Multiplexer method and system for intrinsically safe applications and a multiplexer switch for use therein

A multiplexing system is provided for a plurality of sensors disposed in a tank containing a potentially explosive environment and excited from a common excitation circuit outside of the tank through a corresponding plurality of signal lines that penetrate the tank. The multiplexing system comprises: a multiplexer switch for each sensor, each multiplexer switch disposed outside of the tank in series with the corresponding signal line between the common excitation circuit and the corresponding sensor in the tank. Each multiplexer switch comprises: at least one field effect transistor having a current channel connected in series with the corresponding signal line; an isolation circuit; and a drive circuit coupled to the at least one field effect transistor through the isolation circuit for operating the current channel thereof, the isolation circuit electrically isolating the drive circuit from the at least one field effect transistor to limit energy coupled to the corresponding signal line through the at least one field effect transistor to below levels that could initiate an ignition of the potentially explosive environment of the tank. A multiplexing method is also disclosed.

BACKGROUND OF THE INVENTION

The present invention relates to intrinsically safe circuits and systems, in general, and more particularly to a multiplexing method and system for a plurality of sensors disposed in a tank containing a potentially explosive environment, and a multiplexer switch for use therein.

In systems which supply signals to and receive signals from a contained environment comprising combustible or explosive vapors, restrictions are imposed to limit the energy over the signal lines penetrating the container below levels that could initiate an ignition of such vapors. For example, in a fuel quantity measurement system, sensors are disposed within a fuel tank and when excited, provide response signals indicative of the fuel level in the tank. Accordingly, each sensor has an excitation signal line and response signal line that penetrate the fuel tank. In order to be intrinsically safe, the circuitry that provides the excitation signals and receives the response signals are specially designed to limit the energy over the signal lines penetrating the tank to within safe levels under normal operating and failure conditions.

Having dedicated intrinsically safe excitation and signal conditioning circuits for each sensor of the contained environment is very expensive and complex. Accordingly, system designers have proposed a multiplexing system in which a group of sensors may be excited from a common excitation circuit and the resulting response signals from the sensors received by a common signal conditioning circuit. Thus, the costs and complexity can be reduced by a factor dependent upon the number of sensors in the multiplexed group. Multiplexing systems conventionally include controlled switches in series with the signal lines.

These multiplexer switches pose certain conditions to the designer of an intrinsically safe system which must be considered. For example, any switch added to the system needs to preserve the intrinsically safe nature of the common excitation and signal conditioning circuits and associated signal lines. However, to operate conventional multiplexer switches, a voltage is generally applied directly to the switch from a power source. Thus, if a failure occurs within the switch, this drive voltage may become directly connected to the corresponding signal line, resulting in a defeat of the intrinsically safe design. Moreover, it would be beneficial to render this preservation of intrinsic safety by maintaining the circuitry of each multiplexer switch to a minimum.

Another consideration results from the line capacitance of the signal lines from the excitation circuit to the sensors and from the sensors to the signal conditioning circuit. This line capacitance acts as a load to ground via a cover shield, for example, and may draw a significant amount of current from the excitation signal. For example, at ten volts AC with a frequency of around eighteen thousand hertz, as much as eleven or twelve milliamps may be drawn through the line capacitance. Conventional multiplexer switches have significant “on” resistance such that the current drawn by the signal line capacitance cause an undesirable voltage drop across the switch, resulting in a measurement error which may be in the range of five percent or so.

Further, reactive loading of the excitation signal can often cause an amplifier driving the signal to become unstable. To enhance stability, circuitry is generally added to the amplifier design. However, the addition of significant switch resistance in series with the excitation signal may interfere with the sensitive stability design of the drive amplifier.

Yet another consideration results from the stray capacitance in the signal line from the sensor back to the signal conditioning circuit. As noted above, a conventional multiplexer switch adds significant in series “on” resistance to the response signal. The current of the response signal which flows through this switch “on” resistance results in a voltage drop across the stray line capacitance, that, in turn, diverts some of the sensor response current to ground through the line capacitance. This undesirable diversion of current may also result in a measurement error.

The present invention ensures preservation of an intrinsically safe design and overcomes the drawbacks of conventional multiplexer switches especially in regard to circuit stability, line capacitance and measurement error noted above.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a multiplexer switch comprises: at least one field effect transistor having a current channel connected in series with a signal line that penetrates a tank containing a potentially explosive environment; an isolation circuit; and a drive circuit coupled to the at least one field effect transistor through the isolation circuit for operating the current channel thereof, the isolation circuit electrically isolating the drive circuit from the at least one field effect transistor to limit energy coupled to the signal line through the at least one field effect transistor to below levels that could initiate an ignition of the potentially explosive environment of the tank.

In accordance with another aspect of the present invention, a multiplexing system is provided for a plurality of sensors disposed in a tank containing a potentially explosive environment and excited from a common excitation circuit outside of the tank through a corresponding plurality of signal lines that penetrate the tank. The multiplexing system comprises: a multiplexer switch for each sensor, each multiplexer switch disposed outside of the tank in series with the corresponding signal line between the common excitation circuit and the corresponding sensor in the tank. Each multiplexer switch comprises: at least one field effect transistor having a current channel connected in series with the corresponding signal line; an isolation circuit; and a drive circuit coupled to the at least one field effect transistor through the isolation circuit for operating the current channel thereof, the isolation circuit electrically isolating the drive circuit from the at least one field effect transistor to limit energy coupled to the corresponding signal line through the at least one field effect transistor to below levels that could initiate an ignition of the potentially explosive environment of the tank.

In accordance with yet another aspect of the present invention, a method of multiplexing an excitation signal from a common excitation circuit to a plurality of sensors disposed in a tank containing a potentially explosive environment comprises the steps of: disposing a multiplexer switch is series with each signal line coupling a corresponding sensor of the plurality to the common excitation circuit outside of the tank; isolating each multiplexer switch from a corresponding drive circuit to limit energy coupled to the corresponding signal line through the corresponding multiplexer switch to below levels that could initiate an ignition of the potentially explosive environment; and controlling the drive circuits to multiplex the excitation signal from the excitation circuit to selected sensors of the plurality through the corresponding multiplexer switches.

DETAILED DESCRIPTION OF THE INVENTION

One aspect of the present invention is directed to a multiplexing system and a multiplexer switch for use therein that permits a plurality of sensors to be successively excited and read by a single electronics channel or the entire plurality excited and read by two or more electronic channels. The multiplexer switching function of the present invention can be performed directly in the signal lines of sensor leads even with impedance sensitive sensor signaling, and with circuits that have been stabilized for reactive loads without upsetting the circuit stability. In addition, it can be applied to circuits where it is critical that the sensor signal exactly match a reference signal. More particularly, all of the switching functions of the multiplexer system can be accomplished in circuits or signal lines that have been designed for intrinsically safe applications without compromising the intrinsically safe nature of the circuitry or signal lines.

FIG. 1is a circuit schematic of an exemplary multiplexer switch suitable for embodying an aspect of the present invention. As shown inFIG. 1, the multiplexer switch embodiment is configured as a single pole double throw switch in which one of signal lines1and2may be selected for connecting to signal line3by switches denoted as Q1and Q2. In the present embodiment, each switch Q1and Q2comprises two N channel metal oxide semiconductor field effect transistors (MOSFETs) connected in series source to source to form a bi-directional switch that will operate with both AC and DC signals. Switches Q1and Q2may be an integrated circuit of the type marketed under a part number IRF7341, for example. More specifically, the drain of one of the MOSFETs of each switch Q1and Q2is connected commonly to signal line3. In addition, the drain of the other MOSFET of Q1is connected to signal line1and the drain of the other MOSFET of Q2is connected to signal line2. Each MOSFET may have an in series “on” resistance of 0.043 ohms and a maximum isolation voltage (the maximum voltage that can be turned off reliably) of 55 volts, for example. Other FETs may be selected with “on” resistance and isolation voltage specifications as the application dictates.

The switch embodiment ofFIG. 1includes optical isolators OP1and OP2which may be voltage output isolators of the type marketed under the part number TLP190B, for example. Internally in each isolator OP1and OP2, light output from a light emitting diode is focused on an array of series connected photocells which may be photodiodes. The photocells convert the light to an output voltage across the series connection thereof. There is electrical isolation on the order of 2500 volts, for example, between the light emitting diode circuit and the photocell circuit in each of the isolators OP1and OP2. In the present embodiment, application of a current through the input light emitting diode results in an output voltage of approximately 10V that is electrically isolated from the input of the optical isolator. Resistors R3and R4may be coupled in parallel with the array of photocells of OP1and OP2, respectively. In the present embodiment, each resistor R3and R4may be on the order of 300K ohms, for example.

The voltage output of OP1is connected across the gate to source junctions of both of the MOSFETS of the switch Q1and the voltage output of OP2is connected across the gate to source junctions of both of the MOSFETS of the switch Q2. The light emitting diode of OP1is driven by a control signal C1through a NAND gate G1and resistor R1and the light emitting diode of OP2is driven by a control signal C2through a NAND gate G2and resistor R2. Resistors R1and R2may be on the order of 200 ohms, for example.

In the present embodiment, when the gate to source voltage of the series connected MOSFETs is zero, i.e. no voltage output from an optical isolator, both MOSFETs will be non-conducting and have a high series resistance, thus blocking the connection of the signal lines connected thereto. If the gate to source voltage of the series connected MOSFETs is more than 5V from the voltage output of an isolator, then both FETs will be rendered conducting or turned “on”. Under these conditions, each MOSFET will afford a low series resistance path from the drain of one FET to the drain of the other (i.e. a low “on” resistance contact), thus connecting the signal lines connected thereto. In this manner, Q1and Q2may be controlled by providing either 0V or more than 5V from the common source point to the common gate point of the series connected MOSFETs thereof.

Control of the switch embodiment ofFIG. 1is provided by control signals C1and C2. In the present embodiment, when the control signal C1is low, the output of gate G1is high, (drive state) resulting in current flow through the input diode of OP1. This has the effect of applying a voltage greater than5volts across the gate to source of the MOSFETs of Q1via OP1which renders Q1conducting to connect signal line1to signal line3. At the same time, the control signal2is maintained high which results in a low output on gate G2, resulting in no current flow through the input diode of OP2. Under these conditions, Q2is therefore rendered non-conducting or “off”, and signal line2is isolated from signal line3.

Setting the control signal2to low (true) and signal line1to high (false), reverses all of the logic states. In this state, current flows through the input diode of OP2and no current flows through the input diode of OP1. Thus, Q2is turned “on” and Q1is turned “off” and signal line2is connected to signal line3and signal line1is isolated from signal line3. Note that the “on” resistance of Q1and Q2when conducting is approximately 0.086 ohms for the present embodiment. This value of in series “on” resistance will have an insignificant effect on the signal input source impedance, and will thus have a negligible effect on the signal itself. Other FETs may be selected if a lower “on” resistance is desired.

Combining the voltage output of the optical isolator with the voltage control properties of the MOSFETs results in a switch embodiment that can be turned on and off in response to a current through the input light emitting diode of the optical isolator. Further, since there is no contact between the isolator input diode circuit and output voltage circuit, the MOSFETs are completely isolated from the switch control logic circuitry. The voltage output of the optical isolator has a current capability of less than 50 uA. Thus, even a failure of the MOSFET gate circuitry cannot cause a significant amount of energy to flow in the signal line or circuit as a result of activity in the control circuitry. Since there are no other power sources required in the switch embodiment, the addition of the multiplexer switch in series with the signal line will not affect the intrinsic safety capability of the signal circuitry.

The multiplexer switch embodiment ofFIG. 1may be configured to turn an excitation signal on and off to a selected sensor. For example, the excitation signal may be coupled over signal line1, signal line2may be coupled to ground as shown by the dashed line D1, and signal line3may be coupled to the sensor. In addition, the output of gate G2may be coupled to the inputs of gate G1as shown by the dashed line D2. In this configuration, control C2governs the operation of the switch embodiment. When control signal C2is high, the output of G2is low and no light is coupled across the barrier in OP2. In addition, with C2high, the output of G1is high (double inversion by G2and G1) and light is coupled across the barrier of OP1. In this state, the excitation signal is connected to the sensor via Q1in conduction, and signal line3is isolated from ground by Q2which is open circuited. When control signal C2is low, the output of G2is high and light is coupled across the barrier in OP2. In addition, with C2low, the output of G1is low and no light is coupled across the barrier of OP1. In this state, the excitation signal is isolated from signal line3by Q1which is off or open circuited, and signal line3is held at ground by Q2in conduction to prevent static build up or stray signal paths.

FIG. 2is a block diagram illustration of an intrinsically safe application of the multiplexing system in accordance with another aspect of the present invention. InFIG. 2, a fuel measurement system10is operative to measure the quantity of fuel12in a fuel tank14utilizing a plurality of capacitive type fuel level sensors S1and S2disposed within the tank14. A controller16which may be a programmed digital computer, for example, is coupled to a primary exciter circuit18and a separate and independent secondary or back-up exciter circuit19over signal lines20and coupled to a primary signal conditioning circuit21and a separate and independent secondary or back-up signal conditioning circuit22over signal lines24. The exciter circuits18and19and signal conditioning circuits21and22are common to the plurality of sensors S1and S2. As directed by the controller16, the primary exciter circuit18is operative to generate a primary excitation signal EXC (PRI) and independent thereof, the secondary exciter circuit19is operative to generate a secondary or back-up excitation signal EXC (SEC), both excitation signals provided to a multiplexer circuit26over separate and independent signal lines. Likewise, the primary signal conditioning circuit21is operative to receive a primary response signal RESP (PRI), and independent thereof, the secondary signal conditioning circuit22is operative to receive a secondary or back-up response signal RESP (SEC) over separate and independent lines from the multiplexer circuit26. All of the circuits18-22are designed to afford intrinsically safe signaling to and from the sensors S1and S2for the present embodiment.

To achieve certain multiplexing and switch-over functionality, the multiplexer circuit26is included with optically isolated logic circuits28. The output of the multiplexer26provides one set of excitation and response signals to each sensor S1and S2. The multiplexer26may be governed by control signals A-D from the controller16(or, in the alternative, from pushbutton switches, not shown) to select one of the set of primary excitation and response signal combinations to excite and read one of the capacitive sensors S1and S2as will become more evident from the description found herein below. While the embodiment ofFIG. 2is representative of an intrinsically safe application, it is understood that present invention may be applied to other applications and other sensor types without deviating from the broad principles thereof.

More specifically, a block diagram circuit schematic of an exemplary multiplexer circuit suitable for embodying the multiplexer26is shown inFIG. 3. The embodiment ofFIG. 3includes a plurality of multiplexer switches SW1-SW5, each of the same design as described in connection with the embodiment ofFIG. 1. For example, in switch SW1, switches Q1and Q2may include the same dual series connected MOSFET switch design, the SW Q1control block may include the gate G1with the control signal C1connected to the inputs thereof and the optical isolator OP1with the output voltage thereof applied to the gate to source junctions of the MOSFETs of Q1, and the SW Q2control block may include the gate G2with the control signal C2connected to the inputs thereof and the optical isolator OP2with the output voltage thereof applied to the gate to source junctions of the MOSFETs of Q2. Switches SW2-SW5may be of the same design as SW1.

In the embodiment ofFIG. 3, EXE (PRI) is an intrinsically safe excitation signal from the primary excitation circuit18(seeFIG. 2) while RESP (PRI) is the intrinsically safe sensor response signal which is provided to the primary signal conditioning circuit21. The signals EXE (PRI) and RESP (PRI) in combination make up a primary channel. The EXE (SEC) and RESP (SEC) signals are the same as the aforementioned primary signals, except are associated with a completely independent set of electronics19and22as shown inFIG. 2, and in combination make up a redundant or backup channel. As noted above inFIG. 2, the representative multiplexer circuit26is controlled by logic signals A-D which may be generated by controller16or, in the alternative, manual pushbuttons (not shown). Other controls are equally applicable.

In the present embodiment, logic signals A, B, C, and D are coupled to one input of NAND gates G3, G4, G5, and G6, respectively. Gates G3and G4are configured as one set-reset (S-R) flip-flop circuit and gates G5and G6are configured as another S-R flip flop circuit. The output of gate G5is coupled to an input of NAND gates G7and G10, the output of gate G6is coupled to an input of NAND gates G8and G9, the output of gate G3is coupled to another input of gates G7and G8and to the control input C2of switch SW3, and the output of gate G4is coupled to another input of gates G9and G10and to the control input C1of switch SW3. In addition, the output of gate G7is coupled to the control inputs C1of switch SW1and C2of switch SW2, the output of gate G8is coupled to the control inputs C2of SW1and C1of SW2, the output of gate G9is coupled to the control inputs C1of SW4and C2of SW5, and the output of G10is coupled to the control inputs C2of SW4and C1of SW5.

In this embodiment, the logic gates G3-G10and the switch control SW Q1-Q10make up the optically isolated logic28for the multiplexer26which are powered from a power source which may be around five volts, for example, and a digital ground. Accordingly, the MOSFET combinations Q1-Q10of switches SW1-SW5and their respective photocell drivers are isolated from and floating with respect to the power source and ground of the aforementioned logic circuits. As pointed out above, the maximum current supplied by a photocell is around 50 microamps.

Still referring toFIG. 3, the input sides of Q1and Q3are commonly coupled to EXE (PRI), the output side of Q1is coupled to the excitation lead of S1, and the output side of Q3is coupled to the excitation lead of S2. Similarly, the input sides of Q7and Q9are commonly coupled to EXE (SEC), the output side of Q9is coupled to the excitation lead of SI, and the output side of Q7is coupled to the excitation lead of S2. Note that C3and C4represent the line capacitance of the excitation leads of S1and S2, respectively. Also, the input sides of Q2, Q4, Q8, and Q10are all coupled to ground. The input sides of Q5and Q6are coupled to RESP (PRI) and RESP (SEC), respectively, and the output of SW3is coupled to the response lead(s) of S1and S2. Note that C5represents the line capacitance of the response leads of S1and S2.

In the present embodiment, logic signals A and B select between the primary and secondary or back-up channels, and logic signals C and D control to which sensor the excitation/response signals of the selected channel are coupled. More specifically, as shown inFIG. 4, a pulse at logic signal A will select the primary channel and a pulse at logic signal C will select sensor SI. Referring toFIG. 4, when logic signal A is pulsed low, gate G3is set high represented by the notation “1” and gate G4is set low represented by the notation “0”. With gates G3and G4high and low respectively, all of the switches Q7-Q10of the back-up channel are open circuited and C1and C2of SW3are low and high respectively rendering Q5closed connecting the response lead of S1and S2to the primary circuit21(seeFIG. 2) and Q6open circuited. When logic signal C is pulsed low, gate G5is set high and gate G6is set low. In this state, the inputs to gate G7are both high rendering a low signal at its output and C1of SW1and C2of SW2. Also, the inputs to G8are high and low rendering a high at its output and C2of SW1and C1of SW2. A low C1and high C2of SW1controls Q1closed connecting EXE (PRI) to the excitation lead of S1and Q2open circuited. Likewise, a high C1and low C2of SW2controls Q4closed connecting the excitation lead of S2to ground and Q3open circuited.

With the primary channel selected, a pulse on signal line D will set the output of gates G6and G5high and low respectively as shown inFIG. 5. Referring toFIG. 5, the states of the outputs of G5and G6will not alter the open circuited states of Q7-Q10. However, with G6high and G5low, the outputs of gates G8and G7are rendered low and high respectively, which causes a low signal to be applied to C1of SW2and C2of SW1and a high signal to be applied to C1of SW1and C2of SW2. In this state, Q3is closed connecting EXE (PRI) to the excitation lead of S2, Q4is open circuited, Q2is closed connecting the excitation lead of S1to ground, and Q1is open circuited. Note that the state of SW3is unchanged maintaining the connection of the response leads of S1and S2to the primary channel.

As shown inFIG. 6, pulsing the B logic signal will select the secondary channel and pulsing the C logic signal will connect the excitation/response signals to the sensor S1. More specifically, pulsing the B logic signal will set G4high and G3low rendering C1and C2of SW3, high and low, respectively, causing Q6to be closed connecting the response leads to the secondary channel and Q5to be open circuited. Pulsing the B signal will also render the outputs of gates G7and G8high causing Q1-Q4to be open circuited. Moreover, pulsing the C logic signal will cause the outputs of gates G9and G10to be high and low respectively. In this state, Q9is rendered closed connecting EXE (SEC) to the excitation lead of S1, Q7and Q10are rendered open circuited, and Q8is rendered closed connecting the excitation lead of S2to ground.

By pulsing logic signal D while the secondary channel is selected will connect the signals of the secondary channel to sensor S2as shown inFIG. 7. Referring toFIG. 7, pulsing D sets G6high and G5low which renders the outputs of G9and G10to low and high respectively. Note, however, that the states of Q1-Q6remain unchanged. With G9low, Q7and Q10are closed connecting EXE (SEC) to the excitation lead of S2and the excitation lead of S1to ground, and with G10high, Q8and Q9are rendered open circuited. In this manner, logic signals A and B can select between a primary and back-up or secondary channel of excitation and response signals, thus in the event of a failure or malfunction in one channel, the other channel may be selected. Also demonstrated by the foregoing description is the selection of one of the plurality of sensors for connection to the selected set of excitation and response signals by the logic signals C and D.

The optically isolated logic and switch design of the present embodiment preserves the intrinsically safe nature of the excitation and response signals with a minimal of additional circuitry. A failure in any multiplexer switch of the multiplexer system resulting in a direct connection into the sensor lines which penetrate the explosive vapor containment will not compromise the intrinsically safe design since the control of each switch is performed by an array of photocells of an optical isolator which isolates the switch from and renders the switch floating with respect to the power source of the control logic thereof. The photovoltaic output of the isolators provide sufficient voltage to control the “on” and “off” states of the associated switches, but generate only approximately fifty microamps of current.

In addition, measurement errors caused by in series switch resistance of conventional multiplexer switches and line or stray capacitance as noted above are substantially reduced to within acceptable limits by the present embodiment. For example, the “on” resistance of a multiplexer switch of the present design is under 0.1 ohms and may be further reduced with judicious selection of the MOSFETs. Also, the present multiplexer design does not suffer from the problem in which series switch “on” resistance can interfere with the stability design of the drive amplifier of the excitation signal since the series “on” resistance of each multiplexer switch is commensurate with the resistance of the excitation line itself, thus having a negligible effect on the reactive load being driven by the amplifier.

While the present invention has been described herein above in connection with one or more embodiments, it is understood that such embodiments were presented by way of example with no intention of limiting the invention in any way. Accordingly, the present invention should not be limited by any of the described embodiments, but rather construed in breadth and broad scope in accordance with the recitation of the claims appended hereto.