GB2317018A

GB2317018A - Low drift, temperature compensated accelerometer component

Info

Publication number: GB2317018A
Application number: GB9718231A
Authority: GB
Inventors: Francis Jean-Paul Gibert
Original assignee: Individual
Current assignee: Individual
Priority date: 1996-08-29
Filing date: 1997-08-28
Publication date: 1998-03-11
Anticipated expiration: 2017-08-28
Also published as: GB9718231D0; FR2752942B1; GB2317018B; DE19734923A1; FR2752942A1

Abstract

The invention relates to an accelerometer component comprising a body made up of a housing 16 and a cover 18 constituting the mechanical strength structure of the component, and containing an accelerometer element 10 together with connection wires 20 exiting the body with the purpose of at least transmitting a signal from the accelerometer element. According to the invention, the housing and the cover form an enclosure having low thermal conductivity, and at least one temperature regulator member 12 is located inside the enclosure.

Description

2317018 A LOW-DRIFT ACCELEROMETER COMPONENT The present invention relates

to an accelerometer component having very low zero drift, responsiveness drift, and phase response drift.

BACKGROUND OF THE INVENTION

Accelerometers of various types are already known, e.g. of the variable resistance, variable capacitance, variable inductance, piezoelectric, etc. types, which accelerometers are generally used for static and/or dynamic measurements. They nevertheless suffer from the drawback of possessing zero drift, responsiveness drift, and phase response drift. For example, when such an accelerometer is excited by a physical phenomenon at a frequency of 1 kHz, for example, variations in temperature, and sometimes also is in pressure and humidity, give rise to zero drift, to responsiveness drift, and to phase response drift.

Such drift can completely vitiate any results given by such accelerometers. For example, when performing vehicle testings in Scandinavian countries, accelerometers are mounted on vehicles whose insides are heated to more than 200C and which travel in outside temperatures in the range -200C to -25'C. The results obtained with accelerometers placed in various locations of the vehicle, such as on the wheel or the axle, in the engine compartment, in the cabin, etc., cannot easily be compared because of temper- ature differences. It is known that certain kinds of measuring instrument drift can be corrected by using thermostatically controlled enclosures. 30 For example, French patent No. 2 633 465 relates to an oscillator said to be llultrastable in frequency,, because its responsiveness drift is very low; the oscillator is mounted inside a thermostated enclosure. According to that document, the thermostatted enclosure in which the oscill- ator is placed is heated from the outside by resistances, and the assembly is placed in a housing closed by a cover and designed to provide mechanical protection for the assembly. The dimensions of the resulting instrument are 2 relatively large; it does not constitute a transducer, but on the contrary constitutes a generator of a signal that must be very stable.

Document EP-A-449 721 describes a pilot frequency generator, i.e. an assembly comprising a resonator and an oscillator, which presents signal drift in the presence of variations in humidity, in pressure, and in temperature. That generator is thus a member which generates a reference signal and it is not a transducer. The drift in the signal it creates is reduced by using a sealed housing and a thermostatted enclosure. In that document, a pilot frequency generator including a piezoelectric resonator is placed in a thermostatted enclosure. The enclosure is heated by heater elements placed on the outside, and the is assembly is placed in a housing that provides mechanical protection for the resulting instrument. The dimensions thereof are relatively large and it does not constitute a miniature component.

OBJECTS AND SUMMARY OF THE INVENTION

The present invention relates to applying an enclosure having an internally regulated temperature rather than an enclosure which is externally thermostatted, to acceler ometers for which various kinds of drift give rise to special problems in operation.

More precisely, the invention relates to an acceler ometer component in which the accelerometer element or transducer is placed in an enclosure whose temperature is regulated by a temperature regulator member placed inside the enclosure. It is the internally temperature-regulated enclosure itself which, because of the material from which it is made, also forms the mechanical protection for the element.

The temperature regulator member may either be a heater member which keeps the temperature inside the enclosure at a temperature close to the upper temperature limit of the operating temperature range of the component, or else it is a member that provides cooling, e.g. by using the Peltier effect, and which maintains the temperature 3 close to the bottom limit of the operating temperature range.

The invention presents special advantages when the temperature regulator member is a thermistor having a positive coefficient for variation of resistance with temperature (PTC).

Heater members having a positive temperature coefficient for variation of resistance with temperature are already known and they are used, for example, in hair dryers, in hot-melt glue guns, in heaters for babies' bottle heaters, etc. They have the advantageous property of automatically regulating the quantity of heat they give off. If they give off more heat because they convey more current, then their temperature rises so their resistance is increases so the current flowing through them decreases, thereby reducing the amount of heat given off. This automatic self-regulation is such that under steady conditions, such temperature regulator members maintain a stable temperature.

In the application of the invention, they are particularly advantageous since they can be used without any additional temperature regulator member and can therefore easily be incorporated in a miniature component.

The active elements placed in the housing of the miniature component can then merely comprise an accelerometer chip, e.g. in the form of a chip having a thickness of 3 mm and a side of 6 mm or 8 mm, together with a heating thermistor having a positive coefficient for variation of resistance with temperature, in the form of a disk having a diameter of 7.5 mm and a thickness of 1.3 mm. Preferably, an additional miniature printed circuit with a very small number of components serves to process the signal from the accelerometer; it can also act as a power supply for the thermistor.

Another property of accelerometer components of the invention is that the portion of the body on which the accelerometer element is mounted possesses a high degree of stiffness, i.e. has a resonant frequency much greater than 4 that of the accelerometer element, so that the response thereof is not disturbed.

More precisely, the invention provides an accelerometer component designed to operate in a determined temperature range and having at least one of very low zero drift, very low responsiveness drift, and very low phase drift, and being of the type which comprises: a body constituting the mechanical strength structure of the component having an accelerometer element fixed to at least a portion thereof and defining an enclosure for housing the accelerometer element, and connection wires emerging from the body for conveying at least a signal from the accelerometer element; according to the invention, the portion of the body to which the accelerometer element is is fixed possesses a high degree of stiffness, the enclosure of the body possesses low thermal conductivity, and at least one temperature regulator member is disposed inside the enclosure.

Preferably, the component also contains a circuit for processing the signal from the accelerometer element and/or a power supply circuit for the temperature regulator member.

It is advantageous for the temperature regulator member to be a resistive element having its temperature regulated close to the upper temperature limit of the determined temperature range, preferably being a thermistor having a positive coefficient for variation of resistance with temperature.

In another embodiment, the temperature regulator member is a Peltier effect member.

It is advantageous for the body to comprise a housing and a cover, and at least the housing or the cover to be made of a glass ceramic material or of a thermoplastic material that withstands heat.

It is convenient for the connection wires to exit via the cover.

Metallization is advantageously formed on the outside of the body to provide protection against electromagnetic interference.

In an embodiment, the longest dimension of the 5 component is less than or equal to 20 mm.

BRIEF DESCRIPTION OF THE DRAWING

Other characteristics and advantages of the invention appear better from the following description given with reference to the accompanying drawing, in which:

Figure 1 is a graph showing how resistance varies as a function of temperature for a thermistor having a positive coefficient for variation of resistance with temperature; Figure 2 is a section through an example of an is accelerometer component constituting an embodiment of the invention; and Figures 3 and 4 are plan views of two embodiments of accelerometers corresponding to the section of Figure 2.

MORE DETAILED DESCRIPTION

Before describing an accelerometer component of the invention, the properties of thermistors having a positive coefficient for variation of resistance with temperature, referred to as lIPTC thermistors11 are briefly summarized.

They have the property of being able to operate at a temperature that is practically constant. They exist in various configurations, but they are often very small, having a thickness of millimeter order and an area that is much less than one square centimeter.

Figure 1 shows an example of the operating curve for such a thermistor. It will be observed that as temperature increases (along the abscissa), resistance begins by decreasing (down the ordinate), until it reaches a temperature TO where it has its minimum resistance RO, after which it increases. In practice, it has been observed that it is convenient to use such a thermistor at a temperature T1 where its resistance R1 is more or less twice its minimum resistance RO. Such thermistors have the advantage of not requiring a thermostat, of having a lifetime that is 6 practically unlimited, of including no moving parts subject to wear, of being low in cost, of not generating electrical noise, and of being small in size.

In accordance with the invention, this thermistor with automatic temperature regulation is mounted inside a housing provided with a cover to form an enclosure in which temperature is regulated.

A thermostatted enclosure is normally an enclosure defining a volume in which a temperature is contained at a constant value. To ensure that the temperature is constant throughout the entire volume, there must be no temperature gradient inside the volume so the enclosure is heated or cooled from the outside. The invention does not implement such an enclosure, since it would be too large for is implementing miniature components.

In accordance with the invention, temperature variations inside the enclosure are regulated, i.e. a stable thermal relationship is established, inside the enclosure in which there are placed both the member whose temperature is to be regulated (an accelerometer) and the temperatureregulating member. Clearly a temperature gradient must exist inside the enclosure since temperature is high at the heating thermistor and then forms a very shallow and practically constant gradient inside the enclosure formed by the body, followed by a very steep gradient through the walls of the body. The temperature regulation obtained inside the housing, given the small amount of variation in temperature distribution therein, suffices to solve problems posed by zero drift, by responsiveness drift, and by phase drift of accelerometer transducers.

The embodiment of Figure 2 is now considered in greater detail.

An accelerometer transducer or element, formed in a wafer of silicon with a thickness of a few tenths of a millimeter and including a bridge circuit, is given reference 10. It is housed in a ceramic housing having a wall thickness of 3 mm and in the form of a square with a side of about 8 mm. A second element is the thermistor 12 7 of the above-mentioned type, and a third element, which is merely optional, is a printed circuit 14 carrying components. These elements are placed inside the body of the component which comprises a housing 16 and a cover 18. The cover 18 has a hole for passing wires 20 connected to the various elements.

An important characteristic of the invention is constituted by the body. In the embodiment under consideration, the body comprises a housing and a cover. Nevertheless, it could be implemented in other forms, e.g. by being overmolded on the internal elements. An essential characteristic is that the body must be made of a material that insulates heat sufficiently for the temperature gradient which exists between the temperature regulator element is (e.g. a thermistor) and the outside of the component is located essentially in the wall of the body. As mentioned above, this characteristic is such that the gradient of temperature distribution inside the housing must be very small so that the temperature of the accelerometer element varies very little. Preferably, at least 959k of the total temperature gradient is located in the wall of the body.

Another essential property of the body is the stiffness of the portion on wl-lich the accelerometer element is fixed. It is essential that the response of this element is not disturbed by the body, so the body must transmit the exciting physical phenomenon with practically no change. The mechanical impedance of the body must therefore be very high. It is represented by the ', stiffness" of this portion of the body. The stiffness of this portion of the body must be such that its resonant frequency is much greater than the resonant frequency of the accelerometer element (by a factor of at least 3, and preferably by a factor of 10).

The housing and the cover may be made of materials that are machinable or moldable, thereby facilitating manufacture thereof, and making it easier to mount elements of small dimensions. It is advantageous for the material of the housing and of the cover (whether they are identical or different) to be suitable for being connected together in 8 sealed manner, e.g. by adhesive, or by metallization and soldering.

For example, the housing and the cover may be made of the machinable glass ceramic known as 11Macorll. This material has very good dimensional stability, is completely leakproof, and can be metallized. In addition, it constitutes an excellent electrical insulator at high voltages, high frequencies, and high temperatures. It gives off no gas that could cause aging of the components placed inside the housing. The machineability properties of this material make it easy to fabricate steps inside the housing and on the face of the cover that co-operates therewith, so that it is easy to install and retain the various elements inside the housing.

is It is also possible to use another material known as I'Micatherm11 which is an inorganic thermoplastic used in numerous electronic applications and which can be shaped by molding.

Thus, the housing and its cover can be formed either by machining, or by molding, or by any other appropriate technique.

Although a printed circuit 14 is shown placed inside the component of Figure 2, such a circuit is not essential. The wires 22 of the thermistor 12 and the wires of the accelerometer element 10 can be taken directly to the outside as indicated by reference 20. Nevertheless, it is advantageous for the four wires of the accelerometer element (corresponding to the four corners of the bridge) to be connected to an amplifier circuit 14. This circuit 14 may optionally also provide a power supply for the thermistor 12. Under such circumstances, the wires 20 extending from the component comprise two wires for powering the accelerometer element 10 and the thermistor 12, and two wires for the accelerometer signal.

Figure 2 shows a thermistor 12 which is supported solely by its power supply wires 22. Naturally, the thermistor is stationary as are the accelerometer transducer 10 and the printed circuit 14. To this end, it 9 is possible to use an epoxy adhesive as is well known in the art.

The disposition shown in Figure 2 is merely an example. In another example, the components are disposed at 900 relative to the disposition shown. In this way, they can all be carried by the cover 18, with the housing 16 merely constituting a cap. This disposition facilitates fabrication.

Although an outlet hole is shown for the wires 20 through the cover 18, the wires could naturally leave through any other location, e.g. of the housing.

In a preferred embodiment as shown in Figure 2, designed for car testing, the accelerometer component in plan view has the configuration shown in Figure 3 or in is Figure 4. In Figure 3, the side of the outer square is about 20 mm. The circle represents the cavity formed in the housing, and the inner square represents the ceramic housing of the accelerometer transducer. In Figure 4, the size is about 20 mm. between pairs of opposite sides. An accelerometer component has been made comprising, for example, an accelerometer transducer in the form of a silicon bridge powered at 10 volts and having a resonant frequency lying in the range 1 kHz to 10 kHz. The thermistor 12 is a thermistor designed to operate in the range 5 V to 30 V with a turnover temperature of 7CC, having a diameter of about 7.6 mm and available from Advanced Thermal Products Inc., St. Marys, Pennsylvania. The housing and the cover are made of "Macro" glass ceramic available from Morgan Matroc, Gonesse, France. 30 The entire component is preferably metallized so as to be protected against electromagnetic interference. Although an embodiment is shown in which the thermistor 12 is supported separately inside a housing, it is also possible to place it against the accelerometer transducer. The accelerometer transducer itself comprises a ceramic enclosure. In another embodiment, the chip containing the accelerometer transducer can be placed between two thermistors, with the assembly being disposed in the housing closed by the cover.

The embodiment described has a thermistor and is therefore designed for use by heating, i.e. the temperature inside the accelerometer is close to the upper limit of the temperature range in which the component can operate. In another embodiment, the temperature regulator member is a Peltier effect element, in which case the component operates in the vicinity of the bottom limit of its operating temperature range. Certain types of accelerometer component need to operate in the range -400C to +700C. When the component includes a thermistor, it operates at a temperature in the range 750C to 80C. When the component includes a Peltier effect component, its temperature is is about -400C. Other components are capable of operating in the range - 550C to +125'C. The invention is not limited in any way to a particular operating temperature range.

The invention thus makes it possible practically to eliminate any zero drift, any drift in responsiveness, and any phase response drift in accelerometers by mounting an accelerometer transducer in an enclosure whose temperature is not entirely uniform, but within which temperature variations are very small. This disposition makes it possible to provide a component of very small size which is extremely robust and which can be used in the same manner as an ordinary accelerometer component. The various fields in which the invention can be applied include transport, in particular road, air, and rail transport. 30 Naturally the invention has been described and shown merely by way of preferred example, and any technical equivalents could be provided for its component elements without thereby going beyond the ambit of the invention.

Claims

1. An accelerometer component designed to operate in a determined temperature range and having at least one of very low zero drift, very low responsiveness drift, and very low phase drift, and being of the type which comprises:

a body constituting the mechanical strength structure of the component having an accelerometer element fixed to at least a portion thereof and defining an enclosure for housing the accelerometer element, and connection wires emerging from the body for conveying at least a signal from the accelerometer element, wherein:

the portion of the body to which the accelerometer is element is fixed possesses a high degree of stiffness, the enclosure of the body possesses low thermal conductivity, and at least one temperature regulator member is disposed inside the enclosure.

2. An accelerometer component according to claim 1, also containing a circuit for processing the signal from the accelerometer element.

3. An accelerometer component according to claim 1, also containing a circuit for powering the temperature regulator member.

4. An accelerometer component according to claim 1, wherein the temperature regulator member is a resistive element having its regulated temperature in the vicinity of the upper temperature limit of the determined temperature range.

5. An accelerometer component according to claim 4, wherein the temperature regulator member is a thermistor having a positive coefficient for variation of resistance with temperature.

6. An accelerometer component according to claim 1, wherein the temperature regulator member is a Peltier effect member.

12

7. An accelerometer component according to claim 1, wherein the body comprises a housing and a cover, and at least the housing or the cover is made of a glass ceramic material.

8. An accelerometer component according to claim 1, wherein the body comprises a housing and a cover, and at least the housing or the cover is made of a thermoplastic material that withstands heat.

9. An accelerometer component according to claim 7, wherein the connection wires exit via the cover.

10. An accelerometer component according to claim 1, including metallization formed on the outside of the body to provide protection against electromagnetic interference.

11. An accelerometer component substantially as herein described with reference to and as shown in Figures 2 to 4 of the accompanying drawing.