WHITEPAPER – Rotating Electric Motors for Precision Positioning-- Dr. Nico Bolse --Rotating Electric Motors for Precision Positioning
An application-related comparison of different motor types

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Page 1 of 17

WHITEPAPER – Rotating Electric Motors for Precision Positioning-- Dr. Nico Bolse --Contents
1

Introduction

4

2

DC Motor (DC)4

2.1Standard Configuration

4

2.2

Operating Behavior

4

2.2.1

Continuous Operation

5

2.2.2

Short Time Operation

5

2.3

Position Control

2.3.1

5

Dual-Loop Controller

63

DC Motor with ActiveDrive (PWM)

6

4Brushless DC Motor (BLDC) / Synchronous Servo Motor (SSVM)

6

5

4.1

Motor Design

6

4.2

Operating Behavior

7

4.3

Determination of the Rotor Position

7

4.3.1

Sensor-Aided Determination of the Rotor Position

7

4.3.2

Sensorless Determination of the Rotor Position

7

4.4

Block Commutation

7

4.5

Sinusoidal Commutation

8

4.6

Field-Oriented Control

8Torque Motor (TQM)
5.1

6

Motor Design

2-Phase Stepper Motor (2SM)

8
8
9

6.1

Variable Reluctance Stepper Motor9

6.2

Permanent Magnet Stepper Motor9

6.3

Hybrid Stepper Motor9

6.4

Full-Step and Half-Step Operation

10

6.5

Microstepping Operation

10

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WHITEPAPER – Rotating Electric Motors for Precision Positioning-- Dr. Nico Bolse --6.6
7

Step Loss10

Gear Motors

11

7.1

Worm Gears

11

7.2

Bevel Gears

11

7.3

Spur Gears

11

7.4

Planetary Gears

12

7.5

Belt Gears

12

7.6

Harmonic Drive Gears

128

Suitable Motion Controllers

12

9Other Motor Types

13

10

Summary13

10.1

Comparison of motor types

14

10.2

Standard products overview

15

11

Author17

12

About PI

17

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WHITEPAPER – Rotating Electric Motors for Precision Positioning-- Dr. Nico Bolse --1 Introduction
Rotating electric motors are typical drives for precision
positioning. In positioning systems, they are usually applied
together with a lead screw or ball screw to translate themotor's rotary motion into a linear motion. Every motor typehas different strengths and weaknesses. This white paper
discusses an application-related comparison of different
motor types used by PI and also describes design and functionof these different motors.
Fig. 12 DC Motor (DC)
Direct current motors offer good dynamics over a large speed
range, low heat generation, as well as smooth and vibrationfree operation. For example, in a positioning solution, a drive
screw pitch of 1°mm/per revolution results in a typical travel
velocity of approximately 50 mm/s. However, the wear-pronebrushes used to commutate the motor can be a disadvantage
with a short lifetime of 1,000 to 5,000 hours.
-6
Applications in a vacuum are only possible up to 10 hPa,otherwise the humidity is lacking that is required by the
carbon brushes for commutation. Moreover, the brushes
exude carbon dust; a problematic effect in vacuum, clean
rooms, and optical applications.

2.1

With the help of the right-hand-rule, it is possible to establish thedirection of the force relative to the direction of the current andthe magnetic field.

PI mainly uses permanent DC motors with conductor loops
reeled up on a coil and mounted as a rotor within a stator.
The stator's permanent magnets provide the poles.
In order for the motor to perform a revolution, the applied
direct current is transformed to an alternating current. This isachieved with a commutator; e.g., in its most basic version, asegmented slip ring. Additional carbon brushes turn thecurrent in the rotor winding, which results in an alternating
current that maintains the direction of force with continued
movement. Figure 2 shows an example of a permanent DC
motor design.

Standard Configuration

A DC motor exploits the force effect of the electrical current
via a rotatable conductor loop in a magnetic field. The
movement is caused by the Lorentz force, resulting from thecharge carriers in the electromagnetic field. With the help of
the right-hand-rule, as shown in figure 1, it is possible toestablish the direction of the force relative to the direction of
the current and the magnetic field.

Fig. 2

DC motor design with stator magnet, rotor windings,
commutator, brushes, and motor shaft.

2.2

Operating Behavior

Figure 3 shows a simplified model of the rotor circuit of a DC
motor with inductance, resistor, and voltage source.

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WHITEPAPER – Rotating Electric Motors for Precision Positioning-- Dr. Nico Bolse --higher starting torque, either a thicker copper wire will be
required for the winding or a higher voltage during operation.
Generally, continuous operation is only permissible withlimited torque speed. The intersection between continuous
operation and short term operation with the torque-speed
characteristic curve equates the nominal operating point.

Fig. 3

Simplified rotor circuit equivalent diagram of a DC motor withinductance, resistor of the rotor winding, and rotor voltage.

This results in a motor voltage according to the law of
induction:𝑑𝐼
𝑈=𝐿+ 𝑅𝐼 + 𝐸𝑀𝐹
𝑑𝑡
L is the winding inductance, I the motor current, R thewinding resistance, and EMF is the induced voltage
counteracting the motion. This is described as the backelectromotive force (back-EMF). This force acts proportional
to the rotational speed. When the back-EMF equals the
motor voltage, the maximum rotational speed will beachieved. In DC motors, the voltage drop over the inductancecan be neglected, resulting in a proportionality of voltage
and rotational speed, where the sign of the voltage
determines the direction of rotation:
𝑈 = 𝑅𝐼 + 𝑘𝜙2𝜋𝑛k is the motor constant, 𝜙 is the magnetic flux in the air gap,
and n is the rotational speed. Accordingly, the torque𝑀 = 𝑘𝜙𝐼is proportional to the current. A combination of both
equations results in a decrease of the rotational speed underload that is proportional to the torque. This is illustrated infigure 4 with the torque-speed characteristic.

Fig. 42.2.1

Torque-speed characteristic of a DC motor with idle speed,
nominal operating point, starting torque, short-term, and longterm range.

Continuous Operation

A powerful motor is identified by a flat characteristic curve
because the rotational speed is more resistant to loadchanges. In order to shift the characteristic curve up while
maintaining the gradient, i.e., a higher idle speed and a

2.2.2

Short Time Operation

The maximum permissible overload during short term
operation is limited by the maximum permissible winding
temperature. However, it is possible to irrevocably demagnetize the permanent magnets by very high currents.
Mounting position, convection, ambient pressure (vacuum),
etc. may also pose limitations. Generally, the permissible
residence time in the short term operation is one to threeseconds and is signified by the thermal time constant of thewinding and the extent of the overload. Figure 5 shows anexample of typical values.

Fig. 5Overload periods in multiples of the thermal time constant of thewinding over the overload in multiples of the nominal torque toillustrate the continuous and short-term operation of a DC
motor.

Higher loads result in increased motor losses due to the
proportionality of torque and current, leading to a higher
operation temperature. For a quick estimate of thetemperature changes, the current heat losses
𝑃𝐿 = 𝑅𝐼²
can be multiplied by the thermal resistance from thecorresponding motor data sheet. Should the motor be
operated at its thermal limits, a closer examination isrequired. The motors are limited thermally by thesurrounding temperature as well as maximum winding
temperatures.

2.3

Position Control

DC motors require position encoders, e.g., incremental orabsolute encoders, for positioning tasks. A motor can be
operated in a closed servo loop when encoder signals are
returned to a controller. In this manner, high position

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WHITEPAPER – Rotating Electric Motors for Precision Positioning-- Dr. Nico Bolse --resolution, constant feed, and a large dynamic range can beachieved. Basically, so-called DC servo motors allow for thecontrol of position, torque and velocity. However, due to the
feedback loop, position jitter occurs when the motor tries tokeep a position.
Figure 6 shows an example of a closed loop. Normally, arotary encoder is used on the motor shaft for the speed
control of DC motors. An additional linear encoder on the
motion platform can be deployed for position control.

Fig. 6

2.3.1

Example of a closed loop for DC servo motors.

Dual-Loop Controller

In so-called dual loop controllers, as offered by ACS Motion
Control, the linear encoder can be deployed for both positionand velocity control at the same time. This is an advantage
when a rotary encoder would limit the minimum incremental
motion due to low resolution. However, this also reduces the
bandwidth of the velocity control loop because the linear
encoder registers the oscillations of the motion platform to ahigher degree than the rotary encoder on the motor shaft.

3 DC Motor with ActiveDrive(PWM)
PI has developed the ActiveDrive technology for the control
of motors with a rated power exceeding the controller's
output power to realize higher velocities. To this end, anamplifier is integrated with the motor in a shielded case. The
controller triggers the integrated amplifier via a pulse widthmodulation (PWM). The motor power will be controlled by
the ratio of on/off-duration, as shown in figure 7. Thisrequires an additional power adapter for the amplifier andoptimized heat dissipation to maintain precision.

Fig. 7

4 Brushless DC Motor (BLDC) /Synchronous Servo Motor(SSVM)
Due to their brushless commutation, BLDCs (brushless direct
current) and SSVM (synchronous servo motors) respectively,
provide many advantages compared to brush DC motors:
The lifetime is mainly limited by the bearings andends after several ten thousand hours. As aconsequence, the motors are more reliable.
The brushless design allows for a motor that issmaller, lighter, more efficient, and lowmaintenancewhile
providingthesameperformance; resulting in a large torque/motor size
ratio.
The electronic commutation allows for highdynamics at low temperatures with few vibrations.
These are the reasons why BLDCs and SSVMs are
preferred in industrial applications instead of DC
motors.

4.1

Motor Design

Despite its name, the motor design does not correspond to
the design of brushless DC motors but rather to the design ofa permanent-magnet synchronous machine. This is thereason why BLDC motors are often referred to as SSVM, forexample when used as industrial motors operating in the lowvoltage range. The term BLDC emphasizes the brushlesscommutation whereas the term SSVM stresses the rotor
movement synchronous to the rotating magnetic field.
However, the terms BLDC and SSVM are often usedinterchangeably. Sometimes, a difference is made based on
the control type (block or sinusoidal commutation); in this
scenario, block commutation refers to a BLDC motor, andsinusoidal commutation to an SSVM. Figure 8 shows anexample of the corresponding motor design.

Pulse width modulation (PWM) for various on/off-duration
options.

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WHITEPAPER – Rotating Electric Motors for Precision Positioning-- Dr. Nico Bolse --Apart from this, the operating behavior is similar to that of a
DC motor. This explains the origin of the name of thebrushless DC motor despite the deviating motor design.

4.3

Determination of the Rotor Position

In order to operate the motor synchronously at maximum
torque, the stator and rotor magnetic fields need to beperpendicular to each other at all times. To achieve this, therotor position needs to be known.
Fig. 8Design of a BLDC and SSV motor, respectively with rotor magnet,
stator winding, bearing, and motor shaft.

The stator carries the winding that creates a rotating
magnetic field after electronic activation to drive thepermanent magnet rotor; this means that commutation ispurely electronic. For this reason, this motor is often termed
electronically commutated motor (EC motor), too. Here, therotor moves synchronous to the alternating current of thestator winding, i.e.
𝑓𝑛=𝑝
The speed can only be influenced by the frequency f or thenumber of pole pairs p. Since the number of pole pairs isprescribed by the motor design, the speed setting can only bechanged via the frequency during operation. In a servo
amplifier, this is typically achieved by a change in voltage.

4.2

Operating Behavior

The operating behavior corresponds to a synchronousmachine where the current controller switches relative to therotor dependent on the actual pole position. In this mode, thespeed can perform non-linear changes dependent on theload. Figure 9 illustrates this in the torque-speed
characteristic curve.

Fig. 94.3.1

Sensor-Aided Determination of the Rotor Position

Since stator and rotor have neither mechanical nor electriccontact, the relative rotor position needs to be determinedwith the help of hall sensors, for example. A directed magnet
switches these sensors in the exact moment when the current
in the winding is turned.

4.3.2

Sensorless Determination of the Rotor Position

Alternatively, it is possible to determine the rotor position
sensorless by measuring the back-EMF. However, thismethod requires a specific startup process as the back-EMF
completely vanishes at standstill. Furthermore, this method isonly suited to continuous operation at high speed; otherwise
the amplitude of the back-EMF would be too low. Typically,
the rotor position is determined with the help of sensors.

4.4

Block Commutation

If the rotor position of a three-phase motor is determined bythree hall sensors, offset by 120° to one another, the signal
combinations are clearly defined for each rotor position at
60°. In block commutation, the current between the phases
will be switched in blocks, i.e., in six steps of 60° each for theduration of one complete revolution. It is a disadvantage,
however, that the magnetic stator field is only stable over 60°
while the rotor field continues its revolution due to theturning permanent magnet. Consequently, the magnetic
fields are not always perpendicular to each other. Placing
the hall sensors in the center of the two switchover points will
result in a phase displacement of 30° each, decreasing thetorque by approximately 13,4%. These torque ripples occur
with a six-fold electric rotation frequency of the motor,
causing vibrations and noises. Figure 10 shows an illustrationof block commutation and the resulting torque ripples.

Speed-torque characteristic curve of a BLDC respectively SSVM
motor.

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WHITEPAPER – Rotating Electric Motors for Precision Positioning-- Dr. Nico Bolse --(a)and positioning accuracy to be achieved. This type ofoperation is possible for example, with the PI C-891 controller
as well as controllers from ACS Motion Control.

5 Torque Motor (TQM)
(b)

(c)

Fig. 10(a) Block commutation principle on a BLDC motor. (b) Hall sensor
signals HS1, HS2, and HS3, (c) torque ripple.

4.5

Sinusoidal Commutation

Smoother motion can be achieved with sinusoidal
commutation where the sinusoidal current introduced into
each motor winding is phase-shifted by 120°. When only hallsensors are being deployed for the determination of the rotor
position it is possible to avoid torque ripples for the most partwith the help of interpolation between the switchover points.
Usually however, an additional encoder is used for the rotor
position measurement and, as a consequence, the controller
currents can be controlled in more detail. In this scenario, hallsensors are redundant or may be used for the determinationof the phases respectively. The described operation ispossible with PI's C-891 controller, SMC Hydra controller, as
well as controllers from ACS Motion Controller.

4.6

Torque motors are zero-play drives with often large radial
dimensions. They can have a very flat design. The large radial
dimensions allow for hollow shafts and large apertures,
respectively, e.g., to conduct laser beams and cables. The
zero-play allows high positioning accuracy and high drive
rigidity, resulting in high repeatability. The high drive torqueenables high acceleration and therefore high dynamics.
Additional features include high torsional rigidity, high peaktorques, high degrees of efficiency, as well as very smoothrunning.
Among others, torque motors are suitable for high load
applications on multi-axis or rotation stages thanks to theircompact design with respect to torque and rotational
symmetry.

5.1

Motor Design

Torque motors are permanent-magnet synchronous motors
where the load is directly connected to the rotor, i.e., zeroplay. In other words, a torque motor is similar to a wound up
linear motor or a short-dimensioned BLDC motor, with alarge diameter. In comparison to the other discussed motor
types, the torque motor is usually deployed as a direct drivefor rotation stages. Figure 11 shows the motor design.

Field-Oriented Control

In the case of sinusoidal commutation at high rotor speeds,
the calculated currents may lag behind the actual rotor
position as a result of limited control bandwidth, whichmeans that the stator and rotor field are no longer exactly
perpendicular to each other at all times. This can be avoided
by field-oriented control, also called vector control. This isachieved by controlling the current vector in a rotating
coordinate system of the rotor. This allows extended speed

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WHITEPAPER – Rotating Electric Motors for Precision Positioning-- Dr. Nico Bolse --6 2-Phase Stepper Motor (2SM)

Fig. 11Torque motor design, similar to a wound up linear motor orshort BLDC motor.

Due to the center-based hollow shaft, this motor issometimes referred to as a hollow shaft motor. This usuallyrefers to a drive screw combination, allowing a particularlycompact drive design for linear stages. As a result, this is nolonger a direct drive because the load is no longer directly
connected to motor.
The design of torque motors differs from BLDC motors withrespect to the large number of pole pairs, accounting for hightorques at medium and high speed.
The torque increases according to a so-called rule that applies
to induction machines and therefore to the following
proportionality:
𝑀 ~ 𝐴 ⋅ 𝐵 ⋅ 𝑙 ⋅ 𝐷²
A is the current coverage in the stator winding, B is the flux
density in the air gap, I is the active motor length and D is therotor diameter. For this reason, increased torque in torquemotors results in a large diameter.
In torque motors, the rotor typically carries the magnets andthe stator carries the coils that are embedded in the ironmatrix. Often, the coils are applied in three phases in starconnections. However, a delta connection application is
generally also possible.
A star connection signifies a higher torqueconstant, i.e., more torque is being achieved withthe same amount of current.
The delta connection allows for a higher speed
constant, i.e., the same speed is being achieved
with less voltage.

Stepper motors only take discrete positions within one
revolution. Due to their quantized steps, stepper motors onlyoffer reduced dynamics compared to DC motors. High speeds
can only be achieved with a large number of steps at theexpense of the torque, because the windings need to beenergized against one another in order to set theintermediate steps.
Stepper motors can be set up for applications in vacuum,
have a long lifetime, and can be applied for positioning tasks
without the requirement of an encoder. Open-loop
operation of stepper motors does not cause position jitter
because it is caused by the feedback loop in closed-loop
operation. Often, a mechanical damper is applied in the formof a hand wheel to enhance smooth running and to suppress
resonances.
Stepper motors are available in different configurations.

6.1

Variable Reluctance Stepper Motor

A typical example is the variable reluctance stepper motorfeaturing a stator with energizable windings and a soft
magnetic iron rotor, e.g., made from electric sheet. Poweringa winding pulls at the teeth closest to the magnetized pole
and aligns the rotor. A fine division of the teeth achieves ahigh step resolution. As soon as the stator is no longer
energized, the magnetic field of the soft iron rotor vanishes,i.e., it does not exhibit detent torque. For this reason, thismotor type always requires power to maintain its position.

6.2

Permanent Magnet Stepper Motor

In a permanent magnet stepper motor, the stator is made ofsoft iron and the rotor contains permanent magnets. This
design achieves higher torques, however, at a lower
resolution because the rotor is difficult to make with a large
number of poles.

6.3

Hybrid Stepper Motor

Nowadays, both motor variants are combined in hybrid
stepper motors to reach a compromise between step
resolution and torque. Figure 12 shows a typical design ofthis motor type.

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WHITEPAPER – Rotating Electric Motors for Precision Positioning-- Dr. Nico Bolse --

(b)

(a)

(c)

Fig. 12Hybrid stepper motor design with a soft iron stator and toothed
permanent magnet rotor.

6.4

Full-Step and Half-Step Operation

Typically, PI's 2-phase stepper motors have a step angle of 1.8
in full-step operation. This corresponds to 200 full steps. Fullstep operation is achieved when one phase or two phases,
respectively, are being energized. Energizing two phases
simultaneously results in a torque increase by 30 to 40%,
however, the energy consumption also doubles. Combining
these two energizing variants reduces the step angle by 50%(half-step operation). The higher step resolution accounts fora smoother run of the motor, however, it does not achieve
the full torque of the full-step mode with two energizedphases. The torque decreases by approx. 15% which might bebalanced by a higher motor current while only one phase isbeing energized.
Figure 13 illustrates full-step and half-step operation.

Fig. 13(a) Full-step operation with a powered-on phase, (b) Full-step
operation with two powered-on phases, and (c) half-step
operation.

6.5

Microstepping Operation

The so-called microstepping operation achieves up to several
thousand micro steps between two full steps throughelectronic interpolation and two 90° phase-shifted sinusoidal
current waves. The microstepping principle is illustrated infigure 14. A high step resolution with sinusoidal current feed
results in an even smoother run with fewer vibrations.

Fig. 14Microstepping operation with a 2-phase stepper motor and withquasi-sinusoidal current progression.

6.6

Step Loss

Encoders are not mandatory because of the countablediscreet step and therefore, stepper motors can be operated
in an open servo loop. Step losses are possible and would
result in a positioning error. They can occur due to excessive

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WHITEPAPER – Rotating Electric Motors for Precision Positioning-- Dr. Nico Bolse --loads and accelerations, or resonance effects. In such cases,
the problems are caused by the rotor which can no longer
follow the applied rotation field. However, step loss errors
can only be avoided by closed loop operation, i.e., an
additional encoder will be required.
Please refer to the white paper "Performance of Stepper
Motor Axes" for more information on using stepper motors
for precision positioning tasks.

gear ratios at one translation level, which makes themcomparatively inexpensive. PI often deploys worm gears forrotation stages because they transfer motion at right angles.
In this way, the motor can be aligned lateral to the rotation
stage.

7 Gear Motors
Gear motors are used for precision positioning tasks toprovide higher torques and higher resolutions at lower
speeds. In slow applications, the rotor moves comparably fast
thanks to the gear ratio. Without gears, undesired cogging
torques may occur due to the low rotor speed. Moreover,gears support the holding forces in vertical applications.
Often, less effort is required for the control loop because thegear reduces the load on the motor by the square of the gear
ratio.
However, a geared motor is not play-free and additional
friction reduces efficiency. For this reason, gear
manufacturers often offer suitable lubrication to guarantee
the expected lifetime. The lifetime is largely subject to input
speed and output torque as well as operation, ambient, andinstallation conditions. In customized solutions, the lifetime
can be prolonged by self-lubricating bearings, ball or ceramicbearings, metal gearwheels, and special greases.
Strictly speaking, drive screws also act as gears in gear spindle
positioning systems because speed adjustments are achievedon the expense of the torque in dependence of the screw
pitch. To this effect, a motion platform moves twice as fast at
50% of the torque with a pitch of 2 mm/revolution compared
to a pitch of 1 mm/revolution. For some applications,however, an actual gear is required to act between motor anddrive screw. PI deploys various gear types for this scenario.

7.1

Worm Gears

Worm gears as shown in figure 15 consist of a shaft withhelical mount and a worm wheel. The power is transferred bysliding friction at a right angle. For this reason, the worm
shaft has high self-locking forces, making additional brakes insome applications unnecessary. Sliding friction however,causes a low degree of efficiency, high wear, and potentiallyhigh temperatures. Typically, worm gears can achieve highFig. 15

Worm Gears

7.2

Bevel Gears

Bevel gears as shown in figure 16 consist of a bevel pinion anda toothed wheel with angled teeth. Similar to the worm gear,the energy is transferred at a right angle, but reaches higher
torque capacities. High gear ratios can only be achieved withadditional spur gear stages. The occurring rolling contact isadvantageous. This makes bevel gears low-wear and more
efficient than worm gears. However, they are often more
expensive. PI deploys bevel gears in customized rotation
stages.

Fig. 16Bevel Gears

7.3

Spur Gears

Spur gears as shown in figure 17 consist of two parallel but
different-sized toothed wheels. Thanks to their simple
structure, spur gears are easy to manufacture and robust. Allmetal models fulfill high requirements with respect to aneven and smooth run and particularly play-free models canbe realized for applications where high precision at low
torque is needed. To achieve this, the preload is set byinverse twisting of the gear trains and their tension on the
motor pinion. For PI's linear stages, spur gears are next toplanetary gears the standard choice for gear motors.

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WHITEPAPER – Rotating Electric Motors for Precision Positioning-- Dr. Nico Bolse --Fig. 17

7.4

Fig. 19

Belt Gears

7.6

Harmonic Drive Gears

Spur Gears

Planetary Gears

Figure 18 shows a planetary gear, which consists of a sunwheel connected to a shaft, and other planet wheels, which
are located within a ring wheel. They are suitable fortransferring the highest torque because the load is
distributed via several toothed wheels. In this manner, highgear ratios can be realized in very compact assembly spaces.
Often, the input stage's toothed wheels are made of asynthetic material to reduce high speed noises. However, forapplications in vacuum, high temperatures or for very hightorques, the input stage is preferably made of steel. For PI's
linear stages, planetary gears are next to spur gears thestandard choice for gear motors.

Fig. 18

Planetary Gears

7.5

Belt Gears

Belt gears, as shown in figure 19 feature double-toothed
wheels connected by a belt. In this way, larger shaft distances
can be bridged and higher peripheral speeds can be realized.
Maintenance tasks usually involve a belt change or
restressing of the belt. The belt limits the temperature
range. The occurring push and pull forces usually cause larger
shaft loads. PI often uses belt gears in compact positioning
stages; e.g., Z or linear stages where the drive is folded on-6
the side. Belts are suitable for vacuum up to 10 hPa.

Harmonic drive gears, as shown in figure 20, are characterized
by elasticity properties of the transmission element, allowing
for high gear ratios, high torque capacities, high linear
torsional stiffness, high degrees of efficiency as well as playfree operation. The elastic transmission element is anelliptical disc that deforms a thin steel bush with external
teeth. The steel bush is located on an outer ring with ballbearing and internal teeth. When deformation occurs, theinternal and external teeth interlock play-free in the area ofthe larger elliptical axis. Additionally, harmonic drive gears
offer the possibility of using a central hollow shaft; e.g., forcables, shafts, or laser beams. However, these gears arecomparatively expensive due to their complex, compact, andmaintenance-free setup. PI sometimes uses harmonic drive
gears for customized solutions to achieve particularly highpositioning accuracy and repeatability thanks to the zeroplay characteristic of these gears.

Fig. 20

Harmonic drive gears

8 Suitable Motion Controllers
PI offers various controllers for motors; e.g., C-863 and C-884
for DC motors, C-663 for stepper motors, C-891 for BLDC, aswell as SMC Hydra, ACS or C-885 controllers withcorresponding modules for all motor types.

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

Page 12 of 17

WHITEPAPER – Rotating Electric Motors for Precision Positioning-- Dr. Nico Bolse --9 Other Motor Types
PI uses further motor types like magnetic linear and voice coilmotors, as well as other torque motors. PI develops these
motors in-house under the name of the registered trademark
PIMag®; therefore, these are being discussed in a separate
white paper.

10 Summary
A variety of motorizations are available in the field of
precision positioning.
Different motor types are suited for specific applications: Stepper motors are cost-efficient and suitable forvacuum applications, they offer low speed, low
dynamics, and high resolution;
 DC motors are suitable for cost-efficient, dynamic
applications; ActiveDrive DC motors offer motor performances thatexceed the controller performance, and thus enablehigher velocities;
 Gear motors provide low speed, high torque, highresolution, as well as additional holding forces in verticalapplications; Brushless DC motors and synchronous servo motors are
suitable for industrial applications and offer highdynamics and continuous operation;
 Torque motors used as direct drives for industrial
applications offer high torque, zero-play, high dynamics
and continuous operation, and are mainly found inrotation stages.

The characteristics and specifications of a positioning system
are not only influenced by the choice of a suitable motor but
also depend on a number of other factors. They also have to
be considered when designing a precision positioning
solution: Drive screw Guide Encoder model Encoder type
 Encoder interface
 Limit and reference point switch type
 Cable management
 Ambient conditions
 …
The following table 1 compares the most important motor
characteristics. Table 2 gives an overview of the motorized
standard products.

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

Page 13 of 17

WHITEPAPER – Rotating Electric Motors for Precision Positioning-- Dr. Nico Bolse --10.1 Comparison of motor types
Property

2SM

DC/PWM

BLDC/SSVM

TQM

Velocity

The velocity depends on theswitching frequency. Typically, 20
to 30 revolutions/second

Proportional to the voltage,
higher as with 2SM. Higher
velocities with ActiveDrive.
Typically, 50
revolutions/second (limited bydrive screw and noise)

Proportional to the voltage, higher
as with DC. Typically, 60
revolutions/second (limited by drive
screw)High velocity stability

Smooth running

Possibly vibrations at highvelocity, smooth running throughmechanical damper and sinusoidal
commutation

Quieter than 2SM

Sinusoidal commutation for very smooth running (typical in ACS controllers, PI C891 controller, and SMC Hydra)

Resolution /Precision

Position information without
encoder, high position stability,
very high resolution in
microstepping mode

Requires encoder for positioning information, resolution depends on the encoder, position jitter caused by the control
loop

TorqueLow torque in microstepping
mode, high torque at low speeds

Proportional to the current

High, determined by the number of polepairs

Self-locking

High holding force, self-locking
only at a full-step (loaddependent)

Low, can only be increased by drive gear and precision drive screw

Not available

Lifetime

Higher than DC, comparable to
BLDC/SSVM

Limited by brushes, typically
1,000-5,000 hours

Limited by bearings, typically several 10,000 hours, comparable with magnetic
linear motors

Miscellaneous

Less expensive than DC, long
lifetime, reliable, position error atstep loss

Less expensive than
BLDC/SSVM, lower control
effort required, high dynamics
and smooth running, low heatbuild-up

Low maintenance, efficient and low
noise, light, high reliability anddynamics, low heat build-up and few
vibrations, suitable for industrial
applications

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

Page 14 of 17

Direct drive, high drive rigidity, highrepeatability, high torques, play-free,hollow shaft, flat, suitable for industrial
applications

WHITEPAPER – Rotating Electric Motors for Precision Positioning-- Dr. Nico Bolse --10.2 Standard products overview
Type

Product family

DCLinear actuator (lowprecision)

L-402

x

Linear actuator (highprecision)

M-110 / M-111 /
M-112

DC-G

PWM

BLDC

SSVM

TQM

2SM

x

L-406

x

x

x

L-408

x

x

x

VT-80

x

x

M-406

x

x

M-403

x

x

M-404

x

x

M-413

x

x

M-414

x

x

L-412

x

L-417

x
x

L-505

x

M-122.2DD1

x

x
x

x

MTS-65xx

M-105 / M-106

x

L-509

x

x

x

x

x

L-511

x

x

x

x

x

x

x

x

M-511 / M-521 /M-531

Z Stages

HPS-170

x

x

L-306

x

x

L-310

x

M-501

XY stages

Rotation stages

x
x

x

x

x

UPL-120

x

x

L-731

x

x

L-738

x

x

L-741

x

x

M-116
RS-40

ManualxxLS-180

2SM-G

x
x

x

DT-34

x

x

DT-80

x

x

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

Page 15 of 17

WHITEPAPER – Rotating Electric Motors for Precision Positioning-- Dr. Nico Bolse --Type

Product familyDC

M-060 / M-061 /
M-062
L-611

Linear actor (low
load)

PWM

x

x

x

PRS-200

Goniometer

DC-G

x

BLDC

TQM

x

2SM

x

UPR-120

x

WT-85

x

x

WT-100

x

x

WT-90

x

x

WT-120

x

x
x

M-227

x

x

x

M-230

x

M-232

x

M-235

x

L-239

x

M-238

Manualx

UPR-100

L-220

2SM-G

x

x

M-228 / M-229

Linear actor (high
load)

SSVM

x

x
x

x

x

x

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

Page 16 of 17

WHITEPAPER – Rotating Electric Motors for Precision Positioning-- Dr. Nico Bolse --11 Author

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

WP4016R1 Rotating Electric Motors for Precision Positioning 03/2020. Subject to change. © Physik Instrumente (PI) GmbH & Co. KG 201812 About PI
Well known for the high quality of its products, PI (Physik
Instrumente) 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 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 new
requirements.
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 make 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 Roemerstrasse 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

Page 17 of 17