Cold gas reaction control system design

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Introduction
Reaction control system (RCS) is an active Attitude and Orbit Control subsystem (AOCS) of Spacecraft and Missiles used for
Attitude control during re-entry
Station keeping of spacecraft
Close manoeuvring during docking
Also used for deorbiting
Applications
PSLV uses RCS in its fourth stage for pitch,yaw and roll control during coast phase
IRS CARTOSAT uses RCS for AOCS
RCS thrusters are used in International Space Station as backup and augmentation systems for primary attitude control
Apollo, Soyuz, Mercury,X-15, X-20 and Gemini space shuttles employ RCS
In Aircrafts like V/STOL fighters (YAV-8B Harrier)and NASA X1-B Research plane RCS is used
OBJECTIVE OF MISSILE RCS
In supersonic missiles, high maneuverability is not possible using aerodynamic control surfaces like wing, tail and canard control due to stall conditions. Hence Reaction Control Systems (RCS) are employed for high angle of attack maneuvering.
Working Principle
A secondary motor is added in missiles with RCS which makes use of 15% of the gas generated in the primary motor.
The secondary motor has the same pressure (~70×105N/m2) and temperature (~2000K) of that of primary motor. The secondary motor has a C-D nozzle which can produce thrust from 20 N to 120 N by varying the nozzle throat area using a plug.
The linear motion of a plug varies the throat area which is directly proportional to the thrust developed at the nozzle exit.
This plug is attached to a Ball and Screw mechanism which converts the rotary motion of a Brushless DC motor (BLDC) into micro precise linear motion.
The BLDC motor is driven by Pulse Width Modulated (PWM) signals generated by TMS320C6711 Digital Signal Processor (DSP).

Introduction to DSP

DSP processors are used mainly for real-time signal processing. Analog systems are more affected due to temperature changes than a digital system. Many DSPs especially tailor suited for PWM signal generation are available such as the F2812 processor; the TMS320C6x series are more economical and easily available. Hence the algorithm has been implemented in DSK6711 DSP Starter Kit which is commonly available in every DSP lab. DSP processors are used mostly in the audio frequency range because of the limitations posed by the sampling rate.
Three commonly used PWM techniques include sinusoidal, hysteresis (bang bang) and space-vector (symmetrical or asymmetrical) implementations. Sinusoidal PWM is an effective method to generate sinusoidal currents. Sinusoidal PWM is obtained by comparing a high-frequency carrier with a low frequency sinusoid. The switching instants are determined from the crossing of the carrier and the modulating signal . The carrier signal is a sawtooth wave of fixed frequency and the modulating signal is a sine wave of variable frequency. The characteristics of the output signal depend on the accuracy of the algorithm. More precise the algorithm, more are its processing time and cycles but there is a comparative reduction in the harmonic distortions, ripples present in the output signal.

PWM SIGNALS

PWM signals are widely used in telecommunications, power control systems, voltage regulation and in audio signal processing. This paper deals with the simulation of PWM signals for power control. PWM power control systems became reality with the advent of modern semiconductor switches like MOSFETs and Insulated Gate bipolar transistor (IGBT). PWM signals can be easily generated using microcontrollers rather than Digital signal processors (DSP), but when the accuracy and speed of the generated signal is of particular importance, then microcontrollers can no longer be used and this is where DSP comes into play. Programming a DSP to generate PWM signal is not an easy task since the interfacing codes are not as simple as for microcontrollers. Hence the simulation of the PWM signal has been done to study the efficiency of the C code. This work is a part of a project to develop DSP-FPGA based BLDC motor control.

PWM SIGNAL GENERATION using TMS320C6711 Floating Point Digital Signal Processor

The DSP Starter kit consists of TMS320C6711 Floating Point Digital Signal Processor and a 16 bit dual-channel voice/data codec TLC320AD535 for input/output support. This codec uses Sigma-Delta Technology for Analog to Digital/Digital to Analog conversion. This codec has a fixed sampling rate of 8 KHz. The DSP uses Very Long Instruction Word (VLIW) Architecture. There are many ways of generating PWM signals. The ADC converts an input signal into discrete output digital words in 2’scomplement format that correspond to the analog signal value
Polling Method
The Multichannel Buffered Serial Port (McBSP) can be interrupt driven or can be driven by polling method. A polling-based program (non-interrupt driven) continuously polls or tests whether or not data are ready to be received or transmitted. This scheme is less efficient than the interrupt scheme. Within the output write function mcbsp0_write, SPCR is ANDed with 0x20000 to test bit 17 .
temp = *(unsigned volatile int *)McBSP0_SPCR & 0x20000;
An output data value is sent through the data transmit register of McBSP. The polling technique uses a continuous procedure of testing when the data are ready.
The following lines of code are used to generate a PWM wave
comm_poll();
while(1)
{
y=-ampoframp*ramp1000[i];
i = (i + freqoframp) % 2000; //next sample is + freq in table
x=ampofsine*sine8000[j];
j = (j + freqofsine) % 8000; //next sample is + freq in table
if (y < x)
pwm=-10000;
else pwm=10000;
output_sample(pwm);
}

PWM signal generation using CCS

The PWM signal is generated by comparing the sine wave with the sawtooth wave. The figure shows a PWM signal generated for a sine wave of Fm=320 Hz and a sawtooth wave of Fc=1 KHz

Sawtooth signal generation using CCS

The sawtooth wave has a positive and a fixed slope. In case of generating the wave the step size defines the frequency of the sawtooth wave. The following lines of code are used to simulate a sawtooth waveform.
for(j=0;j<1024;j++)
{
if(y>1.00)
{
w=0.00;
}
sawtooth[j]=y;
y=y+0.5;
}
Figure shows two sawtooth waveforms of different frequencies. The first wave with F=4 KHz and the second wave with F=1 KHz. The step sizes taken were 0.5 and 0.125 respectively