The Development of Neural Network Based System Identification ...

70 CHAPTER 3 AERIAL PLATFORM AND CUSTOM-BUILT FLIGHT TEST SYSTEM
where L 1 and L 2 indicate the length of arm 1 and 2 respectively.
Due to cost constraints, it was decided that positional measurements should be
accurate to 1%. To achieve this accuracy in the XY plane, the encoders must have a
resolution finer than 0.57 ◦ (θ = sin −1 (1%)). The Romuron A6A2-CWZ3E incremental
optical encoder was selected for this project, with sufficient accuracy (θ = 360 ◦ /2000 =
0.18 ◦ revolution) for a prototype. The encoder is compact and has a very small starting
torque, so it will easily fit into confined spaces and will not add any dynamic effects to
the test stand.
In order to measure the altitude of the helicopter, a linear position transducer
is used to monitor the actual altitude reading of the helicopter. The linear position
transducer is mounted at the side of the sliding vertical mechanism as shown in the
Figure 3.11.
The UniMeasure LX-PA-30 linear position transducer was chosen to
measure the displacement of the sliding vertical arm. The unit consists of a precision
potentiometer, attached to a spring loaded string (maximum extension = 750 mm),
which unwinds as the vertical arm is extended. It has linearity of ±0.25% which produce
distance measurement accuracy up to 1.875 mm. There are also dead band regions at
the maximum and minimum allowable displacements which need to be accounted for in
the string length displacement, l calculation. The linear transducer outputs an analog
signal measurement and this signal is converted to digital signal with 12-Bit MCP3221
A/D converter chip with I 2 C interface. The signal received from the A/D converter
chip is used to calculate the length of the string, l by the following relationship:
V in = V dd × A in
l (mm) =
2 12
V in − V o,low
V dd − V o,low − V o,high
× 750 (3.3)
where A in and V dd denote output voltage and excitation voltage supplied to the potentiometer.
V o,low and V o,high indicate the offset voltages of the potentiometer corresponding
to the lowest (initial) and highest (maximum) string displacement. Since the
linear transducer is not placed parallel with the vertical arm, a simple modification is
further made into the altitude measurement from the linear transducer. Figure 3.12