GB2188785A - Method of making a potentiometer - Google Patents

Abstract

In a method of making a rotary potentiometer in which a rotary actuator (2) is arranged to cause a wiper on a carrier in a thermoplastics housing (1) to traverse a resistance track in the housing (1), a thermoplastics member (8) on the surface of the housing (1) is arranged for rotation about the axis of rotation of the wiper carrier. With the actuating means (2) in contact with a stop (12) on the member (8), relative rotation is effected between the member (8) and the housing (1) until a desired output voltage is obtained at a terminal connected to the wiper. The member (8) is then secured to the housing (1) by passing a current briefly through a wire (11) at the interface therebetween to locally melt the thermoplastics material and fuse the parts (1, 8) together. Holes (10) in the member (8) may be slotted and are aligned with slots in the housing (1) to enable the potentiometer to be screwed to a support. <IMAGE>

Description

SPECIFICATION Method of making a potentiometer This invention relates to electrical potentiometers and particularly to a method of making a rotary potentiometer.

Rotary potentiometers are well known in which an arcuate resistance track supported in a housing is traversed by an electrically conductive wiper supported for rotation on a carrier and spindle in the housing. Provision is made for application of a voltage to first and second terminals connected to opposite ends of the track and for obtaining an output voltage at a third terminal connected to the wiper. The output voltage is a function of the position of the wiper along the track and may, for example, rise from zero, with the wiper at one extreme end of the track, to a maximum with the wiper at the other end of the track.

Mechanical end stops are usually provided to limit the traverse of the wiper on the track.

Such potentiometers are finding increasing application in the automotive industry particularly for engine throttle position sensing. An actuating lever may be provided connected to the spindle on the wiper carrier on the potentiometer and arranged to cooperate with the engine throttle mechanism. As the throttle is opened, the potentiometer wiper traverses the resistance track and the output voltage from the potentiometer is a function of the extent of opening of the throttle. Some applications now require that when the potentiometer operating lever is in contact with a mechanical end stop on the potentiometer, the wiper is in a position near one end of the track at which an output of a predetermined accuracy must be guaranteed at the wiper terminal.The required accuracy may be such that it would necessitate the mechanical end stop being correctly positioned within little more than + 1 degree of rotation of the wiper. This is generally impossible with current design, from the mass production aspect, since analysis of manufacturing tolerances for piece parts employed in a potentiometer shows that there is likely to be as much as t5 degrees rotational error for a given output from the wiper with reference to the mechanical stop.

It is an object of the present invention to overcome or minimise this problem.

The present invention provides a method of making a rotary electrical potentiometer of the type comprising a housing having therein an elongate arcuate electrical resistance track; an electrically conductive wiper supported on a carrier for rotation about an axis in said housing and adapted to traverse said track; rotary actuating means external to said housing and cooperating with said rotatable carrier by way of a cylindrical aperture extending through a face of said housing; first and second electrically conductive terminals connected to opposite ends of said track and adapted for connection to a voltage source, said wiper being connected to a third terminal at which an output voltage is capable of being derived whose level is a function of position of said wiper on said track; said method being characterised by; providing external to said housing, in contact with said face of said housing, a member having an opening therein located concentrically with said cylindrical aperture in said face of said housing and allowing the cooperation between said actuating means and said rotatable carrier, said member being adapted and arranged to be rotatable about said axis; effecting relative rotation between said member and said housing, whilst said actuating means is maintained in a predetermined position of rotation relative to said member, until a desired output voltage is obtained at said third terminal, then securing said member to said housing.

Suitably the said housing may be provided with an external cylindrical projection extending from the said face concentrically with the said cylindrical aperture, the said projection being located through said opening in said member such that said member is rotatable on said projection.

The said member may suitably incorporate stop means adapted and arranged to cooperate with said actuating means and such that with said actuating means maintained in contact with said stop means, effecting of relative rotation between said member and said housing results in said wiper traversing said track and thereby causing said output voltage to change, the said member being secured to said housing at a position where said output voltage is at a desired level. This arrangement is particularly advantageous where a potentiometer is required having a predetermined output voltage at the third terminal when the actuating member is near one end of the track. Such an arrangement finds one application in a potentiometer used for automotive throttle position sensing.

The said member is suitably provided with two opposed holes through it in alignment with corresponding holes through the said housing of the potentiometer and such that threaded fasteners are able to be passed through the holes to secure the potentiometer to a support.

In one arrangement the holes in the member are provided of circular form and the holes in the housing are of slotted form. This arrangement permits the necessary degree of relative rotation between the member and housing to be effected while the circular holes in the member facilitate subsequent securing of the potentiometer to a support by means of threaded fasteners. The use of the circular holes also enables a known angular relationship to be achieved between a mechanical datum at the housing of the potentiometer and the actuating means.

In another arrangement the holes in both the member and the housing are provided of slotted form. This has the advantage that when the potentiometer is subsequently secured to a support by threaded fasteners a limited amount of rotation of the potentiometer may be effected during the securing operation. This allows small angular displacement errors in an external mechanism, e.g. an automotive engine throttle mechanism, which may be arranged to drive the actuating means of the potentiometer, to be accommodated to prevent undesirable mechanical force being applied to the potentiometer actuating means and stop member.

If the slots in either the member or housing are displaced towards one side of a line through them and the said axis, a greater degree of relative rotation between the member and housing can be achieved.

The rotary actuating means may conveniently comprise a lever connected at one end to the wiper carrier. The lever may suitably have a portion arranged to contact a stop means on the said member. The other end of the lever is suitably arranged to be contacted by an external mechanism, such as an automotive engine throttle mechanism, to effect rotation of the wiper carrier.

The said member and said housing preferably comprise plastics material. They may, for example, be secured together by means of a suitable adhesive. If the member and housing are formed of thermoplastics material they may be secured together by a technique such as ultrasonic welding. Preferably, however, a loop of thin wire or metal tape is provided between the member and housing and an electric current passed through it to generate heat to cause the thermoplastics material to locally melt; when the current flow is discontinued, the wire or tape cools and member and housing become fused together in the region of the wire or tape. Excess wire or tape protruding from the interface between the member and housing is then removed by severing.

The invention is now described by way of example with reference to the accompanying drawings in which: Figure 1 represents a perspective view of a potentiometer housing and a member for assembly therewith, according to the invention.

Figure 2 represents a perspective view of a potentiometer assembled according to the invention.

In Figures 1 and 2, certain standard parts which are well known in potentiometer technology and which are not essential to an understanding of the invention have been omitted in the interest of clarity and simplicity.

A potentiometer is constructed as follows, according to the invention. A housing 1 of well known rigid thermoplastics material contains the following components (not shown) which are well known in the art; an elongate arcuate electrical resistance track; an electrically conductive wiper supported on a carrier for rotation about an axis A in the housing and adapted to traverse the track; first and second electrically conductive terminals connected to opposite ends of the track and arranged to be connected to an external voltage source. The wiper is connected to a third terminal at which an output voltage is capable of being derived whose level is a function of the position of the wiper on the track.A rotary actuating means comprising a lever 2 (Figure 2) has one end connected to the wiper carrier by means of a screw 3, by way of a cylindrical aperture 4 extending through a face 5 of the housing 1. The wiper carrier suitably has a spindle extending from it through the aperture 4 and to which the lever 2 is secured by the screw 3. The housing 1 is provided with an external cylindrical projection or spigot, 6, extending from the face 5 concentrically with the cylindrical aperture 4. A pair of holes 7 of slotted form are provided through the housing 1 and are preferably displaced towards one side of a line B through them and the axis A.

A member 8 is constructed also of a rigid thermoplastics material and has a cylindrical opening 9 therein dimensioned such that it will fit over and be rotatable on the projection or spigot 6 on the potentiometer housing 1. The member 8 has a pair of cylindrical holes 10 disposed on a diameter C through the cylindrical opening 9. The holes 10 are arranged such that they are capable of alignment with the holes 7 in the housing 1 when the member 8 is located on the projection or spigot 6.

A loop of wire 11 is located over the projection or spigot 6 to contact the face 5 of the housing 1.

With the lever 2 removed, the member 8 is located over the projection or spigot 6. The lever 2 is then fitted in place by means of the screw 3. A stop means 12 moulded on the surface of the member 8 is arranged to make contact with the lever 8 to limit the traversal of the wiper on the track inside the potentiometer. The lever 2 is intended to be acted upon by an external mechanism (not shown) such as a throttle mechanism in an automotive engine. When the lever 2 is displaced by this mechanism the output voltage at the third terminal (not shown) of the potentiometer will change, this output voltage being a function of the position of the external mechanism. For some automotive applications it is required that when the lever 2 is in contact with the stop 12 the wiper is in a position a short distance from one end of the track such that an accurately predetermined output voltage is obtained at the third terminal. This is arranged in the method of the invention by applying a voltage to the track by way of the first and second terminals and monitoring the output voltage at the third terminal. With the lever 2 maintained in contact with the stop 12 the housing 1 is rotated relative to the member 8 about the axis A until precisely the desired output voltage is obtained. The member 8 is then secured to the face 5 of the housing 1.

This is achieved by passing an electrical current through the wire 11 to heat the wire 11 and locally melt the thermoplastics material of the member 8 and housing 1. The current is then interrupted and on cooling the member 8 becomes bonded to the housing 1. The surplus wire 11 protruding from the assembly is then cut off. The slotted holes 7 in She housing 1 allow the necessary relative rotation of the member 8 and housing 1 to be effected while allowing threaded fasteners (not shown) to be subsequently passed through the cylindrical holes 10 and the slotted holes 7 to secure the potentiometer to a required support. The feature, previously referred to, of the slotted holes 7 being displaced towards one side of the line B allows an increase in the degree of relative rotation between the member 8 and housing 1 to be achieved.

If desired, alternative methods, e.g. involving the use of an adhesive or ultrasonic welding, may be used to secure the member 8 to the housing 1.

The provision of the cylindrical holes 10 in the member 8 facilitates mounting of the potentiometer on a support since they act as a guide and hold the threaded fasteners at right angles to the face of the support. This method of mounting using two cylindrical holes is also advantageous for systems that require a known angular relationship between a mechanical datum at the body of the potentiometer and the potentiometer operating lever. There may however be disadvantages in the use of the cylindrical form for the holes 10. If the external mechanism acting on the lever 2 is an automotive engine throttle mechanism, a mechanical stop is usually provided to retain this mechanism in a desired, e.g. idling, position. Ideally when in this position the lever 2 of the potentiometer should also be just in contact with the stop 12 on the member 8.Mechanical tolerances may result in the throttle mechanism bringing the lever 2 into contact with the stop 12 before the throttle mechanism contacts its own mechanical stop. This can subject the potentiometer to excessive mechanical force and cause damage thereto. This situation may be avoided by always ensuring that the throttle mechanism contacts its mechanical stop a few degrees away from the stop 12 on the potentimeter. This may not be an ideal state because it means that the output voltage from the potentiometer may not be at the precisely required level when the throttle mechanism contacts its stop. A solution to the problem is to replace the cylindrical holes in the member 8 by slotted holes as shown by the dotted outline 13. This enables the potentiometer to be rotated while being fitted to its support and prior to tightening the threaded fasteners, such that when the throttle mechanism is in contact with its mechanical stop, the lever 2 on the potentiometer is arranged to be precisely against the stop 12.

Claims (17)

1. A method of making a rotary electrical potentiometer of the type comprising a housing having therein an elongate arcuate electrical resistance track; an electrically conductive wiper supported on a carrier for rotation about an axis in said housing and adapted to traverse said track; rotary actuating means external to said housing and cooperating with said rotatable carrier by way of a cylindrical aperture extending through a face of said housing; first and second electrically conductive terminals connected to opposite ends of said track and adapted for connection to a voltage source, said wiper being connected to a third terminal at which an output voltage is capable of being derived whose level is a function of position of said wiper on said track; said method being characterised by; providing external to said housing, in contact with said face of said housing, a member having an opening therein located concentrically with said cylindrical aperture in said face of said housing and allowing the cooperation between said actuating means and said rotatable carrier, said member being adapted and arranged to be rotatable about said axis; effecting relative rotation between said member and said housing, whilst said actuating means is maintained in a predetermined position of rotation relative to said member, until a desired output voltage is obtained at said third terminal, then securing said member to said housing.

2. A method according to Claim 1 in which the said housing is provided with an external cylindrical projection extending from the said face concentrically with the said cylindrical aperture, the said projection being located through said opening in said member such that said menoer is rotatable on said projection.

3. A method according to Claim 1, or 2, in which the said member incorporates stop means adapted and arranged to cooperate with said actuating means and such that with said actuating means maintained in contact with said stop means, effecting of relative rotation between said member and said housing results in said wiper traversing said track and thereby causing said output voltage to change, the said menoer being secured to said housing at a position where said output voltage is at a desired level.

4. A method according to Claim 1, 2, or 3, in which the said member is provided with two opposed holes through it in alignment with corresponding holes through the said housing of the potentiometer and such that threaded fasteners are able to be passed through the holes to secure the potentiometer to a support.

5. A method according to Claim 4, in which the holes in the- member are provided of circular form and the holes in the housing are of slotted form.

6. A method according to Claim 4, in which the holes in both the member and the housing are provided of slotted form.

7. A method according to Claim 5, or 6, in which the slots in either the member or housing are displaced towards one side of a line through them and the said axis, whereby a greater degree of relative rotation between the member and housing can be achieved.

8. A method according to any preceding Claim in which the rotary actuating means comprises a lever connected at one end to the wiper carrier.

9. A method according to Claim 8 in which the said lever has a portion arranged to contact a stop means on the said member.

10. A method according to any preceding Claim in which the said member and said housing comprise plastics material.

11. A method according to Claim 10 in which the member and the housing are secured together by means of an adhesive.

12. A method according to Claim 10 in which said plastics material is a thermoplastics material.

13. A method according to Claim 12 in which the menber and housing are secured together by means of ultrasonic welding.

14. A method according to Claim 12 in which a loop of thin wire or metal tape is provided between the member and housing and an electric current passed through it to generate heat to cause the thermoplastics material to locally melt, and such that when the current flow is discontinued and the wire or tape cools, the member and housing become fused together in the region of the wire or tape.

15. A method according to Claim 14 in which excess wire or tape protruding from the interface between the member and housing is subsequently removed by severing.

16. A method of making a rotary electrical potentiometer substantially as hereinbefore described with reference to the accompanying drawings.

17. A rotary electrical potentiometer whenever made by the method of any of the preceding Claims.