Accelerometer with piezoelectric element

The seismic mass is fixed to the mounting base by a tension bolt. This bolt passes with an important play through an opening of the piezoelectric transducer arranged between the mounting base and the seismic mass. Under the influence of inertial forces which act perpendicularly to the axis of the bolt on the seismic mass, bending loads occur which result in an increase of the pressure at one side and in a decrease of the pressure at the other side of the piezoelectric transducer. The transducer has halves polarized in opposite directions and subjected to pressures varying in opposite sense and electrodes covering the top and bottom surfaces of the halves, signals of the same polarity being induced under the bending loads. External influences which act uniformly on the transducer are compensated.

BACKGROUND OF THE INVENTION 
The present invention relates to an accelerometer with a seismic mass which 
exerts intertial forces on at least one electromechanical transducer under 
the influence of accelerations. 
Known accelerometers of this kind comprise between a mounting base and the 
seismic mass of the accelerometer an element sensitive to pressure, 
preferably a piezoelectric transducer element. These transducer elements 
are generally in the form of flat-shaped disks which are polarized in the 
same direction as the force to be measured. The surface of the electrodes 
for deriving the charges produced are perpendicular to the direction of 
polarization, i.e., to the direction of pressure. 
Similar accelerometers with transducer elements strained to shear, more 
particularly piezoelectric elements, are known. In this case, the elements 
are flat or annular-shaped and they are polarized in parallel with the 
direction of the force to be measured. The surface of the electrodes which 
derive the charges produced, are oriented in parallel with the direction 
of polarization, respectively the direction of shear. In the case of 
annular elements, the polarization and the direction of the force to be 
measured is mostly axial. 
All known accelerometers use pressure or shear strains which act over the 
full section of the transducer element. Such transducers are relatively 
sensitive to external influences, e.g. electric or electromagnetic fields 
which may induce disturbing signals in the electrodes. 
SUMMARY OF THE INVENTION 
It is the object of the invention to take measures for reducing such 
disturbing influences, with the simplest means and high sensitivity of the 
accelerometer. Embodiments of the invention with their particular 
advantages will be further described with reference to the drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The accelerometer illustrated in FIG. 2 comprises a mounting base 1 and a 
piezoelectric transducer element 12. The seismic mass 5 is screwed by 
means of a bolt 6 onto the mounting base 1 and it is pressed against a 
transducer with such an initial tension that the parts are prestressed and 
immovably held for all conditions of operation and for each possible 
orientation of the accelerometer. Through a housing 7, the active 
mechanical parts are protected and tightly locked. Tight passages 8 for 
terminal pins 9 are provided in the wall of the housing. Fastening holes 
(not shown) for the assembly of the accelerometer are provided in the 
mounting base. The arrow 10 in FIG. 2 shows the axis of sensitivity of the 
accelerometer. If acceleration occurs in this axis of sensitivity, the 
inertial forces of the seismic mass 5 produce bending stresses which 
result in opposite variations of the forces of pressure in the left, 
respectively the right hand side in the transducer parts illustrated in 
FIG. 1 or FIG. 2. As an example, an increase of the pressure will take 
place in the right part of the transducer and a decrease of the pressure 
will occur in the left part. In this manner, variations of strains will 
take place which are directed in opposite directions. With other words, 
the variations of the pressures and of the strains take place in 
push-pull. 
An important novelty with respect to the known embodiments consists however 
in the use of bending moments instead of using, as known, pure forces of 
pressure or shear. More particularly in the case of relatively slender 
constructions of the accelerometer, the gain of charge is more favorable 
for the same geometrical dimensions than in the usual transducer elements 
sensitive to shear or pressure, because the bending strains, for an 
increasing length, increase much more rapidly than the corresponding shear 
or pressure strains. As can be seen from the preceding, the utilization of 
bending strains permits, in the simplest manner, loading the transducer in 
push-pull so that a usual transducer element may be provided simply with a 
particular electrode arrangement for achieving the mentioned advantages. 
In the case of pure pressure or shear strains, such a solution would not 
be possible. It is further possible to increase the sensitivity for 
constant dimensions of the seismic mass in that one reduces the surface of 
the cross section of the transducer. Indication has already been made 
regarding the electrical and geometrical symmetry of the transducer and to 
the advantages which result thereof. One is namely not only independent of 
external disturbing influences but also undesired charges are compensated, 
like the ones which occur due to temperature variations because of the 
pyroelectric effect. Mechanical strains which are produces by the 
elongation of the mounting base and/or the housing (called base strain and 
case strain effects) become largely ineffective because the elongation 
present in the mounting base or in the housing have approximately the same 
effect on both halves of the transducer so that the charges produced 
compensate mutually. 
FIGS. 1 and 2 show an embodiment of the present invention. The essential 
difference consists simply in the arrangement of the electromechanical 
transducer element 12 of piezoceramics. Contrary to piezocrystals, such 
ceramics are polarized by applying a strong electric field under 
determined conditions of temperature, which offers the possibility to 
polarize different zones of the element in different directions. In the 
case of the embodiment according to FIGS. 1 and 2, the two halves of the 
annular transducer element 12 are polarized in opposite directions as 
shown in FIG. 1. The corresponding polarizations are also indicated in 
FIG. 2 on the assembled element. In the neutral condition, that is when 
both halves of the transducer element which are polarized in opposite 
directions are submitted to equal forces of pressure, equal charges which 
compensate mutually appear at the upper side and lower side of the 
element. The transducer is covered in this case with two full 
annular-shaped electrodes 13 so that in the neutral condition the charges 
on these electrodes compensate mutually. However, if different forces of 
pressure act in both halves of the transducer, these forces being due to 
the inertial forces of the seismic mass 5, variations of charges occur 
which add up and result in a summation at the terminals 9. As an example, 
if the pressure increases in the right hand side of FIG. 2, the positive 
charges at the upper electrode will increase and the negative charge at 
the lower electrode will decrease. At the same time, the positive charges 
at the bottom in the left half of the transducer will decrease and the 
negative charges at the top will increase. This results therefore in a 
summation of the variations of the charges. In the case of undesired 
external influences, the compensations mentioned above are again at least 
partially present. The embodiment according to FIGS. 1 and 2 has the 
important advantage of an usual, simple electrode arrangement.