The present invention relates to a metering pump control system. Metering pumps are used in many applications, for example to deliver closely controlled volumes of chemicals to process streams. In the water supply industry for example, it is necessary to deliver chlorine to a flow of water such that the concentration of the chlorine in the water is within desired upper and lower threshold limits. This has been done in the past by relying upon a metering pump in the form of a diaphragm mounted in a housing, the diaphragm being reciprocable relative to the housing. In the known pumps, the diaphragm is reciprocated either by a mechanically driven eccentric or a magnetic solenoid. As the diaphragm moves in one direction chlorine within the housing is forced out into the process flow, and as the diaphragm moves in the other direction chlorine is sucked into the housing. Such an arrangement enables chlorine to be delivered at a reasonably closely controlled rate assuming steady state conditions but it is difficult to rapidly and accurately change the rate at which chlorine is being delivered in response to monitored changes in the condition of the process flow. There are many circumstances in which metering pumps have to deliver fluids which are difficult to handle. For example, viscous fluids are relatively difficult to draw into a metering pump although they are relatively easy to expel. In an application where viscous fluids have to be handled therefore it would be desirable to have a metering pump with a relatively slow suction stroke and a relatively rapid pump delivery stroke so as to reduce the period for which no fluid was being delivered by the pump. It has not been possible in the past to readily control a metering pump to achieve such a desirable characteristic.

It is an object of the present invention to obviate or mitigate the problems outlined above.

According to the present invention, there is provided a system for controlling a metering pump comprising pump housing, a reciprocating pump element which is reciprocable relative to the housing and is arranged to draw fluid into the housing when moved in one direction and to expel fluid from the housing when moved in the other direction, and a double acting piston and cylinder assembly linked to the reciprocating pump element, the control system comprising an electrically controlled proportional valve arranged to control the supply of pressurized fluid to the piston and cylinder assembly and thereby to control reciprocation of the reciprocating pump element, a sensor for monitoring the position of the reciprocating element relative to the housing, and a controller for controlling the proportional valve, the controller comprising means for storing data representative of a desired relationship between the position of the reciprocating element and its motion, and means for applying control signals to the proportional valve in dependence upon an output of the sensor and the stored data to achieve the desired relationship.

The provision of the proportional valve and the ability to control it in direct response to feed back from the metering pump itself makes it possible to design a control system to meet almost any process control requirement. The instantaneous speed and acceleration of the metering pump piston can be closely controlled by reference to its position. High turn down ratios, for example 1000 to 1, are relatively easy to achieve. Different speed versus time relationships can be established as between the delivery stroke and suction stroke of the pump. Thus at low stroking rates, suction can be increased to give reduced "slug dosing" . Given that the position of the piston of the metering pump can be closely controlled, minimizing the

unswept volume of the metering pump and thereby increasing maximum output performance.

The position sensor may comprise a linear positioning device arranged to transmit to the controller a signal representative of the position of a piston of the double acting piston and cylinder assembly. Assuming direct coupling between the piston cylinder assembly this enables the position of the metering pump piston to be indirectly monitored with considerable accuracy.

An embodiment of the present invention will now be described, by way of example, with reference to the accompany drawing.

Referring to the drawing, a metering pump is schematically illustrated as having a housing 1, a diaphragm 2 dividing the interior space of the housing into two mutually isolated compartments, and a reciprocable shaft 3 which is linked to the diaphragm 1. Reciprocation of the shaft 3 causes the diaphragm to reciprocate within the housing. The space to the right of the diaphragm 2 in the drawing is connected to an inlet pipe 4 in which a one way valve is arranged to permit a fluid flow only towards the pump housing. The space to the right of the diaphragm 2 is also connected to an outlet 5 in which a one way valve arrangement is positioned to enable fluid to flow only in the direction away from the housing. Thus movement of the shaft 3 to the left in the drawing causes fluid to be sucked in through inlet 4 and movement of the shaft 3 to the right of the drawing causes fluid to be delivered through- outlet 5.

The shaft 3 is connected to a shaft 6 of a double acting piston and cylinder assembly. The shaft 6 is coupled to a piston 7 which is reciprocable within cylinder 8. The cylinder 8 has two hydraulic inlet pipes 9 and 10 connected to opposite sides of the piston 7 and two hydraulic fluids circulating pipes 11 which return hydraulic fluid to a hydraulic control

pu p unit 12. The hydraulic control pump unit delivers pressurized hydraulic fluid via line 13 to both sides of a proportional servo valve 14. The servo valve 14, which could be for example a Series DIFS proportional valve as supplied by Parker Hannifin Corporation of Ohio, U.S.A., incorporates a spool the position of which determines the distribution fo pressurized hydraulic fluid to the pipes 9 and 10. The spool is displaceable from one extreme position in which a high rate of hydraulic fluid flow is delivered to pipe 9 only and another extreme position in which a high rate of flow of hydraulic fluid is delivered to pipe 10 only. Locating the spool between these extreme positions enables the differential pressure across the piston 7 to be modulated such that the force on the piston 7 can be closely controlled.

The position of the spool of the proportional servo valve 14 is determined by a valve controller 15 which applies control signals on lines 16 to the servo valve 14. The controller 15 receives an input from a linear positioning device 17 mounted on the cylinder 8. Thus the output of the device 17 is a signal proportional to the displacement of the piston 7 from a datum position. Given the direct coupling between the piston 7 and the diaphragm 2 the output of the device 17 is also representative of the position of the diaphragm 2.

The control system is coupled to a memory 18 which may be in the form of a simple look-up table correlating the position and direction of movement of the piston 7 with a desired differential pressure across the piston. The controller 15 receives the output of the device 17 which is indicative of the absolute position of the diaphragm, and scans through the look up table as the diaphragm position changes over time. Data is read out from the look-up table which represents the required differential pressure across the piston (which corresponds to the

acceleration fo the piston) and delivers an output to the servo valve 14 so as to deliver the appropriate differential pressure to the double acting cylinder. Thus the speed and acceleration of the cylinder of the piston 7 can be very closely controlled. The acceleration of the piston 7 does not have to be the same on the delivery and suction strokes of the metering pump and indeed in many applications it will be desirable to have a quick suction stroke and a slow delivery stroke or vice versa.

Thus the described system enables a metering pump to be provided to deliver flow rates and flow characteristics which can be easily and rapidly controlled. Difficult fluids, for example viscous fluids, can be handled by for example having a relatively slow suction stroke and a relative fast delivery stroke. Although in many applications the metering pump once running will have a non-varying, suction and delivery stroke characteristic the characteristics could be readily modified in response to for example inputs representing a change in the associated process conditions simply by suing those inputs to address appropriate sections of the data sorted in the memory. More than one metering pump could be powered from a single hydraulic pump, and the controller could be set up to respond "intelligently" to changes in the characteristics of the fluid being pumped so as to maximize efficiency. This might be appropriate for example if the metering pump is used to deliver slurry the characteristics of which cannot be accurately predicted. The suction speed can be optimized to suit particular valve arrangements that are provided to ensure that the pump in the appropriate direction between the inlet and outlet of the pump. This can extend the pumping ranges.

Limiting devices can be fitted in the hydraulic circuit removing the possible hazards of relief valves opening to release the liquid being pumped.