|Title of Invention||
A PRECISION SLIPRING FOR TRACKING RADARS AND INSTRUMENTS AND A METHOD OF MAKING THE SAME
|Abstract||A precision slipring for tracking radars and instruments is provided. The slipring comprises plurality of co-planner concentric conductor rings supported by an inner support member and embeded in at least one side of an insulator disc made of moulded epoxy resin with only part of thickness of the conductor rings being covered by the epoxy resin, two movable eletrical contacts from opposite sides of each conductor ring and lead wires being taken from each conductor rings and each movable contacts for external connection. A method of making the precision slipring is also provided.|
The invention relates to a precision slipring used for tracking radars and instruments and a method of making the same.
Sliprings are used in radars for quite some time. Various types of sliprings are known in the art. Sliprings which can be operated with high frequency video circuits are required for modern radars. In instruments and radars used with rockets precision sliprings are required for data transfer. The sliprings are stacked and the lead connections are brought outside the housing. Existing sliprings are embeded in glass epoxy providing insulation between the concentric rings and between two layers of concentric rings. The following disadvantages were experienced in the existing sliprings.
The glass epoxy insulator is hygroscopic and hence soaks in water. This brings down the insulation of the sliprings to below 20 Megohm instead of the specified 1000 Hegaohm insulation between rings, when 500 volt DC is charged between them. Due to the absorption of moisture in the disc all the six circuits provided in each disc gets shorted, and not more than 92 circuits can be energised at a time. The brush contact provided on the tip of the contact arm is of a low silver graphite alloy. The
brush is made of a material in which the silver content is not more than 80 %, and the balance is graphite. The problem encountered is that the graphite powder eroded from the brush makes a tail on the surface of the disc and it act as another connection between two circuits and gets shorted. The brush wear is more in the slipring unit and the tip end of brush gets worn-out easily and there by shorting it with adjacent power line causing damage.
In an attempt to overcome these disadvantages a precision slipring was developed. The invention provides a slipring with a capacity to transfer electrical signal from millivolts to 440 volts at a frequency range of 50Hs to 50 MHs. The temperature rise is less than 15oC above ambient during operation. They could be used with shafts ranging from 75 mm to 450 mm and as many as 250 circuits could be used. The epoxy compound used for embeding the conductor rings is non hygroscopic and hence provides improved water resistance. The sliprings made according to the invention can be made in wide range of sizes and the modular structure makes replacement easy. The loss of material during fabrication is minimum and the encapsulation procedure is fast and easy. The sliprings made according to the invention may be used in wide range of applications in various type of instruments.
Accordingly the present invention provides a precision siipring used for tracking radars and instruments said slipring comprising plurality of co-planner concentric conductor rings supported by an inner support member and embeded in at least one side of an insulator disc made* of moulded epoxy resin with only part of thickness of the conductor rings being covered by the epoxy resin, two movable electrical contacts from opposite sides of each conductor ring and lead wires being taken from each conductor rings and each movable contacts for external
The invention also provides a method of making a precision siipring as described hereinabove, comprising the steps of : making plurality of concentric conductors with lead connections; setting these concentric conductors in a mould with each set of concentric conductors in a single plane using re-inforcing spacers; preparing an epoxy resin with good flowability; encapsulating the plurality of conductors by pouring the epoxy resin in the mould; allowing the epoxy resin to cure; releasing the encapsulated conductors from the mould, removing the epoxy resin from the top surface of the conductor rings and fixing the movable contacts on the concentric conductor rings embeded in the epoxy resin.
Preferably two co-planar concentric conductor rings are embeded on both sides of the insulating disc made of moulded epoxy resin. The epoxy resin used provides nonhygroscopic insulation of more than 1000 Megohm between the rings. The resin used for moulding is a modified epoxy polymer resin suitable for insulation embedment applications. The resin used has low viscosity with good flowability.
The conductors are pretreated by cleaning contact points and soldered contact joints by a suitable solvent and dried. Then the contact joints are coated with a formulated resin to provide adequate covering and cured. The conductors are then cleaned by a degreasing solvents and then dried. The conductors are pretreated by contacting with a FeCl3 solution of desired concentration for 2 minutes and then washed under distilled water and dried.
The mould is prepared by applying a release agent based on wax inside the mould and the conductors are fixed inside the mould with reinforcing spacers.
A modified epoxy polymer resin system is used for potting
the conductor rings which is prepared by mixing ingredients of a liquid aromatic epoxy resin, an amine catalyst, a polysulphide resin and a liquid aldehyde namely furfural acting as reactive diluents in fixed ratios and carefully reacting the system in a suitable container for a known duration of time to maintain flowability under controlled conditions of pressure and temperature.
A glass beaker of 2 litres capacity is taken and 350 to 450 gms of an epoxy resin diglycidyl ether of bisphenol-A is taken and then mixed with 80 to 122 gms of Polysulfide resin followed by addition . of 15 to 30 gms of the liquid aldehyde namely furfural. This mixture is thoroughly mixed with a glass rod. To this mixture 12 to 25 gms of an amine catalyst is added and mixed thoroughly again. This is then allowed to stand for 2 to 3 minutes. The mixture is then degassed under vacuum till no foaming and no appreciable quantity of bubbles are seen, This reacting liquid resin is pumped into the mould kept under vacuum and by adopting suitable gravity flow methods and techniques to obtain void free encapsulation and casting. This is allowed to cure for 12 to 24 hours duration at room temperature. The conductor rings are thus embedded in the potting resin.
The encapsulated concentric rings is demoulded, cleaned free of release agent and finished by machining to the required finish and accuracy.
The ingredients and specifications of the modified epoxy resin is given below,
1. Liquid aromatic epoxy resin (Diglicydyl ether of bisphenol-A)
Colour - Clear pale yellow liquid
Epoxy equivalent - 175-210
Volatile matter at 80^, 2hrs - 5%(iDax)
Sp: gravity at 30^C - 1.10 to 1.16.
Viscosity at 30^C - 400 to 800 ops
2. Polysulfide resin
Mercaptan content - 5-6.5%
Viscosity at 30^C - 800-1400 ops
Appearance - Yellow to amber liquid
Purity - 98% (min)
Boiling range (96%) - 150^C to 160°C
4. Amine catalyst
Nitrogen content - 14-16%
Sp:gravity at 30°C - 0.97 to 0.98
The movable contacts are provided with contact brushes made
of a mixture of sliver and graphite. The graphite powder is
coated with silver, mixed with silver powder and compacted into
desired shape. The compacted mixture ofsilver coated graphite
powder and silver powder is sintered in an inert atmosphere and then machiend if desired for obtaining the brush contact of desired dimension and contact surface. Prefered temperature of sintering is between 850^C to 900^C. The brush contacts thus prepared are relieved of the stress by heating preferably at a temperature between 500*^C to 550^C in an inert gas such as argon.
The invention will now be described with reference to the accompanying drawings :
Fig. 1 shows the plan view of a slipring according to the invention in a housing.
Fig. 2 shows the embeded slipring with leads according to the invention.
Fig. 3 shows the sectional view along A-A of fig. 2.
Fig. 4 shows the enlarged view of the portion indicated in
Refering to Fig.l the slipring according to the invention comprises at least a plurality of concentric conductors (Rj^, R2 * R3, . ) embeded in an insulator disc (I) in a single plane made by moulding in an epoxy resin and supported by an inner support member (S). The inner support member (S) is provided with mounting means (M) for mounting plurality of sliprings in a stack. Each slipring preferably have two set of co-planner concentric conductors on each side of the insulator disc (I) made of the epoxy by moulding. The top surface fo each conductor rings are bare conductors free of any insulation. Contacts from the said concentric rings are taken through movable contacts (Bj, B2> B3 • -• )- The movable contacts are provided with spring loaded silver graphite brushes. The silver graphite brushes are made by powder metalurgy from a mixture of silver coated graphite powder and silver powder. The leads (L j^, L2> L3. . ) are taken for providing connection to the tracking circuit.
The precision slip rings according to the invention has the following capabilities.
1. Capacity to transfer electrical signal from milivolt level to 440 V currents with frequency range 50Hz to 50 MHz.
2. Capacity to transfer the signal without temperature rise beyond 15*^ above the ambient.
3. Usability of sliprings for shafts ranging from 75 mm to
450 mm diameter with number of circuits ranging from 24 to 250
with varying electrical and signal array,
4. Superior water resistance capability by encapsulation with modified epoxy resin.
5. The capability to make the sliprings of intricate shapes.
6. The capability to make sliprings with modular
structure, so as to make replacement of any module easy.
7. Capability to make sliprings of any design, complicated or otherwise, irrespective of irregular dimension and shapes.
8. The loss of material in fabrication and manufacture in the sliprings is minimum.
9. The process of realising the sliprings can be adapted
for a wide range of applications.
WE CLAIM :
1. A precision slipring used for tracking radars and instruments said slipring comprising plurality of co-planner concentric conductor rings (R1, R2,R3- • ) supported by an inner support member (S) and embeded in at least one side of an insulator disc (I) made of moulded epoxy resin with only part of thickness of the conductor rings (R1, R2, R3, . ) being covered by the epoxy resin, two movable electrical contacts (B1, B2, 83..,.) from opposite sides of each conductor ring (R1, R2, R3. . . ) and lead wires (L1, L2 , L3. . ) being taken from each
conductor rings (R1, R2 > R3 . . ) and each movable contacts (B1,
B2 , B3. . ) for external connection.
2. The slipring as claimed in claim 1, wherein plurality of co-
planner concentric conductor rings (R1 , R2 , R3 • • ) are embeded
on both sides of the insulating disc (I) made of moulded epoxy
3. The slipring as claimed in claims 1 or 2, wherein the moulded epoxy resin is a non-hygroscopic resin.
4. The slipring as claimed in claim 3, wherein the moulded epoxy resin comprises 350 to 450 gms of diglycidyl ether of bisphenol-A, 80 to 122 gms of polysulphide resin, 15 to 30 gms of liquid aldehyde such as furfural and 12 to 25 gms of an amine catalyst,
5. The slipring as claimed in any one of the claims 1 to 4, wherein the movable electrical contacts are spring loaded silver graphite brushes made by powder metalurgy.
6. The slipring as claimed in claim 5, wherein the silver graphite brushes are sintered brushes from compacted mixture of silver coated graphite powder and silver powder.
7. A method of making a precision slipring as claimed in any
one of the previous claims comprising the steps of : making
plurality of concentric conductors with lead connection; setting
these concentric conductors in a mould with each set of
concentric conductors in a single plane using re-inforcing
spacers; preparing an epoxy resin with good flowability;
encapsulating the plurality of conductors by pouring the epoxy
resin in the mould; allowing the epoxy resin to cure; releasing
the encapsulated conductors from the mould, removing the epoxy
resin from the top surface of the conductor rings and fixing the
movable contacts on the concentric conductor rings embeded in the
8. The method as claimed in claim 7, wherein during encapsulating the conductor rings with epoxy resin vaccum is applied to the mould for removing air pockets.
9. ' The method as claimed in claim 7, wherein the epoxy is
removed from the top surface of the embeded conductors by
10. The method as claimed in any one of the claims 7, to 9
wherein the movable contacts are made by compacting and sintering
a mixture of silver and silver coated graphite powder.
11. A precision slipring used for tracking radars and
instruments, substantially as hereinabove described and
illustrated with reference to the accompanying drawings.
12. A method of making a precision slipring, substantially as
hereinabove described and illustrated with reference to the
|Indian Patent Application Number||544/MAS/1999|
|PG Journal Number||50/2007|
|Date of Filing||11-May-1999|
|Name of Patentee||M/S. INDIAN SPACE RESEARCH ORGANISATION|
|Applicant Address||ISRO HEADQUARTERS, DEPARTMENT OF SPACE, ANTHARIKSH BHAVAN, NEW BEL ROAD, BANGALORE,|
|PCT International Classification Number||H 01 R 39/08|
|PCT International Application Number||N/A|
|PCT International Filing date|