Welcome Again! In this post we will discuss about the main part of the whole Quadcopter/Multirotors system. Here we will learn about RC controller Encoding Decoding system techniques for Drones UAV. Before knowing about RC controller Encoding Decoding system techniques you must know the basics of Quadcopters or UAV or Multirotors. As we are discussing about RC controller Encoding Decoding system techniques for Drones so we must know about the system on which we want to implement it.
RC controller Encoding Decoding system techniques for Drones UAV
At transmitter end signal’s data is in encoded form. Encoding is required because normal signal cannot be transmitted up to a significant distance. Other signal with same frequency can create interference with original signal. There are different technique for encoding and decoding of signal. Some of them are given below:
PPM means pulse position modulation. It is oldest technique. It’s process is as below:-
- It consists the data frames containing a synchronizing pulse. It is followed by shorter pulses equal to the no. of channel.
- For example,if frame duration is 20 ms then data is sent at 50 Hz frequency.
- Transmitter encoding circuit each control potentiometer’s voltage and switch’s position. And convert them in form pulse width.
- The width of each pulse is according to servo’s position.
PCM means pulse code modulation. It follow different process than the PPM system.
- The position of joysticks and switches generates an analog voltage. This analog voltage is first converted in form of digital words of 8 or 10 bits. This is performed using A/D converter.
- No. of bits per frame are about 100-160. It contains 80-100 bits for servos and 16-32 bits for checksum.
- The checksum in the shape of a 16 bit long CRC (Cyclic Redundancy Check). It provides the effective way for bit error reduction
- To reduce failure time frame subdivided for separate CRC check.
- It can only check for errors but cannot correct the error which is serious drawback of this system.
IPD means Intelligent Pulse Decoding. It has its own processor which check for validity and reject the invalid signal. The IPD receiver does not “switch off” the “dirty” signal as field strength declines, but instead widens its tolerance. It works in following way :-
- IPD receiver accepts the signal which has length between 890 µsec to 2350 µsec.
- Receiver analyze the signal and adjust in accordance with field strength.If strength is high then directly passed to servos. Otherwise it is post processed
- IPD receiver calculates the nominal servo position from the last “good” signals ,which it picks.It reduces the interference significantly.
- IPD is faster than PCM because there are no error check cycles.
DRS stands for Digital Signature Recognition. It is used by dual conversion FM receivers. DRS claims for highest level security for crashes when it is used for flight stabilization system. It is operated in following way
- DRS memorizes the actual transmitter’s signal and rejects any other even of same frequency.
- It has a pre-flight interference check which detects and warns pilot that another transmitter is there of same frequency. So it provides the highest level security.
When receiver is switched on it analyze the data stream and check for following factors:
- Valid number of pulses (and stores this)
- Positive or negative shift
- Valid frame length
- Valid pulse widths
If a frame is damaged then it provide three levels of error correction to attempt to restore the data. If the data fails to be restored for 50 consecutive frames, the failsafe mode is enabled.In failsafe mode servos are set to last good frame or preset position by pilot.
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