Technical Information

The following section explains the most important technical terms in precision positioning technology in order to make it easier for you to select the optimum positioning system for your needs.

Accuracy

The accuracy of a linear positioning system can be divided into two categories:

1.The accuracy of the path itself, referred to in technical terms as guide accuracy.

2.The linear positioning accuracy along the path.

The former is a property of the guide system (guide, ball-bearings, crossed-roller bearings, etc.) and characterizes the degree of accuracy with which motion takes place in a defined direction. The latter refers to the conversion of drive into linear motion. The limiting influences are lead-screw and motor properties as well as encoder or other feedback mechanics.

Guide accuracy

Every object has six degrees of freedom (see Fig. 1). These are the linear motions along the three orthogonal axes X, Y and Z as well as the rotations around these axes (θx, θy and θz). The task of a linear guide is to restrict the motion of an object precisely to one of these axes (typically referred to as X-axis). Any deviations from rectilinear motion along the X-axis are the consequences of inaccuracy in the guide system.

Five possible guide errors remain: motion in the X-direc-tion, motion in the Z-direction, rotation around the X-axis (roll), rotation around the Y-axis (pitch) and rotation around the Z-axis (yaw)(see Fig. 2).

With linear positioning elements, the mainly linear motion arises from the interaction of a large number of points of support along the length of the guide. That is why any deviation of the individual guide from a straight line results both in a translational error and an angle error. As the average straightness error in the individual guide is generally extremely small, the angular errors can be neglected for most purposes. On the other hand, even small angular errors do add up over large guide lengths to considerable linearity deviations. The combi-nation of pitch and displacement in the Z-direction yields the flatness and the combination of yaw and displace-ment in the Y-direction yields the straightness (see Fig. 2).

Positioning Accuracy

Most linear positioning systems use leadscrews to transform the rotation of a motor into linear motion. The quality of the leadscrew determines the accuracy of the positioning system. The leadscrew nut should be free from backlash which will reveal itself upon direction reversal.

Accuracy analysis of the leadsrew error distinguishes between the cumulative error over the path and the periodic error which occurs with leadscrew rotation.

High positioning accuracy is attained using interferome-tric measurement, compensation data determination and online axis compensation. In practice, however, control via encoder position feedback has proved effective.

We offer two types of encoders: linear and rotary. Linear encoders eliminate a number of error sources such as leadscrew error. The accuracy of the positioning system is determined in this case by the accuracy of the encoder. Rotary encoders require considerably less space and can therefore be more easily integrated into the positioning elements, and for that reason we use them as standard encoders.

Thermal expansion is a main source of error in high-precision positioning systems. Every degree of tempe-rature difference to the desired temperature (usually 20 °C) results in an error of approx. 12 · 10-5 due solely to thermal expansion. The thermal expansion of the positio-ning element itself must equally be taken into account in order to make use of its high accuracy. When using linear encoders, the thermal expansion of the encoder is the element that determines accuracy.

Reproducibility

The reproducibility of a positioning system refers to the range in which the actual position varies when a particular commanded position is to be approached. We distinguish between unidirectional reproducibility, where a position is always approached from the same direction, and bidirectional reproducibility, which refers to the approach to a position from any direction. As system-inherent inaccuracies such as nut backlash are often not recognized with unidirectional reproducibility, bidirectional reproducibility should always be considered when there is no guarantee that the application allows a unidirectional approach to the commanded position.

A system with high reproducibility shows very little range of variation for motion to a given position, no matter which direction the approach is from.

It is important to make a clear distinction between accu-racy and reproducibility. Figure 3 shows the difference between reproducibility and accuracy

Resolution

The resolution of a positioning system is defined as the smallest position increment produced by an encoder or other feedback device. The mechanical positioning element, the drive and in some cases an electronic controller determine together the total resolution of a positioning system. In stepping motor systems, the resolution is determined for example by the leadscrew pitch, the step angle of the motor and the driver electronics.