Abstract:
In a control arrangement and a method for controlling an alkaline pressure electrolyzer including gas spaces for receiving the H 2  and O 2  gases generated by the pressure electrolyzer, a manual control input and an integrally operating controller for at least one of the control values gas pressure and fill level difference of the gas spaces, and an uncoupling network which provides control inputs for independently controlling at least one of the fill level difference and the gas pressure in the gas spaces, hydrogen and oxygen blowdown valves in communication with the respective gas spaces are controlled such that the blowdown flows of the hydrogen and the oxygen from the respective gas spaces are at a ratio of 2:1 and the sum of the blowdown gas flow volume change is zero.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to a control arrangement for an alkaline pressure electrolyzer including controllers for the control values fill level and gas pressure and to a method of controlling an alkaline pressure electrolyzer.  
           [0002]    An electrolyzer generally includes a plurality of electrolytic cells arranged in series for example as disclosed in DE 196 07 235 C1.  
           [0003]    For a better understanding of the art, FIG. 1 shows a pressure electrolyzer of the state of the art. It shows an electrolyzer block  20  including an alkaline solution and consisting of a stack of a plurality of identical cells, each having an anode  11 , a cathode  12  and an ion-permeable diaphragm  13 . The stacked cells have good electrical conductivity and are interconnected in a gas-tight manner.  
           [0004]    An operating medium is added to an electrolyte, supplied to the electrodes and dissociated into its components (product gases). By the introduction of electrical energy (for example, from renewable sources such as wind or the sun) the electrolysis process is initiated whereby., under high pressure, hydrogen, H 2 , is released at the cathodes  12  and oxygen, O 2 , is released at the anodes  11 . The product gases H 2  and O 2  of all the electrolytes cells are separately collected and conducted away whereby a mixture of H 2  bubbles and an alkaline medium reaches a gas separator  36  by way of the conduit  30  and a mixture of O 2  bubbles and an alkaline medium reaches the gas separator  37  by way of the conduit  31 . In the gas separators  36  and  37 , the gases are separated from the alkaline medium which is returned to the electrolytes block  20  by way of the return line  35 .  
           [0005]    The gas or gases produced thereby can be conducted from the gas separators to a storage (for example, a hydrogen tank, hydride storage element) under pressure. From there, the gas can be retrieved and bottled (not shown).  
           [0006]    The gas separator  36  forms, together with the gas separator  37  and the connection  34 , a system comprising two communicating tubes, which are filled with the alkaline columns  28  and  29  up to the fill levels  24  and  25 . The arrangement separates the H 2  gas space  26  from the O 2  gas space  27  and, because of the movability of the alkaline columns  28  and  29 , permits pressure equalization between the gas spaces  26  and  27 . Such pressure equalization is necessary because of the pressure sensitivity of the diaphragm  13 .  
           [0007]    Control values for the operation of the electrolyzer are the gas pressure in the whole system and the fill level difference in the gas separators. With non-stationary electrolysis operation, there are relatively high requirements for operational quality.  
           [0008]    The gases H 2  and O 2  produced in the electrolyzer block  20  increase the pressure in the gas spaces  26  and  27  and affect, depending on the geometric design of the gas separator  36 ,  37 , more or less the fill levels  24  and  25  and, consequently, the fill level difference.  
           [0009]    During electrolysis, the fill level of the electrolyte in the gas separators is routinely monitored for operational and safety reasons. The average fill level is utilized for controlling the operating medium amount supplied to the electrolyzer. In a water electrolyzer, for example, water is added when the fill level has become too low. Also, the emergency shut down system of the electrolyzer may be based on the fill level of the operating medium. By operating the H 2 , blow-down valve  22  or the O 2  blow-down valve  23 , the gas pressures in the gas spaces  26  and  27  and the fill levels  24  and  25  can be controlled. The fill levels  24  and  25  however must be controlled automatically because of the danger of an oxy-hydrogen explosion as a result of a mixing of H 2  and O 2  when the fill level difference is too large. A suitable control arrangement is provided for this purpose, which also controls the gas pressure, wherein the changes of the gas volume flows in the conduits  30  and  31  affect the control values as disturbance variables.  
           [0010]    From Mohr et al. (P. Mohr, V. Peinicke, and T. Welfouder, 1996, “Konzeption und Realisierung der Druckregelung eines solar betriebenen Elektrolyseurs”, in Automatisierungstechnische Praxis 38, Heft 9, 42-48) a control arrangement for an electrolyzer is known (FIG. 2).  
           [0011]    In addition to the control circuits for the gas pressure and the fill level difference shown in FIG. 2, FIG. 1 shows the electrolysis gas separation system  21 , the H 2  blow-down valve  22  and the O 2  blow-down valve  23  which are also shown in FIG. 2.  
           [0012]    The fill level difference control circuit of FIG. 2 includes the fill level measuring arrangement  51 , which retrieves from the electrolysis gas separation system  21  the momentary information  52  concerning the fill level  25  in FIG. 1 and converts this information to the corresponding electrical actual value signal  54  for the fill level difference. This signal is compared in the comparator  66  with the desired value  68  for the fill level difference. The difference is supplied to the controller  64  for the fill level difference whose control output  70  is connected to the input  58  of the H 2  blow-down valve  22  by way of the jointure  99   a.    
           [0013]    The gas pressure control circuit as shown in FIG. 2 includes a pressure measuring device  55 , which obtains the momentary gas pressure information from the electrolysis gas separation system  21  and converts the information to a corresponding electrical actual value signal  57  for the gas pressure. The signal  57  is supplied to a comparator  67  for comparison with a desired gas pressure value  69 . The differential control signal is supplied to the controller  65  for the gas pressure whose control output  71  is connected to a control action input  59  of the O 2  blow-down valve  23  by way of a connecting point  99   b.    
           [0014]    The controllers  64  and  65  in FIG. 2 are, in accordance with the state of the art, conventional PID controllers with proportional, integral and differential transmission components and, by controlling the blow-down valves  22  and  23 , are to influence the blow-down flows  32  and  33  in such a way that the control values gas pressure and fill level difference do not stray from the predetermined desired values even during instationary operation of the pressure electrolyzer.  
           [0015]    It is however a disadvantage of this control arrangement that, in practice, the control oscillations are not sufficiently damped. The gas pressure spaces  26  and  27  shown in the area  21  of FIG. 1 represent the integral parts of the control system areas, which, as is known in the control technology, cannot be satisfactorily controlled by controllers with integral components since their dynamic quality is already at the stability limit. However, the I component of the PID controller is needed if the disturbances effective on the control values, that is the changes of the electrolytes performance, are to be attenuated effectively.  
           [0016]    Another disadvantage of this control arrangement is that it does not support manual operation of the pressure electrolyzer. Manual operation may be necessary for initial operation or during maintenance of the electrolyzer and is initiated by severing the connecting points  99   a  and  99   b  and supplying manually controllable signals to the control inputs  58  and  59 . However, it is difficult with these manual control signals to adjust the control values gas pressure and fill level difference in a well defined manner or to maintain them constant since both control signals affect at the same time both controlled variables. Manual control is particularly difficult since the system is close to the stability limit anyhow. For safety reasons, a one-to-one coordination of control signals and controlled variables and a stabilization of the control system is therefore desirable.  
           [0017]    It is the object of the present invention to provide a control system which has an improved control behavior and which permits safe manual operation.  
         SUMMARY OF THE INVENTION  
         [0018]    In a control arrangement and a method for controlling an alkaline pressure electrolyzer including gas spaces for receiving the H 2  and O 2  gases generated by the pressure electrolyzer, a manual control input and an integrally operating controller for at least one of the control values gas pressure and fill level difference of the gas spaces, and an uncoupling network which provides control inputs for independently controlling at least one of the fill level difference and the gas pressure in the gas spaces, hydrogen and oxygen blow-down valves in communication with the respective gas spaces are controlled such that the blow-down flows of the hydrogen and the oxygen from the respective gas spaces are at a ratio of 2:1 and the sum of the blow-down gas flow volume change is zero.  
           [0019]    With the control arrangement according to the invention, the operational safety of the electrolyzer and the control quality are substantially improved.  
           [0020]    As means for independently influencing the controlled variables an uncoupling network may be used which includes control inputs for independently influencing the controlled variables fill level difference and gas pressure.  
           [0021]    The uncoupling network provides for an uncoupling of manual control inputs of the control values fill level difference and gas pressure whereby manual control actions can be performed without danger.  
           [0022]    In order to ensure that the controlled variables gas pressure and fill level difference can be controlled independently of one another the uncoupling network comprises transmission blocks, summing devices and, if necessary, a constant voltage source. The transmission blocks provide the transmission factors needed for the uncoupling effect. In each case, two summing devices are provided which form the input signals and the output signals of the uncoupling network. The voltage source is needed for the uncoupling of control circuit sections with so-called equal percent blow down valves. In this way, the uncoupling network becomes usable also for equal-percent characteristic valve performance graphs.  
           [0023]    Particularly at least four summing devices and at least four transmission blocks together with a stationary voltage source can be so utilized that an uncoupled manual control of the fill level difference and the gas pressure can be performed.  
           [0024]    As control means also P-controllers may be provided which stabilize the controlled variables fill level difference and/or gas pressure.  
           [0025]    The controlled variables fill level difference and gas pressure are expediently so influenced that, with the occurrence of disturbances, they react with a finite change of the actual values. This means that the control sections of the fill level difference and the gas pressure provided with P-controllers have a proportional behavior and are consequently far from the stability limit.  
           [0026]    The control system includes expediently an uncoupling network and also a P-controller. The uncoupling network does not need to be, but it may be, utilized together with one or both P-controllers. If both control mechanisms, that is the uncoupling network and one or both P-controllers, are utilized together, the control means support each other, that is, they influence the control values in such a way that they contribute together to an increased operational safety and to the control quality. In this way acceptable control results are achieved even with not optimally dimensioned control mechanisms.  
           [0027]    In another embodiment of the invention, the control arrangements include P-controllers which, besides the inputs for the connection of the PID controllers, include each an additional desired value input. In this way, the control processes can be accelerated past the PID controllers if the integral components of the PID controllers result in unsatisfactorily slow control actions.  
           [0028]    With the method according to the invention, the hydrogen blow-down gas flow and the oxygen blow-down gas flow is adjusted to a ratio of 2:1. This ratio corresponds to the electrolysis gas flows occurring during electrolysis. In this way, the effects on the fill level difference is minimized so that it remains constant.  
           [0029]    A change of the control signal for a control mechanism of the fill level difference designated as a manual control input may control the blow-down volume flows of the gases in such a way that the sum of the blown down gas flow changes is zero.  
           [0030]    If for example the hydrogen blow-down volume flow is in creased by a certain amount, the oxygen blow-down volume flow is reduced at the same time by the same amount. By this method step, the gas pressure is maintained constant.  
           [0031]    If both control steps are utilized together, the control steps are uncoupled and have no influence on each other if gas pressure and fill level difference are to be adjusted at the same time.  
           [0032]    If the P-controller is operated by way of summing devices of the uncoupling network, the control steps are preferably automatically uncoupled. Preferred embodiments of the invention will be described below on the basis of the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]    [0033]FIGS. 1 and 2 shows prior art arrangements,  
         [0034]    [0034]FIG. 3 shows the control arrangement according to the invention, and  
         [0035]    [0035]FIG. 4 shows a control arrangement according to the invention with operational amplifiers. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0036]    [0036]FIG. 3 shows a control arrangement  99  with an uncoupling network according to the invention which comprises transmission blocks  84 ,  85 ,  86 , and  87 , summation devices  88 ,  89 ,  92  and  93  and a voltage source  90 . The uncoupling network, upon maintaining two so-called uncoupling conditions, causes the output signal of the summation device  93  to exclusively control the gas pressure, and the output signal of the summation device  92  to exclusively control the fill level difference.  
         [0037]    The transmission blocks  84 ,  85 ,  86  and  87 , the summation devices  88 ,  89 ,  92  and  93  and the voltage source  90  may consist of common electronic components such as operational amplifiers, resistors and condensers. Transmission blocks are characterized by certain signal transmission factors the summation devices by their summing functions with the transmission factor one and the voltage source by the electrical voltage provided at the outlet.  
         [0038]    The uncoupling rule for the manual control input  95  for the gas pressure is for example: The transmission blocks  85  and  87  and the voltage source  90  are to be so dimensioned that the H 2  blow-down gas flow  132  and the O 2  blow-down gas flow  133  are, like the electrolysis gas flows, at a ratio 2:1. In this way, during manual control input for the gas pressure  95 , the effect on the fill level difference is minimized.  
         [0039]    The dimensioning of the transmission blocks  85  and  87  and of the voltage source  90  depends on the characteristics of the blow-down valves  122  and  123 , which reflect the functional connections of the gas volume flows  132  and  133  with the control voltages  158  and  159 . Assuming the H 2  gas volume flow  132  has the symbol vf 123 ; the valve control voltage, the symbol U 159 , the O 2  volume flow  133  has the symbol vf 133 , the valve control voltage has the symbol U 159 , and the manual control voltage has the symbol U 95 , the output voltage of the voltage source  90  is U 9O  and the pressure differentials at the valves  122  and  123  are P 122  and P 123 . Valve characteristics frequently occurring in chemical plants are linear or equal-percentage types.  
         [0040]    For equal-percentage valve characteristics as functions of the valve lifts Y 122  and Y 123  the following equations are applicable:  
         Hydrogen side:  vf   132   =p   122   ·K   V122   ·exp {( Y   122 −1)· ln ( ST   122 )} 
         
       y 
       122 
       =f 
       s 
       U 
       158  
     
         oxygen side:  vf   133   =p   123   ·K   v123   ·exp {( Y   122 −1)· ln ( ST   123 )} 
         
       y 
       123 
       =f 
       s 
       ·U 
       159  
     
         [0041]    The values for the flow coefficients K v122 , K v123  and the setting ratios ST 122 , ST 123  and also for the control factors f s  are provided in the data sheets of the valves  122  and  123 . If, for simplicity reasons, the transmission factor one is used for the value  87 , U 158 =U 95  is obtained. With the uncoupling rule for 95 vf 132 /vf 133 =2 and the assumption p 122 =p 123 , a relation ship for the control voltage U 159  is obtained:  
         
       U 
       159 
       =U 
       95 
       ·K 
       85 
       +U 
       90  
     
           K   85   =ln ( ST   122 )/ ln ( ST   123 )  
           U   90 ={1− K   85   −ln (2· Kv   123   /Kv   122 )/ ln ( ST   123 )}/ f   s    
         [0042]    Herewith the block  85  obtains the transmission factor K 85  and the voltage source  90  provides the constant value U 90 .  
         [0043]    For linear characteristics, the following equations can be provided:  
         Hydrogen side:  vf   132   =p   122   ·Kv   122   ·f   s   ·U   158    
         Oxygen side:  vf   123   =p   123   ·Kv   123   ·f   s   ·U   159    
         [0044]    The values for the flow coefficients Kv 122 , Kv 123  and the control factors f s  are given in the data sheets of the valves  122  and  123 . With the uncoupling rule for 95 vf 132 /vf 133 =2, the assumption p 122 =p 123  and with U 158 =U 95 , a relationship for the control voltage U 159  is obtained as follows:  
         
       U 
       159 
       =U 
       95 
       ·K 
       85  
     
           K   85   =Kv   122 /(2· Kv   123 )  
         [0045]    The block  87  obtains again the transmission factor one, the block  85  obtains the transmission factor K 85 , and for the voltage source  90 , the voltage value U 90 =0 is obtained.  
         [0046]    The uncoupling rule for example for the manual control input  94  for the fill level difference is: the transmission blocks  84  and  86  are to be dimensioned in such a way that a change of the control signal  94  increases (or decreases) the H 2  blow-down volume flow  132  by a certain amount and decreases (or increases) at the same time the O 2  blow-down volume flow  233  by the same amount, that is, that the sum of  132  and  133  remains constant. The formula for the uncoupling rule  94  is:  
         Δ vf   132   +Δvf   133 =0  
         [0047]    With the assumption that also the uncoupling rule vf 132 /vf 133 =2 for  95  is observed, by a differentiation of the equations for the valve characteristics according to the valve control signals  158  and  159 , a functional connection between the control voltage changes ΔU 158  and ΔU 159  is obtained. If for  86  in accordance with the transmission block  87  for example the transmission factor one is used, ΔU 159 =ΔU 159  is obtained, if ΔU 94  designates the change of the manual control  94 .  
         [0048]    For the equal-percentage valve characteristics, the following applies:  
         Δ U   158   =ΔU   94   ·K   84    
           K   84 =−0.5· ln ( ST   123 )/ ln ( ST   122 )  
         [0049]    As a result, the block  84  obtains the transmission factor K 84 . For linear value characteristics, independently of the uncoupling rule for  95 , the following is obtained:  
         Δ U   158   =ΔU   94   ·K   84    
         
       K 
       84 
       =−Kv 
       123 
       /Kv 
       122  
     
         [0050]    The block  84  obtains the transmission factor K 84 .  
         [0051]    With the uncoupling network consisting of the transmission blocks  84 ,  85 ,  86 , and  87 , the summing devices  88 ,  89 ,  92 , and  93  and the voltage source  90 , the controlled variables fill level difference and gas pressure can be controlled by way of the manual control inputs  94  and  95  in an uncoupled and therefore safe manner. Also, an improved automatic control behavior is achieved thereby. The control values gas pressure and fill level difference are controlled independently of each other.  
         [0052]    Furthermore, the uncoupling network also enhances the control operation if the controllers  82  and  83  included in the control arrangement  99  of FIG. 3 and are effective via the summation devices  92  and  93 .  
         [0053]    The controller  82  is responsible for the stabilization of the fill level difference. It obtains information concerning the deviation of the actual value  154  of the fill-level difference from the desired value  168  by way of the comparison device  80 . The controller  82  is a P-controller and causes the fill level difference, upon deviations, to react with a finite change of the actual value, that is, the control arrangement for the fill level difference provided with a P-controller has a proportional behavior and therefore is far from the stability limit.  
         [0054]    The controller  83  is responsible for the stabilization of the gas pressure and obtains information concerning a deviation of the actual value  157  of the gas pressure from the desired value  169  by way of the comparison device  81 . If it is a p-controller causing the gas pressure upon occurrence of disturbances to react with a finite adjustment of the actual value, the gas pressure control arrangement provided with the p-controller has a proportional behavior and therefore is far from the stability limit.  
         [0055]    The dimensioning of the proportional coefficients  82  and  83  is not problematic and occurs in accordance with methods common in control engineering.  
         [0056]    The stabilizing effects of the P-controllers support also the PID controllers  164  and  165  shown in FIG. 3. It is the object of these controllers to eliminate the residual control deviations of the P-controllers  82  and  83  by supplying the compensation signals  170  and  171  to the comparators  80  and  81 . From the fact that the PID controller of FIG. 3 are connected in a circuit with proportional control arrangements instead of arrangements near their stability limits, as they are known from the state of the art, short control times and relatively small overshoot widths are obtained for the control values fill level difference and gas pressure.  
         [0057]    The fill level difference control circuit of FIG. 3 further includes a fill level measuring arrangement  151 , which obtains from the electrolysis gas separation system  121  the actual information  152  and  153  concerning the fill levels in the H 2  and, respectively, O 2  gas chambers and converts them into the corresponding actual value signal  154  for the fill level difference. This signal is compared in the comparator  166  with the desired value  168  for the fill level difference. The control difference is supplied to the controller  164  for the fill level difference whose control output  170  is connected by way of the comparator  80  of the controller  82  with the control input  158  of the H 2  blow-down valve  122  and with the control input  159  of the O 2  blow-down valve  123 .  
         [0058]    The gas pressure control circuit in FIG. 3 further includes a pressure measuring arrangement  155 , which obtains from the electrolysis gas separation system  121  the actual gas pressure information  156  and converts it to the corresponding electrical actual value signal  157  for the gas pressure. The signal  157  is compared in the comparator  167  with the desired gas pressure value  169 . The control difference is directed to the gas pressure controller  165 . The output  171  of the controller  165  is connected, by way of the comparator  81  of the controller  83 , to the control input  159  of the O 2  blow-down valve  123  and the control input  158  of the H 2  blow-down valve  122 . The blow-down flows are represented by the reference numerals  132  and  133 .  
         [0059]    [0059]FIG. 4 shows a circuit for the control arrangement  99  of FIG. 3 provided with operational amplifiers. The reference numerals of the operational amplifiers, of the eight inputs and the two outputs correspond to the reference numerals of the respective components, inputs and outputs of the control arrangement  99  of FIG. 3. Operational amplifiers of the type OP741 are used. With the resistance R 82 , the amplification of the P-controller  82  is predetermined. With the resistance R 83 , the amplification of the P-controller  83  is determined. With the resistors R 84  to R 87 , the transmission factors  84  to  87  of the uncoupling network are determined. With the potentiometer R 90 , the voltage source  90  is adjusted. All resistors which are not marked have constant values of 10 kΩ.  
         [0060]    In another embodiment, the circuit arrangement of FIG. 4 is implemented as ANSI-C-Code in the integrated digital data acquisition system ADnC812 of the company Analog Devices. This system has eight analog inputs and two analog outputs. For a comfortable generation of the C-code, the circuit is simulated with the simulation program 20-sin 3.1 Pro of the company Controllable Products B.V., which is suitable for control engineering designs. From the operational model of the circuit, the program 20-sim 3.1 Pro generates the ANSI-C-Code. This code is treated and compiled with the development environment WE-dit32 of the company Raisonance S.A. The compiled program is transferred with its own loading program to the data acquisition system ADuC812 and is made there operational.  
         [0061]    Listing of reference numerals concerning FIG. 1 (state of the art)  
                                                       11, 12, 13   Anode cathode diaphragms           20   Electrolysis block           21   Electrolysis gas separating system           22, 23   H 2  blow-down value, O 2  blow-down value           24, 25   Fill level in the H 2 , O 2  gas separator           26, 27   H 2  gas chamber, O 2  gas chamber           28, 29   Alkaline columns in the H 2  and O 2  gas separator           30, 31   Conduits to the H 2  and O 2  gas separator           32, 33   H 2 , O 2  blowdown gas flows           34   Connection between the H 2  and O 2  gas separator           35   Return flow conduit in the electrolysis block           36, 37   H 2  gas separator, O 2  gas separator.                      
 
         [0062]    Additional reference numeral for FIG. 2, (State of the Art)  
                                                       51   Fill level measuring device           52, 53   Information concerning the fill level in the H 2 , O 2                 gas spaces           54   Actual value signal for the fill level difference           55   Pressure measuring device           56   Actual gas pressure information           57   Actual value signal for the gas pressure           58, 59   Control input of the H 2 , O 2  blowdown valve           64, 65   PID controller for fill level difference and gas               pressure           66, 67   Comparator locations (desired value minus actual               value for fill level difference and gas pressure)           68, 69   Desired values for fill level difference and gas               pressure           70, 71   Control outputs of the PID controllers 64, 65 for               fill level difference and gas pressure           99a   Connecting point, interconnects control input 58 of               the H 2  blowdown valve 22 and the control output 70               of the PID controller 64 in the fill level difference               control circuit.           99b   Connecting point, interconnects control input 59 of               the O 2  blowdown valve 23 and the control input 71               of the PID controller 65 in the gas pressure control               circuit                      
 
         [0063]    Reference numerals concerning FIG. 3 (control arrangement according to the invention):  
                                                       80, 81   Comparator locations           82, 83   P-controller           84, 85,   Transmission blocks           86, 87           88, 89,   Summing devices           92, 93           90   Voltage sources           94   Manual control input for the fill level difference           95   Manual control input for the gas pressure           99   Control arrangement (dash-point lines)           121   Electrolysis gas separator           122, 123   H 2  blowdown valve, O 2  blowdown valve           132, 133   H 2 , O 2  blowdown gas flows           151   Fill level measuring arrangement           152, 153   Actual fill level information for the H 2 -, O 2  gas               space           154   Actual value signal for the fill level difference           155   Pressure measuring device           156   Actual gas pressure information           157   Actual value signal for the gas pressure           158, 159   Control input of the H 2 , O 2  blowdown valves           164, 165   PID controllers for the fill level difference and               the gas pressure           166, 167   Comparator location (desired value minus actual               value)           168, 169   Desired values for the fill level difference and               the gas pressure           170, 171   Control outputs of the PID controllers 164, 165 for               the fill level difference and gas pressure                      
 
         [0064]    Additional reference numerals for FIG. 4 (control arrangement with operational amplifiers according to the invention).  
         [0065]    R 82 , R 83 , R 84 , R 85 , R 86 , R 87 —resistors  
         [0066]    R 90 —potentiometer