Technical articles

Aquaticus PWM guide

Introduction

This guide describes principles of PWM for Atmel AVR micro controllers. It is not complete documentation for PWM nor AVR timers but tries to lighten some practical aspects of PWM. Some of the PWM aspects are simplified, some are not mentioned at all. Usability was the main factor when writing this text. Full PWM description can be found in Atmel AVR documentation.

Although the article was written based on ATmega16 and ATmega32 documentation, information is useful for any chip from AVR family.

RS485 communication test

RS485 communication test

This article describes how to test RS485 communication between ROV Control Unit and PC computer with RS485 to RS232 converter. Test programs are provided to test transmission from 2400bps to 115200bps.

There are 2 programs. The first one is prepared for Windows, the second for Atmega16 chip. Windows program sends random data to ROV Control Unit. Atmega16 sends back all data to desktop computer. If sent and received data is the same communication works properly.

Thrusters

Thrusters

The main problem in thruster design is how to isolate engine from water. There are three solutions used to keep engines away from water:

  • O-ring
  • Rotary shaft seal
  • Magnetic coupling

The best and more complicated is solution 3. The engine is perfectly isolated from water because it is connected to the propeller using magnetic coupler.
Solutions 2 and 3 are similar.

Pressure sensor

Pressure sensor

Pressure is used to calculate depth at which rov operates.
Pressure at 1m is equal to 0.098bar, at 100m it is 9.8bar. In real world depth and pressure relationship isn’t constant. It depends on location, water temperature, salinity, etc. You can read more in this article "A Sea Water Equation of State Calculator".

Light

To see anything in deep water, additional source of the light is necessary. Here you have a description of the popular types of lamps.

Halogen lamps

Halogen lamps
Cheap and bright. Easily available in any Home&Garden; shop. 50W/12V halogen lamp costs about 1€. The only problem is power consumption. For 50W/12V lamp you must supply 4A. The way you can limit power consumption is to use PWM, this gives you the possibility to limit the brightness and power according to your current needs.

Compass

Compass

Navigation in the underwater environment is not as easy as on the ground. You have only images from a camera, no sun, no characteristic points, water is not as clear as the air.

Expensive commercial vehicles uses sonar, but for homebuilt ROV an old fashioned compass is enough.

There are two main solutions using:

  • analog compass
  • digital compass

It is worth to mention that you must be careful installing a compass in the robot.

Camcorder

Camcorder

Second hand video camera Nikon VH-3100 (8mm standard). Cost about 40€. I use it as secondary source of video signal.
It has much better lens than PCB camera and can provide colorfull pictures. Has ability to record movies. Much cheaper than colour CCTV camera with comparable lens.

The bad thing is that it needs more power and sensivity is much worst than PCB cam. Compare 4lux/F1.8 for camcorder to 0.05lux/F1.2 for PCB cam. It has built it microphone, but underwater it is useless.

PCB camera

PCB camera

Cheap PCB B/W camera. Intended as primary vision because of high sensibility. Cost 30€. Color cameras with the same prize has only 1lux sensibility, 20 times worst.

Camera

Camera

Video signal

Most cameras have a standard composite video output signal (described as 1Vpp 75 Ohm). You can directly connect camera output to the TV or a video recorder. This is the best choice, cheap and widely used. Because it uses an analog signal, you need an extra equipment to connect it to a computer, but in most cases it is not necessary – you can use standard video monitor. Video to USB converter costs about 50€ , TV or video PCI cards are even cheaper.

RS485

RS485

RS485 is an alternatve for RS232 for long distances. Maximum length of the cable for RS232 is only 15m. For RS485 it is about 1200m. In addition you can connect many devices to RS485 bus (unlike RS232 -- only 2).

Syndicate content