Pulsed Eddy Current Technology

The MTG Box Gauge, based on PEC (Pulsed Eddy Current) technology, opens up new dimensions in aluminium strip gauging. Weak magnetic fields are used for measurement and the gauge is completely safe to use.

The technology is completely insensitive to anything in the measuring zone, except the metal strip. The gauge will, therefore, measure true strip thickness, unaffected by coolant, dirt, steam, air temperature variations, etc.

Since the MTG Box Gauge is material-independent, there is no need for alloy compensation and calibration.

Phase 1: Transmitting

• An electric field is generated with a transmitting coil
  
• The field is applied until it fully penetrates the strip
  
• The field "energy" is stored in the air gap and in the strip
  
• The amount of energy stored in the strip depends on the thickness alone - not on resistivity

Phase 2: Distance measurement

• Fast air response just after transmit field is swithed off
  
• The measurement of air gap energy corresponds to distance between coil and strip
  
• Pulse time t1 is typically less than 5μs

Phase 3: Resistivity measurement

• Response from the surface area of the strip
  
• The measurement starts when the air gap signal has declined
  
• The measurement of the surface response corresponds to resistivity and is dependent on the speed of the field leakage out of the strip

Phase 4: Thickness measurement

• Response from the whole strip
  
• The measurement starts when the air gap signal has declined (phase 3 is included in phase 4)
  
• The measurement of the whole strip corresponds to the complete energy stored in the strip, and continues until the strip response has almost declined (t3)
  
• The measurement corresponds to the strip thickness
  
• The resistivity does not affect the level of the whole strip response
  
• The tail part after t3 is calculated with the use of the resistivity part

The Millmate Thickness Gauging system is based on the Pulsed Eddy Current technology, which uses magnetic fields for measurement of the strip thickness. The interaction between an applied magnetic field and the electrically conductive strip indicates the thickness - without influence from material composition or conditions in the measuring gap.

Phase 1: Transmitting

• An electric field is generated with a transmitting coil
• The field is applied until it fully penetrates the strip
• The field "energy" is stored in the air gap and in the strip
• The amount of energy stored in the strip depends on the thickness alone - not on resistivity

Phase 2: Distance measurement

• Fast air response just after transmit field is swithed off
• The measurement of air gap energy corresponds to distance between coil and strip
• Pulse time t1 is typically less than 5μs

Phase 3: Resistivity measurement

• Response from the surface area of the strip
• The measurement starts when the air gap signal has declined
• The measurement of the surface response corresponds to resistivity and is dependent on the speed of the field leakage out of the strip

Phase 4: Thickness measurement

• Response from the whole strip
• The measurement starts when the air gap signal has declined (phase 3 is included in phase 4)
• The measurement of the whole strip corresponds to the complete energy stored in the strip, and continues until the strip response has almost declined (t3)
• The measurement corresponds to the strip thickness
• The resistivity does not affect the level of the whole strip response
• The tail part after t3 is calculated with the use of the resistivity part

A Millmate Strip Scanner Edge Sensor operates with weak pulsating electromagnetic fields, generated by a number of electrical coils in the edge sensor.

ABB’s patented Pulsed Eddy Current (PEC) technology makes it possible to measure accurately despite variations in material, environment conditions and measurement distance such as wavy edges and strip lift-off.

Three coils are used simultaneously. The partly covered coil at the strip edge measures a combination of strip distance and edge position. Together with the distance measurement of the two other coils a true edge position can be calculated, independent of material and distance.
In order to make measurement range wide enough, a number of measuring coils are arranged in line in the edge sensor. The relevant coils are automatically selected by the system itself.

PEC technology measurement phases

Phase 1: Transmitting

• An electromagnetic field is generated with the coils transmitting
  
• The field “energy” is stored in the air gap and in the strip

Phase 2: Position measurement

• Fast air gap response just after transmit field is switched off
  
• The measurement of air gap energy corresponds to strip edge position and to distance between coil and strip

Phase 3: Material compensation

• The material dependent response from the surface area of the strip begins when the transmit field is switched off
  
• The material signal measurement starts when the air gap signal has declined
  
• The material signal is used to compensate for the surface response during the position measurement, giving true material independent position measurement