| In this circuit, the Bend Sensor® potentiometer is the input of a resistance-to-voltage converter. The output of this amplifier is described by the equation: VoUT = Vref * [-Rg/ RBend Sensor®] With a positive reference voltage, the output ofthe op-amp must be able to swing below ground, from OV to -Vref, therefore dual sided supplies are necessary. A negative reference voltage will yield a positive output swing, from OV to +Vref. Since this is a simple inverse relation between Vout and RBend sensor®, the output equation can be re-arranged to: VoUT = (-Rg * Vref) / RBend Sensor® Vout is inversely proportional to RBend sensor®. Changing Rg and/or Vref changes the response slope. The following is an example of the sequence used for choosing the component values and output swing: For a human-to-machine variable control device, like a joystick, the maximum deflection applied to the Bend Sensor® is about 2". The testing of a typical Bend Sensor® shows that the corresponding RBend Sensor® at 2" is about 4.6 kQ. If Vref is -5V, and an output swing of OV to +5V is desired, then Rg should be approximately equal to this minimum RBend Sensor®. Rg is set at 4.7 kQ. A full swing of OV to +5V is thus achieved. A set of DEFLECTION vs. VoUT curves is shown on Figure 1.O for a standard Bend Sensor® using this interface with a variety of Rg values. The current through the Bend Sensor® should be limited to less than 1 mA/square cm of applied deflection. As with the voltage divider circuit, adding a resistor in parallel with RBend Sensor® will give a definite rest voltage, which is essentially a zero-deflection intercept value. This can be useful when resolution at low deflections is desired. |