BI-DIRECTIONAL CURRENT SOURCE

Abstract: A bi-directional current circuit disposed between two power souces and a load. A first switch and a second switch each connect th load to one of the power sources respectively, whereby when the first switch is closed and the second switch is open current flows to the load in a first direction and when the first switch is open and the second switch is closed current flows to the load in a second direction opposite to the first direction.

Description: BI-DIRECTIONAL CURRENT SOURCE TECHNICAL FIELD This invention relates to bi-directional current circuits. BACKGROUND OF THE INVENTION Airplanes often have an on-board gas turbine engine referred to as an auxiliary power unit that provides electrical power and compressed air to various systems throughout the airplane. When the airplane is on the ground, the auxiliary power unit is the main source of power to drive the environmental control systems, air driven hydraulic pumps, and the starters for the engines. Auxiliary power units may also provide pneumatic and electric power during flight. FIG. 1 shows a typical Auxiliary Power unit (APU) generally denoted by the reference numeral 10. The APU 10 includes in flow series arrangement an air inlet 14, a compressor 16, a bleed port 18 for providing compressed air to the aircraft, a combustor 20 having a primary fuel nozzle 22 and a secondary fuel nozzle 24, a turbine 26 and an gas exhaust 28. Of the two nozzles 22 and 24, only the primary nozzle 22 operates during the initial stages of a startup. The compressor 16 and the turbine 26 are mounted for rotation on a shaft 30 which extends to a gearbox 32. Mounted to the gearbox 32 is a fuel control unit 40 in fluid communication with a fuel source, (not shown), aboard the aircraft. Preferably, the fuel control unit 40 is a single stage, electromechanical fuel metering valve of the type which is well known in the art. The fuel control unit 40 includes an electrically operated torque motor 42 which has a known and repeatable relationship with a power signal from an electronic control unit (ECU) 80 which may be analog or digital. The motor 42 is directly coupled to a metering valve, not shown, and controls the valve position so that a known flow area in the metering valve corresponds to a known power signal from the ECU 80. The ECU 80 communicates with the torque motor 42 through a bi-directional circuit such as an H-bridge and commands the motor 42 to open the valve, when an increase in fuel flow is needed and to close the valve when a decrease in fuel is required. The ECU 80 also controls the speed at which the torque motor opens and closes the valve. From the fuel control unit 40 metered fuel flows through a conduit 46 to a flow divider 50. From the flow divider 50 the stream of fuel splits with a portion flowing through a conduit 52 which leads to the primary fuel nozzle 22 and the remainder through a conduit 54 to a secondary fuel nozzle 24. One type of bi-directional circuit used between the ECU 80 and the torque motor 42 is the conventional H-bridge. This configuration typically uses one power supply and four power switches (i.e. power transistors) to produce the desired current flow through a load. The load is the device to which power is delivered, (i.e. the torque motor 42). It can be resistive or inductive. The advantage of the H-bridge configuration is that it uses a single power source (i.e. the ECU 80). Referring to FIG. 2, the current to the load is controlled by means of the four power switches. During one half cycle, current flows from switch A through the load to switch D. During the other half cycle, current flows from switch B through the load to switch C. There are, however, several disadvantages to using an H-bridge. The H-bridge requires four power switching devices for each load. Since power switching devices are usually the most expensive components of a driver, these types of switches drive up the cost of the ECU 80 in which they are mounted. There is also a size limitation. With four power switching devices, the power driver might grow beyond the size limitations of the overall design. Another disadvantage is that during any half cycle power is always being dissipated in two power switching devices, either A and D or B and C. Accordingly, there is a need for bi-directional current circuit that has fewer switches and thus takes up less board area, costs less, dissipates less power and has fewer parts than an H-bridge. SUMMARY OF THE INVENTION An object of the present invention is to provide an improved bidirectional current circuit. The present invention meets this objective by providing a bi- directional current circuit disposed between two power sources and a load. First and second switches are arranged in series with the load and connect the load to one of the power sources, whereby when said first switch is closed and said second switch is open current flows to the load in a first direction and when said first switch is open and said second switch is closed current flows to the load in a second direction opposite to the first direction. Thus a bi-directional current circuit or source is provided that has fewer switches, takes up less board area, dissipates less power and costs less to manufacture than an H-bridge circuit. These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic of a gas turbine engine referred to as an Auxiliary Power Unit. FIG. 2 is a prior art H-bridge amplifier circuit. FIG. 3 is a circuit diagram of the bi-directional current source contemplated by the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 3, a bi-directional current circuit is generally denoted by reference numeral 100. The circuit 100 includes a transformer 102 with a primary winding or input side 104 and secondary windings or output sides 106, 107. The primary winding 104 receives DC power from the ECU 80 and the secondary windings 106, 107 deliver DC power to the load 108 which may be the torque motor 42 or some other comparable device. Importantly, the transformer 102 converts a single power source into two power sources. It should be appreciated that this splitting of power can be accomplished by other electrical elements. The load 108 is grounded at 109. The direction of the current through the circuit 100 is controlled by switches 110 and 112 which act in a response to a digital command represented by arrow 114 from the ECU 80. A switch 118 couples a pulse width modulator controller 116 to the primary winding 104 of the transformer 102. The controller 116 receives a command 120 from the ECU 80 indicative of how much current should flow. A rectifier 122 is disposed between the switches 110, 112 and the controller 116. The rectifier 122, in a manner familiar to those skilled in the art, enables to circuit 100 to accommodate both negative and positive feedback. A resistor 134 is coupled to both the controller 116 and the switch 118 and causes the switch 118 to open in the event of an over current condition. The switches 110 and 112 are each in series with the load 108. When switch 110 is closed and switch 112 is open, current flows through the secondary winding 106 and to the load 108 in a first direction represented by an arrow 124. In the case of the torque motor 42, the current will cause the motor to rotate in a first direction which results in the metering valve opening. When switch 112 is closed and switch 110 is opened, current flows through the secondary winding 107 and to the load 108 in a second direction represented by arrow 126, opposite from the first direction. This will cause the torque motor 42 to rotate in the opposite direction and the metering valve will close. There is no preferred correlation with the direction of the current and the opening and closing of the valve and the designer is free to arrange either way. The command signal 120 to the controller 116 can be a constant voltage which means constant current to the load 108. Preferably, the command signal is variable and the position of the torque motor will vary with the corresponding variable current. A rectifying diode 128 is disposed between switch 110 and the load 108 and a rectifying diode 130 between switch 112 and the load 108. The switches 110, 112 and 118 are conventional high speed electronic switches. Thus, a bi-directional current circuit or source is provided that has fewer switches and other components when compared to an H-bridge circuit. As a result it takes up less board area, costs less to manufacture, and is more efficient. It will be appreciated by those skilled in the art that various changes and modifications may be made to the illustrative embodiment without departing from the spirit or scope of the invention. For example, throughout the description reference as been made to a torque motor used in the fuel control of a gas turbine engines. This was done to provide an exemplary illustration of the invention, not to limit its applicability. The present invention can in fact be used in any application requiring bi-directional current. Therefore, it is intended that the scope of the invention not be limited in any way to the illustrative embodiment shown and described but that the invention be limited only by the claims appended hereto.

Claims: WHAT IS CLAIMED IS: 1. A bi-directional current circuit (100) disposed between a first and second power sources (106,107) and a load (108) characterized by: a first switch (110) connecting said first power source (106) to said load (108); and a second switch (112) connecting said second power source (107) to said load (108), said first and second switches (110,112) arranged in series with said load (108) so that when said first switch (110) is closed and said second switch (112) is open current flows to said load (108) in a first direction (124) and when said first switch (110) is open and said second switch (112) is closed current flows to said load (108) in a second direction (126) opposite to said first direction (124). 2. The circuit (100) of claim 1 wherein said first and second power sources (106, 107) are first and second output windings of a transformer (102). 3. The circuit (100) of claim 2 further comprising a rectifier (122) disposed between said first and second switches (110, 112) and an input winding (104) of said transformer (102). 4. The circuit (100) of claim 3 further comprising a pulse width modulator (116) disposed between said rectifier (122) and said input winding (104) of said transformer (102). 5. The circuit (100) of claim 4 further comprising a third switch (118) disposed between said pulse width modulator (116) and said input winding (104) of said transformer (102). 6. The circuit (100) of claim 5 further comprising a first diode (128) disposed between said first switch (110) and said load (108). 7. The circuit (100) of claim 6 further comprising a second diode (130) disposed between said second switch (112) and said load (108). 8. The circuit (100) of claim 7 further comprising a first resistor (132) between said first and second switches (110,112) and ground (109). 9. The circuit (100) of claim 8 further comprising a second resistor (134) disposed between said third switch (118) and ground.

Inventors: ZAPPACOSTA ERNEST E II, ZAPPACOSTA, ERNEST, E., II
Applicants: HONEYWELL INT INC, HONEYWELL INTERNATIONAL INC.
Application year: 2001
Main classification: H03K17
IPC codes: H02M3, H02M7, H03K17