METHOD FOR SETTING ELECTRICAL CHARACTERISTICS OF SENSORS AFTER ASSEMBLY

Abstract: A sensor, such as a proximity sensor, having its components sealed within a case, has its sensitivity adjusted with a remotely settable digital potentiometer after assembly of the sensor. The adjustment connection is then destroyed.

Description: METHOD FOR SETTING ELECTRICAL CHARACTERISTICS OF SENSORS AFTER ASSEMBLY BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates generally to sensors whose sensitivity is set via a potentiometer located within the sensor casing and which is encased in a potting compound. DESCRIPTION OF THE PRIOR ART In the assembly of a typical proximity sensor, the operate point, or sensitivity, of the sensor is set before assembly is completed. This is done because in a typical sensor, such as a proximity sensor, the sensitivity is set manually through direct contact with the potentiometer adjustment screw. The potentiometer and associated electronics, usually contained on a PC board, are then fitted within the casing of the sensor and surrounded with a setting, or potting, compound in a liquid state which then hardens to protect the components from outside environmental influences such as shock, moisture, or the like. However, upon this potting operation, the sensitivity or operate point, of the sensor may shift from the initial set point. If the sensitivity shifts far enough so that the sensor is no longer within specifications the sensor must be scrapped. Also, because of this limitation the design and layout of the components may need to be tailored around setting the sensitivity in the middle of the assembly process. Therefore, the assembly process may require methods such as multiple fillings and automated assembly interruption for manual intervention during assembly. These steps result in more time and expense to assemble the sensor. Therefore, it would be desirable to have a method of constructing a sensor such that the potentiometer may be potted and then set to yield the proper sensitivity for the sensor thereby yielding a smoother assembly process and less out of tolerance sensors which must be scrapped. It would further be desirable to render the set point to the potentiometer, and thus the sensor sensitivity, stable against all outside influences once the sensor is completely constructed within specifications. SUMMARY OF THE INVENTION In order to accomplish the above purposes it is desirable to replace a potentiometer of the direct-contact, manually adjustable type with a remotely adjustable potentiometer, whereby the adjustment means may affect the set point of the potentiometer and thus the sensitivity of the sensor, after the sensor is completely constructed with its electronic components sealed within the case, such as by overmolding, sealing in a potting compound, or encased in a snap-together casing which is not easily separated. This may be accomplished by the substitution of a digital potentiometer having remote adjustment means. However, because digital potentiometers may be knocked out of adjustment by having their set point affected by environmental transients, the remote adjustment line, once the proper set point has been obtained, is destroyed so that the sensor remains within specified operating parameters. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more fully and completely understood from a reading of the Description of the Preferred Embodiment in conjunction with the drawings, in which: Figure 1 is a partial cut away view of a proximity sensor according to the present invention generally detailing the environment of the present invention. Figure 2 is a schematic representation of the electrical components of the proximity sensor of Figure 1. Figure 3 is an alternative embodiment of the schematic of electrical components from Figure 2. DESCRIPTION OF THE PREFERRED EMBODIMENT Throughout the Description of the Preferred Embodiment, like components will be identified by like reference numerals. Referencing Figure 1 , a proximity sensor 11 is shown. While the present invention is described in the preferred embodiment with specific reference to this proximity sensor, it will be appreciated by the ordinary artisan that other sensors may like-wise benefit from the utilization of the present invention. The proximity sensor 11 generally comprises a case 13 within which is inserted a PC board 15 to which are affixed electronic components, collectively referenced as 17. The type and amount of electrical components are not considered necessary to an exposition of the present invention beyond that detailed below. The PC board 15 is connected to electrical connectors 19 extending to the exterior of the casing 13. The PC board 15 and electrical components 17 thereon are fixed within the casing during assembly, preferably by application of a fluid potting compound which then hardens to prevent environmental damage to the electrical component 17. The hardened compound 21 is schematically indicated by cross hatching in Figure 1. While the necessary components for operation of the sensing apparatus 11 will be known to the ordinarily skilled artisan, a remotely settable digital potentiometer 23 is shown in Fig. 1 for further understanding of Figures 2 and 3. Preferably, during construction of the sensor 11, the potentiometer 23 will be set to zero before the PC board 15 is inserted in the case 13 so that only the increment function to set the potentiometer need be operated to set the final value of sensor sensitivity. However, the potentiometer could be set to a midpoint and incremented or decremented later as necessary. The potting compound 21, typically an epoxy, will then be applied and allowed to harden and the case 13 will be thereby sealed. Additionally one or more end caps 25 may be applied as necessary for the finished assembly. At this point an electrical connection to the digital potentiometer 23 will still exist through at least one of the plurality of electrical connectors 19. The sensor 11 is then placed in a suitable test apparatus and the potentiometer 23 is adjusted until the sensitivity of the sensor 11 is within the specification. As detailed below, the electrical connection between the remotely adjustable, or settable, digital potentiometer 23 is then severed so that the potentiometer value can not be changed accidentally or through encountering environmental transients. Referencing Figure 2, an electrical diagram for operation of the sensor 11 is shown. While the generally referenced electrical components 17 are shown in Figure 2 as discrete boxes, this is not intended to limit the present invention to any configuration or combination of discrete components. It will be recognized that various functional groupings may be combined into one or more integrated circuits, subassemblies, or the like. The voltage supply 29 and a pulsing circuit 33 are preferably external to the PCB 15 (Fig. 1) and connected thereto via an electrical connector such as a wire, ribbon connector, or the like extending between the electrical connectors 19 and the PCB. A voltage regulator 31, is connected to a fuse 35 which in turn is connected to an enabling circuit 37 and a pulse coupling circuit 38 both of which are connected to, and for the control of, digital potentiometer 23. The fuse 35 is further connected to a fuse blowing circuit 39 which in turn is connected between the supply voltage and ground to enable an overvoltage condition which will draw enough current to blow the fuse without harming the other sensor components. The enabling circuit 37, and pulse coupling circuit 38 are connected to those pins necessary to implement the adjustment of the digital potentiometer 23. The digital potentiometer 23 may be of the type such as part number DS1804 available from Dallas Semiconductor Corporation or the like. Once the proximity sensor has been assembled, the potentiometer is remotely set by applying proper voltage pulses to the circuitry through external connectors 19 to adjust the potentiometer as necessary. Once the operating parameters of the proximity sensor are within the desired specification an overvoltage condition is applied to the electrical circuit of Figure 2 causing the fuse blowing circuit 39 to increase current through the fuse 35 to sufficient to blow, or open, the fuse 35 thereby rendering the set point of the digital potentiometer fixed against accidental pulse application through accidental human intervention or environmental transients. As shown in the alternative embodiment of Figure 3, the circuitry may include a plurality of filters 33 and switches 45 connected between the fuse and the digital potentiometer adjustment pins, indicated at generally 47, to further regulate and control the setting of the potentiometer set point, especially should the remote setting of the potentiometer require a modulated signal or the like. The voltage regulator 31 is indicated in position between the voltage supply 29 and both the digital potentiometer 23 and sensing circuitry 41 in order to control application of the overvoltage condition thereto when opening the fuse 35. It will thus be appreciated that after setting the potentiometer by remote means, i.e. without resort to the direct manual adjustment of the potentiometer, the fuse 35 will be opened and the sensitivity of the sensor will be locked in to its correct state. Major benefits of this method of sensor construction include the elimination of multiple epoxy fillings steps, eliminating the need for manual intervention during automated assembly and elimination of the need for special interface wires or contacts external to the sensor. While the digital potentiometer shown has been described in terms of having a hard wire electrical connection and pulsed voltage adjustment, it will of course be appreciated that alternative embodiments such as adjustment through a modulated carrier signal or application of pulsed electromagnetic waves such as radio frequencies might also be incorporated into the setting circuitry of the digital potentiometer while retaining the desirable benefits of the present invention. While the present invention has been described in terms of a specific embodiment, it will of course be appreciated that many variations will occur to a person have ordinary skill in the art and that the present invention may be implemented in a variety of ways and is not to be limited except by the claims herein.

Claims: What is claimed is: 1. A method of assembling a sensor comprising: (a) placing sensing apparatus inside a casing, the sensing apparatus including a sensing circuit having means for electrical connection to the outside of the case and a potentiometer for setting electrical characteristics of the sensor connected to the electrical connection means; (b) sealing the sensing apparatus within the casing; (c) measuring the electrical characteristics of the sensor; (d) attaching the leads to means capable of emitting a signal for changing the value of the potentiometer, and operating the means for changing as necessary to reset the value of the potentiometer to a pre-selected electrical characteristic for the sensor. 2. A method of assembling a proximity sensor according to claim 1 further comprising the step of destroying the electrical connection between the outside of the sensor casing and the potentiometer. 3. A method of assembling a proximity sensor according to claim 1 wherein: the step of destroying the electrical connection between the potentiometer and the outside of the case includes raising current to a level sufficient to destroy the connection. 4. A method of assembling a proximity sensor according to claim 2 wherein: the step of destroying further comprises opening a fuse supplied in said connection. 5. A method of assembling a sensor comprising: (a) supplying a sensor casing; (b) placing sensing apparatus inside the casing, the sensing apparatus including a sensing circuit having means for electrical connection to the outside of the case and a potentiometer for setting electrical characteristics of the sensor connected to the electrical connection means; (c) setting the potentiometer to an initial value; (d) sealing the sensing apparatus within the casing; (e) measuring the electrical characteristics of the sensor; (f) attaching the leads to means capable of emitting a signal for changing the value of the potentiometer, and operating the means for changing as necessary to reset the value of the potentiometer to a pre-selected electrical characteristic for the sensor and; (g) destroying the electrical connection between the outside of the sensor casing and the potentiometer. 6. A method of assembling a proximity sensor according to claim 4 wherein: the step of destroying the electrical connection between the potentiometer and the outside of the case includes raising a voltage or a current to a level sufficient to destroy the connection. 7. A method of assembling a proximity sensor according to claim 5 wherein: the step of destroying further comprises opening a fuse supplied in said connection. 8. A method of assembling a proximity sensor according to claim 4 wherein the step of operating the means for changing includes selecting switches, or filters, or both, within the means for electrical connection. 9. A method of assembling a sensor comprising: (a) supplying a sensor casing; (b) inserting sensing apparatus inside the casing, the sensing apparatus including a sensing circuit having means for electrical connection to the outside of the case and a digital potentiometer for setting the sensitivity of the sensor apparatus; (c) sealing the sensing apparatus within the casing ; (d) measuring the sensitivity of the sensor; (e) attaching the electrical connection means to means capable of emitting a signal for changing the value of the potentiometer, and operating the means for changing as necessary to rest the value of the potentiometer to a preselected operating point of the proximity sensor; and (f) destroying the electrical connection between the outside of the case and the potentiometer. 10. A method of assembling a proximity sensor according to claim 8 wherein: the step of destroying the electrical connection between the potentiometer and the outside of the case raising voltage or current to a level sufficient to destroy the connection. 11. A method of assembling a proximity sensor according to claim 9 wherein: the step of destroying further comprises opening a fuse supplied in said connection. 12. A method of assembling a proximity sensor according to claim 8 wherein the step of setting the potentiometer to an initial value before it is sealed in the case. 13. A method of assembling a proximity sensor according to claim 8 wherein the initial value is about zero resistance. 14. A method of assembling a proximity sensor according to claim 8 wherein the initial value is about a midpoint of the potentiometers resistance range. 15. A method of assembling a proximity sensor according to claim 10 including the step of adjusting the potentiometer in either direction. 16. A method of constructing a sensor comprising in order: (a) supplying a case; (b) stuffing the case with sensing components, the components including a potentiometer for setting the sensitivity of the sensor, the potentiometer having a means for remote adjustment; (c) filling the case with a fluid-state potting compound to cover and protect the components; (d) letting the fluid potting compound harden; (e) checking the sensitivity of the sensor after the potting compound is hardened; (d) operating an adjustment means to set the value of the potentiometer to set the sensor sensitivity to a predetermined value. 17. A method of assembling a proximity sensor according to claim 12 wherein: wherein the sensor is a proximity sensor. 18. A method of assembling a proximity sensor according to claim 12 wherein: the sensor is a photoelectric sensor. 19. A method of assembling a proximity sensor according to claim 12 wherein: the means for remote adjustment includes a wire extending between the outside of the case and a circuit connected to the potentiometer for receiving an adjustment signal. 20. A method of assembling a proximity sensor according to claim 12 further comprising the step of: destroying the remote adjustment means.

Inventors: HINTZ FREDERICK W, POSTMA STEPHEN J, HINTZ, FREDERICK, W., POSTMA, STEPHEN, J.
Applicants: HONEYWELL INC, HONEYWELL INC.
Application year: 1999
Main classification: H03K
IPC codes: G01D11, G01D18, H01H36, H03K17