Abstract:
The present invention provides a motor drive system that is substantially more robust than those of the prior art. Generally, the motor drive system provides a redundant communication topology with the drives, thereby greatly reducing the chance of failure by a fault in the drive-to-drive communication link. Specifically, the motor drive system includes a plurality of motor drives joined by a communication media. Each motor drive has a motor control circuit configured to control the speed of an electric motor and a media access control unit having two communication modules. Each communications module includes a transmitter and a receiver joined to the communications media. A set of routing switches in the media access control unit joins the communication modules to connect the motor drives in an independent primary and a secondary ring communicating data for controlling the motor drives.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     CROSS REFERENCE TO RELATED APPLICATION 
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     BACKGROUND OF THE INVENTION 
     The present invention relates generally to a control system for medium voltage variable frequency drives, and specifically to a control system for connecting multiple variable frequency drives together with a fiber optic communication ring. 
     In applications requiring multiple medium voltage variable frequency drives (“MV Drives”), there is typically a single drive designated as the main controlling drive. The remaining drives are then designated as followers. In such applications, typically the main controlling drive is responsible for the control of the overall speed (i.e., speed regulator), while the followers are responsible for assisting in providing the torque (i.e., torque regulators) to the motors in the system. In this arrangement, data is transferred from drive to drive over an industrial network such as, for example, DeviceNet, ControlNet or Industrial Ethernet. This network may also be shared with other controllers and drive combinations. 
     The main disadvantage experienced by such existing systems, is the creation of network latencies, that is, delays caused by network propagation delays and processing times in the system controllers and MV Drives. In critical applications, such as conveyors, such communication latency can create undesirable lateral or torsional vibrations. These vibrations can be amplified if they occur at certain natural frequencies. Another disadvantage of existing systems is that each MV Drive operates asynchronously from each other such that regulators add to the vibration in conveyor systems. 
     A secondary drive-to-drive communications link may be added to help alleviate the latencies of a single industrial network. However, such secondary links are typically over copper wire and therefore suffers from some of the same limitations as the industrial network such as lack of fault tolerance, excess latencies, limitations on distances, and the limitations of available bandwidth. 
     In the event of a break in communication with the controlling motor drives, either because of communication media failure or failure of the controlling drive itself, the system controller or PLC must we assign another drive to be the system controller or declare a fault in the system stopping all of the drives. Thus the motor drive system, even with a secondary drive-to-drive communication link, is susceptible to failure of (1) the PLC, (2) the main industrial control network, (3) the drive-to-drive communication link, and (4) the drive designated as the controlling drive. 
     SUMMARY OF THE INVENTION 
     The present invention provides a motor drive system that is substantially more resistant to failure. Generally the motor drive system provides a redundant communication topology with the drives, greatly reducing the chance of failure caused by a fault in the drive-to-drive communication link. Further, the invention designates one of the drives as a “Link Keeper” that can reconfigure the drive-to-drive communication link without intervention of the PLC. This provides integrity of the drive system even in the absence of communication with the PLC or failure of the PLC (1 and 2 above) and substantially reduces failure of the drive-to-drive communication link (3 above). The system readily adapts to optical fiber allowing extremely high-speed communication with low latency. 
     Specifically, the present invention provides a motor drive control system including a plurality of motor drives joined by communication media. Each motor drive has a motor control circuit for controlling the speed of an electric motor and a media access control unit (MACU). The MACU has a first communication module and a second communication module, each having a transmitter and a receiver joined to the communication media. A set of routing switches in the MACU joins the communication modules to connect the motor drives in an independent primary ring and a secondary ring communicating data for controlling the motor drives. 
     It is thus an object of the present invention to provide a communication system for a plurality of MV Drives that provides greater resistance to network failure. 
     In addition, the communication media may be constructed of optical fiber cables. 
     It is thus another object of the present invention to provide a communication system readily adaptable to optical fiber. Because the transmitters and receivers may be joined directly by routing switches, very little communication latency is experienced. 
     The dual communication ring of the present invention may alternatively be constructed of copper wiring at a reduced bandwidth and distance. 
     It is thus an object of the present invention to provide a low-cost alternative means of communication between the communication modules of the present invention. 
     The second communication module of all but one motor drive is coupled to the first communication module of an adjacent motor drive to form the primary ring. 
     It is thus yet another object of the present invention to provide simple method of establishing a redundant communication topologies that accommodate a daisy chaining connection system as well as breaks at any point in the chain. 
     The drive control system may have four routing switches: a first routing switch joining the transmitter of the first communication module with the receiver of the second communication module, a second routing switch joining the receiver of the first communication module with the transmitter of the second communication module, a third routing switch one shunting the transmitter and receiver of the first communication module and a fourth routing switch shunting the transmitter and receiver of the second communication module. During operation, the first and second routing switches may be closed or alternatively the third and fourth routing switches may be closed to change the topology of the primary and secondary ring. 
     It is thus an object of the invention to provide a simple switch network that may pass through or shunt data communications without the need for delaying computer processing of the data stream. 
     The motor drive control system may have one motor drive designated as a Link Keeper to (a) detect breaks in the primary ring upon the failure of the Link Keeper to receive a response to a communication from another motor drive and (b) provide instructions to other motor drives to reconfigure their routing switches detected. 
     It is thus yet another object of the present invention to provide a system where faults may be handled locally without the requirement of communication with the PLC. 
     Furthermore, the system of the present invention may be configured to include a redundant Link Keeper. 
     It is yet another object of the present invention to provide communication system that is less susceptible to failure of a single Link Keeper and thus which is inherently more reliable than a PLC moderated system where there is only one PLC. 
     These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a multiple drive system of the prior art; 
         FIG. 2  is a schematic illustration of a Media Access Control Unit (MACU) of the type used in the present invention; 
         FIG. 3  is a schematic illustration of the system of according to the present invention; 
         FIG. 4  is a schematic illustration of the system of  FIG. 3  wherein a physical break is detected at a point in the system; 
         FIG. 5  is a schematic illustration of the system of  FIGS. 3-4  demonstrating the reconfiguration of the system after a link follower failure; and 
         FIG. 6  is a schematic illustration of the system of  FIGS. 3-5  demonstrating the reconfiguration of the system after a link keeper failure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to the drawings and initially  FIG. 1 , a schematic illustration of a multiple drive system  10  of the type known in the prior art includes a plurality of drives  12  coupled to an industrial control network  14  such as, for example, ControlNet. Each drive  12  is in communication with a motor  16 . In such systems  10 , one of the drives  12  is typically designated as the main controlling drive while the remaining drives are designated as followers. In such applications, typically the main controlling drive is responsible for the control of the speed while the followers are configured to assist in providing torque to the motors  16  of the system. 
     Motion control data is transferred between the drives  12  by way of a dedicated drive-to-drive communication network  15 , while configuration data for the drives  12  and for their communication on drive-to-drive communication network  15  is communicated over the industrial control network  14 . The network  14  may be shared with other controllers and drives. The system  10  further comprises a system controller  20  such as, for example, a programmable logic controller (PLC). The system controller  20  is generally configured to manage the flow of data over the drive-to-drive communication network  15  and to communicate with other elements of the control system over the industrial control network  14 . The system  10  of the prior art further includes an operator interface  18 , which may comprise a computer adapted to allow a user to configure the system  10  for various applications. 
     In this system  10 , each drive  12  can transfer data between the controlling drive and drive followers by going directly through the drive-to-drive communication network  15 , however in the event of failure of this drive-to-drive communication network  15  or any drive  12  communication must be had with the system controller  22  by way of the industrial control network  14 . A physical break in either the industrial control network  14  or drive-to-drive communication network  15  can cause the complete system to fault and thereby halt the process it is currently operating. 
     Turning now to  FIG. 2 , a schematic diagram of a Media Access Control Unit (MACU)  30  of the present invention includes a first communications module  32  and a second communications module  34 . Each communication module  32  and  34  includes a transmitter  29  converting a outgoing electrical signals from the MACU  30  to optical signals to be received by first optical fiber  35   a  and  37   a , respectively, and a receiver  33  receiving optical signals from optical fiber  35   b  and  37   b  respectively to convert them to electrical signals communicated with the MACU  30 . The MACU  30  communicates with the MV Drives  12  of the present invention to communicate data therewith. 
     Each MACU  30  further includes a set of routing switches,  31   a ,  31   b ,  31   c , and  31   d . Switch  31   d  joins the output of receiver  33  in module  34  with the input of transmitter  29  in module  32 . Switch  31   c  joins the output of receiver  33  in module  32  with the input of transmitter  29  in module  33 . Switch  31   b  shunts the output of receiver  33  and the input of transmitter  29  in module  33 . Likewise switch  31   a  shunts the output of receiver  33  and input of transmitter  29  in module  32 . The switches  31  and receivers  33  and transmitters  29  may be directly connected devices, without the intermediary processing of computer circuitry and thus provide extremely low latency communication for example from fiber  35   b  to fiber  35   a  when switch  31   d  is closed. As will be understood the fibers  35  may be replaced by copper conductors for lower speed operation. 
     Now referring to  FIGS. 2 and 3 , and initially to  FIG. 2 , a plurality of MACUs  30  operably coupled to one another to form a dual ring network  42 . In the system  40  of the present invention, the second communications module  34  of each MACU  30  (for example in drive  12   b ) is coupled to the first communications module  32  of an adjacent MACU  30  (for example  12   c ) for each of modules  12   a ,  12   b ,  12   c , and  12   d  to form a primary ring  44 . For each of these modules switches  31   d  and  31   c  are closed and switches  31   a  and  31   b  are open. For module  12   e , switch  31   a  is closed and switch  31   d  and  31   c  are open terminating the primary ring  44 . 
     The second communications module  34  of the module  12   e  is then coupled to the first communications module  32  of module  12   a  to create a secondary ring  46 . 
     The electrical connections between each of the communication modules of the MACUs  30  are electrical switches contained in a field programmable gate array (FPGA) that will change operating conditions of the system  40  depending on the configuration of the link. These switches aid in reducing the costs of the device by eliminating the need for optical switches between fiber optic transmitters and receivers. The configuration of the switches may be set by data communicated on dual ring network  42  from a Link Keeper as will be described. 
     In normal operation of system  40 , data flows around the primary ring  44 , and the secondary ring  46  remains idle. When idle, the light source is removed by the MACU transmitters thereby creating a digital high level at the adjoining receiver. Preferably, system  40  initializes to this configuration automatically upon the application of power to the system  40 . 
     The system  40  will preferably contain at least one MV Drive  12  configured to operate as the Link Keeper  12   a  and the remaining drives  12  will be designated as Link Followers  12   b . In addition, one of the Link Followers  12   b  may be designated as a redundant Link Keeper  12   c . The redundant Link Keeper  12   c  will generally operate as a Link Follower  12   b  unless a failure of the Link Keeper  12   a  is detected or the original Link Keeper  12   a  requests to transfer its role to a Link Follower  12   b , upon which the redundant Link Keeper  12   c  will be configured to take over operation as the Link Keeper until commanded to transfer the role of Link Keeper by either the end user of the system controller. 
     In operation, the Link Keeper  12   a  controls of the state of the routing switches  31  in each of the Link followers  12   b - e . Upon initialization the Link Keeper  12   a  will have all of its switches  31  open and will only use the second communications module  34  for transmitting and receiving messages between the Link followers  12   b  on the primary link at  44 . All Link Followers  12   b - f  excluding the final Link Follower  12   e  will have switches  31   c  and  31   d  closed, effectively creating low latency repeaters along the primary ring  44 . Depending on the type of fiber-optic used, this can allow a distance between drives in the order of kilometers. The last drive  12   e  will have switch  31   a  closed due to sensing an idle condition in the second communication module  34 , thus closing the entire primary ring  44 . 
     Turning now to  FIG. 4 , a break in the fiber optic cable  35  and/or  31  between modules  12   c  and  12   d  may occur compromising communication on the primary ring  44 . In response to this break, the system  40  of the present invention is automatically reconfigured by the Link Keeper  12   a  or by devices  12   d  and  12   e  to communicate on the secondary ring  46  in order to reach the devices  12   d , and  12   e  isolated by the break in the primary ring  44 . A break in the primary ring  44  is preferably detected by monitoring the electrical level of the receiver in the adjacent devices  12   c  and  12   d  for an idle condition. Alternatively, a break in the ring is detected upon the failure to respond on the part of an adjacent Link Follower (e.g.  12   d ) to a Link Keeper&#39;s message. In this latter case the Link Keeper will turn on transmitter  37  in its second communication module  34  placing the secondary ring into an active state. Device  12   e  will detect this change in state and reconfigure its routing switches accordingly. 
     As shown in  FIG. 4 , when the physical break is between devices  12   c  and  12   d , upon detection, the final drive  12   e  will open its switch  31   a  and close switches  31   c  and  31   d  to allow throughput of data from the secondary ring to isolated device  12   d . In addition, it will then take its secondary ring transmitter out of the idle state. Upon detecting this, the Link Keeper will then use both communication modules  32  and  34  for messaging. As such, the fourth drive  12   d  will then assume the state of the last drive on the secondary ring  46  and open up switches  31   c  and  31   d  and close switch  31   b . The third drive  12   c  now assumes the state of the last drive on the primary ring  44  will open switches  31   c , and  31   d  and close switch  31   a , thus completing the primary ring  44 . 
     Now referring to  FIG. 5 , after the failure of a MACU  30  the system  40  reconfigures itself as noted previously to communicate along the secondary ring  46  in order to bypass the failed MACU  30 . In this case, the second drive  12   b  will assume the state of the last follower on the primary ring  44  and open switches  31   c  and  31   d  and close switch  31   a . As before, the Link Keeper  12   a  can close switches  31   c  and  31   d  for peer to peer messaging if supported. 
     Turning now to  FIG. 6 , if there is a failure of the Link Keeper  12   a , then assuming drive  12   b  was designated as the redundant Link Keeper  12   c , the redundant Link Keeper  12   b  takes over the operations of the Link Keeper  12   a  automatically and the system  40  operates without interruption. 
     In the illustrated scenario, only the primary ring  44  will be utilized. If there are any additional breaks or failures in the network, the network will fail as it cannot be reconfigured to use the secondary ring  46 . As such, a third optional communication module may be added to the system  40  as an external component. The addition of a third communication module would effectively disconnect the secondary ring  46  from the second communications module  34  in the original Link Keeper  12   a  and switch it to the first communication module of the new redundant Link Keeper  12   c . Preferably, such a configuration would comprise an optical switch. 
     In addition, the system  40  the present invention may be configured to detect in the addition of a new drive  12  to the primary ring  44 . Accordingly, the newly added drive  12  is provided with a location known to the system  40  and specifically to the Link Keeper  12   a  and the redundant Link Keeper  12   b . During power up, each Link Follower will be assigned a node ID and Port ID depending on their respective location to the Link Keeper. Node IDs and Port IDs differ in that node IDs are used for addressing during messaging, and Port IDs are used for detecting the location of a physical break in the system  40 . As such, Port IDs are fixed after power on configuration, while node IDs may change depending on a change in Link Keeper&#39;s  12   a.    
     In addition, all configuring and reconfiguring of the system  40  of the present invention is accomplished by the drives  12  themselves preferably, this occurs automatically, thus eliminating the need for system controller such as, for example, a PLC, or through a set of commands coming from a system controller. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.