MEV 403 Introduction to Mechatronics

Module 2: Sensors and Actuators

1 Santhakumar Mohan, Assistant Professor, MED, NITC Introduction to Actuators

• Actuators are basically the muscle behind a mechatronics system that accepts a control command (mostly in the form of an electrical signal) and produces a change in the physical system by generating force, motion, heat, flow, and so forth .

2 Santhakumar Mohan, Assistant Professor, MED, NITC Classification of Actuators

• Based on motion • Based on energy supplied – Linear actuator – Electrical – Rotary actuator – Mechanical • Based on number of stable – Electromechanical state outputs – Electromagnetic – Binary – Hydraulic and Pneumatic – Continuous – Smart actuators

3 Santhakumar Mohan, Assistant Professor, MED, NITC Electrical Actuators

• Advantages of Electrical actuators – Electricity is easily routed to the actuators; cables are simpler than pipe work. – Electricity is easily controlled by electronic units – Electricity is clean – Electric faults are often easier to diagnose • Disadvantages of electric actuators – Fire hazard – Poor torque – speed characteristics – Basically Rotary motion and complicated mechanism needed for linear motion – Power to weight ratio is inferior to hydraulic motors

4 Santhakumar Mohan, Assistant Professor, MED, NITC Types of Electrical Actuators

• DC Motor – Wound field – Permanent – Electronic commutation (brushless motor) • AC Motor – – – • – Variable reluctance – Permanent magnet – Hybrid

5 Santhakumar Mohan, Assistant Professor, MED, NITC Stepper motor

• A stepper motor is an electromechanical device which converts electrical pulses into discrete mechanical movements . • The stepper motor is a discrete (incremental) positioning device that moves one step at a time for each pulse command input. • Since they accept direct digital commands and produce a mechanical motion, the stepper motors are used widely in industrial control applications. • They are mostly used in fractional horsepower applications. With the rapid progress in low cost and high-frequency solid- state drives, they are finding increased applications.

6 Santhakumar Mohan, Assistant Professor, MED, NITC Why Stepper Motor?

• Relatively inexpensive • Ideal for open loop positioning control – Can be implemented without feedback – Minimizes sensing devices – Just count the steps! • Torque – Holds its position firmly when not turning – Eliminates mechanical brakes – Produces better torque than DC motors at lower speeds • Positioning applications

7 Santhakumar Mohan, Assistant Professor, MED, NITC Types of Stepper Motor

Permanent Magnet (PM) type Variable reluctance (VR) type Magnetic Non Magnetic, Geared Rotor

Hybrid type Combines characteristics from PM and VR Magnetic, geared rotor

8 Santhakumar Mohan, Assistant Professor, MED, NITC Stepper motor: Principle of operation

Variable Reluctance Type

Permanent Magnet Type

9 Santhakumar Mohan, Assistant Professor, MED, NITC Comparison of different types

10 Santhakumar Mohan, Assistant Professor, MED, NITC Stepper motor characteristics

1. Stepper motors are constant power devices. 2. As motor speed increases, torque decreases. 3. The torque curve may be extended by using current limiting drivers and increasing the driving voltage. 4. Steppers exhibit more vibration than other motor types, as the discrete step tends to snap the rotor from one position to another. 5. This vibration can become very bad at some speeds and can cause the motor to lose torque. 6. The effect can be mitigated by accelerating quickly through the problem speeds range, physically damping the system, or using a micro-stepping driver. 7. Motors with a greater number of phases also exhibit smoother operation than those with fewer phases. 11 Santhakumar Mohan, Assistant Professor, MED, NITC Torque vs. speed characteristics

• Holding torque The maximum torque produced by the motor at standstill. • Pull-In Curve The pull-in curve defines a area referred to as the start stop region. • Maximum Start Rate The maximum starting step frequency with no load applied. • Pull-Out Curve The pull-out curve defines an area referred to as the slew region. • Maximum Slew Rate The maximum operating frequency of the motor with no load applied.

12 Santhakumar Mohan, Assistant Professor, MED, NITC Step angle and Stepping mode

For Permanent Magnet type × Step angle = 360º / (N r Ns ) For Variable Reluctance type × Step angle = (Nr -Ns ) 360º / (N r Ns ) where, N r number of rotor poles and Ns number of phases

The following are the most common drive (stepping) modes. – Wave Drive (1 phase on) – Full Step Drive (2 phases on) – Half Step Drive (1 & 2 phases on) – Micro-stepping (Continuously varying motor currents)

13 Santhakumar Mohan, Assistant Professor, MED, NITC Advantages of stepper motors

• Low cost • Can work in an open loop (no feedback required) • Excellent holding torque (eliminated brakes/clutches) • Excellent torque at low speeds • Low maintenance (brushless) • Very rugged - any environment • Excellent for precise positioning control • No tuning required

14 Santhakumar Mohan, Assistant Professor, MED, NITC Disadvantages of Stepper Motors

• Rough performance at low speeds (unless you use micro- stepping) • Consume current regardless of load • Limited sizes available • Noisy • Torque decreases with speed (you need an oversized motor for higher torque at higher speeds) • Stepper motors can stall or lose position running without a control loop

15 Santhakumar Mohan, Assistant Professor, MED, NITC Applications of Stepper motor

• Cruise control • Auto air vents • Light leveling • Printers • Industrial machines • Automotive gauges • Office equipment • Computer drives • Medical scanners • Scientific Instrumentation

16 Santhakumar Mohan, Assistant Professor, MED, NITC Servo Motors

• A (servo) is an electromechanical device in which an electrical input determines the position of the of a motor.

17 Santhakumar Mohan, Assistant Professor, MED, NITC Types of Servo motors

• AC servo motors , based on induction motor designs; • DC servo motors , based on designs; • AC brushless servo motors , based on synchronous motor designs.

Servo motors are special category of motors, designed for applications involving position control, velocity control and torque control .

These motors are special in the following ways: 1. Lower mechanical time constant. 2. Lower electrical time constant. 3. Permanent magnet of high flux density to generate the field. 4. Fail-safe electro-mechanical brakes.

18 Santhakumar Mohan, Assistant Professor, MED, NITC Difference between Stepper and Servo

• The basic difference between a traditional stepper and a servo-based system is the type of motor and how it is controlled. Steppers typically use 50 to 100 pole brushless motors while typical servo motors have only 4 to 12 poles. A pole is an area of a motor where a North or South magnetic pole is generated either by a permanent magnet or by passing current through the coils of a winding. • Steppers don't require encoders since they can accurately move between their many poles whereas servos, with few poles, require an encoder to keep track of their position. • Steppers simply move incrementally using pulses [open loop] while servo's read the difference between the motors encoder and the commanded position [closed loop], and adjust the current required to move.

19 Santhakumar Mohan, Assistant Professor, MED, NITC Difference between Stepper and Servo

• Stepper motors have many more poles than servo motors. One rotation of a stepper motor requires many more current exchanges through the windings than a servo motor. The stepper motor's design results in torque degradation at higher speeds when compared to a servo. Using a higher driving bus voltage reduces this effect by mitigating the electrical time constant of the windings. Conversely, a high pole count has a beneficial effect at lower speeds giving the stepper motor a torque advantage over the same size servo motor. • Traditional steppers operate in the open loop constant current mode. This is a cost savings, since no encoder is necessary for most positioning applications. However, stepper systems operating in a constant current mode creates a significant amount of heat in both the motor and drive, which is a consideration for some applications. Servo control solves this by only supplying the motor current required to move or hold the load. It can also provide a peak torque that is several times higher than the maximum continuous motor torque for acceleration. However, a stepper motor can also be controlled in this full servo closed loop mode with the addition of an encoder. 20 Santhakumar Mohan, Assistant Professor, MED, NITC Difference between Stepper and Servo

• Steppers are simpler to commission and maintain than servos. They are less expensive, especially in small motor applications. They don't lose steps or require encoders if operated within their design limits. Steppers are stable at rest and hold their position without any fluctuation, especially with dynamic loads. • Servos are excellent in applications requiring speeds greater than 2,000 RPM and for high torque at high speeds or requiring high dynamic response. Steppers are excellent at speeds less than 2,000 RPM and for low to medium acceleration rates and for high holding torque. • Servo control systems are best suited to high speed, high torque applications that involve dynamic load changes. Stepper control systems are less expensive and are optimal for applications that require low-to- medium acceleration, high holding torque, and the flexibility of open or closed loop operation.

21 Santhakumar Mohan, Assistant Professor, MED, NITC Piezoelectric Transducer

Piezoelectric effect • The piezoelectric effect describes the relation between a mechanical stress and an electrical voltage in solids. • The direct piezoelectric effect is that piezo-ceramic generates an electrical charge during mechanical distortion or load. During an inverse piezoelectric effect the piezo- ceramic body changes under the influence of an electrical field.

22 Santhakumar Mohan, Assistant Professor, MED, NITC Piezoelectric materials

Crystals Quartz SiO 2 Berlinite AlPO 4 Gallium orthophosphate GaPO 4 Tourmaline Ceramics Barium titanate BaTiO 3 Lead zirconate titanate PZT Other materials Zinc oxide ZnO Aluminum nitride AlN Polyvinylidene fluoride PVDF

23 Santhakumar Mohan, Assistant Professor, MED, NITC

• A piezo motor is based on the change in mechanical shape of a piezoelectric material when an tension is applied. • The material produces ultrasonic or acoustic vibrations and produces a linear or rotary motion. • A few design exists currently: – Locking mechanisms – Stepping Actions – Single Action

24 Santhakumar Mohan, Assistant Professor, MED, NITC Piezoelectric sensor

• Piezoelectric sensor are devices using the piezoelectric effect to measure acceleration, pressure, strain or force and converting them to an electrical signal. • Piezoelements are suitable for the detection of dynamic processes. • In static applications the piezoelectric charges are too small, in order to be detected. • An amplifier is used to convert the piezoelectric charges into a measurable electrical tension. • Principle of operations – transverse – longitudinal – shear

25 Santhakumar Mohan, Assistant Professor, MED, NITC • Charge generated Q = d × F (longitudinal effect) Q = d × F × a / b (transverse effect) Where, d is the piezoelectric coefficient of the material a is the length of the bar, b is the width of the bar • Voltage produced V = g × t × p Where, g is the crystal sensitivity factor t is the thickness of the bar, p is the pressure or stress V/t is the electric field strength

26 Santhakumar Mohan, Assistant Professor, MED, NITC Piezoelectric actuator designs

Piezo-actuators: (a) stacked; (b) laminated; (c) unimorph; (d) amplified; (e) mooney; and (f ) inchworm motor.

27 Santhakumar Mohan, Assistant Professor, MED, NITC Applications of Piezoelectric Instrumentation

• Aerospace: Modal testing, wind tunnel and shock tube instrumentation, landing gear hydraulics, rocketry, structures, ejection systems and cutting force research. • Ballistics: Combustion, explosion, detonation and sound pressure distribution. • Biomechanics: Multi-component force measurement for orthopedic gait and posturography, sports, ergonomics, neurology, cardiology and rehabilitation. • Engine Testing: Combustion, gas exchange and injection, indicator diagrams and dynamic stressing. • Engineering: Materials evaluation, control systems, reactors, building structures, ship structures, auto chassis structural testing, shock and vibration isolation and dynamic response testing. • Industrial/Factory: Machining systems, metal cutting, press and crimp force, automation of force based assembly operations and machine health monitoring. • OEMs: Transportation systems, plastic molding, rockets, machine tools, compressors, engines, flexible structures, oil/gas drilling and shock/vibration testers.

28 Santhakumar Mohan, Assistant Professor, MED, NITC Some application areas

29 Santhakumar Mohan, Assistant Professor, MED, NITC Advantages and Limitations

Advantages: – Wide frequency range – Compact, often low weight – High stability – Can be mounted with any orientation – Self generating High impedance output – No moving parts, no wear No true DC response – Rugged – Very large dynamic range Limitations: – High impedance output – No true DC response

30 Santhakumar Mohan, Assistant Professor, MED, NITC Fluid power actuators

• Use of fluids to transmit power: – Pumps are power generators – Inverse pumps or cylinders are power drain – Valves used for control • Types of fluid power actuators – Hydraulic (oil as the fluid) • Linear actuators (cylinders) • Rotary actuators (motors) – Pneumatic (air as the fluid) • Linear actuators (cylinders) • Rotary actuators (motors)

31 Santhakumar Mohan, Assistant Professor, MED, NITC Hydraulic Cylinder

• A Hydraulic cylinder (also called a linear hydraulic motor) is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. • Types of cylinders – Single acting cylinder – Double acting cylinder • Single rod end • Double rod end – Telescopic cylinder

32 Santhakumar Mohan, Assistant Professor, MED, NITC Hydraulic Motor

• A hydraulic motor is a mechanical actuator that converts hydraulic pressure and flow into torque and angular displacement (rotation). The hydraulic motor is the rotary counterpart of the hydraulic cylinder. • Conceptually, a hydraulic motor should be interchangeable with a hydraulic pump because it performs the opposite function - much as the conceptual DC is interchangeable with a DC electrical generator. • However, most hydraulic pumps cannot be used as hydraulic motors because they cannot be back driven. Also, a hydraulic motor is usually designed for the working pressure at both sides of the motor.

33 Santhakumar Mohan, Assistant Professor, MED, NITC Types of Hydraulic motor

• Gear motor – Internal gear – External gear • Vane motor • Lobe and gerotor motor • Piston type motor – Axial piston type – Radial piston type

34 Santhakumar Mohan, Assistant Professor, MED, NITC Gear motor

35 Santhakumar Mohan, Assistant Professor, MED, NITC Vane motor

36 Santhakumar Mohan, Assistant Professor, MED, NITC Axial piston type motor

37 Santhakumar Mohan, Assistant Professor, MED, NITC Lobe and Radial piston motor

38 Santhakumar Mohan, Assistant Professor, MED, NITC Internal gear and Gerotor motor

39 Santhakumar Mohan, Assistant Professor, MED, NITC Gear pump External type Internal type Other features (Fixed displacement In-line assembly for multi- • Low cost • Low noise pump units • Low contaminant • Low contaminant sensitivity sensitivity • Compact, low weight • 250cm³/rev, 250bar • Good suction performance • 250cm³/rev, 250bar Vane Fixed displacement Variable displacement In-line assembly for multi- • Low noise • Low noise pump units • Good serviceability • Low cost • 200cm³/rev, 280bar • Good serviceability • Displacement controls • 100cm³/rev, 160bar Piston Fixed and variable displacement

• High efficiency • Integral boost pump and • Good serviceability multipurpose assemblies • Wide range of displacement controls (not bent axis0 • Up to 1000 cm³/rev., 350/400bar • Can use most types in hydrostatic transmissions 40 Santhakumar Mohan, Assistant Professor, MED, NITC Advantages and disadvantages of Hydraulic Actuators

Advantages: Disadvantages: • infinitely variable control of gear-ratio in a • efficiency of a volumetric hydraulic wide range and an opportunity to create the actuator is a little bit lower, than big reduction ratio; efficiency of mechanical and electric • small specific weight, i.e. the weight of a transfers, and during regulation it is hydroactuator is in ratio to transmitted reduced; capacity (0,2...0,3 kg / kW); • conditions of operation of a • opportunity of simple and reliable protection hydraulic actuator (temperature) of the engine from overloads; influence its characteristics; • small sluggishness of the rotating parts, • efficiency of a hydraulic actuator is a providing fast change of operating modes little reduced in the process of (start- up, dispersal, a reverser, a stop); exhaustion of its resource owing to • simplicity of transformation of rotary the increase in backlashes and the movement into reciprocating one; increase of outflow of liquid (falling of volumetric efficiency); • opportunity of positioning a hydraulic engine on removal (distance) from an energy source • sensitivity to pollution of working and freedom in making configuration. liquid and necessity of high culture service.

41 Santhakumar Mohan, Assistant Professor, MED, NITC Advantages and disadvantages of Pneumatic Actuators

Advantages: Disadvantages: • simplicity of realization relatively to small • compressibility of the air ; back and forth motions; • impossibility to receive uniform • sophisticated transfer mechanisms are not and constant speed of the required; working bodies movement ; • low cost; • di fficulties in performance at • high speed of moving; slow speed; • ease at reversion movements; • limited conditions - use of • tolerance to overloads, up to a full stop; compressed air is beneficial up to the definite values of pressure; • high reliability of work; • compressed air requires good • explosion and fire safety; preparation. • ecological purity; • ability to accumulation and transportation.

42 Santhakumar Mohan, Assistant Professor, MED, NITC FACTOR AIR ELECTRICITY HYDRAULICS

Reliability Poor Good Good Weight Light Heavy Light Installation Simple Simple Simple Control Mechanism Valves Switches and solenoids Valves

Difficult, requiring skilled Constant attention necessary Simple Maintenance personnel

High pressure bottle dangerous; broken lines Vulnerability cause failure and danger Good Safe; broken lines cause failure to personnel and equipment

Slow for both starting and Rapid starting, slow stopping Instant starting and stopping Response stopping

Controllability Poor Fair Good Quietness of Operation Poor Poor Good

43 Santhakumar Mohan, Assistant Professor, MED, NITC Shape memory alloy (SMA) actuator

• Shape Memory Alloys (SMA's) are novel materials which have the ability to return to a predetermined shape when heated. • When an SMA is cold, or below its transformation temperature, it has a very low yield strength and can be deformed quite easily into any new shape--which it will retain. However, when the material is heated above its transformation temperature it undergoes a change in crystal structure which causes it to return to its original shape. • If the SMA encounters any resistance during this transformation, it can generate extremely large forces. This phenomenon provides a unique mechanism for remote actuation.

44 Santhakumar Mohan, Assistant Professor, MED, NITC • Shape memory alloys (SMAs) are metals that "remember" their original shapes. • SMAs are useful for such things as actuators which are materials that "change shape, stiffness, position, natural frequency, and other mechanical characteristics in response to temperature or electromagnetic fields" • This material is a lightweight, solid-state alternative to conventional actuators such as hydraulic, pneumatic, and motor-based systems. Shape memory alloys have applications in industries including medical and aerospace. • Material effects – Shape memory effect – Pseudo elasticity effect

45 Santhakumar Mohan, Assistant Professor, MED, NITC Shape memory effect

46 Santhakumar Mohan, Assistant Professor, MED, NITC Pseudo-elasticity effect

• Pseudo-elasticity , sometimes called super-elasticity , is an elastic (reversible) response to an applied stress, caused by a phase transformation between the austenitic and martensitic phases of a crystal. It is exhibited in Shape memory alloys. • Pseudo-elasticity is from the reversible motion of domain boundaries during the phase transformation, rather than just bond stretching or the introduction of defects in the crystal lattice . Even if the domain boundaries do become pinned, they may be reversed through heating. Thus, a pseudo-elastic material may return to its previous shape (hence, shape memory) after the removal of even relatively high applied strains

47 Santhakumar Mohan, Assistant Professor, MED, NITC 48 Santhakumar Mohan, Assistant Professor, MED, NITC Applications of SMA actuators

• Nanomuscles • Surgical instruments – Tissue Spreader – Stents (angioplasty) – Coronary Probe – Brain Spatula • Endoscopy: miniature zoom device, bending actuator • Force sensor • Smart skin (wing turbulence reduction) • Robotics • Aircraft and spacecraft industries

49 Santhakumar Mohan, Assistant Professor, MED, NITC Limitations of SMA actuators

• Heat Dissipation • Range of Motion • Stiffness / Flexibility • Relatively expensive to manufacture and machine compared to other materials such as steel and aluminum. • Most SMA's have poor fatigue properties; this means that while under the same loading conditions (i.e. twisting, bending, compressing) a steel component may survive for more than one hundred times more cycles than an SMA element.

50 Santhakumar Mohan, Assistant Professor, MED, NITC