Apparatus for controlling diaphragm extension in a diaphragm metering pump

A diaphragm metering pump having control over diaphragm extension is described. A position sensor is incorporated in a diaphragm metering pump to indicate the relative position of the diaphragm during flexure. When excessive extension of the diaphragm is sensed by the position sensor, a control valve will provide hydraulic fluid from a reservoir for inhibiting further deflection of the diaphragm in the direction in which it was moving. Diaphragm life is extended as well as the accuracy of metering provided by the pump maintained.

RELATED APPLICATIONS 
This application is related to U.S. Ser. No. 07/424,443 filed Oct. 20, 
1989, now U.S. Pat. No. 5,056,036. 
BACKGROUND OF THE INVENTION 
The present invention relates to diaphragm metering pumps. Specifically, an 
apparatus for monitoring and controlling the extension of a diaphragm 
being actuated via a hydraulic fluid in a metering pump is described. 
Metering pumps find diverse uses in many industrial processes. Diaphragm 
metering pumps operate from flexure of a flexible diaphragm which applies 
pressure to a pumped media, forcing the media through an outlet check 
valve. Reduction of the hydraulic pressure against the diaphragm returning 
to its preflexed state results in the diaphragm creating a pressure 
differential between the pumping chamber and pumping media inlet. A second 
valve permits additional pumping media to fill the pumping chamber. 
The different applications for these metering pumps require diaphragms as 
diverse as stainless steel and Teflon. A major source of failure for 
metering pumps of this type results when the diaphragm ruptures, through 
excessive flexure and overextension. The overextension of a diaphragm 
results when the hydraulic force applied to the diaphragm either pushes or 
pulls it beyond material specific flexural limits. 
Limitations against overextension of the diaphragms in either direction are 
provided by first and second dish plates in the hydraulic fluid chamber 
and pumping chamber. An overextension condition will occur as a result of 
a hydraulic imbalance as can be caused by leakage of hydraulic fluid past 
the piston. During retraction of the piston, which produces the hydraulic 
force for actuating the diaphragm, the diaphragm retracts against the rear 
dish plate before achieving an overextended state. Likewise, when the 
diaphragm is in the forward extended position during forward extension of 
the piston, a forwardly located dish plate retains the diaphragm from 
achieving an overextended state. Contact of the diaphragm with the dish 
plate can result in excessive stress levels and can contribute to 
pre-mature diaphragm failure and is therefore, undesirable. 
The subject of monitoring diaphragm failure has been described in several 
prior art patents. In U.S. Pat. No. 4,781,535 to Mearns, a leak detector 
was provided which essentially detected the occurrence of a rupture in the 
diaphragm after the fact. Although this technique minimizes the amount of 
contamination which results from hydraulic fluid mixing with pumped media 
and otherwise signals corrective action at the earliest possible time, it 
does not control diaphragm deflection to be certain that the deflection is 
within safe limits to avoid the possibility of a rupture and to prolong 
the life of a diaphragm. 
The sensing of diaphragm position has been considered in U.S. Pat. Nos. 
4,619,589 and 4,828,464. In these devices, the position of the diaphragm 
is monitored in an effort to precisely control the amount of fluid being 
pumped. The problem of overextension of the diaphragm in both directions, 
however, has not been completely addressed by the prior art. Experience 
has shown that the rearward dish plate will cause extrusion of some 
diaphragm materials such as Teflon when the diaphragm is drawn against the 
porous dish plate when the piston is retracted. Further, cavitation has 
been experienced wherein an air interface occurs between the diaphragm and 
hydraulic fluid in some extreme circumstances, due to the dish plate 
inhibiting further rearward movement of the diaphragm. The cavitation 
effect reduces the metering accuracy of the pump and is otherwise 
undesirable. 
Given the foregoing difficulties of maintaining metering pump reliability, 
the present invention has been provided. 
SUMMARY OF THE INVENTION 
It is an object of this invention to accurately control deflection of a 
metering pump diaphragm. 
It is a more specific object of this invention to continuously monitor 
diaphragm position and control hydraulic pressure against the diaphragm 
based on the position. 
In accordance with the invention, a diaphragm position indicator is 
incorporated in a metering pump for detecting when a diaphragm has reached 
an overextended position. The hydraulic pressurizing fluid of the metering 
pump is connected via a solenoid-operated valve to a reservoir of 
intermediate pressurizing fluid. A control circuit connected to the 
diaphragm position sensor determines when the diaphragm deflection exceeds 
a maximum safe displacement. At such time, the control circuit will 
energize the solenoid-operated valve, venting the pressurizing chamber to 
the reservoir of intermediate pressurizing fluid. The result of venting 
the pressurizing chamber immediately inhibits further extension of the 
diaphragm. 
Overextension of the diaphragm can occur either during the pressurizing 
stroke, when the piston advances, or during a pressure reduction which 
occurs when the piston retracts and pumping media is forced into the 
pumping chamber. During retraction of the piston, further extension of the 
diaphragm is prevented by operating the solenoid operated valve, 
connecting the pressure chamber to the reservoir, permitting a reverse 
flow of pressurizing fluid from the reservoir to the pressure chamber. 
When the pressurizing stroke of the diaphragm metering pump begins, the 
hydraulic fluid will be inhibited from flowing back through the 
solenoid-operated valve to the reservoir. Pressurizing of the diaphragm 
will then continue such that the diaphragm moves forward, pressurizing the 
pumping chamber and displacing pumped media. The diaphragm position sensor 
will generate a signal to close the valve once the diaphragm has moved 
forward into a region of safe displacement. 
The invention may be implemented to prevent diaphragm over extension during 
the pressurizing stroke. When the diaphragm position is detected to have 
reached a second maximum displacement, a second valve means is operated 
connecting the pressurizing chamber to the intermediate reservoir. This 
will effectively terminate further diaphragm expansion. As the pressure is 
reduced due to the operation of the valve means, the diaphragm returns to 
a safe displacement. The new diaphragm position is detected, closing the 
second solenoid valve means. 
By controlling the effective diaphragm displacement, it is possible to 
avoid overflexing of the diaphragm, thereby prolonging the life of the 
diaphragm and the need for any replacement. Controlling the deflection of 
the diaphragm will result in a predictable life expectancy for the 
diaphragm, permitting its replacement to be made before catastrophic 
failure occurs.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, there is shown a schematic representation of a 
metering pump 7 connected to a pumped media reservoir 12. A check valve 10 
on the inlet of the diaphragm pump 7 and check valve 9 on the outlet of 
the diaphragm pump 7 permit the pumped media to enter and leave the 
pumping chamber 13 under pressure from the diaphragm 11. 
Opposite the pumping chamber 13 is a hydraulic fluid chamber 14 which 
pressurizes the diaphragm 11 during a pumping stroke and creates a partial 
vacuum within the pumping chamber 13 during an intake stroke. The flexure 
of the diaphragm 11 is sensed by a sensor 16 facing a magnet 15 fixed to 
the diaphragm 11. Thus, motion of the diaphragm 11 may be effectively 
monitored by the proximity sensor 16. The sensor 16 may be positioned by a 
positioning member 17 to maintain the sensor 16 at the preferred distance 
from the magnet 15. 
Pressurizing of the hydraulic pressure chamber 14 is accomplished via a 
piston 26 operating within cylinder 20. A reciprocating crosshead 28 will 
position the piston 26 to pressurize the chamber 14 and in a reverse 
motion, spring 25 will return the piston to its starting position as the 
crosshead 28 is retracted. The entire assembly is driven by a crank 27. 
A pressure relief check valve is shown in the hydraulic circuit connecting 
the piston cylinder 20 to the hydraulic pressurizing chamber 14. The check 
valve 21 serves as a pressure relief valve such that an excessive amount 
of pressure causing excessive deformation of the diaphragm 11 and damage 
to the drive mechanism 42 would be avoided. The intermediate media 
reservoir 34 receives the hydraulic fluid passed by the pressure relief 
valve 21. 
There is a solenoid-operated valve 31 connected via a check valve 32 to the 
hydraulic pressurizing chamber 14. When the diaphragm 11 is detected as 
having moved rearwardly to a position where it will be overextended, 
controller 30 will supply an operating signal to the solenoid-operated 
valve 31. Valve 31 opens, permitting the intermediate media hydraulic 
fluid from reservoir 34 to enter the hydraulic pressurizing chamber 14. 
This will inhibit further movement of the diaphragm 11 toward the sensor 
16. 
Thus, the diaphragm 11 will remain in its sensed position until the piston 
26 pressurizes the hydraulic pressure chamber 14, closing check valve 32. 
FIGS. 2A, 2B, 2C and 2D illustrate the operation of the device of FIG. 1. 
As is shown, the crosshead displacement varies from a reference line of 0% 
to 100% forward, and then back to 0%, cyclically. Due to the lost motion 
coupling between the piston 26 and crosshead 28, the piston position 
advances when the crosshead moves from 50% of its stroke length to 100% 
stroke length--dependent on the current mechanical stroke adjustment 
setting. 
The diaphragm position 2B can be shown in response to motion of the piston 
26. The scale on the y-axis of FIG. 2B is shown in units of percentage of 
diaphragm displacement where the 100% value is indicative of the diaphragm 
attached magnet 15 in close proximity to the sensor 16. When the diaphragm 
is being retracted from a forward position rearwardly, where it would 
normally be stopped by a rearwardly located dish plate, the controller 30 
will activate valve 31. This position is illustrated in FIG. 2C as a 
dotted line, and the resulting control signal is shown in FIG. 2D. The 
diaphragm position which will result in operation of solenoid valve 31 is 
experimentally determined and specified to the controller 30 such that the 
diaphragm 11 is not overflexed. This position is represented by the dotted 
line in FIG. 2C and is dependent on the material type and other 
considerations known to those skilled in the art. 
With respect to FIGS. 1 and 2A-2D, the general operation of the preferred 
embodiment has been described. A practical embodiment of the foregoing 
system design is shown in FIGS. 3A and 3B. FIG. 3A is a section-view of a 
diaphragm metering pump employing the system of FIG. 1 for limiting 
diaphragm deflection. Detail "A", shown in FIG. 3B shows the hydraulic 
pressure relief valve 21, positioned to be in communication with piston 
cylinder 20. The embodiment of FIG. 3A provides for an intermediate media 
reservoir 40 which surrounds the pump piston 26. The motor drive 41 and 
gear structure 42 is used to drive the cam 28 to reciprocate the piston 26 
via the cam follower 43, also known as a cross-head. A stroke adjustment 
45 is provided which will limit the rearward travel of the piston 26 when 
pushed rearwardly by spring 25. These structural details regarding the 
driving of the mechanism for the piston 26 are conventional in metering 
pump design, and will not be further described. 
The solenoid valve 31 is shown connected via the conduit 46 to the internal 
intermediate hydraulic fluid reservoir 40. Check valve 32 connects 
hydraulic inlet of solenoid valve 31 to the piston chamber 20. 
The magnet 15 is mounted to the diaphragm 11 and is sensed by the sensor 16 
supported at the outlet of the piston cylinder 20. Sensor 16 may be a Hall 
proximity transducer device which detects the magnetic field of magnet 15 
and which provides a current proportional to the distance between the 
magnet 15 and the sensor 16. Electrical connections 47 from the sensor are 
connected to the controller 30. In the preferred embodiment, the 
controller 30 includes a pair of light indicators 59 and 48 to show the 
status of solenoid valve 31 as being either open or closed. Further, a 
threshold adjustment 49 permits the position threshold at which the 
solenoid valve 31 will be open to be manually adjusted. Thus, for various 
diaphragms, one may set the threshold at a greater or lesser value, 
depending on the limits of deflection sought to be imposed on the 
diaphragm 11. The adjustment of the threshold voltage can be facilitated 
by using a voltage metering device across resistor 51. Thus, as shown in 
FIGS. 3A and 3B, the foregoing preferred embodiment may be implemented in 
a conventional metering pump design. 
The controller 30 is illustrated in greater detail in the schematic drawing 
of FIG. 4. Referring now to FIG. 4, the control circuit can be seen to 
include a first operation amplifier 50 connected via a series resistor 51 
to receive a signal from the Hall effect transducer 16. An internal offset 
control 52 causes amplifier 50 to offset the output signal. A conventional 
internal gain control 53 is also shown for setting at the factory an 
appropriate gain setting for amplifier 50. Those skilled in the art will 
also recognize it possible to provide a volt meter connected to the output 
of amplifier 5 to monitor the diaphragm position. 
Switch 54 is shown for connecting either the output of the amplifier, a 10 
volt reference level, or a floating reference level to the input of 
comparator 56. Selection causes the valve to operate in the automatic, 
forced open or forced closed states. The threshold adjustment control 49 
comprises a potentiometer connected in series with two limiting resistors. 
The output of comparator 56 will change when the Hall effect transducer 
produces a signal on the input of comparator 56 greater than the signal 
provided by the threshold adjustment potentiometer 49. The two states 
provided by comparator 56 represent either the valve open or valve closed 
condition, depending on the proximity of magnet to sensor 16. 
Indicators 59 and 48 are conventional LED diodes, responsive to the signal 
produced by the comparator 56. Comparator 58 conditions the signal to the 
opto-isolators as required by the solenoid valve. Thus, it can be seen 
that the controller for the embodiment of FIG. 3A can be constructed of 
standard electronic components which will provide for an indication of the 
current operating condition of the solenoid valve, thus illustrating 
whether or not an overextension condition is being imposed on the 
diaphragm 11. 
The foregoing description is illustrative of only one embodiment of several 
which may be implemented to avoid overextension of the diaphragm 11. The 
example illustrates diaphragm overextension in the context that diaphragm 
11 and attached magnet 15 are in close proximity to sensor 16. This same 
system may be used to protect diaphragm 11 from overextension in the 
opposite direction--when diaphragm 11 is furthest away from sensor 16. 
This can be accomplished by simply reversing the input to comparator 56 
shown in FIG. 4 and reversing the stop direction of check valve 32 shown 
in FIG. 3A. Such a configuration would prevent the overextension of the 
diaphragm into the pumped media chamber. Additionally, both protection 
mechanisms can be applied simultaneously. 
FIG. 5 illustrates an embodiment in which the diaphragm 11 is protected 
from overextension during the pressurizing stroke. The sensor 16 is 
capable of providing an indication of when the diaphragm 11 exceeds an 
extension threshold. The controller 30, upon sensing the diaphragm 
position beyond the extension threshold, will issue a signal as shown in 
FIG. 6E to control solenoid valve 37. Valve 21, as in the previous 
embodiment, provides a failsafe relief valve in the event an excess amount 
of pressure occurs which is not relieved by valve 37. 
In this embodiment, further pressurizing of chamber 14 ceases as the 
pressure is vented back to the intermediate reservoir when the extension 
threshold has been met. The appropriate operation then for the diaphragm 
is shown in FIG. 6B, wherein the diaphragm position is maintained within a 
retraction limit and extension limit to avoid overstressing of the 
diaphragm in two directions of flexure. During retraction, the embodiment 
of FIG. 5 works as the embodiment of FIG. 1, such that a signal is applied 
from controller 30 to the solenoid-operated valve 31, thus limiting the 
extension of the diaphragm during retraction of the piston. 
Although not illustrated in FIG. 3A, the conventional dish plate structure, 
which normally inhibits rearward movement of the diaphragm 11 may continue 
to be used as a secondary backup means for checking overextension of the 
diaphragm 11 during the intake cycle of the diaphragm pump. 
The foregoing embodiments are not limited to a particular type of diaphragm 
material 11 but may be used on diaphragms of all types with suitable 
changes in the threshold implemented, presenting the maximum safe 
displacement of diaphragm 11. Additionally, it is not limited to a 
particular means of adjusting the pump displacement. Those skilled in the 
art will recognize yet other embodiments as described by the claims which 
follow.