I posted this same post on another thread this morning, but seems that it fits here as well:

Depending on how your climber is driven and geared, running the motors in brake mode can be the easiest and best solution. Your bot slowly lowers, yes, but if done correctly, you can still easily be above the ground at the 5 second mark.

The idea behind brake mode is that the leads of the motors are shorted, and as the robot lowers, it runs the motor in reverse as a generator. You can calculate the rate of falling from the basic power equation:

mgv = EI = E^2/R

where m = mass, g = gravitational field, v = falling speed, E = emf of motor, I = generated current and R = motor resistance.

The emf generated by the motor is proportional to the rotor speed and can be estimated to a very close approximation by realizing that 12 V is generated at the 12 V free speed. The motor resistance can be found by dividing 12 V by the 12 V stall current.

Doing all the math gives the falling speed (I use all MKS units):

v = (mgr^2R)/(G^2k^2n)

where r = radius of drive (winch drum, etc.), G = gear ratio, k = 12 V/motor free speed, n = number of motors

We drove our climber with 2 Falcons on a 40:1 reduction with a 3" diameter drum:

R = 0.044 ohms

k = 0.018 volt seconds/radian

calculated falling speed = 3.4 cm/s

So as long as we lift more than, say, 20 cm off the carpet (7 or 8 inches), we are good. Note that the above assumes no friction, so the calculated value is very conservative. Falling speeds in reality can be slower than the calculated value by quite a bit.

So we just try to make sure that we gear and climb so that brake mode works. We find that easier than a mechanical solution.