aktuatori i deo

Univerzitet u Beogradu, Mainski fakultet HIBRIDNI TEHNIKI SISTEMI

Strana 1 dr Marko Milo; kabinet 136; e-mail: [email protected]

O aktuatorima (sa primerima upotrebe u vazduhoplovstvu) - I deo

Actuator Device that creates mechanical motion (rotary or linear) by converting various forms of energy (Electic, Hydraulic, Pneumatic) Mainly there are three types of actuators:

Electro-Hydraulic Actuators

Hydraulic actuator use the hydraulic fluid power/energy and transform into motion. The available devices are both linear and rotary. Hydraulic actuators are often used when large forces are required.

Electro-Pneumatic Actuators Pneumatic actuator converts power/energy (in the form of compressed air or gases) into motion. The motion can be rotary or linear, depending on the type of actuator. Because low pressure involve so these actuators are limited to low force applications.

Electro-Mechanical Actuators In Electro-Mechanical actuator electrical power/energy is converted into mechanical power or motion with the use of mechanical system i.e. gears, harmonic drive and lead screw etc.



Block Diagram of a General Actuation Module



Diagram for Conventional Methods of Actuation System Design



Aerospace Application Device used for Attitude control by moving Aerodynamic (Fins, Ailerons, Canards..) or Gas dynamic (Vans, tabs) Control Surfaces or by tilting the whole Gimbaled Engine/Nozzle Assembly



About Flight Control Systems Or

How Fying Object Turns in Flight

To perform turns, flying object that use no thrust vectoring must rely on only aerodynamic control surfaces; craft with vectoring still can/must use control surfaces, but to a lesser extent.

A typical aircraft's primary flight controls

(see animation 1 - ControlSurfaces.gif)

Flight control: Aileron [Linear Actuator]

Used on typical airplanes:

Ailerons (A-roll control) Elevators (C-pitch control) Rudders (D-yaw control)



About Flight Control Systems Or

How Fying Object Turns in Flight

Thrust vectoring is the ability of an aircraft, missile or other vehicle to direct the thrust from its main engine(s) in a direction other than parallel to the vehicle's longitudinal axis.

Missile with thrust vectoring (see animation 2 - MovableNozzle.gif

& 3 - TVC_Animation.mpg)

Flight control: Thrust Vectoring / Jet Vane [Rotary Actuator]

Most missiles do not have conventional rudders (D-yaw control), ailerons (A-roll control) or elevators (C-pitch control) like those used on typical airplanes.



Electro-Mechanical Actuators / Basic Concept

Fig. 1 EMA_version 1, Main Parts

As it was written before, three basic solutions should be used in various types of control systems:

Electro-mechanical actuators Electro-hydraulic actuators Electro-pneumatic actuators

The advantages of electro-mechanical actuators in comparison with hydraulic solution (electro-pneumatic actuators are rare used):

Electromechanical actuators are cost effective, compared to electro-hydraulic system, because there is one energy conversion versus two in a hydraulic system.

Installation time is reduced, as the system requires the mechanical installation of the actuator and the connection of two cables (one power and one resolver) only.

There are no periodic maintenance requirements. No need for flexibly hydraulic lines and fittings, no replacements of high-pressure

seals or filters, no leakage of oil. A hydraulic pump unit may require a separate additional space with fluid

containment provisions. Concerning respond speed: when a step function is generated in a hydraulic

system, the hydraulic fluid will exhibit some compression before the actuator begins to move. In an electric system, the electric motor begins operation at peak power as soon as the command is received.

However, for very large loads and very long strokes, etc., hydraulic actuators are often the only solution

Fig. 1 EMA_version 1, Main Parts The system consists of the three major components :

Object (FOR EXAMPLE: Control surface: Vane or Fin) Electric motor with (or without) gearhead Transmission assembly-speed reducer

Fig. 2 EMA_version 1, Main Subassemblies Main subassemblies of actuator are:

1. Motor 2. Housing assembly 3. Transmission assembly 4. Lever with Slides




Fig. 2 EMA_version 1, Main Subassemblies




Fig. 3 EMA_version 2, Linear EMA, Main Subassemblies




Fig. 3a EMA_version 3, Linear EMA, Main Subassemblies

Fig. 3b Rotary EMA, Main Subassemblies



Electro-Mechanical Actuators / How It Works

Fig. EMA_AT3

Fig. EMA_AT4

The primary mechanical scheme of EMA (Fig. EMA_AT3, Fig. EMA_AT4) is relatively simple even the component design could be complex. The design consists of the following major components:

Control surface Electromotor with (or without) gearhead & encoder [Gearhead makes possible control of large load inertia with a comparatively small motor inertia. Without the gearhead, acceleration or velocity control of the load would require that the motor torque, and thus current, would have to be as many times greater as the reduction ratio which is used; DC electric motors produce large output speed with relatively small torque. In actuating system, however, opposite situation is required, so we need relative small speed (angular velocity) and large torque. Because of that, reducing system between electric motor and vane (or fin) may be introduced if it is necessary] Transmission subassembly [Converts rotational input into linear output]

System control could be provided with electronic controller and power supply. Usually, resolvers and associated electronics deliver position feedback to the controller such precise positioning could be achieved.

(see animation 4 - FIN 1_Animation.flv & 5 FIN 2_Animation.flv)

2

1

3

4

5



Electro-Mechanical Actuators / How It Works

Application

Fig. EMA_AT1

Fig. EMA_AT2

Basic principle of work The electromotor and gearhead (Fig. EMA_AT4 pos. 2) in the combination with transmission subassembly - transforms rotation motion (of electromotor) via roller screw (Fig. EMA_AT4 pos. 3) in translation motion of nut (Fig. EMA_AT4 pos. 4). Translational motion of nut is then via fork lever (Fig. EMA_AT4 pos. 4) trans-formed into rotational motion of vanes (Fig. EMA_AT4 pos. 1) shaft.

Fig. EMA_AT5 EMA assembly

Fig. EMA_AT6 EMA subassembly



Electro-Mechanical Actuators / Selection of Electric Motors

Types of Electric Motors Which Should Be Applied In

Electro-Mechanical Actuators DC Motor Drives Features

Field orientation via mechanical commutator Controlling variables are Armature Current and Field Current, measured DIRECTLY from the motor Torque control is direct

In a DC motor, the magnetic field is created by the current through the field winding in the stator. This field is always at right angles to the field created by the armature winding. This condition, known as field orientation, is needed to generate maximum torque. The commutator-brush assembly ensures this condition is maintained regardless of the rotor position. Once field orientation is achieved, the DC motors torque is easily controlled by varying the armature current and by keeping the magnetizing current constant. The advantage of DC drives is that speed and torque the two main concerns of the end-user - are controlled directly through armature current: that is the torque is the inner control loop and the speed is the outer control loop (Fig. 4).

Fig. 4 Control Loop of a DC Motor Drive

Advantages

Accurate and fast torque control High dynamic speed response Simple to control

Initially, DC drives were used for variable speed control because they could easily achieve a good torque and speed response with high accuracy. A DC machine is able to produce a torque that is:

Direct - the motor torque is proportional to the armature current: the torque can thus be controlled directly and accurately. Rapid - torque control is fast; the drive system can have a very high dynamic speed response. Torque can be changed instantaneously if the motor is fed from an ideal current source. A voltage fed drive still has a fast response, since this is determined only by the rotors electrical time constant Simple - field orientation is achieved using a simple mechanical device - a commutator/brush assembly. Hence, there is no need for complex electronic control circuitry, which would increase the cost of the motor controller. Drawbacks

Reduced motor reliability Regular maintenance Needs encoder for feedback

The main drawback of this technique is the reduced reliability of the DC motor; the fact that brushes and commutators wear down and need regular servicing; require encoders for speed and position feedback.

While a DC drive produces an easily controlled torque from zero to base speed and beyond, the motors mechanics are more complex and require regular maintenance.




Basic Motor Theory Summary for Commonly Used DC Motors Commonly used DC motors are:

1. Brush DC motor

Fig. 5 Brush DC Motor

Permanent magnet DC motor converts electrical energy into mechanical energy. This conversion takes place due to interaction of the motors two magnet fields. One of these magnetic fields is created by a set of permanent magnets (on the brush-type motor, the stator usually contains the permanent magnets; the brushless motors magnets are a part of a rotor assembly). The other magnetic field is created by current flowing through the motors windings (the windings of brush-type motor are contained in the armature motor), while the brushless windings are part of the stator assembly. The interaction of these two fields causes a resulting torque; the result of which is motor rotation. As the rotor turns, the current in the windings is commutated, resulting in a continuous torque output. Brush-type motors (Fig. 6) are mechanically commutated (with brushes), while brushless motors are electronically commutated.

2. Brushless DC motors

Fig. 6 Brushless DC Motor

A brushless DC (BLDC, Fig. 6) motor is inherently more reliable that a brush motor. They, of course, have no brushes to wear out, so they have a longer life and less downtime due to brush replacement. Because of lack of brushes, there is no brush arcing or brush bounce. With no brush resistance, they typically provide high speeds than brush motors. And because the winding is typically on the outside element, they offer better heat dissipation.

The absence of brushes also makes the BLDC motor a more quiet (both acoustically and electrically) unit than one with brushes. Advances in electronics and power semiconductors permit cost effective control of a BLDC motor. Conversely, brushless motors are usually more expensive. Also, since there is a need for additional electronics that will convert monophase signal to multiphase, they are more space and also weight consuming.




3. Stepper motors

Fig. 7 Stepper Motor

Stepping motors (Fig. 7) can be viewed as electric motors without commutators. Typically, all windings in the motor are part of the stator, and the rotor is either a permanent magnet, or a toothed block of some magnetically soft material (in the case of variable reluctance motors). All of the commutation must be handled externally by the motor controller, and typically, the motors and controllers are designed so that the motor can be held in any fixed position as well as being rotated one way or other. Most steppers, as they are also known, can be stepped at audio frequencies, allowing them to spin quite quickly, and with an appropriate controller, they may be started and stopped on a dime at controlled orientations

For some application, there is a choice between using servomotors and stepping motors. Both types of motors offer similar opportunities for precise positioning, but they differ in a number of ways. Servomotors require encoder feedback control systems of some type. Typically, this involves an optical or magnetic encoder to provide feedback about the rotor position, and some mix of circuitry to drive a current through the motor inversely proportional to the difference between the desired and the current position.

Stepping motors can be used in simple open-loop control systems; these are generally adequate for systems that operate at low accelerations with static loads, but closed loop control may be essential for high accelerations, particularly if they involve variable loads. If a stepper in an open-loop control system is overtorqued, all knowledge of rotor position is lost and the system must be reinitialized. Servomotors are not subjected to this problem.

Stepping motors can also be used in closed loop systems, much like servos, with the addition of an encoder and feedback drive circuitry. Performance is improved at the expenses of additional cost.




AC Motor Drives AC Drives general characteristics

Small size and robust Simple in design Light and compact Low maintenance and low cost

The evolution of AC variable speed drive technology has been partly

driven by the desire to emulate the performance of the DC drive, such as fast torque response and speed accuracy, while utilizing the advantages offered by the standard AC motor. Features

Controlling variables are Voltage and Frequency Simulation of variable AC sine wave using modulator Flux provided with constant V/f ratio Open-loop drive Load dictates torque level

Unlike a DC drive, the AC drive frequency control technique uses

parameters generated outside of the motor as controlling variables, namely voltage and frequency.

Both voltage and frequency reference are fed into a modulator, which

simulates an AC sine wave and feeds this to the motors stator windings. This technique is called Pulse Width Modulation (PWM) and utilizes the fact that there is a diode rectifier towards the mains and the intermediate DC voltage is kept constant. The inverter controls the motor in the form of a PWM pulse train dictating both the voltage and frequency.

Significantly, this method does not use a feedback device, which

takes speed or position measurements from the motors shaft and feeds these back into the control loop. Such an arrangement, without a feedback device, is called an open-loop drive.

Fig. 8 Control Loop of a DC Motor Drive

Advantages

Low cost No feedback device required simple

Because there is no feedback device, the controlling principle offers

a low cost and simple solution to controlling economical AC induction motors. This type of drive is suitable for applications which do not require

high levels of accuracy or precision, such as pumps and fans. Drawbacks

Field orientation not used Motor status ignored Torque is not controlled Delaying modulator used

With this technique, sometimes known as Scalar Control, field

orientation of the motor is not used. Instead, frequency and voltage are the main control variables and are applied to the stator windings. The status of the rotor is ignored, meaning that no speed or position signal is fed back.

Therefore, torque cannot be controlled with any degree of accuracy.

Furthermore, the technique uses a modulator which basically slows down communication between the incoming voltage and frequency signals and the need for the motor to respond to this changing signal.




Selection of Electric Motor General Considerations Regardless of how simple or complex the application, there are some common requirements to consider for the selection of the proper motor (and/or controller). Some common considerations are:

Required output torque Motor torque is combination of the internal torque loses (a function of motor design) and external torque load. External torque load is a function of load inertia and load acceleration.

Required speed range

How fast should motor run when loaded and unloaded

Available space for motor mounting Motor length and its maximum diameter must be taken into consideration. Also, motor dimensions may be dictated by performance requirements.

Source of power for the motor

Source of power is DC. Current limits and voltage range could be limiting factors.

Special shaft and/or mounting requirements

What length and diameter of shaft are needed, and is a rear shaft extension required (for encoders, brakes, etc.) are the questions that should be kept on mind.

Environmental considerations

- Temperature - Humidity - Shock and vibration - Altitude - Presence of chemicals, contaminants, vapors, etc.

Heat sinking of motor A motor can be heat sinked by mounting it on a mass of thermally conductive material. The material conducts heat away from the motor. Heat sinking has a dramatic effect on motor performance. Effective heat sinking increases the continuous output torque capability of the motor.

Expected velocity profile

A velocity profile is a graph that shoes how quickly the motor accelerates to rated speed, the time the motor runs at rated speed, and how quickly the motor decelerates to zero speed.

Electric motor should be chosen according to required technical requirements for a specific actuator system. The electric motors, which have been taken into consideration are produced by:

MOOG KOLLMORGEN MAXON




Kollmorgen ServoDisc Concept

Servodisc concept (Fig. 9) has many advantages in comparison with motors from other manufacturers, so we will give its detail description. The greatest difference between the ServoDisc motor and conventional DC servos is its ironless disc armature. This difference enables ServoDisc motors to deliver a level of performance, in both incremental motion and continuous speed applications, which is not attainable with conventional ironcore motor designs.

As additional performance advantages, ServoDisc motors have a

unique compact shape that can be an attractive alternative in solving tight packaging problems.

A conventional ironcore motor (Fig. 10) uses a radial design with magnets placed concentrically around the shaft in such a way to produce a radial magnetic field. The armature consists of slotted steel laminations wound with coils of wire, which interact with the magnetic field to produce torque. As the motor rotates a commutator automatically maintains the correct current flow. A ServoDisc motor uses entirely different physical construction. The motor is designed with the magnetic field aligned axially, parallel to the shaft. The conductors in the armature have a current flow, which is perpendicular to the magnetic field (radial to the shaft). This produces a torque perpendicular to both the magnetic field and the current (the left-hand rule). This force rotates the shaft. This construction approach is much more efficient than the radial design of conventional ironcore motors and eliminates the heavy iron armature and the electrical losses associated with it. The large number of commutations makes possible with Kollmorgens unique flat armature to produce dramatically smoother torque output.

In a conventional slot-wound servomotor, the armature is

constructed from a heavy, laminated ironcore wounds with coils of wire. In a ServoDisc motor, the armature has no iron. Instead, it is constructed from several layers of copper conductors in a unique flat-disc configuration.

Fig. 9 Kollmorgen ServoDisc Motor




Fig. 10 Conventional Ironcore Motor

Fig. 11 ServoDisc Motor

Beside armature designs, completely different is also the shape and internal construction. In a conventional servo, the permanent magnets are mounted on the motor shell creating a radial magnetic field, perpendicular to the shaft (Fig. 11). Because the magnet pairs are so far apart, the iron core of the armature is needed to contain and focus the lines of magnetic flux. Motors of this type are typically long, thin and heavy.

In a ServoDisc motor, the magnets are mounted on the end plates

creating an axial magnetic field, parallel to the shaft. This leads to a very small air gap between the magnets, separated only by the thickness of the disc armature-a very clean and effective design approach. Torque is created when the current flowing radially through the copper conductors interacts directly with the field of the permanent magnets (Fig. 11). This configuration is a very efficient way of producing torque. These different approaches produce dramatically different motors (Fig. 12).

Fig. 12




The iron-free ServoDisc armature provides some significant

performance advantages for motion control applications. They are:

Size The ServoDisc armature is much smaller and lighter than bulky

ironcore designs of equivalent output.

Long brush life Because there is no iron, there is nearly no inductance. The result is

no arcing, because there is no stored energy in the armature to be dissipated during commutation. In an ironcore motor, a lot of energy is stored in the magnetic field of each coil. When this field collapses, the energy may be discharged by arcing to the brushes. Arcing, not friction, is the major cause of brush wear. The elimination of arcing leads to very long brush life in most applications. In fact, depending on the application, it is possible for the brushes to last as long as the bearings. Also, if there is some electronics stored in vicinity of a motor, arcing can cause serious problems, due to electrical disturbances. In this case, there is no such a problem.

Acceleration ServoDisc motors accelerate up to 10 times faster than conventional

servomotors. The thin, low-inertia armature design leads to exceptional torque-to-inertia ratios. This translates into blazing acceleration (Fig. 13). A typical ServoDisc motor can accelerate from 0 to 3000 rpm in only 60 degrees of rotation. In some applications, the entire move can be performed in less than 10 milliseconds. This means shorter cycle times, more moves per second and higher throughput. For incremental motion applications, this translates into higher productivity and more profitability. No arcing also means no commutation limits due to speed. In a conventional motor, arcing increases as speed increases and eventually causes motor operation to become erratic. ServoDisc motors do not suffer from this problem and can run to 4000 rpm and above.

Fig. 13

Cogging The ironless ServoDisc armature has absolutely no cogging at any

speed of operation. If you rotate a conventional motor when it is unpowered, you will notice that it pulls into certain preferred positions. This occurs when the iron laminations in the armature line up with the permanent magnets on the stator. This phenomenon is called cogging. It also occurs when the motor is powered and shows up as torque disturbances, which can be a serious problem in critical applications. The ServoDisc armature, being ironless, is not attracted by the magnets and. consequently, has intrinsically zero cogging (Fig. 14). The result is ultra-smooth rotation at any speed.




Fig. 14

Electrical Time Constant A very low electrical time constant results in torque much sooner than

with conventional wire-wound motors. Low inductance provides another advantage-low electrical time constant. This is a measure of how long it takes for current to flow into the armature. For ServoDisc motors, this is much less than one millisecond (Fig. 15). This means full torque almost instantly; a key to fast moves and accurate tracking.

Fig. 15

Torque-Speed Curves With full torque from 0 to full speed, ServoDisc motors solidly

outperform conventional motors. In a conventional motor there are losses associated with rotating the

iron armature in a magnetic field. These losses are increased with speed, so if the motor goes faster, it uses more and more of its available torque just to keep itself in turning.

Consequently, less torque is left to deliver to the output shaft (Fig. 16a). ServoDisc motors do not have these iron-associated losses and, as a result, deliver more torque over their entire speed range. In fact, the torque is almost constant from 0 to 4000 rpm (Fig. 16b). Comparing this performance with the torque-speed characteristics of a conventional motor, when sizing a conventional motor, the torque drop-off with speed may require you to select the next higher size to get sufficient torque at high speed. You will never have this problem with a ServoDisc motor.

Fig. 16




Peak Torque Capability High peak torque capability means more throughputs than is available

from standard servos.

Fig. 17

For rapid acceleration and deceleration, higher level than normal

torque is usually required. To produce this temporary peak torque, a peak current is applied to the motor. In an iron-core design, the magnetic field of the armature can interact with and demagnetize the permanent magnets. Because of this effect, peak current is generally limited to 2 or 3 times the continuous current rating. With the non-magnetic ServoDisc armature and axial magnetic field, this problem is virtually eliminated. Most ServoDisc motors are rated for peak current of 10 times the continuous rating (Fig. 17). Kollmorgen Servodisc motors has many advantages in comparison with electric motors from other manufacturers with classic ironcore design. This motor is faster in comparison with others, and would satisfy all requirements, but it has one big shortage: it has brush. Nevertheless numerous advantages, this makes it inappropriate for specific application.

MOOG electric motors Moog Components Group manufactures wide range of different torque and servomotors electric motors, both brush and brushless. Moog motors are designed to operate over a range of speeds for a wide variety of military, aerospace and industrial applications.

Different applications can require unique configurations of mounting flanges, housings, output shafts and electrical requirements, which can be ordered from this manufacturer.

A variety of shaft configurations can also be ordered from this manufacturer, in dependency of mechanical configuration where this motor will be applied.

Motors from this manufacturer also have high torque-to-power and torque-to-inertia ratios, high linearity and low electrical time constants. These motors are ideally suited for applications that require maximum dynamism in minimum space, and offer other benefits, such as low inductance, high efficiency and extreme reliability.

Fig. 18 MOOG permanent magnet DC motors




MAXON electric motors The Maxon Group is specialized in developing, and manufacturing high-quality drive components and systems under the Maxon motor trade mark. It manufactures precision components for highly dynamic drive systems: small, high-quality, precision direct current (DC) brush and brushless motors. These motors can be used for a wide range of applications from medicine, semiconductor, aerospace & defense, test and measurement, robotics, assembly, manufacturing and many other applications. Maxon motors offers several advantages over conventional DC motors: a low mechanical time constant for fast acceleration, low current consumption for extended battery life, eliminates cogging for smooth rotation, even at low speeds, linear speed-torque constants for simple, accurate control. Maxons electronically commutated (EC) brushless motors enable extremely long motor life since there are no mechanical brushes to wear out. By assembling the motor such that the coils are outside the rotor, good heat dissipation and high overload capability is assured. In addition, the use of high-energy neodymium magnets results in very high torque with minimum overall size. Other advantages are: the rotor's low inertia, minimal detent, robust bearings and compact construction. The use of high-powered permanent magnets ensures high power density, providing great speed stability under load.

Fig. 18 MAXON cross-section of typical brushless DC motors



Electro-Mechanical Actuators / Gearhead Selection

Selection of Gearhead General Considerations The primary reason to use a gearhead is that it makes possible control of large load inertia with a comparatively small motor inertia. Without the gearhead, acceleration or velocity control of the load would require that the motor torque, and thus current, would have to be as many times greater as the reduction ratio which is used. There are extensive motor and gearhead combinations that will satisfy the performance requirements of a specific application. Basically, it is best solution to choose the motor-gearhead combination from same manufacturer (if possible), regardless of what it would, eventually, be. In that way, one can bypass the problem of their compatibility, thus reducing design and production coasts, as well as construction size. Primary consideration in selection of the best combination of motor and gearhead should be output torque and the speed required by application. The no-load speed or peak torque may also be the driving consideration for certain applications. The long and short-term capabilities of a gearmotor vary widely. Furthermore, the nature of the load which is being driven, is a factor which must be considered when selection of the right gearmotor for the application is performed. Shock loads shorten unit life, though the average torque may not exceed the specified rating. Duty cycle will limit the gearmotor capability. Mechanisms that contribute to gearmotor failure often are related to heat and its effect on lubrication. Therefore, long duty cycles which can significantly increase motor temperature over ambient conditions should be avoided.

Fig. 1 Gearheads



Electro-Mechanical Actuators / Gearhead Selection

In selecting a gearhead, one must be mindful that gearhead selection

will impact more than just the output speed and torque level at the output shaft. Some of the considerations to keep in mind should include:

Backlash this is the gearhead characteristic that allows bi-directional shaft play in the shaft. It is measured at the output shaft of the gearhead and can vary typically from some arc minutes up to 6 or 7 degrees, in dependence of load. Backlash can be deduced by using preloaded ball bearings, and by specifying zero-backlash gearheads.

Ball bearings are typically suggested in applications where high

axial loads are present. Be advised, however, that using of ball bearings can increase audible noise in some cases.

Ceramic bearings - for cost-sensitive applications in which

extended life and enhanced load-bearing capabilities are important. These bearing systems allow the user to increase radial loads beyond the levels allowed in traditional sintered bronze bearing systems. Costs of the ceramic bearings are also considerably below above of ball bearings.

Factors affecting gearhead life - the life performance of a

reduction gearhead and motor combination is determined by input speed, output torque, and conditions of operation. Since a multitude of parameters are present in a given application, it is impossible to state actual lifetimes that can be expected from a specific type of gearhead or motor-gearhead combination. A number of options for standard reduction gearheads are available to enhance performance such as ball bearings, all metal gears, special lubricants, etc.

Assembly precaution - whether a gearhead with sintered,

ceramic, or ball bearings is chosen, it must be handled as any piece of precision machinery. Avoid dropping, crushing, or otherwise abusing the gearheads, and keep them away from dirt if they are not assembled to a motor. Getting dirt particles or other foreign matters in the input end of a gearhead will impair its performance and lifetime. Many ball bearing systems are preloaded with calibrated spring washers to lower current consumption and backlash. Exceeding shaft loading specifications or maximum push-on pressures will destroy the preload in the bearings. This can result in decreased performance, increased operating currents, higher audible noise, increased backlash, and shorter gearhead life.

Assembling of motors and gearheads requires that the motors be run

at specific speeds, and that assembly be done according to specific procedures in order to prevent damage of the motor and the gearhead.



Electro-Mechanical Actuators / Transmission System

Preliminary Consideration of Transmission System Some solutions of actuator transmission system design will be considered in the further text. Regarding the way of connection and transfer of electric motor's rotary motion on the object, several types of electro-mechanical actuators (EMA) can be defined. Direct fitting of electric motor (torque-motor) to the object

It is used for applications with large torque and low speed. Its disadvantages are large weight and a need for special shaft of the

object with adequate bearings.

Fig. 2

Connection of electric motor and the object using gear reducer (standard, planetary gearhead, harmonic drive...) Advantages: They allow favourable selection of dynamic characteristics of actuator, and reduction of weight of device. Disadvantages: Because of presence of inevitable teeth gaps, which have unfavorable effect on control loop, it is needed very precise gear production, or special construction with elimination of gaps, which makes production harder and more expensive.

Fig. 3




Connection of electromotor and the object using rolled threads ball screws along with some lever mechanism. This type of screws transforms rotating motion of electric motor to linear motion of nut. However, instead of classic screw-nut conjunction, hardened steel balls rotating between them are used. In that way sliding friction is replaced by rolling friction, thus increasing efficiency of the screw from 30% to around 90%. Except that, they have another advantage-small gap that could be eliminated by preloading (for example with two nuts and preload spacer). Disadvantage is that balls must at the end of nut go out of touch with screw and go back through special tube, thus having external recirculation. Also, they have weak capability of carrying load in radial direction. They are sensitive to impact loads, too.

Fig. 4

Connection of electromotor and the object using " Planetary roller screws" Connection of electromotor and the object using " Planetary roller screws" with lever mechanism or two coaxial cylinders in which screw and nut are placed, making compact construction (resembles to hydraulic cylinder). They work in a similar manner as ones with ball screws, but instead of balls, they have threaded rollers. The helix angle of the roller thread is exactly the same as the nut thread, so the roller does not move axially relative to the nut as it rolls. It has high efficiency 80-90%, because there is no sliding. The absence of recalculation means that the nut is robust and capable of high rotation speed with smooth running. They also have small gap that could be eliminated by preloading. They are less sensitive to impact loads and they can carry higher loads, comparing to those with ball screws. Disadvantages: Higher costs of production due to increased precision, and somewhat larger dimensions.

Fig. 5




Ball / Roller Concept The classic sliding screw typically consists of a steel shaft with trapezoidal thread and bronze nut. The high efficiency screw has rolling elements, balls or rollers, between the nut and the screw shaft. The efficiency of the screw is increased by replacing the sliding friction with rolling friction from 30% to 90%, and provides other essential benefits that are common to all types of high efficiency screw:

Smooth action - no stick slip Excellent repeatability and high reliability Predictable life Low wear ensuring consistent precision

External recirculation ball screw

Ball screws, in which the balls return by a tube after 11/2, 21/2 or 31/2 turns of load carrying, have external recirculation (Fig. 6). This is the least expensive method of recirculation, but has the disadvantage of a relatively large nut. This nut design is matched with economical rolled thread screws.

Fig. 6

Internal recirculation ball screw

The internal recirculation system uses a stainless steel insert in which the balls jump back over the screw thread after only one turn of load carrying (Fig. 24). This is more costly than external tube recirculation, but provides a more compact nut.

In all types of ball screw the load is transmitted from screw shaft to

nut through each ball. According to Hertz's law the load carrying capacity of each ball is a function of its diameter. In a conventional ball screw the ball diameter can never be larger than the lead of the screw normally 60% to 70% of the lead. The load carrying capacity of a high efficiency screw also depends on the number of contact points. More contact points with larger diameter are required therefore, to increase the load carrying capacity. This can be achieved by means of rollers, threads (Fig. 7) or grooves. Thus, roller screws have higher load carrying capacity than ball screws.

Fig. 7




Fig. 8

Fig. 9

Fig. 8 and Fig. 9 represents the design of electro-mechanical actuator with integrated electric motor and SKF recirculation ball screw.

Planetary roller screw The high load Transrol planetary roller screw has threaded rollers (Fig. 10). The helix angle of the roller thread is exactly the same as the nut thread, so the roller does not move axially relative to the nut as it rolls. The absence of recirculation means that the nut is robust and capable of high rotational speeds. Its planetary timing system enables it to run smoothly even in dirty or poorly lubricated conditions.

Fig. 10 Fig. 11 Recirculation roller screw A grooved roller is the basis of the Transrol recirculating roller screw (Fig. 11). The rollers move axially as they roll inside the nut so that recirculation is necessary as in the case of a ball screw. This mechanism permits high loads to be carried with leads as small as one millimeter. The recirculating roller screw facilitates ultimate positioning accuracy and rigidity, often with a simplified transmission. Resolutions as small as 0.025 p have been achieved by this kind of roller screw.



Electro-Mechanical Actuators / Transmission System - Lever Mechanism

Kinematics and Dynamics Analysis of Lever Mechanisms With Ball and Roller Screws Among possible transmission systems which can be applied in an actuating systems, we will consider custom screw-nut transmission system. Custom screw-nut transmission system transform fast rotating motion of electromotor to slower translation motion of nut. That motion is then, in some way, translated to rotational movement of objects shaft, using lever mechanism. Screw is connected to electric motor shaft, and screw is connected to vane the object via lever with slot and slider. The advantages of this mechanism are:

- small overall dimensions of system - simple design and easy manufacturing

The main disadvantage is great friction, and

thus, large losses. However, these shortcomings could be overcome using ball and roller screw drives.

This type of reduction system using ball or roller

screw drives is primarily used in actuating system that require large precision of positioning. Coefficient of efficiency is much larger (up to 0.9) than in conventional screw-nut systems. Friction forces are not dependant of speed. Also, backlash can be practically totally eliminated.

Custom screw-nut transmission system is presented in Fig. 12.

BEARING

FIN

NUTSCREWELECTROMOTOR

m

Mh

lvn

LEVER

Fig. 12 Screw-Nut Transmission System




The following parameters can be recognized on the sketch (Fig. 12): h - screw pitch (lead) vn - axial velocity of nut 2rn - mean thread diameter m - speed of rotation of an electric motor l - lever arm Mh - moment on object - objects deflection Calculation of axial velocity of nut for this transmission system is given bellow:

nmn

n

r2htan

rv

(1.1)

2

hv mn (1.2)

From figure: lcosvn (1.3)

and from that we can get:

i2lcosh m

md

(1.4)

Where reduction ratio is:

hl

cos2i (1.5)

The reduction ratio depends on the objects deflection , i=i(). For small angles , we can take cos=1, thus letting that mistake be 1-2%. However, for angles from 0.35-0.4rad (20-25), this approximation is not allowed, leading to erroneous results.

The equation of system behavior could be done applying law of kinetic energy preservation. Kinetic energy of system equals sum of kinetic energies of rotor, screw, nut, lever and the object:

2fa

2nn

2msmk )JJ(2

1vm21)JJ(

21E (1.6)

or

22fa22

n22

smk J21JJ

21

cosl

m21i)JJ(

21E

(1.7)

Where: Jm - moment of inertia of motor Js - moment of inertia of screw mn - mass of nut Ja - moment of inertia of arm (lever) Jf - moment of inertia of the object J - moment of inertia of rotating elements of system, reduced to the objects axis of rotation

fa22n2

sm JJcos1lm

hl

2JJJ

(1.8)

The power of torques acting in system is:

hmn.fmm.fmm

loadfrictiondeveloped

MMMMP

PPPP (1.9)




Loses in power screw are due to friction forces. They are proportional to torques that are transmitted. If the coefficient of efficiency of power screw is n, then the torque due to friction is:

Mf.n = (1-n)Mm (1.10) Since, according to law of change of kinetic energy

Pdt

dE k from (1.7) follows:

dtdJ

dtdJ

21

dtdE 2k (1.11)

Replacing (1.10) into (1.9), we get:

dtdJ

21MMM)(i

dtdJ fhmn (1.12)

Where Mf is sum of all Coulombs frictions, reduced to fins axis of rotation. The first derivation of J , according to (1.8) is:

2n

2

sm

22

n

2

sm

lmhl

2JJ2dt

dJ

costanlm

hl

2JJ2dt

dJ

(1.13)

Thus, we can conclude that derivation dt

dJ is proportional to

deflection angle and its derivation. From expression (1.12), we can see that this derivation is multiplied with derivation of deflection angle, so this component could be neglected, as small value in respect to other components in equation (1.12).

aktuatori i deo

Documents