MME 3374a - Electrical Foundations for Mechanical Engineers - Unit 4
MME 3374a - Fall 2025 Notes
Overview of Switches and Relays
Electromechanical switches and relays: Utilized across various industrial sectors.
Primarily mechanical, offering simplicity, robustness, and cost-effectiveness.
Recent decline in use due to the rise of electronic technologies that eliminate moving parts along with associated wear or breakage.
Types of Switches
General Function: Switches deliver or direct electrical current between conductors.
Common Types:
Pushbutton
Lever
Rocker
Magnetic
Toggle
Rotary
Actuation Mechanism: The types defined by how they are actuated; the electrical current direction is determined by the contact type.
Contact Types
Open Contact: No current flows between conductors.
Closed Contact: Current flows between conductors.
Nature of Contact:
Momentary: Actuation leads to temporary current flow.
Static: Sustained current flow once switch is actuated.
Current Carrying Capacity
Types Based on Current Carrying:
Low-current switches: Used to signal an event or status; technologies include:
Mechanical
Capacitive
Inductive
Magnetic (Hall-Effect)
Optical
Electrical (transistors)
High-current switches: Energize actuators; consist of:
Mechanical
Semiconductor-based
Advanced Contact Types
Normally Open (NO): No current flows until switch actuated.
Normally Closed (NC): Current flows until switch actuated.
Multiple Contacts: Some switches include multiple contact sets.
Examples of Types:
Pushbutton with NO and NC contacts
Limit switch with NO contacts
Switch Example: Inertia Switch
Example in airbag systems.
Mechanism: Acceleration causes a mass to depress a spring, opening contacts.
Switch Selection Considerations
Criteria for appropriate switch selection include:
Electrical Requirements: Current specifications and necessary plating for high current handling.
Mechanical Specifications: Speed of actuation, actuation forces.
Environmental Specifications: Such as operational temperature and resistance ratings to dust and moisture.
IP Ratings (Ingress Protection Ratings)
IP67 Specification:
Protection against solid objects greater than 1mm.
Immersion protection: Effects of water immersion between 15cm to 1m depth for 30 minutes.
Electrical Principles Related to Switches
Contact Considerations
Wetting Current (Sealing Current): Minimum current needed to flow through a newly-closed switch to break through an oxidation film on contacts.
Occurs more frequently in humid environments.
Arcing Concerns: High current operations may cause arcing when contacts open, utilizing:
Resistor-Capacitor (RC) Snubber: Employed across contacts to mitigate effects of contact arcing.
Contact Bounce
Bounce Phenomenon: Mechanical and some electronic switches may exhibit a bounce effect during actuation, leading to fluctuations between no current and full current.
Control Remedies: Implementing "debouncing" routines in control software, along with RC snubbers.
Hall-Effect Sensors
Discovery: Identified by Edwin Hall in 1879.
Function: Application of a perpendicular magnetic field to a conductor causes charge displacement, creating a Hall voltage proportional to magnetic field strength.
Hall Coefficient: Ratio of induced electric field to the product of current density and magnetic field strength.
Applications: Often includes internal NPN transistor for interfacing with microcontrollers or providing analog output.
Applications of Hall-Effect Sensors
Magnet Detection: Basic function in various systems.
Motor Speed Detection:
A Hall Effect sensor can quantify the rotational speed of a motor with a magnet ring affixed to the shaft.
Optical Switches
Structure: Comprised of an LED transmitter and a photo transistor receiver.
Functionality: Measure voltage based on the presence of an object; voltage configurations fluctuates depending on object presence or absence.
Application in Rotary Encoders
Uses of Optical Switches in Encoders: Provides feedback for position and velocity.
Types:
Incremental Encoders: Outputs series of square waves; resolution determined by line count on the encoder disk.
Absolute Encoders: Each shaft position correlates to a unique binary output pattern.
Incremental Encoders
Operation:
A 500-count encoder generates 500 square waves for one revolution.
Two types:
Tachometer (single-track): Speed measurement through pulse count.
Quadrature (multi-track): Uses dual signal and a second bit for determining direction.
Quadrature Encoder Mechanics
Design: Channels A and B have a 90° separation enabling direction determination.
Clockwise vs. Counterclockwise Rotation: Pulse changes between channels denoting direction of rotation.
Example Calculation:
Given a 2,880 pulse-per-revolution incremental encoder producing 934 pulses:
Change in angle calculated as:
Absolute Encoders
Function: Each position yields a unique output pattern indicative of the shaft's angle.
Resolution Calculation: For a 12-bit encoder outputting 101100010111:
per position.
Relays
Purpose: Switching electrical loads either through mechanical switches or electromagnetic/solid-state relays.
Electromagnetic Relays
Structure: Operate using a low-current solenoid to control high-current contacts.
Isolation: Maintain isolation between control circuits and load circuits.
Relay Symbols
Configuration Types:
SPST (Single Pole Single Throw, Normally Open)
SPDT (Single Pole Double Throw)
DPST (Double Pole Single Throw, Normally Open)
DPDT (Double Pole Double Throw)
H-Bridge Driver Configuration
Designed for motor applications requiring bidirectional control.
Controls direction based on energization of DPDT relay.
Solid State Relays (SSRs)
Characteristics: Used for low-frequency applications, no moving parts, require heatsinking for heat dissipation.
Applications: Commonly found in injection molding machines to manage power delivery to mold heaters.
Conclusion
Questions and Clarifications:
Engage in interactive discussions for further understanding.