A laser range finder was used to sense the obstacles What pa

A laser range finder was used to sense the obstacles. What parameter will the microcontroller record? If an obstacle is 5m away, what will be the recorded value?

Solution

The main function of the microcontroller is to record the time measurement from
the time-measuring unit, average the time measurements and calculate the distance. After the
distance is calculated, the data is transmitted to a PC. The microcontroller sends the signals to start the counter for the time measurement unit and to send the laser light pulse. Also, the microcontroller is used to initiate and set the parameters of the time measurement unit (speed, mode, resolution, etc.) After the time measurement unit calculates the time, the microcontroller reads and stores the time information. At least fifty time measurements are taken from each distance to be measured. The more samples taken the most accurate the results are, but the more time needed to obtain the distance measurement.

Once the microcontroller has obtained the values for the time and computes their
average, it calculates the distance value and transmits it to a PC using USART interface. Each
distance calculation should take less than 20ms. This time depends on the number of time
measurements taken and the duty factor of the pulse laser diode Laser Diode Driver
This section outlines the function of the laser driver. The main function of this driver is
to provide fast, high current pulses to drive the PLD. The PLD is operated in a short pulse mode
in order to achieve high peak powers with fast rise and fall times

The amount of current generated by the driver must be limited not to exceed the
maximum forward current of the PLD. In order to maximize the optical power of the PLD, a collimating lens is used to minimize the divergence of the laser beam.

Receiving Channel Hardware
A small fraction of the light beam emitted by the PLD is received by an optical receiver and focus
on the APD which photo-electrically convert the pulse of light received. The small pulse of current
generated by the APD is converted into a TTL voltage level signal using a two-stage transimpedance
amplifier. This voltage signal serves as the STOP signal for the time counter.
To measure the time of flight of the emitted pulsed of light, an ultra-fast counter as a TDC is used.
TDCs are used to measure the time intervals between two events with a resolution of less than
one nanosecond. The accuracy of a pulsed TOF laser radar depends critically on the performance of its
time measurement unit. Therefore, a high resolution and a high speed TDC is needed to
improve the system performance..
High Voltage Power Supply
To reach the proper gain and responsivity of the APD, a high voltage is used to reverse
bias the APD. This high voltage is generated and stabilized using a high voltage DC-DC converter
which provides up to 400V output from a 5V input. The high voltage DC-DC converter must
provide a low voltage noise and low voltage ripple. Increasing the voltage output of the DC-DC converter increases the gain of the APD. Therefore, it increases the current output of the APD per optical power. A resistor is connected in series with the APD to limit the current it generates in order to protect the receiving channel from saturation and damage.
Pre- and Post-Amplifiers
A small fraction of the reflected focused on the APD is converted to a voltage signal
using a two transimpedance amplifiers. This conversion represents one of the major challenges
of the design because bandwidth, gain, and input referred noise are coupled together.
Furthermore, to control the transimpedance gain such that the variation of the amount of
current generated by the APD does not saturate the system is another challenge.

Optical Lens
The infrared (IR) light pulses emitted by the PLD are collimated by the transmitter lens.
Collimating lens used to reduce to a minimum the divergence of the laser beam such that the
laser light does not disperse with distance The optical power reflected from the measurement target object is collected, focused and filtered by a plano-convex lens and an optical band-pass filter with an AR coating for 600- 1050nm wavelength. The plano-convex lens is used to focus the collimated reflected laser beam of the target on the APD. The band-pass filter is integrated in the APD and it is used to further
block the surrounding light. Using an AR coated lens and a band-pass filter helps to achieve a better signal-to-noise ratio, and minimize the effect of the background light.