1. How do pancake compression load cells work?
A pancake load cell is a type of low-profile load sensor designed for measuring compressive forces though often also capable of measuring tensile forces. Its geometry is shown in the illustration below. The outer frame is fastened to a solid base. A load is applied to the inner section of the load cell, which is connected to an outer frame by 4 beams. The load causes the beams to deform. The magnitude of deformation and therefore the load is measured by 4 strain gauges (2 of which are visible in the illustration) bonded to the beams 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 pancake load cell’s unique geometry provides high sensitivity to axial loads whilst minimizing its sensitivity to off axis loading. Pancake load cells are sometimes mistaken for or assumed to be the same as button type load cells. However, button type load cells are built differently and tend to be smaller, cheaper and significantly less accurate than pancake load cells.
2. General characteristics of pancake compression load cells
Pancake load cells are medium cost sensor with reasonably high accuracy. They can measure a wide range of compressive and tensile loads and are available with full-scale measurement ranges from as little as 1 kg to as large as 100 tonnes. They are relatively compact, and their geometry makes them insensitive to non-axial forces. However, their accuracy is lower than many other types of load cells. Most pancake 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
Pancake 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 1-4 mv/V. Many load cells include built-in amplifiers that amplify the mV signal and convert it to a standard analog output signal 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 pancake compression load cells
Pancake load cells can be used in a large range of tensile and compressive loading applications, particular where low cost and insensitivity to non-axial loading is required. Popular applications include tank weighing, hopper weighing, mechanical overload protection and structural performance testing.
5. Typical specification
|Measurement range||0-1 kg to 0-100 tonnes|
|Zero error||2% F.S.|
|Ambient temperature||-55 to 90 °C (up to 120 °C possible)|
|Output voltage||1-4 mV/V, 0-10 V, (serial available but less common)|
|Supply voltage||5-15 Vdc|
|Active / passive||Active|
|Contact / non-contact||Non-contact|
6. Purchasing tips
- Output sensitivity: Unamplified pancake load cells are typically available with 1-4 mV/V output signals. Larger output signals improve noise immunity and effective resolution.
- Compression only or tension + compression: All pancake load cells are capable of measuring compression. Many but not all are also capable of measuring tension.
- Overload capacity: Load cell data sheets specify an overload capacity, typically 150-300% of 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 frequencies of the system.
- Creep: Creep effects long term output stability, particularly when operating near to the maximum rated load. The majority of pancake load cells have creep induced output variations of 0.02-0.1% F.S. after 30 minutes of loading at their maximum rated load. For applications requiring long term stability, aim to purchase a load cell with minimal creep.
- Built-in amplifier: Pancake 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.
- Height to width ratio: Although you should always aim to prevent off axis loading of load calls, pancake load cells are less effected by off axis loading than other types of load cells. Furthermore, the smaller the height and larger the width of the load cell, the less sensitive it is to off axis loading.
7. Advantages of pancake compression load cells
Pancake load cells:
- Are low cost, particularly when compared to canister load cells
- Have a wide measurement range and high maximum capacity, with some models capable of measuring 100 tonnes or more.
- Are the most suitable load cell for applications with off axis loading.
8. Disadvantages of pancake compression load cells
Pancake load cells:
- Are of lower accuracy than many other types of load cells (though their accuracy is still reasonably high).
9. Application tips
- Overload capacity: When placing the load cell in compression (or tension), the inertia of the system (e.g. the inertia of a mass placed on the load cell) 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 compression to avoid the instantaneous load from surpassing the overload capacity of the load cell. Some data sheets will also provide the maximum allowable side loading.
- 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.
- Off axis loading: The geometry of pancake load cells makes them insensitive to off axis loading. This means that when the load is not perpendicular to the load cell, only the perpendicular component of load will be detected.
- 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 of the sensor and avoiding overload.
- 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 noise caused by mechanical vibration can be filtered from the output signal by analog or digital filtering.
- Load shunting: It may seem obvious, but make sure that all 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.
- Mounting: It is important that load cells are mounted to a solid, stable and flat surface such as a concrete floor. Often tanks are mounted on a supporting frame; keep in mind that multiple tanks supported by a common frame my effect each other’s weight readings.