9a Robotic Systems Sensors

7/29/2019 9a Robotic Systems Sensors

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Robotic Systems(9a)

Dr Richard Crowder

School of Electronics and Computer Science


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Robot Sensors Global objective: Acquire knowledge and resolve uncertainty about the

position or quality of an object and/or its environment.

Application

Robot parameters for the internal control loops

Locating objects

Correcting for errors between the world and the robots model

Detecting and avoiding failures

Detecting and avoiding collisions

Monitoring interactions (force control)

Monitoring the environment for changes (temperature)

Inspection

Guidance


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Sensing within a Gripper

Slip sensorTouchsensor


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Tactile sensing to control a gripper


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Sensing process

TransducerSignal

conditioningADC orsimilar

Datareduction& analysis

Modelanalysis

PhysicalQuantity

ElectricalSignal

Linearised &amplifiedsignal

Digitalsignal

Model

Perception

Depending on the sensor and application, not all steps are required


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Hierarchy Perception analysis of the model to infer

State of the world

Consequence of actions

Collisions

Model formation of a geometric model Data compression

Sensor fusion

Feedback to actuators

Measurement physical model

Conversion of electrical signals to physical quantities

Physical

Interface

Signal conditioning

transducers


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Types

Internal to the robot

Joint position: Encoders, Resolvers

Safety: Joint limit switches

External to the robot

Tactile, Force and Slip

Vision

Chemical (pheromones)

.


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Tactile Sensing

Tactile

The detection and determination of the spatial distribution offorces over an defined area.

Touch

Single point sensing (can be binary touch or no-touch)

Force

Measurement of forces along the principle axes

Slip

Measurement of the loads velocity relative to the sensor


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Typical Characteristics

Sensor surface must be compliant and durable

Spatial resolution should be approximately 1mm

Sensor should be stable, monotonic, and repeatable

Sensors require as low hysteresis as possible

Response time typically 1-10ms

A sensitivity of 0.01N, and a dynamic range of 1000 : 1

A typical array should have 50 - 200 individual elements.

Harmon, L. D. 1982. Automated tactile sensing. Int. J. Robotics Research 1(2):331.


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Sensing Technologies

Strain gauges

=

GFR

R

G

Where GF is the gauge factor (typically 2)and is the strain
http://en.wikipedia.org/wiki/Image:Strain_gauge.svg


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Resistive foam

0 0.2 0.4 0.6 0.8 1

0.5

1

Applied Force

Sensor

Resistance

Protective Surface

Conductive Foam

Dot and Ring

on substrate


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Piezoelectric

PVDF Polyvinylidene Fluoride

Top and bottom surfaces

Coated with aluminium

Chargeamplifier

Long chain molecules

Orientated during manufacture


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Optical

Optical receiver

Optical source

Deformable tube

Spring steel

Reflective surface

Combined optical receiver

and source


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Photelastic

Receiver

fibre

Sourcefibre

Photoelastic

material

Analyser

Polariser


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Tactile technology summary

SENSOR ADVANTAGE DISADVANTAGEMechanical Arrays Simple Complex to manufacture

Poor spatial resolution

Resistive (including FSR) Wide dynamic rangeDurable

Good overload tolerance

HysteresisLimited spatial resolution

Capacitive Wide dynamic rangeRobust

Susceptible to EMITemperature sensitiveLimitation in spatial resolution

Magnetic Wide dynamic rangeRobust

Poor spatial resolutionSusceptible to EMI

Optical (intrinsic and extrinsic) Intrinsically safeVery high resolution possible

Electronics can be complex

Piezoelectric Wide dynamic rangeGood mechanical properties

Pyroelectric effectDynamic response onlyCharge amplifier


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Tactile sensing to control a gripper


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Sensor Functionality

Sensor Location Parameter

Tactile array sensor Outer surface of thefinger tip

Pressure distribution,local shape

Fingertip, force torquesensors

Finger tip structure Contact force andtorque vectors

Finger joint anglesensor

Finger joint or at amotor

Finger tip position,contact location

Actuator effort"

sensor

Motor or joint Motor torque

Dynamic tactile sensor Outer surface of thefinger tip

Vibration, stress,changes, slip


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Touch sensors in manipulation


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Object recognition using tactile sensing

The key is to break the sensing down into a number of primitive steps:surface properties, structural properties andfunctional properties.One approach to the problem could be:

Approach: The end effector moves to grip the object; the points of

contact will give an initial indication of shape. Shape: Movement of the fingers over the object, using a compliant

frame with no specific gaol.

Texture: derived from the mean and standard deviation of thesensor values.

Hardness: Force is applied and the resultant localiseddisplacements are measured.

Thermal: Used to distinguish between metal and other materials.


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Problem with tactile arrays

The resolution is poor, tens of taxels against thousands of pixels.

The background of a tactile image is clear - no object no force.

A tactile sensor suffers from considerable cross talk, while equivalent to visual

blurring it cannot be easily solved in the same manner.

Applied force


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Slip Sensing

Tactile array image

Touch data (photo elastic sensor)

Special purpose sensor

Spring loaded Roller,Connected to aposition sensor


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Tactile sensors - applications

Industrial automation is clearly the area of greatestopportunity in robotics. Such operations as assembly, pick-and-place, grinding, and inspection will becomeincreasingly more automated in the next 10 years. Touchsensing, now extremely primitive, can be expected to evolveinto a highly developed technology in the near future.(Harmon 1982, p. 30)

However this is not been the case the key drivers was notconventional industrial applications.


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Future growth areas

Surgery and Medicine

Very rapid take-up Restore taction in MIS

Disposable equipment Laparoscopy improvements

Soft tissue discrimination Hardness/softness sensing

Health Care and Service RoboticsDemographic projections Personal space manipulation

Enormous demand imminent Mobility aids

Cost reduction essential Automated household tools

Haptic exploration, dexterity Safety

Adaptation, customization Reliability

Low costs User acceptance


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Force Control and Measurement

Force control

Active

The system will respond to sensory information Passive

inherently correct, reliance on springs or compliantsystems


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Remote Centre Compliance - RCC

TheRCCsystem is a passive system, and relies on a springand link structure.

The system can correct for minor error in position, both

rotational and translational.

The design relies on the RCC and the object withstandingthe applied forces


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RCC

Translation Rotation

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9a Robotic Systems Sensors