Electrohydraulic drive

An electrohydraulic drive is provided. High-speed injection valves of hydraulic lift transformers have, for example, a pressure piston driven by a piezoelectric actuator and a reciprocating piston mounted in an axially displaceable fashion in a pressure piston bore and connected to the valve needle. The electrohydraulic drive has a piezoelectric actuator and a hydraulic force/travel transmission of compact design. To ensure the required axial symmetry of the drive despite production-induced tolerances, a balance element supported in a frustoconical depression of the pressure piston is arranged between the piezoelectric actuator and the lift transformer. During assembly of the hydraulics, the element having the form of a spherical segment and produced from high-grade steel can slide freely on the piezoceramic and thus compensate a nonconcentric alignment of the actuator and pressure piston. The ability to rotate freely inside the abutment ensures that the upper part of the actuator always bears with the full surface contact against the pressure piston.

BACKGROUND OF THE INVENTION 
An injection valve is generally known from German Patent No. DE4306073 C1 
that contains a drive having a of compact design and also having very good 
dynamic properties that still operates reliably even at high operating 
frequencies (f&gt;1 kHz). Since the drive permits valve opening and closing 
times in the range of .tau..ltoreq.0.1 ms, it is possible to inject even 
the smallest fuel quantities into the combustion chamber of an engine in a 
precisely metered and reproducible fashion. The main components of the 
drive are a piezoelectric actuator, which generates the primary operating 
travel, and a hydraulic lift transformer which essentially has a pressure 
piston driven by the piezoelectric actuator and a reciprocating piston 
which is mounted in an axially displaceable fashion in a pressure piston 
bore and is connected to the valve needle. The piezoelectric actuator 
arranged in one of the hydraulic chambers is supported on the housing side 
on a spherical cap bearing. This measure ensures that the actuator always 
bears with full surface contact against the pressure piston even should 
its end surfaces are not parallel for production reasons, and that no loss 
of lift occurs. 
The design of this known valve places high demands on the axial symmetry 
and dimensional accuracy of the individual components. In particular, the 
multiply guided reciprocating piston must be produced accurately down to a 
few .mu.m in order to prevent canting or jamming. This complicates mass 
production and makes production of the valve substantially more expensive. 
SUMMARY OF THE INVENTION 
It is, therefore, an advantage of the present to provide an operationally 
reliable electrohydraulic drive which has a compact design, operates 
within a large temperature range and has good dynamic properties. To this 
end, in an embodiment, an electrohydraulic drive is provided having an 
actuator and a lift transformer arranged in a housing filled with a 
hydraulic medium. The length of the actuator varies in a controllable 
fashion. A first piston is arranged in an axially displaceable fashion in 
a housing bore. A second piston acts on a spring element and a control 
element wherein the first piston is driven by the actuator. The first 
piston has an axial bore in which the second piston moves in an opposite 
direction to the first piston. A balancing element is arranged between the 
actuator and the lift transformer wherein the balancing element is formed 
of a spherical segment and is supported in an actuator side depression of 
the first piston and is capable of sliding freely on the actuator. 
The advantage which can be achieved with the present invention consists, in 
particular, in that even a comparatively large decentering of one of the 
multiply guided parts does not impair the functionality of the drive. The 
drive can, therefore be, produced with a substantially lower outlay and in 
a more cost effective fashion.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
The design and method of functioning of the electrohydraulic drive will be 
described in conjunction and with reference to FIGS. 1 and 2 wherein like 
numerals refer to like parts. 
FIG. 1 shows essentially only the components of a high-speed fuel injection 
valve which relate to the drive according to the invention; such a valve 
is disclosed for example, in German Patent No. DE 43 06 073 C1 or 
described in more detail in the German Patent Application DE 4406522. As 
drive element, the injection valve contains an electromechanical actuator 
P which acts on a hydraulic lift transformer DK/HK and which is supplied 
with the required operating voltages via a pressure-tight housing bushing 
LD. Particular consideration is given as the electromechanical actuator P 
to a piezoelectric multilayer stack which still generates comparatively 
large primary lifts even in the case of moderate operating voltages 
(relative changes in length .DELTA.l/l.apprxeq.1.times.10.sup.-3 ; drive 
force F=10.sup.2 to 10.sup.5 N) 
Because of the high mechanical stiffness of the piezoelectric sintered 
body, its electromechanical resonance is in the range of about 10 to 1000 
kHz, with the result that it is possible in principle to achieve response 
times of about 0.001 to 0.1 ms. The response times realized in practice 
are, however, longer and depend, inter alia, on the electrical activation 
and wiring of the piezoelectric stack, as well as on the size of the 
masses driven by the actuator P. Since the electrical capacitance of the 
piezoelectric stack is typically in the range of about C.sub.p =1 to 100 
.mu.F, and the internal resistance of the voltage source assigned to the 
actuator is about R.sub.i =1.OMEGA., values of about .tau.=1 to 100 .mu.s 
result for the charging time constant defined by .tau.=C.sub.p 
.times.R.sub.i. The response times of the piezoelectric actuator P are 
thus 1 to 2 orders of magnitude below those of comparable electromagnetic 
drives, and, in conjunction with a compact valve design and small moving 
masses, this permits extremely short valve opening and closing times. 
In order to initiate the injection of the fuel into the combustion chamber 
of the engine, the actuator P is activated and thereby elongated in the 
axial direction. The change in length .DELTA.l of the actuator P is 
attended by a corresponding upward displacement of the pressure piston DK, 
which is mounted with a clearance fit in a cylindrical bore of the housing 
VG, with the result that an overpressure p.sub.1 is built up in the 
chamber KA1 filled with hydraulic oil, and an underpressure p.sub.2/3 
&lt;p.sub.1 is built up in the chambers KA2 and KA3, which are like-wise 
filled with hydraulic oil and are connected to one another in terms of 
fluid flow by a pressure piston bore B1. As soon as the hydraulic forces 
proportional to the pressure difference .DELTA.p=p.sub.1 -p.sub.2/3 exceed 
a value dependent on the stiffness and biasing of the helical spring SF 
arranged in the chamber KA2, the pot-shaped reciprocating piston HK moves 
downward in the cylindrical pressure piston bore ZY and thereby lifts the 
valve needle VN, which is connected to it, from the sealing fit, and the 
injection operation begins. 
The fuel injection is terminated by the electrical discharge of the 
piezoelectric actuator P. Because of the attendant contraction of the 
actuator P, the pressure piston DK moves back downward into its initial 
position under the compulsion of the restoring force exerted by a strong 
disk spring TF. Supported by the helical spring SF and the pressure 
difference existing between the chambers KA1 and KA2/KA3, the 
reciprocating piston HK carries out a movement upward in the opposite 
direction, with the result that the valve needle VN, which is guided in a 
sealed fashion out of the housing VG, sinks onto the sealing fit and seals 
the injection opening. 
The transient mode of operation of the drive necessitates mechanically 
prestressing the piezoelectric actuator P. The force required for this is 
generated by the disk spring TF which is arranged in the chamber KA1 and 
which also supports the return of the pressure piston DK to its neutral 
position. Flow channels SK in the chamber ensure unhindered inflow and 
outflow of the hydraulic oil into and from the volume enclosed by the disk 
spring TF and the valve housing VG. 
In order to guarantee the required axial symmetry of the system of the 
primary drive and force/travel transmission despite production-induced 
tolerances, a balancing element AE supported in a frustoconical depression 
WL of the pressure piston DK is arranged between the piezoelectric 
actuator P and the lift transformer. The balancing element AE, which has a 
form of a spherical segment, preferably consists of high-grade steel or a 
nickel-chromium-steel. Because of its polished surfaces, the balancing 
element AE can slide freely on the piezoceramic during the assembly of the 
hydraulics, and thus compensate a nonconcentric alignment of the actuator 
P and pressure piston DK. The freedom of the balancing element AE to 
rotate inside the conical abutment WL ensures, furthermore, that the upper 
part of the piezoelectric actuator P, which is mounted securely in terms 
of rotation on the bottom of the housing, always bears with full surface 
contact against the pressure piston DK. The disk spring DF, which 
mechanically prestresses the piezoelectric actuator P, ensures the 
force-closed contact of the parts with one another. 
The force/travel transmission driven by the actuator P comprises two 
coupled hydraulic transformers, wherein the transmission ratio .eta..sub.1 
of the upper lift transformer is given by 
EQU .eta..sub.1 =AD1/AH1 (1) 
wherein 
AD1 is area of the pressure piston top side and 
AH1 is area of the reciprocating piston top side; 
and the transmission ratio .eta..sub.2 of the lower lift transformer is 
given by: 
EQU .eta..sub.2 =AD2/AH2 (2) 
wherein 
AD2 is actuator-side pressure piston area and 
AH2 is actuator-side reciprocating piston area 
Equation (2) holds, however, only under the precondition that the actuator 
P arranged in the hydraulic chamber KA3 has the same volume in the 
elongated and discharged states. Like the piezoelectric stack P which is 
used, electrostrictive and magnetostrictive actuators also display such a 
behavior to a good approximation. 
If the actuator P experiences a change in volume .DELTA.V proportional to 
the change in length .DELTA.l, it can be assigned the effectively active 
actuator area AP:=.DELTA.V/.DELTA.l. In this case, the transmission ratio 
.eta..sub.2 ' of the lower lift transformer is given by: 
EQU .eta..sub.2 ':=(AD2-AP)/AH2 (3) 
In the ideal case, the upper and lower lift transmission ratio should be 
identical (.eta..sub.1 =.eta..sub.2 =.eta.), and this can be achieved 
directly by an appropriate design of the pressure-active end faces of the 
two pistons DK, HK. Thus, the pressure piston DK of the force/travel 
transmission represented in FIG. 1 is of stepped design (AD1&lt;AD2), in 
order to take account of the inequality, caused by the valve needle, 
between the pressure-active reciprocating piston areas AH1&lt;AH2. 
It is a consequence of the hydraulic coupling of the two lift transformers 
that for every change in length of the actuator P, complementary pressures 
build up in the chambers KA1 and KA2/KA3. A displacement of the pressure 
piston DK by .DELTA.l causes causing a displacement of the reciprocating 
piston HK in the pressure piston bore ZY which is in the opposite 
direction and is enlarged in accordance with the hydraulic transmission 
ratio .eta.&gt;&gt;1. 
In order to ensure that the drive is largely independent of temperature, 
the hydraulic chambers KA1, KA2, KA3 are connected, both via one another 
and via the capillary gap KS present between the pistons DK, HK and the 
corresponding cylinder bores, to a balancing volume AV which is at 
overpressure. Temperature-induced changes in volume of the hydraulic oil 
can therefore lead neither to the formation of static pressure differences 
between the chambers KA1 and KA2/KA3 (this would result in undefined 
positions of the reciprocating piston HK), nor to the formation of 
undefined pressure states in the entire system. The connection, effected 
via the housing bore G1, of the annular chamber RV to the balancing volume 
AV has, furthermore, the advantage that no cavitation reducing the maximum 
operating frequency occurs in the hydraulic oil. 
By adapting the flow resistances of the capillary gaps to the viscosity of 
the hydraulic oil employed, it can be ensured that in the relevant working 
temperature range the valve blocks at the frequency prescribed by the 
activating signal and for the desired period. In order to set a large flow 
resistance, it can be recommended, for example, to provide the bore G1 in 
the region of the pressure piston sealing surface. However, it can in 
principle, also be fitted in any other region of the valve housing VG 
provided that flow resistances in the form of orifices, gaps, throttles, 
constrictions etc. ensure that only comparatively slow balancing processes 
take place between the various volumes and chambers. The chambers are, if 
appropriate, to be sealed with respect to one another to such an extent 
that the required blocking times are reached and it continues to be 
ensured that the drive is independent of temperature. A 
temperature-dependent control of the gap flows is possible if the valve 
housing VG and the built-in components (pressure piston DK, reciprocating 
piston HK) are produced from materials having different coefficients of 
thermal volumetric/linear expansion. It can be achieved thereby that the 
gap widths reduce with increasing temperature, and this correspondingly 
increases the flow resistance. Temperature-controlled flow resistances 
can, of course, also be produced as discrete components and be built into 
the corresponding bores G3 or supply lines. 
The drive according to the invention has a range of advantages. Thus, the 
drive permits symmetric, cavitation-free switching with very short 
switching times, extremely short dead times and high operating 
frequencies. Furthermore, because of its comparatively simple and compact 
design and of the large range of operating temperatures, the drive is 
distinguished by a high operational reliability. This is also aided by the 
fact that the actuator P is hermetically encapsulated in one of the 
hydraulic chambers KA3. Good dissipation of the heat generated, and 
optimum protection against environmental influences are thereby ensured. 
The drive is also largely sealed, since the electric connections L of the 
actuator P are led outward through a pressure-tight, electrically 
insulating element LD. 
The reciprocating piston of the force/travel transmission represented in 
FIG. 2 has two parts HK1, HK2. The outer part HK1 is formed in the shape 
of a pot, is open on the valve needle side and rests on the spring SF, 
being guided with a closer tolerance in the pressure piston bore ZY. 
Supported herein is the inner reciprocating piston part HK2, which is 
likewise pot-shaped and open on the actuator side. A screw S connects the 
inner part HK2, which can be displaced transverse to the direction of 
lift, to the valve needle VN. Both parts can also be soldered or welded. 
The horizontal displaceability of the inner reciprocating piston part with 
respect to the outer one guided in the bore ZY ensures that an 
eccentricity present in the pressure piston/reciprocating piston system is 
largely compensated. In the non-actuated state, spring SF arranged in the 
chamber KA2 ensures force-closed contact between the two reciprocating 
piston parts HK1/2. The valve needle VN is supported on the valve seat. 
The force-closed contact is maintained even in the event of deflection of 
the pressure piston DK, since the hydraulic oil exerts a larger force on 
the inner part HK2 than on the annular surface A1 (A.sub.2 &gt;A.sub.1) 
assigned to the outer part HK1. The two stops denoted by AS limit the 
downward deflection of the outer reciprocating piston part AK2. A throttle 
DR present on the bottom of the outer reciprocating piston part HK1 
renders it possible to exchange fluid between the two chambers KA2/KA4 as 
previously set forth. 
Furthermore, it should also be understood that other various changes and 
modifications to the presently preferred embodiments described herein will 
be apparent to those skilled in the art. Such changes and modifications 
may be made without departing from the spirit and scope of the present 
invention and without diminishing its attendant advantages. Therefore, it 
is intended that such changes and modifications be covered by the appended 
claims.