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[Klunk]

I have a landing craft that has a ramp door that opens......herein lies a problem. The small geared 280 motor is attached to aspring that in turn is attached to the pulling chain. The door drops by gravity (I think). The motor is controlled by a robbe speed controller that appears to only go forward only.
question: How, with a new speed controller, do I control the length of the door to drop and in reverse to pull up?
Or is there any other way to do it!

[catengineman]

A servo winch like the type used on the sail boats would sort out your raise lower of the ramp

[Umi_Ryuzuki]

If you are dropping the door free fall, then it stops when it hits the ground.

For stopping the motor when lifting the door, the you will want to place a limit switch to
the motor. When the door closes, it hits the switch, or a rod that triggers the switch that
will cut power to the motor.  

A single pole, double throw micro switch should work.
When the door is closed, the motor is disconnected.
Once you drop the door, the switch will be released, and allow
power to the motor. Once you power up the esc, the motor will
wind the chain till the the door reaches the closed postion, triggering
the switch and cutting power between the esc, and the motor.

:)

I just ran a direct servo rod to the base of the door, and had plenty of throw
from a standard servo to open and close the door.

[malcolmfrary]

A trick used by the model rail lot for many years past for a similar job (automatic stop at a dead end, still be able to back out) was to have a diode (say, a 1N4005) across the limit switch contact.  When driving up, the switch stops the motor by breaking thecircuit.  When you drive the other way, there is a path for the current and the motor can drive past the bit where the switch is cutting the current.  Without the diode, once its up, it stays up.

I recall in the "supermodels" series that Mr Tomahawk did the job by replacing the servo pot with a linear one, and operating it with a slider and a couple of pins.  The slider was in the linkage to the ramp being driven by the motor controlled by the servo electronics.  It looked crude, but was actually very subtle and worked like magic was being performed.