Monday, June 18, 2012

Project ST5000: Finis

The final step in the transition from our old Benmar 14 autopilot to the new ST5000+ was the creation of a place for Ray (yeah, I know it's the obvious name, but still) to live.  The old Benmar was installed in the cockpit coaming, mounted to a decrepitating wood box.  We had gotten used to that position, and in keeping with our desire to not turn Eolian's cockpit into a video game, I decided that the old location would suit.  But the old box had to go.

I bought a sheet of 1/2" UHMW polyethylene and cut the necessary pieces to make a replacement for the wood box.  To make it easier to view the display, I canted the actual autopilot mounting surface back at 20°.  Finally, since nothing sticks to UHMW, I screwed it together using 1" #6 stainless screws.

Ray's new home

Design was a little tricky. The space inside the coaming is not very much larger than the desired size of the new mount.  Thankfully I had the old wood box to use for setting dimensional constraints.  When it was completed, the moment of truth arrived:  Will I be able to get it into the space? 

Yup.  Just.  The 20° taper helped by making the bottom side of the box thinner.
Snug as a bug in a rug
This has been a fun project, but it has gone on for a long time, from inspiration to now.  And now I am finding that there is a gap in my thinking, a gap that used to be filled up with all the various kinds of problem-solving involved with the project.  It's a little unsettling.  But I am confident that another project will soon arise to fill it in.  It's a boat, after all.

Anyone need spare parts for a Benmar 14 autopilot?  I have a bunch...
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10 comments:

Jim N said...

I have been following your autopilot project with great interest. I played with the mating of the 14b drive and a ST4000+ last year but the PWM threw me, great fix with the SSRs. I have ordered some and will continue the project. I don't see much difference with the ST4000+ and the ST5000. Also, any idea on avg amp hours?

bob said...

I think there may be some logic differences between the units. For example, the ST5000+ seems to require the rudder transducer for proper operation, while the ST4000+ apparently gets along nicely without one.

Interestingly, in the dealer setup mode on tghe ST5000+, there is an opportunity to select "ST4000+". I've not tried that. I wonder if the ST4000+ can be told to act as an ST5000+?

I really have no idea on the amp hours. The ST5000+ itself draws so little current as to be negligible. I have a 250 milliamp fuse on mine (remember - it isn't powering the drive). The drive unit can draw up to 15 amps with locked rotor on the motor (measured, actual), but as for amp-hours, well that would depend on the sea state, wind, tidal current, and the tuning of the unit itself. But just for a very rough idea, we seem to draw 5 amps or so when the motor is being actuated, and that happens briefly (0.1 sec?) a couple of times a minute in steady conditions.

bob

Jim N said...

Yes, I guess there will be a lot of variables effecting the draw of the autopilot.

I pulled my 14b from the boat (BTW we are in south Florida on the hard halfway through a full refit) took the unit home to start prodding again. Strangely, despite being the same model 14b drive as yours, the motor has an older 4 wire motor. The same relay driven white and blue wires and two field wires. Black to terminal 3 and red to 4. These will need 12 vdc with a constant polarity. I thought of leaving 12 volts to these field wires when the autopilot is turned on but I don't think this will work for me as field wires draw nearly 2 amps which will start to add up after awhile. I think I will need to find a location for these that will come on with the armature power but have a constant polarity.

bob said...

Jim -

Yes, my documentation mentioned that older drive units were fitted with motors that had field coils instead of permanent magnets.

I think you could turn on the field at the same time as the armature windings with just one more SSR and a pair of diodes to the drive lines - ground the negative side of the new SSR's control, and feed the other side with the cathodes of the two diodes - whichever of the control lines goes positive will feed the SSR. And you'd want another MOV across the field terminals to protect the SSR.

Wouldn't that work? And as you say, it would eliminate the demand for continuous field current.

bob

Jim N said...

Bob,

I was thinking along those lines also. What potential problems do you see with leaving the field neg to -12 VDC and using the diodes off the orange armature wires to get the pos.

Luckily I had ordered 5 SSRs to have a spare, so I have options once the relays arrive.

Jim

bob said...

Jim -

If the diodes were heavy enough to carry the field current (plus surge) then it seems that should work, and it would be way simpler. But then the field current would also be going thru the drive SSR's, so they'd maybe have to beef up a bit or run a little hotter. FWIW, my SSR's are not getting warm at all in the operations I have seen so far.

The field voltage would be reduced by the approx 0.5V forward voltage drop on the diodes... not sure how much effect that would have on the operation of the motor. Probably not too much.

bob

Jim N said...

Bob,

Thanks for the input, I will wait for the bits to arrive and start playing. I will let you know how I make out.

Jim

bob said...

Jim -

Good - given the interest in this project, I'm sure that others will want to get the benefit of your experiences...

bob

Jim N said...

Bob,

Thanks again for the great write up on your autopilot project. I finished mine and could only bench test it until we put in January 2nd. We have been cruising full time since and the system has been rock solid. We are now in the south Bahamas and the warmer temperatures worry me a little, the casing for the motor gets hot in rougher seas. I'm sure it's fine but I think I will mount a computer fan in the bulkhead next to the motor for some air flow.
I still keep an eye out for these 4000+ and 5000+ control heads as they are so versatile. If you want to stop in for a visit we are at svsanibel.com

Fair winds,
Jim

Robert Salnick said...

Hi Jim -

Thanks again for the kind words!

So far mine has never gotten beyond "slightly warm"... but then we don't have swells in Puget Sound, so it is likely that your autopilot is doing more work. Maybe you could reduce sensitivity a little? Also, I do not have a field coil, therefore you have one more SSR than I. The fan sounds like a good idea - and maybe a big aluminum heat sink attached with silicone to the case?

bob

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