Meeting the Challenges of Precision Sensing: Very Small Target Displacement

Fundamental application problem: Inductive prox sensor is latching on (or…failing to turn on)

• The prox sensor gap is set to turn on when the target approaches, but it does not turn off when the target recedes (latching on)
• The prox gap is opened up until sensor turns off at maximum target approach, but it fails to detect the target upon the next approach cycle
• The prox sensor gap is set to turn on when the target approaches, but later on the operation becomes intermittent (prox fails to reliably detect the target)

Solution: High-performance miniature inductive prox sensor

Critical sensing performance specifications:

o   Low variation of switch point from sample to sample
o   Tight repeat accuracy of switch point
o   Low temperature drift of switch point
o   Low maximum hysteresis (distance between switch-on to switch-off)

Example of a real-world precision miniature prox:

o   Effective switching distance (Sr) specification = 1.5 mm ± 0.15 mm

• ± 0.15 mm is the maximum sample to sample variation
• Lowest effective operating distance = 1.5 mm – 0.15 mm = 1.35 mm
• Highest effective operating distance = 1.5 mm + 0.15 mm = 1.65 mm
• Max switch point difference between any two random samples = 0.3 mm
• That’s just one-third of a millimeter or 0.012” (12 thou) difference from sample to sample of different proxes (worst case)
• This low variation allows for standardized tooling design when the sensor-to-sensor tolerance is taken into account

o   Repeat accuracy specification = 5%

• This is the worst-case switch point repeatability for any given prox sample
• Worst-case calculation: compute using max effective switching distance 1.65 mm (repeat accuracy will be better for prox with shorter distance)
• Max switching distance = 1.65 mm + 5% = 1.65 mm + 0.08 mm = 1.73 mm
• Min switching distance = 1.65 – 5% = 1.65 – 0.07 mm = 1.58 mm
• Range of switch point variation = 1.73 mm – 1.58 mm = 0.15 mm

o   0.15 mm = 0.006” (6 thou) (worst-case)

o   Temperature drift specification

• Maximum switch point drift = 1.5 mm ± 0.15 mm
• Min switch point due to drift = 1.5 mm – 0.15 mm = 1.35 mm
• Min switch point due to drift = 1.5 mm + 0.15 mm = 1.65 mm
• Maximum difference between any two random samples = 0.3 mm (worst case)

o   That’s just one-third of a millimeter or 0.012” (12 thou) difference across the specified temperature range

• If the temperature in your application is relatively stable, temperature drift is not even a consideration
• If temperature fluctuation is a concern, the tolerance can be accounted for in the tooling design

o   Hysteresis specification (the difference between switch-on and switch off points)

• Hysteresis = max 15% of effective switching distance (Sr)
• Hysteresis is always additive to the switch-on point
• Max switch-on distance (from above) = 1.65 mm
• Max switch-off distance = 1.65 mm + 15% = 1.65 mm + 0.25 mm = 1.9 mm
• Max differential switch-on and switch off: 1.9 mm – 1.65 mm = 0.25 mm (0.010” or ten thou)

Conclusions

• Worst-case prox behavior / design accommodations
• Switch point variation prox to prox = 0.3 mm (0.012”)

o   Account for 0.3 mm variance in standardized tooling

• Repeatability error = 0.15 mm (0.006”)

o   Process switch point is repeatable within 0.15 mm

• Temperature drift = 0.3 mm (0.012”)

o   Generally, not a consideration for indoor applications

• Hysteresis = 0.25 mm (0.010”)

o   Target movement must be greater than 0.25 mm

o   Assure consistent sensor operation

Stay tuned to this space for more precision sensing challenges and solutions. Miniaturized sensors are also available in photoelectric, capacitive, magnetic cylinder, ultrasonic, and magnetic encoder. Click here to see the whole mini family.