Power Management & Thruster Control

Take a look at the system-level block diagram of the whole Microship power system.

There is an H-bridge controller for the Minn-Kota steering motor. The thruster is deployed via a pivoting black box, into which indexes the heavy fiberglass tube that supports the motor itself and carries its power cables. The goal here is to allow manual or automatic control of thruster angle, calling, of course, for position feedback. Tim Nolan, who designed the whole power-control system, solved this neatly by gluing a couple of magnets into the nylon gear train of the steering assembly, using Hall-effect sensors to let the processor keep track of position and re-center the thrust angle if a crash ever knocks them out of synch.

The diagram shows Hall-effect current sensors (Microswitch) taking all the measurements; we had problems with noise, power requirements, and start-up delays when rapidly cycling to reduce overall power dissipation. In their place are now four shunts from Blue Sea Systems, allowing high-side current measurement with virtually no loss. Blue Sea, by the way, has an excellent product line of marine power distribution hardware... we use their terminal strips, breakers, and fuse blocks exclusively.

A detailed operational description of this subsystem is coming, but basically, the unit performs the tricky task of coming up with an analog value that's passed to the PWM controller driving the Minn-Kota thruster, defining its maximum thrust at any instant as a function of the amount of power left over after the battery and all other system loads are satisfied. This is a bit more complex than it sounds, as there's no way to simply measure how much solar power might be available without actually taking it. As a result, it's a sort of peak-following algorithm, in which the thruster is brought up to speed until the whole system reaches equilibrium, and then tracked around that point. The point, of course, is to allow the relatively power-hungry electric thruster to consume all available leftover power without running down the battery, which it could do in a couple of hours at full load.

Naturally, there's an emergency override mode: if we're on a collision course with a freighter, the future status of the battery is of little interest!

Back to Microshipnet drawing...