Saturday, February 18, 2012

Auto Pilot - Part 1

It's been a while since I've posted, but I've been working on so many different things, and none of them were close enough to completion to warrant a posting. That is about to change.  One of my projects this year is to install an autopilot, so we'll start with that. Five main components make up the system:

1 - The HMI, or human machine interface. Some autopilots are configured with a separate control device, and others use the capability of the multifunction display. Since I installed a Simrad NSE8 with autopilot capability last year, I have nothing further to do for this element of the system. The remaining components will simply plug in to the Simnet network, and the MFD will allow me to control the autopilot.

2 - Rudder Feedback Unit. This device is mechanically connected to the rudder(s), and provides a signal to the autopilot computer, indicating rudder angle. This is one of the inputs used by the computer to calculate steering corrections.

3 - Rate Compass - This device provides instantaneous input to the computer concerning the change in heading of the vessel. The computer uses this data, in conjunction with the rudder feedback signal, to determine the amount of steering correction required from moment to moment. It also facilitates the overlay of radar data on the display.

4 - Autopilot Pump - This is an electrically driven pump that actually creates hydraulic pressure in the steering lines, thereby actuating the steering cylinder. It is connected in parallel with the helm pump, and takes over the steering duties when the autopilot is engaged. The pump is driven by the autopilot computer, below.

5 - Autopilot Computer - This is the "black box" of the system, and contains the actual autopilot computer, as well as the drive electronics for the pump. Based on the data input to the computer, it generates and outputs a drive signal to the pump, thereby steering the vessel.

Today's post will detail the installation of the rudder feedback unit. This device must be rigidly mounted to the structure of the vessel, and then the input arm of the device must be connected to the tiller arm of one of the rudders. So, I first had to fabricate a mounting bracket for the base of the unit. I used 1/2" Delrin to make the bracket components:

The last pic shows the rudder feedback unit mounted to the bracket. Next, I had to machine the tiller arm to accept a mount for the linkage to the feedback unit. I used 1/2" solid brass, and machined threads in each end (1/4"-20 to mount it to the tiller arm, and 10-32 to accept the linkage pivot ball). Once the mount was fabricated, and the tiller arm machined, I had this:

 Lastly, I removed the rudder shelf from the boat, and machined it to accept the bracket. Then, everything was reinstalled back in the boat. If you remove the rudder shelf on your boat, be sure to reseal any screws that penetrate the stringers, or any other hull structure - you don't want to allow any water intrusion into the structure. The completed rudder feedback unit install looks like this:

The cable was routed into the boats wiring harness, for a "factory" look, and secured to the hull every 8 inches, to prevent chaffing, and for a rugged and seaworthy installation. I will continue adding autopilot posts as each subsequent component is installed.

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