Types of Modulation

MODULATION

In this section we are explaining only two types of modulation that are required. Here we are converting a PWM signal, that is received by the quad-copter from the remote controller and converting it into a PPM signal that is required by the ARDU-IMU board to understand the commands.

Note: "ARDU-IMU board does not understand the PWM signal, so the conversion is required."

PULSE WIDTH MODULATION (PWM)

Pulse Width Modulation is a way of simulating an analog output by varying HIGH and LOW signals at intervals proportional to the value. Width of each pulse varies according to the amplitude of the analog signal.

FIGURE 4.1: pulse width modulation

Pulse-width modulation, or pulse-duration modulation, is a commonly used technique for controlling power to inertial electrical devices, made practical by modern electronic power switches. The term duty cycle describes the proportion of 'on' time to the regular interval or 'period' of time; a low duty cycle corresponds to low power, because the power is off for most of the time. Duty cycle is expressed in percent, 100% being fully on.

The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast pace. The longer the switch is on compared to the off periods, the higher the power supplied to the load is.

Duty Cycle:

In a periodic event, duty cycle is the ratio of the duration of the event to the total period.

Duty cycle 

Where,

τ is the duration that the function is active

Τ is the period of the function.

FIGURE 4.2: duty cycle

PULSE POSITION MODULATION (PPM)

Pulse-position modulation is a form of signal modulation in which M message bits are encoded by transmitting a single pulse in one of 2^M possible time-shifts. This is repeated every T seconds, such that the transmitted bit rate is M/T bits per second.

Pulse position modulation (PPM) is a pulse modulation technique that uses pulses that are of uniform height and width but displaced in time from some base position according to the amplitude of the signal at the instant of sampling.

FIGURE 4.3: pulse position modulation

PWM TO PPM CONVERSION IN A QUAD-COPTER

PWM refers to a pulse width modulation signal, where the width of each pulse changes according to the amplitude of an analog signal. PPM on the other hand refers to a pulse position modulation signal, where the width of each pulse remains the same, but each pulse is displaced by a certain position based on the analog signal amplitude.The basic need for conversion of a PWM signal received from a transmitter into a PPM signal arises due to the fact that the main controller board used (ArduIMU) on a quad-copter cant process a PWM signal and hence a converter is required to convert a PWM signal to a PPM signal.The conversion of a PWM signal to a PPM signal is as explained in the below figure -:

FIGURE 4.4: Pwm to ppm conversion

CHIP USED FOR CONVERSION (PWM TO PPM):

The best way to read RC signals is PPM, which is sequential stream where each channel is output in turn on a single wire. The chip used by us can convert up to 8 channels of regular PWM signals to one PPM signal. This board plugs into the servo output ports on an R/C receiver and encodes them into PPM signal.

Block Diagram:

Integration of atmega328p encoder (PWM TO PPM) with the main board (ARDUIMU V2). Atmega328p converts the PWM signal received from RC transmitter into a PPM signal and sends it as input to main board as shown in the figure below.

FIGURE 4.5: block diagram for Pwm to ppm conversion