Basics of Thickness Gauging Systems

Thickness Gauging Systems

Depending on the measuring tasks and the corresponding operating conditions, such as the specific installation situation and location, the required accuracy and environmental conditions you can selected from a wide range of Thickness Gauging Systems.

Static and Dynamic Thickness Gauging

Basically, we distinguish two types of measurement systems. In the case of static measuring system (e.g. LTM-BASE), the measuring point is unchanged with respect to the system. In the case of the dynamic measuring system (e.g. LTM-SMART), the measuring point can change its position and within the system it can be dependent on time or distance.

Measurement modes of Thickness Gauging Systems

Depending on the requirements or the selected Thickness Gauging System, different measuring modes are available. These can be selected in measuring software LTM-S and stored with respect to the specific product. Dynamic measurement systems offer a wider range of measurement modes.

• Line Measuring
  The measuring system remains in the static state. The Y coordinate of the measuring system remains unchanged during the measuring process. As the material travels in the X direction, thickness measuring values are determined along a so formed straight line.
  
• Micro-Traversing
  During micro - traversing, the Y - coordinate of the measuring system changes dynamically. The system oscillates back and forth between two freely selectable Y coordinates (Y1 and Y2) at a constant speed. The resulting traversing path is always smaller than the current coil or material width. As the material moves in the X direction, thickness measurements are taken along a sinusoidal line. Only a partial area (strip or trace) of the material is measured.
  
• Macro-Traversing
  In the case of macro-traversing, the Y coordinate of the measuring system changes dynamically. By doing that, the system oscillates back and forth between two fixed Y coordinates (Y1 and Y2) at a constant speed.
  The two coordinates Y1 and Y2 result from the two edges of the current material to be measured. Only the movement of the material in the X direction produces thickness measurements along a determined sinusoidal line. It is measured over the entire strip or material width.
  
• Macro-Traversing with track measuring
  The macro-traversing with track measurement basically works as described above. In addition, the LTM-S measuring software can be used to subdivide the material into subareas (tracks). Each track has two unique Y coordinates (YS, YE), which determine the track width. It is thus possible to hide tracks (partial areas) during the measurement, or to include only relevant tracks for further processing in the measurement data evaluation.

The Principle of Thickness Gauging via Laser Triangulation

The lasers used in the Thickness Gauging Systems by ROLAND ELECTRONIC function according to the principle of triangulation. That means a distance determination by simple trigonometric function. A laser spot is projected onto the material surface. Depending on the distance of the material surface, the reflected laser beam meets the receiving element of the laser at a certain angle. From this angle and the fixed distance of the laser source to the receiver part, the distance of the laser to the material surface can be determined. Within ROLAND ELECTRONIC Thickness Gauging Systems, a measuring point always consists of two laser sensors, which are positioned exactly opposite one to another between which the material to be measured is guided through. Each laser sensor individually measures the distance to the material surface (A1 or A2).

The thickness „d“ of the material is obtained by subtracting the sum of the measured values (A1 and A2) from the installation distance (Ax).

Mechanical functional dimensions, operation range

For the installation of the measuring system, the knowledge of the mechanical functional dimensions, es- pecially the knowledge of the measuring center points, as well as the system´s matching line is necessary. In contrast to the measuring center, which is a fixed dimensional size, the system´s matching line must be calculated. The operation range is defined as the area (Z - direction) in which the material which must be measured is allowed to move in the working position in order to ensure a plausible measurement data acquisition. The operation range cannot be fully used.

Measuring distance, measuring focus, matching line

The measuring distance is the distance range of the laser to the material (Z - direction) in which a plausible measuring data acquisition is possible. The goal must be that the material to be measured is guided past the lasers with a constant distance.The distance should ideally be laser - or material identical. The measuring focus is defined as the point or the dimension in which both laser beams are meeting, in the Z-direction at their optimal measuring distance. The bottom edge of the system always serves as a reference. The matching line results from the distanceof the reference edge of the system (Z-direction) to the measuring center point minus half the material thickness. As a measure of the material thickness, the most commonly measured nominal thickness is used.

Structure of Thickness Gauging Systems

During the development of Thickness Gauging Systems all necessary requirements in the field of mechanics and electrics were taken into account, which will result from modern production processes and the measuring technology to be integrated over the next few years.

Mechanic design

In the mechanical design, a special emphasis was placed on a rigid, vibration and temperature resistant construction. Mechanical interfaces allow easy mounting and integration into the customer system.

Material transfer system

The dynamic systems LTM-SMART and LTM-MAXI are structurally designed for an upgrade with a material transfer system. This permits, if not feasible from the customers system, the necessary smooth transport of the materials, as well as the proper material guiding.

Control concept of LTM-ECO

The control concept for the thickness gauging system LTM-ECO is based on an embedded solution. This allows a small compact design, making integration into a customer-side system very easy. All necessary components for controlling and regulating the measuring task for the LTM-ECO are integrated in the HMI device. Operation is performed via four integrated buttons, the display of information via a dot matrix display (4x20). To synchronize the determined thickness measurements with the associated material path or associated location on the material (X coordinate), the encoder supplied with the system must be integrated by the customer into the customer-specific system. Communication with the higherlevel control system is via a defined interface (Profibus, Profinet). All connections are pluggable.

The LTM control concept

The basic control principle for LTM-BASE, LTM-SMART, LTM-MAXI and LTM-ULTRA systems is based on a server-client solution. Internally, on the CPU (Beckhoff), the necessary server and the included internal client are installed.
An optional 21.5“ touch monitor is used to operate the measuring system. A customer mouse and keyboard can optionally be connected via USB.
Additional clients can be connected to the controller via Ethernet if required and do enable an almost open operating concept.
To synchronize the determined thickness measurements with the associated X-coordinate (material path or associated location on the material), a separate path signal must be provided by the customer or the encoder, which is optionally included in the scope of delivery and must be integrated into the customer-specific system.
Communication with the higher-level control system is performed through a defined interface (Profibus, Profinet). Optionally, there is the possibility save the determined thickness measurements values additionally to the customer´s network, e.g. as CSV format.

Measuring accuracy

The resolution of a thickness measuring system provides information about the accuracy with which the desired metrological detection is carried out or realized and how the specified tolerances can be reliably monitored. In many areas of industry, the rule is still „The Golden Rule of Measurement“, which is used as a criterion for preselection of measuring means is used and which states that the maximum permissible error limit (measuring accuracy) of the measuring system may only be 10% of the tolerance to be tested. Even though the correctness of this rule is often discussed today, it still has its justification and can also be used to pre-select the appropriate Thickness Gauging System.

Example calculation using the „golden rule of metrology“:
For a sheet metal, the compliance with a material thickness of 3 ± 0.1 mm shall be monitored. According to the dimension, the tolerance is 0.2 mm. When applying the golden rule of metrology (rule of thumb), the thickness gauge must accordingly have a measurement deviation of less than 0.02 mm to ensure compliance with the specified tolerance monitor.
The measuring accuracy of a thickness measuring device is determined by a sum of influencing variables with different weighting, whose control has received a great deal of attention especially during the development of the ROLAND Thickness Gauging Systems.

These factors are:

• The bending and vibration stiffness of the mechanical construction and its elements with high resistance to temperature variations.
  
• The selected laser sensors in terms of linearity, repeatability, sampling rate, etc.
  
• The electric and mechanic hardware matching to the sensors.
• The Thickness Gauging Software with the corresponding measuring and electrical filter elements, mathematical-statistical functions etc.

On the other hand, the measurement deviation is defined by the mechanical system (loading deformation, manufacturing tolerances, etc.). Any incorrect measurements that may occur are ecluded by the Thickness Gauging Software LTM-S with its integrated statistical methods and measuring filters. The linearity error is assumed to be constant in the second illustration below. However, this assumption implies that the material to be measured moves at a constant, fixed distance within the operational range of the laser. When the distance fluctuates, the corresponding linearity error will also change. Accordingly, when measuring thickness with laser technology, special care must be taken that the material to be measured is optimally guided at the measuring center point of the system and at a constant distance. (Vibration free material transport)

Calibration system

The regularly performed calibration of the system can have a great influence on the measurement result. A calibration unit is integrated in every ROLAND Thickness Gauging system. Calibration cycles are implemented in the software and enable the required metrological adjustment of the system. Additionally the adjustment of the measuring system is performed on a certified test piece (calibration piece) and also includes the zero adjustment of the laser sensors.

Testing the measuring capability

In order to maintain the quality of measurement, the accuracy respectively correctness of the measurement, during the current production process, the ROLAND Thickness Gauging Systems LTM-BASE, LTM-SMART and LTM-MAXI possess a corresponding software tool for performing the so-called „Measurement system analysis according to method 1“. Thereby, a calibrated standard is integrated in the ROLAND Thickness Gauging System and measured at least 25 or 50 times.

As required by the ISO norm, the influence of the operator is excluded by the system. The recorded measured values are then according to ISO methods defined and the capability factors Cg and Cgk are calculated. On the basis of these ability characteristics Cg and Cgk it can be decided whether the thickness gauging
device using a certified standard is suitable for the intended use under the given operating conditions and can be accepted. The advantage of the ISO methods is that they give a decision support of the equipment measuring capability for the lesser experienced.

Factors and conditions:

1. Surface quality of the material to be measured.
   The surface quality of the material which has to be measured has a limited influence on the measurement results. Depending on the surface, color, coating and amount of oil, the reflection behavior of the material may vary slightly. Accordingly, the received light intensity in the laser can change. In the above case the lasers are designed to automatically adjust the light intensity. Furthermore by choosing the calibration standard-piece with a similar surface, this effect is taken into account and compensated for.
   
2. Material speed
   Thickness measurement using laser technology can only be performed when a relative movement between the material to be measured and the lasers takes place.
   
3. Measuring density
   According to the customer‘s application, each LTM Thickness Gauging System is dimensioned accordingly and each measuring laser has a defined sampling rate. With the aid of the sampling rate [Hz] and the material speed [m/s] of the customer, the measuring density (number of measured values per track) can be calculated.
   
4. Material guiding
   The material guiding has a decisive influence on the measurement result (see section work area or measuring distance).
   Therefore, in the vicinity of the laser sensors a guided vibration free material transport must be guaranteed. The installation location of the measuring systems must be selected in order to fulfill these requirments.

Special solutions

In general, all Thickness Gauging Systems of the ROLAND LTM product series can be combined with each other easily. Therefore it is possible to measure distances greater than 1000mm, or only to measure the edge area e.g. Use our experience. We will also find a solution for your challenge!

• Tandem solution edge measurement with combined central line measurement (2 x LTM-MAXI and 1 x LTM-BASE)
  
• Tandem solution with a bandwidth of 1600mm  (2 x LTM-MAXI)

LTM product series

We are developed a wide range of Thickness Gauging Systems for a variety of applications or for metrological tasks.
The ROLAND Thickness Gauging Systems differ not only from the possibility of static
or dynamic data sampling, but also in the data sampling rate and accuracy by which data is captured and provided for further use.
The operating options and the different operating modes serve as selection criteria to help you to choose the most suitable thickness gauging system from our LTM portfolio.

The best suited system for your application

We help to clarify all important questions during the design phase and will develop the requirements profile for a customized and efficient solution together with you.
In order to choose the best suited system for your application, some questions have to be clarified beforehand, such as:

• Material type, shape and surface finish
  
• Geometric dimensions such as material width, length, etc.
  
• Thickness range incl. thickness tolerances which must be measured or monitored, Measurement capability analysis etc.
  
• Desired operating modes such as line measurement, traversing, etc.
  
• Kinematic sizes, e.g. transport speed, cycle time, etc.
  
• Ambient and material temperature
  
• Mounting conditions and installation site, material guiding and smooth material transport
  
• Operating concept
  
• Electrical interface, type of data backup and respectively, cable lengths etc.