Apparatus for determining the position and velocity of a moving object

An apparatus for determining the position of a movable object is disclosed. A pair of fixed elements having spaced electrically conductive surfaces form a capacitor. A movable element having an electrically nonconductive composition is connected to the movable object. The movable element is disposed between the fixed element pair. In response to the movement of the movable element, the capacitance value of the variable capacitor changes. A circuit detects the change in capacitance and determines the relative position of the movable object.

TECHNICAL FIELD 
This invention relates generally to apparatus for determining the position 
of a moving object and, more particularly, to an apparatus for determining 
the position of a moving object using capacitive sensing. 
BACKGROUND ART 
Sensors are used to provide positional information for use by 
servomechanisms. Servomechanisms are used in a plurality of control 
systems that utilize feedback control. Such sensors comprise various 
technologies, including: optical, inductive, mechanical, acoustic, or 
capacitive-type technologies. However many of these sensor technologies 
cannot survive in the harsh environments typically exposed to work 
vehicles, e.g. extreme temperature, humidity, dust, oil, moisture, 
vibration, shock, etc. 
One example of a servomechanisms for a work vehicle is an electrohydraulic 
valve system. Electrohydraulic valve systems provide the muscle for 
high-force applications. An electromechanical actuator provides the 
necessary linear or rotary motion to displace the spool of an hydraulic 
valve to a desired position. Typically a position sensor measures the 
position of the actuator armature to achieve feedback control. 
The most common method of determining the position of the armature is to 
connect an external sensor to the actuator. Such sensors often take the 
form of linear voltage differential transformers (LVDT). While the 
addition of the LVDT provides the desired information, the excessive cost 
of the sensor due to the associated complex electronic circuitry and EMI 
shielding requirements make the LVDT undesirable. 
Due to the inherent simplicity of capacitive technology, it may be 
desirable to use such technology in work vehicle applications. Capacitive 
technology includes the advantages of a non-contacting sensor design that 
lends to long term reliability. Additionally, capacitive technology can be 
used in an hydraulic environment to which the entire sensor can be fully 
immersed in hydraulic fluid and still provide good accuracy. 
However existing capacitive sensor technology has several drawbacks. For 
example, such capacitive sensors utilize moving electrically conductive 
elements to give positional information. Unfortunately the moving elements 
tend to cause poor sensor reliability because the movement weakens the 
electrical connections. Further the moving elements also introduce 
unwanted "noise" to the control system. Other sources of problems with 
existing capacitive sensor technology include the addition of circuitry to 
deliver an "input" signal to the capacitive elements, and circuitry to 
condition the "output" signal. The additional circuitry adds excessive 
cost and complexity to the sensor. 
The present invention is directed to overcoming one or more of the problems 
as set forth above. 
DISCLOSURE OF THE INVENTION 
In one aspect of the present invention an apparatus for determining the 
position of a movable object is disclosed. A pair of fixed elements having 
spaced electrically conductive surfaces form a capacitor. A movable 
element having an electrically nonconductive composition is connected to 
the movable object. The movable element is disposed between the fixed 
element pair. In response to the movement of the movable element, the 
capacitance value of the variable capacitor changes. A circuit detects the 
change in capacitance and determines the relative position of the movable 
element.

BEST MODE FOR CARRYING OUT THE INVENTION 
The present invention is particularly suited for determining the position 
and velocity of a moving object. For example FIG. 1 is a cross sectional 
view of the preferred embodiment of the present invention. As shown the 
present invention 100 includes a pair of fixed elements 105 that have 
electrically conductive surfaces 110,115. The electrically conductive 
surfaces 110,115 are spaced apart and are parallel to one another. One of 
the surfaces 110 receives electrical energy of one polarity, while the 
other surface 115 receives electrical energy of a second polarity. A 
movable element 120, which is composed of an electrically nonconductive 
material, is disposed between the fixed element pair 105. Preferably the 
movable element 120 is composed of a dielectric material having a higher 
dielectric value than that of the medium surrounding the elements 105,120, 
e.g. air or hydraulic fluid. Advantageously the fixed element pair 105 and 
movable element 120 form a variable capacitor 125, where the movement of 
the movable element 120 relative to the fixed element pair 105 varies the 
effective capacitance of the variable capacitor 125. A coupling shaft 130 
connects the movable element 120 to the moving object to provide the 
necessary movement of the movable element 120. 
In the preferred embodiment, the fixed element pair 105 and movable element 
120 each are of a cylindrical shape, which is aptly illustrated by FIG. 2. 
Although a cylindrical configuration is shown, the present invention is 
not limited to the illustrated geometrical shape as a myriad of other 
geometrical shapes may be apparent to those skilled in the art. For 
example, the variable capacitor 125 may instead consist of planar 
elements. However, the illustrated configuration is chosen because the 
design achieves a high capacitance value per unit area and is desirable 
for a particular application to be discussed infra. 
The relationship between the capacitance of the variable capacitor 125 and 
the relative position of the moving element 120 to the fixed element pair 
105 is now discussed. The total capacitance, C.sub.tot, of the variable 
capacitor 125 is shown by the following equation: 
EQU C.sub.tot =(C.sub.m * X)+(C.sub.s * (L-X)) Eq. 1 
where the quantity (C.sub.m * X) corresponds to the capacitance value 
associated with the moving element 120 and the quantity (C.sub.s * (L-X) 
corresponds to the capacitance value associated with the medium occupying 
the space between the fixed elements, e.g. air or hydraulic fluid. 
Eq. 1, however, may be simplified by the following relationship: 
EQU C.sub.tot =a*X Eq. 2 
where, a, is the linear coefficient that is associated with the dielectric 
material of the moving element 120 and X represents the amount of 
displacement of the moving element 120. 
Although the above equation does not directly compensate for temperature 
variations of the variable capacitor 125, those skilled in the art may 
readily make the necessary adjustments since the capacitance value may 
change in response to varying temperatures. 
A block diagram of the electronic circuitry that is associated with the 
present invention is now shown with reference to FIG. 3. A timing means 
305 produces a frequency modulated position signal in response to 
capacitance value of the variable capacitor 125. Preferably, the timing 
means 305 is a circuit manufactured by National Semiconductor as part no. 
LM 555. The period, T, of the position signal is related to the 
capacitance value, C.sub.tot, of the variable capacitor 125 by the 
following equation: 
EQU T=(0.693* (R1+R2))* C.sub.t =b* C.sub.tot Eq. 3 
substituting C.sub.tot in Eq. 2, Eq. 3 becomes 
EQU T=b* (a*X) Eq. 4 
since the constants, a,b, are known values the period, T, represents the 
relative position of the movable element, X. 
A converting means 310 receives the frequency modulated position signal and 
transforms the position signal into pulse width modulated (PWM) form. The 
PWM position signal is then delivered to a control means 315. 
Preferably the control means 315 is a microprocessor based circuit that 
employs either a look-up table or an empirical equation to determine the 
position of the movable element 120. Thus, the control means 315 may 
include an EPROM to store empirically determined data that relates a 
plurality of position signal magnitudes to a plurality of displacement 
values. For example, the control means 315 receives the PWM position 
signal and retrieves the stored characteristics from the EPROM and 
compares the characteristics to the representative signal to determine the 
position of the moving element 120 with respect to the fixed element pair 
105. A two-dimensional look-up table of a type well-known in the art may 
be used to complete the comparison and select the value. The number of 
characteristics stored in memory is dependent upon the desired resolution 
of the system. Interpolation may be used to determine the actual value in 
the event that the measured and calculated values fall between the 
discrete values stored in memory. 
Once the positional information is determined, the control means 315 may 
then determine the velocity of the moving element 120 via differentiation 
techniques that are well known in the art. 
The block diagram of FIG. 3 depicts a complete working model of the present 
invention. The specific circuit configuration to carry-out the invention 
is a matter of design choice and is not critical to the present invention. 
INDUSTRIAL APPLICABILITY 
The operation of the present invention is best described in relation to its 
use in relation to work vehicles. For example reference is now made to 
FIG. 4, where an electrohydraulic valve system is shown. Although the 
present invention is described in relation to an electrohydraulic valve 
system, it is understood that the present invention may be used in a 
variety of other work vehicle applications where positional and velocity 
information is desired. 
As shown, an electromagnetic actuator 405 is used to position a spool 410 
of an hydraulic valve 415. The electromagnetic actuator 405 includes a 
coil 420 and an armature 425. A driving circuit 430 delivers electrical 
energy to the coil, which responsively energizes. The energized coil 
causes the armature 425 to displace, which positions the spool 410. In the 
illustrated embodiment the present invention provides positional 
information to the control means 315, which controls the magnitude of the 
driving signal to yield the desired spool position. 
For example the moving element 120 is connected to the armature 425. As the 
armature 425 moves so does the movable element 120. The movement of the 
movable element 120 causes a change in capacitance of the variable 
capacitor 125. The timing means 305 detects the capacitance value of the 
variable capacitor 125, and produces a position signal having a period 
proportional to the capacitance value, which is proportional to the 
displacement of the moving element 120. The converting means 310 receives 
the frequency modulated position signal and produces a PWM position signal 
to the control means 315. The control means 315 performs the necessary 
comparisons and determines the position of the moving element 120, which 
is proportional to the position of the spool 410. The control means 315 
then controls the driving circuit 430 to deliver the proper driving signal 
to the coil 420 to achieve the desired spool position. 
Thus, the determined position of the moving element 120 usable in 
connection with a number of control and diagnostic systems. As shown 
above, the positional information can be used to provide a closed-loop 
control for a solenoid by providing the controller with an actual position 
that can be compared with a desired position in order to responsively 
change the magnitude of the coil current accordingly. Alternatively, the 
positional information can be used in a diagnostic system to determine 
whether the device being actuated by the armature is operating properly in 
response to the desired function being indicated by the control system. 
Other aspects, objects and advantages of the present invention can be 
obtained from a study of the drawings, the disclosure and the appended 
claims.