Quality Magazine
NDT - Probes

Gaging Probes Get Results

Variations in LVDT gage head technology address different manufacturing automation applications.

June 10, 2013
gaging probes lvdt precision ndt
Electronic gaging probes provide the data collection capability for many of today’s automated dimensional data acquisition systems. Widely used in machine tool inspection and gaging equipment, electronic gaging sensors serve as important components of quality assurance systems, providing dimensional feedback used for online quality control and post-process statistical analysis. Typical users of electronic gaging probes include machine manufacturers and quality control departments that use these sensors as part of complete systems for go/no-go inspection of complex mechanical parts.
The industry standard design of gage heads consists of a linear variable differential transformer (LVDT) assembly that includes a spring-loaded movable armature coupled to a shaft that is supported in a high-precision linear bearing. The probe shaft is guided in a sleeve bearing retained in a case that also encloses the LVDT coil windings. 
Often, the term “spring-loaded LVDT position sensor” is used to refer to gaging probes with LVDT assemblies. Physically in contact with the part when under measurement, LVDT gaging probes typically have a range of ±0.5 millimeter to ±50.0 millimeter (±0.020 inch to ±2.000 inches). While the resolution of an LVDT is considered to be infinitely small, the resolution of a gage head is somewhat limited by the mechanical repeatability of the bearing system and its supporting electronics.
Figure 1 is an example of a gage head that consists of an LVDT assembly whose core is connected to a spring-loaded probe shaft having a removable tip. The plunger is guided in a bearing within a case that encloses the LVDT coil windings. As no physical contact is made between the core and coils, the LVDT is highly repeatable and reliable. 
Gage heads are available in both AC-operated and DC-operated versions that contain electronics within the assembly of the gaging probe. For applications where sensors must operate at temperatures as high as 425 C, the sensing element of an AC-operated LVDT gage head can be segregated from the electronic circuitry. Connected by cables, AC-operated spring-loaded LVDTs can work with remotely-located electronics that power the sensors, amplify and demodulate their output. Output is then displayed on a suitable readout and/or input into a computer-based data acquisition system for statistical process control. 
The DC-operated LVDT gage head, on the other hand, can eliminate the volume, weight and cost of conventional external AC excitation, demodulation and amplification equipment. However, temperature exposure is limited by the properties of the materials in the electronic signal-conditioning module that is contained within the sensor. 
LVDT gage heads are typically categorized according to performance:
  • Economy gage heads: Reasonable level of performance at modest cost
  • Precision gage heads: Good linearity and excellent repeatability
  • Ultra-precision gage heads: Lowest error 

Economy Gage Heads

Providing good performance at a moderate price, economy gage heads are ideal for cost-sensitive applications such as student projects, low overhead machines and fabrication shops, as well as R&D labs on a limited budget. By incorporating cost-sensitive arrays, these spring-loaded LVDT position sensors offer medium accuracy (linearity of ±0.5% of full range output) dimensional gaging at very economical prices with good life (life cycles exceeding 5 million). 
Because unit shafts are loosely fitted in a sleeve bearing, units can freely rotate, resulting in core skewing and rotation during operation that can result in a somewhat larger repeatability error around 0.0001 inch. 
Economy gaging probes function well in relatively benign environments of quality assurance laboratories, or in a protected inspection jig or fixture located away from machines and manufacturing processes. Economy gage heads can be utilized with low cost signal conditioner to make a cost effective position measuring system. 

Precision Gage Heads

More rugged and robust than economy gage heads, these hermetically sealed sensors operate in adverse environments containing moisture, dirt and other contaminants. Constructed entirely of stainless steel, these gaging sensors consist of a spring-loaded shaft running in a precision sleeve bearing and connected to an LVDT core. Use of a precision sleeve bearing results in excellent measurement repeatability of 0.00005 inches (1.2 microns) or better, and maximum linearity error of ±0.25% of full range output. 
Precision gage heads can be connected to any standard LVDT signal conditioner, and then passed to a gaging column display, digital recorder or computer-based data acquisition system. Because of their ability to operate in harsh industrial environments such as shop floors where dirt, oil, water and machine coolants may be present, precision gage heads are often used for dimensional or position measurement in automotive factories, forges and foundries (large shaft TIR measurements), metal fabricating shops, paper and/or plastic film plants, industrial process automation applications and all types of motion control systems. 

Ultra-Precise Gage Heads

Engineered for highly precise and repeatable measurements in quality control and metrology applications, ultra-precise gage heads combine a special LVDT with a selectively fit sleeve bearing to produce linearity of up to 0.05% of full range and repeatability of less than 0.000006 inch (0.15 microns). 
The probes’ exceptional repeatability is due, in part, to the use of a linear ball bearing assembly precisely fitted to a hardened-and-ground, non-rotating probe shaft that minimizes radial play and the effects of side loading. One end of the probe shaft is coupled to the core of a friction-free LVDT sensor, which produces an output voltage linearly proportional to the probe shaft position. The use of an LVDT for probe shaft displacement sensing eliminates any additional transducer errors due to friction, stiction, or mechanical hysteresis. 
These premier gaging probes provide ultra-precise measurement of the position of a targeted surface in a wide variety of quality control and industrial metrology applications, including online parts inspection, servo-loop positioning systems, and manufacturing process control. The ultra-precision gage heads measure the position of a target surface, with an unusually high degree of accuracy. Complete systems for go/no-go inspection of complex mechanical parts can be assembled using several spring-loaded LVDT linear position sensors mounted in suitable fixtures. 

Special Gage Head Configurations

Pneumatically Actuate Gage Heads: In this design configuration, the probe spring returns the probe shaft to its fully retracted position. During measurement cycles, the shaft is extended by introduction of a low-pressure, clean and dry air supply. Probe contact force can be set by adjusting air pressure. Air outflow through the probe's bearing aids the exclusion of dirt, dust, oil, solvents, and other contaminants from entering the unit.
With the release of pressure, an internal spring returns the probe to its fully retracted position. The retracted probe provides room for manual or automated part lead in. 
As with all sensors, LVDT gaging probes can be manufactured in different configurations (compact), materials (stainless steel, Inconel, etc.), and connectors to meet specific applications.