Advances in direct-drive rotary servos

Inside machines: In performance and market growth, direct-drive servo motors are outpacing conventional servo motors and gear motors. Advanced designs and new technologies can add efficiency.

09/04/2012


Figure 1: Direct drive rotary (DDR) torque motor cutaway shows how DDR motors transmit mechanical power in the form of torque and speed directly without the need for any power transmission elements, such as gears, belts, pulleys, or chains. Courtesy: Etel

Today’s direct drive rotary (DDR) motors and drives offer many critical performance features and improvements to a wide range of factory automation machine builders. They also are providing better solutions in medical equipment, energy (generators and alternators), and transportation traction applications, in addition to traditional automation applications. While gear motors of all types have a much larger overall U.S. market share, the direct drive rotary (DDR) servos market continues to grow at a much higher pace, according to IMS Research. By definition, DDR motors transmit mechanical power in the form of torque and speed directly without the need for any power transmission elements represented by gears, belts, pulleys, or chains.

Many U.S. factories and machine builders require more accurate positioning and variable motion capabilities. Gear trains have loss motion limitations in the form of backlash and compliance that severely limit position accuracy. At the same time, power efficiency and energy savings are other concerns attracting many machine builders to using DDR motors. (Note: DDR motors are technically lower efficiency if run at low speed.)

Today’s DDR motor customers come in three categories.

1. The first buys off-the-shelf housed DDR motors. They are used for automation applications that include robot and machine tools’ end users along with some original equipment manufacturers (OEMs).

2. The second category specifies custom DDR motors. These customers buy frameless motor kits, encoder kits, bearings, and customer mechanical components. The customer assembles the motor for very specific applications.

3. The third category is similar to the customers in the second category, but they employ a third party integrator to develop a finished DDR motor design and build the motor. Each DDR motor manufacturer concentrates on one or two categories.

Motor technologies


The basic configuration of a DDR motor is a ring shape with a large outside diameter and a large inside diameter. The stator (nonrotating) device consists of a multiple coil winding interspersed in a number of soft magnetic iron or steel teeth. The rotor consists of a hub with rare earth magnets positioned on the rotor hub’s surface or buried within the hub’s soft iron magnet flux return structure. (See Figure 1.)

These current DDR motors are classified into two types of radial air gap DDR motors. The first is the surface permanent magnet (PM) type. The second type is the buried magnet type. The permanent magnets are located on the magnetic rotor hub. This construction generates reluctance and PM torque that increases overall shaft torque per motor volume (torque density). Special motor design tools are led by finite element analysis (FEA) models for optimizing these motor designs. This construction is a brushless PM synchronous motor, also called a brushless dc motor. The stator winding configuration is a concentrated winding type with each copper coil wound directly around each stator tooth. This winding configuration minimizes the coil resistance leading to lower copper (I2R) losses and increases the DDR motor’s power efficiency. The DDR motor can be supplied in framed (housed) or frameless (unhoused) configurations depending on customer needs.

An option to extend motor torque performance is building water or fluid cooling channels on the housing outside diameter. Water cooling will increase continuous torque values by 2 to 4 times normal ratings. Other industry drivers include position accuracy and resolution, large through-hole and thin-ring profile, and pressure for lower unit cost. (DDR motors are typically higher in cost than standard cylinder shaped brushless PM motors of the same power levels.)

DDR motor suppliers

While the larger DDR motor manufacturers possess standard product lines, the typical customer asks the DDR motor suppliers to customize the motor to a specific application. More than 40 years ago, Inland Motors (now Kollmorgen, Radford, Va.) supplied brush dc torque motors to the U.S. defense market. The company’s switch to brushless PM motors in housed form replaced the older brush dc motor technologies. Today, Kollmorgen supplies a cartridge-based DDR motor that simplifies assembly into the customers’ equipment. Early applications focused on machine tool table positioning systems that were overwhelmingly housed DDR. Today, Kollmorgen technology specialist Jeff Arnold identified general market trends from customers that include higher position accuracy, increased reliability, and lower maintenance costs.

Etel, a Swiss company, started a DDR motor business in 1974 and has developed three standard product families of slotted brushless DDR brushless PM torque motors. (See Figure 2.)

 

Figure 2: Standard direct drive rotary (DDR) torque motor performance table

Company

 

Brand, family, series

 

Housed, frameless

 

Slotted, slotless

 

Outer diameter range (mm)

 

Continuous torque range (Nm)

 

Peak torque range (Nm)

 

Comments

 

Aerotech

 

S

 

Frameless

 

Slotless

 

50 to 180

 

0.2 to 29.1

 

0.82 to 116.4

 

Zero cog

 

Allied Motion (Emotek)

 

CMS SE

 

Housed

 

Slotted

 

66.3 to 170.2

 

0.22 to 27.6

 

 

 

 

 

Allied Motion (Emotek)

 

Megaflux MF

 

Housed

 

Slotted

 

186 to 355

 

3.89 to 213.4

 

 

 

 

 

Allied Motion (Emotek)

 

HT

 

Frameless

 

Slotted

 

19.3 to 127

 

.007 to 8.4

 

.04 to 46.99

 

 

 

Allied Motion (Emotek)

 

Megaflux MF

 

Frameless

 

Slotted

 

170 to 792

 

3.5 to 2020

 

 

 

Ultra thin

 

Applimotion

 

UTH

 

Frameless

 

Slotted

 

29 to 533

 

.05 to 74.2

 

.15 to 223.5

 

 

 

Applimotion

 

ULT

 

Frameless

 

Slotted

 

101.6 to 165

 

.05 to .763

 

.15 to 2.12

 

Ultra thin

 

Applimotion

 

UTS

 

Frameless

 

Slotless

 

29 to 240

 

.035 to 3.11

 

.105 to 9.33

 

Zero cog

 

Applimotion

 

UTO

 

Frameless

 

Slotted

 

29 to 533

 

.007 to 4.05

 

.021 to 12.15

 

Outer rotor

 

Aspen Motion

 

RM

 

Frameless

 

Slotted

 

17.8 to 350

 

.02 to 300

 

.06 to 1500

 

Inner rotor and outer rotor configurations. Custom designs of direct drive motors and electronics to solve specific problems for volume OEM applications.

 

Etel

 

TMK

 

Frameless

 

Slotted

 

198 to 395

 

81 to 1050

 

119 to 2240

 

Water cooled

 

Etel

 

TMB

 

Frameless

 

Slotted

 

160 to 1290

 

19 to 16,200

 

38 to 31,200

 

IPM rotor available

 

Etel

 

TML

 

Frameless

 

Slotted

 

235 to 581

 

24 to 1080

 

127 to 5000

 

 

 

Etel

 

TMM

 

Frameless

 

Slotted

 

140 to 540

 

8 to 1090

 

38 to 5000

 

 

 

Kollmorgen

 

Cartridge

 

Frameless

 

Slotted

 

108 to 350 sq

 

4.6 to 510

 

12.3 to 1017

 

 

 

Kollmorgen

 

KBM

 

Housed

 

Slotted

 

60-782

 

0.3 to 3445

 

1.2 to 12,800

 

 

 

Kollmorgen

 

D(H)

 

Housed

 

Slotted

 

176-362

 

5.3 to 339

 

16.9 to 1340

 

 

 

Courtesy: Incremotion, Phoenix Tech, Control Engineering

Kevin Derabasse, president of Etel USA, explained that his latest TMK design improves its speed by as much as 8 times in the larger DDR torque motors and continuous torque ratings by 30%. Etel’s largest sized custom DDR torque motor exceeds 3 meters (118 in.) outside diameter and develops 31200 Nm (23000 lb-ft) of continuous torque. It drives and positions a large astronomical telescope.

The Emoteq division of Allied Motion, Tulsa, Okla., is another major supplier of DDR torque motors. It manufactures framed and frameless DDR rotary brushless PM motors under the Megaflux brand name. A significant portion of Emoteq’s customers are from the military and aerospace markets.

Dave Hawes, director of engineering, explained that Emoteq can provide DDR motors with very low cogging and high stator copper fill. The company’s largest size DDR motor is 792 mm (31.2 in.) outside diameter and develops a continuous torque of 2020 Nm (1490 lb-ft).

Joseph Profeta, director of Control Systems Group at Aerotech in Pittsburgh, Pa., reported that a large majority of its DDR motors supplied to customers are frameless types: “The DDR motor user is asking for longer life cycles and improved accuracy.”

The Aerotech DDR product families are aimed at the smaller sized applications that include cardiac stent and photovoltaic cell manufacturing machines. Accuracy is a key parameter in manufacturing these devices.

Figure 3: Transverse flux motor cross-section shows stator, magnets, and other elements of this design. South Korea Electric Research Institute has built these for more than 11 years. Courtesy: Incremotion, Phoenix Tech, and Control EngineeringApplimotion, Loomis, Calif., is a more recent addition to DDR torque motor suppliers, and is, perhaps, the fastest growing U.S. supplier of DDR torque motors and custom direct drive motor assemblies. The company provides a wide range of slotted and slotless frameless DDR motor product families including the ULT, UTH, UTS, and UTO series with large through-holes. The largest standard unit currently produced is the UTH series that is 533 mm (21.0 in.) in outside diameter and develops torque of up to 68 Nm (50 lb-ft) continuously, and as small as 12 mm (.472 in.) in diameter and axial lengths as short as 8 mm (.314 in.).

Another version of the brushless DDR motor offered by Applimotion is configured in a slotless, ironless configuration that eliminates cogging, for the smoothest motion available. It also can provide a benefit of low eddy current losses at higher motor shaft speeds. The performance trade-off is an approximately 25% lower shaft torque due to the larger magnetic air gap between rotor and stator.

Aspen Motion Technologies, Radford, Va., division of Pentair, makes custom DDR motors which range in diameter from under 1 in. to 39 in. and power ratings from a few Watts to 65 kW. A custom approach to motor design results in the least expensive solution for volume applications. All features (laminations, magnets, motor geometry, mounting, shaft, etc.) can be customized to meet OEM requirements and constraints, said Ron Flanary, director of engineering, Aspen Motion Technologies.

New motor technology

The transverse flux motor is a recent development moving from research and development departments into production. It possesses a very high torque capability in a smaller volume, thereby maximizing torque density. It has a complicated magnetic structure that requires 3D-FEA simulation capability. Electric Torque Machines, Flagstaff, Ariz., has spent a number of years testing various configurations and is now moving toward production with an initial number of frame sizes. Scott Reynolds, director of electric torque machines at Electric Torque Machines, indicated that these transverse flux motors can exceed the current DDR motor’s continuous torque density by almost 2 to 1.

Rare earth opportunities

Rare earth magnets, primarily Neodymium and the thermal stabilizer Dysprosium, have been undergoing significant pricing spikes that have resulted in extensive research in ascertaining if other motor types, including alternate PM motors, can be used. The factory automation and allied markets continue to demand motion solutions that improve performance and increase throughput.

- Dan Jones is president, Incremotion Associates Inc., and Muhammad Mubeen is owner, Phoenix Tech. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering and Plant Engineering, mhoske(at)cfemedia.com.

ONLINE

Advanced Motor Design: New Motors Reach New Applications 

 

Servo Vs. Induction Motors: Which Should Be Considered For What Applications? 

Tuesday, Oct. 23, 2012 at 2 p.m. ET/1 p.m. CT/11 a.m. PT

 


Control Engineering - machine and motion control - motor-driveInduction motor-based systems, with appropriate design, can have servo-like performance at what can be a lower price point. Which should be considered for what applications? Are there areas where either might apply? Criteria will be provided for deciding between them. An exam and certificate are available for one professional development hour (PDHs), according to Registered Continuing Education Program rules (from the American Council of Engineering Companies). Learn more or register to see the servo vs. induction motor webcast from Control Engineering.

 



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