# Selection of Optimal Path Control Algorithms for Probe Heads Used on Five-Axis Measuring Systems
## 2 Path Control Algorithms of the Articulating Probing System  
The software responsible for path control and the determination of measured points coordinates plays a crucial role in the proper performance of the five-axis coordinate system. Most metrological programs allow the five-axis coordinate system to be used as a classic CMM equipped with a articulating probe, which retains a fixed orientation during point probing. Currently the most popular software, which allows the ability to fully use the rotational moves capabilities of a probing system, is Modus software by Renishaw. The programming environment is based on dimensional measuring interface specification (DMIS), but it is also possible to combine it with external applications in order to enrich the software capabilities. To control the probing system's performance, Modus uses an option called \"tilt\" and \"advance\", which defines the orientation of the probe during measurements. Tilt defines the rotation of the probe away from the plane, perpendicular to the measured surface normal vector [16]; and advance specifies the rotation around surface normal, while the negative value of this parameter means that the head is behind the tip [16]. As can be seen, the definitions are not exactly clear. However, the code created in the DMIS format uses, directly, the values of the A and B angles during measurement, or utilizes the Euler angles notation when the orientation is given in the part coordinate system (PCS). During the measuring path determination, Modus visualizes the location of the measured points, and the probe's stylus orientation corresponding to them, by drawing a leader line from each measured point to the probing system's central point [16]. Depending on the geometrical feature chosen for measurement, four different methods can be applied to manipulate the probe orientation: Guide point (GP), guide rail (GR), guided curve (GC), and fixed tilt and advance (FTAA). All mentioned methods are shown in Figure 2.  
Figure 2: _Cont._  
Guide point is the option used for minimizing machine motion during probing. The centroid, which determines the position of the center point of the probing system, is calculated from points included in the inspection path. It is offset along the normal direction to the plane in which the measured points are defined. The user can also specify the position to be taken by the head during measurements. The guide rail method can be used to define the path for the center point of the probing system during measurement. It consists of two segments between three reference points, of which the position can be defined by the user. By default, the position of the starting and ending points of the rail is calculated as offset, in normal direction to the plane, on which the measured points lie at the two extreme points of the inspection path. The offset has the same length as the utilized stylus. The guided curve is calculated, taking into account all points in the inspection path, by projecting them along the normal direction to the plane, on which the inspection path lies. Fixed tilt and advance can be used to ensure that each measured point would be inspected using the same tilt and advance values. In the case of the measurement of rotational features, it results with big changes in the probing system's center point position during measurements. Table 1 shows which method can be applied for the measurements of different geometrical elements.