direct drive torque motors for machine tool applications

8/8/2019 Direct Drive Torque Motors for Machine Tool Applications

1/10

TERM PAPER

OF

ELE-102

Submitted by:

MANMEET SINGH Submitted to:

Roll.no- RE2801A26 Lect. NITIKA

Reg.no- 10801620

Course- B Tech. M Tech. IT


2/10

ACKNOWLEDGEMENT

I take this opportunity to express my gratitude to our

guide Miss. Nitika who encouraged me with her critical

and constructive suggestions in the project work.

I am also thankful to my FRIENDS for their invaluable

guidance in this project. They benefited me with their

experience and skills in the project.

I again thank all the people who helped me in my

project for their support and encouragement.

Manmee

t Singh

Student

sign


3/10

Direct Drive Torque Motors for Machine Tool Applications

Introduction:

As the requirements for machine tool productivity, accuracy and dynamic performance haveincreased, direct drive technology has emerged as an ideal way to meet these demands. Direct

drive torque motors, in particular, have been demonstrated to provide significant machine tool

performance improvements. In addition to providing high dynamic performance, torque motorscan reduce machine cost of ownership, simplify the machine design, and reduce wear and

maintenance.

What is a Torque Motor?

Torque motors are a special class of brushless permanent magnet servomotors. This type ofmotor is also commonly referred to as a permanent magnet synchronous motor or a brushless DC

motor. This motor technology has many advantages over other types, for example:

Very small electrical time constant High dynamic response Large mechanical air gap (0.5 1.5mm) Easy mounting and alignment

High efficiency due to the use of permanent magnets

Torque motors are frameless kit motors. Theyconsist of a permanent magnet rotor and a laminated


4/10

stator.

From the point of view of general electrical theory of operation, torque motors are not differentfrom their more conventional counterparts. Therefore, the electrical control requirements are

fundamentally the same. However, in many ways the similarities end there. It is their specific

differences that give torque motors their unique advantages for

machine tool applications. The most unique feature of a torque motor concerns the physical

dimensions. They have a relatively large diameter to length ratio, and they also have a rather

short axial length. Additionally, torque motors can simultaneously have both a very large OD

(outer diameter) and ID (inner diameter), resulting in a motor that is a thin ring. One important

outcome of this characteristic is that the mass is quite low as a function of the diameter. Also, the

large diameter allows very high torque to be developed. As an extreme example, a torque motor

for a telescope drive was constructed with a diameter of 2.5m, and a length of less than 50mm.

This motor produces a continuous torque exceeding 10,000 N-m. For most machine tool

applications, diameters of 1m or less are more typically encountered. Torque motors are a type of

frameless motor. This means that the motor does not include a housing, bearings, or feedback

device. In this sense the motor is a kit motor, meant to be an integral part of the machine

structure. To assist in integrating torque motors, they can be provided with a reusable assembly

aid called a bridge. The bridge is set at the factory to ensure the proper alignment of the rotor

and stator for assembly. The bridge also keeps the magnetic field contained within the motor,

thereby eliminating the need for a special non-ferrous area for assembly, and protecting the rotor

from damage from metal scraps or loose screws.

Torque motors can be provided with a Bridge,which is a reusable assembly tool to maintain thealignment of the rotor and stator during assembly.

The Direct Drive Advantage:

Torque motors are designed to be used as direct drives. That is, they eliminate the need for

gearboxes, worm-gear drives, or other mechanical transmission elements and enable a directcoupling of the payload to the drive. This enables a drive with high dynamic response without

hysteresis. Angular stiffness can be extremely high, on the order of 100 N-m/arc-sec for a motor


5/10

with a peak torque of 2500 N-m. The large inner diameter of a torque motor is advantageous for

machine tool construction. Essentially a large hollow shaft, it gives the machine designer great

flexibility in locating the motor. In most cases the motor can be optimally located with respect tosupport bearings, feedback devices, and the payload. It enables the motor to be integrated in the

machine without adding excessive moving mass or inertia

Torque Motor Size Ranges:

Torque motors are available in a wide range of sizes.

Torque motors are available in a wide range of sizes, with diameters from smaller than 100mm

to greater than 2m (though 1.2m is typically the largest for machine tool applications). The motor

diameter is analogous to the frame size of a conventional brushless DC servomotor. For agiven diameter, there are several axial lengths available. This enables the machine designer a

wide range of physical motor sizes to satisfy the torque requirements for a given application.

Torque-Speed Characteristics:

Their relatively large number of magnetic pole-pairs distinguishes torque motors. There areconsequently a large number of permanent magnets on the rotor. It is this characteristic thatenables the motor to be constructed as a thin ring. It also enables torque motors to achieve very

smooth velocity regulation, with low ripple. However, eddy current losses in a brushless motor

increase with increased pole-pairs, so this sets a design constraint on their maximum practical

number. As a result, torque motors are primarily designed for low speed applications, generallybelow 1000 RPM. However, this is more than adequate for many machine tool axis drives.

Torque motors can produce very high torque at stall, and they are capable of high dynamic

stiffness. Hence the common name of torque motor. However, the motor alone does notdetermine high dynamic stiffness and precision. The benefits of a direct drive motor system are

only realized if the machine tool is built to the necessary standards of precision and stiffness, and

if the system incorporates a high performance control system.

Feedback Considerations:

High precision, high-resolution feedback is essential for optimal performance of a direct drive.Because the load is directly coupled to the drive higher accuracy is possible, but the positioning

resolution is also in direct relation to the resolution of the feedback system. One needs an optical


6/10

encoder with a high line count (typically 9000 lines per revolution and above), combined with a

high-resolution interpolation factor. Such feedback devices are available from several

manufacturers. System resolution below 1 arc-sec is generally required.

Thermal Considerations:

Like all DC servomotors, torque motors generate heat in operation. This heat must be effectivelyremoved to eliminate thermally induced machine distortions. Machine tool grade torque motors

include provision for liquid cooling to remove heat generated in the stator. Since the liquid flows

in close proximity to the stator windings, this is a very efficient method to remove heat. The useof liquid cooling also effectively increases the continuous torque rating of the motor. Air-cooling

is also an option. However, it is much less effective than liquid for heat transfer. An important

difference between a direct drive system and one driven by a conventional DC servomotor and

gearbox, for example, is that the torque motor is mounted internally in the axis and is an integral

part of the machine. Therefore, it is much more important to remove the heat. The conventionalmotor is typically mounted in a less critical location (e.g. at the end of the worm gear), so the

heat removal is less of an issue, and the motor can be allowed to run at a higher temperature.

Selecting the Right Torque Motor: Sizing

The sizing and selection of a torque motor is not fundamentally different from the sizing of a

conventional brushless DC servomotor. However, because of the aforementioned thermal

considerations, one needs to pay increased attention to certain parameters associated with heatgeneration. It is not the objective of this paper to present a detailed treatise on servomotor sizing.

There are several excellent references on this subject. (See, for example, reference 2) Also, manymanufacturers provide excellent applications support and calculation aids to assist in selecting a

servomotor. The objective here is to present the particular characteristics that are important toconsider for a torque motor. It is critically important to calculate the thermal power dissipated by

the motor. The first step in selecting a motor is to determine the torque and speed required for the

application. The RMS torque should be calculated to be able to estimate the power dissipation ofthe motor.

Comparing Motors: The Motor Constant, Km:

Note the use of the very important term, Km, the Motor Constant in the preceding calculations.

Km is very useful as a figure of merit for comparing the relative efficiency of motors fromvarious manufacturers. It shows the relationship between torque produced and resulting power

losses. A motor with a higher value of Km is a more efficient generator of torque. Km is

probably the most important factor for comparing torque motors for a machine tool application.

Km is determined by the design and construction of the motor. It relates to factors such as thepacking efficiency of the windings (i.e. copper fill), the type and design of the laminations, and

the electromagnetic circuit design. Therefore, it is a better indicator of motor performance than,

say, the torque constant, Kt (Nm/ Amp), which relates the torque output to supplied current. Kt is


7/10

readily adjusted by changing the wire gage. It is useful for matching a motor to a servo amplifier,

but it doesnt give information about the efficiency.

Putting the Pieces Together: The Torque Motor in a System:

Torque motors are but one element in a complete system. The complete axis requires a

mechanical structure of high rigidity, bearings, and feedback device. It is the overall integration

of these elements that determines the system performance. However, with a good overall design,the resulting performance can significantly exceed the performance achieved with a more

conventional solution. To illustrate how one could consider the integration of a torque motor.

The advantages of direct drive in a machine tool rotary axis implementation are nicelysummarized in the table below. In this example a comparison was made considering several

important performance criteria, including cost. The advantage of the torque motor compared to a

conventional drive is readily apparent. Other experience has shown that torque motors can haveup to 20 percent higher initial cost versus a conventional alternative drive, but much lower

lifetime operating costs, while also providing many performance benefits.

Case Studies and Candidate Applications:

Probably the best way to illustrate the value of direct drives in machine tool applications is to

show some real-world examples of their successful implementation. In general, two major

applications benefit best from direct drive motors: those requiring high acceleration and accuracyunder high torque and low speed loads, and those that cannot tolerate gears and couplings that

require frequent maintenance and replacement.

Case Study 1: Precision gear grinding machine:

The grinding of precision gears requires a machine of very high accuracy and high dynamic

performance to enable accurate profile generation. Surface finish and profile accuracy are key

determinants of gear noise. In this type of machine, the torque motor is used to drive theworkpiece turntable during complex interpolated moves to make the gear profile. The use of the

torque motor enables the production of a substantially quieter gear.


8/10

View of the direct drive rotary table on a midsizedgear grinder, with an example workpiece.

The use of the direct drive torque motor enables the rotary table to achieve typical dynamic

following errors of less than 0.00020 deg (0.72 arc-sec) at a rotational speed of 400 deg/min.,

which is a typical gear grinding speed. The torque motor has been tested with servo drives fromSiemens, Bosch and ETEL.

Example of the large gears one can grind on a Example of gear grinder for parts up to 100kg.The B-axis incorporates a torque motor

direct drive table. Gears weighing more than 5000kgcan be machined.

Case Study 2: Precision multi-axis machining center:

In this application the use of multiple torque motor driven axes enables double milling in a singlemachine structure with either synchronous machining (two identical workpieces), or independent

machining. The machine incorporates a unique double-rotary table configuration. This table is


9/10

driven by four torque motors with a gantry-mode balanced tilting movement. The 480mm

diameter tables can support workpieces of up to 775mm diameter.

Double Tilt-rotary table incorporating four torque motors

Milling machine incorporating a Double Tilt-rotary tablewith torque motor drive

Typical applications of the machine include the precision machining of turbofans, and other

complex parts in hard to machine alloys.

Case Study 3: Combination CNC machining and turning center:

This application demonstrates the innovative ways machine tool builders are implementing

torque motors to expand the versatility of machining centers in ways that were previouslyunattainable with conventional drives. In this case, the torque motor enables a single machine

that is capable of both multi-axis CNC machining and turning. This revolutionary machine has

many benefits, including reduced setup time and higher productivity.


10/10

A combination CNC milling and turningmachine

The machine tool builder carefully engineered the integration of the torque motor in the machine.

The result is a compact, high performance axis that is an ideal example of the enablingtechnology benefits of torque motors.

Cutaway view showing torque motor integration

direct drive torque motors for machine tool applications

Documents