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Description  |
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FIELD OF THE INVENTION
The present invention relates to a pump and in particular to a micropump
having a chamber, an intake valve, and a discharge valve.
BACKGROUND OF THE INVENTION
A publication by Zengerle, MEMS 1992, Travemunde, IEEE Catalog No.
92CH3093-2, pp. 19-24, describes a micropump having a working chamber, one
intake valve, and one discharge valve that are structured as silicon
wafers. The pump action is achieved by an electrostatically produced
change in the volume of the working chamber. This valve is particularly
suited for liquids.
SUMMARY OF THE INVENTION
The present invention provides a device and method for pumping a gas or
fluid. A micropump according to the present invention has a first plate
having a chamber disposed therein. The first plate includes an intake
valve at a first portion of the chamber for movement between a first
position at which the gas flows into the chamber and a second position
spaced from the first position. The micropump also has a second plate
coupled to the first plate. The second plate includes a discharge valve at
a second portion of the chamber for movement between a third position at
which the gas flows out of the chamber and a fourth position spaced from
the third position. Further, the micropump includes a heating element at a
third portion of the chamber for controlling a temperature of the gas in
the chamber.
The present invention includes a method for operating the micropump.
Accordingly, the present invention includes a method for pumping a gas (or
fluid) by the steps of (a) increasing the temperature of a heating element
to increase the pressure of the gas inside the chamber and to open a
discharge valve of the chamber, which causes the gas to flow out of the
chamber until the discharge valve closes, (b) upon closing of the
discharge valve, decreasing the temperature of the heating element to
decrease the pressure of the gas inside the chamber and to open an intake
valve of the chamber, which causes the gas to flow into the chamber until
the intake valve closes, and (c) repeating steps (a) and (b) until a
predetermined volume of the gas is pumped.
An advantage of the micropump according to the present invention is that
the applied pump principle allows gases to be pumped effectively. The
micropump is small in size and suited for producing pressures of a few
hundred millibars. Also considered as advantageous are the relatively low
power consumption and the relatively fast time constant of the micropump
according to the present invention.
A heating element is designed quite simply as an ohmic resistor. The power
dissipation is reduced and the reaction rate of the micropump is improved
by mounting the heating element on a carrier having a low thermal capacity
and low thermal conductivity. The carrier can be composed of a material
having a low thermal conductivity, or the thermal capacity and the thermal
conductivity of the carrier can be reduced by constructing the carrier as
a thin membrane. A support is used to stabilize the carrier, which
increases the mechanical stability of the micropump. In particular, the
support suppresses any change in the volume of the working chamber caused
by pressure. By forming the supporting structures out of silicon, such
supporting structure can be produced without incurring significant
additional expenses. In the case of a pulse-shaped heating operation, the
amount of gas delivered can be advantageously controlled by controlling
the temperature and/or the time interval between the heating pulses.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a first exemplary embodiment of the micropump according to the
present invention.
FIG. 2 shows the discharge valve of the micropump of FIG. 1 in a closed
position.
FIG. 3 shows the discharge valve of the micropump of FIG. 1 in an open
position.
FIG. 4 shows a second exemplary embodiment of the micropump according to
the present invention.
DETAILED DESCRIPTION
Referring to FIG. 1, formed out of two silicon plates 4, 5 are one intake
valve 2 and one discharge valve 3, which open to volumes 21 and 22,
respectively, separated by a wall 20. The working chamber 1 is created
from a cut-out in the silicon plate 4 and is sealed on its top side by the
plate-shaped carrier 7 of the heating element 6.
The intake valve 2 is designed to open when the pressure prevailing in the
working chamber 1 is less than that on the outside. The discharge valve 3
is designed to open when the pressure prevailing in the working chamber 1
is greater than that on the outside. Both valves are designed to open even
at low pressure differences. The air in the working chamber 1 is heated by
means of the heating element 6. The heating element 6 can consist of, for
example, deposited metallic layers that are heated by a current flowing
through them. FIG. 1 shows a cross-section through such metallic printed
conductors, which are applied on the carrier 7 in a meander form or as
spirals. The gas trapped in the working chamber 1 is heated by the heating
element 6. The heating effect of the heating element 6 increases as the
heat lost through the carrier 7 or the silicon plates 4, 5 decreases.
Therefore, in the exemplary embodiment of FIG. 1, the carrier 7 is
composed of glass that has an especially low thermal conductivity. Such
glass is known, for example, by the commercial name, Pyrex, from the firm,
Corning Glass.
The micropump according to the present invention works on the basis of the
thermal expansion of gases. In the first step of a pump cycle, the
micropump is in the state depicted in FIG. 1. Both valves are closed and
the gas inside of the working chamber 1 has essentially the same
temperature as the gas outside of the working chamber 1. The heating
element 6 is then heated by a current, so that the gas in the working
chamber 1 is heated. Based upon the ideal gas equation, which applies here
in a first approximation, the product of pressure and volume (i.e.,
pressure x volume) in the working chamber 1 is constant in relation to the
temperature of the gas in the working chamber 1. Since the volume of the
working chamber 1 does not change, a pressure increase in the working
chamber 1 is caused by the heating of the gas in the working chamber 1. As
a result of this pressure increase, the discharge valve 3 opens and a
portion of the gas in the working chamber 1 is forced out of the working
chamber 1 into volume 22. Thereafter, when an equilibrium is attained
between pressure and temperature, the discharge valve 3 closes.
In the next cycle step, the heating of the heating element 6 is switched
off. This is associated with a cooling of the gas that is present in the
working chamber 1. Associated with this cooling of the gas is a decrease
in the pressure prevailing in the working chamber 1. As a result of the
diminished pressure in the working chamber 1, the intake valve 2 opens,
and gas flows into the working chamber 1 from volume 21 until this
difference in pressure is equalized, at which time the intake valve 2
closes again. The micropump again enters the state shown in FIG. 1, and a
new pump cycle can begin. Thus, the micropump pumps gas from volume 21
into volume 22. By having appropriate supply lines leading to volumes 21,
22, the micropump can be used to pump gases in any desired manner.
To manufacture the valves, silicon plates 4, 5 are worked on from both
sides using etching processes. Thin membranes are produced in the etching
process, starting from the one side of the silicon plates 4, 5. By
dividing these thin membranes in an etching process from the other side,
the intake opening of the intake valve 2 and the valve flap 11 of the
discharge valve 3 are constructed out of the silicon plate 5. In the same
way, the valve flap 11 for the intake valve 2, the cut-out for the working
chamber 1, and the opening for the discharge valve 3 are constructed out
of the silicon plate 4. The two silicon plates 4, 5 and the carrier 7 are
joined together so as to form the working chamber 1, which is sealed in a
gas-tight manner. European No. EP-A1-369 352, for example, describes
methods for joining the silicon plates 4, 5 and the carrier 7, and methods
for establishing an electrical contact with the heating elements 6.
In FIGS. 2 and 3, the discharge valve 3 of FIG. 1 is shown in an enlarged
view. This discharge valve 3 is structured out of the silicon plates 4, 5.
For this purpose, each of the silicon plates 4, 5 has an opening. However,
in FIG. 2, this opening is sealed by the valve flap 11. In FIG. 2, the
discharge valve is shown in the state in which the pressure in the working
chamber is less than or equal to the outside pressure. In this case, the
valve flap 11 is closed. In FIG. 3, the discharge valve 3 is shown in a
state in which a higher pressure prevails inside the working chamber 1
than outside the micropump. In this case, the discharge valve 3 is open,
i.e., the valve flap 11 is bent in a way that allows air to flow out of
the working chamber 1. The intake valve 2 functions in an analogous
fashion.
FIG. 4 illustrates another exemplary embodiment of the micropump according
to the present invention. This embodiment likewise has an intake valve 2,
a discharge valve 3 and a working chamber 1 that are etched out of silicon
plates 4, 5. On its top side, the working chamber 1 is sealed off by a
carrier 7, and a heating element 6 is mounted on the carrier 7. However,
in contrast to FIG. 1, the carrier 7 is diminished in its thickness in the
vicinity of the heating element 6. As a result of this reduction in the
thickness of the carrier 7, the thermal conductivity and the thermal
capacity of the carrier 7 are reduced. Thus, with this refinement of the
carrier 7, the heating capacity of the heating element 6 is improved. In
this manner, with lower electric power, this heating element reaches the
same temperature as the heating element shown in FIG. 1. Furthermore, with
this measure, the time required to heat the heating element 6 is reduced
and, consequently, the heating of the gas in the working chamber 1 is
likewise accelerated. In comparison with the micropump shown in FIG. 1,
the micropump shown in FIG. 4 provides a lower power consumption and a
faster reaction.
Care must be taken, however, that the membrane 8 on which the heating
element 6 is mounted is not at all, or is only slightly, deformed by the
pressure difference produced in the working chamber 1. Otherwise, the pump
capacity would again be reduced as a result of too great a deformation of
the membrane 8. Therefore, the membrane 8 must be designed to be thick
enough. Furthermore, the membrane 8 can be stabilized by one or more
supports 9, with FIG. 4 illustrating the use of a single support 9. The
support 9 can be structured out of the silicon plate 4. The advantage of
this is that the manufacturing of the support 9 does not require any
additional process steps. In the cross-sectional view of the micropump
shown in FIG. 4, a cross-section through the support 9 is illustrated. The
areas of the working chamber 1 situated in FIG. 4 to the right and left of
the support 9 are joined with one another, however, so that gas can flow
unhindered from the intake valve 2 to the discharge valve 3.
The pump capacity, i.e., the flow rate produced through the micropump, can
be controlled in different ways. One such way is by controlling the
temperature of the heating element 6. In every pump cycle, the quantity of
pumped air depends on the temperature of the heating element 6. The pump
capacity is increased by raising the temperature of the heating element 6.
It is also feasible to control the flow rate through the micropump by
altering the time intervals of the individual pump cycles. The pump
capacity can likewise be controlled by shortening the time between the
individual pump cycles.
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