Burster 8402 Series User manual

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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

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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.

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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.

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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.

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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.

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4. Installation
Caution!
Risk of damage to sensor!
The sensor cable contains components that affect the sensitivity of the sensor.
Never shorten the sensor cable.
4.1 Surrounding mechanical parts and fastening
4.1.1 Adaptation
Mounting surface
A high quality measurement depends on a defined deformation of the measuring element under
load. In order to be certain of excluding any unwanted deformations, the sensor must lie flat on
the mounting surface.
The mounting surface must satisfy the following requirements:
•adequately stable
•hardened, minimum hardness 60 HRC
•polished, preferably lapped, surface quality: N3 (Rz 1), evenness 2 µm
•not coated in any material
•must contain no holes or milled slots, this also means centering holes.

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Applying the force
¾Apply the force in the center.
This means that you must apply the force exactly along the axis of symmetry of the
sensor. The force must act in a direction perpendicular to the sensor.
¾Eliminate any lateral forces and torques.
These are generated by an eccentric load, force applied at an angle and by friction. You
can avoid friction by using parts with an adequate surface quality and hardness.
Parts that are used to apply the force to the sensor must satisfy the following requirements:
•flat surface
•surface hardness 60 HRC
•polished, preferably lapped, surface quality: N3 (Rz 1), evenness 2 µm
•not coated in any material
•must contain no holes, not even centering holes for lathes
•perpendicularity to centre line 0.01°
4.1.2 Mounting
Caution!
The spring body will be damaged if you clamp the load cell by its sides or
press-fit the load cell in a hole!
Only fix the sensor in place as specified in these instructions.
The model 8402 sensor is solely intended for measuring compression forces. Fastening
components are therefore only suitable for the task of fixing the sensor in place.
There are various fixing options depending on the application:
Initial load
¾Connect the sensor in situ to a measuring instrument and monitor the measurement signal
while mounting the sensor.
The measurement signal must remain below the rated force during and after the mounting
operation.

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¾Position the sensor correctly on the mounting surface.
¾Now apply the initial load.
The sum of initial load and maximum measured load must be less than the rated load.
F
Figure 4: When using a spring element to mount the sensor, an initial load is applied to
hold the sensor in position.
Adhesive bonding
Note:
If mounting the sensor in a recess as a means of centering and fixing the sensor, a minimum
radial gap of 0.5 mm must be maintained around the sensor case.
¾Connect the sensor in situ to a measuring instrument and monitor the measurement signal
while mounting the sensor.
The measurement signal must remain below the rated force during and after the mounting
operation.
¾Position the sensor correctly on the mounting surface.
¾Now bond the sensor, from above, by its flange firmly onto the mounting surface.
Note:
There must be no adhesive on the mounting surface of the sensor. There must be no adhesive
underneath the sensor.
Silicone or wax
Figure 5: Bond the sensor fast from above. Do not let any adhesive run underneath the sensor.

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Technical Product information
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Screw fastening
Since the sensor case and hence also the measuring element must not be loaded during
mounting, a mounting plate fitted to the sensor flange provides a suitable fixing method. You can
fasten this mounting plate to the mounting surface using up to three screws. Use screws of
diameter M4 or smaller.
A radial gap of 0.5 mm must be maintained between the measuring element and the mounting
plate.
¾Connect the sensor in situ to a measuring instrument and monitor the measurement signal
while mounting the sensor.
The measurement signal must remain below the rated force during and after the mounting
operation.
¾Position the sensor correctly on the mounting surface.
¾Place the mounting plate on the sensor flange.
In this position, there must be a minimum radial gap of 0.5 mm between the sensor
measuring element and the mounting plate.
¾Screw the mounting plate firmly onto the mounting surface (finger tight).
ø D1
Measuring element
Flange
M4 or smaler
finger tight
Mounting plate
min. gap 0.5 mm
Figure 6: Sensor 8402 with mounting plate; there must be a minimum gap of 0.5 mm between
measuring element and mounting plate.

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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]
4.2 Electrical system, evaluation instrumentation
Caution!
If the cable sheath is damaged, fluids can penetrate and damage the
sensor.
For this reason:
Take suitable precautions to relieve the bending stress exerted by
the connecting cable on the cable bushing at the sensor housing. In
particular, pulsating or alternating loads acting on this bushing will
damage it and hence the cable will come loose.
Make sure that you lay the cables with sufficiently large bend radii.
Avoid vibration on the cables.
Protect the cables from strain.
Protect the sensor from vapors and liquids.
The output signal of the 8402 normally lies between 2,5 and 10 mV. Hence for a measurement
accuracy of 1 % you will need a resolution of approx. 10 µV or better. Hence interference will be
picked up by the sensor, cables and measurement electronics.
The following points apply to the sensor's electrical connection:
¾Locate the sensor, the cable and the measuring instrument outside the fields of
high-energy equipment.
This includes transformers, motors, contactors, frequency converters and so forth.
Otherwise the electromagnetic fields from such equipment will act with their full effect on
the measuring chain, causing incorrect measurements.
¾Lay the measuring lines separately from high-power cables.
If the measuring lines are laid parallel to such cables, interference will be coupled in
inductively and capacitatively.
In some cases it will be helpful to place an extra shield as additional protection over the
measuring cable, or to lay it in a metal tube or pipe