Patent Publication Number: US-9410554-B2

Title: Controlling a gas compressor having multiple magnetic bearings

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The application claims the benefit of U.S. provisional patent application Ser. No. 61/975,466, filed Apr. 4, 2014, which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally pertains to centrifugal gas compressors, and is more particularly directed toward a control system for magnetic bearings within an integrated motor driven centrifugal gas compressor. 
     BACKGROUND 
     Magnetic bearings are bearings that use electromagnetic forces to support a load. Magnetic bearings may support moving machinery without physical contact. For example, they can levitate a rotating shaft, providing for rotation with very low friction and no mechanical wear. Active magnetic bearings use electromagnetic suspension, and may include an electromagnet assembly, power amplifiers configured to drive the electromagnets, a controller, and sensors (e.g., gap sensors) with associated electronics. The power amplifiers drive electromagnets on opposing sides of the shaft. The sensors provide feedback to control the position of the rotor within the gap. The controller offsets the current to drive the electromagnets as the rotor deviates from its desired position. 
     U.S. Pat. No. 5,578,880 issued to Lyons et al. on Nov. 26, 1996 discloses a fault tolerant active magnetic bearing system that comprises a magnetic bearing having a rotor mounted for rotation within a stator and for coupling to a shaft. An electric power distribution system is energized from a multi-phase switched reluctance machine supplying three independent DC power buses. Each of the power buses is coupled for supplying power to a respective pair of diametrically opposite electromagnets of the magnetic bearing so as to establish multiple magnetic control axes. Multiple power controllers are each operatively connected in circuit with a separate respective power bus. The power controllers include independent power control systems each coupled to a respective pair of diametrically opposite electromagnets for independently controlling energization of each one of the pair of diametrically opposite electromagnets. 
     The present disclosure is directed toward overcoming one or more problems discovered by the inventors or that is known in the art. 
     SUMMARY OF THE DISCLOSURE 
     A method for controlling a gas compressor is disclosed herein. In one embodiment, the method includes communicating feedback data about two magnetic bearings to a computer including a multi-core processor via a communication link. The method also includes processing the feedback data about the two magnetic bearings where the feedback data for each of the two magnetic bearings is processed by different bearing control modules on separate cores of the multi-core processor in parallel and issuing a bearing control command to each of the two magnetic bearings in response to the feedback data. The method further includes communicating the bearing control commands to the two magnetic bearings from the computer via a communication link. 
     A control system for a centrifugal gas compressor is also disclosed herein, the centrifugal gas compressor including a compressor driver and a magnetic bearing system including a first magnetic bearing and a second magnetic bearing. In one embodiment, the control system includes a bearing input/output terminal, and a computer. The bearing input/output terminal includes an input/output device. The input/output device is configured to receive signals from a first sensor of the first magnetic bearing and a second sensor of the second magnetic bearing, and to transmit control commands to a first magnetic bearing driver of the first magnetic bearing and a second magnetic bearing driver of the second magnetic bearing. 
     The computer includes a multi-core processor, a first bearing control module, and a second bearing control module. The multi-core processor includes a first core and a second core. The first bearing control module is configured to process a first feedback signal from the first sensor on the first core and issue a first bearing control command to the first magnetic bearing driver. The second bearing control module is configured to process a second feedback signal from the second sensor on the second core and issue a second bearing control command to the second magnetic bearing driver in parallel to the first bearing control module. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cutaway illustration of an exemplary centrifugal gas compressor. 
         FIG. 2  is a cross-sectional view of an alternate embodiment of a centrifugal gas compressor within an integrated machine. 
         FIG. 3  is a cross-sectional view of an alternate embodiment of a centrifugal gas compressor within an integrated machine. 
         FIG. 4  is a block diagram of an exemplary system for controlling magnetic bearings in the centrifugal gas compressor of  FIG. 1 . 
         FIG. 5  is a functional block diagram of an exemplary system for controlling the centrifugal gas compressor of  FIG. 1 . 
         FIG. 6  is a schematic illustration of an embodiment of the driver sensing system of  FIG. 2 . 
         FIG. 7  is a flow chart of an exemplary method for controlling magnetic bearings in the centrifugal gas compressor of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to the control of a gas compressor having a magnetic bearing system including multiple magnetic bearings. In particular, the present disclosure relates to a control system and method of control where a computer, such as an industrial personal computer (PC), including a multi-core processor is configured to control the operation of two or more magnetic bearings in parallel operations with the multi-core processor. The multi-core processor may also be configured to control other systems of the gas compressor in parallel operations. In embodiments, a first core of the processor is configured to perform the calculations related to a first magnetic bearing, a second core of the processor is configured to perform the calculations related to a second magnetic bearing, and a third core of the processor is configured to control other systems of the gas compressor. Using separate cores for the calculations related to the first and second magnetic bearings may allow these calculations to be performed in parallel and may reduce delay in the system, which may provide for a more accurate and responsive control of the magnetic bearing system. 
       FIG. 1  is a cutaway illustration of an exemplary centrifugal gas compressor  700 . Some of the surfaces have been left out or exaggerated (here and in other figures) for clarity and ease of explanation. In addition the centrifugal gas compressor  700  is shown in isolation from its driver and flow path. 
     This disclosure may generally reference a center axis  95  of rotation of the centrifugal gas compressor, which may be generally defined by the longitudinal axis of its compressor shaft  720 . The center axis  95  may be common to or shared with various other concentric components of the centrifugal gas compressor. All references to radial, axial, and circumferential directions and measures refer to center axis  95 , unless specified otherwise, and terms such as “inner” and “outer” generally indicate a lesser or greater radial distance from the center axis  95 , wherein a radial  96  may be in any direction perpendicular and radiating outward from center axis  95 . 
     In addition, this disclosure may reference a forward and an aft direction. Generally, all references to “forward” and “aft” are associated with the flow direction, relative to the center axis  95 , of the compressed gas. In particular, the suction end  97  of the centrifugal gas compressor is referred to as the forward end or direction, and the discharge end  98  is referred to as the aft end or direction, unless specified otherwise. 
     The centrifugal gas compressor  700  includes a compressor housing  710 , a suction port  711 , discharge port  712 , a compressor shaft  720 , a compressor bearing system  730 , an inlet  740 , a rotor  750 , a diffuser  760 , and a collector  770 . The rotor  750  may include one or more centrifugal impellers  751 . The compressor shaft  720  may also include a suction end and a discharge end associated with the suction end  97  and the discharge end  98  of the centrifugal gas compressor  700 . The compressor shaft  720  may be a single shaft or dual shaft configuration. In a dual shaft configuration, compressor shaft  720  may include a suction end stubshaft and a discharge end stubshaft. 
     The compressor shaft  720  and attached elements are supported by the compressor bearing system  730 . In the embodiment illustrated, the compressor bearing system  730  includes three magnetic bearings, a suction end radial bearing  731 , a discharge end radial bearing  732 , and a thrust bearing  733 . Suction end radial bearing  731  and discharge end radial bearing  732  are radial magnetic bearings and support axial ends of the compressor shaft  720 . The thrust bearing  733  is an axial magnetic bearing and counteracts axial forces applied to the compressor shaft  720 . In other embodiments, the compressor bearing system  730  includes more radial/axial magnetic bearings. 
     The radial magnetic bearings, such as suction end radial bearing  731  and discharge end radial bearing  732 , are configured to magnetically levitate the compressor shaft  720  and the thrust bearing  733  is configured to maintain a thrust collar  721  within a gap in the thrust bearing  733 . The compressor bearing system  730  is configured to operate with very low friction and little to no mechanical wear. Additionally, the compressor bearing system  730  may also include auxiliary or backup bearings. 
     During normal operation, the process gas  15  enters the centrifugal gas compressor  700  at the suction port  711  and is routed to the inlet  740 . The process gas  15  is compressed by one or more centrifugal impellers  751  mounted to the compressor shaft  720 , diffused by one or more diffusers  760 , and collected by the collector  770 . The compressed process gas  15  exits the centrifugal gas compressor  700  at a discharge port  712 . 
     According to one embodiment, the process gas  15  may be controlled at or proximate the centrifugal gas compressor  700 . In particular, one or more flow control devices may be integrated into the centrifugal gas compressor  700  as part of a compressor monitoring system. In addition, one or more flow control devices may be part of a process control system separate from the centrifugal gas compressor  700 . 
     Moreover, the process gas  15  may be controlled and/or metered coming into or leaving the centrifugal gas compressor  700 . This may include controlling gas flow, gas pressure, gas temperature, inlet pressure, outlet pressure, etc. For example, the centrifugal gas compressor  700  may be controlled with one or more valves (e.g., yard valves), or other flow metering devices, located proximate the suction port  711  and/or the discharge port  712 . Also for example, the centrifugal gas compressor  700  may be controlled using one or more pressure regulators configured to regulate pressure of the process gas  15 . Also for example, the centrifugal gas compressor  700  may be controlled with one or more temperature regulators (e.g., heat exchangers) configured to regulate the temperature of the process gas  15 . 
       FIG. 2  is a cross-sectional view of an alternate embodiment of a centrifugal gas compressor  700  within an integrated machine  100 . The integrated machine  100  includes the centrifugal gas compressor  700  and a compressor driver  600  within a single housing  110 . The housing  110  may include a first end adjacent the compressor driver  600  and a second end adjacent the centrifugal gas compressor  700 . 
     In the embodiment illustrated, the compressor driver  600  is an electric motor and includes a motor can  611 , motor windings  612 , motor laminations  613 , and driver shaft  620 . Motor can  611  may be cylindrically shaped and may be contained within housing  110 . Motor windings  612  may be wound about driver shaft  620  at each end of motor can  611  and may extend through motor laminations  613 . Motor laminations  613  may be centrally located within motor can  611  and may be located axially between the end windings of motor windings  612 . Driver shaft  620  may extend through motor can  611 . 
     The centrifugal gas compressor  700  within the integrated machine  100  also includes a compressor shaft  720 , an inlet  740 , an collector  770 , a rotor  750  including centrifugal impellers  751 , and diffusers  760 , which may be the same or similar as those described in conjunction with  FIG. 1 . 
     In the embodiment illustrated, the compressor driver  600  is supported by a driver bearing system  630  and the centrifugal gas compressor is supported by a compressor bearing system  730 ; the driver bearing system  630  is distal to the centrifugal gas compressor, adjacent first end, and the compressor bearing system  730  is distal to the compressor driver  600 , adjacent the second end. 
     In the embodiment illustrated, driver shaft  620  and compressor shaft  720  are joined by a tierod  724  and may not need a coupling. Driver shaft  620  and compressor shaft  720  may also be joined/bolted together by bolts  722 , or by other coupling means. 
       FIG. 3  is a cross-sectional view of an alternate embodiment of a centrifugal gas compressor  700  within an integrated machine  100 . In the embodiment illustrated in  FIG. 3 , housing  110  includes a driver housing  114  and a compressor housing  112  coupled together to form housing  110 . The driver shaft  620  extends at least partially through the driver housing  114  and is joined to compressor shaft  720 , such as by a tierod. 
     In some embodiments, as illustrated in  FIG. 3  driver bearing system  630  is a combination bearing including a driver magnetic bearing  631  and a second driver magnetic bearing  632  within a single bearing housing  633 . In the embodiment illustrated, the driver magnetic bearing  631  is a radial bearing and the second driver magnetic bearing  632  is a thrust bearing. In the embodiment illustrated, compressor bearing system  730  is a single radial magnetic bearing. Driver bearing system  630  may be located adjacent the compressor driver  600  and distal to the centrifugal gas compressor  700 , and compressor bearing system  730  may be located adjacent the centrifugal gas compressor  700  and distal to compressor driver  600 . 
     The integrated machine  100  may also include a central bearing system  690  located between the compressor driver  600  and the centrifugal gas compressor  700 . In the embodiment illustrated, central bearing system  690  is a single radial magnetic bearing. 
     Any of the bearing systems and any combination of the bearing systems within the integrated machine  100  including driver bearing system  630 , compressor bearing system  730 , and central bearing system  690  may be a combination bearing and may include a radial magnetic bearing and a thrust bearing within a single bearing housing  633 . 
       FIG. 4  is a block diagram of an exemplary system for controlling magnetic bearings in the centrifugal gas compressor  700  of  FIG. 1 . In particular, the control system  800  is shown along with the centrifugal gas compressor  700  and with a compressor driver  600 . The control system  800  is configured for magnetic bearing control, but, as discussed below, may be configured for additional control functions. For clarity, single elements may be represented where multiple elements may be, and are used. 
     Regarding the centrifugal gas compressor  700 , magnetic bearings in the centrifugal gas compressor  700 , such as suction end radial bearing  731 , discharge end radial bearing  732 , and thrust bearing  733 , may each include an electromagnet assembly  737 , a magnetic bearing driver (e.g., a set of power amplifiers  738  configured to supply current to the electromagnets), and one or more sensors  739  with associated electronics to provide the feedback required to control the position of the levitated member (e.g., the compressor shaft  720  and/or the thrust collar  721 ) within the gap. One or more of the electromagnet assembly  737 , the power amplifier  738 , and the sensor  739  may be combined into a single device or shared with another device. 
     Regarding the compressor driver  600 , the compressor driver  600  may be any device configured to drive the centrifugal gas compressor  700 . In particular, the compressor driver  600  may be mechanically joined/coupled to the compressor shaft  720  of centrifugal gas compressor  700 , and configured to transmit a driving torque. For example, the compressor driver  600  may be an electric motor, a gas turbine engine, a reciprocating engine, etc. 
     Moreover, the compressor driver  600  and the centrifugal gas compressor  700  may have any convenient configuration. For example, the compressor driver  600  and the centrifugal gas compressor  700  may have individual housings, a common housing as illustrated in  FIG. 2 , or a joined or partially shared housing as illustrated in  FIG. 3 . Similarly, the compressor driver  600  and the centrifugal gas compressor  700  may have separate joined drive shafts, a single or common shaft, or a combination thereof. Moreover, the compressor driver  600  and the centrifugal gas compressor  700  may have no shaft or only a partial shaft. For example, the one or more centrifugal impellers  751  ( FIGS. 1 and 2 ) may be stacked together such that no shaft is needed there between. In some embodiments, the compressor driver  600  is integral to the centrifugal gas compressor  700  and is located between the suction end radial bearing  731  and the discharge end radial bearing  732 . 
     As illustrated, the compressor driver  600  may include a driver motor  610 , a driver shaft  620 , a driver bearing system  630 , a power output coupling  640 , and a driver sensing system  650 . Here, the driver motor  610  is embodied as an electric motor configured to apply torque to the driver shaft  620 . The driver shaft  620  is mechanically coupled to the compressor shaft  720  of the centrifugal gas compressor  700  via the power output coupling  640 . The driver shaft  620  may be entirely supported by the driver bearing system  630 . Alternately, and as illustrated, the driver shaft  620  may be partially supported by the driver bearing system  630 . In this configuration, the driver shaft  620  may then also be supported by the compressor bearing system  730  of the centrifugal gas compressor  700  via the power output coupling  640 . 
     According to one embodiment, the driver bearing system  630  may include one or more driver magnetic bearings  631 . The one or more driver magnetic bearings  631  are configured to levitate the driver shaft  620  and/or a thrust collar within a gap there between. Likewise, the driver magnetic bearings  631  may each include an electromagnet assembly  637 , a magnetic bearing driver (e.g., a set of power amplifiers  638  configured to supply current to the electromagnets), and one or more sensors  639 , one or more of which may be combined into a single device or shared with another device. Additionally, the driver bearing system  630  may also include auxiliary or backup bearings. 
     According to one embodiment, the magnetic bearings in the compressor driver  600  and the centrifugal gas compressor  700  may be controlled together. In particular, the control system  800  may be communicably coupled and configured to control at least two magnetic bearings of compressor bearing system  730 , the driver bearing system  630 , or any combination thereof. Moreover, the control system  800  may be configured to control both the driver bearing system  630  and the compressor bearing system  730  as a single magnetic bearing system. For example, the control system  800  may be configured to receive feedback from the sensors  639 ,  739  in both the driver bearing system  630  and the compressor bearing system  730 , respectively. The control system  800  may be further configured to process the feedback, and then issue control commands to the power amplifiers  638 ,  738 , in both the driver bearing system  630  and the compressor bearing system  730 , respectively. In some embodiments, the control system  800  may also be configured to control other bearing systems, such as the central bearing system  690  illustrated in  FIG. 3 , and may be configured to control all of the bearing systems as a single magnetic bearing system. One or more of these bearing systems may be a combination bearing, such as the driver bearing system  630  illustrated in  FIG. 3 . 
     The control system  800  may include a computer  810 , a communication link  830 , and a bearing input/output (“I/O”) terminal  840 . In particular, the computer  810  is communicably coupled to the bearing I/O terminal  840  via the communication link  830 . The bearing I/O terminal  840  is then communicably coupled to each magnetic bearing system to be controlled. In addition, the control system  800  may be dedicated to control of the magnetic bearing systems, or may also control other components and systems, as discussed herein. 
     The computer  810  may be any computer having real time control capability. In particular, the computer can include a multi-core processor  870 , a memory  812 , a communication device  813 , a power supply  814 , a user output  815  (e.g., a display), and a user input  816  (e.g., a keyboard). According to one embodiment, the computer  810  may be an industrial PC. For example, the computer  810  may be rack mountable (e.g., 19-inch (48.26 cm) or 23-inch (58.42 cm)) and in conformance with one or more industrial PC standards (e.g., EIA/ECA-310-E). Also for example, the computer  810  may be a ruggedized INTEL processor-based industrial PC. In addition, the computer  810  may be configured as a front-end to another control computer in a distributed processing environment. In addition, the computer  810  may be dedicated for control of the compressor bearing system  730  and/or the driver bearing system  630  (“the magnetic bearing system”), or shared with one or more additional control functions. 
     The multi-core processor  870  is a single computing component with at least two cores, a core being an independent central processing unit (CPU) configured to read and execute program instructions. In the embodiment illustrated, multi-core processor  870  includes four cores, a first core  871 , a second core  872 , a third core  873 , and a fourth core  874 . Other amounts of cores within multi-core processor  870 , such as two, six, and eight cores, may also be used. 
     The multi-core processor  870  may include a general purpose multi-core processor or any multi-core processor capable of receiving data from the sensors, determining whether and what adjustment should be made to at least two magnetic bearings, and communicating any desired commands. A general-purpose multi-core processor can be a microprocessor, but in the alternative, the multi-core processor can be any processor, controller, microprocessor, or microcontroller with multiple cores. In embodiments, a combination of processors with at least one multi-core processor may also be used, where the multi-core processor is used to control at least two magnetic bearings. 
     The multi-core processor  870  is configured to control two or more magnetic bearings such that at least one core performs the calculations related to a first magnetic bearing, such as the suction end radial bearing  731 , and another core performs the calculations related to a second magnetic bearing, such as the discharge end radial bearing  732 . The multi-core processor  870  may also be configured to receive data from the two or more magnetic bearings or sensors. In particular, the multi-core processor  870  may be communicably coupled to the sensor(s)  639 ,  739  of the two or more magnetic bearings via the communication link  830 . Likewise, the multi-core processor  870  may be configured to issue commands to the two or more of the magnetic bearings or their components. In particular, the multi-core processor  870  may be communicably coupled to the power amplifier(s)  638 ,  738  of the two or more magnetic bearings via the communication link  830 . 
     The memory  812  may include RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, video tape and/or any other form of machine or computer readable storage medium. According to one embodiment, the memory  812  may have a volatile memory storage capacity greater than 2 GB. 
     The multi-core processor  870  and the memory  812  are configured to work together to implement the functionality of the control system  800 . In particular, the memory  812  can be coupled to the multi-core processor  870  such that the multi-core processor  870  can read information from, and write information to the storage medium. According to one embodiment, memory  812  is configured to record instructions for one or more modules of the control system  800 . 
     The communication device  813  may include any piece of equipment, hardware, or software configured to move data to and from the computer  810 . In particular, the communication device  813  is configured to transmit control commands from the multi-core processor  870  to the bearing I/O terminal  840  via the communication link  830 . Also, the communication device  813  is configured to receive digital feedback signals from the bearing I/O terminal  840  via the communication link  830 . 
     According to one embodiment, the communication device  813  may be configured for data packet communications across a communication network. In particular, the communication device  813  may be configured to communicate control commands and feedback data in accordance with a standardized fieldbus communication protocol. For example, the communication device  813  may be configured to communicate data across an Ethernet based communication network using standard IEEE 802.3 Ethernet frames. Also for example, the communication device  813  may be configured to communicate EtherCAT (Ethernet for Control Automation Technology) communications with the bearing I/O terminal  840 . Furthermore, the communication device  813  and any associated hardware or software may be configured to operate as an EtherCAT master controller. 
     The communication device  813  may be embodied as a dedicated device, such as a network interface card, or may have shared or distributed functionality with other components of the computer  810 . The communication device  813  may be configured for wired, wireless, and/or optical communications. Furthermore, the communication device  813  may be configured for full-duplex and/or half-duplex communications across one or more communication links  830 . 
     The power supply  814  may include any hardware configured to supply power to the computer. In particular, the power supply  814  is configured to provide uninterrupted power during bearing operation. According to one embodiment, the power supply  814  may be configured to receive power from an uninterrupted power source (e.g., facility power) shared with one or more of the compressor driver  600 , the centrifugal gas compressor  700 , the electromagnet assemblies  637 ,  737 , etc. 
     The communication link  830  may be any convenient link, including a wired, wireless, and/or optical link. The communication link  830  is configured to support digital communications between the computer  810  and bearing I/O terminal  840 . For example, the communication link  830  may be use twisted-pair cables for the physical layer of an Ethernet computer network, or any other Ethernet compliant cable. 
     In addition, the communication link  830  may provide for the computer  810  to be located at a remote location as opposed to a DSP controller proximate or collocated with magnetic bearings. In particular, the communication link  830  may extend ten or more feet (&gt;3 meters) between the bearing I/O terminal  840  and the computer  810 . For example, the computer  810  may be located at user-friendly location, such as in a control room, while the communication link  830  extends back to the bearing I/O terminal  840 . The bearing I/O terminal being in much closer proximity to the centrifugal gas compressor  700 . This may be beneficial in that operators may have greater access to the controller in general and/or may access the controller without being exposed to the working machinery. In addition, greater resources may be available in the remote location, such as processors, communication networks, climate control, etc. 
     The bearing I/O terminal  840  may include a terminal housing  841 , an I/O device  842 , and a communication device  843 . The terminal housing  841  may enclose the I/O device  842  and the communication device  843 , which may be coupled to each other therein. In addition, the I/O device  842  and the communication device  843  may be embodied as two units, as a single unit, or have a distributed and/or shared architecture. According to one embodiment, the bearing I/O terminal  840  may be configured to receive power from or be powered by an uninterrupted power source. Moreover, the uninterrupted power source may be common or shared with the computer  810 . 
     The bearing I/O terminal  840  may be fixed to, within or located proximate the centrifugal gas compressor  700  (such as in a control cabinet of the centrifugal gas compressor  700 ). Where the terminal housing  841  is located in or on the centrifugal gas compressor  700 , it may be sealed or otherwise include additional environmental protections. 
     In general, the bearing I/O terminal  840  is configured as a communication conduit between the computer  810  and the magnetic bearings. In particular, the bearing I/O terminal  840  may be communicably coupled to components/systems of the magnetic bearings via the I/O device  842 . For example the I/O device  842  may be wired to the electromagnet assemblies  637 ,  737 , the power amplifiers  638 ,  738 , and the sensors  639 ,  739 . 
     The I/O device  842  may be configured to receive signals from sensor(s)  639 ,  739  of two or more magnetic bearings, and further configured to transmit control commands to the power amplifier(s)  638 ,  738  of two or more magnetic bearings. In particular, the I/O device  842  may include any convenient device of any architecture/distribution that is configured to perform analog-to-digital (A/D) conversion, digital-to-analog (D/A) conversion, signal sampling, electronic filtering and/or other signal conditioning. For example, the I/O device  842  may include an A/D converter configured to digitize signals from the at least one sensor  639 ,  739  of the magnetic bearing system or other devices of the compressor driver  600  and/or the centrifugal gas compressor  700 . Similarly, the input/output device  842  may include a D/A converter configured to convert control commands to analog signals for the power amplifiers  638 ,  738  of the magnetic bearings. Also for example, the bearing I/O terminal  840  may be embodied as an ASIC interfaced with the sensors  639 ,  739 , power amplifiers  638 ,  738  and/or other devices. 
     The communication device  843  may include any piece of equipment, hardware, or software configured to move data to and from the bearing I/O terminal  840 . In particular, the communication device  843  is configured to transmit digital feedback signals from the I/O device  842  to the computer  810  via the communication link  830 . Also, the communication device  843  is configured to receive control commands from the computer  810  via the communication link  830 . According to one embodiment, the communication device  813  of the computer  810  and the communication device  843  of the bearing I/O terminal  840  are configured to communicate with an input/output delay of less than 60 microseconds. 
     Like the communication device  813  of the computer  810 , the communication device  843  of the bearing I/O terminal  840  may be configured for data packet communications across a communication network. In particular, the communication device  843  may be configured to communicate control commands and feedback data in accordance with a standardized fieldbus communication protocol. For example, the communication device  843  may be configured to communicate data across an Ethernet based communication network using standard IEEE 802.3 Ethernet frames. Also for example, the communication device  843  may be configured to communicate EtherCAT (Ethernet for Control Automation Technology) communications with the computer  810 . Unlike the communication device  813  of the computer  810 , however, the communication device  843  may be configured as an EtherCAT slave controller communicably coupled to devices such as sensors  639 ,  739  and power amplifiers  638 ,  738  via the I/O device  842 . 
     The communication device  843  of the bearing I/O terminal  840  may be embodied as a dedicated device, such as ASIC, or may have shared or distributed functionality with other components of the bearing I/O terminal  840 . The communication device  843  may be configured for wired, wireless, and/or optical communications. Furthermore, the communication device  843  may be configured for full-duplex and/or half-duplex communications across one or more communication links  830 . 
     According to one embodiment, communication device  843  may be configured to selectively communicate data. In particular, the communication device  843  may be configured to communicate different classes of data separately. For example, classes of data may be distinguished by data source (e.g., control commands from the computer  810  versus feedback data from sensors  639 ,  739 ). Also for example, multiple classes of data may be used. According to one embodiment, distinct data classes may be provided for: feedback from each magnetic bearing, control commands to each magnetic bearing, environmental data, and data associated with other devices or systems (discussed below). 
     In selectively communicating data, the communication device  843  may be configured to selectively communicate data within data packets at separate times. In particular, the communication device  843  may communicate a first data packet for a first class of data and second data packet for a second class of data. For example, the communication device  843  may be configured to communicate a first data packet for feedback signals and second data packet for control commands. Also for example, in the EtherCAT configuration, the EtherCAT telegram may only include updates to Datagrams from a first class of signal (e.g., feedback signals) or to Datagrams from a second class of signal (e.g., control commands) but not to both at the same time. Thus, the communication device  843  is configured to selectively communicate a first and a second EtherCAT telegram with either a first set of Datagrams based on a first class of signal or with a second set of Datagrams based on a second class of signal, respectively. 
     According to one embodiment the EtherCAT telegram may be reduced in size to reflect only one class of data traveling at a time. In particular, the communication device  843  may be configured to alternate signal classes in one or more shared Datagrams. 
       FIG. 5  is a functional block diagram of an exemplary system for controlling the centrifugal gas compressor of  FIGS. 1-3 . In the embodiment illustrated, the control system  800  for the magnetic bearing system is shown configured to also include control functionality for the centrifugal gas compressor  700  and the compressor driver  600 . While the control system  800  may control a driver bearing system  630 , a central bearing system  690 , and a compressor bearing system  730  together, for convenience only a compressor bearing system  730  is illustrated. 
     The control system  800  includes the computer  810 , the communication link  830 , and the bearing I/O terminal  840  described above. In addition, the control system  800  may include a compressor I/O terminal  850  and a driver I/O terminal  860 . The compressor I/O terminal  850  and a driver I/O terminal  860  may be communicably coupled to the computer  810  via a compressor communication link  832  and a driver communication link  834 . The compressor communication link  832  and/or the driver communication link  834  may be separate from, or integrated with each other. Furthermore, the compressor communication link  832  and/or the driver communication link  834  may be separate from or integrated with the communication link  830  to the bearing I/O terminal  840 . 
     The compressor I/O terminal  850  is communicably coupled to the centrifugal gas compressor  700 . The compressor I/O terminal  850  may be fixed to, located within, or located proximate the centrifugal gas compressor  700  (such as in a control cabinet of the centrifugal gas compressor  700 ). Where the compressor I/O terminal  850  is located in or on the centrifugal gas compressor  700 , it may be sealed or otherwise include additional environmental protections. 
     The driver I/O terminal  860  is communicably coupled to the compressor driver  600 . The driver I/O terminal  860  may be fixed to, located within, or located proximate the compressor driver  600  (such as in a control cabinet of the compressor driver  600  and or/the centrifugal gas compressor  700 ). Where the driver I/O terminal  860  is located in or on the compressor driver  600 , it may be sealed or otherwise include additional environmental protections. 
     The compressor I/O terminal  850  may include a compressor I/O module  851  and a compressor communication module  852 . The compressor I/O module  851  and the compressor communication module  852  may be communicably coupled to each other, and may be configured as a communication conduit between the computer  810  and the centrifugal gas compressor  700 . In particular, the compressor I/O module  851  may be communicably coupled to one or more components/systems of the centrifugal gas compressor  700  and the compressor communication module  852  may be communicably coupled to the computer  810 . In addition, the compressor I/O module  851  and the compressor communication module  852  may be embodied as two units, as a single unit, or have a distributed and/or shared architecture. 
     According to one embodiment, the compressor I/O module  851  may be configured to communicate signals with one or more compressor sensors (e.g., measuring valve position, inlet/outlet pressure, gas flow rate, temperature, heat exchanger status, etc.). Also for example, the compressor I/O module  851  may be configured to communicate commands to one or more flow control devices (described above), or other devices configured to control flow to and/or from the centrifugal gas compressor  700 . In addition, the flow control device may include sensors configured to provide feedback regarding the flow metering device (e.g., inlet/outlet pressure, flow rate, temperature, etc.) to the compressor I/O module  851 . The compressor I/O module  851  and the compressor communication module  852  may be embodied as an ASIC, interfaced with one or more sensors, flow metering device and/or other devices. 
     The driver I/O terminal  860  may include a driver I/O module  861  and a driver communication module  862 . The driver I/O module  861  and the driver communication module  862  may be communicably coupled to each other, and may be configured as a communication conduit between the computer  810  and the compressor driver  600 . In particular, the driver I/O module  861  may be communicably coupled to one or more components/systems of the compressor driver  600  and the driver communication module  862  may be communicably coupled to the computer  810 . In addition, the driver I/O module  861  and the driver communication module  862  may be embodied as two units, as a single unit, or have a distributed and/or shared architecture. 
     According to one embodiment, the driver I/O module  861  may be configured to communicate signals with one or more driver sensors (e.g., measuring power, power bus voltage, power bus current, temperature, torque, rotational speed, etc.). Also for example, the driver I/O module  861  may be configured to communicate commands to a local controller such as a variable-frequency drive (VFD), or other devices configured to provide power management and control for the compressor driver  600 . Accordingly, driver I/O module  861  may be configured to operate the local controller rather than the compressor driver  600  directly. The driver I/O module  861  and the driver communication module  862  may be embodied may be embodied as an ASIC, interfaced with one or more sensors, a local controller of the compressor driver  600 , and/or with other devices. 
     Returning to the computer  810  described above, the computer  810  may further include one or more modules configured to control each magnetic bearing, all or part of the centrifugal gas compressor  700 , and all or part of the compressor driver  600 . In particular, the computer  810  includes at least two bearing control modules. In the embodiment illustrated in  FIG. 5 , the computer includes a first bearing control module  821 , a second bearing control module  822 , a third bearing control module  823 , and a fourth bearing control module  824 . Each bearing control module may be configured to control one or more magnetic bearings. In one embodiment, the first bearing control module  821  is configured to control a first magnetic bearing, the second bearing control module  822  is configured to control a second magnetic bearing, the third bearing control module  823  is configured to control a third magnetic bearing, and the fourth bearing control module  824  is configured to control a fourth magnetic bearing. Each bearing control module may be configured to provide conventional automated operation processing control (“control algorithms”) of their respective magnetic bearing. 
     The magnetic bearings may be radial or thrust magnetic bearings, such as those described in  FIGS. 1-4 . In some embodiment, the first magnetic bearing, the second magnetic bearing, and the third magnetic bearings are radial magnetic bearings, while the fourth magnetic bearing is a thrust magnetic bearing. In one of these embodiments, the fourth magnetic bearing and one of the first magnetic bearing, the second magnetic bearing, and the third magnetic bearing are within a single bearing housing forming a combination bearing. 
     In the embodiment illustrated in  FIG. 5 , the computer  810  also includes a compressor control module  825 , a driver control module  826 , and a communication module  827 . The compressor control module  825 , and/or the driver control module  826  (“control modules”) may be configured to provide control algorithms of their respective systems. Control algorithms are generally known in the art for controlling magnetic bearings, as well as centrifugal gas compressors and driver motors. Similarly, the communication module  827  may be configured to provide conventional communications between the bearing control modules/control modules, and the bearing I/O terminal  840 , the compressor I/O module  851 , and the driver I/O module  861  (“I/O terminals”). In addition, the control modules may be configured to allow user control to and/or provide user feedback from the I/O terminals. 
     According to one embodiment, the bearing control modules and the control modules may be configured to communicate with each other. In particular, feedback and/or control commands may be shared amongst the bearing control modules and the control modules. For example, feedback directed toward the compressor control module  825  (e.g., valve position, inlet/outlet pressure, gas flow rate, temperature, heat exchanger status, etc.) may be shared with the each bearing control module. Also for example, feedback directed toward the driver control module  826  (e.g., power, power bus voltage, power bus current, temperature, torque, rotational speed, etc.) may be shared with each bearing control module. Similarly, feedback directed toward the control modules may be shared with the compressor control module  825  and the driver control module  826 . 
     According to one embodiment, the bearing control modules/control modules may be further configured to use data from another module in its own control algorithms. In particular, the shared feedback and/or control commands from one control module may be used to modify commands of another control module. For example, the third bearing control module  823  may be configured to use pressure sensor feedback directed toward compressor control module  825 , or a determination from the compressor control module  825  indicating aerodynamic loading of the rotor  750 , to offset or otherwise adjust a control command to the thrust bearing  733 . 
     The multi-core processor  870  and the at least two bearing control modules are configured such that the calculations required to control two magnetic bearings are performed in parallel on different cores of the multi-core processor  870 . Each of the two or more bearing control modules can be configured to be implemented or performed with one of the cores of the multi-core processor  870 . The control modules and the communication module  827  may be configured to be implemented or performed with a core not used by the bearing control modules or may be divided amongst the cores with portions of each being implemented or performed on the various cores. In one embodiment, the first bearing control module  821  is configured to control and perform the calculations for the suction end radial bearing  731 , and is implemented or performed on the first core  871 ; the second bearing control module  822  is configured to control and perform the calculations for the discharge end radial bearing  732 , and is implemented or performed on the second core  872 ; the third bearing control module  823  is configured to control and perform the calculations for the driver magnetic bearing  631  and is implemented or performed on the third core  873 ; the control modules and the communication module  827  are implemented or performed on the fourth core  874 ; and the fourth bearing control module  824  is configured to control and perform the calculations for the thrust bearing  733  and is implemented or performed on one or more of the first core  871 , the second core  872 , the third core  873 , and the fourth core  874 . 
     The fourth bearing control module  824  along with the other module operating on the one or more shared cores may be threaded and optimized to reduce any time delay in the calculations. In yet other embodiments including a fourth magnetic bearing, the multi-core processor  870  may be configured with an additional one or more cores to control and perform the requisite calculations for the fourth magnetic bearing on a separate core. Embodiments including any additional magnetic bearings or including a smaller number of cores may be implemented in a similar manner. 
     In some embodiments, such as when the compressor driver  600  is a motor, it may be desirable to determine the speed of the driver shaft  620  as well as the direction of rotation of the driver shaft  620 .  FIG. 6  is a schematic illustration of an embodiment of the driver sensing system  650  of  FIG. 4 . In the embodiment illustrated, the driver sensing system  650  includes a first driver sensor  651 , a second driver sensor  652 , and a sensed feature  653 . First driver sensor  651  and second driver sensor  652  are offset and adjacent driver shaft  620 . First driver sensor  651  and second driver sensor  652  are radially spaced apart such that a first angle  658  between the sensors is not equal to a second angle  659  between the sensors. First driver sensor  651  and second driver sensor  652  are each configured to detect the sensed feature  653 . The sensed feature  653  is a feature detectable by the sensors, such as a notch or a protrusion. 
     In the embodiment illustrated, the driver sensing system  650  may determine the direction based on the difference in time it takes the sensed feature  653  to travel a first time from the first driver sensor  651  to the second driver sensor  652  and a second time from the second driver sensor  652  to the first driver sensor  651 . If the first time is less than the second time, the driver shaft  620  is rotating in a first direction, and if the first time is greater than the second time, the driver shaft  620  is rotating in a second direction, opposite the first direction. More sensors may also be used provided that the angle between two adjacent sensors is different than all of the other angles between adjacent sensors. 
     In other embodiments, the driver sensing system  650  may include a single driver sensor with multiple sensed features  653 , such as three sensed features  653 , unequally spaced about driver shaft  620 . If the unequal spaces are detected in a first order, the driver shaft  620  is rotating in the first direction, while if the unequal spaces are detected in a second order, the driver shaft  620  is rotating in the second direction. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure generally applies to a control system in an industrial gas compressor. The described embodiments are not limited, however, to use in conjunction with a particular type of gas compressor (e.g., centrifugal, axial, etc.). Gas compressors such as centrifugal gas compressors are used to move process gas from one location to another. Centrifugal gas compressors are often used in the oil and gas industries to move natural gas in a processing plant or in a pipeline. Centrifugal gas compressors are driven by gas turbine engines, electric motors, or any other power source. 
     In some instances, embodiments of the presently disclosed control system are applicable to the use, operation, maintenance, repair, and improvement of centrifugal gas compressors, and may be used in order to improve performance and efficiency, decrease maintenance and repair, and/or lower costs. In addition, embodiments of the presently disclosed control system  800  may be applicable at any stage of the centrifugal gas compressor&#39;s life, from design to prototyping and first manufacture, and onward to end of life. Accordingly, control system  800  may be used in conjunction with a retrofit or enhancement to existing centrifugal gas compressors, as a preventative measure, or even in response to an event. 
     There is a desire to achieve greater efficiencies and reduce emissions in large industrial machines such as centrifugal gas compressors. Installing magnetic bearings in a centrifugal gas compressor may accomplish both desires. Centrifugal gas compressors may achieve greater efficiencies with magnetic bearings by eliminating any contact between the bearings and rotary element. Contact between the bearings and the rotary element generally causes frictional losses to occur. Magnetic bearings may use electromagnetic forces to levitate and support the rotary element without physically contacting the rotary, element eliminating the frictional losses. 
     Using magnetic bearings may reduce or eliminate production of undesirable emissions. These emissions may be produced by leaking or burning a lubricant such as oil. Eliminating the contact and frictional losses between the rotary element and bearings by supporting the rotary element with magnetic bearings may eliminate or reduce the need for lubricants in centrifugal gas compressors. With this elimination or reduction of lubricants or oil, the emissions in centrifugal gas compressors may be reduced or eliminated. Eliminating lubricants may also eliminate the need for the valves, pumps, filters, and coolers associated with lubrication systems. 
     Control of magnetic bearings in an industrial compressor requires high speed communications between feedback sensors and the controller. In particular, excessive input-to-output delays may lead to phase lag, which may lead to reduced damping. PC control may provide for previously unseen benefits. 
     Control of each magnetic bearing may require complex calculations. Performing all of the magnetic bearing calculations in series may cause delays from receipt of the feedback signal from a magnetic bearing to the transmission of the control signal to the magnetic bearing, which may further lead to phase lag and reduced damping. Using a multi-core processor to perform the calculations of two or more magnetic bearings in parallel may reduce the time delays and the phase lag, and improve damping. 
       FIG. 7  is a flow chart of an exemplary method for controlling the centrifugal gas compressor of  FIGS. 1, 2, and 3 . The centrifugal gas compressor  700 , the compressor driver  600 , and particularly the driver bearing system  630 , the central bearing system  690 , and the compressor bearing system  730  can be controlled by a computer  810  with one or more of the following steps of a method  900 , with reference to  FIG. 1-6 . The steps of method  900  may be performed in the order presented or out of the order presented. In addition, the steps of method  900  may be performed in parts. For example, one step may be performed in part, followed by one or more subsequent steps, and then completed. 
     In step  910 , the bearing I/O terminal  840  receives feedback data about at least two magnetic bearings. In particular, the I/O device  842  may receive feedback data from multiple sources over multiple inputs. The feedback data may be in any form of signal (e.g., analog, digital, optical, etc.). Also, the feedback data may be from at least one sensor  639 ,  739  of each of the magnetic bearings or other source(s). 
     For example, the bearing I/O terminal  840  may receive feedback data from the suction end radial bearing  731  and the discharge end radial bearing  732 , and more particularly from the sensor(s)  739  of the suction end radial bearing  731  and the discharge end radial bearing  732 . Also for example, sensor input corresponding to the compressor shaft  720  and/or other rotating members, such as the driver shaft  620  (e.g., position, speed, rotational direction, vibration, angle, etc.) may be received. Also for example, ancillary input corresponding to environmental conditions (e.g., temperature, available power, etc.), compressor performance (e.g., compressor supply, compressor demand, compressor output, etc.), bearing performance (e.g., current, voltage, applied force, etc.), and other ancillary input may be received. 
     In step  912 , the bearing I/O terminal  840  may convert analog feedback data to digital feedback data. In particular, the I/O device  842  may perform A/D conversion, signal sampling, electronic filtering and/or other signal conditioning. In addition, the I/O device  842  may include one or more digital inputs and communicate digitally inputted feedback data along with converted digital feedback data. 
     In step  920 , the bearing I/O terminal  840  digitally communicates the feedback data to the computer  810 . In particular, the communication device  843  may transmit digital feedback signal from the I/O device  842  to the computer  810  via the communication link  830 . For example, the communication device  843  may communicate feedback data across an Ethernet based communication network. Also for example, the communication device  843  may communicate the feedback data to the computer  810  in accordance with a standardized fieldbus communication protocol, such as EtherCAT. 
     In step  922 , the bearing I/O terminal  840  may selectively communicate data. In particular, the communication device  843  may communicate different classes of data on separate paths. For example, the communication device  843  may communicate an EtherCAT telegram with either a first set of Datagrams based on a first class of signal or with a second set of Datagrams based on a second class of signal. Step  922  may further include creating and/or identifying one or more classes of data as discussed above. According to one embodiment, digitally communicating the feedback data from the bearing input/output terminal to a computer may include selectively communicating the feedback data. 
     In step  930 , the computer  810  processes the feedback data and issues bearing control commands. In particular, the computer  810  processes the feedback data about at least two magnetic bearings with the feedback data for each of the two magnetic bearings being processed by a different bearing control module on a different core of the multi-core processor  870 . For example, the computer  810  may process first feedback data from a first sensor about a first bearing using a first bearing control module  821  on a first core  871  of the multi-core processor  870  and issues a first bearing control command to a first magnetic bearing driver, and processes second feedback data from a second sensor about a second bearing using a second bearing control module  822  on a second core  872  of the multi-core processor  870  and issues a second bearing control command to a second magnetic bearing driver. Similarly, the computer  810  may process third feedback data from a third sensor about a third bearing using a third bearing control module  823  on a third core  873  of the multi-core processor  870  and issues a third bearing control command to a third magnetic bearing driver. Further, the computer  810  may process fourth feedback data from a fourth sensor about a fourth bearing using a fourth bearing control module  824  on a fourth core  874  of the multi-core processor  870  and issues a fourth bearing control command to a fourth magnetic bearing driver. In some embodiments, the fourth core  874  may be primarily dedicated to other processes, such as conventional operational processing and control of the centrifugal gas compressor  700  and the compressor driver  600  (as described below with reference to steps  932  and  934 ). In such embodiments, the computer  810  may process the feedback data about the fourth bearing using the fourth bearing control module  824  on one or more cores of the multi-core processor  870  and may be divided amongst numerous cores of the multi-core processor  870 , such as the first core  871 , the second core  872 , the third core  873 , and the fourth core  874 . 
     Each bearing control module may provide conventional operational processing and control of its corresponding magnetic bearing by performing calculations on its corresponding core based on the feedback data about its corresponding magnetic bearing. For example, using its corresponding core of the multi-core processor  870 , one of the bearing control modules may issue commands directing the power amplifier  738  of its corresponding magnetic bearing to increase or decrease magnetic attraction of the levitated member along one or more axes. In addition, using its corresponding core of the multi-core processor  870 , one of the bearing control modules may calculate bearing control commands based on the feedback received from its corresponding magnetic bearing, preset data libraries, and/or adaptive learning. Furthermore, the multi-core processor  870  may calculate bearing control commands based on a minimum 10 kHz sample rate (100 microseconds scan time), and/or on a 60 microsecond input-to-output delay. Dedicated assignment for magnetic bearing control on parallel cores may reduce delays and improve system performance. 
     In step  932 , the computer  810  may provide conventional operational processing and control of the centrifugal gas compressor  700 , for example, in the compressor control module  825 . Similarly, in step  934 , the computer  810  may provide conventional operational processing and control of the compressor driver  600 , for example, in the driver control module  826 . The operational processing and control of the centrifugal gas compressor  700  and the compressor driver  600  may be performed using a compressor control module  825  and a driver control module  826  on a dedicated core of the multi-core processor  870 , such as a fourth core  874 , or may be divided amongst numerous cores of the multi-core processor  870 . 
     In addition, the method  900  may include interactions between the bearing control modules, the compressor control module  825 , and the driver control module  826  within the computer  810 . In particular, the bearing control modules/control modules may communicate with each other, for example, feedback and/or control commands may be shared amongst the control modules. Also, the control modules may incorporate data from another module in its own operational processing and control functions, for example, shared feedback and/or control commands from one bearing control module/control module may be used to modify commands of another bearing control module/control module. Also, the bearing control modules/control modules may be dynamically adjusted, for example, the shared feedback and/or control commands from a first bearing control module/control module may be used to modify control algorithms of a second bearing control module/control module. 
     In step  940 , the computer  810  digitally communicates the bearing control commands to the bearing I/O terminal  840 . In particular, the communication device  813  may transmit digital control commands from each bearing control module to bearing I/O terminal  840  via the communication link  830 , similar to the digital communications of step  920 . 
     In step  942 , the computer  810  may selectively communicate data, communicating the different classes of data separately, similar to the selective communications of step  922  (e.g., at separate times). In addition, step  942  may include creating and/or identifying one or more classes of data as discussed above. Also, the communication device  813  may communicate as an EtherCAT master controller, whereas the communication device  843  of the bearing I/O terminal  840  may communicate as an EtherCAT master slave device. According to one embodiment, digitally communicating the bearing control command to the bearing I/O terminal  840  may include selectively communicating the bearing control command. 
     In step  950 , the bearing I/O terminal  840  transmits the bearing control command to the corresponding power amplifier  638 ,  738 . In particular, the bearing I/O terminal  840  may then convert the bearing control command to voltage levels corresponding to a predetermined power level of the corresponding magnetic bearing. For example, the communication device  843  may receive the bearing control command transmitted across the communication link  830 , and communicate the bearing control command to I/O device  842 . The I/O device  842  may then issue the bearing control command to the corresponding power amplifier  638 ,  738 . In addition, at step  952 , the I/O device  842  may convert any digital bearing control commands to analog control command as required, similar to step  912 . 
     A computer  810  including a multi-core processor  870  may improve on the current DSP controllers, as the computer  810  including the multi-core processor  870  may have superior performance, flexibility, memory, applications/features, support, human-to-machine interface (HMI), etc. Moreover, the computer  810  including the multi-core processor  870  may be user-modified by reprogramming software via a conventional user interface. In addition, once the magnetic bearings are controlled by the computer  810  including the multi-core processor  870 , synergistic benefits may be realized. In particular, the entire compressor system (centrifugal gas compressor, the compressor driver, and magnetic bearing) may reside on the same platform. Accordingly, the control system could be designed so the compressor, magnetic bearing, and engine or motor all share the same electric power supply and UPS. 
     Those of skill will appreciate that the various illustrative logical blocks, modules, and algorithm steps described in connection with the embodiments disclosed herein can be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the design constraints imposed on the overall system. Skilled persons can implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the invention. In addition, the grouping of functions within a module, block, or step is for ease of description. Specific functions or steps can be moved from one module or block without departing from the invention. 
     The preceding detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. The described embodiments are not limited to use in conjunction with a particular type or combination of driver and driven machine. For example, the driver may be an electric motor, a gas turbine engine, a reciprocating engine, or other rotating machine. Also for example the driven machine may be a gas compressor, a generator, or other rotatingly driven machine. Hence, although the present embodiments are, for convenience of explanation, depicted and described as being implemented in a centrifugal gas compressor driven by an electric motor, it will be appreciated that it can be implemented in various other types of drivers and driven machines, and in various other systems and environments. Furthermore, there is no intention to be bound by any theory presented in any preceding section. It is also understood that the illustrations may include exaggerated dimensions and graphical representation to better illustrate the referenced items shown, and are not consider limiting unless expressly stated as such.