1. How do S-type load cells work?
S type load cells, often referred to as S beam or Z beam load cells, are a class of load sensor used for measuring tensile and sometimes compressive forces. They consist of strain gauges, bonded to a metal S shape (hence the name) frame in a Wheatstone bridge arrangement. A strain gauge is a thin laminated foil pattern used to measure mechanical strain. Applying a force to the load cell causes the metal frame to deform. This in turn stretches (in the case of a tensile force) the strain gauge, causing its length to increase and its cross sectional area to decrease, thereby increasing its electrical resistance. By measuring the change in electrical resistance, it is possible to determine the amount by which the strain gauge has stretched. The configuration of the S type load cell makes it suitable for measuring axial forces only. Any large off axis forces and moments will cause the sensor to output a ‘nonsense reading’.
2. General characteristics of S-type load cells
S Type load cells are highly accurate, medium cost sensors capable of measuring a wide range of tensile and compressive loads. They are available with full scale ranges from as little as 0.5 kg to as large as 50 tonnes. They are extremely compact, though their geometry also makes them unsuitable for applications with off axis forces and moments. Most load cells provide a mV/V unamplified analog output, though amplified outputs are also available. There are no moving parts within the load cell and so they have a long lifetime.
3. Input and output signals
S-type load cells require fixed 5-15 Vdc supply voltages. The strain gauge elements are arranged in a full bridge Wheatstone bridge formation, providing temperature compensation, common noise cancellation and a high signal to noise ratio. The sensor’s output signal takes the form of millivolts per volt (of supply voltage). Most sensors have outputs in the range of 2-3 mv/V. Many load cells include built-in amplifiers that amplify the mV signal and convert it to a standard output signal form such as 0-10 V or 4-20 mA. If the sensor is unamplified then the the process controller must include a built-in amplifier or an external amplifier must be used.
4. Applications of S-type load cells
S type load cells can be used in a large range of tensile and compressive loading applications where compactness is important and loading is axial. Popular applications include crane scales, hanging scales, tank weighing, mechanical overload protection and structural performance testing.
5. Typical specification
|Measurement range||0-0.5 kg to 0-50 tonnes|
|Zero error||1% F.S.|
|Ambient temperature||-55 to 90 °C (up to 120 °C possible)|
|Supply voltage||5-15 Vdc|
|Output voltage||2-3 mV/V, 0-10 V, (serial available but uncommon)|
|Passive / active||Active|
|Contact / non-contact||Contact|
6. Purchasing tips
- Output sensitivity: Unamplified S-type load cells are typically available with either 2 or 3 mV/V output signals. Larger output signals improve noise immunity and effective resolution.
- Tension only or tension & compression: All S type load cells are capable of measuring tension. Many but not all are also capable of measuring compression. Keep in mind that S type load cells perform very poorly in the presence of off axis loading. Off axis forces and moments can often be minimized by securing large objects using several mounting points, each with its own load cell.
- Overload capacity: Load cell data sheets specify an overload capacity, typically 150-300% of the rated load. This is the maximum load that can be applied without damaging the load cell.
- Natural frequency: The load cell is sensitive to mechanical vibrations (as strain gauges are also used in vibration sensors) and so it is important to choose a sensor whose natural frequency does not overlap with the vibration frequency of the system.
- Creep: Creep effects long term temporal output stability, particularly when operating near to the maximum rated load. The majority of S type load cells have creep induced output variations of 0.02-0.06% F.S. after 30 minutes of loading at their maximum rated load.
- Thread type: S type load cells include a female thread on each side for mounting. The thread size will differ depending on the size and rated load of the load cell.
- Ingress protection: S type load cells can be difficult to seal and so whilst most S type load cells do have IP67 ingress protection, many have significantly lower ingress protection. Keep this in mind if your application requires high ingress protection.
- Built-in amplifier: S-type load cells are available with built-in amplifiers which amplify the mV output of the strain gauges. Such load cells normally provide either a 0-10 V or 4-20 mA output signal.
7. Advantages of S-type load cells
S type load cells:
- Are of low cost compared to other load cells of similar accuracy (cheaper than load pin load cells).
- Are compact, lightweight and simple to integrate. Many are also available with IP67 ingress protection.
- Provide high measurement accuracy (typically 0.03% of full scale) over a wide range of loads.
8. Disadvantages of S-type load cells
S type load cells:
- Are highly sensitive to off axis loading. Off axis loads will be detected by the load cell but will be measured incorrectly, possibly leading to a ‘nonsense output’.
9. Application tips
- Overload capacity: When placing the load cell in tension or compression, the inertia of the system (e.g. the inertia of a mass picked up by a crane) will cause a sudden and temporary increase in load to above the steady state load. It is important that the load cell is gradually placed into tension or compression to avoid the instantaneous load from surpassing the overload capacity of the load cell.
- Multiple sensors: For weighing applications, it is often necessary to use multiple load cells because the object being weighed requires multiple support points. The analog outputs of the load cells can be summed by a junction box before the process controller.
- Rod end bearings: S-type load cells are very sensitive to off axis loading. When measuring tension, off axis loading can often be prevented by mounting the load cell using rod end bearings. Maintaining the upper and lower rod end bearings at an angle of 90 degrees from one another, can reduce swaying.
- Creep: Due to the time dependence of mechanical strain, permanent deformation of the load cell can occur, which may increase readings by up to 0.1% within 30 minutes. The creep error can be accounted for by remeasuring the zero error of the load cell. The rate of creep can be reduced by utilizing only part of the full scale measurement range of the sensor. This will improve long term stability but will not significantly improve accuracy as the reduction in creep will be offset by a reduction in accuracy caused by not using the full measurement range.
- Lifetime: Load cells have a high lifetime, typically millions of cycles. The lifetime can be improved further by not using the entire full scale measurement range and avoiding overload of the sensor.
- Mechanical vibrations: Mechanical vibrations cause periodic mechanical deformation of the load cell and are therefore detected by the strain gauge, resulting in high frequency noise. The noise amplitude can be reduced by choosing a load cell with a natural frequency far from the vibration frequencies of the system. Furthermore, the signal noise caused by mechanical vibration can be filtered by analog or digital filtering.
- Load shunting: It may seem obvious, but make sure that all of the force is passing through the load cell, otherwise the load will be underestimated. It is not possible to account for load shunting by using a fixed zero offset.