Sensors Chris Davidson Ari Kapusta Optical Encoders and Linear Variable Differential Transformers

Overview Optical Encoders – What is an optical encoder – Types of optical encoders – Components – How do they work? LVDT (Linear Variable Differential Transformer) – What is a LVDT – Types of LVDT – How do they work? – Applications

Optical Encoders An electro-mechanical device that senses angular (or linear) position or motion

Types of Encoders Rotary: Converts rotational position/velocity to an analog or digital signal Linear: Converts linear position/velocity to an analog or digital signal Absolute: Gives absolute position and knowledge of the previous position is not needed Incremental: Measures displacement relative to a reference point

Fundamental Components Light Source(s): Light provided by LED and focused through a lens Photosensor(s): Photodiode or Phototransistor used to detached light Opaque Disk (Code Disk): One or more tracks with slits to allow light to pass through them Masking Disk: Stationary track(s) that are identical to the Code Disk.

Fundamental Components

Absolute Encoders Advantages A missed reading does not affect the next reading Only needs power when on when taking a reading Disadvantages More expensive/complex

Binary Encoding AngleBinaryDecimal Note: Simplified Encoder (3 Bit)

Gray Encoding AngleBinaryDecimal Notice only 1 bit has to be changed for all transitions.

Gray Code to Binary Code 1.) Copy MSB 2.) Perform XOR (Exclusive OR) between b i and g i+1 3.) Repeat

Example: Gray Code to Binary Code Convert the gray code value of 0101 to binary code. Solution: 0110

Incremental Encoders Advantages Cost Disadvantages Must be “zeroed” at a reference location for each startup

Incremental Disk

Quadrature Quadrature describes two signals 90° out of phase Used to determine direction of measurement Only two possible directions: A leads B or B leads A Provides up to 4 times the resolution

Encoder Resolution

Example: Resolution What is the best resolution you can get on an incremental encoder with 500 windows per channel?

Error and Reliability Quantization Error – Dependent on sensor resolution Assembly Error – Dependent on eccentricity of rotation (Is track center of rotation the same as the center of rotation of disk) Manufacturing Tolerances – Code printing accuracy, sensor position, and irregularities in signal generation Mechanical Limitations – Disk deformation, physical loads on shaft, rotation speed (bearings) Coupling Error – Gear backlash, belt slippage, etc… Ambient Effects – Vibration, temperature, light noise, humidity, dust, etc…

Applications Old Computer Mice Speed Feedback from Motor/Gearbox Angular Position of Robotic Arm

LVDT (Linear Variable Differential Transformer) Presenter: Ari Kapusta What is a LVDT How do they work? Types of LVDT Applications

What is a LVDT Linear Variable Differential Transformer Electrical transformer used to measure linear displacement

Construction of LVDT One Primary coil Two symmetric secondary coils Ferromagnetic core The primary coil is energized with a A.C. The two secondary coils are identical, symmetrically distributed. Primary coil Secondary coils Ferromagnetic core

How LVDT works A current is driven through the primary, causing a voltage to be induced in each secondary proportional to its mutual inductance with the primary.

How LVDT works The coils are connected in reverse series The output voltage is the difference (differential) between the two secondary voltages

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.

Moving Core Left If the core is moved closer to S1 than to S2 More flux is coupled to S1 than S2. The induced voltage E1 is increased while E2 is decreased. The differential voltage is (E1 - E2).

Moving Core Right If the core is moved closer to S2 than to S1 More flux is coupled to S2 than to S1. The induced E2 is increased as E1 is decreased. The differential voltage is (E2 - E1).

Output 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.

LVDT Types - Distinction by : - Power supply : - DC - AC -Type of armature : - Unguided - Captive (guided) - Spring-extended

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

Power supply : AC LVDT Small size Very accurate –Excellent resolution (0.1 μm) Can operate with a wide temperature range Lower unit cost

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

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

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

LVDT Applications LVDT measures absolute position. Can be sealed against environment. LVDT is useful for any application where you want linear position. – Machine tool position – Robot arm position – Forklift/hydraulic actuator position Often used for position feedback

LVDT Applications Crankshaft Balancing Testing Soil Strength Automated Part Inspection Automotive Damper Velocity

Questions? Presenter of Optical Encoder: Chris Davidson Presenter of LVDT: Ari Kapusta

References new-applications-air-ground-and-sea Sensors Lecture: Fall ME