Repeatability

The positioning system will be driven to an arbitrary position on the travel path. An external measurement system (laser interferometer, measurement probe) will record this position. The positioning system drives to the same position repeatedly from the same direction whilst the measurement system is running. The external measurement system repeatedly measures this position. After a stipulated number of repetitions of this process there is a range of measured values available for this position, from which the two-fold standard deviation can be calculated. This value represents the unidirectional repetition accuracy for the respective position on the travel path.

In order to achieve a higher level of safety with the determination of this value, the unidirectional repetition accuracy is determined at multiple positions and the largest value arising from this is quoted as the characteristic value for the positioning system.

The positioning system will be driven to an arbitrary position on the travel path. An external measurement system (laser interferometer, measurement probe) will record this position. The positioning system drives to this position from both directions whilst the measurement system is running. The external measurement system repeatedly measures this position. After a defined number of repetitions of this process there is a range of measured values available for this position, from which the two-fold standard deviation can be calculated. This value represents the bidirectional repetition accuracy for the respective position on the travel path.In order to achieve a higher level of safety with the determination of this value, the bidirectional repetition accuracy is determined at multiple positions and the largest value arising from this is quoted as the characteristic value for the positioning system. 

The repeatability (also referred to as reproducibility or repeat accuracy) describes the deviation with which a specified position can be achieved during a repeated movement. The repeatability is distinguished into bidirectional and unidirectional repeatability. The former describes the scattering of the position which is created when repeatedly moving to the same desired position from both the direction and opposite direction. This value is always greater than the one-sided, as in unidirectional repeatability, for many systems have mechanical clearance or there may also be hysteresis effects present. The unidirectional repeatability is defined as the scattering of the position when always repeatedly moving from the same movement direction.

Repeatability describes how much the measured position value scatters when always moving to the same desired position. In this case, it is not relevant how far away the actually value that was moved to is from the desired value.

The accuracy, however, describes how far away one really is from the desired position. The repeatability is thus better (as in smaller in value) or at the very least as good as the accuracy, since it does not include all sources of error. For example, the spindle threat error or pitch and yaw errors have no effect on repeatability, but they do on the clearance in the components.

Accuracy is less relevant for most positioning tasks. Often it is not necessary to move to an exact length value in the form of 234,445 mm, but you move until the desired point (e. g. provided by a prototype) is reached and then save it. If the repeatability is high, then this position can be moved to over and over again. This process is called “teaching” and is a form of calibration in the widest sense. The advantage is that the repeatable systems are much more cost-effective in their production than accurate systems.

Accurate tables must cover across the route a fraction of the distance “meter” as defined in ISO. This means that a part, whose length is known as precisely as possible, must be fitted on the linear table. Such parts are expensive. An example would be the installation of highly accurate and thus expensive scales or the use of precision spindles. A highly accurate table also always places high demands on the tracking system due to the influence of pitch and yaw.

If a table is, however, only designed for repeatability, then you can use significantly cheaper measuring systems and the demands into the track system are reduced. Repeatable systems can also be precisely designed for the application in the overall system by using teaching or compensation.

Specifically mechanical clearance, unevenness in the track system and drive train, as well as deviations in the control system interfere with the repeatability. To that end, for mechanical aspects, the prerequisite is the precise production of parts and high quality components as well as experience and skill in the assembly of such systems. Despite advances in control technology, the exact production and craftsmanship are indispensable. Controller deviations are prevented by sufficiently high resolution with fast signal periods in the feedback system, sufficient controller frequency and ideal adjustment to PID parameters (“tuning”). Sufficient motorization is also a prerequisite for a system working in a repeatable manner.

External vibrations as they occur in production halls or caused even just by a person passing interfere with the position of the system. The controller has to be in a position to react to it and to offset the fault. This means that the measuring system registers the fault and the controller reacts faster than the fault itself.  A further prerequisite is that the object to be positioned is not caused to vibrate by external faults.  The first natural frequency of the load has to be significantly higher than the frequencies occurring during positioning and due to faults. Since it is almost impossible to predict the frequency content of real faults, measures have to be taken in order to limit the highest interfering frequency. This is done with low-pass arrangements such as the heavy granite base positioned on air bellows in a high quality positioning system.