# Patent

## Title
On-the-fly state synchronization in a distributed system

## Abstract

A new process is synchronized to an existing process in the face of a communications delay (d) by collecting a history of delayed measurements and states of the existing process. This history and predetermined information regarding the behavior of the existing process are used to simulate the existing process forward in time to a current time, thereby computing a current process state. Once the current state is computed, the new process, driven by the same information, maintains synchronization with the existing process. In a document processor the method of synchronizing can be applied to tightly coupled modules. For example, a print media transport system includes a plurality of transport modules. Each transport module includes a plurality of transport actuators and an associated controller. Print media may be driven by actuators of plural modules contemporaneously. Modules about to receive media are synchronized to modules already transporting the media.

## Background
CROSS REFERENCE
The following applications, the disclosures of each being totally incorporated herein by reference are mentioned: U.S. patent application Ser. No. 11/102,910, filed, for Coordination in a Distributed System by Lara S. Crawford, et al. U.S. patent application Ser. No. 11/102,899, filed, for Synchronization in a Distributed System by Lara S. Crawford; et al., and U.S. patent application Ser. No. 11/102,355, filed, for Communication in a Distributed System by Markus P. J. Fromherz, et al.
BACKGROUND
There is illustrated herein in embodiments, an architecture including methods and systems for synchronizing between elements in a distributed system. For example, a distributed system may include a collection of modules, each with its own function. The collection of modules may be interconnected to carry out a particular function or functions. The interconnection may be physical and/or logical in nature. Modules may be connected by a network or other communications scheme. Communications media may include wire, coaxial cable, fiber optics and/or radio frequency (RF) transmissions. The network or communications scheme may be associated with communication delays. Synchronizing controllers or processes in the face of such delays can be problematic. Some document processors are implemented as distributed systems and embodiments will be described with reference thereto. However, embodiments of the methods and systems described herein may be beneficially applied in a wide variety of control system environments.
Document processors include, for example, printers, copiers, facsimile machines, finishers and devices for creating documents, such as word processors and desk top publishers. In some instances, document processors provide the services of two or more of these devices. For instance, document processors that provide printing, copying, scanning, and faxing services are available. Printers and copiers can include feeders that supply print media and finishers that staple, shrink wrap or otherwise bind system output. Finishers may also fold or collate documents.
In order to increase throughput, some printers and copiers are being developed which include two or more marking engines. For example, U.S. patent application Ser. No. 10/924,113 filed Aug. 23, 2004 by Jonas M. M. dejong, et al. for a Printing System with Inverter Disposed for Media Velocity Buffering and Registration; U.S. patent application Ser. No. 10/924,106 filed Aug. 23, 2004 by Robert M. Lofthus, et al. for a Printing System with Horizontal Highway and Single Pass Duplex; U.S. patent application Ser. No. 10/924,459 filed Aug. 23, 2004 by Barry P. Mandel, et al. for a Parallel Printing Architecture Consisting of Containerized Image Marking Engine Modules; U.S. patent application Ser. No. 10/860,195 filed Jun. 6, 2004 by Robert M. Lofthus, et al. for a Universal Flexible Plural Printer to Plural Finisher Sheet Integration System; U.S. patent application Ser. No. 10/881,619 filed Jun. 30, 2004 by Daniel G. Bobrow for a Flexible Paper Path Using Multidirectional Path Modules; U.S. patent application Ser. No. 10/761,522 filed Jan. 21, 2004 by Barry P. Mandel, et al. for a High Print Rate Merging and Finishing System for Parallel Printing; U.S. patent application Ser. No. 10/785,211 filed Feb. 24, 2004 by Robert M. Lofthus, et al. for a Universal Flexible Plural Printer to Plural Finisher Sheet Integration System; and U.S. patent application Ser. No. 10/917,768 filed Aug. 13, 2004 by Robert M. Lofthus for a Parallel Printing Architecture Consisting of Containerized Image Marking Engines and Media Feeder Modules, all of which are incorporated herein by reference, describe aspects of tightly integrated document processing systems including a plurality of marking engines.
Additionally, some printers and copiers are being developed using a hypermodular structure to increase modularity and flexibility. These systems may possess a number of distributed processors, sensors, and actuators. For example, U.S. patent application Ser. No. 10/357,687 filed Feb. 4, 2003 by David K. Biegelsen, et al., for Media Path Modules; U.S. patent application Ser. No. 10/357,761 filed Feb. 4, 2003 by Markus P. J. Fromherz, et al., for Frameless Media Path Modules; U.S. patent application Ser. No. 10/740,705 filed Dec. 19, 2003 by David K. Biegelsen, et al., for a Flexible Director Paper Path Module; and U.S. patent application Ser. No. 10/812,376 filed Mar. 29, 2004 by David G. Duff, et al., for a Rotational Jam Clearance Apparatus, all of which are incorporated herein by reference, describe aspects of tightly integrated document processing systems including hypermodules.
Some systems, including some document processing systems, are based on a centralized control architecture wherein a single computational platform controls all system actuators and receives all system feedback information. These architectures work well where the systems are relatively small and are of a fixed or unchanging configuration. However, as system size increases, the computational capabilities of a single platform can be overwhelmed. Additionally, providing individual interfaces between the single computational platform and each of the sensors and actuators of the system can be impractical. Furthermore, where it is desirable to assemble or reconfigure a system from various subcomponents, the direct interfacing of sensors and actuators to the central platform becomes problematic.
These factors have led to the development of systems based on network communications. For example, U.S. Pat. No. 6,615,091 B1 to Birchenough, et al. for a Control System and Method Therefore allegedly disclosed an embodiment of a distributed control system including a main control coordinator, three local process station controllers and a designated number of process module controllers, each associated with a process module. The control system allegedly provides a real time operating system and has a communication bus platform provided via an EthernetTM communication bus and a second bus to connect the controllers in a distributed control network. The EthernetTM bus connects the main control coordinator and each of the local process station controllers and a continuous motion conveyer controller. Each of the process module controllers are connected via the second bus to designated local process station controllers.
In the system of Birchenough, the main controller agent interacts with each of the process station agents, and each of the process station agents interacts with each of the process module agents that are assigned thereto. During normal manufacturing operation, the main controller coordinator agent sends article notice messages to the process station agents to notify the process station agents of the oncoming articles of manufacture. A process station normally will not process the article of manufacture unless the process station agent which controls a particular process module has received an article notice message indicating that it should do so and the continuous feed indexer has returned a report that it is in proper position. In response, the process station agent notifies the designated process module agent to initiate its programmed process operation. Once the process module has completed its intended operation, the process module agent issues a work report message which is sent to the process station agent. The process station agent then broadcasts the work report message to other process stations as well as to the main control coordinator.
It appears that in the system of Birchenough, et al., a single entity (e.g., the main coordinator) is aware of and maintains information regarding each task, object or workpiece being processed by the system, and is thereby able to issue commands orchestrating the activities of system components. However, this may limit the scalability of the system. For example, as the size of the system increases, the capabilities and/or resources of the main control coordinator (or processor running the main control coordinator) may be overwhelmed. Therefore, it may be desirable to distribute some of this functionality over a number of processors or controllers.
However, as machines become more complex and contain larger numbers of embedded processors, instances of tightly coupled distributed control systems are becoming more common. In a tightly coupled system, controllers may interact through fast physical or informational coupling. That is, the actions of one controller may have an impact on an ability of a second controller to perform its function. Therefore, there is a desire for coordination and communication among the various controllers. One aspect of the coordination problem is how to synchronize a newly activated process or controller, which has been activated in order to address a particular portion of a process, to the status or state of the ongoing process in the face of communication delays.
United States Patent Application Publication No. U.S. 2002/0194269 A1, published Dec. 9, 2002 by Owada, et al. entitled “Distributed Process System, Distributed Processing Method and Client Terminal Capable of Using the Method,” allegedly discloses a distributed processing system wherein a user terminal receives event information generated in other user terminals and transferred from a server. During a period that the event information is transmitted in a network, a model in a processing server becomes different from a model in the user terminal. Then, a state change compensation portion continuously changes a state model processed in a processing portion so that it becomes the same as the state of the model in the processing server, whereby an influence of delay generated by a communication can allegedly be reduced. The application appears to be directed toward compensating for network delays in a multi-player video game environment.
United States Patent Application Publication No. U.S. 2002/0178292 A1, published Nov. 28, 2002 by Mushkin, et al., entitled “Distributed Synchronization Mechanism for Shared Communications Media Based Networks,” allegedly discloses a distributed synchronization mechanism in which a synchronization loop of each station on a shared media based network considers only synchronization signals received having a time phase earlier than the time phase of its internal clock. Therefore, the station with the fastest internal clock effectively functions as an ad hoc synchronization master for all stations in a given connected group.
However, the phase selecting technique of Mushkin is not applicable to the more complex synchronizations required in and between control processes. The video game synchronizing of Owanda is temporary in that synchronization is not necessarily maintained after an initial synchronization event.
Therefore, there is a desire for systems and methods for synchronizing a second process to a first process in the face of communications delays.
BRIEF DESCRIPTION
A method for synchronizing a second process to a first process, wherein state data regarding input to and output of a model of the first process is available to the second process after a delay period, can include beginning a data collection period, receiving delayed state data points regarding the input to and output of the model, storing the delayed state data points received during the data collection period, ending the data collection period after receiving and storing delayed state data that represents the state of the input to and output of the model at a point in time after the beginning of the data collection period and determining a current state of the model of the process based on at least some of the stored state data points and predetermined information regarding a behavior of the state of the model. Additionally, the method for synchronizing can include setting a current state of the second process according to the determined current state of the model, thereby synchronizing the second process to the first process.
Delayed state data points regarding the input to and output of the model can include delayed process sensor information that was used as an input to the model and/or delayed model output information that was used as an input to the model for determining a next state of the model. In some document processing systems receiving delayed sensor information can include receiving delayed sheet position information from a sensor of a sheet handling system and/or receiving delayed sheet state output information from a model of a sheet handling process.
Determining a current state of the model can include initializing a copy of the model with a portion of the stored information that represents input to the model at a first point in time after the beginning of, and before the end of, the data collection period, and forward propagating the copy of the model based on at least one calculated next state of the model. In some embodiments determining the current state of the model includes calculating a past state of the model based on a first portion of the stored information and the predetermined information regarding the behavior of the model and calculating the current state of the first process based on the calculated past state and a second portion of the stored information.
Embodiments useful in a document processing system can include a method for synchronizing a second sheet transportation process to a first sheet transportation process, wherein state data regarding input to and output of a model of the first sheet transportation process is available to the second sheet transportation process after a delay period. The method can include beginning a data collection period, determining a data collection state count to be a number of state times having a total duration at least as long as the delay period, receiving delayed state data points regarding the input to and output of the model, wherein the output of the model includes at least one of a sheet position, a sheet speed and a sheet trajectory, storing the delayed state data points received during the data collection period, ending the data collection period after receiving and storing a delayed state data point after the data collection period has persisted for a number of state times at least as large as the data collection state count and determining at least one of a current position, speed and trajectory of the sheet, from a current state of the model calculated from at least some of the stored state data points and predetermined information regarding a behavior of the state of the model. Additionally, the method can include setting a current state for an output value of the sheet transportation controller according to the determined at least one of a current position, speed and trajectory of the sheet, thereby synchronizing the second sheet transportation process to the first sheet transportation process.
More generally, embodiments can include a method for synchronizing a second process to a first process, wherein state data regarding input to and output of a model of the process is available to the controller after a delay period. The method can include beginning a data collection period, receiving delayed state data points regarding the input to and output of the model, storing the delayed state data points received during the data collection period, ending the data collection period after receiving and storing required information for determining a current state of the model based on forward propagation, and using the stored required information and information regarding the behavior of the model to forward propagate the model from a state at a point after the beginning of the data collection period to the current state, thereby determining the current state of the model. Additionally, the method can include setting a current state of the second process according to the determined current state of the model, thereby synchronizing the controller to the process.
For instance, the data collection state count can be d state periods and receiving delayed state data points regarding the input to and output of the model can include receiving at least a state of the output of the process model at a period d state periods prior to a current period represented as t', the time d state periods prior to the current period being represented as t'−d.
For example, in document processing embodiments, determining a current state of a model, such as determining current position, speed and trajectory of the sheet estimated by the model, can include entering the state of the output of the process model at the period d state periods prior to the current period into a state function that is operative to calculate a next state based on an entered state, thereby calculating a state of the model at a first subsequent period, the first subsequent period being represented as t'−d+1. The method can also include iteratively entering subsequent calculated states of the model, starting with the calculated state at the first subsequent period t'−d+1, into the state function, thereby calculating at least one additional subsequent state of the model, until a state for the current period t'=t'−d+n is calculated.
A system that is operative to control a process can include a model of the process, a communications path associated with a communications delay, a controller that is operative to control a portion of the process and a supervisory element. The supervisor can be operative to activate the controller at a time appropriate for the controller to prepare for controlling the portion of the process, wherein, the controller receives information regarding states of the model of the process over the communications path after the communications delay, and wherein the controller is operative to initialize and maintain a local copy of the model for use in determining appropriate control actions, wherein the controller is operative to initialize the local copy of the model by using delayed information regarding prior states of the model to determine a starting prior state of the model and to forward propagate the model from the starting prior state to a current state of the model, thereby synchronizing the local copy of the model to the model of the process.
Where the communications delay is a maximum of d state periods long, the controller can be operative to initialize the local copy of the model by a process comprising receiving at least a state of the output of the model at a period d state periods prior to a current period represented as t', the time d state periods prior to the current period therefore being represented as t'−d, and entering the state of the output of the process model at the period d state periods prior to the current period into a state function that is operative to calculate a next state based on an entered state, thereby calculating a state of the model at a first subsequent period, the first subsequent period being represented as t'−d+1. Additionally, the controller can be operative to iteratively enter, until a state for the current period t'=t'−d+n is calculated, subsequent calculated states of the model and the additional information used as input to the model at the subsequent state periods, starting with the calculated state period t'−d+1 and additional information used as input to the model at t'−d+1, into the state function, thereby calculating at least one additional subsequent state of the model, until a state for the current period t'=t'−d+n is calculated.
A document processing system embodiment includes a xerographic marking engine, a sheet transport system that is operative to at least transport a sheet of print media to or from the first xerographic marking engine, a model of a sheet transportation process, a communications path associated with a communications delay, a controller that is operative to control a portion of the sheet transportation process and a supervisor that is operative to activate the controller at a time appropriate for the controller to prepare for controlling the portion of the sheet transportation process, wherein, the controller receives information regarding states of the model of the sheet transportation process over the communications path after the communications delay, and wherein the controller is operative to initialize and maintain a local copy of the model of the sheet transportation process for use in determining appropriate control actions, wherein the controller is operative to initialize the local copy of the model of the sheet transportation process by using delayed information regarding prior states of the model of the sheet transportation process to determine a starting prior state of the model and to forward propagate the model from the starting prior state to a current state of the model, thereby synchronizing the local copy of the model of the sheet transportation process to the model of the sheet transportation process.
Some embodiments include at least a second marking engine wherein the sheet transport system is further operative to transport a sheet of print media to or from the at least a second marking engine.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a system wherein second processes or controllers are synchronized to first processes or controllers.
FIG. 2 is a more detailed block diagram of a portion of a system wherein a second process or controller is in a synchronization state and is being synchronized to a first process or controller.
FIG. 3 is a flow chart outlining a method of synchronizing second processes or controllers to first processes or controllers.
FIG. 4 is a simplified state diagram showing a relationship between four possible states of a process or controller.
FIG. 5 is a timing diagram illustrating transitions between states illustrated in FIG. 4.
FIG. 6 is a block diagram of a document processing system wherein elements of the system may be synchronized according to the methods of FIG. 3.



DETAILED DESCRIPTION
Referring to FIG. 1, distributed systems (e.g., 104) often include a communications network for carrying communication between system elements (e.g., 108, 110, 112, 114, 116, 118, 120, 122, 124, 126, 128, 130, 132, 160, 170, 180). Communication in such networks is subject to communication delays. The delays can be significant when compared to system update periods, especially where systems are tightly coupled and system elements need to behave in a cooperative manner. In such systems, some mechanism is needed to ensure that the efforts of one controller or process are synchronized to the efforts of another system element or controller.
One method for ensuring cooperative control efforts is for each cooperating element to be constantly updated as to the activities of the other cooperating elements, and/or as to the status of progress of a task or workpiece. However, such methods require a great deal of inter-element communication, which may over-burden a system network or require the inclusion of a more expensive, higher bandwidth network. An alternative method for ensuring cooperative system element activities is to assign cooperative goals and constraints to relatively autonomous cooperating system elements, and synchronize the activities of the cooperating system elements to each other.
A goal describes a task to be performed. For example, a goal might be to move a workpiece from point A to point B, to move a workpiece at a specified speed or to deliver a workpiece to a particular location. Other examples of goals might include set points, such as a temperature set point, actuator operation, such as to open or close a valve or set a flipper to a first or second position, or to move an actuator at a particular speed.
A constraint is some description regarding how the goal is to be achieved. If goals and constraints are determined by some first or supervisory element that has knowledge regarding goals and constraints sent to the cooperating system elements, then cooperative activities can be ensured. For example, a constraint on the goal of moving a workpiece from point A to point B might be a deadline for delivering the workpiece to point B. By requiring that an element meet the deadline or constraint, the first or supervisory element can ensure that the workpiece is available at point B when a third element will be ready to receive it from point B. If point B will be occupied by another workpiece at a point in time prior to the deadline mentioned above, an additional or alternative constraint might be provided. For example, the constraint on the goal of moving the workpiece from point A to point B might be—do not deliver the workpiece prior to a given time—. Other kinds of constraints may also be employed. For example, a constraint may allocate a portion of a system resource to a system element that is assigned a task. For instance, the goal of moving a workpiece from point A to point B might be associated with a constraint limiting a peak power consumption associated with the task. Such a constraint might ensure that other cooperating controllers are able to draw enough power from a shared system power source to perform their assigned tasks or achieve their respective goals.
Referring to FIG. 1, a first system 104 embodiment includes a plurality 106 of controllers. For example, the plurality 106 of controllers includes a first, second, third, fourth and fifth controller 108, 110, 112, 114, 116. The controllers may, for example, be associated with actuators and sensors. For instance, the first, second and third controllers 108, 110, 112 are associated with first, second and third sets of actuators 118, 120, 122, and first, second and third sets of sensors 124, 126, 128. The fourth controller 114 is associated with a fourth set of actuators 130. The fifth controller 116 is associated with a fourth set of sensors 132. The actuators 118, 120, 122, 130 and sensors 124, 126, 128, 132 manipulate or sense objects in, or aspects of, respective portions of the system 104. For example, the first set of actuators 118 and first set of sensors 124 are associated with a first portion 140 of the system 104. The second set of actuators 120 and the second set of sensors 126 are associated with a second portion 142 of the system 104. The third set of actuators 122 and the third set of sensors 128 are associated with a third portion 144 of the system 104. The fourth set of actuators 130 are associated with a fourth portion 146 of the system 104 and the fourth set of sensors 132 are associated with a fifth portion 148 of the system 104.
Some or all of the system portions may be tightly coupled. Tightly coupled systems or system portions are those wherein the performance or activities of a first system portion has an effect on the performance or activities of a second portion. In such configurations, if the activities of the first portion and the second portion are not coordinated, they may interfere with or disrupt each other. For instance, in an automotive system, an engine/transmission subsystem may be considered to be tightly coupled with a braking subsystem because an uncoordinated application of the braking system may interfere with or prevent the engine/transmission system from propelling a vehicle. In the embodiment illustrated in FIG. 1, first, second and third elements of system dynamics 152, 154, 156 tightly couple the second system portion 142 to the third system portion 144, tightly couple the third system portion 144 to the fourth system portion 146 and tightly couple the fourth system portion 146 to the fifth system portion 148. The first system portion 140 is illustrated as having only a loose or minimal interaction with the second system portion 142 and is not tightly coupled thereto.
The first system 104 may also include a high level element 160. For example, the high level element 160 may be a scheduler and/or a planner. The high level element 160 determines which tasks are to be performed, or which workpieces are to be processed, and activates, spawns or instantiates a separate coordinator for each task or workpiece. For example, a first coordinator 170 is activated or spawned in association with a first task or workpiece, and a second coordinator 180 is activated or spawned in association with a second task or workpiece. The coordinators 170, 180 are activated and initialized in such a manner as to prevent interference between the coordinators.
For example, if the first task or workpiece and the second task or workpiece both require the services of the first, second, third, fourth and fifth system portions 140, 142, 144, 146, 148, then, for example, the first coordinator 170 is activated and takes control of first system portion 140 by communicating with the first controller. The activation of the second coordinator 180 may be delayed until the first coordinator 170 no longer requires the services of the first system portion 140. Alternatively, the second coordinator 180 is activated early and directed to wait or idle until such a time as the first coordinator 170 no longer needs the services of a first system resource (e.g., 140).
The first coordinator 170 releases the first controller 108 when the first task or workpiece no longer needs the services of the first system portion 140. The first coordinator 170 may then send commands requesting the services of another system resource (e.g., the second system portion 142) for accomplishing a second subtask. Alternatively, the first coordinator 170 may begin requesting services from the second resource before the first resource has completed a first subtask. In either case, the first coordinator 170 sequentially sends commands to the controllers (e.g., 110, 112, 114, 116) requesting services of their respective system portions (e.g., 142, 144, 146, 148). When appropriate, the first coordinator 170 sends coordinating commands to a plurality of controllers. For example, if a subtask requires coordinated activity between two or more system portions at once, then the coordinator generates and communicates commands to two or more controllers associated therewith.
In FIG. 1, the first system 104 embodiment is depicted at a point in time wherein the first task or workpiece requires the services of the fourth system portion 146 and the first coordinator is communicating with the fourth controller 114. Proximate to issuing commands to, or taking control of, the fourth controller 114, the third controller 112 may have been deactivated or released from the control of the first coordinator 170. For example, commands previously issued to the third controller 112 might have been associated with an expiration parameter. The expiration parameter may have been, for example, a time limit or a processing milestone. When an event occurs that matches or surpasses the value of the expiration parameter, the third controller 112 may be deactivated or released from the control of the first coordinator 170.
Alternatively, the first workpiece or task may require simultaneous services of both the third system portion 144 and the fourth system portion 146. In that case, the first coordinator generates and communicates coordinated or cooperative commands to the third 112 and fourth 114 controllers.
At an appropriate point, the first coordinator will generate and transmit or communicate demands requesting services of the fifth system portion 148. If the services of the fifth system portion are required contemporaneously with the services of fourth 146 and/or third 144 system portions, then the first coordinator 170 generates and communicates cooperative commands to the fifth 116, fourth 114 and/or third 112 controllers.
FIG. 1 also illustrates the second coordinator 180 to be in communication with the second controller 110. For example, the second coordinator 180 is requesting services of the second system portion 142. The first controller 108 is being, or has been, released from serving the second coordinator 180, and the second coordinator 180 is preparing or will prepare to take control, or request the services of, the third system portion 144 through the third controller 112. Since the second 142 and third system portions are tightly coupled 152, the second controller may generate and communicate cooperative commands to the second 110 and third 112 controllers, thereby directing them to perform cooperative operations or processes on the second task or workpiece.
When the first controller 108 is released or deactivated, it becomes available to execute commands of yet another coordinator (not shown) which the high level element 160 may activate, spawn or instantiate, to coordinate and orchestrate a third task or workpiece processing.
To maintain system resource allocation flexibility and to minimize demands on system communication resources, when controllers (e.g., 108-116) are released from the control of a coordinator (e.g., 170, 180) the controllers (e.g., 108-116) transition to an idle or off state. In the idle or off state, the controllers (e.g., 108-166) do not receive status information regarding processes of the system. It may even be unknown as to which of a plurality of processes or tasks being conducted by the system will next need the services of the controller. Therefore, when a coordinator (e.g., 170, 180) or other supervisory element needs to assign a subtask to a controller, that controller must first be synchronized or made aware of a current state of a process the newly activated controller is about to take part in.
Referring to FIG. 2, in order to keep track of the state of a process wherein information regarding the state of the process is communicated over a network 210 which is associated with network delays, a first process or controller 214 maintains or has access to a process model 218. For example, the model 218 predicts a next state (x(t+1)) from a function (e.g., f(x(t),y(t−d),t) of a current state x(t), delayed sensor 222 data y(t−d) and time t. As mentioned above, the network 210 is associated with transmission delays. Therefore, the process model 218 is adapted to accept as input sensor 222 data that is delayed by a maximum delay period (d). The sensor 222 data is represented as a function of time y(t). Sensor data delayed by the delay period is represented as y(t−d). The process or controller 214 includes a clock 226 that makes the current time t available to all process or controller 214 components, including the process model 218 and a control section 230.
For example, the control section 230 may generate a control output u(t) that is also a function of the current state x(t) of the process model 218, delayed sensor 222 data y(t−d) and the current time t.
A current state 234 of the first process or controller 214 is indicated as a “computational” or a “drive” state. The drive state of a process or controller is one in which the process or controller is actively performing a function, such as controlling an actuator or process portion 238. The computational state of a process or controller is one in which the process or controller is calculating drive output levels (e.g., u(t)) in preparation for transitioning to a drive state.
As indicated above, in systems (e.g., 104) where separate processes or controllers need to behave in a coordinated manner, such as in tightly coupled systems which may act on a workpiece simultaneously or contemporaneously, it can be necessary for a newly activated process or controller to operate based on the same information as the processes or controllers with which the newly activated controller is to cooperate. However, because of the delays associated with the network 210, a second, or newly activated, process or controller 242 (for example, a process or controller activated by a coordinator or supervisory element 170, 180, 244) cannot immediately know the current state (e.g., x(t)) of the process or process model 218 of the first process or controller 214. All that can be available to the second process or controller 242 is delayed state information (e.g., x(t−d), y(t−d)) and the current time t (from the second processes or controller's own internal clock 246). However, since the process model 218 is a function of state, measurement and time, it is possible to calculate a current state of the process model if one can collect enough information regarding historical states, measurements and times which led to the current state, as long as one knows the function of the model or process being modeled (e.g., f(x(t), y(t−d), t)). Therefore, in a “synchronization” state 250, the second process or controller 242 includes a process history collector 254 and a model initializer 258 for initializing a copy 262 of the process model 218. The function f(x(t), y(t−d), t) of the model 218 is predetermined information regarding the behavior of the state of the model 218. Therefore, this model behavior information is or can be made available to the second process or controller 242 before or during the activation process.
The process history collector 254 collects delayed state data. The delayed state data is received from the network 210 and stored until at least enough information is collected for the model initializer 258 to calculate a current state of the process model 218 and to initialize the copy of the process model 262 to the same or an equivalent state.
Referring to FIG. 3, a method 310 for synchronizing a second process to a first process includes beginning 314 a data collection period, receiving 318 delayed state data points, storing 322 the received delayed state data points, ending 326 the data collection period after receiving the information required to determine a current state of the model 218 and determining 330 the current state of the model 218 using at least a portion of the stored 322 data.
Receiving 318 delayed state data points can include, for example, a process history collector 254 receiving 318 delayed state output data of the process model (e.g., {x(t−d), x(t−d+1), x(t−d+2), . . . , x(t−d+n)}). For instance, each delayed model 218 output state (e.g., {x(t−d), x(t−d+1), x(t−d+2), . . . , x(t−d+n)}) may have been used by the model 218 as input for a calculation or determination of a subsequent state. Additionally, or alternatively, receiving 318 and storing 322 delayed state data points can include receiving delayed information regarding other inputs to the process model 218. For example, receiving 318 and storing 322 delayed state points can include receiving 318 and storing 322 delayed sensor 222 information (e.g., {y(t−d), y(t−d+1), y(t−d+2), . . . , y(t−d+n)}) that was used as input to the process model 218 to arrive at a current state of the model (e.g., x(t)).
The data collection period can be ended 326 when sufficient data has been collected to determine 330 a current state of the model. As will be explained in greater detail below, when a forward propagation technique is used, the data collection period can be ended 326 after receiving 318 and storing 322 delayed state data that represents the state of the input to and output of the model (e.g., 218) at a point in time after the beginning 314 of the data collection period. Often the data collection period can be ended 326 when a delayed state data point is received and stored 322 after the data collection period has persisted for a period of time or for a number of state times at least as long as the delay period (d) associated with the network (e.g., 210).
Determining 330 the current state of the model (e.g., 218) using at least a portion of the stored 322 data can include using a form of forward propagation to calculate a current state of the model (e.g., 218) using some of the stored 322 data and predetermined information regarding the behavior of the state of the model (e.g., f(x(t), y(t−d), t)).
Synchronized Control Processes
Consider a set of control processes {p0, . . . , pn-1} where each process pi runs the following state based iterations over time t=0, 1, 2, . . .

xi(t+1)=f(xi(t),yi(t−d),t); xi(0)=xi0 

ui(t)=g(xi(t), yi(t−d), t)

where xi is the state of an ith process or model of the process, ui is the control output of an ith process or controller (e.g., 214), yi is measurement input, d is some nonnegative fixed integer delay (e.g., network 210 delay), and f and g are some functions of state, measurement and time. Note that we make no assumptions about the spaces over which x, u or y are defined: they could be numbers, symbols, discrete or continuous. At every time step t, each process receives a new measurement input yi(t−d), and uses the recursions above to compute the next state xi(t+1) and the current control output ui(t). [For t<d, we assume that f and g are functions of only x and t, and that they do not depend explicitly on y. Hence, we can take yi(t)=Ø (undefined) for t<d.]

The evolution of the states and the controls is completely determined by the initial states xi0 and the measurement inputs {yi(t−d)|t≧0}. Thus, it is clear that if the initial conditions are all equal and the processes are driven with the same measurements, then the states and control outputs are identical for all time. In other words, if

xi0=x0; ∀i

yi(t)=y(t); ∀i, t,

then the processes then all run the same recursion:

x(t+1)=f(x(t), y(t−d),t); x(0)=x0 

u(t)=g(x(t), y(t−d),t).  (1)

Note that this is true for any functions f and g of x, y and t. We refer to such a set of processes, where the xi(t) are identical for all i and all time, as synchronized. We also refer to ui(t) that are based on, or are functions of, synchronized functions, such as, xi(t), as synchronized.

We derive a method (e.g., 310) for synchronizing a new or second process pn (e.g., 242), which starts at some time t'≧d, to the existing processes {p0, . . . , pn-1}, for all time t≧t' as follows. We assume pn (e.g., 242) knows f and perhaps g, but not x0. We would like this method to work for any choice of functions f and g of x, y and t.
Synchronization with Delayed Measurements
At the heart of our development is the following property of the state recursions in eq. (1): The current state captures all the past. Specifically, for any time t', future values of the state {x(t)|t≧t'} only depend on the current state x(t') and future inputs {y(t−d)|t≧t'}. All the effects of past inputs are “summarized” in the current state.
The property above suggests the following method for synchronization. For any t'≧0, it follows immediately from eq. (1) that a sufficient condition for pn (e.g., 242) to be synchronized with {p0, . . . , pn-1} for all time t≧t', and for any functions f and g of x, y and t, is to set

xn(t′)=x(t′); at time t'

yn(t−d)≡y(t−d); ∀t≧t'  (2)

In fact, this condition is also necessary, since it is easy to construct simple examples of functions f and g for which synchronization for all t≧t' fails, if any part of condition (2) does not hold. We call this method instantaneous initialization, since pn (e.g., 242) receives x(t') instantly, without any delay.

Now suppose that we can set yn(t−d)≡y(t−d) for all t≧t' but, because of the delay (such as the communications delay of the network 210), the second or new process pn (e.g., 242) cannot receive the current state x(t') immediately. Instead, as mentioned about with regard to FIG. 2, the second or new process pn (e.g., 242) only has access to the delayed history of the states and measurements:

ld(t′)={(x(0), y(0)), (x(1), y(1)), . . . , (x(t'−d), y(t'−d))},

which does not explicitly contain x(t').

It turns out that synchronization based on ld(t') is still possible, albeit with a little more effort. Observe that x(t') can be computed from the information in ld(t') by first performing d iterations of the state recursion in eq. (1):




  
    
      
        
          
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We refer to operations such as the one illustrated in eq. (3) as forward propagating the state from x(t'−d) to x(t'), and we represent it using the following shorthand notation

x(t′)=Φ(x(t'−d)|y(t'−2d), . . . , y(t'−d−1))

[As before, we take y(t)=Ø for t<d.]

Note that, provided that t'≧d, all of the information required for the forward propagation operation, namely x(t'−d) and {y(t'−2d), . . . , y(t'−d−1)}, is available in ld(t'). Furthermore, this is the only information that we need from ld(t'). In other words, we only need to receive 318 a delayed history that is d time steps deep. And from that, the only value of the states (of the process or model (e.g., 218) to which the new or second process or controller pn (e.g., 242) is being synchronized) that we need is the most recently received 318, namely x(t'−d).
Thus, for t'≧d, a sufficient condition for pn to be synchronized with {p0, . . . , pn-1} for all time t≧t', and for any functions f and g of x, y and t, is to set

xn(t′)=Φ(x(t'−d)|y(t'−2d), . . . , y(t'−d−1)); at time t'

yn(t−d)≡y(t−d); ∀t≧t'  (4)

Once again, it can be shown that this condition is also necessary, as it is easy to construct simple examples of f and g where synchronization would fail if any part of the conditions (4) were not true.
We summarize our findings in the following:

    
    
        Proposition: Let {p0, . . . , pn-1} be a set of processes running (1). A new process pn, which knows f (and, in some cases, g), can be synchronized with the given set from time t' onwards, and for any functions f and g of x, y and t, if and only if, the following conditions hold: pn receives the same input measurements {y(t−d)|t≧t'} and either:
        
            1. t'≧0 and, at time t', pn has access to x(t'), for synchronization via instantaneous initialization (2);
            2. t'≧d and, at time t', pn has access to x(t'−d) and {y(t'−2d), . . . , y(t'−d−1)}, for synchronization via forward propagation (4).
        
        
    
    


Asynchronous Delayed Measurements and Histories
We now consider the problem of synchronization with asynchronous measurements. By asynchronous measurements, we mean that at certain times, some of the elements of the measurement sequence {y(t−d)|t≧0} could be missing, but the ones that arrive do so in the right order. Also, some of the states could be missing from the history ld. We will use the symbol Ø to denote missing measurements or states; it should be interpreted as meaning “no information”.
This asynchronous measurements scenario can still be modeled by equation (1) as follows: at each time t, define y(t−d) as:




  
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Synchronization with Asynchronous Delayed Measurements and Histories
Since we have shown that the asynchronous delayed measurements scenario can be modeled by equation (1), the conditions for synchronization are given in our Proposition. We will now apply the conditions of the Proposition to this specific asynchronous measurements context.
It follows immediately from the Proposition that synchronization using instantaneous initialization always works in this asynchronous case.
Now consider forward propagation. In this case, the Proposition states that synchronization (e.g., 310) at a time t'≧d using forward propagation is possible if and only if: the second or new process pn (e.g., 242) receives 318 the same measurements for all t'≧d and, at time t', the second or new process pn (e.g., 242) has access to (e.g., receives 318 and stores 322) x(t'−d) and {y(t'−2d), . . . , y(t'−d−1)}. Note that, due to missing measurements or states, in general, at a given time t', the delayed state and measurement history (e.g.,. the data stored 322 by the process history collector 254) ld(t') will have the form:




  
    
      
        
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where “state” in (5) refers to the delayed state x(t'−d). From (5), we see that for all t'≧d, Id(t') will always contain the information {y(t'−2d), . . . , y(t'−d−1)}. Entries of Ø for y pose no problem, since they represent what was actually used in f and g in (1) for {p0, . . . , pn-1}. However, (5) also shows that it could happen that, at certain times t'≧d, ld(t'), does not contain x(t'−d). At such times, synchronization using the forward propagation technique in (4) is not possible, and one would have to wait until a time t′′>t', when x(t′′−d) is available, to use forward propagation.

Thus we conclude that in the asynchronous case, synchronization using forward propagation is only possible at times t' when delayed state x(t'−d) is available. Otherwise it is necessary to wait to end 326 the historical data collection period until a time at which the delayed state is available.
State Machine Implementation
This section gives an example of how the synchronization mechanism can be used in practice. The goal in this example is to synchronize pn (e.g. 242/262) to {p0, . . . , pn-1} (e.g., 214/218) from time tDrive until a time tOff. This will be accomplished by embedding the process in a finite state machine (FSM), e.g., 234, 250.
Referring to FIG. 4, the FSM is described by a state chart 410. The state chart 410 is event driven, for example, by the clock tick events, which occur at integer multiples of Ts, a control sample period. At each clock tick, the FSM performs actions based on which state it is in. Usually, this is the action specified in the “do” statement. However, upon the assertion of certain guard conditions, the state machine may transition to another state. If this is the case, then the overall transition operation will consist of three steps: performing the exit actions of the current state, changing the name of the state, and performing the entry actions of the new state.
Referring to FIG. 5, it is assumed that all the processing takes place very quickly and this is shown in a timing diagram 510 by black slabs 514 on the time axis 518. Processing includes all the discrete state machine operations such as accepting inputs, exporting outputs, checking guard conditions, entry and exit actions, changing state, etc., as well as the continuous operations such as control computation and forward propagation to determine 330 a current state of a process or controller (e.g., 214, 218) being synchronized to, using equations (1) and (3), etc. To keep things simple in this example timing diagram 510, we assume the delayed y measurements and x states always arrive (or fail to arrive) simultaneously, thus the y symbols 520 shown denote (x,ym) pairs, and (Ø,Ø) pairs are omitted for clarity.
The FSM has four states: off 540, synch 544 (e.g., 250), compute 548, and drive 552 (e.g., 234). In the off-state, the FSM waits until a time tOn or a command or message from a supervisory or coordinating element (e.g. 244, 170, 180), at which point it transitions to the synch-state. The time tOn is chosen to be sufficiently in advance of tDrive, to provide enough time for a process history collector (e.g., 254) to receive 318 and store 322 the required delayed state data measurements and for a model initializer (e.g. 258) to initialize a copy (e.g. 262) of an appropriate process model before tDrive. For example, in the process history collector 254, the synch-state collects measurements, until a time when it has a delayed measurement and state history that is d time steps deep and a state measurement arrives. At that point it exits the synch-state and the model initializer 258 initializes the process model (e.g., 262) by forward propagation. Then the new or second processor controller 242/250 transitions to the compute-state, and executes the entry action, namely performing a first iteration of (1). It then continues to perform the control computation of equation (1), as shown in a do-statement equation 556, until a time tDrive or another command is received from the supervisory element (e.g., 244, 170, 180), at which point it transitions to the drive-state. The drive-state is very similar to the compute-state, except that, as mentioned above, the control is actually applied to the target system element. Then, at a time tOff, the FSM turns itself off or is commanded to the off state 540. This whole process is illustrated in the timing diagram of FIG. 5.
Hence by embedding the new or second process pn (e.g., 242) in an FSM, the desired synchronization can be accomplished in practice.
A numerical example may be helpful in understanding forward propagation and embodiments of the method 310 for synchronizing a second process to a first process. While in an off state (e.g., 540) a process or controller (e.g., 242) is dormant except for determining whether its time to move to the synchronization state (e.g, 544, 250). For example, the second process or controller (e.g., 242, 108-116) may monitor a network (e.g., 210) for activating commands from a supervisory device (e.g., 244, 170, 180). Alternatively, the process or controller (e.g., 242, 108-116) may have instructions to switch to the synchronization state (e.g, 544, 250) at a predetermined time (e.g., tOn) and be set to transition upon the arrival of that time.
When the second process or controller (e.g., 242, 108-116) transitions to the synchronization state (e.g, 544, 250) a data collection period 314 begins and a process history collector (e.g., 254) begins receiving 318 and storing 322 delayed state data. The data collection period may end 326 when the process history collector (e.g., 254) has received and stored sufficient data to perform forward propagation. For instance, referring to the example of FIG. 5, tOn=2 and the network delay is d=3 control sample periods long. At t=2. delayed state data points (y(2−d), x(2−d))=(y(−1), x(−1)) are received 318 and stored 322 by a process history collector (e.g., 254) in a memory device associated with a new or second process or controller (e.g., 242). At t=3, (y(0), x(0)) are unavailable and null values are stored 322. At t=4, (y(1), x(1)) are received 318 and stored 322. At t=5, (y(2), x(2)) are unavailable and null values are stored. At t=6, (y(3), x(3)) are received. 318 and stored 322.
Also, at t=6, sufficient data to perform forward propagation has been collected and the current state of the model to which this second process or controller (e.g., 242, 108-116) is being synchronized can be determined 330. For example, From equation (3) and the stored data, the model initializer (e.g. 258) calculates x(6−3+1)=f(x(6−3),y(6−3−3),6−3) or x(4)=f(x(3),y(0),3). The model initializer (e.g. 258) is able to do this because the model initializer (e.g. 258) has access to predetermined information regarding the behavior of the state of the process or model (i.e.; the model initializer (e.g. 258) has access to f(x(t),y(t−d),t) of the first process or controller or the process model thereof (e.g. 214/218) and x(3) was stored at t=6. At t=3, y(0) was unavailable (to both the first and second process or controllers) and so a null value was used as input to the first process or controller model 218 and was stored 322 at t=3 and is used as input to this stage of the forward propagation. Next, the model initializer (e.g. 258) calculates x(5)=f(x(4),y(1),4). This is possible because x(4) was calculated above and y(1) is a portion of the data received 318 and stored 322 during the data collection period at t=4. At this point, the current value of the first process or model (e.g., 214/218), x(6)=f(x(5), y(2),5), is calculated from x(5), which was calculated above, and from the null value for y(2) which was stored 322 at t=5. A controller output value u(6)=g(x(6),y(3),6) can also be calculated. For example, x(6) was just calculated above and y(3) was received 318 stored 322 at t=6.
The new or second process (e.g., 242) now has enough information to transition to the computation state (or to the drive state). In the computation state (comp) the new or second process (e.g., 242) uses (1) to maintain synchronization. For example, in anticipation of t=7, both the first 214/218 and second 242/254 processes or controllers can calculate x(7)=f(x(6), y(3),6). The model output x(6) was calculated by both processes or controllers as described above, and y(3) was stored by both process or controllers at t=6. Future states and outputs can be calculated as new delayed measurements (y(t−d)'s) are received and new x's are calculated by local models (e.g., 218, 254).
Role of Synchronization in Tightly Coupled Printing Systems
We will now describe the role of the synchronization technique described above can play in the distributed control architecture of a tightly coupled printing system. In such systems, some transport actuators are referred to as “nips.” The “nips” are the rollers which move the paper or print media through the system. Different nips may be controlled by independent nip controllers. All nips that are touching a sheet of paper or print media at a given time must be synchronized. New nip controllers must be able to join in the control process when the paper arrives at nips associated with the new nip controllers while other nip controllers may be deactivated when a sheet of paper is no longer in contact with them. All communication of measurements and states to the nip controllers can be across a network (e.g., 210), with a worst case delay of d. The measurements are asynchronous because they are triggered by edge crossings (sheets of paper interacting with edge detection sensors).
Referring to FIG. 6, an embodiment of a document processing system 604 includes a high level element 608, a first marking engine 610, a second marking engine 612 and a transportation system 614.
For example, the first and second marking engines 610, 612 may be xerographic marking engines. Alternatively, one or more marking engines of an embodiment may be of other technologies, such as, but not limited to, ink jet marking technology.
The transportation system 614 transports print media such as a first sheet 616 and a second sheet 618 between the first marking engine 610 and the second marking engine 612. In the illustrated system 604, the transportation system includes a plurality of transport modules. For instance, the plurality of transport modules includes a first, second, third, fourth, fifth, sixth and seventh transport module 620, 622, 624, 626, 628, 630, 632. The system 604 may include additional modules. For example, the additional modules may include a media or paper feeder 633, which delivers sheets of print media or paper to one or both of the marking engines 610, 612. Additional modules (not shown) may transport print media from either or both marking engines 610, 612 to other devices, including, but not limited to, additional marking engines and/or output devices such as paper trays, stackers, collators, staplers and other binders. Furthermore, the plurality of transport modules may form paths that branch off from the illustrated path (620, 622, 618, 624, 626, 630, 632) to transport sheets to other marking engines (not shown) or other devices.
In the illustrated document processing system 604, each transport module 620-632 includes transport actuators. For example, the transport modules 620-632 include motor driven nips 634 for driving or urging print media through the transport system 614. Additionally, or alternatively, the modules 620-632 may include flippers or gates for redirecting print media toward other portions (not shown) of the transportation system 614. Furthermore, the modules may include other kinds of transport actuators. For instance, air jets and/or spherical nips may be included in the transport modules (e.g., 620-632). For the purposes of illustration, the modules of FIG. 6 are associated with the nips 634 depicted to their left. The transport modules 620-632 of the document processor system 604 include sensors (e.g., 222). For instance, the sensors may be sheet presence or position sensors. Sensors that report speed or trajectory may also be included instead or in addition. Alternatively, such parameters may be calculated from a series of position measurements reported by a series of sensors. As illustrated, each module 620-632 includes a left side sensor 636 and a right side sensor 638.
Each transport module 620-632 also includes or is associated with a respective module controller 640, 642, 644, 646, 648, 650, 652 (i.e., embodiments of first and second processes or controllers 214, 242). For example, the module controllers 640-652 control the actions of the transport actuators (i.e., embodiments of first and second process portions 238, 270) of their respective modules 620-632 and receive and relay information from their respective sensors 636, 638.
The high level element 608 (e.g., an embodiment of high level element 160) is operative to generate sheet processing task descriptions or itineraries describing respective sheet processing tasks, to activate respective sheet coordinators (e.g., a first sheet coordinator 660 and a second sheet coordinator 664, which are embodiments of supervisory elements or coordinators 170, 180, 244) and to communicate the respective sheet processing task descriptions to the respective sheet coordinators (e.g., 660, 664). For example, the supervisory element 608 receives a job description 670. The job description 670 may include descriptions of sheets or pages. The descriptions may include images, or references to images stored elsewhere and indications as to an order in which the images are to appear on sheets of print media. For example, the job description 670 includes page description language describing text and fonts and graphic items as well as their location on particular pages of a document. The high level element 608 activates, instantiates or spawns a sheet coordinator (e.g., 244) for each sheet or page (a sheet may have two sides and may, therefore, comprise two pages). The high level element 608 analyses the job description 670 and may schedule or plan operations to create the document described in the job description 670. In so doing, the supervisory element 608 generates respective sheet processing task descriptions or itineraries for the transportation and processing of sheets between system resources.
For instance, regarding the transportation of a sheet between system resources, an example itinerary or sheet processing task description may have the following form:

    
    
        Itin 1 1 11
        feeder1 feed 19.544
        me 1 print image27 20.201
        m1 left2right 23.341
        m2 left2right 23.495
        m3 left2right 23.625
        m4 left2right 23.755
        m5 left2right 23.885
        m6 left2right 24.015
        m7 left2right 24.145
        me2 print image28 24.275
        finisher1 stack 27.415
    
    


The first line is, for example, an itinerary or sheet processing task description identifier. The rest of the itinerary specifies, for example, that a component named feeder1 (e.g., 633) should feed a sheet at time 19.544, then a component named me1 should execute a print action on an image named image27 at a later time, then a component named m1 should execute an action (move the sheet left to right) at a still later time, and so on.
The respective sheet coordinators (e.g., 660, 664) are operative to receive the respective sheet processing task descriptions or itineraries and, based on those respective descriptions, identify a plurality of respective sheet processing subtasks to be performed in order to complete the respective sheet processing tasks, identify respective controllers (e.g., 214, 242, 640-652, 674-682) for controlling respective process actuators to perform the respective sheet processing subtasks, generate respective commands for performing the respective sheet processing subtasks and communicate the respective commands to the respective module controllers as appropriate to the respective subtasks. Additionally, the respective sheet coordinators (e.g., 660, 664) may identify respective information sources that are able to provide progress information regarding the performance of the respective subtasks, collect the respective progress information from the respective subsets of information sources and communicate the respective progress information to the respective module controllers as appropriate to the respective sheet processing subtasks.
For example, the information sources may include the sensors 636, 638. Additionally, or alternatively, the module controllers themselves may maintain models (e.g., 218, 254) or estimators of the progress of respective subtasks. Such models are referred to as sheet observer models. In this regard, the module controllers or the estimates (e.g., x(t)) or models of the module controllers may be considered information sources.
For instance, in the illustrated document processing embodiment 604, subtasks for a first sheet may have included matching a speed of nips 634 of the first module 620 to a speed of a sheet exiting the first marking engine 610 and receiving the first sheet 616 therefrom. A second subtask might have been for nips 634 of the second module 622 to match the speed of the first sheet 616 as it exited the first module 620. A subtask of the third module 624 may have been to match the speed of the first sheet 616 as a leading edge thereof exited the second module 622. Yet another subtask may have been for the nips 634 of the first, second and third modules 620, 622, 624 to accelerate or to begin to accelerate the first sheet 616 to a higher transportation system 614 transport speed.
Additional subtasks associated with the fourth, fifth and sixth modules 626, 628, 630 may have included matching associated nip 634 speeds to the speed of the first sheet 616 as it entered each module 626, 628, 630 and/or continuing to accelerate the sheet 616.
The transfer or movement of a sheet from module to module must be done in a coordinated manner. In the document processing embodiment 604, the modules 610, 612, 620-633 are tightly coupled by their relationship to a sheet. For example, at any given point in time, a plurality of modules may be in contact with the same sheet. If the nips 634 of modules contacting a sheet are driven at different speeds or with different rates of acceleration or deceleration, the sheet (e.g., 616, 618) may be damaged or distorted in a manner that causes a jam in the transportation system 614 or system 604 as a whole. The sheet coordinators (e.g., 660, 664) ensure cooperative or coordinated actuation of the actuators or modules (e.g., 610, 612, 620-633). For example, at the instant depicted in FIG. 6, the first sheet 616 is in contact with portions of the fourth, fifth and sixth modules 626, 628, 630. The first sheet coordinator 660 is shown in communication with the fourth, fifth, sixth and seventh module controllers 646-652. For example, the first sheet controller 660 may be sending commands to the fifth and sixth module controllers 648, 650 that result in the fifth and sixth modules 628, 630 driving the first sheet 616 in a cooperative manner. For instance, the fifth and sixth module controllers 648, 650 may be directed to begin decelerating the first sheet 616. Additionally, the first sheet coordinator 660 may be requesting or receiving sensor information or sheet observer model information from the fourth module controller 646. For instance, the first sheet coordinator 660 may be requesting to be notified when a trailing edge of the first sheet 616 passes the left sensor 636 of the fourth module. Additionally, the first sheet coordinator 660 may be asking or receiving sensor information from the sixth module 630. For instance, the first sheet coordinator 660 may be requesting to be notified when a leading edge of the first sheet 616 passes or enters a field of view of the right sensor 638 of the sixth module.
This sensor information may be relayed by the sheet coordinator to the seventh module controller 652. Additionally, or alternatively, the first sheet coordinator 660 may update a model, such as a world observer model of the task or of the subtasks based on the information from the information sources or sensors (e.g., 636, 638).
In addition to possibly relaying sensor information, the first sheet coordinator 660 may be sending commands directing the seventh module controller 652 to prepare the seventh module 632 to receive the first sheet 616. For instance, the seventh module controller 652 may be directed to synchronize (e.g., 310) itself to the world observer (not shown) of the first sheet coordinator or to a sheet observer (e.g., similar to process model 218) of one of the other controllers (e.g., the sixth module controller 650) and to prepare to drive nips 634 of the seventh module 632 at a speed compatible with the speed of the first sheet 616 as the leading edge thereof exits the sixth module 630. As a result, the seventh module controller 652 begins 314 a data collection period and a process history collector (e.g., 254) receives 318 delayed state data points and stores 322 the received delayed state data points as described above. Additionally, when sufficient data is collected to determine a current state of the world observer model or the sheet observer model, the data collection period can be ended 326 and a model initializer (e.g., 258) determines 330 the current state of the world or sheet observer model, thereby synchronizing the new or seventh controller 652 or process to the sheet transportation process or to the activities of the sixth controller 650.
Additionally, the fifth, sixth and seventh module controllers 648, 650, 652 may be receiving commands directing that they begin decelerating the first sheet in preparation for its entry into the second marking engine 612. The first sheet coordinator 660 may also be transmitting commands to the fourth module controller 646 releasing it from service or subtasks related to the transportation of the first sheet 616. Alternatively, prior commands may have included an expiration event, such as a time limit or sensor reading, the occurrence of which automatically deactivates or releases the fourth module controller from services related to the first sheet 616.
At a point later in time than the instant depicted in FIG. 6, the first sheet 616 may enter the second marking engine 612 for processing. For example, the second marking engine may be used to print an image on a second side of the first sheet or may apply color markings that the first marking engine 610 did not apply. Prior to that, the first sheet coordinator may activate the second marking module controller 674 and direct it to synchronize 310 itself to the world model of the coordinator or to an observer model (e.g., similar to process model 218) of, for example, the seventh module controller 652 in a manner similar to the synchronization of the seventh module controller 652 described above.
At some point in time, the first sheet will no longer be in contact with the fourth module 626 and the trailing edge of the first sheet will be about to exit the fifth module. The fourth module controller 646 may have already been released (as described above) from subtasks associated with processing the first sheet and may have begun performing subtasks associated with processing the second sheet 618. The fifth module controller 648 may be about to be similarly released.
The sixth and seventh transport modules 630, 632 and the second marking engine (or module) 612 are likely all in contact with the first sheet 616. Therefore, the first coordinator 660 is generating or has generated and will communicate or has communicated commands for the sixth and seventh transport modules 630, 632 and the second marking engine 612 or a marking engine module controller 674. The commands may be cooperative in nature. For example, the transport modules 630, 632 may be directed to slow the sheet to a speed compatible with capabilities of the marking engine 612. Additionally, commands for the second marking engine controller 674 may direct it to control the second marking engine 612 to accept the first sheet at the compatible speed and to place specified marks on portions of the first sheet 616. As the first sheet 616 continues into the second marking engine 612, the sixth and seventh module controllers 650, 652 will be released from subtasks associated with the first sheet 616, or deactivated. Eventually, the first sheet 616 will exit the second marking engine or module 612 and be delivered to other modules (e.g., transport modules, finishers, stackers and/or other print engines). The first coordinator will continue to send appropriate commands to the subsequent modules, directing them to synchronize (e.g., 310), relay progress information and release or deactivate controllers, in the sequential manner described above, until the task described in the task description, or itinerary, received when the first coordinator 660 was activated is completed. When the task is completed, the first coordinator 660 may be deactivated.
Similar processing occurs with regard to the second 664 and subsequent (not shown) coordinators and second 618 and subsequent (not shown) sheets. For example, as depicted in FIG. 6, the second sheet 618 is within the first, second and third modules 620, 622, 624. The second sheet coordinator 664 is depicted as in communication with the first, second, third and fourth module controllers 640-646. For example, the second sheet coordinator 664 may be directing the second and third module controllers 642, 644 to drive the second sheet 618 at the same speed and/or with the same acceleration, receiving or requesting sensor information from the sensors 636, 638 of the first module 620 and/or the third module 624, releasing the first module controller 640 from tasks associated with transporting the second sheet 618, and/or directing the fourth module controller 646 to synchronize 310 itself and prepare to receive the second sheet 618 by driving nips 634 of the fourth module 626 at a speed appropriate to, or compatible with, a speed of the second sheet 618, as a leading edge thereof exits the third module 624 and enters the fourth module 626. As the sheets 616, 618 are transported through the system 604, the sheet coordinators deactivate or release module controllers no longer processing their respective sheets and send commands to downstream controllers directing them to synchronize 310 to respective processes and preparing them to receive their respective sheets.
Prior to the moment depicted in FIG. 6, the first coordinator generated and sent commands to a feeder module controller 678 directing it to control the feeder 633 to deliver the first sheet 616 to the first marking engine (or transport modules on a path thereto (not shown)) and may have generated and sent commands to a first marking module controller 682 instructing it to synchronize 310 to the feeder module controller 678 and to control the first marking engine 610 to place particular marks on portions of the first sheet 616 and deliver the first sheet 616 to the first transport module 620. As mentioned above, when their respective tasks, as described in their respective sheet processing task descriptions or itineraries, are completed, the respective sheet coordinators (e.g., 660, 664) are deactivated. For instance, they are de-instantiated or placed in an idle mode to await re-initialization with information from a new sheet processing task description.
Supervisory element (e.g., 170, 180, 244, 660, 664) and module controller embodiments (e.g., 640-652, 674-682) may be made substantially in software stored in computer storage devices, such as memory elements, and run by computational platforms, such as microprocessors, microcontrollers, and digital signal processors. Alternatively, supervisory elements and module controllers may be embodied in various combinations of hardware and software.
In a prototype, the transport module controllers were each embodied in separate computational platforms associated with transport modules on a one-to-one basis. Each transport module included a plurality of nips and flippers. Marking engines are known to include their own controllers. The high level element 608 and activated or spawned sheet coordinators (e.g., 660, 664) were software elements run by a single computational platform. However, embodiments wherein the sheet coordinators are offloaded to a second device and/or wherein the sheet coordinators, or activating data associated with the sheet coordinators, move from module controller to module controller (e.g., 640-652, 674-682) as their respective sheets move from module to module (e.g., 633, 610, 620-632, 612), are contemplated.
In this application, the f and g functions in (1) are those of the LQG (linear quadratic Gaussian) algorithm. The LQG algorithm uses the measurements to compute controls for the nips to ensure that the paper accurately follows some desired trajectory. Thus, each process pi runs an LQG controller and is responsible for controlling (the motor of) one nip. Each process is in turn embedded in an FSM which enables it to synchronize with the other processes.
It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.

## Claims

1. A method for synchronizing a second process to a first process, wherein state data regarding input to and output of a model of the first process is available to the second process after a delay period, the method comprising:
beginning a data collection period;
receiving delayed state data points regarding the input to and output of the model by a controller of the second process;
storing the delayed state data points received during the data collection period;
ending the data collection period after receiving and storing delayed state data that represents the state of the input to and output of the model at a point in time after the beginning of the data collection period;
determining a current state of the model of the process based on at least some of the stored state data points and predetermined information regarding a behavior of the state of the model; and
setting a current state of the second process according to the determined current state of the model, thereby synchronizing the second process to the first process.

2. The method of claim 1 wherein receiving delayed state data points regarding the input to and output of the model comprises:
receiving delayed model output information that was used as an input to the model for determining a next state of the model.

3. The method of claim 1 wherein determining a current state of the model comprises:
initializing a copy of the model with a portion of the stored information that represents input to the model at a first point in time after the beginning of, and before the end of, the data collection period; and
forward propagating the copy of the model based on at least one calculated next state of the model.

4. The method of claim 3 wherein forward propagating the copy of the model further comprises:
forward propagating the copy of the model based on additional portions of the stored information that represent input to the model at points in time after the first point in time.

5. The method of claim 1 wherein determining the current state of the model comprises:
calculating a past state of the model based on a first portion of the stored information and the predetermined information regarding the behavior of the model; and
calculating the current state of the first process based on the calculated past state and a second portion of the stored information.

6. The method of claim 1 wherein receiving delayed state information comprises:
receiving delayed sensor information used as input to the model.

7. The method of claim 1 wherein receiving delayed state information comprises:
receiving delayed output information from the model.

8. The method of claim 6 wherein receiving delayed sensor information comprises:
receiving delayed sheet position information from a sensor of a sheet handling system.

9. The method of claim 7 wherein receiving delayed state information comprises:
receiving delayed sheet state output information from a model of a sheet handling process.

10. A method for synchronizing a second sheet transportation process to a first sheet transportation process, wherein state data regarding input to, and output of, a model of the first sheet transportation process is available to the second sheet transportation process after a delay period, the method comprising:
beginning a data collection period;
determining a data collection state count to be a number of state times having a total duration at least as long as the delay period;
receiving delayed state data points regarding the input to and output of the model, wherein the output of the model includes at least one of a sheet position, a sheet speed and a sheet trajectory by a controller of the second sheet transportation process;
storing the delayed state data points received during the data collection period;
ending the data collection period after receiving and storing a delayed state data point after the data collection period has persisted for a number of state times at least as large as the data collection state count;
determining at least one of a current position, speed and trajectory of the sheet, from a current state of the model calculated from at least some of the stored state data points and predetermined information regarding a behavior of the state of the model; and
setting a current state for an output value of the sheet transportation controller according to the determined at least one of a current position, speed and trajectory of the sheet, thereby synchronizing the second sheet transportation process to the first sheet transportation process.

11. The method of claim 10 wherein the delay period is d state periods and receiving delayed state data points regarding the input to and output of the model comprises:
receiving at least a state of the output of the process model at a period d state periods prior to a current period represented as t′, the time d state periods prior to the current period being represented as t′-d; and
receiving additional information used as input to the model at period t′-d and at d subsequent periods.

12. The method of claim 11 wherein receiving additional information comprises:
receiving information reported by sheet position sensors.

13. The method of claim 10 further comprising:
instantiating a copy of the model that is accessible by the second process without a significant delay;
setting an initial state of the copy of the model, to be equal to the current state of the model;
updating the copy of the model according to the received delayed state data points, thereby generating updated copy model outputs that are synchronized with updated model outputs of the model;
updating of an output value of a controller according to the output of the updated copy of the model; and
driving a sheet handling device according to the updated controller output value.

14. A method for synchronizing a second process to a first process, wherein state data regarding input to and output of a model of the first process is available to a controller of the second process after a delay period, the method comprising:
beginning a data collection period;
receiving delayed state data points regarding the input to and output of the model by the controller;
storing the delayed state data points received during the data collection period;
ending the data collection period after receiving and storing required information for determining a current state of the model based on forward propagation;
using the stored required information and information regarding the behavior of the model to forward propagate the model from a state at a point after the beginning of the data collection period to the current state, thereby determining the current state of the model; and
setting a current state of the second process according to the determined current state of the model, thereby synchronizing the controller and the second process to the first process.

15. A system that is operative to control a process, the system comprising:
a model of the process;
a communications path associated with a communications delay;
a controller that is operative to control a portion of the process; and
a supervisor that is operative to activate the controller at a time appropriate for the controller to prepare for controlling the portion of the process, wherein, the controller receives information regarding states of the model of the process over the communications path after the communications delay, and wherein the controller is operative to initialize and maintain a local copy of the model for use in determining appropriate control actions, wherein the controller is operative to initialize the local copy of the model by using delayed information regarding prior states of the model to determine a starting prior state of the model and to forward propagate the model from the starting prior state to a current state of the model, thereby synchronizing the local copy of the model to the model of the process.

16. The system of claim 15 wherein the communications delay is a maximum of d state periods long and the controller is operative to initialize the local copy of the model by a process comprising:
receiving at least a state of the output of the model at a period d state periods prior to a current period represented as t′, the time d state periods prior to the current period therefore being represented as t′-d; and
entering the state of the output of the process model at the period d state periods prior to the current period into a state function that is operative to calculate a next state based on an entered state, thereby calculating a state of the model at a first subsequent period, the first subsequent period being represented as t′-d+1.

17. The system of claim 15 wherein the communications delay is a maximum of d state periods long and the controller is operative to initialize the local copy of the model by a process comprising:
entering the state of the output of the process model at the period d state periods prior to the current period and the additional information used as input to the model at period t′-d into a state function that is operative to calculate a next state based on an entered state and additional information, thereby calculating a state of the model at a first subsequent period, the first subsequent period being represented as t′-d+1, and
entering iteratively, subsequent calculated states of the model and the additional information used as input to the model at the subsequent state periods, starting with the calculated state period t′-d+1 and additional information used as input to the model at t′-d+1, into the state function, thereby calculating at least one additional subsequent state of the model, until a state for the current period t′ is calculated.

18. The system of claim 15 wherein the communications delay is a maximum of d state periods long and the controller is operative to maintain the local copy of the model by a process comprising:
receiving, at the controller, data regarding updated inputs to the model during a same period as the updated inputs are received by a device maintaining the model;
updating the copy of the model according to received data, thereby generating updated copy model outputs that are synchronized with updated model outputs of the model; and
updating a state for an output value of the controller according to the output of the updated copy of the model.

19. The system of claim 18 wherein the controller is further operative to drive an actuator according to the updated output value of the copy of the model.

20. The system of claim 18 wherein the controller is further operative to drive a sheet handling device according to the updated state for the output value.

21. The system of claim 15 wherein the controller is a second controller operative to control a second portion of the process and the supervisor is a first controller that is operative to control a first portion of the process, wherein the model is maintained by the first controller.

22. The system of claim 15 wherein the controller is a second controller operative to control a second portion of the process and the supervisor is further operative to activate a first controller that is operative to control a first portion of the process at a time appropriate for the first controller to prepare for controlling the first portion of the process.

23. The system of claim 22 wherein the model is maintained by the first controller and the model is a copy of another model maintained by another device.

24. The system of claim 15 wherein the controller comprises:
a nip controller.

25. The system of claim 16 wherein the controller is operative to control a sheet transportation process.

26. A document processing system comprising:
a first xerographic marking engine;
a sheet transport system that is operative to at least transport a sheet of print media to or from the first xerographic marking engine
a model of a sheet transportation process;
a communications path associated with a possibly variable communications delay;
a controller that is operative to control a portion of the sheet transportation process; and
a supervisor that is operative to activate the controller at a time appropriate for the controller to prepare for controlling the portion of the sheet transportation process, wherein, the controller receives information regarding states of the model of the sheet transportation process over the communications path after the communications delay, and wherein the controller is operative to initialize and maintain a local copy of the model of the sheet transportation process for use in determining appropriate control actions, wherein the controller is operative to initialize the local copy of the model of the sheet transportation process by using delayed information regarding prior states of the model of the sheet transportation process to determine a starting prior state of the model and to forward propagate the model from the starting prior state to a current state of the model, thereby synchronizing the local copy of the model of the sheet transportation process to the model of the sheet transportation process.

27. The system of claim 26 wherein the model of the sheet transportation process is operative to estimate at least one of a position, speed and trajectory of the sheet being transported by the sheet transport system.

28. The system of claim 26 further comprising:
at least a second marking engine wherein the sheet transport system is further operative to transport a sheet of print media to or from the at least a second marking engine.

29. The system of claim 15 wherein the controller receiving information regarding states of the model of the process over the communication path after the communications delay comprises at least one of:
receiving delayed sensor information used as input to the model; and
beginning a data collection period, receiving delayed state data regarding the input to and output of the model, storing the delayed state data points received during the data collection period and ending the data collection period after receiving and storing delayed state data that represents a state of the input to and output of the model at a point in time after the beginning of the data collection period.