Burster 8402 Series User manual

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Technical Product information
Commissioning, Function, Fitting Specifications
Model 8402
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burster präzisionsmesstechnik gmbh & co kg ■Talstr. 1-5 ■76593 Gernsbach ■Tel.: [phone] ■www.burster.de ■[e-mail]
2.1 Unpacking
¾Inspect the sensor carefully for damage.
If you suspect that the unit has been damaged during shipping, notify the delivery
company within 72 hours. Keep all packaging materials for inspection by the
representative of the manufacturer or delivery company.
Only transport the model 8402 sensors in their original packaging or in packaging of
equivalent a .
2.3 Grounding and potential connection
All connecting wires (including the cable screening braid) are electrically insulated from the
sensor body.
The insulation resistance between the connecting wires and the sensor body is measured.
The test certificate states that the minimum resistance is greater than Ω(test voltage 45 V).
•w
Miniature Load Cell
Model 8402
1. Introduction
The load cells in the model 8402 series are primarily designed for the measurement of force in
production equipment, using Newtons (N) as the unit of measurement.
The local gravitational acceleration (g ≈9.81 m/s²) must be taken into account when determining
masses.
2. Preparing for use
¾
qu lity
2.2 Using the instrument for the first time
¾Only connect the sensor to instrumentation amplifiers that are fitted with a safety
transformer according to EN 61558.
Note:
Transmitters connected to the outputs or other devices that are galvanically connected to the
sensor's signal lines must also be fitted with safety transformers according to EN 61558.
30 M

Technical inqueries:
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Technical Product information
Commissioning, Function, Fitting Specifications
Model 8402
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burster präzisionsmesstechnik gmbh & co kg ■Talstr. 1-5 ■76593 Gernsbach ■Tel.: [phone] ■www.burster.de ■[e-mail]
2.4 Storage
¾The sensor must be stored under the following conditions only:
•dry
•no condensation
•temperature between 0 °C and 60 °C
Note:
Provided the storage conditions have been observed, no special steps need to be taken after
storage and prior to commissioning.
3. Principle of operation
The sensor operates with the aid of a spring element. The force to be measured deforms the
spring element elastically.
This deformation is transformed into an electrical signal by strain gauges. They, together with the
spring element, constitute the measuring element of the sensor.
3.1 Spring element
The spring element is the most important mechanical part of a load cell. Its purpose is to take the
force being measured and convert it into a linear extension. It relies on the elastic properties of
the material in order to determine the force indirectly.
The materials of which load cells are made must satisfy other conditions in addition to their
elastic properties. As a result, only a small number of carefully selected materials come into
question for high-quality load cells.
burster is going one step further than this, using mainly materials from the aeronautical industry
that satisfy additional quality requirements in the place of DIN materials.

Technical inqueries:
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Technical Product information
Commissioning, Function, Fitting Specifications
Model 8402
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burster präzisionsmesstechnik gmbh & co kg ■Talstr. 1-5 ■76593 Gernsbach ■Tel.: [phone] ■www.burster.de ■[e-mail]
3.2 Mechanical design
The spring element in the 8402 is implemented as a hollow compression body.
F
strain gauge
Figure 1: the measuring element in the 8402 is formed by a compression body with strain
gauges
A key feature of the 8402 is its relatively compact design for its rated force. The combination of
compact size and high spring constant ensures a high resonant frequency, allowing the 8402 to
make rapid measurements.
3.3 Function of the strain gauge
The electrical resistance of a wire rises with increasing length and falling cross-section. When a
wire is pulled, it becomes thinner and longer – both of these effects result in an increase in its
electrical resistance.
This is the principle on which the function of strain gauges is based. In practice, however, strain
gauges do not consist of a single wire, but of a metal foil laminated onto a carrier material. The
metal foil is etched to create a meandering structure (see Figure 2).
Figure 2: Foil strain gauge
Special techniques are used to mount strain gauges manufactured in this way onto the surface of
the spring element.

Technical inqueries:
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Technical Product information
Commissioning, Function, Fitting Specifications
Model 8402
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burster präzisionsmesstechnik gmbh & co kg ■Talstr. 1-5 ■76593 Gernsbach ■Tel.: [phone] ■www.burster.de ■[e-mail]
3.4 Strain gauge wiring
In order to reduce undesirable influences on the measurement, the strain gauges in the load cells
are connected as a Wheatstone bridge. Figure 3: illustrates this wiring in a simplified form.
white
yellow
green
brown
-ε
-ε
ε
ε
Figure 3: Full-bridge strain gage
In addition to the four strain gauges shown here, compensation resistors to reduce the effect of
temperature and balancing resistors to balance the bridge circuit are also incorporated.
Depending on the sensor model, further resistors for the purpose of standardizing the rated
characteristic are integrated into the cable or the connector.
The output voltage of the sensor at maximum load, Ua, is calculated as follows:
U = c x U
ab
•U
b: excitation voltage
•c: characteristic value (sensitivity) of the sensor.
The sensitivity parameter "c" can be found in the test report. It is standardized to
1.5 mV/V.
If excitation voltages (Ub) in the range 2.5 to 5 V are used, 0...100 % loading of the sensor yields
output voltages (Ua) between 0...3.75 and 0...7.5 mV.
3.5 Deflection full scale
For sensors with a compression body, the load compresses the measuring element, changing
the overall height of the sensor. This deformation, known as the rated measurement travel, is so
small that it cannot be seen with the naked eye. It can range up to 50 µm at max. load and is
directly proportional to the load.

Technical inqueries:
(+49) 07224 / 645 - 0
Technical Product information
Commissioning, Function, Fitting Specifications
Model 8402
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burster präzisionsmesstechnik gmbh & co kg ■Talstr. 1-5 ■76593 Gernsbach ■Tel.: [phone] ■www.burster.de ■[e-mail]
3.6 External forces
Caution!
Sensor will be damaged!
Avoid vibrations, even if the loads caused by these vibrations are less than the
rated maximum. Design your measuring system in such a way as to prevent
these external forces.
Note:
External forces, acting on the sensor, adulterate the result materially.
External forces here refer to any force that acts outside the sensor's axis of symmetry -
transverse forces, bending moments and twisting moments in particular.
3.7 Overload protection
Compression bodies are relatively robust to overloading.
The short measurement travel (up to 50 µm) means that protecting the sensor directly from
overload by restricting its travel is scarcely feasible.
¾Install an overload protection mechanism.
Place the sensor on a spring using a guided base support. The spring deflection increases
the measurement travel. It is then easier to implement a mechanism for restricting travel.
¾Provide pressure limiters for any hydraulic or pneumatic cylinders acting on the sensor.
¾Absolutely avoid any load that applies a sudden force (pulse).
The high spring constant of the sensor results in high negative accelerations for moving
objects. This generates extremely high forces.
If, despite precautions, the sensor has been overloaded, you will notice this immediately by a
change in the output signal for the zero point.
¾Get the sensor checked out if you notice a change in the signal of 5 % or more (higher or
lower) while no load is present.