Actuators

Instructor: Shuvra DasMechanical Engineering Dept.

University of Detroit Mercy

Summary

• Actuators

• Some actuator examples

• Switches

• Electric motors

• Piezo-actuators

• Mechanisms

Flowchart of Mechatronic Systems

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

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

Armature

Field Coils

Brushes

Commutator

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= KTI; I=armature current,KT 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= KE; = motor speed in rpm.

• KT and KE 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

Permanent magnet DC Motors

• Armature current, I= (V – E)/R. R is the armature resistance and E is back emf.

• The Torque therefore is T= T= KTI = KT (V – E)/R = KT (V – KE)/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= KTI = KT (V – E)/R = KT (V – KE)/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