Abstract:
A control system for regulating a system parameter includes three sensing elements to independently sense a single system parameter and provide respective feedback signals. Three substantially identical controllers are responsive to these feedback signals with the controllers each having a bridge network of three resistors and being connected to a common resistor. In response to the feedback signals and signals representative of the imbalance in the bridge networks, the controllers act to cause bridge balance by collectively supplying current to the common resistor to effect a balance and to thus control the system parameter in accordance with the middle valued of the three feedback signals.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to control circuits and more particularly to a control circuit for supplying a load with electrical power proportional to the middle valued of three input signals. 
     In the discipline of process control, it is often desirable to develop an output signal (e.g., a control signal) which represents the middle valued of three input signals. These input signals are commonly feedback signals of a system parameter which is being controlled with the three feedback signals being the result of independent sensing of the same parameter. For example, a parameter closely monitored in a gas turbine is the internal temperature of the combustion chamber. Because of the criticality of this measurement, since excessive temperature or excessive rate of temperature rise can be extremely detrimental to the turbine, it is common to use a plurality of sensors. These sensors provide independent feedback signals which, although the sensors are of the same type, vary due to variances in the sensors themselves, differences in the amount of degradation of the sensors and the circuitry involved and/or other factors such as one of the sensors may fail. It is, therefore, commonly known to consider these sensors in groups of three and to select the sensor providing the middle value of the three as being the &#34;most correct&#34; and to regulate the temperature to that value. In a gas turbine, temperature is largely regulated by controlling the fuel flow to the mid-value. 
     A variety of mid-value circuits are known. Most of these circuits are either very expensive or have a common active component(s) the failure of which will result in a complete loss of the system. As an example, it is known to have an operational amplifier as the common point with the circuitry associated with the individual feedbacks all feeding the common operational amplifier. It is apparent in this case that should this amplifier fail, the control system will either fail or at least be seriously degraded. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a new and improved mid-value circuit. 
     It is another object to provide a reliable mid-value circuit which employs only passive elements at its point of commonality. 
     It is a further object to provide a mid-value circuit employing a passive element at its point of commonality and which allows for failure of one portion while still providing a valid output. 
     It is an additional object to provide a system for the control of a single parameter employing plural feedbacks supplied to an improved mid-value circuit. 
     The foregoing and other objects are achieved, in accordance with the present invention, by providing a mid-value circuit for developing an output signal to be applied to a load which output signal is proportional to the middle in a value of first, second and third input signals. This circuit includes three resistive bridge circuits, of substantially identical configuration, each of which includes, as one leg, a common resistive element. Each of the bridge circuits is capable of furnishing electrical current to the load. First, second and third control means, associated respectively with each of the bridge circuits control the amount of current that its respective bridge circuit supplies to the load. Each control means comprises a circuit which is responsive to an electrical condition of the load to develop a signal proportional thereto. By comparing this signal with a signal representative of one of the input signals, and combining the result of this comparison with a signal which is proportional to the electrical imbalance of its respective bridge, there is provided a control signal which determines the amount of current that bridge furnishes to the load. Since each of the three circuits senses the same load condition and reacts to the amount of bridge imbalance, the result is a control based upon the middle valued (mid-value) of the three input signals. 
     Employing the invention as a process control, the load is the means to utilize the final output of the mid-value circuit and the plural feedbacks are representations of individual sensings of a single parameter to be controlled, for example, temperature, voltage, current, etc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     While the present invention is described in particularity in the claims annexed to and forming a part of this specification, a better understanding thereof can be had by reference to the following description taken in conjunction with the attached drawing in which: 
     FIG. 1 is a high level block diagram of the control system of the present invention and further illustrating one possible environment for use of the present invention; 
     FIG. 2 is a schematic diagram of the mid-value circuit of the present invention in accordance with a preferred embodiment; and, 
     FIG. 3 is a schematic diagram of a portion of the mid-value circuit illustrating an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Reference is now made to FIG. 1 which shows a high level block diagram of a control system of the present invention further including one possible environmental implementation thereof. As shown in FIG. 1, three sensors, 10, 12 and 14, located external to the circuitry of the present invention, provide feedback signals FB 1 , FB 2  and FB 3  on lines 20, 18 and 16, respectively. These feedback signals typically represent the same operating parameter. As an example, this operating parameter might be the internal temperature of the combustion chamber of a gas turbine. As such, the feedback signals on lines 16, 18 and 20 may be of the same value or they may differ due to variances in the sensors, associated circuitry, etc. The three feedback signals are applied, respectively, to three identical controllers 22, 24 and 26, also designated controller X, controller Y and controller Z. Each of the controllers includes an input section which receives a respective feedback signal as well as other input signals which are not pertinent to the present invention and are included here only for purposes of illustrative completeness. Thus, controller 22 includes input section 28 which receives the feedback signal FB 1  and other inputs via line 30. In a similar manner, controller 24 has an input section 32 which receives the FB 2  signal and other inputs via line 34 and controller 26 has input section 34 which receives the feedback signal FB 3  and other inputs on line 36. Each of the controllers is also shown as including a central processing unit (CPU). CPUs 38, 40 and 42 of controllers 22, 24 and 26, respectively, receive signals from their respective input sections and perform apppropriate computations to provide, insofar as the present invention is concerned, respective outputs V X , V Y  and V Z  (scaled or adjusted feedback signals representing the feedback signal FB 1 , FB 2  and FB 3 ) for application to the mid-value circuits of the present invention. Thus, it is shown in FIG. 1 that the V X  signal is applied to the mid-value circuit 44 of controller 22 while the V Y  signal is applied to a similar circuit 46 of controller 24. The V Z  signal is applied to mid-value circuit 48 of controller 26. 
     It is recognized that it is not technically correct to state that the mid-value circuits 44, 46 and 48 are totally enclosed within the boxes as indicated since, as will be better understood with respect to FIG. 2, a resistor 50, shown included within a passive sensing network 52 external to the circuits, forms a common part of each circuit. In this respect, an output is shown from each of the mid-value circuits 44, 46 and 48 to a node 54 which is connected to one end of the common resistor 50. The other end of resistor 50, at node 56, is connected to a second resistor 60, the other end of which (at node 61) is connected to a process control 58. In accordance with the present invention, control 58 serves to control the parameter being sensed by the sensors 10, 12 and 14. The voltage across the second resistor 60 is applied as a signal to each of the mid-value sensing circuits 44, 46 and 48. Basically, what is provided by the circuitry of FIG. 1 is that the signals representing the parameter sensed by the three sensors 10, 12 and 14 are supplied as feedback signals to the controllers to develop three input signals to the mid-value circuits 44, 46 and 48 (which actually include as a portion thereof the resistor 50). An output of the mid-value circuits at node 56 is applied by way of a sensing resistor 60 to the process control 58 to thus control the process. The mid-value circuits serve to control the process in accordance with the value of the three input signals V X , V Y  and V Z  (i.e., representative feedback signs FB 1 , FB 2  and FB 3 ) accordingly to the one having the middle value. It will be noted that in FIG. 1 the load condition being sensed; i.e., the parameter which is furnished to the process control 58, is the current supplied thereto. This is done, as illustrated by using, as a feedback signal, the voltage across resistor 60 (nodes 56 and 61) which is proportional to the current through that resistor. This current is the combined output of the three mid-value circuits. 
     Reference is now made to FIG. 2 which is a detailed schematic diagram of the mid-value circuitry of the present invention. Shown in FIG. 2, within dashed line blocks, are the three circuit portions 44, 46 and 48 which in FIG. 1 are labeled mid-value circuits X, Y and Z respectively. Also shown are the two external resistors 50 and 60 as well as a load labeled 58&#39;. The latter corresponds to the process control 58 of FIG. 1. As will become apparent as this description precedes and as was earlier indicated, resistor 50 actually forms a portion of the mid-value circuits but is shown externally to the dashed line depiction because it is common to all three of those circuits. Looking now specifically to FIG. 2, only that circuit within dashed line block 44 and its relationship to elements 50, 56, 60, 61 and 58&#39; will be described in detail since the composition and operation of the three circuits 44, 46 and 48 are identical and thus repetitious description is believed unnecessary. 
     It is seen first that a resistive bridge network comprised of resistors 100, 102, 104 and 50 is supplied with electrical power from a source of electrical energy (V + ) applied at terminal 106. The amount of current applied to this bridge circuit is controlled by suitable means such as the field effect transistor (FET) 108. Depending upon the degree of conduction by FET 108, current flows through the resistive bridge and the output of this bridge, at node 56, is applied to a resistor 60 in series connection with load 58&#39;. 
     The voltage across resistor 60 (nodes 56 and 61) is applied to the two inputs of a differential amplifying network. Node 56 is connected to the non-inverting input of an operational amplifier 110 by way of a resistor 112 while node 61 is connected to the inverting input of operational amplifier 110 by way of an input resistor 114. The inverting input is further connected to ground by way of resistor 116 and a feedback resistor 118 is connected between the output and the non-inverting input of the operational amplifier 110 such that, as is well known in the art, a differential amplifier is provided. The output of the amplifier 110 is a condition signal proportional to the voltage across resistor 60. This signal is representative of a load condition which, in this case, is load current (I). 
     The output of amplifier 110 is applied via resistor 122 to the non-inverting input of an operational amplifier 120 of a second differential amplifying network. This non-inverting input is further connected to ground by way of a resistor 124. The output of amplifier 120 is connected to its inverting input by way of resistor 126 and has further applied thereto, by way of resistor 128, the V X  signal. The V X  signal, as described with respect to FIG. 1, is the signal which is proportional to the apparent desired value according with the feedback signal applied to controller 22, feedback signal FB 1 . Thus, the output of operational amplifier 120 which appears at node 130 is an error signal which is proportional to the difference between the apparent desired level of operation as represented by the signal V X  and the condition signal of the load. 
     The error signal at node 130 is applied via resistor 132 to the inverting input of an amplifier 134 which is connected in the summing mode. The non-inverting input of amplifier 134 is connected via resistor 136 to ground and its output is connected to its inverting input by way of a resistor 138. A second input to the inverting input of operational amplifier 134 is by way of a series connection of a resistor 139, a delay means 140, and a diode 142 from the output of an additional operational amplifier 144. The non-inverting input of amplifier 144 is connected via resistor 146 to ground and to the junction of bridge resistors 102 and 104. Its output is connected to its inverting input by way of a series combination of a diode 148 and a resistor 150 such that there is provided a positive feedback thereto which tends to cause the amplifier output to go to zero. The inverting input of amplifier 144 is connected to the junction point of bridge resistors 102 and 50. Thus, the output of amplifier 144 (referred to as a bridge amplifier) at node 153 will be a signal proportional to the imbalance of the bridge. This signal, as was earlier indicated, is applied by way of diode 142, delay means 140 and resistor 139 to the inverting input of the summing amplifier 134. The purpose of the delay is merely to prevent race conditions. 
     The output of the operational amplifier 134 at node 152 is a control signal which is applied to the gate electrode of the FET 108, the drain electrode of which is connected to the positive potential V+ at terminal 106. The source electrode of FET 108 is connected to the bridge circuit at the junction point of resistors 100 and 102. The control signal at node 152, proportional to the sum of the error signal at node 130 and the bridge imbalance signal at node 153, controls the current supplied through the resistor bridge (resistors 100, 102, 104 and 50) and hence the amount of current I furnished to load 58&#39; through resistor 60. 
     The other two mid-value circuits 46 and 48 are, as was earlier stated, identical to circuit 44 and will not be described in detail here. The connections and operations of these circuits being essentially identical to that described with each contributing to total load current in accordance with the relationship among the input signals (V X , V Y  and V Z ) and the value of the voltage across resistor 60. 
     With the foregoing in mind, the following will explain the operation of the mid-value circuit of the present invention. For purposes of this description, it will be assumed that the V X  signal has the highest value and that the V Z  signal has the lowest value. The V Y  signal has a value intermediate V X  and V Z . This selection is entirely arbitrary as each of the circuits is identical and it will be used merely for purpose of illustration. The overall operation of the system is the same regardless of the order values of the input signals. With V X  being the highest signal, amplifier 120 will sense that the condition signal from amplifier 110 is less than V X  value and thus will generate a negative going signal to the input of the summing amplifier 134. Amplifier 134 will, in turn, provide a control signal output to the gate of FET 108 turning that device on to a high degree such that the bridge network of circuit 44 attempts to dominate the load current. That is, the current through the bridge network 100, 102, 104 and 50 will be at a high level. (At this point the relative values of the resistors of the bridges becomes important. As a typical example, if resistor 102 has a value of R, resistors 100 and 104 would be ten times that value or 10R. Resistor 50 would be one-half of that value of R/2.) With the FET 108 full on, the upper bridge network will attempt to supply total load current resulting in a heavy imbalance of the bridge network by virtue of the differential resistive value of resistors 102 and 50. This imbalance will, in turn, cause a large output signal from the bridge amplifier 144. This output signal (positive) is applied to summing amplifier 134 and will tend to cause this latter amplifier to decrease its output and to thus reduce the current supplied to the bridge and hence the load. This decrease in current will continue until it reaches a value corresponding to the middle valued input signal, signal V Y  in the present example. As soon as this occurs, the circuit 46 which has the middle value will sense that its condition output from the amplifier corresponding to 110 (amplifier 210) is less than that required by its input signal V Y  and will begin to supply current through its bridge circuit including common resistor 50 to the load. These two circuits with the high and the middle value input signals (V X  and V Y ) will then stabilize at the middle value with each providing one-half of the load current to resistor 50 at which time these bridges will be in a balanced state. 
     The circuit with the low value input which in the present illustration is circuit 48 does not supply current. Since it has the lowest input value, the result of the comparison of its input signal V Z  with its condition signal from amplifier 310 will result in no current through its bridge network. Thus, it is seen that even if the signal V Z  were to go to zero, indicating a complete fault or failure of this circuit or an input thereto, the other two circuits would consider that as the low value and stabilize on the lower of the remaining two values. This adds both consistency and reliability to the overall system. 
     FIG. 3 illustrates one possible modification of the present invention in which the load voltage rather than load current is employed as the load condition to be monitored. FIG. 3 shows for orientation purposes, the common resistor 50, node 61, load 58&#39; and the differential amplifier 120 and its associated circuitry. The difference here is that rather than sensing the load current as by the amplifier 110 and its related circuitry, the voltage at node 61 (the voltage of the load with respect to ground) is supplied to the non-inverting input of a simple gain operational amplifier 170. The inverting input of that operational amplifier is connected to ground by way of series connected resistors 174 and 172 with the junction of those resistors being connected to the amplifier output. The output of amplifier 170, which forms the system condition signal serves as the input, via resistor 122, to differential amplifier 120 and in all other respects the operational circuit is identical to that earlier described. It is, of course, to be realized that similar circuits would be utilized in each of the three mid-value circuits 44, 46 and 48 such that once again they are identical in format and operation. In all other respects these circuits remain the same. 
     While there have been shown and described what are at present considered to be the preferred embodiments of the invention, modifications thereto will readily occur to those skilled in the art. It is not desired, therefore, that the present invention be limited to the specific embodiments as shown and discussed and it is intended to cover within the appended claims all such modifications as fall within the true spirit and scope of the invention.