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Oct 26, 2015
Chapter 4Actuators INTRODUCTIONActuators are the muscles of robots.
Several types of actuator noteworthy Electric motors Servomotors Stepper motors Hydraulic actuators Pneumatic actuators Shape memory metal actuators
Chapter 4Actuators INTRODUCTION actuator
Weight, Power-to-Weight Ratio, Operating Pressure The weight of the actuating system and Power-to-Weight Ratio is important.stepper motor < servomotor pneumatic : lowest, hydraulic : highest Pneumatic and hydraulic system are composed of two system, actuating system and power system. Operating pressurehydraulic : 55~5,000 psi pneumatic : 100~120 psi Chapter 4Actuators CHARACTERISTICS OF ACUATING SYSTEM Characteristics of actuating system is criteria of selecting the kind of actuator.
Stiffness the resistance of a material against deformation hydraulic systems : very stiff & noncompliant pneumatic systems : easily compressed -> compliant Chapter 4Actuators CHARACTERISTICS OF ACUATING SYSTEM Use of Reduction Gears To increase the torque and make it to slow it down to manageable speeds. It increase the cost, number of parts, backlash, inertia of the rotating body, and resolution of the system.
Inertia and torque relationship between a motor and a load. Chapter 4ActuatorsEffect of reduction gear
It offers a high power-to-weight ratio , large forces at low speeds compatibility with microprocessor and tolerance of extreme hazardous environments. A rotary hydraulic actuator. where, A is the effective area of the piston, p is the working pressure. Chapter 4ActuatorsHYDRAULIC ACTUATOR
Schematic of a hydraulic system and its components.Chapter 4Actuators HYDRAULIC ACTUATOR where, q is the flow rate, C is a constant, x is the spools displacement, y is pistons displacement and A is the area of the piston (d/dt denoted as D).
Chapter 4ActuatorsBlock diagram of the hydraulic system with proportional feedback control scheme Schematic of a spool valve in neutral position.
It is very similar to hydraulic system, but power-to-weight ratio is much lower than hydraulic system. Because the air is compressible, it compressed and deforms under load. Chapter 4Actuators PNEUMATIC DEVICES
Basic Concept of Electric Motors Sorts of electric motors used in robotics DC motors Reversible AC motors Brushless DC motors Stepper motors Important design factor is like below. Dissipation of heat -> Size, Power of motors
Heat dissipation is generated from below. Resistance of the wiring to electric current Heat due to iron losses Eddy current losses, hysteresis losses friction losses, brush losses, short-out circuit losses Basic Principle of all electric motors; Chapter 4ActuatorsELECTRIC MOTORS
Heat dissipation path of motors.Chapter 4Actuators What is important is the path that the heat must take to leave the motor since if the heat is dissipated faster more generated heat can be dissipated before damage occurs.
Chapter 4Actuators DC motor operating principles
Chapter 4ActuatorsComponent of DC motor(brush-type)
The changing flux is provided by the AC current(60Hz) and commutators and brushes are eliminated. AC motors can dissipate heat more favorable than DC motors, yielding more power. Chapter 4ActuatorsAC MotorsBrushless DC Motors This types of motors are a hybrid between AC motors and DC motors. It is not necessary that line frequency as 60Hz. For smooth operation and almost constant torque, the rotor usually has three phases in it. Direct-Drive Electric Motors These motors are designed to deliver a very large torque at very low speeds and with very high resolution, but to be used directly with a joint without any gear reduction.
Scheme of a servomotor controller. Chapter 4ActuatorsServomotors The back electromotive force or back-emf is most important factor for all electric motors. Motor equivalent circuit
Chapter 4ActuatorsStepper motors Stepper motors are long-lasting, versatile and used without feedback. Basic principle of operation of a stepper motor. It has permanent magnet rotors and multiple winding stator housing. Most industrial steppers run between 1.8 to 7.5 degrees at full stepping. The number of poles means more precise resolution, but it has physical limitation.
Chapter 4ActuatorsOperation of unipolar stepper motorFull step phase sequenceHalf step phase sequence
Chapter 4ActuatorsConfiguration of stepper motor
Chapter 4ActuatorsHybrid Stepper Motors These motors are made with two coils, each with four poles. Application of unequal divisions for measuring lengths as in a caliper. Basic operation of a hybrid stepper motor.
Chapter 4Actuators A robot is supposed to be a manipulator that is controlled by computers or microprocessor and actuated by the electric motors with controller, which is computer or microprocessor. PWM timing. Sine wave generation with PWM. MICROPROCESSOR CONTROL OF ELECTRIC MOTORS Pulse Width Modulation PWM is used for DC motor speed control with microprocessors. It requires only high voltage(5V) and one output bit. Average output voltage of PWM is like below.
Chapter 4ActuatorsPWM velocity feedback controlPWM voltage and motor currentPWM feedback control
Chapter 4Actuators Application of H-bridge for motor direction control Direction Control of DC Motors with an H-Bridge PWM It is desirable to change the direction of current flow in a motor for changing its direction of rotation with only two bits of information. It means one should change the direction of the flow by changing bit information from the microprocessor.
Chapter 4Actuators One particular metal alloy, called Biometal shortens about 4% when it reaches a certain temperature. The major disadvantage of the wire is that the total strain happens within a very small temperature range and it is very difficult to accurately control the strain.SHAPE-MEMORY-TYPE METALS