System and method for estimating a change point time in a manufacturing process

• US 5,841,676 A
• Filed: 11/04/1996
• Issued: 11/24/1998
• Est. Priority Date: 11/04/1996
• Status: Expired due to Fees

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Faltin, Frederick William, Ali, Fatma Ozden Gur, Doganaksoy, Necip
• Primary Examiner(s): Stamber, Eric W.
• Application Number:
  US08/743,069
• Time in Patent Office: 750 Days
• Field of Search: 364/552, 364/554, 364/551.01, 364/468.15, 364/468.16, 364/468.17, 364/468.18
• US Class Current: 702/84
• CPC Class Codes: G06Q 10/06 Resources, workflows, human or project management; Enterprise or organisation planning; Enterprise or organisation modelling

Abstract

The present invention discloses a system and method for estimating a change point time in a manufacturing process. The change point estimate is estimated by a change point estimator after receiving an alarm from a statistical process control chart. The change point estimate is estimated by estimating the point of a process before and after a particular point in time and declaring the point with the maximum absolute difference to be the change point time. In the present invention there are three versions for estimating the change point estimate. After estimating the change point time any other likely change point times are then determined and used to match the dates of those changes to any possible changes in the product variables that occurred during the manufacturing process. A source causing the change in the process is then identified, allowing an engineer to provide the necessary corrective action to remedy the change.

Claims

1. A system for estimating a change point time in a manufacturing process producing various products made on multiple lines in several lots, the system comprising:

a test device for measuring a plurality of product variables representing various properties of the manufactured products for each product made on a line in a lot;

a storage device for storing the plurality of measured properties from the test device in various groupings of lots, products, and lines;

a detector for detecting changes occurring in the groupings of properties in the storage device for each product made on a line in a lot, the detector comprising at least one control chart containing control limits for each respective grouping, the detector combining the plurality of measured properties in each grouping and determining if a control limit has been exceeded for any product made on a line in a lot;

a change point estimator for estimating a change point time for each grouping that has exceeded a control limit, the change point time indicating when a process change occurred which subsequently caused the control limit to be exceeded, the change point estimator estimating the change point time by estimating each point of the manufacturing process with a combination of an exponentially weighted moving average (EWMA) and an average of points before and after each particular point in time, the change point time being the particular point in time with a maximum absolute difference between at least one of the EWMA of points before and after the particular point, the average of points before and after the particular point and the EWMA of points before the particular point and the average after the particular point; and

a diagnostic analyzer coupled to the detector and the change point estimator for helping to identify sources causing the control limit to be exceeded, and to provide a basis for selecting actions to correct the sources.

2. The system according to claim 1, wherein the change point time is the particular point in time in the manufacturing process with the maximum absolute difference between the EWMA of points before and after.

3. The system according to claim 1, wherein the change point time is the particular point in time in the manufacturing process with the maximum absolute difference between the EWMA of points before and the average after.

4. The system according to claim 1, further comprising means for determining other likely change point times for each grouping.

5. The system according to claim 4, further comprising means for matching the other likely change point times to changes occurring in any factor of the manufacturing process.

6. The system according to claim 1, further comprising means for deciding whether the control limit has been exceeded by error.

7. A method for estimating a change point time in a manufacturing process producing various products made on multiple lines in several lots, the method comprising the steps of:

measuring a plurality of product variables representing various properties of the manufactured products for each product made on a line in a lot;

storing the plurality of measured properties in various groupings of lots, products, and lines;

detecting changes occurring in the groupings of properties for each product made on a line in a lot using at least one control chart, each of the at least one control charts containing control limits for each respective grouping;

combining the plurality of measured properties in each grouping and determining if a control limit has been exceeded for any product made on a line in a lot;

estimating a change point time for each grouping that has exceeded a control limit, the change point time indicating when a process change occurred which subsequently caused the control limit to be exceeded, the change point time estimated by estimating each point of the manufacturing process with a combination of an exponentially weighted moving average (EWMA) and an average of points before and after each particular point in time, the change point time being the particular point in time with a maximum absolute difference between at least one of the EWMA of points before and after the particular point, the average of points before and after the particular point and the EWMA of points before the particular point and the average after the particular point; and

identifying sources causing the control limit to be exceeded.

8. The method according to claim 7, wherein the change point time is the particular point in time in the manufacturing process with the maximum absolute difference between the EWMA of points before and after.

9. The method according to claim 7, wherein the change point time is the particular point in time in the manufacturing process with the maximum absolute difference between the EWMA of points before and the average after.

10. The method according to claim 7, further comprising determining other likely change point times for each grouping.

11. The method according to claim 10, further comprising matching the other likely change point times to changes occurring in any factor of the manufacturing process.

12. The method according to claim 7, further comprising deciding whether the control limit has been exceeded by error.