Checkweigher Control Module Signals Process Control Problems

Hi-Speed Checkweigher XS Controller Optional Statistical Process Control module signals process control problems to ensure productivity.

The METTLER TOLEDO Hi-Speed Statistical Process Control Module (CW-SPC™) automatically calculates, plots, and displays the sample X-bar (mean), and R (range) on the large 15" XS user interface. The user can configure a unique Sample Size, Interval Type (piece count or time), and sampling intervals for each stored product, along with Upper and Lower Caution Limits, and Upper and Lower Action Limits. Process Capability Index (Cp) and Critical Process Capability Index (Cpk) values are also calculated and displayed to deliver a clear can concise value of process potential and health. Sampling data combined with Cp and Cpk values are presented graphically in real time on the CW-SPC Trend Monitoring Screen of the XS control.

The Upper and Lower Caution Limits (UCL & LCL) give the operator a visual indication of when the sample average goes outside the preset control limits. When the preset control limits are exceeded, the background of the display will change from green to yellow.

Upper and Lower Action Limits (UAL & LAL) is a secondary alert level which when crossed by the sample mean, cause the background color to change to red, and this coincides with a process alarm signal. This alarm can be tied to an external control, a horn, or similar item to alert away the user at or from the production line.

Both "Control" and "Action" limits will stay at the elevated caution levels until the process is back in control. When the process is back in control, the background color changes back to green. The change - from Green, to Yellow, to Red - permits the line operator to immediately see when a process control issue arises. Compared to a traditional, once-an-hour SPC process, the line operator would wait for the next interval to measure a sample, and plot that on the time chart, with CW-SPC, the measurements are "Real time", and the color aspect brings visible attention to the situation. CW-SPC allows the line operator to easily monitor the process and proactively react to unfavorable changes in production. Further the value of the real time CW-SPC Module is enhanced since the on-screen tool can automatically, and much more frequently, perform measurements that are normally done by hand, saving valuable time and reducing costly out of specification product. This permits a quicker recognition of a potential fault area. Since most CW-SPC measurements are done on an hourly basis, the CW-SPC module monitors production operating as a process watchdog. This means a process that strays five minutes after the last sample measurement, doesn't result in 55 minutes of faulty production.

For processes trending "out of control" there are several functions that can occur to alert the operator of the condition. The screen color change is the first indicator to the operator plus a 24VDC output signal tied to the Action Limit settings can enable the CW-SPC Module to initiate an action based on the monitored state. Also system status and error messages on the control home screen can also be displayed (see above). Finally changes in Cp and Cpk values provide enhanced indication of process capability and health. Statistical Process Control Module (CW-SPC) for XS-Series Checkweighers enables maximum uptime and productivity.

Features & Benefits

• Real Time Information allows users to monitor the process on a minute-by-minute basis rather than one hour intervals. This reduces product and packaging material loss when the process strays between hard-copy measurements
• Full Color, 15"Graph Display with Green-Yellow-Red alerts users to status of process health. The screen is visible across the production floor. Saving time as personnel can view several lines at once, rather than line by line.
• Customer Set CW-SPC parameters allows selection of sample size, sampling frequency, and process limits to either match customer quality process – or – at levels tighter than process requirements. Matching parameters means the operator is aware of process status concurrent with quality. Tighter parameters can provide an "early warning" of a possible process condition.

SOURCE: Mettler-Toledo Hi-Speed, Inc