Mechatronic for Health Sciences Lecture 4 PDF

FACULTY OF APPLIED HEALTH SCIENCES Mechatronic for Health Sciences [MEC 141] Lecture No. 4 Associated Professor Dr. Reda Ahmed Khalf-Allah Lecture No. 4 ACTUATORS Robot Actuators & Feed Back Components ACTUATORS Actuators are the devices which provide th...

FACULTY OF APPLIED HEALTH SCIENCES Mechatronic for Health Sciences [MEC 141] Lecture No. 4 Associated Professor Dr. Reda Ahmed Khalf-Allah Lecture No. 4 ACTUATORS Robot Actuators & Feed Back Components ACTUATORS Actuators are the devices which provide the actual motive force for the robot joints. Actuators are the muscles of robots. If you imagine that the links and the joints are the skeleton of the robot, the actuators act as muscles, which moves or rotate the links to change the configuration of robots. The actuators must have enough power to accelerate and decelerate the links and to carry the loads, yet be light, economical, accurate, responsive, reliable and easy to maintain. Actuators in robotic system basically consists of : A power supply. A power amplifier. A servomotor. A transmission system.  Robot Actuators & Feed Back Components Actuator system Pp : Primary source of power (Electric, Press.fluid, POWER compress. Air) SUPPLIES Pc : Input control power usually electric. Pa : Input power to motor Electric, Hydraulic, or Pneumatic. Pm: Power output from motor. PP Pu : mechanical power required POWER Pa MOTOR Pm Pu Pc OR TRANSMISSION AMPLIFIER SERVO MOTOR Pda Pds Pdt Pds, Pdt , Pda : Powers lost in dissipation for the conversion performed by the Amplifier, Motor, Transmission Robot Actuators & Feed Back Components CHARACTERISTICS OF AN ACTUATOR Load (e.g. torque to overcome own inertia) Speed (fast enough but not too fast) Accuracy (will it move to where you want?) Resolution (can you specify exactly where?) Repeatability (will it do this every time?) Reliability (mean time between failures) Power consumption (how to feed it) Energy supply & its weight.  Robot Actuators & Feed Back Components TYPES OF ACTUATORS Based on the source of Input Power actuators are classified in to three groups : 1. Pneumatic Actuators. These utilize pneumatic energy provided by the compressor and transforms it into mechanical energy by means of pistons or turbines. 2. Hydraulic Actuators. These Transform the energy stored in reservoir into mechanical energy by means of suitable pumps. 3. Electric Actuators. Electric actuators are simply electro-mechanical devices which allow movement through the use of an electrically controlled systems of gears Pneumatic and Hydraulic Actuators Both these actuators are powered by moving fluids. In the first case, the fluid is compressed air and In the second case, the fluid is pressurized oil. Pneumatic systems typically operate at about 100lb/in2 Hydraulic systems at 1000 to 3000 lb/in2. ▪ Both Hydraulic and Pneumatic actuators are classified as linear Actuators (Cylinders). Rotary Actuators (Motors). Pneumatic and Hydraulic Actuators linear Actuators Pneumatic and Hydraulic Actuators linear Actuators The simplest power device could be used to actuate a linear joint by means of a moving piston. There are two relationships of particular interest when discussing actuators: 1. The velocity of the actuator with respect to input power and 2. Force of the actuator with respect to the input power. Pneumatic and Hydraulic Actuators linear Actuators Pneumatic and Hydraulic Actuators Rotary Actuators Pneumatic and Hydraulic Actuators Rotary Actuators There is a relationship of particular interest when discussing Rotary actuator: The angular velocity, ω, and Torque, T. R, outer radius of the vane., r, inner radius., h, thickness of the vane., ω, angular velocity., T, torque. Advantages and limitations of Pneumatic actuators ADVANTAGES LIMITATIONS It is cheapest form of all actuators. Since air is compressible, precise control Components are readily available and of speed and position is not easily compressed air normally is an readily available facility in factories. obtainable unless much more complex Compressed air can be stored and electro mechanical devices are conveyed easily over long distances. incorporated in to system. They have few moving parts making If mechanical stops are used resetting the them inherently reliable and reducing system can be slow. maintenance costs. They have quick action and response If moisture penetrates the units and time thus allowing for fast work cycles. ferrous metals have been used then No mechanical transmission is usually damage to individual components may required. happen. These systems are usually compact thus the control is simple e.g: mechanical stops are often used. Advantages and limitations of Hydraulic actuators ADVANTAGES LIMITATIONS High efficiency and high power to Leakages can occur causing a loss size ratio. in performance and general Complete and accurate control over contamination of the work area. speed position and direction of There is also a higher fire risk. actuators are possible. The power pack can be noisy No mechanical linkage is required typically about 70 decibel or i.e., a direct drive is obtained with louder if not protected by an mechanical simplicity. acoustic muffler. They generally have a greater load Changes in temp alter the carrying capacity than electric and viscosity of hydraulic fluid. Thus pneumatic actuators. at low temperatures fluid Self lubricating and non corrosive. viscosity will increase possibly Hydraulic robots are more capable causing sluggish movement of of with standing shock loads than the robot. electric robots.  Electric and Stepper Motors There are a variety of types of motors used in robots. The most common types are Servomotors and Stepper motors. Electric actuators are simply electro- mechanical devices which allow movement through the use of an electrically controlled systems of gear. ELECTRIC MOTORS Electric motors usually have a small rating, ranging up to a few horsepower. They are used in small appliances, battery operated vehicles, for medical purposes and in other medical equipment like x-ray machines. Electric motors are also used in toys, and in automobiles as auxiliary motors for the purposes of seat adjustment, power windows, sunroof, mirror adjustment, blower motors, engine cooling fans.  STATOR ROTATING (COMMUTATOR) ARMATURE Brushes  COMPONENTS OF DC ELECTRIC MOTOR The principle components of an electric motor are: North and south magnetic poles to provide a strong magnetic field. Being made of bulky ferrous material they traditionally form the outer casing of the motor and collectively form the stator. An armature, which is a cylindrical ferrous core rotating within the stator and carries a large number of windings made from one or more conductors. A commutator, which rotates with the armature and consists of copper contacts attached to the end of the windings. Brushes in fixed positions and in contact with the rotating commutator contacts. They carry direct current to the coils, resulting in the required motion.  ELECTRIC MOTORS DC motors :In DC motors, the stator is a set of fixed permanent magnets, creating a fixed magnetic field, while the rotor carries a current. Through brushes and commutators, the direction of current is changed continuously, causing the rotor to rotate continuously. AC motors : These are similar to DC motors except that the rotor is permanent magnet, the stator houses the windings, and all commutators and brushes are eliminated. A Servomotor is a DC,AC, brushless, or even stepper motor with feedback that can be controlled to move at a desired speed (and consequently, torque), for a desired angle of rotation. To do this, a feedback device sends signals to the controller circuit of the servomotor reporting its angular position and velocity.  COMPONENTS OF DC ELECTRIC MOTOR A simple DC electric motor: when the coil is powered, a magnetic field is generated around the armature. The left side of the armature is pushed away from the left magnet and drawn toward the right, causing rotation. The armature continues to rotate, When the armature becomes horizontally aligned, the commutator reverses the direction of current through the coil, reversing the magnetic field. The process then repeats. STEPPER MOTOR When incremental rotary motion is required in a robot, it is possible to use stepper motors. A stepper motor possesses the ability to move a specified number of revolutions or fraction of a revolution in order to achieve a fixed and consistent angular movement. This is achieved by increasing the numbers of poles on both rotor and stator Additionally, soft magnetic material with many teeth on the rotor and stator cheaply multiplies the number of poles(reluctance motor) STEPPER MOTOR ADVANTAGES & LIMITATIONS OF ELECTRIC ACTUATORS ADVANTAGES LIMITATIONS Wide spread availability of power Electric actuators often require supply. some sort of mechanical transmission system this The basic dive element in an electric increases the unwanted motor is usually lighter than that for movement, additional power fluid power. and may complicate control. High power conversion efficiency. Due to increased complexity of No pollution of working environment the transmission system additional cost is incurred for The accuracy and repeatability of their procurement and electric power driven robots are maintenance. normally better than fluid power Electric motors are not robots in relation to cost. intrinsically safe. They cannot Easily maintained and repaired. therefore be used in for example explosive The drive system is well suited to atmospheres. electronic control. APPLICATIONS Stepper motors can be a good choice whenever controlled movement is required. They can be used to advantage in applications where you need to control rotation angle, speed, position and synchronism. These include : printers plotters medical equipment fax machines automotive and scientific equipment etc.  Comparison of actuating systems Hydraulic Electric Pneumatic + Good for large robots + Good for all size of + Many components are and heavy payload Robots usually off-the-shelf +Highest Power/Weight +Better control, good for +Reliable components. Ratio high precision robots +Stiff system, High +Higher Compliance that +No leaks or sparks accuracy, better response Hydraulics +Inexpensive and simple +No reduction gear +Reduction gears used needed reduce inertia on the +Can work in wide range motor +Low pressure compared of speeds without to hydraulics +does not leak, good for difficulty clean room + Good for on-off +Can be left in position applications and for pick +Reliable, low without any damage and place maintenance  Comparison of actuating systems Hydraulic Electric Pneumatic - May leak. Not fit for clean +Can be spark-free. Good for +Complaint systems. room application explosive environment. -Requires pump, reservoir, -Noisy systems. -Low stiffness motor, hoses etc. -Can be expensive and noisy, -Needs reduction gears, - Require air pressure, filter, requires maintenance. increased backlash, cost, etc. weight, etc. -Viscosity of oil changes with -Difficult to control their -Motor needs braking device temperature linear position when not powered. Otherwise, the arm will fail. -Very susceptible to dirt and -Deform under load other foreign material in oil constantly -Low compliance - -Very low stiffness. Inaccurate - -High torque, High pressure, response. large inertia on the actuator. - -Lowest power to weight ratio