring sensors for rotary motion and for linear motion when used in conjunction with mechanical measuring standards such as leadscrews, and convert rotary motion (incremental or absolute) into electrical signalsThey are both effective and low cost feedback devices.

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Incremental Encoders

It is based on the photoelectric principleIt consists of a disk with two traces where transparent and opaque zones are alternatedThe presence of two traces allows to identify the rotation directionN: number of steps (number of light/dark zones per turn)Since the two signals are ¼ step shifted, resolution is 360°/4N

Notch to define an absolute mechanical zero

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Absolute Encoders

It is a disk with transparent and opaque areas, placed on concentric ringsFor an N-bit word there are N ringsResolution: 360°/2N

To avoid reading ambiguities binary codes with single variations (Gray code) are usedIn robotic applications at least 12 rings are used (360°/4096)

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What is perceived?

Sensor may be classified according to what they measure

– distance– proximity– contact– force and torque– vision – position

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Distance Perception

Measure the distance between a reference point and object placed in front of the sensorHuman beings use stereo-vision, while other animals (like bats, dolphin, and whales) use echolocationKnowing the distance of the surrounding objects is useful for obstacle avoidance and for more complex planning activity

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Distance Perception: Reflective Optosensors

An ease way to compute distance is to use triangulationReflective optosensors are active sensors

– emitter: a source of light (LED, light emitting diodes)– detector: a light detector (photodiode or

phototransistor)

The emitter scans the surface with a beam of lightThe detector measure the angle corresponding to the maximum intensity of lightCalling s the distance between the emitter and the detector, the distance from the object is computed as

d=s

tanαi

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Distance Perception: Kinect

Kinect is a motion sensing input device built by Microsoft for Xbox 360A cheap device that provides several sensing informationUsed in many robotic research studies Provides

– 30Hz 8-bit RGB camera (640x480)– 3D scanner

● Infrared projector● Infrared camera (11-bit 640x480)● Range 1.2 – 3.5 m (up to 0.7-6 m)● Angular field of view: 57° h, 43° v

– Multi-array microphone

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Distance Perception: Phase Shift Telemeter

The light emitted is split into two parts– against the object– against a mirror placed inside the sensor

The beam follows different optical paths and the two reflected waves have different phasesThe distance of the object must lead to phase displacement within [0°;360°]Laser wavelength is around 1e-6mAcoustic waves are not directionalThe solution is to modulate the laser light with a wave characterized by a long wave length

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Distance Perception: Time-of-Flight Telemeter

It measures the time between the instant the emitter produces the signal and the instant the detector receives its reflectionThe distance covered by the signal is 2dThe time is ΔT = 2d/cThe speed of light is too high for robotic applicationsAcoustic waves are better (v=340 m/s)

– are characterized by low directionality (20 – 40°)– the reflection is dumped and the signal is largely

affected by noise– Polaroid ultrasonic sensors

● range 0.3 – 10m● accuracy 0.025m● cone opening 30°● Frequency 50 KHz

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Proximity Perception

Proximity sensors measure the presence of objects within a specified distance rangeThey are used to grasp objects and avoid obstaclesSensors

– ultrasonic (low cost)– inductive (perceive only ferromagnetic materials under

the distance of 1mm)– Hall effect (perceive only ferromagnetic materials, may

be small, robust, and cheap)– Capacitive (perceive any object, binary output, high

accuracy only when calibrated for a particular object)– Optical (infrared light, binary output)

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Tactile Sensors

These sensors are used for manipulation purposesTwo main categories

– binary● are realized by switchers ● typically they are placed on the fingers of a manipulator● they may be arranged in arrays● may be placed also on the external side of the hand to

avoid obstacles– analogical

● soft devices that produce a signal proportional to the local force

● typically realized with a spring coupled with a shaft● otherwise soft conductive material that change its

resistance according to its compression● there are sensors that measure also movements

tangential to the sensor surface

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Force and Torque Sensors

Typically these sensors are used at joint level and in the wristFor joints driven by DC motors the force is measured by the currentThe measure of the strain is based on elasticity6 parameters in the Cartesian space

– 3 forces along axes– 3 torques around axes

Very expensive

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Position Sensor

The main sensor to determine the absolute position of a robot is the Global Positioning System (GPS)

– 21 satellites– it is based on the flying time of a radio signal– At least 4 sensors must be perceived– Measuring rate is 2Hz– Accuracy is about 1.5m– with DGPS accuracy arrives at about 2cm

Unfortunately GPS sensors may not be used in indoor environments, underground, underwater, or in urban situations with skyscrapers

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Inertial Sensor

Gyroscopes– Angular velocities

Accelerometers– Gravitational vector

Magnetometers/compass– Magnetic field vector

Used in many mobile and console devices

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Sensor Fusion

A man with one watch knows what time it isa man with two watches isn't so sure

To have a better representation of the world we need to combine measurements from multiple sensors that present also redundancySensor fusion is a complex problem

– different sensor accuracy– different sensor complexity– contradictory information– asynchronous perception

Cleverness is needed to put this information together