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A Prototype Optical Tracking System Investigation and Development - page 82 / 170

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82 / 170

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Black Spot Hardware

3V3

Black Spot 1 TX

Black Spot 2 TX

D1

R1 10K

Hub RX

FLOW_CONTROL

1

2

BUS_SYNC

3

4

5V0

UART_TX

5

6

UART_RX

7

8

COMMS

GND

9 10

GND

GND

Header 5X2 Power and Comms GND IDC Header

GND

D2

Black Spot 3 TX

D3

Figure 5.8 The Blackfin transmit line is pro- tected against two or more modules using the bus at once by a Shottky diode and a resistor.

Figure 5.9 The communications header pro- vides the Black Spot module with power and communications with the hub.

a four layer board means that the two inner layers can be used for power and ground planes. This helps to reduce switching noise from high speed signals by providing inter- plane decoupling capacitance. The four layers from top to bottom are a top routing layer, an internal power plane, an internal ground plane and a bottom routing layer. The power plane is broken into 5 V, 3.3 V, and 1.2 V areas. The Blackfin I/O operates at 3.3 V so the majority of the power plane is 3.3 V. Care was taken to ensure that the high speed signals travel as short a distance as possible on the PCB. The image data bus was also routed with each bit next to each other as these signals are likely to change state together and, there- fore, probably will not affect each other. The 3.3 V I/O signals on the top and bottom layers were routed so that they run over the 3.3 V plane area, again to reduce noise.

5.4.7

Optical design

The optical components of the system consist of a miniature S mount lens, a custom CS mount lens adapter, and an optical filter. The miniature lens is a small lens supplied by Edmund Optics and has a focal length of 6 mm. It screws into the CS mount lens adapter

(Figure 5.10a).

narrow-band green filter from Edmund Optics is attached to the adapter

(Figure 5.10b). This filter has a bandwidth of 10 nm (P/N 510FS10-25). The advantage of using a filter is that the SNR can be improved by attenuating light at wavelengths other

than LED’s predominant wavelength.

disadvantage with using the filter is that the

brightness of objects at the edges of the FOV is severely reduced.

6 mm miniature lens has been chosen for the design. The lens must be able to focus on

an LED in the range of 0.5 m to 3 m.

s the distance to the LED is much larger than the

focal length, the LED is approximated to be at infinity and the thin lens equation (Equation 3.3 presented in the Chapter 3) states that an image is formed at the focal length of the lens,

i.e., 6 mm.

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