Coordinate measuring machines (CMM) probes are transducers that convert physical measurements into electrical signals by using numerous measuring systems within the probe structure. CMM probes are a broad category of instruments that use diverse technologies for direct and comparative measurements. CMM probes are available in three main designs: touch-trigger or discrete point, displacement measuring or scanning probes, and proximity or non-contact probes.
Touch-trigger probes or discrete point probes are the most common type of CMM probe. As their name suggests, they literally touch the surface of the workpiece. Upon contact, these CMM probes send a signal with the point’s coordinates to the CMM. The probe is then moved to the next location, where the process is repeated.
Displacement measuring probes or scanning probes are another basic type of CMM probe. As these CMM probes pass over a target’s surface, they transmit a continuous flow of information to the measurement system. Scanning contact probes may use linear variable differential transformer (LVDT) or optoelectronic position sensing.
Proximity probes or non-contact probes function similarly to displacement-measuring CMM probes. The difference is that proximity probes use laser, capacitive, or video measurement technology instead of LVDTs.
CMM probes use many different sensor technologies to attain their measurements. Each technology offers its own strengths that may be specifically desired for a given application. The most common technologies include kinematic or switching, strain sensing, piezoelectric, LVDT, optoelectronic, laser triangulation, capacitive, and video imaging.
Kinematic or switch technologies are available in a wide range of CMM probes. In terms of size, they are the smallest of the CMM probe types. They offer low over-travel force, simple interfacing, and robust and universal filament.
Strain-sensing CMM probes offer fewer lobing errors, long operating life, wide operating speed range, and long stylus carrying. They are ideal for peck, or stitch scanning.
Piezoelectric, LVDT, and optoelectronic CMM probes are commonly available. Features for piezoelectric devices include very few lobing errors, large stylus-carrying capability, multi-mode sensor operating, extended versatility, and restricted operating-speed range. LVDT sensor types provide a high degree of accuracy and large stylus carrying capacity. Optoelectronic devices offer a higher degree of accuracy than kinematic sensor types and high data rates.
CMM probes can use laser triangulation, capacitive, and video imagine sensor types. Laser triangulation sensor types are equipped with single axis profile measurements. The laser may be reflected by surface reflectivity. Capacitive CMM probes provide another form of non-contact technology. Material types may affect profile, form, surface flaw measurements, or surface chemistry; therefore, they use a fixed stylus. Video imaging sensor types are suitable for 2-D, flexible parts, and automatic edge detection; however, they may be affected by surface reflectivity and ambient light.