Digital motor feedback for a position actuator

A linear actuator (42) includes a housing (44), and a linearly displaceable piston (46) having a shaft (48) with a gradated position scale (60) and a gradated direction scale (62) disposed by ion implantation on a casing (50) of the shaft, in axial alignment (49) with the shaft stroke. Proximity sensors (64, 66) mounted in the housing sense movement in the position of the scales to provide sensed shaft position signals (68) and sense shaft direction signals (70).

DESCRIPTION 
1. Technical Field 
This invention relates to linear actuators, and more particularly to 
sensing piston displacement therein. 
2. Background Art 
As known, closed loop control of linear actuators requires accurate sensed 
position feedback signals. In prior art actuators the position feedback 
signals are analog, and are produced by a potentiometer or a variable 
differential transformer which is geared to the actuator piston. The 
differential transformers may include either linear variable differential 
transformers (LVDT) or rotary variable differential transformers (RVDT). 
However, LVDT devices are most popular for high accuracy applications. 
LVDT sensors are both cumbersome and expensive, and incorporating them into 
an actuator involves a high degree of complexity. The LVDTs are mounted 
within the actuator piston and, therefore, define the piston's dimensions, 
which generally exceeds the sizes otherwise required for mechanical 
strength. The piston, and the entire actuator, could otherwise be made 
smaller. In addition, the LVDT mounting requires extensive machining of 
the supporting parts within the actuator. Although the RVDT and 
potentiometer devices are mounted external of the actuator and, therefore, 
do not impact actuator geometry to the same extent, they are similarly 
complex and costly. 
Furthermore, when the actuator is used in a system providing a flight 
critical function, such as controlling air inlet position for a gas 
turbine engine, or for fuel metering control within a gas turbine engine, 
the feedback sensors must be duplicated to provide redundant sensing and 
increased reliability. This is at the expense of doubling the complexity 
and cost of signal sensor configurations. 
3. Disclosure of Invention 
The object of the present invention is to provide a method and apparatus 
for sensing position and slew direction of a linear actuator, using a 
digital signal format. 
According to the present invention, a linear actuator comprises a housing 
and a piston which is movable through the housing, over a stroke range 
directed along a displacement axis, the piston includes a shaft having a 
graduated position scale and a graduated slew direction scale disposed 
therealong in axial alignment with the displacement axis, the scale 
gradations comprising ion implanted material, the actuator further 
comprising proximity sensing devices associated with each scale, each 
sensor located in the trajectory of its associated scale for providing a 
pulse signal whenever a scale gradation passes in proximity to the sensor, 
whereby the pulse signals provide indications of piston position and slew 
direction. 
In further accord with the present invention, the gradations of the 
position scale are at fixed intervals, defining equal piston stroke 
increments, and the slew direction scale gradations are spaced at 
increasing distances from one end of the scale to the opposite end of the 
scale, so as to allow sensed discrimination between the piston slew 
direction. 
The linear actuator of the present invention provides a digital signal 
indication of actuator position and slew direction. The motion of the 
piston is detected by passage of the ion implanted scale gradations under 
the proximity sensors. The sensors provide a pulsed signal output which 
can be pulse counted to determine actual position, and also frequency 
counted to provide an indication of slew direction. 
The elimination of analog feedback signal devices, such as variable 
differential transformers or potentiometers, results in greater noise 
immunity, and integrity of the feedback signal. These and other objects, 
features and advantages of the present invention will become more apparent 
in light of the following detailed description of a best mode embodiment 
thereof, as illustrated in the accompanying Drawing.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring first to FIG. 4, in a cross sectioned illustration of a prior art 
linear actuator 12, the actuator includes a barrel assembly 14, and a 
piston assembly 16 which is housed within the barrel and having a hollow 
shaft 18 adapted for reciprocal linear motion along a displacement axis 
19. The shaft includes a casing 20 having a load connector assembly 22 at 
the other end. The load connector, which is shown as having a threaded 
surface 23, is adapted for connection to an actuator load, such as an 
aircraft's stator vane assemlby, engine fuel valve, etc. 
As known, in the fluidic actuator of FIG. 4, displacement is due to the 
differential pressure across the piston 16; between the internal chambers 
26, 28. The piston is in a quiescent position when the differential 
pressure across the piston is at equilibrium. The actual pressure 
difference is controlled by controlling the fluid delivery and fluid 
discharge (i.e. fluid pressure) in chambers 26, 28, as provided through 
fluid orifices (not shown) in the barrel. 
The piston stroke extends from a fully retracted position, phantom piston 
position 29, to a fully extended position, as shown by phantom piston 
position 30. Information on the piston's sensed position is provided by a 
linear variable differential transformer (LVDT) 31, mounted within the 
piston shaft 18. The LVDT is a known type such as the LVDT Model 
2640XS-1923 LVDT manufactured by Schaevitz Engineering Company, 
Pennsauken, N.J. The LVDT is a telescoping device, with a fixed section 32 
mounted through a threaded connection 33 to an anvil 34, and a movable 
section 36. The anvil is secured within the barrel by a shear ring 
positioned in a groove formed by the barrel and the anvil. The movable 
section 36 is connected to the piston load connector assembly 22 through a 
link and socket assembly 37. 
The movable section 36 travels in unison with piston stroke. This causes 
movement of the LVDT magnetic core, changing the magnetic inductance 
between the LVDT primary and dual secondary windings, and producing an 
output signal as the differential voltage between two secondary windings. 
The output signal is provided on lines 38 through the back plate 40 of the 
barrel housing. 
To satisfy avionic equipment reliability requirements the LVDT is a dual 
redundant device. It has duplicate core and winding assemblies within the 
LVDT housing. The dual redundant configuration causes the devices to be 
cumbersome. Incorporating them into the actuator assembly is costly, since 
they require a large envelope and extensive machining of supporting parts 
for installations within the piston assembly. They are also susceptible to 
failure in a vibration environment. 
Referring now to FIG. 1, which illustrtes in cross section a linear 
actuator 42 according to the present invention. The actuator includes a 
barrel housing 44, and a piston assembly 46 having a hollow shaft 48 
adapted to linearly extend and retract along a displacement axis 49. The 
shaft includes a casing 50 with a load connector assembly 52 on the outer 
end. As with the prior art actuator of FIG. 4, shaft displacement is 
determined by the differential pressure across the piston 46, between 
internal chambers 56, 58 of the barrel. 
Thus far the description of the present actuator 42 is similar to that of 
the prior art actuator of FIG. 4. The point of departure of the present 
actuator from that of prior art linear actuators is in the sensing of 
piston position. In the present actuator, piston position and the 
direction of piston travel (extend/retract) is provided by graduated 
scales deposited on the piston shaft casing 50. A position scale 60 and a 
slew direction scale 62 are disposed in axial alignment with displacement 
axis 48, on the piston casing. The coordinate location of the scales on 
the casing perimeter may be selected to suit a desired application. In a 
best mode embodiment, the position and slew direction scales are located 
on proximate opposite sides of the piston casing, i.e. nominal 180 degree 
spacing. This allows for the optional placement of a second set of scales, 
i.e. redundant scales, at a proximate 90 degree spacing from the primary 
scale's locations. 
The scales 60, 62 are disposed along a length L of the piston shaft; a 
distance which is at least as long as the piston stroke range. The scale 
gradations, as described in detail in FIG. 2 for the position scale 60 and 
in FIG. 3 for the slew direction scale 62, comprise ion implanted 
material. Proximity sensors 64, 66 are mounted on the end of the barrel 
housing, in proximity to the piston shaft, and in the trajectory of the 
position scale and slew direction scale, respectively. The sensors are a 
known type, such as P/N 6251800 manufactured by C&A Transducer Inc., 
Garden Grove, Calif. The signal output from the sensors 64, 66 are 
produced on output lines 68, 70. 
Referring now to FIG. 2, which illustrates the gradated position scale 60. 
In a best mode embodiment, the scale includes a plurality of ion implanted 
dots 74, disposed at a center-to-center stroke spacing (d) along the 
length L of the scale. The dots comprise a detectable material such as 
cobalt or nickel, which are disposed on the piston shaft casing by known 
ion implantation techniques. The deposited ion implanted dots change the 
magnetic properties of the shaft casing material, which itself may 
comprise stainless steel AMS 5737. This produces a "relative" change in 
the magnetic properties of the shaft casing in the proximity of the dot 
locations, which are then sensed by the proximity sensors 64, 66 to 
provide the sensed position signal and sensed direction signal indications 
on the lines 68, 70. Radioactive material may also be implanted on the 
piston shaft. This technique would require an appropriate type of sensor. 
As known, ion implantation introduces impurities into the near surface 
region of solid materials, by directing a beam of ions at the solid's 
surface. The ions penetrate the surface material and come to rest within 
the near surface region. An ion source provides the ion stream through 
plasma discharge. As the ions are extracted from the plasma and 
accelerated through a high voltage acceleration field (typically 10,000 to 
500,000 volts), they are passed through a variable transverse magnetic 
field, which allows a lateral sweeping of the beam along the length L of 
the scale. A precision template having the desired dot pattern geometry is 
masked onto the casing surface, thereby allowing the implantation to occur 
only in the defined dot areas. The region between the ion source and the 
masked piston shaft casing is maintained under vacuum to prevent beam 
attenuation. 
The advantages of the implantation process includes precise control of the 
type of impurity to be introduced into the solid, the amount of impurity 
introduced, and the depth of the impurity distribution. The use of 
templates allows precise geometric definition of the piston shaft area to 
be exposed to the ion beam, thereby allowing precise control over the 
geometric pattern of the scale. By integrating the current flow of the 
beam, the total ion charge is obtained (using the known charge per ion), 
such that the number of ions implanted in the source target can be 
precisely controlled. 
The number of dots (N) and the maximum center-to-center spacing (d) between 
dots is determined by the sensed position resolution requirements for the 
particular application. A typical stroke range, i.e. length L, is from two 
to three inches. For a two inch stroke, a 0.020 inch center-to-center 
spacing of the dots provides a 1% scale resolution. This requires 100 dot 
implants, each at a nominal diameter of 0.010 inch. 
FIG. 5, illustration (a) shows the sensed position signal output on lines 
68 from the sensor 64, under a slewing condition of the actuator. The 
illustration is on a split X axis, showing a waveform 76A for a retracting 
slew direction 78A and a waveform 76B for the extension slew direction 
78B. The sensed signal waveforms are signal conditioned, through known 
techniques, which compare the waveform peak amplitudes to a threshold 
V.sub.Th 80. Peak amplitudes which exceed the threshold are converted into 
a pulse. 
FIG. 6, illustration (a) shows a series of pulses 80A, 80B which, for the 
purposes of this description, are assumed to be derived from the waveforms 
76A, 76B. Each pulse marks a gradation (d) on the position scale, and when 
integrated over time, provides a sensed piston position feedback signal. 
FIG. 3 illustrates the slew direction scale 62. The direction scale 
similarly includes a plurality of implanted ion dots 82, but located on a 
variable gradated scale, as opposed to the fixed gradations of the 
position scale 60. In the direction scale the center-to-center spacing of 
the dots implanted towards a first end 84 of the scale is smaller in 
dimension than that of the dots implanted toward a second end 86 of the 
scale. The first and second ends may correspond to either the fully 
extended or fully retracted positions of the actuator. 
The spacing of the dots 82 increases incrementally, from the first end 84 
to the second end 86. The greater or lesser spacing between the sensed 
pulse output from the direction sensor 66 distinguishes the piston slew 
direction, from a first direction (e.g. extend) to a second direction 
(e.g. retract). The actual change in spacing is selectable; depending on 
the sensed accuracy requirements. In FIG. 3, the spacing (S) is shown to 
increase linearly, from a space S.sub.1 to S.sub.1 +.DELTA. to S.sub.1 
+2.DELTA. etc. However, the manner in which the spacing is gradated is not 
limited to any one progression, but may be selected by those skilled in 
the art, as may be necessary for a particular application. 
The dots 82 are implanted in the same manner as the position scale dots 74, 
but using a different geometry template for the scale gradations. FIG. 5, 
illustration (b) shows the sensed pulse output from the sensor 66 for two 
different slew directions. Waveform 88A corresponds to X direction 90A and 
waveform 88B is associated with direction 90B. As shown, the peak to peak 
spacing of the waveforms differ; the spacing of waveform 88A increasing 
with X distance 90A and that of waveform 88B decreasing with increasing X 
distance 90B. 
The waveforms 88A, 88B may be processed in a similar manner as the position 
scale waveforms 76A, 76B. The peak amplitudes are compared against a 
reference threshold voltage 92 (FIG. 5) to provide a conditioned pulse 
equivalent signal, as shown with the pulsed signal of FIG. 6(b). The 
pulses 94A, 94B correspond to the sensed direction waveforms 88A, 88B 
(FIG. 5(b)). When the pulses 94A, 94B are plotted against time (t), the 
waveform 94A has a lower frequency and the waveform 94B has a higher 
frequency. 
As stated hereinbefore, by comparing the sensed position pulsed signals 
80A, 80B (FIG. 6(a)) with the sensed direction pulsed signals 94A, 94B, 
the slew direction may be determined. Sensed direction pulsed signal 
frequencies which are lower than the sensed position frequency indicate 
one slew direction, and those that are higher indicate another slew 
direction. 
Referring again to FIG. 1, antirotation rod 96 provides alignment of the 
trajectory of the scales 60, 62 with the sensors 64, 66 during piston 
travel. The rod is inserted through a hole 98 in the piston 46 into a bore 
100 in the barrel housing. The hole 98 includes an O-ring pressure seal to 
prevent fluid leakage between the chambers 56, 58. The opposite end of the 
rod is located in a second bore 102 in the anvil assembly 103. The two 
bores fix the rod in position. To prevent displacement of the anvil due to 
ballistic forces on the piston during slew, a key 104 is inserted in the 
barrel housing cover 106. The key is fixed in position by a key-slot 108 
in the barrel housing. 
The linear actuator of the present invention provides both sensed piston 
position information, and slew direction, in a digital signal format. This 
format, comprising pulsed signal formation, leads itself to digital signal 
processing techniques and ease of information manipulation by the parent 
control system. Furthermore, the signal format provides improved noise 
immunity and simplification of hardware requirements over that provided, 
or required, by analog formatted approaches. 
It should be understood by those skilled in the art that the use of ion 
implanted positions and slew direction scales on the piston shaft casing 
is not limited to hydraulic type actuators. Any other type of linear 
actuator, including electric or pneumatic, may similarly incorporate the 
casing scales. 
Although the present invention has been shown and described with respect to 
a best mode embodiment thereof, it should be understood by those skilled 
in the art, that various other changes, additions, and deletions may be 
made therein, without departing from the spirit and scope of the invention 
.