Mechatronics Introduction • Rapid advances in microprocessors have led to a dramatic increase in electronically controlled devices and systems. • All of these systems require sensors and actuators to be interfaced with the electronics • A mechatronic system include – Sensors – Actuators – Controls Actuators • Linear Actuators • Pneumatic and Hydraulic Actuators • Rotary Actuators • Flow-Control Valves • Sound and Optical Devices (Speakers, LEDs…) Linear Actuators • The task of a linear actuator is to provide motion in a straight line • Basic three ways to achieve linear motion: – Conversion of rotary motion into linear motion: slider-crank ,screw threads – Use of a fluid pressure to move a piston in a cylinder: • Pneumatic system: air as working fluid • Hydraulic: liquid as working fluid – Electromagnetic Slider–Crank Mechanism • Used in – generating a reciprocating linear motion – converting linear motion to rotary motion Screw-drive linear motion • A common means for translating rotary motion into linear motion is a lead screw • A lead screw has helical threads that are designed for minimum backlash to allow precise positioning • The rotary motion of the lead screw is translated into linear motion of the nut, with the torque required to drive the lead screw directly related to the thrust the particular application requires Pneumatic Actuators • A pneumatic cylinder use compressed air as the energy source to create linear motion • In general, the purpose of a pneumatic cylinder is to provide linear motion between two ο¬xed locations Hydraulic Actuator • A hydraulic system or component uses incompressible oil as the working ο¬uid • An example is the power steering on an automobile • Hydraulic ο¬uid is supplied at an elevated pressure from the power steering pump. • When a steering input is made, the rotary valve allows highpressure ο¬uid to enter the appropriate side of the piston, and aid in turning the wheels Solenoid Actuator • An electromagnetic device that is employed to create linear motion of a plunger • When the electromagnet is actuated, the resulting magnetic force pulls the plunger into the C-frame • The initial force available from a solenoid is The Magnetic Field • A current-carrying wire produces a magnetic field in the area around it (Ampere’s law) • A time-changing magnetic field induces a voltage in a coil of wire if it passes through the coil (Faraday’s law)(transformer) • A current-carrying wire in the presence of a magnetic field has a force induced on it (Laplace force)(motor) • A moving wire in the presence of a magnetic field has a voltage induced in it (Lorentz force)(generator) Magnetic Field • The basic law governing the production of a magnetic field by a current is Ampere's law: π» β ππ = πΌ • where H is the magnetic field intensity produced by the current I and dl is a differential element of length along the path of integration. • With N turns winding, and lc mean path length π»ππ = ππΌ • H unit: Amp-turn/m Magnetic Flux Density • The relationship between the magnetic field intensity H and the resulting magnetic flux density B produced ππΌ within a material is given by π΅ = ππ» = π ππ • Unit of B: Tesla (T) or Gauss • π: magnetic permeability of material • The permeability of free space is called π0 , and its value is π0 = 4π × 10−7 H/m • The permeability of any other material compared to the permeability of free space is called its relative π permeability: π = π π0 Magnetic Flux • The total flux in a given area is given by π= π΅ β ππ΄ π΄ • If the flux density is constant throughout the area, π = π΅π΄ • Unit: Webers (Wb) πππΌπ΄ π = π΅π΄ = ππ Production of Induced Force on a Wire πΉ = π(π × π΅) F ο½ ilB sin ο± i Faraday’s Law ο¦ dο¦ eind ο½ ο N dt - eind i B + v l - N eind + ο¦ dο¦ ο½ B ο dA ο½ Bldx ο½ ο Blvdt dο¦ eind ο½ ο dt ο¦ ο Blvdt οΆ eind ο½ οο§ ο· ο½ Blv ο¨ dt οΈ Linear DC Machine • Basic physics equations: • Starting a linear dc machine: Starting a linear dc machine The Linear DC Machine as a Motor The Linear DC Machine as a Generator Example • The linear dc machine has a battery voltage of 120 V. an internal resistance of 0.3ο. and a magnetic flux density of 0.1T a) What is this machine's maximum starting current? What is its steady-state velocity at no load? b) Suppose that a 30-N force pointing to the right were applied to the bar. What would the steady-state speed be? How much power would the bar be producing or consuming? How much power would the battery be producing or consuming? Is this machine acting as a motor or as a generator? c) Now suppose a 30-N force pointing to the left were applied to the bar. What would the new steady-state speed be? Is this machine a motor or a generator now? Rotary Actuators • A rotary actuator is an actuator that produces a rotary motion or torque • The motion produced by an actuator may be either continuous rotation, or movement to a fixed angular position • The torque motor, does not necessarily produce any rotation but merely generates a precise torque • Electric powered – Step Motor – Servo Motor • Fluid powered A Simple Loop in a Uniform Magnetic Field • The Voltage Induced in a Simple Rotating Loop The Torque Induced in a Current-Carrying Loop Stepper Motor • A stepper 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 • Each incoming pulse results in the shaft is turning a specific angular distance • Stepper motors can control velocity, distance and direction of mechanical load Types of Stepper Motors • Permanent magnet type – Used for low speed and relatively high torque applications • Variable reluctance type – Generally used for low torque applications – Induces non-permanent magnetic poles on the ferromagnetic rotor – Torque is generated through the phenomenon of magnetic reluctance • Hybrid type – Multi toothed stator poles and permanent magnet rotor – Has high static and dynamic torque Two-phase stepper motors • There are two basic winding arrangements for the electromagnetic coils in a two phase stepper motor • Unipolar – Has one winding with center tap per phase – Each section of windings is switched on for each direction of magnetic field – A magnetic pole can be reversed without switching the direction of current • Bipolar – Bipolar motors have a single winding per phase – The current in a winding needs to be reversed in order to reverse a magnetic pole, so the driving circuit must be more complicated Two-phase stepper motors Basic Stepper Motor Operating Principles • • • • Consider a unipolar winding: each coil is center tapped to the ground The rotor has one north and one south pole permanent magnet. The stator has four-pole, two-phase winding with four switches At any given time either switch S1 or S2, and S3 or S4 can be ON to affect the polarity of electromagnets. • For each switch state, there is a corresponding stable rotor position. NEMA 17 • A NEMA 17 stepper motor is a stepper motor with a 1.7 x 1.7 inch (43.2 x 43.2 mm) faceplate. Connection Servo Motor • A servomotor is a rotary actuator or linear actuator that allows for precise control of angular or linear position, velocity and acceleration • Consists of a small DC motor, potentiometer (sensor) for feedback and a control circuit • The motor is attached by gears to the control wheel. As the motor rotates, the potentiometer's resistance changes, so the control circuit can precisely regulate the movement • The desired position is sent via electrical pulses through the signal wire Types of Servo Motors • There are two types of servo motors: – AC servos: can handle higher current surges and tend to be used in industrial machinery. – DC servos: • are not designed for high current surges and are usually better suited for smaller applications. • Generally speaking, DC motors are less expensive than their AC counterparts DC Servo Motors • DC servo motors are controlled by DC command signals applied directly to coils. • The magnetic fields that are formed interact with permanent magnets and cause the rotating member to turn. • One type of PM uses a wound armature and brushes like a conventional DC Motor, but uses magnets as pole pieces. • Another type uses wound field coils and a permanent magnet rotor. Induced Torque in the Rotating Loop • Suppose a battery is now connected to the machine • The approach to take in determining the torque on the loop is to look at one segment of the loop at a time and then sum the effects of all the individual segments Types of DC Motors • The separately excited DC motor • The shunt DC motor • The permanent-magnet DC motor • The series DC motor • The compounded DC motor Equivalent Circuit of a DC Motor Internal generated voltage EA=KΟω, Induced torque by machine τind=KΟIA Separately excited DC motors Shunt DC Motors Speed Control of Shunt DC Motors Change Field Resistance Change Armature Voltage • Adjust field resistance RF • Adjust terminal voltage • Adjust VA • • • • • • • • • RF ↑, IF (=VT/RF) ↓ IF ↓, Ο↓ Ο ↓, EA (=KΟω)↓ EA ↓, IA (=(VT -EA)/RA)↑↑ IA ↑↑, τind (=K Ο↓ IA↑↑) ↑ τind > τload , ω ↑ ω ↑, EA ↑ EA ↑, IA ↓ τind = τload equilibrium, ω ↑ • • VA ↑, IA (=(VA -EA)/RA)↑ IA ↑, τind (=K Ο IA) ↑ • • • • τind > τload , ω ↑ ω ↑, EA ↑ EA ↑, IA ↓ τind = τload equilibrium, ω ↑ • Example: A 50-hp, 250-V, 1200r/min, DC shunt motor with compensating windings has an armature resistance of 0.06 Ohm. Its field circuit has a total resistance Radj+RF of 50 Ohm, which produce a non-load speed of 1200r/min. There are 1200 turns per pole on the shunt field winding. • a. Find the speed when its input current is 100A • b. Find the speed when its input current is 200A • c. Find the speed when its input current is 300A 1. VT and RF are constants, so IF is constant 2. No armature reaction effects So the flux is constant. 3. At no load, the armature current is zero. EA=250V @ 1200 rpm. Series DC Motor A series DC motor’s field windings consist of a relatively few turns connected in series with the armature circuit. τind = KΟ IA = KcIA2 , where Ο = cIA ο΄ ind VT ο½ K ο¦ο· ο« ο·ο½ Kc VT 1 Kc ο΄ ind ο ο¨ R A ο« RS ο© R A ο« RS Kc Speed Control of Series DC Motors • There is only one efficient way to change the speed of a series dc motor, which is to change the terminal voltage of the motor • VT ο,ωο • The speed of series dc motors can also be controlled by the insertion of a series resistor into the motor circuit , but this technique is very wasteful of power and is used only for intemittent periods during the start-up of some motors Compound generator Compounded DC Motor Long shunt ο Cumulatively ο§ Differentially VT ο½ E A ο« I A ( R A ο« RS ) I A ο½ IL ο IF VT IF ο½ RF ο net ο½ ο F ο± ο SE ο ο AR N SE ο AR I ο½ IF ο« IA ο NF NF * F Short shunt The Torque-Speed Characteristic of a Cumulatively Compounded DC Motor • There is a component of flux which is constant and another component which is proportional to its armature current • The cumulatively compounded motor has a higher starting torque than a shunt motor (whose flux is constant) but a lower starting torque than a series motor • The cumulatively compounded dc motor combines the best features of both the shunt and the series motors. • Like a series motor, it has extra torque for starting • Like a shunt motor, it does not overspeed at no load. The Torque-Speed Characteristic of a Differentially Compounded DC Motor • The shunt magnetomotive force and series magnetomotive force subtract from each other. • As the load ο, IA ο, and the flux in the motor decreases. • But as the flux decreases, the speed of the motor increases • This speed ο the load ο, IA ο, flux ο―, and the speed ο again • It is unstable and tends to run away, so bad that a it is unsuitable for any application
