Glossary Positioning Systems
Part I - Fundamentals
Absolute accuracy
The absolute accuracy is defined as the deviation between the actual position and the desired one (real designation: Inaccuracy). If a system is to move by 100 mm but the system really only moves by 99.99 mm (measured through an ideal scale), then the inaccuracy is 0.01 mm. The permanent position-ing error along an axis is designated as accuracy, after all the other linear deviations have been eliminated. Linear (or systematic) deviations are, for example, cosine deviation, screw pitch error, angular error at the measuring point, and deviation due to thermal expansion. In graphic representation, these deviations are shown in a protocol of position and deviation by the slope of a compen-sating straight line. Referring to the slope of this straight line the abso-lute accuracy can be approximated by calculation in the following way:
Absolute accuracy =Accuracy along the axis of movement + slope x traverse
In positioning units equipped with, for example, glass scales, the slope is close to zero, and the absolute accuracy equals the accuracy along the axis of movement. The stated values apply only under the condi-tion of a clamping area according to DIN 876/00 and a mounting screw torque of 0.15 Nm (at PMT 160 two center screws).
Resolution
Resolution means the minimum encoder count the system is able to detect. Resolution is also called “resolution of the encoder”, and normally determined by the encoder specification. Due to variations in the drive assembly, such as hysteresis, backlash and distortion, the minimum step width in most systems is higher than the resolution.
Permissible load
For linear tables, the load is the admissible force resulting from a mass applied onto the axis of movement and on the middle of the slide. In rotary measuring tables, the direction of the admissible force acts along the axis of rotation. The load of measuring tables depends primarily on the design of the guideways and preload.
Axial load
Axial load is the maximum load being applied along the drive train. Special attention must be given for tables mounted vertically. As a rule, the axial load depends on the load-carrying capacity of motor and drive screw.
Dynamic load
Dynamic load is the sum of all static loads and the dynamic resistance to movement. Dynamic features such as friction and inertia have to be considered as well.
Lateral load
This term characterises the maximum load acting rectangularly onto the axis of movement along the slide surface. The lateral load is also determined by the load-carrying capacity of the guide-ways and in most cases, equals the vertical load.
Vertical load
Vertical load is the maximum load permitted to be applied centrically on the moving slide.
Acceleration
Speed variation per unit time.
Acceleration = Speed / time (a = v / t)
DC-Motor
A dc-motor (direct current motor) is put into rotation by applying direct voltage. In contrast to the stepping motor, the dc-motor does not need to be equipped with an external commutation device. The rotation is maintained by automatic energising of the rotor windings through the commutator. The power is supplied via brushes. In unloaded state, the speed of the dc-motor is nearly proportional to the applied voltage. As the dc-motor does not rotate through defined steps, an incremental linear encoder or rotary one is required for position feedback.
Control sensitivity
This parameter stands for the minimum control quantity, which initiates movement, also described as the ratio of resulting movement and causing drive.
The guiding error
The guiding error is the linear portion of a deviation from the axis of movement, and composed of two orthogonal components: straightness (deviation inside of the plane of the slide) and flatness (deviation outside of the plane of the slide).
Guiding accuracy
Due to inaccuracies in the guideways (for example, tolerances of the rolling elements) the top surface of a linear stage does not move along an ideal straight line. Typical deviations are the straightness runout Gz and flatness runout Gy. These errors are also defined as absolute and relative deviations. The relative deviation is the deviation related to the medium straight line (compensating straight line). Error of straightness Gz (yawing). Error of flatness Gy (pitch).
Accuracy
Accuracy describes the expected deviation of the actual position from the desired one. Positioning accuracy depends, among others, on the measuring method applied for detecting the actual position.The given value is half peak to peak value.
Speed
Travel variation per unit time
Speed = Travel : time (v = s : t)
The actual speed depends on the screw pitch, motor speed and, if included, transmission gears.
Speed stability
Speed stability is the ability to keep a movement running at constant speed; this capability depends on the mechanical construction of the measuring table, control mechanisms, and control algorithms, encoders and motor selection.
Static friction
Static friction means that friction which must be overcome to move a body from its static position of rest. As the static friction is always great-er than the sliding friction, a higher force has to be applied to move a body from its position of rest than to keep it moving. Thus a body, at first, does not move when force is applied, but then starts moving with a nonlinear “jump” when forced above a non-reproducible threshold; this “jump”, however, can be compensated for by electronics (stick-slip effect).
Concentricity runout and error of drunkenness
For a rotary table, concentricity (eccentricity) defines the runout of the center of rotation from its middle position measured during a single turn. In a perfectly centered rotary measuring table there would be no eccentricity while rotating. Drunken-ness of a rotary measuring table is the angular runout of the axis of rotation measured over a single turn.
MTBF
MTBF or Mean Time Between Failures is the parameter the reliability of a measuring table is gauged with.
Nano technology
The syllable "Nano" (Greek: Gnome) characterises an order of dimension, which is 1000-times smaller than the structures of present components used in the micrometer technology (1 nm corresponds to the billionth part of a metre). The nano technology deals with systems whose components, due to their miniature structure (such as nm-thin layers, nm-smooth precision bodies, nano-based scaled single structures, materials made of nano-sized particles as well as analysis methods basing on nano-size scaled effects), enable creating entirely novel properties and functions for products and techniques.
Among scientists, R&D-results of nano technology have been discussed for years to be the key for future technologies and products. In order to profoundly understand effects in nanometer dimensions, knowledge from many specialised disciplines is necessary to create comprehensive solutions covering different industrial branches and research fields. Understanding of procedures running in atomic dimensions is the pre-requisite for product optimisation as well as development of new products and techniques.
Experts expect enormous progress in the areas of new resource-saving materials, intelligent surfaces, high-precision form bodies as well as ultra-dense data memories. Having complete control over analysis, production and structuring technologies in nm-scale as well as being able governing manipulations in the atomic and molecular fields are preconditions for long-term developments in vehicle, machine, optical and analytic engineering as well as chemical, biology environmental and medical device engineering industries.
Positioning accuracy (dx)
Positioning accuracy is the deviation of the really reached actual position XACTUAL from the desired rated one XRATED
Dx = XACTUAL - XRATED
The positioning accuracy, in contrast to the position resolution, represents the actual accuracy of the system, influenced mainly by the following criteria:
- Screw pitch error
- Bearing play
- Pitch error of the encoder
- Loads acting on the system
- Applied control system
Position resolution
The position resolution has been defined as minimum translational or encoder resolution.
Position stability
The position stability is the ability to keep a precise position constant over a longer period of time. The deviation from the stable position is called “Drift”. Drift occurs due to wear, lubricant displacement and temperature variation.
Precision
Precision describes that position range which 99.7% of the final positions over a number of repeated positioning movements are ranging in. Precision is also called "Repetitive accuracy".
Friction
Friction is defined as resistance between contacting surfaces during movement. Friction may be constant or speed dependent. Various factors have an impact on the total friction of a system, for example, sliding friction or wear, and lubricant viscosity.
Repeatability
Repeatability determines that area in which an actual position varies when a defined position is approached under identical conditions as often as desired. Unidirectional and bi-directional repeatability are distinct.
Rotary encoder
As standard, the dc-motors are equipped with a high-resolution electro-optical pulse transducer. Such transmitted-light, metal discplate orifice-based transducer generates two output signals phase-offset by 90°. Each turn of the encoder supplies, for example, 500 pulses. Electronic evaluation of both the signals delivers two outputs: The number of pulses is quadrupled to 2000 pulses per turn, on the one side, and the direction of movement is detected, on the other side. Given a screw pitch of 1 mm, each control pulse delivers a change of the rated position of 0.0005 mm, that means, a maximum resolution of 0.5 µm.
Step width, minimum
The minimum step width is the smallest encoder resolution a device is able to reliably pass. Do not confuse this parameter with the resolution of the display unit which may be considerably smaller than the real step width of the system!
Systematic errors of positioning systems
Each positioning table enjoys 5 degrees of freedom: Two straight- line movements along Y and Z as well as the rotation around these two axes and the positioning. The explained movements refer, in most cases, to a right-hand coordinate system in which each movement can be considered as composed of various straight-line movements along and /or rotation around the coordinate axis. Tables made by Feinmess Dresden GmbH are characterized by an extremely pre-cise movement, which means that the positioning table is prevented from moving in another direction while traversing along a desired straight-line direction of the axis of movement.
Tilting and drunkenness
Tilting and drunkenness are explained as that deviation from the axis of movement, which is measured in angular terms. Both errors are composed of three orthogonal components often designated as roll,pitch and yaw. These errors mostly impact the overall geometrical error of multiaxes positioning systems.
Backlash
Backlash is a positioning error occurring upon change of direction. Backlash can be caused by insufficiently preloaded thrust or inaccurate meshing of drive components, for example, gear teeth. In most cases, backlash occurs uniformly and can be compensated by installing suitable electronic equipment. Steinmeyer ball screws and ball screw assemblies are characterized by an extremely small backlash.
Inertia and inertia moment
Inertia and inertia moment represent the measure of resistance of a mass against a rotational velocity variation. The higher the inertia or inertia moment the greater is the force (torque) required for acceleration or deceleration of a load. The inertia (inertia moment) depends on the mass (and shape) of the load.
Repeatability
Repeatability determines that area in which an actual position varies when a defined position is approached under identical conditions as often as desired. Unidirectional and bi-directional repeatability are distinct.