Method and device for measuring and controlling the dosage of small quantities of fluid by means of a resonating needle, and resonating needle suitable for this purpose

Abstract

A resonating needle is adapted to be used in a device for measuring and controlling the dosage of a small quantity of fluid, includes: a needle (1) adapted to contain said small quantity of fluid; a resonating element of the piezoelectric type (2), coupled to said needle and adapted to be energized in order to detect variations of oscillation parameters of the needle and of the resonating element for the purpose of determining the dosage measurement and control.

Claims

1. A device for measuring and controlling a quantity of fluid, said device comprising: a needle adapted to contain a fluid, the needle comprising a first end, a second end, a tip opposite the first end, and a length that extends from the first end to the tip; and a resonating element disposed on one side of the first end of the needle so that the resonating element does not encircle the needle, the resonating element comprising a laminar piezoelectric bender, the resonating element being constrained to the first end of the needle at a first point of constraint and at a second point of constraint, the second point of constraint being spaced apart from the first point of constraint so that the resonating element is not constrained to the needle between the first point of constraint and the second point of constraint, the resonating element being configured to subject the needle to a flexural mechanical impulse that causes the needle to oscillate and also being configured to detect variations of oscillation parameters of the needle.

2. The device according to claim 1, wherein said needle is a capillary needle made of metallic material.

3. The device according to claim 2, wherein the metallic material comprises stainless steel.

4. The device of claim 1, wherein the resonating element is constrained to the needle at the first point of constraint by a glue.

5. The device of claim 4, wherein the resonating element is constrained to the needle at the second point of constraint by a glue.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further objects and advantages of the present invention will become apparent from the following detailed description of a preferred embodiment (and variants) thereof and from the annexed drawings, which are only supplied by way of non-limiting example, wherein:

(2) FIG. 1 schematically shows an example of embodiment of the measuring device of the present invention, with frames showing enlarged parts thereof.

DETAILED DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION

(3) FIG. 1 shows an example of embodiment of a resonating needle according to one aspect of the invention. The resonating needle essentially comprises a needle 1 and a piezoelectric resonating actuation element 2 coupled to the needle 1. As depicted in FIG. 1, needle 1 has an upper portion or first end 10 that terminates at a top 12 and a free portion or second end 14 that terminates at a tip 16. Tip 16 is disposed opposite of top 12. Needle 1 is tubular and has a sidewall 18 that encircles a channel 20 that extends from top 12 to tip 16. Sidewall 18 is depicted as having a first side 22 that extends between top 12 and tip 16 and an opposing second side 24 that extends between top 12 and tip 16.

(4) The piezoelectric resonating actuation element 2 (also referred to herein as a resonating element) is preferably a leaf-shaped piezo-bender element positioned alongside the needle 1.

(5) The needle 1 is stressed in order to measure its oscillation-induced deformation by using a bender-type piezoelectric material, wherein the intensity of the electric field determines the amplitude of the differential contraction between two plates, which causes inflection of the element.

(6) A piezo-bender is a laminar piezoelectric material of the piezo-ceramic type that can be used as a sensor or as a source of vibrational mechanical energy; when coupled to a structure, it can apply vibrational mechanical stress thereto or detect the deformation thereof.

(7) In particular, the piezo-bender used in this context is composed of two push-pull feeded laminar piezoelectric elements, contracting and elongating, which cause the needle 1 to oscillate by bending. Thus, piezoelectric resonating actuation element 2 can comprise a laminar piezoelectric bender.

(8) The response of a piezo-ceramic material to a change in the electric field or a deformation is extremely fast, and vibrations in the kHz range can be produced or detected.

(9) The piezoelectric resonating actuation element 2 is fixed to the upper portion 10 of the needle 1 (opposite to the needle tip 16). The fixing operation is carried out by taking into account the nature of the component and the desired effect upon the needle 1. Preferably, the fixing is done by means of a suitable glue 4 (e.g., Araldite 2014-1), which creates a strong mechanical junction with the top of the needle 1 at a first point of constraint 26. A second point of constraint 7 to the needle 1 is found at the opposite end of the piezoelectric resonating actuation element 2, e.g., by means of the same glue, in order to induce inflection thereof. Thus, as depicted in FIG. 1, resonating element 2 is disposed on first side 22 of upper portion 10 of needle 1 so that resonating element 2 does not encircle needle 1. The resonating element 2 is constrained to upper portion 10 of needle 1 at a first point of constraint 26 and at a second point of constraint 7. The second point of constraint 7 is spaced apart from the first point of constraint 26 so that resonating element 2 is not constrained to needle 1 between the first point of constraint 26 and the second point of constraint 7.

(10) Such mutual arrangement of the needle and the piezoelectric element provides the utmost capability of detecting the oscillation frequency.

(11) Preferably, an external protection 3 consisting of a plastic cylinder surrounds the piezoelectric element and at least that part of the needle which faces towards it.

(12) There are also electric conductors for energizing the piezoelectric element and detection of the electric signals generated by it through the effect of the oscillation of the coupled elements, i.e., needle and piezoelectric element. Thus, resonating element 2 is operable between an energized state and a non-energized state.

(13) The needle is normally used for taking and then releasing a very small quantity of fluid 5 (of the order of microlitres, e.g., <10 microlitres), which positions itself within the needle in the terminal part 1 thereof, i.e., in the tip area.

(14) According to one aspect of the invention, the resonance frequency measure is strictly related to the mass of fluid present in the terminal part of the needle.

(15) The flexural oscillation motion of the needle is regulated by an equation such as:
f(t)=s*k+ds/dt*b+m*ds2/dt2

(16) where

(17) s is the movement of the terminal part of the needle;

(18) k is the elastic constant of the needle, which is generally made of stainless steel having a high and stable elastic behavior.

(19) b is the damping,

(20) m is the massThe mass consists of the total of the mass of the needle itself and the mass of the fluid concentrated in the terminal part of the needle,

(21) f(t) is the externally applied oscillating force.

(22) The resonance frequency fr of the needle is given by the following relation:
fr=0.159*(3*E*I/(mf+3/8mt)/Lc{circumflex over ()}3){circumflex over ()}0.5
wherein

(23) E is the Young's modulus of the needle material,

(24) I is the inertia of the needle cross-section,

(25) mt is the distributed mass of the needle,

(26) mf is the concentrated mass corresponding to the fluid within the terminal part of the needle,

(27) Lc is the free inflection length of the needleActually, the piezo-bender also participates in the oscillation, although it is much more rigid than the free part of the needle. The resulting effect is that the equivalent length of the beam (as defined above) is slightly greater than the actual free length of the needle, and the experimental frequency is slightly less than that estimated with Lc.

(28) In one example of embodiment, a conditioning and driving electronics 6 periodically subjects the needle, by means of the piezo-bender, to a flexural mechanical impulse. In response to said impulse, the needle oscillates; by measuring the oscillation frequency and amplitude, it is possible to detect the mass variations due to the presence of fluid in the terminal part of the needle itself. That is, in view of the above and as depicted in FIG. 1, resonating element 2 causes a flexural bending of needle 1 so as to move tip 16 of needle 1 when resonating element 2 is moved from the non-energized state to the energized state. More specifically, wherein when resonating element 2 is in the non-energized state, needle 1 has a central longitudinal axis 28 that extends along the length thereof and that is disposed along a fixed linear axis 30. When resonating element 2 is in the energized state, resonating element 2 causes flexural bending of needle 2 so that at least a portion of central longitudinal axis 28 of needle 1 bends away from the fixed linear axis 30. Thus, as also depiced in FIG. 1, when the resonating element 2 is moved from the non-energized state to the energized state, tip 16 of needle 1 moves laterally away from the fixed linear axis 30.

(29) The variation of the damping induced by the characteristics of the fluid into which the needle is immersed can supply information about the viscosity of the fluid and the contact therewith.

(30) During supply and/or delivery, the continuous measurement of the frequency variation allows the pump 8 to be adjusted for taking and/or delivering exactly the desired mass quantity of fluid.

(31) Therefore, the piezo-bender element is not used for delivering the fluid, but for measuring its quantity, so that the external pump can deliver or take the correct quantity of fluid through the metallic needle.

(32) The needle, preferably a capillary one, must be made of a suitable material having typical metal characteristics in terms of flexural stability, preferably stainless steel, although another material may possibly be used, provided that it has similar flexural stability characteristics. Glass or plastic would not be suitable for this purpose, since they lack such characteristics.

(33) Considering the typical needle dimensions, it is possible to detect frequencies of the component in the range of 100 to 400 Hz, with maximum percent frequency variations of 10% due to the taking of small quantities of fluid.

(34) The operation of the electronics is based on the alternate use of the piezo-bender element as an actuator and as a sensor.

(35) During a period of a few hundreds of microseconds, the electronics generates the actuation impulse that induces flexure of the piezo element and the oscillatory response of the needle. Immediately afterwards, the signal generated by the piezo element through the effect of the oscillation of the needle is picked up, whose frequency and oscillation amplitude variations allow detecting the mass variations.

(36) From the value and frequency of said signal, it is possible to derive the quantity of mass of the fluid that is participating in the component's characteristic motion.

(37) The repetition of the test every fraction of a second allows measuring the fluid as it is being aspirated, and detecting any anomalous conditions such as duct obstruction or gas bubble formation.

(38) The same procedure can be used at the delivery point in order to verify the progressive release of the fluid.

(39) In another example of embodiment, the impulse-based electronics can be replaced by a continuously operating electronic system wherein the piezo-bender acts as a resonating element in the electronic circuit as well (like a quartz element in a tuned oscillator). In this case, the oscillation is continuously maintained at the resonance frequency of the mechanical part, thereby allowing a continuous measurement of mass flows.

(40) The above-described example of embodiment may be subject to variations without departing from the protection scope of the present invention, including all equivalent designs known to a man skilled in the art.

(41) From the above description, those skilled in the art will be able to produce the object of the invention without introducing any further construction details.