BACKGROUND OF THE INVENTION Pumps which are designed to pump a predetermined amount of fluid from an associated reservoir are used in many applications. One such application involves the dispensing of detergents in set amounts to a commercial dishwasher or laundry machine. Clearly, the accuracy of the set amount dispensed is important as the efficacy of the associated cleaning process will be affected or alternatively detergent will be wasted. Commonly a peristaltic pump is used which transfers fluids by employing a peristaltic action on a flexible tube whereby the alternating pattern of squeezing and releasing the flexible tube moves the detergent through the pump.

A significant disadvantage of peristaltic pumps is that the flexible tube will eventually wear resulting in the tube hardening and the capacity of the pump being correspondingly reduced. The rate of wear of the flexible tube will depend on a number of factors including the tube type, the chemicals being pumped and furthermore the time the tube is subjected to mechanical forces whilst pumping. As a consequence these tubes require regular replacement.

When a new tube is installed, the peristaltic pump is calibrated by pumping a fluid for a set time. By determining the amount of fluid pumped during this time an initial flow rate is calculated. This flow rate is then typically entered into a microcontroller which controls the duration of any pumping operation.

Clearly, as the tubes begin to wear the initial calibrated flow rate will not be valid. As the amount of detergent dispensed is determined by the microcontroller actuating the pump for a set time, assuming the initial calibrated flow rate, the incorrect amount will be dispensed.

One means to overcome this problem is to change the tubes regularly before they become worn. However, this is an expensive and time consuming process and due to the variability in tube lifetime, perfectly operational tubes will be replaced in some instances.

Another attempt to address this problem is to program the microcontroller to increase pumping times as a function of time from the last tube replacement.

However, this system must assume an overall wear rate and will not take into account the substantial variability in these rates that occur between individual tubes of the same type. Additionally, as the wear rate also varies even more substantially with tube type, the microcontroller will require reprogramming if a different type of tube is used as the assumed rate of wear will be incorrect.

A similar problem occurs when the type or mix of chemicals being pumped is changed as this will also directly affect the wear rate of a tube.

It is an object of the present invention to provide a method and apparatus which addresses the issue of variability in pumping performance for systems which dispense predetermined amounts of fluid.

SUMMARY OF THE INVENTION In a first aspect the present invention accordingly provides a method of compensating for variation in pumping rate in a liquid dispensing system, said system including a reservoir having a predetermined capacity, a pump for pumping a plurality of predetermined amounts of fluid from said reservoir to a predetermined destination, said method including the steps of:

determining an initial pumping rate for said pump; determining a current pumping rate for said pump; compensating said pump for pumping said predetermined amount of fluid, wherein said step of compensating includes comparing said initial pumping rate to said current pumping rate and adjusting for the difference between said rates.

Clearly, this method provides an effective way to overcome the disadvantages of the prior art as outlined herein. As the current pumping rate is updated regularly the pump can be compensated for any changes in the pumping rate which occur due to alteration in pump behaviour.

Preferably, said step of determining a current pumping rate includes the step of measuring an elapsed time to dispense said capacity of said reservoir. This provides an effect way to calculate the current pumping rate as the capacity of the reservoir is known.

Preferably, said step of measuring an elapsed time includes the step of determining that the reservoir is empty by detecting bubbles in a fluid flow passage that transports said predetermined amounts of fluid from said reservoir to said predetermined destination. A bubble detector is one means to reliably sense when the reservoir is empty.

Preferably, said step of adjusting includes the step of varying a time to pump said predetermined amount of fluid. Optionally, said step of adjusting includes varying a pumping rate of said pump. Depending on the type of pump, either of these methods will effectively compensate for changes in pump behaviour and deliver the correct dose.

In a second aspect the present invention accordingly provides an apparatus for compensating for variation in pumping rate in a liquid dispensing system, said system including a reservoir having a predetermined capacity, a pump for pumping a plurality of predetermined amounts of fluid from said reservoir to a predetermined destination, said apparatus including: first pumping rate measurement means for determining an initial pumping rate of said pump; second pumping rate measurement means for determining a current pumping rate of said pump; processing means to calculate a difference between said initial and current pumping rates; and pump controller means to adjust said pump to compensate for said difference in pumping rates.

BRIEF DESCRIPTION OF THE DRAWINGS An illustrative embodiment of the present invention will now be discussed with reference to the accompanying drawings wherein : FIGURE 1 is a functional block diagram of a pumping system according to a preferred embodiment of the present invention ; FIGURE 2 is a functional block diagram of a detector according to a preferred embodiment of the present invention; and FIGURE 3 is a flowchart illustrating a method according to a preferred embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the Figures, there is illustrated in Figure 1 a functional block diagram of a system 10 for dispensing predetermined amounts of detergent to an industrial washing machine 60 according to a preferred embodiment of the present invention. Clearly, this invention is applicable to any pumping system

which provides individual predetermined amounts of fluid from a reservoir by a fluid flow passage to a predetermined destination. Pumping system 10 includes a reservoir 20 containing detergent 25, a detector 30, a pump 40 and associated controller 50 in addition to plastic tubing 70 acting as a fluid flow passage to transport the detergent 25. Reservoir 20 has a known capacity and in this embodiment is a drum which is periodically replaced. The capacity of this drum is 5 litres and assuming normal use of washing machine 60, the drum would be expected to be replaced every 3 to 10 days.

Pump 40 is a peristaltic pump having a design pumping flow rate between 50 - 1200 ml/minute. In general these pumps have either AC or DC motors driving a plastic rotor with two or three rollers attached. These rollers squeeze the flexible tube against a plastic housing enabling liquid to be moved through the tube by a peristaltic action. The different pumping capacities of the units allow for their use in a wide range of applications ranging from domestic to large scale industrial requirements. As referred to in the "Background of the Invention"the flexible tube regularly requires replacement due to continual flexing of the tube in order to generate the peristaltic pumping action.

For the pump employed in this preferred embodiment, the flexible tube consists of a 22 cm length of 9-16 mm diameter silicone rubber or neoprene material. This tube would be expected to have a meantime lifetime of 200 days with a variation of approximately plus or minus 150 days. When an old tube is replaced, pump 40 is recalibrated by pumping for 30 seconds and measuring the volume of fluid dispensed. This volume is then used to calculate a flow rate. The flow rate value is then entered into controller 50 which consists of a microcontroller having a programmable interface which in this preferred embodiment is a Microchip PIC 16F872 operating at 4 MHz.

Controller 50 also performs the function of actuating pump 40 for set times to dispense the required amounts of detergent 25 to washing machine 60.

Detector 30 provides essential feedback to controller 50 by detecting when reservoir 20 is empty of detergent 25. In this embodiment detector 30 is a bubble detector. Referring now to Figure 2, there is shown a detailed functional block diagram of the bubble detector 30 employed which operates by monitoring a section of glass tubing 70 for the appearance of a suitable sized bubble 130, thus indicating that there is no detergent 25 remaining in reservoir 20.

To detect bubble 130, detector 30 consists of a Light Emitting Diode (LED) 120 which is driven at 10 KHz by oscillator 100 and a photo-transistor 140 positioned to detect changes in the transmission of light through glass tubing 70 caused by bubble 130. The detailed frequency and shape of the waveform used for excitation of the LED 120 is unimportant. However, the centre- frequency must be matched to that of the receiver circuit, which in this embodiment includes photo-transistor 140 and amplifier 135 in combination with a 10 KHz band pass filter 150. The output from band pass filter 150 is sent to a peak detector 160 whose output is sent to threshold comparator 170.

When the peak value of the detected signal exceeds the threshold set in threshold comparator 170 a signal is generated. To this signal, hysteresis is added by a Schmidt trigger 180 before the signal is sent to controller 50 indicating that a suitable size bubble has been detected and hence reservoir 20 is empty. Although in this embodiment a sophisticated bubble detector is employed to detect whether reservoir 20 is empty, equally other means of determining this are considered to be within the scope of the invention.

In operation, controller 50 will measure the total elapsed time to fully dispense the contents of reservoir 20 whose volume is known. As both the total time for dispensing the contents and the volume are known, a new flow rate may be calculated by the controller 50 on emptying of reservoir 20 which is indicated by detector 30. With the new calculated flow rate the controller can adjust the set times appropriately to take into account any tube wear.

Equally, for pumps with variable controlled pumping rates the pumping rate can be increased to compensate for any tube wear Referring now to Figure 3 there is depicted a flow chart illustrating this compensating method in detail. On initial calibration 210, after fitting of a new tube, an initial flow rate is determined and assuming a predetermined dose an associated initial dose time Tdoseis calculated and entered into the system. As the capacity of the reservoir is also known, a predicted time Tpred to empty the reservoir is also calculated and entered into the microcontroller.

The Tube Compensation Factor (TCF) is also initially set to one 220 on initial calibration of a new tube. Tdose is modified by this factor to calculate the actual dose times 230 and it is the TCF which is modified to compensate for tube wear.

During operation the system checks for the presence of a bubble at 240. If no bubble is detected the total time spent dispensing doses is accumulated and stored as Tacc at 270. If a bubble is detected, a check at 250 is made whether Tacc falls between predicted time Tpred and this time multiplied by a scalar factor K. This takes into account an allowable variation in flow rate for a given container. As an example, this may be 10% thus yielding a scale factor K of 1.1. A significant increase in Tacc may indicate that the tube is beginning to fail and requires replacement.

If Tacc does not fall within bounds 250, a further check 280 is performed to determine whether Tacc < Tpred. If this condition is satisfied then a warning flag is set 290 indicating that the bottle was replaced early. In the case where Tacc is much larger than Tpred then it is assumed that a bubble was not detected at the last container change 300. In any case for both of these conditions TCF is unaltered.

If Tacc falls within the predetermined bounds at step 250, then TCF is adjusted at step 260 by the ratio Tacc to Tpred. Tpred is also updated by scaling by TCF to take into account the expected increase in the predicted time to empty reservoir 20. Clearly, as would be apparent to those skilled in the art, in the case of a pump having variable pumping rate control, the pumping rate of the pump could be increased by a similar proportion to also compensate for tube wear. If the newly calculated TCF at 310 is greater than a maximum value, then a new tube is fitted and a new initial calibration at 210 performed.

Although, in this embodiment the present invention has been applied to a peristaltic pump whose pumping capacity diminishes over time, the invention may also be applied to any pumping system which dispenses predetermined amounts of fluid and whose performance may vary over time, whether increasing or decreasing.

Although a preferred embodiment of the method and system of the present invention has been described in the foregoing detailed description, it will be understood that the invention is not limited to the embodiment disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention as set forth and defined by the following claims.