1 1540 Introduction To Mechatronics: Slide 1Hugh Durrant-Whyte Introduction to Mechatronics Mech-1540 Sensors 1 1540 Introduction To Mechatronics: Slide 2Hugh Durrant-Whyte Industrial Sensors • Proximity – Mechanical – Optical – Inductive/Capacitive • Position/Velocity – Potentiometer – LVDT – Encoders – Tachogenerator • Force/Pressure • Vibration/acceleration 1540 Introduction To Mechatronics: Slide 3Hugh Durrant-Whyte Definitions • Accuracy: The agreement between the actual value and the measured value • Resolution: The change in measured variable to which the sensor will respond • Repeatability: Variation of sensor measurements when the same quantity is measured several times • Range : Upper and lower limits of the variable that can be measured • Sensitivity and Linearity 1540 Introduction To Mechatronics: Slide 4Hugh Durrant-Whyte Proximity Sensors • Widely used in general industrial automation – Conveyor lines (counting,jam detection, etc) – Machine tools (safety interlock, sequencing) • Usually digital (on/off) sensors detecting the presence or absence of an object • Consist of: – Sensor head: optical, inductive, capacitive – Detector circuit – Amplifier – Output circuit: TTL, solid state relay 1540 Introduction To Mechatronics: Slide 5Hugh Durrant-Whyte Mechanical Proximity Switches • Essentially a mechanical switch • On/off operation only • Two general modes – Normally Open (NO) – Normally Closed (NC) • Come in a wide variety of mechanical forms • For a wide range of uses Actuator Common Normally Closed Normally Open 1540 Introduction To Mechatronics: Slide 6Hugh Durrant-Whyte Example Mechanical Proximity Switches 2 1540 Introduction To Mechatronics: Slide 7Hugh Durrant-Whyte When to Use Mechanical Proximity Switches • Where physical contact is possible • Where definitive position is required • In operation-critical or safety-critical situations • Where environment conditions preclude the use of optical or inductive sensors 1540 Introduction To Mechatronics: Slide 8Hugh Durrant-Whyte Applications and Use of Mechanical Proximity Switches • Easy to integrate into machinery of all types • Requires contact (thus wear) • Range of voltages: DC 0-1000V, AC, etc. • Very robust (explosion proof if required) • Usually used as: – Limit switch – Presence/absence indicator – Door closed/open 1540 Introduction To Mechatronics: Slide 9Hugh Durrant-Whyte Places You Find Mechanical Proximity Switches ! 1540 Introduction To Mechatronics: Slide 10Hugh Durrant-Whyte Optical Proximity Sensors • Consist of a light source (LED) and light detector (phototransistor) • Modulation of signal to minimize ambient lighting conditions • Various models: 12-30V DC, 24-240V AC, power • Output: TTL 5V, Solid-state relay, etc. Demodulator Amplifier Modulator Power Output Mixer Signal Power Supply Load 1540 Introduction To Mechatronics: Slide 11Hugh Durrant-Whyte Operational mode: 1540 Introduction To Mechatronics: Slide 12Hugh Durrant-Whyte Operational Modes • Through Beam: – Long range (20m) – Alignment is critical ! • Retro-reflective – Range 1-3m – Popular and cheap • Diffuse-reflective – Range 12-300mm – Cheap and easy to use 3 1540 Introduction To Mechatronics: Slide 13Hugh Durrant-Whyte Example Optical Proximity I Optical Fibre Delivery System 1540 Introduction To Mechatronics: Slide 14Hugh Durrant-Whyte Example Optical Proximity II Slot Beam Systems 1540 Introduction To Mechatronics: Slide 15Hugh Durrant-Whyte When to use an Optical Proximity Sensor • Non-contact, no moving parts, small. • Fast switching, no switch bounce. • Insensitive to vibration and shock • Many configurations available • Alignment always required • Can be blinded by ambient light conditions (welding for example) • Requires clean, dust and water free, environment 1540 Introduction To Mechatronics: Slide 16Hugh Durrant-Whyte Applications of Optical Proximity Sensors • Stack height control/box counting • Fluid level control (filling and clarity) • Breakage and jam detection • And many others… http://www.omron-ap.com/application_ex/index.htm http://www.sick.de/english/products/products.htm http://content.honeywell.com/sensing/prodinfo/ 1540 Introduction To Mechatronics: Slide 17Hugh Durrant-Whyte Other Optical Devices Light Curtain Collision Detection 1540 Introduction To Mechatronics: Slide 18Hugh Durrant-Whyte Ultrasonic Proximity Sensors • Use sound pulses • Measures amplitude and time of flight • Range provides more than on/off information • Frequencies 40KHz-2MHz Pulse Echo Vibrating Membrane (metal or ceramic) Sensor Object 4 1540 Introduction To Mechatronics: Slide 19Hugh Durrant-Whyte When to use Ultrasonic Sensors • Provide range data directly: • Level monitoring of solid and liquids • Approach warning (collisions) • Can (usually) work in heavy dust and water • Ambient noise is potentially an issue http://www.automationsensors.com/ 1540 Introduction To Mechatronics: Slide 20Hugh Durrant-Whyte Example Applications Car Wash Application Paper roll Thickness Monitor Waste water flow volume 1540 Introduction To Mechatronics: Slide 21Hugh Durrant-Whyte Inductive and Capacitive Proximity Sensors • Inductive sensors use change in local magnetic field to detect presence of metal target • Capacitive Sensors use change in local capacitance caused by non-metallic objects • Generally short ranges only • Regarded as very robust and reliable 1540 Introduction To Mechatronics: Slide 22Hugh Durrant-Whyte Example Inductive Sensors I Detection of open/close functions Detection of rotation 1540 Introduction To Mechatronics: Slide 23Hugh Durrant-Whyte Example Inductive Sensors II Bulk mounted inductive sensors. Detect presence of object without contact. Range 3mm +/- 10% 1540 Introduction To Mechatronics: Slide 24Hugh Durrant-Whyte Example Capacitive Sensors Panel Mounted Capacitive Sensor. Can detect wood, plastic and metal. Range 3mm-25mm Flat mounted Capacitive Sensor. Used for detecting panels of glass. Range=10mm +/- 10% 5 1540 Introduction To Mechatronics: Slide 25Hugh Durrant-Whyte Position and Velocity Measurement • Position and velocity measurement is often required in feedback loops • For positioning, and velocity control • Position measurement: – Potentiometers – LVDT – Encoders • Velocity Measurement: • Tachometer 1540 Introduction To Mechatronics: Slide 26Hugh Durrant-Whyte Potentiometers R V in V out An analog sensor Works as a voltage divider 1540 Introduction To Mechatronics: Slide 27Hugh Durrant-Whyte Types of Potentiometer • Wirewound – Wiper slides along coil of Ni-chrome wire – Wire tends to fail, temperature variations • Cermet – Wiper slides on conductive ceramic track – Better than wire inmost respects • Plastic film – High resolution – Long life and good temperature stability 1540 Introduction To Mechatronics: Slide 28Hugh Durrant-Whyte Linear Potentiometers 1540 Introduction To Mechatronics: Slide 29Hugh Durrant-Whyte When to use a Potentiometer • Require analog signal for control • Require absolute positional information • Low cost • Temperature and wear variations • Not in dusty or wet environments 1540 Introduction To Mechatronics: Slide 30Hugh Durrant-Whyte Linear Variable Differential Transformer (LVDT) • An LVDT consists of a magnetic core that moves in a cylinder • The sleeve of the cylinder contains a primary coil that is driven by an oscillating voltage • The sleeve also contains two secondary coils that detect this oscillating voltage with a magnitude equal to displacement • The automatic nulling that can be achieved using two coils makes LVDTs very accurate (submillimetre) V in sin ω t V out sin( ω t+ φ) Phase measurement 6 1540 Introduction To Mechatronics: Slide 31Hugh Durrant-Whyte LVDT Signal Conditioning • Uses AC modulation, demodulation and phase comparison • Available in a single monolithic package Power Supply Carrier Oscillator Amplitude Control LVDT Current Amplifier Demodulator Phase Shifter Zero Set AC Power 1540 Introduction To Mechatronics: Slide 32Hugh Durrant-Whyte Example LVDTs Spring-loaded Standard for use In hydraulic cylinders Free core LVDTs for use in hostile environments And total emersion 1540 Introduction To Mechatronics: Slide 33Hugh Durrant-Whyte When to use an LVDT • High accuracy • Linear operation (synchro resolver is equivalent rotary LVDT) • Harsh environment • Analog position control • Embedding (in cylinder for example) 1540 Introduction To Mechatronics: Slide 34Hugh Durrant-Whyte Optical Encoders • Encoders are digital Sensors commonly used to provide position feedback for actuators • Consist of a glass or plastic disc that rotates between a light source (LED) and a pair of photo- detectors • Disk is encoded with alternate light an dark sectors so pulses are produced as disk rotates 1540 Introduction To Mechatronics: Slide 35Hugh Durrant-Whyte Encoder Internal Structure 1540 Introduction To Mechatronics: Slide 36Hugh Durrant-Whyte Incremental Encoders • Pulses from leds are counted to provide rotary position • Two detectors are used to determine direction (quadrature) • Index pulse used to denote start point • Otherwise pulses are not unique 7 1540 Introduction To Mechatronics: Slide 37Hugh Durrant-Whyte Absolute Encoders • Absolute encoders have a unique code that can be detected for every angular position • Often in the form of a “grey code”; a binary code of minimal change • Absolute encoders are much more complex and expensive than incremental encoders 1540 Introduction To Mechatronics: Slide 38Hugh Durrant-Whyte Encoder processing • Need a squaring circuit to digitise signal • A counter and index monitor • Generally available in monolithic form • Often with algorithms for control externally programmable 1540 Introduction To Mechatronics: Slide 39Hugh Durrant-Whyte When to Use an Encoder • Require accurate position information: – 10,000 line incremental – 360 line absolute • Digital feed-back loop • Compact and reasonably rugged (not as good as inductive) • Linear encoders also available 1540 Introduction To Mechatronics: Slide 40Hugh Durrant-Whyte Tachometers • Measurement of rotary speed using a DC generator • Essentially a motor running in reverse • Used to be common to have these attached to motors to enable direct analog feedback • Much less common now with digital control (use incremental encoders) Tacho generator for large industrial plant (GE) 1540 Introduction To Mechatronics: Slide 41Hugh Durrant-Whyte Force and Pressure • Force and Pressure generally measured indirectly through deflection of an alternate surface • Mechanism include: – Physical motion and measurement using (eg) an LVDT – Strain gauges (metal that changes resistance when stressed) – Piezo electric materials that generate a current when deformed 1540 Introduction To Mechatronics: Slide 42Hugh Durrant-Whyte LVDT Load Cell Table Force Spring or Piston LVDT Outer Platform 8 1540 Introduction To Mechatronics: Slide 43Hugh Durrant-Whyte Strain Gauge Bridge Tension Strain Gauges 1540 Introduction To Mechatronics: Slide 44Hugh Durrant-Whyte Example Load Cells Reaction torque load cell Axial load cell http://www.entran.com/ltoc.htm Subminature Load cells 1540 Introduction To Mechatronics: Slide 45Hugh Durrant-Whyte Sub-miniature Load cells All signal conditioning and amplification integrated with the sensor Load cell bridge structure 1540 Introduction To Mechatronics: Slide 46Hugh Durrant-Whyte Piezo Load Cells • Distortion of crystal, either quartz or BaTiO 3 • Used for accurate measurement of small loads • Come in the form of: • single axis load washers • or multiple axis load washers and tables 1540 Introduction To Mechatronics: Slide 47Hugh Durrant-Whyte Pressure • Pressure measured by: – Pitot tube and – Deformation of fixed membrane • Deformation measured using same methods as for force: • Spring (manometer) • Piezo distortion • Strain gauges Miniature Industry IP69 High Temperature 1540 Introduction To Mechatronics: Slide 48Hugh Durrant-Whyte Acceleration • Acceleration is also measured via the force exerted by an accelerating mass • Distortion of a piezo • Motion of a cantilever • Strain on mass restraints • Accelerometers mainly used to measure vibration Single Axis, 10,000g Shielded for Severe environment EMI shielded 9 1540 Introduction To Mechatronics: Slide 49Hugh Durrant-Whyte Tri-axial Accelerometers • Triaxial accelerometers used in mobile systems – In high-performance cars – Inside rotating elements of turbines – In aircraft elements • Provide vibration information • Provide short-term position data Triple axis Accelerometer For racing cars 1540 Introduction To Mechatronics: Slide 50Hugh Durrant-Whyte Silicon Machined Accelerometers Cantilever beams Used in eg air-bags 1540 Introduction To Mechatronics: Slide 51Hugh Durrant-Whyte Silicon Gyroscopes • Structural arrangement of silicon which records centrifugal acceleration and thus angular speed • Use strain-gauge bridges and/or piezo structure to record deformations • Multiple component elements to calibrate other accelrations 1540 Introduction To Mechatronics: Slide 52Hugh Durrant-Whyte Inertial Systems • Many different types of accelerometer and gyroscope systems • Mechanical bodies, fibre optic, etc • Together in an orthogonal arrangement of accelerometers and gyroscopes, these comprise an inertial measurement unit (IMU) • An IMU that is used for navigation is called an inertial navigation system (INS) • These are widely used in aircraft and missile navigation and guidance 1540 Introduction To Mechatronics: Slide 53Hugh Durrant-Whyte Aerospace INS http://www.littongcs.com/products/2guidance/space/overview.html Aircraft Ballistic Missile . 1 1540 Introduction To Mechatronics: Slide 1Hugh Durrant-Whyte Introduction to Mechatronics Mech-1540 Sensors 1 1540 Introduction To Mechatronics: . stability 1540 Introduction To Mechatronics: Slide 28Hugh Durrant-Whyte Linear Potentiometers 1540 Introduction To Mechatronics: Slide 29Hugh Durrant-Whyte When to