Patent Publication Number: US-5838222-A

Title: Multiturn potentiometer

Description:
This application is a continuation of application Ser. No. 08/587,155, filed Jan. 16, 1996, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     Multiturn potentiometers are preferred for many applications because they have the capacity to output a higher useable resolution as compared to a single turn potentiometer which is limited to a resolution that can be achieved in a single turn. Multiturn potentiometer normally use some form of mechanical arrangement to move a contact along a helical coil or a resistance path. As a result multiturn potentiometer are relatively expensive to produce as compared to a single turn potentiometer and typically multiturn potentiometer have limited mechanical travel. Multiturn potentiometer also can exhibit dithering or backlash. Dithering is a fluctuation in potential output that can be caused by minor back and forth movement of the potentiometer rotor wiper on the resistance element, such as may be caused by vibration. Backlash is defined as the maximum difference in contact position when the drive shaft is moved to the same actual output position from opposite directions. Backlash is caused by lost motion between two or more members arranged for transmission of motion from one to the other when direction of movement is reversed. This can be due to necessary clearances or friction, such as between an electric contact and a helical coil. Backlash usually increases as the respective members wear at their points of mutual contact resulting in greater looseness and inaccuracy. Both dithering and backlash are particularly undesirable characteristics in precision potentiometer in which the contact must be precisely moved to a particular point on the resistance path to output a particular potential and precisely hold that potential. 
     Examples of multiturn potentiometer of the general type discussed above can be found in U.S. Pat. Nos. 3,469,225; 3,833,783; 4,459,711; 4,110,721 and 4,716,395. U.S. Pat. No. 3,426,307 describes efforts to overcome the backlash problem encountered with multiturn potentiometer. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the problems encountered with multiturn potentiometer and relates to potentiometer that provide the simplicity of construction found in single turn potentiometer but also provide the high resolution of signal output associated with multiturn potentiometer. 
     In accordance with the invention there is provided an improved multiturn potentiometer comprising a housing defining an interior and having circuit means including an output terminal and a first resistance element electrically connected to the output terminal. At least one other resistive element is provided which is in electrical contact with a source of electrical potential. A rotor assembly is rotatably mounted in the housing interior and a wiper is carried by the rotor assembly to complete an electrical circuit between the output terminal and the first resistance element. The wiper is movable along the first resistance element to vary the output potential as the rotor assembly is rotated in the housing. At least one switch in the housing completes a circuit between the first resistance element and the other resistive element or elements. The switch includes a contact for movement on the other resistive element to periodically modify the magnitude of potential to the first resistance element responsive to the rotation of the rotor assembly. Means, such as a drive shaft is provided for rotating the rotor assembly. 
     In operation the first resistance element is maintained at a selected potential through electrical contact with a resistive element and rotation of the rotor assembly in one direction moves the wiper along the first resistance element to provide a succession of increasing increments of the potential to the output terminal and movement in the opposite direction provides a succession of decreasing increments of the potential. The switch is periodically activated by rotation of the rotor assembly to modify the magnitude of potential to the first resistance element. 
     The invention will be more fully understood from the following description of the preferred embodiment of the invention taken in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded view, without the upper housing member of a potentiometer constructed in accordance with the invention; 
     FIG. 2 is a side section elevation of the device of FIG. 1 fully assembled; 
     FIG. 3 is a top plan view of the upper section of the rotor assembly of the device shown in FIG. 1; 
     FIG. 4 is a side section of FIG. 3 taken along line 4--4; 
     FIG. 5 is a top plan view of the lower section of the rotor assembly of the device shown in FIG. 1; 
     FIG. 6 is a side sectional view of FIG. 5 taken along line 6--6; 
     FIG. 7 is a top plan view of the substrate of a device constructed in accordance with the invention; 
     FIG. 8 is a bottom plan view of the substrate of FIG. 7; 
     FIG. 9 is a top plan view of the lower section of the rotor assembly of the device shown in FIG. 1 with the lower rotor component installed; and 
     FIG. 10 is a simplified schematic diagram of the circuitry of the device shown in FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The potentiometer shown in FIGS. 1 and 2 consists of an upper and lower housing member 10 and 12 which form a potentiometer housing 14 which defines the interior of the potentiometer. As shown the upper and lower housings members, 10 and 12, are annularly shaped and provided with central openings which are aligned when the upper and lower housing members are joined to define the upper and lower-opening of a center passage 16 which extends through the interior of the housing assembly 14. The potentiometer illustrated in FIG. 1 is adapted to be used to determine the position of a point on a rotatable shaft which is rotatably received in the center passage 16 and on which the annular potentiometer is mounted. It will be clear, however, that the potentiometer of the present invention is not limited to annular form and may be operated by a small drive shaft or the like which is utilized to operate the potentiometer. A portion of the upper housing member 10 and the lower housing member 12 is elongated and a perpendicularly extending open ended terminal pin housing 17 is formed on the upper housing member to receive a series of terminal pins 20. A rotor assembly 21, consisting of of an upper component 52 and a lower component 54 is coaxially, rotatably mounted in the interior of the housing 14. A locking ring 24 carrying resilient gripping fingers 26 is mounted on the rotor assembly 21 for grasping a drive shaft (not shown) so that the rotor assembly rotates with the shaft for operation of the potentiometer. 
     As mentioned above, the embodiment illustrated receives a rotable drive shaft through the center passage 16 and as the shaft rotates the rotor assembly 21 also rotates which creates changes in resistance and thus, the potential output by the potentiometer. Switching means are provided in order to increase or decrease voltage depending upon the relative position of a point on the rotating shaft. The extending terminal pins 20 are adapted for connecting the potentiometer through suitable circuitry to a controller (not shown) for receiving the potentiometer&#39;s signal and utilizing that signal in the control of a process. One illustrative application for such a device is in a steering wheel turns application for automobiles in which the number of turns of the steering wheel is sensed by the potentiometer which outputs a representative voltage signal to a controller. The potentiometer of the invention finds use in any application requiring the precision control of voltage by a potentiometer. 
     As shown in greater detail in FIGS. 2, 3, and 4, the housing 14, which is formed from the upper housing member 10 and the lower housing member 12, defines an interior and the upper and lower opening of the center passage 16 through which a rotatable shaft (not shown) can extend. A portion of the upper housing member 10 surrounding the central opening is turned downwardly to define the upper end portion of the center passage 16 and is turned radially inwardly to define a clamping surface 18 for securing the rotor assembly 21 in cooperation with the lower housing member 12. A substrate 22 is disposed in the housing 14 on which is screened the circuitry for the potentiometer. The required resistors and output terminals 28 are also carried on the substrate 22. Surrounding each of the output terminals 28 is a helical spring 30 which is affixed to the substrate 22 in electrical contact with its respective output terminal 28 to form a resilient conductive socket for a terminal pin 20. Each of the terminal pins 20 extend through corresponding openings 32 in the upper housing member 10 and the terminal pins 20 are supported in their upright position by the upper body member 10. The extending ends of the terminal pins 20 terminate within the terminal pin housing 17 defined on the upper member 10. The terminal pin housing 17 is open at its upper end to receive a plug for providing electrical connection from the circuitry of the potentiometer to a suitable controlling device (not shown) through the terminal pins 20. 
     As more clearly shown in FIGS. 7 and 8, the substrate 22 is configured to be received in the housing and is provided with an opening 34 which forms part of the center passage 16 in the housing 14. The substrate 22 is formed of any printed circuit material, such as alumina, epoxy or polyester imide resin, and the circuitry is screened on in a conventional fashion. As is most clearly shown in FIG. 8, the undersurface of the substrate 22, the surface facing the lower housing member 12, is provided with a conductor ring 36 immediately adjacent the periphery of the opening 34 and a coaxial resistive annular strip 38 which is divided into three segments 38a, 38b, and 38c. Three smaller switching annuli 40 are disposed on the undersurface of the substrate 22 in radially spaced relationship to each other and to the resistive annular strip 38. The location and the spacing of the annuli 40 is not critical and are dependent on the location of the switching means and the size and location of the segments 38a, 38b and 38c. As shown, the segements 38a, 38b and 38c are of equal length and the annuli are equally spaced apart. A conductive button 42 is disposed in the center of each of the switching annuli 40. 
     In FIG. 9, three rotary switching members 44 are mounted in the lower housing on posts 45 and each is aligned with a corresponding one of the switching annuli 40 to form a potentiometric switch. Each of the rotary switching members 44 consists of an upper star-shaped section 46 and a lower star-shaped section 48. Each of sections 46 and 48 contains an equal number of arms. The number of arms is equal to the number of switching positions provided by the rotary switching members 44. In the embodiment illustrated, the rotary switching members have seven switching positions, thus there are seven arms on each star-shaped section 46 and 48. The arms of the upper star-shaped section 46 are angularly offset with respect to those of the lower section 48. Each of the upper sections 46 of the rotary switching members 44 contains a contact 50 for completing a circuit between one of the segments, 40a, 40b or 40c of the smaller switching annuli 40 and the conductive button 42 as the switch is rotated. 
     Referring to FIGS. 3, 4, 5 and 6, the rotor assembly 21 consists of upper and lower componants 52 and 54. The upper component 52 in the embodiment illustrated includes a central cylinder 56 which extends into the interior of the housing 14. The bore of central cylinder 56 defines a portion of the center passage 16 of the housing 14. An annular collar 58 is formed on the central cylinder 56 and, in the embodiment shown, carries at its outer edge an annular magnet 60 which is used as part of a digital output device (not shown), the form and function of which does not form a part of the present invention. The annular collar 58 defines an upper surface 62 which slidingly bears against the lower surface of the clamping member 18 of the upper housing. The central cylinder 56 extends through the lower housing member 12 and terminates adjacent the periphery of the center passage 16 therein. The diameter of the center passage 16 in the housing member 12 is smaller than the diameter of the central cylinder 56 and the surrounding portion of the housing member 12 defines a low friction bearing surface for the bottom edge of the central cylinder 56. The rotor assembly 21 is completed by the lower component 54, which is also referred to as the potentiometer rotor. The potentiometer rotor 54 is fitted over the bottom end of the central cylinder 56 of the upper housing member 10 for rotation therewith. The potentiometer rotor 54 describes three concentric overlying annular sections of decreasing outside diameter, the upper section 64 having the largest outside diameter, the middle section 66 the next largest and the lower section 68 having the smallest outside diameter. The top section 64 defines an upper surface which is immediately adjacent to the conductor ring 36 and the resistive annular strip 38 of the substrate 22. A wiper 70 on the upper surface makes electrical contact between the conductor ring 36 and the annular resistive strip 38 as the potentiometer rotor 54 rotates. The outside diameter of the lower section 68 is such that the outer surface of the lower section is immediately proximate the inwardly extending arms of the lower star-shaped sections 48 of the rotary switching members 44, while the outer surface of the middle section 66 is immediately proximate the inwardly extending arms of the upper star-shaped section 68. Thus, the rotor assembly 21 is free to rotate in the housing 14 while the switching rotors 44 are prevented from rotation because of the proximity of the wall of the middle section 66 to the arms of the upper star section. A slot 72 extends through the upper section 64 and middle section 66 of the potentiometer rotor 54 and a lug 74 is formed on the lower section 68 in alignment with the slot 72. FIG. 5 shows lug 74 in a top view looking down through an opening next to wiper 70. The slot 72 provides clearance into which a point of the upper star-shaped section 46 of a switching member 50 can extend to provide the clearance to permit rotation of the switching member responsive to contact between the point of the lower star-shaped section 48 of the switching member and the lug 74 as the rotor assembly 21 rotates. 
     FIG. 10, is schematic diagram illustrating the circuitry of the potentiometer of the invention. As shown, the annular resistance strip 38 is provided with taps 75 0 , 75 1  and 75 2  for electrical connection through lines 76, 78, and 80 to operational amplifiers 82, 84 and 86, each of which are also electrically connected to a corresponding one of the smaller switching annuli 40. The taps 75 0 , 75 1  and 75 2  define the boundaries of the segments 38a, 38b and 38c and the voltage at each tap is different. Therefore, the voltage in each of the segments 38a, 38b and 38c is an interpolation of the tap voltage at its boundaries. The rotary switching members 44 are schematically illustrated as contacts 92, 94 and 96 which are in electrical contact with a corresponding one of the switching annuli 40 and a corresponding one of the operational amplifiers 82, 84 and 86. Lines 88, 90 and 92 connect operational amplifiers 82, 84 and 86 to a corresponding one of the switching annuli 40. The wiper 70 connects the annular resistive strip 38 and the conductor ring 36 and the output is conducted through lines 100 and 102 to terminal pins 20. Power input to the switching annuli 40 is through a circuit defined by lines 102, 104 and 106. 
     As shown in FIG. 10, the wiper 70 is illustrated as positioned in segment 38a to output an increment of the segment voltage as defined between taps 75 0  and 75 1 . As the wiper rotates towards segment 38b (the next higher voltage segment), contact 96 is moved along the annulus 40, as described above, to a next higher voltage and thus sets tap 75 2 , through operational amplifier 86, to a higher voltage. It will be seen that contacts 92 and 94, which control the tap voltages of boundary taps 75 0  and 75 1  of segment 38a are not moved while the wiper 70 is in segment 38a. This increases the reliability of the output from the potentiometer and eliminates switching noise because the rotary switching members are never moved while their output is active. As the wiper 70 moves into segment 38b, taps 75 1  and 75 2  become the active taps and contact 92 is moved to a higher voltage on its annulus 40. Reversing the direction of wiper 70 results in reducing tap voltages in the manner described. From the foregoing it will be seen that the potentiometer is capable of precise voltage output with a minimum of output noise because of the buffering effect of the operational amplifiers. In addition, the rotary switching members are not moved while their output is active thus further reducing output noise and assuring reliability. The rotation period of the potentiometer of the present invention is equivalent to a multiturn potentiometer and has a rotational period greater than 360° provided by the successive switching of voltages at the taps. 
     As will be understood by those skilled in the art, various arrangements which lie within the spirit and scope of the invention other than those described in detail in the specification will occur to those persons skilled in the art. It is therefore to be understood that the invention is to be limited only by the claims appended hereto.