Chapter Electromechanical Systems 6

Objectives Discuss the five basic subsystems that are common to all automated robotic systems. Describe mechanical systems as they relate to robotics. Explain how sensing, timing, and control systems are used in the operation of robots. Discuss rotary motion systems used for robotics.

Automated Systems and Subsystems System is combination of components that work together Electromechanical system uses mechanical motion to transfer power Work occurs when energy is transformed into mechanical motion, heat, light, chemical action, or sound

Automated Systems and Subsystems Subsystems are various components of a complete robot Synthesized system is combination of subsystems

Automated Systems and Subsystems Basic parts of automated system Goodheart-Willcox Publisher

Energy Source Energy source provides system power Alternating current power, electrons flow in one direction and then in opposite direction Cycle is each repeated pattern of direction change Single-phase is one alternating current Three-phase is three alternating currents varying by 120°

Energy Source Goodheart-Willcox Publisher

Energy Source Direct current, electrons flow in only one direction Rectification converts alternating current to direct current

Transmission Path Transmission path is channel for transfer of energy Load is part designed to produce work May be alternate transmission paths in system

Control Control alters flow of power Causes operational change in system Control devices within transmission path Anywhere between energy source and load device

Load Load is part designed to produce work Electric motors, heating systems, lighting systems, alarms, and mechanical actuators Largest portion of energy consumed by load device

Indicators Indicator displays operating conditions at points throughout system Digital meters, pressure gauges, tachometers, and thermometers

Mechanical Systems Mechanical motion transfers power to do work Rotary, linear, and reciprocating motion Electrical or fluid power energy source Changing pressure, direction, force, and speed is control Indicators measure physical quantities

Electrical Systems Sensing Timing Control Providing rotary motion

Sensing Systems Sensing system signals response to particular form of energy Light systems most common Light pipes transfer light energy Flexible, fiber-optic rods can extend long distances

Sensing Systems Detector responds to source and outputs signal to control load device Control in sensing system Interrupting light beam between source and detector Altering intensity, focus, shape, or wavelength

Sensing Systems Counter on conveyor line Goodheart-Willcox Publisher

Timing Systems Timing systems turn devices on or off Delay timing provides lapse before load is energized Interval timing uses specified time periods after load is energized Cycle timing may include both interval and delay timing

Control Systems Control systems continually make adjustments to alter machine operations Control unit determines robot flexibility and efficiency

Control Systems Non-servo/open-loop control systems are most basic Two open-loop variations: Full control turns system off or on Partial control alters system operations

Control Systems Closed-loop system allows control unit and controlled element interaction Feedback provides interaction information in closed-loop system Activated by electrical, thermal, light, chemical, or mechanical energy

Control Systems Closed-loop system incorporates feedback Goodheart-Willcox Publisher

Control Systems Closed-loop system with automatic correction control Comparator compares feedback signal to reference signal or standard Correction signal sent to control unit

Control Systems Comparator in closed-loop control system Goodheart-Willcox Publisher

Control Systems Digital system components process numeric information Instructions Variations in pressure, temperature, or electric current

Rotary Motion Systems Electric motor converts electrical energy to mechanical movement Armature is electromagnet placed between permanent magnets Poles of armature and magnets aligned

Rotary Motion Systems Poles repel and cause armature to rotate Goodheart-Willcox Publisher

Rotary Motion Systems Torque is turning force Common parts of a motor: Strength of magnetic fields and amount of current Common parts of a motor: Stator is stationary portion Rotor is rotating component

Basic Parts of DC Motors Goodheart-Willcox Publisher

DC Motors Commutator switches direction of current flow alternately Produce one-directional direct current through armature windings Brushes rub against commutator and allow current to flow through

DC Motors Field windings wrap around electromagnets Create magnetic field of stator Armature of DC motor rotates and generates its own voltage Counter electromotive force (cemf)

DC Motors Goodheart-Willcox Publisher

DC Motors Horsepower is measure of amount of work performed Horsepower rating represents power of a motor Percentage of speed regulation Difference between no-load motor speed and rated full-load motor speed Better speed regulation capabilities with lower values

Categories of DC Motors Permanent-magnet Series-wound Shunt-wound Compound-wound

DC Motors Permanent-magnet dc motor power supply connected directly to conductors of rotor Magnetic field produced by permanent magnets mounted in stator

DC Motors Permanent-magnet dc motor used for low-torque applications Goodheart-Willcox Publisher

DC Motors Series-wound dc motor armature and field circuits connected in series arrangement One path for current to flow Only dc motor that also operates using ac power

DC Motors Series-wound dc motor produces high torque, but poor speed regulation Goodheart-Willcox Publisher

DC Motors Shunt-wound DC motor field windings connected in parallel with armature Relatively high resistance Small amount of current flows through Field current changes result in corresponding changes in electromagnetic field flux

DC Motors Shunt-wound dc motors have effective speed control characteristics Goodheart-Willcox Publisher

DC Motors Compound-wound dc motor has two sets of field windings One set in series with armature One set in parallel with armature High torque and good speed regulation

DC Motors Compound-wound dc motor Goodheart-Willcox Publisher

AC Motors Single-phase ac motors use single-phase ac power source Three types: universal, induction, and synchronous

AC Motors Universal motors powered by either ac or dc source Built like series-wound dc motor Portable tools and small equipment

AC Motors Goodheart-Willcox Publisher

AC Motors Single-phase induction motor has a solid rotor Rotor set in motion by auxiliary method Speed based on speed of rotating magnetic field and number of stator poles Difference between synchronous speed (stator speed) and rotor speed is slip More torque produced with greater slip

AC Motors Goodheart-Willcox Publisher

AC Motors Three-phase ac motors use three-phase ac power source Two basic types: induction and synchronous

AC Motors Three-phase induction motors have squirrel cage rotor Voltage applied to stator Good starting and running torque Used in industrial applications

AC Motors Goodheart-Willcox Publisher

AC Motors Three-phase synchronous motors deliver constant speed Direct current applied to wound rotor produces electromagnetic field Three-phase ac power applied to stator External means needed to start Zero slip at synchronous speed

Servo Systems Servo systems respond to system feedback or error signals Input is reference source Load responds Error detector receives data from input source and output device Correction signal amplified and applied to actuator

Servo Systems Servo systems follow closed-loop control path Goodheart-Willcox Publisher

Servo Systems Servomotor produces controlled shaft displacements to achieve precise rotary motion Two types: synchronous and stepping

Servo Systems Adept

Servo Systems Synchronous motor comprised of rotor and stator assembly No brushes, commutators, or slip rings Requires carefully maintained air gap Speed directly proportional to ac frequency and number of pairs of stator poles

Servo Systems Single-phase ac synchronous motor construction Goodheart-Willcox Publisher

Servo Systems Two-phase synchronous motor with four poles per phase Superior Electric Co.

Servo Systems DC stepping motors change electrical pulses into rotary motion Rotor has permanent magnet More torque than synchronous servomotors Stator coils wound using bifilar construction Four-step switching sequence

Servo Systems

Rotary Electric Actuators Rotary motion controls angular position of shaft Transmit rotary motion without direct linkage Computer signals applied to actuators and translated into rotary motion