Monday, January 16, 2012

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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Chuck and Jackie on SV Aria said...

I have a ST4000+; I think that Autohelm also varies ( reduces ) the voltage for minor adjustments and can sense the current (amps) for feedback. Does the Benmar vary the speed as well? Your relays may result in a unpleasant action. Worth a try in any regard.

bob said...

Hi Chuck & Jackie -

I think the ST4000+ and the ST5000+ may actually the same unit... At least in my ST5000+ dealer setup, I can configure it to identify itself as an ST4000+.

I confess that I am a little worried about (what I assume is) PWM modulation of the output power. That is part of the reason for the extended testing before I go for a real install.

I think I have a some choices:
o configure for hard start and hydraulic drive
o get a very fast (solid state) relay that will essentially pass the PWM thru
o take my lumps and/or build some custom circuitry.

I'll try them in roughly this order...

But first I gotta get a new multimeter.


Anonymous said...

I am eagerly awaiting your results. I have a very good benmar sitting here installed. Periodically I have to clean the 'points'. Then it is working great again. The control head seems to be the issue with most of them going T/U. I would like to interface the drive unit with a modern unit as a back up to my windvane system.

bob said...

Anon -

I plan to be doing the initial testing this week, so stay with me...

As far as the points go, their failure is due to a design defect. Replacing the relays will solve that. but hold off on the relay replacement for just a little while - I may be recommending the use of high-speed solid-state relays instead, as a part of this project.


Anonymous said...

I'm in Newport Beach, CA and have converted quite a few BENMAR drives to be used by newer pilots (H-Bridge/Reversing/PWM type) Anyway...too easy. Just have to remove guts of BENMAR controller board inside and isolate the 2 wires from your motor and the 2 for the clutch drive. The clutch is merely a solenoid actuator. 12 volt non polarized when ever you WANT TO use the pilot. Off when you don't. I've used these BENMARs to do sailboats where a linear drive won't fit or isn't rated to drive high load or, merely easiest to incorporate. A Raytheon ST5000 should work as long as you use a "rudder feedback" unit and perform the necessary rudder-time calculation...You want the BENMAR be able to move from Port to Stbd in 10-15 seconds. Any slower and you "chase" yourself. ST5000 had no fast compass (ala "rate" type) so it's a trade off. Faster=less often/less significant rudder corrections
A Robertson AP300 "type" with a J300-40amp control box is what I always used (or built around)....great all around pilot. A diode & relay box could be used to augment power & Left/Right the motor instead, fed from the output of RayST5000. Gotta use a feedback! Way better with! Experimenting is good.

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