Module 2 – Actuators PDF

Module 2 – Actuators LEARNING OBJECTIVES After successful completion of this module, you should be able to: Describe basic parts of an electric motor; Explain the differences between series, shunt, compound, permanent magnet, and stepper DC motors; Identify and...

Module 2 – Actuators LEARNING OBJECTIVES After successful completion of this module, you should be able to: Describe basic parts of an electric motor; Explain the differences between series, shunt, compound, permanent magnet, and stepper DC motors; Identify and describe the components used in hydraulic and pneumatic systems; Differentiate hydraulic and pneumatic systems. Actuators Actuators are the devices which are used to produce motion or action, such as linear motion or angular motion. Some of the important actuators used in mechatronic systems are solenoids, electric motors, hydraulic pumps, and hydraulic cylinders and pneumatic cylinders. These actuators are instrumental in moving physical objects in mechatronic systems. TYPES OF ACTUATORS: Electromechanical Actuators Hydraulic Actuation System Pneumatic Actuation System ELECTROMECHANICAL ACTUATORS SOLENOIDS AND RELAYS A solenoid consists of a coil and a movable iron core called the armature. When the coil is energized with current, the core moves to increase the flux linkage by closing the air gap between the cores. The movable core is usually spring-loaded to allow the core to retract when the current is switched off. Figure 36. Solenoids An electromechanical relay is a solenoid used to make or break mechanical contact between electrical leads. A small voltage input to the solenoid controls a potentially large current through the relay contacts. Applications include power switches and electromechanical control elements.  Electrical relays however, are basically electrically operated switches that come in many shapes, sizes and power ratings suitable for all types of applications. Relays can also have single or multiple contacts within a single package with the larger power relays used for mains voltage or high current switching applications being called “Contactors”. Figure 37. A relay Figure 38. A magnetic contactor ELECTRIC MOTORS An electric motor is a machine that converts electrical energy into mechanical torque. Electric motors are the most widely used electromechanical actuators. They can either be classified based on functionality or electromagnetic characteristics. Basic Parts of Electric Motors: Stator - is the stationary outer or inner housing of the motor that supports the material that generates the appropriate stator magnetic field. Field coil - is the portion of the stator that is responsible for generating the stator (field) magnetic flux. Rotor - is the rotating part of the motor. Armature - is the rotor winding that carries current and induces a rotor magnetic field. Commutator - is a part of the rotor of a DC motor that delivers and controls the direction of current through the armature windings. Carbon Brush - is the part of a DC motor through which the current is supplied to the armature (rotor). CLASSIFICATION OF ELECTRIC MOTORS Figure 39. Classification of electric motors DC MOTOR The DC motor is a type of electric motor that are powered from direct current source. DC motors can vary in size and power from small motors in toys and appliances to large mechanisms that power vehicles(such as electric bikes, cars and light trains), pull hoists, and other applications that uses direct current(DC). Figure 40. The DC motor Brushed DC Motor With this type of DC motor, magnetic field is produced by the current passing at the commutator through the carbon brushes. Hence, they are called Brushed DC Motors. The following figures shows the different kinds of brushed DC motors. Figure 41. The brushed DC motor https://www.researchgate.net Brushed DC motor can be connected in three ways: Series Shunt Compound(short shunt or long shunt) (a) Series (b) Shunt (c.1) Short Shunt Comound (c.2) Long Shunt Compound Figure 42. Wiring connections for brushed DC motors In some cases, brushed DC motor uses permanent magnet in stator. This is shown in the figure below. Figure 43. Brushed DC motor with permanent magnet stator https://cecas.clemson.edu Brushless DC Motor Brushless DC motors typically consist of a rotor made of permanent magnets and a stator made of wound coils. This arrangement eliminates the use of carbon brushes in the rotor. The speed and direction of rotation is usually controlled by Pulse Width Modulator(PWM) circuits. In some cases, the inner part of the brushless DC motor that contains wound coils is stationary and outer part which is made of permanent magnets is the rotating part. Figure 44. The brushless DC motor (with permanent magnet in the inner part) https://www.orientalmotor.com  Figure 45. The brushless DC motor (with permanent magnet in the outer part) https://www.overclock.net STEPPER MOTOR A stepper motor (or step motor) is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor's position can then be commanded to move and hold at one of these steps without any feedback sensor (an open-loop controller), as long as the motor is carefully sized to the application. Figure 46. The stepper motor http://tech-story.net VARIABLE RELUCTANCE STEPPER MOTOR Figure 47. The schematic diagram Figure 48. The variable reluctance stepper motor https://all3dp.com https://www.motioncontroltips.com AC MOTORS AC motors are types of electric motor that are powered from alternating current source. Traditional ac machines fall into one of two categories: synchronous and induction. In synchronous machines, rotor-winding currents are supplied directly from the stationary frame through a rotating contact. In induction machines, rotor currents are induced in the rotor windings by a combination of the time-variation of the stator currents and the motion of the rotor relative to the stator. SYNCHRONOUS MOTOR In synchronous motor, the stator and rotor both are excited separately. The excitation is the process of inducing the magnetic field on the parts of the motor with the help of an electric current. The rotor is excited by the DC supply. The DC supply induces the north and south poles on the rotor. As the DC supply remains constant, the flux induces on the rotor remains same. Thus, the flux has fixed polarity. The north pole develops on one end of the rotor, and the south pole develops on another end. When the three-phase supply is given to the stator, the rotating magnetic field developed between the stator and rotor gap. The field having moving polarities is known as the rotating magnetic field. The rotating magnetic field develops only in the polyphase system. Because of the rotating magnetic field, the north and south poles develop on the stator. The rotor acting like a bar magnet will turn to line up with the rotating magnetic field. The rotor gets locked to the rotating magnetic field and rotates at synchronous speed under all conditions. Figure 49. The synchronous motor https://circuitglobe.com/synchronous-motor.html Synchronous motor is not self-starting. This type of motor can be started by external prime mover or by incorporating damper windings in its design. INDUCTION MOTOR Induction motor works on the principle of induction where electro-magnetic field is induced into the rotor when rotating magnetic field of stator are cut by copper or aluminum bars embedded in the rotor. Induction machines are by far the most common type of motor used in residential, commercial or industrial settings. Induction motors can be designed either for single-phase or three-phase use. Characteristic features: Simple and rugged construction Low cost and minimum maintenance High dependability and sufficiently high proficiency Needs no additional starting motor and necessity not be synchronized Figure 50. Dismantled parts of induction motor Source: electrotechnical-officer.com THREE-PHASE INDUCTION MOTOR The three-phase AC induction motor is a rotating electric machine that is designed to operate on a three-phase supply. These types of motors are known as self-starting induction motors but larger motors will need starting methods that will reduce starting current. Three-phase induction motors can be squirrel-cage type or wound rotor type. The stator of both types is constructed similarly. They differ only on how rotors are constructed. SQUIRREL-CAGE INDUCTION MOTOR In squirrel-cage type, copper or aluminum bars are embedder in the rotor and are connected by end discs or rings on both ends as shown in the figure. A picture of a complete three-phase induction motor is also shown. Figure 51. Rotor of a squirrel-cage induction motor  Figure 52. Three-phase induction motor WOUND ROTOR INDUCTION MOTOR The rotor of a wound rotor induction motor is also shown below. Three sets of wires usually made of copper are placed in the slots of the rotor, one end of each set are connected together internally while the other end of the three sets are brought out through the use of three sets of slip rings. Carbon brushes are attached to the slip rings and are connected to a three- phase resistance. This is to vary the resistance of the rotor circuit to control speed and torque of the motor. Figure 53. Rotor of a wound rotor induction motor SINGLE-PHASE INDUCTION MOTOR The single-phase induction motor is a rotating electric machine that is designed to operate on single-phase supply. These types of motors are not as self-starting motors and usually of squirrel-cage type. TYPES OF SINGLE-PHASE MOTORS: Split-phase motor. Capacitor start motor. Permanent capacitor run motor. Capacitor start capacitor run motor. Shaded pole motor. SPLIT-PHASE MOTOR Split-phase motors have two stator windings, a main winding (also referred to as the run winding) which we will refer to with the subscript “main” and an auxiliary winding (also referred to as the start winding) which we will refer to with the subscript “aux.” The auxiliary winding has a higher resistance-to-reactance ratio than the main winding, with the result that the two currents will be out of phase, as indicated in the phasor diagram, which is representative of conditions at starting. These two windings are connected in parallel across the supply. Due to the inductive nature, current through main winding lags the supply voltage by a large angle while the current through starting winding is almost in phase with voltage due to resistive nature. Hence there exists a phase difference between these currents and thereby phase difference between the fluxes produced by these currents. The resultant of these two fluxes produce rotating magnetic field and hence the starting torque. Figure 54. Split-phase motor: (a)connections (b)phasor diagram at starting (c) sample picture CAPACITOR START INDUCTION MOTOR This motor is similar to the split phase motor, but in addition a capacitor is connected in series to auxiliary winding. The capacitor-start motor is also a split-phase motor, but the time- phase displacement between the two currents is obtained by means of a capacitor in series with the auxiliary winding, as shown. Again the auxiliary winding is disconnected after the motor has started, and consequently the auxiliary winding and capacitor can be designed at minimum cost for intermittent service. Figure 55. Capacitor start motor: (a)connections (b)phasor diagram at starting (c) sample picture PERMANENT CAPACITOR INDUCTION MOTOR This motor is also called as a capacitor run motor in which a low capacitor is connected in series with the starting winding and is not removed from the circuit even in running condition. Due to this arrangement, centrifugal switch is not required. Figure 56. Permanent capacitor motor: (a)connections (b)phasor diagram at starting (c) sample picture CAPACITOR START CAPACITOR RUN MOTOR These motors are also called as two-value capacitor motors. It combines the advantages of capacitor start type and permanent capacitor type induction motors. This motor consists of two capacitors of different value of capacitance for starting and running. A high value capacitor is used for starting conditions while a low value is used for running conditions. Figure 57. Permanent capacitor motor: (a)connections (b) sample picture SHADED POLE MOTOR The shaded-pole induction motor usually has salient poles with one portion of each pole surrounded by a short-circuited turn of copper called a shading coil. Induced currents in the shading coil cause the flux in the shaded portion of the pole to lag the flux in the other portion. The result is similar to a rotating field moving in the direction from the unshaded to the shaded portion of the pole; currents are induced in the squirrel-cage rotor and a low starting torque is produced.  Figure 58. Shaded pole motor: (a)connections (b) sample picture UNIVERSAL MOTOR The universal motor is a type of electric motor that can be supplied by either AC or Dc source. The rotor of this type of motor is almost the same as the rotor of a DC motor because of the presence of armature and commutator segments. A sample picture is shown below. This type of motor is usually used in blenders, electric hand drills and vacuum cleaners. Figure 59. A universal motor HYDRAULIC ACTUATION SYSTEM Hydraulic systems are designed to move large loads by controlling a high-pressure fluid in distribution lines and pistons with mechanical or electromechanical valves. A hydraulic pump is usually driven by an electric motor (e.g., a large AC induction motor) or an internal combustion engine. Typical fluid pressures generated by pumps used in heavy equipment (e.g., construction equipment and large industrial machines) are in the 1000 psi (6.89 MPa) to 3000 psi (20.7 MPa) range. Figure 60. Hydraulic system components VALVES Are devices that regulate the flow of fluid(gases, liquids, fluidized solids or slurries) by opening, closing, or partially obstructing variou4s passageways. Valves are technically pipe fittings but are usually discussed as a separate category. In an open valve, fluid flows in a direction from higher pressure to lower pressure. SWITCHING SYMBOLS FOR VALVES: Note: The number of squares corresponds to the number of switching positions Lines indicate the flow paths Arrows indicate the flow direction Closed ports are represented by two lines drawn at right angles to one another TYPES OF VALVES Pressure regulator valves On-off valves Directional valves Flow rate regulator valves Figure 61. Pressure regulator Figure 62. ON-OFF valve Figure 63. Control valve Figure 64. 4/3 valve schematic Figure 65. Check and Poppet valves Hydraulic Cylinders Figure 66. Single-acting and double-acting cylinders Figure 76. Double-acting cylinder with 4/3 control valve Figure 68. Example mechanisms driven by hydraulic cylinders PNEUMATIC ACTUATION SYSTEM Pneumatic systems are similar to hydraulic systems, but they use compressed air as the working fluid rather than hydraulic liquid. A compressor is used to provide pressurized air, usually on the order of 70 to 150 psi (482 Kpa to 1.03 MPa), which is much lower than hydraulic system pressures. As a result of the lower operating pressures, pneumatic actuators generate much lower forces than hydraulic actuators. Figure 69. Pneumatic system components Note: Symbols used for pneumatic valves and cylinders are almost the same with hydraulic valves and cylinders Figure 70. Pneumatic actuator Figure 71. Single-acting cylinder Figure 72. Single-acting cylinder Figure 73. A Festo brand double-acting cylinder SELF ASSESSMENT 1. What form of electric motor is most widely used in high-power applications? A. DC motors. B. AC motors. C. Stepper motors. 2. Many wrist watches use electric motors to move conventional hands. What form of motor is most suited to this application? A. DC motor. B. AC motor. C. Stepper motor. 3. Which type of motion is transmitted by hydraulic actuators? A. linear motion B. rotary motion C. both a and b D. none of the above 4. What is the function of electric actuator? A. converts electrical energy into mechanical torque B. converts mechanical torque into electrical energy C. converts mechanical energy into mechanical torque D. none of the above 5. Why are hydraulic cylinders cushioned? A. cushioning decelerates the piston of a cylinder B. stress and vibrations can be reduced C. both a and b D. none of the above 6. Heavy lifting work is often accomplished by shifting fluids in big machines. The power system of such machines can be described as A. Reciprocating B. Pneumatic. C. Hydraulic. D. Hybrid. 7. Pneumatic and other power systems can support three kinds of motion; they are A. linear, reciprocating, and random motion. B. linear, flowing, and rotary motion. C. linear, zigzag, and spiral motion. D. linear, reciprocating, and rotary motion. 8. A single acting cylinder can be pressurized externally from one direction only. A. True B. False 9. A one-way valve that lets air into the reservoir of a compressor, but doesn't let it out, is a A. check valve. B. receiver valve. C. control valve. D. three way valve. 10. A 5/2 way single solenoid valve has A. 2 ports 2 positions. B. 5 ports 2 positions. C. 5 ports 5 positions. D. 2 ports 5 positions.