Low-drag electrical contact arrangement for maintaining continuity between horizontally movable members

This invention is a low-drag electrical contact arrangement for establishing continuity between upper and lower spaced members which are subject to relative horizontal movement. In one aspect, the invention comprises an electrical commutating arrangement which includes a horizontally disposed linear electrical commutator. A horizontally movable electrically conductive pedestal is positioned below the commutator and defines a clearance therewith. The pedestal is formed with a cavity confronting the commutator. In the cavity is a bead of electrical conductive liquid, the bead being characterized by an upwardly convex meniscus portion which extends across the clearance and contacts the commutator. The surface tension of the bead is sufficient to maintain the bead intact when the commutator and pedestal are displaced horizontally at speeds from zero to at least twelve inches a minute. This arrangement provides a significant advance in highly precise machining processes, such as diamond-turning, where precision is limited by the drag imposed by conventional commutators of the carbon-brush type.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to electrical contacts for maintaining 
electrical continuity between members subject to relative horizontal 
motion. More particularly, it relates to a contact arrangement for 
maintaining electrical continuity between upper and lower spaced surfaces, 
one of which is movable horizontally with respect to the other, while 
imposing minimal drag. This invention was made as a result of a contract 
with the United States Department of Energy. 
2. Problem 
This invention was made in response to a problem which arose during testing 
of an experimental system for automatically positioning a diamond-tipped 
cutting tool to a precision of .+-. 0.6 microinch. The cutting tool was 
mounted on a carriage which in turn was supported and guided along an axis 
by fluid bearings. The carriage was to be driven by a commercially 
available d.c. linear electric motor including a permanent magnet which 
was affixed to the carriage and which extended freely about an elongated 
horizontally extending stationary armature containing a series of axially 
spaced, separate windings. Mounted on the bottom of the armature was a 
horizontally extending linear commutator whose segments were connected to 
the windings. The permanent magnet carried a plurality of standard 
spring-loaded carbon brushes for contacting the commutator segments and 
selectively energizing the armature windings so that, for any position of 
the carriage, only the winding nearest the magnet was energized. The 
linear motor was connected in a conventional continuous-path closed-loop 
position-control system generally similar to that disclosed in U.S. Pat. 
No. 4,221,995, issued on Sept. 9, 1980, to W. E. Barkman. Unfortunately, 
tests of the experimental positioning system showed that because of 
excessive friction resulting in stick-slip problems, the carriage could be 
positioned with a precision of only about 35 microinches. 
3. Previous Related Work 
The above-referenced patent to Barkman discloses a positioning system in 
which the carriage is driven by a linear electric motor which does not 
utilize brushes or a commutator. The motor is relatively bulky and 
generates appreciable heat. U.S. Pat. No. 4,171,496, to Eriksson, 
describes an electrical contacting arrangement including two conductors 
which are disposed for relative rotation and are each formed with an 
annular ring. The rings extend respectively into electrically 
interconnected grooves provided in an auxiliary rotor. The rotor is driven 
to force liquid metal into the grooves, thus connecting the rings on the 
two conductors. U.S. Pat. No. 3,870,914, to Walker, describes an 
electrical contact arrangement for a vertically disposed rotatable 
electric machine in which mercury is vaporized from a heated reservoir in 
the base of the machine and condenses in a vertically extending capillary 
formed by a fixed contact ring and a contact affixed to the rotor of the 
machine. The machine volume exposed to mercury vapor is sealed from 
atmosphere. U.S. Pat. No. 3,916,235, to Massar, describes a contact 
arrangement in which two relatively rotatable conductors form an annular 
gap. Liquid metal from an external supply is pumped through bores to 
completely fill the gap and thus connect the conductors. Preferably, a 
metallic ring is provided in the annular gap to float on the liquid metal. 
U.S. Pat. No. 2,845,554, to Schwab et al, describes a contact arrangement 
for interconnecting a rotatable member and two spaced, fixed members. The 
rotatable member is formed with a ring of triangular section, which 
rotates in the annular space between the fixed members. An electrically 
conductive liquid is circulated through the annular clearances between the 
fixed and rotating members, through a cooler, and back to the clearances. 
None of these arrangements is well suited for use with upper and lower 
elongated members which are subject to relative horizontal movement. 
SUMMARY OF THE INVENTION 
In one aspect, the invention is an electrical contact arrangement that 
includes electrically conductive upper and lower members which are 
separated by a clearance and are mounted for relative horizontal movement. 
The lower member has a horizontal top surface which defines a cavity 
confronting a bottom surface of the upper member. Disposed in the cavity 
is a body of an electrically conductive liquid having a convex meniscus 
portion which extends across the clearance contacts against the 
aforementioned bottom surface. The surface tension of the liquid is 
sufficient to maintain the liquid body intact when the members are 
subjected to relative horizontal movement at speeds up to at least twelve 
inches a minute. In another aspect of the invention, the upper member is a 
horizontally disposed linear electrical commutator and the lowest member 
is an electrically conductive pedestal for supporting the liquid body. In 
another aspect, the invention comprises a low-friction tool-carriage 
arrangement. The arrangement includes a bed which carries fluid-film 
bearings for supporting a carriage which is reciprocatable along a 
horizontal axis. The carriage is reciprocated by a linear electric motor 
which includes an elongated stator having axially spaced windings which 
are connected to a horizontally disposed linear commutator carried by the 
stator. The motor also includes a permanent magnet which extends freely 
about the stator and is affixed to the carriage; the motor moves the 
carriage along the aforementioned axis when a selected stator winding is 
energized. The carriage assembly carries brushes for respectively 
contacting the commutator segments. Each brush assembly includes an 
electrically conductive pedestal which supports a bead of electrically 
conductive fluid having an upwardly convex meniscus portion which extends 
upwardly above the pedestal a sufficient distance to make electrical 
contact with the commutator. 
OBJECTS 
It is an object of this invention to provide a novel electrical contact 
arrangement. 
It is another object to provide an electrical contact arrangement for 
maintaining low-friction contact between a fixed member and member spaced 
therefrom which define an operating clearance, one of the surfaces being 
horizontally movable. 
It is another object to provide a novel electrical commutating arrangement 
utilizing a linear commutator. 
It is another object to provide a novel low-friction carriage arrangement 
for positioning a tool--such as a diamond knife, a probe, or a gauging 
member. 
Additional objects, advantages and novel features of the invention will be 
set forth in part in the description which follows, and in part will 
become apparent to those skilled in the art upon examination of the 
following or may be learned by practice of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to the experimental positioning system described above under 
"Background", we have found that the aforementioned stick-slip problem is 
due in large part to friction, or drag, between the linear commutator and 
the spring-loaded carbon brushes bearing thereon. To overcome this 
deficiency, we have developed a special brush which reduces that friction 
to close to the absolute minimum. The brush also reduces the transmission 
of vibrations across the brush-to-commutator interface. As a result, the 
desired positioning precision (.+-. 0.6 microinch) can be obtained at 
carriage speeds up to many inches a minute. 
The invention is illustrated as incorporated in a low-friction carriage 
arrangement designated as 7. The carriage arrangement is designed for use 
in microinch machining operations (e.g., diamond turning) and for carriage 
speeds on the order of from zero to, say, twelve inches a minute. For 
simplicity, the arrangement will be described in terms of movement along 
only one axis. 
Referring to FIGS. 1 and 2, the illustrative arrangement includes a 
horizontally disposed tool-carriage 9, or slide, which is mounted for 
reciprocation along an axis 11. The carriage is supported by fluid-film 
bearings 13 (only one of which is in view); the bearings mate respectively 
with a V-type bearing way 15 and a flat bearing way 17. As shown, the ways 
are supported by a stationary bed 19. The above-mentioned components are 
of conventional design. The carriage 9 is positioned incrementally along 
the axis 11 by means of a linear electric motor 21, which is incorporated 
in any suitable automatic position-control loop (not shown). The linear 
motor is of the linear-commutator type and is of conventional design, with 
the exception that its standard spring-loaded carbon brushes are replaced 
with low-friction brushes of a special design (to be described). 
The motor 21 includes an elongated stator 23, which extends parallel to the 
axis 11. As shown, an end of the stator is rigidly supported by a bracket 
20 affixed to the bed 19. The stator includes a series of axially spaced, 
internal windings (not shown), whose ends are connected to a linear 
commutator recessed in the bottom of the stator and extending parallel to 
the axis 11. A typical bar 25 of the commutator is shown in FIG. 2. 
The motor also includes an open-centered permanent magnet 27, which is 
affixed by machine screws 28 to the underside of the carriage 9. The 
magnet extends about a section of the stator, defining a continuous narrow 
clearance therewith. 
Referring to FIG. 2, the base of the magnet 27 is formed with a series of 
vertical bores 29, positioned for sequential registry with the commutator 
bars. Cylindrical insulators 31 are respectively mounted in the bores, 
each insulator being threaded internally to receive a brush assembly. The 
brush-and-commutator arrangement is such that when a d.c. positioning 
pulse of a given polarity is received, it is impressed across whichever 
stator winding is nearest the center of the magnet 27. The magnetic field 
generated by that winding reacts with the field of the permanent magnet to 
effect incremental movement of the magnet-and-carriage assembly in a 
selected direction along the axis 11. A pulse of the opposite polarity 
moves the assembly in the opposite direction. 
In accordance with the invention, the low-friction carriage arrangement 7 
utilizes special brush assemblies whose contact with the commutator is 
characterized by high conductivity and minimal drag. FIG. 3 illustrates a 
typical special brush assembly 33, which includes an electrically 
conductive T-shaped pedestal 35. A wire 36 is connected to the lower end 
of the pedestal to connect the same to any suitable electrical circuit for 
generating drive pulses for the motor. The pedestal, which includes a 
threaded shank 37 and an upper tip 39, is threaded into an insulator 31, 
the tip 39 defining a selected narrow operating clearance with the 
commutator. The tip is formed with a depression 41, or cavity, which 
confronts the commutator 25. As shown, the depression contains a small 
body or bead 43 of mercury having an upwardly convex meniscus portion 
which extends above the tip and across the aforementioned operating 
clearance to contact the commutator 25. (See FIG. 2) Preferably, the 
pedestal is threaded into the insulator 31 to a point where the meniscus 
portion of the mercury bead is flattened somewhat by contact with the 
commutator. If desired, a continuity tester may be connected across the 
pedestal and commutator to indicate when contact is established. 
We have found that the mercury bead 43 resists displacement from the cavity 
41 and breakaway of the meniscus portion at carriage speeds which are 
suitable for microinch machining operations. That is, at such speeds, 
gravity maintains the bead in the cavity, and the surface tension of the 
mercury is sufficient to maintain the integrity of the bead. This was not 
predictable. As compared with the spring-loaded brushes normally used with 
the motor 21, the new arrangement provides an electrical contact which is 
characterized not only by less drag but also by high conductivity, less 
arcing, and less transmission of vibration. Referring again to the tip 39, 
we prefer to promote retention of the bead in the tip by forming the upper 
portion of the cavity sidewall with an upward flare. If desired, retention 
also can be promoted by fabricating the basal portion of the cavity of a 
material which is wetted by mercury. 
The following is a more detailed description of the above-described form of 
the invention. 
EXAMPLE 
A low-friction carriage arrangement of the kind shown in the drawings was 
fabricated and tested. The linear electric motor 21 was a commercial 
design (Model FM 1903, manufactured by Inland Motor Division, Kollmoger 
Corp., of Radford, Virginia). The motor commutator bars were composed of 
copper alloy. The motor was energized by a closed-loop position-control 
system similar to that described in above-referenced U.S. Pat. No. 
4,221,995, with the exception that the position-command pulses were not 
fed into a microprocessor but into a conventional General Electric Mark 
Century 7500 Machine Controller. The carriage 9 was supported by standard 
pressurized oil-film bearings. The insulators 31 were composed of Bakelite 
and the brush pedestals 35, of aluminum. The pedestal cavity 41, which was 
formed by drilling and countersinking, had a depth of 0.100" and a maximum 
diameter of 0.125". The mercury bead 43 was of spheroidal shape and had a 
diameter of 0.125". When out of contact with the commutator, the meniscus 
portion of the bead extended at a maximum of about 1/8" above the tip 39. 
The operating clearance between the pedestal and the commutator was 
approximately 35 mils. With a load of 2100 pounds on the carriage, the 
control system positioned the carriage to the target precision of .+-. 0.6 
microinch, at carriage speeds in the desired range (about 0 to 0.200" per 
minute). These values are suitable for high-precision gauging, 
diamond-turning, and the like. No stick-slip problems were observed. In 
contrast, when spring-loaded carbon brushes were used in the same system, 
the stick-slip condition caused by the brushes made the system unusuable 
for precision gauging, diamond-turning and the like. The improved system 
was tested intermittently over a period of three months with no evidence 
of loss or deterioration of the mercury or of corrosion or wear of the 
commutator bars. The current density in the typical mercury bead was 
estimated to be about 100 amps/in.sub.2. As compared with carbon brushes, 
much less arcing occured at the brush-to-commutator interface. 
The foregoing description of a preferred embodiment of the invention has 
been presented for purposed of illustration and description. It is not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed, and obviously many modifications and variations are possible in 
light of the above teaching. For example, the pedestal 35 need not be 
composed throughout of electrically conductive material, so long as it 
includes a suitable conductor for maintaining the bead 43 in circuit with 
the electrical input for the motor. The phrase "electrically conductive" 
as used herein with respect to the pedestal is intended to include both 
designs. The pedestal cavity 43 may be of various shapes and depths: for 
example, its open end may define a rectangle. We prefer that the body of 
mercury be sized to provide a convex meniscus portion which extends at 
least 1/16" above the pedestal. We have found that beads of mercury having 
a diameter of from about 0.06" to 0.125" are especially suitable; thus, 
the amount of mercury inventory and the amount of mercury exposed to 
atmosphere is very small. At bead diameters exceeding about 0.250", the 
meniscus portion tends to flatten appreciably. The bead 43 may be any 
current-conducting fluid (e.g., mercury or amalgam) which is characterized 
by a meniscus portion of the kind described and which is liquid at the 
desired operating temperature. Preferably, the commutator bars are 
composed of a material which is not wet by the liquid. Preferably, the 
operating clearance between the pedestal and commutator is in the range of 
from 10 to 50 mils. 
Our tests of the type of brush illustrated in FIG. 3 have been conducted at 
carriage speeds consistent with certain diamond-turning operations. Based 
on these tests, it is our opinion that the brush should perform 
satisfactorily at carriage speeds up to at least fifty inches per minute. 
It is intended that the scope of the invention be defined by the claims 
appended hereto.