The calibration check signal (CC signal) is a low-active, digital switching signal used to generate a defined detuning of the measuring bridge. The magnitude of this detuning relative to the zero point can be specified as early as the force transducer design stage. In doing so, the dynamic range of the measuring amplifier used must be taken into account.

For signal transmission, we recommend measuring amplifiers with a current output. The required load resistance is connected to the input of the evaluation unit. This resistance is used to generate the required input voltage from the transmitted current signal. Current signals are particularly suitable for transmission over longer cables, as they are less sensitive to voltage drops along the cable.

The zero-point current can also be used to check the signal line. A current signal greater than 0 mA allows a broken line to be easily detected.

How the calibration check signal works

In a load cell with an internal calibration check, the CC signal is activated by connecting the CC contact to earth (GND). When the sensor is unloaded, this generates a defined signal current at the output of the sensor amplifier, which corresponds to the signal at full load, for example 9 mA or 20 mA.

This allows the function of the downstream cut-off circuit to be checked without actually having to apply a load to the sensor.

Example of using the calibration test signal to check the shutdown sequence

Activating the calibration test signal induces a defined detuning directly at the measuring bridge. This results in a defined signal amplitude at the output of the measuring amplifier. 

This stroke reaches at least the signal value required for tripping, so that the downstream trip relays enter the overload state.

In this way, the entire trip chain can be tested either via an external unit or by means of a manual test.

The following functions are checked:

• Correct connection to the DMS measuring bridge
• How the measuring amplifier works
• Load limit cut-off function
• Correct external wiring of the cut-off circuits

Tip: Calibration test signal for online monitoring

If designed appropriately, the calibration check signal can also be used for online monitoring of the sensors. This requires that the measuring range of the evaluation unit be selected to be sufficiently wide.

Example of a sensor with a 4 … 20 mA amplifier:

When the sensor is at full load, the output signal is 20 mA. If the CC signal is also activated, this generates a signal swing of 16 mA.

This gives:

20 mA + 16 mA = 36 mA

In this case, the measuring range of the downstream signal processing unit must be capable of handling a signal of at least 36 mA. Provided this is the case, the CC signal can be used for continuous monitoring of the sensor and the connected signal processing unit.

Example of how to determine the required measurement range

The following applies to a sensor with a 4 … 20 mA amplifier:

• Output signal at zero point: 4 mA
• Output signal at full load: 20 mA
• CC signal: +16 mA
• Maximum signal when the CC signal is active: 36 mA

The required measurement range of the evaluation unit must therefore be designed accordingly.

Please do not hesitate to contact us should you have any queries or comments.