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Interfacing the Flexiforce Sensor
Setting up the amplifier circuit for the Flexiforce sensor [2] may be straightforward enough.
However, when interfacing it with various DAQ-systems one may encounter small surprises. Thus, we had a curcuit which, while, connected to Labjack U12 DAQ-system [3], gave a very good signal, whereas the same system interfaced with National Instruments E-card series displayed a completely worthless output (fig. 1). After a careful consideration of the grounding issue we found the solution to the problem for the NI card. Using voltage dividers we get from the 20 V output from the power source ±10 V for driving the operational amplifier, and – 5 V for the FlexiForce input (fig. 3). The midpoint of the voltage divider defines the floating GND of the circuit. This is connected to the non-inverting input of the op amp, and to the AISENSE terminal of the NI-DAQ card.
Frank Borg (borgbros@netti.fi) Jyväskylä University, Chydenius Institute, Finland.
Fig. 1 Signal via NI-Daq E-series card in referenced single ended mode.
Fig. 2 Signal after properly grounding the leads in the non-referenced single ended mode.
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The NI-Daq card is configured in the non-referenced single-ended (NRSE) mode. Resistors in the range of 10k – 100k are connected between AISENSE and AIGND, and between ACH and AIGND (fig. 4). In our case we got a good signal with the choice of R = 100k (fig. 2).
Frank Borg (borgbros@netti.fi) Jyväskylä University, Chydenius Institute, Finland.
Fig. 4 A discussion of the different configuration modes of the NI-Daq cards can be found in [1].
ACH
AISENSE
AIGND 10 k ... 100 k
NI Daq card E-series.
Configured in NRSE-mode.
Fig. 3 Voltage values are given relative the floating ground which thus will be “0 V”.
FlexiForce
- 5 V
- +
Power Source Mascot
- 10 V + 10 V
- 5 V
100 k 1 k
1 k
2 k
Floating GND
Vout -Vc
+Vc
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1. “Field wirings and noise considerations for analog signals”. National Instruments online document at www.ni.com, developer zone.
2. www.tekscan.com 3. www.labjack.com
Frank Borg (borgbros@netti.fi) Jyväskylä University, Chydenius Institute, Finland.
