Compressed air system and method of control

#This invention relates generally to compressed air systems, and more particularly to a compressed air system for a locomotive.

#Compressed air systems are used to provide energy for driving a variety of devices in a variety of applications. One such application is a railroad locomotive where compressed air is used to power locomotive air brakes and pneumatic control systems.

#A typical compressed air system will include a reservoir for storing a volume of compressed air. A motor-driven compressor is used to maintain the air pressure in the reservoir within a desired range of pressures. The reservoir pressure may be higher than the demand pressure for a device supplied by the system, in which case a pressure regulator may be used to reduce the pressure supplied to the device. The stored volume of compressed air in the reservoir provides an inertia that allows the compressor to be sized smaller than would otherwise be necessary if the compressor supplied the individual devices directly. Furthermore, the stored volume of compressed air in the reservoir allows the compressor to be cycled on and off less frequently than would otherwise be necessary in a direct-supply system. This is important because the electrical and mechanical transients that are generated during a motor/compressor start-up event may severely challenge the compressor motor and associated electrical contacts.

#FIG. 1 is a schematic diagram of a compressed air system.

#FIG. 2 illustrates the steps embodied in logic in the controller of the compressed air system of FIG. 1.

#FIG. 3 illustrates pressure verses time for two different operating conditions in the compressed air system of FIG. 1.

#An improved compressed air system 10 as may be used on a locomotive or other application is illustrated in FIG. 1. The system includes a compressor 12 that is driven by an electrical motor 14 to provide a flow of compressed air to a reservoir or storage tank 16. A power supply may be coupled through a relay 18 or other such electrical switching device to energize the motor 14. The relay 18 is selectively positioned to energize or to de-energize the motor 14 in response to a motor control signal generated by a controller 20. The flow of compressed air is directed to the reservoir 16 when a bypass valve 22 in the compressed air supply line is closed, i.e. in a compressor loaded position or mode. The flow of compressed air is vented to atmosphere when the bypass valve 22 is open, i.e. in a compressor unloaded position or mode. A check valve 24 prevents compressed air in the tank 16 from escaping through the compressed air supply line. The controller 20 provides a control signal to the bypass valve 22 to command the desired bypass valve position.

#The compressed air system of FIG. 1 further includes a pressure transducer 26 for providing a pressure signal responsive to the air pressure in the reservoir 16. The pressure signal is provided as an input to the controller 20, and that signal is used in combination with a time parameter measured by a timer 28 to determine a parameter related to pressure in the reservoir, as will be discussed more fully below.

#FIG. 2 illustrates exemplary steps in a method 50 that may be implemented by logic executed in the controller 20 (FIG. 1) in a control module 51 to reduce the duty cycles experienced by the compressor motor. Such logic may be stored in a memory device and/or embodied in software or firmware, and the controller may be a personal computer, a digital or analog processor, or other such device known in the art. The method may begin with a decision step 52 wherein the pressure in the reservoir (P), as measured by the pressure transducer 26 (FIG. 1), is compared to a predetermined lower specification limit (LSL) set point. If the actual pressure has dropped below the lower set point, the controller 20 will produce an appropriate motor-on signal to position the relay 18 to energize the motor at step 54. At this point the bypass valve 22 (FIG. 1) is open and the motor 14 starts the compressor 12 in an unloaded mode. A predetermined time later, such as approximately 2 seconds later once the compressor has come up to speed, the controller 20 will produce a valve-close signal at step 56 to position the bypass valve to load the compressor. The compressor will deliver a flow of compressed air to the reservoir until, as determined at decision point 58, the pressure P in the reservoir exceeds an upper specification limit (USL) set point, at which time the bypass valve will be signaled to open to place the compressor in the unloaded mode and a timer function will be set to T=0, as indicated at step 60. It is known to run the compressor in the unloaded mode for a predetermined cool down period, typically 30 seconds, following its operation in the loaded mode in order to cool the compressor head and motor relay contacts. A method embodying aspects of the present invention will allow the compressor to run in the unloaded mode for a longer period of time when a measured parameter indicates a likelihood that the flow of compressed air from the compressor will again be required within a selected time period.