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Mechanical Overview of Servo Motors

Servo motors have serval distinct characteristics that sperate them from their stepper counterparts. The biggest is the lack of direct gearing between the rotor and the output shaft. This eliminates the backlash and cogging behaviors found ins steppers, where there is period of slop between the gear teeth before movement actually begins, and where the shaft continues to move after the Delta servo motor has stopped. This can lead to jerky starts and stops, as well as a time delay in movement. This does not impede static positioning performance markedly, but it presents major issues when on-the-fly velocity changes or hard starts/stops are needed.

A model of a typical radial brushless DC servo motor is shown before in figure 1.1 For a long time, servo motors used brushes to transfer current from the static winding to the rotor, but this would lead to wear on the brushes, in turn shortening the lifespan of the motor. With the advent of electronic motor controllers, the brusheless design was adopted, which uses control electronics to vary the currents phases to the motor’s windings in the same way the brushes do. For the rest of this paper, all mention of mitsubishi servo drives will be of the brusheless type.

Looking at figure 1.1 below, there are several objects of interest. First are the armature windings (held by the stator), which create a magnetic field that travels through the air gap to the permanent magnets on the rotor. Even though there are normally no gears in a servo motor, cogging can still exist, as there are gaps between the magnets on the rotor where the flux decrease, though this only becomes noticeable at low speeds. This type of congging in servos is perhaps more accurately termed “detent torque.” There are two ways to minimize this type of cogging, the most common being the addition of some gearing to the drive shaft. This allow the motor to run at a higher speed out of its cogging region, but does not compromise power output or precision, thought it can induce some backlash. The other way of minimizing cogging is to skew the magnets on the rotor so that a radial line from the center of the rotor always intersects a magnet at least once. When using a motor without gearing, it is known as a direct drive motor. This allows for the best transfer of power to the load, and avoids any of the negative aspects of gearing previously mentioned. A feature in newer servo motors (including the Bodine models used in this thesis) is the use of an ironless stator, which eliminates iron saturation, a situation where the magnetic properities of the iron limit how much current can be applied to the windings. Inducing iron saturation too ofen will cause overheating and possibly damage the winding or magnets. With an ironless stator, rotor magnet skewing is not necessary, as the magnetic fields aren’t influenced by the material of the stator. Also, since the only mechanical connection between the shaft and the body is through the bearings, friction is very low (especialy when using ball bearings).

In high torque motors such as the ones used in this thesis, the rotor actually consists of two plates of permanet magnets sandwiching the stator, which allows for a major increase in torque. This feature only exists in axial flux motors, due to the design where the stator lies in between the rotors, whereas in radial flux servos, the rotor is completely enclosed by the stator. The majority of the heat dissipated from a servo motor comes from the stator, so its outside location adis in cooling. In fact, the main limiting fator in the power of a servo motor is the heat capacity of the stator and the armature windings.

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Introduction of Induction Servo Motors

Induction Servo Motors are the most commonly used motors in many applications. These are also called as Asynchronous Motors, because an Induction Leadshine servo motor always runs at a speed lower than synchronyous speed. Synchronous speed means the speed of the rotating magnetic field in the stator.

Principle of Induction Servo Motor:

When a three phase supply is given to the stator, a rotating field produces induced e.m.f. in rotor windings which cause induced currents tend to propose the action, producing them and therefore they circulate in such a manner that a torque is produced in the rotor tending it to cause it to flow the rotating field and thus reduce the relative motion which is producing the induced currents.

Induction Servo motor speed

Induction Servo motor works as follows: Electricity is supplied to the stator, which produces a magnetic field. This magnetic field moves around the rotor at synchronous speed. Rotor currents produce secondary magnetic field, which is trying to fight the stator magnetic field, which causes the rotor to rotate. However, in practice, the servo motor driver never runs at synchronous speed but the “base rate” is lower. The difference between the two speeds is the “slip / slide” that increases with increasing load. Slip only occurs in an Induction Servo motor. To avoid slip ring can be fitted a sliding / slip ring, and the motor is called “motor slip ring / slip ring motor”.

The following equation can be used to calculate the percentage of slip / slide (Parekh, 2003):

% Slip = (Ns – Nb) / Ns x 100

Where:
Ns = synchronous speed in RPM
Nb = base speed in RPM

The relationship between load, speed and torque
graph torque vs. speed three-phase AC Induction Servo motor with the current set. When the motor (Parekh, 2003):
• Start turns lights are flame high initial currents and low torque (“pull-up torque”).
• Achieve 80% of full speed, the torque is at the highest level (“pull-out torque”) and the current begins to drop.
• At full speed, or synchronous speed, torque and stator currents down to zero.

Construction

The stator of an Induction Servo motor consists of poles carrying supply current to induce a magnetic field that penetrates the rotor. To optimize the distribution of the magnetic field, the windings are distributed in slots around the stator, with the magnetic field having the same number of north and south poles. Induction Servo motors are most commonly run on single-phase or three-phase power, but two-phase motors exist; in theory, Induction Servo motors can have any number of phases. Many single-phase motors having two windings can be viewed as two-phase motors, since a capacitor is used to generate a second power phase 90° from the single-phase supply and feeds it to the second motor winding. Single-phase motors require some mechanism to produce a rotating field on startup. Cage Induction Servo motor rotor’s conductor bars are typically skewed to reduce noise.

Moving servo motors

Servo motor is an engine that controls the operation of mechanical components in the servo system.Servo motor can make the control speed, position accuracy is very accurate, can be converted to torque and speed voltage signals to drive the control object. Servo motor rotor speed by the control signal input, and can respond quickly, in automatic control system that is used for the implementation of components, and has small electromechanical time constant, high linearity, initiating voltage characteristics, the received signal is converted into the motor shaft angular velocity or displacement output. DC and AC yaskawa servo motor is divided into two major categories, the main feature is that when the signal voltage is zero without rotation, speed with the increase of torque and uniform decline.

Telling a servo motor to move to specific angle is easily accomplished using write(). The Arduino will do all the nessary caculation;determining the length of the pulse to generate and sending the pulse on time:

servo.write(angle);

The angle parameter is an integer number,from 0 to 180, and represents the angle in degrees.

If you require precision, you can specify the length of the pulse by using the writeMicroseconds () function. This eliminates the need for calcuation by the Arduino and specifies the exact pulse length, an integer, expressed in microsenconds:

servo.write Microseconds (microseconds);

It does not matter what the original position waw, the servo motor ASD-B2-0421-B automatically adjusts its position. The Arduino does not need to calculate this either; all the intelligence is embedded inside the motor assembly. It does, however, keep the last angle that it was instructed to use, and this value can be feched with read();

int angle * servo.read()

Remember that servo motors can receive only instructions and not return information. The value returened by read() is the value inside the Arduino. When connecting a servo motor, there is no way to know what position it was in initially. It can be helpful to set a servo motor to a default position before starting your application. (For exapmle, a remote-controlled car should probably have the wheels turn so that they are at 90 degrees; withoust adjusting the steering, the owner would expect the car to go straight and not at an angle.)

Servo motors and other physical objects take time to get to where you want them to be, so it’s considered good practice to give your motor a bit of time to get where it wants to go. Some motors move faster than others, if you’re unsure of how much time you’ll need, it’s best to check your motor’s documentation.

Four Basic Types of Gear reducers

There are four basic types of gear reducers on the market today:helical gears,parellel shaft helical gears,helical bevel gears,and helical worm gears. The latter three types of gear reducers are often used in the theatre industry. Both parallel shaft and helical bevel Gear reduction motors are very efficient in transmittting power from the motor to the output shaft. The helical worm gear reducer is inefficient in its transmission of power from the motor to the output shaft. The helical worm design is the mostly widely used gear reducer type, offering long service life overload and shock tolerance.

gear motor

Efficient gear reducers are used on most of today’s packaged winch systems. They are also used on most fire curtain motor systems as they are easy to back drive, which is the reverse of the normal operation. The output shaft is used to turn the input shaft. Many fire curtain release systems release the brake on the motor, using the weight of the fire curtain to turn the output shaft of the gear reducer, which turns the motor. A hydraulic dampener attached to the motor controls the descent speed of the fire curtain.

An inefficient gear reducer design does, however, have advantages in the threatre world. A helical worm gear reducer with a gear ratio greater than 60:1 statistically cannot be back-driven. This means that a load on the output shaft will remain staionary even if the motor brake is released. Most line shaft, drum, and counterweight assist winches use this type of planetary gear reducer because of this feature. Helical worm gear reducers are a simple design that is very cost-effective to produce. This design lends itself to the lower output RPM and higher gear ratios used in the theatre industry.

Gear reducers are filled with oil and are vented because the oil will expand as the reducer is used. The oil helps to keep the reducer cool. The reducer will have breather vent and a drain plug. While it may look like as if they can be mounted or oriented in any position, it is very important to be certain that the vent is at the top and drain at the bottom when the reducer is mounted in its final position. The amount of oil with which the reducer is filled varies with the orientation that is used. The orientation is also important regarding the bearings that are installed into the reducer. The manufacturer will install different bearing depending on whether the bearing is below or above the oil level in the reducer. Thus it is important to order a gear reducer for the specific orientation for which it will be used.

Gear reducers also have their own service factor, which is defined by the American Gear Manufactures Association (AGMA). AGMA adjusts a reducer’s ratings relative to the individual load charateristics of the reducer. AGMA’s ratings are based upon time duration. For winches used for between three and ten hous per day the service factor is 1.25. For winches used for more than ten hours per day the service factor is 1.5.

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Select the proper gears for the expected load charateristics

Numerous industrial machines need energy at slow speeds and higher torque. Conveyors and concrete mixers are typcial examples of machinery with such specifications. At speeds of 780 rpm and much less, the following drives might be prudently utilized: chain drive, belt drive, separate speed reducer coupled towards the motor, or gear motor.

The gear reducer motor is really a speed-reducing motor that provides a direct energy drive from a single unit. The gear motor offers an very compact, effective, packaged energy drive. A gear motor generally consists of a standdard AC or DC motor along with a sealed gear train properly engineered for the load. This assembly is mounted on a single base as a 1 package, enclosed pwoer drive. The benefit of this unit is its intense compactness. A gear motor really is smaller sized than a low-speed regular motor from the exact same horsepower.

The motor-shaft pinion of the worm gear motor drives the gear or series of gears in an oil bath that is linked with the output shaft. This type of arrangement is usually the most economical and convenient way to obtain low speeds of approximately 1 rpm to 780 rpm.

One-unit gear motors are available with the following options:(1)shafts parallet to each other or at right angls, (2) polyphase, single-phase, or DC voltages, and (3) horsepower ratings ranging from approximately 1/6 hp to 200 hp.

Gear motor are accessible in numerous from the regular motor kinds like squirrel cage or wound rotor induction motors, operating at either continuous or adjustable speeds. The manage gear for the motor is chosen exactly the same as for any other motor from the exact same kind.

When choosing a gear motor, an essential consideration will be the degree of gear service and gear life primarily based around the load circumstances to which the motor will probably be subjected. Gear motors are divided into 3 classes. Every class utilizes various gear sizes to deal with particular load circumstances. Every class give concerning the exact same life for the gears. The American Gear Manufactures’s Assocation has defined 3 operationg conditons generally discovered in industrial service and has established 3 regular gear classifications to meet these circumstances.

Class I: For steady loads within the motor rating of 8 hours per day duration, or for intermittent operation under moderate shock conditions.

Class II: For 24-hour operation at steady loads within the motor rating, or 8-hour operation under moderate shock conditions.

Class III: For 24-hour operation under moderate shock conditions, or 8-hour operation under heavy shock conditions.
For conditions that are more severe than those covered by Class III gears, a fluid drive unit may be incorporated in the assembly to cushion the shock to an acceptable value.

To achieve multiple speeds, separate units are available with a transmission comparable to that of an automobile. These units must be assembled with the motor and the driven machine. Because the amount of power lost in gearing is very small, the multiple drive has essentially constant horsepower. In other words, as the output speed is decreased, the torque is increased. Generally, this means that larger shaft sizes are needed for the output side.

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What is Worm Gear Motor

A worm gear assembly resembles a single threaded screw that turns a modified spur gear with slightly angled and curved teeth. Worm Gear reduction motors may be fitted with either a right-, left-hand, or hollow output (drive) shaft. This correct angle gearing kind is utilized when a sizable speed reduction or perhaps a big torque improve is needed inside a restricted quantity of space. Figure 1 shows a single thread (or single begin) worm along with a forty tooth worm gear resulting inside a 40:1 ratio. The ratio is equal towards the quantity of gear teeth divided by the amount of starts/threads around the worm. A comparable spur gear set having a ratio of 40:1 would need a minimum of two stages of gearing. Worm gears can attain ratios of much more than 300:1.

Worm gears have an inherent design advantage over other gear sets; the worm can easily turn the dc worm gear motor, but the gear cannot turn the worm. In lifting applications, this feature acts as a secondary brake due to limited back drivability.

Worms can be made with multiple threads/starts as shown in Figure 2. The pitch of the thread remains constant while the lead of the thread increases. In these examples, the ratios relate to 40:1, 20:1, and 13.333:1 respectively.

Worm gear sets can be self-locking: the worm can drive the gear, but due to the inherent friction the gear cannot turn (back-drive) the worm. Typically only in ratios above 30:1. This self-locking action is reduced with wear, and should never be used as the primary braking mechanism of the application.

27RPM DC24V 2.45NM 25kg.cm Turbo Worm Gear Motor GW31ZY DIY Robot

14672810763

Feature:
1.Every product has a unique Manufacturing Part Number label on the inner package that proves it has been qualified,which include Part Number,Model Number and inspection date information;
2.If you have any questions about the item,please provide us the Manufacturing Part Number for checking,your profits will be guaranteed.
3.Use for labeling machines, remote control curtains, automatic voltage electricity, grills, ovens, washing machines, garbage disposal machines, household appliances, slot machines, the banknote recognition, automatic actuator, coffee machine, towel disposal, lighting, etc.
4.Low speed structure,high power,large output torque,stable performance,small installation space and self lock,etc.
5.This type is low speed Worm Gear DC Motor,self locking.
6.Widely used in various of occasions that require special install size.

The worm gear is usually bronze and the worm is steel, or hardened steel. The bronze component is designed to wear out before the worm because it is easier to replace.

The hoist motor can transmit motion through the gear reducer, but the load cannot transmit motion back through the gear reducer.

Regulates the lowering speed of the load
The load will not be allowed to free fall
OSHA [1910.179] recognizes this as an approved means of controlled braking

In an industry where ‘load brakes’ and ‘regenerative braking’ are widely accepted, Electrolift offers the better option by using a worm drive gearbox as the secondary braking function.

The main advantages of Electrolift’s non-load brake worm drive gear reducer:

Constant load on the load block. Load brake hoists are prohibited from doing this because it does not allow the load brake to release causing overheating and premature failure.
Long lifts – Long lifts cause load brakes to overheat and prematurely fail
Faster lifting speeds – Load brakes accumulate excessive heat as lifting speeds increase.
Inherently safe – Worm drives do not need controls or high maintenance mechanisms to ensure safe lifts.
Low Maintenance – Less moving parts than gearboxes that require complex load brake mechanisms

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