While Measuring Cents of µInches: Passive & Active Scanning: 6 Pros & vs 6 Cons

Years ago metrology specialists were viewed with suspicion by their own production coworkers. Today their work is widely recognized and valued as First Article Inspection verification or *FAI is the only way to validate the serial process production. Since High Speed Scanning Measuring Technology is more accurate and faster than Single probing point Probeheads for the same workpiece characteristic measurement: the 3x3D known State of the Art Scanning Technologies are here shown:

Passive Scanning Technology Probe Heads

#1· Simple, compact mechanisms
#2· No motor drives
#3· No locking mechanism
#4· No taration system
#5· No electromagnets
#6· No electronic damping: Springs generate contact force which varies with deflection

vast_xxtPassive scanning probes use a simple spring mechanism to sense stylus deflection in 3 axes.
During measurement the springs in the suspended probe mechanism generate a force to match the deflection.
This contact force varies with deflection, but it is highly repeatable.
The majority of scanning on CMMs involves measuring defined features: precision geometric forms whose size, position and shape is known to lie within a small tolerance band.
These features are scanned to check that they fall within the required tolerances.
Passive scanning systems use the CMM’s drives to accommodate surface variations.
When performing a scan using a pre-defined scan path, the probe must accommodate form errors.
These errors will cause the probe deflection to vary, resulting in slight variations in contact force.

Adaptive Scanning Technology Probe Heads

CMM moves around feature
Adaptive scanning keeps deflection variation to a minimum close to zero
Small form errors accommodated by sensor mechanism
Small force variation due to deflection range

vast_XPRIf adaptive scanning algorithms are used, the scan path is continuously adjusted based on the probe deflection, resulting in a still smaller force variation.

The result is a variation in contact force that differs little from that seen with active sensors. Furthermore, tensor calibration ensures that any variation in contact force does not affect measurement results.

Active Scanning Technology Probe Heads

#1· 3x Force coils in where the injected current is proportional to the probing contact force, they are also used to tare the probe head deflection to zero
#2· 3x ±n mm Probe Deflection Measurement coils are used, n=3mm or 2mm for HSS or VAST, XT & XTR Probe Heads respectively
#3· 3x Clamping motors at the zero probe deflection position for HSS Probe Heads
#4· 3x mechanic damping mechanism
#5· 6x electromagnets (HSS) + 6x permanent magnets (HSS)
#6· 5x force deflection springs (HSS)

vast_xtActive scanning systems use their 3 axis force coils to control the contact force vector constant between the stylus and the component during the ±2mm Scan.

Probing force is controlled from 100 mN to 1N within a narrow range by the Proportional to the Axis Projection Current Inyection Algorithm, but here there are some small variations. Unknown Contours ore Self Centering Probing Points are then possible to be measured.
The scan involves servoing the stylus carrier over the small range of deflections necessary to accommodate the errors in the form of the feature. Usually these are small measured in tenths of a micrometer; that is why in most commonly used to measure friction mechanisms or shaft/holes fits and its adjustment degree. This technology allow us to preview the basic fit even before mounting the workpieces.

*FAI=First Article Inspection


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