US3743869A

US3743869A - Transducer with ground isolation

Info

Publication number: US3743869A
Application number: US00120777A
Authority: US
Inventors: H Hugli
Original assignee: Kistler Instrument Corp
Current assignee: Kistler Instrument Corp
Priority date: 1971-03-03
Filing date: 1971-03-03
Publication date: 1973-07-03
Anticipated expiration: 1990-07-03

Abstract

Disclosed is a piezoelectric transducer which is isolated from ground by a rigid metallic insulator to improve the high frequency response of the transducer. The insulator takes the form of an aluminum or aluminum alloy ring or sleeve that is coated with an aluminum oxide. The oxide coating is electrically insulating but hard and rigid to provide a rigid mount for the transducer making it useful at frequencies up close to its natural resonant frequency.

Description

United States Patent [1 1 Hugli TRANSDUCER WITH GROUND ISOLATION [75] Inventor: Hans W. Hugll, Williamsville, NY.

[73] Assignee: Kistler Instrument Corporation,

Clarence, NY.

[22] Filed: Mar. 3, 1971 [21] Appl. No.: 120,777

Related US. Application Data [63] Continuation of Ser. No. 862,381, Sept. 30, 1969.

52 us. Cl. 3l0/8.4, 310/9.1

51 int. Cl H04r 17/00 [58] Field of Search 310/8, 8.3, 8.4, 310/s.7, 9.1, 9.4

[56] References Cited UNITED STATES PATENTS 2,972,006 2/1961 Shoor 310/8.4 X

3,031,591 4/1962 Cary et al. 3lO/8.7

3,042,744 7/1962 Shoor 310/8,4 X

Orlacchio 3 10/84 [451 July 3, 1973 3,171,989 3/l965 Hatschek 3l0/8.7 3,218,488 ll/l965 Jacke 3l0/8.7 X 3,307,054 2/l967 Shoor 3l0/8.4 3,424,930 1/1969 List et al 3 lO/8,7 3,489,932 1/1970 Kopel et al. 3l0/8.7 X

Primary Examiner-J. D. Miller Assistant Examiner-Mark O. Budd Attorney-LeBlanc & Shur [5 7] ABSTRACT Disclosed is a piezoelectric transducer which is isolated from ground by a rigid metallic insulator to improve the high frequency response of the transducer. The insulator takes the form of an aluminum or aluminum alloy ring or sleeve that is coated with an aluminum oxide. The oxide coating is electrically insulating but hard and rigid to provide a rigid mount for the transducer making it useful at frequencies up close to its natural resonant frequency.

1 Claim, 11 Drawing Figures Patented July 3, 1973 3 Sheets-Sheet l INVENTOR HANS W. HUGLI ATTORNEYS Patented Jul 3, 1973 3,743,869

3 Sheets-Sheet 3 8 INVENTOR HANS W HUGLI ATTORNEYS TRANSDUCER WITH GROUND ISOLATION The present application is a continuation of my copending application Ser. No. 862,381, filed Sept. 30, 1969, for TRANSDUCER WITH GROUND ISOLA- TION.

This invention relates to piezoelectric transducers and more particularly to a simplified arrangement for isolating transducers of this type from ground.

Piezoelectric transducers utilizing quartz, barium titanate, or other ceramic materials are well known and are used in a variety of applications. Among others, the transducers are used to measure acceleration, pressure, shock and related phenomena. One of the most widely used is the piezoelectric accelerometer in which a seismic mass acts on a plurality of quartz or ceramic wafers which are prestressed or preloaded so as to accurately follow the input to produce an electrical output signal indicative of the acceleration force acting on the transducer.

A problem is encountered when piezoelectric transducers and particularly piezoelectric accelerometers are used in conjunction with other electrical equipment. If the transducer is not carefully isolated from ground, the transducer is subject to what is commonly referred to as electrical ground loops which have an adverse effect on the output. Very often these electrical ground loops appear as 60 cycle interference signals which are superimposed on the electrical output signal of the transducer and can be very annoying and difficult to eliminate.

In order to provide ground loop isolation, it has been the custom to insert an isolator between the transducer base and the support, such as a shaker table or the like upon which the accelerometer is mounted. Unfortunately, most insulating materials, such as paper, plastics, and the like, have relatively poor physical properties and lack the strength and hardness required for properly mounting an accelerometer or other piezoelectric transducer. For example, in order to obtain optimum band width, a typical quartz accelerometer may be operated up to frequencies close to its resonant point, i.e., in the neighborhood of 40 kHz. However, in order to obtain this high frequency response, the accelerometer needs a strong, rigid support with essentially direct contact between the base and the surface to which it is mounted. That is, the cushioning or resilient effect of the insulating material used to isolate the base from its support significantly reduces the high frequency response of the transducer.

This problem is overcome by the present invention because the transducer is mounted to its support by a rigid metallic insulator and more particularly by a metallic insulator having one or more surfaces coated with a very hard insulating oxide. In the preferred embodiment, the insulator takes .the form of an aluminum sleeve or washer which has its surfacecontacting the transducer support provided with an aluminum oxide coating. The insulator not only provides good electrical insulation between the transducer and support, but also is quite strong and hard to provide an essentially direct connection of the transducer to the support so that the transducer evidences a good high frequency response up to close to its resonant frequency, typically in the neighborhood of 40 kHz. Thus, the overall band width of the transducer may be increased from a range of typically 0.1 Hz to kHz to 0.1 Hz to approximately 40 kHz. Thus, the present invention provides an increase in response band width for the transducer of approximately four times that of prior ground isolated constructions.

It is, therefore, one object of the present invention to provide a transducer incorporating ground isolation.

Another object of the present invention is to provide a transducer which is isolated from ground and which has an increased frequency response band width.

Another object of the present invention is to provide a piezoelectric transducer having at least one surface provided with a metallic oxide electrically insulating coating which has increased strength and rigidity to impart to the transducer and support combination an improved high frequency response.

Another object of the present invention is to provide a piezoelectric accelerometer or other type of piezoelectric transducer which is mounted to a support by a metallic sleeve or ring having at least its outer surface coated with an aluminum oxide.

These and further objects and advantages of the invention will be more apparent upon reference to the following specification, claims, and appended drawings, wherein:

FIG. 1 is a cross section through a piezoelectric quartz accelerometer constructed in accordance with the present invention;

FIG. 2 is an end view of the accelerometer of FIG. 1;

FIG. 3 is a view of the mounting assembly for the accelerometer of FIG. 1;

FIG. 4 is a cross section through the mounting assembly taken along line 4-4 of FIG. 3;

FIG. 5 is a detailed view of the mounting stud for the accelerometer of FIG. 1;

FIG. 6 is a view of the mounting stud taken at right angles to the view of FIG. 5;

FIG. 7 is a detailed view of the accelerometer base forming a part of the accelerometer of FIG. 1;

FIG. 8 is a detailed view of the accelerometer base taken at right angles to the view of FIG. 7;

FIG. 9 is a perspective view of the mounting sleeve adapted to receive the head of the mounting stud of FIGS. 5 and 6;

FIG. 10 is a cross sectional view of a pressure gage constructed in accordance with the present invention and incorporating insulating metal sleeves; and

FIG. 11 is a cross sectional view of a modified pressure gage which may be insulated in the manner illustrated in FIG. 10.

Referring to the drawings, the novel transducer of the present invention takes the form in FIG. 1 of an accelerometer, generally indicated at 10, including a housing 12 made of stainless steel or other suitable material which has mounted in one end a conventional coaxial connector indicated at 14. Mounted within the housing or casing 12 and spaced from its walls is a piezoelectric quartz module 16 including a seismic mass 18, preferably formed of tungsten and connected to the module base 20 by a stainless steel preloading sleeve 22. Sandwiched between module base 20 and seismic mass 18 are a plurality of quartz wafers 24 and between the quartz wafers are gold or silver discs 26 which form electrodes developing an output electrical signal in response to acceleration forces along the axis 28 of the accelerometer. Included in the module is a temperature compensating disc or wafer 30 for matching the quartz to the stainless steel or other material forming the module base 20. One side of the quartz wafers are connected by tabs 32 through disc 30 and base 20 to the case 12 which is, in turn, in electrical communication with the outer conductor of coaxial connector 14 forming one side of the electrical output. The other side of the quartz wafers are connected by similar bent over tabs on the gold or silver discs to a conductive pin 34 electrically connected by lead 36 to the inner conductor 38 of coaxial connector 14 which forms the other side of the electrical output for the accelerometer 10.

Preloading sleeve 22 is provided with a shoulder 40 received over the reduced end portion 42 of the seismic mass and the preloading sleeve is welded at its other end, preferably by a series of spot welds, to module base 20. Base 20 is threaded over a projection 44 forming a part. of an accelerometer base 46 also received within housing 12. The two bases are also preferably connected together and to the prestressing sleeve 22 by epoxy, as indicated at 48 and 50. Received within accelerometer base 46 is the head of a mounting stud 52 having a threaded shank 54 for mounting the accelerometer to a suitable support, such as a shaker table or the like, generally indicated by the hatch lines at 56. Accelerometer base 46 is mounted to support 56 by way of an electrically insulating washer 58 which serves to isolate the transducer from support 56 and, therefore, from any ground loop currents which may be flowing through the support.

FIGS. 3 and 4 illustrate the manner in which the accelerometer is isolated from ground. Base 46, which is preferably made of stainless steel, is provided with a circular cavity 60, best seen in FIGS. 7 and 8, and received in this cavity is the head 62 of mounting stud 52. Head 62 is of generally circular configuration but is cut away on both sides to provide the flats 64 and 66, best seen in FIG. 6. Surrounding the upper portion of threaded shank 54 of the mounting stud is a stud mounting sleeve 68 which is are welded as at 70 to base 46. Sleeve 68 is cut away, as best seen in FIG. 9, to provide projecting

portions

65 and 67 which overlie the flats on the head of the mounting stud 62 to keep it from turning. Electrically insulating the stud from the base 46 and from the mounting sleeve 68 are several layers of paper insulation soaked or impregnated with epoxy as indicated at 72. Finally, slipped over the shank 54 of the stud is the insulating washer 58 having an electrically insulating coating 74 over at least one surface.

It is an important feature of the present invention that the washer 58 provides electrical insulation for the base 46 from its support and at the same time has sufficient strength and hardness to insure a tight rigid contact between the support 56 when stud 52 is threaded into the support and an annular ridge 76 formed on the end 78 of the base, which ridge has a substantial extension of flat surface to make good physical contact with insulator 58. Insulator 58 is preferably made of aluminum or aluminum alloy in which at least the surface in contact with support 56 of FIG. 1 is provided with the oxide coating 74. Coatings of this type may be formed by what is referred to as the Sanford process which produces a coating of aluminum oxide (M from aluminum or an aluminum alloy. This coating is formed partly from penetration and partly from dimensional buildup and produces a smooth surface that is extremely hard. The coating is preferably from about 0.001 to about 0.002 inch thick and, in addition to its hardness, evidences excellent insulating properties. For example, coatings of 0.0015 inch thickness have withstood 1,200 volts breakdown with an insulation resistance in excess of 500 megohms. The coating hardness is between 7 and 9 on the Mohs scale or between 60 and on the Rockwell C scale. Aluminum and aluminum alloys provided with a coating of this type are presently available from Duralectric Incorporated, Natick, Massachusetts.

In attaching the stud 52 to the transducer, the paper insulators are first soaked with epoxy and placed over the head of the stud and along the upper portion of the shank. Epoxy is also preferably applied to the interior of the base cavity 60 and to the inside of sleeve 68. The sleeve is then placed over the stud and the stud inserted into the cavity where the epoxy is allowed to cure. Sleeve 68 is then heli-arc welded to the base 46 as indicated at 70 and when completed, the insulation of the epoxy impregnated paper insulators should be a minimum of IO ohms. Insulator 58 is then slipped over the threaded end of the stud and the stud is threaded into its support until the insulator 58 is tightly clamped between the support 56 of FIG. 1 and the annular ring 76 on the end of base 46 to securely and positively mount the accelerometer to its support.

FIG. 10 is a partial cross sectional view through a piezoelectric pressure transducer constructed in accordance with this invention. This transducer of FIG. 10 comprises a base 80 welded as at 82 to a stainless steel sleeve 84. Mounted on the end of sleeve 84 is a stainless steel pressure diaphragm 86 which exerts pressure on a piezoelectric module 88, similar to the module 16 of FIG. 1. An electrical output is taken from a microdot coaxial connector 90 at the other end of the transducer, which connector comprises the threaded end 92 of the body forming the outer coaxial connector and an inner coaxial receptacle 94 mounted within base 80 by Teflon insulating rings 96 and 98. Electrical connection is established from module 88 to the inner conductor 94 of the coaxial connector by way of a lead 100 passing through a central insulating sleeve 102.

The pressure transducer of FIG. 10 is adapted to be mounted to a suitable support (not shown) by a pair of threaded adaptors 104 and 106 which are threaded together as at 108 with the former also having external threads 110 for the purpose of mounting the transducer. These adaptors surround a pair of aluminum or aluminum alloy sleeves 112 and 114 which are provided at least on their outer surfaces with an aluminum oxide coating which is the same as the coating 74 on insulator 58 illustrated in FIG. 3. Insulating sleeve 114 is preferably epoxied to body 80 and insulating sleeve 112 is epoxied to stainless steel sleeve 84 and also to body 80. Body 80 includes an annular flange 116 forming a pair of shoulders against which abut aluminuminsulating rings 118 and 120, which rings preferably have all surfaces provided with an oxide coating as previously described. When adaptors 104 and 106 are tightened together, the adaptors press a pair of soft copper sealing rings 122 and 124 against insulating rings 118 and to insure a tight seal between the adaptors and the insulating sleeves 112 and 114. Thus, in FIG. 10 the aluminum oxide coating 74 of FIG. 3 is applied to the outer surfaces of insulating sleeves 112 and 114 and to all surfaces of the insulating

rings

118 and 120 to isolate body 80 forming the outer coaxial conductor from the support engaged by threads 110 of adaptor 104.

FIG. 11 shows a modified pressure transducer construction adapted to be mounted within the insulating sleeves 112 and 114 of FIG. and secured by insulating washers or rings 118 and 120. The transducer 130 of FIG. 11 again comprises a base 132 welded as at 134 to a stainless steel sleeve 136. Secured to the end of the sleeve is a stainless steel diaphragm 138, similar to the diaphragm 86 of FIG. 10, and, as before, the diaphragm bears against a piezoelectric quartz wafer module 140, similar to the module 16 of FIG. 1. Also welded to base 132 is a connector sleeve 142 which carries at its outer end coaxial connector 144. Received within connector sleeve 142 is a mounting sleeve 146 within which is mounted an electronic package, generally indicated at 148. Package 148 preferably comprises an amplifier or impedance converter of the type shown and described in assignees copending application Ser. No. 746,700, filed July 9, 1968, which includes an insulated gate field effect transistor for matching the impedance of quartz module 140 to the lower impedance of a conventional detector, such as an oscilloscope or the like, or to a conventional coaxial line.

Electrical connection to the outer element of coaxial connector 144 is again by way of body 132 and conductive conductor sleeve 142. The other side of the output from quartz module 140 is fed by way of an electrical lead 150 through insulating sleeve 152 and header 154 to the center conductor of coaxial connector 144. This connection, as indicated at 162, is preferably through the gate lead 156 of a field effect transistor, indicated by dashed lines at 170, contained in electronic package 148, as described in assignees copending application Ser. No. 746,700, filed July 9, 1968. Header 154 is preferably mounted within the transducer by epoxy as indicated at 160.

It is apparent from the above that the present invention provides a simplified and inexpensive structure for isolating all types of piezoelectric transducers from socalled ground loop currents which might otherwise be superimposed upon and adversely affect the transducer output electrical signal. An important feature of the present invention resides in the fact that those surfaces of the transducer which are adapted to be tightly clamped in contact with a suitable support are provided with an insulating oxide coating having increased strength and hardness over previously proposed constructions. This provides a rigid and tight physical contact between the transducer and its support and significantly increases the high frequency response of the transducer so that the quartz crystal transducers are usable in environments requiring elevated frequency responses up to in the neighborhood of 40 kHz. Thus, the oxide coated aluminum or aluminum alloy provided for mounting the transducer makes it possible to utilize the transducer at frequencies close to the natural resonant frequency of the quartz module and its associated structure.

What is claimed and desired to be secured by United States Letters Patent is:

1. A piezoelectric transducer comprising an electrically conductive annular metal casing, a plurality of quartz crystal wafers mounted in said casing, an electrical output connector on said casing, conductive means including said casing for electrically coupling said wafers to said output connector, and a metallic insulator in contact with at least a portion of said casing for electrically isolating said casing from a support, said insulator having an aluminum oxide coating on at least one surface from about 0.001 to about 0.002 inch thick with a hardness between 60 and on the Rockwell C scale, said insulator comprising at least one aluminum sleeve surrounding at least a portion of said casing and a pair of threaded adaptors surrounding said casing, said sleeve spacing at least one of said adaptors from said casing.