The Gps Based Autopilot

Published Date: 02 Nov 2017

Abstract- Speed and angular measurement of direction plays an important role in the functioning of most of the ships based on autopilot system. Basically the autopilot system uses gyro and speed log for accurate measurement of heading (i.e. direction) and speed of the ships respectively. However these gyro and speed log can be replaced by GPS (global positioning system) which gives speed over ground and course over ground similar to the input fed by speed log and gyro respectively. Data at any baud rate from GPS will be received by an embedded system which will processed and then transmitted at fixed baud rate to monitoring devices. This paper presents an efficient marine autopilot system based on GPS that ensures optimal accuracy, portability and economical as compared to autopilot system which are fed through gyro and speed log.

Key words: speed log, speed over ground, course over ground.

I. INTRODUCTION

One of the most important equipment used in ships today is an ‘Autopilot’. Apart from leading the ship on a desired trajectory, autopilots are also used to raise safety level of the journey and to control the ship economically. An efficient autopilot can help to avoid undesired situations on maneuvering and remarkably reduce the number of ship operators. It can shorten the length of the journey to a considerable extent and thus reduce the fuel consumption especially in bad weather condition. In the last few decades, taking advantages of drastic development of microelectronics and embedded systems, several new and improved methods have been proposed and developed for designing ship autopilot. Ship autopilot’s design is based on gyro and speed log which are reliable and efficient but are not economical and consumes a large space. Also, in various environmental conditions their performance is not as good as desired.

Fig. 1. A Typical Example of Ship Autopilot System

The design of autopilot for ships is based on GPS which can replace the conventional autopilot system based on gyro and speed log. The advantages of using GPS based ship autopilot are that space and cost of the autopilot system is reduced up to a desirable amount thus providing the optimal efficiency. The paper is organized as follows: the first section gives problems associated with it and its conventional autopilot system, the second section deals with GPS for autopilot and their subsequent working. Third section deals with serial communication protocols. The fourth section deals with control mechanism for controlling the speed and heading using embedded system. The last section draws conclusions from this paper based on ‘GPS based Autopilot’ [2].

II. GPS FOR AUTOPILOT

The GPS (global positioning system) is a space-based satellite navigation system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites. It is maintained by the United States government and is freely accessible to anyone with a GPS receiver [1].

A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the Earth. Each satellite continually transmits messages that include

the time the message was transmitted

satellite position at time of message transmission

The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite using the speed of light. Each of these distances and satellites' locations define a sphere. The receiver is on the surface of each of these spheres when the distances and the satellites' locations are correct. These distances and satellites' locations are used to compute the location of the receiver using the navigation equations. This location is then displayed, perhaps with a moving map display or latitude and longitude; elevation information may be included. Many GPS units show derived information such as direction and speed, calculated from position changes.

Fig.2. Display showing COG & SOG using GPS

GPS sends data in the following form: $GPRMC (global positioning recommended minimum configuration), time, latitude, north/south, longitude, east/west, SOG (speed over ground), COG (course over ground), date, checksum [4].

From this for autopilots in ships we require COG and SOG. COG is calculated in degrees and SOG in knots. The data from GPS is continuously obtained in the ship using the embedded system autopilot checks the values of rudder and propeller in the ship for the control of heading and speed respectively.

III. SERIAL COMMUNICATION PROTOCOL

A serial communication protocol assumes that bits are transmitted in series down a single channel. A serial protocol has to address the following issues:

How does the receiver know when to start looking for information?

When should the receiver look at the channel for the information bits?

What is the bit order?(MSB or LSB first)

How does the receiver know when the transmission is complete?

These four issues must be agreed upon by both transmitter and receiver prior to sending any information. Serial communication protocol may be described as a set of rules for exchange of digital message formats in or between computing systems and telecommunications. These protocols may include signaling authentication and error detection and correction capabilities [6].

Many digital types of equipment which are supporting serial communication mainly include RS-232, RS-422, RS-485 and so on which are widely used in the industries. Among all thee serial communication protocols,RS-232 and RS-422 prominently supports half-duplex while the maximum communication of RS-232 is only less than 15 meters while the maximum communication of RS-422 is up to 1500 meters. However, the interfacing of RS-232 is simpler and the computer receives the data usually through the RS-232 serial port. Hence, only RS-232 standard needs to be considered for serial communication in autopilot systems[6].

The RS-232 serial port communication is very popular in monitoring and controlling system. Basically RS-232 is an asynchronous serial communication protocol used for data transfer between electronic devices. It can transfer a single byte of data over a serial cable having 3 to 22 signals and running at speeds from 100 to 20k baud. Common baud rates used are 2.4k, 9.6k, and 19.2k the cable length can be up to 50feets. Higher baud rates are used but not covered by the standard they still work though. To transfer a block of data, individual bytes are transmitted one after another [5].

Fig.3. Serial Communication using RS-232

Data is transmitted serially in one direction over a pair of wire. Data going out is labeled Tx(indicating transmission) while data coming in is labeled Rx(indicating reception). To create a two way communication system a minimum of three wires are needed Tx, Rx and GND (ground).

Each byte can be transmitted at any time (as long as the previous byte has been transmitted). The transmitted byte is not synchronized to the receiver then it is an asynchronous protocol i.e. there is no clock signal. For this reason software at each end of the communication links must be set up exactly the same so that each serial decoder chip can decode the serial data stream. For a baud rate of 24000(2400 bps) the frequency is 2400Hz and the bit period is 1/2400 or 416.6 microseconds. This is the information that a receiver uses to recover the bits from the data stream.

MAX232 is an integrated circuit, which converts signals from an RS-232 serial port to signals suitable for use in TTL compatible digital logic circuits. It is a dual driver/receiver and typically converts the Rx, Tx, CTS, RTS signals. MAX232 has a power supply of +5v volts with two receivers and two transmitters used for conversion from RS232 to TTL level [3].

IV. CONTROL MECHANISM

The heading in a ship is controlled by a device called rudder and the speed is controlled by propeller. The autopilot controls the rudder and propeller of the ship when in operation, thus reduces a pilot’s work [2].

In a GPS based autopilot, the data obtained from GPS is fed into the embedded system. COG (course over ground) and SOG (speed over ground) from the GPS is obtained every minute which shows the current heading and speed of the ship. The autopilot which is loaded with a particular cut-off of direction and speed compares it with the heading and speed obtained from GPS and autopilot starts operating if the heading or speed in below or above the set cut-off[3].

V. CONCLUSION

Autopilot using GPS technology is an efficient design of ships autopilot which can replace the conventional autopilot system based on gyro and speed log mechanism. This would ensure minimum cost and space requirement providing optimal efficiency. Features like simplicity, flexibility, user friendly and low cost have been highlighted through the use of GPS in ships. Such autopilot systems must be implemented into ships for practical and economical use [1].

ACKNOWLEDGEMENTS

We would like to thank our H.O.D Prof. Mrs. Suman Wadkar and our project guide Prof. Mrs. Suchitra Patil for providing us with their patience, guidance and profound knowledge.