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Technical Documents - Documentos Técnicos: Servomotors - Permanent-Magnet DC Servomotors - Brush-Type PM DC Servomotors - Disk-Type PM DC Motors

Permanent-Magnet DC Servomotors

Tools & Equipment - DC & AC Electric Motors

Permanent-magnet (PM) field DC rotary motors have proven to be reliable drives for motion control applications where high efficiency, high starting torque, and linear speed–torque curves are desirable characteristics. While they share many of the characteristics of conventional rotary series, shunt, and compound-wound brush-type DC motors, PM DC servomotors increased in popularity with the introduction of stronger ceramic and rare-earth magnets made from such materials as neodymium–iron–boron and the fact that these motors can be driven easily by microprocessor-based controllers.

Fig. Cutaway view of a fractional horsepower permanent- magnet DC servomotor.

The replacement of a wound field with permanent magnets eliminates both the need for separate field excitation and the electrical losses that occur in those field windings. Because there are both brush-type and brushless DC servomotors, the term DC motor implies that it is brushtype or requires mechanical commutation unless it is modified by the term brushless. Permanent-magnet DC brush-type servomotors can also have armatures formed as laminated coils in disk or cup shapes. They are lightweight, low-inertia armatures that permit the motors to accelerate faster than the heavier conventional wound armatures.

The increased field strength of the ceramic and rare-earth magnets permitted the construction of DC motors that are both smaller and lighter than earlier generation comparably rated DC motors with alnico (aluminum– nickel–cobalt or AlNiCo) magnets. Moreover, integrated circuitry and microprocessors have increased the reliability and costeffectiveness of digital motion controllers and motor drivers or amplifiers while permitting them to be packaged in smaller and lighter cases, thus reducing the size and weight of complete, integrated motioncontrol systems.

Brush-Type PM DC Servomotors

The design feature that distinguishes the brush-type PM DC servomotor, as shown in the figure above, from other brush-type DC motors is the use of a permanent- magnet field to replace the wound field. As previously stated, this eliminates both the need for separate field excitation and the electrical losses that typically occur in field windings.

Permanent-magnet DC motors, like all other mechanically commutated DC motors, are energized through brushes and a multisegment commutator. While all DC motors operate on the same principles, only PM DC motors have the linear speed–torque curves shown in the following figure, making them ideal for closed-loop and variable-speed servomotor applications. These linear characteristics conveniently describe the full range of motor performance. It can be seen that both speed and torque increase linearly with applied voltage, indicated in the diagram as increasing from V1 to V5. The stators of brush-type PM DC motors are magnetic pole pairs.

Fig. - A typical family of speed/torque curves for a permanent- magnet DC servomotor at different voltage inputs, with voltage increasing from left to right (V1 to V5).

When the motor is powered, the opposite polarities of the energized windings and the stator magnets attract, and the rotor rotates to align itself with the stator. Just as the rotor reaches alignment, the brushes move across the commutator segments and energize the next winding. This sequence continues as long as power is applied, keeping the rotor in continuous motion. The commutator is staggered from the rotor poles, and the number of its segments is directly proportional to the number of windings. If the connections of a PM DC motor are reversed, the motor will change direction, but it might not operate as efficiently in the reversed direction.

Disk-Type PM DC Motors

The disk-type motor shown exploded view in the following figure has a diskshaped armature with stamped and laminated windings. This nonferrous laminated disk is made as a copper stamping bonded between epoxy–glass insulated layers and fastened to an axial shaft. The stator field can either be a ring of many individual ceramic magnet cylinders, as shown, or a ring-type ceramic magnet attached to the dish-shaped end bell, which completes the magnetic circuit. The spring-loaded brushes ride directly on stamped commutator bars.

Fig. - Exploded view of a permanent-magnet DC servomotor with a disk-type armature.

These motors are also called pancake motors because they are housed in cases with thin, flat form factors whose diameters exceed their lengths, suggesting pancakes. Earlier generations of these motors were called printed-circuit motors because the armature disks were made by a printed-circuit fabrication process that has been superseded. The flat motor case concentrates the motor’s center of mass close to the mounting plate, permitting it to be easily surface mounted. This eliminates the awkward motor overhang and the need for supporting braces if a conventional motor frame is to be surface mounted. Their disk-type motor form factor has made these motors popular as axis drivers for industrial robots where space is limited.

The principal disadvantage of the disk-type motor is the relatively fragile construction of its armature and its inability to dissipate heat as rapidly as iron-core wound rotors. Consequently, these motors are usually limited to applications where the motor can be run under controlled conditions and a shorter duty cycle allows enough time for armature heat buildup to be dissipated.

Cup- or Shell-Type PM DC Motors

Cup- or shell-type PM DC motors offer low inertia and low inductance as well as high acceleration characteristics, making them useful in many servo applications. They have hollow cylindrical armatures made as aluminum or copper coils bonded by polymer resin and fiberglass to form a rigid “ironless cup,” which is fastened to an axial shaft. A cutaway view of this class of servomotor is illustrated in the following figure:

Fig. Cutaway view of a permanent-magnet DC servomotor with a cup-type armature.

Because the armature has no iron core, it, like the disk motor, has extremely low inertia and a very high torque-to-inertia ratio. This permits the motor to accelerate rapidly for the quick response required in many motion-control applications. The armature rotates in an air gap within very high magnetic flux density. The magnetic field from the stationary magnets is completed through the cup-type armature and a stationary ferrous cylindrical core connected to the motor frame. The shaft rotates within the core, which extends into the rotating cup. Spring-brushes commutate these motors.

Another version of a cup-type PM DC motor is shown in the exploded view in the figure below. The cup type armature is rigidly fastened to the shaft by a disk at the right end of the winding, and the magnetic field is also returned through a ferrous metal housing. The brush assembly of this motor is built into its end cap or flange, shown at the far right. The principal disadvantage of this motor is also the inability of its bonded armature to dissipate internal heat buildup rapidly because of its low thermal conductivity. Without proper cooling and sensitive control circuitry, the armature could be heated to destructive temperatures in seconds.

Fig. - Exploded view of a fractional horsepower brushtype DC servomotor.

Brushless PM DC Motors

Brushless DC motors exhibit the same linear speed–torque characteristics as the brush-type PM DC motors, but they are electronically commutated. The construction of these motors, as shown in the following figure, differs from that of a typical brush-type DC motor in that they are “inside-out.” In other words, they have permanent magnet rotors instead of stators, and the stators rather than the rotors are wound. Although this geometry is required for brushless DC motors, some manufacturers have adapted this design for brush-type DC motors.

Fig. - Cutaway view of a brushless DC motor.

The mechanical brush and bar commutator of the brushless DC motor is replaced by electronic sensors, typically Hall-effect devices (HEDs). They are located within the stator windings and wired to solidstate transistor switching circuitry located either on circuit cards mounted within the motor housings or in external packages. Generally, only fractional horsepower brushless motors have switching circuitry within their housings.

The cylindrical magnet rotors of brushless DC motors are magnetized laterally to form opposing north and south poles across the rotor’s diameter. These rotors are typically made from neodymium–iron–boron or samarium–cobalt rare-earth magnetic materials, which offer higher flux densities than alnico magnets. These materials permit motors offering higher performance to be packaged in the same frame sizes as earlier motor designs or those with the same ratings to be packaged in smaller frames than the earlier designs. Moreover, rare-earth or ceramic magnet rotors can be made with smaller diameters than those earlier models with alnico magnets, thus reducing their inertia.

A simplified diagram of a DC brushless motor control with one Halleffect device (HED) for the electronic commutator is shown in the figure below:

Fig. - Simplified diagram of Hall-effect device (HED) commutation of a brushless DC motor.

The HED is a Hall-effect sensor integrated with an ampli-fier in a silicon chip. This IC is capable of sensing the polarity of the rotor’s magnetic field and then sending appropriate signals to power transistors T1 and T2 to cause the motor’s rotor to rotate continuously. This is accomplished as follows:

1. With the rotor motionless, the HED detects the rotor’s north magnetic pole, causing it to generate a signal that turns on transistor T2. This causes current to flow, energizing winding W2 to form a southseeking electromagnetic rotor pole. This pole then attracts the rotor’s north pole to drive the rotor in a counterclockwise (CCW) direction.

2. The inertia of the rotor causes it to rotate past its neutral position so that the HED can then sense the rotor’s south magnetic pole. It then switches on transistor T1, causing current to flow in winding W1, thus forming a north-seeking stator pole that attracts the rotor’s south pole, causing it to continue to rotate in the CCW direction.

The transistors conduct in the proper sequence to ensure that the excitation in the stator windings W2 and W1 always leads the PM rotor field to produce the torque necessary keep the rotor in constant rotation. The windings are energized in a pattern that rotates around the stator. There are usually two or three HEDs in practical brushless motors that are spaced apart by 90 or 120º around the motor’s rotor. They send the signals to the motion controller that actually triggers the power transistors, which drive the armature windings at a specified motor current and voltage level.

Fig. - Exploded view of a brushless DC motor with Hall-effect device (HED) commutation.

The brushless motor in the exploded view seen in the figure above illustrates a design for a miniature brushless DC motor that includes Hall-effect com-mutation. The stator is formed as an ironless sleeve of copper coils bonded together in polymer resin and fiberglass to form a rigid structure similar to cup-type rotors. However, it is fastened inside the steel laminations within the motor housing.

This method of construction permits a range of values for starting current and specific speed (rpm/V) depending on wire gauge and the number of turns. Various terminal resistances can be obtained, permitting the user to select the optimum motor for a specific application. The Halleffect sensors and a small magnet disk that is magnetized widthwise are mounted on a disk-shaped partition within the motor housing.

Position Sensing in Brushless Motors

Both magnetic sensors and resolvers can sense rotor position in brushless motors. The diagram in the following figure shows how three magnetic sensors can sense rotor position in a three-phase electronically commutated brushless DC motor. In this example the magnetic sensors are located inside the end-bell of the motor. This inexpensive version is adequate for simple controls.

Fig. - A magnetic sensor as a rotor position indicator: stationary brushless motor winding (1), permanent-magnet motor rotor (2), three-phase electronically commutated field (3), three magnetic sensors (4), and the electronic circuit board (5).

In the alternate design shown in the following figure, a resolver on the end cap of the motor is used to sense rotor position when greater positioning accuracy is required. The high-resolution signals from the resolver can be used to generate sinusoidal motor currents within the motor controller. The currents through the three motor windings are position independent and respectively 120º phase shifted.

Fig. - A resolver as a rotor position indicator: stationary motor winding (1), permanent- magnet motor rotor (2), three-phase electronically commutated field (3), three magnetic sensors (4), and the electronic circuit board (5).

Brushless Motor Advantages

Brushless DC motors have at least four distinct advantages over brushtype DC motors that are attributable to the replacement of mechanical commutation by electronic commutation.

• There is no need to replace brushes or remove the gritty residue caused by brush wear from the motor.

• Without brushes to cause electrical arcing, brushless motors do not present fire or explosion hazards in an environment where flammable or explosive vapors, dust, or liquids are present.

• Electromagnetic interference (EMI) is minimized by replacing mechanical commutation, the source of unwanted radio frequencies, with electronic commutation.

• Brushless motors can run faster and more efficiently with electronic commutation. Speeds of up to 50,000 rpm can be achieved vs. the upper limit of about 5000 rpm for brush-type DC motors.

Brushless DC Motor Disadvantages

There are at least four disadvantages of brushless DC servomotors. • Brushless PM DC servomotors cannot be reversed by simply reversing the polarity of the power source. The order in which the current is fed to the field coil must be reversed.

• Brushless DC servomotors cost more than comparably rated brushtype DC servomotors.

• Additional system wiring is required to power the electronic commutation circuitry.

• The motion controller and driver electronics needed to operate a brushless DC servomotor are more complex and expensive than those required for a conventional DC servomotor.

Consequently, the selection of a brushless motor is generally justified on a basis of specific application requirements or its hazardous operating environment.

Characteristics of Brushless Rotary Servomotors

It is difficult to generalize about the characteristics of DC rotary servomotors because of the wide range of products available commercially. However, they typically offer continuous torque ratings of 0.62 lb-ft (0.84 N-m) to 5.0 lb-ft (6.8 N-m), peak torque ratings of 1.9 lb-ft (2.6 N-m) to 14 lb-ft (19 N-m), and continuous power ratings of 0.73 hp (0.54 kW) to 2.76 hp (2.06 kW). Maximum speeds can vary from 1400 to 7500 rpm, and the weight of these motors can be from 5.0 lb (2.3 kg) to 23 lb (10 kg). Feedback typically can be either by resolver or encoder.

Basic of servomotor control

This information explains the difference between a servomotor and a stepper motor when connected to a servo driver. It covers the terms used in controlling the pulse train supplied to servomotors by a PCL series controller.

- The difference between a stepper motor and a servomotor configuration is shown below. The design and construction of the motors are also different.

- Stepper motor operation is synchronized by command pulse signals output from the PCL or a pulse generator (strictly speaking it follows the pulses). In contrast, servomotor operation lags behind the command pulses.

I. Connection and operation differences in stepper motors and servomotors

II. Advantages and disadvantages of stepper motors and servomotors

1. Stepper motor

<Advantages>

(1) Since stepper motor operation is synchronized with the command pulse signals from a pulse generator such as the PCL, they are suitable for precise control of their rotation.

(2) Lower cost.

<Disadvantages>

(1) Basically, the current flow from a driver to the motor coil cannot be increased or decreased during operation. Therefore, if the motor is loaded with a heavier load than the motor's designed torque characteristic, it will get out of step with the pulses.

(2) Stepper motors produce more noise and vibration than servomotors.

(3) Stepper motors are not suitable for high-speed rotation.

2. Servomotor

<Advantages>

(1) If a heavy load is placed on the motor, the driver will increase the current to the motor coil as it attempts to rotate the motor. Basically, there is no out-of-step condition. (However, too heavy a load may cause an error.)

(2) High-speed operation is possible.

<Disadvantages>

(1) Since the servomotor tries to rotate according to the command pulses, but lags behind, it is not suitable for precision control of rotation.

(2) Higher cost.

(3) When stopped, the motor’s rotor continues to move back and forth one pulse, so that it is not suitable if you need to prevent vibration.

Both motors have advantages and disadvantages. The selection of which type to use requires careful consideration of the application’s specifications.

Below is a summary of the comparison of stepper motors and servomotors

* ppr = Pulses per revolution

III. What is a deflection counter?

The servomotor rotation lags behind the command pulses from the PCL. This means that when the PCL completes outputting a number of pulses equivalent to the preset amount, the encoder will take some time to return all of the pulses. That is why the servo driver includes a "deflection counter."

=> This counter compares the number of command pulses from the PCL and the number of pulses returned from the encoder.

=> If the number of pulses returned from the encoder is smaller than the number of command pulses output, the driver will try to rotate the motor some more.

If the number of pulses returned from the encoder is larger than the number of command pulses output, the driver will attempt to run the motor backward.

When the number of command pulses output from the PCL and the number of pulses returned from the encoder match, the motor stops.

(In other words, the driver attempts to rotate the motor until the deflection counter is zero.

IV. Output signals from an encoder

An encoder is a kind of pulse generator. It outputs three types of pulse signals: A phase, B phase, and Z phase (Index signal)

1. A phase and B phase signals

In order to make the pulse per rotation resolution finer and to set the direction of rotation, two pulse trains with the same cycle length are phase shifted.

This half-pulse deviation is the key. For example, if the A phase pulse rises first, this means the motor rotation is clockwise (CW). If the B phase rises first, this means the rotation is counter-clockwise (CCW).

That is how to tell the direction of rotation.

2. Z phase (Index signal)

In our example, we will assume that the encoder has 1000 pulse per revolution (1000 ppr) and that it outputs 1000 A and B phase pulses (1000 rising edges per revolution). But, it outputs a Z phase pulse only once per one revolution.

If you want to execute a zero return precisely, a stop using only the ORG sensor may cause a deviation of plus or minus a few pulses each time. Therefore, after the ORG turns ON, count a specified number of Z phase signals and make this the official zero position.

Supply these A phase, B phase, and Z phase signals from the encoder to the PCL.

(The corresponding terminal names on the PCL50xx series, PCL61xx/PCL60xx series are: EA, EB, and EZ)

V. Multiplication

Since stepper motors have full step and half step operation modes, the number of encoder divisions can be enhanced.

For example, an encoder which puts out a nominal 1000 pulses per revolution (1000 ppr) can be multiplied as follow:

- 1x => 1000ppr = 0.36°/ pulse

- 2x => 2000ppr = 0.18°/ pulse

- 4x => 4000ppr = 0.09°/ pulse

(The multiplication rate is specified in the servo driver.)

You can specify the multiplication rate to apply (1x, 2x, and 4x) in the PCL. Shown below are the bits used by 3 typical PCL models.

* Multiplication settings of the encoder A/B phase signals

- PCL3013/5014: Set bits 10 and 11 in R16 (environment setting register 1)

- PCL61xx series: Set bits 16 and 17 in RENV2 (environment setting register2) (Bit names: EIM0 and EIM1)

- PCL60xx series: Set bits 20 and 21 in RENV2 (environment setting register2) (Bit names: EIM0 and EIM1)

 

 

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