Glossary Positioning Systems

Partl II - Mechanics 

Aluminium 

Aluminium is a low-density material, resistant against cold flow, offers a favourable ratio between rigidity and weight as well as good thermal conductivity. Therefore, aluminium is especially suited for applications featuring thermal gradient, or those ones in which rapid adaptation to temperature variation is required. Aluminium is easy to machine. Raw material is available in different shapes and sizes making it cut out for components incorporated in measuring tables. Even with untreated surface, there is no interfering corrosion in normal environment. Anodised aluminium has a surface, which is very resistant to wear. Anodised surfaces, however, show a porous texture and thus, are not suited for vacuum applications.

Surface texture of aluminium 

Anodised aluminium features a surface being resistant against corrosion and mechanical impacts. Most optical devices are black surfaced to reduce dazzling. Anodising makes the surface harder, and improves resistance against scratches and wear.

Actuators 

The actuators described herein are characterised by a slide, which is traversed through moving in and moving out of a shaft. This shaft, in turn, is driven by an electro-magnetic unit. To minimise backlash and hysteresis, in many cases the slide is pre-loaded against the drive shaft by spring.

Rotating shaft 

In many actuators the torque of the motor is utilised for rotating and moving out a screw. With this method, the actuator centre point is rotating on the surface of the slide causing problems on the centre such as eccentricity and unevenness, which can be minimised by rounding the actuator centre point, that means, reducing the contact area. One of the advantages of a rotating shaft drive is its compact profile size. In the tables made by Feinmess Dresden GmbH this advantages has been realised through a special

Non-rotating shaft 

Actuators such as piezo-based ones and electrostrictive units or even actuators featuring a special shaft design are the better solution.
Despite piezo-based drives are excellently suited for micro positioning, their repetitive accuracy is rather poor due to the high hysteresis and long-time drift.
Consequently, electrostrictive drives have been developed which assure fine positioning even with far less hysteresis and drift. The minimum increment of movement, which can be obtained, is 5 nm in these drive assemblies.
Both electrostrictive and piezo-electric materials work according to the same principle. Depending on the applied voltage, the materials expand or contract. The electrostrictive drive, however, makes use of a stack of PMN-crystals whilst the piezo-electric one relies on a stack of lead-zirconate-titanate crystals (PZT). The PMN-stack consists of many single layers whose thickness varies from about 120 µm to approximately 250 µm, which are connected to each other by diffusion bonding. The total displacement results from the deformation of the single layers.
As regards PMN-materials, the length variation behaves proportionally to the square of the applied voltage, and also in the same magnitude as with PZT-materials. In contrast to piezo-electric materials, the PMN-ceramics are not poled. Positive and negative voltage variations make the material expanding in direction of the electrical field - independently of the polarity. As PMS is not poled, the material is considerably more stable and not subject to long-time drift.

Drives, manual 

Manual drives mostly consist of a fine-pitch adjusting screw with knurled rotating handle knob. The setting accuracy depends on the screw pitch. All manual drives manufactured by Feinmess Dresden GmbH deliver an adjusting accuracy of 1 µm. In most drives, the diameter of the handle knob is 30 mm while the screw pitch amounts to 1 mm. The real adjusting accuracy of 0.1 µm is obtained by reduction gears.

Drive systems 

The following drives are usually built in linear and rotary precision positioning assemblies: Screw, ballscrew and worm gearing. Axis couplings and gear trains are mounted between motor and drive, that means, components which have an impact on the dynamics of the system (for example, speed, load capacity, backlash, and stiffness of the drive assembly). The gears often are integrated inside of the motor. The design of the piezo-ceramic motor, however, is different as herein only a ceramic gib represents the core part.

Expansion, thermal 

Temperature variations cause alterations of size and shape of components. The alteration depends on the size of the component, construction of the product (bi-metal effect), temperature variation and applied material. The length alteration follows the below formula:
ΔL = αLΔT
α = Material-specific thermal expansion coefficient.

Absolute Encoder 

Absolute encoders are equipped with several detectors and a number of black-white-sample tracks. Each encoder position is assigned a specific binary signal to, that means, the axis position is picked up on an absolute basis. Absolute encoders deliver the absolute position value directly after powering on or failure.

Incremental Encoder 

Applications that demand high resolution or relative few pieces of information only, incremental encoders are the better choice for. The electronic output signals are produced by the movement of a scale or disc featuring a graduation.
Many incremental encoders generate a so-called index pulse. In rotary encoders, this pulse is output once every turn marking an absolute mechanical reference position of the 360° circular graduation, and assigned exactly to one measuring step. The index pulse links the relative co-ordinates of the respective device to the absolute system co-ordinates.

Linear encoder 

Linear encoders are used if direct control of the positioning accuracy, resolution and repetitive accuracy are the preferences. In this way, for example, variations in mechanical measuring tables caused by backlash, hysteresis and screw pitch error can be compensated for (see also "Length measuring system").

Optical Encoder 

Optical encoders are generally applied as rotary or linear encoders to detect movements and positions. A disc or straight scale, respectively, having transparent and non-transparent sections is moved between a light source and detector while the light beam is interrupted. From the varying light intensity the actual position is derived from. The electrical signals generated in this way are transmitted to a controller, which computes position and speed from the received signals. Regarding optical encoders, absolute and incremental ones are distinguished.

Quadrature-Encoder 

Quadrature encoders represent a special type of incremental encoders supplying at least two output signals, frequently designated as channel ‘A‘ and ‘B‘. The second channel is for detecting the direction of movement. This capability comes into effect when the encoder rotation stops at the flank of a pulse. Without this direction detection, counters can only pick up the ascending signal flank and loose position.
Further advantage of the quadrature signals:
The number of pulses per revolution can be electronically increased. In 1-cyle mode, the pulses are generated on channel ‘A‘ through the ascending flank. In 2-cycle mode, the ascending and descending flanks are used on channel ‘A‘ as well. In 4-cycle mode, however, the ascending and descending flanks are used on channel ‘A‘ as well as channel ‘B‘, that means, the resolution gets quadrupled. Encoders basing on sine-shaped output signals, a very high resolution can be achieved by interpolation.

Guide-ways 

Guide-ways, as a rule, enable smooth rotational or linear movements generating little friction only between two bodies. Guide-ways are characterised either by friction or anti-friction movement. The load-carrying capacity as well as guiding accuracy of a rotary or linear measuring table are primarily determined by the type of guide-way.

Cross roller bearings 

Cross roller bearings do not only offer all named advantages of ball-based guide-ways but also improved load-carrying capacity and rigidity due to the point contact between ball and guide-way as well as line contact between cylindrical rollers and guide-way.
Cross roller bearings require higher manufacturing expenditure. They are built in those applications in which maximum stability, rigidity and resistance to wear have absolute priority.

Ball bearings 

In ball guide-ways the sliding friction is replaced by rolling friction, which in turn, reduces the friction as such. The balls are kept in place between Vee-guide-grooves made of hardened steel. The play is eliminated by pressing the guide-ways from outside onto the balls, the so-called 'pre-load'. Even with such pre-load applied friction remains so low enabling smooth-running and jerky-free positioning travels.
Ball guide-ways are relatively insensitive to foreign particles as ball and guide-way touch in a single point. Any foreign matter is pushed aside.
The load-carrying capacity of Vee-shaped ball guide-ways is lower than the one of cross-axis roller guide-way as the contact area to absorb the load is smaller. To achieve same load carrying capacity in ball guide-ways, the balls have to be bigger or greater in number.
Considering Gothic-arc guide-ways, the shape matches far better to the form of the ball radii, that means, smaller balls can be inserted as with straight guide-ways. The arc-shape is very close to the form of a Vee-groove, and the load is split into two components under an angle of 45°.

Gearing 

Gear transmissions are used for transferring rotational movements and torque. In many applications, gears are installed to reduce speeds or to achieve higher resolution, which are difficult to be obtained, or even cannot be generated at all by standard motors. When looking at a reduction gearbox with a ratio of 10:1, then ten revolutions of the driving shaft make the driven shaft turning by 1 revolution. A 200-stepping motor could, combined with that gear transmission, obtain an effective resolution of 2000 pulses per turn.
Gears featuring speed increasing or reducing ratios change both the output torque and output speed as a screw pitch does. Considering the above given example, the torque is increased and speed decreased.

Lead screw 

To move loads, the axial linear movement of a nut located on a rotating screw is made use of. Screws are highlighted by their self-locking ability, efficient price, simple manufacture and large variety of materials they can be made of (see also "Ballscrews").

DC and steeping motors (operating principle) 

DC and stepping motors are fundamentally distinguished by their functional principle. Applying a voltage to a dc-motor both torque and rotation are produced, whilst in a stepping motor the torque is the only parameter generated. In order to get the stepping motor rotating the current has to be commutated.
Commutation means the inversion of the current flow direction of the coil while the motor takes a single turn. A brush-type dc-motor is self-commutating whilst the stepping motor cannot automatically commutate.
There are three types of stepping motors:

• Permanent magnet motors,
• variable reluctance motors and
• hybrid motors.

Brush-type dc-motors 

A brush-type dc-motor is basing on a cylindrical rotor whose coils are perpendicularly arranged related to its cylinder axis. Interactions between the coils and magnetic field of the stator put the rotor into rotation when applying voltage. In order to generate a smooth-running continuous movement at constant speed, the force and alignment of the rotor and stator fields must be maintained constant. For that reason, several coils are arranged on the rotor, which are electrically commutated through two brushes.
Smooth running at high speed is one of the main characteristics of dc-motors. A dc-motor keeps on running as long as current flows through its coils. For precise and reliable positioning an encoder has to be attached for position feedback.

Granite 

Due to its outstanding physical properties and progress made in machining technologies, granite structures are extra-ordinarily well suited for basic constructions. In many cases, flatness of the surface exerts essential impact on both guiding and repetitive accuracy of the entire system. Polished granite surface finish of such structures, typically at ± 15 µm per square metre, is considered the most plane surfaces available on the market. Geometrically perfect surface finish is obtained by hand lapping as the surface is substantially more plane. Such surface quality can never be achieved by machines.
Extreme hardness is another important property of granite to get the granite lapped to very close tolerances. Studies have revealed that granite has higher resistance to wear and shock than steel does have. Referring to its special strength, granite is excellently suitable for big systems absorbing high static loads. In addition, granite is non-magnetic and thus, predestined for electron-beam applications and moreover, does react with few chemicals only.
By reason of its extra-ordinary dimensional stability granite is used for precision structures. Due to missing inner stress granite behaves uniformly and calculable under temperature variation. The high mass imparts granite high thermal inertia making experiments and other processes hardly reacting to rapid temperature variations. For applications, which do not require extreme flatness, steel constructions are the better alternative due to their more favourable ratio between mass and stiffness. Completed with dampers, steel constructions can be optimised to match to defined frequencies.

Hybrid stepping motors 

Hybrid stepping motors unite the advantages of motors with variable reluctance with those of permanent magnets. These motors are equipped with multi-teeth stator poles and same rotor, offering big holding moment as well as excellent dynamic and static torque combined with high stepping rates.
As a rule, hybrid motors are equipped with two coils located on a stator pole making this pole working as north pole or south pole according to the direction of current. Hybrid motors available now-a-days cover a wide range of torque.

Instability of materials 

Instability describes the alteration of physical size over the time, also designated as cold ageing or creeping. Aluminium, brass or stainless steels are very resistant against such impacts.

Ball screws 

Ballscrews are, so to speak, screws equipped with a number of ball bearings and internal ball recirculation between screw and nut. The profile of the screw track has been rounded to provide for better adaptation to the recirculating balls. The main advantage of ballscrews is their higher degree of efficiency, that means, the ratio between utilisable energy and applied one. Other advantages of ballscrews are long life and low wear. As ballscrews have no self-locking capability brakes have to be attached where necessary to prevent returning.
In practically every application the ballscrew contributes its important part to the comprehensive performance and determines, to very large extent, the price of the final product as such ballscrew belongs generally to the most expensive purchased parts. Steinmeyer has gained excellent reputation in the field of ballscrew technology. Highly sophisticated production engineering in connection with years-long experience in this matter put Steinmeyer in a position to keep up pace as regards technical development and cost efficiency. Steinmeyer holds an unique position on the market for his flexibility and large-scale product line. The production program includes a standard range of miniature ballscrews in the diameter bandwidth from 3 mm to 16 mm as well as various standardised screw designs of nominal diameter from 16 to 100 mm. Due to the outstanding and innovative nut technology such as internal ball recirculation and patented pre-loading system UNILOCK, Steinmeyer is capable manufacturing nuts of minimum outer diameter for special applications. This is particularly true for those users whose drive assemblies have already been dimensioned.
Steinmeyer offers to his customers individual consultation and optimum solution for drive problems of all kinds.

Ball screw pre-load 

Steinmeyer produces ballscrews with four different modes of pre-load at option to be able covering all applications. Basically, pre-load is used to eliminate backlash out of the ballscrews as well as to improve stiffness. As pre-load has great impact on torque and actual lifetime these parameters are recommended to be specified very carefully.

Double nut 

Especially for miniature ballscrews Steinmeyer has developed the spring-force pre-loaded double nut. Through this kind of pre-load, lowest friction moments combined, at the same time, with practically entirely constant pre-load can be achieved. Technical details of the double nut series 1510/1530 can be taken from the Steinmeyer catalogue "Miniature ballscrews".
The Steinmeyer-make double nut completed with the UNILOCK pre-loading system is a patented development in the field of double nuts. This solution includes two nuts, which are positively clamped together to each other by an inside ring made of epoxy resin. In this way, pre-load can be steplessly adjusted without any spacer rings being needed.
UNILOCK makes manufacture of compact double nuts offering increased stiffness and improved precision possible. Pre-load can be adjusted in uncomplicated and precise manner, and keeps durable for long period of use. The Steinmeyer UNILOCK pre-loading system has proved in thousands of practical applications.

Single nut 

Single nuts are deliverable with axial play or pre-load set by oversize of the balls (without axial play). The play-free single-nut design can be the optimum solution in those cases when the screw features large pitch, but only little space to build in a double nut is available. Same fits to special demands for positioning accuracy in ultra-precise applications in which the single nut with its four-point contact pattern often offers advantages. Steinmeyer has perfected the technology of pre-loaded single nut designs. Running qualities have become reality, which stand every comparison to those achieved by double nuts.
When looking at improved efficiency and stiffness the two-point contact pattern provides optimum solutions. Two-point contact is created in the pitch-shift nut by axial pitch shift (limited enlargement of the pitch when the nut undergoes internal grinding).
This design is used when the available space is not wide enough to house a double nut. The technical data, however, correspond to those of an equivalent double nut.

Couplings, flexible 

Couplings transmit force and movement between two offset shafts. Generally, flexible couplings compensate for small offset and angular displacement of parallel axes. According to the wide variety of designs, these couplings compensate for more or less strong misalignment, and are the recommended solution when higher rigidity, load-carrying capacity, and speeds stand in the foreground.

Conductibility, thermal 

If the component is subject to irregularly distributed temperature variation, specific materials such as aluminium are recommended to be preferred. When the heat cannot be dissipated as quickly as required through the material itself, then considerable deformation due to the temperature gradient may occur. Deformation caused by non-uniform temperature variation behaves proportionally to the thermal expansion coefficient divided by the thermal conductibility
Relative thermal deformation = α / c

Brass 

Brass is a copper-based alloy characterised by higher density than steel. Brass can be easily be machined and is very resistant to wear. Often this material is applied as partition stuff to prevent cold welding between bolts and shafts made of ordinary steel or stainless one. Brass is also used in many high-precision applications in which an especially high resistance against cold flow is required. Additionally, brass is very suitable for those cases where an especially smooth surface finish desired.

Brass specification 

Compared with aluminium and steel, brass offers an unfavourable ration of rigidity to density. The thermal expansion coefficient behaves similarly to the one of aluminium, the thermal conductibility, however, is only half that big.

Surface texture - brass 

For applications in optical engineering laboratories brass often gets black finished. To improve durability brass can be coated with chromium or nickel.

Motor characteristics 

Permanent-magnet motors 

Permanent-magnet motors are basing on a permanent-magnet rotor whose field is aligned orthogonally to the axis or rotation. Due to excitement in four subsequent fields the rotor is put into rotation by the changing magnetic field. As typical characteristic, permanent-magnet motors work with step angles of 45° and 90°. Their stepping rate is relatively small but the torque high and damping property excellent.

Variable reluctance motors (VR-motor) 

Other than permanent-magnet motors, the variable reluctance ones are highlighted by a multi-teeth rotor whereby each tooth represents a separate magnet. With the motor at rest, the magnets change to inherent holding positions to generate a stronger holding moment.

Piezo ceramic motor 

These drives combine the outstanding positional resolution of conventional piezo-based drive assemblies with the wide control ranges at high traversing rate. The drives are of extremely compact profile and enable space-saving installation. These units do not require mechanical intermediate elements such as gear transmissions as well as screws and thus, their usability is not restricted by backlash.
 
The drive consists of a stator component (mostly stationary) housing piezo-ceramic oscillating bars. These oscillators resiliently act onto an armature gib mounted on the moving part of the slide. The shaft-shaped piezo elements oscillate electrically excited in two superimposed vibration forms, which are longitudinal and bending vibrations. These vibrations are excited by a bi-modal natural frequency of 40 kHz creating an upright waveform inside of the bars. Superimposing these waveforms initiates micro-elliptic movements on the bar ends. As the bars are resting on the armature gib under mechanical pre-load they transfer a driving pulse onto this armature by friction. This procedure runs in half the cycle time (cycle time T=25 µs). Continuing the phase of movement, the contact bars return into their initial position without any force being applied. Due to the high frequency quasi a constant feed force acts onto the armature depending on the control voltage.
 
The control voltage applied to the motor driver module determines the vibration amplitude and consequently, the force, respectively, speed whilst the frequency as system magnitude remains unchanged. The speed depends on the applied load and drops nearly linear with the force.
 
The motors reach a maximum speed of more than 350 mm/s. In dead state the motor effects as brake applying a defined maximum holding force. The force range depends on the number of piezo oscillators.
 
To increase the force several motors can be arranged on a single axis, synchronously driven by a driver module. The piezo oscillators are tipped with sliding shoes made of hard ceramics running on lapped ceramic gibs serving as tracks. This tribological mating ensures long lasting operation over 20,000 hours at a load variation rate of 50% of continuous duty.
 
Rotary drives can be composed of either radial installation of the motors on the circumference of a cylinder (ceramic ring as track) or axial assembly acting on a ceramic disk.
 
The motors are characterised by absolutely missing interfering magnetic fields, and on the other side, are insensitive against external magnetic fields.
 
The motors of constructional series HR-1/8 succeed the ones of the series SP-1/8. The motors distinguish by the modified mechanism for setting the mechanical pre-load to essentially facilitate assembly and adjustment.

Position measurement, indirect and direct 

The place at which the feedback module takes measurement is of great importance as it has direct impact on the quality of the data sent to the control electronics. The closer the feedback module is located to the parameter to be checked the more exact are the date and the more precisely the desired position can be approached to. For example, when directly measuring the linear position of the measuring slide, the data fed back will be more precise as if the angle position of the driving screw would be picked up, and the position of the measuring slide calculated. The first method is called "Direct measurement" and eliminates inaccuracies emerging in the drive chain such as backlash, hysteresis and distortion which may occur with indirect measurement. Stages manufactured by Feinmess Dresden GmbH are highlighted by very precise drive chains.

Reference and limit switch 

The reference switch defines a repetitively approachable reference point. Reference switches are available as mechanical execution (ON/OFF-switch) or optical element. If desired, precision limit switches can be installed for reference value determination.
 
Limit switches are used for limiting the traversing range, in many installations mounted at the end of the traverse directly before the mechanical limit stop. Limit switches cut off the power supply to the drive as soon as the limit position is reached. Normally, limit switches are installed in linear tables, but may also be assembled in rotary measuring tables to avoid cable twisting.
 
Limit switches are used for limiting the traversing range, in many installations mounted at the end of the traverse directly before the mechanical limit stop. Limit switches cut off the power supply to the drive as soon as the limit position is reached. Normally, limit switches are installed in linear stages, but may also be assembled in rotary measuring tables to avoid cable twisting.
 
As standard, the tables made by Feinmess Dresden GmbH are equipped with contactless electronic switches, which do not suffer from those disadvantages.

Clean room compatibility 

Feinmess Dresden GmbH disposes of that equipment required for preparation of the products for clean-room applications. A large number of methods, regulations, techniques and material specifications for clean-room applications are similar to the ones current for vacuum preparation. Nevertheless, the requirements direct to each individual application are different. Therefore address our product manager to discuss your specific requirements.

Transducer 

The main task of a transducer is to convert a physical parameter into an electrical signal to be evaluated by the control electronics. In many applications, encoders are used for position detection and transmission, tachometers for speed feedback, and acceleration pickups for acceleration transduction.

Worm gearing 

The worm gearing is basing on meshing of a worm with a worm wheel to change a rotation into an opposite rotation. Rotating the worm makes the worm wheel rotating as well due to the teeth of the worm wheel meshing with the spiral of the worm. The worm gearing drive provides for higher speeds and load transmission compared with direct drive assemblies.

Stepping motors 

Stepping motors operate according to the principle of magnetic attraction and repulsion, converting electrical pulses into mechanical axis rotation. The angle of rotation is proportional to the number of input signals whilst the speed depends on the pulse frequency. For stepping motors, a permanent magnet and/or an iron rotor as well as stator are the characteristic components. The torque necessary to make the stepping motor rotating is generated through commutation.

Steel 

Steel owns a high coefficient of elasticity, and is characterised by high stiffness (three times higher than aluminium) and stability. The thermal expansion is only half the one of aluminium. Thus steel is very suited for an environment subject to uniform temperature variation. Stainless steel can be used in vacuum applications without any problem.

Specification Steell 

The machining of steel requires considerably more expenditure compared to aluminium. Thus components made of steel are more expensive than the ones made of aluminium. Alloyed steels are resistant to corrosion.

Surface texture steel 

Construction components made of steel are, in many cases, coated or painted. Mostly chromium, nickel, paints or powders are used for coating. Bolts and assembly parts are often black finished to improve resistance to corrosion. Alloyed stainless steels require nearly no protective coating.

Stiffness 

Stiffness describes that force to be applied to deform a body. The ratio of force and deformation remains constant over a wide area, and can be explained with the following equation:

"F" is the force, "x" the deformation and "k" a material-dependant constant quantity. The higher the value for "k" the more stiff is the material.

Tachometer 

For applications requiring speed control, the speed can be measured either directly or through position information supplied from the encoder. Tachometers help to precisely control the speed while generating a voltage or current behaving proportionally to the motor speed. The tachometer feedback immediately indicates any speed variation giving the chance to rapidly correct and precisely control the speed via the control electronics.

Vacuum preparation 

All measuring tables output by Feinmess Dresden GmbH can be used without any preparatory expenditure in vacuum applications down to 10^-2 Torr.
On request, Feinmess Dresden GmbH is able preparing all measuring tables equipped with stepping motors for full-step mode or with piezo ceramic motor for vacuum applications down to 10^-6 Torr. Special driver modules, which are compatible to our controllers, are available to move these measuring tables.

Vacuum compatibility 

Measuring tables, which are applied from 10^-6 Torr, need special preparation. Many materials, which are used in standard applications, evolve gas in a high-vacuum environment. This "virtual leak" makes it very difficult to create and maintain the desired vacuum.
 
The techniques applied by Feinmess Dresden GmbH to prepare tables for vacuum environment ensure that our products will operate without any problem down to 10^-6 Torr, and contaminate the vacuum environment to very small extent only.  
For appropriate preparation, however, the ambient and working conditions have to be known. This is particularly true for the operating pressure, but also the admissible level of gas evolution, loss of mass and condensation may vary according to application, pump capacity, temperature, and the like.
 
These materials are especially selected for vacuum applications, that means, suitable metals, ceramics, coatings, lubricants, glues, rubbers, synthetics, electrical components, etc. Porous anodised aluminium surfaces, for instance, contain numerous air molecules causing considerable gas evolution. Thus only non-anodised aluminium is used in high-vacuum applications. Same fits to motors, which have to be especially prepared for vacuum operation. When producing motors, care has to be taken that small surface areas are created only, which are capable easily absorbing gases and other foreign matters, and to release them when working under vacuum environmental conditions. Furthermore, it has to be made sure that gas has not been occluded in hollow assembly spaces. Special rules have to be complied with material selection, manufacture, cleaning, handling, assembly and packing. In order to minimise contamination from the air these operations are carried out in clean environment.
 
For example, heat is poorly dissipated only, therefore the permanent operation duty of the motor has to be shortened, which in turn, may limit the maximum speed. If for your application vacuum preparation is necessary then please address to our contacts to discuss the specific requirements resulting from your application.