# Sensor amplifier

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## Sensor signal

Instrumentation amplifier topology for amplifying the sensor signal

The signal induced on the sensor coil is very weak and requires amplification before further processing can be performed. Careful amplification is required in order to prevent loading and distortion of the sensor signal which would degrade the system performance. An instrumentation amplifier based design is used. A diagram of the circuit schematic is shown below:

The INA128 is a general purpose instrumentation amplifier from Texas Instruments. The sensor wires are directly connected to the inputs of the INA128. The gain of the amplifier is set using a 10kΩ potentiometer (not shown). The 47kΩ resistors at the op-amp inputs are necessary in order to provide the necessary bias currents to the amplifier.

The TL081 is a general purpose operational amplifier configured as an inverting unity gain buffer using two 10kΩ resistors. The precision of these resistors is not important as the device is a simple buffer. The output of this buffer is connected to the data acquisition unit (DAQ).

The Eagle PCB project files and Gerber CAM files for this circuit can be found in the Anser project repository.

## Current signal

Current summer circuit used to detect the phase of the current through the coils

A current summer amplifier is required to efficiently sample the current in each of the emitter coil PCBs. The composite signal is demodulated in an identical manner to the sensor signal. Data regarding the phases of the signals are extracted to provide information regarding the orientation of the sensor. This is used by the position and orientation algorithm (P&O). Without this, the sign of the ${\displaystyle n_{A}}$ vector cannot be determined. (i.e. Two solutions exist for the orientation angle, ${\displaystyle \theta }$ and ${\displaystyle \theta +\pi }$ The phase information decides which orientation is valid). The circuit schematic of the summer circuit is shown below

${\displaystyle V1}$ to ${\displaystyle V8}$ represent the current-sense voltage signals from each emitter coil. These voltage signals are generated from the high-speed amplifier shown in section 2a. Capacitors ${\displaystyle C_{DC}}$ block any DC bias from being amplified. Resistors ${\displaystyle Rin}$ set the gain of the amplifier in conjunction with ${\displaystyle R_{f}}$. Both ${\displaystyle R_{f}}$ and ${\displaystyle C_{f}}$ also form a low-pass filter to block any high frequency noise from entering amplifier. A single TL081 operational amplifier is used in the Anser system, although this particular model is not required. The resulting output voltage ${\displaystyle V_{sum}}$ is given by:

${\displaystyle V_{sum}=-{\frac {R_{f}}{R_{in}}}\left(V_{1}+V_{2}+...V_{8}\right)}$

The precision of this circuit not vital for accurate system operation. The purpose of ${\displaystyle V_{sum}}$ is to provide the phase information of the coil current. i.e. magnitudes are not important. The amplifier gain should be chosen such that it can be accurately sampled by the data acquisition system (DAQ).