Actuators Elements that can execute physical action Electromechanical elements - receive input from controllers Controller could be dedicated or embedded in software Software control needs D/A signal conversion
Role of Actuators Electrical actuation signal (from controller) Actuator (pneumatic, hydraulic, motor, switch, etc.) Mechanisms (belts, pulleys, gear trains, etc.) Mechanical load
Actuators: Typical Actions Move a load Open a valve to increase flow Rotate a shaft …….
Actuators: Examples Hydraulic or Pneumatic cylinders Control valves Electric motors Switches Relays electric motors are most common actuators but for high power requirements Hydraulic or pneumatic ones are used.
Types of Actuators Hydraulic Actuators Mechanical Actuators Electrical Actuators
Switching devices: mechanical systems, relays or solid-state devices, control signal switches an electrical device on or off Solenoid type devices: current through a coil activates an iron core that controls a hydraulic or pneumatic valve Drive systems: D.C. and A.C. motors, where a current through a motor produces rotation
Electromagnetic Relays A mechanical switch can be closed or opened as a result of control signal When the coil is energized it pulls the plunger closing mechanical contact Used in activating motors or heating elements Demagnetization leads to contact loss NO: normally (unenergized) open NC: normally (unenergized) closed
Solenoid relays Electrical Energy converted to linear mechanical motion De-energized state: Plunger half-way inside the coil Energized state: Plunger pulled in completely e.g. car door locks, opening/closing valves disadvantage: stroke very short
Solid State Relay Input signal: typically 5V DC, 24V DC, or 120V AC Input circuit works like EMR (electro-magnetic relay) Output circuit works like EMR as well output circuit can be AC and electronic switch capable of supporting large currents LED and phototransistor pair optically coupled, i.e. light activates electrical signal in photo transistor
Solid State Relay The amplifier boosts the signal to a suitable level to trigger the triac Triac: electronic switch that supports current in both directions Input => LED => phototransistor => amplifier=> triac => actuation output Separates high power output side from low power input side
Electronic Vs. Mechanical Switches Disadvantages (elec.) False triggering through noise Failure unpredictable when on-not 100% short; when off -not 100% open Advantages(electronic) No contact-no wear No contact bounce No arcing Faster Maybe driven by low- voltage
DC Motors Current carrying conductor in magnetic field experiences force (Lorentz effect) A conductor moved in a magnetic field generates (back) emf that opposes the change that produces it. (Faraday/Lenz’s law) Back emf rate of change of flux Current due to back emf in closed circuit will create a flux opposite the magnetic flux motor direction is reversed by reversing the polarity of voltage
DC Motors Armature coil is free to rotate in the magnetic field Loop of wire is connected through the commutator to the brushes (brushes stationary, commutator rotates) Current flows when power is supplied to brushes
DC Motors Opposite forces on opposite sides generates a torque Commutator changes current direction when the plane of wire is vertical Torque direction remains unchanged Multiple wires are wound in a distributed fashion over cylindrical rotor of ferromagnetic material Multiple loops increases and also evens out the torque
Permanent magnet DC Motors Permanent magnet provides a constant value of flux density. For an armature conductor of length L and carrying a current I the force resulting from a magnetic flux density B at right angles to the conductor is B I L.
Permanent magnet DC Motors With N conductors the force is F=N B I L. The forces result in a torque about the coil axis of Fc, if c is the breadth of the coil, T= (NBLc)I. Torque is thus written as T= K T I; I=armature current,K T is based on motor construction.
Permanent magnet DC Motors Since the armature coil is rotating in a magnetic field, electromagnetic induction will occur and a back emf will be induced. The back emf E is related to the rate at which the flux linked by the coil changes. For a constant magnetic field, is proportional to the angular velocity of rotation. Back emf is related to flux and angular rotation (in rpm) E= K E ; = motor speed in rpm. K T and K E depend on motor construction
The motor circuit can be represented as: The current in the circuit is I = (V – E)/R Permanent magnet DC Motors V E R
Armature current, I= (V – E)/R. R is the armature resistance and E is back emf. The Torque therefore is T= T= K T I = K T (V – E)/R = K T (V – K E )/R At start-up, back emf is minimum therefore I is maximum and Torque is maximum. The faster it runs the smaller the current and hence the torque.
Permanent magnet DC Motors T= K T I = K T (V – E)/R = K T (V – K E )/R T speed V
Other types DC motors Separately excited armature windings: –series wound motor –shunt wound motor –compound motor Non-DC motors: AC motors
Servo motors Consists of DC motor, gear train and built in pot (and circuitry) for shaft position indication
Servo Motors A servo motor is a DC or AC component coupled with a position sensing device. A DC servo motor consists of a motor, gear train, potentiometer, limit stops, control circuit. Three wires: ground, power, control signal. The control signal is in the form of a pulse width signal. As long as the control line keeps receiving the signal the servo holds the position of the shaft. With the change of the coded signal the position of the shaft changes.
Servo motors Input is pulse width modulated signal (PWM) Pulse duration is based on a coded number from 0- 255 (programmed into microcontroller) The PWM is used to turn an electric switch on and off such that a fixed DC source is intermittently applied to the motor. This reduces the effective voltage seen by the motor
Servo motors The servo has some control circuit and a pot. Once the final position is reached the circuit turns the power off. The output shaft can travel between 0 and 180. A servo expects to see a pulse every 20 ms. The duration of the pulse determines how far the servo will travel. A 1.5ms pulse makes it travel by 90 degrees. For a longer pulse the travel is closer to 180 and for a shorter pulse it is closer to 0.
Servo Motors When the new position is reached (coresponding to the duration of PWM signal) motor is shut off by the control circuitry This position is maintained until the PWM signal input is unchanged Most common servos use 5 volts of input supply
Servo motor Amount of power to motor distance the servo needs to travel Control wire is used to send the PWM signal Servos are usually small but extremely powerful for its size Futaba S-148 has 42oz.inches of torque
Stepper Motors Moves in discrete steps rotor is permanent magnet When electromagnets are energized the rotor aligns itself properly Step sizes can be obtained from 0.9 through 90 degrees
Stepper Motors Common uses: dot matrix printer paper advance, positioning read-write heads of disk-drives Advantage: Can be used in open-loop control mode without shaft position recorder (if the number of steps taken is recorded). No sensors needed! Disadvantage: for heavy loads steps could be missed; without feedback this cannot be recovered