Autopilot drive interface

[DArcy]

My autopilot is old, really old. I'd like to replace the autopilot but keep the drive, mostly because it is incredibly robust and I really don't want to replace the quadrant. The quadrant and bevel drive are in very good shape and there is not a lot of room to work in there. The drive I have is a Benmar S Power Unit chain drive. I'm replacing the original motor driver with a much more modern driver, interfaced to an Arduino so I can make the drive interface pretty much whatever I want it to be. Currently the Benmar uses a 0 to 8V reference where 4V is straight, 0V is turn one direction, 8V is turn the opposite direction. I made it reversible with a jumper so 0V can be either port or starboard (just in case I messed up the directions).

The question is, what drive interface do autopilots put out to the drive head? I could build a Pi-Pilot but I'd like to see what commercial options are out there.

 

[dlochner]

I believe the drive units are not smart. My drive (B&G Ram) has 2 wires, the AP computer simply reverses polarity to cause the ram to move in one direction or the other. There is a rudder sensor that provides rudder position info to the AP computer as well as fancy (read expensive) sensor sends pitch, yaw, rotation speed, and magnetic compass data.

Edit: A while back I had a RM Wheel Pilot that used the same basic system, I learned this when the installation manual had the wires backwards. Try to turn to port, the boat would go to starboard. Made for an exciting time in the harbor trying to calibrate the AP computer. Switched the connections and all was good.

 

[jviss]

Are you asking about the rudder reference signal?

 

[DArcy]

Are you asking about the rudder reference signal?

No, the drive unit. I don't currently have a rudder angle sensor.

 

[jviss]

No, the drive unit. I don't currently have a rudder angle sensor.

So, I'm at a loss understanding how this works.

 

[DArcy]

Usually an autopilot has three main units
- control head
- compass
-drive unit

There is often a rudder sensor and there may be a junction box that wires these all together and also could connect to the NMEA bus for your MFD.

What I'm wondering about is the interface between the control head and the drive unit. It may be proprietary for each make, just hoping someone before me has figured this out. Likely two wires that tell the drive unit to turn port or starboard and, I suspect, 1 or 2 wires that enable the clutch. My drive unit uses a 12V signal for the clutch.

 

[jviss]

I know very well how typical autopilots work, I am just confused about what you have said about yours.

For the drive unit, it's typically a 12V clutch control, and plus or minus 12V to the motor to turn it one way or the other.

What are you talking about when you said 4V in the center, 0V at one extreme and 8V at the other, if not the rudder position?

 

[DArcy]

Oh, yes the Benmar drive unit is a bit odd with the 0V, 4V, 8V signals. That's why I was asking what a "typical" drive unit signal is. I can adapt mine to whatever I need since I'm modifying it anyway.

I replaced the driver boards with a Cytron MDC106 13A motor driver (10A motor) that takes a PWM and direction signal. I am using an Arduino to drive the motor controller and, with appropriate voltage dividers, can take whatever signal I can get to control the drive unit. Another nice thing about this setup is I can make a soft start so it isn't jerky motion.

 

[jssailem]

Do not have an answer for you.
Have you tried contacting the company?
Reading the website, it appears to me the design was made for large power boats. Would explain why so beefy in design.

Benmar Marine
25510 Foster Lake Rd.
PO Box 4007
Idyllwild, CA 92549-4007


Phone: (951) 659-8510
Fax: (951) 659-8576
E-Mail: BenmarMarine@aol.com

 

[DArcy]

I did reach out to Benmar by email to verify the interface to my current autopilot but no response.

What I want to do is replace the old Benmar with a more modern autopilot and I didn't see anything appropriate on their site. If I had received a reply I might be more encouraged to work with them.

 

[jviss]

Do you have any documentation for that drive unit? I'm really curious about how that interface works.

 

[DArcy]

I do have the manuals which include schematics although the drive unit is only a block diagram.
The first image is the autopilot schematic, second is the drive unit

 

[jviss]

Yikes! I might look into hot-wiring the drive board to get to the motor +/- and clutch, as conventional drives are interfaced. It looks like they split autopilot logic functions across the two boards. Definitely old technology - like, 1970's/1980's technology - and before there was a "dominant design" for autopilot drive units.

 

[DArcy]

Well, that was what the S Power Unit block diagram was, this is what it is now. I'm using the Arduino to interpret the PHC_OUT signal which is the 0V/4V/8V from the Pilot House Computer. The PHC actually gets the reference voltage from the Power Unit hence the 4V and 8V regulators. The motor driver is an off the shelf Cytron MDC106 which takes PWM and direction signal from the Arduino. J2 on the right is the jumper to switch Port/Starboard.

 

[jviss]

I confess that re-reading this thread I'm still no clear on your goal. In the first post you asked:

what drive interface do autopilots put out to the drive head?

Yet, you don't seem to be pursuing this, instead you're building an Arduino interface to I don't know what!

What are you trying to accomplish?

 

[jssailem]

WOW... It has been 45 years since I played with electronic diagrams. As I remember I needed a magnifying glass and a box of colored pencils so I could make sure all the drawn lines went to the correct connections.

I looked at your revised drawing of the Arduino Nano, which I gather is a self designed board. The manufacture states it accepts different voltages or a specific voltage. Looks like you have 2 voltages being input. 5 volts on the 5V and 8 volts on the VIN. Is one power and the other signal as the 8V is coming from your 8V Enable out?

 

[DArcy]

What are you trying to accomplish?

I'm considering buying a new autopilot that I would like to interface to my existing Benmar drive unit.

I just want to know what input typical Raymarine/Simrad/B&G/etc. drive units take. Is it a simple +V left, 0V center, -V right interface or do they use an NMEA or proprietary serial interface? How hard would it be to adapt the output from a modern autopilot to my old but modified drive unit? As you can see from the above post I'm comfortable with some DIY electronics but I don't want to start reverse engineering an NMEA 2000 interface.

I already have the Arduino working on the 0V/4V/8V input on a breadboard but I'd like to know what options to include on the board for other interfaces before I order the PCB.

 

[DArcy]

Looks like you have 2 voltages being input. 5 volts on the 5V and 8 volts on the VIN. Is one power and the other signal as the 8V is coming from your 8V Enable out?

Since my current systems needs an 8V signal and the Arduino can run off 5V to 12V I'm using the 8V power supply to power the Arduino. I don't want to run straight of ships 12V because there can be nasty spikes on ships power that could damage the Arduino. 5V is output from the Arduino to the limit switches and the direction selector. The Arduino monitors signals back from the switches and direction selector jumper to set the motor drive direction and on/off.

 

[dlochner]

On the APs I’ve owned it has been a simple 2 wire solution. The speeds were constant. The current AP computer monitors turn speed and adjusts the time the AP is active and returns the helm to neutral.

 

[jssailem]

Raymarine uses 2 wires to power a the wheel pilot. It uses a direct power DC motor and a positive/negative switching system to move the rudder port/starboard.