I think Autohelm, now Raymarine calls it a type 2. It drives a Hynautic ram. It's a pretty universal style as it also works with other manufacturers autopilots. I do know they make a 'linear' actuator as well. We've been very happy with our decision to get one. I purchased a backup set of brushes but took the old ones out after 6 years to check them and they looked new.

Our old Autohelm got so hot in a following sea that after 2 days the solder holding the brushes melted and the brushes fell off. There's nothing like tearing an electric motor apart while at sea and rebuilding it. Did it on the main saloon floor with a ring of towels an plastic boxes to hold the parts fromc getting away. The only way we kept it from melting again was to use it for 2 hours and let it cool for the next two. Thw Accu-steer has been flawless.

http://www.accu-steer.com/