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Digital Capacitive and Strain Gage Load Cell Technologies Compared


The basics

Capacitive and strain gage load cells both relies on an elastic element which is deformed by the load to be measured.

The material used for the elastic element is normally aluminum for low cost load cells and stainless steel for load cells used in corrosive industrial applications.

A capacitive respectively a strain gage sensor measures the deformation of the elastic element and the output of the sensor is converted by an electronic circuit to a signal which represents the load.

A capacitive sensor is a conductor which, placed at a small distance from the elastic element, measures the deformation without any contact with the elastic element while a strain gage is an insulated resistance foil which is bonded directly to the elastic element and which is consequently exposed directly to the shocks and overloads, frequently met in industrial applications.

Comparison Capacitive and Strain Gage Load cells
Comparison Capacitive and Strain Gage Load cells

Sensitivity

Furthermore a capacitive sensor is very sensitive with a change of capacitance of 10% while the foil strain gage normally only has a change of resistance of 0.1%.

Because of the much higher sensitivity of a capacitive sensor,
a much lower deformation of the elastic element is needed and the elastic element of a capacitive load cell is therefore strained around 5 to 10 times lower than the elastic element of a strain gage load cell.

The low strained element combined with the fact that a capacitive sensor is non-contacting, provides the very high shock resistance and overload capability of the capacitive load cell compared to the strain gage load cell.

Cabling and sealing

The high change of capacitance lends itself to provide a digital output signal which in the Eilersen capacitive load cells is a high speed signal, representing the load directly in g, kg or Newton.
 
A low cost coaxial cable with a one wire hermetically sealed connector supplies the load cell with power and transmits the high speed digital signal back to the instrumentation which may be placed several hundred meter away. 

In a standard analog strain gage load cell, the power supply and the low level analog signal are normally conducted through a rather expensive 6 wire cable to the instrumentation where the analog signal is converted to a digital signal.

In a digital strain gage load cell, an amplifier and also the A/D conversion are placed in the housing and the power supply and the digital signals are normally conducted through a rather expensive 6 or 7 wire cable to the instrumentation.
With the 6 or 7 wire cables used in strain gage load cells it is difficult to obtain the necessary IP68 protection for industrial environments without complicated water traps or expensive multi pole hermetic feed through connections.

In contrast to this, is the uncomplicated IP69K single pole connector used in the Eilersen capacitive load cells which furthermore has the logistically important advantage that the cable may be mounted just before delivery with length and PVC/Teflon type according to customer specifications.

Conclusion

With reference to the above-mentioned merits of the Eilersen capacitive and the strain gage load cells the following can be concluded:

The Eilersen capacitive load cell provides a tolerance to shock and overloads of up to 10 times nominal load compared to the 2 times nominal load for strain gage load cells.

This is an obvious advantage in industrial environments and especially for the lower capacity load cells where the risk of damage because of shocks and overloads is high.

The Eilersen capacitive load cell provides a tolerance to side loads and eccentric loads of up to 10 times higher than for strain gage load cells
 
These features means that expensive mounting kits with side load- and overload stops – which often cost as much as the load cell itself and needs maintenance – are normally not needed with the Eilersen capacitive load cells which as a consequence offers a simple and inexpensive mechanical mounting in the application.

The factory calibrated, digital high speed signal, conducted through the well shielded coaxial cable, provide simple plug and play installation and at the same time the EMC protection and the protection against welding voltages needed in industrial applications.

The high speed signal, sent at 230 kBaud, with the load value in 24 bits and with address and diagnostic codes, is necessary for the 1000+ readings/second in dynamic weighing applications and for the very short update time of 2 mS necessary for four loads cells in platform and cargo weighing, mixing, dosing and hopper applications.

As a final conclusion, the Eilersen capacitive load cells have important advantages compared to the digital strain gage load cells which still are strain gage load cells.

10 Reasons for using Eilersen digital load cells

The Eilersen shockproof load cells hammer test: