Build-up system

Excerpts from the manual of physical-technical force measurement with kind permission of the author Dieter Theiss

Special areas of force measurement
measurement technology and Multi-component force transducer
The measurement technology with several force transducers

Introduction to the metrological situation
From the wide spectrum of force measurement technology, many tasks can only be completed by using a network of several force transducers solvable. In contrast to tensile force measurement, where one force transducer is often sufficient, for example when measuring in ropes or tie rods, force measuring systems for measuring compressive forces usually require several force transducers. The reason lies essentially in the requirement for stability of the force measuring arrangement and the often unclear line of action of the force to be measured.

In practice, the force-transmitting component is supported on several points. Examples of this include the distribution of axle forces between two bearings or the distribution of the container weight over three or more support points. In these cases, the power flow bifurcates. Since the force distribution is usually not known and the line of force can shift, the forces in each partial force flow must be measured. The total force in the effective direction of interest is obtained by adding the corresponding signals of the individual forces. In order to ensure task-appropriate function and accuracy of the measuring arrangement, the aim is to use force transducers with the same nominal force Fn with a calibrated output signal Ua, i.e. the same transmission coefficient Ua/Ue.

When measuring weight forces, ie in weighing technology, one speaks of load cells according to VDI/VDE 2637. The term “transfer coefficient” is called “characteristic value C”. The vectorial character of the physical quantity “force” must be taken into account in every case. There are still no ideal force transducers that are only sensitive in the measuring direction. The known measuring principles and constructions have major differences in this regard. Therefore, special emphasis must be placed on the design goals of force application. For measurements with higher accuracy (< 5 ‰), adapted installation parts from the force transducer manufacturer are recommended to avoid or reduce measurement errors.

Disturbances such as lateral forces and moments lead to parasitic effects on the force transducers and result in pseudo measurement signals. A separation into useful and interference signals is not possible. If the system is tied up, significant transverse forces arise in response to imposed transverse displacements, for example during thermal expansion processes. Decoupling the load introduction part in the transverse direction avoids such errors. If the effective direction of the force to be measured deviates from the measuring direction of the force transducer, for high-precision measurements, a component decomposition must be achieved and the respective individual component must be measured (see Sect. 4.2 Multi-component measurement). In addition to the design-related force branches, there are a number of special measurement cases that also require the use of several separate force transducers. In this way, force measurement units are created which can be viewed as self-contained measuring elements (input variable force - output variable electrical signal). Applications for this include triple arrangements to expand the force measuring range in testing machines and normal load machines with the help of master force transducers or the coupling of two force transducers for extremely precise tensile-compression force measurements. Due to the outstanding importance of the force measurement method using strain gauges (DMS), all considerations mentioned here refer specifically to DMS force transducers; This applies in particular to accuracy specifications and electrical interconnection techniques. The following statements naturally also apply to all other converter principles.

Combination when using several force transducers

Force transducers are electromechanical transducers with the mechanical input variable “force” and an electrical output variable. For a combination of several
• In principle, a distinction must be made between force transducers
• input side, ie mechanical and
• output side, ie electrical interconnection.

Both can be carried out in parallel or in series. DMS force transducers are to be viewed as very stiff, elastic springs and how they are mechanically connected to each other. Parallel connections are used to measure partial force connections in force branches. Example: Distribution of the force across several supporting forces with a parallel force line, see section. 4.1.4 (Change in force distribution) Series connections are rarely used because all force transducers are subjected to the same force. Example: Use of several force transducers for safety reasons (redundancy) From an application perspective, the question of electrical interconnection is not of primary importance.

The strain gauges in the force transducer are electrically connected as a Wheatstone bridge and can therefore be connected to each other in parallel or in series. The type of connection on the supply side is determined by the choice between constant current or constant voltage operation. An impressed voltage, common to all force transducers, requires a parallel connection of the input side, an impressed current requires a series connection. The most common output-side circuit (measuring side), especially with DC voltage supply, is the parallel circuit. A series connection requires galvanically isolated voltage sources, which requires considerable effort with direct voltage. With alternating voltage supply, this is achieved by separate secondary windings. The simplest electrical determination of the measurement signal is certainly the measurement of the open-circuit voltage on the output side when supplied with constant voltage. The measuring amplifiers available today for strain gauge applications have such a high input resistance that the force transducers are operated virtually idle (measuring current is negligibly small). The other possible operating modes for measuring the output signal of strain gauge force transducers will not be discussed here. The mechanical arrangement and the electrical switching options can be combined as desired and independently of each other. The most favorable combination for the given application is determined by the measurement task and the practical possibilities of the user's electrical measurement technology.

Mechanical and electrical parallel connection

Only pressure force sensor up to 450kN

The majority of all tasks in force measurement technology, in which several force transducers have to be used, can be solved with complete parallel connection, ie both mechanically and electrically (on the input and output sides). The basic idea is to determine the sum of the partial forces acting in the measuring direction by adding the individual output signals in order to obtain an output signal that is proportional to the total force. For error-free summation, it is important that all force transducers have the same transmission coefficients C (measurement sensitivity) and internal resistances Ri (for more details, Sect. 4.1.4). A shift in the line of action, ie a change in the point of force application within the support area, then does not lead to any disruptive errors. Strain gauge force transducers usually work with a completely symmetrical bridge circuit. The internal resistance of the force transducer RiKA is then independent of the internal resistance of the supply source Rie and the compensation resistances RKe on the input side; it is only determined by the bridge resistances themselves. Modern precision force transducers usually have a standardized internal resistance with a very small tolerance (< 200 ppm).

Application example for a mechanical and electrical parallel connection of several force transducers

For cost reasons, hydraulic testing machines with so-called hydraulic testing machines are often used for precision measurements of large forces Master cells, i.e. particularly precise, time-stable and low-creep force transducers. The measurement quality is now so high that such machines are used as normal load machines for measurement accuracies better than 2 x 10-4, based on the actual value. The stable and statically determined three-arrangement of individually calibrated precision force transducers is ideal as a master cell. It is essentially a measuring platform. The construction has become known as a “build up” system [3] and offers several advantages:

• Measuring range extension by a factor of 3 compared to the nominal force of the single KA,
• the individual KA can be tested and calibrated on normal load machines with direct mass effects [4),
• no loss of accuracy compared to single KA,
• no centering problems of the test specimen; Off-center force introductions do not cause any relevant measurement errors,
• statically determined system,
• stable support for the examinee,
• no force shunts, since the system is self-centering,
• mechanical use and electrical output signal like a single KA,
• For forces of around 500 kN or more, there can be a cost advantage over individual force transducers.

The three precision force transducers are arranged offset at 120° on a pitch circle. The centers form an equilateral triangle. Ideally, the line of action of the test force is located in its center of gravity, which is identical to the center of the pitch circle. Figure 4.5 shows the force distribution among the individual force transducers.

force transducer

Since the system only stands on three points in the plane, it is statically determined, which means that the distribution of forces is independent of deflections and deformations and thus ensures a largely uniform load. In order to exclude any force shunt, such three-way arrangements should definitely be equipped with self-centering force introduction parts. An important requirement for the three-way arrangement is the location-dependent introduction of force into the system that does not influence the measurement signal, ie, a force Fex acting outside the center of gravity of the system should produce the same output signal as a force Fz of the same size acting in the center of the triangle. Due to the use of pressure force transducers, the line of force action should remain within the triangular area. The three force transducers are electrically connected in parallel.

Application example for the mechanical series connection of several force transducers

This is used for testing force transducers and load cells in so-called master cell operation. Here, the master cell and the test specimen are mechanically connected in series. Electrically, a separate evaluation of the individual measurement signals is required, since in the case of an interconnection, signal separation of the master cell and the test object is no longer possible. An assignment of the acting force via the master cell signal is no longer possible. An individual measurement signal evaluation is also required for the mechanical series connection of two identical force transducers for the purpose of measurement reliability (redundancy). Another application for the mechanical series connection is to use several force transducers (usually two) with different nominal forces.

Terminal box or junction box

This is intended to ensure that smaller forces can be measured with higher resolution or measurement sensitivity in a measuring range specified by the force transducer with a larger nominal force. If the nominal force range of the “smaller” force transducer is exceeded, an overload protection device located parallel to it must generally be triggered. Or the force transducer is designed to be mechanically overloadable, but the measuring signal for F > Fn is faulty. When the measuring limit is reached, the output signal of the “larger” force transducer is switched over. In this case, electrical interconnection of the force transducers is not functionally possible. The measurement signals from each force transducer must be processed separately. The seamless transition from one measuring range to the other is difficult; unless two separate measuring amplifiers are used. However, a certain degree of discontinuity remains due to the hysteresis effect.

Electrical connection of several force transducers

8-channel measuring amplifier for force and torque sensors. USB and analogue outputs as standard. Optionally with CANbus or EtherCat fieldbus

An electrical interconnection of several force transducers always occurs when a total force is to be determined from partial force measurements. This is the case with all force transducers connected mechanically in parallel. Due to the galvanic connection between the supply and measuring signal voltage side of strain gauge force transducers, only a few of the basic interconnection options can be used. Parallel connection is mainly used on the input and output sides. When making precision measurements, pay attention to the same transmission coefficients and internal resistances. In order to avoid loading the force transducers (idle operation), the
Input resistance of the electronic evaluation devices must be very high-resistance (> 1 Gn). Modern digital voltmeters (for DC voltage measurement) or carrier frequency measuring amplifiers meet this requirement many times over. In most cases, force transducers connected mechanically in series are not connected electrically. The measurement signals are processed separately. Either each force transducer will have its own Evaluation electronics assigned, or each measurement signal is sampled sequentially (scanning method) and fed to a common evaluation device.

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Interfaceforce – Build-Up Systems

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