GB1563132A - Highest wins circuit - Google Patents

Description

(54) HIGHEST WINS CIRCUIT (71) We, ROLLS-ROYCE LIMITED, a British Company of 65 Buckingham Gate, London, SWIPE 6AT, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to "highest wins" circuits and control systems incorporating such circuits. A highest wins circuit is a signal selection circuit which can receive a plurality of input signals but which passes only the strongest signal to its output side.More particularly, the invention relates to the type of highest wins circuit which includes a plurality of "perfect" diodes connected between respective input lines and a common output line the diode receiving the greatest signals conducts and provides reverse bias to the other diodes which are switched off. For a "perfect" diode, the current passing through the diode in the conducting direction is substantially proportional to the applied voltage even at low voltage levels.

Such circuits find application, for example, in control systems technology, such as fuel control systems for gas turbine engines, where they are used to select a particular control signal from a plurality of control signals and to pass the selected signal to a subsequent stage where it is used to control the fuel flow.

In order to reduce the disturbing effect of transients in the various control loops which pass through the highest wins circuit, active (operational) integrators are included in the control loops to apply an electrical gain including an integral term to each control loop. An active integrator usually comprises an amplifier with a negative feed back loop having capacitative and resistive elements in series; in practice therefore the electrical gain is applied to a control loop is not purely integral, but is a ratio of integral to proportional gain.

In the prior art, active integrators have been included either in the input lines to the highest wins diode circuit or in the common output line. In the former case an integrator is provided in each input line before its respective perfect diode whilst in the latter case one integrator acts for all the control loops.These two arrangements have their own disadvantages; in the first case reaction time is slow because the outputs of those active integrators not in an active control loop but still receiving a control signal decline until they reach a limiting value, but then take a finite time to increase when their particular loop is selected by the highest wins circuit; in the second case the same ratio of integral to proportional gain is applied to all the control loops, which does not provide a satisfactory response on all loops because the design of the integrator must be a compromise between the different requirements of the different control loops.

According to the present invention, a highest wins circuit includes a plurality of perfect diodes, each comprising an inverting amplifier provided with diode feedback and a further diode connected in series to the output side of the amplifier, each perfect diode being connected between one of a corresponding plurality of input lines and an output line common to all the perfect diodes, wherein each perfect diode is provided with a negative feedback loop which includes an integral term.

Preferably, the negative feedback loop includes a capacitance connected in series with a resistance.

Also according to the present invention, a control system has a plurality of control loops, a perfect diode included in each control loop, and means operative to apply a signal gain including an integral term to each control loop, each perfect diode comprising an inverting a amplifier provided with diode feedback and a further diode connected in series to the output side of the amplifier, the perfect diodes being connected to a common output line and together forming a highest wins circuit for the control loops, wherein said means operative to apply a signal gain including an integral term to each control loop includes an integrating negative feedback loop across each perfect diode.

Circuits and control systems according to the invention have the advantage that each integrator can be designed to suit the individual requirements of the particular control loop with which it is associated, whilst allowing rapid response in the event of changeover between control loops when the circuit selects a different signal.

An embodiment of the invention will now be described by way of example only with reference to the accompanying circuit diagram.

The diagram illustrates a highest wins circuit of a type suitable for use in a fuel control system for a gas turbine engine. The circuit has three input lines It, 12, I3 for respective control signals xl, x2, x3. The resistances seen in the input lines by the rest of the circuit are represented by respective resistances R11, R2 1, R3 1.

Each input line is connected to a respective perfect diode D1, D2, D3 as defined within the broken lines. Perfect diodes D1, D2, D3 are all connected to a common output line 0, and thus collectively form the highest wins circuit. The arrangement of the circuit will now be described with reference to the control loop through D1 only, since the loops through D2 and D3 are similar to D1, as are the perfect diodes themselves.

Perfect diode D1 comprises an inverting amplifier Al provided with a diode D12 feedback and connected in series with a further diode D11 on its output side. The inverting amplifier Al has a high open loop gain (for example 106 or higher) and the combination of the amplifier Al with diodes Dl 1 and D12 makes the "perfect" diode, which has a substantially linear characteristic even at very low voltage levels.It is possible of course to construct a highest wins circuit in which an ordinary diode such as D11 or Dl 2 is used instead of perfect diode D1, but such an arrangement is not suitable for use in situations where the circuit is to be used in a control system which requires sensitive operation at low diode voltage levels, since ordinary diodes have a "dead" band from zero to about half a volt in which their characteristics are very nonlinear.

It should be noted that perfect diode D1 has a positive output by virtue of the arrangement of diodes Dll and D12, and thus in order to pass through D1, signal xl must have a negative value. However, if it were desired to use S1 to pass a positive signal, diodes D11 and D12 would be reversed to give D 1 a negative output.

Electrical gain must be applied to each control loop in the system; i.e. signal xl must be amplified to operate subsequent units in the system whenever it is selected by the highest wins circuit, and this of course is at least partly accomplished by amplifier Al. Further, the gains in the control loop through D1 and the common output line 0 must include integral terms in order to reduce the disturbing effect of transients, such as "hunting" due to small fluctuations in the control signal xl. As explained above, these two requirements have hitherto been met by including an active integrator in the control loop either in the input line I1, before perfect diode D1, or after the highest wins circuit in the common output line 0.These two arrangements have the disadvantages already stated.

However, the circuit diagram shows an arrangement in which an integral term is included in the gain through the control loop by combining the perfect diode D1 with an integrator in parallel. The integrator takes the form of a negative feedback loop comprising a capacitor C1 connected in series with a resistor R12, which latter component, together with the input resistance R11 seen in line I1, serves to define the proportional gain. The negative feedback through C1 and R12 makes the perfect diode D1 in a form of half-wave operationa amplifier, i.e. an "operational" or "active" integrator.

Assuming signal xl is dominant and of negative voltage, the amplifier Al output is positive and hence diode Dii conducts and diode Dl 2 does not. With current flowing throug Dii, amplifier Al has a complete negative feedback loop through capacitor C1 and resistor R12.

This arrangement achieves an integral plus proportional operational amplifier which applies integral plus proportional gain to the signal xl in the control loop through D1 and shares a common amplifier Al with perfect diode D1.

The integrator can be designed to match only the requirements of the control loop through D1 and can thus have optimum response.

The operation of the highest wins circuit is discussed as follows. It is assumed that the amplifiers Al, A2, A3 each have the same open loop gain and that the values of the various capacitances and resistances are the same at corresponding positions in different control loops. With signal xl dominant and negative, as above, the signals x2, x3 will be either positive or less negative than xl. If they are positive, amplifiers A2, A3 have negative outputs, and hence the respective feedback diodes D22, D32 conduct, thereby limiting the outputs to about zero volts and keeping summing junctions S2, S3 at approximately zero potential. Diodes D21, D3 1 also do not conduct and hence the amplifier outputs are isolated from common line 0. However, feedback capacitors C2, C3 (the integrators) are connected between the summing junctions S2, S3 and the common line 0. The capacitors C2, C3 are thus maintained in a charged state by virtue of the output voltage from D1 and are immediately ready to take over control when the circuit selects a different control loop, i.e. their response time is very fast.

If signals x2, x3 are less negative than xl, amplifiers A2, A3 will give a positive output and feedback diodes D22, D32 will not conduct.

Diodes D21, D31 will also not conduct because the voltage in the common line from D1 is greater than the outputs from A2, A3, and capacitors C2, C3 will remain charged up to the output voltage of do as before.

If, say, signal x3 becomes more strongly negative than xl, or xl becomes less strongly negative than x3, the output from diode D3 1 will become dominant and the control loop through D3 will immediately take over from the control loop through D1.

The foregoing description of the circuit operation assumes that there is an output load resistance in line 0, which provides a discharge path for the integrating capacitors. The embodiment described would, in a practical control system, always be part of an active control loop.

Although it has been assumed in the foregoing description that circuit components in different control loops but at corresponding positions therein are identical, this need not necessarily be the case. For example, the feedback resistances R12, R22, R23, and capacitances C1, C2, C3 could be made different from each other if required, so that differing proportions of proportional to integral gain could be applied to each control loop.

Again, amplifiers Al, A2, A3 could be of different gains if it were desired to apply different gains to different control loops.

The signal in the common output line 0 may be further amplified if necessary to operate subsequent elements of the control system.

Further, although the present circuit has been described as having only three control loops as are needed for any particular application.

Other modifications to the circuit of the present invention will be readily thought of by one skilled in the art.

WHAT WE CLAIM IS: 1. A highest wins circuit including a plurality of perfect diodes, each comprising an inverting amplifier provided with diode feedback and a further diode connected in series to the output side of the amplifier, each perfect diode being connected between one of a corresponding plurality of input lines and an output line common to all the perfect diodes, wherein each perfect diode is provided with a negative feedback loop which includes an integral term.

2. A highest wins circuit according to claim 1 in which the negative feedback loop includes a capacitance connected in series with a resistance.

3. A control system including a highest wins circuit according to claim 1 or claim 2.

4. A control system having a plurality of control loops, a perfect diode included in each control loop, and means operative to apply a signal gain including an integral term to each control loop, each perfect diode comprising an inverting amplifier provided with diode feedback and a further diode connected in series to the output side of the amplifier, the perfect diodes being connected to a common output line and together forming a highest wins circuit for the control loops, wherein said means operative to apply a signal gain including an integral term to each control loop includes an integrating negative feedback loop across each perfect diode.

5. A control system according to claim 4, in which the integrating feedback loop includes a capacitance connected in series with a resistance.

6. A highest wins circuit substantially as hereinbefore described with reference to and as shown in the accompanying drawing.

7. A control system substantially as hereinbefore described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. amplifiers A2, A3 will give a positive output and feedback diodes D22, D32 will not conduct. Diodes D21, D31 will also not conduct because the voltage in the common line from D1 is greater than the outputs from A2, A3, and capacitors C2, C3 will remain charged up to the output voltage of do as before. If, say, signal x3 becomes more strongly negative than xl, or xl becomes less strongly negative than x3, the output from diode D3 1 will become dominant and the control loop through D3 will immediately take over from the control loop through D1. The foregoing description of the circuit operation assumes that there is an output load resistance in line 0, which provides a discharge path for the integrating capacitors. The embodiment described would, in a practical control system, always be part of an active control loop. Although it has been assumed in the foregoing description that circuit components in different control loops but at corresponding positions therein are identical, this need not necessarily be the case. For example, the feedback resistances R12, R22, R23, and capacitances C1, C2, C3 could be made different from each other if required, so that differing proportions of proportional to integral gain could be applied to each control loop. Again, amplifiers Al, A2, A3 could be of different gains if it were desired to apply different gains to different control loops. The signal in the common output line 0 may be further amplified if necessary to operate subsequent elements of the control system. Further, although the present circuit has been described as having only three control loops as are needed for any particular application. Other modifications to the circuit of the present invention will be readily thought of by one skilled in the art. WHAT WE CLAIM IS:

1. A highest wins circuit including a plurality of perfect diodes, each comprising an inverting amplifier provided with diode feedback and a further diode connected in series to the output side of the amplifier, each perfect diode being connected between one of a corresponding plurality of input lines and an output line common to all the perfect diodes, wherein each perfect diode is provided with a negative feedback loop which includes an integral term.

2. A highest wins circuit according to claim 1 in which the negative feedback loop includes a capacitance connected in series with a resistance.

3. A control system including a highest wins circuit according to claim 1 or claim 2.

4. A control system having a plurality of control loops, a perfect diode included in each control loop, and means operative to apply a signal gain including an integral term to each control loop, each perfect diode comprising an inverting amplifier provided with diode feedback and a further diode connected in series to the output side of the amplifier, the perfect diodes being connected to a common output line and together forming a highest wins circuit for the control loops, wherein said means operative to apply a signal gain including an integral term to each control loop includes an integrating negative feedback loop across each perfect diode.

5. A control system according to claim 4, in which the integrating feedback loop includes a capacitance connected in series with a resistance.

6. A highest wins circuit substantially as hereinbefore described with reference to and as shown in the accompanying drawing.

7. A control system substantially as hereinbefore described with reference to the accompanying drawing.