[0001] This invention relates to a rotary potentiometer that is particularly, but not exclusively,
intended for mechanical connection to a rotatable mechanism to be monitored and which
gives an electrical output voltage of magnitude proportional to the degree of rotation
of the mechanism. A particular application of the potentiometer is to a rotatable
throttle or butterfly shaft used in an internal combustion engine.
[0002] The present invention provides a rotary potentiometer comprising: a housing; an arcuate
film electrical resistance element supported in said housing; electrically conductive
terminals connected to opposed ends of said element for application of an electrical
potential across said resistance element; a wiper of electrically conductive material
supported on a rotatable carrier in said housing and arranged to traverse said element;
means to electrically connect said wiper to a further terminal in said housing so
that an output voltage at said further terminal indicates the angular position of
said wiper; stop means for said wiper carrier arranged whereby a predetermined position
of restraint of said wiper at an end region of said arcuate element is obtained; resilient
means to return said wiper to said position of restraint from a position of rotation;
and a film of electrically insulating material overlying said element at the said
end region thereof where said stop means is provided and such that in said region
said wiper is electrically insulated from said element.
[0003] The stop means may be adjustable so that the position of restraint of said wiper
relative to said resistance element can be varied and the angle through which the
wiper has to be rotated from the position of restraint before it leaves said film
of electrically insulating material may be preset.
[0004] The said resilient means suitably comprises a helical spring of conductive material
and also electrically connects said wiper to said further terminal in said housing.
[0005] An operating spindle is preferably provided, arranged for rotation of said wiper
and adapted to be secured to an external rotatable mechanism to follow .rotation thereof.
[0006] Suitably said spindle is hollow and dimensioned and arranged to receive therein a
matching portion of said external mechanism.
[0007] Suitably said hollow spindle has an end region of split form and is provided at said
end region with a spring collar whereby said spindle is adapted to grip said external
mechanism.
[0008] Fine adjustment of the electrical resistance of said element at the end thereof where
said insulating film is provided may be effected by means of a narrow radial cut formed
through the resistance element and extending from an edge of said element. This adjustment
may be effected either by cutting through the insulating film, or a suitable portion
of the element may be left uncovered by the insulating film where said adjustment
is required.
[0009] The aforementioned rotary potentiometer may be operatively connected to a microprocessor
control system responsive to the voltage output of the wiper so that when the wiper
is on the insulating layer the microprocessor is in a quiescent state and the microprocessor
converts to its operative state on detecting the abrupt voltage change when the wiper
leaves said insulating layer.
[0010] The invention is now described by way of example with reference to the accompanying
drawings in which:
Figure 1 is a plan view of a rotary potentiometer according to the invention with
part of the housing removed;
Figure 2 is a section through the potentiometer of Figure 1 but with the housing complete;
Figure 3 is an exploded view of wiper carrier stop means employed in the potentiometer
of Figures 1 and 2;
Figure 4 is a plan view of an arcuate resistance element employed in the potentiometer
of Figure 1; and
Figure 5 is a spring collar arrangement for use in securing an external mechanism
in the operating spindle of the potentiometer of Figure 2.
[0011] Referring to the drawings, a rotary potentiometer has a two part housing 1 formed
of a plastics material and provided with mounting flanges 2. An arcuate film electrical
resistance element 3, eg. of electrically conductive polymer material, is provided
on a support on the inner surface of a base part lA (Figure 2) of the housing. Electrically
conductive terminals 4 and 5 are secured in the housing and are electrically connected
by leads 6 and 7 and film conductors 8 and 9 (Figure 4) to the ends of the resistance
element 3. A multi-fingered wiper 10 of resilient electrically conductive material,
which may be a base or precious metal, is supported on a rotatable carrier 11, the
carrier 1.1 having a spindle 12 which rotatably fits into a cylindrical aperture 13
in the base part lA of the housing. As the carrier 11 is rotated in the housing, the
wiper 10 is caused to traverse the arcuate resistance element 3.
[0012] An adjustable stop means is provided for the wiper carrier 11 and comprises a threaded
screw 14 and nuts 15 retained by appropriately shaped regions 16 and 17 of the two-part
housing. The wiper carrier 11 is arranged to come into contact with the end of the
screw 14 of the stop means so that the wiper 10 is restrained in a position at an
end of the resistance element 3. This position of restraint of the wiper relative
to the element 3 can be precisely determined by adjusting the screw 14 by means of
a small screwdriver inserted through an aperture in the housing 1.
[0013] A helical spring 18, eg. of beryllium-copper, is provided, encircling a portion of
the wiper carrier 11 and electrically connected at one end to a further terminal 19
mounted in the housing. The other end of the spring is electrically connected to the
wiper 10. This spring 18 serves a dual purpose. It provides the necessary electrical
connection between the wiper 10 and terminal 19 in the housing. It also acts as a
return spring for the wiper and carrier assembly such that when the wiper carrier
11 is rotated by application of an external influence to cause the wiper 10 to traverse
the resistance element 3 to fulfil a desired potentiometric function, upon release
of the external influence the wiper carrier 11 is returned to its initial position
in contact with the screw 14 of the stop means.
[0014] A film 20 (Figure 4) of electrically insulating material such as an epoxy resin is
applied, eg. by screen printing, on top of the resistance element 3 at an end region
thereof such that when the wiper 10 is in its initial position of restraint with the
carrier 11 in contact with the screw 14 of the stop means, the wiper 10 is electrically
insulated from the resistance element 3 by the film 20. This arrangement effectively
provides a switch incorporated in the potentiometer. If a voltage supply is connected
to terminals 4 and 5 and an output voltage obtained from terminals 4 and 19, then
when the wiper carrier 11 is in contact with the stop means and the wiper 10 is in
contact with the insulating film 20, the wiper 10 is in an open circuit condition
and no output voltage will appear between terminals 4 and 19. If the wiper and carrier
assembly is rotated, then at the point where the wiper leaves the insulating film
20 and contacts the resistance element 3 an output voltage is obtained between terminals
4 and 19. As the wiper and carrier assembly 10, 11 is continued to be rotated, the
output voltage increases up to a maximum value. If an electrical resistor of appropriately
high value (eg. 50,000 ohms) is connected between the wiper terminal 19 and the voltage
supply, then when the wiper is in its initial position in contact with the insulating
film, an output voltage corresponding to the maximum value is obtained between terminals
4 and 19. As the wiper and carrier assembly is rotated, the output voltage switches
to a low level as the wiper leaves the insulating film 20 and contacts the resistance
element 3. The output voltage then continues to rise, up to a maximum value, as the
wiper and carrier assembly is further rotated.
[0015] The angle through which the wiper and carrier assembly is rotated before the wiper
leaves the insulating film 20, can be precisely determined using the adjustable stop
means 14, 15.
[0016] Fine adjustment of the electrical output obtained from the potentiometer at the point
where the wiper leaves the insulating film and contacts the resistance element can
readily be effected by selective removal or abrasion of a portion of the resistance
element underlying the insulating film in Figure 4. To achieve this, a narrow radial
cut 21 is suitably formed through the insulating film 20 and underlying resistance
element 3 and extending from an edge of the element for a distance sufficient to provide
the desired adjustment of the resistance value of the portion of the element underlying
the insulating film.
[0017] In order to avoid having to cut through the insulating film, a portion of the resistance
element in the region where the cut 21 is to be made may be arranged to be left uncovered
by the insulating film when the latter is applied. Adjustment of the resistance element
will be effected in a region with which the wiper will not come into contact.
[0018] A further stop 24 is provided with which the carrier 11 comes into contact when the
wiper 10 reaches the opposite end of the resistance element to that where the insulating
film 20 is provided.
[0019] For mechanical connection of the potentiometer to a rotatable external mechanism
(not shown) whose rotation is to be followed and monitored by the potentiometer, the
spindle 12 of the potentiometer is suitably of hollow construction and shaped to receive
a shaft or spindle (eg. of D shaped section) of the external mechanism. One or more
slots 22 are provided in the spindle 12 and a spring collar 23 (Figure 5) is provided
to encircle the slotted end of the spindle 12. When the shaft or spindle of the external
mechanism is fitted into the hollow spindle 12, the presence of the spring collar
23 ensures that the spindle 12 securely grips the received shaft or spindle.
[0020] In one particular application, the external mechanism comprises a throttle or butterfly
shaft in an internal combustion engine.
[0021] The aforementioned potentiometer may form part of a microprocessor controlled engine
management system. The microprocessor is in a quiescent state when the engine is running
at tickover speed and a triggering or switching-on signal is required for the microprocessor
when the throttle is opened. Furthemore, the microprocessor must begin its operation
virtually at the instant when a driver applies his foot to the accelerator pedal.
The required triggering is achieved by arranging for a fast rate of change of output
voltage from the potentiometer to occur as the wiper 10 leaves the insulating layer
20 and contacts the resistance element 3. At this point the output voltage drops rapidly
from the maximum level to a low level (which may typically be about 11% of the maximum).
Thereafter, the microprocessor controlled electronics utilises the level of output
voltage from the potentiometer as an indication of the position of rotation of the
throttle.
1. A rotary potentiometer comprising a housing (1), an arcuate film electrical resistance
element (3) supported in said housing (1), a wiper (10) of electrically conductive
material supported on a rotatable carrier (11) in said housing and arranged to traverse
said resistance element (3), means to connect said wiper (10) to a first terminal
(19) in said housing (1), stop means (14, 15) for said wiper carrier (11) arranged
to define a predetermined position of restraint of said wiper (10) at an end region
of said resistance element (3), and resilient means (18) to return said wiper (10)
from a position of rotation to said position of restraint characterised in that second
and third electrically conductive terminals (4, 5) are connected to opposed ends of
the resistance element (3) for application of an electrical potential across the resistance
element (3) so that an output voltage at the first terminal (19) indicates the angular
position of the wiper (10) and a film of electrically insulating material (20) overlies
the resistance element (3) at a region thereof corresponding to the position of restraint
of the wiper (10) so that in said region the wiper (10) is insulated from the resistance
element (3).
2. A potentiometer according to claim 1, wherein said stop means (14, 15) is adjustable
so that the position of restraint of said wiper (10) relative to said resistance element
(3) can be varied and the angle through which the wiper (10) has to be rotated from
the position of restraint before it leaves said film (20) of electrically insulating
material may be preset.
3. A potentiometer according to claim 1 or 2, wherein the said resilient means (18)
is a helical spring of conductive material that also electrically connects said wiper
(10) to said first terminal (19).
4. A potentiometer according to any preceding claim, wherein an operating spindle
(12) is provided, arranged for rotation of said wiper carrier (11) and adapted to
he secured to an external rotatable mechanism to follow rotation thereof.
5. A potentiometer according to claim 4, wherein said spindle (12) is hollow and dimensioned
and arranged to receive therein a matching portion of said external mechanism.
6. A potentiometer according to claim 5, wherein said hollow spindle (12) has an end
region of split form (22) and is provided at said end region with a spring collar
(23) whereby said spindle (12) is adapted to grip said external mechanism.
7. A potentiometer according to any preceding claim, wherein fine adjustment of the
electrical resistance of said element (3) at the end thereof where said insulating
film (20) is provided is effected by means of a narrow radial cut (21) formed through
the resistance element (3) and extending from an edge of said element (3).
8. A potentiometer according to claim 7, wherein said adjustment is effected by cutting
through said insulating film (20).
9. A potentiometer according to claim 7, in which a suitable portion of said element
(3) is left uncovered by the insulating film (20) in the region where said adjustment
is required.
10. An internal combustion engine including a throttle or butterfly shaft fitted with
a potentiometer according to any one of the preceding claims.
11. A rotary potentiometer as claimed in any of claims 1 to 9 connected to a microprocessor
control system responsive to the voltage output of the wiper (10) so that when the
wiper (10) is on the insulating layer (20) the microprocessor is in a quiescent state
and the microprocessor converts to its operative state on detecting the abrupt voltage
change when the wiper (10) leaves said insulating layer (20).