Liquid flow meter

A positive displacement liquid flow meter uses two variable volume chambers 16,18 arranged in parallel, each with a reciprocable piston 24,26. While liquid from one chamber is being fed to the outlet 11, the other chamber is being charged with the liquid from the inlet 10. A sensing piston 32 in the outlet line 11 indicates when flow variation occurs and allows a feed back signal to be produced to produce the necessary alteration in movement of the piston from the chamber which is feeding the outlet line. Valves A, B, C, D are provided on either side of each chamber, so that when a chamber is feeding it is isolated from the liquid supply.

This invention relates to a positive-displacement liquid flow meter for 
metering the volume flow of a liquid. The meter is particularly, but not 
exclusively, adapted to be used as a fuel flow meter in the testing of 
internal combustion engines. 
With the great emphasis being placed nowadays on fuel economy, it is 
important for engine designers and manufacturers to be able to quantify 
accurately the fuel flow requirements of engines, so that they can decide, 
for example, if a particular change in an engine parameter results in an 
increase or decrease in fuel consumption. All engine development requires 
this information, but currently available meters do not possess sufficient 
accuracy to be able to provide engineers with the accurate and detailed 
information they require. Apart from the self-evident requirements of 
accuracy and repeatability of readings, it is also very important that a 
meter used for this purpose should have a very quick response time so that 
it can follow sudden changes in fuel demand by the engine, for example 
when the engine is accelerated sharply. It is also desirable that it 
should be able to provide fuel flow information over extremely short time 
periods. 
According to the present invention, there is provided a 
positive-displacement liquid flow meter having an inlet and an outlet, two 
variable-volume chambers arranged in parallel between the inlet and the 
outlet, a pressure sensing device downstream of the chambers, and valves 
on either side of each chamber, wherein the chambers each comprise a 
cylinder with a moveable piston and drive means, responsive to the 
pressure sensing device, for moving the piston to vary the chamber volume, 
the meter also including control means for opening and closing the valves 
in a predetermined sequence to provide a continuous liquid flow from the 
outlet, and an output device for measuring piston movement to indicate the 
quantity of liquid flowing through the meter. 
In one embodiment, the chambers are independent of one another and each has 
its own piston. However in a preferred embodiment, the chambers are formed 
at opposite ends of a common cylinder, with a single, double-acting piston 
dividing the cylindrical cavity into two parts and bounding each of the 
two chambers. 
The pressure sensing device is preferably formed by a sensing piston 
floating on a column of liquid which is in communication with the meter 
outlet. 
In operation, one chamber is being filled from the inlet, with the piston 
being correspondingly moved to increase the chamber volume, whilst the 
other chamber is feeding into the outlet. Once the chamber feeding into 
the outlet is empty, then the valves are switched over so that the other 
chamber, which is now full, feeds into the outlet while the first chamber 
is refilled from the inlet. In practice, to ensure a continuous regular 
flow, the opening and closing of the valves preferably overlap. 
It is a principle of operation of the meter that the pistons exert no 
pressure on the liquid. Normally only one chamber is connected to the 
outlet, and so it is the movement of the piston associated with that 
chamber which is monitored to give a reading of liquid flow rate. The 
piston is moved by a solenoid arrangement using a voice coil, and the 
signal passed to the voice coil is derived from the sensing piston. While 
the fuel flow rate is constant, the sensing piston will not move and thus 
the signal passed to the voice coil will remain constant and the piston 
movement will remain uniform. If the liquid demand goes up, then the 
sensing piston will fall and a corresponding signal will be passed to the 
piston in the feeding chamber to speed up that piston. 
Piston movement needs to be detected accurately in order to give the 
necessary accuracy of measurement, and it is preferred to use a Moire 
transducer to effect this measurement. 
The output device receives signals responsive to piston movement. In the 
normal state when only one chamber is feeding, the signal from the other 
piston will be zero. However whilst the chambers are changing over, 
signals from both pistons will be positive and will be passed to the 
output device for summation. Additionally however there is a small flow 
associated with the sensing piston. When, for example, the flow rate 
diminishes, the sensing piston rises and the volume of liquid downstream 
of the chambers increases. As a steady state is once again attained, this 
small extra volume of liquid is fed into the outlet line, and preferably 
the output device also receives a signal from the sensing piston (movement 
of the sensing piston can also be picked up by a Moire transducer) and 
this signal is then summed with the signals from the main metering pistons 
to give the final volume flow rate reading.

The meter shown in FIG. 1 is connected in a liquid supply line so that one 
part 10 of the supply line becomes the inlet to the meter and another part 
11 becomes the outlet from the meter. The line 10 splits into two branches 
12 and 14. The branch 12 leads to a first variable-volume chamber 16 and 
the branch 14 leads to a second variable-volume chamber 18. From the 
chambers 16 and 18, branches 20 and 22 respectively lead into the common 
outlet line 11. 
Solenoid valves A, B, C and D are arranged in each of the branches 12, 20, 
14 and 22 respectively. 
The chambers 16 and 18 each include moveable pistons 24 and 26 which can 
reciprocate in the chambers to vary the chambers volumes. The pistons 24 
and 26 are moved electromagnetically by voice coils 28, 30. 
In the outlet line 11 a sensing piston 32 is arranged which floats on a 
column of liquid in a communication with the liquid in the line 11. 
The two pistons 24 and 26 work in opposition, with piston 24 taking in fuel 
from the supply 10 (inlet valve A open, piston 24 moving towards bottom 
dead centre) whilst piston 26 is delivering fuel via its exhaust port 
(valve D open) by moving towards top dead centre. At BDC/TDC respectively 
the inlet valve A to piston 24 closes and the outlet valve B opens and for 
piston 26 the inlet valve C opens and the outlet valve D closes. 
There are no leakage problems with this meter because it is ensured that 
the exhaust valve (B or D) is always closed when the corresponding inlet 
port (A or C) is open. 
The volume flow is obtained by measuring the displacement of the pistons 24 
and 26. This is done to a high degree of accuracy using Moire transducers 
34 and 36 which sense piston movement. Moire transducers essentially 
consist of a pair of optical gratings aligned at a small angle to each 
other. This produces a series of millimeter-sized opaque and transparent 
bands from a micron-sized grating pitch. A micron-sized horizontal 
movement of one grating with respect to the other then produces 
millimeter-sized movement of the opaque and transparent bands, which 
alternately allows and prevents the passage of light from a light source 
to a photodetector. In this way a micron-scale movements can conveniently 
be sensed by available LED light sources and photodiode detectors with 
millimeter-scale active dimensions. In this application, one grating will 
be attached to the moving piston shaft whilst the other is held 
stationary. 
Displacement is therefore measured by counting the passage of bright and 
dark bands. The method is digital in nature, and of inherent high 
repeatability and reliability. 
In operation, the amount of flow delivered by each of the pistons is 
directly available from the Moire transducer rigidly coupled to each 
piston. The output flow from both pistons is combined and enters the 
outlet line 11 in which the sensing piston 32 is located. This sensing 
piston also carries a Moire transducer and fits with a close mechanical 
tolerance in a vent tube 38. If the sum of the flows from the chambers 16 
and 18 exceeds the flow demand, the pressure drop across the sense piston 
32 drives it upwards. Similarly, if the flow provided is less than the 
demand, the piston is driven downwards. The position of the sense piston 
32 can therefore be used to control the flow delivery from the two pistons 
via a servo loop to match the engine flow demand. This volumetric control 
approach employs a digital, drift free transducer (ie the sensing piston 
32 with its Moire encoder), rather than an analogue pressure transducer, 
and can also be arranged to provide fuel overflow if the sensing piston 
moves out of range due to servo failure. If a sealed system using a 
pressure transducer was employed, servo failure could result in damage to 
the transducer. 
When the meter is at the cross-over point where flow transfers from chamber 
18 to chamber 16, a simple abrupt transition in flow could be used by 
simultaneously switching over all the valves A, B, C and D. The valves 
could in this case be simple on/off valves. However this would undoubtedly 
produce irregularities in the flow since neither the piston 24 or 26 can 
be accelerated or decelerated instantaneously. It is preferred therefore 
to have a short time during which both pistons deliver fuel and during 
this short period the fuel supply may temporarily exceed or under provide 
the fuel demand. However this excess or shortfall will be registered by 
the sensing piston 32, and volume displacement of the sensing piston will 
be equal to the error. Therefore at all times the actual flow taken by the 
engine is given by 
##EQU1## 
It is therefore unnecessary to wait until the time-averaged position of the 
sense piston is near zero before an accurate measure of flow can be 
provided. 
Further refinement to the metering process can be provided at the 
cross-over point, if desired. The velocity (and therefore flow) of piston 
24 can be gradually decreased below that needed to satisfy the demand, and 
that of piston 26 can be gradually increased, so that the sum of the flows 
from the two pistons closely equals the flow demand. This can be done 
either by 
a sin.sup.2 (at) and cos.sup.2 (at) showing a flow between pistons 24 and 
26 (as shown in FIG. 2) or 
a 1/2(1-at) and 1/2(1+at) showing a flow between Pistons 24 and 26 
where t=time and a=constant. 
Then total flow is given by 
##EQU2## 
The flow delivered remains equal to the flow demand at all times during 
changeover. This also holds true, of course, when the flow demand, F, is 
time-dependent, F=F(t). 
This ensures a minimised volume displacement of the sense piston 32, which 
therefore can be made with small stroke and cross-section, giving very 
rapid response. The shareout of flow at the cross-over point is controlled 
from the positions of pistons 24 and 26 within their strokes, as measured 
by their Moire encoders, by means of an `inner` servo loop. This 
refinement is not essential, but is available to give accuracy and quick 
response. 
In order to further improve the sense piston response time, the meter can 
be arranged within a flooded chamber so that liquid is present on both 
sides of the sensing piston 32. In this figure the inlet line 10 leads 
into an outer chamber 40 which is entirely flooded with liquid 42. The 
meter itself is mounted in a wall of an inner enclosure 44, and the meter 
is indicated schematically in FIG. 3 at 46. Liquid flowing from the outer 
enclosure 40 into the inner enclosure 44 flows through the meter 46 and 
out through the outlet line 11. The sensing piston 32 is now influenced by 
pressure from the enclosure 40 on one side and pressure from the enclosure 
44 on the other side. 
If the meter delivers an excess of flow, then the sensing piston 32 is 
driven upwards, as before. If it delivers a shortfall, the liquid pressure 
in the outer enclosure 40 drives it downwards. This avoids the need to 
rely upon the mass of the sensing piston itself or on an auxiliary 
magnetic field to return the sensing piston to its zero position. 
The servo control system for the voice coil drive and for forming the 
output metered flow is shown in FIG. 4. It will be seen that the output 
from the sensing piston provides signals to the drives for the pistons 24 
and 26. A feed back system in each drive circuit ensures that the correct 
piston drive rate is maintained. 
Signals from the Moire transducers 34 and 36 are passed to a summation unit 
48, and a third signal 50 derived from sensing piston movement is also fed 
to the same summation unit. The output signal from this unit 48 therefore 
represents the total flow for the meter. 
A control 54 for the solenoid valves A, B, C and D is also connected into 
the circuit. The two `inner` loops of the servo control the `sharing` of 
the flow demand between pistons at various points during the meters cycle; 
the `outer` loop ensures that the sum of flows from both pistons matches 
the flow demand. 
FIG. 5 shows a second version of a meter. Only that part between the 
solenoid valves A, B, C, D is shown, Other parts of this meter will be as 
shown in the other figures. The meter is formed in a single body with a 
central cylindrical cavity 60 divided by a single double-acting piston 62 
into upper and lowr chambers 64 and 66. As in the embodiment shown in FIG. 
1, a voice coil drive 68 moves the piston and a Moire transducer 70 is 
used so that the movement can be read. 
The piston 62 therefore both intakes liquid and delivers liquid at all 
points during its stroke. This reduces the piston velocities (and 
therefore acceleration) necessary to achieve a given flow, so that the 
forces that must be supplied to enable a given rate of flow delivery can 
be reduced. 
This form of the meter requires shaft seals to be made to the piston shaft 
at both ends. Although two voice coils 68 and two transducers 70 are 
shown, the meter could work with one coil and one transducer only. 
The single-acting meter of FIG. 1 can achieve the following performance 
flow range of 700:1 from 0.1 kg/hr to 70 kg/hr 
response time &lt;0.1 s 
accuracy of +/-0.25% for flows of 0.1 kg/hr in times above 0.7 s. 
Above 0.7 kg/hr, accuracy of +/-0.25% is possible in 0.1 s. 
ability to meter to +/-0.25% accuracy, a flow changing from 0.1 to 33 kg/hr 
in 1 s. 
The double-acting meter shown in FIG. 5 can achieve the following 
performance 
flow range of 700:1 from 0.1 kg/hr to 70 kg/hr 
response time below 0.1 s 
accuracy of +/-0.25% in 0.2 s at 0.1 kg/hr if required. 
Above 0.2 kg/hr accuracy of +/-0.25% can be achieved in 0.1 s 
ability to meter to 0.25% accuracy a flow changing from 0.1 to 35 kg/hr in 
1 s 
The meter described is able to meet the desired criteria of reliability by 
the use of active components below their upper limits of capability, eg 
solenoid valves, voice coil drive etc. and by the avoidance of side loads 
on, or mechanically operated porting by the pistons. 
Repeatability of readings from the meter will be assured by employing 
direct measurement of fuel flow by the techniques of displacement and time 
measurement, by using digital transducer techniques that avoid drift and 
by metering the `error flow` sensed by the sensing piston 32. 
If desired, more than one meter could be used in parallel to extend flow 
range capability, and this is helped by the use of digital pickoff 
techniques.