Abstract:
A unit for detecting an addition of a cleaning agent in a beverage dispenser, in particular in a coffee machine. In order to guarantee that a cleaning agent has been added when automatic cleaning is carried out, a measurement device is provided as well as a method of determining whether a cleaning agent has been added to the aqueous cleaning liquid. Whether a cleaning agent has been added to the aqueous cleaning liquid can be determined in dependence upon the measured conductivity and the measured electrical resistance of the cleaning liquid, respectively.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a beverage dispenser, in particular a coffee machine, comprising a means for initiating and executing a cleaning process with an aqueous cleaning liquid. 
   BACKGROUND OF THE INVENTION 
   The most frequent failure cause in the case of beverage dispensers used on a commercial basis, such a coffee machines used on a commercial basis, is insufficient cleaning. Al-though a regular execution of machine cleaning processes can be guaranteed by designing a suitable control software, it is impossible to guarantee that cleaning agents will actually be used during these cleaning processes. In fully automatic cleaning processes, liquid cleaning agents are used, among other substances. The cleaning agent is fed to the brewing unit at fixed times through a timer or in that a cleaning process is manually triggered by means of a pump. Such a means is complicated and expensive and, in addition, it is difficult to super-vise the flow of the cleaning agent. 
   SUMMARY OF THE INVENTION 
   It is therefore the object of the present invention to provide a beverage dispenser in which a correct addition of cleaning agents during the cleaning process can be checked. 
   According to the present invention, this object is achieved by a method and a measurement device for determining whether a cleaning agent has been added to the aqueous cleaning liquid. 
   Due to the fact that the aqueous cleaning liquid has provided therein a measurement device which is able to determine whether a cleaning agent has been added to the aqueous cleaning liquid, a correct cleaning process can be guaranteed. If it is detected that the aqueous cleaning liquid does not contain any cleaning agent, the cleaning process can be discontinued, if desired. An unintentional and unnoticed execution of an insufficient cleaning process without any cleaning agent can be prevented in this way. 
   According to a preferred embodiment, the measurement device is configured for determining the conductivity and the electrical resistance of the cleaning liquid. If cleaning agents of any kind are added to water, the conductivity of said water will increase markedly due to the addition of ionizable dissolved substances. The presence of cleaning agents in water can be measured in this way. Water containing no cleaning agent will have a resistance which is about ten times that of water containing a cleaning agent so that, in dependence upon the measured conductivity, it can reliably be differentiated whether or not a cleaning agent has been added to the aqueous cleaning liquid. 
   The measurement device preferably comprises two spaced-apart electrodes which are arranged in the cleaning liquid. This embodiment is inexpensive and easy to realize. 
   The means for initiating and executing a cleaning process comprises a control unit and a manual or an automatic feeder for the cleaning agent. According to a special embodiment, the beverage dispenser comprises a brewing unit, the measurement device being arranged downstream of the brewing unit in an advantageous manner. The device according to the present invention permits the measurement device to be arranged also downstream of the brewing unit, since even the presence of e.g. coffee powder will change the conductivity of the cleaning liquid only to a minor extent so that, in spite of the presence of coffee powder, it can be determined whether or not a cleaning agent has been added to the cleaning liquid. 
   According to an advantageous embodiment, the measurement device is fed by a constant current source for measuring then the voltage or the resistance between the electrodes. The measurement device can be integrated in an LCR measuring bridge or in a voltage divider. The electrodes may also have applied thereto an alternating voltage or a pulsed direct voltage so as to avoid oxidation and contamination. 
   According to a preferred embodiment, the measurement device is connected to the control unit and the control unit is configured in such a way that, if it is determined that no cleaning agent has been added, it will output a cleaning-process stop signal. This guarantees that no cleaning process without cleaning agent will be carried out. Furthermore, an additional indicator unit can be provided, which will then demand an addition of cleaning agents once more. The control unit may, however, also be configured in such a way that, when it has been determined that no cleaning agent has been added, the cleaning process will be continued, the control unit comprising an error memory for storing the absence of the cleaning agent. 
   Furthermore, the method according to the present invention comprises the step of determining at the beginning of the cleaning process whether a cleaning agent has been added to the cleaning liquid. The cleaning process can thus be stopped in time. According to an ad-vantageous embodiment, the output signal of the measurement is detected in analog form, e.g. as an analog voltage signal. 
   In the following, the present invention will be explained in detail making reference to the figures enclosed, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic, simplified representation of a beverage dispenser according to the present invention. 
       FIG. 2  shows a device for determining whether a cleaning agent has been added to an aqueous cleaning liquid. 
       FIG. 3  shows measurement results of various media measured by the device according to the present invention. 
       FIG. 4  shows an equivalent circuit diagram for voltage measurement. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows, in a greatly simplified representation, the schematic structural design of a coffee machine according to a preferred embodiment of the present invention. As can be seen in  FIG. 1 , the coffee machine according to the present invention comprises a water-supply line  1  through which water is conducted into a water heater  3  where it is heated. The hot water can run via a brewing line  4  into a brewing unit  5  to which coffee powder has been fed by a dosing means, which is not shown. The brewed coffee can then run via a 3/2-way valve  6  either into a coffee pot  8  or it can be discharged through a discharge line  7 . Such coffee machines must be regularly cleaned with a cleaning agent. Cleaning is carried out either automatically or manually. In so doing, cleaning agent is fed to the brewing unit via a cleaning agent feeder  2 . The cleaning process is initiated and executed by a control unit  10 . In the present embodiment, the discharge line  7  has incorporated therein a measurement device  9  for measuring the conductivity and the electrical resistance of the cleaning liquid flowing off through the discharge line  7 . As can be seen from  FIG. 2 , the measurement device  9  comprises two spaced-apart electrodes  9   a,    9   b  which extend into the aqueous cleaning liquid. In the present embodiment, the two electrodes  9   a,    9   b  extend up to the center line M of the tube section of the discharge line  7 . The electrodes may consist e.g. of titanium or of V2A steel. The electrode spacing ranges preferably from 5 to 30 mm, in the preferred embodiment it is e.g. 16 mm. The size of the electrode surface ranges between 10 and 50 mm2, in the present embodiment it is e.g. 35 mm2. The media temperature is between 90 and 100°. 
   Via the lines  12 , a current is applied to the electrodes  9   a  and  9   b  via a constant-current source or an a.c. voltage source (e.g. 100 Hz or in the KHz range), which is not shown. The control unit  10  is connected to the measurement device  9  and interprets the measurement value detected by said measurement device  9  so as to find out whether or not cleaning agent is contained. The control unit  10  is additionally connected to an indicator unit  11  capable of outputting an optical and/or an acoustic signal. 
   At the beginning of the cleaning process, i.e. when the cleaning process has been initiated by the control unit  10 , a measurement is carried out by the measurement device  9 . If cleaning agents of any kind are added to water, the conductivity of water increases markedly due to the addition of ionizable dissolved substances. The presence of cleaning agents in the aqueous cleaning liquid can be measured in this way with the aid of the two electrodes  9   a ,  9   b , i.e. the ohmic resistance R of the measurement arrangement changes in accordance with the addition of cleaning agents, since R=ρ×I/A (wherein ρ is the specific resistance of the solution, I the electrode spacing and A the effective electrode surface). The quotient I/A is the resistance capacitance for the measurement arrangement, which is a constant value. It follows that the conductivity of the cleaning liquid can be ascertained through the measurement of the ohmic resistance and through the measurement of the voltage between the electrodes  9   a  and  9   b , respectively. 
   The output signal of the measurement device is detected in analog form by the control unit  10 . The analog output signal can also be converted into a digital signal via a threshold switch. The output signal of the measurement device is detected via the normal wiring for resistance measurement; the measurement device can be integrated either in an LCR measuring bridge or in a voltage component. 
     FIG. 4  shows an example for an equivalent circuit diagram for voltage measurement. As has already been mentioned, the electrodes  9   a  and  9   b  are supplied with alternating voltage via lines  12  through the oscillator (I  1 ). In order to prevent deposits on the electrodes, an alternating voltage in the KHz range, here 4 KHz, is applied to the electrodes in this embodiment. It is also imaginable to apply a pulsed direct voltage to the electrodes. An alternating voltage will also prevent an oxidation of the electrodes. In  FIG. 4 , the resistor R 2  corresponds to the ohmic resistance in the measurement solution which is to be determined. The resistors R 1  and R 2  define a voltage divider. The signal from the voltage divider, whose magnitude depends, in turn, on the conductivity of the measurement solution, is then ap-plied to a voltage follower. The voltage-divided signal is here integrated in the integrator (I  2 ). The analog output signal can be converted into a digital signal via a threshold switch, or it can be evaluated directly via the analog input of the control unit  10 . According to the present invention, the measurement device, for example, comprises electrodes  9   a ,  9   b  with an electrode spacing of 16 mm and a respective electrode surface of 35 mm2. In the case of a measured conductivity of various drinking waters in the range of 500 to 1000 μS, the following holds true:
   R=L/ ( A *ρ)=16 mm/(35 mm 2 *500 μS)=914 Ω 16 mm/(35 mm 2 *1000 μS)=457 Ω 
   With the electrode dimensions chosen, a resistor of R 1 =180 ohm is recommended. In the case of a voltage divider of 180 ohm to 914 ohm and 457 ohm, respectively, this will, in the case of drinking water, result in an output voltage or measurement voltage of 4.2 volts to 3.6 volts when a voltage of 5 volts is applied. 
   The control unit  10  interprets the measurement value so as to find out whether or not cleaning agent is contained, i.e. the measurement value is compared e.g. with reference values stored in the control unit  10 , whereupon it is decided whether the measurement value corresponds to a conductivity with or without cleaning agent. If the measurement value corresponds to a conductivity with cleaning agent, the cleaning will be continued. If the measurement value corresponds to an electric conductivity for a cleaning liquid without a cleaning agent, which means that a cleaning process has been initiated but no cleaning agent has been supplied to the tap water via the cleaning agent feeder  2 , the cleaning process will be discontinued by the control unit  10  in that the control unit outputs a cleaning-process stop signal. Simultaneously, the control unit can pass on a signal to the indicator unit  11  which will demand an addition of cleaning agents once more. The indicator unit  11  can out-put an optical an/or an acoustic signal. It is, however, also possible that the control unit  10  is configured in such a way that, even if it is detected that no cleaning agent has been added to the cleaning liquid, the cleaning process will be continued nevertheless. The absence of the cleaning agent is, however, stored in an error memory, which is not shown, and, if desired, an error message can appear. 
     FIG. 3  shows the voltages between the electrodes  9   a  and  9   b  of various media, which have been measured by the arrangement shown in  FIG. 1 . As can be seen from  FIG. 3 , the measured voltages will drop markedly, when a cleaning agent is added to the cleaning liquid. A possible presence of coffee powder in the brewing unit  5  will not corrupt the measurement result, since the respective voltage measured is markedly higher than the voltages measured in the presence of cleaning agents. Only a very large amount of coffee corresponding to 26 g of coffee powder will reduce the measured voltage to such an extent that it corresponds to a range which is equal to the conductivity range of cleaning liquid with cleaning agent. This will, however, not interfere with the measurement, since 26 g of coffee powder exceed the normal amount of coffee powder by far. Such large amounts will not be used in practice and, if this should actually be the case, the control unit would indicate a malfunction of the motor of the brewing unit so that the cleaning process could not even be started. 
   Summarizing, it should therefore be stated that the beverage dispenser according to the present invention as well as the method according to the present invention permit reliable cleaning.