Transducer for Measurement Bridges

Operating Instructions

Connecting a measurement bridge to a DIN-rail module

The above figure shows how to connect a resistive measurement bridge to a DIN-rail module. Note that the impedance of the bridge power supply cables should be as low as possible. The voltage drops could cause a slight reduction in the bridge supply voltage, especially for larger bridge currents.

Connecting a measurement bridge to a printed circuit board module

The above comments are also generally applicable to the printed circuit board modules.

Testing a bridge amplifier module (SIGS 10/32, SCM 70/82, unipolar)

A voltage calibrator can only be connected to the SIGS 10/32, SCM 70/82 modules with unipolar power supplies, when a DC voltage (common-mode voltage) is used to raise the signal voltage inputs to at least 1 V. This can be achieved, e.g., by connecting a second voltage source between connectors 6 (GND) and 7 (In-). Another possibility is illustrated in the neighbouring diagram. The 10 kOhm resisters reduce the bridge voltage to 5 V, which can then be used as the common-mode voltage. A similar procedure can be carried out for a printed circuit board module.

Note: The signal inputs 7 and 8 must always have a DC path to ground (true differential amplifier). This is automatically the case when a measurement bridge (with bridge supply 5,6) is connected. If sensor fault monitoring is installed (available as an option for modules SIGS 15 and SCM 90), then a voltage equivalent to half the bridge supply voltage must be applied to connector 7 (±20%). For a test, this can, for example, be accomplished as shown in the above diagram.

Programming the SIGS 15 and SCM 90 modules

The programmable switches 1a to 8a and 1b to 8b are located inside the module on the back of the printed circuit board. Carefully remove the plexiglass cover. The printed circuit board can now be removed by pulling gently on the screw terminals.

Setting the cut-off frequency in the SIGS 15S and SCM 90S modules

The cut-off frequency is set using a small SMD potentiometer located next to switch b. There are two test holes directly underneath the switch, between which the resistance of the potentiometer can be measured using an ohmmeter. This simplifies the setting of the cut-off frequency. The table below shows the relationship between the cut-off frequency and the potentiometer resistance for the upper range (15 kHz - 330 Hz, switch 4a "off") and for the lower range (340 Hz - 3 Hz, switch 4a auf "on").

The response time (to 1% of the final value by a jump in the input signal) varies in the upper range between approx. 40 us (15 kHz) and 2.1 ms (330 Hz) and in the lower range between 2.1 ms (340 Hz) and approx. 300 ms (3.3 Hz).

Distortions in the output signal at high frequencies

a) Voltage output

A sine curve will be amplified without distortion over the entire range (-10 V to +10 V) up to around 2 kHz. Above this frequency, distortions occur around the zero point and at approx. ±0.8 V (with respect to the output voltage). At larger output amplitudes (> 1 V) the sine shape can also be distorted. A load resistor of between 1k and 10k can practically eliminate the distortions when the output amplitude is not too large. Note, however, the output impedence of approx. 50-70 Ohm.

b) Current output

There is no distortion up to 20 kHz for output current > 2mA (range 0-20 mA) or > 6 mA (range 4-20 mA). At around 2 or 6 mA, respectively, the distortion is limited to a few percent.

For calibration of measurement range and zero point see General Operating Instructions

For EMC and cabling guidelines see Cabling and Installation Requirements