Actuators are the muscles of robots.

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Rudolph Fitzgerald
5 years ago
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1 6.1 INTRODUCTION Actuators are the muscles of robots. Several types of actuator noteworthy? Electric motors? Servomotors? Stepper motors? Direct-drive electric motors? Hydraulic actuators? Pneumatic actuators? Shape memory metal actuators? Magnetostrictive actuators

2 6.2 CHARACTERISTICS OF ACUATING SYSTEM? Characteristics of actuating system is criteria of selecting the kind of actuator.? Weight, Power-to-Weight Ratio, Operating Pressure? The weight of the actuating system and Power-to-Weight Ratio is important.? Pneumatic and hydraulic system are composed of two system, actuating system and power system.? The weight of the actuating system and Power-to-Weight Ratio is important.? Stiffness vs. Compliance? The weight of the actuating system and Power-to-Weight Ratio is important.? Pneumatic and hydraulic system are composed of two system, actuating system and power system.? The weight of the actuating system and Power-to-Weight Ratio is important.? 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.

3 Fig. 6.1 Inertia and torque relationship between a motor and a load. 1 1 T m? I m? m? bm? m? T1? Im? m? bm? m? ( Il? l? bl? l ) N N 1 N 1 N IEffective? I 2 l I Total? Il? I 2 m

4 6.4 HYDRAULIC ACTUATOR? It offers a high power-to-weight ratio, large forces at low speeds compatibility with microprocessor and tolerance of extreme hazardous environments. da? t? dr T? r2 r2? r 1 r1? p? r? da? p? r? t? dr Fig. 6.3 A rotary hydraulic actuator.? pt? r 2 r1 r? dr? pt( r? where, A is the effective area of the piston, p is the working pressure. 2? r )

5 6.4 HYDRAULIC ACTUATOR? q? Cx?? q( dt)? d (vol)? A( dy)? Cx ( dt)? A( dy)? C y? x? AD? where, q is the flow rate, C is a constant, x is the spool s displacement, y is piston s displacement and A is the area of the piston (d/dt denoted as D). Fig. 6.4 Schematic of a hydraulic system and its components. Fig. 6.5 Schematic of a spool valve in neutral position.

6 6.5 PNEUMATIC DEVICES? 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.

7 6.6 ELECTRIC MOTORS? Basic Principle of all electric motors; F? I? B? 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 Fig Basic Concept of Electric 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

8 ? 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. Fig Heat dissipation path of motors.

9 6.6.1 DC motors? DC motors is reliable, sturdy and relatively powerful. DC motor Generator The stator is a set of fixed permanent magnets, creating a fixed magnetic field, while the rotor carries a current. IF the rotor is rotated within the magnetic field, a DC current will develop, and the motor will act as a generator. T M? T M? kt? k t k f I I rotor rotor I stator Fig Schematic of a disk motor.? Mathematical expression of DC motors

10 6.6.2 AC Motors? 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 Brushless 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.

11 6.6.5 Servomotors? The back electromotive force or back-emf is most important factor for all electric motors.? Back-emf voltage V emf? nke? The motor s velocity V in? IR? V? IR? emf nke Fig Scheme of a servomotor controller.? The motor s torque T? TL? T f? nkd? IK T

12 6.6.6 Stepper motors? Stepper motors are long-lasting, versatile and used without feedback.? 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. Fig Basic principle of operation of a stepper motor.

13 Canstack Motors? These motors used for application with low vertical clearance because of their construction, flat shape.

Hybrid Stepper Motors? These motors are made with two coils, each with four poles. Fig. 6.

14 Hybrid Stepper Motors? These motors are made with two coils, each with four poles. Fig Application of unequal divisions for measuring lengths as in a caliper. Fig Basic operation of a hybrid stepper motor.

15 Unipolar, Bipolar, and Bifilar Stepper Motors? Unipolar motors are designed to work with one power source.? Bipolar motors have two power source, one is for the drive circuitry and the other is for the motor windings and its polarity can be switched. Fig Stepper motor lead configurations. Fig Schematic drawing for unipolar and bipolar drive circuits.

16 Stepper Motor Speed-Torque Characteristics? Stepper Motors do not require any feedback because the signals from stepper motors generated with a known angle every time.? Stepper Motors develop their maximum torque, called holding torque, at zero angular velocity, when the rotor is stationary (The torque developed with no power is called detent torque.). Fig A typical speed-torque curve. Fig Application of (a) a diode, and (c) a zener diode with stepper motors for increasing the maximum velocity.

17 Stepper Motor Control? Stepper Motors may be driven by microprocessors (or microcontrollers) either directly or through driver circuits.? Another way of stepper motor control is using a dedicated integrated-circuit chip. Fig Schematic of a typical stepper motor translator. Fig Application of (a) a diode, and (c) a zener diode with stepper motors for increasing the maximum velocity.

18 Stepper Motor Control? Another way of stepper motor control is to provide the impulse train to the translator by means other than a microprocessor. Fig Schematic drawing of the application of a translator to run a stepper motor with a microprocessor. Fig Schematic drawing of a timer circuit that creates an impulse train for driving a stepper motor indexer.

19 6.7 MICROPROCESSOR CONTROL OF ELECTRIC MOTORS? 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 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. V out? V CC t t 1 Fig PWM timing. Fig Sine wave generation with PWM.

20 6.7.2 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. Fig Application of H-bridge for motor direction control

21 6.8 MAGNETROSTRICTIVE ACTUATORS? A terfenol-d is placed near a magnet, this special rare-earth-iron material will change its shape slightly, which is called magnetostriction effect and is used to make linear motors with micro inches displacement. 6.9 SHAPE-MEMORY-TYPE METALS? 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.

22 6.10 SPEED REDUCTION? Speed reduction includes fixed-axis and planetary gear trains, with many types of gears.? In planetary gear trains, there are normally four basic elements: The sun gear, the ring gear, the arm and the planet.?? A L? ( N ( N F F? 1) NL? N L ) Fig Schematic drawing of a planetary gear train. Fig Schematic of the Harmonic Drive gear train.

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