WHITEPAPER – PIMag®: Voice Coil, Linear, and Torque Motors Developed In-House-- Dr. Nico Bolse --PIMag®: Voice Coil, Linear, and Torque Motors Developed In-house-- Dr. Nico Bolse -Individual design of magnetic direct drives for flexible, competitive, and application-specific positioning solutions

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstr. 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

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WHITEPAPER – PIMag®: Voice Coil, Linear, and Torque Motors Developed In-House-- Dr. Nico Bolse --acts bidirectionally depending on the direction of the current.

1 Introduction
Positioning and scanning systems with magnetic direct drives
are suitable for applications that require high dynamics andrepeatability and that work reliably even at high operating
cycles.
PI has many standard and customer-specific positioning
solutions on offer, which are equipped with magnetic directdrives, and the company has many years of experience in thedesign, construction, and manufacture of the necessary
mechanical and electronic components, including proprietary
encoders, guides, and motion controllers.
PI can also develop proprietary motors if required. For
example, if standard series or customer-specific solutions
need to achieve specific performance characteristics thatcannot be reached by using drive components currently
available on the market. Here, the main focus is on voice coilmotors, ironless and iron-core linear motors, and torquemotors.
PI can provide customers with a competitive and flexible
solution that is precisely tailored to the requirements of theirapplication thanks to its expertise in motor development andits high vertical range of manufacturing.
The motors developed in-house are identified by the PIMag®
brand name. PI's positioning systems and scanners that use
these motors are labelled: Driven by PIMag®.
This paper explains the functional principle and properties ofvoice coil, linear, and torque motors. Furthermore, it uses
examples to show how the performance characteristics of thevarious motor types can be adapted to the specificrequirements of the positioning solution through anindividual design, e.g., to achieve a high force density or acompact design.

Direct drives offer advantages, especially in regards to wearand dynamics, compared to common drive screw-based
technologies. Since they use as few mechanical components
as possible, less friction and zero backlash are the result andthey, therefore, achieve a higher precision.

2.1

Cylindrical voice coils are constructed according to theplunger coil principle, i.e., the winding body sits in a field
assembly as indicated in Figure 1. Either the winding body(“moving coil”) or the pot (“moving magnet”) can be moved.
These voice coils can also be designed with a hollow shaft
which can be used as a feedthrough or can be used tointegrate a linear guide.

Abb. 1

Voice coil, linear, and torque motors work due to different
physical principles. For example, voice coil drives are basedon the Lorentz force in which the force generated is
proportional to the magnetic field strength and to the currentof the current-carrying conductors. The electrical energy isconverted here into mechanical energy. The generated force

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstr. 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

PIMag® cylindrical voice coil motor for use in medicalendoscopes

Besides cylindrical designs, flat designs can be realized. In thiscase, the winding body moves over parallel magnets as shownin Figure 2.

Abb. 22 Magnetic Direct Drives

Voice Coil Motors

Voice coil motor with a flat design

Voice coil motors are suitable for positioning tasks with highdynamics and high forces. They are used primarily in scanningapplications with travel ranges up to several tens ofmillimeters. For example, the V-522, V-524, and V-528 linear
stage series (as shown in Figure 3) are based on voice coilmotors from PI.

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WHITEPAPER – PIMag®: Voice Coil, Linear, and Torque Motors Developed In-House-- Dr. Nico Bolse --Performance
characteristics

Size 1Size 2Size 3

Unit

Travel range

101520mm

Peak force

80614

1772

N

Nominal force

20149

380

N

Motor constant

7.5

3157

N/√W

Voice coils can also be optimized for force or motor constant.
The motor constant denotes the ratio of force to power loss.
The higher the motor constant, the less heat is producedwhen a certain force is generated. It, therefore, also describes
the efficiency of the motor in regard to converting electricalinto kinetic energy. However, as the temperature rises, thewinding resistance and, therefore, also the power loss
increase, which is why the motor constant is temperaturedependent.

Force constant

22.6

88

98

N/A

Back EMF

22.6

88

98

Vs/m

Current at peak
force

3.5

718

A

Current at nominalforce

0.9

1.7

3.9

ADC resistance

9.2

8.1

2.9

ohmInductance

4.5

7.5

3.4mH

PI has developed cylindrical motors, as shown in Figure 4, tomaximize the motor constant.

Electrical time
constant

0.49

0.93

1.17

ms

Mechanical time
constant

1.71

0.66

0.54

ms

Runner mass

95630

1800g

Stator mass

235

2530

8800g

Abb. 3

Driven by PIMag®: V-522, V-524, and V-528 linear stage series
with flat-design voice coil motors from PI

Tab. 1

Abb. 4

PIMag® cylindrical voice coil motors

Examples of cylindrical voice coil motors developed in-house

The force depends on the position since the coils move inrelation to the permanent magnets along the travel range.
This can be seen in Figure 5.

They are characterized by a maximized ratio of motor
constant to installation space and can be manufactured invarious sizes. Cylindrical motors are used, for example, in
positioning solutions for focusing devices in order todynamically move a measuring head or an optical system
vertically by means of a flexure. Table 1 shows theperformance characteristics of three examples in different
sizes

Abb. 5Force-displacement diagram of a cylindrical PIMag® voice coilmotor

In order to drive the force into the motor as quickly aspossible, the voltage can be increased as shown in Figure 6. In
this way, the current is then made available correspondingly
faster. The acceleration then increases in the same ratio.
Highly dynamic applications are, therefore, possible through afast increase in acceleration (jerk).

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstr. 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

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WHITEPAPER – PIMag®: Voice Coil, Linear, and Torque Motors Developed In-House-- Dr. Nico Bolse --motor dynamics. It is also possible to use typical industrial
servo amplifiers with high voltages.
The magnetic tracks used in PI linear motors are available invarious lengths. They can be used as modular elements andarranged in series in order to realize any desired travel range.
Single-sided or U-shaped magnetic tracks are available. Ushaped magnetic tracks achieve higher magnetic fieldstrengths and higher forces than single-sided magnetic tracks.
If the magnets are additionally arranged as a Halbach array,the magnetic field strength can be increased by about 10%compared to a North Pole to South Pole arrangement. In
addition, the iron counterplate can be omitted in a Halbach
array, making these magnetic tracks significantly lighter. The
advantages of using a Halbach array also apply to single-sided
magnetic tracks. In this case, the use of Halbach arrays avoids
the generation of high stray fields on the back of the singlesided magnetic track. PI can provide carbon supports forapplications that require ultra-light magnetic tracks.
Abb. 62.2

Force-time diagram of a cylindrical voice coil motor developed
in-house

Linear Motors

PI manufactures both ironless and iron-core linear motors.
For example, motors of both types are used in the V-508
linear stage series. An example of a linear stage of this series
is shown in Figure 8.

A classical 3-phase linear motor is basically a series of at least
three (or a multiple of three) voice coil motors which arecontrolled, i.e. commutated, according to a positiondependent, fixed pattern. A motor consisting of three coils ina U-shaped magnetic track is illustrated in Figure 7.

Abb. 8Abb. 7Structure of a linear motor: Three current-carrying magnetic coils
move in a magnetic track or vice versa

Driven by PIMag®: Linear stage of the V-508 series based on PI's
proprietary linear motor with Halbach array for a particularly flat
design with both low weight

2.2.1 Iron-Core Linear Motors
Optionally, PI encapsulates its linear motors under vacuum.
This results in an improved heat dissipation, whereby higher
nominal forces can be achieved. In addition, the sealing
compound ensures that the motor is encapsulated andtherefore protected against external damage, e.g., duringassembly.
For special applications that require high velocities or fastcurrent rise times, PI can design motors for very highoperating voltages of up to 600 VDC. In this regard, linear
motors benefit from the same effect that was previously
described for voice coil motors: Due to the higher voltage, thecurrent can be made available faster, resulting in increased

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstr. 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

Iron-core linear motors are suitable for applications requiringhigh forces and accelerations with limited installation space.
The iron maximizes the magnetic forces and contributes tohigh thermal stability. To reduce eddy current losses, the ironis laminated and it is mostly made of stacked and insulatedtransformer plates. The disadvantage of iron-core motors isthe attraction force that arises between the iron and themagnets arranged on the opposite side. This is increased
further still if a linear guide is used. “Cogging” is also a
problem since the displacement force varies over the travel
range – while this can be minimized by means of special tooth
geometries, it cannot be completely eliminated. An example
of an iron-core linear motor is shown in Figure 9.

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WHITEPAPER – PIMag®: Voice Coil, Linear, and Torque Motors Developed In-House-- Dr. Nico Bolse --Tab. 2

Sealed linear motors with three or six coils

To ensure a constant force, the linear motors can reach a
specified maximum speed. This is shown in Figure 11 with theexample of an iron-core linear motor. In addition to othermotor data, this maximum speed also depends on the motor
voltage provided.
Abb. 9Example design of an iron-core linear motor without a magnetictrack

The sealed motor shown in Figure 10 is an example of a
proprietary linear iron-core motor developed by PI.

Abb. 11 Force-speed diagram of an iron-core linear motor

2.2.2 Ironless Linear Motors
Abb. 10 PIMag® linear motor with laminated magnetic steel and epoxyresin sealing

The iron-core linear motors, for example, are constructedwith three or six coils. The performance characteristics areillustrated by two examples in Table 2.
Performance
characteristic

3 coils

6 coils

Unit

Max. voltage

48

48

V

Nominal force(with/withoutcooling)

5.8 / 12.5

9.0 / 22.7

N

Nominal current(with/withoutcooling)

1.2 / 2.6

1.0 / 2.5

A

Motor constant

4.59

6.27

N/√W

Force constant

4.81

9.08

N/A

Back EMK(Phase-Phase)

3.75

7.5

Vs/m

Resistance (phasephase)

2.1

4.1

ohm

Inductance (phasephase)

1.64

3.79

mH

Electrical time
constant

0.78

0.92

ms

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstr. 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

Ironless linear motors are suitable for positioning tasks withthe highest demands on precision. Unlike with iron-corelinear motors, cogging does not occur. This makes ironlessmotors ideally suited for applications requiring high travel
accuracy and speed stability because there are no varying
forces on the linear guide or cogging-related disturbances forthe controller. They are also suitable for the smallest
installation spaces thanks to their particularly flat design.
Power and dynamics requirements can be met by increasing
the number or dimension of the motor coils.
In most cases, ironless motors achieve lower nominal andpeak forces than iron-core motors. This is due to the lack ofthermally conductive metals in the design and the resulting
limited heat dissipation from the coils. However, the motors
are protected against overload by means of additionaltemperature sensors. An example of an ironless linear motor
is shown in Figure 12.

Abb. 12 Example design of an PIMag® ironless linear motor without amagnetic track

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WHITEPAPER – PIMag®: Voice Coil, Linear, and Torque Motors Developed In-House-- Dr. Nico Bolse --The flat motor with a U-shaped magnetic track shown in
Figure 13 is an example of a proprietary ironless linear motor
developed by PI.

2.3

Torque Motors

A torque motor can simply be thought of as a radiallydesigned linear motor. In an alternative design, the rotor canalso be represented as a rolled-up, single-sided magnetic
track, while the stator houses the coils which are embeddedin an iron matrix. An example of a torque motor developed by
PI is shown in Figure 14.

Abb. 13 Flat PIMag® linear motor with three coils and u-shaped magnetic
track

The ironless motors, for example, are also constructed withthree or six coils. The performance characteristics areillustrated by two examples in Table 3.
Performance
characteristic

3 coils

6 coils

Unit

Max. voltage

48

48

V

Nominal force(with/withoutcooling)

4.1 / 9.8

8.1 / 18.4

N

Nominal current(with/withoutcooling)

0.5 / 1.2

1.0 / 2.3

AMotor constant

4.566.45

N/√W

Force constant

7.9

7.9

N/A

Back EMK(Phase-Phase)

6.5

6.5

Vs/m

Resistance (phasephase)

6.1

3.1

ohm

Inductance (phasephase)

0.87

0.42

mH

Electrical time
constant

0.14

0.14

ms

Abb. 14 Extremely flat PIMag® torque motor with large aperture

Tab. 3

Especially flat motors with three or six coils

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstr. 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

Like linear motors, torque motors are well-suited topositioning tasks in which the highest demands for precisionneed to be combined with high torques and accelerations.
While the magnet length scales linearly, the torque scales
quadratically with the diameter. In addition, large diameters
allow apertures, e.g., for the passage of laser beams orcables.
Torque motors are zero-play direct drives which are,therefore, suitable for positioning applications that require
the highest accuracy. The high stiffness leads to a highrepeatability. The high drive torque enables high accelerationand leads to high dynamics.
In precision positioning, torque motors are mainly used fordirect drive rotation stages. Such a rotation stage is shown inFigure 15, stacked on a hexapod. This example shows an
extremely flat, highly dynamic, and stable torque motor
rotation stage with a very large aperture. The common
aperture of the rotation stage and the hexapod can be used,e.g., as a feedthrough.

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WHITEPAPER – PIMag®: Voice Coil, Linear, and Torque Motors Developed In-House-- Dr. Nico Bolse --mode” in digital slide scanning processes.

Abb. 15 Flat PIMag® torque motor rotation stage with large aperture
stacked on a PI hexapod

Another example is the torque motor rotation stage, shown in
Figure 16, which is mounted on the H-811 PI hexapod for use
in highly automated production systems.

Abb. 17 Autofocus application with voice coil on the Z axis and linear
motor on the X axis

However, thanks to PI's vertical range of manufacturing andtechnological range, PI is the ideal partner even whencustomers only require one single component of a positioningsolution.

Abb. 16 PIMag® torque motor rotation stage stacked on a PI hexapod forhighly automated production systems

3 SummaryWith the in-house expertise to develop proprietary motors
and with the core technologies needed for a complete
positioning solution, such as sensors, guides, and motion
controllers, PI offers its customers competitive solutions withperformance characteristics that are optimally adapted to therequirements of the application. Figure 17, for example,
shows an application that combines different motor types.
The multi-axis setup for autofocus applications shown,consists of an X and a Z axis. The X axis, for example, holds aworkpiece that is to be machined on a V-508 linear stage. As asupplement to the commonly used piezoceramic drives, thevoice coil Z axis enables long travel ranges up to several
millimeters. This is important, among other things, for lasermaterial processing. Travel ranges of 1 to 7 mm are typicallyalso required for multiphoton fluorescence microscopy anddeep tissue microscopy. Furthermore, voice coils offer
particularly high maximum speeds which can, for example, beused to increase throughput when using “scanning-on-the-flyPhysik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstr. 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

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WHITEPAPER – PIMag®: Voice Coil, Linear, and Torque Motors Developed In-House-- Dr. Nico Bolse --4 Author

Dr. Nico Bolse, Product Manager for PIMag® magnetic drives
and motorized positioners at Physik Instrumente (PI) GmbH &Co. KG in Karlsruhe, Germany.

5 PI in Brief

WP4018E PIMag® Motors 06/2019 0 Subject to changes. © Physik Instrumente (PI) GmbH & Co. KG 2019

Well known for the high quality of its products, PI (PhysikInstrumente) has been one of the leading players in the global
market for precision positioning technology for many years. PI
has been developing and manufacturing standard and OEM
products with piezo or motor drives for more than 40 years.
Continuous development of innovative drive concepts,products, and system solutions and more than 200
technology patents distinguish the company history today. PI
develops, manufactures, and qualifies all core technology
itself: From piezo components, actuators, and motors as wellas magnetic direct drives through air bearings, magnetic andflexure guides to nanometrological sensors, control
technology, and software. PI is therefore not dependent oncomponents available on the market to offer its customers
the most advanced solutions. The high vertical range ofmanufacturing allows complete control over processes andthis allows flexible reaction to market developments and newrequirements.
By acquiring the majority shares in ACS Motion Control, aworldwide leading developer and manufacturer of modular
motion controllers for multi-axis drive systems, PI can also
supply customized complete systems for industrial
applications that have the highest demand on precision anddynamics. In addition to four locations in Germany, the PI
Group is represented internationally by fifteen sales andservice subsidiaries.

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstr. 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

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