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Fig. 1: Torque motors are frameless kit motors. They consist of a permanent magnet rotor and a laminated stator.

Direct Drive Torque Motors for Machine Tool Applications

Arthur Holzknecht Vice President, General Manager

ETEL, Inc. 333 E. State Parkway

Schaumburg, IL 60173 Aholzknecht@etelusa.com

Tel: 847.519.3380

Introduction As the requirements for machine tool productivity, accuracy and dynamic performance have increased, 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 motors can 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 of motor 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 From the point of view of general electrical theory of operation, torque motors are not different from 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 (1). 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

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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. 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 direct coupling 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 with a peak torque of 2500 N-m (1). 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 to support 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, 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 a given 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 are consequently a large number of permanent magnets on the rotor. It is this characteristic that enables 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

Fig. 3: Torque motors are available in a wide range of sizes.

Fig. 2: Torque motors can be provided with a “Bridge”, which is a reusable assembly tool to maintain the alignment of the rotor and stator during assembly.

Bridge Rotor Stator

Measured stiffness (in the moving direction) of an ETEL motor with a DSA2-PL-D servodrive and a moving mass of 250kg

100

1000

10000

1 10 100 1000

frequency in Hz

Stif

fnes

s in

N/u

m

Natural stiffness given by a 250 kg moving mass (calculated)

Measured stiffness of the closed loop system

DSA2.010.JMV 19/09/96-Drive DSA2-PL-D-300V-35A-Linear motor 2*LMA22-1808

Fig. 4: Direct drives increase the dynamic stiffness of machine tools (in

this example: >400 N/µm at all frequencies, and >1000 N/µm below 20 hz)

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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, generally below 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 not determine 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.

TMA 0210-050-3TB

0

20

40

60

80

100

120

140

160

180

0 200 400 600 800 1000 1200

Speed [rpm]

Tor

que

[Nm

]

Tcw105 [Nm] Drive 300V Tcw105 [Nm] Drive 600V

Tp [Nm] Drive 300V Tp [Nm] Drive 600V

Torque motor with OD = 230mm and Length = 90mm

TMA 0530-100-3VD

0

500

1000

1500

2000

2500

0 20 40 60 80 100 120 140 160 180

Speed [rpm]

Tor

que

[Nm

]

Tcw105 [Nm] Drive 600V Tp [Nm] Drive 600V

Torque motor with OD = 565mm and Length = 160mm

Fig 6: Two examples of Torque-Speed curves for torque motors typically used for machine tool applications. Data is for liquid cooled motors and 600 VDC bus amplifier. (Courtesy ETEL S.A.)

Intermittent duty

Continuous duty

0 500 1000 1500 2000 2500 3000 3500 4000

TMA 0530-030

TMA 0360-100

TMA 0530-050

TMA 0360-150

TMA 0530-070

TMA 0760-030

TMA 0530-100

TMA 0760-050

TMA 0990-030

TMA 0530-150

TMA 0760-070

cont. Torque @ 80° cont. Torque @ 130° with cooling peak torque

Fig 5: Example torque ratings (N-m) for motors with Diameter

from 565mm to 795mm and Stator Length from 90mm to 210mm (Courtesy ETEL S.A.)

Intermittent duty

Continuous duty

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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 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 effectively removed 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 use of 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 conventional motor 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 heat generation. 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, many manufacturers 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 to consider 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 of the motor.

Customerhousing

A

Water CoolingInlet / Outlet

EtelStatorFrame

Seal( O-Ring )

B

Fig. 7: Torque motors typically require cooling to remove generated heat.

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Dynamic torque: Tdyn = I · α (Nm) I: inertia (kg·m2) α: acceleration (rad/s2)

Friction, machining torque, axis configuration (static force due to the weight of offset loads) and external perturbations generate additional torque:

Tadd (Nm)

Total Torque: Ttot = Tdyn + Tadd (Nm)

Peak torque: Tmax = MAX( Ttot ) (Nm)

Thermal equivalent continuous torque: ( )Nm t

dtT

= Tcycle

t

0

2tot

cont

cycle

∫

For many systems, the motion can be considered as taking place over discrete intervals, allowing the following calculation:

With N discrete values: ( )Nm t

t T = T

cycle

N

1kk

2k

cont

∑=

⋅

Tk: discrete element of torque #k

tk: time during which Tk is working

Peak power estimation : ( )W KT

= P2

m

maxp

Continuous power : ( )W K

T = P

2

m

contc

The Motor Constant Km:

=

WNm

or WN

losses CopperTorque or Force

Km

The heat produced by motor power dissipation must be removed by the cooling system to avoid heating of the machine elements. Under static conditions with an applied load, one must perform more detailed calculations to be sure that one motor phase does not get disproportionately hot, since power dissipation is not shared equally among all three phases in this case.

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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 from various 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 the packing 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 (N-m/Amp), which relates the torque output to supplied current. Kt is readily adjusted by changing the wire gage. It is useful for matching a motor to a servo amplifier, but it doesn’t 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, an example is given in figure 7. The advantages of direct drive in a machine tool rotary axis implementation are nicely summarized 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 (3). Other experience has shown that torque motors can have up to 20 percent higher initial cost versus a conventional alternative drive, but much lower lifetime operating costs, while also providing many performance benefits. Conventional Mechanical Drive Direct Drive Cost (normalized at conventional drive = 100%)

100% 97%

Mounting/assembly time 88 hr. 12 hr. Position Index time 1 sec 0.33 sec Position repeatability 2.5 arc-sec 1 arc-sec Feedback system resolution - 0.18 arc-sec Stiffness 7.2 x 106 N-m/rad 13 x 106 N-m/rad

Bearing

Stator frame

Housing

Rotor

Encoder

Fig 8: Example design of a direct drive rotary table

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Fig. 9: View of the direct drive rotary table on a mid-sized gear grinder, with an example workpiece.

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 accuracy under high torque and low speed loads, and those that cannot tolerate gears and couplings that require frequent maintenance and replacement. (5) Case Study 1: Precision gear grinding machine (Courtesy Hoefler Maschinenbau GmbH, Ettlingen, Germany) 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 the workpiece turntable during complex interpolated moves to make the gear profile. The use of the torque motor enables the production of a substantially quieter gear.

Summary of Key Torque Motor Characteristics for Machine Tool Applications ü Direct Drive (Direct coupling of the load to the motor, without

intervening power transmission elements) ü High accuracy and repeatability ü High dynamic performance ü Servo stiffness ü High bandwidth ü No hysteresis

ü Eliminates mechanical power transmission elements (gearboxes, worm gear drives, etc.) ü High reliability ü Reduced wear and maintenance

ü High torque and low speed ü Very smooth motion, with excellent velocity regulation ü Ultra-low speed operation possible ü High torque at stall and over the complete speed range

ü Large Diameter to length ratio, short axial length ü Ring configuration enables optimal configuration of the

machine ü Low mass and inertia

ü Liquid cooling to eliminate heat transfer to the machine structure

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Fig. 10: Example of the large gears one can grind on a direct drive table. Gears weighing more than 5000kg

can be machined.

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 from Siemens, Bosch and ETEL.

Case Study 2: Precision multi-axis machining center (O.M.V. S.r.l., Caltana di S. Maria di Sala, Italy) In this application the use of multiple torque motor driven axes enables double milling in a single machine structure with either synchronous machining (two identical workpieces), or independent machining. The machine incorporates a unique double-rotary table configuration. This table is driven by four torque motors with a gantry-mode balanced tilting movement. The 480mm diameter tables can support workpieces of up to 775mm diameter.

Fig. 12: Double Tilt-rotary table incorporating four torque motors

Fig. 13: Milling machine incorporating a Double Tilt-rotary table with torque motor drive

Fig. 11: Example of gear grinder for parts up to 100kg.

The B-axis incorporates a torque motor

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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 (Deckel Maho Gildemeister) This application demonstrates the innovative ways machine tool builders are implementing torque motors to expand the versatility of machining centers in ways that were previously unattainable 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.

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 enabling technology benefits of torque motors.

Fig. 14: A combination CNC milling and turning machine 50

0

300

200

400

100

RPM

20002000

18001800

12001200

14001400

10001000

800800

600600

400400

200200

2020

17,517,5

1515

12,512,5

1010

7,57,5

55

2,52,5

22,522,5

Powerin kW

Torquein Nm

S1-100 %

16001600

21,422

19,418,8

680

Fig. 15: Both high torque machining and high power turning are accomplished using torque motors

Fig. 16: Cutaway view showing torque motor integration

Torque Motor

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References: 1. Wavre, N., et.al. “Drive Systems for Demanding Applications”, Swiss Quality Production, Carl

Hanser Verlag, Munich, Germany, 1998 2. Raskin, C. “Designing with Motion Handbook”, Technology 80, Inc. Minneapolis, MN, USA, 1997 3. Zirn, O. and Baldini, G., “Direct Drives in Machine Tools” (Direktantriebe in Werkzeugmaschinen),

Werkstatt und Betrieb, Vol. 130, No. 4, Carl Hanser Verlag, Munich, Germany, 1997 4. Favre, E., et.al. “Permanent Magnet Syncronous Motors: A Comprehensive Approach to Cogging

Torque Suppression”, IEEE Transactions on Industry Applications, Vol. 29, No. 6, Nov./Dec. 1993. 5. “Direct Drive Motors: Fast and Accurate”, Machine Design Magazine, Basics of Design Engineering,

February 11, 1999 6. Banon, L., Feusi, H., “Servos with High Torque Motors for Direct Drive”, Automation, Motion Drives

and Control (AMD&C) International Magazine, May 1997 Acknowledgements: The author would like to gratefully acknowledge the support and cooperation of Hoefler Maschinenbau GmbH, of Ettlingen, Germany, O.M.V. S.r.l., of Caltana di S. Maria di Sala, Italy, and Deckel Maho Gildemeister for the case studies presented in this paper. About the author: Arthur Holzknecht is the Vice President and General Manager of ETEL, Inc., a leading supplier of direct drive motion control solutions for machine tools and other high tech applications. Prior to joining ETEL, Mr. Holzknecht held positions of Vice President, Sales and Marketing at Etec, a manufacturer of test equipment for MEMS devices, and Director of Marketing and Business Development at Anorad Corp., a manufacturer of sub-micron positioning systems for the semiconductor industry. Mr. Holzknecht holds a B.S.M.E. from the University of Michigan at Ann Arbor.