I have had a reply from the ram manufacturers that does not fill me with confidence that they know what they are talking about

Yes, it would have been good if they said why it won't work, not just that it won't with a condescending comment.
It's true it's a basic circuit, but the switching frequencies are low, so a more elaborate circuit should not be needed.
And all H-bridge circuits can be considered to have a "totem pole" output.

You might try asking them if a "non-basic" (whatever that may be to them) H-bridge circuit would work to drive the unit and, if not, why not.

 

Yes, it would have been good if they said why it won't work, not just that it won't with a condescending comment.
It's true it's a basic circuit, but the switching frequencies are low, so a more elaborate circuit should not be needed.
And all H-bridge circuits can be considered to have a "totem pole" output.

You might try asking them if a "non-basic" (whatever that may be to them) H-bridge circuit would work to drive the unit and, if not, why not.

That is exactly what I already went back to them and asked.

There are some H-Bridge speed controllers on FleaBay that might be suitable but I am struggling to find out exact specifications mainly due to the usual Chinese/English issues and a lack of datasheets.

 

There are some H-Bridge speed controllers on FleaBay that might be suitable

One like this may work.

 

That looks similar to the ones I have been looking at:
https://www.ebay.co.uk/itm/DC-5-12V...027523?hash=item1a5a2caf03:g:XacAAOSwL3Jatnsd
The motor connection side of things is quite clear but I am struggling to work out from the description how the two wire 12v PWM from the Autohelm 3000 would be connected to get it to work. (I did say that I was no electronics guru!)

 

how the two wire 12v PWM from the Autohelm 3000 would be connected to get it to work

It would appear you would connect one control wire to IN1 and the other to IN2.

But it is a PWM controller and I'm not sure it can be operated with an external pulse input in a non-PWM mode. (?)

 

Thank you . That is what I was trying to understand - whether the control input was PWM or the power output was PWM!

So what I am looking for is a H-Bridge that doesn't care that the control input is PWM 12v and just uses the signal to output a straight (non PWM) 12v 20A/30A supply to the ram motor?

 

So what I am looking for is a H-Bridge that doesn't care that the control input is PWM 12v and just uses the signal to output a straight (non PWM) 12v 20A/30A supply to the ram motor?

Yes.
My understanding is the the controller already pulses the power in a PWM fashion, so all you need is an H-bridge circuit that turns on and off from the input signals.

 

Something like this looks like a possibility then?
https://www.ebay.co.uk/itm/High-Pow...092230&hash=item25e8de7290:g:COMAAOSwk~ZaQcn9

 

So I had a reply from the ram manufacturers. Rather condescending again and typical of the sort of attitude of those in the marine industry I find. I think that I may have upset him by suggesting that he needs to go back to his college books! Here is the reply:

Thank you for your email which has been passed for my attention.

You are absolutely correct, please accept my apologies it is a H bridge, I was obviously trying to do too many things at once when discussing it with one of our support team. It is of course made up of 2 totem poles although I suspect this is academic in such a discussion.

Anyway, I will of course take on board your suggestion to review my college books and despite my 30 years experience of autopilots having helped design them for 2 manufacturers and being one of the few people left in the industry around when the ST3000 was designed and built, will in future restrain my comments to areas of my expertise - professionally manufactured equipment rather than speculative custom circuit designs downloaded from internet 'experts'.

In view of this I am sure you will understand if I decline to elaborate why the basic circuit you initially proposed does not work and is nothing more than a simple training aid for the classroom (you should review a typical Raymarine/Simrad/Cetrek H Bridge) or indeed why any external professional amplifier will not work as you propose or discuss the intricacies of back EMF which like PWM is essential to the design of your autopilot.

If you require any further technical information on the drive unit please do not hesitate to contact our support team.

Regards,

Tony Eaton

'technical manager'

 

I assume your boat is using a hydraulic steering system? If it does check up on a RayMarine S1000 auto pilot. It uses a small reversible hydraulic pump that only draws 7A. It splices into the original hydraulic system. I do have the manual on PDF file, you should be able to google it. It is driven by a H bridge very similar to yours.

 

I assume your boat is using a hydraulic steering system?

No - it is a cable steering system - not hydraulic.

 

Sounds to me that when he says
"I decline to elaborate why the basic circuit you initially proposed does not work and is nothing more than a simple training aid for the classroom (you should review a typical Raymarine/Simrad/Cetrek H Bridge) or indeed why any external professional amplifier will not work as you propose or discuss the intricacies of back EMF which like PWM is essential to the design of your autopilot."
it's because it indeed is not his "area of expertise" and he doesn't really know what he's talking about.

 

it's because it indeed is not his "area of expertise" and he doesn't really know what he's talking about.

Which was exactly my first thoughts. In fact his communications after this were even ruder and even more uncustomer friendly. I wrote to the ram manufacturer who had passed my first question on to him and complained about him. I am now in communication with another one of their agents and when I mentioned the first chaps name he commiserated me for having had to deal with him - so he has previous!

The second agent seemed to think that it was a feasible solution but could not guarantee how well it would steer as he had not seen an ST3000 connected to a ram before - but he could see no reason why not to try it. He suggested testing it on an electric motor before committing to buying a ram. It also turns out that he sells the same ram £150 cheaper than the first idiot.

Now I just have to work out the feasibility of the ram installation before going any further.

 

Now I just have to work out the feasibility of the ram installation before going any further.

Sounds like you are making progress.
So let us know how it all turns out.

 

If you get this working as you describe, in order to steer manually, you would have to use a jog function on your autopilot or disconnect the cylinder from the rudder.
This reminds me of a thread with a similar situation.
This uses your original autopilot motor with power assist.
At the end of the other thread, he decided to use idler sprockets mounted on a pivoting plate that would swing one way or the other depending on which side of the chain was pulled tight. If the autopilot motor pulled the slack out of one side, the plate would move, operating a DPDT switch to move the cylinder in that direction until the plate / switch moved to the neutral off position.




I THINK, when you turn the autopilot off, if you swap polarity on the input power, it will act like power steering in manual mode. Due to the wheel taking out the slack instead of the motor.

 

The ram I am considering has a clutch in it. When the clutch is powered on the ram works and when it is powered off the manual steering is able to push the ram back and forth with minimal drag so there is no need to disconnect it from the steering quadrant

At the moment the issue that is stopping me going ahead with this is the fact that the ram needs to be installed so that it never reaches its maximum travel in either direction. This is to ensure that it is never at full extension or retraction and then has excess loads applied beyond its limits by water pressure on the rudder or a gorilla on the steering wheel. Unfortunately the opposite of this is if it never reaches its limit then it can apply hundreds of kilos of force to the steering quadrant when the steering has reached its maximum limit/stops. Modern autopilots program in virtual stops which mean that the rudder and the ram can never reach their maximum limits when under autopilot control and if the ram is installed correctly and can never reach its limits when on manual steering then nothing breaks or goes through the side of the boat! The Autohelm ST3000 does not have the ability to apply virtual stops which could end up being very expensive.

 

the ram needs to be installed so that it never reaches its maximum travel in either direction. This is to ensure that it is never at full extension or retraction and then has excess loads applied beyond its limits by water pressure on the rudder or a gorilla on the steering wheel. Unfortunately the opposite of this is if it never reaches its limit then it can apply hundreds of kilos of force to the steering quadrant when the steering has reached its maximum limit/stops.

Surely external limit switches added to both the ram and the quadrant could overcome this?

 

I guess it might be possible to have the limit switches just cut the power in the Autohelm 3000 wire that tells the ram to go in that direction and then it can still be powered by the opposite direction wire and so be able to move away from the limit back towards centre.
I will have to have a think about how it could work with the manual steering and autopilot ram needing different maximum stop distances. I would need a switch that triggers early but still has plenty of movement in the switch mechanism after it has triggered. It might also be necessary to consider some sort of delay as otherwise the ram might just judder rapidly back and forth when it hits the limit switch.
I would also have to consider that then I am becoming dependent on a limit switch and if it fails the ram could snap off the steering quadrant or worse. Not something you want when offshore!

 

the ram could snap off the steering quadrant or worse.

Can't you attach the ram at a suitable radius on the quadrant such that at maximum ram extension the quadrant is not quite at its maximum position?

 

Can't you attach the ram at a suitable radius on the quadrant such that at maximum ram extension the quadrant is not quite at its maximum position?

Yes but that is a high degree of accuracy to try to achieve while upside down in a cockpit locker. Access is quite poor and also bear in mind that nothing on a boat is square! These rams are powerful things and so just 1 or 2mm out would put huge loads on the aluminium steering quadrant or ram mount. On the other side of the equation the sea is a very powerful force and it could put huge loads on the expensive ram if things were just 1 or 2mm out the other way. Aluminium expands and contracts considerably with temperature too just to add to the problem of accuracy.