Sensors Bryson Cook James Wyler Hao Phan 1

Bryson Cook Outline • Optical Encoders: Theory and applications –Types of encoders –Fundamental Components –Quadrature –Errors –Applications • LVDT (Linear Variable Differential Transformer) –What is a LVDT –Types of LVDT –How do they work? –Applications 2

Bryson Cook What are Encoders • For our class, an encoder is a device that senses position or orientation for use as a reference or active feedback to control position. – Most are either: • Rotary: converts rotary position to an analog or electronic signal. • Linear: converts linear position to an electronic signal. – Are also either absolute or incremental. • Absolute gives the absolute position and knowledge of the previous position is not needed. • Incremental encoders is more ambiguous and requires counting of cycles to determine absolute position. 3

Bryson Cook Optical Encoders • Use light & photosensors to produce digital code • Most popular type of encoder. • Can be linear or rotary. 4

Bryson Cook Optical Encoders: Components • Light source(s) – LEDs or IR LEDs provide light source. – Light is collimated using a lens to make the beams parallel. • Photodetector(s) – Either Photodiodes or Phototransistors. • Opaque disk (Code Disk) – One or more “tracks” with slits to allow light to pass through. 5

Bryson Cook Optical Encoders: Theory LED Code Disk Photosensor 6

Bryson Cook Rotary Optical Encoder Types • Incremental Encoders: Mechanical motion computed by measuring consecutive “on” states. • Absolute Encoders: Digital data produced by code disk, which carries position information. Incremental Encoder code Disk Absolute Encoder code Disk 7

Bryson Cook Binary and Gray Encoding • In some devices, Binary Encoding is used to keep track of the various positions. The areas of the disk are named counting in binary. – This can cause problems since multiple bits can change from one successive area to the next, such as in 011 to 100 all three bits change. • Gray Encoding is a binary system where the adjacent areas only differ in one bit. 8

Bryson Cook Standard Binary Encoding Angle Binary Decimal 0 -45 000 0 45 -90 001 1 90 -135 010 2 135 -180 011 3 180 -225 100 4 225 -270 101 5 270 -315 110 6 315 -360 111 7 *Note: Extremely simplified encoder 9

Bryson Cook Gray Encoding Angle Binary Decimal 0 -45 000 0 45 -90 001 1 90 -135 011 2 135 -180 010 3 180 -225 110 4 225 -270 111 5 270 -315 101 6 315 -360 100 7 Notice only 1 bit has to be changed for all transitions. 10

James Wyler Quadrature • Quadrature describes two signals 90° out of phase • Used to determine direction of measurement • Only two directions possible, A leads B or B leads A 11

James Wyler Quadrature • Standard Encoder Track – Gives velocity and position but not direction • Quadrature Encoder Track – Gives velocity, position AND direction 12

James Wyler Quadrature – How It Works • Grey Encoding • Identical tracks – Phase offset of 90º • Two sensors • Current state vs. next state 13

James Wyler Quadrature – Rotary Encoders • Examples of Quadrature Rotary Encoders 2 Bit Wheel 64 Bit Wheel 14

James Wyler Optical Encoder Errors 1. Quantization Error – Dependent on resolution of sensor 2. Assembly Error – Disk not positioned correctly with respect to sensor 3. Manufacturing Error – Tolerances of sensor positioning and code printing lead to inaccurate signals 15

James Wyler Optical Encoder Errors – Cont. 4. Structural Limitations – Loading on shaft or disk deformation 5. Coupling Error – Gear backlash, belt slippage, etc… 6. Ambient Effects – Vibration, temperature, light noise, humidity, etc… 16

James Wyler Optical Encoder Applications • Coordinate Measuring Machine (CMM) • Digital Calipers • CNC Machining • Electric Motors • Robotics 17

James Wyler LVDT • • What is a LVDT Types of LVDT How do they work? Applications 18

James Wyler What is a LVDT • Linear Variable Differential Transformer • Electrical transformer used to measure linear displacement 19

James Wyler Construction of LVDT • One Primary coil • Two symmetric secondary coils • Ferromagnetic core 20

Hao Phan Types of LVDT • Power supply : – DC – AC • Type of armature : – Free (Unguided) – Captive (Guided) – Spring-extended 21

Hao Phan Power supply : DC LVDT • • Easy to install Signal conditioning easier Can operate from dry cell batteries High unit cost 22

Hao Phan Power supply : AC LVDT • • Small size Very accurate –Excellent resolution (0. 1 μm) Can operate with a wide temperature range Lower unit cost 23

Hao Phan Armature : Free Core (Unguided) • Core is completely separable from the transducer body • Well-suited for short-range applications • high speed applications (high-frequency vibration) 24

Hao Phan Captive Core (Guided) • Core is restrained and guided by a low-friction assembly • Both static and dynamic applications • Long range applications • Preferred when misalignment may occur 25

Hao Phan Spring-Extended Core • Core is restrained and guided by a low-friction assembly • Internal spring to continuously push the core to its fullest possible extension • Best suited for static or slow-moving applications • Medium range applications 26

Hao Phan How do they work? • An alternating current is driven through the primary, causing a voltage to be induced in each secondary proportional to its mutual inductance with the primary. 27

Hao Phan How do they work? • The coils are connected in reverse series • The output voltage is the difference (differential) between the two secondary voltages 28

Hao Phan Null Position • When the core is in its central position, it is placed equal distance between the two secondary coils. • Equal but opposite voltages are induced in these two coils, so the differential voltage output is zero. 29

Hao Phan In Phase Voltage • Displacing the core to the left causes the first secondary to be more strongly coupled to the primary than the secondary. • The higher voltage of the first secondary in relation to the secondary causes an output voltage that is in phase with the primary voltage. • The phase of the voltage indicates the direction of the displacement. 30

Hao Phan Out of Phase Voltage • Displacing the core to the right causes the secondary to be more strongly coupled to the primary than the first secondary. • The greater voltage of the secondary causes an output voltage to be out of phase with the primary voltage. 31

Hao Phan How do they work? • The magnitude of the output voltage is proportional to the distance moved by the core, which is why the device is described as "linear". • Note that the output is not linear as the core travels near the boundaries of its range. 32

Hao Phan LVDT Applications • • Crankshaft Balancing Testing Soil Strength Automated Part Inspection Automotive Damper Velocity 33

References • • • http: //www. macrosensors. com/lvdt_tutorial. html http: //zone. ni. com/devzone/cda/tut/p/id/3638#toc 3 http: //en. wikipedia. org/wiki/Linear_variable_differential_transformer http: //prototalk. net/forums/showthread. php? t=78 http: //www. transtekinc. com/support/applications/LVDT-applications. html http: //www. sensorsmag. com/sensors/position-presence-proximity/modern-lvdtsnew-applications-air-ground-and-sea-7508 http: //www. macrosensors. com/lvdt_tutorial. html http: //zone. ni. com/devzone/cda/tut/p/id/3638#toc 3 http: //en. wikipedia. org/wiki/Linear_variable_differential_transformer Sensors Lecture: Fall ME 6405 2009 http: //electricly. com/absolute-optical-encoders-rotary-encoders 34