[DG] [DG] Autohelm ST7000

[Jasra]

TSDY TOPIC: Autohelm ST7000

After an extended journey from La Rochelle and some 3500nm on the clock, I finally arrived in Portimao, Algarve. The only problem was on leaving Sines the autopilot which had worked without a hitch for many hours on the trip down from England, malfunctioned and the warning signal flashed up on the Control Unit. I disengaged same to Standby and hand steered to Portimao. On stripping down the Rotary Drive Unit, it was apparent that one of the studs inside the electric motor had slackened back and the remaining stud subsequently sheared under the torque.

Easily replaced same and Loctite on all studs. Re-installed the motor in the Drive Unit, ensured cabling was as before and tested steering by hand all turning smoothly. Engaged auto at the Control Unit, momentary chattering then steering went hard over and didn’t appear to want to stop. Disengaged and ran through calibration settings and all as per the book.

Additionally, in auto mode +10 degrees of heading change commanded, then the wheel and rudder/drive moves to port. Similarly, when -10 degrees of heading change commanded, the wheel and rudder/drive moves to starboard. When switched into auto mode and no heading change commanded the wheel and rudder/drive makes small chattering moves and then goes hard over to starboard or port alternatively! I have read the posts on the Raymarine forum and understand that this may indicate that the drive motor phasing is incorrect, however the wiring is unchanged from before and don’t understand why the rudder/drive makes small chattering moves and then goes hard over to starboard or port alternately when no course change is requested. All this whilst securely moored to the jetty.

Would seem the Rotary Drive Unit is getting an erroneous signal from the Control Unit or the Course Computer but unsure how or why. The Rotary Drive Unit had two wires to power the motor and a pair of very small gauge wire to the electronic clutch which disengages on standby and engages on auto. The Control Unit and the Course Computer have not been touched but may have had some residual memory of the motor failing and the Rotary Drive Unit not operating?

Any thoughts gratefully received as unwilling to embark on my next leg to Morocco and the Canaries on hand steering!

[Derek - Raymarine - Moderator]

HI
Thank you for your enquiry,
Which studs in the drive unit failed? the bolts that hold the motor onto the main chassis plate?

It sounds like a cable may have been swapped at some point, however, it should be easily solved.
First, select STANDBY and turn the steering wheel by hand to Starboard and check the rudder angle displayed moves to starboard too, repeat turning the wheel to port and check the rudder angle displayed moves to Port as well. If this happens, then the rudder reference unit is all OK and working correctly.

Next, select AUTO and then check the clutch engages - the steering will feel locked.
Then press 2 x +10 and the wheel should turn to Starboard, if it does not and turns all the way to Port, then MOTOR wires are reversed and simply reversing them where they connect to the computer will fix this. It is possible they were reversed from installation and swapped when commissioning maybe. maybe the wires from the motor were reversed at the drive connection, but in any case, there is no residual memory or anything like that in the computer.

I am sure it is a simple swap has happened at some point and caused this - swap the motor wires as suggested and all should be OK.
Thanks
Derek