Project ST5000: And so it begins - thinking out loud

The old Benmar 14B
(Remember when schematics came with electronics?)

You may remember that a long, long time ago I posted a picture showing the bench testing of a Raymarine ST5000+ autopilot (sadly, that post seems to be gone...).

Well today I start in earnest the process to graft the ST5000+ onto the old Benmar drive.  I am thinking out loud here, while I write this up, so bear with me...

Why am I keeping the old Benmar drive, you might ask?  Well, for three reasons:

• It is built like a brick sh%*#house
• I have a spare
• It is already installed - no small task by any means

Basically, to interface with the drive unit, I need to supply +12V on either of two leads to get the drive to steer to port or starboard and a -12V common, and +/-12V to engage the clutch.

That's it.

Well, there is one slightly tricky part...  As I just mentioned, the Benmar drive unit has two wires for direction control - basically a port and a starboard wire - and a common -12V connection, while the ST5000+ has  just two terminals, which reverse polarity for port/starboard.  Since relay coils don't care which way the current is flowing thru them, a pair of diodes will take care of interfacing this, presuming that the -12V directional control common on the drive unit is isolated.

As far as current requirements that need to be met by the ST5000+, everything in the drive unit is driven by relays, so the ST5000+ (which was designed to feed small hydraulic or linear drive motors directly) won't be taxed at all.  For example, the ST5000+ can supply 1-2 amps on the clutch line, but the clutch relay in the Benmar only needs 100 mA.  So the ST5000+ will be coasting.

The larger issues are these:

• I want to make the initial installation a "trial" install.  Just in case something doesn't work out.
• I need to find a place near the roll/yaw/pitch center of the boat to mount the compass.  But unfortunately, that is very close to the engine.  And guess what - you're not supposed to mount the compass near any large masses of iron.  I guess I'll be trying compass locations out...
• Running the wires will not be easy at all.  Especially considering that I will want to keep the old wiring in place, at least for a while, so I can't use the old wiring as messengers to pull in the new.
• Mounting the rudder reference transducer will not be fun, hanging headfirst into the compartment beneath the aft berth.  Neither will be stringing wire to it.

So today, I started by making current measurements.  Well I tried to make current measurements.  Sadly, it seems that my multimeter no longer has a functioning current circuit (no, its not the fuse).  The numbers I quoted above are based on the component values I found on the schematic.

So stick with me here while I try to figure out my next moves...

• Get a new multimeter and measure current flows for real.  Check to see if the -12V common lead for directional control is isolated.
• Trial install #1:
• Temporarily hook up the ST5000 to the drive unit, with the compass but without the rudder reference.
• First, ensure that engaging the autopilot causes the clutch to engage.
• Next, have the autopilot request a course correction - the drive should operate.
• This trial can be done in the slip, with short wires.
• Trial install #2:
• Basically a parallel install with the existing Benmar controller.  This will require a lot more wire to get the ST5000 into the cockpit, where I can actually use it on the water.  At this point, I will also have to have the rudder reference installed.
• Final install:  
• Remove the Benmar controller.  
• Make a proper mount for the ST5000 in the cockpit.
• Locate the optimum compass location
• Clean up the wiring.

At least that's the way it looks from here, right now.

{Time passes}

I just went over the schematic yet again...  Unfortunately, I have now convinced myself that the Benmar drive unit has the common -12V lead on the directional control tied to the negative buss inside the drive.  I can easily lift this, but it will require opening up the drive.  Maybe Trial #1 should take advantage of that spare drive...  That would have the further advantage of the trial being completely a bench test, with no alteration whatsoever to the existing autopilot setup.

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