Tension Dynamometer / Tension Load Cell

Sensors for force measuring devices or appliances for force measurement are called force transducers. Force transducers can have a wide variety of shapes and sizes. Force transducers with a flat cylindrical shape are often also called load cells. Today, the term load cell is also used for other designs. There are force transducers that make one whole piece with the force display, but there are also force transducers that can be connected to various force measuring devices or force displays. External force transducers have an advantage that the measuring point and display point can be spatially distant and thus, they can also be used when the space is very limited. In case of some electrical measuring principles, the signals of several force transducers can be evaluated together. This option is often used in weighing technology. Since the mass can be determined from the weight force when a gravitational value is known, the force transducers are used for a large number of scales. Special force transducers can record several force directions along one axis or the values for several force axes at the same time.
The guideline VDI 2638 regulates parameters for the force transducers. According to the VDI press release, by defining the parameters for force transducers, the guideline enables "the consumer to clearly assign the product properties and make a selection based on requirements."

Force transducers with spring force
Simple mechanical force transducers can be made from metallic coil springs. If the coil spring is pressed or pulled along the axis, the change in length within the measuring range is proportional to the force exerted. When the spring is released, the force transducer returns to its original shape. Depending on the material, material thickness, type and number of windings, the springs can be designed for different measuring ranges. The spring way is shown on a scale as a force value. Some spring dynamometers also pull along a separate drag pointer when loaded, which does not return to the starting position when unloaded. Thus, the maximum value of a series of measurements is saved.

Load cell with hydraulic fluid
Hydraulic force transducers often have the form of a load cell. In the most common form, the force acts on a piston which usually protrudes a little bit and is pressed into the cylinder filled with the hydraulic fluid. There are special ring pistons for the measuring points where the load cell is positioned and held by means of a shaft.
Since liquids are difficult to compress, they pass on the force applied in the system further. The display can, for example, be done mechanically via pressure gauges, pressure transmitters or special contacts. A hydraulic load cell does not require any auxiliary energy, is robust and for the appropriate hydraulic fluid can be used in wide temperature ranges.

Load cell with strain gauge
Force transducers with strain gauges also use the elastic deformation of metal wires. However, it is not the change in length that is recorded here, but the change in electrical resistance caused by it. When the wire is stretched, the resistance increases and it decreases when the wire is compressed. The thin wire, which is usually applied in a meandering shape to a carrier film, is known as a strain gauge. The base body to which the strain gauges are applied is called a spring body because it deforms elastically too.
By using several cross-connected resistors, the measurement errors due to shear forces, bending moments and temperature changes can be avoided. This is achieved by means of the Wheatstone bridge circuit, in which four strain gauges are attached to the spring body of the load cell or the force transducer in such a way that two are compressed and two are stretched when the force is applied. If the temperature changes, the resistance changes in all the strain gauges, but due to the opposite alignment there is no output voltage. With the help of this circuit, very precise force sensors can be manufactured at a relatively low cost. Many different measuring ranges can also be covered, depending on the shape and material of the spring body.

Force transducer with electromagnetic compensation
In the case of the force transducers with electromagnetic compensation (also EMFC for electromagnetic force compensation), a magnetic field is generated around a coil that is energized. A position sensor detects when the coil lowers due to the application of force. The sensor sends the signal to a controller that increases the coil current until the original position is reached again. The position is therefore the same in the loaded and unloaded state, because the force transducer electromagnetically compensates the force effect. EMFC force transducers can also measure very small forces precisely. The current required is proportional to the load.

Force transducers with piezoelectric elements
Because of the load, a charge appears on the surface of the piezoelectric crystals, the amount of which is proportional to the applied force. This effect on the certain crystals with polar axes was discovered in 1880 by Pierre and Jaques Curie. Because this charge only remains for a short time, the principle is mainly suitable for rapid load changes, short-term load peaks or for continuous measurements with continuous recording. Even dynamic loads with the frequencies up to 100Hz can be measured with the piezoelectric force transducer. If several elements are positioned accordingly, multi-axis measurements are also possible.
A charge amplifier is connected after the force sensors, which are often made of quartz crystals. It records the changes in charge, converts them and passes them on to the displays, interfaces or storage media. Piezoelectric force transducers measure dynamic loads very quickly and with high sensitivity.

The following basic questions help to find a suitable force transducer:
- Which type of force should be registered by the force transducer?
- Do you need special adapters for the measuring task?
- How high are the maximum forces?
- What accuracy is expected?
- Do the measurement results have to be saved?
- Will the force transducer be used under special environmental conditions?

The design of the force transducers differs depending on the measuring task. It is important whether static or dynamic forces are to be measured. A distinction is then made between:
- force transducers that measure only tensile or only compressive forces,
- force transducers that measure bidirectional tensile and compressive forces
- force transducers that can record the torques.

Some force transducers offer the possibility to connect special adapters. In addition to the connection option for the force display on the upper and lower parts, the DMS force transducers with S-shape have metric threaded holes. Different measuring tips with defined circular, spherical or conical surfaces for pressure measurement or the hooks, eyelets or clamping elements for attaching the samples for tensile measurements on the force transducer can be fixed to them.

Maximum force is specified for each load cell and each force transducer. The specified measuring accuracy can be achieved up to this load. Some force transducers will be destroyed if this load is exceeded by a few percent. For the other force sensors, a factor, that defines how much overload the load cell can withstand without damage, is specified. One can differentiate between nominal load, limit load and breaking load. Some force transducers or force gauges have separate overload protection so that the force sensors were not destroyed. When selecting, not only the intended measuring range must be taken into account, but also whether some unintentional overloads are possible.

Depending on the design and measuring range, force transducers can achieve different levels of accuracy. It should be noted that the force transducers with high measurement accuracy can only achieve this accuracy if the specified environmental conditions are met. The measurement accuracy and the display resolution are not identical. This must be taken into account both when selecting the force transducers and when evaluating the measurement results.

For some measurements, the direct display is sufficient, for example, when weighing fruit. However, if the measured values are to be saved directly or used for calculations, such force transducers are available that can transfer the data. A simple spring force transducer or a hydraulic load cell can store the maximum value of a series of measurements via the drag pointers. Electronic force sensors offer far more options. With the help of the electrical variables that they transmit to the display and evaluation unit, the measurement values can be saved, converted and passed on. The signals of the electrical force transducers can be evaluated directly and forwarded to the alarm transmitters or switching contacts or displayed and saved on a PC. When measuring dynamic forces, it must be taken into account at what time interval the force transducer can detect the changes in force.

Depending on the measuring principle, force transducers only work reliably under certain conditions with regard to temperature, air humidity, electrical or magnetic charging. A force transducer in which the electrical quantities are evaluated is much more sensitive than a mechanical load cell. Digital displays for force transducers and load cells can only be used if the temperature and humidity values are suitable. For some applications, mechanical and hydraulic systems are therefore suitable.

Electrical signal may be “received” by the load cell as a result of mechanical deformation at compression and thus, the sensor detects the level of transformation upon impact of a mechanical tensile or compressive force, and transfers this electrical signal (analogue or digital) to the weight indicator.

Nowadays load cell is considered to be the main part of the highly accurate weighing equipment. Being good for measuring both, static and dynamic tension, load cells do not distort the values and provide accurate results even under harsh conditions and when applied in transportation vehicles. An important thing to remember about load cells is that as any electronic equipment, they require correct installation and proper maintenance, as well as timely calibration – based on that, they ensure that the user obtains the best results possible.