Bruce,
Take a look at the website, there are several pictures of the doors and the dome. These are BIG doors.

We can monitor motor current with no problem, we get "35 pulses per inch" from little reed switches in the actuators.If we do use the linear actuators, I have some doubts that we can distinguish the excess current running into an appendage until it REALLY hurts.....There is a very high reduction ratio in these actuator things.

I am proposing to split the system into four little (cheap) PWM poweramps driving the motors at variable speed. The most obvious approach is to start a counter counting at a rate slightly less than flat out for the acuators, lets say we can screw 1 inch in one second, then we increase our internal counter at 30 counts per second. Then there are four software position servos. One of each pair is the master servo.

The master sees an error between its counter and the internal counter and drives the motor at 75% of full speed in such a way as to reduce the error. The slave does the same, except it checks that the difference between the slave and master position is less than 35 counts (one inch of crab) If the error increases, the slave speeds up, if it decreases the slave slows down. If the error increases to more than 35, then there is a problem, and something needs to be switched off quick !

Simple 16 bit arithmetic throughout, its not like I have a very high speed servo or anything.

We COULD use only one motor, but with very much increased mechanical complexity. And I have this 25mm crabbing spec too. How do we correct a crabbing door when it is rigidly attached to essentially a curtain opener ?

There would be nearly 60 metres of drive cables to tension. Then detecting overloads could be done fairly easily by measuring the drive belt tension.

Steve