FEATURE NON-CONTACT MEASUREMENT & INSPECTION ThreadChecker in operation:
A NON-CONTACT thread checking sensor
A non-contact sensor designed for thread presence detection is now available from Ixthus Instrumentation. Here the company examines its operation, use, application concerns, and how to use the sensor’s ‘teach’ mode set-up
A
vailable in the UK from Ixthus Instrumentation, Kaman’s
ThreadChecker Universal is a non-contact sensor range designed to check thread presence for critical parts manufacture and automated assembly line applications. It comprises a choice of eddy current probes and an electronics module with a Teach Mode set-up that features both output and visual indication for thread presence. But how does it operate and what is it used for? Let us take a look at the Inductive
‘eddy current’ principles of operation: An oscillating electro-magnetic field is produced in the sensor tip; any conductive material engaging this field will have ‘eddy current’ induced in the surface, setting up a corresponding electro- magnetic field. As the gap between the sensor and the conductive target material changes, the influence of the eddy current field on the sensor field changes. The electronics produces an analogue voltage proportional to the gap between the sensor and the target. With normal displacement operation, the sensor uses the portion of the electro-magnetic field radiating from the end of the sensor. As the target moves, the output voltage decreases. With eddy current technology, instead of using the axial portion of the electro- magnetic field, thread detection uses the radial portion of the field. For thread presence/absence operation, the sensor uses the portion of the electro-magnetic field radiating radially from the sensor. An untapped hole results in a lower voltage output than that of a tapped hole.
APPLICATION CONCERNS: Thread pitch – Coarse threads are easier to detect. Finer threads require tighter insertion repeatability: • Eddy currents induced in the surface follow the contour of the threads
• Output voltage indicates the average of the major and minor thread diameters, essentially the pitch diameter of the threads • The coarser the threads, the greater
10 MARCH 2016 | INSTRUMENTATION
the voltage difference between tapped and untapped holes.
• Finer threads warrant tighter radial insertion repeatability of the sensor. Pitch diameter vs. sensor diameter –
Bigger gaps provide less sensitivity. Smaller gaps require tighter insertion repeatability: • Although the output is linear, the sensitivity is greater close in to the sensor
• Too small a gap can cause rubbing between the sensor body and thread ID resulting in irreparable damage to the sensor
• Too large a gap and eccentricity between the sensor and hole becomes an error source • It is best to stay within the published recommendations for the thread size being checked. Axial insertion repeatability: Long
thread length is easier. Short thread lengths require tighter insertion repeatability.
• The output changes as the sensor enters the tapped hole
• If not fully inserted, minor depth changes will result in output changes. • Longer thread lengths provide best reliability
• Thread lengths shorter than the electro-magnetic field length require very repeatable axial insertion. Radial insertion repeatability: Big
hole/small sensor is easier. Less gap requires tighter insertion repeatability: • The output changes as the sensor moves laterally in a hole, tapped or untapped.
• Hole/sensor eccentricity can be an error source
• Better radial insertion repeatability will provide greater headroom, resulting in more reliable operation. Thermal environmental changes: Sensor temperature changes can/will cause a change in the output: •Spec is 0.05% ˚C • Output is 0-10 VDC • With thread voltage @ 4.500 VDC and unthreaded voltage at 4.300 VDC, when taught, the switch is set to
Thread detection Zone 2
• Sensor enters the tapped hole
• Radial portion of the electro-magnetic field engages the threaded hole
• Output begins to decrease
Thread Detection Zone 3
• Sensor is completely in threaded section of the tapped hole
• Electro-magnetic field is fully engaged
• Output indicates pitch diameter of threads
Thread detection Zone 1: • Sensor in air
* Output saturated high
Thread Detection Zone 4
• Sensor transitions to untapped section of the hole
• Electro-magnetic field transitions from threads to tap drill diameter
• Output decreases
Thread Detection Zone 5 • Sensor’s electro- magnetic field is completely in untapped section of the hole
• Output indicates untapped hole diameter
Thread Detection Zone 6 • Sensor’s electro-
magnetic field begins to engage with bottom of the hole
• Output continues to decrease as it does in a normal displacement measurement
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