Digital position sensor

A sensor for providing an indication of the movement of one component of a system with respect to another includes a plurality of discrete electrodes and a common electrode affixed to the non-moving component along the axis of movement of the moving component and a pointer affixed to the moving component also along the axis of movement. As the moving component moves, it positions the pointer at consecutive ones of the plurality of discrete electrodes forming a capacitor structure with each in succession. When interrogated, the capacitor provides a signal indicative of the position of the moving component. The sensor is useful for indicating linear movement, angular displacement and pressure, strain and temperature when adapted to structures which indicate such parameters as a function of displacement. Alternative structures include "Bar Codes" with a laser as a pointer and magnetic stripes with a magnetic read head as the pointer. The output of the sensor may be employed as a feedback signal to a position controller for, for example, a computer-driven machine tool or for a digital hydraulic control system.

FIELD OF THE INVENTION 
This invention relates to a digital position sensor and more particularly, 
to such a sensor useful to indicate, for example, the displacement of a 
piston in a cylinder or the angle of rotation of a shaft. 
BACKGROUND OF THE INVENTION 
Digital sensors are well known in the art for sensing the level of a fluid. 
U.S. Pat. No. 5,138,880 for example, discloses a level sensing probe with 
two concentric cylinders. The probe is formed with a set of capacitors and 
employs the fluid to be measured as the dielectric. The capacitors are 
arranged along the axis to he measured, each capacitor representing a 
discrete level increment and is assigned a unique time slot in a switching 
sequence. In operation, an AC signal is applied across each capacitor (in 
it's time slot) and the result is compared with the inverted signal 
applied across the first level capacitor. A logic "1" results when the 
fluid is present at the interrogated capacitor. All the capacitors are 
grounded except the reference and the interrogated capacitor. 
U.S. Pat. No. 3,935,739 also discloses a level sensing capacitive probe 
with a common electrode and individual opposing electrodes. An AC signal 
is impressed on all the capacitive elements simultaneously and the 
presence of the fluid (dielectric) material yields a current at each 
capacitor which is greater than the current produced in the absence of the 
fluid. The AC currents are rectified or summed to produce an analog output 
representing the height of the fluid column measured. 
U.S. Pat. No. 3,343,415 discloses a cylindrical capacitor with a common 
inner electrode and discrete electrodes spaced apart along the vertical 
axis of the fluid column measured. The signal output from two adjacent 
capacitors are compared. If both were immersed in fluid or if both were 
above the fluid, the outputs would be the same. If only one were immersed, 
the outputs would be different providing an indication of the fluid level. 
BRIEF DESCRIPTION OF THE INVENTION 
The invention is based on the recognition that a set of discrete sensors 
such as the electrodes of a capacitor of the above-noted fluid level 
sensors could be used to determine, for example, the angular displacement 
of a shaft or the displacement of a piston in a cylinder in the absence of 
a fluid. If, for example, the discrete electrodes were disposed along the 
axis of a non-moving component, such as on the inner surface of a cylinder 
and a common electrode were disposed on the outer surface of the moving 
component, such as the piston, a pointer extending from the common 
electrode to the discrete electrodes would provide a moving electrode 
positioned by the movement of the moving component to successive ones of 
the discrete electrodes. The pointer forms a capacitor with the discrete 
electrode located at the position of the pointer, air between the discrete 
electrode and the pointer providing the dielectric. The discrete 
electrodes are operative as switches activated by the pointer. 
For the determination of the angular displacement of, for example, a shaft 
such as in a throttle of an automobile, the discrete electrodes are 
arranged in an annular geometry about the inner surface of the throttle 
casing and the common electrode and the pointer are arranged in an annular 
geometry about the outer surface of the throttle shaft. The rotation of 
the throttle moves the pointer to the position of consecutive discrete 
electrodes which produce characteristic signals representative of the 
position of the pointer. 
The discrete electrodes may be arranged in a coded configuration which 
generates a code representative of the position of the moving component in 
response to the movement of the pointer to the position of the discrete 
electrode. In such an embodiment, the codes need not only be formed by 
electrode configurations but also can be formed to be read optically or 
magnetically. For example, instead of discrete electrodes, a set of bars 
such as a bar code can be formed on the moving piece part along the axis 
of movement. The bar code is read out by a laser as is now in common usage 
at super market check out counters.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THIS INVENTION 
FIG. 1 shows an arrangement 10 in accordance with the principles of this 
invention in which, for example, a machine includes first and second 
components one of which moves with respect to the other and where the 
position of the moving component is to be determined. Such an arrangement 
is particularly useful in digital hydraulic control systems where the 
piston is driven to specified positions in a cylinder as, for example, in 
a computer-driven machine tool and the position of the piston at any given 
instant is fed back to the controller. 
Specifically, FIG. 1 shows a portion of a stationary or fixed-position 
component 12 of a machine which may be, for example, a cylinder in which a 
piston moves. A plurality of electrically-isolated "position" electrodes, 
13A, 13B, 13C - - - 13N, is formed, for example, on the interior face of 
component 12 along with a spaced-apart common electrode 15. A multiplexer 
16 has an output connected to each of the position electrodes as shown in 
the figure. 
An output of an excitation generator 17 is connected to the input of 
multiplexer 16. Also, outputs of logic circuit 19 are connected to inputs 
to multiplexer 16. Logic circuit 19 is operative to permit excitation 
interrogation of the position electrodes one at a time (in sequence) and 
to signal on its output (20) the position of the moving component such as 
the piston. 
FIG. 1 also shows a moving shaft 21 which is connected to the piston. A 
"floating" pointer 22 is connected to the shaft and thus moves up and down 
as viewed, in concert with the movement of the piston. The pointer can be 
seen to bridge the space between the common electrode and the linear 
arrangement of position electrodes overlying one position electrode at a 
time as shown. The position electrode corresponding to the pointer 
produces a signal when interrogated. 
The reason the signal is produced is due to the fact that the pointer is a 
metal element and forms two capacitors in series with the corresponding 
position electrode and the common electrode. An input to a detector 23 is 
connected to the common electrode ,15, and the output of detector 23 is 
connected to the input of logic circuit 19. The instant position of the 
piston is indicated at output 20 from logic circuit 19 as that of the 
position electrode (of the linear set of position electrodes) which 
produces a signal during an interrogation cycle. The output signal may be 
applied to a digital display or to a controller represented by block 24. 
The arrangement of FIG. 1 includes both position electrodes and a common 
electrode on a non-moving component of a machine. In another embodiment 
only position electrodes need be formed on the non-moving component. The 
pointer may itself comprise the common electrode by being electrically 
connected to the detector. Such an arrangement is shown in FIG. 2 where a 
set 28 of position electrodes is attached to one component and the pointer 
29 is connected to the other component (the moving one). 
FIG'S. 3 and 4 show cross sectional views of the position electrodes and 
pointer organizations for the embodiments of FIG'S. 1 and 2. In FIG. 3, a 
position electrode 30 and the common electrode 31 are bridged by pointer 
32 which moves with shaft 33. In FIG. 4, position electrode 40 is formed 
on one component (without the common electrode). Pointer 41 (the common 
electrode) moves with shaft 42. 
FIG. 5 illustrates an embodiment where the rotational position of a moving 
(driven) component is measured with respect to an arcuate arrangement of 
position electrodes with respect to which it rotates as in this case, for 
example, in a vehicle throttle mechanism. FIG. 5 specifically illustrates 
the rotation of pointer 50 with shaft 51 of a throttle. The arrangement of 
position electrodes may be defined on the inner face of the sleeve within 
which the throttle rotates as indicated at 52 or on the inner face of a 
case attached to the throttle housing. The position electrodes are 
arranged from zero degrees (closed) to fully open, illustratively, in ten 
degree increments as shown in FIG. 5. 
The degree of resolution shown in FIG'S. 1 or 5 can be defined arbitrarily 
by increasing the number of position electrodes and by any one of a number 
of coded positioning arrangements well known in the art. 
It should be clear at this juncture that a technique useful for liquid 
level measurements and requiring the presence of a liquid for operation is 
adapted herein for measuring the position of a moving piece part in a 
machine in the absence of a liquid. The arrangement employs a pointer on 
the moving piece part with an air gap providing the insulation for the 
capacitor formed by an opposed position electrode and the pointer. 
Alternatively, the pointer may be affixed to the non-moving piece part 
with discrete electrodes on the moving piece part. 
This approach may also employ an optical source (i.e. a laser) which may be 
attached to a non-moving piece part to, in effect, function as a pointer 
and coded indications 58 (i.e. a series of bar codes) could be attached to 
an associated movable piece part functioning as position "electrodes". A 
detector 59 (attached to the non moving piece) would then be operative as 
a bar code reader, where each bar code is associated with a specific 
position as represented in FIG. 6. 
Alternatively, the codes of the embodiment of FIG. 6 may comprise magnetic 
stripes and the detector (59) may comprise a magnetic stripe read head. 
The simple expedient of placing position indicators on a non-moving (or 
moving) component of a machine and the placement of a pointer (electrical, 
optical, or magnetic) on a moving (or non-moving) component of a machine 
not only provides an inexpensive and reliable position indicator for 
providing feedback, for example, for a digital hydraulic control system, 
but also provides a mechanism for sensing pressure, strain, force, 
acceleration, temperature or any other parameter that can be translated 
into a linear or rotational displacement. 
FIG. 7 illustrates an example in which pressure is sensed by a position 
sensor in accordance with the principles of this invention. The sensor, in 
this embodiment, includes a position-coded strip 60 spring loaded by 
spring 61 and free to move within cylinder 62 responsive to pressure 
applied in the direction of arrow 63. The pressure is applied in a 
direction to compress spring 61. Thus, coded strip 60 moves along the axis 
of cylinder 62. The sensor also includes, for example, a laser (and a 
detector) 65 affixed to cylinder 62 and operative under the control of 
control circuit 66 to read a position code on code strip 60. Code strip 
60, in this embodiment, may be encoded in a manner analogous to the 
encoding of optical disks (compact disks). 
Pressure (or force) can be applied directly to a shaft (arrow 63) and may 
be calibrated by the load (spring) on the position code strip. The applied 
pressure is indicated at display 67 in FIG. 7. 
FIG. 8 illustrates an example wherein pressure (force) manifests itself by 
angular displacement. In the embodiment of FIG. 8, a position coded disk 
70 rotates about on axis 71. Sensor 72 is positioned to read codes (not 
shown) arranged along the edge of disk 70. A lever arm (or gear assembly) 
represented by line 74 is connected to disk 70 to rotate the disk (against 
a load) to an angular position which is a function of the force applied. 
Once again, disk 70 is encoded, for example, as are optical disks, and 
sensor 72 is a laser (and a detector) operative, under the control of 
control circuit 76, to provide a pressure indication at display 77. 
Position sensors, in accordance with the principles of this invention,are 
particularly useful, for example, in feedback loops for 
computer-controlled machine tools or digital hydraulic control systems.