Synchronizing controllers linked by a communications network

• US 7,149,606 B2
• Filed: 03/01/2005
• Issued: 12/12/2006
• Est. Priority Date: 03/01/2005
• Status: Active Grant

• Current Assignee: Fanuc Robotics America Incorporated (Fanuc Corporation)
• Original Assignee: Fanuc Robotics America Incorporated (Fanuc Corporation)
• Inventors: Krause, Kenneth W.
• Primary Examiner(s): PATEL, RAMESH B
• Application Number:
  US11/069,126
• Publication Number:
  US 20060200254A1
• Time in Patent Office: 651 Days
• Field of Search: 700 2- 3, 700/20, 700/14, 700/170, 700/248, 700/306, 702/79, 702/89, 702/176, 713/375, 713/400, 318/705, 375/106, 375/145, 375/149, 375/240.28
• US Class Current: 700/306
• CPC Class Codes: G05B 19/0421 Multiprocessor system
• CPC Class Codes: G05B 2219/25473 Compensation variable cycle time, synchronized processes
• CPC Class Codes: G05B 2219/25483 Synchronize several controllers using messages over data bus
• CPC Class Codes: G05B 2219/33097 Variable ticks, align clocks, to synchronise cycles with other machine, robot

Abstract

A control system includes controllers that are coupled mutually by a communications network, on which information, transmitted from a master controller to a slave controller, is used to make timing corrections on the slave controller in order to synchronize event timers on the slave controller with that of the master controller. Timing accuracy for the occurrence of the event commanded by each controller is synchronized in narrow range of time, preferably within a few milliseconds depending on the specific application and system.

Claims

1. A system for synchronizing controllers comprising:

a first controller for incrementing a master count of ticks produced at periodic intervals, and for executing event information when the master tick count matches a target tick count;

a second controller for adjusting, with reference to the master tick count, a incremented shadow tick count of ticks produced at periodic intervals, and for executing second event information when the shadow tick count matches the target tick count, the executions by said first and second controllers being mutually synchronized; and

a communications network interconnecting said first and second controllers.

2. The system of claim 1 wherein said first controller further comprises a counter for maintaining the master count, said first controller further determining a target tick count at which an event is to occur on said first controller, and associating the target tick count with the first event information before the master tick count matches the target tick count.

3. The system of claim 1 wherein said second controller further increments the shadow tick count at periodic intervals, determines a target shadow tick count at which an event is to occur on said second controller, and associates the target shadow count with the second event information before the shadow tick count matches the target tick count.

4. The system of claim 1 wherein said first controller further comprises a counter for maintaining the master count, said first controller further determining a target tick count at which an event is to occur on said first controller, and associating the target tick count with the first event information before the master tick count matches the target tick count;

and said second controller further incrementing the shadow tick count at periodic intervals, determining a target shadow tick count at which an event is to occur on said second controller, and associating the target shadow count with the second event information before the shadow tick count matches the target tick count.

5. The system of claim 1 further comprising a first robotic actuator operating under control of said first controller;

and a second robotic actuator operating under control of said second controller.

6. The system of claim 1 wherein the communications network is at least one of a wired network and a wireless network.

7. In a system of intercommunicating controllers, a method for synchronizing the occurrence of an event on each controller, the method comprising the steps of:

a. maintaining on a master controller a count of ticks produced by a first clock;

b. sending an inquiry from a slave controller to the master controller for the tick count on the master controller;

c. sending a response from the master controller to the slave controller that includes the tick count on the master controller;

d. adjusting a shadow tick count on the slave controller on the basis of the tick count sent from the master controller;

e. incrementing the shadow tick count on the slave controller when ticks are produced by a second clock;

f. establishing a target tick count at which an event is to occur on the master controller and a target shadow tick count at which an event is to occur on the slave controller;

g. using the master controller to command an event when the tick count matches the target tick count; and

h. using the slave controller to command an event when the shadow tick count matches the target shadow tick count.

8. The method of claim 7 wherein:

step b. further includes determining the time when the inquiry is sent from the slave controller; and

step d further includes;

determining the time when the response is received on the slave controller; and

adjusting the shadow tick count on the slave controller by a difference between the tick count received from the master controller and the tick count on the slave controller, plus one-half the length of the period that begins upon sending the inquiry from the slave controller and ends upon receiving the response on the slave controller.

9. The method of claim 7 further comprising a step of maintaining on the slave controller a count of ticks produced by the second clock on the slave controller;

and wherein said step e. further comprises using ticks produced by the second clock to increment the shadow tick count.

10. The method of claim 7 wherein said step h. further comprises the steps of:

identifying event information to be executed on the master controller at a future event;

associating the event information with a corresponding target tick count; and

using the master controller to execute the event information that is associated with the corresponding target tick count upon the occurrence of the target tick count on the master controller.

11. The method of claim 7 wherein said step g. further comprises the steps of:

identifying event information to be executed on the slave controller at a future event;

associating the event information with a corresponding target shadow tick count; and

using the slave controller to execute the event information that is associated with the corresponding target shadow tick count upon the occurrence of the target shadow tick count on the slave controller.

12. The method of claim 7 wherein said step b. further comprises:

determining from prior adjustments of the shadow tick count on the slave controller a desired interval length since the last adjustment on the slave controller;

determining the duration of a current period that began upon making the last adjustment on the slave controller; and

determining a fractional tick adjustment from the length of the current period and the desired interval length.

13. The method of claim 7 wherein said step b. further comprises:

repetitively determining from prior adjustments of the shadow tick count on the slave controller an average of interval lengths since the last adjustments on the slave controller;

repetitively determining from prior adjustments of the shadow tick count on the slave controller an average of the magnitude of said prior adjustments; and

determining a fractional tick adjustment from the average of said interval lengths and an average of the magnitude of said adjustments.

14. A method for synchronizing events produced by multiple controllers, the method comprising the steps of:

a. connecting a master controller to a communications network;

b. connecting a slave controller to the communications network;

c. synchronizing an incremented tick count on each controller using the communications network;

d. determining a target tick count on each controller at which an event is to occur;

e. preparing event information on the master controller and slave controller to be executed at the next event; and

f. executing the respective event information on the master controller and slave controller when the tick count on the respective controller matches the target tick count.

15. The method of claim 14 wherein:

said step b. further comprises connecting multiple slave controllers to the communications network;

said step e. further comprises preparing event information on each slave controller to be executed at the next event; and

said step f. further comprises executing the respective event information on each slave controller when the tick count on the respective controller matches the target tick count.

16. The method of claim 14 wherein robotic actuators are subject to the control of at least one of the controllers, and said step f. further comprises controlling the operation of the robotic actuators in response to executing the respective event information on the at least one controller when the tick count on the at least one controller matches the target tick count.

17. The method of claim 14 wherein multiple robotic actuators are subject to the control of each of the controllers, and said step f. further comprises controlling the operation of the robotic actuators in response to executing the respective event information on the master controller and the slave controllers when the tick count on the respective one of the controllers matches the target tick count.

18. The method of claim 14 wherein said step e. further comprises the steps of:

sending an inquiry from the slave controller to the master controller for the tick count on the master controller;

sending a response from the master controller to the slave controller that includes the tick count on the master controller; and

adjusting a tick count on the slave controller on the basis of the tick count sent from the master controller.

19. The method of claim 14 wherein said step e. further comprises the step of receiving on the slave controller event information sent on the communications network from the master controller before the tick count matches the target tick count.

20. The method of claim 14 wherein said step c. further comprises the steps of:

referencing the target tick count to the respective event information; and

receiving on the slave controller the event information sent on the communications network from the master controller before the tick count matches the target tick count.