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