The sensors of a coordinate measuring machine are used to pick up the primary signal from the workpiece. They are designed using mechanical and, in some cases, opto-electronic and software components of varying complexity. The sensors must be selected on the basis of the conditions on and near the workpiece, the touch sensitivity of the object, the size of the features to be measured, the requirements of the measurement plan and the number of measured points. Thus, the selection of the sensor or sensors basically depends on the measuring task at hand.

Coordinate measuring machines can be equipped with tactile or optical trigger and measuring (usually called dynamic or scanning) sensors. Trigger sensors only produce a trigger signal after detecting a measuring point. This causes the measuring system of each machine axis to be read out, determining the coordinates of the point in space. Movement in the axes is absolutely essential in order to determine the coordinates of an object point (dynamic measurement principle). Measuring sensors have an internal measuring range of up to several millimeters. An object point is determined by superimposing the measured values of the sensor over the coordinates read out by the measuring machine. It is thus possible to determine a point even when the coordinate measuring machine is standing still (static measurement principle), as long as the magnitude of the object point is located within the measuring range of the sensor.

Another important criterion for differentiating between sensors is the physical principle of transmission of the primary signal. In this regard, the sensors commonly used today can be divided into two groups – optical and tactile. The location information of a measured point is transmitted to an optical sensor by light in such a way that it can be used to determine the corresponding coordinates. In the case of a tactile sensor, this information is generated by touching the workpiece with a probing element, which in most cases is a stylus tip.

Another important application-specific feature is the number of dimensions of the sensor. This factor determines whether the sensor can pick up information in one, two or three coordinate axes. For sensors with less than three degrees of probing freedom, the remaining coordinates are determined from the previously measured position of the sensor probing point within the machine coordinate system. However, this approach restricts the system’s applicability in connection with complex, three-dimensional objects (e.g. a 1-D laser cannot measure the cylindrical form of a bore or an X-Y touch probe cannot measure the flatness of a plane in the Z-direction).