Seafox ROV
In this section we will discuss in more detail the specific pieces of hardware that make up this project and how they are interconnected. Figure 1 shows how each part of the system is connected. Microcontroller 1 communicates to the joystick control pad and to microcontroller 2 via the tether. Onboard the ROV are microcontroller 2, switching electronics and the thrusters.
Figure 1, System connection
Joysticks
As mentioned in the introduction
here, the ROV is controlled by
means of a Sony® PlayStation® DUALSHOCK®2
joystick control pad. In order to get the control pad to operate, a little
bit of 'reverse engineering' was needed to understand how the
PlayStation®2 console communicated with the control pad and vice versa.
Once the transmission protocol was understood, a program was written for
microcontroller 1 that attempted to emulate the console.
Microcontroller 1
The microcontrollers used for this project are PIC16F877s, they are manufactured by Microchip®. These PICs as they are commonly called, are 8-bit microcontrollers that come in a variety of packaging. The ones used here are 40-pin dual inline package types (DIP). The function of microcontroller 1 is to communicate with the control pad, perform operations on the data and then send the data through the tether to microcontroller 2. The data is sent to microcontroller 2 according to the RS-485 serial communications protocol. This protocol can span distances of up to 1200m (4000ft). A pin diagram of the PIC is shown to the right.
Tether
The tether provides the physical link, power and communications to the ROV. The tether used for this project has two conductors for 12V and ground as well as two conductors for communication to the ROV. At this stage there is no communication from the ROV to the microcontroller. The tether is 10m (33ft) long and has an outside diameter of 5mm (0.2in).
Microcontroller 2
Microcontroller 2 receives the data from microcontroller 1 and operates the thrusters. It configures the order in which the thrusters will operate so that only one thruster is on at any given time according to the project description here. The speed of the thrusters and hence the amount of thrust they produce is also be controlled by the microcontroller by a method known as pulse width modulation (PWM).
Switching electronics
The switching electronics consist of 6 separate power MOSFETS that are used to switch the 6 thrusters on and off. They are manufactured by STMicroelectronics. These MOSFETs operate much like a relay, in that a small voltage from the microcontroller is used to switch a larger voltage. Because these MOSFETs can be switched on and off at quite high frequencies, if we vary the ratio of time that they are switched on compared to being switched off, we can control the average voltage 'seen' by the thrusters. That is, if the switching frequency is say 1000Hz, corresponding to a time period of 1ms, and the thrusters are on for half this time, 0.5ms and off for the other 0.5ms, a duty cycle of 50%, then the voltage 'seen' by the thrusters is half that of the supply voltage. Although the picture shows an MTP3055V made by Fairchild, the ones used for this project are MTP3055E's however they look identical.
Thrusters
The
thrusters used for the Seafox are
rule® 1890LPH (500GPH) 12V submersible
bilge pumps. They draw up to 1.8A @ 12V and can provide up to 1.6N
(0.36lb) of thrust.
All trademarks remain the property of their respective owners
Site created by Curtis Schur, last updated 27/10/04