BECKHOFF AL2000 User Manual

Beckhoff Drive Technology

Designing your Application
with AL2000

Last change: 08.11.2001

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Contents

Designing your Application with AL2000

1. Introduction 5

2. System considerations 6

Power Supply 6
Protective Earth 7
Heat dissipation 8
Mounting frame: solidity and stability 9
Positioning 10
Measurement unit 11

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Servo-controller 12
Accuracy 13
Braking 14
Contamination 15
Cables 16
Bearings 17
Vertical applications 18
Coupling of Coil Units 19

3. Linear motor dimensioning 20

Introduction 20
Physical background 21
Used symbols 22
Used formulas 23
The case 25
Dimensioning force and speed 26

Step 1: Worst case cycle 26
Step 2: Kinematic analysis 28
Step 3: Force analysis 29
Step 4: Motor sizing 30
Step 5: Amplifier sizing 31

4. Appendix 32

Heat transfer and temperature 32
Influences of accuracy 34

5. Technical data 35

List of specifications AL series 35

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Peak force - velocity diagrams 36
DC link diagram 38
Technical data of some optical linear rulers 39

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1. Introduction

With the series AL2000 Beckhoff Industrie Elektronik introduced a wide, standard range of high quality linear
motors. Due to a far-reaching standardization it is possible for designers to select the linear motors themselves.
However specialized knowledge is required for making the right design decisions. This knowledge is provided
here step by step departing form a practical case.

A linear motor of Beckhoff Industrie Elektronik is not a system in itsself. Usually a linear motor is build within a
total machine concept or a working unit. Depending on the application choices have to be made concerning the
specifications and the sizing of the motorsystem. To assure a faultless operation all the components of the mo­torsystem must comply to strict requirements. In this document the relevant choices and requirements are dis­cussed step by step.

Several stages can be distinguished in a motor system analysis and design proces. In this leaflet you will be
guided through these stages. First of all some system considerations have to be made. These considerations
will provide you with relevant practical information, for instance about the powersupply, heat dissipation, stabil­ity, accuracy and braking. As a designer of linear motor systems one should be familiar with some theoretical
physical laws and formulas. This information will be provided together with the step by step analysis and design
proces. Special attention is given to mechanics.

This leaflet is the second of a series of three, concerning Beckhoff Industrie Elektronik AL2000 series Linear
Motors. The series consists of the following titles:

1. A Primer of Linear Motors

2. Designing your Application with AL2000 series Linear Motors

3. Installing AL2000 series Linear Motors

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2. System considerations

Power Supply

The performance of a linear motor is depending on the powersupply. Therefore a linear motor is specified for an
appropriate voltage. The motor’s servo-amplifier can be connected to different voltage power supplies: 230 to
480V, one- or three phase. By means of capacitors and a rectifier bridge the power is transformed in a DC volt­age link. For high forces and velocities a DC link is required of at least 560 V. For limited forces and velocities a
310 V DC link will do. For more information about the DC voltage link and the powersupply - motorforce relatio,
please see the appendix - ’Peak force - velocity diagrams’ and - ’DC Link diagram’

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Protective Earth (PE)

To prevent hazardous situations in case of an electrical failure, all metal components must be earthed. The
coilunit housing and ironcore are earthened by the PE of the motor’s power cable. The cable shield is con­nected to the housing but is not appropriate as PE. Earthen the cable shield on the servo-amplifier to prevent
EMC problems. Follow instructions of the servo-amplifier.

The magnet plates which are bolted to the frame, have to be electrically connected to the frame through the
bolts. The stainless cover of the magnetplate is earthened through the magnetplates. The earthing has to be
checked according to demands with respect to electrical machine safety.

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Heat dissipation

Heat dissipation is a very important but difficult item. Every linear motor produces heat. The heat will mainly be
dissipated in the coil unit. There are two aspects to consider:

1. This heat must flow away to ambient. If possible, the route of the heat conduction should be traced.

2. Heat generation causes temperature differences. That can be unacceptable for accuracy or other reasons.
For your system, especially for the coil unit, the allowed temperature increase should be determined.

The coil unit is fitted with a temperature sensor. In some designs the coil unit is not capable enough of transfer­ring the heat to the surrounding air. This counts especially for heavy loaded motors with a high ratio between
motorforce and coil unit size. Also when the motor is thermally isolated or when ventilation is prevented by a
hood. In these cases active cooling, like water cooling, is needed.

Without cooling an unacceptable heat up of the coil unit could occur. This could result in lower performance,
thermic safety stops and even damage to your motor system. Water-cooling of the motor is very effective to
reduce the heatflow and to obtain a constant temperature of the body. Water-cooling requires a water condition­ing unit containing at least a pump and a cooler. Beware of leaking. The temperature sensor can be used to
detect failures of a watercooling system. An additional flow-sensor for controlling the cooling is strictly not nec­essary, but yields extra safety and system information. If heat up of particular parts of the system is critical, a
thermal insulation between the coil unit and the critical parts of the slide can be considered. This can reduce,
but not nullify, the temperature rise. Sometimes small heat up also takes place in the magnet plates.

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Mounting frame: solidity and stability

The propelling forces of a linear motor are relatively high. Therefore the frame needs sufficient dynamic stiff­ness. Because of the required accuracy the frame should be insensible to shocks and vibrations.

A linear motor system gaines its accuracy by means of a high bandwidth feedback control loop. In this loop all
mechanical parts such as load, frame and mountings are involved as well as the characteristics of the servo­controller and the linear encoder. The loop can be compromised by the characteristics of the construction. A
bad construction can even cause total loss of control of the linear motor system.

Attention should be paid to the machine’s natural frequencies. Especially vibrations between 50 - 500 Hz in
driving direction can be harmful for accuracy. To meet main problems a rule of thumb is: the motorsystem
should be rigidly connected to a massive and rigid body of at least 3 times the mass of the accelerated load.

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Positioning

LM applications require a sophisticated position and velocity feedback. A linear encoder and a servocontroller
are taken up in the positioning system. The position of the slide is detected by a measurement unit, a ruler­probe combination. The unit’s linear encoder returns the information to the servocontroller. The accuracy of the
motor system depends strongly on this positioning system.

Most measurement units return incremental position information. So the linear motor has to do without the
absolute position of the slide. Especially when starting a motor operation this could be problematical. Herefore
the slide is activated to some minimal testmovement. This ‘magnetic alignment’ supplies the positioning system
with the required information.

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Measurement unit

The series AL2000 motors use a wireless linear encoder for communication, speed and position control. The
performance of the linear motor depends on the characteristics of the applied linear encoder. The use of an
encoder system with a wire is not possible due to the bad dynamic characteristics. The unit is to be connected
and shielded with care. Any disturbance of the positioning signal could lead to positioning failures and system
oscillations.

Several types of measurement units can be applied, such as encoder kits by Heidenhain, Renishaw, Siko and
Numerik Jena. Mostly the resolution should lie between 0.1 and 5 µm. The accuracy of the measurement unit
must at least be better than the required accuracy of the motor system. Depending on the dynamics of the ap­plication it can be a factor 2 to 10.

The position of a moving body is measured with respect to the frame. Depending on its stiffness the frame acts
more or less as a reaction force body. In the measurement appearing rotations or vibrations should be dis­garded. Therefore the mounting of the measurement unit is critical. The probe should be mounted as close as
possible to the mass centre of the complete moving unit (slide + load), whereas the ruler needs to be placed
near the centroid of the frame. The presence of rotations or vibrations is surmountable but their measurement
and feed back should be avoided.

In exceptional cases stick or slip effects are present. If so the measurement can be placed best between the
bodies between that cause these effects.

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Servo-controller

The position information as well as the current in the coils is fed back to a servocontroller. Here the information
is processed and translated into a proper inputsignal for the linear motor. Because the linear motor needs a
powerful input the signal is provided by a servo-amplifier.

The slide’s movement is directed by a three phase voltage pulse to the coil unit. The phase depends on the
actual and the desired position of the slide. Hereby the motion’s directing quantity is the pulse width. In fact the
servo-amplifier is a pulse width modulation amplifier with a fixed voltage and a fixed switching frequency.

AL2000 linear motors can ideally be combined with our servo-controller series AX2000. With prepared motor-,
feedback- and thermal protection cables we offer a complete and flexible linear motor system.