FEATURE SENSORS & SENSING SYSTEMS
ADDRESSING DIFFERENT manufacturing automation applications
Lee Hudson of Macro Sensors, explains how some of the variations in LVDT technology can aid manufacturers
E
lectronic 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. 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 an LVDT (linear variable differential transformer) 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. 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. Gage heads are available in both AC operated and DC operated versions that
Left: LVDT gaging probes can be manufactured in different configurations (compact), materials (stainless steel, inconel, etc) and connectors to meet specific applications
Right: spring loaded, AC operated LVDT position sensors interface with single channel LVDT signal conditioners that provide either a low noise 0-10V DC or 4-20mA output for computer readout of dimensional or
position measurement
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 categorised 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 well suited 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 and dimensional gaging at economical prices with good lifecycles. 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.
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. The DC operated LVDT gage head, on
12 APRIL 2014 | AUTOMATION
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. 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, precision gage heads are often used for dimensional or position
measurement in all types of motion control systems. Constructed entirely of stainless steel, these hermetically sealed 19mm diameter LVDT gaging probes provide long service life. Hermetically sealed, spring loaded LVDT position sensors can withstand the effects of dirt, water, steam and other corrosive elements without affecting performance.
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 < 0.000006 (<0.15μm). The probes’ 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 minimises radial play and the effects of side loading.
SPECIAL GAGE HEAD CONFIGURATIONS In this design configuration, the probe spring returns the probe shaft to its fully retracted position. During measurement cycles, the shaft is extended by the 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.
Macro Sensors
www.macrosensors.com T: 01256 884 902
Enter 203 /AUTOMATION
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