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ZhuoHe Intrumentation HomePage > Technical article > >Basis weight sensor designed by Thermo

Basis weight sensor designed by Thermo

2009-12-23 16:23   Article Source:N/A   View Times:

Thermo's Basis Weight Sensor provides fast, accurate profiles of a moving web. The transmission sensor(sensor transmitter), utilizing a Strontium 90 source, is suitable for medium to heavy weight plastic and rubber sheet, coated substrates, paper, textiles and composite products. With reliable density information, the sensor can provide a noncontacting thickness measurement as well.

Thermo's Basis Weight Sensor

The Basis Weight Sensor is designed to provide high cross-direction resolution at fast scan speeds, without compromising absolute sensor accuracy. Unique optics enhances the cross-direction resolution of the sensor, allowing streaks as narrow as 1.5 mm to be detected in a single traverse.

The intelligent weighing sensor design permits advanced compensation techniques to be executed in realtime, providing accurate basis weight measurements scan-to-scan, roll-to-roll, and runto- run. High weighing accuracy (± 0.25%) provides tighter specifications, consistent control, and a repeatable, reliable online quality reference.

FEATURES AND BENEFITS

* Transmission sensor measures the basis weight of a suspended web

* 100 millicurie Strontium 90 source provides excellent signal-to-noise characteristics and high repeatability

* Digital Signal Enhancement improves signal-to-noise ratio by a factor of four

* Sensor exhibits 50 - 70% less noise than conventional designs

* Unique elongated source and receiver optics provide 1.5 mm streak detection

* Measurement plateau eliminates sensitivity to dynamic offset between source and receiver

* Periodic standardization continually compensates full calibration range for dirt, environment, and sensor drift

* Time-based model compensates for source decay, ensuring continual accuracy over complete system life cycle

DESCRIPTION

The Basis Weight Sensor measures the weight per unit area of a suspended web by monitoring the intensity of beta radiation, which is transmitted through the product. A Strontium 90 isotope emits beta particles (excited electrons) as a function of nuclear decay. The concentration of beta particles transmitted through the web into the ion chamber is related to the basis weight of the product. The ion chamber signal is monitored by the distributed processor based upon position feedback from the scanner encoder. Discrete measurements are allocated into profile segments for display, control, and reporting. Profiles are converted into customer units according to product specific calibration data.

High Intensity Source

A 100-millicurie Strontium source provides high signal strength, resulting in excellent repeatability (0.1% repeatability). The weight sensor exhibits 50-70% less noise than conventional designs. The unique source geometry, which is elongated in the machine direction, allows high cross-direction resolution without compromising absolute accuracy.

Streak Resolution

The weigh sensor exhibits a time constant of less than 15 milliseconds; allowing high-resolution profiles to be generated at fast scan speeds. The weigh sensor processor utilizes a unique enhancement algorithm, which improves the signal-to-noise ratio by a factor of 4 without a degradation of sensor response. The digital enhancement techniques, fast sensor response, and elongated source enable detection of streaks as narrow as 1.5 mm in a single traverse.

Measurement Plateau

Proprietary weight sensor optics provide a measurement plateau that exceeds the mechanical frame run out tolerances by 300%. The plateau eliminates sensitivity to X, Y, and Z spatial deviations between the source and receiver heads which can occur as the weight sensors traverse.

Temperature Compensation

Infra-Ray Temperature Compensation monitors the temperature at four points and corrects for air density changes in real-time. Two highspeed thermotectic devices, smooth-sealed in each sensor head, continually monitor the temperature in the air gap. The temperature of the source and receiver head is monitored as well, allowing the weigh system to compensate for air density changes, which can occur in the internal air columns.

Standardization

In order to ensure accuracy scan-to-scan, rollto- roll, and run-to-run, the system periodically executes a standardization procedure, which compensates for dirt buildup on the windows, environmental and electrical dynamics, and source decay.

Open-shutter air gap and closed-shutter measurements monitor the effect of dirt, environmental changes, and electronic drift. A time-based model calculates the impact of source decay, so that the full calibration curve can be modified in order to compensate for any dynamic effects. This compensation ensures accuracy throughout the full weight sensor measurement range, guaranteeing that the system always provides a standard quality reference.




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