A reluctor sensor measures the ability of magnetic fields to pass through adjacent structures and it measures the changes in the magnetic field lines. The presence of metal allows the field lines to readily pass, while nonmagnetic materials impede the passage. Aluminum with embedded steel set screws or a sprocket style all steel reluctor can provide the required environment for a reluctor sensor to measure changes in magnetic fields.
One commercially available reluctor ring (Picture DSC01888) is a 1.50 inch diameter and 9/16 inch thick Aluminum ring with four symmetrical steel set screws. The heads of the 5/16-8 set screws are 0.25 inch across and when tightened are nearly flush with the ring surface. If only two set screws are used, then a two pulse per revolution signal can be produced. A companion reluctor sensor can be mounted adjacent to the motor or affixed to the face of the Warp motor (Picture DSC02023 below).
Picture DSC01888 of an Aluminum ring with four set screws. The heads of the screws are nearly flush with the ring surface when installed. When used with a companion electrical sensor, the RPM of a rotating shaft can be measured.
Picture DSC02023 showing the mounting plate that positions the sensor adjacent to the reluctor ring.
Picture DSC02024 showing a close up of the sensor and Aluminum ring. For clarity the picture shows a large gap between the sensor tip and the steel set screws that are threaded into the Aluminum ring. When operational, the gap is reduced until almost eliminated to enhance both the magnitude and the quality of the signal.
Drawing 20141010 by David of DMF Consulting of a first generations square tooth reluctor. The final outline of the cut was the composite of three separate square end mill cuts.
Picture DSC01840 face view of the steel reluctor prior to mounting on the Warp 11 0.875 shaft.
Picture DSC01843 edge view showing the 0.50 inch thick piece with a 10/24 set screw.
Picture DSC01847 of the installed first generation all steel reluctor along with the sensor and mounting bracket.
An alternative more elegant second generation reluctor was machined by David starting with 2 inch diameter steel rod stock. After a 0.5 inch thick disk was cut from rod stock, it was bored out to either a 0.875" or 1.125" center hole, and then it was machined to produce a six tooth sprocket.
Drawing by David of the second generation reluctor that can be used with the 1.125" shaft found on current Warp-11 motors. It was produced from a 0.5 inch thick section of rod stock using a 0.5' diameter mill bit.
Picture DMF-DSC0285 showing David's setup on his mill using a Collet Indexer and a 0.5" Ball-End Four Flute Carbide End Mill (McMaster part 88805A45). The center of the steel stock was held with an Expanding-End Mandrel (McMaster part 2779A22). The final outline of the cut was identical to the outline of the mill bit
Picture of a Collet Indexer of the style that was used to prepare the reluctors shown in picture DMF-DSC0285 above and DMF-DSC01289 below.
Picture DMF-DSC01289 of the final 6 tooth reluctors prepared by David using a mill bit and a 2" diameter x 0.5" thick (McMaster Carr cat # 7786T12 , $3.03) steel disk. The left unit was prepared for a 0.875" shaft (for an older model Warp-11 manufactured prior to 7/1/2012) , and the right unit was prepared with a 1.125" shaft opening (for Warp-11 manufactured after 7/1/2012). The set screws were 10-32 x 3/16 for the 0.875 shaft and 10-32 x 5/16 for the 1.125 shaft on the right.
Alternative use of the BMW Harmonic Damper
Interestingly, it is possible to create a reluctor using the original BMW OEM harmonic damper that originally has 58 teeth and two missing teeth. If a rotary table is used on a mill to symmetrically remove patterns of the teeth, it is possible to create a reluctor with 20 or 15 or 12 or 10 or 6 or 5 or 4 or 3 or 2 or 1 remaining tooth.
Picture of the original BMW OEM harmonic damper prior to modification.
Picture showing the appearance of the BMW OEM harmonic damper after removing pairs of two teeth and alternating with 1 remaining tooth followed by 2 missing teeth, etc. In this example the modified harmonic damper has a total of 20 teeth. If it is intended to use the pulley that is integral to the harmonic damper, then this modification would allow RPM measurement without the need to purchase additional hardware.
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