1. How do canister load cells work?
A canister load cell is a type of load sensor intended for measuring compressive forces. Its geometry is illustrated below. It consists of one or more vertical columns connected to a solid base. A compressive load applied to the column(s) causes them to contract. The magnitude of contraction and therefore the load is measured by 4 strain gauges (2 of which are visible in the illustration) bonded to the columns in a full bridge Wheatstone bridge arrangement.
A strain gauge is a thin laminated foil pattern used to measure mechanical strain. Applying a compressive force to the load cell causes the strain gauges to contract and therefore their lengths to decrease and their cross sectional areas to increase, which decreases their electrical resistance. By measuring the change in electrical resistance, it is possible to determine the amount by which the strain gauge has contracted.
2. General characteristics of canister load cells
Canister load cells are high cost, high accuracy sensors. They can measure a wide range of compressive loads, up to extreme loads of over 3,000 tonnes. The load cell consists of one or more vertical columns which are placed under load. This design enables canister load cells to achieve extreme loading and makes them insensitive to off-axis loading, particularly if they have multiple columns. Most canister 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
Canister 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 1-2 mv/V. Many load cells include built-in amplifiers that amplify the mV signal and convert it to a standard 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 canister load cells
Canister load cells are used in many compressive loading applications. They provide higher accuracy and larger maximum loads than pancake load cells but are more expensive. Popular applications include truck scales, tank weighing, hopper weighing, and structural performance testing.
5. Typical specification
|Measurement range||0-0.1 tonnes to 0-3,000 tonnes|
|Accuracy||0.02 % F.S.|
|Hysteresis||0.02 % F.S.|
|Linearity||0.02 % F.S.|
|Zero error||1% F.S.|
|Ambient temperature||-55 to 90 °C (up to 120 °C possible)|
|Supply voltage||1-2 mV/V, 0-10 V, (serial available but less common)|
|Output voltage||5-15 Vdc|
|Passive / active||Active|
|Contact / non-contact||Non-contact|
6. Purchasing tips
- Output sensitivity: Unamplified canister load cells are typically available with 1-2 mV/V output signals. Large output signals improve noise immunity and effective resolution.
- Over 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. Some data sheets will also provide the maximum allowable side loading.
- Natural frequency: Load cells are sensitive to mechanical vibrations (strain gauges are also used in vibration sensors) and so it is important to purchase 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. Most canister 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: Canister 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 aim to prevent off axis loading where possible, canister load cells are less effected by off axis loading than most other types of load cells. Choosing a canister load cell with multiple columns and a small height to width ratio, minimizes off axis loading sensitivity.
7. Advantages of canister load cells
Canister load cells:
- Provide high measurement accuracy (typically 0.02% of full-scale) over a wide range of loads.
- Can measure extremely large loads, up to 3,000 tonnes or more.
8. Disadvantages of canister load cells
Canister load cells:
- Cost more than most other types of load cells
- Single column canister load cells may be 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’. Multi column canister load cells are significantly less sensitive than single column versions, meaning that off axis loads will be ignored.
9. Application tips
- Overload capacity: When placing the load cell in compression, 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.
- Multiple sensors: For weighing applications, it is often necessary to use multiple load cells because the object being weighed may require multiple support points. The analog outputs of the load cells can be summed by a junction box before the process controller.
- 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 their entire full-scale measurement range 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 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 the force is passing through the load cell, otherwise the load will be underestimated. It is not possible to account for load shunting with a fixed zero offset.