Hall effect based electronic gauging is known to be used in thickness gauging. A typical Hall Effect probe deploys a magnetic field applied at right angles to a semiconductor element carrying a current. This combination includes a voltage in another direction perpendicular to the current flow and magnetic field. The magnetic circuit of the measurement system comprises a Hall sensor and a magnetic source such as magnets within a probe, and an external magnetic flux field. A ferromagnetic target such as a steel ball of known mass, placed in the magnetic circuit, alters the reluctance of the magnetic circuit, the magnetic flux density through the Hall sensor, and hence the induced voltage is changed. As the target is moved further away from the magnet, the magnetic reluctance is increased and hence the induced voltage is changed in a predictable manner. If these changes in the induced voltage are plotted, a curve can be generated which compares induced voltage to the distance of the target from the probe.
To make a measurement, a Hall sensor probe is simply placed on a first side of the test object, such as a blow-molding part, to be measured. A ferromagnetic target, frequently a small steel ball bearing, is placed opposite the probe on a second side of the test object. An electronic gauging instrument connected to the probe measures the probe voltage and displays the corresponding distance between the target and the probe tip, which is wall thickness. The target moves freely in all other dimensions except being drawn by the magnetic force of the Hall probe to stay aligned with the Hall probe on the opposite side of the wall of the test target.
A Hall probe is often favored for thickness gauging and quality checking for applications with non-magnetic test objects and thin walls such as molded plastic containers, automobile dashboard, fiber reinforced plastics and titanium castings.
Existing Hall probes have been seen in thickness gauge products, such as Magna-Mike 8600 by Olympus NDT. The Hall probes come in as one piece in an integral housing, including an un-exchangeable probe wear tip. Inside the housing, there's a magnet, a Hall sensor, electrical interconnections and a concentrator tip having high magnetic permeability and low magnetic remanence.
However, the existing Hall probes present some problems including an issue attributed to the unexchangeable probe tip, or wear tip. It is understandable that the tips of the Hall probes are often used to be slid over abrasive surfaces such as titanium castings and fiberglass layups, and are subject to wear. Probe tip wear changes the induced voltage to distance curve and the accuracy of thickness measurements compared to the original curve. Excessive wear can also damage the integrity of Hall sensor and electrical interconnections between the Hall sensor other electrical circuits. A tip damaged by other means such as impact from dropping or simply banning on the test object also changes the induced voltage and accuracy of thickness measurements. Damage or wear that degrades measurement accuracy or integrity of the probe requires expensive repair or replacement.
Another aspect of the problems presented by the existing probes involves direct force of the probe. The magnetic flux passing through a portion of the circuit comprising the Hall sensor and concentrator tip is dependent on the magnetic reluctance of the tip. The reluctance of the tip is reciprocally related to its magnetic permeability, a material characteristic that is a function of pressure and temperature. A force holding an object or a target on a tip that is opposed by an equal force extending through the concentrator exerts a pressure in the concentrator that can change the circuit reluctance and hence the induced voltage. Therefore any force exerted on the concentrator has an undesirable effect on the measurement accuracy.
Yet another factor affecting the measurement accuracy of existing Hall sensor probe relates to temperature variation within the existing probe. The induced output voltage of a Hall sensor is dependent on the input current, the flux passing through it, and the temperature. The flux passing through the Hall sensor is dependent on the temperature dependent reluctance of the concentrator tip. Heat transferred between an object or environment at one temperature and the outer surface of the tip at a second temperature will cause a change in the temperature of the tip. The heat flowing through the tip results in a temperature gradient throughout the tip. The temperature of a Hall sensor that is in intimate contact with an inner surface of the concentrator tip will change in response to temperature changes at that inner surface. Means for compensating temperature dependent output could be devised with on one or more discretely located temperature sensors that indicate localized conditions. However, in terms of transient thermal gradients within the magnetic circuit affecting measurement accuracy, it may be necessary to maintain physical contact until thermal equilibrium is achieved through the continuous volume of the concentrator. Therefore, it would be desirable that the probe's mechanical design functions to minimize the thermal gradients through the wear tip which can be minimized by increasing the thermal resistance and restricting heat flow through the probe.
Test object surfaces often include small radii or recessed contours preventing access of large probe tips to sharply curved surfaces. Test objects with severe contour restrictions such as automobile safety bag deployment tear seams require custom tip geometries.
Existing efforts have improved wear resistance of concentrator tips by a number of surface treatments or coatings, for example boride, nitride or carbide diffusion processes, or coating deposition by physical vapor deposition (PVD), chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD) among others that harden surfaces and reduce wear. The Borofuse Process™ is an example of a commercially available method of case hardening a variety of metals that has been used to substantially improve surface properties of concentrator tips. However, the improvement in wear life remains limited by the thinness of the surface treatment and the softness of the substrate concentrator material. Coatings do not protect the probe from impact damage and pressure or heat applied to a coated tip is readily transferred to the concentrator affecting tip permeability and Hall output voltage.
Another existing effort involves using hard, wear resistant caps made of ceramic or ceramic/metal composite material (cemented carbide) which are permanently adhered to concentrator tips to improve wear resistance. Ceramics offer superior wear resistance but ceramics are brittle and subject to fracture during impact. Cemented carbides offer improvement in fracture toughness relative to ceramics at the expense of lower wear resistance. Cemented carbides with a high cobalt content having moderately high magnetic permeability can serve as the concentrator and wear tip. Although wear is reduced by ceramic or cemented carbide tips relative to metal or coated metal tips, the tips have a finite wear life and present a series of other problems. For instance, probes with worn, permanently adhered wear tips may need expensive repair or replacement. Pressure or heat applied to a ceramic or cemented carbide tip is readily transferred to the tip affecting tip permeability and Hall output voltage. In addition, a hard cap rigidly adhered to a concentrator having different thermal expansion will be subject to varying stress with a change in temperature. Changing stress may change concentrator permeability resulting in Hall sensor output voltage and thickness measurement variation.
Similar issues have been experienced with existing replaceable, conformal plastic wear caps used to protect probe concentrator tip from wear. Those include a limited wear life, little protection from impact damage, needed adjustments to indicated thickness to compensate for plastic material lost progressively to wear. Plastic wear caps provide minimal increase in thermal resistance and pressure applied to the plastic is readily transferred to the tip affecting tip permeability and Hall output voltage.
Therefore the service life of Hall sensor probe assemblies for thickness measurement would be much improved with an exchangeable tip, provided that a range of challenges are addressed. One of the foremost is to make sure the exchangeable wear tip does not interfere with measurement accuracy in terms of its magnetic susceptibility and residual magnetization. Another challenge is to assure the exchangeable wear tip provides consistent tolerance, enduring incidences normal to regular field use, such as banging, pushing and occasionally dropping. A further challenge is to provide a measurement reference surface at a fixed distance from the concentrator tip. An ancillary challenge is to provide a reference surface at fixed distance from the tangent of a small concave surface. Yet a further requirement is to conserve measurement range greater than the test object thickness. An additional challenge is to oppose and withstand the measurement force without increasing the pressure on the concentrator tip. A supplemental challenge is to reduce temperature gradients within the probe by restricting heat flow between the probe and object being measured.
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