Snap-in force sensor with bellows

A snap-in LVDT force sensor includes a housing having separate chambers for enclosing an LVDT and an assembly of circuit components. One end of the housing is coupled for movement with one link of a two-link frame which is subject to the forces to be measured. A plunger is coupled for movement with the other link and includes a shaft which extends into one of the chambers to support an LVDT core for reciprocal movement within the LVDT coil assembly. A bellows is sealingly and flexibly coupled between the plunger and housing to provide protection from the environment. An optional spring may provide additional stiffness between the plunger and housing.

BACKGROUND OF THE INVENTION 
The present invention relates to a linkage and transducer assembly for 
sensing forces. 
Various systems of linkages and transducers have been proposed to sense 
forces in members such as the links of the hitch for an agricultural 
implement. For example, U.S. Pat. No. 4,253,331 described a diamond-shaped 
frame with a load cell device inserted therein. Similarly, U.S. 
application, Ser. No. 332,577, filed Dec. 21, 1981 and assigned to the 
assignee of the present application, describes such a sensor system having 
a "snap-in" LVDT transducer. However, the "snap-in" design was expensive 
and difficult to manufacture and assemble due to the multiplicity of 
parts. For example, the separate sealing and biasing functions are 
performed by separate rubber boot and spring elements. Furthermore, in 
that design, the LVDT coil assembly is slidably received by one of the 
housing components and is therefore subject to wear caused by friction 
between the relatively movable parts. Accordingly, it would be desirable 
to have a simple and inexpensive "snap-in" type LVDT force sensor in which 
the LVDT coil assembly is protected from wear. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a simple and 
inexpensive "snap-in" type draft sensor. 
Another object of the present invention is to provide a snap-in type draft 
sensor assembly wherein the sealing and biasing functions are performed by 
a single bellows element. 
A further object of the present invention is to provide a "snap-in" type 
draft sensor with LVDT components which are protected from wear. 
These and other objects are provided by the present invention which 
includes a housing having inner and outer chambers enclosed thereby. An 
LVDT coil assembly is enclosed within the inner chamber. An LVDT circuit 
board is located in the outer chamber. An end of the housing is coupled 
for movement with one of the links of a two-link frame. A plunger is 
coupled for movement with the other link and includes a shaft which 
slidably extends through the housing and into the inner chamber. The shaft 
supports the LVDT core for reciprocal movement within the hollow 
cylindrical coil assembly. A spring urges the plunger and housing apart 
and a rubber bellows is sealingly and flexibly coupled between the plunger 
and the housing. Alternatively, the separate spring and bellows may be 
replaced by a single flexible stainless steel bellows.

DETAILED DESCRIPTION 
A draft sensor 10 includes a frame 12 comprised of a pair of links 14 and 
16 connected together at opposite ends, as described in detail in U.S. 
application, Ser. No. 228,440, filed Jan. 26, 1981. A transducer 20 is 
inserted in the opening between the links to sense variations in the 
separation between the links. The frame 12 may be inserted in any member 
where it would be desirable to measure forces, such as in the link or 
links of a hitch for an agricultural vehicle (not shown). 
Referring now to FIG. 2, the links 14 and 16 each have blind bores 22 and 
24 drilled therein. Support pins 26 and 28 are press-fitted into the bores 
22 and 24 and include heads 30 and 32 with rounded bearing surfaces. 
The transducer 20 has a plastic or non-magnetic cylindrical housing 34 
which cooperates with the shaft 52 to form a transducer chamber which 
completely encloses the coil assembly 36 of a conventional linear variable 
differential transformer (LVDT), such as are available from Schaevitz 
Engineering or Trans-Tek, Inc. The housing also forms an annular outer 
chamber 38 which surrounds the transducer chamber. A circuit board 40 may 
be positioned in the outer chamber 38 for mounting thereon the components 
of a conventional LVDT circuit, such as described on page 17-56 of Fink's 
"Electronics Engineers' Handbook", McGraw-Hill, 1975. One end of the 
housing 34 forms a socket 42 for engaging the head 32 of support pin 28. 
In this manner, the circuit board 40 is entirely protected from the 
environment by being located in the outer chamber 38. The other end of the 
housing 34 has a bore 44 extending therethrough and communicating with the 
transducer chamber. 
The transducer 20 also has a non-magnetic plunger 46 with a head 48 having 
a rounded recess 50 engaging the head 30 of support pin 26. The plunger 
has a shaft 52 which extends from head 48 through bore 44 to an end to 
which is fixed the hollow cylindrical ferrite core member 54 of the LVDT. 
The shaft 52 supports the core 54 for reciprocal movement with the coil 
assembly 36. A spring 56 is mounted over the shaft 52 and urges the 
plunger 46 and housing 34 away from each other so that they are maintained 
in contact with their respective support pins 26 and 28. A flexible 
exterior bellows 58 is sealingly fixed at opposite ends to the plunger 46 
and the exterior of housing 34 to prevent contamination of the components 
of the transducer 20 from the environment. The bellows 58, housing 34 and 
plunger member 46 enclose an end chamber 57 which is separated from the 
transducer chamber by a wall 59 of the housing 34. A silicon rubber 
material is suitable for the bellows 58. However, as shown in FIGS. 3 and 
4, the bellows 58a and 58b may be formed of stainless steel. Then, the 
bellows 58a and 58b may perform both the sealing function and the biasing 
function of the spring 56. In such cases, the spring 56 may be eliminated, 
as shown in FIGS. 3 and 4, unless additional stiffness beyond that 
provided by the stainless steel bellows 58a and 58b is desired. Note, in 
FIG. 3, the bellows 58a has a generally C-shaped cross-section, viewing 
the left half of the section, and the housing 34a has an axially extending 
annular groove 60 which surrounds the bore 60 and which receives an 
annular tab 61 on one end of the bellows 58a. In the alternative shown in 
FIG. 4, the bellows 58b has a generally S-shaped cross-section, viewing 
the left-hand half of the section. The radially inwardly extending end of 
the bellows 58b is sealingly coupled to a flange portion of the plunger 
52, while the radially outwardly extending end of the bellows 58b is 
sealingly coupled to the other end of the housing 34b by means of a snap 
ring 62. In either case, as forces are applied to the ends of the frame 
12, it deforms, and the separation of the links 14 and 16 varies, as does 
the position of core 54 relative to coil assembly 36. The LDVT and the 
circuit on board 40 can thus provide signals representing the forces 
applied to the frame 12. An electrical cable (not shown) is run through an 
access opening (not shown) in the housing 34 for connecting to the circuit 
(not shown). 
The transducer 20 may be inserted into or removed from the frame 12 merely 
by compressing the bellows, the spring or both, depending upon the 
particular embodiment, to allow the recesses 42 and 50 to be pulled away 
from their respective support pins 28 and 26. 
While the invention has been described in conjunction with a specific 
embodiment, it is to be understood that many alternatives, modifications, 
and variations will be apparent to those skilled in the art in light of 
the aforegoing description. Accordingly, this invention is intended to 
embrace all such alternatives, modifications, and variations which fall 
within the spirit and scope of the appended claims.